1.1
INTRODUCTION
The word ‘polymer’ was derived from the classical Greek word ‘poly’ meaning
“many” and ‘meres’ meaning “parts”. If simply stated, polymers are large molecules
formed by the repetition of small and simple chemical units called monomers. Polymers
may be natural, synthetic or semi-synthetic in origin. They are synthesized by the process
called polymerization and if the combining monomers which may be two or more, are
different then the resultant is called copolymer. Lord Todd, President of Royal Society of
London, answered to the question, ‘What do you think has been the chemistry’s biggest
contribution to science, to society?’ “I am inclined to think that the development of
polymerization is, perhaps, the biggest thing chemistry has done, where it has the biggest
effect on everyday life. The world would be totally different place without artificial fibers,
plastics and elastomers. Even in the field of electronics, what would you do without
insulation?, and there you get back to polymers again”1.
The macromolecular concept came into existence about 90 years ago, thereafter
tremendous advancements in terms of fundamental aspects and technological
developments in the field of polymers have taken place. With the inception of polymer
science, a number of relatively simple processes, that permit the translation of
polymerization and polycondensation reactions into large scale industrial applications,
had resulted due to systemic engineering and scientific efforts, over a period of time. Predetermined properties in variable combinations could be incorporated into the end
product with high accuracy by fair understanding of polymerization mechanism,
synthesis and their kinetics. Modification of natural or synthetic polymers with the aim of
imparting specific properties to the product has given a great thrust to macromolecular
science. Properties such as melting point, glass transition temperature, crystallinity,
solubility, elasticity, permeability and chemical reactivity of the existing polymers could
be modified as per the specific requirements by graft copolymerization. It provides a tool
for the fundamental investigation in structure-property relationship in order to meet the
requirements of the specialized applications. This has lead to a new class of materials
called specialty polymer that play an important role in many frontline technological
applications. Polymers for future use include the high volume commodity plastics and
1
specially designed low volume functional specialty polymers. New polymers with
modern techniques of polymerization and characterization have been developed and
elaborated for numerous applications.
Cellulose occurs in abundance in nature, therefore it gains scientific interest for
maximum judicious utilization. On graft copolymerization cellulose offers chemical
resistance, radiation stability and other desired properties. These copolymers possessing
large hydrophilic surface area could be utilized for the separation of metal ions by way of
binding, adsorption, chelation and ion exchange processes. Potential of cellulosics could
further be improved by cross-linking, radiation grafting and surfactant adsorption. High
degree of reactivity and longer stability could be imparted by grafting of suitable
monomers for hydrophilic and hydrophobic properties2-6.
1.2
Concept of Copolymerization
A polymer may be known as homopolymer if it has similar monomeric units,
whereas, if two different monomers are the repeat units, the resultant product is called
copolymer and the reaction may be termed as copolymerization. Reactivity of a particular
monomer decides the sequence in which the different monomers attach themselves onto
the trunk polymer. Common methods to induce grafting are radiation source initiation,
chemical initiation and physical or mechanical treatment (thawing or beating). Grafting
may proceed either through ionic or free radical reaction mechanism.
Since the probability for the production of newer monomers at low cost remains bleak,
so the modification of the exitisting synthetic and natural polymer through graft
copolymerization, by incorporating specific properties through comonomers has gained
much scientific attention. Improvement in dyeing, printing, chemical resistance, water
repellency, fiber strength, abrasion resistance, crease resistance and impact strength of the
thermoplastics and rubber are some of the advantages of the graft copolymerization
2-6
.
Sequencing copolymer exhibit many properties of each constituent homo-polymer.
Copolymers with substantially large polymer sequences of diverse chemical
compositions are designed in such a manner that the desirable properties are retained
while eliminating the rest. Mechanical properties of polymers such as tensile strength,
extensibility and impact-strength exhibit a correlation with the percent grafting (Pg). On
2
grafting crystal lattice of the polymer gets disrupted but the strength of the added material
helps to reinforce the structure7,8. However, if crystallinity does not get distorted on
grafting, then continuous increase in strength could be obtained with the increase in Pg9.
Copolymers can be classified into different types:
1.2.1
Random Copolymer
There lies no definite sequence of distribution of monomer units on the polymer
back-bone. The monomer units are randomly placed along the polymeric chain and could
be examplified as:
-A1-A2-A1-A1-A1-A2-A1-A2-A1-A2-A2-A1-A1-A1-A11.2.2
Alternate Copolymer
In these copolymers, monomers are placed orderly forming a polymeric chain and
having a perfect alternate arrangement of monomer units, for example, poly (styrene-altmaleic anhydride) where styrene (A1) and maleic anhydride (A2) are copolymerized in a
regular fashion to give an alternate sequence as shown below:
-A1-A2-A1-A2-A1-A2-A1-A2-A1-A2-A1-A2-A1-A21.2.3
Block Copolymers
They are composed of chemically dissimilar extended lengths or blocks or segments
terminally connected to one another. They could be examplified as:
-A1-A1-A1-A2 -A2 -A2-A1-A1-A1-A2-A2-A21.2.4
Graft Copolymers
Graft copolymers involve a preformed polymeric back-bone to which other
polymeric chains of different chemical nature get attached at several points. Stannett10
defined graft copolymers as: “Graft copolymers consist of a polymer back-bone with
lateral covalently linked side chains.” Both the back-bone and the side chain polymers
3
could be homopolymers or copolymers. Graft copolymers are made of molecules with
one or more block of species having different configurational features connected to main
chain as side chains11. Alfrey and Bandel12 were the first to synthesize the graft
copolymers in 1950. They polymerized vinyl acetate (VA) in presence of styrene (Sty)
and vinylidene chloride (ViCl). The resulting graft copolymer assumed its shape
depending upon the structural format of the back-bone polymer. The generalized view of
graft copolymer has been shown below, where the monomer (A2) has been incorporated
onto the back-bone polymer, of building units (A1).
------A1-A1-A1-A1-A1-A1-A1----│
│
│
(A2)n (A2)n (A2)n
Monomer (A2) could be grafted as a unitary system or as a binary mixture with other
monomers. Although, random and alternate copolymers show better properties than the
constituent homopolymers yet they are not found suitable for their applications as
specialty polymers. On the other hand, both block and graft copolymers have been found
to show many technologically advanced characteristics.
