EFFECTS OF COMBUSTION TEMPERATURE AND TIME ON
THE PHYSICAL AND CHEMICAL PROPERTIES OF RICE
HUSK ASH AND ITS APPLICATION AS EXTENDER IN PAINTS
BY
IGWEBIKE-OSSI, CLEMENTINA DILIM
REG. NO: PG/Ph.D/02/33269
A RESEARCH WORK PRESENTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE AWARD OF
THE DEGREE OF DOCTOR OF PHILOSOPHY (Ph.D) IN
INDUSTRIAL CHEMISTRY (POLYMER CHEMISTRY) IN THE
DEPARTMENT OF PURE AND INDUSTRIAL CHEMISTRY
FACULTY OF PHYSICAL SCIENCES
UNIVERSITY OF NIGERIA NSUKKA
AUGUST, 2011
1
APPROVAL PAGE
This research project has been approved for the Department of Pure and
Industrial Chemistry, Faculty of Physical Sciences, University of Nigeria,
Nsukka
BY
.………………….
DR P.A OBUASI
Head Of Department
………………………
PROF. J.A. IBEMESI
Project Supervisor
DATE
DATE
…………………………..
PROF. O. OGBOBE
External examiner
DATE
2
DEDICATION
This research project is dedicated to my Love, my Joy, my Inspiration, my
Strength and the Essence of my life- the Lord God Almighty, for always being
there for me.
“The impossibilities of today will be the reality of tomorrow, yet the reality of tomorrow CAN remain the
impossibilities of today, if they remain untried. ”
~ Andreas Lara
“Success doesn’t mean the absence of failures; it means the attainment of ultimate objectives. It means
winning the war, not every battle. ”
~ Edward Bliss
“Courage doesn’t always roar - sometimes courage is the quiet voice at the end of the day saying, ‘ I will try
again tomorrow.’ ”
~ Unknown
3
CERTIFICATION
IGWEBIKE-OSSI CLEMENTINA DILIM, a postgraduate student in
the Department of Pure and Industrial Chemistry with Registration
Number,
PG/PhD/02/33269,
has
satisfactorily
completed
the
requirements of research work for the award of the degree of Doctor of
Philosophy (Ph.D) in Industrial Chemistry (Polymer Chemistry option).
The work embodied in this project has not been submitted in part or
whole for any other degree of this or any other university.
…………………
……………………
DR. P.A OBUASI
PROF. J.A IBEMESI
Head of Department
Project Supervisor
Date
Date
ACKNOWLEDGEMENT
4
The success of this work was achieved not only by my unwavering focus, passionate zeal
and dogged determination, but also by the invaluable contributions of persons and
organizations that rendered assistance to me in one way or the other.
My profound gratitude goes first and foremost, to the Almighty God, who in the midst of
the quietude, solitude and serenity of Nature, inspired this work and also saw me through
challenging moments to its ultimate success.
I am immensely grateful to my erudite and academically-distinguished supervisor,
Professor John Akolisa Ibemesi, whose innate meticulousness and intensive academic
tutelage, particularly during my master’s programme, equipped me with the necessary
research skills that has launched me into a world of independent academic research; who
articulately guided this work and painstakingly yet expeditiously, read through my
voluminous Ph.D thesis, constructively and articulately making suggestions and corrections
where necessary.
I am appreciative of the motivational role of my recently- turned -octogenarian father, Sir
Clement Igwebike, a retired Senior Meteorologist, a lover of education and a firm believer
in women empowerment and emancipation.
My appreciation also go to my friends, well-wishers and close associates who were morally
supportive in the course of this work, prominent amongst whom are Prof. Nicholas
Akwanya, former Dean of PG school, UNN, Prof. I. Onyido of UNIZIK, Prof. C.O
Esimone of UNIZIK, Prof. C.O. Okafor of UNN, Prof. O. Ekpe of EBSU, Prof. E.I
Obiajunwa of CERD, OAU, Prof. Osisiogu of EBSU, Prof. F.E Okieimen of UNIBEN,
Prof. O. Ogbu of EBSU, Prof. S. Elom of EBSU, Prof. C. Diribe of EBSU, Engr. Dr. Idenyi
of EBSU, Dr. U.C Okoro of UNN, Rev. Fr. Dr. Odina, Rev. Fr. Dr. Obuna and His
Lordship, the Bishop of Abakaliki Diocese, Dr. Michael Okoro.
I acknowledge with a feeling of indebtedness, the indispensable role of the following
establishments by way of giving me unrestrained access to their research facilities The Biotechnology Research Lab., Ebonyi State University, where the combustion of the
rice husks was carried out using the furnace and oven installed therein; Centre for Energy
Research and Development (CERD), OAU, Ife, where the PIXE analysis was carried out;
Chemical and Allied Products (CAP) Plc, Lagos, where the formulation, production and
qualitative evaluation of the paint products were carried out. I am particularly grateful to
the Managing Director of CAP Plc., Omolara Elemide (Mrs) for affording me the
5
wonderful opportunity of using the Paint Research and Development facilities for the most
crucial aspect of this research. I am also appreciative of the laboratory staff, who, not only
rendered technical assistance, but also provided a congenial working atmosphere for me,
amongst whom are Charles, Busayo, Sandra, Lawrence, Lilian, Dapo, Niyi, Ajibade and
Gbenga. I also acknowledge the contribution of some management staff namely Kemi,
Ogunmadewa and Yemitan.
Thanks must be accorded to the Head, Department of Industrial Chemistry, Ebonyi State
University, Dr. Frank Nwabue for his kind consideration in reducing my academic
workload so as to enable me devote more time and attention to this work and consequently
to expedite action on it. I am equally appreciative of the kind gestures and administrative
roles of Dr. P.O Ukoha and Dr. P.A Obuasi the immediate past and present Heads,
respectively, of Dept. of Pure and Industrial Chemistry, UNN towards the completion of
this program.
Finally, I am thankful for the love, patience and understanding of my family members, who
had to endure periods of my absence and absent-mindedness occasioned by research
pursuits and preoccupation, particularly my daughter, Yvonne who ‘pestered’ me to finish
the work as quickly as possible. To my mum and siblings, I owe gratitude for their constant
encouragement throughout this work.
IGWEBIKE- OSSI, CLEMENTINA, DILIM
Abstract
Rice husk ash (RHA) samples were obtained by controlled incineration of rice husks (RH)
in a muffle furnace at varied combustion temperatures of 400 to 10000C and combustion
duration of one to six hours. The ash samples were withdrawn from the furnace at hourly
6
intervals and the effects of combustion temperature and time on the colour, surface
morphology and yield of the RHA samples were determined. The ash samples were
subjected to elemental compositional analysis using the relatively new, and highly sensitive
analytical technique, Particle Induced X–ray Emission (PIXE) spectrometry from which the
silica and metallic oxide concentrations of the ash samples were obtained. The granular
RHA was reduced to finer particle size and the RHA flour thus obtained, was used to
formulate the paint products.
The RHA yields obtained at the specified incineration conditions were found to be in the
range of 19 to 24%. Plots of ash yields as a function of temperature, indicated that lower
combustion temperatures (400 to 6000C) gave higher ash yields than higher temperatures
(7000C to 10000C). The ash yields, however did not vary significantly with prolonged
combustion time (5 – 6hrs) at both lower and higher temperatures, showing a greater
dependence of higher ash yields on the temperature of incineration than on the duration.
The colour of the ash, which is of relevance in paint formulation, was affected by the
temperature and duration of the combustion process as black and brown ash samples were
obtained at 4000C while ‘white’ ash was obtained from 5000C (3rd hour) to 8000C.The RHA
colour at high temperatures (9000C and 10000C) was found to be largely dependent on the
heating rate, as low heating rates of 15.520C/min and 9.800C/min produced homogeneously
white ash at 9000C and 10000C, respectively, while higher heating rates of 320C/ min and
270C/ min at same incineration temperatures, gave predominantly grey ash due to high
combustion inefficiency resulting in a high residual carbon. The morphological study of the
surface of the magnified, pyrolized RHA particles, revealed numerous button-like
protuberances (bumps) interspaced with craters (pores) at all the combustion temperatures
and time. Shrinkage of the pyrolyzed RHA particles along the transverse section was also
observed for all the combustion conditions
The silica (SiO2) content of the ash samples, which is of significant importance in paint
formulation, was found to be in the range of 68-91%. The results indicated lower silica
contents at lower incineration temperatures (400 to 6000C) and at shorter combustion time
of 1 to 4hrs while higher silica contents were obtained at increased combustion time of 5 to
6hrs and at higher temperatures of 800 and 9000C.Above 7000C, combustion duration had
little or no effect on silica yield.
A total of fifteen elements (expressed as oxides) were detected in the RHA samples – Na,
Ma, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Zn, Sr and Rb using the PIXE technique. Similar
elements were detected in the virgin rice husks except for Sr and Rb. The presence of
7
elements like Ti, Sr, and Rb, which had not been previously reported, is probably due to the
higher sensitivity of the PIXE technique to trace elements than other spectroscopic
techniques.
The applicability of RHA flour as paint extender was investigated by incorporating it in the
formulation of five different paint products, namely, emulsion, textured finish, red oxide
primer, matt wood finish and cellulose matt finish. The RHA- based paints were
qualitatively evaluated in comparison with those produced with standard commercial fillers
(calcium carbonate, silica flour and fumed silica) in relevant attributes such as thickening
effect, bulk increase effect, flatting effect, stability and settling resistance. In emulsion and
textured finish, RHA displayed a remarkably superior thickening effect, far surpassing that
of calcium carbonate (the standard) and silica flour, of same particle size range while kaolin
ranked a close second. RHA however, ranked second to calcium carbonate in bulk (volume)
increase effect while silica flour showed no appreciable increase in this regard. The RHAbased emulsion paints showed better resistance to settling than the other two.
In red oxide primer, RHA displayed excellent flatting (matt) effect superior to that of
calcium carbonate (the standard) and silica flour. This superiority, which was visually
evident on the dry film, was substantiated by gloss measurements, in which the RHA-based
primer had the lowest gloss value (highest flatting effect) of the three. RHA, similarly
exhibited good flatting properties in matt wood finish and cellulose matt finish as it
outperformed silica flour in this attribute and ranked second to fumed silica (the standard),
due to the finer particle size of the latter, which gave it an advantage. The RHA-based
paints as well as silica flour, calcium carbonate and fumed silica, showed good stability
after nine months of stability monitoring.
TABLE OF CONTENTS
PAGE
8
TITLE PAGE
i
APPROVAL PAGE
ii
DEDICATION
iii
CERTIFICATION
iv
ACKNOWLEDGEMENT
v
ABSTRACT
vii
LIST OF TABLES
viii
LIST OF FIGURES
ix
CHAPTER ONE
1.0 INTRODUCTION
1
1.1 Environmental Issues
2
1.1.1 Abatement Measures for Global Warming
and Environmental Pollution
6
1.2 Rice Husk, a Vast Renewable Bio-resource
for Sustainable Industrial Development
8
1.2.1 Lignocellulosic Materials
10
1.2.1.1 Components of Lignocellulosic Materials
11
1.3. Global Rice Production
16
1.3.1 Rice Production in Sub-Saharan Africa,
Nigeria and Ebonyi State
17
1.4 The Chemical Nature of Silicon, Silica and Silicates
19
1.4.1 Silica flour
24
1.5 Particle Induced X-Ray Emission (PIXE) Spectrometry
25
9
1.5.1 Development of PIXE Analytical Technique
27
1.5.2 X-Ray Emission Theory
27
1.5.3 Ion Beam Analysis (IBA) Principles
28
1.5.4 Ion Beam Analysis Methods
28
1.5.5 PIXE Analysis
29
1.6 Paint Technology
29
1.6.1 Classification of paints
31
1.6.2 Composition of Paints
32
1.6.2.1 Binder (Resin)
33
1.6.2.2 Pigments
33
1.6.2.3 Solvents
35
1.6.2.4 Solvency
36
1.6.2.5 Pigment Extenders
38
1.6.2.5.1 Calcium Carbonate (Chalk)
39
1.6.2.5.2 Aluminium Silicates (Clays)
40
1.6.2.5.3 Barium Sulphate
41
1.6.2.5.4 Hydrated Magnesium Silicates (Talcs)
41
1.6.2.5.5 Silicas
41
1.6.2.5.6 Calcium Sulphate
42
1.6.2.6 Paint Additives
43
1.6.2.6.1 Driers
44
1.6.2.6.2 Antioxidants
46
1.6.2.6.3 Catalysts, Activators and Accelerators
46
1.6.2.6.4 Thickeners
47
1.6.2.6.5 Flow Aids
48
1.6.2.6.6 Flatting Agents
49
1.6.2.6.7 Fungicides and Bactericides
49
1.6.2.6.8 Dispersion Aids
49
1.6.3 Paint formulation
50
1.6.4 Emulsion Paints
52
1.6.5 Paint Defects
53
1.7 Research design
55
CHAPTER TWO
2.0 LITERATURE REVIEW
10
2.1 The Chemical Nature and Composition of Rice Husks
57
2.2 General uses of Rice Husks
61
2.3 Composition of Rice Husk Ash
63
2.4 Amorphous and Crystalline Forms of RHA Silica
67
2.5 Factors That Influence Rice Husk Ash Properties
68
2.6 Determination of Amorphous Silica in RHA
72
2.7 Thermal Degradation of Rice Husk and its Effect
on Rice Husk Particle Shape, Structure and
Surface Morphology
73
2.8 Effect of Chemical Treatment of Rice Husk
and Rice Husk Ash
77
2.9 Effect of Combustion Temperature and Time
on Chemical Composition of Rice Husk Ash
79
2.10 Pyrolysis Products of Rice Husk
81
2.11 Effect of Combustion Temperature and Duration
82
on Rice Husk Ash Yield
2.12 Effect of Combustion Temperature and Duration
83
on Rice Husk Ash Colour
2.13 Research–based Applications of Rice Husk Ash
84
2.14 Potential uses of Rice Husk Ash
92
2.15. Justification for study
94
11
2.16 General Objectives of Study
97
2.16.1.Specific Objectives
98
CHAPTER THREE
3.0 MATERIALS AND METHODS
3.1 MATERIALS
3.1.1 Materials for Production of Emulsion
and Textured Finishes
99
3.1.2 Materials for Production of Red Oxide Primer
100
3.1.3 Materials for Production of Matt Wood Finish and
Cellulose Matt Finish
100
3.1.4 Equipment for Combustion Studies on Rice Husks
100
3.1.5 Equipment for Determination of RHA Colour
and Surface Morphology
101
3.1.6 Equipment for Determination of Elemental Composition
101
of Rice Husk Ash
3.1.7 Equipment for Production and Testing of Paint Samples
102
3.2 Determination of Rice Husk Particle Size
102
3.3 Combustion Studies of Rice Husks
102
3.3.2 Combustion of Rice Husks in a Muffle Furnace
103
3.4 Determination of Heating Rate of RH
Combustion Process
103
3.5 Determination of Volatile content of Rice Husks
104
3.6 Determination of RHA Colour
104
3.6.1 Bulk Sample Photograph
104
3.6.2 Colour Scale
104
3.7 Determination of Surface Morphology
of Rice Husk Ash Particles
105
12
3.7.1 RHA Particle Magnification
105
3.7.2 RHA Cluster Magnification
105
3.8 Effect of Heating Rate on RHA Colour
105
3.9 Large-Scale Production of RHA
105
3.10 Preparation of RHA Flour
106
3.11 Separation of RHA Flour into Particle Sizes
106
3.12 Determination of Specific Gravity of RHA
106
3.13 Determination of the Elemental Composition
107 of RHA using PIXE Analytical Technique
3.13.1 Sample preparation
107
3.13.2 Ion Beam Analysis (IBA) Set-Up
107
3.13.3 The End Station
107
3.13.4 The Detector
108
3.14 Conversion of Concentration of Elements (in ppm)
108
to their oxides Expressed in Percentage %
3.15 Calculation of conversion factors
111
3.16 RHA AS EXTENDER IN EMULSION PAINTS
112
3.16.1 Production of Emulsion Paints Using RHA Flour
as Extender in Comparison with Silica flour,
Kaolin and Calcium Carbonate (standard)
114
3.16.2 Formulation of Blue and Green Emulsion
Paint Variants
115
3.16.3 Determination of In-Can Appearance of Emulsion
Paint Products
3.16.4 Determination of pH of Emulsion Paint Products
115
115
3.16.5 Determination of weight per litre (specific gravity)
116
13
of Emulsion Paint Products
3.16.6 Determination of Viscosity of Formulated Emulsion Paints
116
3.16.7 Determination of Opacity of Emulsion Paints.
116
3.16.8 Determination of Washability (Scrub resistance)
of Emulsion Paints
116
3.16.9 Calculation of Pigment Volume Concentration (PVC)
of Emulsion Paints
117
3.16.10 Application of Emulsion Paint Products
117
3.17 RHA AS EXTENDER IN TEXTURED FINISHES
3.17.1 Production of Textured Finishes using RHA Flour
as Extender in Comparison with Silica Flour
and Calcium Carbonate (standard)
117
117
3.17.2 Formulation of Deep Pink, Light Purple and
Pale Ivory Textured Finish Variants
119
3.17.3 In-Can Appearance of Textured Finishes
120
3.17.4 Determination of pH of Textured Finishes
120
3.17.5 Determination of weight per litre (specific gravity)
of Textured Finishes
120
3.17.6 Determination of Gel strength (viscosity)
of Textured Paints
120
3.17.7 Application of Textured Finishes
121
3.18 RHA AS EXTENDER IN RED OXIDE PRIMER
121
3.18.1 Production of Red Oxide Primer using RHA Flour
New Filler in Comparison with Silica Flour
and Calcium Carbonate (Standard)
121
3.18.2 Thinning of Red Oxide Primer
123
3.18.3 Wt per litre Determination
123
3.18.4 Determination of Film Appearance
123
3.18.5 Determination of Drying Time of Red Oxide Primers
123
3.18.6 Determination of Matt Effect of RHA
in Red Oxide Primer
123
3.19 RHA AS EXTENDER IN MATT WOOD FINISH
124
14
as
3.19.1 Production of Matt wood finish using RHA Flour
as Extender in Comparison with Silica Flour and
Fumed Silica (Standard )
124
3.19.2 Thinning of Matt Wood Finish
125
3.19.3 Determination of Wt per litre of Matt Wood Finishes
125
3.19.4 Determination of Dry Film Properties of Matt Wood Finish
126
3.19.5 Determination of Efflux Time of Matt Wood Finish
126
3.19.6 Determination of Flatting (Matt) Effect of wood Finishes
Produced Using fumed Silica (standard), Silica flour
and RHA
126
3.19.7 Determination of Gloss and Matt Values
of Matt wood Finish
126
3.19.8 Determination of Settling Resistance of Matt wood finishes
127
3.19.9 Determination of Drying Time of Matt Wood Finish
127
3.20 RHA AS EXTENDER IN CELLULOSE MATT FINISH
127
3.20.1Production of Cellulose Matt Finish using RHA flour
as New Extender in Comparison with Silica Flour
and Fumed Silica (Standard)
129
3.20.2 Thinning of Cellulose Matt Finish
129
3.20.3 Determination of Wt per litre of Cellulose Matt Finishes
129
3.20.4 Determination of Viscosity (Efflux Time)
of Cellulose Matt Finishes
129
3.20.5 Determination of Flatting (Matt) Effect of Cellulose
Matt Finishes
129
3.20.6 Determination of Dry Film Appearance
129
CHAPTER FOUR
4.0 RESULTS AND DISCUSSION
15
4.1 Combustion of Rice Husks in a Muffle Furnace
130
4.2 Particle Sizes of Milled Rice Husks
130
4.3 Heating Rates of RH Combustion Process
131
4.4 Determination of Volatile Content of Rice Husks
131
4.5 Specific Gravity of RH, RHA ,RHA Flour and Commercial Fillers
131
4.6 Effects of Combustion Temperatures and Time on
Ash Yields of Rice Husks
132
4.7 Effect of Combustion Temperature and Time on
RHA colour
135
4.7.1 Photographs of RHA Samples obtained at Different
Combustion Conditions Showing their Colour
139
4.7.2 Effect of Reduced Heating Rate on Colour of RHA
Samples Obtained at 900oC and 10000C
146
4.8 Effect of Combustion on Surface Morphology
of RHA Particles
147
4.8.1 Photographs of Magnified RH and RHA Particles showing their
Surface Morphology at Different Combustion Conditions
150
4.9 Effect of Combustion Temperature and Time on the Elemental
Composition of RHA
165
4.9.1 Elemental Composition of RHA at Different Combustion
Temperatures and Time Using the PIXE Technique
165
4.9.2 Effect of Combustion Temperature and Time on
Silica Content of RHA
166
4.9.3 Effect of Combustion Conditions on Metallic Oxides
Components of RHA
168
4.9.3.1 Sodium Oxide Content of RHA at Different Combustion
Conditions
169
4.9.3.2 Potassium Oxide Content of RHA at Different Combustion
Conditions
169
4.9.3.3 Magnesium Oxide Content of RHA at Different Combustion
Conditions
171
16
4.9.3.4 Calcium Oxide Content of RHA at Different Combustion
Conditions
173
4.9.3.5 Aluminium Oxide Content of RHA at Different Combustion
Conditions
174
4.9.3.6 Iron (III) Oxide Content of RHA at Different Combustion
Conditions
176
4.9.3.7 Loss on Ignition (LOI) of RHA at Different Combustion
Conditions
177
4.10 Comparison of Properties of Emulsion Paints Produced
Using RHA Flour, Silica flour, Kaolin and Calcium
Carbonate (standard) as Extenders
179
4.10.1 Extender Level and Particle Size
4.10.2 In-Can Appearance
179
180
4.10.3 Effect of Extender Colour on Dry Film Appearance
180
4.10.4 Pigment Volume Concentration (PVC)
181
4.10.5 Thickening Effect
184
4.10.6 Wt per litre Values
188
4.10.7 Bulk Increase Effect
190
4.10.8 pH of Emulsion Paints
192
4.10.9 Opacity of Emulsion Paints
193
4.10.10 Settling Resistance of Emulsion Paints
193
4.10.11 Scrub Resistance
193
4.10.12 Stability Monitoring
194
4.10.13 Dry Film Sheen
194
4.11 Comparison of Properties of Textured Finishes Produced
using RHA Flour, calcium carbonate (standard)
and silica flour as Extenders
194
4.11.1 In-can Appearance
4.11.2 pH
194
195
4.11.3 Wt Per Litre (S.G)
195
4.11.4 Thickening Effect of the Extenders
195
4.11.5 Dry Film Appearance of Textured Finishes
195
4.11.6 Opacity
196
4.11.7 Drying Time
196
17
4.12
Comparison of Properties of Red Oxide Primers Produced using
Calcium Carbonate (standard),Silica Flour and RHA
as Extenders and Flatting Agents
198
4.12.1 In-can Appearance
198
4.12.2 Viscosity (Efflux Time)
198
4.12.3 Wt per Litre
199
4.12.4 Dispersion Level
200
4.12.5 Drying Time
200
4.12.6 Flatting (matt) Effect of Red Oxide Primers
200
4.13 Comparison of Properties of Matt Wood Finishes Produced using
Fumed Silica (Standard),Silica Flour and RHA as
Extenders and Flatting Agents
201
4.13.1 In -can Appearance
4.13.2 Viscosity (Efflux Time)
201
4. 13.3 Wt per Litre Values
202
4.13.4 Drying Time
202
4.13.5 Flatting (Matt) Effect of Wood Finishes
202
4.13.6 Settling
204
4.14 Comparison of Properties of Cellulose Matt Finishes
(CMF) Produced Using RHA, Fumed Silica ( standard)
and Silica Flour as Flatting Agents
205
4.14.1 In-Can Appearance
205
4.14.2 Wt per Litre
205
4.14.3 Viscosity Efflux Time
205
4. 14.4 Dispersion Level
206
4.14.5 Flatting (Matt) Effect of Cellulose Matt Finishes
206
4.14.6 Film Appearance
208
4.14.7 Drying Time
208
4.14.8 Stability
208
CONCLUSION
208
REFERENCES
209
18
APPENDIX
LIST OF TABLES
TABLE
TITLE
Table 1.1
Typical Composition of Various
LignocellulosicMaterials
11
Table 1.2
Rough Rice and Husk production for
Countries Producing more than 1 million
tons of rough rice, in order of Decreasing
Production for 2000.
18
Table 1.3
Pigments for Surface Coatings
35
Table 1.4
Drier Recommendations for Alkyds
46
Table 1.5
The PVC Range for Various Paints
52
Table 2.1
The Chemical Constituents of Wood Flour
and Rice Husk Flour
63
Table 2.2
Typical Rice Husk Analysis
64
Table 2.3
Elemental Composition of Five RHA
Samples Using RH obtained from
Benue and Kogi States
69
Table 2.4
Chemical Composition of RHA
from Various Locations
70
Table 2.5
PAGE
Chemical composition of RHA, before
83
and after burning at 7000C for 3 and 6hrs.
Table 3.1
Colour Scale Showing Colour Numbers
For Various Transition Colours of
19
105
Rice Husk Ash During Combustion
Table 3.2
Oxide Conversion Factors for Conversion of
Elements to their Oxides ( Element Fraction)
113
Table 3.3
Control Recipe( 0% Extender) for Emulsion
Paints
114
Table 3.4
Formulations of White Emulsion Paint
Finishes Produced with Different Extenders
at 2% to 12% Levels
115
Table 3.5
Codes for White Emulsion Paints Containing
Different Levels of Extenders
115
Table 3.6
Formula for Textured Finishes Showing
the Components and their Weight
Percentages
120
Table 3.7
Formula for Red Oxide Primer
Showing the Components and
their Weight Percentages
124
Table 3.8
Formula for Matt Wood Finish Showing the
Components, their functions and
Weight Percentages
126
Table 3.9
Formula for Cellulose Matt Finish
Showing the Components, their functions
and Weight Percentages
130
Table 4.1
Heating Rates of RH at Different Temperatures
131
Table 4.2
Specific gravity Values of RH, RHA Flour and
Commercial Extenders at Different
Particle Size Ranges
131
Table 4.3
Average RH Ash Yields (in %) at Different
Combustion Temperatures and Time (hr)
133
Table 4.4
Heating Rates, Colour, and Colour Numbers
of RHA Obtained at Different Combustion
Temperatures and Duration (hr)
137
Table 4 5
Silica (SiO2) Content of RHA obtained
at different Combustion Temperatures
and Time
166
Table 4.6
Sodium oxide (Na2O) content of RHA
obtained at different Combustion
Temperatures and Time
169
20
Table 4.7
Potassium oxide (K2O) Contents of RHA
obtained at Different Combustion
Temperatures and Time
170
Table 4.8
Magnesium Oxide (MgO) Content of RHA
Obtained at different Combustion
Temperatures and Time
171
Table 4.9
Calcium oxide content (CaO) of RHA
obtained at different Combustion
Temperatures and Time
173
Table 4.10
Aluminium Oxide (Al2O3) Content of RHA
Obtained at Different Combustion
Temperatures and Time.
174
Table 4.11
Iron (III) Oxide (Fe2O3) Contents of RHA
176
at Different Combustion Temperatures
and Time
Table 4.12
Loss on Ignition (LOI) Values of RHA
at Different Temperatures and Time
178
Table 4.13 :
PVC Values of White Emulsion Paints (WEP)
Produced with Different Extenders at 2-12% Level
183
Table 4.14
Summary of Viscosity Values of White
Emulsion Paint Products Produced
with different Extenders at 2-12% Levels
Using an Analog Rotothinner (viscometer)
185
Table 4.15
Summary of viscosity values of white
Emulsion Paint Products Produced
with Different fillers at 2-12% Levels
using a Digital Rotothinner (Viscometer)
187
Table 4.16
Summary of wt per litre (specific gravity)
values of White Emulsion paints
Produced with Different Extenders at
2-12% Levels
189
21
Table 4.17
Summary of Bulk Increase Values of
White Emulsion Paints Produced
with Different Extenders at 2-12% Levels
191
Table 4.18
pH Values of Emulsion Paints Formulated with
Different Extenders at 2-12% Levels
192
Table 4.19:
Gloss and Matt Values of Red Oxide Primer
Produced with Different Extenders at 20% Level
200
Table 4.20
Gloss and Matt Values of Matt Wood Finishes
(MWF) Produced with Different Extenders as
Flatting Agents at 3% Level
203
Table 4.21
Gloss and Matt Values of Cellulose Matt
Finishes (CMF) Produced with Different Extenders
Flatting Agents at 3.2% Level
207
LIST OF FIGURES
FIGURE
TITLE
PAGE
Fig. 1.1
Representation of Lignin Structure Showing
some of the Various possible Linkages
between the Phenyl Propane Units
Fig. 1.2
Structures of α - D-glucose, Open- Chain
13
14
Glucose and β -D- glucose
Fig. 1.3
Structure of Cellulose
15
Fig. 1.4
Tetrahedral Structure of Silicate
24
Fig. 2.1
Two - dimensional diagram of a Random
68
Silica (SiO2) Network
Fig. 2.2
Two- dimensional diagram of an Ordered
22
68
Silica (SiO2) Network
Fig. 2.3
Temperature-dependent Transformation of Silica
from Amorphous to Crystalline state
71
Fig. 3.1
The CERD Accelerator Showing the Various
Components
109
Fig. 3.2
The End-Station Showing Four Ports and a
Window
110
Fig.4.1
Plots of Rice husk Ash (RHA) Yield Versus Combustion 133
Temperature for Different Combustion Time of 1 to 6 hrs
Fig. 4.2
Plots of Rice husk Ash Yield Versus Combustion Time for 134
Different Combustion Temperature of 400 to 10000C
Fig, 4.3
Photographs of (a) milled rice husks( b)
RHA-MPSC-400-1A and (c) RHA-MPSC-400-1B
Ash samples
139
Fig. 4.4
Photographs of (a) RHA-MPSC-400-2,
(b) RHA-MPSC400-3 and (c)RHA-MPSC 400-4
Ash Samples
139
Fig. 4.5
Photographs of (a) RHA-MPSC-400-5, and
(b) RHA-MPSC-400-6 Ash Samples
139
Fig. 4.6
Photographs of (a) RHA-MPSC-500-1,
(b) RHA-MPSC- 500-2 and (c) RHA-MPSC- 500-3
Ash Samples
140
Fig. 4.7
Photographs of (a) RHA-MPSC-500-4,
(b) RHA-MPSC- 500-5 and (c) RHA-MPSC- 500-6
Ash Samples
140
Fig. 4.8
Photographs of (a) RHA-MPSC-600-1,
(b) RHA-MPSC- 600-2 and (c) RHA-MPSC- 600-3
Ash Samples
140
Fig. 4.9
Photographs of (a) RHA-MPSC-600-4,
(b) RHA-MPSC- 600-5 and (c) RHA-MPSC- 600-6
Ash Samples
141
Fig. 4.10
Photographs of (a) RHA-MPSC-700-1,
(b) RHA-MPSC- 700-2 and (c) RHA-MPSC- 700-3
Ash Samples
141
Fig. 4.11
Photographs of (a) RHA-MPSC-700-4,
(b) RHA-MPSC- 700-5 and (c) RHA-MPSC- 700-6
Ash Samples
141
23
Fig.4.12
Photographs of (a) RHA-MPSC-800-1,
(b) RHA-MPSC- 800-2 and (c) RHA-MPSC- 800-3
Ash Samples
142
Fig. 4.13
Photographs of (a) RHA-MPSC-800-4,
(b) RHA-MPSC- 800-5 and (c) RHA-MPSC- 800-6
Ash Samples
142
Fig. 4.14
Photographs of (a) RHA-MPSC-900-1,
(b) RHA-MPSC- 900-2 and (c) RHA-MPSC- 900-3
Ash Samples
142
Fig. 4.15
Photographs of (a) RHA-MPSC-900-4,
(b) RHA-MPSC- 900-5 and (c) RHA-MPSC- 900-6
Ash Samples
143
Fig. 4.16
Photographs of (a) RHA-MPSC-1000-1
(b) RHA-MPSC- 1000-2 and (c) RHA-MPSC- 1000-3
Ash Samples
143
Fig. 4.17
Photographs of (a) RHA-MPSC-1000-4,
(b) RHA-MPSC- 1000-5 and (c) RHA-MPSC- 1000-6
Ash Samples
143
Fig. 4.18
Photographs of (a) RHA-MPSC-900-1,
(b) RHA-MPSC- 1000-1 Ash Samples obtained at
low heating rate and (c) Ground RHA (RHA flour
144
Fig. 4.19
Plots of Colour Number Versus Combustion
Temperature of RHA at Combustion
Time of 1 to 6hrs
145
Fig. 4.20
Plots of Colour Number Versus Combustion Time
of RHA at Varied Incineration Temperatures of
4000C to 10000C.
146
Fig. 4.21:
Photographs of (a) magnified single rice husk (x40)
(b) magnified milled rice husk cluster (x40) and
(c) magnified isolated rice husk particles (x40)
150
Fig. 4.22:
Photographs of RHA-MPSC-400-1A
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
150
Fig. 4.23:
Photographs of RHA-MPSC-400-1B
(a) magnified particle cluster(x40) and
(b)magnified isolated particles(x40)
150
24
Fig. 4.24:
Photographs of RHA-MPSC-400-2,
(a) magnified particle cluster(x40) and
(b) magnified isolated particles(x40)
151
Fig. 4.25:
Photographs of RHA-MPSC-400-3
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
151
Fig. 4.26:
Photographs of RHA-MPSC-400-4
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
151
Fig. 4.27:
Photographs of RHA-MPSC-400-5
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
152
Fig. 4.28:
Photographs of (a) RHA-MPSC-400-6
magnified particle cluster (x40) and
(b) magnified isolated particles(x40)
152
Fig. 4.29:
Photographs of x RHA-MPSC-500-1
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
152
Fig. 4.30:
Photographs of RHA-MPSC-500-2
(a) magnified particle cluster (x40) and
(b) magnified isolated particle(x40)
152
Fig. 4.31:
Photographs of RHA-MPSC-500-3
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
153
Fig. 4.32:
Photographs of RHA-MPSC-500-4
(a) magnified particle cluster (x40) and
(b) magnified isolated particles(x40)
153
Fig. 4.33:
Photographs of RHA-MPSC-500-5
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
153
Fig. 4.34:
Photographs of RHA-MPSC-500-6
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
154
Fig. 4.35:
Photographs of RHA-MPSC-600-1
(a) magnified particle cluster (x40) and
( b) magnified isolated particles(x40)
154
Fig. 4.36:
Photographs of RHA-MPSC-600-2
154
25
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
Fig. 4.37:
Photographs of RHA-MPSC-600-3
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
155
Fig. 4.38:
Photographs of RHA-MPSC-600-4,
(a) magnified particle cluster (x40) and
(b) (b) magnified isolated particles (x40)
155
Fig. 4.39:
Photographs of RHA-MPSC-600-5
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
156
Fig. 4.40:
Photographs of RHA-MPSC-600-6
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
156
Fig. 4.41:
Photographs of (a) RHA-MPSC-700-1,
(b) magnified particle cluster (x40) and
(c) magnified isolated particles (x40)
156
Fig. 4.42:
Photographs of RHA-MPSC-700-2 (a)
magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
157
Fig. 4.43:
Photographs of RHA-MPSC-700-3
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
157
Fig. 4.44:
Photographs of RHA-MPSC-700-4
(a) magnified particle cluster(x40) and
(b)magnified isolated particles (x40)
157
Fig. 4.45:
Photographs of RHA-MPSC-700-5
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
158
Fig. 4.46:
Photographs of RHA-MPSC-700-6
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
158
Fig. 4.47:
Photographs of RHA-MPSC-800-1
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
158
Fig. 4.48:
Photographs of RHA-MPSC-800-2
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
159
26
Fig. 4.49:
Photographs of RHA-MPSC-800-3
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
159
Fig. 4.50:
Photographs of RHA-MPSC-800-4
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
159
Fig. 4.51:
Photographs of RHA-MPSC-800-5
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
160
Fig. 4.52:
Photographs of RHA-MPSC-800-6
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
160
Fig. 4.53:
Photographs of RHA-MPSC-900-1
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
160
Fig. 4.54:
Photographs of RHA-MPSC-900-2
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
161
Fig.4.55:
Photographs of RHA-MPSC-900-3
(a) magnified particles cluster (x40) and
(b) magnified isolated particles (x40)
161
Fig. 4.56:
Photographs of RHA-MPSC-900-4
(a), magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
161
Fig. 4.57:
Photographs of RHA-MPSC-900-5
(a) magnified particle cluster (x40) and
(b) (b) magnified isolated particles (x40)
162
Fig. 4.58:
Photographs of RHA-MPSC-900-6
(a) magnified particle cluster(x40) and
(b) magnified isolated particles (x40)
162
Fig. 4.59:
Photographs of RHA-MPSC-1000-1
(a) magnified particle cluster (x40) and
(b) magnified isolated particles(x40)
162
Fig. 4.60:
Photographs of RHA-MPSC-1000-2,
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
163
Fig. 4.61:
Photographs of RHA-MPSC-1000-3
163
27
(a), magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
Fig. 4.62:
Photographs of RHA-MPSC-1000-4
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
163
Fig. 4.63:
Photographs of RHA-MPSC-1000-5
(a) magnified particles cluster (x40) and
(b) magnified isolated particles (x40)
164
Fig. 4.64:
Photographs of RHA-MPSC-1000-6
(a) magnified particle cluster (x40) and
(b) magnified isolated particles (x40)
164
Fig. 4.65:
Photographs of RHA-MPSC-900-1
(a) magnified particle cluster (x40) and
(b) magnified isolated particles
obtained at low heating rate
164
Fig. 4.66:
Photographs of RHA-MPSC-1000-1
(a) magnified particle cluster(x40) and
(b) magnified isolated particles(x40)
obtained at low heating rate
165
Fig. 4.67:
Photographs of (a) magnified RHA flour particle cluster
(x40)and (b) magnified RHA flour particles (x40)
165
Fig. 4.68
Plots of Silica (SiO2) Conc of Rice Husk Ash (RHA)
Versus Combustion Temperature at Varied
Combustion Duration of 1 to 6 hrs
167
Fig. 4.69
Plots of Silica (SiO2) Conc of Rice Husk Ash (RHA)
Versus Combustion Time at Varied
Combustion Temperatures of 4000C to 10000C
167
Fig. 4.70
Plots of Potassium Oxide (K20) Concentration in RHA
Versus Combustion Temperatureat Varied
170
Combustion Time of 1 to 6hrs
Fig 4.71
Plots of Potassium Oxide (K2O) Concentration in RHA
Versus Combustion Temperature at Varied
Combustion Time of 1hr to 6hrs
Fig 4.72
Plots of Magnesium Oxide (MgO) Concentration in RHA 172
Versus Combustion Temperature at Varied Combustion
Temperature of 1hr to 6hrs
Fig 4.73
Plots of Magnesium Oxide (MgO) Concentration in RHA 172
Versus Combustion Time at Varied Combustion
28
170
Temperature of 400 to 10000C
Fig 4.74
Plots of Calcium Oxide Concentration in RHA Versus
Combustion Temperature at Varied Combustion Time
of 1 to 6hr
173
Fig 4.75
Plots of Calcium Oxide Concentration of RHA Versus
Combustion Time at Varied Combustion Temperatures
of 400 to 10000C
174
Fig 4.76
Plots of Aluminium Oxide Concentration of RHA Versus 175
Combustion Temperature at Varied Combustion Time
of 1 to 6hrs
Fig 4.77
Plots of Aluminium Oxide Concentration of RHA Versus 175
Combustion Time at Varied Combustion Temperatures
of 400 to 10000C
Fig 4.78
Plots of Fe2O3 Concentration of RHA Versus
Combustion Temperature at Varied Combustion Time
of 1 to 6hrs
Fig 4.79
Plots of Fe2O3 Concentration of RHA Versus Combustion 177
Time at Varied Combustion Temperatures
of 4000C to 10000C
Fig 4.80
Plots of Loss on Ignition (LOI) Values of Rice Husks
Versus CombustionTemperature at Varied Combustion
Time of 1hr to 6hrs
178
Fig 4.81
Plots of Loss on Ignition (LOI) Values of Rice Husks
Versus Combustion Time at Varied Combustion
Temperatures of 4000C to 10000C
179
Fig. 4.82
Calcium Carbonate White Emulsion Paint Film
(standard) CCWEP-8
182
Fig. 4.83
Silica Flour White Emulsion Paint Film
SFWEP-8
182
Fig. 4.84
RHA White Emulsion Paint Film
RHAWEP-8
182
Fig. 4.85
Kaolin White Emulsion Paint Film
KWEP-8
182
Fig. 4.86
Calcium Carbonate White Emulsion Paint Film
(Standard) CCWEP-12
182
29
176
Fig. 4.87
Silica Flour White Emulsion Paint Film
SFWEP-12
182
Fig .4.88
RHA White Emulsion Paint Film
RHAWEP-12
182
Fig. 4.89
Kaolin White Emulsion Paint Film
KWEP-12
182
Fig. 4.90
Calcium Carbonate Green Emulsion Paint
Film (standard) CCGEP-10
182
Fig. 4.91
Silica Flour Green Emulsion Paint Film
SFGEP-10
182
Fig. 4.92
RHA Green Emulsion Paint Film
RHAGEP-10
182
Fig. 4.93
Kaolin Green Emulsion Paint Film
KGEP-10
182
Fig. 4.94
Calcium Carbonate Blue Emulsion Paint
Film (standard) CCBEP-6
182
Fig.4.95
Silica Flour Blue Emulsion Paint Film
SFBEP-6
182
Fig. 4.96
RHA Blue Emulsion Paint Film
RHABEP-6
182
Fig. 4.97
Kaolin Blue Emulsion Paint Film
KBEP-6
182
Fig. 4.98
Plots of PVC Value Versus Extender Level
for Different Extenders
183
Fig. 4.99
Plots of PVC Value versus Extender Type
at 2-12% Extender level
183
Fig. 4.100
Plots of Viscosity (poises) Versus Extender Level (%)
for White Emulsion Paints Produced With Different
Extenders: SFWEP, RHAWEP, KWEP, CCWEP
Using Analog Rotothinner
186
Fig. 4.101
Plots of Viscosity (poises) Versus Emulsion Paint Type
for White Emulsion Paints Produced With Different
Extenders at 2-12% Extender Level
Using Analog Rotothinner
186
Fig. 4.102
Plots of Viscosity (poises) Versus Extender Level (%)
for White Emulsion Paints Produced With Different
187
30
Extenders: SFWEP, RHAWEP, KWEP, CCWEP
Using Digital Rotothinner
Fig. 4.103
Plots of Viscosity (poises) Versus Emulsion Paint Type
for White Emulsion Paints Produced With Different
Extenders at 2-12% Extender Level
Using Digital Rotothinner
188
Fig. 4.104
Plots of Weight per Litre Values of White Emulsion
Paints Produced with Different Extenders Versus
Extender Levels of 2% to 12%
189
Fig.4.105
Plots of Weight per Litre Values of White Emulsion Paints 190
Produced with Different Extenders Versus Emulsion Paint
at 2% to 12 % Levels
Fig. 4.106
Plots of Bulk Increase Versus Extender Level for
different Extenders
191
Fig. 4.107
Plots of Bulk Increase Versus Emulsion Paint Type
at Varied Extender Levels of 2% to12%
192
Fig. 4.108
Calcium Carbonate Textured Finish-White
(Standard) CCTEXF-W
197
Fig. 4.109
Silica Flour Textured Finish-White
SFTEXF-W
197
Fig. 4.110
RHA Textured Finish-White
RHATEXF-W
197
Fig. 4.111
Calcium Carbonate Textured Finish-‘Deep Pink’
(Standard) CCTEXF-DP
197
Fig. 4.112
Silica Flour Textured Finish- ‘Deep Pink’
SFTEXF-DP
197
Fig. 4.113
RHA Textured Finish-‘Deep Pink’
RHATEXF-DP
197
Fig. 4.114
Calcium Carbonate Textured Finish-‘Gentle
Lavender’ (Standard) CCTEXF-GL
197
Fig. 4.115
Silica Flour Textured Finish- ‘Gentle Lavender’
SFTEXF-GL
197
Fig 4.116
RHA Textured Finish-Gentle Lavender
RHATEXF-GL
197
Fig. 4.117
Calcium Carbonate Textured Finish-‘Blossom White’
197
.
31
(Standard) CCTEXF-BW
Fig. 4.118
Silica Flour Textured Finish- ‘Blossom White’
SFTEXF-BW
197
Fig. 4.119
RHA Textured Finish-‘Blossom White’
RHATEXF-BW-20
197
Fig. 4.120
RHA Textured Finish-‘Blossom White’
RHATEXF-BW-10
197
Fig 4.121
Calcium Carbonate Red Oxide Primer (Standard) CCROP
201
Fig 4.122
Silica Flour Red Oxide Primer SFROP
201
Fig 4.123
RHA Red Oxide Primer RHAROP
201
Fig 4.124
Fumed Silica Matt Wood Finish Film (Standard) FSMWF
204
Fig 4.125
Silica Flour Matt Wood Finish Film SFMWF
204
Fig 4.126
RHA Matt Wood Finish Film RHAMWF
204
Fig 4.127
Fumed Silica Cellulose Matt Finish Film (Std) FSCMF
207
Fig 4.128
Silica Flour Cellulose Matt Finish Film, SFCMF
207
Fig 4.129
RHA Cellulose Matt Finish Film RHACMF
207
32
33
34
1.0
INTRODUCTION
Rice husks (or hulls), the major bio-resource used in this study are generated in substantial
quantities in the rice-producing countries of the world as agro-waste during the milling
(dehusking) of rough rice (paddy) to obtain white rice. Rice husks have a few known and
established uses but numerous potential industrial applications, which are yet to be fully
explored. Presently, the domestic and industrial demands for it are extremely low, thus the
rate and volume of its production far surpasses that of consumption, and coupled with its
intrinsic slow-biodegradability, it accumulates in the environment taking up arable land,
constituting a major disposal problem and a colossal environmental nuisance in riceproducing areas. Thus, the utilization of rice husks as a biomaterial for industrial and
possibly domestic uses, through research-driven efforts will ameliorate its environmental
impact and reduce minimally, its nuisance value. The increased global sensitivity to
environment preservation issues due to the recurrent climatic catastrophes linked to global
warming and the need to restore bio-diversity in the eco-system, has triggered a transition
from non-renewable (fossil-based) to renewable (plant-based) resources for sustainable
development. Consequently, there is an ongoing massive, research-driven search,
worldwide, in this regard for the development of more environment-friendly fuels and
chemicals.1-3 Rice husks present an attractive, cheap, abundant, renewable bio-resource, to
be explored, both in the raw form and when converted into ash.
The paint industry uses silica (silicon dioxide) both natural and manufactured, as extender
(filler) in some paint products. Natural silicas include silica flour (100% crystalline silica),
obtained by grinding quartz sand as well as diatomaceous earths, which consist of the
siliceous remains of diatoms and other aquatic creatures deposited in primordial seas over
long periods of time. Silica is used as flatting and consistency-control agent and to impart
special properties to the paint such as settling resistance 4. Rice husk ash (RHA), derived
from rice husks by burning, has a high silica content of 80-95% and therefore presents a
potentially valuable, cheap alternative to the more expensive natural and synthetic silica for
paint production and for numerous other industrial applications. The polymer industry is
constantly in search of new, cheaper, more effective and more recently, renewable rawmaterials. The paint industry is a part of the polymer industry, thus this research into the
applicability of rice husk ash as extender in paint products, could constitute a means of
utilizing huge quantities of this bio-resource in the polymer industry This quest has
stimulated a new-found interest in fillers from waste materials having potential
’recyclability’. This new class of fillers includes fillers from natural sources e.g natural
fibres, industrial by-products (e.g saw dust, rice husks) and a recent entry in the form of rice
35
husk ash (RHA), an industrial agro-waste product obtained by burning rice husks. Their
recyclability, abundant availability and low-cost has become a major driving factor in their
acceptance and utilization as industrial raw -materials.
. Since rice husk generation, accumulation and disposal pose an environmental problem and
environmental issues and challenges have become of great concern globally and have
assumed prominence in international discourse because of the far-reaching consequences of
environmental degradation which is being experienced today, it is considered pertinent to
briefly discuss the prevailing environmental situation, which was one of the considerations
for embarking on this study as well as the abatement measures adopted globally, to palliate
the damaging impact of man’s diverse activities on the environment.
1.1
ENVIRONMENTAL ISSUES
Our planet earth is endangered as it is glaringly beleaguered by environmental problems,
most of which are connected to man’s numerous and varied anthropogenic activities in his
quest for survival, comforts of modern living and technological advancement. Man’s
expanding population, industrialization, urbanization, intensive agriculture and overexploitation of natural resources have exacerbated the situation resulting to environmental
perturbations evident mostly in climatic distortions as recorded globally in recent times.
These changes in climatic patterns have been attributed to the menace of ‘global warming’
which in turn, has been linked to the phenomena of ‘ozone layer depletion’ in the
stratosphere and ‘greenhouse effect’ arising from increased generation of greenhouse gases
(CO2, CH4, water vapour and the man-made chlorofluorocarbons). The greenhouse gases
strongly absorb and trap the infra-red radiation (heat energy) from the sun (like the glass
roof of a greenhouse) and do not allow the earth to re-radiate the heat into space.
5-7
The
heat therefore, does not escape. The net result is the warming up of the earth’s surface by
this phenomenon called the ‘‘greenhouse effect’’. Under normal circumstances, the
greenhouse gases, particularly CO2 and water vapour are responsible for keeping our planet
warm and thus sustaining life on earth. If the greenhouse gases were less or totally absent in
the earth’s atmosphere, the average temperature of the earth would have been at sub-zero
levels. However, if the concentration of greenhouse gases exceeds normal levels, they may
trap too much heat from the sun, which may threaten the very existence of life on earth.
Greenhouse gases have been continuously increasing because of deforestation,
industrialization, increased burning of fossil fuels, mining, increasing number of
automobiles and other anthropogenic (man-made) activities. Burning of fossil fuel is by far,
36
the most important and significant source of atmospheric CO2. With the escalation of
population and rapid industrial growth, the demand for fossil fuel has greatly increased to
such an extent that it contributes about 5 billion metric tones of carbon dioxide to the
atmosphere annually.6 It is estimated that the atmospheric CO2 content has increased by
25% during the last two centuries. This is mostly attributed to the industrial revolution over
these two centuries. This is one of the reasons for the slight increase in global temperature
(about 0.50C) 5.
