i
CHEMICAL MODIFICATION OF WOOD FIBER TO ENHANCE THE
INTERFACE BETWEEN WOOD AND POLYMER IN WOOD PLASTIC
COMPOSITES
A Thesis
Presented in Partial Fulfillment of the Requirements for the
Degree of Master of Science
with a
Major in Forest Products
in the
College of Graduate Studies
University of Idaho
by
Smith T Sundar
April 2005
Major Professor: Armando McDonald, Ph.D.
iii
ACKNOWLEDGMENTS
I would like to express my deepest gratitude and appreciation to my major professor Dr.
Armando McDonald for his guidance and encouragement. Grateful appreciation is made
to my graduate committee members, Dr. Steven Shook and Dr. Michael Wolcott for their
invaluable suggestions and time. I would also like to thank the staff, faculty and graduate
students in the Department of Forest Products for their support and friendship.
iv
ABSTRACT
Novel wood based materials and composites that have economic and environmental
values are sought after for the function of substituting for traditional wood based
applications. Wood plastic composites (WPC) proved to be a promising group for the
afore mentioned qualities. Most polymers especially thermoplastics are non-polar
(hydrophobic) substances which are not compatible with polar (hydrophilic) wood fibers.
This results in a poor adhesion between polymers and wood fiber. The aim of this study
is to improve the adhesion properties by chemically modifying the maple wood fiber
surface to enhance its miscibility within the plastic matrix. This was achieved by using a
reactive diisocyanate (diphenyl-methane-4,4’-diisocyanates, MDI; toluenediisocyanate,
TDI) linker to couple the micron sized (300 mesh) wood fiber with either a fatty alcohol
(ethanol, octanol, dodecanol, octadecanol) or amine (octadecylamine) to tentatively form
core-shell like structures with weight gains of between 35 and 60%. The reactions were
characterized by a combination of liquid chromatography mass spectrometry (LC-MS)
and FTIR spectroscopy. The effect of differing alkyl group lengths (C2 to C18) on the
modified wood fiber were examined after compounding in a torque rheometer) with high
density polyethylene (HDPE) at 10, 30 and 50% wood loadings. Wood fiber modification
was shown to significantly improve the compoundability of WPC systems over controls,
as assessed by torque rheometry. Wood modification was shown to increase both the
crystallization temperature and extent of crystallization of HDPE in WPC systems as
compared to controls.
v
TABLE OF CONTENTS
Acknowledgements........................................................................................................... III
Abstract ............................................................................................................................. IV
Table of Contents................................................................................................................V
List of Tables ................................................................................................................... VII
List of Figures .................................................................................................................VIII
Chapter 1.0...........................................................................................................................1
1.1 Introduction........................................................................................................1
Chapter 2.0 Background Information and Literature Review .............................................4
2.1 Wood..................................................................................................................4
2.1.1 Physical composition ..........................................................................4
2.1.2 Wood ultra structure ...........................................................................5
2.1.3 Chemical composition ....................................................................... 7
2.1.4 Extractives...........................................................................................9
2.2 Polymers ..........................................................................................................10
2.2.1 Chemical composition of polyethylene
2.3 Wood plastic composites .................................................................................12
2.4 Coupling agents used in WPC .........................................................................14
2.4.1 Categorization of coupling agents ....................................................15
2.5 Methods of modification..................................................................................16
2.5.1 Cross-linking.....................................................................................16
2.5.2 Acetylation........................................................................................17
2.5.3 Graft co polymerization ....................................................................18
2.5.4 Isocyanate coupling agents ...............................................................18
2.6 Variety of Isocyantes and their uses ...............................................................19
2.6.1 Toluene diisocyanate ........................................................................19
2.6.2 MDI - Diphenyl methane diisocyanate .............................................20
2.7 Reactions of Isocynates...................................................................................21
2.8 Core shell structures........................................................................................22
2.9 Property enhancement by modification ..........................................................23
2.9.1 Mechanical properties....................................................................23
2.9.2 Biological resistance ......................................................................23
Chapter 3.0 Materials and Methods ............................................................................25
3.1 Synthesis of wood core shell structures: approach 1. ......................................25
3.1.1 Initial synthesis and characterization of modified isocyanate
derivatives ......................................................................................25
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3.1.2
3.1.3
Thin layer chromatography analysis (TLC)...................................25
High performance liquid chromatography (HPLC) and mass
spectrometry...................................................................................26
3.1.4 Wood fiber preparation ..................................................................26
3.1.5 Wood fiber modification................................................................27
3.1.6 Fourier Transform Infrared Spectroscopy .....................................28
3.1.7 Compounding/Torque Rheometery ...............................................28
3.1.8 Differential scanning calorimetry (DSC).......................................29
3.1.9 Mechanical Property evaluation ....................................................29
3.2 Synthesis of wood core shell structures: approach 2 and 3 ...........................30
3.2.1 Wood-isocyanates (MDI) modified with alcohols (approach 2) ...30
3.2.2 Wood-isocynates (MDI, TDI, ODI) modified with amine
(approach 3) ...................................................................................30
Chapter 4.0 Results and Discussion...................................................................................32
4.1 Modification of wood fiber and characterization.............................................32
4.1.1 Synthesis of core shell wood fiber structures (approach 1) ...........32