1.3
Grafting methods
Induction of monomeric polymerization, if carried-out by an active site generated
along a polymer back-bone, it is also called as ‘grafting from’ technique that finally ends
in a graft copolymer:
A2
^^^^A1 ^^^^A1 *
^^^^^^A1
│
A2*
Whereas, if there has been an attachment of the growing chain (A2) onto polymer
back bone (A1), thereby, resulting in a graft copolymer, it may be named as ‘grafting
onto’ technique:
^^^^^^A1 ^^^^^^A1
*
4
A2* + nA2 (A2)*n+1
^^^^^^A1 + (A2)*n+1 ^^^^A1
*
│
(A2)n+1
Grafting could be achieved through free radical, anionic or cationic mechanism13,14 .
1.4
Synthesis of Graft copolymers
Since last decades, numerous methods have been explored for the preparation of graft
copolymers by conventional chemical techniques15-17. The active sites are created on the
polymeric back-bone for the synthesis of graft copolymers, either a free-radical or a
chemical group that may get involved in an ionic polymerization or in a condensation
process. Ionic polymerization has to be carried-out in presence of anhydrous medium or
in presence of considerable quantity of alkali metal hydroxide. Usually, low molecular
weight graft copolymers are obtained in ionic grafting, while in case of free radical
grafting high molecular weight copolymers could be prepared 15-17. A number of methods
are used for the generation of active sites on the polymeric back-bone and are broadly
classified as:
1.4.1 Physical Methods
Active sites could be generated on the parent polymer by swelling it with suitable
solvent that ruptures the bond leading to the formation of free radicals. Williams and
Stannett18 utilized freezing and thawing of polymer-monomer mixture to effect the
grafting of vinyl monomers onto wool. Preparation of polymers with basic pendent
moieties were carried-out by grafting dimethyl amino-ethylmethacrylate onto linear low
density polyethylene (LDPE)19.
1.4.2
Chemical Methods
Conventional radical initiators20,
21
, benzoic peroxide (BPO)22, ceric ammonium
nitrate (CAN)23-25, potassium persulphate (KPS)26,27, Fenton’s reagent (FAS-H2O2)28,29,
potassium permanganate (KMnO4)30, β-di ketones of transition metal alone31-33 and in
5
conjugation with additives34,35 , Lewis acids36, strong bases37 and metal carbonyl38, have
been successfully used for the chemical grafting. Misra et al. have successfully grafted a
wide variety of monomers onto wool39-43, cellulose31,33, 44, starch45and gelatin46 by use of
the radical initiators.
1.4.3
Physico-Mechanical Methods
In this method, the application of stress on parent back-bone caused segmental
motions and molecular flow, that leads to bond-scission and consequent formation of
active sites on the back-bone. Free-radicals may also be generated by mastication and
milling of back-bone polymers47.
1.4.4
Radiation Methods
The arena of graft copolymerization has been completely revolutionized by the
radiation grafting of vinyl monomers onto the polymeric back-bone. When the
electromagnetic radiations were passed through the reactants, there was a decrease in the
intensity of the incident ray while the absorbed radiations created active sites on the
polymer back-bone. Generally, UV and high-energy γ-radiations have been employed for
generating the active sites where grafting of appropriate monomer takes place. In addition,
low cost regulation over structural factors, such as the number and length of the grafted
chain, has been done by careful selection of dose and dose rate47, 48 .
1.4.5
Plasma Radiation Induced Grafting
Plasma radiation induced grafting has gained much scientific attentions in the recent
years. Plasma conditions attained through slow discharge offered the same possibilities as
with ionizing radiation49. Electron induced excitation, ionization and dissociation are
some of the main processes. High energy accelerated electrons are the motivating agents
that induce cleavage of chemical bonds in polymeric structure to form active sites.
1.4.6
Enzymatic Grafting
In this method, an enzyme initiates the chemical or electro chemical grafting
reaction50, for example, phenol was converted to o-quinone by tyrosinase, that underwent
6
subsequent non-enzymatic reaction with chitosan. Cosnier et al.51 reported the enzymatic
grafting on poly(dicarbazole-N-hydroxy-succinimide) film, thionine and toluidine blue
have been inversely bound to the poly(dicarbazole) back-bone and grafting of polyphenol
oxidase (PPO) on poly(dicarbazole) are some of the examples.
1.5 Factors affecting Graft copolymerization
The percentage of grafting and grafting efficiency are usually affected by the nature
of the monomer, concentration, reaction time, reaction temperature, reaction medium,
additives and structure of the back-bone. Some of these factors affecting the graft yield
have been discussed underneath:
1.5.1 Role of Inorganic ions on Grafting
The free radical formation gets induced by the addition of some additives that
increase the graft yield. In case of cellulose, Cu2+ ions help in complex formation52.
Only a small concentration of inorganic ions is needed to enhance the graft yield,
whereas the higher concentration may adversely affect it. Huglin and Johnson53 reported
the effect of Cu2+, Fe2+, Fe3+ ions on radiation induced grafting in case of nylon. The
reactivity order observed was Cu2+> Fe2+> Fe3+. Grafting efficiency of polar monomers
was increased by facilitating monomer access on reaction site, by decreasing the
crystallinity of cellulose and by the activation of the monomer through donor or acceptor
complex formation with monomers after the addition of ZnCl2 salt.
Different transition metals such as Zn, Fe, V, Co, Cr and Al have been used in the
preparation of metal acetyl acetonates and other diketones. Metal acetyl acetonate could
be used as an initiator for the vinyl polymerization. Metal chelates generally decompose
on heating with the generation of free radicals that abstract hydrogen atoms from the
polymeric back bone producing active sites.
Metal chelates are better initiators as compared to other redox systems. Since the
reaction could be carried-out even at low temperatures, so the undesired reactions due to
chain transfer, homo-polymerization could be avoided54.
7
1.5.2 Effect of Acid in Graft Copolymerization
The strength of the acid used in the process has a great impact on the magnitude of
graft yield. In radiation induced grafting of cellulose, following changes are supposed to
occur55:
Cell-OH + H+ CellO+ H2
(I)
where, (I) could capture an electron to give (II).