The ozone layer present in the stratosphere acts as a protective shield for the earth as it
strongly absorbs ultra violet radiation from the sun in the region of 220-230nm, thereby
protecting life on earth from the harmful effects of the absorption of solar U.V radiation
such as DNA mutation, skin cancer, cataract, sun burns etc. Thus, only a small fraction of
U.V. radiation reaches the lower atmosphere and the earth’s surface. Ozone is constantly
formed in the stratosphere but is destroyed by the man-made chlorofluorocarbons (CFCs)
used as coolants in refrigerators, air-conditioners, propellants in aerosol sprays and in
polyurethane foam production. The CFC molecules escape into the atmosphere and
accelerate the depletion of the ozone layer by releasing chlorine in the stratosphere, which
attacks the ozone converting it to oxygen.5, 6 By this process, the thickness of the layer is
reduced much faster than its reduction by natural processes. The increased solar UVradiation also activates the greenhouse effect, which in turn affects the global energy and
radiation balance. The ozone layer, if not protected would enormously affect the stability of
eco-systems causing environmental disequilibria.
The twin plagues of ‘‘greenhouse effect’’ and “ozone layer depletion” have culminated in
global warming which is assuming a nightmarish proportion as it has often spiraled into
natural disasters of unprecedented levels with the accompanying casualties, as recorded in
recent times. Tsunamis, tornadoes, hurricanes (also known as cyclones or typhoon),
landslides, mudslides, avalanche, monsoon, floods and storms have become a recurrent
feature in weather reports and forecasts of today.
Deforestation, through bush - burning, indiscriminate felling of trees and urbanization,
disrupts bio-diversity and consequently, the natural balance in the environment thereby
aggravating environmental degradation, contributing to an increase in global temperature.
Equally worrisome is the fact that life-threatening problems of drought, famine, and
desertification are traceable to deforestation. Pollution, in its diverse forms, has contributed
substantially to environmental devastation. Man-made air pollutants e.g. CO, CO2 NO2
37
SO2, hydrocarbons and particulates have surpassed the pollutants contributed by nature,
(volcanic eruptions, decay of vegetation, sand or dust storms) a thousand-fold. The
magnitude of the problem of air pollution has increased alarmingly due to population
explosion, industrialization, urbanization, increased use of automobiles and other human
quests for greater comfort. Majority of water pollutants are either man-made materials or
originate from man’s activities. These include organic wastes such as pesticides, detergents,
organic solvents, pharmaceuticals, insecticides and various industrial chemicals; inorganic
pollutants such as mineral acids, inorganic salts, trace elements, various metals and metallic
compounds; radioactive materials arising from increasing uses of radioactive isotopes in
research, agriculture, industrial, medical (diagnostic as well as therapeutic) applications,
nuclear power plants and nuclear reactors, from testing and use of nuclear weaponry etc.
All industrial processes lead to the generation of some waste, which goes into the
environment as water, air or soil pollutant. The chemical industry alone releases about 5
billion tons of chemical wastes annually to the environment.5 It also spends over 300 billion
dollars annually for treatment, control, and disposal of these chemical wastes5. This is an
unacceptably huge amount to be expended on waste treatment alone. This situation poses a
formidable challenge for chemists, and chemical engineers to review the existing design,
processes, techniques and raw-material feedstock in the chemical industry and to introduce
cleaner alternatives through research-driven efforts. This suggests that every major, new
industrial revolution such as the emerging science of nanotechnology will require a focus
on environmental impact and sustainability. The search for alternative, renewable, ecofriendly resources becomes imperative 11. This need has also brought into prominence, the
new philosophy of ‘Green chemistry’.
It is evident from the foregoing, that man’s activities on earth and his ignorance of and/or
non-adherence to the laws of nature are largely responsible for the gradual but obvious
deterioration in the environment. These activities create disequilibria in the ecological
system, Nature, then reacts violently, often times, with disastrous consequences in an
attempt to balance the imbalance and restore the ecological dynamic equilibrium. Man then
attempts to palliate the effects of the devastation triggered off by his activities. A vicious
cycle is thereby created! The cycle can only be broken by a drastic reduction in or
elimination of these environment-unfriendly practices that results to climatic perturbations
and catastrophes.
38
It has been projected 5,6,7 that if the present emission trends of greenhouse gases continue, a
global warming of 3.5 to 4.5 degree is likely to occur. This may seem to be a small and
insignificant temperature change, however, geologic historical records have indicated that
average global temperatures have varied over a range of no more than 20 in the last 10,000
years. Any such increase in the global temperature beyond the present 150C value would
have adverse environmental implications. The effect of global warming is most severe on
global climate but it also adversely affects plants, human health aquatic organisms and
wildlife. The projection from computer modeling regarding the climatic changes that could
be triggered off by global warming reveals alarming scenarios as it may cause increased
thermal expansion of ocean water and melting of glaciers in Antarctica which may result in
the sea level rising by 20cm to 1.5m by the later part of the 21st century. Higher sea levels
increase the frequency and severity of floods, damage coastal areas, cause loss of soil
replenishment, seawater intrusion into river and other aquatic systems near the ocean. Thus,
coastal cities like London, Venice, Bombay, Miami, Bangkok and Leningrad may become
extremely vulnerable. Defences against the rising sea levels and expanding oceans are very
difficult and expensive to build and many nations of the world cannot afford them. A global
temperature rise of even 1.50C is likely to wreak havoc on the environment in the form of
floods, hurricanes, tornadoes etc in addition to raising sea levels. On the 5th of April, 2009 a
BBC news item on weather, reported that the “ice bridge holding an ice shelf the size of
Jamaica had collapsed in Iceland”. This represents a huge volume of water introduced into
the ocean. The ripple effect can only be imagined.
The said climatic distortions arising from global warming may also cause shorter, wetter
and warmer winters and longer, hotter and drier summers particularly in mid-continental
areas. Increased global temperature will cause more water to evaporate from aquatic system
and create water vapours. The extra water vapour will amplify global warming because
water vapour is itself a greenhouse gas. Patterns of rainfall would change worldwide
causing large shifts in agriculturally productive areas-dry areas may become drier and
humid areas like the Amazon may suffer more intense tropical storms. A rise in global
temperature could cause the creation of unstable lakes by melting glaciers. These lakes
could burst their banks endangering the lives of tens of thousands of people. Increased
global temperature will slow down photosynthesis, lower the nitrogen content of plant,
increase the rate of decomposition of dead plant matter and soil organic matter (which
would yield more CO2), reduce the moisture content of soil, hence its fertility; and increase
the threat of insecticide-resistant pests. With respect to human health, an increase in the
39
average global temperature will adversely affect human health due to the likely increase in
the incidence of diseases such as malaria, dengue, typhoid and yellow fever,
schistosomiasis, sleeping sickness etc since a global temperature increase is suspected to
extend the range of vectors. With respect to aquatic organisms, an increase in ocean
temperature due to global warming can cause the death of corals and lead to algae colony
being wiped out. Wildlife would also be affected since as trees and plants die out and
habitats disappear, so will the animals that depend on these .On the CNN ‘Connect to the
World’ news of 17th Oct. 2009, it was announced that a whopping sum of 100 billion
dollars is the estimated amount required to combat global warming through finding
alternatives to fossil fuels.
1.1.1 Abatement Measures for Global Warming and Environmental Pollution
The threat of global warming is as real as it is terrifying! The effects are no longer mere
projections of a future scenario but are already creeping in, as the impact is being felt today,
globally, and particularly, as observed in the changing climatic patterns. The consequence
of this disturbing situation is a global outcry against those human activities and practices,
which if unchecked, could worsen the condition of an already threatened environment.
There is therefore, the realization of the need to regulate such activities and also adopt
mitigating measures against the environmental degradation already caused by pollution as
well as the adverse effects of global warming. Such abatement measures are geared towards
the protection and preservation of human lives and the environment for now and for the
future. Certain natural phenomena are known to offset the adverse implications of the
greenhouse effect and global warming. However, the focus here is on the strategies adopted
by man to palliate these effects. These are succinctly discussed hereunder.
i.
The worldwide ban of the use of chlorofluorocarbon (CFCs) found to be largely
responsible for the depletion of stratospheric ozone. This emanated from the Montreal
protocol in 1987 and was signed by twenty-four countries. It was aimed at 35%
reduction in the global production of CFCs by the year 19995. The alternatives to
CFCs, which have been found effective include hydrochlorofluorocarbons (HCFCs)
and methyl cyclohexane (MCH). These do not diffuse up to stratospheric levels so
cannot attack the ozone layer.
ii.
The call for a drastic reduction in the use of fossil fuels (petroleum, natural gas and
coal), which have been found to generate large quantities of greenhouse gases into the
environment, thereby contributing significantly to global warming.
40
iii.
As a consequence of (ii), the encouragement of intensive search for alternative
sources to fossil fuels (such as biogas, biofuel and biodiesel) as well as sustainable
sources of energy generation e.g. solar, wave/tidal, wind, geothermal, hydro power
and biomass.
iv.
Imparting of formal and non-formal environmental education, as is evident in the
vigorous sensitization of the public through the electronic and print media, of the
importance of environment protection practices and the adverse effects of
deforestation practices like bush burning, indiscriminate felling of trees etc
v.
The introduction of stiffer penalties for non-compliance to anti-pollution regulations
such as improper disposal of refuse and waste products (both toxic and non-toxic),
non-treatment of industrial effluents etc.
vi.
The ban on nuclear weapons and explosions, which produce toxic emissions that have
far-reaching deleterious health and environmental impact.
vii. Conservation of forests and extensive afforestation particularly in desertificationprone areas to help maintain the natural balance of CO2. Community forestry is also
encouraged.
viii. Effective check on population growth globally which is essential to reduce the
atmospheric burden of greenhouse gases.
ix.
Development of environmentally-compatible technologies through interdisciplinary
research
x.
Reduction in the use of automobiles since automobile exhausts account for 30% of
CO2 emissions in developed countries.
xi.
Introduction of the relatively new vehicles with biofuel-based engines and hybrid
vehicles having biofuel-petrol and ethanol-petrol compatible engines (‘flex’ cars) as
substitutes to the petrol-based internal combustion engines so as to reduce the CO2
emissions. Research into the development of more fuel-efficient automobiles with
extremely low emissions such as the fuel-efficient hybrid electric vehicle (HEV),
which combines the internal combustion engine with the electric motor and battery
storage of an electric vehicle. In addition, there is an ongoing research into the
ultimate fuel-efficient zero-emission vehicle powered by a fuel cell 5.
xii. The introduction by Government, of the principle and practice of the three R’s (since
1991) with respect to waste – Reduce, Re-use and Recycle. This principle is aimed at
reducing as much as feasible, the generation of waste products, re-use of the supposed
waste materials by conversion into useful products for industrial and domestic uses
and the recycling of solid waste products for the recovery of re-usable materials. By
41
this principle, a waste should be considered a potentially useful resource. These have
led to the emergence of slogans like “waste to wealth”, ‘trash to treasure’, ‘trash to
fortune’ etc.
xiii. The introduction of the principle and practice of the new philosophy, “Green
chemistry” for clean technology, which is the use of chemistry techniques and
methodologies to reduce or eliminate the use or generation of substances (feedstocks,
products, by-products, solvents or reagents) that are hazardous to human health and
the environment.8-10 It focuses on the design, manufacture and use of chemicals and
chemical processes that have little or no pollution potential or environmental risk and
are both economically and technologically feasible. Green chemistry, also called
benign or clean chemistry, seeks to prevent or reduce pollution at source.
xiv. The gradual shift from petroleum-based raw material feedstock to biological resource
(or biomass) to ensure a renewable (non-depleting) supply of raw materials for
industries with little or no environmental implications. One of the twelve principles of
Green chemistry formulated by Anastas and Warner in 199811 is the use of renewable
(non-depleting) feedstock rather than non-renewable (depleting).
1.2
RICE HUSK, A VAST RENEWABLE BIO-RESOURCE
SUSTAINABLE INDUSTRIAL DEVELOPMENT
FOR
Man’s over-dependence on fossil fuels, amongst other factors earlier mentioned, has
contributed tremendously to the damage on the environment. There are increasing concerns
that a continued reliance on petroleum-based resource is not sustainable due to the finite
nature of this fossil resource that could become limiting in the first half of the 21st century
12
. Other concerns are stimulated by its adverse environmental impacts that range from the
effect of fossil fuels on global warming to the accumulation of non-biodegradable
petroleum-derived packaging polymers that constitute a massive disposal problem.
Consequent upon these concerns, there is now a growing interest in obtaining fuels and
chemicals from renewable resources. This transition is however, challenging because the
competing resource, petroleum is cheap while biological materials are often expensive to
process relative to petroleum12. Furthermore, a half-century of investment in petroleum
processing has brought about efficient technologies for the conversion of petroleum into a
staggering array of products. As a result, products from renewable resources have had little
success competing against petroleum-derived product for low-value applications such as
fuels and simple chemicals. Nevertheless, extensive research towards this transition is on
42
worldwide, as this seems to be the means of saving the environment from further
devastation.
In the light of the foregoing, there is a global consciousness that any future developmental
activity has to be considered from the perspective of its ultimate environmental impact.
This global awakening to the need to preserve and protect the environment for the present
and future generations has given rise to the now-frequently-used term “sustainable
development’. The World Commission on Environment, in its report to the United Nations
in 1987, defined sustainable development as “meeting the needs of the present without
compromising the ability of future generations to meet their own needs”. In other words, it
is a pattern of resource use that aims at meeting human needs while preserving the
environment so that those needs can be met not only for the present but also for future
generations.13 Thus, environmental protection constitutes an integral part of the present
developmental process and unsustainable patterns of production and consumption are being
eliminated. Sustainability also implies that fewer materials should be extracted from the
earth and less should go into manufacturing, thereby conserving materials and energy for
generations to come. Over-exploitation of natural resources must therefore cease and
dependence on renewable resources must commence for environmental needs to be
equitably met. This awareness, coupled with health considerations, has triggered the
channeling of research and technological interests towards the use of biomass (bioresource) as a source material for the development of various industrial products. Hence,
the gradual shifts from petroleum-based feedstock to biomass (or biomaterials). ‘Biomass’
is the term used to describe the materials that are produced by the food and agricultural
industries (e.g. starch and lignocellulosics) and were discarded as waste in the past.14,15 It
refers to raw, biological material that is used as a starting material in industrial processes.
The utilization of biomaterials therefore guarantees a renewable supply of raw materials for
industries with little or no environmental implications.
Rice husk (or hull), an agricultural by-product of the rice-milling industry belongs to this
category of biomass/bio-resource material. Rice husk has great potentials, as a bio-resource
as it is generated in huge quantities in the rice-producing countries of the world, hence
constitutes a steady, sustainable and renewable resource convertible to a wide variety of
industrial products. Renewable (inexhaustible) resources have the inherent ability to
reappear or replenish themselves by recycling, reproduction or replacement.15 Thus,
renewable sources include sunlight, plants, animals, soil, water and living organisms. The
non-renewable (exhaustible) resources are the earth’s geologic endowments i.e. minerals,
43
fossil fuels, non-mineral resources and other materials which are present in fixed amounts
in the environment. Unlike renewable resources, non-renewable resources are finite in
quantity and quality.
The industrial revolution and increasing human population is fast depleting the natural
(non-renewable) resources due to over-exploitation and this is causing environmental
imbalance leading to disastrous consequences. The solution to over-dependence on fossil
fuels lies in harnessing and utilizing the vast quantities of lignocellulosic materials
available and devising eco-friendly processes and technologies for converting them into
useful industrial products.
This shift to renewable materials has also kindled so much interest in other disciplines that
in the area of biotechnology, researchers all over the world are currently testing a number
of technological strategies to deal with large-scale wastes such as lignocellulosics and toxic
substances that persist in ecosystems. Thus, the term ‘bioremediation’ has been introduced
to describe the process of using biological agents (microorganisms) to clear the
environment of contaminating substances.16 In the chemical sciences, the conversion of the
lignocellulosics into useful industrial feedstock or products is the focus.
1.2.1 Lignocellulosic Materials
Rice husk is a lignocellulosic material. Lignocellulosics constitute a vast biomass that is
often a waste product of agriculture, timber processing and other human activity and needs
to be disposed of in a safe and efficient manner or used as a resource. From this
perspective, lignocellulosic materials have been grouped into three classes:14
i.
Primary cellulosics include plants that are harvested specifically for cellulosic
content, structural use or feed value e.g. cotton, timber and hay.
ii.
Agricultural waste cellulosics are the plant materials that remain after harvesting
and processing e.g. straw, corn stoves, rice husks, sugar cane bagasse, animal
manures and timber residues
iii.
Municipal waste cellulosics encompass waste paper and other discarded paper
products
Rice husk is the hard, outer, protective sheath of the rice grain (seed) that protects the seed
during the growing season. The rice grain commonly called a seed consists of the true fruit
or brown rice (caryopsis) and the husk (hull), which encloses the brown rice. A single rice
grain weighs about 10-45mg at 0% moisture, grain length, width and thickness vary widely
44
among varieties. The husk weight averages about 20% of total grain weight16, thus during
the milling of rough rice (paddy), 20% of the whole rice on weight basis is removed as
husk17. Therefore, a ton of rough rice generates approximately 200kg of husk. The
lignocellulosic components of rice husk have been found to be 30.24% cellulose, 21.34%
hemicellulose, 21.44% lignin, 1.82% extractives, 8.11% water, and 15.05% mineral
ash 18, 19.
Rice husk is an attractive lignocellulosic material to be fully explored as an industrial
resource because of its numerous advantages which include chemical stability, granular
structure, high surface to volume ratio, low bulk density (about 200kg/m3)
20
low specific
gravity, local and abundant availability, low or no cost, high ash content (about 20%) 21 and
not requiring regeneration because of its low production cost.
1.2.1.1 Components of Lignocellulosic Materials14
Lignocellulosics are composed of the polymers, lignin, hemicellulose and cellulose, which
combine in various proportions to form the “lignocellulosic” structural support system for
all terrestrial plants. Table 1.0 shows the composition of different lignocellulosic materials.
Table 1.1 Typical composition of Various Lignocellulosic Materials14
Amount (%)
Raw material
Lignin
Cellulose
Hemicellulose
Pine wood
27.8
44.0
26.0
Birch wood
19.5
40.0
39.0
Sugar cane bagasse
18.9
33.4
30.0
Rice straw
12.5
32.1
24.0
Cotton
None
80-95
5-20
Rice husk19, 20
21.44
32.24
21.34
Lignocellusic materials are chemically active and decompose thermochemically between
150 and 5000C; hemicellulose between 150 and 3500C; cellulose between 275 and 3500C
and lignin between 250 and 5000C19,21 For most industrial applications, the lignin is
separated from the cellulose. Wood, a lignocellulosic material has three major materials:
cellulose, lignin and oils and resins. To produce wood pulp, for instance, the cellulose must
be separated from the lignin and it is done by a chemical process known as digestion. In
digestion, the chemical reaction involved is hydrolysis whereby the addition of water under
proper conditions, breaks down the lignin polymer into alcohols and organic acids
45
containing long chains of carbon atoms23. The hydrolysis proceeds only in a slow, digestive
process in the presence of other chemicals. In the sulphate process, the digestion liquor for
the hydrolysis of lignin is NaOH and Na2S in aqueous solution.
a. LIGNIN13, 24-27
Lignin is a complex polymeric material that acts as a matrix to bind the fibres of cellulose
together. It is most commonly derived from wood and constitutes a quarter to a third of the
dry mass of wood. Lignin is one of the most abundant organic polymers on earth, exceeded
only by cellulose, employing 30% of non- fossil organic carbon and constituting from a
quarter to a third of the dry mass of wood 26. As a biopolymer, lignin is unusual because of
its heterogeneity and lack of a defined primary structure. Lignin fills the spaces in the cell
wall between cellulose, hemicellulose and pectin components. In plants, it is covalently
bonded to hemicellulose, wraps around fibres composed of cellulose, and thereby crosslinks
different plant polysaccharides, conferring mechanical strength to the cell wall and by
extension, the plant as a whole In other words, lignin is responsible for the rigidity of plants
and for their resistance to mechanical stress and degradation by micro-organism. Lignin can
thus be described as a three – dimensional, globular, irregular, insoluble, high molecularweight (>10,000) polymer made up of phenylpropane subunits with no chains of regular
repeating units or any bonds that are easily hydrolysed either enzymatically or chemically.
14,27
The lignin polymer molecule has many different types of chemical linkages between
aromatic phenyl propane units. The physical and chemical characteristics of lignin are
generally attributed to the last step in lignin biosynthesis, the non-enzymatic free-radical –
based joining of the phenyl propane units in a more or less random fashion.14 Fig. 1.0 is
representative of lignin structure showing some of the various possible linkages between
the phenyl propane (a C6 aromatic group attached to a C3 alkyl chain) units. The phenyl
propane units are linked in an unorganized, non-repeating fashion.
46
H2COH
H2COH
HC
CO
HCOH
CH2
H2COH
OMe
MeO
HC
H2COH
CH
O
OMe
HC
HC
OMe
OH
OMe
H2COH
O
O
HC
HCOH
OC
CH2
HC
CH
HC
HCOH
MeO
O
OMe
O
Fig. 1.1 Representation of Lignin Structure Showing some of the
Various possible Linkages between the Phenyl Propane Units
b. HEMICELLULOSE13, 28-29
Hemicelluloses are short-chain heterogeneous polymers that contain both hexoses (sixcarbon sugars such as glucose, mannose and galactose) and pentoses (five-carbon sugars
such as xylose and arabinose) 14. A hemicellulose is a polysaccharide, related to cellulose.
However, while cellulose is crystalline, strong and resistant to hydrolysis, hemicellulose
has a random, amorphous structure with little or no strength and is easily hydrolysed by
dilute acid or base as well as myriad hemicellulose enzymes. Also, while cellulose contains
only anhydrous glucose as repeat units, hemicellulose contains many different sugar
monomers. Unlike cellulose, hemicellulose consists of shorter chains of 500 to 3000 sugar
units as opposed to 7000 – 15,000 glucose molecules per polymer chain seen in cellulose 28,
, 29
. Hemicellulose is a branched polymer while cellulose is unbranched.
c. CELLULOSE 23, 25, 30-33
Cellulose, the simplest of the components found in lignocellulosic materials is the most
abundant organic compound on earth24, 25. It is the chief structural material of plants, giving
the plants strength, rigidity and form30. Depending on the source, cellulose is present in the
47
biomass at levels of 40 – 60% dry basis) Cellulose, like starch, is a polymer of glucose and
has the general formula (C6H1005)n. Both contain d-glucose unit as monomers and each can
contain as many as 5000 glucose units. However in starch, all the cyclic glucose units are αD-glucose units while in cellulose, they are β– D – glucose23. The monomer, glucose has
two different ring forms, depending on whether the – OH group created from the aldehyde
by the closing of the ring lies above or below the plane of the ring. These two ring forms of
D-glucose are called α-D-glucose and β-D-glucose. In aqueous solution, these two ring
forms interconvert via the open-chain glucose form and cannot be separated. However, they
can be isolated separately in crystalline form.
O
H
H
HO
Ring opens here
4
6
CH2OH
H
H
3
H
2
HO
3
H
2
HO
H
O
5
OH
1
*
H
C
1
H
OH
C
C
4
C
5
C
H
OH
HO
Ring opens here
4
6
CH2OH
O
5
H
H
OH
H
2
HO
OH
OH
3
H
6
CH2OH
α-D-glucose β
β-D-glucose
Open-Chain D-glucose
Fig. 1.2 Structures of α - D-glucose, Open- Chain Glucose and β -D- glucose
In cellulose, the D-glucose units are linked by the 1,4-β-glycoside bonds. Several thousand
glucose units are linked to form one large molecule and different molecules then interact to
form a large aggregate structure held together by hydrogen bonds
31
.The structure of
cellulose is a linear chain of glucopyranose units attached to each other by β-glycosidic
linkages from carbon 1 of one unit to the to the hydroxyl group on carbon 4 of another unit.
The structure consists of long chains of six- membered rings in the most stable chair
conformations with all the larger substituents in the equatorial position.24 The glucose
chains in cellulose are arranged in a manner that permits them to pack together in a crystallike structure that is impervious to water. Consequently, the cellulose polymer is both
insoluble and resistant to hydrolysis, but it can be broken down chemically into its glucose
units by treating it with concentrated acids at high temperature. Cellulose is a
polysaccharide.
48
1
*
H
OH
H
H
H
O
H
HO
OH
HO
CH2OH
O
H
H
O
CH2OH
O
O
H
H
O
OH
H
O
HO
CH2OH
H
OH
H
H
H
H
H
Fig. 1.3 Structure of Cellulose
Polysaccharides are carbohydrates in which tens, hundreds or even thousands of
monosaccharide units (simple sugars) per molecule are linked together through glycoside
bonds. Cellulose is tasteless, odourless, hydrophilic, chiral and biodegradable. Humans
cannot digest cellulose because they lack the enzyme, cellulase, necessary to break the trans
glycosidic linkages in cellulose to give glucose. It is often referred to as ‘dietary fibre’ or
‘roughage’, acting as a hydrophilic bulking agent for faeces34. However, termites, a few
species of cockroaches and ruminant mammals such as cows, sheep, goats and camels have
the proper internal chemistry for this purpose- cellulase can be produced in their gut &
stomach by bacteria 25
.
Cellulose is a form of stored glucose, so it is the component of lignocellulosics that has the
greatest potential for conversion into a variety of useful compounds such as alcohol. In
natural environments cellulose is almost never obtained as pure cellulose but associated
with other components such as lignin and hemicellulose. Therefore, before cellulose can be
utilized, it must be released from its complex with lignin and hemi-cellulose. For most
lignocellulosic materials, this separation requires treatment with either a strong acid or a
strong base or the use of high temperature and pressure. One of the obstacles to microbial
degradation of lignocellulosics to produce cellulose therefore is the structural combination
of cellulose with these other lignocellulosic materials
27,35
. Radiation pretreatment on
cellulosic wastes give some advantages for the degradation of these materials 35
In India and other rice-producing Asian countries, rice husk is used as fuel for boilers to
produce steam used for processing paddy. Some power–generating plants are also fuelled
by rice husks as well as specially designed stoves and industrial furnaces. RH therefore, is
used as fuel to provide heat and electrical energy for domestic and industrial uses. In the
rice producing locations of Nigeria, the husks are dumped as refuse in the open, forming
gigantic heaps and in some cases, incinerated in the open causing health and environmental
49
hazards. It is also used in landfill and for a few domestic purposes like poultry bed.
However, in all of these, only a small proportion of RH generated is utilized, thus the global
production of RH far exceeds its consumption. With the ongoing global transition from
fossil- to bio-resource, RH presents a high-potential, vast renewable biomass for sustainable
industrial development, both in the raw form and when converted to ash. The high silica
content of the ash (85-95%) provides an abundant, cheap alternative source of silicon
dioxide convertible to various industrial products.
1.3 GLOBAL RICE PRODUCTION
The rice plant is of such antiquity that the exact time and place of its development will
perhaps never be known. However, archaelogical findings reveal that this important plant
was first grown in Asia17, 36 as far back as 10,000 B.C. Rice has fed more people over a
longer period then has any other crop. The domestication of rice is one of the most
important developments in history. Oryza Sativa L., the principal cultivated species of rice
is believed to have been domesticated nearly 10,000 years ago in an area that includes
northeastern India, Bangladesh, Burma, Thailand, Laos, Vietnam and southern China. Rice
spread from its area of primary diversity throughout Southeast Asia and adjacent Islands of
the pacific region. The only cultivated species of rice, O. glabberima steud is indigenous to
the upper valley of the Niger river in West Arica35 Although, still cultivated in small areas
of west tropical Africa, O. glabberima is being replaced by improved cultivars of O. Sativa.
From early, perhaps separate beginnings, in different parts of Asia, the process of diffusion
has carried rice in all directions until today, it is cultivated on every continent except
Antarctica17.
African rice,Oryza glabberima is endemic to West Africa. It was selected and established in
parts of West Africa more than 3,500 years ago. O. glaberrima` may have originated or was
first domesticated in the inland delta area of the Nigeria river from where it spread, through
the upper Niger valley and delta to other parts of West Africa. Oryza sativa was introduced
into East Africa a little more than 2,000 years ago, when sea trade between East African
and Madagascan Ports and India was flourishing. Portuguese traders introduced Asian rice
either directly from India or from East Africa and Madagascar into Senegal, Guinea Bissau
and Sierra Leone when they returned from expeditions to India.
37
The Asian rice
established well in many parts of West African, spreading quickly to where the African
rice, oryza glaberrima originated.
50
Rice is the staple food for the largest number of people on earth as it is eaten by nearly half
the world’s population. Considering that the present world’s population is estimated at 6.76
billion people as at 200938 (and still increasing), it implies that about 3.38 billion people
have rice as a staple food in their diet.According to the United Nations FAO 2008, the
annual world rice production for 2007 was estimated at 649.7 million tons,39 the husk
constitutes approximately 20% of it. Another report gave the total world rice production as
approximately 600 million metric tones annually40. The largest rice-producing countries
namely China, India, Indonesia, Bangladesh, Vietnam and Thailand, together, account for
more than three-quarters of world rice production. Since RH constitutes about 20 wt% of
paddy (unhusked rice), about 120 million metric tons of rice husk is generated annually, the
world over. This figure is close to that of 100 million quoted by Nakoo in 199939
Considering that 20% of the grain is husk and about 20% of the husk after combustion is
converted into ash, a total of 20 million tons of ash is obtainable globally. Table 1.2
16
shows the rice-producing countries of the world in order of decreasing quantity of rough
rice production for the year 2000. Only the countries that produce more than 1 million tones
of rough rice annually are listed. The corresponding quantities of rice husk generated are
also indicated.
1.3.1 Rice production in Sub-Saharan Africa, Nigeria and Ebonyi State
In 2006, the total paddy rice production in Sub-Saharan Africa (SSA) was estimated at 14.2
million tons, out of which West Africa was projected to produce 9.32 million tons and East
Africa 4.60 million42 SSA comprises west, central, eastern and southern Africa. West and
East Africa are the main rice-producing sub-regions of SSA. These two regions account for
95% of the total rough rice produced in SSA Nigeria is the largest rice producing country
in West Africa. Since Nigeria produced an estimate of 3,277 million tons of rice in 2000,
with an average annual growth rate of 3.23% per annum from 1961 to 200542, it is
estimated that the amount of rough rice that will be produced in Nigeria in 2010 will be 4.5
million tons, which will yield 900,000 tons of husk. In 2009, the President of Nigeria,
Umaru Musa Yar’Adua during the 26th ordinary session of the Africa Rice Centre
(WARDA) in Abuja, announced that Nigeria’s rice output increased from 3.4 million tons
in 1999 to 4.3million tons in 2006. These figures are in close agreement with the figures
given and projected respectively in Table 1.2.
51
Table 1.2 16 Rough Rice and Husk production for countries producing more than 1
million tons of rough rice, in order of decreasing production for 2000.
S/N
COUNTRY
Rough Rice
Production (000)
% Of World
Rice
Rice Husk
Generation (000)
% Of World
Husk
1.
2.
China
India
190,168
134,150
31.8
22.4
38,034
26,830
31.8
22.4
3.
Indonesia
51,000
8.5
10,200
8.5
4.
Bangladesh
35,821
6.0
7,164
6.0
5.
6.
7.
Vietnam
Thailand
Myanmar
32,554
23,403
20,125
5.4
3.9
3.4
6,511
4,681
4,025
5.4
3.9
3.4
8.
Philippines
12,415
2.1
2,483
2.1
9.
Japan
11,863
2.0
2,373
2.0
10
Brazil
11,168
1.9
2,234
1.9
11
USA
8,669
1.4
1,734
1.4
12
South Korea
7,067
1.2
1,413
1.2
13
Pakistan
7,000
1.2
1,400
1.2
14
Egypt
5,997
1.0
1,199
1.0
15
Nepal
4,030
0.7
806
0.7
16
Cambodia
3,762
0.6
752
0.6
17
Nigeria
3,277
0.5
655
0.5
18
Sri Lanka
2,767
0.5
553
0.5
19
Iran
2,348
0.4
470
0.4
20
Madagascar
2,300
0.4
460
0.4
21
Lao PDR
2,155
0.4
431
0.4
22
Colombia
2,100
0.4
420
0.4
23
Malaysia
2,037
0.3
407
0.3
24
North Korea
1,690
0.3
338
0.3
25
Peru
1,665
0.3
333
0.3
26
Argentina
1,658
0.3
331
0.3
27
Ecuador
1,520
0.3
304
0.3
28
Australia
1,400
0.2
280
0.2
29
Italy
1,300
0.2
260
0.2
30
Uruguay
1,175
0.2
235
0.2
31
Cote d’Ivoire
1,162
0.2
232
0.2
598,852
100%
119,770,400
100%
World
*Based on 20wt% of rough rice (paddy)
Ebonyi, one of the south-eastern states of Nigeria is one of the largest producers of rice and
other agricultural products such as yams, cassava and palm produce. Several rice mills and
de-stoning machines are located in Abakaliki, the state capital, where the milling of rough
rice to produce white rice as well as the de-stoning of the white rice to produce stone-free
52
white rice take place. The variety of rice cultivated in Ebonyi state is mostly improved
species of oryza sativa (Asian rice) An estimated 45,000 tons of rough rice is produced
annually in Ebonyi State43, which yields about 9,000 tons of rice husks yearly. The husks
are dumped in heaps at various dumpsites littered around the mill, as they are far in excess
of the low domestic demands for it. This situation is not different from other rice-producing
locations.
Rice is a remarkably resilient crop as it is produced in a wide range of locations and under a
variety of climatic conditions, from the warmest areas in the world to the driest deserts. It is
produced along Myanmar’s Araken coast, where the growing season records on average of
more than 5,100mm of rainfall and at AI Hasa oasis in Saudi Arabia, where annual rainfall
is less than 100mm16. Temperatures under which rice is produced vary greatly too – in the
upper Sind in Pakistan, the rice season averages 330C; in Otaru, Japan, the mean
temperature for the growing season is 170C.The crop is produced at sea level on coastal
plains and in delta regions throughout Asia, and at a height of 2,600m on the slopes of
Nepal’s Himalaya. Rice is also grown under an extremely broad range of solar radiation,
ranging from 25 to 95% of potential. The contrasts in the geographic, economic and social
conditions under which rice is produced are amazing.
Paddy on an average consists of about 72% of rice, 5 – 8% of bran and 20-22% of husk44 It
is estimated that every ton of paddy produces about 200kg of husk and every ton of husk
produces about 180 to 200kg of ash depending on the variety, climatic conditions and
geographical locations.
1.4. THE CHEMICAL NATURE OF SILICON, SILICA AND SILICATES
1.4.1 SILICON 45-48
Silicon, like carbon belongs to group 4 of the periodic table, is non-metallic in character
and therefore forms predominantly covalent compounds.45 This is due to the fact that an
enormous amount of energy is needed to remove four valence electrons from the silicon
atom. Similarly, the gain of four electrons to give the 4-valent anion, is energetically
impossible. Covalent compounds formed by Silicon are silicon hydrides or silanes (SiH3,
SiH4), silicon nitride (Si3N4), silicon carbide(SiC) and silicon halides(SiX4).46, 47 SiO2 is
formed by strong directional covalent bonds and has a well-defined local structure: four
oxygen atoms are arrayed at the corners of a tetrahedron around a central silicon atom.
53
Unlike carbon, silicon does not form double or triple bonds. Silicon is also one of the few
elements that is capable of forming long chains, and in these chains, silicon is always
combined with oxygen. The Si-Si bond is relatively weak so they are very sensitive to
oxidation and reaction with OH- ions forming Si-OH and Si-O-Si bonds. The Si-O-Si bond
is thermodynamically strongly favoured. To a great extent, the chemistry of silicon is the
chemistry of the silicon-oxygen bond. Just as carbon forms unending C-C chains, the Si-Ogrouping repeats itself endlessly in a wide variety of silicates, the most important minerals
on the planet, and in silicones, synthetic polymers that have many applications.48
Pure silicon has the same crystal structure as diamond but acts as a semiconductor, the
energy gap between its valence and conduction bonds keeping the conductivity low at room
temperature.25 Silicon occurs very widely as silica, SiO2 (sand and quartz) and in a wide
variety of silicate minerals and clays.The element Si is obtained by reducing SiO2 with high
purity coke. There must be an excess of SiO2 to prevent the formation of silicon carbide,
SiC. Si has a shiny blue-grey colour and has an almost metal-like luster, but it is a semiconductor, not a metal. High purity Si, (for the semiconductor industry) is made by
converting Si to SiCl4, purifying this by distillation and reducing the chloride with Mg or
Zn.49
SiO2 + 2C
Si + 2CO
Si + 2Cl2
SiCl4
SiCl4 + 2Mg
Si + 2MgCl2
The electronics industry requires small quantities of ultrapure silicon and Ge (with a purity
better than 1:109) These materials are insulators when pure, but become p-type or n-type
semiconductors when doped with a Group 15 or Group 13 element respectively. These are
used as transistors and semiconductor devices. Very pure silicon is also used to make
computer chips. Group 4 elements typically form four bonds. Carbon can form pπ-pπ
double bond hence CO2 is a discrete molecule and is a gas. Silicon cannot form double
bonds in this way using pπ-pπ orbitals. Silicon compounds are now known to contain pπ-dπ
bonds in which the Si atom appears to use d orbitals for bonding.25
1.4.2 SILICA 23, 49
Two oxides of silicon, silicon monoxide (SiO) and silicon dioxide (SiO2) have been
reported. Silicon monoxide is thought to be formed by high temperature reduction of SiO2
with Si, but its existence at room temperature is in doubt.49
54
SiO2 + Si
2SiO
Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer
approx Inm or 10Ao) of so – called ‘nature oxide’ is formed on the surface when silicon is
exposed to air under ambient conditions. Higher temperatures and alternative environments
are used to grow well-controlled layers of silicon dioxide on silicon, for example at
temperatures between 600 and 1200oC,using the so-called ‘dry’ or ‘wet’ oxidation with O2
or water respectively. The thickness of the layer of silicon replaced by the dioxide is 44%
of the thickness of the silicon dioxide layer produced. Silicon dioxide is commonly called
silica and it is widely found as sand and quartz. Silica is hard, chemically inert and has a
high melting point. It exists in at least 12 different forms. The main ones are quartz,
tridymite, cristobalite, each of which has different structures at high and low temperatures.
ᾳ– quartz is by far the most common and is a major constituent of granite and sandstone.
Pure SiO2 is colourless, but traces of other metals may colour it, giving semi-precious
gemstones such as amethyst (yiolet), rose quartz (pink), smoky quartz (brown), citrine
(yellow) and non-precious materials such as flint (often black due to carbon), agate and
onyx (banded).
49
Silicon forms an infinite three-dimensional structure. In all the different
forms of silica, each Si is tetrahedrally surrounded by four 0 atoms. Each corner is shared
with another tetrahedron, thus giving an infinite array. The difference between those
structures is the way in which the tetradedral SiO4 units are arranged.23 α-quartz, the most
common form of silica, is the most stable form at room temperature and in this, the
tetrahedra form helical chains that are interlinked. In cristobalite, the Si atoms have the
same arrangement as the carbon atoms in diamond, with oxygen atoms midway between
them. Heating any form of silica to its softening temperature, or slow cooling of molten
SiO2 gives a glass- like solid. This is amorphous and contains a disordered mixture of rings,
chains and three-dimensional units.
SiO2 is formed by strong directional covalent bond and has a well-defined local structure:
four oxygen atoms are arrayed at the corners of a tetrahedron around a central silicon
atom.It is the oxygen bridge bonds between Si atoms that give SiO2 many of its unique
properties.The bond angle Si-O-Si is nominally about 145 degrees but can vary from about
100 to 170 degrees with very little change in bond energy. Furthermore, rotation of the
bond about the axis is almost completely planar. Silica in any form is unreactive. It is an
acidic oxide so does not react with acids. However, it does react with HF, forming silicon
tetrafluoride, SiF4. This reaction is used in qualitative analysis to detect silicates; when the
55
SiF4 comes into contact with a drop of water, it is hydrolysed to silicic acid. This can be
seen as a white solid forming on the surface of the drop of water.
H 2O
SiO2 + 4 HF 
→ SiF4 + H 2O 
→ HF + SiOH 4 or SiO2 .2 H 2O
SiO2, being an acidic oxide dissolves slowly in aqueous alkali and more rapidly in fused
alkalis, MOH or fused carbonates, M2CO3, forming silicates
SiO2 + NaOH
(Na2SiO3 )n and Na4SiO4
This reaction accounts for ground glass stopper sticking in reagent battles containing
NaOH. Of the halogens, only fluorine attacks SiO2 forming silicon tetrafluoride.
SiO2 + 2F2
SiF4 + O2
Silica is one of the most valuable inorganic, multipurpose chemical compounds. It can exist
in gel, crystalline and amorphous forms.
Manufactured forms of silica
Silica is manufactured in several forms including:
i.
Glass (a colourless, high-purity form is called fused silica)
ii.
Synthetic amorphous silica, silica gel
iii.
Fumed silica (also known as pyrogenic silica, colloidal silica, silica fumes)
iv.
Precipitated silica, produced by precipitation from a water glass solution by
acidification
v.
Silica aerogel
vi.
Silica nanosprings (nano-size, helical-shaped silica) are produced by vapourliquid-solid (VLS) method at temperatures as low as room temperature.
Applications of silica 50, 51-53
Silica is used in the production of various products.
i.
Glass: Inexpensive soda-lime glass is the most common and typically found in
drinking glasses, bottles and windows.
ii.
Optical fibres: majority of optical fibers for telecommunication are made from
silica.
iii.
Ceramics: silica is a raw-material for many whiteware ceramics such as
earthenware, stoneware and porcelain.
iv.
Portland cement: SiO2 is a raw-material for the production of Portland cement.
56
v.
Food additive: SiO2 is used primarily as a flow agent in powdered foods or to
absorb water
vi.
Diatomaceous earth: Silica is the primary component of diatomaceous earth
which has many uses, ranging from filtration to insect control
vii.
Microelectronics: An oxide layer grown on silicon is hugely beneficial in
microelectronics. It is a superior electric insulator, with high chemical stability.
In electrical applications, it can protect the silicon, store charge, block current
and even act as a controlled pathway to allow small currents to flow through a
device. The traditional method of manufacturing such an oxide layer has been to
react the silicon in a high-temperature furnace within an oxygen ambient
(thermal oxidation).
viii.
Aerogel: Silica is used as a raw material for aerogel in the stardust spacecraft.
ix.
Used in the extraction of DNA and RNA due to its ability to bind the nucleic
acids under the presence of chaotropes
x.
Defoamer: As hydrophobic silica, it is used as a defoamer component
xi.
Tooth paste: As hydrated silica is used in tooth paste as an abrasive to remove
plaque
xii.
Protective wear: As a high-temperature thermal protection fabric
xiii.
Cosmetics: Silica is used in cosmetics for its light-diffusing properties and its
absorbency
xiv.
Wine & Juice Industry: Liquid silicon dioxide (colloidal silica) is used as a
wine and juice fining agent
xv.
Pharmaceutical aid: As a glidant in pharmaceutical products, silicon dioxide
aids powder flow when tablets are formed
xvi.
Thermal enhancement: Used in thermal grouts for the ground source heat
pump industry.
1.4.3 SILICATES
Silicon and oxygen, the two most abundant elements in the earth’s crust are combined in
the crust as silicate minerals. Such minerals all contain SiO44- ion in which four oxygen
stoms are arranged tetrahedrally around a central Si atom. The SiO44- ions are the
fundamental building blocks for all silicate minerals. About 95% of the earth’s crust is
composed of silicate minerals, aluminosilicate clays or silica. These make up the bulk of all
57
rocks, sands and their breakdown products, clays and soil. In silicate minerals, these
tetratedra typically share one or more oxygen atoms to form chains, sheets and rings. The
silicates show beautifully, how organization at the molecular level manifests in the
properties of macroscopic substances.
0 = Oxygen
0
= Silicon
Fig. 1.4 Tetrahedral structure of silicate
1.4.3.1 Silicate Minerals48, 49
The simplest structural class is the chain silicates which occurs when each SiO4 unit shares
two of its oxygen corners with other SiO4 units, forming a chain with only weak
intermolecular forces between sheaths, the material occurs in fibrous strands as is the case
in asbestos. In sheet (layer) silicate, each SiO4 unit shares three of its four oxygen corners
with other SiO4 units to form a sheet. Double sheets arise when the fourth oxygen is shared
with another sheet. In talc, the softest mineral, the sheets interact through weak forces, so
talcum powder feels slippery. If Al substitutes for some Si, or if Al(OH)3, layers interlink
with silicate sheets, an aluminosilicate results, such as the clay, kaolinite (or kaolin). The
final structural class of silicates, the framework silicates, arises when SiO4 units share all
four oxygen corners to give a three-dimensional silicate structure, such as silica (SiO2)
which occurs most often as x-quartz. Some of silica’s twelve crystalline forms exist as
semi-precious gems. Feldspars, which comprise 60% of the earth’s crust, result when Al
replaces some Si. Granite consists of microcrystals of feldspar, mica and quartz. Natural
zeolites also belong to this class.
Soluble silicates like sodium silicate can be prepared by fusing an alkali metal carbonate
with sand in an electric furnace at about 14000C
1400C
Na2CO3
CO2 + Na2O
SiO2
Na4SiO4, (Na2SiO3) n and others
1.4.4 SILICA FLOUR 54-56
Silica flour (100% crystalline silica) is obtained by grinding pure silica sand to a fine
powder. Silica sand deposits are normally exploited by quarrying and the material extracted
may undergo considerable processing before sale. The objectives of processing are to
reduce impurities and increase the grade of silica present and to produce the optimum size
58
distribution of the product depending upon end use.54 After processing, the sand may be
sold in the moist state or it may be dried. Dry grinding in rotary mills, using beach pebbles
or alumina balls as grinding media is the most common way to produce silica flour (ad
cristobalite) flour. Silica flour is also known as silica sand, quartz flour, crystalline silica,
quartz and silicon dioxide.
Applications
Silica flour is used as glazing media in ceramic industry, cementing in oil well drilling,
filler in paints and powder coats, additive in construction chemicals and sealant, and as
filler in plastics.
1.4.4.1 Silica Flour Technical Data 54-56
High purity and fine silica flour have particle sizes ranging from 8 to 100microns and mesh
size ranging from 140 to 635 .Other technical information are given below.
Grain shape: Angular, irregular (fine powder)
Colour :
Light Beige
Hardness :
knoop-820; mohs-7.0
Specific Gravity:
2.635g/cm3 to 2.660g/cm3
PH
6.8 to 7.2 or 7.0 to 8.5
:
Fusion point :
3135oC
Typical Application : Filler for resin systems
Melting point :
1610oC
Boiling point:
2230oC
Odour :
odourless
Solubility:
negligible in water, soluble in hydrofluoric acid
Stability and Reactivity : chemically stable, no particular incompatibility
Typical Chemical Analysis
A typical analysis of pure grade silica flour gives the following components (in wt%) SiO2
(99.8), Fe2O3(0.035), Al2O3(0.05), TiO2 (0.02), CaO(0.01), MgO( <0.01), K2O(<0.01),
Na2O (<0.01) and loss on ignition (0.10)
59
1.5 PARTICLE INDUCED X-RAY EMISSION (PIXE) SPECTROMETRY
Particle – Induced x-ray emission or proton – induced x-ray emission (PIXE) spectrometry
is a powerful, non-destructive analytical technique used to determine the elemental
composition of a solid, liquid, thin film and aerosol filter sampes57, 58
PIXE can detect all elements from sodium to uranium, giving a total of seventy-two
elements (excluding Po, At, Fr, Ra, Ac, Pa and the inert gases)
58
detectable using this
method. PIXE technique relies on the analysis of the energy spectra of characteristic x-rays
emitted by the deexcitation of the atoms in the sample bombarded with high-energy (1-3
Mev) protons with the aid of a suitable energy dispersive detector59. This technique was
first proposed in 1970 by Sven Johansson at Lund university, Sweden and developed over
the next few years with his colleagues, Roland Akselsson and Thomas B. Johansson.57 Like
other spectroscopic techniques used for elemental analysis, PIXE is based upon the physics
of the atom as it involves both the excitation of the atoms in the sample to produce
characteristic x-rays and a means of detection in order that their intensities may be
identified and quantified. However, the use of proton beams as an excitation source offers
several advantages over other x-ray techniques. Among these are58
i.
Higher rate of data accumulation across the entire spectrum which allows for
faster analysis
ii.
Better overall sensitivities, especially for the lower atomic number elements
However, the major advantage of the PIXE technique is its high sensitivity for trace
element determination58. This is due to a lower Bremstrahlung background resulting from
the deceleration of ejected electrons as compared to electron excitation and the lack of a
background continuum compared to XRF analysis 58. Because of the ever-increasing need
for elemental analysis of very small sample amounts (e.g. 0.1g – 1mg) as is the case with
aerosol filters, the PIXE technique has rapidly gained acceptance as a valuable analytical
tool. Samples (or materials) which their elemental make-up can be determined using the
PIXE technique include oils and fuels, plastics, rubbers, textiles, pharmaceutical products,
foodstuffs, cosmetics, fertilizers, minerals, ores, coals, rocks and sediments, cements,
ceramics, polymers, inks, resins, papers, soils, ash, leaves, films, tissues, forensics,
catalysts
57,58
etc. The most extensive use of the PIXE method however is in elemental
analyses of atmospheric aerosol samples, dust and fly ash samples, different biological
materials, archaeological and artistic artifacts. The need for a small particle accelerator has
60
confined PIXE in the main, to nuclear physics laboratories and is yet to be widely
recognized in the analytical community.
1.5.1 Development of PIXE Analytical Technique57
This new analytical method, which became known under the acronym PIXE, was tested and
applied in many nuclear physics laboratories during the 1970s. PIXE developed rapidly.
There were several reasons for its rapid development; first the growing interest in
environmental problems created a need for efficient methods of elemental analysis examples are air pollution studies and the determination of toxic elements in the
environment and in humans. Because PIXE is well suited for the determination of trace
elements in a matrix of light elements, it is ideal for studies of this kind. Second, small
accelerators became available in many nuclear physics laboratories, where they were the
standard equipment in the early days of fundamental nuclear physics. Soon, however,
interest shifted to higher energies and the small machines became obsolete as far as nuclear
physics was concerned. An alternative to scrapping them was to use them for research in
applied science and PIXE was one of the most popular options. A contributing reason was
that, not only the accelerator, but also auxiliary equipment such as detectors, electronics,
and computer facilities were available in most laboratories thus, feasibility tests were
carried out in many laboratories. PIXE is capable of detecting elemental concentrations
down to parts per million, however, the technology is still relatively new and untested in
wider avenues of chemical research.60
1.5.2 X-Ray Emission Theory
PIXE is an analytical method based on x-ray emission 57. When samples are bombarded
(irradiated) with high-energy protons (or x-rays in the case of XRF) the interaction of the
protons with the electrons of the atoms in the sample causes ejection of the electrons in the
innermost shells in atoms of the specimen.