4.1.2 High performance liquid chromatography (HPLC).......................34
4.1.3 Mass spectrometery .......................................................................35
4.1.4 Wood fiber modification................................................................37
4.1.5 Fourier transform infrared spectroscopy (FTIR) ...........................38
4.1.6 Torque rheometry...........................................................................40
4.1.7 Differential Scanning calorimetry (DSC) results...........................41
4.2 .........................................................................................................................42
4.2.1 Synthesis of core shell wood fiber structures (approach 2) ...........42
4.2.2 Fourier Transform Infra red Spectroscopy (FTIR) ........................42
4.2.3 Compounding/torque rheometery ..................................................44
4.2.4 Differential scanning calorimetry (DSC).......................................45
4.3 .........................................................................................................................46
4.3.1 Synthesis of core shell wood fiber structures (approach 3) ...........46
4.3.2 Fourier transform infra red spectroscopy (FTIR) ..........................47
4.3.3 Torque rheometry...........................................................................47
4.3.4 Differential scanning calorimetry (DSC).......................................48
Chapter
5.0 Summary of findings/Conclusion..............................................................50
References..........................................................................................................................52
Appendix............................................................................................................................57
vii
LIST OF TABLES
Chapter-4 ..........................................................................................................................32
Table 4.1 Summary of the positive ion electrospray-mass spectral ions
(pseudomolecular) of the mono and di-substituted alcohol-MDI adducts.......33
Table 4.2 Yields of sieved ball milled maple wood flour and particle size analysis.......33
Table 4.3. DSC results for crystallinity and crystallization temperature for WPC made
from modified wood fiber (30% loading) using approach-1 ...........................34
Table 4.4 DSC results for crystallinity and crystallization temperature for WPC made
from modified wood fiber (30% loading) using approach-2 ...........................35
Table 4.5. Summary of DSC results for WPC (30% loading) made from the amine
modified wood fibers .......................................................................................36
viii
LIST OF FIGURES
Chapter 2 ............................................................................................................................4
Figure 2.1
Section of a dicot stem ................................................................................4
Figure 2.2
Structure of wood cell, showing Middle lamella (ML), the primary wall
(P), the outer (S1), the middle (S2) and inner (S3) layers of secondary cell
wall and the warty layer (W) .......................................................................6
Figure 2.3
(A)- Coumaryl alcohol- Lignin precursor in Gramine, (B)-Coniferyl
alcohol Lignin precursor in conifers, (C)-(Synapyl alcohol) Lignin
precursor in angiosperms .............................................................................9
Figure 2.4
Polyethelene repeating unit .......................................................................11
Figure 2.5
A representation of the amorphous and crystalline regions of HDPE..........12
Figure 2.6
Reaction of an isocyanate with an alcohol ...................................................21
Figure 2.7
Structure of a urethane linkage.....................................................................21
Figure 2.8 Reaction of an isocyanate with an amine to forma urea linkage...................22
Chapter-3 ..........................................................................................................................25
Figure 3.1 Reaction sequences of wood with an alcohol modified isocyanate ................27
Figure 3.2 Experimental setup of for wood modification.................................................28
Figure 3.3 Reaction sequence in modification of wood surface by isocyanate and
attachment of long chain amine .........................................................................................31
ix
Chapter-4 ..........................................................................................................................32
Figure 4.1 Combinations of products obtained when MDI reacted with DBA
and alcohol .......................................................................................................33
Figure 4.2 Thin layer chromatogram of DBA derivatized ethanol-MDI
reaction mixture ...............................................................................................33
Figure 4.3 HPLC chromatogram of the DBA derivatives of octanol-MDI reaction
products.........................................................................................................34
Figure 4.4 HPLC chromatogram of the DBA derivatives of octadecanol-MDI reaction
products............................................................................................................35
Figure 4.5 Electrospray-MS spectrum of the DBA derivatized monosubstituted-ethanol
MDI adduct ......................................................................................................36
Figure 4.6 Electrospray-MS spectrum of the DBA derivatized monosubstituted
dodecanol-MDI adduct ....................................................................................36
Figure 4.7. FTIR spectra of the ethanol MDI-adduct modified wood fiber (top) and
extractives free maple fiber (bottom)...............................................................39
Figure 4.8 Torque rheometery data for compounding WPC made from modified wood
using approach-1..............................................................................................40
Figure 4.9 Reaction scheme for preparing wood core-shell structures by approach-2......42
Figure 4.10 FTIR spectra of the ethanol MDI-adduct modified wood fiber by
approach-2.......................................................................................................43
Figure 4.11 Torque rheometery data for compounding WPC made from modified
x
wood using approach-2 ....................................................................................44
Figure 4.12 Reaction scheme for preparing wood core-shell structures by
approach 3.....................................................................................................46
Figure 4.13. FTIR spectrum of TDI- Octadecyl amine modified wood fiber
(approach 3) ..................................................................................................47
Figure 4.14. Torque rheometery data for compounding WPC made from modified wood
using approach-3...........................................................................................48
1
CHAPTER 1
1.1 Introduction
Composites are materials consisting of two or more identifiable constituents but
not mixed to an atomic scale (1). Composite materials are governed by the properties of
individual constituents and their interaction. The major constituents of wood plastic
composites (WPC) are the fillers (wood or natural fibers) and matrix (thermosetting or
thermoplastics resins) and other chemical additives.