CellO+ H2 + e- (Cell-OH)* + H*
(I)
(II)
Structure (II) decomposes into different radical species, that offer sites for the attachment
of graft chains.
(Cell-OH)* Cell-O* + H*
Moreover, Cell and OH* may also be formed depending on the dissociation point (II).
H2SO4 has been reported to be the best acid amongst the series of acids to enhance the
graft yield.
When ceric ammonium nitrate (CAN) was used as a source of ceric ions, the presence
of nitric acid has been found to play a significant role 56. In the absence of nitric acid no
grafting was observed because of the large size ceric ions which were unable to form
complex with the polymer back-bone. However, an increase in concentration of nitric
acid resulted in the formation of complexes with functional groups on the polymer backbone. The complex later decomposes to give free radicals on the back-bone where the
grafting occurs56.
1.5.3 Nature of the Substrate
The rate of grafting has been found to increase with the increase in the surface area
of the backbone. Therefore, the nature of the substrate as fiber, film or powder form plays
an important role. Stannett10 reported that grafting of styrene onto cellulose acetate film
decreased with increase in thickness of the film. Moreover, in case of cellulose, lignins
adversely effect graft yield57. Polymer analogous reactions of cellulose also affect
grafting efficiency58 and affect the properties like swelling behavior59, redox nature or the
8
complex forming ability60. Sometimes the groups introduced on the polymer back-bone
decrease the graft yield due to increase in hydrophobicity, that blocks the diffusion of
monomer and the initiator to the active sites.
1.5.4
Type of monomer and its concentration
Presence of aromatic rings, monomer polarity and the length of alkyl chain are
among the structural factors that affect the graft yield and graft efficiency. Decrease in
grafting efficiency has been reported as a function of steric and polar effects by Verma et
al61. They had reported the decreasing reactivity order of various acrylates as
methylacrylate (MA) > ethylacrylate (EA) > butylacrylates (BuA) > methylmethacrylate
(MMA). Moreover, in case of simultaneous irradiation method, if monomer has high
sensitivity to radiation than back-bone polymer, homo-polymerization is favored instead
of graft copolymerization62 . A higher concentration of monomer may result in increased
polymerization but at the same time graft copolymerization may not be the preferred
process. In case of a redox reaction, graft copolymerization always starts from complex
formation between monomer and the substrate that depends upon the concentration of
monomer52.
1.5.5
Effect of Initiator Concentration
Initiator concentration plays an important role to determine the graft yield. Generally
at a molar ratio of 1:6 of FAS : H2O2 maximum grafting was observed. However, further
increase in the molar ratio deteriorated the graft yield. This could be due to the fact that at
higher concentration more Fe3+ ions are produced that served as chain terminator63, 64.
Flax-(M)*n+1 + Fe3+ Flax-(M)n+1 + Fe2+
The amount of initiator, monomer and the back bone polymer are the major factors
responsible to determine the rate of grafting65. There are various empirical relationships
governing the dependence of grafting efficiency on the initiator concentration66. It has
been observed that once a certain initiator concentration is achieved, higher concentration
of initiator do not further effect the graft yield67.
9
Solubility of the initiator in the grafting medium shows a profound impact on the
yield. Ideally, the initiator should be highly soluble in grafting medium in order to initiate
the grafting through monomers. Nakamura et al.68 studied the grafting of the styrene,
vinyl acetate, MMA and methacrylic acid on sericin (obtained by scouring silk fiber). It
has been observed that water-soluble initiators were superior than water insoluble
initiators to obtain graft copolymers having homogenous molecular weight distribution
for the grafting methacrylic acid to the sericin.
1.5.6
Effect of Reaction Temperature and Reaction Time
The graft yield gets enhanced with the increase in reaction time and temperature.
After reaching the optimum value, further increase in either, results in the decline of
percentage grafting. It occurs due to the increase in various hydrogen–abstraction
reactions and high viscosity of the reaction medium, due to homopolymerization. High
viscosity of the medium creates hindrance in the path of free radicals to reach the active
sites on the polymer back-bone. Moreover, a further increase in temperature, results in
the dissolution of water soluble components of the fiber69 thereby, affecting the overall
graft yield.
1.5.7
Effect of the pH
The pH of the reaction medium plays a key role during the grafting process.
Kaith et al. 63, 69 observed maximum graft yield at pH 7.0 and further increase in pH
resulted in decreased graft yield. It could be due to decrease in the concentration of the
free radicals on back-bone polymers as *OH ions served as the chain terminator:
Flax-(M)n+1* + OH* Flax-(M)n+1-OH
1.6
Effect of Grafting on the properties of the back-bone
The properties of the back-bone appreciably changes on grafting with vinyl
monomers. As grafting involves covalent attachment of a monomer to a pre-formed
polymeric back-bone, the physical nature and the chemical compositions play an
important role in the process70.
10
Grafting modifies the back-bone and makes it amorphous by the incorporation of
monomer units. Stanett et al.71 reviewed the properties of graft copolymers and reported
that grafting of hydrophilic monomers lead to increase in wettability, adhesion, dying and
rate of release of oil stains. On the other hand, hydrophobic monomers affect the
properties adversely. Grafting of acrylic acid (AA)72, methacrylic acid (MAA)73 and
vinyl acetate (VA)74 onto the polymer back-bone increased water absorbance. However,
grafting of styrene75 increased the water repellence and so did the grafting of isoprene76,
acrylonitrile and vinylidene chloride (ViCl)77.
Cellulose graft copolymers of styrene, acrylates and methyl methacrylate have been
reported to be resistant to acids78. Monomers containing phosphorous, chloride and
nitrogen as n-Vinyl pyrolidone79 impart flame resistance to cellulosics. Dyeing properties
to cellulose was imparted by graft copolymerization of styrene80, acrylamide (AAm)81,
glycidyl methacrylate (GMA)82, acrylic acid or methacrylic acid83 and allyl acrylate80.
1.7 Graft copolymerization onto lingo-cellulosics
Since the chapter deals with the grafting of vinyl monomers onto cellulosic fibers, so
it was pertinent to discuss in brief the chemistry of cellulose and review of literature
related to the modification of cellulose through graft copolymerization.