61, 62
This creates a hole (vacancy) in the inner
shell, converting it to an ion thereby putting it in an unstable state. To restore the atoms to
more stable states,i.e. their original configuration, the holes in the inner shells (or orbitals)
are filled by electrons from outer shells. Such transitions from higher to lower energy levels
are accompanied by energy emission in the form of x-ray photon, for instance an L-shell
electron fills the hole in the K-shell. Since an L-shell electron has a higher energy than a Kshell electron, the surplus energy is emitted as x-rays. In a spectrum, this is seen as a line.
The energy of the x-ray emitted when the vacancies are refilled depends on the difference
in the energy of the inner shell with the initial hole and the energy of the electron that fills
61
the hole. The emitted x-ray radiation is characteristic of the element from which they
originate since each atom has its specific energy levels
63
. The number of X-rays is
proportional to the amount of the corresponding element within the sample. An energy
dispersive detector is used to record and measure these x-rays and their intensities are then
converted to elemental concentrations. An atom emits more than just a single energy (or
line) because different holes can be produced and different electrons can fill up these holes.
The collection of emitted lines is characteristic of the element and can be considered a
fingerprint of the element.
1.5.3 Ion Beam Analysis (IBA) Principles
PIXE is one out of four of the ion beam analysis methods. IBA consists of whole methods
of studying materials based on the interaction at both atomic and the nuclear level, between
accelerated charged particles (ions) and the bombarded material (sample).
When a charged particle moving at high speed strikes a material, a number of events can
take place. The ion can interact with the electrons and nuclei of the material atoms, slows
down and possibly deviates from its initial trajectory. This can lead to the emission of
particles and/or radiations (X and γ-rays), whose energy is characteristic of the elements
that constitute the sample.
1.5.4 Ion Beam Analysis Methods
The spectrometic analysis of the various secondary emissions lead to the various IBA
techniques:57, 61, 64
1.
PIXE (Particle Induced x-ray Emission) is based on atomic fluorescence and the
analysis is performed with characteristic x-rays. PIXE is well adapted for the analysis
of trace elements ranging from Na to U.
2.
PIGE (Particle-Induced Gamma-ray Emission is based on nuclear reaction and the
analysis is performed with characteristic Gamma-ray PIGE is particularly useful for
analyzing light elements such as F and lighter elements which are inaccessible by
PIXE
3.
NRA (Nuclear Reaction Analysis) is based on nuclear reaction and the analysis is
performed with charged particles. NRA has demonstrated its usefulness in the study
of the oxidation and deposition of hydrocarbon residue on metallic surfaces
62
4.
RBS (Rutherford Backscattering Analysis) is based on nuclear scattering and the
analysis is performed by charged particles. RBS has proved its efficacy in identifying
and localizing thin layers.
1.5.5 PIXE ANALYSIS
Basic principles: As a charged particle moves through a material, it loses energy primarily
by exciting electrons in the atoms that it passes by. Electrons in the inner shells of the atom
(predominantly the K and L shells) are given enough energy to cause them to be ejected,
resulting in an unstable atom. Electrons from higher shells in the atom then ‘drop down’ to
fill the vacancies and in so doing, give off excess energy in the form of X-rays.61 The
energies of these X-rays are characteristic of the element and therefore can be used to
identify elemental composition. Also, by measuring intensities of characteristic X-ray lines,
one can determine concentrations of almost all elements in the sample down to
approximately 1 PPM (part per million)
Sample preparation
There is virtually no need for sample preparation. Most samples are analyzed in their
original states; aerosol filter, archeological samples, soil, ash, biological samples etc.
However, as PIXE technique probes only the top 10 to 50 micrometers of the sample
(depending on the material, energy of the incident beam and most importantly, on the
energy of characteristic X-rays), it is very important that the area/volume irradiated by the
beam (usually a circular area with the diameter of 1 to 10mm is representative of the whole
sample. Therefore if the sample is obviously not homogeneous, for example some pottery
and geological samples, it is advisable to grind the sample to a fine powder (preferably with
particle size smaller than 1-2 micrometers), thoroughly mix it with 20% analytical grade
carbon powder and consequently press into pellets.
1.6 PAINT TECHNOLOGY
General
Paints are products primarily designed to protect and decorate surfaces. All objects are
vulnerable at their surfaces since it is the surface that makes continual contact with the
corroding (or oxidizing) air. The surfaces of objects left in the open bear the brunt of the
sun, rain, fog, dew, ice and snow. Under these conditions, iron rusts, wood rots (or shrinks
and cracks) and road surfaces crack and disintegrate. These, and more sheltered objects,
suffer the wear of daily use, scratches, dents and abrasions – at their surfaces. To prevent or
63
minimize damage, man applies to these surfaces various coatings designed to protect them,
coatings can also be used to decorate the articles, to add colour and luster and to smooth out
any roughness or irregularities caused by the manufacturing process. There are many
surface coatings that do this: wallpaper, plastic sheet, chrome and silver plating. However,
no coating is as versatile as paint which can be applied to any surface, however awkward its
shape or size, by one process or another. This is because the fluidity of paint permits
penetration into the most intricate crevices.’Paint’ is a loosely used word covering a whole
variety of materials such as enamels, lacquers, varnishes, undercoats, primers, sealers,
fillers etc. Paint can be defined as a fluid material which, when spread over a surface in a
thin layer, will form a solid cohesive and adherent film.65
If pigment is omitted from the paint, it is usually called a varnish. The pigmented varnish –
the paint – is sometimes called an enamel, lacquer, finish or topcoat, meaning that it is the
last coat to be applied and they are seen when the coated object is examined. Paint applied
before the top coat is called an undercoat. Some undercoats may be briefly defined as
follows 65,66
i.
Fillers or stoppers are materials of high solid content, used to fill holes and deep
indentations or irregularities and to provide a level surface for the next coat. Stoppers
are very highly pigmented so that they dry hard without shrinking and can be rubbed
down to a smooth surface before application of further coats of paints.
ii.
Primers are applied to the filled or unfilled surface to promote adhesion, to prevent
absorption of later coats by porous surfaces and to give corrosion resistance over
metals. Special pigments like red oxide, improve the anti-corrosive properties.
iii.
Surfacers (called undercoats in the house painting trade) are highly pigmented
materials containing large quantities of extender to give good obliterating power They
are easily rubbed smooth with abrasive paper. They provide body (build or thickness)
to the paint film, level out minor irregularities in the substrate and stick well to primer
and topcoat.
iv.
Primer-surfacers are surfacers that can be applied directly to the object’s surface (the
substrate)
v.
Sealers are clear or pigmented materials applied in thin coats to prevent the passage
of substances from one coat of paint to another or from the substrate into later coats.
They can be required to improve adhesion between coats, where this is otherwise
weak.
64
1.6.1 CLASSIFICATION OF PAINTS 66-68
Materials and practices in the field of paint and coating are diverse and changing rapidly
with the shift from natural to synthetic materials. Due to the above reason, the classification
of paints and coating materials are related to their uses, source of materials, and functions
including the properties they exhibit.204 However, the classification here has been broken
into four different subtitle which are according to, mode of film formation, uses, type or
source of binders and specialty purposes.
i. CLASSIFICATION BASED ON MODE OF FILM FORMATION
In this classification, the paint system is considered to undergo some changes the change
may be either physical or chemical. viz:
a. Convertible coatings: This is a coating or paint that goes through a chemical change
upon drying to form a non-reversible film. It produces a material chemically different from
its liquid state and cannot be reversed back by the use of its original thinner (solvent). The
binders of such coating are thermosetting. They are sometimes known as thermosetting
paint e.g. alkyd paints. Some resins or binders that can give this type of paints include alkyd
resin, amino resin, epoxy resin, phenolic resin, silicone resin, polyurethane resin, oil such as
linseed and tung oil.
b. Non-convertible coatings: This type of coating has the concept of a purely reversible
film. They undergo a physical change upon drying. In this type of paint system, the solvent
evaporates leaving only the binder or resin in its original form. Such binder or resin can be
collected and reversed to the liquid form by the application of its original solvent in the
system. The binder or resins of this type of paints are thermoplastic e.g. Emulsion paints.
ii. CLASSIFICATION BASED ON APPLICATION
Based on this classification, there are three major categories of paints:
a. Trade sales paint: They are popularly called architectural paints. They are paints
in small packages for sale to the public or professional painters in paint stores.
They are used on the interior and exterior surfaces of residential, commercial and
institutional buildings. They are mainly for protective and decorative use.
Examples are gloss, aluminium, floor, and emulsion paints.
b. Industrial paints: They are paints generally applied to materials being
manufactured. Industrial finishes are applied to a wide variety of materials and
65
include coatings for automobiles, aircraft, rubber and plastic items, electrical
appliances and furniture.
c. Maintenance paint: They are used in the maintenance and upkeep of factories,
utilities and buildings. Many trade sales paints are used as maintenance paint.
iii.
CLASSIFICATION BASED ON TYPE OF SOLVENT AND BINDERS
Paint can be classified according to the type of solvent and binder into two broad classes:
water-based and oil - based paints.
a. Water-based paints: Emulsion paint is called water-based paint because the
polymer is formed through an emulsion polymerization whereby the monomers
are emulsified in a water continuous phase. The binder commonly used in waterbased paint is usually polyvinyl acetate (PVA). They are widely used on walls
and ceilings of buildings and other places where excellent light reflectance, fast
drying and low cost of materials are needed.
b. Oil-based paint: They are paints whose solvent or dispersion medium is
predominantly oil or oil-based varnish or hard resin depending on whether the
drying mechanism is by oxidation or solvent evaporation. The binder usually
used in oil-based paint is alkyd resin. Oil based paints are also known as solventthinned paints because they can be dissolved or thinned by only their solvents.
They are highly glossy and are therefore called gloss paints.
iv. CLASSIFICATION BASED ON SPECIALTY APPLICATION
Classification of paints and allied materials by their function is of greatest value and the
most important types are: Fillers and stoppers, sealers, primers, undercoats, varnish and
lacquers. The first five have been described in section 2.1. Varnishes and lacquers are
paints for specialty purposes with the former drying by oxidation and the latter by
evaporation of the solvent.
1.6.2 COMPOSITION OF PAINTS
The ingredients of surface coatings are known as “raw materials” in the paint industry,
generally there are three primary components of paints; pigment, binder, diluents and a
secondary component, the additives. However, only the binder is absolutely required. The
diluents serve to adjust the viscosity of paint, it is volatile and does not become part of the
paints film.
66
1.6.2.1 BINDER (RESIN)
The binder is the part, which eventually solidifies to form the dried paint film. Binders are
the oils, resins and plasticizers, which go to make up the protective films. They are also
called film-formers and non-volatile vehicles. The word “vehicle” denotes all the liquid
portion of a surface coating both volatile and non-volatile. Typical binders include synthetic
or natural resins such as alkyd resins, acrylics, polyurethanes, polyesters, melamines, oils or
latex. The binder also is the liquid components of paint that forms the film on application to
substrate and is largely responsible for the protective and general mechanical properties of
the film. Film properties depend also on the nature of the pigments, the extent the pigment
is dispersed in the binder (pigment-binder ratio). The binder becomes converted from a
liquid or semi-liquid layer on the substrate to a hard, coherent solid film as the paint. There
are three main mechanisms by which surface coatings dry.66,
69
The first is by solvent
evaporation. After the paint has been applied to the substrate by brushing or some other
means,the solvent constituent evaporates, leaving a coherent film.In this process, drying is
thus a purely physical phenomenon, no chemical reaction taking place.The principal
products that dry in this way are emulsions and lacquer-based e.g nitrocellulose wood
finish. The second mechanism of drying is by chemical reaction with atmospheric oxygen
i.e by autoxidation. The oxidation process proceeds via formation of peroxides and
hydroperoxides accompanied by simultaneous polymerization. 70, 71 Alkyd resins and other
synthetic resins contain in their molecule, unsaturated long-chain fatty acids. On exposure
to air, a series of complex autoxidation reactions take place. Gloss paints dry by this
process. The third mechanism of drying is by chemical reaction between the film-forming
constituent of the paint and another constituent known as a curing agent. Generally, it is
necessary to heat the painted object to a fairly high temperature before the reaction will
proceed at a satisfactory rate. Sometimes, solvents are present in these coatings so that
evaporation precedes curing, but quite frequently, solventless formulations are used. Paints
that dry by chemical curing are used almost exclusively for coating industrial products such
as car bodies, refrigerators and other kitchen equipment, toys, bicycles etc
1.6.2.2 PIGMENTS
Pigments are coloured organic and inorganic insoluble substances used to give color,
opacity, consistency, build, durability, and other properties to surface coatings
4,70
However, the function of pigments is not only to provide an aesthetically pleasing coloured
surface,the solid particles in some paints reflect many of the destructive light rays and thus
help to prolong the life of the paint 68.In general, pigments should be opaque to ensure good
67
covering power and chemically inert to secure stability,hence long life. Pigments should be
nontoxic, or at least of very low toxicity, to both the painter and the inhabitants. Finally,
pigments must be wet by the film-forming constituents and be of low cost.70 Different
pigments possess different covering power per unit weight.
Pigments serve to give the paint : 66
i. Color
ii. Opacity
iii. Improved strength and adhesion of the paint film
iv. Improved body
v. Improved durability and weathering properties
vi. Reduced gloss
vii. Improved flow and application properties
In choosing a pigment to carry out a given selection of these functions, the following
properties of the pigments must be known:
i.
Tinting strength.
ii.
Light fastness.
iii.
Hiding power.
iv.
Bleeding characteristics.
v.
Refractive index.
vi.
Particle size and shape.
vii. Specific gravity.
viii. Chemical reactivity
ix.
Thermal stability
Different types of pigments are given in Table 1.3. Pigments have particle sizes varying
from 0.01µm (carbon blacks) to 10µm while some extenders have particle size of up to
50µm .66 No sample of any pigment contains particles all of an identical size; rather there is
a mixture of sizes with an average diameter. It is interesting to note that while 1g of rutile
titanium dioxide white (particle diameter 0.2 – 0.3 µm) has a surface area of 12m2, 1g of
fine silica (particle diameter 0015 – 0.02 µm) has a surface area of 190 m2 – about the area
of a singles tennis court. Different pigments possess different covering power per unit
weight. Among the white pigments, titanium dioxide is prominent as it is unrivalled in
respect of colour, opacity, stain resistance and durability.
68
Table 1.3 Pigments for Surface Coatings 68,72
PIGMENTS
FUNCTION
White Pigment
Titanium dioxide
Zinc oxide
Zinc sulphide
Lithopone
Antimony
Yellow Pigment
Hansa Yellows
Ferrite Yellows
Lead or zinc chromate
Litharge
Black Pigments
Carbon black
Lampblack
Graphite
Iron black
Orange Pigments
Basic lead chromate
Cadmium orange
Molybdenum orange
Blue Pigments
Green Pigments
Ultramarine
Copper phthalocyanine
Iron blues
Chromium oxide
Chrome green
Hydrated chromium oxide
Phthalocyanine green
To strengthen the film and
impart an aesthetic appeal.
Pigments should possess the
following properties: opacity
and good covering power,
wettability by oil, chemical
inertness, nontoxicity or low
toxicity, reasonable cost.
Red Pigments
Red lead
Iron oxide
Cadmium Reds
Toners and Lakes
Brown Pigments
Burnt sierra
Burnt umber
Vandyke brown
Metallics
Aluminium
Zinc dust
Bronze powder
Metal Protective Pigments
Red Lead
Blue lead
Zinc, basic lead and barium
Potassium chromates
1.6.2.3 SOLVENTS
The solvents also called thinners and volatiles, are liquids added to most surface coatings to
make them fluid enough for proper application and to control viscosity changes during the
application and film formation processes.4, 65 They evaporate leaving a residue of pigment
and binder to form the decorative and protective films by various drying and hardening
processes
69
. The word ‘solvent’ is often used as a general term to describe all non-film
69
forming liquids used in a paint. By strict definition, a solvent is a liquid, which produces a
solution of the binder component whilst a liquid is simply the fluid component of the
system. Generally, the solvency of a solvent for a particular binder is governed by the
chemical nature of both components. Normally a liquid is a solvent for a binder if it is
chemically similar such that the binder attracts the solvent. Selection of volatile
components affects the occurrence of such film defects as popping, sagging and leveling
and can affect such properties as adhesion, corrosion protection and exterior durability. 73
The most important physical properties of solvents in paints are:
1.
Solvency
2.
Evaporation rate and boiling point
3.
Specific gravity and costs
4.
Chemical nature – reactivity, toxicity etc
5.
Flash point.
The general properties required of a solvent or solvents before selection in paint application
is that it should dissolve the binder as efficiently as necessary, thereafter to evaporate at the
rate required from the film to give the best air drying or stoving properties; this should be
achieved at the lowest possible cost.
1.6.2.4.Solvency
Under normal conditions, the solvent must dissolve the binder, the major exception to this
is in emulsion paints where the binder is carried as a dispersion in the liquid. The overall
efficiency of a solvent is difficult to rate but one method attempts to rate all solvents by
assessing hydrogen bonding and establishing a solubility parameter. By this method all
solvents can be grouped into three groups for hydrogen bonding 73 :
a) Weak (hydrocarbons, chloro and nitro paraffins)
b) Moderate (ketones, esters, ethers and other alcohols)
c) Strong (alcohols and water)
The function of the solvent in alkyd and acrylic resins is to dissolve the binder efficiently,
thereafter to evaporate from the film at the required rate to give the best air-drying or
stoving properties. Since solvent effectiveness is still governed by the nature of the resin to
be dissolved, in practice, solvents are selected by consideration of the binder type:
Long oil alkyds
-
aliphatic hydrocarbons
Short oil alkyds
-
aliphatic/aromatic hydrocarbons
Acrylic resins (thermosetting) -
aromatic hydrocarbons/esters and ketones
Acrylic resins (thermoplastic) -
esters and ketones/aromatic hydrocarbons
Nitrogen resins
alchohols/aromatics hydrocarbons
-
70
Epoxy resins
-
aromatic hydrocarbons
Cellulose
-
esters, ketones, aromatic hydrocarbons
and alcohols
The chemical classes of liquids or solvents used in paint can be grouped for convenience
and described in practical terms as follows 4, 66:
1.
Water
This is the liquid phase in all emulsion paints used for decorative purposes. It is also
used to dissolve water-soluble resins in specific products. It has advantages of being
cheap, readily available, non-toxic and non-flammable, whilst its major
disadvantage is that it only has limited compatibility with most other solvents and
therefore its properties cannot be readily modified.
2.
Aliphatic Hydrocarbons
These are obtained as distillation products from the petroleum industry and always
consist of a mixture of hydrocarbons such as hexanes, heptanes and octanes which
are mainly aliphatic but can contain a proportion of aromatic products. These
mixtures are usually supplied and quoted according to the total boiling range which
normally covers the range 20-400C. These mixtures have the lowest solvency of all
solvents but are acceptable for long and medium oil alkyds.The most commonly
used aliphatic hydrocarbon solvent in the paint industry is white spirit and kerosene.
3.
Aromatic Hydrocarbons
Aromatic hydrocarbons are supplied either as fractions or pure compounds such as
xylol (xylene) and toluol (toluene). These have higher solvency than aliphatic
hydrocarbons with which they are frequently used to give increased solvency.
4.
Esters, ketones and Ethers
This group of solvents is generally used as primary solvents for dissolving lacquertype binders such as nitrocellulose and acrylics. The properties of esters and ketones
of comparable molecular weight are very similar and they are frequently
interchangeable – although odour can vary. Generally, the higher the molecular
weight the poorer the solvency of these solvents. Low molecular weight ethers are
rarely used as they are expensive and have very low flash points, however the
higher molecular weight versions (cellosolves etc) are regularly used as efficient
high boiling solvents i.e. they promote flow in lacquer or short oil length alkyd
films. Commonly used esters include ethyl acetate and butyl acetate; ketones
71
include metyl ethyl ketone (MEK), methyl iso-butyl ketone (MIBK) , while ethers
are diethyl ether
5.
Chloroparaffins and Nitro paraffins
These solvents are not normally used because of cost and toxicity except in special
circumstances. However chlorinated solvents such as trichloethylene having very
high specific gravity as liquid and vapour and being non-inflammable are used
where these properties are beneficial e.g. dip primers.
There are no absolute rules to be used for solvent selection except to use a balanced
blend considering the binder system involved, the film properties required and
finally the cost.
1.6.2.5 PIGMENT EXTENDERS (FILLERS)
Pigment extenders, also known as fillers, generally are inert, inorganic solid compounds
that are added to paints for the purpose of increasing bulk, reducing cost, and imparting
some special properties to the product
65, 68.
They are sometimes incorporated into paint to
partially replace true pigments. All extenders have a low refractive index, which is close to
that of the binders commonly used and this means that they do not contribute opacity or
specific colour to the paint.
Although originally, extenders were simply used to reduce the cost of paint by replacing
expensive pigments they are now used to impart certain specific properties, namely 4, 65, 68:
1.
Sanding
-
improved ease and efficiency of application
2.
Build
-
improved build by way of reduced sinkage
3.
Rheology
-
providing false body to reduce settlement
4.
Flow
-
less sagging on application
5.
Durability
-
improved resistance to weathering
6.
Adhesion
7.
Gloss
-
intercoat adhesion to substrate and topcoat can be
improved
level of gloss can be controlled
The term ‘auxiliary pigment’ might be more appropriate for extenders because of the
specialty properties they confer on paint products.202 Extenders can be obtained from
natural sources followed in certain instances by chemical treatment, or by chemical
precipitation. The shapes of extender pigments vary, depending upon the crystal shape and
method of preparation. Generally three shapes are found 4:
Nodular
-
round irregular
72
Acicular
-
needle or rod-like
Lamellar
-
plate-like
Individual extenders considered can consist of either mixtures or single shaped particles.
The shape of the extenders do impart certain specific, identifiable properties, for instance:
Nodular
-
pack together can therefore impart more hard settling
Acicular
-
do not pack therefore do not settle, but can “protrude” through film
surface giving low gloss but good intercoat adhesion
Lamellar
-
by overlapping the film is reinforced to resist cracking and reduce
moisture permeation, “lubrication” of the paint is obtained, giving
easy brushing.
In the main, extenders fall into the following major chemical groups 4, 69 :
1.
Carbonates
2.
Aluminium Silicates
3.
Barium Sulphates
4.
Magnesium Silicates
5.
Silicas (Silicon Dioxides)
6.
Calcium Sulphate
Within these groups apart from differences due to the varied natural sources the extenders
are frequently graded according to colour and particle size.
1.6.2.5.1 Calcium Carbonate (Chalk)
This type, which constitutes the major type of extender used, is widely available and is
generally produced from natural chalk or limestone. Three different basic types of untreated
calcium carbonate can be obtained, which can be classified as 4,65, 69:
1. Whiting, a natural grade of chalk (calcium carbonate) which is produced when
natural chalk is reduced to a powder
2. Calcite, a crystalline form of calcium carbonate, harder than the chalks and is useful
where a hard extender with low specific gravity is required.
3. Dolomite , a calcium magnesium carbonate (CaCO3 MgCO3) which occurs in
crystalline state and is used as a general purpose extender.
4. Lubestine Talc (calcium/magnesium carbonates plus magnesium silicate)
Most grades of calcium carbonate are basically nodular although somewhat irregular in
shape. The properties of all these grades are fairly similar being cheap, readily available
and reducing floating of coloured pigments. The disadvantages are that they have poor acid
resistance, although dolomite is slightly superior in this respect. In addition, coated grades
73
of calcium carbonate are available which are extremely fine grades coated with stearic acid;
these have good anti-settling properties.
1.6.2.5.2 Aluminium Silicates (Clays)
Clays are chemically composed of mainly aluminium silicates .There are various types of
clays used in paint production. These include 4,65, 69:
i. China Clays (hydrated aluminum silicates) are the most common of this class,
being of granite origin. It is also known as kaolin and is usually assigned the
formula Al2O3.2SiO2.2H20 Their colour is generally poor, but they contribute
some opacity, give good brushing properties and resist settlement to a certain
extent. Clays are generally lamellar in shape. At low levels, they show a major
advantage of reducing cracking but at higher levels mud - cracking can occur. If
used in primers, some acceleration of corrosion can take place. In addition, they
are specifically introduced to give even mattness. The major variations between
the grades are in particle size and therefore ease of dispersion.
ii. Bentonite type clays, these are hydrated aluminum silicates complexed with
magnesium and calcium oxides. It is assigned the
general formula,
Al2O3.4SiO2.2H2O, but the aluminium is often replaced to varying degrees by
magnesium and sodium . These clays have the specific property of producing a gel
in water, thus are used as thickeners in certain types of water-based paints. Of
more common use are amine modifications of these clays called bentones, which
can form gels in solvent systems and are therefore used specifically as bodying and
anti-settling agents.
ii. Slate powders are impure hydrated aluminium silicates obtained as a dark grey or
greenish grey powder from slate, which is a highly compressed naturally-occurring
silicate being a complex mixture of talc, mica and clay. It is very hard and
coarse,but cheap and is used to produce dark-coloured fillers and stoppers because
it facilitates rubbing down and does not clog the paper. It’s poor colour, is
undesirable for most paint finishes but it has a generally lamellar structure which
imparts good crack and moisture resistance to products.
iii Mica is a hydrated potassium aluminium silicate, the composition of which can be
represented by the formula K2O.Al2O3.6SiO2.2H2O, but many variants occur.Mica
is an inert material and is used in paint because of the ability of the particles to leaf
or to orientate themselves parallel with a paint surface and to overlap. This property
74
confers improved moisture resistance and anti-penetrating properties and in some
cases,hard settlement of thepigment in the liquid paint is prevented.
1.6.2.5.3 Barium Sulphate
This extender is available as natural barytes or precipitated blanc fixe, the major difference
is that the precipitated version is cleaner and more easily dispersed. Barytes shows colour
variation, but has the advantage of being chemically stable and inert to all paint media. It is
widely used in undercoats and primers, fillers and stoppers in which it gives physical
reinforcement to the film , imparts good blister resistance and sanding properties. Its
disadvantages are that it is difficult to disperse and can settle badly. Blanc fixe ,as a
precipitated product has a finer texture and higher oil absorption than barytes. In water
paints, the finer texture of blanc fixe gives it greater opacity than ordinary barites and
confers smoother working properties on the paint 65.
1.6.2.5.4 Hydrated Magnesium Silicates (Talcs)
The most common form of this extender is talc, which has a ‘greasy’ feel and can be
represented by the formula, 3MgO 4SiO2 .H2O .A fibrous form exists as asbestine but its
use in the paint industry has been discontinued for health reasons due to its structural
similarity to asbestos. Ground talcs contain a variety of particle shapes, but lamellar
particles predominate. Talc is a useful general-purpose extender for solvent-based paints. It
is hydrophobic and consequently disperses without difficulty .In water however, talc has a
great tendency to flocculate. The major advantages of talc is that it prevents cracking in
high PVC exterior paints, reduces water penetration, improves brushing properties and has
good sanding properties in undercoats 4. However the colour of talc is variable but
generally poor, high levels can give chalking on exterior paints.
1.6.2.5.5 SILICA
The paint industry uses silica obtained from natural sources as well as manufactured
hydrated silicas. The former tend to be harder and more coarse than the latter. The various
grades of silica are introduced into paints to take advantage of their low specific gravity,
giving non-settling properties.
A.Natural Silicas
i. Quartz : This is a crystalline form of silica and is the most common form of natural
silica obtained from quartz sands. It is a pure grade of silica, also known as silica flour
75
and is used in paint products for its transparency and good settling resistance The main
use of quartz extender is in fillers and stoppers.
ii. Flint :The ground form of quartz, sometimes used in fillers and stoppers is known as
flint. It is not as hard as quartz
iii. Kieselguhr : This a member of the family known as diatomaceous earths.These
consist of the siliceous remains of diatoms and other tiny prehistoric aquatic creatures
which were deposited in primordial seas over long periods of time. It varies in colour
from white to pink or grey
69
.The deposit is amorphous, readily powdered and highly
absorbent. It is therefore used as a flatting and consistency control agent ,to give body,
and reduce settlement. in paints and as a filter aid in oil processing.
iv. Celite : These comprise a range of diatomaceous earths and are characterized by
excellent colour.They are marketed in various grades ranging 2 to 20µm mean particle
size and with oil absorption from 75 to 110.
65
They are used for flatting and for
viscosity control in paints.
v. Diatomite : This is a diatomaceous hydrated silica obtainable in a range of particle
sizes with oil absorptions from 60 to 100. The specific gravity is 1.95 and refractive
index 1.46. Diatomite is used as a flatting agent and it is claimed to improve intercoat
adhesion.
B. Manufactured Silicas
These are generally of very fine particle size and low specific gravity. They are marketed
under a number of trade names. Synthetic silicas used are produced from two sources;
precipitated silicas are produced from solution and have higher particle size than the vapour
phase silicas (fumed silica). Their uses however are similar in that they are generally used
to control gloss and produce structure. A typical member is produced by flame hydrolysis
of silicon tetrachloride
Manufactured silicas are chemically inert and by virtue of their large surface areas, they
are used for flatting and viscosity control in solvent type paint. They are also used to
boost viscosity in emulsion.
Toxicity of Silicas
Amorphous grades are classified as ‘nuisance particulates’, but crystalline quartz of fine
partcle size is more hazardous and can lead to silicosis.
76
1.6.2.5.6 Calcium Sulphate
Calcium sulphate is obtained from natural sources such as Gypsum and Plaster of Paris. It
has high water solubility therefore its use is limited although it has certain advantages when
used in sealers.
Summary
In general the selection of extenders for normal uses is based initially on the following
considerations:
Calcium Carbonates
-
Low cost, poor acid resistance
Clays
-
Talcs
-
Poor colour, good brushing, good
resistance to settlement
prevents cracking, reduce water penetration
Barium Sulphate
-
good blister resistance good sanding, bad settling
Silica
-
mainly for gloss control, settling resistance and
rheology
1.6.2.6 PAINT ADDITIVES
In addition to the major components, a number of so-called additives are used in paint to
fulfill a particular function. Additives are materials usually employed in small proportions
of the total composition of surface coatings to improve or modify the performance
characteristics in various ways 73. Driers, skinning inhibitors, fungicides and wetting agents
come under this heading.
Additives when added to paint in amounts that can be as little as 0.001% and seldom more
than 5%, have a profound influence on the physical and chemical properties of the paint..
One employs additives to provide non-setting properties, non-floating properties or other
properties. The only difference between an additive and any other raw materials is the
amount used. Normally, the amount of an additive in a formulation is in the vicinity of 0.1
to 2.0%. Some properties that can be controlled by the use of additives are the following 4,
65, 66
i.
Viscosity can be increased or decreased.
ii.
Settling can be prevented.
iii.
Drying can be accelerated.
iv.
Foaming or bubbling can be eliminated.
v.
Flocculation can be created or eliminated.
vi.
Flow and leveling can be increased or decreased.
77
vii.
Stability can be obtained.
viii. Brushing can be improved.
ix.
Adhesion to a poor surface such as rust can be improved.
x.
Penetration can be increased or decreaed.
The above list gives an idea of the types of properties that can be controlled by the use of
additives. These properties show that there are lots of chemicals, which are employed for
the formulation of paints and other surface coatings. Some of the additives are either for
oil-based paint or water thinned paints.
1.6.2.6.1 Driers
A drier is an additive which catalyses the oxidation and polymerization of oil, alkyd and
oleoresinous films. Driers are metallic soaps of certain organic acids, which are added to
certain types of paints (e.g gloss) to bring about the change from liquid to solid state in a
reasonable time. Without driers, the rate of drying would be so slow as to be impracticable.
The metals used are multivalent and include lead, manganese, cobalt, calcium and
zirconium, while the usual compounds are naphthenates, tallates and octoates. Naphthenate
driers are made from naphthenic acid, tallate driers from tall oil while the octoates are made
synthetically. These metals are divided into two categories: active or participating driers
and auxiliary (or through) driers70.Active driers promote oxygen uptake, peroxide
formation and peroxide decomposition e.g cobalt and manganese.Consequently, they
promote rapid surface or top drying, but show poor through-drying. Auxiliary driers do not
show catalytic activity themselves, but appear to enhance the activity of active drier metals
by improving the mobility or solubility of the active drier metals in the reactive medium,
examples are lead, zirconium, barium. They have weak surface drying but good through
drying. Driers are normally supplied at the highest possible metal content level. The acid
radical is largely irrevelant since in the process of drying, it is the metal cation which acts
as an oxidation initiator.
The drying process takes place in three steps: 71
i.
an induction period in which any inhibitors (antioxidants) present are destroyed.
ii.
absorption of oxygen and formation of hydroperoxides and peroxides
iii.
Decomposition (cleavage) of hydroperoxides to form free radicals and
subsequent crosslinking of the fatty acid chains
Driers act in a combination of the following reactions 4, 70, 71
1. They oxidize antioxidants present naturally in the film and thus eliminate an
inductive period before drying commences.
78
2. Cobalt and manganese type cations act as oxygen “carriers” making oxygen
available where required at reactive sites.
3. Lead-type
driers
accelerate
through-drying
by
promoting
hydroperoxide
decomposition and accelerating polymerization.
4. Driers increase the overall capacity for the system to absorb oxygen.
The anionic portion of the drier is specially selected to ensure that the drier is completely
miscible with all binder systems at all stages of drying. Four major groups of acids are
employed in the manufacture of driers; these are naphthenates, octoates, tallates and
dodecanates.
The metals generally available and their particular properties are as follows 4, 70:
1. Cobalt – the most reactive metal, promotes surface dry and provides the “tack free”
drying. Too much cobalt gives too rapid surface dry and can produce wrinkling.
2. Lead – this promotes drying through the film. Used at relatively high levels, lead is
nearly always used in combination with another metal such as cobalt. Lead has the
disadvantages of sometimes reacting with acidic vehicles, being also toxic and
reducing sulphide resistance of paint films.
3. Manganese – This metal promotes both surface- and through- dry but can impart
brittleness when used alone. It can replace cobalt in combination with lead, whilst a
combination of manganese and cobalt gives increased hardness.
4. Calcium – this is a useful auxiliary drier, it can replace lead in combination with
cobalt and manganese. Also it can be used to stabilize lead in reactive vehicle
systems.
5. Iron – for high temperature reaction, iron is useful although its reactivity is limited
at room temperatures. The dark brown colour also limits the usefulness of this drier.
6. Zinc – this is also an auxiliary drier, it has the particular property of slowing the
initial surface dry and allowing more complete through dry without wrinkling. Zinc
is reputed to prevent floating and flooding and to improve gloss.
7. Zirconium – This is normally used to completely replace lead in combination with
cobalt and manganese, part of which it can replace.
Selection of driers is dependent upon the type of vehicle used and the drying properties
required. The sort of levels required is illustrated in Table 1.4
Various specific problems can occur related to drier choice, for instance:
79
1. Loss of dry - Certain pigments, notably carbon black will absorb driers causing reduced
drying on storage. This can be partly overcome by dispersing the drier into the paint (but
not cobalt due to high reactivity). Litharge ground into the mill base can be a solution to
this problem. Certain semi-soluble “feeder” driers are becoming available and are
considered to solve this problem.
Table 1.4 : Drier Recommendations for Alkyds 4
Alkyd
Modifier
(Oil)
Length
Driers (%) Metal on non-volatile content of Vehicle
Ca
Co
Pb
Mn
Dehydrated
Short
-
0.05
0.3-0.6
0.0-0.03
caster
Medium
0.1
0.05-0.07
0.4-0.7
-
Long
-
0.05-0.10
Short
-
0.05
0.3-0.5
0.03
Medium
-
0.03-0.05
0.3-0.5
0.03
Long
-
0.02-0.05
0.5-0.8
0.02-0.03
Short
0.1
0.05
0.1
0.03
Medium
0.1
0.01-0.05
0.2-0.3
0.0-0.3
Long
-
0.03-0.07
0.3-0.6
0.0-0.3
Tall oil
Medium
0.2- 0.5
0.05-0.10
0.0-0.2
0.0-0.2
2. Seeding
-
Linseed
Soyabean
3. Skinning
-
Sometimes caused by reaction of lead driers with acid
vehicles.
- normally due to excessive driers, more specifically
cobalt. Antioxidants and stronger solvents can overcome this
problem.
4. Loss of gloss -
one of many causes of this defect can be excess cobalt
in the presence of PE based alkyds.
1.6.2.6.2 Antioxidants
As mentioned in the section on driers, it is possible for oxidative paints to produce an
insoluble skin of oxidized material. Where this is not due to excessive drier such as cobalt,
it can be overcome by addition of certain additives, which will inhibit surface reaction.
These products are usually of the phenolic or oxime type, methyethyl ketoxime being the
most used. Sometimes specific solvents, which are not chemically antioxidants but are slow
boiling, strong solvents can have the same effect as true antioxidants.
80
1.6.2.6.3. Catalysts, Activators and Accelerators 4, 69
By definition a catalyst initiates but does not actually take part in a reaction, thus in paint
formulation, a catalyst is added as a minor ingredient to cause a reaction which converts a
liquid paint into a solid film. A major example of this is the use of a peroxide to initiate the
polymerization of an unsaturated polyester. A wide range of peroxides is commercially
available of which benzoyl, methyl ethyl ketone, cyclohexanone and cumene are the most
popular. None of these products can react at room temperatures with polyesters sufficiently
fast enough to be viable commercially, however, they all require an accelerator, which will
speed up the peroxide decomposition. Normally cobalt driers are used with the ketone
peroxides whilst amines are used with benzoyl peroxide, this latter combination however
can lead to yellowing. Activators actually take part in a reaction and examples are
diamines, which react with epoxy resins and isocyanates reacting with alkyd or hydroxy
acrylic resins.
1.6.2.6.4 Thickeners 4, 65
All paints have an inherent viscosity which whether Newtonian or Non-Newtonian may
need to be increased for improved application properties etc. There are various methods of
achieving this:
1. Increased pigment level – by achieving closer packing of the pigment particles, flow
is reduced i.e viscosity is increased. However this method normally has major
disadvantages of giving poorer gloss, higher cost etc.
2. Introduction of silica – fine particule silicas have extremely high surface area and if
dispersed into the paint physically reduce the flow potential of the paint. Bentonite
(A1203, 2SiO2, 2H2O) in water- based paints has a slightly different mechanism in
that it swells by absorption of water. Bentones contain ammonium groups and react
with various organic solvents to different degrees to swell in a similar fashion.
3. Cellulose incorporation – cellulose either in the acetate buyrate or ethyl form are
high molecular weight solids, which can be dissolved to give highly viscous
solutions. Ethyl cellulose (or variants) being soluble in water are used as thickener
in emulsion paints. Cellulose acetate butyrate can be used in solvent paints where
the solvents are strong enough.
4. Bodied oils – high viscosity oils can be obtained by polymerising drying oils either
by heating in the presence of catalyst (stand oils) or oxidizing with oxygen (blown
oils). These oils can be added to oleoresinous or alkyd finishes to give increased
viscosity.
81
5. Amide resins – it is possible to modify alkyd resins with amide groups during
processing, these groups provide strong hydrogen bonding in the resin, which is
evident as thixotropy. However, these resins suffer from the problems of being
expensive, difficult to handle and sometimes prone to yellowing.
1.6.2.6.5 Flow Aids
Agents termed flow aids are frequently added to paint for two purposes:
i.to improve flow in general i.e. reduce mottle, orange peel etc.
ii.to overcome cissing and cratering which can occur in paint films.
Flow aids can be divided into two major types:
i. Silicone Type
Silicones or silicone polymers , also known as polyorganosiloxanes or simply polysiloxanes
contain a backbone chain made up of silicon atoms alternating with oxygen atoms instead
of the usual carbon chain found in the backbone of most polymers.77, 78 The side groups
attached to silicon are organic in nature and it is therefore a semi organic polymer of the
type
R
Si
R
R
O
Si
R
R
O
Si
R
R can be either hydrogen or paraffin-type chain.
Silicones simply lower the surface tension of the film thus giving increased flow – this
results in improved film appearance. Silicone solutions however are extremely critical in
the level at which they operate, excess silicone reduces surface tension too much and results
in a film which can itself crater in addition to giving rise to cratering in other films which
might be contaminated. Various silicones of variable molecular weights are available with
somewhat varied recommended uses as detailed by the supplier.
ii. Surface Active Agents (Surfactants) 4, 65
Surfactants are added to paint formulations to assist in dispersing the pigment in the
medium and to modify the properties of the coatings. Surfactants are in effect soaps i.e.
they normally contain a polar “end” to the molecule with a long chain non-polar tail. Thus
one part of the molecule is attracted to polar molecules whilst the other is attracted to nonpolar molecules, the effect of this is that when a paint film is contaminated with dust etc.
82
the surfactant reduces the interfacial tension between the paint and the dust allowing the
dust to be absorbed into the film.
1.6.2.6.6 Flatting Agents 4,65
Where a fully matt or non-glossy film is required this is normally achieved by using
conventional extenders at such level that the film is fully integral but has no gloss.
To achieve semi-matt pigmented, or more particularly non-pigmented films is less easy and
formulation is more critical. In these cases it is more usual to use a mettalic “soap” or a
coated extender, which has a given particle size and operates by being present in the surface
of the film and reducing light reflection at the surface. In certain cases, flatting agents can
be stirred into the finish but more usually a predispersed intermediate is used. Zinc stearate
and silica coated with silica fluoride are examples of flatting agents.
1.6.2.6.7 Fungicides and Bactericides 4
The binders and thickeners used in emulsion paints are susceptible to attack in the container
by bacteria and in the film by fungus, for this reason additives are added to kill or prevent
the growth of bacteria and to stop the growth of fungus on the film.
These additives cover a wide spectrum of chemical types such as phenyl mercury salts,
sulphur compounds, metaborates, isothiazolones plus many varied recently introduced
compounds. The major problem associated with selection is the toxicity of these
compounds mainly in terms of handling during manufacturing.
1.6.2.6.8 Dispersion Aids
During the process of dispersing pigments it is frequently necessary to incorporate an agent
that will assist the solvent and vehicle to replace the air and moisture absorbed on the
surface of the pigment aggregates. These agents termed dispersing or wetting aids not only
make dispersion easier, but impart a certain degree of dispersion stability.
Dispersion aids are generally characterized by consisting of molecules with two separate
components. One component is soluble in solvent or compatible with vehicle, the other
component is rejected by the solvent or vehicle but attracted to the pigment surface.. Thus
not only does the solvent/vehicle rejected component become concentrated around the
pigment displacing the air and moisture, which allows the pigment particles to be separated
more easily but the presence of the other component being dissolved in the total vehicle and
solvent system stabilizes the resultant dispersion.
83
In practice pigments are either hydrophilic, hydrophobic or occasionally neutral, this has
the effect that hydrophilic pigments are relatively easily wetted by water but with difficulty
by non polar organic media, for hydrophobic pigments the situation is reversed. Most
inorganic pigments are hydrophilic whilst organic pigments are hydrophobic. Thus the
necessity for dispersion aids does vary to an extent, depending upon the pigment type and
liquid medium.
Dispersion aids for non-aqueous systems can normally be classified into the following
types 4:
i. Long chain polar compounds
ii. Fluorocarbon compounds
iii. Silicone compounds
Typical long chain polymer compounds are hydrocarbons with end chain polar groups such
as – COOH, - NH2, - OH, - CONH2, - SH, - SO3H and salts or esters of fatty acids.
Flurocarbons with polar groups have the specific advantage as do the silicone compounds
of reducing interfacial tension at the pigment surface, which also assists in the wetting
action.The dispersion aids or surfactants, as they are normally termed for aqueous systems
are far more varied although they can be classified into four types:
a. Anionic – long chain carries negative charge in solution.
b. Cationic – long chain carries positive charge in solution.
c. Nonionic – do not ionize in solution.
d. Amphoteric – ionize in solution but charge depends upon pH of solution.
Anionic surfactants form probably the largest range of surfactants for aqueous systems, the
most common types are the water- soluble salts of long chain carboxylic acids. These acids
are obtained naturally and can be saturated or unsaturated with normally between 12 and 22
carbon atoms in the chain. Sodium and potassium are the metals most used to form the
salts. The other common type of anionic surfactant is the class of sodium alkylbenzene
sulphonates whilst a third lesser class are the sulphated vegetable oils. Cationic surfactants
are normally long chain primary, secondary, tertiary or quaternary amines but are rarely
used in aqueous systems (i.e. emulsion paints). Non-ionic and amphoteric are also rarely
encountered in paint formulation.
84
1.6.3 PAINT FORMULATION
Proper paint formulation centers around the specific requirements of the particular
application .These requirements are hiding power, colour, weather resistance, washability,
gloss,
metal
anticorrosive
properties
and consistency as
related
to
type
of
application(brushing, dipping, spraying or roller coating. Individual requirements are met
by proper choice of pigments, resins, extenders, and vehicles by the formulator. Since the
techniques of paint technology formulation are still largely empirical, it is difficult to
predict the properties of a specific formulation and this often means that a considerable
number of trials have to be run before the desired properties are obtained
1.6.3.1 REQUIREMENTS OF A GOOD PAINT
The general requirements of a good paint can be summarized as follows 72 :
(1) The paint pigment should be opaque to ensure good covering power.
(2) It should be chemically inert to secure stability and hence long life.
(3) The paint should have a good colour and high hiding power.
(4) It should be weather- resistant.
(5) It should have good washability and metal anti-corrosive property.
(6) Its consistency should be suitable to appreciation by the types of application such as
brush dipping, spraying or roller coating.
From the modern viewpoint, it is believed that the most important concept, which control
various factors, such as gloss, washability, durability and reflectance and rheological
properties are largely controlled by pigment volume concentration (PVC), which is defined
as 68,
%PVC =
vol of pigment in the pa int
x 100
vol of pigment in the pa int + vol of non − volatile constituents in pa int
The inherent vehicle requirements of the pigment-extender combination being applied,
however, affect the PVC used in a given formulation. As a consequence, there is usually a
range of PVC for a given paint, as indicated in Table 1.5
85
Table 1.5 The PVC Range for Various Paints 73
TABLE (1)
Paint
PVC range
Paint
PVC range
Flat paint
50-70%
Exterior House Paint
28-36%
Semigloss Paints
35-45%
Metal Primers
25-40%
Gloss Paints
25-35%
Wood Primers
35-40%
1.6.3.2 IMPORTANCE OF PVC
(1) The gloss decreases as the PVC increases. This is due to the fact that when volume of
pigment increases relative to the non-volatile vehicle, gloss decreases until the finish or
gloss of the paint becomes flat. (2) With increase in PVC, adhesion as well as durability,
both decrease. If volume of pigment increases as compared to the volume of binder, the
film will loose cohesion. The paint will be in powdered form and obviously will have little
durability, (3) when extenders are added, the PVC increases and gloss decreases.
The addition of pigment initially reinforces and improves the film-forming properties of the
paint, until a critical PVC concentration (CPVC) is reached. At this point and beyond, the
resistance properties of the paint decrease, as the film becomes porous, causing the paint to
weather faster and lose abrasion resistance and flexibility.
1.6.4
Emulsion Paints 65, 75
The most popular emulsion paints are based on the resin, poly (vinyl acetate) and acrylic
lattices. The former are prepared by the emulsion polymerization of vinyl acetate,
CH2=CHOCOCH3 and are stabilized by a combination of surfactants and protective
colloids. Although, PVA is a relatively soft polymer,like polymethyl acrylate,its glass
temperature is above room temperature.Emulsion paints must coalesce to form a film at
room temperature,so PVA must have its glass transition temperature lowered,either by
addition of a lacquer plasticizer, such as dibutyl phthalate, or by copolymerization with
other monomers e.g ethyl acrylate, 2-ethyl hexyl acrylate.In acrylic lattices, the
‘hard’monomer,is methyl methacrylate and the plasticizing monomer, an acrylate, such as
butyl acrylate or one of the acrylate comonomers mentioned above.Acrylic lattices usually
contain copolymers of acrylic or methacrylic acid as colloids and thickeners, these being
solubilized by neutralization with base. Whatever, the type of latex, coalescing solvents are
also normally added to improve film formation. These may or may not be water miscible
and include alcohols, glycols,ether-alcohols, ether-alcohol esters and even hydrocarbons.
Pigment is dispersed in the continuous phase with suitable surfactants, additives and a
86
water-soluble thickener,e.g hydroxyethyl cellulose.The dispersion and latex are carefully
blended with efficient stirring to form the paint. It is most stable if just alkaline. The
additives include fungicides, to prevent mould growth feeding on cellulosic or other
colloids in the dry film, and biocides, to prevent bacterial degradation of colloid and
thickener molecules in the paint can. The bacteria or enzymes produced by them, rapidly
reduce molecular weights of these molecules, leading to a dramatic decline in viscosity and
malodorous by-products. Susceptibility to bacterial attack is dependent not only latex
colloid and thickener chemistry, but also on pH and whether or not other chemicals with a
biocidal action,such as certain coalescing solvents or traces of unpolymerised monomer are
present. Emulsion paint is easy to apply and dries quickly without unpleasant smells.
Brushes and rollers can be cleaned with water. However, it has not been possible to date, to
produce a satisfactory full gloss version. To get a smooth surface of high gloss, fine particle
lattices must be used and relatively large quantities of polymer in solution. Such paints give
less than perfect flow on application. In addition, it is hard to keep a water-based coating
fluid for long enough to give good merging of brushed overlaps, and application over
porous subtrates can lead to low gloss. Acrylic lattices and vinulidene chloride copolymer
lattices are used in water-based gloss finishes.
1.6.5. PAINT DEFECTS 4, 65, 74
A high proportion of paint defects and failure results from inadequate surface preparation,
but other factors can also be involved. These include poor application technique or
incorrect method of application, application under unsuitable conditions e.g low
temperature, humidity, insufficient coating thickness, the use of paint unsuited to the
substrate or to the exposure conditions. In each case, there is a special term used to describe
it. Some of the common painting defects are described below.
i. Bittiness
Bittiness in the paint coating is nearly always due to extraneous matter picked up during
painting by use of dirty brushes or carelessly cleaned spray apparatus,by the incorporation
of broken skins during stirring and failing to strain paint on which skin has formed.