The use of fillers in thermoplastic industry is a common practice to augment
stiffness and strength to plastics. It is estimated that about 5.5 billion pounds of fillers
were used in the North American polymer industry during 2001 (2). Inorganic fillers
were greatest in demand, especially calcium carbonate (2.2 billion lb), glass fiber (1.7
billion lb), and other filler such as clay talc and mica. However, the quantity of natural
fibers used was estimated at 400 million lb (2). The perceived environmentally benign
nature of organic fillers has attracted the industry and has led to a major shift in choosing
filler types. The growth of WPCs has averaged more than 25% per year since 1998 and is
considered as a fast paced stride in the plastic industry (3). The acceptance of WPC,
which are currently used in decking, siding, roof tiles, window and door frames
encouraged the industry to adhere to organic fillers (4). It is speculated that
approximately 1.1 billion pounds of WPCs will be used in North America in 2006 (2).
Recently, there has been an increased interest in the production of WPCs in order to find
a viable alternate use for residual plant fiber material and thermoplastics. Over the past
decades, there have been numerous studies on wood fiber plastic composites that indicate
that composites can be readily formed using compounding and compression molding
technologies (5). Wood fiber and polymer matrix composites are normally made by
mixing wood fiber with polymer, or by adding wood fiber as a filler in a polymer matrix
and pressing or molding it under high pressure and temperature. The term ‘WPC’ covers
a wide range of composite materials using plastics ranging from polypropylene (PP) to
2
polyvinylchloride (PVC) and binders/fillers ranging from wood flour to flax fibers (6).
The first generation of ‘WPC’ was a combination of recycled wood flour or chips and
binders. Unfortunately, such composites products were ideal for relatively less
demanding applications.
The new and rapidly developing generation of WPCs exhibits superior
mechanical properties, high dimensional stability and can be used to produce complex
shapes. They are tough, stable and can be extruded to high dimensional tolerances. The
new WPC materials are high-end technology products for demanding engineered
applications (4). Mixing wood flour with plastic produces the most common types of the
WPCs. The advantage of WPCs is that it can be processed just like a plastic but has the
best features of wood and plastics. WPCs have the following benefits:
•
They are true hybrid materials and combine the best properties of both
wood and plastics.
•
They use low cost and wide range of raw materials. Wood waste and
recycled plastics become assets instead of causing environmental concerns
by converting them into WPC.
•
They are competitively priced and are competitive with traditional
materials such as timber, medium density fiberboard (MDF) and PVC.
•
They are easily produced and easily fabricated using traditional wood
processing techniques.
•
They are available in a broad range of finishes and appearances.
Despite the advantages of WPC over wood and plastic, the interaction between
wood and plastic at fiber-matrix interface does not necessarily yield strong bonds (8). The
major reason for weak bonds has been attributed to the polar nature of the wood
(hydroxyl groups) while some plastics such as PE and PP are non-polar polymers (9). In
order to develop composites with superior bonds at the interface, it is necessary to modify
3
the wood fiber by chemical reaction with a suitable coupling agents or coating the fiber
with appropriate resins/chemicals. Such surface modifications of the wood fiber would
improve the interfacial bond strength, which is a critical factor for obtaining better
mechanical properties of composites (17).
The purpose of this study is directed at modifying wood fibers in order to enhance
and improve fiber interaction with plastic, thereby yielding improved processability and
performance. Three distinct approaches were trailed to synthesize wood core shell
structures by chemically modifying wood fibers as follows:
1. Synthesize aliphatic isocyanates derivatives, which can then be linked to the wood
fiber.
2. Activate the wood fiber surface with a diisocyanate, which can then be reacted
with an aliphatic alcohol.
3. Activate the wood fiber surface with a diisocyanate, which can then be reacted
with an aliphatic amine.
4
CHAPTER 2
2.0 BACKGROUND INFORMATION AND LITERATURE REVIEW
2.1 Wood
2.1.1 Physical composition
Wood (xylem fiber) is composed of elongated cells; they are oriented in the longitudinal
direction of the stem (Figure 2.1). The ends are connected through openings, and these
openings are called pits. These cells vary in function and differ in shape. They perform in
the transport of liquid and act as food reserves. They also provide necessary mechanical
support to the tree.
Figure 2.1. Diagram showing a section of a dicot stem.
5
The xylem or wood is organized in concentric growth rings (Figure 2.1). Wood
contains rays in horizontal direction. The central portion of the stem is called pith. As
growth takes place, the heartwood gets deposited towards the center of the stem. The
cambial zone is a thin layer consisting of live cells between the wood (xylem) and inner
bark (phloem); the cell division and radial growth of the tree takes place in this region.
The wood substance in softwoods is comprised of two different types of cells: tracheids
(90-95%) and ray cells (5-10%). Tracheids give softwood the mechanical strength and
provide a water transport function, which occurs through the thin walled early wood
tracheids and through the bordered pits. Resin canals are intercellular spaces building up
a uniform canal network in the tree. The resin canals are surrounded by epithelial
parenchyma cells, which secrete oleoresin into the canals. Most pines have abundant
resin canals where they are largely concentrated in the heartwood of the stem and roots.
Hard wood cells contain several cell types, each specialized for different functions. They
comprise of vessels, parenchyma, and libriform cells (11).
2.1.2 Xylem Ultra structure
The xylem cells consist mainly of cellulose, hemicellulose, and lignin. Cellulose
is comprised of a crystalline structure, while and hemicellulose has a semi-crystalline
structure while lignin is an amorphous polymer. The cell wall is built up by several
layers, namely the middle lamella (ML), primary wall (P), outer layer of the secondary
wall (S1), middle layer of secondary wall (S2) and warty layer (W) (Figure 2.2). These
layers differ from each other based on their chemical composition and their structure. The
ML is located between the cells and serves the function of binding wood cells together.