Cellulose has been the most abundant form of terrestrial biomass85. It finds
applications in many spheres of modern industry. Cellulose, especially cotton linters, has
immense use in the manufacture of nitro cellulose, used in smokeless gunpowder.
Cellulose could be processed to make cellophane and rayon and other textile derivatives.
It has also been used in the laboratory as a solid substrate for thin layer chromatography
and a major constituent of paper. In the recent years, many attempts have been made to
modify cellulose by graft copolymerization of vinyl monomers that help in the
incorporation of desirable properties without drastically affecting its inherent traits. The
formation of graft copolymers with long polymeric sequences of diverse chemical
composition opens the ways to afford specialty polymeric materials 84, 85.
11
1.7.1
Cellulose chemistry
The existence of cellulose, as a plant cell wall material was first recognized by
Anselm Payen in 183886. It occurs in pure form in cotton fiber and in combination with
other materials, such as lignin and hemicellulose in wood, plant leaves and stalks.
Cellulose exists as a natural polymer, a long chain made by the linking of smaller
molecules. The links in the cellulose chain are a type of sugar, β-D-glucose87. The sugar
units are linked when water is eliminated by combining the –OH group and hydrogen linking just two of these sugars produce a disaccharide entity called cellobiose88.
It took decades for the identification of cellulose by Payen. It was found to have a
long chain polymer with repeating units of D-glucose, a monosaccharide. In the cellulose
chain, the glucose units are in 6-membered rings, called pyranose. They are joined by
single oxygen atom (acetal linkage) between the C-1 of the one pyranose ring and the C-4
of the next ring. Since a molecule of water gets liberated, when an alcohol and hemiacetal
reacts to form an acetal, the glucose units in the cellulose polymer are referred to as
anhydro-glucose units.
OH
OH
O
O
1
HO
O
4
O
OH
OH
O
OH
O
OH
O
m
HO
OH
where, m =2,000 - 26,000
1,4 beta acetal
The stereochemistry of these acetal linkages is such that the pyranose rings of the
cellulose molecule have all the groups larger than the hydrogen stretching-out from the
periphery of the rings (equatorial positions). The arrangement of the carbons at 2, 3, 4
and 5 of the glucose molecules are fixed; but when glucose forms a pyranose ring, the
hydroxyl at C-4 may approach the carbonyl at C-1 from either side, resulting in two
different stereochemistry at C-1. When the hydroxyl group at C-1 is on the same side of
the ring as the C-6 carbon, it is said to be in the α -configuration. In cellulose, the C-1
oxygen is in the opposite or β-configuration (cellulose is poly [β-1,4-D-
12
anhydroglucopyranose]). This β-configuration, with all functional groups in equatorial
positions, cause the molecular chain of the cellulose to extend in a more or less straight
line and a good fibrous polymer is formed.
The equatorial positions of the hydroxyl groups on the cellulose chain, protrude
laterally along the extended molecule that makes them readily available for hydrogen
bonding. The strong inter-chain hydrogen bonds in the crystalline regions increase its
strength and insolubility in most solvents. In the less ordered regions, the chains are
further apart and more available for the hydrogen bonding with other molecules, such as
water. Thus, cellulose swells but does not dissolve in water.
In cellulose, the component that does not dissolve in 17.5% solution of sodium
hydroxide at 20oC consists of Alpha cellulose, true cellulose. The portion that dissolves
and then precipitates upon acidification signifies Beta cellulose and the part that dissolves
but does not precipitate upon acidification remains as Gamma cellulose. Cellulose has
very high melting point and in fact, decomposes before beginning to melt. A number of
different thermal degradation reactions are known to occur within cellulose at different
temperatures. Recent work89 showed that there has been a continuous interest in the
thermo analysis of cellulose. Acidic degradation90 of cellulose gives D-glucose and
fermentative production of ethyl alcohol. In alkaline degradation, cellulose is subjected to
β-elimination to produce degradation products91.
The cellulose molecule contains three different kinds of anhydroglucose units
However, unlike simple alcohols, cellulose reactions are usually controlled by the steric
factors than would be expected on the basis of the inherent reactivity of the hydroxyl
groups. C-2, C-3 and C-6 hydroxyls and C-H groups are active sites in cellulose for the
incorporation of polymeric chains through grafting. In grafting it has been reported that
the reactivity of the hydroxyl group at C-6 was far less than those at C-2 and C-3.
1.9
Literature review
Graft copolymerization is a suitable technique that has all the modification of
cellulose. Several workers have stimulated the graft copolymerization of a variety of
vinyl monomers onto cellulose92,93. Chemically induced grafting of itaconic acid onto
cellulose fiber has been studied by Sabaa and Mokhtar94 . Roman-Aguirre et al.95 reported
13
the grafting of MMA onto wood-fiber in dispersed media. SEM showed the presence of
small agglomerate regularity distributed on the surface of the modified fiber and IR
analysis of the acid hydrolysis products of the modified fiber showed a clear
characteristic signal of carbonyl group at 1734 cm-1. AAm has been grafted onto fully
bleached kraft pulp (BKP) and thermo mechanical pulp (TMP) fiber via dielectricdischarge treatment at various treatment dosages. The increased dielectric barrier
discharge treatment leads to the increased polymerization and incorporation of AAm onto
fiber surfaces. Incorporation of poly(AAm) occurred on the BKP fibers at a higher rate
than the TMP under similar conditions96. Mishra and Tripathy97 reported the grafting of
vinyl monomers onto natural fibers using peroxy diphosphate–tartaric acid redox system.
The effect of the reaction conditions on the graft copolymerization of MA onto ethyleneoxy-cellulose has been studied by Kislenko and Berlin98. IR spectroscopy confirmed the
formation of graft copolymers and it was concluded that free radicals played an important
role in the process of grafting. Graft copolymers were synthesized by irradiating original
cellulose and hydroxyethyl cellulose, both in film-form, in presence of styrene and were
analyzed by UV and IR spectroscopy99. Bayazeed et al.100 investigated the graft
copolymeristaion of methacrylic acid (MAA) onto cotton using thiocarbonate persulphate
redox system under a variety of conditions. The graft yield was significantly favored in
acidic media as well as by increasing reaction time, persulphate and MAA concentration.