Insufficiently dispersed pigment can also manifest in the form of bits in the paint.
ii. Bleeding
This term describes the discoloration or staining of newly –applied paint by a constituent of
the old paint, or other material, over which it is applied. It is commonly encountered when
painting over bituminous or tar products.
87
iii. Brushmarking (Ropiness, Ribbiness, Tramlines)
Brushmarks result from the failure of the paint to flow out after application i.e poor flow. It
is not in the nature of certain types of paints to flow to a great extent and a smooth surface
,free from brushmarks , cannot be expected. This is a common fault with highly pigmented
materials such as decorative undercoats in which pigment/extender content is high or where
there is intense pigment fluctuation. The poor flow is a feature of pseudo-plastic
consistency and this, in turn, is controlled by the type and relative amounts of the extenders
When not due to the nature of the paint, ropiness can usually be traced to one of the
following : uneven or careless application of undercoat or finish, overbrushing, using a
paint too thick, using dirty clogg.
iv. Blistering
Blistering is usually caused by the trapped evaporation of trapped moisture or solvent. If
surfaces are painted shortly after washing down,even if time is allowed for surface
drying,blistering may occur as a result of moisture absorbed into the old paint. The paints
most prone to blistering on prolonged exposure to water are the decorative types based on
alkyds epoxy esters and oleoresinous finishes.
v. Blooming (Haze, Milkiness)
Blooming is the formation of a surface mist or haze considered to be due to the presence of
moisture or impurities in the atmosphere. It is a deposit on glossy films resembling
the‘bloom’on a grape. The haziness can be removed by polishing with a soft rag, or by
washing, but is liable to return under certain atmospheric conditions.
vi. Chalking
This term is used to describe surface powdering of the paint film. It can occur when paint
deficient in binder or vehicle is used on surfaces exposed to the attack of weather or when a
normal paint is applied over an insufficiently sealed , porous surface Highly-pigmented oil
paints are likely to disintegrate in this manner. Chalking may also occur if the paint film has
been adversely affected by alkalinity, moisture etc., during drying.
vii. Cissing
This is the shrinkage or drawing away of the new coating from large or small areas of the
work. It may become apparent as small ‘craters’ or circles, or it may result in almost
complete lack of adhesion over a large area, where the paint gathers in blobs. It occurs
88
more readily with varnish but does happen occasionally with gloss finishes. It is nearly
always due to greasiness of the undersurface or to’ sweating’ of an over oily undercoat.
viii. Crazing (Cracking)
Crazing is the breakdown of a paint or varnish usually due to ageing. Progressive chalking
or powdering of the paint film is called chalking and is caused by the destructive oxidation
of the oil after drying of the paint on the surface to be coated is called flaking or peeling
and caused by the presence of dirt or grease on the surface or water entering from below the
paint. If the centre portion remains attached to the surface and the portion around the centre
peels off, a term ‘alligatoring’ is employed. Fine surface cracking is called ‘checking’ and
is due to the absence of plasticizers in the paint.
Paint defects and failure can be generally avoided by careful mixing of the constituents or
ingredients in specified proportions, proper preparation of the surface to be coated before
the paint is applied, using a paint suitable for the surface, applying the paint under the right
conditions and using a primer coat before the application of the paint.
1.6.6 Renewable Resource in Paint Industry
The gradual global transition from non-renewable (fossil-based) raw-materials to renewable
(plant-based) has intensified the search for alternative industrial raw-materials. The paint
industry is not left out in this as several plant-based materials have been introduced,
particularly as fillers. These include wood flour, walnut shell flour, corn cob flour,
cellulose, wheat flour, rice flour, rice hull flour, pecan shell flour etc 76
THE RESEARCH DESIGN
This research is in two dimensions- Basic Strategic Research and Applied Research. The
basic strategic study is aimed at investigating the effects of systematic variation in
combustion temperature and time on some physical properties of rice husk ash specifically,
the colour, surface morphology and yield as well as the chemical properties of RHA,
involving its silica and metallic oxide composition at the varied conditions. The Applied
research, which is on product development, involves exploring the applicability of rice husk
ash as a new paint extender, which can totally (or partially) replace some standard
commercial extenders. In this area, RHA will be tested as extender, thickening and flatting
agent in some decorative and industrial paints.
89
The findings from the basic research will undoubtedly, be useful to other workers and
prospective workers carrying out various types of studies on rice husks and rice husk ash
while the results of the product development will be of significant importance to the paint
industry, which is constantly in search of cheaper, more effective and in more recent times,
renewable alternative raw- materials for sustainable development of the polymer industry.
90
CHAPTER TWO
2.0
LITERATURE REVIEW
2.1 The Chemical Nature and Composition of Rice Husks
Many plants have the ability to absorb and accumulate various minerals and silicon
compounds from the earth into their systems; a typical example being rice (oryza sativa 45,79
Inorganic materials, especially silicates are found in higher proportions in annually grown
plants such as rice, wheat and sunflower, than in long-lived trees. The inorganic materials
are found in the form of free salts and particles of cationic groups combined with the
anionic groups of fibres in the plant.80 The silica occurs in several forms within the rice
husks and is associated with water. It is this silica, which, when concentrated in the husk by
burning, that makes the ash so valuable.
Earlier investigations carried out to determine the chemical nature of rice husks revealed
that it contains a high proportion of lignin and about 20% of their outer surfaces are
covered with silica81,82 which is deposited onto this surface as monosilicic acid to form a
silicon- cellulose membrane.83 Other investigations confirmed that the exterior of rice husk
is composed of dentate rectangular elements which themselves are composed mostly of
silica coated with a thick cuticle and surface hairs. The mid region and inner epidermis
contain little silica.40 Bharadwaj et al
84
determined the scanning electron microscopic
image of a virgin rice husk particle and observed that the structure resembled that of a
composite material with fibers regularly interspaced in the matrix. The fiber was found to
be silica and the matrix consisted largely of cellulose, hemicellulose and lignin. The
transverse (cross-section) of the rice husk particle was found to be dense with no visible
porosity. The diameter of the longitudinally aligned silica fibers was found to be between 5
and 10µm. Rice husk is an irregular boat-like particle and is about 8-10mm in length, 2-3
mm in width and 0.2mm in thickness.16 It is very light, with a packing density of 122kg/m3
and is also characterized by a high volatile content, a high ash content and a nearly uniform
size.
Several researchers have used RH as a source material for various studies either in its raw
form or after conversion into ash by burning the husks. In the course of these studies,
efforts were made to establish the chemical composition of RH by proximate analysis,
ultimate analysis and lignocellulosic content. Thus Muntohar
85
stated that the chemical
composition of RH typically corresponds to the following cellulose (40-45%), lignin (2530%) ash (15-20%) and moisture (8-15%) Daifullah et al17 and Arnesto et al 52 presented
91
the main constituents of RH as 64-74% volatile matter, 12-16% fixed carbon and 15-20%
ash. The results of the RH composition determined by Govindaro18 did not quite correspond
with that of Muntohar since he obtained 32.24% cellulose, 21.34% hemicellulose, 21.44%
lignin, 1.82% extractives, 8.11% water and 15.05% mineral ash. This variance was
reflected in another report,
87
which gave the cellulosic content of RH as 55-60wt%
(including cellulose and hemicellulose) and lignin as 22wt%. These variations in
lignocellulosic contents are probably due to differences in rice crop species as reflected in
the work of Balakrishnan
87
who determined the proximate analysis and chemical
composition of rice husks from four different varieties of rice obtained from Malaysia.
Results of the proximate analysis (% dry basis) gave volatile contents of 66.4, 67.3, 63.00,
67.30 and 66.08 for the four species; fixed carbon contents of 13.6, 13.9, 12.40, 14.20 and
13.53 respectively and ash contents of 20.00, 18.80, 24.60, 18.20 and 20.20 respectively.
The cellulosic contents (%) of the four species were found to be 29.20, 33.47, 25.89, 35.50
and 31.02; their hemicellulose contents, 20.10, 21.03, 18.10, 21.35 and 20.15 respectively
and their lignin contents , 30.70, 26.70, 31.41, 24.95 and 28.44 respectively. This variation
is further revealed in other works.
Simonov et al,
88
in their study of the catalytic combustion of rice husk, carried out a
general analysis of the RH composition (in %) and obtained the following: volatile
substances (70.2), fixed carbon (14.1) and ash (15.7). His determination of the elemental
composition of RH gave (in %): C (43.5), H (5.5), O (35.2), N (0.05), and Cl (0.01). In
comparison, Natarangan and Sundaram
89
obtained a similar volatile content of 70.6, a
considerably lower value of 2.97% for fixed carbon, and an ash content of 17.09%. Their
elemental analysis (in %) gave values that do not vary widely from those of Simonov as
follows: C (50.45), H(6.58), O(41.46), N(1.49), S(0.23), moisture content (9.45) and a
calorific value of 19807KJ/kg. The investigation by Mahvi et al 90, of the potential use of
RH for phenol removal in aqueous systems necessitated their characterization of the husk
used for the study. The results they obtained for the proximate analysis of the RH did not
vary widely from those of Simonov and Natarangan
89
(in wt%): volatiles (59.5), moisture
(7.9) and ash (17.1); those of ultimate analysis (in wt%) gave carbon (44.6), hydrogen (5.6)
and oxygen (49.3) while the component analysis (wt%) gave cellulose (34.4), hemicellulose
(29.3) and lignin (19.2). The cellulosic content is evidently close to those obtained by
Balakrishnan but the hemicellulose value is appreciably higher and the lignin content
significantly lower. It is probable that differences in experimental methods are contributory
to the variation in results apart from differences in rice variety earlier mentioned, for
92
instance the volatile content could be determined at differently selected temperatures. Kim,
et al
22
,in their study on thermogravimetric analysis of rice husk flour-filled thermoplastic
polymer composites, determined the ash and lignocellulosic content of rice husk flour vis-avis that of wood flour and obtained the results presented in the table below.
Table 2.1 The Chemical Constituents of Wood Flour and Rice Husk Flour22
Lignin
Holocellulose
Others
Ash
Rice Husk Flour
21.6
60.8
5.0
12.6
Wood Flour
26.2
62.5
10.9
0.4
Rice Husk Flour*
20.6
59.9
6.3
12.2
*specification from Saran Filler Co.
Holocellulose is the total polysaccharide fraction of wood i.e. the mixture of cellulose and
hemicellulose that is obtained as a fibrous residue when the extractives and the lignin are
removed from the natural material.91
Another report92 using rice husk from India, gave the elemental composition of RH
(expressed in mass fraction %) as C (41.44), H (4.94), O (37.32), N (0.57), Si (14.66),
K(0.59), Na(0.035), S(0.300, P(0.07), Ca(0.06), Fe(0.006) and Mg (0.003).The values for
C, H and O do not differ widely from that of Simonov et al.
In their investigation of the utility of rice husk as a nutrient medium for the growth of
bacteria, Chockalingam et al
93
carried out elemental analysis of the RH sample also
sourced from India and obtained the following results: C : 45.5%, H : 7.3%, N : 1.1%, S :
0.92%, and O: 44.18%. The organic composition of the components dissolved from the rice
husk was found to be (i) 1.66% lignocellulosic material (ii) 24% xylose + arabinose and
(iii) 10% galactose. The inorganic elemental analysis of the rice husk revealed the presence
of potassium, magnesium, sodium, calcium, manganese, iron, boron, copper, zinc,
phosphate and nitrate. Similar results on ultimate analysis, to that of Chockalingam were
presented in another report
16
(in wt%) as C (45.28), H (5.51), N(0.67), S(0.29) and
Cl(0.19). In another report, by Janvijitsakul and Vladimir 94 , the proximate analysis of Thai
rice husk gave a moisture content of 11.00 and an ash content of 17.64% while the ultimate
analysis gave the following. C (37.5), H (5.20), 0 (39.0), N (0.31), and S (0.05).
Kwong et al
95
, from environmental impact considerations, studied the combustion
performance and pollutant emission of a combination of coal and rice husk as fuel. For this
93
purpose, the proximate and ultimate analyses of coal vis-à-vis rice husk were determined
and gave the following percentages on dry basis;
Coal (wt%)
RH (wt%)
Moisture
15.86
10.61
Volatile matter
28.45
64.44
Ash
14.63
8.93
Fixed carbon
41.06
16.02
C
69.35
46.57
H
4.40
4.61
N
1.40
0.73
O
9.21
39.04
S
1.01
0.12
Proximate Analysis:
Ultimate Analysis
Calorific value (CV)
27464
16054
KJ/ kg
It is evident from the result that the nitrogen and sulphur contents of RH, which directly
affect NO2 and SO2 emissions during combustion, are lower than those of coal. This
substantiates the fact that RH, a biomass is more environment-friendly than coal, a fossil
fuel. The result however, showed an unusually low RH ash content (8.93) considering that
the ash content is known to be about 20%.
A typical analysis of rice husks gives the result presented in Table 2.2
Table 2.2: Typical Rice Husk Analysis40, 96
Property
Range
Bulk density, kg/m3
96-160
Length of husks, mm
2-5
Hardness (Mohr’s scale)
5-6
Ash %
22-29
Carbon %
~ 35
Hydrogen %
4-5
Oxygen %
31-37
Nitrogen %
0.23-0.32
Sulphur %
0.04-0.08
Moisture %
8-9
94
The results show there could be fairly wide variations in bulk density and ash contents of
rice husks, probably due to paddy variety and other factors.
2.2
GENERAL USES OF RICE HUSKS
There is ongoing research in major rice-producing countries of the world to find
uses for rice husks. The husks are used in the raw form or converted into ash before
use. The current applications of rice husk are enumerated below.97
1.
Pet Food Fibre: Rice husk is an inexpensive by-product of food processing. They
serve as a source of fibre and filler in cheap pet foods because of their low cost and
cellulosic content
2.
Fertilizer: Rice hulls are organic materials that can be composted. However, their
high lignin content can make this a slow process. Thus, sometimes earth - worms
are used to accelerate the process using vermi-composting techniques, the husks can
be converted to fertilizer in about four months.
3.
Building material: Rice husks are a class A insulating material because they are
difficult to burn and less likely to allow moisture to propagate mould or fungi. The
high silica content when burned provides excellent thermal insulation.
4.
Silicon carbide (SiC) production: Rice hulls are a low – cost material from which
silicon carbide whiskers can be manufactured. The SiC whiskers are then used to
reinforce ceramic cutting tools, increasing their strength tenfold. SiC is also used as
a semi-conductor
5.
Fuel for Boilers and Power Generators: Rice husk is used mainly as fuel for
heating in Indian homes and industries. Its heating value of 13-15MJ/ kg
98, 99
is
lower than those of most woody biomass fuels. Despite that, it is extensively used in
rural India because of its widespread availability and low cost. The annual
generation of RH in India is 18-22 million tons100 and this corresponds to a power
generation potential of 1200MW.
101
RH is used as fuel in boilers for processing of
paddy and in power-generating plants. A few rice-husk based power plants with
power-generating capacities of between 1 and 10MW are already in operation in
India and other parts of Asia. These plants are based either on direct combustion or
through fluidized bed combustion. In Punjab, India, rice husks are used to fire the
textile mill’s boiler and produce process steam.
102
The steam runs a turbine that
generates electricity.
6.
Fuel for Domestic Stoves and Industrial Furnaces: Researchers have developed a
simple gas stove, which uses small-scale rice husk gasification technology turning
95
rice husk into efficient fuel.103 The burner generates a clear, blue flame. In Mekong
Delta, Vietnam, massive piles of rice husks are used to fuel brick and pottery
furnaces .104
Production of Lightweight Bricks: Chiang et al,
7.
105
produced lightweight bricks
by sintering mixtures of dried water-treatment sludge and rice husks. Samples
containing up to 15wt% RH sintered at 1100oC produced low bulk density and
relatively high-strength materials that were compliant with standards for lightweight
bricks. They also reported that RH addition increased the porosity of sintered
samples and higher sintering temperatures increased compressive strengths.
High-tech Insulator (aerogel): Halimaton Hamdan, a university of Cambridge-
8.
trained Chemistry professor from Malaysia, announced in Jan 19, 2008 that she had
discovered a cheap way of converting discarded rice husks into a high-tech material
(aerogel) that could reduce electricity bills, protect buildings from bomb blasts and
make airplane and tennis rackets lighter.106 She added that her process cuts the cost
of producing aerogel by 80%, making it so affordable that it could become a
commonplace material with widespread use. Aerogel, the lightest solid known to
man was invented in 1931, but its high cost has limited its use.
Adsorbent for Wastewater Surfactants: The use of rice husks as an adsorbent for
9.
the removal of surfactants in wastewater was investigated by Hosseina et al.73 The
results showed RH to be effective for adsorption of surfactants, however the
efficiency was affected by factors like nature of surfactants (anionic or nonionic),
pH and time.
10.
Pillow stuffing 97: Rice hulls are used as pillow stuffing. The pillows are loosely
stuffed and considered therapeutic as they retain the shape of the head. In China,
these pillows have become quite popular.
11.
Brewing 97: Rice hulls can be used in brewing beer to increase the lautering ability
of a mash
12.
Juice Extraction 97: Rice hulls are used as a ‘press aid’ to improve the extraction
efficiency of apple pressing
13.
In Agriculture: Rice husks have been shown to be biologically inactive and to
make an excellent hydroponic matrix in which disease-free plants can be raised and
seeds germinated 108.
14.
Ethanol Production
The appreciable cellulosic content of RH makes it a good starting material for ethanol
production after treatment to remove its lignin content, using appropriate enzymes or
96
microorganisms. Thus, Patel et al 109, in their preliminary study on the bioconversion of
rice husk and bagasse into usable forms such as reducing sugars for ethanol production,
used five different fungi for pretreatment of the rice husk and saccharoyces cerreviseae
for fermentation. They discovered that rice husk treated with Aspergillums awamori and
Pleurotus sajor- caju, resulted in a good ethanol yield of 8.5g / litre.
15. Particle Board Production
Particle boards are composed of distinct particles of wood or other lignocellulosic
materials which are bonded together with synthetic phasing adhesive under the
influence of heat, pressure and additives .A wide variety of raw materials and particles
of various sizes and shapes is used in the manufacture of particle boards. Rice husk, due
to its advantage of heat and fire resistance has been found useful in particleboard
production. Consequently, it is used for household appliances, building material, heat
absorber and cold storage. In their study on the effect of RH particle size on some
mechanical properies (modulus of elasticity, modulus of rupture and internal bond) and
physical properties (thickness, swelling and water absorption), Osarenwinda and
Nwachukwu110 found that as the particle size increased, the mechanical and physical
properties decreased. They concluded that the smaller the particle size, the better the
mechanical properties.
16. Source of Rice Husk Ash
Rice husk (RH) can be converted to rice husk ash (RHA) by burning. It is well known
that RH contains a high ash content (about 20%) and the silica (SiO2) content of this ash
is also high (~ 85-90%) as detailed in section 2.3. It is the high content of silica in the ash
that makes it attractive for numerous research-driven industrial applications as an
abundant and cheap alternative source of silicon dioxide (SiO2).
2.3 COMPOSITION OF RICE HUSK ASH (RHA)
Rice husk ash (RHA), obtained by combustion of rice husk (RH), has stimulated a great
deal of research interests because of its high silica content as well as the abundant
availability of RH, the starting material, at low cost. The husks removed from the rice grain
during the milling of paddy contains about 75% organic volatile matter and the balance
25% of the weight of this husk converted into ash during the burning process, is known as
RHA 111. In other words, RH is about 20% by weight of rice paddy, so for every 1000kg of
paddy milled, about 200kg of husk is produced and when this husk is burnt in the boilers,
about 50kg (25%) of RHA is generated.
97
Several researchers, in the course of using RHA as a source material for their
investigations, necessarily determined the chemical composition of the RHA used in their
study. Generally RHA contains around 87 to 97% amorphous silica with small amounts of
akalis and other trace elements .44 Simonov et al,89 in their study of rice husk oxidation in
the vibrofluidized bed of catalyst or an inert material, used rice husk sample obtained from
Vietnam. The composition of the RHA as determined was 94% Si, 2.5% K, 0.6% Ca, 0.4%
Mg and 0.2% Mn. The contents of Na, Al, Fe, P and S in the ash was less than 0.1% mass.
Similarly, in their work on conversion of RHA into soluble sodium silicate, Foletto et al78
characterized the RHA sample obtained from a local industry in Brazil by X-ray diffraction
(XRD) and by chemical analysis using Atomic Absorption spectrometry. The chemical
composition was found to be (in wt%), 94.4(SiO2), 0.61(AI2O3), 1.06(K2O), 0.77 (Na2O),
0.59 (MnO), 0.83 CaO, 1.21 (MgO), 0.03 (Fe2O3). They, however, stated that differences in
RHA composition may occur due to geographical factors, type of ground, year of harvest,
sample preparation and analyses methods.
Muntohar85 utilized RHA obtained in an uncontrolled combustion condition for soil
improvement studies. The husk was sourced from Yogyakarta Indonesia and the
compositional analysis revealed the presence (in %) of SiO2 (89.08), AI2O3 (1.75) Fe2O3
(0.78), CaO (1.29), MgO (1.17), Na2O (0.85), K2O (1.38), MnO (0.14), TiO2 (0.00) P2O5
(0.61) , H2O (1.33) and the loss on ignition was 2.05.
Oyetola and Abdullahi113 also determined the composition of the RHA sample,sourced
from Minna, which they used in the production of low cost sandcrete blocks. The RHA was
found to contain SiO2 (67.3%), AI2 O3 (4.9%) Fe2O3 (0.95%), CaO(1.36%), Mgo (1.81%)
The loss on ignition (LOI) was 17.78, which is comparatively high. The relatively low
content of silica in the ash suggests a high carbon content due to incomplete combustion.
However, the combustion temperature and duration as well as the analytical method were
not specified.
Omotola and Onojah,
114
using X-ray fluorescence (XRF) technique determined the
elements present in five RHA samples produced at 500 and 10000C. The husks were
sourced from Kogi and Benue States. The ash samples obtained at 5000C were dark in
colour, indicating the presence of substantial quantities of unburnt carbon while the ones
heated at 10000C were milky-white. The determination by XRF, of the elemental
98
composition (in %) of the samples heated to 10000C showed the presence of thirteen
elements in all, as presented in Table 2.3 for the five samples.
They however stated that the variation in the composition of the sample could be due to the
difference in soil chemistry of the locations where the samples were collected, paddy
varieties and type of chemical fertilizers 115, 116
Table 2.3 Elemental Composition (%) of Five RHA samples Using RH obtained from
Benue and Kogi States
Elements
K
Ca
Cr
Mn
Fe
Ni
Cu
Zn
Sr
Br
I
As
Cl
Total impurity
Total purity
Sample 1
0.5451
0.3070
0.0004
0.0422
0.1603
0.0008
0.0063
0.0057
0.0010
0.0298
1.0986
98.9014
Sample 2
1.9044
0.9002
0.0169
0.1731
1.0836
0.0325
0.0139
0.0312
0.0060
0.0064
0.2178
0.0043
1.1751
5.5654
94.4346
Sample 3
2.4514
0.8707
0.0230
0.2041
0.7219
0.0368
0.0220
0.0435
0.0031
0.0075
0.1735
1.0048 `
5.5623
94.4377
Sample 4
2.0009
0.7869
0.0312
0.2156
1.2093
0.0422
0.0256
0.0237
0.0036
0.0090
0.0481
1.1827
5.5788
94.4212
Sample5
2.4860
0.4825
0.0236
0.2545
0.6801
0.0346
0.0207
0.0326
0.0047
0.0102
0.0439
1.1348
5.5682
94.4318
Interestingly, their results did not show the presence of Na, Mg, Al, P, as is commonly found
in RHA samples. Additionally, the results presented the components as pure elements
rather than as oxides, which is the form in which they are found after combustion.
In the synthesis of zeolite using RHA as a source of silica, Prasetyoke et al
79
used white
RHA as obtained from a rice field in northern Malaysia which on characterization was
found to contain (in wt%) SiO2 (94.0), AI2O3 (0.19), CaO (0.32), K2O (1.64). Again, other
oxides like MgO, Na2O, Fe2O3, were not reported.
Recently, Habeeb and Fayyadh39, using X-ray fluorescence (XRF) spectrometry determined
the composition of RHA obtained by incinerating the RH at a temperature not exceeding
7000C. The X-ray diffraction (XRD) analysis was used to determine the silica structural
form in the ash and it was found to be mainly in the amorphous form. The composition of
the ash was (in wt%) SiO2 (88.32), AI2O3 (0.46) Fe2O3 (0.67), CaO (0.67) ,MgO (0.44),
Na2O (0.12), K2O (2.91) and the loss on ignition (LOI), 5.81. The relatively high value of
loss on ignition (LOI) was attributed to the amount of unburnt carbon in the ash. Marvi et
al90 found the composition of RHA obtained by heating RH at 4000C for 3hrs to be 1.88%
99
carbon and 79.27% silicon dioxide. The relatively low content of silica suggests incomplete
oxidation of the carbonaceous materials in the husk.
Other researchers in different countries who have carried out analysis of RHA for their
80
various studies include Basha et al
Fournier119, Al-Khalif and Yousif
120
, Teshima et al
117
, Bui et al
118
, Bouzouba and
, Chandrasekar et al121 and Zhang et al.122 The results
of the compositional analyses (in %) of the various RHA samples are tabulated in Table
2.1. In many of the reported compositional results, the combustion temperatures and
duration were not stated, so as to be able to deduce if it had an effect on the ash
composition.
Table 2.4 Chemical Composition of RHA from Various Locations
Malaysia46
Brazil83
Netherlands84
India85
Iraq86
USA87
Canada88
Silica as SiO2
93.10
92.90
86.90
90.70
86.80
94.50
87.2
Alumina as AI2O3
0.21
0.18
0.84
0.40
0.40
Trace
0.15
Iron as Fe2O3
0.21
0.43
0.73
0.40
0.19
Trace
0.16
Calcium as CaO
0.41
1.03
1.40
0.40
1.40
0.25
0.55
Potassium as K2O
2.31
0.72
2.46
2.20
3.84
1.10
3.68
as 1.59
0.35
0.57
0.50
0.37
0.23
0.35
Sodium as Na2O
*
0.02
0.11
0.10
1.15
0.78
1.12
Sulphur as SO3
*
0.10
*
0.10
1.54
1.13
0.24
Loss on ignition
2.36
*
5.14
4.80
3.30
*
8.55
Magnesium
MgO
Note: * not reported
It is evident from these results that ash produced from the burning of rice husks contains a
great amount of silica (SiO2) and small amounts of other elements considered as impurities.
The most common trace elements in RHA are sodium, potassium, calcium, magnesium,
iron, copper, manganese and zinc.
Another report from India
92
presented the range for the components of RHA on dry basis
as follows:
Element
Mass Fraction (%)
Silica (SiO2)
80 – 95%
Alumina (Al2O3)
1.0 – 2.5
Ferric Oxide (Fe2O3)
0.5
Titanium dioxide (TiO2)
Nil
Calcium Oxide (CaO)
1.0 – 2.0
100
Magnesium Oxide (MgO)
0.5 – 2.0
Sodium Oxide (Na2O)
0.2 – 0.5
Potash (K2O)
0.2
Loss on Ignition (LOI)
10 – 20
The fairly wide range of the LOI, which is indicative of the amount of unburnt components,
suggests that the efficiency of the combustion process, which depends on the combustion
temperature and time, affects the LOI value, and consequently, the silica content
particularly. This is because the higher the LOI value resulting from incomplete
combustion, the lower the SiO2 content and vice versa. The implication is that LOI values
lower than the above-stated range are obtainable as seen in Table 2.4.
2.4 Amorphous and Crystalline Forms of RHA Silica
The silica in RHA undergoes structural transformation from amorphous to crystalline forms
depending on temperature range and duration of incineration (burning) of the husk. The
crystalline and amorphous forms of silica have different properties, thus it is important to
produce ash with correct specifications for specific end use. Generally, the amorphous
forms of silica are composed of silica tetrahedrally arranged in a random three-dimensional
network without regular lattice structures. Due to the disordered arrangement, the structure
is open with holes in the network where electrical neutrality is not satisfied and the specific
surface area is also large.123 This helps to increase the reactivity, since large surface area is
available for reaction to take place. The structure of crystalline silica is built by repetition
of a basic unit, the silicon tetrahedron in an oriented three-dimensional framework. In a
framework type structure, (for example quartz) the silicon tetrahedrons are joined through
the vertices of oxygen, each of which is linked to two silicon atoms. The oxygen to silicon
ratio equals to 2:1 thus electrical neutrality is attained.123 Figs. 2.1 and 2.2 show the random
and ordered structures of amorphous and crystalline silica, respectively.The incinerating
conditions (temperature range and time) for these structural transformations are discussed
in section 2.5.2 .
101
Fig. 2.1124 Two - dimensional diagram of a Random Silica (SiO2) Network
Fig. 2.2124 Two- dimensional diagram of an Ordered Silica (SiO2) Network
2.5 Factors That Influence Rice Husk Ash Properties
It is understood that RHA production in uncontrolled conditions may not be useful for any
effective applications because of the structural transformation from amorphous to
crystalline silica, which is temperature and duration dependent. These two forms of silica
differ in properties as a result of structural differences. The factors that influence the ash
properties are the incinerating conditions (temperature and duration), rate of heating, colour
102
and fineness of the ash.124 Since RHA is obtained from the husks of the rice plant, the
quality of the ash is also dependent on geographical factors, such as location of the rice
crop, the rice crop variety and crop year. Among these, the incinerating conditions make the
greatest impact on the quality of the final product.
2.5.1 Geographical Factors
Studies have shown that the physical and chemical properties of ash are dependent on the
soil chemistry where the rice crop is grown, paddy variety and climatic conditions.40,
121
Studies have also shown that differences may also arise due to fertilizers applied during rice
cultivation.62,
87
The chemical compositions of RHA from various locations presented in
Table 2.4, show that the variation in chemical composition, especially silica content, is not
high (85 to 95%). All the other constituents of RHA except potassium and magnesium are
available in a very small range i.e. less than 1%
2.5.2 Incinerating Temperature and Time
Studies have shown that burning rice husk below 7000C yields amorphous ash and that a
temperature greater than 8000C, gives crystalline ash.40, 80, 119 Joseph et al 125 specified 550
to 8000C, as the temperature range within which amorphous ash is formed but at a
temperature greater than this, crystalline ash is formed. According to Sugita, 126 the change
from amorphous to crystalline ash occurs at approximately 8000C, although the process is
often incomplete until 9000C. All the combustion processes devised to burn rice husks,
however, have to remain below 14400C, which is the RH melting point. Mehta
115
reported
that a completely amorphous silica could be obtained by maintaining the combustion
temperature below 5000C under oxidizing conditions for prolonged periods, or up to 6800C
with a hold time less than 1 min.
Yoeh et al 128 stated that RHA remains in the amorphous form at a combustion temperature
of up to 9000C for one-hour duration or less, while crystalline silica is produced at 10000C
with a combustion time of more than 5mins. Sampalo et al,129 conducted a thermal analysis
of RHA and concluded that a furnace temperature of approximately 6500C could produce
amorphous silica. Similarly, Asavapisit and Ruengrit
0
130
conducted tests on husk burnt at
0
different temperatures ranging from 400 C – 800 C for 1hr duration and also reported
6500C as the optimum temperature for producing reactive (amorphous) RHA based on
strength - activity index. The said researchers however maintained a constant duration (1hr)
103
of incineration for all the samples. They stated that apart from the temperature range, the
duration of burning is equally important in controlling the characteristics of the ash.
Chopra et al
131
burnt RH at 700oC, evaluated the ash through X-ray diffraction (XRD)
technique and found that it was amorphous. Further heating of this ash at the same
temperature transformed some of it into the crystalline state. Habeeb and Fayyadh132
prepared RHA for their investigation by burning the husk using a Ferro-cement furnace at
an incinerating temperature not exceeding 7000C. The ash was ground using a mill. The
XRD analysis performed to determine the silica form of the RHA, showed it was mostly in
the amorphous form. These findings showed the direct effect of time (at constant
temperature) on the nature of prepared RHA. Ankra et al
133
reported that the burning
environment equally affects the surface area, therefore it must also be considered for
efficient RH pyroprocessing. He further added that if pyroprocessing occurs in the range
450oC to 550oC, the residual carbon, though amorphous in nature, cannot be removed on
subsequent thermal treatment. Real et al
134
also reported that a homogeneous size
distribution of nanometric silica particles is obtainable by burning RH at 873 to 1073K(600
to 800oC) in a pure oxygen atmosphere. Della et al
135
observed that active silica, with a
high specific area could be produced from RHA after heat – treating at 973K (700oC).
2.5.3 Incinerating Methods
‘Incineration’ is the term used to describe combustion of husk without the extraction of
energy. It encompasses open burning such as deliberately setting fire to piles of dumped
husk and enclosed burning which typically involves a chamber made from fire-resistant
bricks with openings to allow air to enter and flue gases to leave. The technologies of ash
production, therefore, vary from open-heap burning to specially designed incinerators and
furnaces, including fluidized beds. Different researchers have devised various incineration
methods for burning rice husks as well as monitoring the combustion process. Thus, Maeda
et al
62
conducted incinerating tests by changing the temperature and duration of holding
using a rotary kiln and suggested that a temperature below 5500C with prolonged duration
may produce reactive RHA (r-RHA). He also designed a stirring furnace for the production
of high quality RHA. Nehdi et al
136
made use of a new technology for combustion of rice
husk, which involved using a torbid reactor, and indicated that r-RHA could be obtained at
a combustion temperature between 750 and 8500C with a jet of external air on the side
walls of the reactors. Sugita
126
introduced a two-step burning method. In this method, the
rice husk was preheated to 300 – 3500C for a fixed period and then the temperature was
increased to a fixed temperature, higher than the flashpoint, and held for a specific period.
104
In an attempt to develop a process for large-scale burning of RH, Mehta
137,
designed a
furnace into which RH was sucked under the negative pressure maintained by an exhaust
fan. The hot gases from the furnace, being mixed with ash, were taken to a boiler and
finally separated by a multicone separator. In addition, this process had the provision to
recover the heat produced by combustion of the husk. Similarly, Shar et al
104
conducted
studies at pilot plant level and fabricated a low-cost incinerator to produce RHA for cement
manufacture. The factor of burning atmosphere was duly considered in the design as the
RH combustion environment was controlled by varying the air flow through a central tube.
On the other hand, Yeoh and co – workers
128
used a modified Yamamoto paddy drier and
carried out field studies to produce amorphous ash. However, their studies were centred
round the feasibility of Mehta system. The Rice Growers Co-operative Society
139
also
developed a fluidized bed furnace or combustor in which two tons of RH per hour could be
burnt to produce amorphous RHA. The heat generated during combustion was used to dry
citrus pulp. Huang, et al l40 developed a thermal model to obtain silica white from rice husk
in a fluidized bed.
Another study by Fang et al
141
on combustion of rice husk in a circulating fluidized bed
reported that a fluidizing velocity of 1.2m/s and air split of 7:3 are reasonable to obtain a
combustion efficiency of above 97%. However, a detailed study is important, to list the
advantages of fluidized bed combustion of rice husk so that the process may be
commercialized.
The transformation of silica from the amorphous to the crystalline state is illustrated in the
equation below (Fig. 2.3) where the transition temperature is about 8000C .124
o
o
o
o
573 C
870 C
1470 C
1750 C
α − quartz 
→ β − quartz 
→ β − tridymite 
→ β − cristobalite 
→ Melts
Fig. 2.3 Temperature-dependent Transformation of Silica
from Amorphous to Crystalline state
2.5.4 Rate of heating
The rate of heating is another important factor in controlling the quality of RHA. If the
heating rate is high, the potassium in rice husks does not volatilize, but reacts with silica,
turning into potassium polysilicate combined with carbon. Thus, rapid burning of rice husks
causes high residual carbon in the ash.96 However, this chemical interaction between
potassium and silica in rice husks is expected to be dramatically reduced if potassium is
removed from rice husks by chemical treatment before subjecting the husks to combustion
as discussed in section 2.7.
105
2.5.5 Fineness
Generally, chemical reactivity is favoured by increasing the fineness of the reacting
materials since this increases the surface area. But the reactivity of RHA is attributed to its
high content of amorphous silica and its porous nature.40, 142 Thus, RHA, being porous in
nature, has an extremely high surface area while its average size still remains fairly high
compared to silica fume with average particle size of 0.1µm.Thus, RHA with particle size
around 45µm has three times higher surface area 87. Mehta argued that, since, RHA derives
its pozzolanicity (cementing property) from its internal surface area, grinding of RHA to a
high degree of fineness should be avoided.103 Bui et al 118 also found that at a certain stage
of grinding the RHA, the porous structure of particles would collapse resulting in reduced
surface area. Bouzoubaa and
Fournier
119
concluded that the optimum grinding time of
RHA was 140s when it was ground in a pulverizer. Other studies show that RHA ground in
a ball mill for a period of 15 to 30mins resulted in particles sizes of around 45mm. RHA in
amorphous form is a soft and fragile material so may not require prolonged grinding and
high energy to achieve the fineness of around 45µm.
2.5.6 Colour
The conversion of RH to RHA by burning is accompanied by colour changes. These colour
changes are associated with the completeness of the combustion process as well as the
structural transformation of silica in the ash from the amorphous to the crystalline state.
Ash of white colour is an indication of complete oxidation of the carbon, which is also an
indication of availability of a large portion of amorphous silica in the ash. At high
temperatures, strong interaction between potassium and silica ions cause the formation of
potassium polysilicate combined with carbon, resulting in grey colour ash.124 If, however,
the husk is pretreated with acid, a major portion of the potassium will be removed and ash
will not attain grey colour.
121
At still higher temperatures, along with prolonged burning,
the result is ash with lilac pink colour, representative of silica in crystalline form, such as
cristobalite and tridymite.142 The three colours distinctly stand for amorphous, transition
and crystalline states of ash formation
2.6 Determination of Amorphous Silica in RHA
Different methods and conditions of preparation of RHA result in silica with different
structure and surface morphology. A proper method of identification of the structural state
of silica in RHA is by the use of various instrumental techniques such as scanning electron
106
microscope (SEM) and X-ray diffraction method (XRD). 40, 119, 123 However, a few workers
have devised other workable analytical methods. One of them, proposed by Mehta, is that
the degree of amorphousness of silica is estimated by calculating the percentage of
available silica that is dissolved in an excess of boiling 0.5M sodium hydroxide, in a threeminute extraction period.109 Standard test methods for evaluating reactive silica are
somewhat tedious, since these include stages of filtration, calcinations, etc. Paya et al
144
introduced a rapid analytical method based on bringing the siliceous, non-crystalline
fraction of pozzolan into solution as glycerosilicate. The advantage of this titrimetric
method is its speed.
Luxan et al,145 suggested an electrical conductivity method for the evaluation of
pozzolanicity of materials. The electric conductivity of 200ml saturated calcium hydroxide
solution at 40oC is measured and then 5g of pulverized pozzolanic material is added to the
solution .The variation in electric conductivity (ms/cm) after 2min is correlated with the
pozzalinicity. When the conductivity value is greater than 1.2, pozzalinicity is good; less
than 0.4, pozzalinicity is zero; and in between 0.4 and 1.2, pozzalinicity is variable. Surana
and Joshi
146
introduced a simple conductometric technique for measuring the activity of
pozzolanic materials based on the dissolution of 200g of pozzolana sample in 0.7%
hydrofluoric acid solution and measuring the conductivity after 30min .The specific
conductivity is directly correlated to the amount of silica present in the samples. However,
in this method, constituents other than reactive silica present in pozzolana may cause
significant influence on the results.
2.7 Thermal Degradation of Rice Husk and its Effect on Rice Husk Particle Shape,
Structure and Surface Morphology
Research findings have revealed that there are two distinct stages in the thermal
decomposition of rice husk – carbonization and decarbonization. Carbonization is the
decomposition of volatile matter in rice husk at a temperature greater than 3000C, which
releases combustible gas, and tar to form char. Decarbonization is the combustion of fixed
carbon in the presence of oxygen. The melting point of RHA is estimated at 14400C
124
,
which is the temperature at which silica melts A diagrammatic representation of the two
processes is given in Fig. 2.4
107
Heat
Rice
R husk
O2
Rice husk ash
Char
Combustible
gas and tar
Heat + CO2
Carbonization
Decarbonization
Fig. 2.4 Diagrammatic Representation of Rice Husk Combustion Process showing the carbonisation
and decarbonisation stages
The amount of work recorded in literature involving efforts by researchers to establish the
micro-structural changes that occur in the RH particles in the course of thermal degradation
and the mechanism of the process is minimal due to difficulties associated with such a
study. Most of the observations of pyrolysis of rice husk were from bulk studies on RH and
pyrolysed products from gasifiers as it was not possible to conduct in situ investigations on
the pyrolysis of individual particles under controlled heating and atmospheric conditions.
The expected difficulties in carrying out such studies have been overcome with the
availability of the hi-tech confocal scanning laser microscope (CSLM), which combines the
advantages of confocal optics with He-Ne laser, thereby enabling observations of samples
at high resolution and at elevated temperatures. The confocal optics enables the detection of
strong signals from objects on the focal plane. The utilization of a laser results in higher
illumination intensity that overcomes the noise generated by the thermal radiation present
in materials at elevated temperatures. However the only recorded extensive work carried
out using CSLM appears to be that of Bharadwaj et al, who carried out a comprehensive
study on the pyrolysis of rice husk for the purpose of observing the structural changes in
individual rice husk particles under controlled heating and atmospheric conditions using
first, a CSLM followed by scanning electron microscopic observations of the pyrolized
structure of the particles.84 Pulverised and sieved rice husk samples with size in the range
between 250 and 300mm were used in the study. The individual particles were heated to
155, 432, 710 and 8100C .The micro-structural observations revealed the following :
i. There is virtually no change in size or shape of the particle until 2000C.
ii. This is followed by a rapid shrinkage between 200 and 4000C.
108
iii. Above 400oC the particle shrinks at a slow rate and stops shrinking after about
8000C.
Two distinct stages in the shrinkage of RH particles were observed. The first stage, they
suggested, was due to rapid thermal degradation of the RH particles, which manifested in
volatiles escaping from the particle. The process was complete when the temperature rose
to 4000C. The second stage was due to char combustion as a result of oxygen and gases
generated from the first stage. This stage was sluggish and less pronounced in rice husk.
Mansaray and Ghaly
147
reported a similar two-stage weight loss in thermogravimetric
experiments for different heating rates and atmospheres. Bharadwaj et al also enunciated
that an important feature of this shrinkage is that the size reduction along the transverse
direction of the largely longitudinal husks was significantly larger than along the
longitudinal (vertical) axis. Their observations support the hypothesis that there is a
preferential direction for particle shrinkage and this may be due to the structure of silica
skeleton in rice husk particles. The cellulose and lignin components of the husk were
preferentially consumed in geometrically arranged pores and channels. Further heating to
15000C did not show any change in the particle size or shape. This suggests that the RH
residue is structurally strong and stable. This phenomenon appears to be unique to rice husk
and some other straws since most other biomass fuels do not exhibit this behaviour. The
particle finally softened and melted at temperatures above 15250C.84 To gain additional
insight into the changes in RH particle structure during pyrolysis, Bharadwaj et al
performed scanning electron microscopic (SEM) observations of a RH particle heated to a
temperature between 350 and 8500C in a tube furnace under an inert atmosphere of argon.
The SEM image of a rice husk particle heated to 3500C showed the presence of a large
number of button-like structures or bumps interspaced with small pores. These were not
present initially on the virgin particles. The presence of bumps and pores was attributed to
the escaping volatiles from the surface of the rice particle as a result of rapid thermal
degradation between 250 and 4000C. The bumps were due to the obstruction of silica fibre
to devolatilization. The craters present appeared to be where the volatiles escaped from the
particles. The transverse section of the particle clearly showed the presence of pores
confirming the porous nature of the ash. It was obvious that pyrolysis of the lignocellulosic
material, (rice husk) had selectively consumed the matrix leaving the silica fibres behind.
Similar exploratory experiments carried out with sawdust particles exhibited complete
combustion with little residue left. Habeeb and Fayyadh 132 confirmed the porous nature of
109
RHA when they scanned RHA samples heated to 700oC, using SEM to reveal the
multilayered and microporous surface of the ash particles.
The surface of the RH particle heated to 8500C showed a large number of pores and bumps.
It was similar to that of 3500C except that the number of pores was more and their size
larger. The pores appeared as channels from where the cellulosic material was
preferentially removed during pyrolysis and char combustion. From their CSLM
experiments combined with SEM observations, Bharadwaj and his co-workers 84 concluded
that the structure of a virgin rice husk was similar to a composite material with silica tubes
filled with cellulose materials, constituting the strong phase and the matrix consisting of
lignin. There were no pores in the RH structure before gasification, however, after exposure
to higher temperatures, the silica cylinders remained unaffected with a large number of
pores that are the remnants of pyrolysis and char combustion. The pores appeared to have
been arranged almost geometrically suggesting preferential consumption of the matrix
material during pyrolysis. Kaupp
148
also reported the inertness and rigidity of the silica
skeleton after thermal degradation. The results on the microstructure of hollow fibres as
seen in the transverse section of rice husk after exposure to high temperatures and the
structure of virgin rice husk with elongated silica fibres, confirm the earlier observations of
other researchers about the inertness of silica. Based on their observations, Bharadwaj, et al
suggested that the role of silica is more than just a geometrical shield to the combustible
material in the sample and that if silica were geometrically shielding the carbon, fluidized
bed combustion could have yielded better carbon conversion, since it improves the
comminuting of particles in the bed, thus exposing fresh surfaces for gasification. Their
postulation then was that silica probably forms molecular bonds with carbon, which are not
easily broken at the gasification temperatures. Reactions leading to the formation of silicon
carbide from silica involve very high temperatures (above 25000C) and gasification
temperatures are not high enough for such reactions to take place. They finally suggested
that full carbon conversion in rice husk may not be achievable during pyrolysis.
Studies of surface morphology, chemical reactivity and surface area measurements revealed
the formation of amorphous silica during ashing. The reactivity was found to be maximum
at an ashing temperature range of 4000C – 6000C and hold time of 6 – 12hrs, but was found
to decrease with ashing temperature and hold time. The comparison of the X-ray
photoelectron spectroscopy of natural silica with those of RHA showed differences in peak
widths which was attributed to variation in immediate chemical environment of silica and
oxygen in the RHA.150
110
2.8 Effect of Chemical Treatment of Rice Husk and Rice Husk Ash
Some researchers were interested in investigating the effects of subjecting the RH to
chemical pretreatment before the thermal decomposition process. Others studied the
compositional effects of acid and alkali treatment on RHA, particularly with regards to
silica content improvement.
Thus, Chakraverty 151 et al studied the effects of various acid treatments of rice husk on the
removal of its metallic components and in addition, the effect of different combustion
temperatures on treated and untreated RH in the production of amorphous silica (white
ash). The I.R results revealed that leaching of husk in dilute HCL (IM) was effective in
substantially removing most of the metallic components and producing ash completely
white in colour. The ash residues obtained from complete combustion of acid-treated husk
samples were completely white in colour. On the other hand, under similar conditions, the
ash residues obtained from untreated husk remained light brown. They also observed that
the acid treatment of husk did not affect the amorphicity of the silica. They finally
concluded that irrespective of the treatments given to the ash, the minimum temperature
required for complete combustion within a reasonable period was 5000C. The combustion
time varied from 5hrs at 5000C to 1.5hr at 7000C.
Chandresekhar and co-researchers 152 investigated the effect of calcination temperature and
heating rate on the optical properties of acid-treated and untreated RH sourced from India.
The raw husk and its acid-treated forms were calcined at different conditions such as
temperature, soaking periods, and heating rates. They reported that silica of high purity,
chemical reactivity and white colour can be produced from RH by controlling the heating
conditions and that the properties of the ash were dependent upon various pretreatments and
calcination conditions. Their study confirmed that a high potassium content in the husk
inhibited the carbon removal during ashing which affected the colour as well as reactivity
of the ash.
Ndazi et al,
153
carried out a related investigation by studying the chemical and thermal
stability of rice husks against alkali treatment, using a thermal gravimetric analysis
instrument. They found that the proportion of lignin and hemicellulose in rice husks treated
with NaOH ranging from 4 to 8% decreased significantly by 96 and 74%, respectively. The
thermal stability and final degradation temperatures of the alkali-treated rice husks were
also lowered by 24 – 260C due to degradation of hemicelluose and lignin during alkali
treatment. This led to their conclusion that alkali treatment of rice husks with more than 4%
NaOH causes a substantial chemical degradation of rice husks, which subsequently
111
decreases their thermal stability. Nizami,154 improved the silica content of RHA by
digesting it with 5M HCl for the purpose of removing the alkali and alkaline earth oxide
impurities. He observed that the acid lost its leaching efficiency with the passage of time
and that it became very slow after 120mins .The original concentration of the ash (obtained
by heating RH at 500oC for 8hrs) and after subjecting to 120mins of acid leaching which he
obtained, are respectively as follows: SiO2 (92.01 to 96.50%), Al2O3(1.48 to 0.91%) ,Fe2O3
(0.40 to 0.06%) CaO(1.52% to trace) ,MgO(0.41% to trace) ,Na2O( 0.63 to 0.18%), K2O
(1.53 to 0.21%). Nizami also treated the ash with HNO3 to remove the residual carbon (or
organic matter) and he reported that the silica content increased slightly from 92.01 to
92.91% showing that some of the c,arbon had been oxidized by the acid. Kalapathy et al 121
used a different method to produce silica with lower sodium content by adding silicate
solution to hydrochloric, citric or oxalic acid of pH 1.5 until a pH of 4.0 was obtained.
According to Procter and his co-workers
156
dissolving RH in sodium hydroxide and then
titrating with acid, produced silica gel.
Alkali treatment of cellulosic materials is viewed as one of the widely employed chemical
treatment techniques for the removal of lignin and hemicellulose and for surface
modification of cellulosic fibres for the purpose of improving their adhesion properties with
various thermosetting binders.157 This treatment involves partial removal of the cementing
materials (hemicellulose and lignin) and other impurities from the fibre.
158
It was found
0
that leaching of rice husks with HCl at 75 C for 1hr prior to combustion produces
amorphous silica of complete white colour with 92% purity and average diameter of 0.04 to
0.05.125
In their observation of the changes in morphology of RH flakes, Singh et a1l60 reported that
the micrograph of phosphoric – acid activated RH flakes showed cavities, which become
predominant with increasing temperature .At high temperatures (800 and 9000C), there was
cavitations collapse, which they attributed to solid state reaction between phosphoric acid
and silica resulting in the formation of calcium phosphosilicate as evident in sharp XRD
peaks.