Though it contains pectin in the initial stages it becomes lignified in later stage of life.
The middle layer (S2) forms a major portion of the cell wall. Its thickness in soft
wood tracheids varies between 1-5µm and it may thus contain between 30 to more than
150 lamellae.
6
Figure 2.2. Structure of wood cell, showing the middle lamella (ML), primary wall (P),
outer (S1), middle (S2) and inner (S3) layers of secondary cell wall and the warty layer
(W) (12).
The primary cell wall (P), secondary wall (S1), secondary wall (S2) and
secondary wall (S3), are distinguished by the orientation of cellulose fibrils in the walls.
The primary cell wall cellulose fibrils are arranged in slopes perpendicular to the fiber
direction. In the S2 layer, fibrils are arranged closely in the fiber direction. The P and S3
layers have less cellulose when compared with S1 and S2 layers. The change in wall
thickness between early wood and late wood is mainly because of the changes in S2
thickness, where majority of cellulose is found (12).
Compression and tension wood is abnormal wood as a that result from mechanical
forces and seasonal changes in the growing tree. Angiosperms develop tension wood and
gymnosperms develop compression wood. Highly lignified cells characterize
compression woods. The fibrils of the S2 layer are arranged at about a 45o angle relative
to the longitudinal direction of the cell. Tension woods contain fewer vessels than normal
7
wood, and cells compensate for the lack of fewer vessels through the addition of a new
gelatinous layer (G layer). This layer may be present instead of the S2 or the tertiary wall.
The gelatinous layer consist of crystalline cellulose aligned in the direction of the fiber
2.1.3 Chemical composition
Wood is a lignocellulosic material and is composed of approximately 50-65%
cellulose, 20-25% lignin and 1-10% extractives and traces of ash. The ratio of
constituents differs based on species.
Cellulose is a homopolysacchride composed of repeating β-D-glucopyranosyl
units, which are linked together by β (1-4) glycosidic bonds. Although the chemical
structure of cellulose is understood in detail, the nature of celluloses super molecular
state, crystalline and fibrillar structure is still being investigated (11). Cellulose molecules
are linear and ribbon like, and together with available hydroxyl groups create the strong
tendency to form intra and inter molecular hydrogen bonds. Bundles of cellulose
molecules are thus aggregated together in the form of microfibrils in which the crystalline
region alternates with the amorphous region. Microfibrils build up fibrils and finally a
cell wall. As a consequence of this fibrous structure and strong hydrogen bonds, cellulose
exhibits a high tensile strength and is insoluble in most solvent systems. Hydroxyl groups
on cellulose are largely responsible for its reactive nature. The absorption of water by
cellulose depends on the number of hydroxyl groups that are not linked with other
hydroxyl groups (11). Therefore water absorption is mainly concentrated in the
amorphous region rather than in the crystalline region. Usually, the crystalline structure
of cellulose will not take part in chemical reaction because of it unavailable hydroxyl
groups but only the amorphous region.
8
Hemicelluloses were originally believed to be intermediates in the biosynthesis of
cellulose. Today, hemicelluloses are grouped as heterogeneous polysaccharides, which
are formed through biosynthetic routes that differ from that of cellulose. Hemicellulose
acts as a supporting material in the cell wall. There is considerable difference in the
composition of cellulose obtained from different part of the tree such as stem, branches,
root and bark. Galactoglucomannans are the principal hemicellulose component in
softwoods followed by arabino-4-O-methylglucuronoxylan. Even though there are
variations among the hemicellulose obtained from hardwoods, it mostly comprised of Oacetyl-4-O-methylglucuronoxylan (11).
Lignin is a polymer that is a 3-dimensional highly branched network. Lignin is
relatively an inert material, which act as a bonding and stiffening agent within the wood
cell wall and the ML. Monomeric lignin precursors are trans p-coumaryl, coniferyl, and
sinapyl alcohols that undergo dehydrogenative polymerisation by peroxidase and/or
laccase activity to form macromolecular lignin by random coupling (Figure 2.3). The
reactivity and levels of the lignin precursors govern the final constitution of lignin.
Softwoods contain lignin made up of guiacyl units, while hardwood lignin is built up
from both guiacyl and syringyl units.
Sophisticated analytical techniques show that lignin polymer contains methoxyl
groups, phenolic hydroxyl group, and some terminal aldehyde groups in the side chain
(11). There is variation of lignin originated from these precursors depends on whether it
is obtained from hardwood, soft wood or species. Generally, softwoods have larger
percentage of lignin than hardwoods, which accounts for 23-33% in softwoods and 1625% in hard woods (11).
9
(A)
(B)
(C)
Figure 2.3. (A) Coumaryl alcohol- lignin precursor in Gramine, (B)coniferyl alcohol
lignin precursor in conifers, (C)-(synapyl alcohol) lignin precursor in angiosperms.
2.1.4 Extractives
Extractives from wood are comprised mainly of two forms that differ in their
solubility in water or organic solvent. They contain large number of compounds either as
lipophilic or hydrophilic in nature. Extractives are considered non-structural wood
constituents, mostly composed of extra cellular and low molecular weight compounds.
Extractives occupy certain morphological sites in the wood structure. For instance, resin
acids are located in the resin canal, where as fat and waxes, they are in the ray
parenchyma cells. Phenolic extractives are present mainly in heartwood and in bark.