Effect of different parameters on polymer loading, polymerization efficiency as well
percent grafting has been investigated by El-Meadawy and El-Ashmawy101 in order to
optimize the grafting of AAm onto wood pulp by xanthate method. The different factors
affecting the graft copolymerization of AAm onto methylcellulose back-bone by using
potassium persulphate as an initiator in aqueous solution were studied by Bardhan et
al.102 and found that grafting increased steadily with increase in monomer / polymer ratio
at a temperature upto 40 oC. Graft copolymerization of MMA onto cellulose pulp was
found to be dependent on the parameters like initiator and monomer concentrations,
temperature and reaction time103. Grafting of vinyl monomers onto cellulose in the
presence of sodium sulphate-sodalime glass as an initiator has been reported by Mansour
and Nagaty104. With the sodium suphate-sodalime glass system, the reactivity decreased
in the order: MMA > EA >AN, which is inconsistence with the arrangement of the
14
acceptor monomer with decreasing electron–donating ability. Mansour et al.105 changed
the structure of cellulose by treating with different concentration of alkali, using 5-30%
NaOH. Reactivity of cellulose towards graft copolymerization has been lowered due to
decreased crystallinity. Low graft yield and efficiency were obtained on using 15-25%
NaOH. Biermann and Narayan106 reported a new synthetic route to cellulose graft
copolymerization by nucleophillic displacement of mesylate groups from mesylcellulose
acetate by the polystyrylcaboxylate anion. This method of grafting overcomes the
drawback of radical polymerization method and allows precise control on the molecular
weight distribution in grafted side chains, higher degree of substitution on the cellulose
back-bone, the number and nature of grafted side chains and overall, better control with
high reproducibility in the graft yield. Graft copolymerization of VA-MA mixture onto
cellulose using ceric ion as an initiator was examined by Fernandez et al107. They studied
the effect of addition of NaNO3 and NaCl on graft copolymerization and observed that
NaNO3 enhanced the cellulose oxidation and leaves the homo-polymerization reactions
almost unaffected. NaCl was also found to reacts in similar manner. Chauhan et a1.108
graft copolymerized binary mixture of styrene and maleic anhydride onto cellulose,
extracted from Pinus roxburghii needles. The grafting reaction was initiated by gamma
rays in air by simultaneous irradiation method. Grafting parameters and reaction rate
were optimized with styrene and maleic anhydride in 1:1 ratio. Thermal behavior of graft
copolymers showed that they exhibited single stage decomposition with characteristic
transitions at 161-165oC and 290-300oC. Graft copolymerization of MA, EA and Ethyl
methacrylate on carboxymethyl cellulose by the use of ceric ion in aqueous medium at
35oC has been carried-out by Okiemen and Ogbeifun109. Thermal stability of the textiles
has been found to increase in acrylic acid grafted polymers110. Karlsson and
Gatenholm111 had prepared cellulose fiber-supported hydrogels of HEMA using ozoneinduced graft copolymerization. Abdel- Razik112 reported the homogenous grafting of
dichloro dimethyl silane onto ethyl cellulose. Grafting of some acrylates and
methacrylates onto cellulose and silk, in the absence of radical initiator was studied. It
has been found that polymerization of methacrylates proceeded more easily than other
acrylates and graft yield was much higher in silk than cellulose113. 1, 1 dihydro
15
perfluorobutylacrylate was graft copolymerized onto cellulose fabric by using γ-rays
irradiation to produce oil and water repellence in cellulosic fabric114.
Graft copolymerization of acrylic monomers onto cotton fabric using an activated
cellulose thiocarbonate–azo-bis-isobutyronitrile redox system has been investigated by
Zahran et al115. It has been found that a reaction medium of pH 2.0 and temperature of
70oC constituted the optimum conditions for grafting. MMA has been found to be
superior than other monomers. Chemically modified cellulosic depicts higher grafting
with MMA as compared to unmodified cellulose. Oxidized cellulose was grafted with
2-methyl-5-vinyl pyridine using a H2O2-thioureadioxide redox system. Considerable
grafting took place on oxidized cellulose on excluding thio-ureadioxide from the system
whereas it offsets grafting onto cotton cellulose116. Lenka et al.117 studied the grafting of
MMA onto modified cellulose at 60 oC using peroxydiphosphate as an initiator. They
determined the rate of grafting by varying initiator, monomer, nature of the substrate and
temperature. MMA was graft copolymerized onto cotton-cellulose using hexavalent
chromium, Cr(VI) as an initiator. Graft copolymers were subjected to various chemical,
mechanical and thermal evaluation and the results showed that there has been a
significant improvement in properties117. Graft copolymerization of acrylonitrile onto
wheat straw using FAS- H2O2 as an initiator has been reported118.
Graft copolymerization of Styrene onto cellulose was investigated under variable
reaction conditions using H2O2 as an initiator. Cellulose carbamates having less than
1.1% nitrogen showed lower graft yield than the unmodified cellulose119. Huang et al.120
graft copolymerized styrene onto cellulose in the form of cotton linters, cotton cloth and
rayon by ionizing radiation. The graft copolymers thus obtained were characterized by
removing the cellulose back-bone on hydrolysis and determining the molecular weights
of the residual poly(styrene). Molecular weights of the grafted poly(styrene) were found
to be considerably higher than the homopolymer polystyrene or the polystyrene formed
by irradiating styrene in bulk. Radiation induced graft copolymerization of AAm, styrene
and a mixture of AAm and styrene onto cellulose acetate films was investigated by
Bhatacharyya and Maldas121. They studied the permeability of sodium chloride and
sodium sulphate through grafted and ungrafted acetate films. They observed that
permeability of sodium chloride increased with increase in the extent of grafted styrene
16
onto cellulose acetate. Similar trend in permeability has been observed when cellulose
acetate was grafted by binary monomer mixtures. Grafting of MMA onto polypropylene
in the presence of additives using UV and ionizing radiation were reported. Charge
transfer monomer complexes used as additives have been directly grafted onto cellulose
through radiations122. Abdel–Razik123 investigated the graft copolymerization of
polystyrene macro-monomers onto ethyl cellulose homogenously using UV irradiations
in the range of 310-460 nm. Hongfei et al.124 studied the radiation grafting of styrene
onto cotton cellulose w.r.t. molecular weight (Mn), molecular weight distribution (Mw/Mn)
of the grafted side chain and reaction kinetics. Radiation induced grafting by mixture of
styrene and n-butylacrylate in various compositions onto cellulose and cellulose triacetate
fibers at 0 oC in air was reported by Matsuzaki et al125. It has been observed that
monomer reactivity ratios of the graft copolymers were different from those of the ungrafted copolymer or those of AIBN-initiated copolymers. Graft copolymerization of
styrene onto cotton fabric by direct radiation method has been studied by Dessouki et
al126.