Studies on the formation of black (carbon fixed) particles on rice husks by Krishnarao et al
161
revealed that the black particles in the silica obtained by calcination of raw untreated
rice husks is higher than that in acid-treated husks. They reported that by treating with 3.0
M HCl, the formation of black particles could be avoided. They observed that there was no
effect of heating rate on the formation of black particles in silica from acid-treated rice
112
husks as was the case with the untreated. Treatment with HCl was found to decrease the
oxidation (burning of carbon) at a lower temperature of 4000C.
2.9 Effect of Combustion Temperature and Time on Chemical Composition of Rice
Husk Ash
It has been observed from literature that several researchers who used RHA as a starting
material for various studies subjected the ash to a specific incineration temperature and
duration or used the ash as- obtained from the rice mills (as a by-product) of boilers and
RH-based power generators. There appeared to be little information on the systematic
monitoring of the RHA composition at varied temperatures and time. However a few
researchers studied the effect of combustion temperature and duration on the silica, metallic
oxides and trace element contents of RHA
Thus Thuadaij and Nuntiya,
162
in their work of preparing nanosilica from RHA, produced
the ash sourced from Thailand, by burning at 7000C for 3hr and 6hr combustion time. They
found that the silica content obtained after heat treatment at 3 and 6hrs were 97.86 and
98.14, respectively, showing a higher SiO2 content after a prolonged heating period.
However, the levels of the alkali and trace metals were slightly reduced. The details of the
ash composition are shown in Table 2.5.The instrumental technique used in the
determination of the ash composition was not specified.
Table 2.5 Chemical composition of RHA, before and after burning out
at 7000C for 3 and 6hrs.
Components
expressed as oxides
RHA – as
received
RHA after burning
out at 7000C for 3hrs
RHA after burning out at
7000C for 6hrs
SiO2
AI2O3
Fe2O3
CaO
ZrO
MgO
P2O3
Mn2O3
SO3
LOI
96.51
0.15
0.17
0.66
0.05
0.77
0.21
0.21
0.04
n/a
97.86
n/a
0.07
0.52
0.01
0.29
n/a
0.16
0.07
0.01
98.14
n/a
0.07
0.46
0.03
n/a
n/a
0.16
0.07
0.02
Singh et al 150, in another study found that the RHA samples prepared by burning RH in air
at different temperatures showed increasing concentration of silica in contrast with
decreasing concentration of K, Ca and Fe, which were the only elements considered. They
113
suggested that K, Ca and Fe could be present in some form of hydrated salts which sublime
with increasing process temperature especially K2O which is known to sublime slowly with
increasing temperature, resulting in the observed decrease in its concentration.On the
increase in the concentration of silicon with increased temperature, they postulated that K,
Ca and Fe were probably in some complex form, which on increasing the temperature
comes out of the silica matrix leading to apparent silicon enhancement. They concluded
that the husk burning temperature does have an effect on RHA chemical composition.
Earlier, Nizami 154 in an attempt to obtain a high yield of amorphous silica, explored varied
pyroprocessing conditions by heating the RH samples in an electric furnace at 450, 500,
550, and 600oC at two- hourly intervals of 2, 4, 6, 8, 10 and 12hrs.The results of the
compositional analysis of the ash samples, which was carried out with the aid of Atomic
Absorption Spectrometry (AAS) and thermogravimetric methods, are presented in Table
2.6. The results show that the ash prepared by heating rice husks at 500oC for 8 hrs yielded
the highest percentage of silica (92.01%) while RH samples thermally treated at 450oC for
2 hrs had the lowest silica content of 84.48% and was blackish grey in colour, indicating
the presence of a large quantity of unburnt carbon. The table shows a slight increase in
silica yield with increasing temperature, particularly from 450 to 6000C. The increase is
however marginal between 500 and 6000C. A similar trend is observed with combustion
time, where the silica yield is seen to increase with increase in combustion duration from
the second hour to the eighth hour. Between the sixth and eight hour, the increase in silica
yield is marginal and appears to remain constant beyond the eighth hour, implying that
prolonging the combustion time beyond 8hrs has no effect on silica yield.
Table 2.6 Silica Contents of RHA at Different Pyrolysis Conditions
Pyrolysis Silica Contents (%) of RHA Obtained at Pyrolysis Time of 2 to 12hrs
Temp
(0C)
2
4
6
8
10
12
450
84.48
87.23
88.99
90.22
90.91
90.86
500
86.87
89.33
91.42
92.01
91.94
91.95
550
87.68
90.11
91.45
91.85
91.86
91.95
600
90.42
90.77
91.38
91.90
91.92
91.99
Nizami reported the presence of only eight oxides in the ash samples, namely, Al2O3,
Fe2O3, CaO, MgO, Na2O, ,K2O, TiO2 and P2O5 which were in trace quantities.
114
The study of Krishnarao et al
161
confirmed that the tendency to form black particles
(unburnt carbon) increases with increase in the heating rate and the temperature of
calcination of untreated rice husks.
2.10 Pyrolysis Products of Rice Husk
Among the possible renewable energy options, agricultural and forestry residues (generally
called biomass residues) can be used as raw materials to generate energy. Pyrolysis is one
of the thermo chemical processes, which convert the solid biomass into liquid (bio-oil), gas
and char. Biomass pyrolysis converts essentially 80 to 95% of the feed material to gases
and bio-oil. Thus, Natarajan and Sundaram
89
carried out pyrolysis experiments on rice
husks between 400oC and 600oC. The IR spectrum of the liquid obtained at the pyrolysis
temperature of 500oC (gave highest liquid yield), particle size of 1.18 -1.80mm, heating rate
of 60oC/min and 300mm reactor length, indicated the presence of phenols and alcohols,
alkanes, ketones or aldehydes, alkenes, carbonyl components and aromatic groups. The
mass spectroscopy analysis indicated the presence of acetic acid, phenol, 1,2 benzenediol,
1-hydroxy-2-butanone, furfural, phenol-3-methyl, 2,5-dimethyl phenol, benzene, 1-ethyl-4methoxy and 2,5-dimethyl-benzoic acid present in different percentages in the rice husk
pyrolysis liquid. They observed that the liquid component of the pyrolysis experiments was
dominated by oxygenated species and concluded that the high oxygen content reflected by
the presence of mostly oxygenated fractions such as carboxyl and alcohol groups, were
produced by pyrolysis of the cellulose while the phenolic and methoxy groups were
produced by pyrolysis of the lignin in rice husks. With regards to the chemical nature of the
gaseous emissions during pyrolysis of rice husks, it was observed that most of the research
studies were devoted to the inorganic pollutants. However, volatile organic compounds and
polycyclic aromatic hydrocarbons (PAHs) are also emitted as revealed in the work of
Janvijisakul and co-workers, 94 which entailed studying the size distribution of PAHs in the
fly ash emitted from a fluidized bed combustor, firing Thai rice husk. They were able to
detect and quantify the concentrations of eleven PAHs namely naphthalene, phenanthrene,
anthracene, fluoranthene, pyrene, benzo(b)fluoranthene, benzo(c)fluoranthene, benzo(a)
pyrene, dibenzo(gh)anthracene, indenol (1,2,3 – cd) pyrene and benzo (g, h, i) perylene.
PAHs are a family of volatile, semi-volatile and non-volatile organic species made up of
carbon and hydrogen atoms. Basically, they are formed during the initial stage of the
combustion, following devolatilization of fuel particles. Some PAHs with greater number
of aromatic rings or with higher molecular weights are known to be strong carcinogens and
115
mutagens. Basically, PAHs emitted from combustion systems are present in two phases of
combustion products, particulate matter and gaseous phase. Low molecular weight (2 and 3
– ring) PAHs such as naphthalene, phenanthrene and fluoranthene are generally associated
with gaseous combustion products, whereas high molecular weight (4- 5- and 6- ring)
PAHs are primarily emitted via fly ash and soot particles, commonly termed particulate
matter. Regarding the components of the gaseous emissions, pyrolysis of rice husks carried
out by Williams and Bester
129
in a thermogravimetric analyzer in nitrogen atmosphere,
showed that the major volatiles evolved during lower temperature pyrolysis were CO, CO2,
and H2O and at higher temperatures, lower concentrations of CO, CO2, H2O, H2, CH4 and
C2H4.
2.11 Effect of Combustion Temperature and Duration on Ash Yields of Rice Husks
Rice husk is known to have a relatively high ash content when burnt, compared to other
lignocellulosic materials. There have been variations in the figures quoted by researchers as
the ash content of RH, the most frequently quoted theoretical figure being in the
neighbourhood of 20%. A few researchers, in the course of their studies, determined the ash
content of RH samples used in their work while others quoted theoretical values seen in
literature. Thus, Allen
108
stated that between 17% and 23% by weight is left as ash when
RH is burnt. Bharadwaj,84 similarly, gave the ash content as 18 to 22% by weight of husk.
Mansaray and Ghaly 147, stated that RH contains 20% of ash when burnt. Subbukrishna et al
20
gave the range as 20 to 25% . On the other hand, Omatola and Onojah 114 experimentally
determined the ash yield of RH samples sourced from Benue and Kogi states of Nigeria by
subjecting them to combustion at 500oC and 1000oC. The ash yields obtained at 500oC were
20.96, 26.8, 23.4, 21.56, and 22.98 for samples 1, 2, 3, 4 and 5, respectively. For samples
subjected to 1000oC, the ash yields (in %) were 16.26, 22.65, 18.85, 17.95 and 19.47 .The
results suggest a dependence of ash yield on combustion temperature, since the values
obtained at 500oC were all higher than those of 1000oC. Much earlier, Nizami
154
had
determined the ash content of RH samples sourced from Pakistan after thermal treatment at
500oC for varied duration. The ash contents obtained were 21.00, 20.85, 20.69 and 20.60%
for 1, 2, 3, and 4hrs duration, respectively. Their results again are suggestive of ash yield
dependence on combustion duration, since the value obtained after 1hr combustion time
was highest and that obtained after 4hrs was the lowest, however the values do not show
wide variation. Houston
81
while Ikram and Akhter
had earlier reported that RHA content varies from 16% to 26%
164
based on their study, found it to be 17.06% and Mehta
115
obtained a value of 20%. Since RHA is derived from RH, it is probable that variations in
116
ash content may arise due to differences in rice variety, soil chemistry, climatic conditions
and sample preparation. The rice husk pyrolysis study of Natarajan and Sundaram
89
revealed that the solid yield (ash) of RH significantly decreased from 39.98% to 32.53 %
when the pyrolysis temperature was increased from 4000C to 600oC. This, they attributed to
greater primary decomposition of the sample at higher temperature. The decrease of liquid
yield and sudden increase of gas yield observed at higher temperature was adduced to
secondary cracking of the pyrolysis into gaseous product at higher temperature.
2.12 Effect of Combustion Temperature and Duration on Rice Husk Ash Colour
There is very little recorded information on the colour changes of RHA with variation in
temperature. In most cases, researchers simply describe the ash obtained as ‘white’ or
‘black/ grey’,based on the predominant colour of the ash sample. Burning of RH in air
always leads to the formation of silica ash, which varies from grey to black depending on
inorganic impurities and unburnt amounts. Thus, Omotola and Onojah 114 described the ash
they obtained at 500oC as black, while that obtained at 1000oC was described as milky white. In like manner, Nizami
154
described the ash obtained at 450oC as blackish grey.
Houston had earlier classified RHA into high-carbon char, low-carbon grey ash and carbonfree pink or white ash. He apparently did not take cognizance of combinations of these
colours, which could arise from varied combustion temperature and duration. Considering
that the colour of RHA is controlled by incinerating conditions, it is likely that a systematic
monitoring of the ash colour with increasing temperature would reveal interesting colour
mixtures, shades and tinges, such that ‘white’ may not be all white, technically, but could
be mixed or tinged with black or grey particles. Also, ’black RHA’ may not be all black,
but could be mixed or tinged with varying shades of white ash particles. The so-called
‘white RHA’ could also have varying degrees of whiteness such as off-white, milky-white,
cream, ivory etc. ‘Black’ could also encompass a preponderance and / or blends of dark
grey, grey, deep brown, brown and other dark shades. Additionally, between the two
extremes of ‘white’ and ‘black’ on the colour scale are shades of grey and ash colours,
which could manifest in the RHA particles with changing combustion conditions. A close
monitoring of colour changes with systematic variation in incinerating conditions would
reveal such technical details of colour changes. There is no recorded work from literature
on this colour changes associated with varying incineration conditions.
117
2.13 RESEARCH–BASED APPLICATIONS OF RICE HUSK ASH
The incineration of rice husk either as fuel to generate heat and electrical energy, for
research or disposal purposes, results in the combustion by-product known as rice husk ash
which may be grey due to incomplete combustion or white (complete combustion). RHA is
a waste product and also poses disposal problems like the husk. However, its high content
of silica (85 – 90%) makes it valuable as a cheap and alternative source of silicon dioxide,
which is useful for many industrial applications. Several researchers have made attempts to
utilize silica from rice husk ash for various investigations, which are enumerated below.
1. Production of Special Cement and Concrete Mixes
There is a growing demand in the construction industry for fine amorphous silica for the
production of special cement and concrete mixes, such as high performance, high strength
concrete, low permeability concrete for use in bridges, marine environments, nuclear power
plants etc. Silica fumes or micro silica (very fine, non-crystalline silicon dioxide) currently
fills this market.165 RHA has been found to have excellent pozzolanic properties.
Pozzolanas are materials containing reactive silica and/or alumina, which on their own have
little or no (cementitious) binding property, but when mixed with lime in the presence of
water and in the finely divided form, will set and harden like cement.166 They are important
ingredient in the production of an alternative cementing material to ordinary Portland
cement (OPC). Nowadays, a wide variety of siliceous or aluminous materials are used for
producing pozzolans, the common materials being calcined clays, pulverized fly ash,
volcanic ash and ash from agricultural residues such as rice husks.167,168 RHA, from
research findings, can be used as a substitute for silica fumes or microsilica at a much lower
cost, without compromising quality. Adding RHA to the concrete mix even in low
replacement dramatically enhances the workability, strength and impermeability of
concrete mixes, while making the concrete durable to chemical attacks
135
, abrasion and
reinforcement corrosion and increasing the compressive strength by 10 – 25%. 136
Al – Khalif and Yousif 171 investigated the effects of using RHA in concrete production and
observed that water requirement decreases as fineness of RHA increases, and that the
higher the RHA content, the lower the compressive strength. They also reported that the
most convenient and economic temperature for conversion of RH into RHA is 5000C.
Similarly, Oyetola and Abdullahi 172 studied the effects of using RHA in the production of
low-cost sandcrete block. The results of their tests revealed that the compressive strength of
the sandcrete blocks produced with OPC/RHA, increased with age of curing and decreased
as the percentage of RHA increased. An optimum RHA replacement level of 20% was
118
suggested. In a related study, Oyekan and Kamiyo
173
observed that 10% RHA content in
the sand-cement mix is the optimum for improved structural performance, but added that
concrete made with 20% RHA content was suitable for low-cost housing development.
Cook and Suwantitaya
174
, in their study of the compressive strength of cement – lime –
RHA mortar mixes reported that the highest strengths are achieved if the RHA and cement
are ground together for about two hours while Cisse and Laguerbe
175
observed that the
mechanical resistance of sandcrete blocks obtained from the addition of unground RHA
was better in performance than the classic mortar blocks. Recently, Habeeb and Fayyadh,
132
confirmed that inclusion of RHA in the concrete mix resulted in increased water demand
and provided similar or enhanced mechanical properties when compared to the ordinary
OPC mixture, with the finer RHA giving better improvement. It has been found that OPC
on hydration, (setting) contains about 25% Ca(OH)2, which is responsible for poor acid
resistance of built structures. Mixing of appropriate amount of RHA with OPC results in
acid-proof mortars, which are even superior to commonly sold acid - resistant cements.
Such blends have been successfully used to construct floors of food and chemical process
industries. This superiority has also been observed in the improvement of the long-term
performance of such mixes in artificial sea water (5% NaCl solution) as demonstrated by
Anwar et al.169
2. Green Concrete
It is estimated that every ton of cement manufactured for use in concrete emits a ton of the
greenhouse gas, CO2 into the atmosphere. Worldwide, cement production accounts for
about 5% of all CO2 emissions related to human activity. Recently, Rajan Vempati of Chk
Group Inc., in Plano, Texas and a team of researchers reported that heating rice husks to
800oC in a furnace, drives off carbon leaving fine particles of nearly pure silica behind.176
They added that the ash makes concrete stronger and more resistant to corrosion. The team
speculated that RHA could enhance performance by replacing up to 20% of the cement
typically mixed into concrete in the construction of skyscrapers, bridges and any structure
built on or near water. The researchers are currently working on a pilot operation to test and
refine their new method, which if proven successful, will trigger the construction of a full
sized furnace that can produce 15,000 tons of RHA annually. They opined that this could be
a good effort to limit emissions of CO2 in the concrete industry and lead to a boom in green
construction, hence the name ‘Green Concrete’
119
3. As an Adsorbent
The sorption properties of RHA have also been studied by some workers, either by using
the ash directly or converting it first to activated carbon. Since the main components of rice
husks are carbon and silica, it has a good potential applicability as an adsorbent. Thus,
Mahvi et al
56
studied the potential of RHA for phenol adsorption from aqueous solution
and from their results concluded that RHA can be used as an efficient adsorbent material
for removal of phenol from water and wastewater. Activated carbons prepared from rice
husks have been found useful as an adsorbent for metal ions and can be employed as a lowcost alternative to commercial activated carbons. This was substantiated by the work of
Okieimen et al177 in which activated carbon from rice husks was produced by steeping rice
husks in saturated ammonium chloride solution followed by pyrolysis at 5000C. The
product was effective in adsorbing metal ions from aqueous solutions. Its capacity was
found to be comparable to commercial activated carbons as has been reported in similar
studies. Similarly, activated carbon prepared by Malik178 was found effective in the
removal of acid dyes (Acid Yellow 36) from aqueous solution. Also, the work of Feng et
al179 in which RHA was used in the adsorption of lead and mercury from water and that of
Naiya et al
180
who reported a feasible and spontaneous adsorption of Pb (II) on RHA,
confirmed its suitability for this purpose.
4. Synthesis of Heterogeneous catalysts
RHA can be used as a support for heterogeneous catalysis. Thus, Ahmed and Adam181
successfully synthesized RHA – Ca, RHA – In and RHA - Fe from RHA at room
temperature using a sol-gel technique. The catalysts were found to be amorphous, porous
and had good textural properties. When used in the benzylation of benzene with benzyl
chloride, they found that the activity of the catalysts depended primarily on the redox
potential of the supported metal. The catalysts are reusable and stable to leaching.
Similarly, Balakrishnan,
87
using RHA silica as support material prepared RHA-Fe and
RHA-Ru heterogeneous catalysts by the sol-gel technique using an aqueous solution of a
metal salt in 3.0M nitric acid. The prepared heterogeneous catalysts were effective as they
showed high activity in the reaction between toluene and benzyl chloride.
5. Silica Source for Zeolite Synthesis
Due to their high cation-exchange capacity (CEC) as well as small size particles, zeolites
are widely used in water treatment (for softening of water), sugar refining, as catalysts e.g
in the catalytic cracking of crude oil and particularly for incorporation into detergents for
the sequestration of calcium from wash water. However, the cost of the zeolite must be low,
120
demanding the utilization of cheap raw materials for the production process. as a source of
silica. Zeolites are hydrated, crystalline aluminosilicates with open three-dimensional
framework structures made up of SiO4 and AIO4 tetrahedral linked by sharing their oxygen
atom to form regular intracrystalline cavities and channels of atomic dimensions.45 They
carry a negative charge on their framework, which is compensated by neighbouring cations
such as alkali and alkaline-earth metal ions. Zeolites have exceptionally high degree of
thermal and acid stability and high selectivity in certain catalytic conversions, thus they are
used for the said industrial applications. Some workers have successfully synthesized
zeolite using RHA.Thus, Bajpai et al182 obtained mordenite, Wang et al
183
obtained ZSM-
48 type zeolite and Ramli et al184 synthesized zeolite Y. Other types of zeolite synthesized
include ZSM-5 by Rawtani and Rao185 who used RHA as the only source of silica and
alumina for the synthesis of this special zeolite, which has an unusually high silica/alumina
ratio. Ramli and Bahruji
186
also synthesized ZSM-5 while Rubin
187
and Prasetyoko
79
obtained zeolite-beta .
6. Production of Refractory Bricks: Due to its insulating properties and good radiation
absorption properties, crystalline RHA is used in the manufacture of refractory bricks,111, 114
for the construction of furnaces, which are exposed to extremely high temperature e. g blast
furnaces used for producing molten iron and cement clinker. It is also useful in the steel
industry and in the ceramic industry as a glazing media.
7. Oil absorbent 111
Rice husks burnt for prolonged periods have been successfully used as oil absorbent.
8. For soil Improvement
When geotechnical engineers are faced with clayey soils, the engineering properties of
these soils may need to be improved to make them suitable for construction. Soil
improvement could either be by modification or stabilization or both. Soil modification is
the addition of a modifier (cement, lime etc) to a soil to change its index properties while
soil stabilization is the treatment of soils to enable their strength and durability to be
improved such that they become suitable for construction beyond their original
classification.188 Waste materials such as fly ash or pozzolanic materials have been used for
soil improvement. The good pozzolanic activity of RHA makes it a potential material to be
utilized for soil improvement. Alhassan 188 stabilized lateritic soil with RHA and found that
RHA has a little potential for strength improvement of A – 7 – 6 lateritic soil when used at
6 – 8% levels. Muntohar,
85
similarly carried out a soil stabilization study using lime and
121
RHA. He observed that lime-RHA decreased the swell of expansive soil and improved its
strength and bearing capacity. Basha et al
80
also examined the possibilities of improving
residual properties by mixing RHA and cement in suitable proportion as stabilizing agents.
9. Oil Industry 111
Today, the oil industry uses silicon dioxide to help refine crude oil into usable fuel such as
gasoline. RHA has also been found useful as an oil spill absorbent.
10. As Fertilizer
A case study on organic farming groups indicates that RHA could be utilized as silicate
supplement for farmland soils.189
Engineering Corporation of Japan,
The National Agricultural Research Center and Takata
190
jointly developed a technology for creating fertilizer
that contains highly soluble silicic acid from RHA. In the course of their invention, they
observed that when rice husks are burnt at high temperatures, the silicic acid that accounts
for over 90% of the ash becomes insoluble, precluding its use as a fertilizer. However, rice
husks burnt at low temperatures (around 500oC) produces a useful fertilizer, which contains
highly soluble silicic acid and on application of this ash fertilizer, paddy fields yield a rich
harvest of rice.
11. Water purification
RHA contains silica and carbon that help in removing colour, odour, suspended particles
and microorganisms, thus has been found suitable for water filtration. Thus a RHA – based
water filter having RHA as filtering medium, cement as binder and pebbles as support for
the matrix was designed by Tata, Research Development and Design centre (TRDDC),
India as a simple and low-cost method of providing safe drinking water in rural India.191
The best performance in terms of the elimination of water-borne bacteria and suspended
matter was obtained with RHA of an average fineness of 200microns.
12. Source of Nanosilica
Recently, nanotechnology has attracted considerable scientific interest due to the new
potential uses of particles in nanometric scale. Thus industries may be able to re-engineer
many existing products that function at unprecedented levels. At present, nanoscale
materials are prepared using several methods, including vapor-phase reaction, sol-gel and
thermal decomposition technique. The powder production of silica is a high-energy
intensive process unlike the economically viable and high-grade nanosize amorphous silica
from RH, which can be produced simply by burning under appropriate conditions. Using
the precipitation method, Thuadaij and Nuntiya162 prepared stable nanosilica powder from
122
RHA. The ash was first purified to obtain pure grade silica by boiling with sodium
hydroxide to remove the metallic oxide impurities before converting to nanosilica by first
refluxing with 6N HCl and dissolving in 2-3N NaOH.
13. Source of Sodium Silicate
Sodium silicate has been produced by reacting RHA with sodium hydroxide.112 Sodium
silicate is used as a raw material for several purposes – silica gel production, preparation of
catalysts, inks, load for medicines, concrete hardening accelerator, component of detergents
and soaps, refractory constituent and deflocculant in clay slurries. 158
14. Pesticides for Soybean Pest Control
RHA has been found useful, as a carrier for pesticides.111 Experiments carried out by Naito
159
on the control of pests (bruchid beetles) in stored soybean showed that the inclusion of
RHA in the pest control mixture had a marked effect on the pests’ mortality rate.
15. As Filler in Rubber and plastics
It has been established that silica as filler in rubber, has a weaker polymer-filler interaction
than carbon black and are therefore extensively used where a high degree of reinforcement
is not essential.194 However, it was found to improve the processability of rubbers. Thus
Arayapranee et al
195
found that the incorporation of RHA as a filler into natural rubber
(NR) improved its hardness, but decreased tensile strength and tear strength. RHA also
resulted in lower mooney viscosity and shorter cure time of the NR. RHA was
characterized by a better resilience property than that of silica and carbon black. Other
properties of the rubber such as Young’s modulus and abrasion loss showed no significant
change. The overall results confirmed that RHA could be used as cheaper filler for NR
materials where improved mechanical properties are not critical. Black RHA, when added
as filler to silicone rubber gave results that suggest very good adherence matrix-fillers as
reported by Sereda and co-workers.196 RHA was also used as a filler by Ferro,197 in
polyamides in an attempt to improve the dimensional stability as well as electrical,
mechanical and thermal properties of Nylon 6 and Nylon 6,6
15. Fertilizer Production
The National Agricultural Research centre and Takata Engineering Corporation of Japan198,
199
jointly developed the technology for creating fertilizer that contains highly soluble
silicic acid from rice husk ashes. Rice husks burnt at low temperatures of around 5000C
were found useful in producing fertilizer that contains highly soluble amorphous silicic
123
acid. The application of this ash fertilizer to paddy fields was found to increase the
concentration of silicic acid in the soil, yielding a rich harvest of rice.
16. Production of Eco-friendly Heat – Absorbing Glass
The glass industry obtains sand from seacoasts and riverbeds for glass production. This
causes decline on the ecology of nektons and other organisms. In this research, RHA was
used in place of sand in the glass composition to produce a grey coloured heat - absorbing
glass,
200
which has helped to solve a number of environmental problems and to stabilize
building environment. Heat-absorbing glass enables better heat control in buildings
therefore, this special glass as well as the manufacturing process are eco-friendly.
17. As an Insulator
RHA has very good insulating properties and is consequently used for the insulation of
molten metal in tundish and ladle in slab caster. 201 The temperature of molten metal in the
ladle is around 1400oC. When this metal flows from ladle to tundish, the temperature drops
to around 1250.oC. This reduction in temperature leads to choking and causes breakdown in
the slab caster. When RHA is spread as a coating over the molten metal in the tundish and
ladle, the temperature is maintained and does not cool down , hence reducing the
breakdown time of the casting.
18. Pure Grade Silica From Rice Husk Ash
High-grade silica can be obtained from RHA by removing the trace metals, alkali and
alkaline –metal oxide impurities and residual carbon. The high-grade silica can then be
directly converted to other chemical products like silicon, silicon carbide, silicon nitride etc,
which are very useful in the electronics industry. Thuadaij and Nuntiya162 produced pure
silica by removing the said impurities from RHA. This, they carried out by boiling RHA
with 3.0N NaOH for 3hrs, followed by filtration of the solution, acidification with 5N
H2SO4 and neutralization with NH4OH to pH 8.5 .The filtrate was then dried in an oven at
120oC for 12hrs. More recently the Indian Space Research Organization 202 successfully
developed a technology for producing high purity silica from RHA that can be used in the
manufacture of silicon chips, which is the heart of the computer world.
19. Silicon from Rice Husk Ash
The element Silicon (Si) is obtained by reducing SiO2 with high purity coke203. There must
be an excess of SiO2 to prevent the formation of silicon carbide (SiC). The electronics
industry uses small quantities of ultra pure silicon (and Ge). Silicon is an insulator when
124
pure, but become p-type or n-type semiconductors when doped with a Grp 15 or Grp 13
element respectively 204. They are used as transistors and semiconductor devices. Very pure
silicon is also used to make computer chips.
RHA can be purified to obtain high-grade silica, which can subsequently be reduced to
produce silicon. Mishra et al
205
prepared polycrystalline silicon of reasonable purity by
metallothermic reduction of purified RH white ash (amorphous silica) using calcium as the
reducing agent. Acharya and co-workers 206 obtained silicon by directly reducing RHA with
magnesium powder at 600 to 650 oC. Bose
207
, while following the process of Acharya,
made several improvements and reported to have produced silicon with a purity level of
99.5%. Ikram and Akhter164, in another work, prepared RHA at 620oC, reduced it by a
metallo- thermic process and the silicon thus obtained was purified subsequently. The total
impurity content after purification was found to be less than 500ppm. Similarly, Hussain
and co-workers208 utilized RHA as the starting material for their extraction of solar grade
silicon. They reported that depending on the silica content of prepared RHA, 18: 27 (Mg:
SiO2 ) was found to be the most suitable ratio for reduction. The resulting silicon powder
was leached with different acids and evaluated as 99.8% pure. In a related study, Patel and
co- workers,
209
in an attempt to produce solar grade silicon, subjected RH to thermal
treatment at 1000oC and different durations and attempted to retain the amount of RH
carbon necessary for carbothermic reduction of accompanying SiO2. This experimentation
resulted in 99% pure silicon, which can be used in solar cell production technology. Solar
cells are devices used to convert solar energy to electrical energy.
20. Silicon carbide from Rice Husk Ash
Silicon carbide, known as carborundum is widely used for specialty applications in modern
industries owing to its unique properties of extreme hardness, high thermal conductivity
chemical inertness and refractoriness.210 Consequently it is used in applications such as car
brake discs, ceramic plates in bullet-proof vests, as abrasives (sand paper) and in obrasive
machining processes such as grinding, honing, water-jet cutting and sandblasting. Silicon
carbide is also a semiconductor, which can be doped n-type by nitrogen or phosphorus and
p-type by aluminium, boron, gallium or beryllium. In fact, there is currently much more
interest in its use as a semiconductor material in electronics, where its high thermal
conductivity, high electric field breakdown strength and high maximum current density
make it more promising than silicon for high-powered devices.
125
Silicon carbide is manufactured by electromelting high purity silica (SiO2) sand with good
quality coke (carbon) at a high temperature (about 22000C).211 Hence the process involves
high -energy requirement and this makes it an extremely expensive process. RHA is a new
and potential source of raw material for SiC production. The technology for this process as
proposed by Vlaslov et al
212
involves treating the husks with mineral acid to remove
undesirable impurities of alkali and alkaline-earth metallic oxides, subjecting the treated
husks to pyrolysis at 600-8000C (to produce active carbon), calcining the RHA obtained at
1400 – 18000C in a current of inert gas. The carbon at a rather low temperature (1200 –
13000C) begins to reduce the SiO2 with the formation of SiC.
2.14 POTENTIAL USES OF RICE HUSK ASH (RHA)
RHA can be used in place of commercial silica in several other areas, as a cheap, abundant
and sustainable alternative, however, intensive research has to be carried out in order to
establish satisfactory applicability of RHA in the proposed areas which are described
below.
Electronics Industry: Silicon dioxide is one of the most commonly encountered
substances in both daily life and in electronics manufacturing. Crystalline silicon dioxide,
(in several forms: quartz, cristobalite, tridymite) is an important constituent of a great many
minerals and gemstones, both in pure form and mixed with related oxides. Today, the
modern electronics world greatly depends on silicon dioxide for the manufacture of
semiconductors, wire insulations and fibre optic cables. Its high melting temperature
(1440oC) makes it a perfect ingredient for insulating wires. Since quartz (SiO2) has
piezoelectric properties, this makes silica an ever more valuable compound to modern
electronics.213 Piezoelectric means that it has the capability of converting mechanical
energy to electrical energy and electrical to mechanical. This property of quartz allows
radio and TV stations to transmit, stabilize and receive signals. Sonar also uses this
piezoelectric property to detect vibrations. Wrist watches, such as the famous Rolex, use
quartz to help keep accurate time.
1. Silicon nitride (Si3N4) from Rice Husk Ash
In recent years, there has been an increasing interest in silicon nitride, which may be
ascribed to its thermal shock resistance, high wear resistance, chemical inertness and high
dielectric properties.214 These outstanding properties make it the material of choice for
many applications. Si3N4 can go from extreme high to cold temperatures repeatedly without
reducing its effectiveness and is also harder wearing and more durable than alternative
126
metals.215 Silicon nitride also has better high temperature capabilities than most metals,
combining retention of high strength and creep resistance with oxidation resistance. In
addition, its low thermal expansion coefficient gives good thermal fatigue resistance
compared with most ceramic materials. Although, the original goal for developing silicon
nitride, which was producing ceramic engines, was not achieved, Si3N4 has found use in
niche market applications such as in reciprocating engine components bearings, metal
cutting and shaping tools. For silicon nitride to move from a niche to a major industrial
market, there has to be a drastic reduction in production cost. Engine technology is the area
likely to benefit most from future growth, as reduced fuel consumption and reduced
emission become ever-stronger motivating factors. The use of pure grade RHA has the
potential of providing a far cheaper, abundant and non-depleting alternative source to
commercial silica.
2. Filler in soaps and detergents
RHA can be used as inert filler in soaps and detergent in place of the more expensive
commercial fillers such as kaolin, sodium sulphate, calcium carbonate etc used in these
products.
3. Extender (filler) in paint products
RHA has a high potential of providing a cheap alternative to commercial paint extenders
such as silica flour (a transparent pure grade of silica), used as an extender in paints for
imparting special properties in paint products. Such properties that can be imparted in paint
products by the incorporation of RHA include settling resistance, improvement of structure,
gloss control and enhancement of viscosity. Other paint extenders, that RHA can substitute,
include kaolin, calcium carbonate, barytes etc. Research- driven efforts are required to
explore the applicability of RHA in this area as is done in this work.
4. Flame Retardants
The intrinsic low-flammability of RHA due to its high silica content could be a useful
property in flame-retardants.
5. Toothpaste formulation
RHA, after purification to remove the alkali and alkaline-oxide impurities can be used for
toothpaste production due to its hardness and abrasiveness, which can help to scrub away
plaque on teeth.
127
6. Defoamer component
RHA can be included in defoamer formulation due to its hydrophobic nature.
7. Cosmetics
RHA can be useful in cosmetics formulation particularly, powders and make-up
preparations based on its light- diffusing properties and absorbency.
8. Engineering
Silicon dioxide is also often applied in the field of engineering. Silicon dioxide is used in
glasses for windows and various other glass materials, metal alloys, pipe flux, sand for
foundations, sealants, sandblasting and many more. RHA can be a useful alternative for
these applications.
9. Production of Silicon Tetrachloride ,Chlorosilanes, Silanols and Silicones
High grade RHA can be reduced to silicon, in an electric furnace, using proper reducing
agents
216, 217
and the silicon used in the production of silicon compounds such as silicon
tetrachloride (SiCl4), chlorosilanes, (Si Rn X4-n), silanols (RSi (OH)3, R2Si (OH)2 and
R3(SiOH) and silicone polymers (also known as silicones or polysiloxanes).130 SiCl4 can be
used to produce silica fumes (or fumed silica),which is a very fine particulate form of SiO2,
by burning in an oxygen-rich hydrocarbon flame to produce a ‘smoke’ of SiO2
SiCl4 + 2H2 +O2
SiO2 + 4HCl
10. Component of Adhesives for Roof and Floor Tiles
The potentials of RHA include its use as a filler in adhesives for roof tiles, floor tiles and
related applications.
2.15 JUSTIFICATION FOR STUDY
The justification for our study is based on the following factors, namely:
1. The Principle of the 3 R’s - Reduce, Re-use and Re-cycle of waste products, as
discussed in section 1.1.1
2. Resource for Sustainable Development
Sustainable development is ‘meeting the needs of the present without compromising the
ability of future generations to meet their own needs’. It is a pattern of resource that aims at
meeting human needs while preserving the environment so that those needs can be met not
128
only for the present but also for future generations. Sustainability also implies that fewer
materials should be extracted from the earth and less should go into manufacturing, thereby
conserving materials and energy for generations to come. The use of rice husk ash (derived
from rice husks) will help to conserve the natural sources of silica namely quartz silica,
which is mined, and diatomaceous earths (silica obtained from remains of aquatic
organisms), thereby maintaining bio-diversity and promoting sustainability.
3. Green Chemistry Philosophy of Renewable resource
One of the principles of Green Chemistry advocates that chemical processes be developed
based on use of renewable (non-depleting, inexhaustible ) i.e plant-based raw-materials
rather than non-renewable (depleting, exhaustible), i.e fossil-based for chemical processes
so as to reduce the pernicuous environmental impact of the latter. Rice husk is obtained
from the rice plant and is an inexhaustible bio-resource because for as long as rice is eaten
as a staple food globally, the cultivation, harvest and milling (de-husking) of rice will
always generate the husk, thus, global rice husk resource cannot be depleted, as it is
constantly regenerated (replenished) Thus, this work, which explores the potential
applicability of rice husk ash, obtained from rice husks as pigment extender in paint
products promotes the ‘Green Chemistry’ principle.
4. Vastness of Rice Husk Resource
The global, annual production of rice is about 650 million which yields about 13million
tons of rice husk (constitutes about 20% by weight of rice harvested). RH therefore presents
a vast, renewable bio-resource,with numerous industrial potentials yet to be extensively
explored. The use of RH by conversion into the ash through combustion is one of several
ways of exploring its industrial applicability. In this study its potentials as paint extender is
investigated.
5. Cheap, Alternative, source of silica
The industrial applicability of RH can be investigated both in its raw form and on
conversion into ash by burning. The high silica (SiO2) content of the ash (80 – 95%) makes
it a most valuable cheap and alternative source of silicon dioxide for various industrial
applications. Silica flour (100% crystalline silica) is used as extender in specialty paint
products, but is a relatively expensive material due to the processing involved.Also, the
manufacture of silica is energy intensive. RHA, therefore presents a cheap and abundant
129
alternative to silica flour, synthetic silica and possibly other fillers like calcium carbonate,
kaolin, barites etc .
6. Attractive properties
Other attractive properties of RH, which can be exploited for various industrial purposes
include chemical stability, low bulk density (or low specific gravity), high ash content
(22%), local abundance and availability, low or no cost and an appreciable cellulosic
content. In this work, the high ash content and high silica content will be exploited. Its low
specific gravity, which enhances settling resistance when used in paints will in addition, be
exploited.
7. Re-cycle of RHA Waste Product
It is desirable that incineration of rice husks be done in a closed system such that the steam
and fumes emitted can be trapped and used gainfully, such as in power generation to avoid
any health and environmental hazards. Rice husk ash (RHA) can be obtained from RH by
burning (incineration or combustion). In India and other parts of Asia, rice husk is used as
fuel in boilers to generate steam for the processing (parboiling) of paddy and as fuel for
power-generating plants as reported in the literature review. Specially designed gas stoves,
which use small-scale RH gasification technology are also in use. In all these combustion
processes, the RHA produced as waste product also poses disposal problems like the husks,
as it is dumped as refuse in the open. The RHA obtained from these sources can be
subjected to further combustion due to its high carbon content and utilized in paint
formulation. Utilization of the ash is another form of waste re-cycle.
8. Novel Instrumental Analytical Technique (PIXE)
The instrumental techniques used for the determination of RHA composition and
microstructure as reported in the literature review include :
i.
Atomic Absorption Spectrometry (AAS)
ii. X-ray fluorescence spectrometry for elemental composition determination.
iii. X-ray Diffraction (XRD) technique for the determination of crystalline structure of
silica in RHA
iv Scanning electron microscope (SEM) for micro-structural determination and
The uncommon high-tech Confocal Scanning Laser Microscope (CSLM) for
combustion under controlled heating and atmospheric conditions.
130
XRF, which is the commonly used technique has inherent machine design limitations so is
unable to detect some elements. The proposed use of the highly sensitive particle-induced
x-ray emission (PIXE) would be the first of its kind in RHA compositional determination.
9. Novel Application of RHA
From the research – based applications of RHA enumerated in section 2 of chapter 2, it is
evident that there is no recorded work on the applicability of RHA as paint extender, either
as a partial or total substitute for synthetic silica, silica flour and other commercial paint
fillers such as kaolin, calcium carbonate, barites etc. This therefore justifies the novelty of
these investigations.
10. Generation of Basic Strategic Research Information
It has been established that RHA colour, morphology and composition is affected by
incinerating conditions, particularly the temperature and duration. It was observed from
literature reports that several researchers who used RHA as a source material for various
studies obtained the ash as a by-product of boilers and power generators fuelled by RH, or
produced the ash by burning the husk at a specific temperature and time from which
deductions could not be made on the effects of incineration conditions on the ash yield and
silica content. The RHA composition quoted by different researchers, therefore varied,
particularly the silica (SiO2) content, which is the major component of the ash.. This
investigation involving systematic variation of incineration conditions of rice husks and the
effects on RHA colour, surface morphology, yield and elemental composition would
produce results that will provide basic strategic information for other research
investigations.
1.5 GENERAL OBJECTIVES OF STUDY
The general objectives of our study are:
- to determine the colour, particle surface morphology and yield of RHA samples obtained
at systematically varied combustion temperatures and time
- establish the elemental composition of the ash samples at the varied conditions, using a
highly sensitive, relatively new and uncommon analytical technique, Particle-Induced Xray Emission (PIXE) spectrometry and
- utilize the ash as extender in the formulation of different paint products.
131
1.5.1 SPECIFIC OBJECTIVES
The specific objectives of the study are:
1.
To determine the effect of the various incineration temperatures (at specific time) on
the colour of RHA and surface morphology of the ash particles.
2.
To determine the effect of the various incineration temperatures (at specific time) on
the yield of RHA from rice husks.
3.
To determine the effect(s) of different combustion temperatures on the silica and
metallic oxides composition of RHA.
4.
To establish the effect(s) of systematically varied combustion time, (at specific
temperatures) on the compositional levels of silica and metallic oxides in RHA.
5.
To determine the elemental composition of virgin rice husks
6.
To establish the combustion conditions (temperatures and time) that produce RHA of
satisfactory whiteness for use as a paint extender (filler).
7.
To determine the thickening effect, bulk increase effect and anti-settling resistance, of
RHA as extender in decorative paints.
8.
To determine the flatting (gloss- control) effect of RHA as extender in selected
industrial paint products namely red oxide primer, matt wood finish and cellulose
matt finish
9.
To qualitatively evaluate RHA-based paint products in comparison with those
produced using standard commercial fillers.
10.
To establish standard formulations & specifications for RHA – based paint products
that conform with specifications of the Standards Organization of Nigeria (SON)
11.
Provide accurate baseline information on the elemental composition of RHA for other
workers carrying out research on RHA.
132
CHAPTER THREE
3.0
MATERIALS AND METHODS
3.1 MATERIALS
The paint raw -materials used for the formulation of the paint products were obtained from
Chemical and Allied Products Plc, 2, Adeniyi Jones Avenue, Lagos, a former subsidiary of
Imperial Chemical Industries (ICI), UK and producers of ‘Dulux’ International brand.The
specific supplier of each material is indicated, where applicable. Comprehensive testing of
the formulated products were carried out using the equipment and other facilities in the
CAP Plc Paints Research and Development laboratory, which have been enumerated
hereunder, with the make specified, where applicable.
3.1.1 Materials for Production of Emulsion and Textured Finishes
Water (solvent) ; CAP Plc
Calgon PT (wetting agent); supplied by Cormart Nig. Ltd., Lagos
Acticide Bx ( in-can preservative); Cormart Nig. Ltd, Lagos
Acticide Ep (dry-film preservative) ; Cormat Nig., Ltd.
Berol 09 (surfactant); Emmyson Nig. Ltd.
Antifoam (Defoamer) ; Emmyson Nig. Ltd Lagos
Coatex (Dispersant); Emmyson Nig. Ltd., Lagos
Titanium Dioxide(Pigment); Chizzy Nig. Ltd., Lagos
Calcium Carbonate (Extender); Cormart Nig. Ltd
Silica flour (Extender); Camic Nig. Ltd., 3, Ojike Lane, Aba
Rice Husk Ash Flour (Extender)
Kaolin ; Camic Nig. Ltd., 3, Ojike Lane, Aba
Styrene – acrylic resin (Binder); Nycil Nig. Ltd., Sango Otta, Ogun State
Natrosol (Cellulosic thickener); Cormart Nig. Ltd., Lagos
Ammonia (pH adjuster); Individual suppliers
Texanol (coalescing agent); Emmyson Nig., Ltd., Lagos
Pigment Pastes; Clariant, UK; Cormart Nig. Ltd, Lagos; Nicsol Nig. Ltd., Lagos
Sand (Texturizer); Independent Suppliers
133
3.1.2 Materials for Production of Red Oxide Primer
Medium oil Alkyd resin(Binder) Nycil Nig. Ltd.,Sango Otta, Ogun State
Kerosene; Petroleum Marketers
Soya lecithin (Dispersant); Chizzy Nig. Ltd., Lagos
Easi gel (Anti-settling agent) ; Chizzy Nig. Ltd., Lagos
Red Oxide (pigment / anti-corrosion agent); Chizzy Nig. Ltd., Lagos
Calcium drier ; Chizzy Nig. Ltd., Lagos
Cobalt drier; Chizzy Nig. Ltd., Lagos
Zirconium drier; Chizzy Nig. Ltd., Lagos
Silicone fluid (Flow aid); Emmyson Nig. Ltd., Lagos
Methyl Ethyl Ketoxime (MEKO) (Anti-skinning agent); Emmyson Nig. Ltd., Lagos
3.1.3 Materials for Production of Matt Wood Finish and Cellulose Matt Finish
Short oil alkyd resin (binder); Nycil Nig. Ltd., Sango Otta, Ogun State
Methyl Isobutyl Ketone (MIBK); Petroleum Marketers
Toluene (Solvent); Petroleum Marketers
Isopropyl Alcohol (IPA); Petroleum marketers
Nitrocellulose Solution( resin/binder)
Dioctyl Phthalate (DOP)(plasticizer); Emmyson Nig. Ltd., Lagos
Xylene(solvent); Petroleum Marketers
Silicone Fluid; Emmyson Nig. Ltd
White mill base (Pigment); Clariant, UK; Cormart Nig. Ltd,
Yellow Oxide mill base (Pigment); Clariant, UK; Cormart Nig. Ltd,
Black mill base (Pigment); Clariant, UK; Cormart Nig. Ltd,
Fumed silica (Flatting agent); African Paints, Lagos
Silica flour ; Camic Nig. Ltd., 3, Ojike Lane, Aba
Rice Husk Ash Flour
3.1.4 Equipment for Combustion Studies on Rice Husks
-
Muffle furnace; Labline
-
Laboratory Electric Oven
-
5.5 KVA Generator; TEC
-
Automated sieve
-
Household sieves
134
-
Standard stainless steel sieve of aperture 63 MIC BS410
-
Standard stainless steel sieve of aperture 212 MIC BS410
-
Stainless steel pot
-
Camping gas stove
-
Plastic buckets
-
Plastic trays
-
Long metal tongs
-
Large screw driver
-
Asbestos gloves
3.1.5 Equipment for Determination of RHA Colour and Surface Morphology
-
Digital camera, Kodak 8.2 mega pixels, AF 3x Optical Aspheric lens
- Palaeontology microscope; Vickers M1OA
3.1.6 Equipment for Determination of Elemental Composition of Rice Husk Ash
The RHA samples were first pelletised using a hydraulic press and the elemental
composition determined using a Tandem Pelletron Accelerator, the components of which
are listed below and diagram shown in Fig. 3.1
i.
Hydraulic press
ii.
1.7 MV Tandem Pelletron Accelerator, model 5SDH built by the National
Electrostotics Corporation (NEC), USA and recently acquired by the Centre for
Energy Research and Development (CERD), Obafemi Awolowo university, IleIfe
iii.
End station for IBA analysis, designed and built by the Materials Research
Group (MRG) at Themba Labs, Sommerset West, South Africa (Picture shown
in Fig.3.2)
iv.
Si(Li) PIXE detector Model ESLX30-150)
135
3.1.7 Equipment for Production and Testing of Paint Samples
Wt per litre (specific gravity) cup
Analog Rotothinner (viscometer): Sheen
Digital Rotothinner viscometer sheen 455N
Doctor’s blade: sheen 150 µm
pH meter Mettler Toledo MP 220
Gloss meter, Sheen Tri-Gloss Master (20-60-85)
Washability Tester: Elcometer 1720
Digital Weighing scale: sauter
Rotothinner Gel strength Tester (viscometer): sheen
Morest chart (black and white striped paper)
K – bar , Sheen
Mini stirrer, Diaf A/S Copenhagen NV. Denmark
Siphon feed Spray gun, U - mate CH – 202
Brush-out cards (6” x 4”)
Gloss panels (6” x 4”)
Metal panels (6” x 4”)
Ply wood boards (6” x 4”)
Hagemann Gauge (5 – 60 MIC range) G125 ICI PAT. PEND
METHODS
3.2 Determination of Rice Husk Particle Size
The milled rice husks obtained from the rice mill were composed of different particle sizes.
To determine the particle sizes, the husks were placed in an automatic sieving machine on
which were stacked sieves of various aperture sizes (microns) ranging from 200 to
1000microns (0.2 to 1.0mm). When the machine was switched on, it began to vibrate and
the vibration automatically separated the RH particles into sizes, each size being retained in
the corresponding sieve.
.
3.3. Combustion of Rice Husks in a Muffle Furnace
3.3.1
Washing and Drying of Rice Husks
The milled rice husks obtained from the rice mill were placed in plastic buckets, washed
several times to remove sand and stone contaminants which settled at the bottom of the
buckets during washing and was easily separated from the husks with the aid of household
136
sieves. The washed rice husks were then spread on plastic trays and other extraneous
materials like broken rice grains present were removed by handpicking. The wet rice husks
were then dried at 1000C to a constant weight and then stored in a dessicator.