10
There are three types of lipophilic wood extractive compounds: terpenes (and
terpenoids), aliphatics (fatty acids and their esters) and phenolic compounds. Aliphatic
compounds include- alkanes, fatty alcohols, fatty acids, fat esters and waxes. Terpenoids
include monoterpenes (turpentine) and resin acids. Phenolic compounds include tannins,
flavnoids, lignans, stilbines and tropolones. Extractives are typically removed from wood
through steam distillation, solvent extraction, and water or alcohol extractions. Steam
distillation is used to remove the volatile terpenes and solvent (eg ether) extraction is to
remove resin acids, fatty alcohols, fatty acids, and waxes. Tannins are extracted with the
help of alcohol, while carbohydrates and other inorganic materials are water-soluble (12).
2.2 Polymers
Polymers are molecules made up of monomeric building block to form long
chains and are typically of high molecular weight. The individual building block of a
polymer is termed a monomer. Polymers can be obtained from natural materials as well
as synthetically processed. Addition polymerization (chain polymerization) and
condensation polymerisation (step wise polymerisation) are the two steps involved in
synthetic processing of polymers. In chain polymerization, a free radical is utilized with
the assistance of an initiator to polymerize the monomer units. The major steps include
initiation, propagation and termination. In this reaction the molecular weight of the
monomer is not altered. In condensation reaction the functional group react with one
another to eliminate a water molecule.
According to the curing properties of polymers they are classified as either
thermosets, thermoplastics or elastomers. Thermosets are polymers that do not melt once
they have been heated and cured. When thermoset polymers cool after heating, crosslinks are formed between the polymers that limit polymer mobility. Thermoplastics have
a unique property of repeated flow characteristics. They can be further classified as
amorphous and semi-crystalline. Semi-crystalline polymers, such as PE and PP, have
both amorphous and crystalline regions. The amorphous region settles in a disorderly
11
form when it cools while the crystalline region maintains an order. Another property
shown by the semi-crystalline polymer is that at glass transition temperature (Tg) the
amorphous region melts and flows and the crystalline region melts at its melting point
(Tm). Elastomers are polymers having low Young’s modulus during initial stretching and
there onwards the modulus increases (39). These polymers can be stretched before they
fail. Examples of thermoplastics are poly ethelene, poly vinyl chloride and so on. PE was
used in this study
2.2.1 Chemical composition of Polyethylene
Polyethylene (PE) was first synthesized by the German chemist Hans von
Pechmann, who prepared it by accident in 1898 while heating diazomethane. When his
colleagues Eugen Bamberger and Friedrich Tschirner characterized the white waxy
subsance he had created and they recognized that it contained long methylene (-CH2-)
repeating units in the chain and was termed polymethylene (Figure 2.4). There are two
types of PE namely low density (LD) and high density (HD) PE. HDPE is a linear
molecule and it is harder, stronger and a little heavier than LDPE, but less ductile. LDPE
has less hardness, stiffness and strength compared to HDPE, but better ductility. HDPE is
opaque and only thin foils can be transparent. Sometimes some of the carbons, instead of
having hydrogen attached to them, will have long chains of polyethylene attached to
them. Due to its molecular structure, PE is a semi-crystalline thermoplastic with both
amorphous and semi-crystalline regions (Figure 2.5).
Figure 2.4. A partial chemical structure of polyethylene.
12
LDPE is a branched polyethylene but when there is no branching, it is called
linear polyethylene, or HDPE. Linear polyethylene is much stronger than branched
polyethylene, but branched polyethylene is less costly and easier to produce. HDPE and
LDPE are linear molecule with a carbon backbone and hydrogen side groups. Crystalinity
of LDPE can range from 40-55% and that of HDPE ranges from 60-75%. Linear
polyethylene is normally produced with molecular weights in the range of 200,000 to
500,000, but it can be produced in excess of 500,000 (42).
Figure 2.5. A representation of the amorphous and crystalline regions of HDPE (42)
2.3 Wood plastic composites
The acronym ‘WPC’ covers an extremely wide range of composite materials that
use plastics ranging from PP to PVC and binders/fillers ranging from wood flour to
natural fibers (e.g. flax) (6). WPCs are true composite materials and have properties of
both wood and plastic. WPCs exhibit stiffness and strength properties between those for
plastic and wood, but the density is generally higher than the individual components. The
13
properties of WPCs come directly from their structure; they are an intimate mix of wood
particles and plastic. The plastic coats the wood particle as a thin layer. The properties of
WPCs can be tailored to meet the product requirements by varying the species and
geometry of wood or plastic. For example, PE based products are cheaper and have a
higher heat distortion temperature than the PVC based products but the PVC products are
easier to paint and post treat (13). Pigments, UV stabilizers and fire retardants can all be
added to the WPC raw material before extrusion to improve specific properties. WPCs
have good stiffness and impact resistance properties, dimensional stability, resistance to
rot, excellent thermal properties and low moisture absorption (25).
WPCs are coined environmentally friendly because it they do not contain toxic
wood preservatives (such as copper-chrome-arsenic) Furthermore WPC based products
are intended to be replace pressure treated solid wood in exterior applications (eg decking
material). The environmental pressures on industries with respect to of recycling and
sustainability are growing daily. In light of this situation there is a need to extend the life
cycle of traditional building materials such as wood and to improve the properties and to
recycle the raw material waste that occurs during manufacturing. For users of plastic
products there is also a requirement to reduce the dependence on petrochemicals with
their rising and repeatedly changing raw materials costs. WPCs aim to increase the
efficiency of wood usage by up to 40% compared to traditional wood processing. WPCs
also provide other environmental benefits, such as:
•
They use residual wood (eg sawdust) and recycled plastic.