Graft copolymerization of vinyl monomers onto allylated, alkali treated and
untreated cotton using tetravalent cerium as an initiator was investigated. The alkali
treated cotton showed higher grafting than allylated and untreated cotton during the initial
stages of reaction while reverse trend has been obtained for the later stages127. Iwakura et
al.128 studied the graft copolymerization of styrene onto cellulose using ceric ion. It was
found that the graft copolymers were mixture of block and graft copolymers. MA was
graft copolymerized onto cellulose in the alkaline solution by using potassium
ditelluratoargentate (III) redox system. The evidence of grafting was obtained from IR
spectra and gravimetric analysis. Thermal stability, crystallinity and morphological
changes of the graft copolymers were focused129. Narita et al.130 studied graft
copolymerization of styrene onto microcrystalline cellulose initiated by ceric ion in
acrylonitrile and water solution. AAm was graft copolymerized onto cellulose acetate
using ceric ion. The order of reactivity and activation energy with respect to monomer
and initiator concentration were determined131. AAm/MA copolymers were graft
copolymerized onto cellulose using ceric ammonium nitrate as an initiator in the presence
of nitric acid. The reactivity ratio of AAm and MA were determined by Mayo and Lewis
17
method that were found to be 0.65 and 1.07, respectively. An alternate arrangement of
copolymer blocks in the graft polymer chain was proposed because the product of
reactivity ratio was less than unity132. Vinyl acetate (VA), methyl acrylate (MA) and VAMA mixture was graft copolymerized onto cotton cellulose by ceric ion initiator system.
The structural changes introduced in amorphous and crystalline regions of the fiber
through grafting were studied by X-ray diffraction. It was found that the degree of
crystallinity varied significantly between different samples of cotton. Crystallinity of the
cellulose in the grafted fibers decreased with increase in grafting133. Ranby and
Sundstrom134 studied the grafting of AN onto native cellulosic substrate using Mn2+ ion
as an initiator that resulted in 80 - 90% grafting efficiency at 30 oC for 2-3 hours in N2
atmosphere.
Agave lechuguilla and fourcroydes fiber on grafting were thermally more stable135.
Naguaib136 grafted itaconic acid onto sisal fibers using potassium persulphate as an
initiator. IR spectroscopy and SEM provided the evidence for grafting. TGA, tensile
strength of the grafted and ungrafted samples were investigated. Graft copolymerization
of AAm onto oil palm empty fruit bunch (OPEFB) fiber using H2O2 as an initiator and
MA as a co-monomer was studied. MA facilitated the incorporation of AAm monomer
onto OPEFB fiber. Fineman Ross plot was used for the determination of the reactivity
ratio of both monomers137. MMA and EA were graft copolymerized onto cellulosic chain
of textile with oxidized sites. Mechanical strength, thermal stability and resistance to
chemical and biological agents of cellulose based textile were improved upon grafting
with acrylic monomers138. Grafting of ethyl acrylate has been found to enhance thermal
stability of cellulose139.
Oven and lyophilized products show almost identical values of graft yields. SEM
showed some morphological differences between both type of products that led to
different physical properties140. Halab-Kessira and Ricard141 used the trial and error
method for the optimization of graft copolymerization of the cationic monomer onto
cellulose for graft copolymerization in order to establish the conditions for the synthesis
of cellulose graft cationic copolymers with less than 10% grafting.
Monomer molecules penetrate deep into the amorphous regions rather than the
ordered crystalline part. So, the mechanical properties like elongation at break, rigidity,
18
tensile strength and flexibility were not changed to a great extent. The crystallinity of the
cellulose content in the grafted fibers decreased with increase in grafting of VA and MA
onto cellulose by ceric ion system133. Kaizerman et al.142 investigated the mechanical
properties of rayon grafted with AN and its tensile strength was found to increase. The
mechanical properties of the continuous cellulose filaments did not change much with
polyacrylic acid grafting143. Mechanical properties were found to improve on grafting of
styrene onto cellulosic fiber under certain reaction conditions135. Moreover, grafting of
acrylic monomers onto cellulose-based textile has improved the mechanical strength138.
1.10
Composite
The term ‘composite’ has been used in material science refers to a material made up
of a matrix containing reinforcing agents. The beginning of composite materials may
have been the bricks, fashioned by the ancient Egyptians from mud and straw. Nearly 70
years ago, a number of technical products and other commodity materials were derived
from natural resources e.g., textile ropes, canvas and paper were made of local natural
fibers such as flax and hemp. Emergence of polymers in the beginning of the nineteenth
century inculcated the new era of research based on exploring the viability of natural
fibers and their applications in more diversified fields. At the same time, interest in
synthetic fibers due to its superior dimensional properties, gained attention and slowly
replaced the natural fibers in major avenues. With the passage of time, the accumulation
of the hazardous synthetic byproducts and waste, started polluting the environment and
once again led the scientists towards natural fibers due to their distinct advantages. Thus,
the renewed interest in the natural fibers resulted in a large number of modifications in
order to bring it equivalent and even superior to synthetic fibers. After tremendous
changes in the quality of natural fibers, they emerged as a substitute for the traditional
building materials including lumber steel, portland cement and lime. Considering the
high performance standard of composite materials in terms of durability, maintenance
and cost effectiveness, applications of natural fiber reinforced composites as construction
material, have done wonders and are environment friendly material for the future.
Many shortcomings due to high density and poor recycling properties were seen in
glass fiber reinforced plastics. Moreover, glass fiber dust produced during processing
19
triggers allergic skin irritation. The possible substitution of glass fiber by natural fiber in
exterior application raised the question about mechanical properties of the material,
flammability and effect of weathering. Natural fibers offer several advantages over glass
fibers:
(i)
Plant fibers are renewable and their availability is unlimited.