3.3.2
Combustion of Rice Husks in a Muffle Furnace
2.00g of the washed and dried rice husks were weighed into four medium-size porcelain
crucibles, which were then placed inside the muffle furnace without the covers. The furnace
was then switched on and left to attain the desired combustion temperature. As the
temperature of the furnace increased, the time it took to attain the desired combustion
temperature (heating rate) was monitored and recorded. Once this was attained, heating was
continued until the required experimental combustion time for the ash sample was
exhausted. The ash samples were then withdrawn from the furnace, allowed to cool in a
dessicator and the ash samples weighed to obtain the weights, from which the yields were
calculated. Four samples were obtained for each temperature and time and the average yield
taken. The batches adopted are coded as follows:
Example RHA-MPSC-400-1
where RHA stands for Rice Husk Ash; MPS for Mixed Particle Size; C –400 represents the
combustion temperature of 400, while the last number represents the combustion time in
hours. Thus, there are altogether forty- two (42) batches designated as:
RHA-MPSC-400 (1 to 6)
RHA-MPSC-500 (1 to 6)
RHA-MPSC-600 (1 to 6)
RHA-MPSC-700 (1 to 6)
RHA-MPSC-800 (1 to 6)
RHA-MPSC-900 (1 to 6)
RHA-MPSC-1000 (1 to 6)
3.4 Determination of Heating Rate of RH Combustion Process
The heating rates of the RH combustion process were determined for each combustion
temperature by monitoring the length of time it took to attain the specific temperathure .
The heating rate was then calculated using the formula:
Heating Rate (0 C / min) =
Temperature Attained
Time taken to attain the temp.
137
3.5 Determination of Volatile content of Rice Husks
The method used for the volatile content determination was that of Simonov et al, 88 which
involved heating the rice husks at 8500C for 7mins after which the ash was allowed to cool
at room temperature and then weighed. The loss in weight of the husks after incineration
was taken as the volatile content.
3.6 Determination of RHA Colour
3.6.1 Bulk Sample Photograph
A small quantity of RHA (~1.5g) sample was placed in a petri dish and the dish was briskly
moved forward and backward on a flat hard surface for an even spread of the ash. A digital
camera was used to take snapshots of the RHA – bulk samples so as to record the colour of
ash obtained at different combustion temperatures and time.
3.6.2 Colour Scale
The colour ‘white’ comes in various shades, which in the paint industry, are generally
described as ‘brilliant white’ (for sparkling white colour), white (for not- so- brilliant
white), off-white (for milky white), cream (for slightly yellowish white) and ivory (for
slightly deeper than cream). The colour scale is designed to assign quantitative values to the
different colours observed during the thermal degradation of rice husks, taking cognizance
of the various shades of white. The colour description is the predominant colour of the ash
while the colour number is the figure assigned to the colour on the colour scale.
Table 3.1:Colour Scale Showing Colours and Colour Numbers For Various Transition
Colours of Rice Husk Ash During Combustion
Colour
Colour Number
Colour
Colour Number
Black
10
Black
10
Dark Brown
8
Dark Grey
8
Brown
7
Grey
7
Light Brown
6
Light Grey
6
Ivory
5
Ivory
5
Cream
4
Cream
4
Milky/off-white
3
Milky/off white
3
White
2
White
2
Brilliant white
1
Brilliant white
1
Note: The lower the value, the closer it is to white and farther from black.
138
3.7 Determination of Shape and Surface Morphology of Rice Husk Ash Particles
Pyrolized RHA particles are porous and shrunken along their cross-section according to
Bharadwaj et al, who also reported the presence of bumps and craters on the surface of the
particles .For the morphological examination of the RHA particles, they were viewed under
a palaeontology microscope at a magnification of 40.
3.7.1 RHA Particle Magnification
A few grains of RHA were placed in a clean petri dish. The dish was then placed under the
viewing lens and incident light of the microscope and the lens adjusted to pick just three or
four particles, which were subsequently magnified by 40. A digital camera was placed
against the lens of the microscope and snapshots of the magnified particles taken. This was
done for all the forty-two (42) ash samples produced at the various incineration conditions.
3.7.2
RHA Cluster Magnification
0.1g of RHA was weighed into a petri dish. The dish was moved to and fro severally on a
hard surface to ensure a fairly even spread of the ash on the dish. The dish was placed under
the viewing lens and incident light of the microscope to obtain magnified clusters of RHA
particles (x40). A digital camera was placed against the lens of the microscope and
snapshots of the magnified particle clusters taken. This was done for all the forty-two (42)
ash samples produced at the various incineration conditions.
.
3.8 Effect of Heating Rate on RHA Colour
The heating rate of the rice husks was reduced from 320C to 15.520C (for 9000C) and 270C
to 9.80C (for 10000C) by reducing the furnace setting from a maximum of 100 to 70 to
determine the effect of lower heating rate on the colour of RHA obtained at 900oCand
1000oC
3.9 Large-Scale Production of RHA
The use of porcelain crucibles for the combustion study of rice husks is scientifically
appropriate. However, the only snag is the very small quantities of ash produced, since only
2g of husks per crucible is used and this yields only about 0.4g of ash. For paint
formulation and production, a sizeable amount of the ash is required, thus the use of
crucibles becomes inadequate. Consequently, a two-stage method was adopted for this
purpose similar to that used by Sugita and co-workers.126 In this method, washed and dried
rice husks were put in a stainless steel pot and incinerated with a camping gas with stirring
139
until all the volatiles had escaped and there was no further emission of fumes. This lasted
for about 30mins. The product was black RH char, which is highly carbonized. This
constitutes the carbonization stage. The RH char was then thinly spread on the floor of the
muffle furnace and heated at 6500C for 4hours to obtain white ash (amorphous silica). This
represents the decarbonization stage. The temperature at 6500C used has been reported as
the most economical and convenient temperature for RHA production.
3.10 Preparation of RHA Flour
The ash produced from the large-scale method was placed in a porcelain mortar and ground
with a pestle until the ash was reduced to fine particles size that had a powdery texture.
3.11 Separation of RHA Flour into Particle Sizes
The RHA flour obtained was sieved with three standard sieves, with different aperture sizes
of 32, 63 and 106microns to obtain RHA flour of three different particle size ranges of 2032, 32-63 and 63-106 microns, respectively, for the formulation of different paint products.
3.12 Determination of Specific gravity of Rice Husks, Rice Husk Ash and RHA Flour
Powdery substances like RHA contain voids of air, which have to be filled for the specific
gravity (S.G) to be accurately determined. This can be achieved by filling them with a
liquid that does not affect the powder e.g n-hexane or more common solvents like kerosene,
white spirit. In this case, kerosene was used as a solvent. The S.G of the kerosene was
determined by expressing the weight of 100cc of kerosene over the weight of an equal
volume of water. The specific gravity value of the granular RHA and RHA flour were
determined as follows: 218
The weight of a 100cc measuring cylinder was obtained, 10g of RHA was added and
kerosene poured into the cylinder until the 100cc mark was reached. The weight of the
empty cylinder was subtracted to obtain the weight of known volume of RHA and kerosene
(W2).The weight of the RHA (10g) was subtracted to give the weight of kerosene used
(W1) The volume of kerosene (V1) was obtained by dividing its S.G by its mass (W1) and
the volume of RHA (V2) was calculated by subtracting V1 from 100cc. The S.G of RHA
was then calculated by expressing the mass (10g) over the volume (V2).
140
3.13
Determination of the Elemental Composition of RHA using Particle-Induced
X-ray Emission (PIXE) Spectrometric Technique
3.13.1 Sample preparation
The RHA samples were ground in a porcelain mortar to reduce the particle size and ensure
a uniform distribution of the particles. The ground ash samples were then pelletised using a
hydraulic press before subjecting then to ion beam analysis using the PIXE technique
described below.
3.13.2 Ion Beam Analysis (IBA) Set-Up and Method
The Tandem Pelletron Accelerator is equipped with a RF charge exchange Ion source
(Alphatross) to provide both proton and helium ions. Positive ion beam is extracted from a
plasma in the RF source and accelerated at 4.6 Kev for protons (6 Kev for alphas) into the
charge exchange cell, where a portion (1-2%) is converted to negative ions by means of
rubidium vapour. These negative ions are extracted and then accelerated to the desired
energy by the tandem pelletron accelerator, which has a maximum terminal voltage of 1.7
MV. At the terminal, in the center of the accelerator, a nitrogen stripper gas converts the
negative ions to positive ions and they undergo a second-stage of acceleration. Thus the
accelerator can deliver a proton beam of 0.6-3.4 Mev or an alpha beam of up to 5.1 Mev.
The accelerater tank is filled with SF6 insulating gas at a pressure of 80 psig. Two ultrahigh
vacuum turbo molecular pumps rated at 3001/sec, one at the low energy (LE) end and the
other at the high energy (HE) end of the accelerater maintain the ultrahigh vacuum which
could reach 2 x 108 Torr inside the accelerater tube and ~ 10-7 Torr in the beam line
extension. Control of the accelerator is facilitated by a computerized control panel with
digital and analogue displays.
The accelerator has provision for five beam lines but only one was in use. The beam line in
use (+ 150) is equipped with a multi-purpose End-station for broad beam IBA analysis Fig.
3.1 shows a general view of the IBA facility.
3.13.3 The End Station
The end-station consists of an Aluminum chamber of about 150cm diameter and 180cm
height. At 90cm height, the chamber has four parts and a window. Port 1 at 165% is for the
RBS detector, port 2 at 1350 is for the PIXE detector, port 3 at 300 is for the ERDA
detector, the window at 00 is for observing the beam position and size, while port 4 at 2250
is for PIGE. The chamber has a sample ladder that can accommodate eleven 13mm
141
samples. The end-station has a turbo pump and a variable tantalum beam collimator (1 mm,
2mm, 4mm and 8mm diameter) to regulate beam size and an isolation value. Fig.3.2 shows
a general view of the end station.
3.13.4 The Detector
The PIXE detector is a canberra Si (Li) detector with 30mm2 active area, a 25mm thick Be
window, and 150 ev FWHM energy resolution at 5.9 Kev. There is a wheel in front of the
Pixe detector to hold up eight different absorbers (four are installed) to cut off low energy
peaks if necessary and/or reduce the count rates of the low Z elements. Canberra Genie
2000 (3.1) software is used for the simultaneous acquisition of the PIXE and RBS data.
GUPIXWIN is the computer code used for PIXE data analysis.
3.14 Conversion of Concentration of Elements (in ppm) to their Oxides Expressed
in Percentage %
The PIXE analysis gives the concentration of the elements in parts per million (ppm).
However, since the analyzed samples are ash samples, the elements are in the form of
oxides in the samples. To convert the concentration of the elements expressed in ppm to
their oxides (also in ppm), the former is divided by a conversion factor, which is obtained
from a ratio of the element to its oxide as follows:
Oxide of element (in ppm) =
Conc. of element in ppm
Conversion factor
To convert from ppm to percentage:
1
1 ppm =
1, 000, 000
In % =
1
1
× 100 =
1, 000, 000
10, 000
To convert Xppm to X %
=
X
10, 000
For example in RHA-MPSC-500-1, to convert the value of Mg expressed in ppm
(4164.5ppm) to MgO expressed in percentage concentration (% conc):
MgO % Conc =
=
=
4164.5 ppm
1
×
0.6031
10, 000
6905.16 ppm
=
10,000
142
=
0.6905% of MgO
SWITCHING
MAGNET
SCATTERING
CHAMBER
ION SOURCE
CONTROL DESK
ACCELERATOR
TANK
QUADRUPOLE
MAGNET
Fig. 3.1 The CERD Accelerator, OAU, Ife Showing the Various Components
143
Fig. 3.2. The End Station Showing Four Ports and a Window; Port 1 at 1650 for
RBS Detector, Port 2 at 1350 for PIXE Detector, Port 3 at 30o for ERDA Detector,
Port 4 at 2250 for PIGE and Window at 00 for observing Beam Position and Size
144
QUA
3.15 Calculation of conversion factors
The conversion factors (element) presented in Table 3.2 were calculated from the ratios of
the atomic weights of the pure elements to their oxides. For example for SiO2, the
conversion factor is calculated as follows :
Si = 28.086
O = 15.999
SiO2 = 28.086 + (15.99 x2) = 60.084
Ratio of Si in SiO2 =
28.86
= 0.4674 conversion factor for Si to SiO2
60.08
This calculation was carried out for all the elements found present in RHA and are
presented in Table 3.2.
Table 3.2 Oxide Conversion Factors for Conversion of
Elements to their Oxides
Compound
SiO2
Al2O3
Fe2O3
FeO
CaO
Na2O
MgO
K2O
TiO2
MnO2
P2O5
H2O
CO2
ZrO2
SrO
ZnO
Rb2O
*Element fraction
0.4674
0.5326
0.6994
0.7773
0.7147
0.7419
0.6031
0.8301
0.5994
0.6320
0.4364
0.1119
0.2729
0.7403
0.8456
0.8034
0.9144
*used in conversion from element to oxide
145
Oxygen fraction
0.5326
0.4707
0.3006
0.2227
0.3006
0.2581
0.3969
0.1699
0.4006
0.5636
0.8881
0.7271
0.2597
0.1544
.
3.16 RICE HUSK ASH AS EXTENDER IN EMULSION PAINTS
3.16.1 Production of Emulsion Paints Using RHA Flour as Extender in
Comparison with Silica flour, Kaolin and Calcium Carbonate (standard)
White and coloured emulsion paint products of premium quality were formulated using
RHA flour (as new extender), silica flour, kaolin and calcium carbonate. The filler levels
were varied from 2 to 12% for the purpose of comparatively determining the viscosity
effects of the said fillers in emulsion paint products as well as the colour impact effects of
the fillers in the emulsion paints. Since the particle size (surface area) of fillers affects the
viscosity of the paint products, a uniform particle size was maintained by sieving the four
different fillers with a sieve of aperture 63microns(µm) to obtain particle size range of 3263 µm. The particle size of fillers used in emulsion paint formulation (in CAP Plc) is below
75 µm, hence the choice of 63 µm.
The Control for the emulsion paint sample, which did not contain any extender was
produced using the formulation given in Table 3.3:
Table 3.3 Control Recipe (0% Extender) for White Emulsion Paints (WEP)
Components
Function
Control (CONWEP)
Wt %
Wt in g
Water
Solvent
45.15
135.45
Calgon PT
Wetting agent
0.15
0.45
Acticide Bx
Preservative
0.50
1.50
Berol 09
surfactant / emulsifier
0.20
0.60
Antifoam
Defoamer
0.20
0.60
Coatex/ Bermodoll
Dispersant
0.20
0.60
Filler
Bulk and Viscosity increase
0.00
0.00
Titanium dioxide
Pigment
19.00
57.00
Styrene acrylic resin
Binder
33.00
99.00
Natrosol
Thickening agent
0.50
1.50
Ammonia
PH adjuster
0.10
0.30
Texanol
coalescing agent,
co-solvent
1.00
3.00
100%
300g
146
Table 3.4 Formulations of White Emulsion Paint Finishes Produced with Different
Extenders at 2% to 12% Levels
WEP-2
WEP-4
WEP-6
WEP-8
WEP-10
WEP-12
Water
43.15
41.15
39.15
37.15
35.15
33.15
Calgon PT
0.15
0.15
0.15
0.15
0.15
0.15
Acticide Bx
0.50
0.50
0.50
0.50
0.50
0.50
Berol 09
0.20
0.20
0.20
0.20
0.20
0.20
Antifoam
0.20
0.20
0.20
0.20
0.20
0.20
Coatex
0.20
0.20
0.20
0.20
0.20
0.20
Filler
2.00
4.00
6.00
8.00
10.00
12.00
Titanium
dioxide
Styreneacrylic resin
Natrosol
19.00
19.00
19.00
19.00
19.00
19.00
33.00
33.00
33.00
33.00
33.00
33.00
0.50
0.50
0.50
0.50
0.50
0.50
Ammonia
0.10
0.10
0.10
0.10
0.10
0.10
Texanol
1.00
1.00
1.00
1.00
1.00
1.00
100%
100%
100%
100%
100%
100%
Components
The white emulsion paint products (WEP) produced using different fillers namely calcium
carbonate (standard), Kaolin, RHA flour and silica flour were coded as follows:
Table 3.5 Codes for White Emulsion Paints Containing Different Levels of Extenders
Name
of Code
2%
4%
6%
8%
10%
12%
sample
Silica
flour
white
emulsion paint
RHA
white
Emulsion
paint
SFWEP
SFWEP-2
SFWEP-4
STWEP-6
SFWEP-8
SFWEP-10
SFWEP-12
RHAWEP
RHAWEP-2
RHAWEP-4
RHAWEP-6
RHAWEP-8
RHAWEP-10
RHAWEP-12
Kaolin white
emulsion paint
KWEP
KWEP-2
KWEP-4
KWEP-6
KWEP-8
KWEP-10
KWEP-12
Calcium
carbonate
white
emulsion paint
Control white
emulsion paint
CCWEP
CCEWP-2
CCWEP-4
CCWEP-6
CCWEP-8
CCWEP-10
CCWEP-12
CONWEP
-
-
-
-
-
-
147
Production Procedure for Emulsion Paints
The same procedure was used in the production of all the emulsion paint products. 300g of
each paint sample was produced and this was obtained by multiplying the wt% values of
each component by 3 to give a total weight of 300g. The procedure is described using 2%
filler level as an example.
The following components were loaded into a vessel:
Water (part)
15.15g
Calgon PT
0.45g
Acticide Bx
1.50g
Berol 09
0.60g
Antifoam
0.30g
Dispersant
0.60g
They were stirred at low speed by means of a mini stirrer until calgon PT dissolved (approx
5 mins). The following ingredients were added into the same pot with low-speed stirring:
Filler
6.00g
Titanium dioxide
57.00g
The mixture was then stirred at high speed (pigment dispersion stage) for about 20mins
while intermittently scraping the sides of the vessel. When the dispersion was satisfactory,
the stirring speed was reduced and the following components were added with slow
stirring.
Styrene-acrylic resin
99.00g
Water
30.00g
Slurry of the following was prepared in a separate container and added with stirring to the
main pot
Water
84.3g
Natrosol
1.5g
The following components were finally added and the mixture stirred until natrosol
dissolved (approx 25mins).
Antifoam (part)
0.30
Ammonia
0.30g
Texanol
3.00g
The pigment dispersion was checked with the aid of a small piece of a 100-mesh sieve
rolled into the shape of a funnel. A small quantity of the sample paint was poured into the
funnel and water passed through the sieve. The absence of particles showed a welldispersed pigment.
148
3.16.2 Formulation of Blue and Green Emulsion Paint Variants
The white emulsion paint products obtained with different fillers were tinted with
appropriate colour pigment pastes to obtain blue and green emulsion paint variants. The
blue colour (‘First dawn’) was obtained by tinting as follows:
wt of white emulsion paint = 100g
wt of blue pigment paste = 0.03g
wt of yellow oxide paste = 0.08g
The green emulsion paint variants (‘well being’) were obtained by tinting as follows:
wt of white emulsion paint = 100g
wt of green pigment paste = 0.02g
wt of yellow oxide paste = 0.05g
The colours were selected from the Dulux paints colour chart. The blue and green emulsion
paint samples produced with different extenders were coded as follows:
Name of Blue Emulsion Paint
Code
Calcium carbonate Blue emulsion paint
CCBEP
Silica flour Blue emulsion paint
SFBEP
Rice Husk ash Blue emulsion paint
RHABEP
Kaolin Blue Emulsion paint
KBEP
Name of Green Emulsion Paint
Calcium carbonate Green Emulsion Paint
CCGEP
Silica Flour Green Emulsion Paint
SFGEP
Rice Husk Ash Green Emulsion Paint
RHAGEP
Kaolin Green Emulsion Paint
KGEP
3.16.3 Determination of In-Can Appearance of Paint Products
The formulated paint products were visually observed and assessed in terms of colour,
homogeneity/consistency and smoothness.
3.16.4 Determination of pH of Emulsion Paint Products
The pH meter was switched on and calibrated by immersing the electrode in a buffer
solution of pH 7.0 until a pH meter reading of 7.00 was obtained. The electrode was
removed from the buffer solution, rinsed with water and dried with tissue paper. It was then
dipped into the paint sample and the ‘read’ button pressed. When there was a ‘beep’ sound,
the pH reading was taken. The electrode was then cleaned with water and dried with tissue
paper for subsequent pH readings.
149
3.16.5 Determination of weight per litre (specific gravity) of Emulsion Paint Products
The weight per litre cup was first weighed on a digital weighing scale (W1). The paint
sample was poured into the cup and any excess paint cleaned off from the hole in the lid.
The weight of cup and paint was recorded (W2). The wt per litre value of the paint was
obtained by deducting the wt of the cup (W2-W1) alternatively, the wt of the cup was
‘tared’ after weighing such that when the paint was poured into the cup and weighed, the
reading gave the direct wt per litre value of the paint.
3.16.6 Determination of Viscosity of Formulated Emulsion Paints
The paint sample was poured into the sample can to a level of about 2.5cm from the top of
the cup. The can was then placed on the turntable of the rotothinner after it had been
switched on. The disc was immersed into the emulsion paint inside the sample can. The
disc with sample was allowed to rotate until the peak viscosity value was obtained. The
viscosity reading was taken from the graduated scale around the turntable. The disc was
raised and the sample can removed, following which the disc was thoroughly cleaned with
a brush and water.
3.16.7 Determination of Opacity of Emulsion Paints.
The Morest chart (black and white striped paper) and a K-bar (stainless steel bar) were used
for the opacity test. The paint sample was scooped with a palate knife and spread evenly
across the width of the paper about 4cm from the edge of the paper. The paint was then
evenly applied down the length of the paper by means of the K-bar and left to dry. A
second coat was then applied using the same application technique but with a space of 5cm
left from the edge of the first coat.
3.16.8 Determination of the Washability (Scrub resistance) of Emulsion Paints
Two coats of the paint under test were applied to an asbestos panel of dimension 6”x 12”
using a brush. The paint was allowed to dry under ambient conditions and allowed to age
for seven days. The test panels, were mounted on the washability (scrub resistance) testing
machine fitted with nylon brushes. The machine was set at 5,000 cycles, which is the
standard, to determine if the paint would be washed off the asbestos panel after brushing
the surface 5,000 times. The number of cycles that the paint withstands before any scratch
was observed and recorded.
150
3.16.9 Calculation of Pigment Volume Concentration (PVC) of Emulsion Paints
The PVC of the emulsion paints produced with different levels of extenders were calculated
using the expression :
% PVC =
Vol. of pigment in pa int
x 100
vol of pigment in pa int + vol of non − volatile constituents in pa int
The total solids content of Styrene – acrylic resin is in the range 49 – 51%. An average
value of 50% was used in the calculations
3.16.10 Application of Emulsion Paint Products
A paintbrush was dipped into the paint sample and the paint applied on a brush-out card in
vertical strokes to give the first coat. When the first coat was dry (after about 15mins), the
second coat was applied using the same application technique.
3.17 RHA AS EXTENDER IN TEXTURED FINISH
3.17.1 Production of Textured Finishes using RHA Flour as Extender in Comparison
with Silica Flour and Calcium Carbonate (standard)
Textured paint contains most of the components present in emulsion paint in addition to the
texturizer (sand) and a higher extender level. Since the particle size of filler used in textured
finishes need not be as fine as that for emulsion paints, the three fillers were sieved with a
standard sieve of aperture 106microns (0.106mm) to ensure fairly uniform particle size
range of 63-106µm for all the extenders used.
The formula (recipe) adopted for the production of the textured finishes is given in Table
3.6:
151
Table 3.6
Formula for Textured Finishes Showing the Components, their
Functions and Weight Percentages
Components
Function
Wt%
Weight (g)
Water
solvent
11.20
56.00
Calgon PT
Wetting agent
0.15
0.75
Acticide
Preservative
0.50
2.50
Berol 09
Emulsifier
0.20
1.0
Antifoam
Defoaming
0.30
1.5
Texanol
Titanium dioxide
Co-solvent, coalescing 1.00
agent
Bulk and viscosity 15.00
increase,
Pigment
16.00
80.00
Styrene acrylic
Binder
30.00
150.00
Natrosol
Thickening agent
0.50
2.50
Ammonia
pH adjuster
0.15
0.75
Sand
Texturizer
25.00
125.00
100%
500g
Extender
Total
5.00
75.00
The fillers used in producing the textured finish were calcium carbonate (standard) silica
flour and RHA flour. The products were coded as follows:
Name of Textured Finish Sample
Code
Calcium carbonate white textured finish
CCWTEXF
Silica flour white textured finish
SFWTEXF
RHA white textured finish
RHAWTEXF
Production Procedure for Textured Finish
The following products were loaded into a vessel:
Water (part)
40.00g
Calgon PT
0.75G
Acticide Bx
2.50g
Berol 09
1.00g
Antifoam
0.75g
They were stirred at low speed with the aid of a mini stirrer until calgon PT dissolved
(about 5mins). The following ingredients were added into the same vessel with low-speed
stirring:
152
Filler
75.00g
Titanium dioxide
80.00g
The mixture was then stirred at a high speed for about 20 mins (pigment dispersion stage).
Intermitterntly, the sides of the vessel were scraped. When the disperson was satisfactory,
the stirring speed was reduced and the following components were added with slow
stirring:
A slurry of the following was prepared in a separate container and added with stirring to the
main pot:
Natrosol
2.50g
Water
316.00g
The following components were added and the mixture stirred rapidly until natrosol
dissolved completely
Antifoam
0.75%
Ammonia
0.75%
Finally, the following was added with slow stirring
Sand
125.00g.
3.17.2 Formulation of Deep Pink, Light Purple and Pale Ivory Textured Finish
Variants
The white textured finish products obtained with different types and levels of fillers were
tinted with the appropriate pigment pastes to obtain deep pink, light purple (‘Gentle
lavender’) and pale Ivory (‘Blossom white’).
The deep pink colour was obtained by tinting with the following:
Wt of white textured finish = 100g
Wt of bright red pigment paste = 0.5g
Wt of purple pigment paste = 0.1g
The ‘Gentle lavender’ coloured finish was obtained with the following:
Wt of white textured finish = 100g
Wt of bright red pigment paste = 0.05g
Wt of purple pigment paste = 0.08
The ‘Blossom white’ coloured finish were obtained as follows:
Wt of white textured finish = 100g
Wt of yellow oxide pigment paste = 0.15g
Wt of bright red pigment paste = 0.03g
153
The coloured textured finishes produced with calcium carbonate, silica flour and RHA were
coded as follows:
Name of Textured Finish
Calcium carbonate Textured finish –Deep pink
Silica flour Textured finish – ‘sexy pink’
RHA Textured Finish – ‘sexy pink’
Code
CCTEXF-DP
SFTEXF-DP
RHATEXF-DP
Name of Textured Finish
Code
Calcium carbonate Textured finish – ‘Gentle CCTEXF-GL
lavender’
Silica flour Textured finish – ‘Gentle lavender’
SFTEXF-GL
RHA Textured Finish – ‘Gentle lavender’
Name of Textured Finish
RHATEXF-GL
Code
Calcium carbonate Textured finish – ‘Blossom CCTEXF-BW
white’
Silica flour Textured finish – ‘Blossom white’
SFTEXF-BW
RHA Textured Finish- ‘Blossom white’
RHATEXF-BW
3.17.3 In-Can Appearance of Textured Finishes
The physical appearance of the textured finishes in-can was described in terms of the
attributes listed in the case for emulsion paints in section 3.16.3
3.17.4 Determination of pH of Textured Finishes
The pH for textured paint finishes were determined as described for emulsion paint
products in section 3.16.4
3.17.5 Determination of weight per litre (specific gravity) of Textured Finishes
The wt per litre values of the textured paint finishes were determined as described for
emulsion paint products in section 3.16.5
3.17.6 Determination of Gel strength (viscosity) of Textured Paints
The sample can was filled with the textured paint to within 2.5cm of top of can. The
rotothinner gel strength tester was switched on. The sample can was then placed on the turn
table of the rotothinner and the paddle immersed into the paint inside the sample can. The
154
rotor with the sample was allowed to rotate until the peak value was obtained. The reading
was taken from the graduated scale around the turntable. The paddle was raised and the
sample can removed. The paddle was cleaned with water and a brush in readiness for
subsequent tests.
3.17.7 Application of Textured Finishes
A paintbrush was dipped into the textured finish and some quantity scooped with the brush
and applied evenly on the brush-out cards using the vertical strokes technique. After
application of the finish, the tip of the brush was used to make some designs on the applied
paint similar to the design made by a roller when the paint is applied on the wall. The paint
was then left to dry under ambient conditions.
3.18 RHA AS EXTENDER IN RED OXIDE PRIMER
3.18.1 Production of Red Oxide Primer using RHA Flour as New Filler in
Comparison with Silica Flour and Calcium Carbonate (Standard Filler)
Red oxide primer is an undercoat, containing ferric oxide as pigment, hence the name. It is
applied on metal surfaces to promote adhesion of the topcoat, usually, a gloss paint and to
prevent corrosion of the surfaces. To maintain particle size range uniformity, the fillers
were sieved with a standard sieve of aperture 63 microns.
The formula used in the production of the red oxide primer is shown in Table 3.7
The extenders used in the formulation were calcium carbonate, which is the standard filler,
silica flour and RHA flour. The primer products were coded as follows:
Name of Textured Finish
Code
Calcium carbonate red oxide
(standard)
Silica flour red oxide primer
RHA flour red oxide primer
155
primer CCROP
SFROP
RHAROP
Table 3.7 Formula for Red Oxide Primer Showing the Components,
their functions and Weight Percentages
Component
Function
Wt%
Wt(g)
Medium oil alkyd resin
Binder
44.20
221.00
Kerosene
Solvent
25.3
126.50
Soya lecithin blend
Dispersant
0.50
2.50
Easi gel
Anti-settling agent
2.5
12.50
Red oxide
Pigment
4.00
20.00
Extender
Matt effect, bulk increase
20.00
100.00
Calcium drier
Surface dry
0.30
1.50
Cobalt drier
Through - dry
0.25
1.25
Zirconium drier
Through -dry
0.25
1.25
Silicone fluid
Flow aid,coalescing
0.20
1.00
2.50
12.50
100%
500.00g
agent
Methyl Ethyl Ketoxime
Anti-skinning agent
Production Procedure for Red Oxide Primer
The following components were loaded into a vessel
Medium oil alkyd (part)
71.00
Kerosene (part)
76.50
Soya lecithin blend
2.50
Easigel
12.50
The following ingredients were added with slow stirring:
Red oxide pigment
20.00
Filler
100.00
The pigment and filler were dispersed at high speed with bits by means of a mini stirrer.
When the dispersion was satisfactory (20 – 30microns), the mixture was run at low speed
and the following materials were added
Medium oil alkyd
150.00
Kerosene
50.00
Calcium drier
1.50
Cobalt drier
1.25
Zirconium drier
1.25
156
Silicone fluid
1.00
Methyl Ethyl Ketoxime
12.50
Total wt :
500g
3.18.2 Thinning of Red Oxide Primer
The red oxide primers after production were found to be so viscous that they exceeded the
maximum readings of both the analog and digital rotothinners. It was therefore necessary to
thin them down with kerosene to the application viscosity of 3.0 poises. This was carried
out by weighing 100g of the primer into a sample can, then gradually adding a weighed
amount of kerosene and checking the viscosity in the rotothinner until the required
viscosity of 3.0 poises was attained. The weight of kerosene used to achieve this viscosity
was recorded.
3.18.3 Wt per litre Determination
The wt per litre values of the red oxide primers were determined as described in section
3.16.5
3.18.4 Determination of Film Appearance
Two coats of the primer were applied on 6” x 4” steel panels, allowing 10 mins interval
between coats. The film on drying was observed for presence/absence of haze, bits and
seeds.
3.18.5 Determination of Drying Time of Red Oxide Primers
The red oxide primer was applied on 6” x 4” steel panels and the time it took for the applied
coat to become tack-free (non-sticky to the touch) was recorded. The time it took for the
coat to dry hard was also recorded.
3.18.6 Determination of Matt Effect of RHA in Red Oxide Primer
i Film Appearance: The red oxide primer, which had been thinned down to the application
viscosity of 3.0 poises using kerosene was applied on 6”x 4” iron metal panels by means of
a paint brush. When the first coat had dried, a second coat was applied. The paint was then
left to dry hard at room temperature before visual examination for flatting (matt) effect.
ii Determination of Gloss Value of Red Oxide Primer
The red oxide primer, which had been thinned down to the required application viscosity of
3.0 poises was scooped with a pallet knife and applied generously across the width of the
glass panel about 2cm from the edge of the glass panel. With the aid of a doctor’s blade, the
157
paint was applied evenly along the length of the glass panel by holding the two ends of the
blade and moving it down the length of the glass panel. The primer was allowed to dry hard
under ambient conditions for 16hrs. The gloss meter was switched on and allowed to warm
up for a few minutes and then set to the required angle for the gloss measurement (200, 600
or 850), the gloss meter was then placed on the surface of the paint applied on the glass
panels and the reading taken. The gloss meter was placed at different positions on the gloss
panels to obtain four readings
3.19 RHA AS EXTENDER IN MATT WOOD FINISH
3.19.1 Production of Matt wood finish using RHA Flour as Extender
in Comparison with Silica Flour and Fumed Silica (Standard )
Matt wood finish is applied as topcoat on wood and produces a flatting (low gloss) effect
on wood. The standard flatting agent used in the formulation of the wood finish to give the
matt effect is fumed silica (microsilica), which has a particle size of 20microns.The matt
effect of FS was compared with those of RHA flour and silica flour (SF) used as alternative
flatting agents. RHA flour and SF were sieved with a standard sieve of aperture 32microns
to obtain a particle size range of ~32microns since there is no standard sieve with aperture
of 20MIC.A control for the wood finish was formulated without extender.
The recipe used in the production of the wood finish is given in Table 3.8
Table 3.8 Formula for Matt Wood Finish Showing the Components, their Functions
and Weight Percentages
Matt Wood Finish
Control
Component
Function
Wt%
Wt(g)
Wt%
Wt (g)
MIBK
Solvent
12.00
36.00
12.00
36.00
Toluene
Solvent
35.00
109.50
36.50
109.5
Nitrocellulose
solution
Short oil alkyd
Binder
12.00
36.00
12.00
36.00
Binder
12.00
36.00
12.00
36.00
Dioctyl Phthalate
Plasticizer
2.00
6.00
6.00
18.00
Extender
Gloss control
3.00
9.00
0.00
0.00
Xylene
Solvent
23.50
70.50
25.00
75.00
Silicone fluid
Flow aid
0.50
1.50
0.50
1.50
100.00g
300.00g
100.00g
300.00g
158
The formulated matt products were coded as follows:
Name of Matt Wood Finish Sample
Code
Fumed silica matt wood finish
FSMWF
Silica flour matt wood finish
SFMWF
Rice husk ash matt wood finish
RHAMWF
Control matt wood finish
CONMWF
Production Procedure for Matt Wood Finish
The following components were loaded into the vessel and slowly stirred:
MIBK (part)
18.00g
Toluene (part)
105.00g
Nitrocellulose solution
36.00g
Short oil alkyd
36.00
DOP
6.00g
The extender (flatting agent) was added under high speed
Flatting agent
9.00g.
When dispersion was satisfactory, the following components were added:
Xylene
70.50g
Silicone fluid
1.50g
MIBK
18.00g
Toluene
4.50g
The mixture was then stirred thoroughly until homogeneous.
3.19.2 Thinning of Matt Wood Finish
The matt wood finish was thinned down to an application viscosity (efflux time) of 23secs
since the specified application viscosity is 23 - 27secsBSB4 cup/ 250C. 100g of the wood
finish was weighed into a sample can and methyl isobutyl ketone (MIBK) was gradually
added, while the efflux time was checked intermittently, until the desired viscosity was
obtained.
3.19.3 Determination of Wt per litre of Matt Wood Finishes
The wt per litre values (specific gravity) of the wood finishes were determined as described
in section. 3.16.5
159
3.19.4 Determination of Dry Film Properties of Matt Wood Finish
The wood finish was strained through 200-mesh nylon and poured out onto glass panel for
observation of film properties.
3.19.5 Determination of Efflux Time of Matt Wood Finish
The flow cup (BS B4) was placed on the viscosity stand in a draught-free position and
such that the top of the cup was level. The forefinger was used to cover the orifice at the
bottom of the flow cup. The paint sample which had been thoroughly stirred and at a temp
of 27±20C was poured into the flow cup to the brim such that the excess flowed into the
groove around the top of the cup. A stop clock was used to record the time of flow. The
finger was removed with simultaneous pressing of the knob of the timer for the
commencement of timing. As soon as the last drop of the paint was out, the timing was
stopped, by pressing the knob on the stop clock. The time it took for the primer to flow out
of the flow cup was recorded.
3.19.6 Determination of Flatting (Matt) Effect of Wood Finishes Produced Using
Fumed Silica (standard), Silica flour and RHA
i. By Application on wood panels
Four pieces of 6” x 4” plywood were first smoothened with sand paper, then coated
with sanding sealer to fill the pores on the surface of the wood. The sealer was then
allowed to dry for about 20 mins. The surface of the wood was further smoothened with
‘smooth’ sand paper, after which the wood finish was applied by spraying, with the aid
of a spray gun.
ii. By Gloss value Measurement
The gloss level of the wood finish was measured directly with a gloss meter as described in
section 3.18.6 for red oxide primer to obtain the gloss values.
3.19.6.1 Determination of Gloss and Matt Values of wood Finish
The gloss value of a paint quantifies the extent of its light- reflecting ability while the matt
value gives the degree of its light-scattering ability. A paint with high light-scattering
ability will have a high gloss value and a shiny (glossy) appearance, while one with a low
gloss value will invariably have a high matt value and a flat (non-glossy) appearance.Since
the two effects are opposites, the matt value(in %) of a paint can therefore be determined
indirectly by subtracting the gloss value from 100
% Matt = 100 − % Gloss
160
3.19.7 Determination of Settling Resistance of the Matt Wood Finishes
The matt wood finish is prone to settling after some hours due to the high level of solvent
and resin. Thus, the settling property of the wood finishes was examined by allowing the
product to stand for 24hrs and then stirring with the aid of a pallet knife.
3.19.8 Determination of Drying Time of Matt Wood Finish
The wood finish was applied on brush-out cards by means of a brush. The time it took for
the wood finish to dry tack-free and hard were recorded.
3.20 RHA AS EXTENDER IN CELLULOSE MATT FINISH
3.20.1 Production of Cellulose Matt Finish using RHA flour as New Extender in
Comparison with Silica Flour and Fumed Silica (Standard)
Cellulose matt finish is a topcoat applied on expensive furniture and in coating metallic
substrates such as file cabinets, refrigerators etc. It is categorized as an industrial coating.
The recipe used in producing the cellulose matt finish is given in Table 3.9
Formula for Cellulose Matt Finish Showing the Components, their
Table 3.16
functions and Weight Percentages
The cellulose matt finish was produced using three different extenders for comparative
evaluation of their flatting (matt) effect. The extenders used were fumed silica (the
standard), silica flour and RHA flour (as new flatting agent). A sieve mesh size of 32
microns was used to sieve the extenders to maintain fairly uniform particles size of
<0.032mm. The matt finishes were coded as follows:
Cellulose Matt Finish
Code
Fumed silica cellulose matt finish (standard)
FSCMF
Silica flour cellulose matt finish
SFCMF
RHA cellulose matt finish
RHACMF
Control cellulose matt finish (0% Extender)
CONCMF
161
Table 3.9 Formula for Cellulose Matt Finish Showing the Components, their
Functions and Weight Percentages
Components
Function
Cellulose
Finish
Wt%
Matt
Control
Wt (g)
Wt%
Wt(g)
Short-oil alkyd resin
Resin
26.33
78.99
26.33
78.99
Xylene
MIBK
Flatting agent
IPA
DOP
Silica fluid
Nitrocellulose solution
White mill base
Yellow oxide mill base
Black mill base
Solvent
Solvent
Gloss control
Solvent
Plasticizer
Flow aid
Resin
Pigment
Pigment
Pigment
8.65
9.21
3.20
2.99
0.57
0.34
26.59
21.00
1.12
Trace
25.95
27.63
9.60
8.97
1.71
1.02
79.77
63.00
3.36
Trace
8.65
11.41
0.00
3.99
0.57
0.34
26.59
21.00
1.12
Trace
25.95
34.23
0.00
11.97
1.71
1.02
79.77
63.00
3.36
Trace
100.00%
300.00g
100.00% 300.00g
Production Procedure for Cellulose Matt Finish
The following components were loaded into the vessel and stirred thoroughly using the
mini stirrer.
Short oil alkyd
78.99g
Xylene (part)
16.98g
MIBK (part)
12.00g
The fumed silica (9.60g) was added gradually under high speed to ensure proper dispersion.
The following components were added with slow stirring:
Isopropyl alcohol (IPA)
8.97g
Xylene(balance)
8.97g
Dioctyl phthalate (DOP)
1.71g
Silicone fluid
1.02g
Nitrocellulose solution
79.77g
When the mixture was homogeneous, viscosity was adjusted with
MIBK( balance)
15.63g
The cellulose finish was then tinted to the desired shade (magnolia) with the following
White millbase
63.00g
Yellow oxide mill base
3.36g
Black mill base
trace
Total wt :
300g
162
3.20.2 Thinning of Cellulose Matt Finish
The cellulose matt finish was thinned down with methyl isobutyl ketone, MIBK to the
application viscosity of 23 secs since the specification is 21-25 secs/BSB4 cup /250C before
application, by spraying on the substrate.
3.20.3 Determination of Wt per litre of Cellulose Matt Finishes
The wt per litre values of the cellulose matt finishes produced with fumed silica, silica flour
and RHA flour were determined as described in section 3.18.6 for red oxide primer.
3.20.4 Determination of Viscosity (Efflux Time) of Cellulose Matt Finishes
The viscosity values of the matt wood finish produced with fumed silica, silica flour and
RHA flour were determined as described for matt wood finish in section 3.19.5
3.20.5 Determination of Gloss and Flatting (Matt) Values of Cellulose Matt Finishes
The flatting (matt) effect of the cellulose matt finishes produced with fumed silica, silica
flour and RHA flour were determined as described for the wood finish in section 3.18.6
3.20.6 Determination of Dry Film Appearance
The cellulose matt finish was applied on metal panels by means of a spray gun and when
dry, the film was observed for presence/absence of bits
163
CHAPTER FOUR
4.0 RESULTS AND DISCUSSION
4.1
Combustion of Rice Husks in a Muffle Furnace
The following general observations were made during the combustion study of rice
husks to obtain rice husk ash (RHA):
60 – 2000C: Slow emission of light brown fumes (volatiles) commenced at about 600C with
the rate and volume of (emission) fumes increasing gradually with increase in temperature.
200- 3000C: There was a more rapid emission of fumes with increased volume of fumes. At
about 250-2600, there was a flash sound indicating that the flash point of the components of
the volatiles is in that neighbourhood.
300 – 4000C: There was more profuse emission of fumes. The rate of emission and volume
of fumes emitted increased appreciably (appeared to have tripled)
400 – 5000C: Profuse emission continued with the colour of the fumes becoming whitish
>5000C: The rate and volume of emission began to drop gradually and stopped at about
5500C. After combustion, tar-like black deposit was observed on the door of the furnace.
These observations are similar to those made by Bharadwaj et al 84 from which they stated
that the first stage in thermal degradation of RH particles was accompanied by the emission
of volatiles from the particles, which was completed at about 4000C. Mansaray and
Ghaly147 also reported a similar two-stage weight loss during the thermogravimetric study
of RH at different heating rates and atmospheres. The fume emission pattern reflects the
two-stage process of carbonisation and decarbonisation during which rice husk char
(accompanied by emission of volatiles) and RHA (after cessation of emission), respectively
are formed.
4.2 Determination of Particle Sizes of Milled Rice Husks
The different particle sizes present in the milled RH used in the combustion study was
found to be in the range of 300 to 1000 microns i.e 0.3 to 1.0 mm
[
4.3. Heating Rates of RH Combustion Process
The heating rates calculated by dividing each combustion temperature with the time to
attain the temperature, are given in Table 4.1
164
Table 4.1: Heating Rates of RH at Different Temperatures
Temp(oC)
Time to attain Temp(mins)
Heating Rate(oC/min)
400
8
50
500
10
50
600
14
43
700
18
38.9
800
22
36.4
900
28
32
900
58
15.52
1000
37
27
1000
102
9.8
4.4 Volatile Content of Rice Husks
The volatile content values obtained for rice husks were found to be 76.17, 76.21, 76.14
and 69.75%. These values are close to those quoted in literature 65. The volatile content is
the amount of volatile materials in the rice husk, which are emitted as fumes during the
combustion process.
4.5 Specific Gravity of RH, RHA, RHA Flour and Commercial Fillers
The specific gravity values determined for the milled rice husks, RHA grains, RHA flour
and kaolin, calcium carbonate and silica flour are presented in Table 4.2
Table 4.2 Specific gravity of RH, RHA Flour and Commercial Extenders
Specific Gravity Values for Various Extenders
Particle size range
Milled RH RHA Grains RHA Flour kaolin Calcium
Silica
(microns)
Carbonate Flour
Mixed Particle Size
1.21
1.41
32-63 microns
1.54
1.76
2.24
2.18
32-106 microns
1.55
1.80
2.26
2.15
32-212 microns
1.56
1.88
2.28
2.13
The values obtained for RH and RHA grains support the literature reports of the low bulk
density of RH, and the relatively high porosity of RHA,
84,132
which is evident in its lower
S.G (1.41) than RHA flour (~1.55) The observed increase in SG on grinding RHA to
165
produce RHA flour, which is of finer particle size, is as a result of increase in surface area
and elimination of most of the pores as the particles become more closely packed. The
lower SG values of RHA flour at different particle size ranges, compared with the other
commercial extenders, is expected to influence the paint properties with regards to
viscosity, since it implies a greater volume of extender for the same mass .The results in
Table 4.2 seem to show that particle size has little effect on the S.G values, probably
because we are dealing with a distribution of particle sizes. The S.G values for calcium
carbonate, silica flour, kaolin and RHA used as extenders in this study are lower than the
values of 2.5-2.8, 2.8, 2.6 and 2.1,65 respectively, quoted in literature because the extenders
used in this work were sieved to a finer particle size range of 32-106 microns, as required
for paint production, thereby eliminating larger-sized particles while the literature values
have a wider particle size range, inclusive of bigger particles of each extender.
4.6 Effects of Combustion Temperatures and Time on Ash Yields of Rice
Husks
Combustion conditions influence the chemistry (composition) and amount of ash produced
(yield) This is because combustion causes an expulsion of water, the conversion of
combustible carbon into CO2 and steam, the loss of carbon dioxide from carbonates, the
conversion of metal sulphides into metal oxides, metal sulphates and sulphur oxides and
other chemical reactions.The average ash yields obtained from four samples combusted at
the same temperature and time are given in Table 4.3 Plots of RHA yield as a function of
temperature and time are shown in Figs.4.1 and 4.2 respectively.
The results revealed generally that, lower combustion temperatures (400, 500 and 6000C),
gave higher ash yields than higher combustion temperatures (700 to 1000oC). The highest
ash yield of approximately 23.81% was obtained at the lowest combustion temperature of
4000C (at 1hour duration) while the lowest ash yield of 19.78 was obtained at the highest
temperature of 1000oC. Similarly RH ash yields of 21.79%, and 22.41%, were obtained at
500, 600 and 7000C combustion temperatures respectively at 1hr combustion duration
while yields of 20.02, 20.12 and 19.78% were obtained at 7000C, 8000C and 10000C
respectively at the same duration. The ash yields were found to fall within the range of 19
to 24% as shown in Fig. 4.1.
166
Table 4.3 Average RH Ash Yields (in %) at Different Combustion Temperatures
and Time (hr)
Combustion
Temp (0C)
Combustion Time (hr)
1
23.81
21.79
22.34
22.41
20.20
20.12
19.78
400
500
600
700
800
900
1000
2
23.74
22.03
21.70
21.64
20.57
19.89
20.05
3
24.16
21.35
22.48
21.78
20.63
20.65
20.11
4
23.68
21.77
21.88
20.83
20.27
19.94
19.96
5
24.13
22.96
21.71
21.53
20.29
20.50
20.01
6
23.48
22.29
21.42
20.92
20.30
20.01
19.95
30
25
RHA Yield (%)
1
20
2
3
15
4
5
10
6
5
0
400
500
600
700
800
900
1000
Combustion Temperature (0C)
Fig. 4.1 Plots of Rice husk Ash (RHA) Yield Versus Combustion Temperature for
Different Combustion Time of 1 to 6 hrs
. This trend is similar to that observed by Omatola and Onojah114 in which higher ash yields
were obtained at 5000C than at 10000C. Nizami 154 reported similar results. Nataranjan and
Sundaram,98 similarly reported a significant decrease in solid yield from 39.98 to 32.52%
with increase in temperature from 400 to 600oC in their study of the pyrolysis of rice husks.
This, they attributed to enhanced secondary decomposition of rice husks with increasing
temperature during the decarbonisation process which involves the conversion of the highly
carbonized RH char to CO2. This trend suggests improved carbon conversion efficiency
with increase in combustion temperature.
167
Plots of RHA yields as a function of duration of combustion are given in Fig. 4.2. This
figure shows that RHA did not vary much with time of combustion for the various
combustion temperatures. For instance, ash yields (%) of 23.48, 22.29, 21.42, 20.92, 20.30,
20.01 and 19.95 obtained at 400, 500 600, 700, 800, 900 and 10000C after 6hrs duration did
not vary much with the yields of 23.81, 21.79, 21.13, 22.81, 20.02, 20.12 and 19.78%
obtained respectively after 1hr duration. In addition, ash yields did not vary much with
combustion duration within a specific combustion temperature as seen from Fig. 4.2. For
instance, at 5000C the ash yields for 1, 2, 3, 4, 5 and 6hrs are 21.79, 22.03, 21.35, 21.77,
22.96 and 22.29%, respectively, thus showing that prolonged combustion time has little
effect on RH ash yield.
30
RHA Yield (%)
25
400
500
20
600
15
700
800
10
900
1000
5
0
1
2
3
4
5
6
Combustion Time (hr)
Fig. 4.2 Plots of Rice husk Ash Yield Versus Combustion Time for
Different Combustion Temperature of 400 to 10000C
This little effect of time on RHA yield suggests that both carbonisation (rice husk char
formation) and decarbonisation processes occur rapidly to produce the ash product so that
prolonging the combustion time has little or no effect on the amount of ash obtained,
particularly at higher temperatures that cause faster rate of combustion. Also, within the
few seconds time lag between the start of the combustion process and the attainment of the
combustion temperature, substantial amount of volatiles have escaped starting from about
600C and formation of char has commenced. Subsequent carbon conversion
(decarbonization) commences at about 4000C so that by the time the combustion
temperature was attained, some reactions had occurred such that lengthening the
combustion time will have little effect on ash yield, particularly at higher temperatures.