•
WPCs contain no formaldehyde or volatile organic compounds.
•
WPCs are potentially recyclable since it can be reground and processed.
•
WPCs are considered nonhazardous waste and can be disposed of by
standard methods. The basic material structure of WPCs shows that
leaching from WPCs is minimal to non-existent (13).
14
The high moisture resistance of WPCs (water absorption of 0.7%) compared to
17.2% for pine is a direct result of structure (14). Moisture can only be absorbed into the
exposed sections of wood and is not transmitted across the plastic boundaries. This
suggests that WPCs are extremely moisture resistant, have little thickness swell in water
and are resistant to fungal or insect attack. The properties of composite materials are
determined by the interaction of individual constituents. In the case of WPCs, the
mechanical properties of the wood are not only dependent on the fiber properties, but also
to the level of adhesion between the wood fibers and the polymer matrix. An important
issue is that WPC are made up from two incompatible components and phases (wood
fiber being hydrophilic in nature while the plastic matrix is hydrophobic in nature) and
the interaction of these components occurs at the interface, which can directly affect the
strength properties. In this conjecture it become inevitable to enhance the interfaces to a
compatible phase between lignocellulosic/wood fiber and the polymer matrix. To
improve binding properties, the unanimity between the two phases needs to be more
continuous. This can be achieved by transforming the hydrophobic phase or the
hydrophilic phase into a mutual interphase (15). This challenge is best confronted by
chemically modifying the wood or polymer.
2.4 Coupling agents used in wood plastic composites
Chemical modification of either wood or polymer can be done to achieve a
mutual interphase. Chemical modification is defined as chemical reaction between some
reactive part of a lignocellulose material and a simple single chemical reagent, with or
without catalyst, to form a covalent bond between the two. This excludes a simple
chemical impregnation treatment that does not form covalent bonds (16). The chemical to
be used for this purpose is generally called a coupling agent. However, coupling agents
are comprised of bonding agents and surfactants (including compatibilizers and
dispersing agents). Bonding agents bridge the thermoplastic polymer and wood by either
covalent bonding, polymer chain entanglement or by secondary bonding such as
hydrogen bonding (17). Compatibilizers enhance miscibility of polymer with wood by
reducing the interfacial tension (18). Dispersing agents reduce the interfacial energy at
15
the wood fiber matrix interface to assist in the uniform dispersion of wood fiber in the
polymer matrix without aggregation and thereby facilitate the formation of the formation
of new interfaces (19).
There are several methods available to chemically modify lignocellulose and
polymers. Imparting hydrophobicity to wood fiber or hydrophilicity to thermoplastic
matrix turned out to be the most appealing methods of chemical alteration. Recently, the
later approach is been applied to wood fiber plastic composites because of the ease of
application, specifically maleated polymer is co-compounded with a base polymer
together with wood fiber to form products with improved mechanical and water
absorption properties (7). The alternate approach has been to modify the wood surface
with a hydrophobic coating. Various coupling agents such as polymeric isocynates,
silanes, and acid anhydrides have been evaluated and have shown improvements in
mechanical properties of the final product (20)
Coupling agents date back to 1963 when Ford Motor Company invented a method
to graft olefinic unsaturated monomers onto wood fiber using a catalyst system
containing ferrous cations and hydrogen peroxide that enhances the compatibility
between the wood fiber and the thermoplastic polymer (21). Further studies led to series
of patents for modification of wood fiber by coupling agents such as maleic anhydride
(MA) and various isocynates. Dalvaph et al. used MA as a coupling agent in cellulose
and polypropylene composites (22). Kokta patented polymethelene (polyphenyl
isocynates) [PMPPIC] for cellulose fiber and polyethylene composites (23).
2.4.1 Categorization of coupling agents
Different categories of coupling agents have been used for the purpose of wood
modification, these coupling agents includes organic, inorganic, and organic inorganic
16
group. Organic agents include isocynates, anhydrides (pthalic, succinic, maleic,
propionic, and butyric), amides, acetaldehyde, formaldehyde, difunctional aldehydes,
alkyl chlorides, beta-propiolactone, acrylonitrile, imides, acrylates, chlorotriazines,
epoxides, organic acids, polymers, and copolymers (16). Inorganic coupling agents
mainly include silicates (16). Organic-inorganic includes silanes and titanates (21).
Organic coupling agents, which are used in modifying the interface of wood plastic
composites, have bi- or multi-functional reactive groups. The primary aim of chemical
modification of wood is to reduce the number of hydroxyl groups and to enhance the
cross-linking with the polymer matrix. Which results in a hydrophobic interface.
Functional groups such as isocyanates [-N=C=O], maleic anhydride [-(CO)2-O-] and
dichlorotriazine [-Cl-], derivatizes the polar hydroxyl group of the wood to form a
covalent bond or hydrogen bond (23). The chemical bonds formed by this process
determine the stability of the composite. These bonds influence physical and mechanical
properties of the composites. Covalent bonds are generally formed during the
modification of wood fiber. The polymer matrix can also be tailored by graft
copolymerisation, which can result in better miscibility and cross-linking at the interface.
Inorganic coupling agents act as dispersing agents to counteract the surface polarity of
wood fiber and polymer (22). Organic-inorganic agents are hybrid compounds in
structure and the organic moiety determines the efficacy of coupling at the interface.