(ii) When natural fiber reinforced plastics are subjected to combustion or landfill at
the end of their life cycle, the released amount of carbon dioxide is less with
respect to that assimilated during its life cycle.
(iii) Natural fibers are less abrasive and can be easily processed as compared to glass
fiber.
(iv) Natural fiber reinforced plastic, consisting of biodegradable polymer matrix
are environment friendly and can be composted easily.
1.11 Review on Natural Fiber reinforced Composites
Natural fibers are being used by the mankind for centuries together for various
applications in order to meet the basic requirement of shelter, food and clothes. In most
of the countries, people have explored the possibilities of using natural fibers such as
cotton stark, rice-husk, rice-straw, baggase, cereals-straw and kenaf for a wide range of
domestic and industrial applications. Most of these fibers have primarily been used for of
particle boards. Work on reinforcement of composite with barley stalk has been reported
by Bouhicha et al144. Importance of indigenous natural fibers in preparation of
composites over synthetic fibers has been reported by Paramasivam and Kalam145. Lee et
al.146 developed the novel short silk reinforced polybutylene succinate bio-composites by
compression molding method and observed that there was improvement in tensile and
flexural properties. Joseph et al.147 fabricated composites reinforced with banana fibers by
varying fiber length and fiber loading. The analysis of textile, flexural and impact
properties of these composites revealed that the optimum length and type of banana fiber
was different in phenol-formaldehyde resole material. Interlocking between the banana
fiber and phenol formaldehyde resins was much higher than that between glass and
phenol formaldehyde resin.
20
Askargorta et al.148 studied the wetting behavior of flax fiber and possible
replacement of polypropylene by flax fiber as reinforcement for the preparation of
composites. Escamilla et al.149 observed that grafting of polymethyl methacrylate or
polybutylacrylate on the cellulosic fiber resulted in lower mechanical strength than those
of ungrafted cellulosic fibers on their use in composites as reinforcing agent. Donnell et
al.150 made use of vacuum infusion process for the preparation of composite panel out of
plant oil based resin and natural fibers. They used recycled paper as cheap source of
cellulose fiber and used mats of flax, cellulose pulp and hemp as reinforcement. These
low cost natural composites were found to possess mechanical strength and other
properties suitable for the application in house construction materials, furniture and
automotive parts. Cement paste and mortar reinforced with different lengths of jute fiber
were studied for direct tension, flexural, axial compression and impact strength. Jute
fibers could be used to develop low cost construction materials particularly for roofing,
wall panels and other building materials151. Reinforcement of cement mortar with about
80% sisal pulp by mass provided 50-60 fold increase in fracture toughness over the neat
matrix152. Use of lignocellulosic fibers as reinforcement in thermoplastic and
thermosetting resin for developing the low cost and light weight composites was
reviewed by Mishra et al153. Physico-chemico-mechanical properties of plant cell wall as
fibrous lignocellulosic composites were studied in relation to shrinkage and water
swelling behavior154. Khanbashi et al.155 studied the potential of using date palm fibers as
reinforcement in polymeric materials. An empirical formula to predict the ultimate
strength of fiber reinforced concrete was given by Shanmugam and Swaddiwudhipong156.
Feasibility of low cost yet strong engineering materials from natural sources was
explored by Castino et al.157 for preparing the laminates of urea formaldehyde reinforced
with natural fibers. Satyanarayana et al.158 found new uses of natural fibers as
reinforcement for polyester based composites. Performance of these composites was
evaluated after exposure to indoor and outdoor weathering by both destructive and non
non-destructive testing methods. Dweib et al.159 developed natural composites roof for
housing facility, structural panels and unit beams out of soybean oil based resin using
vacuum assisted resin transfer molding technology. These composites as building
construction materials introduced many advantages such as high strength and stiffness
21
with weight, susceptibility to severe weather conditions, desired ductility, fatigue
resistance and design flexibility. McMullen160 reported the development of composites
consisting of gelatin and starch reinforced with cellulose fibers for use in aircraft
structures. Composites consisting of short wheat structure fibers with better resin
penetration and fiber wetting have been found to depict high strength. The flexural
strength increased by 40% on butyrated lignin adhesion161. In case of flax fiber loaded
with approximately 8 to 10% by mass, flexural strength increased that was comparable to
Pinus radiata fiber reinforced cement mortars. However, the fracture toughness values
were approximately half to that of the P. radiata composites. Hence, flax fibers were not
effective for asbestos fiber replacement as are P. radiata fiber162. Rousion et al.163
manufactured hemp/kenaf fiber-unsaturated polyester composites using resin transfer
molding process. Their tensile and flexural strength were analyzed. Reinforcement of
cement mortar with bamboo mesh imparted considerable ductility, toughness as well as
its tensile, flexural and impact strength.164 Maffezzoli et al.165 formulated cardanol matrix
based bio-composites reinforced with natural fiber. Bledzki et al.166 improved the
properties of epoxy resins and polypropylene composites through reinforcement with
mercerized flax and hemp fibers. Mechanical properties of the fiber were monitored by
focusing on mercerization parameters. Triglyceride oils derived from plants were used in
the preparation of polymers with a wide range of physical properties and their
reinforcement with 20% hemp fibers displayed a tensile strength of 35 MPa. In case of
reinforcement with flax fiber, tensile and flexural strengths were found to be in the range
between 20-30 and 45-65 MPa, respectively. However, hybrid composites like
reinforcement with both glass and natural fibers showed properties in between the two167.
Aggarwal168 developed bagasse-cement composites and physico-mechnical testing
showed that these composites met most of the requirements of various standards on
cement-bonded particle-boards and had high performance even in moist conditions.