168
This observation is similar to that of Nizami in which there was a small reduction in ash
yields with increasing time, although not significantly as shown by his RHA values of
21.00, 20.85, 20.69 and 20.60% for 1, 2, 3 and 4 hrs, respectively.
In conclusion, increase in combustion temperature (700 to 1000oC)) tends to decrease RHA
yield, though with increase in ash quality. Prolonging the duration of combustion (5-6hrs)
seems to have little effect on the ash yield, particularly at higher temperatures. Thus, ash
yields of rice husks therefore seem to depend largely on combustion temperature than on
combustion duration.
4.7. EFFECT OF COMBUSTION TEMPERATURE AND TIME ON RICE HUSK
ASH COLOUR
The colour of ash used as extender is important in paint production for colour-matching
purpose. White extenders are preferable as they give perfect (‘A’) colour matches, while
extenders that are darker in colour constitute a problem as they prevent the desired colour
match. Hence the need to establish the sets of conditions that would give satisfactorily
‘white’ amorphous ash (silica). White crystalline ash (silica), obtained at higher
temperatures, is undesirable for paint production because it is harder, with smaller surface
area and is less reactive due to electrical neutrality.123
Table 4.4 gives the colours and colour numbers of RHA samples obtained at the various
combustion temperatures and time. The colour stated is the predominant colour of the ash
obtained, but may be tinged with one or more colours, usually a shade of grey, with the
extent depending on the incineration condition. Snapshots of the colours given in Table 4.4
are shown in Figs. 4.1 to 4.16.
Table 4.4 shows that the RHA colours generally become lighter with increasing
incineration temperature and time up to a certain limit, as seen in the colour trend of
black/dark brown, ivory, cream, cream, for 400, 500, 600 and 7000C, respectively, after 1
hr combustion time. At 4000C, for instance, the colour trend is black, dark brown, brown,
light brown, light brown and ivory after 1, 2, 3, 4, 5, and 6hrs combustion time,
respectively. The colour numbers, which correspond to the various colours, decrease with
increasing combustion temperature and time, due to the increasing extent of the
decarbonisation process, resulting in ash of lighter colours and improved whiteness. The
exception to this general trend is combustion at 900 and 1000C at high heating rates (32
and 270C/min, respectively), during which grey coloured ash samples were obtained at all
169
the incineration time regimes as a result of inefficient combustion. The highest colour
number values were obtained at 400C, corresponding to the dark coloured ash obtained at
this temperature due to incomplete combustion. ‘White’ ash is however not obtained at
4000C, even with prolonged combustion time of 6hrs, probably because the temperature is
not sufficiently high for the efficient combustion that would yield ‘white’ ash. This is
evident in the formation of white ash at temperatures higher than 4000C.
Interestingly, two types of ash in terms of colour variation were reproducibly produced at
4000C after one-hour combustion time as shown in Fig.4.3.. One type, 400–1A was mostly
black with some dark brown particles (Fig. 4.3b) while the other type, 400-1B was
predominantly dark brown with some brown particles as shown in figures 4.3c. The
preponderance of dark-coloured particles in both cases is a clear indication of incomplete
combustion of the carbonaceous materials in the rice husk char produced in the first stage
of the thermal decomposition process. This may be attributed to the fact that 4000C seems
to be the transition temperature between the two stages of the combustion processcarbonization and decarbonization as reported by Bharadwaj et al
84
who, in their study,
observed that at 4000C, the emission of volatiles ceased and the decarbonisation of the
highly carbonized char to produce white ash commenced. The presence of the said
black/dark brown and brown particles is a manifestation of the formation of black RH char
and its transformation to white ash in the course of the oxidation process. However, the 1hour combustion duration is insufficient for this conversion to have substantially occurred,
hence the presence of only a few ‘white’ grains in both samples. The relatively low silica
content of 400-1A (72%) as shown in Appendix 2 signifies that the extent of combustion is
minimal. The ash colour however improved with combustion time as seen in Figs 4.2 to 4.4
after 2-6hrs combustion time.
‘White’ ash is obtainable from 500-8000C, with the degree of whiteness increasing with
combustion time to a limit. Thus at 5000C, ‘white’ ash (cream coloured) is obtainable from
the 3rd hour beyond which there’s no change in the ‘whiteness’. At 6000C, the same cream
coloured ash is obtained from the first hour to the 5th hour, while at 700 and 8000C, ash
samples of slightly improved ‘whiteness’ (milky white) were obtained from the 3rd and 2nd
hour repectively, but there was no further improvement in ash quality beyond the said
duration. These observations suggest that the decarbonisation process, during which ‘white’
ash is formed is enhanced by increase in the combustion temperature and time, but in the
case of combustion time, there appears to be for each temperature, an optimum duration for
170
the formation of white ash, beyond which there is no further improvement in the
‘whiteness’ of the ash.
Table 4.4 Heating Rates, Colour and Colour Numbers of RHA obtained at Different
Combustion Temperatures and Combustion Time of 1 to 6hrs
Colours and Colour Numbers of RHA Samples obtained at Combustion Time
of 1 to 6hrs
1
2
3
4
5
6
Comb
Temp (0C)
Heating
Rate
Colour
Colour
Colour
Colour
50
Colour no.
500
50
Colour no
600
43
Colour no.
700
38.9
Colour
.
(0C/min)
400
Colour
Black,
Dark
Brown
9
Dark
brown
Brown
Light
brown
Light
brown
Ivory
8
7
6
6
5
Ivory
Cream
Cream
Cream
Cream
Cream
5
4.5
4
4
4
4
Cream
Cream
Cream
Cream
Cream
4
4
4
4
4
Milky
White
3
Cream
Cream
Milky
White
Milky
White
Milky
White
Milky
White
4.5
4
3
3
3
3
Cream
Milky
White
Milky
White
Milky
White
Milky
White
Milky
White
4
3
3
3
3
3
Dark
Grey
8
Grey
7
Light
Grey
6
Light
Grey
6
Light
Grey
6
Light
Grey
6
Grey
Grey
Grey
Grey
Grey
Grey
7
7
7
7
7
7
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Milky
white
3
Colour no.
800
36.4
Colour no.
900
32
Colour no.
1000
27
Colour no.
900
15.52
Colour no.
1000
Colour no.
9.8
Fig, 4.3 shows coloured snapshots of brown milled rice husks before incineration; the
predominantly black and dark brown colour variants of RHA-MPSC-400-1A and RHAMPSC-400-1B, respectively, obtained after 1 hr combustion time. Figs. 4.4 and 4.5 show
coloured photographs of the dark brown, brown, light brown, light brown and ivory ash
samples of RHA-MPSC-400-2, RHA-MPSC-400-3, RHA-MPSC 400-4 and RHA-MPSC
171
400-5, indicating incomplete , but improved combustion efficiency with increasing time.
Thus, ash of satisfactory ‘whiteness’ is not obtainable at 4000C.
Fig. 4.6 and 4.7 show the predominantly cream colour of the ash samples obtained at 5000C
at 2-5 hrs combustion time, due to more efficient combustion process as a result of increase
in temperature. In the case of 1hr, ivory -coloured ash particles were preponderant because
of the shortness of the combustion time. The ‘whitest’ ash colour (milky- white) was
obtained after 6hrs period,. However, the cream coloured ash samples obtained at 3 -5hrs
are of satisfactory ‘whiteness’ for paint production and for most industrial purposes.
Fig. 4.8 and 4.9 show the predominantly cream colour of the ash samples obtained at 6000C
at all the combustion duration, indicating complete, effective and efficient combustion at
this temperature. The colour of the ash obtained at 6hrs, however, appeared ‘whitest’and
was therefore described as ‘milky white’. Figs 4.10 and 4.11 show the cream coloured ash
samples obtained at 7000C after 1 and 2hrs and the milky- white samples obtained at 7000C
at 3-6hrs due to complete combustion. It seems therefore that at 7000C, prolonging the
combustion time beyond 3hrs, has no effect on ash ‘whiteness’. Fig. 4.12 and 4.13 show the
predominantly milky-white colour of ash samples obtained at 8000C at all the combustion
hours, indicating complete combustion and also showing that at 8000C, combustion
duration has little or no effect on ash colour. However, there’s a great likelihood of the
formation of the less desirable crystalline silica at 8000C.
Fig 4.14 - 4.16 show the grey coloured ash samples obtained at 900 and 10000C at all the
combustion time regimes, due to combustion inefficiency arising from high heating rates.
The shades of grey varied slightly at 9000C, becoming lighter with time. At 10000C, the
grey shades remained virtually the same for 1-6hrs. Fig. 4.18 shows the ‘white’ ash
obtained at 900 and 10000C after 1hr at lower heating rates of 15.52 and 9.80C /min,
respectively, and photograph of ground RHA (RHA flour).
172
4.7.1 Photographs of RHA Samples Obtained at Different Combustion
Conditions Showing their Colours
(a)
(b)
(c)
Fig, 4.3 Photographs of (a) milled rice husks( b) RHA-MPSC-400-1A and (c) RHA-MPSC-400-1B Ash
samples
(a)
(b)
(c)
Fig. 4.4 Photographs of (a) RHA-MPSC-400-2, (b) RHA-MPSC400-3 and (c)RHA-MPSC 400-4 Ash
Samples
(a)
(b)
Fig. 4.5 Photographs of (a) RHA-MPSC-400-5, and (b) RHA-MPSC-400-6 Ash Samples
173
(a)
(b)
(c)
Fig. 4.6 Photographs of (a) RHA-MPSC-500-1, (b) RHA-MPSC- 500-2 and (c) RHA-MPSC- 500-3 Ash
Samples
(a)
( b)
(c)
Fig. 4.7 Photographs of (a) RHA-MPSC-500-4, (b) RHA-MPSC- 500-5 and (c) RHA-MPSC- 500-6 Ash
Samples
(a)
( b)
(c)
Fig. 4.8 Photographs of (a) RHA-MPSC-600-1, (b) RHA-MPSC- 600-2 and (c) RHA-MPSC- 600-3 Ash
Samples
174
(a)
( b)
(c)
Fig. 4.9 Photographs of (a) RHA-MPSC-600-4, (b) RHA-MPSC- 600-5 and (c) RHA-MPSC- 600-6 Ash
Samples
(a)
( b)
(c)
Fig. 4.10 Photographs of (a) RHA-MPSC-700-1, (b) RHA-MPSC- 700-2 and (c) RHA-MPSC- 700-3 Ash
Samples
(a)
( b)
(c)
Fig. 4.11 Photographs of (a) RHA-MPSC-700-4, (b) RHA-MPSC- 700-5 and (c) RHA-MPSC- 700-6 Ash
Samples
175
(a)
( b)
(c)
Fig.4.12 Photographs of (a) RHA-MPSC-800-1, (b) RHA-MPSC- 800-2 and (c) RHA-MPSC- 800-3 Ash
Samples
(a)
( b)
(c)
Fig. 4.13 Photographs of (a) RHA-MPSC-800-4, (b) RHA-MPSC- 800-5 and (c) RHA-MPSC- 800-6 Ash
Samples
(a)
( b)
(c)
Fig. 4.14 Photographs of (a) RHA-MPSC-900-1, (b) RHA-MPSC- 900-2 and (c) RHA-MPSC- 900-3 Ash
Samples
176
(a)
( b)
(c)
Fig. 4.15 Photographs of (a) RHA-MPSC-900-4, (b) RHA-MPSC- 900-5 and (c) RHA-MPSC- 900-6 Ash
Samples
(a)
( b)
(c)
Fig. 4.16 Photographs of (a) RHA-MPSC-1000-1, (b) RHA-MPSC- 1000-2 and (c) RHA-MPSC- 1000-3 Ash
Samples
(a)
( b)
(c)
Fig. 4.17 Photographs of (a) RHA-MPSC-1000-4, (b) RHA-MPSC- 1000-5 and (c) RHA-MPSC- 1000-6 Ash
Samples
177
(a)
(b)
(c)
Fig. 4.18 Photographs of (a) RHA-MPSC-900-1, (b) RHA-MPSC- 1000-1 Ash Samples obtained at low
heating rate and (c) Ground RHA (RHA flour)
White ash is obtainable at 900 and 1000C at low heating rates of 15.52 and 9.80C,
respectively, from the first hour. Chandresekhar et al152 in a related study, had reported that
a high potassium content in the husk will inhibit the carbon removal during ashing. Another
report 130 had it that if the heating rate is high, it will prevent volatilization of the potassium
in rice husks, such that it will react with silica to produce potassium polysilicate combined
with carbon, thereby causing a high residual carbon in the ash. Thus, the relatively high
heating rate is probably a major contributory factor to the formation of grey ash at 900 and
1000C at high heating rates, as Krishnarao et al
161
confirmed that the tendency to form
black particles (unburnt carbon) increases with increase in the heating rate and temperature
of calcination of untreated rice husks. The formation of white ash at 900 and 1000C at
lower heating rate as shown in Fig.4.16 buttresses the fact that high heating rate at the said
temperatures is the factor largely responsible for this phenomenon..
Plots of colour number as a function of combustion temperature and time are presented in
Figs.4.19 and 4.20 respectively. The graph trends are in line with the colour trends
summarised in Table 4.4. For instance, it can be seen from Fig. 4.19 that the highest colour
numbers were obtained at 4000C and at 900-1000 0C due to the formation of black/ dark
brown ash and grey coloured ash, respectively. Beyond 4000C, there is a significant drop in
the colour numbers, which stabilises between 600 and 8000C, at which temperatures,
‘white’ ash was obtained. This suggests that increasing the combustion temperature beyond
6000C has little or no effect on the ‘whiteness’ of the ash. The increase in the colour
numbers at 900 and 10000 C is due to the formation of grey ash at high heating rate.
178
Fig. 4.20 shows that higher colour numbers were generally obtained at shorter combustion
duration (1-2hrs) than at longer (3-6hrs) for each combustion temperature. This is
attributable to incomplete decarbonisation of the ash due to the shortness of the combustion
duration, resulting in darker coloured ash. A significant drop in colour number is observed
beyond 2hrs. This decrease in colour number is seen to stabilise between 3-6hrs, as there is
no further change in the value beyond 3hrs, indicating that prolonging the incineration time
beyond 3hrs has, little or no effect on the whiteness of the ash, particularly for 600-8000C.
Generally, lower colour numbers were obtained for higher combustion temperatures (6008000C), except in the cases of 900 and 10000C at high heating rates, where high colour
numbers are observed due to the formation of grey ash as a result of combustion
inefficiency.
10
9
Colour Number
8
1
7
2
6
3
5
4
4
5
6
3
2
1
0
400
500
600
700
800
900
1000
Combustion Temperature(0C)
Fig. 4.19 Plots of Colour Number Versus Combustion Temperature of RHA a
at Combustion Time of 1hr to 6hrs
179
10
Colour Number
9
8
400
7
500
6
600
5
700
4
800
3
900
2
1000
1
0
1
2
3
4
5
6
Combustion time(hr)
Fig. 4.20 Plots of Colour Number Versus Combustion Time of RHA at Varied Incineration
Temperatures of 4000C to 10000C.
4.7.2 Effect of Reduced Heating Rate on Colour of RHA Samples Obtained
at 900oC and 10000C
In an attempt to unravel the factor responsible for the formation of predominantly grey ash
instead of ‘white’ ash at 900 and 10000C, the heating rates were reduced from 320C/min to
15.52 oC/min (for 9000C) and 270C/min to 9.8 oC /min for 10000C, so as to further
investigate the effect of heating rate on the colour of the ash samples incinerated at high
combustion temperatures. In both cases, homogeneously off- white ash with no trace of
grey, were obtained even at one–hour duration as shown in Figs 4.18a and 4.18b for 9000C
and 10000C respectively. This finding suggests that high heating rate with resultant carbon
conversion inefficiency, was responsible for the high residual carbon observed in ash
samples heated at 900 and 1000 oC, which were predominantly grey in colour. A probable
explanation is that at a high heating rate, the combustion process proceeds so rapidly that
the heat does not get to spread out uniformly in the RH char particles. The result of this
uneven distribution of heat is that the particles within the efficient combustion zone are
white while those outside the zone are grey. There’s also the possibility of potassium
reacting with silica to form potassium polysilicate rather than potassium oxide due to a high
heating rate. The heating rate of 27oC/min used in this study is comparable to those studied
by Natarajan and Sundaram- 20oC/min, 40oC/min and 60oC/min during the pyrolysis of rice
husks.
180
These results conclusively show that milky-white /off-white amorphous ash suitable for
paint production is obtainable from 500 (3rd hour) to 8000C. At 8000C, however, some
quantity of crystalline ash is likely to be present, since amorphous white ash is desirable for
paint production, the temperature range selected should be such as to reduce minimally the
formation of crystalline silica. In consideration of the factors of energy cost and availability
of completely amorphous silica, the combustion temperature range should therefore be 5006500C while the combustion time should be 3 to 4hrs (for 5000C) and 2 to 3hrs (for 6000C).
The results also show that white ash is obtainable at 900 and 10000C at slow heating rates,
but at these high temperatures, the ash is predominantly crystalline in nature
40,80,119
and is
less desirable for paint production.
4.8 Effect of Combustion on the Surface Morphology of RHA
Particles
According to Bharadwaj et al,
84
the structure of a virgin RH is similar to a composite
material with silica tubes filled with cellulose materials, constituting the reinforcing
(discontinuous) phase and the matrix (binder) consisting of lignin. The discontinuous phase
imparts significant reinforcement to a composite material, similarly in the RH, the silica
tubes are responsible for its strength and rigidity. Subjecting the rice husk to pyrolysis
(combustion), therefore disrupts this composite structure leading to its eventual collapse
with the resultant product, RHA, which is of high porosity and having a different surface
morphology from the virgin rice husks. These microstructural changes can be observed
with a scanning electron microscope (SEM) as carried out by Bharadwaj et al 84, but in this
study, these changes were observed using simple instrumentation in the form of a
paleontology microscope and a digital camera. The findings were similar to those made by
Bharadwaj and co-workers, who used the hi-tech and more sophisticated SEM, despite the
limitations of the simple instruments.
Fig. 4.21 shows the surface morphology of a virgin rice husk, revealing the flat and
relatively smooth surface when compared to the surface of the pyrolized rice husks shown
from Figs. 4.22 (b,c) to 4.66 (b,c). The clusters and isolated RHA particles are represented
in Figs. 4.22 (a) to 4.66 (a) and 4.22(b) to 4.66(b), respectively’ in each diagram. The
clusters constitute snapshots of magnified RHA particles clustered together while the
isolated particles are made up of a few spaced-out RHA particles for a closer view of the
surface form.
181
The varied shapes and sizes of RHA particles seen in the clusters and isolated particles are
a reflection of the various sizes and shapes of rice husk particles present in the milled rice
husks. The use of a mixed particle size of a fairly wide range (300-1000microns) was based
on cost considerations, as the processing of the husks into a specific particle size range
would be cost ineffective especially on a large (industrial). Since this study is aimed at
investigating the industrial applicability of rice husks, an attempt was made to simulate
industrial conditions as much as possible. In addition, the selection of a particular size
range would imply that the findings are restricted to that particular size range.
The surface of the magnified pyrolized RHA particles obtained at all the combustion
temperatures generally revealed the presence of a large number of round, button-like
protuberances (or bumps) interspaced with small craters (pores), which made them
resemble the surface of a corncob. These bumps and craters were found to be absent in the
virgin rice husks, which had a flat surface morphology as evident in Fig. 4.21(a,b,c). These
observations were made by Bharadwaj et al,84 using the SEM. They attributed the presence
of the bumps to obstruction of silica fibres to devolatilization and the craters, to the point of
escape of volatiles. The magnified RHA particles also reflected the colour of the ash, thus
the RHA particles obtained at 4000C after one hour (Fig. 4.22 to 4.23 a,b) were dark in
colour making the surface look like that of a roasted corn cob while the particles obtained at
500 to 8000C (Figs. 4.29 to 4.52a,b) were white in colour and looked like a boiled corn
cob.
All the RHA particles (4.22-4.66b) showed a curved shrinkage in size along the horizontal
(transverse) axes at all the combustion temperatures and time, similar to the folding of a
sheet of paper along its length, as is evident in their generally narrower widths, compared
with the wider and relatively flat shape of the virgin rice husks. This reduction in size was
also reported by Bharadwaj et al.
There was no noticeable difference in the shape and surface morphology of the RHA
particles obtained at lower combustion temperatures (400 to 6000C) as shown in Figs 4.22
(b) to 4.40(b) The RHA surface morphology also appeared unaffected by combustion time
at the said temperatures. This is attributable to the fact that the escape of volatiles at the
carbonization stage, during which the bumps and craters are formed, occur at about 4000 C
and is completed within a short time, so that at a prolonged combustion time the
morphology remains essentially the same.
182
At higher temperatures, (700 to 10000C) and higher heating rates, some of the particles
became sickle-shaped i.e were slightly curved along the vertical axes, owing to a reduction
in size (shrinkage) of the particles along this axes. Some curved particles were observed at
700-8000C to a lesser degree, as is evident in Figs.4.42a and 4.50b, but this was more
pronounced from 900 to 10000C as shown in Figs.4.56 to 4.58b, 4.62 to 4.64b. This vertical
shrinkage, however, occurred to a lesser degree in comparison to that along the horizontal
axes as was observed by Bharadwaj et al, who postulated that “there is a preferential
direction for particle shrinkage, which could be due to the structure of silica skeleton and
that this size reduction occurred along the vertical axis, but to a lesser degree than at the
horizontal.” The sickle-shaped particles were not observed in the ash produced at high
temperatures (900 –10000C) and low heating rates (15.52 and 9.80C/min), suggesting that
high heating rate was a factor in this deformation of the RHA particle shape. This formation
of sickle-shaped particles can be linked to the observed formation of grey RHA at high
temperatures (900-10000C) and high heating rates, which is postulated to be as a result of
rapid and uneven temperature distribution within the RHA particle, resulting to a high
carbon conversion inefficiency and consequently, high residual carbon.
RHA is known to be highly porous and to have low bulk density. These can be attributed to
the formation of the said craters at the points of escape of volatiles from the lignin matrix,
which appear like pores in the RHA particles during the thermal degradation process. These
craters were not so obvious from the coloured snapshots due to the instrumental limitation.
The presence of pores, which are tiny holes, in the matrix increases the effective volume,
thereby lowering the bulk density when compared with a composite system of same mass
but with no pores. In addition, the selective consumption of the cellulosic materials within
the silica tubes, during pyrolysis of rice husks, introduces hollowness into the discontinuous
phase of the composite, thereby increasing the porosity of the RHA particle, which can be
said to have a collapsed composite structure.
183
4.8.1 Photographs of Magnified RH and RHA Particles showing their
Surface Morphology at Different Combustion Conditions
(a)
(b)
(c)
Fig. 4.21: Photographs of (a) magnified single rice husk (x40) (b) magnified milled rice husk
cluster (x40) and (c) magnified isolated rice husk particles (x40)
(a)
(b)
Fig. 4.22: Photographs of RHA-MPSC-400-1A (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.23:Photographs of RHA-MPSC-400-1B (a) magnified particle cluster(x40)
and (b)magnified isolated particles(x40)
184
(a)
(b)
Fig. 4.24: Photographs of RHA-MPSC-400-2, (a) magnified particle cluster(x40)
and (b) magnified isolated particles(x40)
(a)
(b)
Fig. 4.25: Photographs of RHA-MPSC-400-3 (a) magnified particle cluster(x40)
and ( b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.26: Photographs of RHA-MPSC-400-4 (a) magnified particle cluster
(x40) and (b) magnified isolated particles (x40)
185
(a )
(b)
Fig. 4.27: Photographs of RHA-MPSC-400-5 (a) magnified particle cluster
(x40) and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.28: Photographs of RHA-MPSC-400-6 (a) magnified particle cluster
(x40) and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.29: Photographs of RHA-MPSC-500-1 (a) magnified particle cluster (x40)
and (c) magnified isolated particles (x40)
186
(a)
(b)
Fig. 4.30: Photographs of RHA-MPSC-500-2 (a) magnified particle cluster (x40)
and( b) magnified isolated particle(x40)
(a)
(b)
Fig. 4.31: Photographs of RHA-MPSC-500-3 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.32: Photographs of RHA-MPSC-500-4 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
187
(a)
(b)
Fig. 4.33: Photographs of RHA-MPSC-500-5 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.34: Photographs of RHA-MPSC-500-6 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.35: Photographs of RHA-MPSC-600-1 (a) magnified particle cluster (x40)
and( b) magnified isolated particles(x40)
188
(b)
(c)
Fig. 4.36: Photographs of RHA-MPSC-600-2 (a) magnified particle cluster
(x40) and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.37: Photographs of RHA-MPSC-600-3 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.38: Photographs of RHA-MPSC-600-4, (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
189
(a)
(b)
Fig. 4.39: Photographs of RHA-MPSC-600-5 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.40: Photographs of RHA-MPSC-600-6 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.41: Photographs of (a) RHA-MPSC-700-1 ash sample, (b) magnified particle
cluster (x40) and (c) magnified isolated particles (x40)
190
(a)
(b)
Fig. 4.42: Photographs of RHA-MPSC-700-2 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.43: Photographs of RHA-MPSC-700-3 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.44: Photographs of RHA-MPSC-700-4 (a) magnified particle cluster(x40)
and (b)magnified isolated particles (x40)
191
(a)
(b)
Fig. 4.45: Photographs of RHA-MPSC-700-5 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.46: Photographs of RHA-MPSC-700-6 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.47: Photographs of RHA-MPSC-800-1 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
192
(a)
(b)
Fig. 4.48: Photographs of RHA-MPSC-800-2 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.49: Photographs of RHA-MPSC-800-3 (a) magnified particle cluster(x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.50: Photographs of RHA-MPSC-800-4 (a) magnified particle cluster(x40)
and (b) magnified isolated particles (x40)
193
(a)
(b)
Fig. 4.51: Photographs of RHA-MPSC-800-5 (a) magnified particle cluster (x40)
and magnified isolated particles (x40)
(a)
(b)
Fig. 4.52: Photographs of RHA-MPSC-800-6 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.53:Photographs of RHA-MPSC-900-1 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
194
(a)
(b)
Fig. 4.54: Photographs of RHA-MPSC-900-2 (a) magnified particle cluster(x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig.4.55: Photographs of RHA-MPSC-900-3(a) magnified particles cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.56:Photographs of RHA-MPSC-900-4(a), magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
195
(a)
(b)
Fig. 4.57:Photographs of RHA-MPSC-900-5 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.58: Photographs of RHA-MPSC-900-6 (a) magnified particle cluster(x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.59: Photographs of RHA-MPSC-1000-1(a) magnified particle cluster (x40)
and (b) magnified isolated particles(x40)
196
(a)
(b)
Fig. 4.60: Photographs of RHA-MPSC-1000-2, (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.61: Photographs of RHA-MPSC-1000-3 (a), magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.62:Photographs of RHA-MPSC-1000-4 (a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
197
(a)
(b)
Fig. 4.63:Photographs of RHA-MPSC-1000-5 (a) magnified particles cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.64: Photographs of RHA-MPSC-1000-6(a) magnified particle cluster (x40)
and (b) magnified isolated particles (x40)
(a)
(b)
Fig. 4.65:Photographs of RHA-MPSC-900-1 (a) magnified particle cluster (x40)
and (b) magnified isolated particles obtained at low heating rate
198
(b)
(a)
Fig. 4.66: Photographs of RHA-MPSC-1000-1 (a) magnified particle cluster(x40)
and (b) magnified isolated particles(x40) obtained at low heating rate
(a)
(b)
Fig. 4.67:Photographs of (a) magnified RHA flour particle cluster (x40) and (b) magnified
RHA flour particles (x40)
4.9 Effect of Combustion Temperature and Time on the Elemental
Composition of RHA
4.9.1 Elemental Composition of RHA at Different Combustion Temperatures and
Time Using the PIXE Technique
Even though the same stock of rice husks was used in the entire study the elemental
composition of ash obtained from furnace combustion was found to vary with combustion
conditions (temperature and time). These variations substantiate the fact that the
combustion conditions influence the chemistry and amount of ash produced (yield). The
combustion process, by its intrinsic nature, exerts this influence because it involves several
199
chemical reactions, which cannot be controlled; these include oxidation of fixed carbon,
conversion of metals into their oxides and non-metallic oxides into gases and formation of
decomposition products.
The detailed result of the elemental compositional determination of virgin rice husks using
the PIXE analytical technique is presented in Appendix A.1 while those for the various
RHA samples obtained at different combustion temperatures and time are given in
Appendix A.2 to A.45, giving a total of forty-two (47) PIXE results.
A total of fifteen elements were detected in RHA samples by the PIXE analytical
technique: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, Mn, Fe, Zn, Rb and Sr. In some ash samples,
however, the concentration of some of the trace elements were below the detection limit.
Similar elements were detected in the virgin rice husks except for Sr and Rb. The presence
of the elements Ti, Rb and Sr in RHA, have not been previously reported in literature using
other analytical techniques, showing the high sensitivity of the PIXE technique to trace
elements. The SiO2, Na2O, K2O, MgO, CaO, Al2O3
and Fe2O3 levels at different
combustion conditions are presented in Tables 4.64 to 4.69. The major component of RHA
is silica. This high level of silica(68-91%) as shown in Table 4.10 is responsible for the
attractiveness of RHA to researchers as a potential industrial bioresource.
4.9.2 Effect of Combustion Temperature and Time on Silica Concentration of RHA
The silica concentrations of RHA at various incineration conditions are given in Table 4.5.
Plots of RHA concentration versus temperature and time are shown in Figs.4.68 and 4.69
respectively
Table 4.5: Silica (SiO2) Content of RHA obtained at different Combustion
Temperatures and Time
Combustion
Temp (0C)
Silica content of RHA at 1 to 6hr combustion time
400
1
71.93
2
83.59
3
74.01
4
77.42
5
73.72
6
79.04
500
600
700
800
900
1000
67.70
83.42
78.024
88.36
70.72
76.59
77.39
71.02
73.43
80.45
83.35
67.67
71.43
75.34
87.28
86.14
75.06
76.66
73.18
74.19
86.83
79.64
85.03
72.37
81.56
76.67
89.04
91.42
87.91
81.97
86.15
85.31
88.38
76.71
85.98
74.64
200
100
90
RHA Silica Conc (%)
80
70
1
60
2
3
50
4
40
5
30
6
20
10
0
400
500
600
700
800
900
1000
Combustion Temperature (0C)
Fig. 4.68 Plots of Silica (SiO2) Conc of Rice Husk Ash (RHA) Versus
Combustion Temperature at Varied Combustion Duration of 1 to 6 hrs
100
90
RHA Silica Conc (%)
80
400
70
500
60
600
50
700
40
800
900
30
1000
20
10
0
1
2
3
4
5
6
Combustion Time (0C)
Fig. 4.69 Plots of Silica (SiO2) Conc of Rice Husk Ash (RHA) Versus
Combustion Time at Varied Combustion Temperatures of 4000C to 10000C
The silica contents of RHA at various combustion temperatures and time ranged from 68 to
91%. The highest silica concentrations were generally obtained at higher combustion
temperatures (7000C to 9000C) as evident in Figs 4.68 except for 10000C at which the silica
concentration was observed to have dropped, possibly due to the formation of polysilicate,
therefore reducing the concentration of available silica. Lower silica values were obtained
at lower temperatures (4000C to 6000C). Prolonged combustion duration of 5hrs to 6hrs
generally increased the concentration of silica at all the combustion temperatures, but
201
particularly at lower temperatures. For instance, at combustion temperatures of 400, 500,
600 and 7000C, the highest silica concentrations of 79.04, 86.15, 85.31 and 88.38% were
obtained at the 6th hour combustion duration compared to 71.93,67.70,83.42, and 78.02%
obtained for the same temperatures, respectively, after 1hr incineration period.. Thus, this
effect of longer combustion periods on yields of silica at lower temperatures is probably
because longer periods of incineration are required for the decarbonization process to be
complete due to slower rate of carbon conversion. At higher combustion temperatures of
800, 900 and 10000C, the combustion duration appears to have little or no effect on silica
content. This is evident in the silica yields of 88.36, 70.72 and 76.59 obtained for 800, 900
and 10000C respectively after 1hr combustion duration, which compare favourably with
76.71, 85.98, and 74.64% obtained respectively, after six hours combustion period. This
suggests that at higher temperatures the decarbonization process takes place more rapidly
due to faster reaction rate, such that prolonging the combustion time has little or no effect
on silica yield. Elevated temperature therefore increases the rate of the reaction such that
the carbonization and decarbonization processes are completed within a short period while
at lower temperatures, the reaction is slower and more time is required for the completion
of the reaction.
In conclusion, the results indicate that lower incineration temperatures of 400 to 6000C at
short duration,(1 to 4hrs) give lower concentrations of silica while higher temperatures (700
to 9000C) give higher levels. However, at lower temperatures, prolonged combustion
duration of 5 to 6hrs increases the silica yield, while at higher temperatures, time of
incineration has little effect. This trend is reflected in the ash yields where lower
combustion temperatures gave higher ash yields at short combustion periods due to
incomplete carbon conversion while higher combustion temperatures gave lower ash yields
due to more efficient carbon conversion (decarbonization) process.
4.9.3 Effect of Combustion Conditions on Metallic Oxides Components of RHA
The major component of RHA is silica. This high level of silica (68-91%) as shown in
Table 4.5 is responsible for the attractiveness of RHA to researchers as a potential industrial
bioresource. Other metallic oxides present in RHA are often regarded as impurities by most
researchers due to their extremely low and (in some cases) trace levels and thus, are in
some cases removed to obtain pure RHA. Due to their low levels, they are not of any
commercial importance. However, in this work, the effects of combustion conditions on the
202
levels of these metallic oxides are examined for the purpose of research interests since they
constitute part of the chemical composition of RHA.
4.9.3.1 Sodium Oxide Content of RHA at Different Combustion Conditions
The sodium oxide concentrations of RHA samples obtained at different combustion
temperatures and time are given in Table 4.6
Table 4.6 : Sodium oxide (Na2O) concentration (%) of RHA at different Combustion
Temperatures and Time
Combustion
Temp. (0C)
400
500
600
700
800
900
1000
Sodium oxide (Na2O) content of RHA at 1hr to 6hr combustion Time
1
0.728
0.1060
0.1114
ND
0.1075
ND
ND
2
0.1232
ND
0.0435
0.0813
ND
ND
0.14
3
N.D
0.0415
ND
0.1215
ND
0.1101
ND
4
0.0677
ND
0.0950
0.0443
ND
ND
ND
5
ND
0.1297
ND
ND
0.0625
0.0241
0.0569
6
0.0228
ND
ND
0.0897
ND
ND
ND
ND- not detected
Sodium oxide (Na2O) was found present in very low concentrations, which ranged from
0.02 to 0.13% and was detected mostly at lower temperatures (400, 500, 600 and 7000C).
However, at higher temperatures (800, 900 and 10000C), it was in several cases, not within
the detectable limit for most of the combustion periods, notwithstanding that the melting
point of Na2O is about 12750C. 219
4.9.3.2 Potassium Oxide Content of RHA at Different Combustion Conditions
The potassium oxide concentrations of RHA samples obtained at different combustion
temperatures and time are given in Table 4.66 while plots of K2O cocentration as a f
unction of temperature and time are shown in Figs. 4.55 and 4.56 respectively.
203
Table 4.7: Potassium oxide Contents (%) of RHA at Different Combustion
Temperatures and Time
Combustion Potassium oxide (K2O) contents of RHA at 1hr to 6hr combustion
Time (0C)
time
1
0.1410
0.2699
0.2862
0.1237
0.2930
0.1365
0.1752
400
500
600
700
800
900
1000
2
0.1553
0.2846
0.2298
0.2663
0.1328
0.2496
0.0639
3
0.2946
0.2527
0.1587
0.1510
0.2592
0.2204
0.0762
4
0.3549
0.2710
0.2531
0.2969
0.2380
0.2231
0.2218
5
0.2653
0.2871
0.2419
0.2836
0.3685
0.2565
0.0827
6
0.1560
0.3041
0.2748
0.3004
0.2406
0.1090
0.0920
Potassium Oxide Conc (%)
0.4
0.35
1
0.3
2
0.25
3
0.2
4
0.15
5
0.1
6
0.05
0
400
500
600
700
800
900
1000
Com bustion Tem perature (0C)
Fig. 4.70 Plots of Potassium Oxide (K20) Concentration in RHA Versus
Combustion Temperatureat Varied Combustion Time of 1 to 6hrs
Potassium Oxide Conc (%)
0.4
0.35
400
0.3
500
0.25
600
0.2
700
800
0.15
900
0.1
1000
0.05
0
1
2
3
4
5
6
Combustion Time(hr)
Fig 4.71 Plots of Potassium Oxide (K2O) Concentration in RHA Versus
Combustion Temperature at Varied Combustion Time of 1hr to 6hrs
204
Potassium oxide (K2O) was found present in low concentrations that ranged from 0.06 to
0.37%. Highest K2O values (0.2-0.37 %) were detected at 400 –8000C, while the lowest
values (0.06 – 0.22 %) were obtained at 10000C. The low values obtained at 10000C for all
the combustion duration may be attributed to the probable formation of potassium
polysilicate, thereby reducing the free potassium oxide content. It has been suggested
124
that at high temperatures, the low – melting oxides (like K2O) fuse with silica on the
surface of the rice husk char and form glassy or amorphous phases, preventing the
conversion of carbon. It has also been suggested that at high heating rates, which is
applicable here, this formation of potassium silicate takes place leaving a high residual
carbon96. The presence of K2O, which has a melting point of about 3500C
220
, at all the
combustion temperatures and time suggests that it is in a form that makes it inaccessible for
melting and volatilization. Combustion duration had little effect on the level of potassium
oxide since the values of 0.14 – 0.35 obtained at 1hr duration compare favourably with
values of 0.15- 0.30 obtained at 6hrs.
4.9.3.3 Magnesium Oxide Content of RHA at Different Combustion Conditions
The magnesium oxide concentrations of RHA samples obtained at different combustion
temperatures and time are given in Table 4.8 while plots of MgO concentration as a f
unction of temperature and time are shown in Figs. 4.72 and 4.73 respectively.
Table 4.8: Magnesium Oxide (MgO) Content of RHA Obtained at different
CombustionTemperatures and Time
Combustion
0
Temp( C)
400
500
600
700
800
900
1000
MgO content (%) of RHA at 1hr to 6hr combustion time
1
0.5599
0.6905
0.6597
0.5041
0.6571
0.5623
0.4927
2
0.3367
0.5004
0.5431
0.5996
0.5472
0.5318
0.4099
3
0.5895
0.6975
0.5752
0.7664
0.6283
0.5811
0.4061
205
4
1.1650
0.6824
0.6419
0.7614
0.6314
0.6471
0.6369
5
0.7081
0.7892
0.5545
0.8451
1.0674
0.4569
0.3430
6
0.4897
0.6493
0.7238
0.9229
0.6830
0.3732
0.3356
Magnesium Oxide Conc (%)
1.4
1.2
1
1
2
0.8
3
0.6
4
5
0.4
6
0.2
0
400
500
600
700
800
900
1000
Combustion Temperature (0C)
Fig 4.72 Plots of Magnesium Oxide (MgO) Concentration in RHA Versus
Combustion Temperature at Varied Combustion Temperature of 1hr to 6hrs
Magnesium Oxide Conc (%)
1.4
1.2
400
1
500
0.8
600
700
0.6
800
900
0.4
1000
0.2
0
1
2
3
4
5
6
Combustion Time (hr)
Fig 4.73 Plots of Magnesium Oxide (MgO) Concentration in RHA Versus
Combustion Time at Varied Combustion Temperature of 400 to 10000C
MgO was detected in all the ash samples obtained at the various combustion temperatures
and time as is expected since it has a high melting point of 28520C 221 and should therefore
be unaffected by the combustion temperatures studied, which are far below its melting
temperature. The level ranged from 0.34 to 1.17% and highest values were obtained at 700
and 8000C while the lowest values were obtained at 900 and 10000C as shown in Fig.
4.72 Combustion duration had an effect on the MgO level as lower values ranging from
0.50-0.70 were obtained at shorter time of 1-3hrs while higher values ranging from 0.341.17 were obtained at longer time of 4-6hrs as shown in Fig. 4.73.
206
4.9.3.4 Calcium Oxide Content of RHA at Different Combustion Conditions
The calcium oxide concentrations of RHA samples obtained at different combustion
temperatures and time are given in Table 4.9 while plots of CaO concentration as a function
of temperature and time are shown in Figs. 4.74 and 4.75, respectively. CaO was detected
in all the RHA samples obtained at different combustion temperatures and time since its
melting point is 25720C
222
.The level of CaO found at the various combustion conditions
ranged from 0.12 to 0.18% as shown in Table 4.9
Table 4.9: Calcium oxide content (CaO) of RHA obtained at different
Combustion Temperatures and Time
Combustion
Temp(0C)
400
500
600
700
800
900
1000
CaO Content of RHA at 1hr to 6hrCombustion time
1
0.1933
0.1345
0.1456
0.1497
0.1573
0.1454
0.1619
2
0.1842
0.1271
0.1355
0.1373
0.1670
0.1214
0.1383
3
0.1191
0.1359
0.1713
0.1571
0.1547
0.1460
0.1574
4
0.1850
0.1420
0.1333
0.1446
0.1449
0.1611
0.1320
5
0.1391
0.1525
0.1272
0.1517
0.1804
0.1558
0.1636
6
0.1871
0.1556
0.1449
0.1635
0.1522
0.1674
0.1511
Calcium Oxide Conc (%)
0.25
0.2
1
2
0.15
3
4
0.1
5
6
0.05
0
400
500
600
700
800
900
1000
Combustion Temperature (0C)
Fig 4.74 Plots of Calcium Oxide Concentration in RHA Versus Combustion
Temperature at Varied Combustion Time of 1 to 6hr
207
Calcium Oxide Conc (%)
0.25
400
0.2
500
600
0.15
700
800
0.1
900
1000
0.05
0
1
2
3
4
5
6
Combustion Time (0C)
Fig 4.75 Plots of Calcium Oxide Concentration of RHA Versus Combustion
Time at Varied Combustion Temperatures of 400 to 10000C
Combustion temperature had little effect on CaO content of RHA since the values obtained
at lower temperatures (400 to 6000C), which is in the range of 0.12 to 0.17% are close to
the values obtained at higher temperatures (700 to 10000C), which is in the range of 0.14 –
0.18% ,except in the case of 4000C, which had the highest values in the range of 0.17-0.19.
Combustion time appeared to have little effect too on CaO level since the values ranging
from 0.13-0.19% obtained after 1hr, are comparable to the values in the range of 0.140.19% obtained after 6hrs. The temperature and time of incineration therefore have little
effect on the CaO content of RHA.
4.9.3.5 Aluminium Oxide Content of RHA at Different Combustion Conditions
The aluminium oxide concentrations of RHA samples obtained at different combustion
temperatures and time are given in Table 4.10 while plots of Al2O3 concentration as a
function of temperature and time are shown in Figs. 4.76 and 4.77, respectively.
Table 4.10: Aluminium Oxide (Al2O3) Content of RHA Obtained at Different
Combustion Temperatures and Time.
Combustion
Temp 0C
400
500
600
700
800
900
1000
Al2O3 Content (% conc) of RHA at 1hr to 6hr Combustion Time
1
0.0440
0.1060
0.0560
0.0554
0.0565
0.0358
0.0608
2
0.0493
0.0418
0.0453
0.0679
0.0514
0.0517
0.0333
3
0.0742
0.0496
0.0468
0.0656
0.0508
0.0465
0.0457
208
4
0.0440
0.0467
0.0565
0.0561
0.0548
0.0619
0.0457
5
0.0569
0.0554
0.0486
0.0611
0.0680
0.0587
0.0519
6
0.0467
0.0604
0.0695
0.0574
0.0573
0.0477
0.0480
Aluminium Oxide Conc (%)
0.08
0.07
1
0.06
2
0.05
3
0.04
4
0.03
5
0.02
6
0.01
0
400
500
600
700
800
900
1000
Com bustion Tem perature (0C)
Fig 4.76 Plots of Aluminium Oxide Concentration of RHA Versus Combustion
Temperature at Varied Combustion Time of 1 to 6hrs
Aluminium Oxide Conc (%)
0.08
0.07
400
0.06
500
0.05
600
0.04
700
0.03
800
0.02
900
0.01
1000
0
1
2
3
4
5
6
Combustion Time (hr)
Fig 4.77 Plots of Aluminium Oxide Concentration of RHA Versus Combustion
Time at Varied Combustion Temperatures of 400 to 10000C
Al2O3, which has a melting point of about 20540C
223
was found present in trace
quantities in all the ash samples. The combustion temperature appeared not to have any
effect on the oxide levels, which ranged from 0.03 to 0.07%. Values obtained at lower
temperatures (400 to 6000C) ranged from 0.04 to 0.07% and are comparable to values
obtained at higher temperatures (700 to 10000) which are in the range of 0.03 to 0.07%.
The duration of incineration, similarly, appear not to have much effect on the Al2O3
level, as the values obtained after 1hr duration, which ranged from 0.04 to 0.06%
compare favourably with the values obtained after 6hrs, which ranged from 0.05 to
209
0.07% The levels of Al2O3 therefore seem not to be significantly affected by combustion
conditions as shown in Figs. 4.76 and 4.77
4.9.3.6 Iron (III) Oxide Content of RHA at Different Combustion Conditions
The ferric oxide concentrations (in %) of RHA samples obtained at different combustion
temperatures and time are given in Table 4.11. Plots of Al2O3 concentration as a function of
temperature and time are shown in Figs. 4.78 and 4.79, respectively.
Table 4.11.
Iron (III) Oxide (Fe2O3) Contents of RHA at Different Combustion
Temperatures and Time.
Combustion
Time 0C
Fe2O3 Contents of RHA at Combustion Time of 1hr to 6hrs
1
0.0885
0.1083
0.1068
0.0722
0.0565
0.0465
0.1245
400
500
600
700
800
900
1000
2
0.0961
0.0962
0.1164
0.1017
0.0688
0.1025
0.1002
3
0.0867
0.0982
0.0639
0.1113
0.1294
0.1414
0.1573
4
0.1216
0.1055
0.1187
0.1371
0.1139
0.1469
0.1215
5
0.1235
0.1220
0.0995
0.1196
0.1448
0.1177
0.1059
6
0.0870
0.1169
0.1204
0.1244
0.1657
0.0947
0.0996
0.18
Iron (III) Oxide Conc (%)
0.16
0.14
1
0.12
2
0.1
3
0.08
4
5
0.06
6
0.04
0.02
0
400
500
600
700
800
900
1000
Com bustion Tem perature (0C)
Fig 4.78 Plots of Fe2O3 Concentration of RHA Versus Combustion
Temperature at Varied Combustion Time of 1 to 6hrs
210
0.18
Iron(III) Oxide Conc(%)
0.16
0.14
400
0.12
500
600
0.1
700
0.08
800
0.06
900
0.04
1000
0.02
0
1
2
3
4
5
6
Com bustion tim e(hr)
Fig 4.79 Plots of Fe2O3 Concentration of RHA Versus Combustion
Time at Varied Combustion Temperatures of 4000C to 10000C
Fe2O3, was detected in all the ash samples produced at different combustion temperatures
and time since it has a high melting point of approximately 13900C.
224
The levels ranged
between 0.04 and 0.16%. The combustion temperature and time appear to have no effect on
the Fe2O3 yield as the values shown in Table 4.11 reveal no particular trend. For instance,
the Fe2O3 yield in the 4th hour for 400, 500, 600, 700, 800, 900 and 10000C, are 0.1216,
0.1055, 0.1187, 0.1371, 0.1139, 0.1469 and 0.1215.This is also the case with Fe2O3 yield
with combustion duration as the values for 5000C, for instance, at 1. 2, 3, 4, 5 and 6hrs are
0.1083,0.0962, 0.0982, 0.1055, 0.1220 and 0.1169%, respectively, which show little
variation.
4.9.3.7 Loss on Ignition (LOI) of RH at Different Combustion Conditions
The loss on ignition (LOI) refers to the mass loss of a combustion residue whenever it is
heated in an air or oxygen atmosphere to high temperatures. The mass loss can be due to
loss of moisture, carbon, sulphur, chlorine from the decomposition or combustion of the
residue 225. LOI gives an estimate of the combustible or unburned carbon in the sample. A
higher LOI value is suggestive of low combustion efficiency, with a relatively high level of
unburnt materials, while a low level suggests high combustion efficiency with low level of
unburnt compounds.The LOI values at various combustion temperatures and time are given
in Table 4.12. Plots of LOI as a function of temperature and time are shown in Figs.4.80
and 4.81 respectively. Despite the zig-zag pattern of the graphs, the results reveal that high
LOI values (9 –25%), signifying inefficient combustion, were obtained at lower
combustion temperatures of 400, 500 and 6000C at 1 to 4hr combustion duration. However,
211
the values obtained at 1000oC for all the combustion duration were the highest (16-30 %),
suggesting that the highest combustion inefficiencies occur at 10000C at the high heating
rate of 27oC/min used.
Table 4.12: Loss on Ignition (LOI) Values of RHA at Different Combustion
Temperatures and Time
Combustion
Temp (0C)
LOI Contents of RHA at Combustion Time of 1hr to 6hrs
1
25.4813
28.0059
12.5873
19.9181
7.2008
26.5536
20.4253
400
500
600
700
800
900
1000
2
13.6088
19.5079
25.2477
23.0796
17.1001
13.5718
30.2196
3
22.7115
24.8331
21.7444
8.2798
9.9419
21.4407
21.1423
4
16.6601
23.1846
21.8390
9.0819
16.8548
11.3127
22.6613
5
22.397
13.9662
20.3743
6.8358
3.0006
8.9716
16.0164
6
17.9568
9.5749
10.2421
6.8272
19.6902
11.6732
23.4171
Loss on Ignition Value(%)
35
30
1
25
2
20
3
15
4
5
10
6
5
0
400
500
600
700
800
900
1000
Combustion Temperature(0C)
Fig 4.80 Plots of Loss on Ignition (LOI) Values of Rice Husks Versus Combustion
Temperature at Varied Combustion Time of 1hr to 6hrs
212
Loss on Ignition Value (%)
35
30
400
25
500
600
20
700
15
800
10
900
5
1000
0
1
2
3
4
5
6
Combustion Time (hr)
Fig 4.81 Plots of Loss on Ignition (LOI) Values of Rice Husks Versus
Combustion Time at Varied Combustion Temperatures of 4000C to 10000C
. The lowest LOI values (6-9%), signifying efficient combustion were obtained at 700, 800,
and 900oC from 3 to 6hr duration; the 1 to2hr duration at these temperatures, however, had
higher values. At low heating rates of 15.520C/min and 9.80C/min for 9000C and 10000C
respectively, homogeneously white ash was obtained, which signifies high carbon
conversion efficiency. The LOI values would expectedly be lower than those of higher
heating rates.