2.5 Methods of modification
The chemicals, that are used to react with the hydroxyl group of wood, can be
categorized as those that react with the hydroxyl group and polymerize afterwards, and
those, which react with the, single hydroxyl group (24).
2.5.1 Cross-linking
Cross-linking of hydroxyl groups in sugar moieties can be achieved by various methods.
That is a hydroxyl group on a sugar molecule or several hydroxyl groups on different
17
sugar molecules can be cross-linked by appropriate chemicals. Formaldehyde is been
widely used in this scenario (24) and its interaction with wood can be described as
follows.
Wood-OH + H-CH=O
Wood - O -CH2- O -H
Wood - O -CH2- O -H + HO – Wood
Wood - O - CH2 - O - Wood
The active hydroxyl groups that are hygroscopic can be altered by this way to
impart hydrophobicity and in turn reduce moisture absorption.
2.5.2 Acetylation
Acetylation of wood is characterized by formation of ester (R-O-(CO)-R) bonds. Acetic
anhydrides are used for reaction with hydroxyl group of the wood (25). Wood
components like hemicellulose and lignin are more prone to acetylation since they are
amorphous polymers and the acetylating reagent can ready diffuse and react within the
wood cell wall structure. Acetylation adds an acetyl group to each hydroxyl group
available.
Wood – OH +CH3 –CO – O – CO – CH3
Wood - O- CO- CH3 + CH3COOH
The resultant product will have a reduced number of polar and active hydroxyl
groups in the wood, thus making it less hygroscopic. This modified wood has been
assessed for use in wood plastic composites (26). The use of this wood particles results in
unanimity between interphase of thermoplastic polymer. If this modification is done in
blocks of woods there may arise a problem for penetration of water based resin and
adhesives. The acetylating process is largely recommended for small pieces of wood and
wood flour.
18
2.5.3 Graft Co-polymerization
Graft co-polymerization is another method employed to modify the polymer
materials used in the production of composites. Graft copolymers are comprised of
polymer backbone, grafted with functional groups, which react or interact with other
polymers. This method is widely used for modification of chemical properties and
polarity of the wood based fillers and polymer matrices. The Graft co-polymerization
reaction in the case of cellulose based composite, is initiated by free radicals of the
cellulose molecule (26). Cellulose-based compounds are initially exposed to high-energy
radiation for the formation of free radicals. Afterwards, the radical sites of the cellulose
are treated with suitable solution (compatible with the polymer matrix) such as
polystyrene (25). Polystyrene is thereby covalently linked to the cellulose and reduced
the hydrophilic nature of the wood fiber. The product thus formed exhibits properties of
both filler and thermoplastic matrix. This results in improved compatibility of interface
with the polystyrene matrix (26). Maleic anhydride (MA) is commonly used as a grafting
agent (27, 28). MA grafted on synthetic polymers has been shown to form both covalent
ester linkage and hydrogen bonding when reacted with hydroxyl groups in the wood
surface (29). Maleic anhydride polypropylene-copolymers (MAPP) grafted to wood
fibers can be tuned to manufacture of composites with improved adhesion, wettability
and fiber dispersion. It has been noted that the moisture absorption and swelling of the
composite is considerably reduced (30).
2.5.4 Isocynates coupling agents
Among the different coupling agents used, polyisocynates (PIC) have shown
considerable improvement in mechanical properties of the final product (31). PIC forms
urethane structure from the formation of covalent bond with the wood hydroxyl group. A
significant improvement in strength has been achieved using PMPPIC in wood–HDPE
composites (31). The delocalized π electrons in the benzene rings of the PMPPIC results
in slight polarity (26). If used with polystyrene (PS), which contain benzene ring that can
interact with PMPPIC to form strong bonds (31). PMPPIC makes a bridge between
19
polymer and wood fiber matrices in the interphase region giving an opportunity to
transfer stress between two physically immobilized phase (32). However, it has been
observed that concentration of PMPPIC should be optimum for maximum strength
properties (32). Addition of more PMPPIC may results in interfacial saturation and even
high concentrations may results in formation of by products (33). Isocynates were used in
the present study to modify wood fibers for better mutual interfacial bonding between
wood and thermoplastic.
2.6 Variety of isocynates and their uses
Isocyanates are a group of low molecular weight aromatic and aliphatic
compounds containing the isocyanate group (-NCO). In industry, isocyanates are used for
the production of polyurethanes, which are polymers formed by urethane links of the NCO groups with the hydroxyl (OH) groups of polyols. Since at least two -NCO groups
are required for such reaction, diisocyanates are the principle monomers. They are widely
used in the manufacture of flexible and rigid foams, fibers, coatings such as paints and
varnishes, and elastomers. Isocynates, which are commercially used, are toluene diisocyanate (TDI), diphenyl methane diisocyanate (MDI), hexamethylene diisocyanate
(HDI), naphthalene diisocyanate (NDI), methyl isocyanate (MIC), and other variants.
2.6.1 TDI - Toluene diisocyanate
TDI is a liquid, commercially available as a mixture of 2,4 and 2,6 isomers, which
is used to produce flexible foams, insulation, elastomers, polyurethane paint coatings,
varnishes enamels, adhesives, and sealants. TDI is often seen commercially in its pure
form. Therefore, as it can be seen by its melting point (20-22°C), it often solidifies when
exposed to low ambient temperatures. It is also widely sold as a mixture (80% 2,4-TDI /
20% 2,6-TDI and 65% 2,4-TDI / 35% 2,6-TDI) where its melting point may be in the
12°C range (41).