Coutts and Ni169 reported bamboo fibers as a potential reinforcement for autoclaved
cement building material. On fiber loading of 14% by mass, the composite gave flexural
strength greater than 18 MPa and 1.3gcm-1 density but the fracture toughness was less
then 0.50 KJ m-2 . Zhu et al.170 reported the fabrication of banana fiber reinforced cement
composites and fiber loading of 8-16% by mass resulted in increase in flexural strength
22
by 20 MPa for use as commercially viable building materials. Chemically treated sisal
fiber used as reinforcement in low density polyethylene (LDPE) in composite showed a
constant increase in the electric constant with increase in fiber loading. The dielectric loss
factor of treated sisal fiber reinforced LDPE composites were found to be lower than that
of untreated sisal fiber LDPE composites171. A higher sisal fiber loading of rubber
composite showed better resistance to aging, especially in presence of a bonding agent.
Acetylation of fiber and fiber orientation was found to reduce the extent of degradation.
The doses of gamma-radiation were found to influence the extent of retention/
degradation172. Ismail et al.173 prepared the oil palm wood flour (OPWF) reinforced
epoxide composites. The cure (t90) and scorch times of all filler have been found to
decrease with increase in OPWF content. Sanjuan and Filho174 studied the free plastic
shrinkage and cracking sensitivity at drying mortars, reinforced with low modulus sisal
and coconut fibers. Compressive shear strength of the aged cellulose fiber reinforced
cement composite was related to the micro-structures within the composites175. Ghavami
et al.176 prepared the composite
blocks reinforced with sisal and coconut fibers.
Dynamic–mechanical analysis of flax and hemp fiber reinforced polypropylene, revealed
that the elastic properties of composites were dependent upon the type of coupling agent.
Other influencing parameters were the specific surface area and the content of the fiber
added177. Pathan and Thomas178 investigated the interfacial properties of the banana fiber
reinforced polyester composites. The fiber was chemically modified prior to
reinforcement and was found to have different polarity. Sain and Panthapulakkal179
explored the possibility of using the wheat straw as reinforcing material in thermoplastics. Treatment of flax fibers with combination of alkali and dilute epoxy depicted a
significant improvement in flexural properties. The longitudinal strength was found to
increase by 40%, the transverse strength to 250% and transverse modulus improved up to
500%180 . Wambua et al.181 manufactured sisal, kenaf, hemp, jute and coir reinforced
polypropylene composites by compression molding using film stacking method.
Joshi et al.
182
reviewed natural fiber reinforced composites with its impact on the
fibers. Nechwatal et al.183 reported the theoretical approaches and practical results of the
young’s modulus of the reinforcing fibers used for the preparation of composites.
Mohanti et al.184 fabricated chopped hemp fiber and cellulose-ester biodegradable
23
plastics through compression molding and extraction molding followed by injection
molding. Bio-composite obtained through later process containing 30% hemp by weight
showed an improvement in modulus by 150% over the virgin matrix polymer. The
coefficient of thermal expansion decreased by 60% whereas the heat deflection
temperature improved by 30%, thereby indicating the superior thermal behavior of the
bio-composites. Brahma Kumar et al.185 reported a good interfacial bonding between the
fiber matrix in case of coconut fiber possessing waxy surface. Silane chemical
modification as well as morphological improvement of the natural fiber enhanced the
matrix fiber wetting and improved the physico-chemical interaction at the fiber-matrix
interface186. Singh et al.187 studied the physico-mechanical properties of the jute
reinforced composites under different humidity, thermal and weathering conditions. Flax
fiber length effected the mean fiber strength and limit strain was determined through
single fiber fragmentation test188. Sapuan and Maleque189 fabricated banana woven fabric
reinforced
epoxy composites for their applications as household telephone stand.
Li et al.190 reviewed the properties of sisal fiber and its use in composites as
reinforcement. In recent developments, the versatility of graft copolymerization of
cellulose by vinyl monomers has rendered magnificent results with the use of latest
advancement in its synthesis, processing, characterization and applications. The use of
cellulose and its graft copolymers as reinforcement in composites have shown such
distinguished results in term of mechanical strength and compatibility (green composites)
that it makes the topic a pioneer arena of research till ages 191-200.
1.12
Hibiscus sabdariffa Linn. (Roselle, Jamaica Sorrel, Red Sorrel)
The plant belongs to family Malvaceae. It is an annual erect shrub with red or green
stem, practically unbranched with a few branches near the base, stem glabrous or slightly
hairy with minute tubercles, leaves serrate, lower leaves ovate and undivided, upper ones
palmately 3-5 lobed, flowers large, yellow with dark crimson eye, calyx connate below,
free above. It is a native of Africa and Asia, cultivated through out the tropical region. Its
fiber is silky, soft, lustrous, white to pale yellowish brown in color with chemical and
physical properties similar to jute. It has attained predominance as a jute substitute in the
areas not favorable for jute production.
24
It has various uses like its tender leave and stalks are eaten as salad in seasoning
curries. They are acid to taste and may be used for the preparation of jelly, flavoring
extracts, syrup, soups, puddings, cakes and whine. It has diuretic and chloretic properties
that acts as an intestinal antiseptic, mild laxative, used in heart, nerve disease, high blood
pressure and calcified arteries200-205.
1.13
Aims and Objectives
From the foreongoing discussion, it is evident that there is a considerable scope in
studying the graft copolymerization of methyl acrylate, ethyl acrylate, butyl acrylate and
acrylonitrile and their binary mixtures with different vinyl monomers onto H. sabdariffa
fiber and their application as reinforcement for the preparation of phenol-formaldehyde
matrix based composites. The aims of the present study are:
1. To synthesize graft co-polymers of Hibiscus sabdariffa fiber using methyl
acrylate, ethyl acrylate, butyl acrylate and acrylonitrile as principal monomers
along-with different vinyl monomers as binary mixtures.
2. To characterize the graft co-polymers by different physico-chemical techniques
such as FTIR, X-RD, SEM and TG-DTA.
3. To subject the various graft co-polymers for the evaluation of different chemical
and physical properties such as swelling behavior in different solvents, moisture
absorbance, dying uptake, chemical and thermal resistance.
4. To synthesize the phenol-formaldehyde resin for its use in the preparation of
polymer matrix based composites using graft co-polymers as reinforcing materials
and evaluating their various mechanical properties such as wear resistance, tensile
strength, compressive strength, hardness, modulus of rupture, modulus of
elasticity and stress at the limit of proportionality.
5. To make the effective use of renewable bio-mass like H. sabdariffa fiber as
reinforcing material for the preparation of phenol-formaldehyde matrix based
composites.
25
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