4.10 Properties of Emulsion Paints Produced Using RHA Flour as
Extender in Comparison with Silica flour, Kaolin and Calcium Carbonate
(standard)
4.10.1 Extender Particle size
A fairly uniform particle size was maintained in all the extenders studied by sieving each
one with a standard sieve having a mesh size (aperture) of 63 microns. This was to ensure
that a particle size range of 32 - 63 microns ( i.e 0.032 to 0.063mm) was maintained. This
is because the particle size (surface area) of the extender affects the thickening effect of the
extender as well as other properties.
213
4.10.2 In-Can Appearance
All the formulated emulsion paint samples had a homogenous, creamy, viscous appearance,
typical of emulsion paint products. Some air bubbles were found in most of the samples,
probably due to foam formation, arising from high-speed stirring during the pigmentdispersion stage. This is in spite of the defoamer incorporated to prevent or reduce
minimally foam formation.
4.10.3 Effect of Extender Colour on Dry Film Appearance
The white emulsion paints containing different extenders when applied on 6” x 4” brush out
cards showed a satisfactory film consistency, The colour of the extender expectedly, had an
impact on the colour of the finished paint products. Consequently, the Control, which
contained no extender but only the white pigment, titanium dioxide came out brilliant white
in colour. Calcium carbonate, the ‘whitest’ of all the extenders also gave brilliant white
emulsion paint; silica flour and RHA both of which are off-white in colour gave white
emulsion paints while kaolin, which is a deep cream colour gave an off-white colour. These
colour characteristics of the paint products are illustrated with the white emulsion paint
samples containing 8% extender level namely CCWEP-8, SFWEP-8, RHAWEP-8 and
KWEP-8 are shown in figs 4.82, 4.83, 4.84 and 4.85, respectively and 12% extender levels
coded CCWEP-12, SFWEP-12, RHAWEP-12 and KWEP-12 are shown in Figs. 4.86, 4.87,
4.88 and 4.89, respectively. The implication of this observation is that only calcium
carbonate extender can be used to produce brilliant white emulsion and other colours, silica
flour and RHA can be used to produce white and other light and dark colours and their
shades. The darker colour of kaolin restricts it to off-white and some selected colours since
its colour might interfere in the colour match of the standard and prevent an ‘A’ match. The
high level of titanium dioxide (19%) in the samples, however, reduced the impact of the
colour of the extenders. In a standard quality paint, which contains a much lower level of
titanium dioxide and a larger quantity of extender, the colour of the extender will probably
have a greater impact than in the premium.
The white emulsion paints were tinted with the appropriate pigment pastes to obtain green
(‘well-being’) and blue (‘First dawn’) colour variants in order to examine the impact of the
extender colour on other light colours apart from white. In the two cases, silica flour and
RHA gave colour matches similar to calcium carbonate, the standard, while kaolin was
slightly off-spec as it gave darker shades, hence with kaolin the colour match was not an
214
‘A’ match. The dry films of the applied green emulsion paint variants namely CCGEP-10
SFGEP-10 , RHAGEP-10 and KGEP- 10 are shown in figs 4.90, 4.91, 4.92 and 4.93,
respectively while those of the blue emulsion paint variants namely CCBEP-6, SFBEP-6,
RHABEP-6 and KBEP-6 are shown in figs 4.94, 4.95, 4.96 and 4.97, respectively. The
results show that the dry films of the RHA-based emulsion paints are comparable to those
of the standards, produced with calcium carbonate and consequently can be used to produce
the said colours as well as other light colours and dark colours on a commercial scale. Its
only limitation is in the production of brilliant white colour due to its intrinsic milky white
(off-white) colour , which would be imparted on the paint.
The results of the comparative evaluation of the physical properties of the white emulsion
paints produced with the different extenders are presented in Tables 4.13 (Pigment Volume
Concentration), 4.14 (viscosity values), 4.15 (wt per litre values), 4.16 (bulk increase
values) and 4.17 (pH values). Other standard paint properties studied are opacity, settling
resistance, scrub resistance, stability and dry film sheen which are discussed in section
4.10.9 to 4.10.13.
4.10.4 Pigment Volume Concentration (PVC)
The PVC values of white emulsion paint products produced with different extenders at 212% levels are presented in Table 4.13. Plots of PVC values versus extender level and
extender type are shown in Figs. 4.98 and 4.99, respectively. The PVC values range from
25.6 to 43.2 %. The two graphs show that RHA flour has the highest PVC values at all the
extender levels (2-12%) due to its low S.G, followed closely by kaolin. Silica flour and
calcium carbonate rank third and fourth, respectively. This trend conforms with that of the
specific gravity (shown in Table 4.4), which has an inverse relationship with PVC, such
that RHA flour with the lowest S.G, has the highest PVC value while calcium carbonate
with the highest S.G has the lowest PVC value. This trend is also reflected in their
thickening effect as discussed in section 4.12.
215
Dry Films of White, Green and Blue Emulsion Paints Produced with Calcium
Carbonate, Silica Flour, RHA and Kaolin as Extenders
Fig 4.82 Calcium
Carbonate White emulsion
paint (Std) CCWEP -8
Fig 4.83 Silica Flour White
Emusion Paint
SFWEP-8
Fig 4.86 Calcium Carbonate
White Emulsion Paint
(Standard) CCWEP-12
Fig 4.87 Silica Flour White
Emulsion Paint
SFWEP-12
Fig 4.84 RHA White
Emulsion Paint
RHAWEP-8
Fig 4.88 RHA White
Emulsion Paint
RHAWEP-12
Fig 4.85 Kaolin White
Emulsion Paint
KWEP-8
Fig 4.89 Kaolin Based
White Emulsion Paint
KWEP-12
[
Fig 4.90 Calcium
Carbonate Green Emulsion
Paint (Std) CCGEP-10
Fig 4.94 Calcium
Carbonate Blue Emulsion
Paint (Standard) CCBEP-6
Fig 4.91 Silica Flour Green
Emulsion Paint
SFGEP-10
Fig 4.95 Silica Flour Blue
Emulsion Paint
SFBEP-6
Fig 4.92 RHA Green
Emulsion Paint
RHAGEP-10
Fig 4.96 RHA Blue
Emulsion Paint
RHABEP-6
216
Fig 4.93 Kaolin Green
Emulsion Paint
KGEP-10
Fig 4.97 Kaolin Blue
Emulsion Paint
KBEP-6
Table 4.13 : PVC Values of White Emulsion Paints (WEP) Produced
with Different Extenders at 2-12% Level
Type of
PVC Values of Emulsion Paints at 2-12(%) Extender level
Extender
2
4
6
8
10
12
Silica Flour (SFWEP)
25.58
28.51
31.25
33.79
36.86
38.32
RHA Flour (RHAWEP) 26.83
30.82
34.39
37.59
40.52
43.18
Kaolin (KWEP)
26.30
29.84
33.09
36.05
38.73
41.22
Calcium
Carbonate (CCWEP)
25.49
28.39
31.05
33.52
35.77
37.99
50
45
PVC Value (%)
40
35
Silica Flour
30
RHA Flour
25
kaolin
20
calcium carbonate
15
10
5
0
2
4
6
8
10
12
Extender Level (%)
Fig. 4.98 Plots of PVC Value Versus Extender Level for Different Extenders
50
45
PVC Value (%)
40
2
35
4
30
6
25
8
20
10
15
12
10
5
0
Silica Flour
RHA Flour
kaolin
calcium carbonate
Type of Extender
Fig. 4.99 Plots of PVC Value versus Extender Type at 2-12% Extender level
217
Since the lower the S.G, the higher the PVC value, it implies that for an extender with a
low S.G, the volume of extender particles in the paint will be larger than for one with
higher, for the same weight of filler. The increase in volume of extender means that the
extender particles spread out more in the paint matrix, thereby occupying more space than
an extender with a higher S.G (lower PVC). This invariably increases the viscosity of the
paint. This factor is responsible for the remarkably good thickening effect of RHA when
compared with other extenders as discussed in section 4.12
The PVC values of the emulsion paints containing 2-8% extender levels generally ranged
from 25 to 34 %, except for RHA emulsion paints, which had slightly higher values of 2838% (due to low S.G). These values are within the PVC range for exterior house paints (2836%)
73
. However, the variants containing 10 and 12% extenders, with PVC values in the
range of 35- 45% respectively are closer to semi-gloss paints (35-45%). This implies that
the dry paint films of the emulsion paints possess a small degree of gloss (sheen),
attributable to the relatively high level of resin (33%) in the formulations, which places
them as premium quality paints. This higher resin level, in addition to aesthetic appeal
enhancement, confers some desirable properties to the paint such as reduced staining,
improved adhesion, improved washability, weatherability and durability, which are
dependent on the type and level of the resin.
4.10.5 Thickening Effect
The viscosity values of white emulsion paint products produced with different extenders at
2-12% levels using Analog Rotothinner Viscometers are given in Tables 4.14. Plots of
viscosity as a function of extender level are shown in Figs. 4.100 while plots of viscosity
versus emulsion paint are shown in Figs.4.101
All the extenders expectedly, had a thickening effect on the emulsion paint products but to
varying degrees. This effect was determined by viscosity measurement. The thickening
effect of the extenders evident in the viscosity of the paints was generally found to be more
pronounced at higher filler levels (8 - 12%) than at lower levels (2 - 6%) as is evident in the
graphs shown in Figs. 4.100 and 4.101. Silica flour showed a sharp increase in viscosity
from 10 to 12% also evident in the said graphs. The better thickening performance of all the
fillers at higher levels (8 to 12%) than at lower (2 to 6%), implies that higher levels of
extenders are required in the paint formulation for a significant increase in the viscosity of
218
the paints. The disparity in extender viscosity values was also more pronounced at higher
extender levels than at lower levels as seen in Fig. 4.100 and 4.101.
RHA was found to exhibit the highest thickening effect followed closely by kaolin. Silica
flour ranked third while calcium carbonate, the standard filler, ranked fourth. However,
silica flour was comparable to RHA and kaolin at higher levels of 10-12%, suggesting that
its thickening effect improves at higher levels. This low viscosity effect is probably why
CaCO3 is used for bulk increase in emulsion paints, but not for thickening purposes. On the
other hand, amorphous silica of fine particle size is used as a thickener in emulsion paints.
The low bulk density (low mass to volume ratio), low specific gravity, high surface to
volume ratio and amorphous nature of RHA are probably responsible for its thickening
superiority to CaCO3 and silica flour, which is predominantly crystalline α-quartz obtained
from milling quartz sand. The low S.G bulk density implies that for the same weight of
RHA, CaCO3 and SF, RHA, will have a greater volume, thus greater number of particles,
which will occupy/ fill more space in the paint system than the other two. In addition, the
expanded conformation and disorderly nature of amorphous materials like RHA confers
greater surface area, flexibility and reactivity on the materials than in crystalline materials
where an ordered crystal lattice reduces its surface area and imposes some rigidity to the
structure and less reactivity on the materials.
Table 4.14 Summary of Viscosity Values of white Emulsion Paint Products Produced
with different Extenders at 2-12% Using Analog Rotothinner (Viscometer)
Viscosity (poises) of White Emulsion Paints at 2-12%
Filler Level (wt%)
2
4
6
8
10
12
Silica flour (SFWEP)
1.6
2.0
2.4
3.2
4.5
5.6
RHA Flour (RHAWEP)
2.1
2.8
3.0
3.8
4.3
5.4
Kaolin (KWEP)
2.0
2.7
3.5
3.8
4.3
5.0
Calcium Carbonate (CCWEP)
1.9
2.5
2.8
3.0
3.2
4.2
-
-
-
-
-
-
Control
219
0
1.5
7
Viscosity (poises)
6
5
SFWEP
4
RHAWEP
3
KWEP
CCWEP
2
1
0
2
4
6
8
10
12
Extender Level (%)
[
Fig. 4.100 Plots of Viscosity (poises) Versus Extender Level (%) for White
Emulsion Paints Produced With Different Extenders: SFWEP, RHAWEP,
KWEP, CCWEP Using Analog Rotothinner
Viscosity (poises)
7
6
2
5
4
4
6
3
8
2
10
1
12
0
SFWEP
RHAWEP
KWEP
CCWEP
Emulsion Paint
Fig. 4.101 Plots of Viscosity (poises) Versus Emulsion Paint Type for White
Emulsion Paints Produced With Different Extenders at 2-12% Extender Level
Using Analog Rotothinner
220
The viscosity values of white emulsion paint products produced with different extenders at
2-12% levels using the Digital Rotothinner Viscometer are given in Table 4.15.The Digital
Rotothinner was used for the purpose of comparing its sensitivity with that of the Analog.
Plots of viscosity as a function of extender level are shown in Fig. 4.102 while plots of
viscosity versus emulsion paint are shown in Figs. 4.103. Figs. 4.102 and 4.103 show
similar trends with 4.100 and 4.101, respectively, obtained for the Analog Rotothinner
except for the slightly higher values of the Digital. Kaolin is comparable to RHA in
thickening effect while calcium carbonate shows superiority to silica flour at low extender
levels of 2-% , but at higher levels (8-12%), silica flour supercedes.
Table 4.15: Summary of viscosity values of White Emulsion Paint Products
Produced with Different fillers at 2-12% Levels using Digital Rotothinner
(Viscometer)
Viscosity (poises) of White Emulsion Paints at 2-12%
Filler Level (wt%)
2
4
6
8
10
12
Silica flour (SFWEP)
2.2
2.5
3.0
3.8
5.0
6.5
*RHA Flour (RHAWEP)
2.8
3.5
3.8
4.3
4.9
6.4
Kaolin (KWEP)
2.8
3.4
4.1
4.4
5.0
5.9
Calcium carbonate (CCWEP)
2.6
3.2
3.5
3.6
4.0
5.0
Control (0% filler)
-
-
-
-
-
-
0
2.8
7
Viscosity (poises)
6
5
SFWEP
4
RHAWEP
KWEP
3
CCWEP
2
1
0
2
4
6
8
10
12
Extender Level(%)
Fig. 4.102 Plots of Viscosity (poises) Versus Extender Level (%) for White
Emulsion Paints Produced With Different Extenders: SFWEP, RHAWEP,
KWEP, CCWEP Using Digital Rotothinner
221
7
Viscosity (poises)
6
2
5
4
4
6
8
3
10
2
12
1
0
SFWEP
RHAWEP
KWEP
CCWEP
Em ulsion Paint
Fig. 4.103 Plots of Viscosity (poises) Versus Emulsion Paint Type for White
Emulsion Paints Produced With Different Extenders at 2-12% Extender Level
Using Digital Rotothinner
4. 10.6 Wt per litre Values of Emulsion Paints
The wt per litre (specific gravity) values of white emulsion paints produced with different
extenders at 2-12% levels are shown in Table 4.16. Plots of wt per litre versus extender
level and emulsion paint are shown in Figs 4.104 and 4.105, respectively.
The wt per litre values of the paint, which is synonymous with the specific gravity of the
paint, has a direct relationship with the weight of the paint but an inverse relationship with
the volume. Thus, in theory, as the wt per litre value of the paint increases, the volume of
the paint decreases at constant weight and vice versa.
Extenders generally increase the bulk (volume) of the paint to varying degrees, therefore as
the extender level increases, the volume of the paint is expected to increase proportionately.
However, there are many factors that could affect the wt per litre values of paints. These
include the specific gravity of the pigment and extender used , the level of dispersion of the
pigment, the extent of foam formation, the pigment level, the level of cellulosic thickener
(natrosol) etc. The interferences of these factors are reflected in the wt per litre values,
which seem to be non - dependent on extender levels as shown in Figs 4.104 and 4.105.
The values were within the range of 0.93 to 1.23. The decrease in wt per litre with
increasing extender level (increasing volume) was observed only in the case of kaolin and
222
silica flour. Calcium carbonate, and RHA did not reveal any meaningful trend in this regard
as a zig-zag pattern was observed. Another contributory factor is the variation in solvent
(water) level, since as the extender level was increased the solvent level was decreased
proportionately, while other components were kept constant, so as to maintain a total
weight % value of 100. SON has no established specification for the wt per litre values of
paints probably due to many variables. However, the range as observed for CAP PLC
emulsion is 0.90 to 1.04.
Table 4.16: Summary of wt per litre (specific gravity) values of White Emulsion
Paints Produced with Different Extenders at 2-12% Levels
Emulsion Paint
Wt per litre values of white emulsion paints at 0-12%
extender level
0
2
4
6
8
10
12
Silica flour (SFWEP)
-
1.17
RHA Flour (RHAWEP)
-
1.16
Kaolin (KWEP)
-
Calcium carbonate (CCWEP)
Control 0% filler) CONWEP
1.23
1.23
1.19
1.19
1.13
1.00
1.13
1.19
1.16
1.04
1.08
1.10
1.15
0.99
1.01
-
0.98
0.99
1.03
0.93
1.03
1.04
1.17
-
-
-
-
-
-
1.4
Wt per Litre
1.2
1
SFWEP
0.8
RHAWEP
0.6
KWEP
CCWEP
0.4
0.2
0
2
4
6
8
10
12
Extender Level (%)
Fig. 4.104 Plots of Weight per Litre Values of White Emulsion Paints Produced
with Different Extenders Versus Extender Levels of 2 to 12%
223
1.4
1.2
2
Wt per Litre
1
4
0.8
6
0.6
8
10
0.4
12
0.2
0
SFWEP
RHAWEP
KWEP
CCWEP
Emulsion Paint
Fig. 4.105 Plots of Weight per Litre Values of White Emulsion Paints Produced
with Different Extenders Versus Emulsion Paint at 2 to 12 % Extender Levels
4.10.7 Bulk Increase Effect
The bulk increase values of white emulsion paints produced with different extenders at 212% levels are shown in Table 4.17. Plots of bulk (volume) increase as a function of
extender level and emulsion type are shown in Figs. 106 and 107, respectively. The bulk
increase determination is a theoretical estimate of the bulk (volume) increase of the
extenders when incorporated into the emulsion paint at varied levels. The values were
calculated from the relationship volume = mass/sp gravity. This theoretically implies that as
the wt per litre (specific gravity) of the paint decreases, the volume increases and vice versa
at constant mass. The extent of volume increase approximates to the bulk increase of the
paint. The emulsion paint Control formulation that contained 0% filler, (CONWEP) is
therefore expected to have the highest specific gravity and the lowest bulk (smallest
volume) theoretically. Thus, the extent of bulk increase was calculated by deducting the
specific gravity (or volume) values of the emulsion paint samples from that of the Control.
This estimation revealed that calcium carbonate had the highest bulk increase effect while
kaolin came second and RHA third as shown in Figs 4.106 and 4.107. Silica flour had no
significant bulk enhancement effect, as the values were all negative (slightly less than that
of the Control) except for the 4%. It is probable, however, that the high level of TiO2 (19%)
used as pigment and its much higher specific gravity of approximately 4.00
65
have a
dominant effect on the paint properties and generally suppress the bulk increase effect of
the extenders, due to the relatively low levels of the extenders as well as their lower S.G,
resulting to a possible net lowering of bulk (volume) by TiO2. This lowering effect is
224
expected to be more pronounced at lower extender levels (2-6%) than at higher (8-12%).
Thus, hypothetically, the bulk–lowering effect of TiO2 compete with the expected bulkincrease effect of the extenders, with the former predominating. These competing factors
and consequent volume- decrease effect are expected to be more pronounced if the extender
has a lower SG than a higher. Thus, RHA with a considerably lower S.G (1.55) showed
marginal bulk increase, while calcium carbonate and kaolin with S.G of 2.24 and 1.76,
respectively showed higher bulk increase. However, silica flour with a high S.G of 2.18
showed negative (no) bulk increase at all levels due to its crystalline nature and reduced
surface area, which cause low or no volume increase, when compared with amorphous
silica. These factors probably explain why calcium carbonate is generally used for bulkincrease purpose in paints, kaolin as flatting and thickening agent, crystalline silica (silica
flour) as extend,er in fillers and stoppers and amorphous silica as thickening and gloss
control agent.65
Table 4.17: Summary of Bulk Increase Values of White Emulsion paints
(WEP) Produced with different Extenders at 2-12% Levels
Bulk Increase Values of white Emulsion paints at 2-12%
Emulsion Paint
0
2
4
6
8
10
12
Silica flour (SFWEP)
-
0.00
27.00
-6.00
-6.00
-2.00
-2.00
RHA Flour (RHAWEP)
-
1.00
4.00
17.00
4.00
-2.00
1.00
Kaolin (KWEP)
-
13.00
9.00
7.00
2.00
18.0
16.00
carbonate -
19.00
18.00
14.00
24.0
4.00
13.00
Calcium
(CCWEP)
Bulk (volume) Increase (%)
30
25
20
15
SFWEP
RHAWEP
10
KWEP
5
CCWEP
0
-5
2
4
6
8
10
12
-10
Extender Level (%)
Fig.4.106.Plots of Bulk Increase Versus Extender Level for different Extenders
225
30
Bulk Increase(%)
25
20
2
15
4
6
10
8
5
10
0
12
SFWEP
-5
RHAWEP
KWEP
CCWEP
-10
Type of Em ulsion Paint
Fig 4.107 Plots of Bulk Increase Versus Emulsion Paint Type at Varied Extender
Levels of 2% to12%
4.10.8 pH of Emulsion Paints
The pH values of the emulsion paint products produced with different extenders at 2-12%
level are presented in Table 4.17.The values generally range from 9.00-9.50, which is close
to the Standards Organisation of Nigeria (SON) specification of 8.0-9.0. The pH values of
the finished paint products are affected by factors such as the pH of the resin, water and
amount of ammonia used. The pH of a paint product, however, can be adjusted to the
desired level by adjusting the level of ammonia added to the paint, which is often used as
the pH adjuster. The pH values seem unaffected by the extender level as pH values at 2%
extender level are same as those of 4-12%.
Table 4.18 pH Values of Emulsion Paints Formulated with
Different Extenders at 2-12% Levels
Type of
Extender
Silica Flour (SFWEP)
pH Values
2
4
6
8
10
12
9.10 9.10 9.16 9.16 9.24 9.22
RHA Flour (RHAWEP) 8.99 9.08 8.98 8.93 8.88 8.88
Kaolin (KWEP)
9.21 8.96 9.15 9.13 9.44 9.47
Calcium
Carbonate (CCWEP)
9.51 9.49 9.05 9.45 9.39 9.25
226
4.10.9 Opacity of the Emulsion Paint Products
The opacity of the paint, which is also known as the hiding or obliterating power, is largely
dependent on the nature and amount of pigment in the paint formulation since it is the
pigment that confers this property on the paint. A titanium dioxide pigment level of 19.00%
was maintained in all the emulsion paint samples and this manifested in the hiding power as
they all covered in two coats, thereby conforming with the Standards Organization
specification for premium quality paints. The extender level did not make much impact on
opacity because extenders have little or no hiding power, thus the 12% extender level did
not differ noticeably from the 4% in opacity in the case of calcium carbonate (CC), kaolin
(K) and silica flour (SF). However, RHA had a slightly better hiding power than CC, SF
and K. This is attributable to the trace quantity of titanium dioxide in RHA.which was more
pronounced in paints containing 12% filler levels. Extenders have refractive indices close to
that of the resin, thus have little or no colour impact in the paint product unlike the
pigments which have refractive indices widely different from that of the resin.
4.10.10 Settling Resistance of Emulsion Paints
Emulsion paints have an intrinsic tendency to settle at the bottom when left to stand for
days/weeks due to the high level of pigment and extenders, which have higher density than
the solvent. This phenomenon manifests in the form of phase separation in which the
solvent (water) forms the top layer, however, on stirring, the paint regains its homogeneity.
This property was assessed for the emulsion paints produced with the different extenders,
by leaving them to stand for 10 weeks and observing for degree of phase separation. RHA
was found to surpass the other extenders in anti-settling resistance as the least phase
separation was observed for the 10 months period of monitoring. Calcium carbonate was
lowest in this property while silica flour came second.
4.10.11 Scrub Resistance (Washability)
All the emulsion paints showed excellent scrub resistance of above 5000 cycles, which
implies that the dry paint films can be washed (scrubbed) over 5000 times without loss of
the paint from the substrate. This is due to the relatively high level of resin (33%) in all the
emulsion paints compared to lower (standard) quality emulsion paints.
227
4.10.12 Stability Monitoring
The emulsion paints formulated were all stable after 10 months of stability monitoring as
there were no changes in the physical state of the products such as discoloration, viscosity
loss, development of offensive odour.
4.10.13 Dry Film Sheen
The dry films of the emulsion paint all had a characteristic sheen (luster or gloss) for all the
extenders used due to their relatively high resin levels which placed them under the
category of semi-gloss paints based on their PVC levels as discussed in section 4.8.8.
However, RHA film was visibly lowest in this property, probably due to its excellent
flatting effect, which partially suppressed the film gloss or sheen .The crystalline nature of
silica flour is also largely responsible for the slightly better sheen of silica flour-based
emulsion films while the mattiness of RHA-based film can be attributed to its amorphous
nature.
4.11 Properties of Textured Finishes Produced using RHA Flour as New
Filler in Comparison with calcium carbonate (standard) and silica flour
An extender level of 15% was used in all the textured finishes as such products contain a
large amount of fillers. However, in the case of RHA, an additional variant containing 10%
level was included because of its remarkably high thickening effect as discussed below.
The particle size of extenders used in the formulation of textured finishes is larger than that
of emulsion. This is because textured finishes are not smooth like emulsion paint but
contain texturizers, which impart ‘roughness’ on the product, thus fineness of the extender
is not required. Thus, the extenders studied namely calcium carbonate, silica flour and RHA
were sieved with a larger mesh size (aperture) of 106 microns to maintain a fairly uniform
particle size ranging from 63 to 106microns in the said extenders used in the formulation of
the textured finish.
4.11.1 In-can Appearance
The textured finishes produced with the different extenders all possessed a characteristic
creamy paste-like consistency, with the texturizer (sand) imparting a ‘gritty’ texture. The
colour of the extender imparted slightly on the finished product. Thus, calcium carbonate
228
gave a more brilliant whiteness than silica flour and RHA. This implies that RHA and silica
flour can be used to produce white textured finish and other colours, but not brilliant white.
4.11.2 pH
The textured finishes were all slightly alkaline as required of such products. The pH values
(at 270C) of the calcium carbonate- based (CCTEXW), silica flour- based (SFTEXW),RHA
-based (RHATEXW-20) and RHATEXW-10 were 9.18, 8.94, 9.11 and 9.08, respectively
which are in conformity with SON specification of 7.5-9.0. The pH values can be altered by
adjusting the level of ammonia used in the formulation.
4.11.3 Wt Per Litre (S.G)
The wt per litre values obtained for the textured finishes were 1.44, 1.47, 1.49 and 1.23 for
CCTEXW, SFTEXW, RHATEXW-20 AND RHATEXW-10, respectively. The lower
value of 1.23, obtained for RHATEXW-10 is due to its lower solids content.
4.11.4 Thickening Effect of the Extenders
The gel strength (viscosity) of the white textured finishes, CCTEXW, SFTEXW,
RHATEXW-20 and RHATEXW-10 were 75, 142, 180 and 135 poises. As in the case of
emulsion paints, the superior thickening effect of RHA is evident in the textured finish, as
the viscosity of RHATEXW ((180 poises) far surpasses that of CCTEXW (75poises),
which is the standard. Even the RHATEXW-10, which contains only 10% filler still
showed a remarkably good viscosity (135 poises), far higher than that of CCTEXW.
The silica flour-based finish (SFTEXW) ranked second with a viscosity of 140 poises. A
reduction of RHA level to 10% still gave a remarkably high viscosity of 135 poises, which
was still far higher than that of CCTEXW. These findings imply that the use of RHA in the
formulation of textured finish would provide a far more cost-effective product and yield a
bigger profit margin as desired by paint manufacturers.The Standards Organisation of
Nigeria( SON ) specified a minimum viscosity of 45 poises for textured finishes.
4.11.5 Dry Film Appearance of Textured Finishes
The white textured finishes containing the different extenders were applied on brush-out
cards to simulate real application appearance. The applied finishes revealed no flaking,
cracking, chipping or run as required by SON standard. The dry film samples of CCTEXW,
229
SFTEXW and RHATEXW are shown in Figs. 4.108, 4.109 and 4.110 respectively. The
pink colour variants (‘Deep pink’) of the finishes namely CCTEX-DP, SFTEX-DP and
RHATEX-DP are shown in figs 4.111, 4.112 and 4.113, respectively while the light purple
(“Gentle lavender”) variants namely CCTEX-GL, SFTEX-GL and RHATEX-GL are
shown in figs 4.114, 4.115 and 4.116, respectively. The ‘Blossom white’ variants, CCTEXBL, SFTEX-BL AND RHATEX-BL are presented in Figs. 4.117, 4.118 and 4.119,
respectively.
It is evident from these films that the RHATEX films are comparable to those of CCTEX,
the standard, thereby making RHA a good substitute for calcium carbonate. Its superior
thickening effect also enhances its value as a good extender.
4.11.6 Opacity
All the textured finishes containing the different extenders expectedly covered in one coat.
This is because the same extender pigment and resin levels were maintained in all the
products.
4.11.7 Drying Time
The drying time of the textured finishes were found to be as follows: Touch dry time (min)
of CCTEXW, SFTEXW, RHATEXW-20 RHATEXW-10 were 42, 45, 37 and 40mins,
respectively while the hard dry time(hr) were 3.3, 3.25, 3.3, 3.10 and 3.20 for the same
respective samples. The textured finishes therefore all dried within the Standards
Organisation of Nigeria (SON) stipulated drying time of 2-3hrs (max) for touch /surface
dry and 4 to 5hrs (max) for hard dry.
230
Dry Films of ‘White’, ‘Deep Pink,’ ‘Gentle Lavender’ and ’Blossom White’ Textured
Finishes Produced with Calcium Carbonate, Silica Flour and RHA as Extenders
Fig 4.108 Calcium
Carbonate Textured Finishwhite (Std) CCTEXF-W
Fig 4.109 Silica Flour
Textured Finish-White
SFTEXF-W
Fig 4.111 Calcium
Carbonate Textured Finish‘Deep Pink’ (Standard)
CCTEXF-DP
Fig 4.112 Silica Flour
Textured Finish-‘Deep Pink’
(Standard) SFTEXF-DP
Fig 4.114 Calcium
Carbonate Textured Finish
‘Gentle Lavender’ (Std)
CCTEXF-GL
Fig 4.115 Silica Flour
Textured Finish- ‘Gentle
Lavender’ SFTEXF-GL
Fig 4.117 Calcium Carbonate
Textured Finish – ‘Blossom
White’ (Standard) CCTEXFBW-20
Fig 4.118 Silica Flour
Textured Finish – ‘Blossom
White’ SFTEXF-BW-20
231
Fig.4.110 RHA Textured
Finish-white
RHATEXF-W
Fig 4.113 RHA Textured
Finish-‘Deep Pink’
RHATEXF-DP
Fig 4.116 RHA Textured
Finish – ‘ Gentle Lavender’
RHATEXF-GL
Fig 4.119 RHA Textured
Finish ‘Blossom white’
RHATEXF-BW-20
Fig 4.120 RHA Textured
Finish – ‘Blossom white’
RHATEXF-BW-10
4.11.10 Application Properties
The textured finishes retained their patterns on application and did not sag or crack in
conformity with SON requirements.
It is evident from the results of using RHA in the formulation of textured finish, that RHA
presents a far cheaper and superior extender to calcium carbonate and silica flour in
addition to its other advantages of local and abundant availability and ‘renewability’.
4.12 Comparison of Properties of Red Oxide Primers Produced using Calcium
Carbonate (standard), Silica Flour and RHA as Extenders and Flatting Agents
The red oxide primer, being on undercoat, is applied on metal surfaces to prevent corrosion
as well as fill tiny crevices on the surface of the substrate this helps to ensure smoothness of
the top coat (gloss) when applied. For this purpose, a high extender level of about 20% is
used in the formulation as most extenders have flatting (matt) effect. A uniform particle
size range of 32 – 63 microns(<0.063mm) was maintained in the three extenders used for
this study-calcium carbonate (standard), silica flour and RHA flour by sieving with a
standard sieve of mesh size (aperture) 63 microns.
4.12.1 In-Can Appearance
The red oxide primers produced with the different extenders had a reddish-brown colour
due to the ferric oxide pigment used in the product
4.12.2 Viscosity (Efflux Time)
The red oxide primers, after production, all had a high viscosity of >15 poises, so had to be
thinned down to the required application viscosity of 3 poises/ RT/ 25C before their
physical properties, such as wt per litre, drying time gloss values etc were determined .This
viscosity is the required viscosity for proper spread of the finish during spraying. A higher
or lower viscosity is likely to affect the consistency of the dry film.
232
4.12.3 Wt per Litre
The wt per litre values of the formulated primers, CCROP, SFROP, RHAROP were 1.04,
1.06 and 1.04 respectively
4.12.4 Dispersion Level
The dispersion levels of the primers were all found to be 25 microns, which is the required
level for satisfactory film properties in a primer.
4.12.5 Drying Time
The tack-free (touch-dry) time of the primers , CCROP, SFROP, and RHAROP were 13, 11
and 9mins respectively while the hard-dry time were 23, 19 and 18mins respectively. The
values are relatively close due to same levels of driers and solids contents.
4.12.6 Flatting (matt) Effect of Red Oxide Primers
The matt (flatting) effect in paints is caused by scattering of the light rays incident on the
applied paint film by solid particles in the paint. This means diffraction of the light that
illuminates the substrate, otherwise a perfect reflection would produce a gloss effect. The
light-scattering, which results in matt effect is due to the micro-roughness of the paint film
brought about by extender particles dispersed in the paint. The concentration and particle
size of the extender will invariably affect the degree of matt of the paint.
i. Visual Observation
The primers prepared with the three different extenders were applied on 6” x 4” panels and
6” x 4” brush out cards with the aid of a paint brush and allowed to dry. The RHA based
primer (RHAROP) showed superiority in flatting effect that was clearly visually observable
and which surpassed that of calcium carbonate (standard) and silica flour.
ii. Gloss Measurement
The flatting effects of the primers were also determined by gloss measurements, since the
higher the gloss value, the lower the matt effect and vice versa. Interestingly, RHAROP had
the lowest gloss values of 0.3, 3.2 and 1.9 %, which corresponds to highest matt values of
99.7, 96.8 and 98.1, respectively at 200, 600 and 850 angles of measurement, substantiating
its superior matt effect to CCROP and SFROP. SFROP ranked second in flatting effect
with gloss values of 1.5, 10.5 and 7.3% as shown in Table 4.18. The Control, on the other
hand, had high gloss values and consequently low matt values due to the complete absence
233
of extender. The outstanding flatting effect of RHA can be attributed to its relatively low
specific gravity (1.54), which produces paints of generally higher PVC values than those of
calcium carbonate and silica flour, which have higher S.G of 2.24 and 2.18 respectively.
The higher PVC values implies a greater concentration of the RHA particles in the paint,
This larger number of particles per unit volume as well as the closer packing of the particles
in RHA cause a greater degree of light-scattering, (which is responsible for matt effect) and
light absorption, than in the case of calcium carbonate and silica flour, which have fewer
particles due to higher S.G and lower PVC values.
Table 4.19: Gloss and Matt Values of Red Oxide Primer Produced with
Different Extenders at 20% Level
Calcium
carbonate(std)
CCROP
Silica flour
RHA flour
Control
SFROP
RHAROP
CONROP
Av. Gloss Value (%)
3.1
1.5
0.3
72.85
Matt value (%)
Gloss values (%): 600
96.9
98.5
99.7
27.15
Av. Gloss values (%)
16.4
10.5
3.2
90.93
Matt Value
Gloss values (%) : 850
83.6
89.5
96.8
9. 07
Av. Gloss value (%)
14.7
7.3
1.9
110.23
Matt value (%)
85.3
92.7
98.1
-10.23
Av. Gloss value (%)
2.8
1.3
0.4
Matt value (%)
97.2
98.7
99.6
16.4
10.2
3.7
83.6
89.8
96.3
18.0
10.8
3.4
82
89.2
96.6
Type of Extender
Gloss values (after 24hrs)
200
Gloss values (after 3 days)
200
0
60
Av. Gloss values (%)
Matt Value (%)
850
Av. Gloss values (%)
Matt Value (%)
234
The dry film samples of CCROP, SFROP and RHAROP applied on brush-out cards are
shown in figs 4.121, 4.122 and 4.123, respectively.
Dry Films of Red Oxide Primers Produced with Different Extenders
Fig 4.121 Calcium
Carbonate Red Oxide Primer
(Standard) CCROP
Fig 4.122 Silica Flour Red
Oxide Primer SFROP
Fig 4.123 RHA Red Oxide
Primer RHAROP
The gloss values of the three red oxide primers were found to show no significant change
after three days, indicating stability of the matt with time. This superiority in flatting effect
of RHA is attributable to its low bulk density (low specific gravity) relative to SF and
calcium carbonate such that at same particle size, RHA has a much larger volume hence
greater spread (coverage) of the RHA filler within the binder matrix. This manifests in
better matt effect.
4.13 Comparison of Properties of Matt Wood Finishes Produced using Fumed Silica
(Standard), Silica Flour and RHA as Extenders and Flatting Agents
The wood finish is predominantly a mixture of solvents (70.5%), resin (24% ) and a small
quantity of extender (3.0%), used as the flatting agent. The standard extender used in the
formulation is fumed silica of 20 microns particle size. However, the particle size of the
silica flour and RHA used was <32 microns because there was no standard sieve of mesh
size 20 microns to yield same particle size as the standard i.e. fumed silica.
The results of the comparative qualitative assessment of the matt wood finishes, formulated
with different extenders as flatting agents are discussed hereunder.
4.13.1 In -can Appearance
The Control, which contained no extender was clear in appearance while the matt wood
finishes produced with the different extenders all had a slightly cloudy appearance.
235
4.13.2 Viscosity (Efflux Time)
The matt wood finishes produced with the different extenders were first thinned down
to an efflux time of 23 seconds before their physical properties, such as wt per litre, drying
time, gloss values etc were determined .This viscosity is the required viscosity for proper
spread of the finish during spraying. A higher or lower viscosity is likely to affect the
consistency of the dry film.
4. 13.3 Wt per Litre Values
The wt per litre values of the formulated wood finishes, CONMWF, FSMWF, SFMWF and
RHAMWF were 0.89, 0.91, 0.91 and 0.91 respectively. The Control which contained no
extender, expectedly had a slightly lower value due to less solids content while the other
extenders had same wt per litre values due to their similar solids contents .In spite of the
differences in the specific gravity of the extenders, the high level of solvents (70.5%) and
resins (24%) used in the wood finish, appear to have masked the S.G effect of the flatting
agent (extenders) which is of relatively low level (3.0%) in the wood finishes.
4.13.4 Drying Time
The tack-free (touch-dry) time of CONMWF, FSMWF, SFMWF and RHAMWF were 20,
9, 10 and 8mins respectively while the hard dry time were 30, 20,22 and 20mns
respectively after spraying on wood. The closeness in the values can be attributed to their
having same viscosity values and solids contents.
4.13.5 Flatting (Matt) Effect of Wood Finishes
The level of flatting effect of the three extenders incorporated into the wood finish was
studied by means of visual observation and gloss measurement as discussed in section
4.11.3 and 4.11.4. The gloss values are presented in Table 4.20
i. Visual Observation
The wood finishes were sprayed on wood panels by means of a spray gun and then allowed
to dry and the level of matt effect observed. The Control (CONMWF) expectedly, had the
highest visually observable gloss since it contained no extender (flatting agent). The fumed
silica wood finish (FSMWF) had the highest matt effect and consequently lowest gloss
value, followed closely by RHA wood finish (RHAMWF) while silica flour finish
(SFMWF) was next to the control in highest gloss values out of the four.
236
ii. Gloss Values
The matt levels of the primers were obtained indirectly from the gloss level measurements
of the wood finishes after application on 6” x 4” glass panels. The control had the highest
gloss values (%) of 47.8, 65.6 and 59.3 and consequently, lowest matt values of 52.2,65.6
and 40.7 for 200,600 and 850 respectively as shown in Table 4.20. The fumed silica-based
finish (FSMWF) showed excellent matt effect evident from its lowest gloss values of 0.93,
4.20 and 2.20 corresponding to highest matt values (%) of 99.07, 95.8 and 97.8 for 200, 600
and 850 respectively followed closely by RHA – based (RHAMWF) having matt values of
97.4, 88.9 and 84.9 for 200, 600 and 850 respectively.
Table 4. 20 Gloss and Matt Values of Matt Wood Finishes (MWF) Produced with
Different Extenders as Flatting Agents at 3% Level
Type of Extender/ Control
Flatting agent
CONMWF
Fumed silica
(standard)
FSMWF
Silica Flour
RHA Flour
SFMWF
RHAMWF
Gloss values(%): 200
Av.Gloss value (%)
47.83
0.93
20.93
2.60
Matt value (%)
52.17
99.07
79.07
97.4
Av.Gloss value (%)
65.60
4.20
31.45
11.10
Matt value (%)
34.4
95.8
68.55
88.90
59.33
2.20
13.80
15.10
40.67
97.80
86.20
84.90
Gloss value (%): 600
Gloss value (%): 850
Av.Gloss value (%)
Matt value (%)
237
The dry film samples of FSMWF, SFMWF and RHAMWF are shown in figs., 4.124, 4.125
and 4.126, respectively.
Dry Films of Matt Wood Finishes Produced with Different Extenders
Fig 4.124 Fumed Silica Matt
Wood Finish Film
(Standard) FSMWF
Fig 4.125 Silica Flour Matt
Wood Finish Film
SFMWF
Fig 4.126 RHA Matt Wood
Finish Film RHAMWF
Silica flour-based finish (SFMWF) ranked third from the matt values of 79.07, 68.55 and
86.20,showing a high degree of mattness and a small degree of gloss. FSMWF showed the
highest degree of mattness, due to its smaller particle size of 20MIC and extremely low S.G
(looks like fine, white dust) which will increase the concentration of the FS particles in the
paint. The increased surface area of the particles are all factors responsible for the
superiority of fumed silica to RHA and silica flour in matt effect. Interestingly, however,
despite the larger particle size, RHAMWF ranked a close second to FSMWF implying that
at the same particle size, it would probably be at par with FSMWF in matt effect. The low
bulk density of RHA particles (high volume) compensates partially for the larger particle
size (reduced surface area) compared to fumed silica.
The significantly high cost of the fumed silica (Ν 1750/ kg as at March 2010), makes it
expensive in comparison to the low cost of rice husks. Since there are varying degrees of
mattness, A maximum of 10% on 600 is the specification of CAP Plc.
4.13.7 Settling
The matt wood finish is susceptible to settling when left to stand however, on thorough
mixing the solids are re- dispersed in the solvent. This is attributable to the high solvent and
resin level as well as the higher specific gravity of the flatting agent. The sediment is
usually re-dispersed on stirring. All the four red oxide primers were observed to settle at the
238
bottom when left to stand for 24hrs, however, the settlement was hardest in the case of SF
while fumed silica (the standard) was the least hard due to its smallest particle size. RHA
ranked second to fumed silica.
4.14 Comparison of Properties of Cellulose Matt Finishes (CMF) Produced Using
RHA, Fumed Silica (the standard) and Silica Flour as Flatting Agents
The cellulose matt finish, being a topcoat is applied on metal and wood surfaces after
application of the primer. The level of extender, which was used as the flatting agent in the
cellulose matt finishes, is 3.2%. The standard flatting agent used in the product is fumed
silica (also known as micro silica) which is ultra fine with a particle size of 20 microns.
However, the particle size of the silica flour and RHA used in the production of the
cellulose matt finish was ~32 microns (having been sieved by a standard sieve of 32mic)
because there was no standard sieve of mesh size 20 microns to yield same particle size as
the standard (fumed silica.) .Thus the particle size of the RHA particles and silica flour used
was greater than that of fumed silica.
The result of the comparative qualitative assessment of the cellulose matt finishes
formulated with different extenders as flatting agents are presented hereunder.
4.14.1 In-Can Appearance
The Control as well as all the cellulose matt finishes formulated with the different extenders
were ivory-coloured (magnolia) viscous liquids due to same type and levels of pigments
4.14.2 Wt per Litre
The wt per litre values of the cellulose matt finishes before thinning were 1.55, 1.59, 1.56
and 1.59 for CONCMF, FSCMF, SFCMF AND RHACMF. The Control, which contained
no extender had a slightly lower value due to less solids content while the other extenders
had same wt per litre values due to their similar solids contents
4.14.3 Viscosity (Efflux Time)
The cellulose matt finishes were very viscous after production as their viscosities were
found to be >15 poises i.e greater than the maximum value on the Rotothinner scale.
Consequently, they were thinned down to the required application viscosity of 23secs
239
determined by efflux time measurement, before the other properties such as dispersion level
and gloss were determined.
4. 14.4 Dispersion Level
The cellulose matt finishes all had a dispersion level of <5microns which is the level
required for a satisfactory film appearance. The dispersion level is affected by the particle
size of the pigment and extender in the product and same degree of dispersion was obtained
for all the finishes since the particle size range of the extenders used are the same .
4.14.5 Flatting (Matt) Effect of Cellulose Matt Finishes
The matt effect of the extenders in cellulose matt finishes were visually observed and also
quantified by gloss measurements as presented in Fig. 4.21
i. Visual Observation
On application of the cellulose matt finishes on metal panels, the control was observed to
have the highest gloss due to the absence of extender/ flatting agent in the formulation. The
standard (FSCMF) had the lowest gloss value (and consequently, the highest matt effect),
followed closely by RHACMF and lastly SFCMF as shown in Figs.4.127, 4.128 and 4.129
ii. Gloss Values
The visually observed semi-gloss and semi-matt nature of the finishes was substantiated by
gloss measurements which showed a similar trend from the values recorded as shown in
Fig. 4.21. The Control, (with 0% extender) expectedly, had the highest gloss level of 57.1,
83.9 and 94.3% corresponding to matt values of 42.9, 16.1 and 5.7% for 200, 600 and 850
respectively, confirming a partial mattness and partial gloss of the product. The relatively
high matt level is probably due to the titanium dioxide pigment used in the formulation,
which has excellent hiding power so that despite the absence of extender, the flatting
property was pronounced. The fumed-silica based finish (FSCMF) had the lowest gloss
levels of 1.8, 9.6 and 9.7% corresponding to high matt values of 98.2, 90.4 and 90.3 %, for
200, 600 and 850 respectively, followed closely by RHACMF, which had low gloss values of
5.3, 29.5 and 45.9 %, thus, matt values of 94.7,70.5 and 54.1% for the same respective
geometry.
240
Table 4.21: Gloss and Matt Values of Cellulose Matt Finishes (CMF) Produced
with Different Extenders as Flatting Agents at 3.2% Level
Flatting agent
( Extender)
Control
Silica Flour RHA flour
CONCMF
Fumed
silica (std)
FSCMF
SFCMF
RHACMF
Av.Gloss value (%)
Matt value (%)
600
57.1
42.9
1.8
98.2
13.1
86.9
5.3
94.7
Av. Gloss value (%)
Matt value (%)
850
83.9
16.1
9.6
90.4
48.3
51.7
29.5
70.5
94.3
5.7
9.7
90.3
62.2
37.8
45.9
54.1
Gloss values after 24 hrs:
200
Av. Gloss value (%)
Matt value (%)
The dry films of cellulose matt finishes produced with fumed silica, silica flour and RHA
are shown in Figs. 4.127, 4.128 and 4.129, respectively.
Dry Films of Cellulose Matt Finishes (CMF) Produced With
Different Flatting Agents
Fig 4.127 Fumed Silica
Cellulose Matt Finish Film
(Standard) FSCMF
Fig 4.128 Silica Flour
Cellulose Matt Finish Film,
SFCMF
Fig 4.129 RHA Cellulose
Matt Finish Film
RHACMF
The silica flour-based finish (SFCMF) ranked third with gloss values of 13.1, 48.3 and
62.2% representing matt values of 86.9, 51.7 and 37.8% at 200, 600 and 850. The superior
matt effect of RHA to silica flour is due to its lower S.G, and higher PVC, which produces
a greater number and closer packing of particles than is the case for SFCMF. This causes a
greater degree of light-scattering resulting in the observed superior matt effect These
factors are more pronounced in fumed silica, due to its ultra fine nature. Thus the superior
matt effect of fumed silica is equally displayed in the cellulose matt finish.
241
4.14.6 Film Appearance
The films of the four finishes were completely free of bits. The high dispersion level of
<5microns is an indicator of the fineness of the particles.
4.14.7 Drying Time
The time it took for the cellulose finishes, CONCMF, FSCMF, SFCMF and RHACMF to
dry hard after application on metal panels are 53, 45, 48 and 43 mins, which are within the
range of 1- 1.5hrs specified by SON
4.14.8 Stability
All the samples remained stable in-can, after ten months of production in terms of absence
of any changes in colour and appearance, loss or reduction in viscosity, change in colour,
phase separation .The applied dry films also showed stability over time.
4.15 CONCLUSION
The findings from this research have shown the various colours and colour blends of RHA
obtainable by variation of the incineration temperature and time and thus established the
sets of conditions at which ash of satisfactory ‘whiteness’ can be obtained for paint
production and other industrial purposes. The study also showed the morphological changes
that occur on the surface of the ash particles due to pyrolysis, the ash yields of rice husks at
various combustion conditions and the silica and metallic oxides concentration of RHA at
different combustion temperatures and duration. These findings will undoubtedly, be useful
to other workers and prospective workers carrying out various types of studies on rice
husks and rice husk ash.
The application of RHA in the formulation of different paints has revealed its superior
thickening effect, improved opacity and anti-settling resistance in emulsion and textured
finishes over silica flour and calcium carbonate (the standard extender), its excellent flatting
effect in red oxide primer, surpassing that of calcium carbonate (the standard) but
comparable to fumed silica (standard flatting agent) in matt word finish and cellulose matt
finish. These results have firmly established RHA flour as a good, far cheaper, abundant,
readily available and renewable paint extender that can be used as an alternative to silica
flour (obtained from sand-milling), calcium carbonate, kaolin and fumed silica.
242
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