20
2.6.2 MDI - diphenyl methane diisocyanate
MDI is a solid, pale yellow in color, commercially available in the form of a
concentrated solution usually used to make insulators. Modified MDI is made by
converting some of the isocyanate groups into carbodiimide groups that react with the
excess isocyanate, which at the end liquefies the low melting MDI. Liquid MDI (also
called prepolymers) is also made by the reaction of the diisocyanate with small amounts
of glycols. Liquid MDI is used in RIM ("reaction injection molding") polyurethane
elastomers (41). Monomeric MDI (MMDI) is a purified material distilled from a
polymeric MDI mixture. MMDI consists of over 97% 4,4'-MDI with small amounts of
2,4'-MDI and traces of the 2,2' isomer. It is a solid with a melting point of about 38°C
and it starts to decompose at 230°C. It is used in thermoplastic and cast elastomer
applications, coatings, adhesives, sealants, and synthetic fibers. Purified MDI is used for
high performance polyurethane elastomers and spandex fibers.
PMDI or PMPPI are crude products that vary in their exact composition. The
main constituents are 40-60% 4,4'-MDI, the remainder being other isomers of MDI (2,4'
& 2,'), trimeric species and higher molecular weight oligomers. These are always found
commercially in a liquid state (brownish liquid). Their main use is in the manufacture of
rigid polyurethane foam eg (construction, refrigeration), polyurethane coatings, and used
as a resin in some wood composites (41).
Many isocyanates are now formulated as pre-polymers by partial reaction with
polyols. These compounds are of lower volatility than TDI or HDI and contain less free
isocyanate, thus reducing the vapor hazard. The formation of pre-polymers does not
substantially reduce the hazard when aerosols are used in spray painting.
21
2.7 Reactions of Isocynates
Isocyanates have a highly reactive functional group that readily reacts with
various other functional entities (36). They readily react with amines and alcohols
forming urea and urethane bonds, respectively Figures 2.6 to 2.8. Urethane bonds are
unique in their strength properties. Polyurethanes can hydrogen bond readily, and thus
can be very crystalline. For this reason they are often used to make block copolymers
with soft rubbery polymers. These block copolymers have properties of thermoplastic
elastomers.
Figure 2.6 Reaction of an isocyanate with an alcohol.
Figure 2.7 Structure of a urethane linkage.
22
Figure 2.8 Reaction of an isocyanate with an amine to form a urea linkage.
2.8 Core shell structures
Core-shell polymers systems are becoming wide spread in their applications such
as rubber, latex based coatings and plastics. To illustrate the core-shell structure a latex
polymer system has been developed using polystyrene core (<100 nm in diameter) and
encapsulated with polymethylmethacrylate shell (45). Typically, the core is made up of a
material with a higher glass transition temperature (Tg) than the shell. Therefore the core
acts as reinforcement for the shell. Upon processing, such as melt fusing or annealing the
core structures are packed and dispersed within the outer shell phase (47).
Other applications of core-shell systems have been used as impact modified
thermoplastics. Bucknall (48) had made core-shell polymers with a polybutadiene (PB) or
polybutyl acrylate-co-stryene) (PBA) cores and a grafted PMMA shell to which they
were blended into a polymer matrix (polyvinyl chloride, polycarbonate). The core-shell
blends were shown to have improved impact resistance compared to conventional rubber
reinforced plastic due to the improved bonding between the various polymers within the
network.
23
The rheological behavior of these multiphase polymeric systems is governed
primarily by their structure, and in particular the size and shape of the dispersed phase
domains plus the interfacial tension between the phases (49). It has been also observed
that the melt viscosities of the core-shell modified matrices are similar to those of the
pure matrix over the shear rates typically used during extrusion/injection molding
processing.
2.9 Property enhancement by modification
2.9.1 Mechanical properties:
Strength properties of wood fibers are dependent on moisture content of the cell
wall since water acts as a plasticizing agent (34). Chemical modification of wood results
in a reduction of available hydroxyl groups and thus reduces the hygroscopicity of wood.
This modification also aids in compounding of the wood particles within the polymer
matrix (34). The resultant increase in strength properties can be explained on the basis of
improved wettability (compatibility) of the wood fibers with the polymer matrix. The
increased compatibility is obtained by reducing the polarity of the wood fiber surface
nearer to the polymer matrix (34). Dimensional instability of wood due to moisture
absorption is a significant challenge in lignocellulose composites. The rate of thickness
swelling in fiberboards made from acetylated and untreated fiber has shown significant
reduction in swelling for the modified fibers (16). As the control fiberboards swelled
from 18 to 45%, the acetylated fiberboards showed swelling from 3 to 10% only (16).
2.9.2 Biological resistance:
Biological agents such as fungi and bacteria can cause a detrimental effect to
lignocellulosic based composites. Chemically modified composites have been tested for
fungal decay in several ways (35). Chemically modified particleboards treated with a
variety of chemicals like butylenes oxide, propylene oxide, methyl isocyanate, acetic
24
anhydride and betapropiolactone were exposed to brown and white rot fungi. The
chemically modified particleboards were highly resistant against the fungal attack except
for the propylene oxide treated material (35).
25
CHAPTER-3
MATERIALS AND METHODS
This chapter is concerned with analytical methods and the development of wood
core shell structures by attaching a long chain alkyl group coupled to the wood surface
via a diisocyanate linker for use in WPC.
3.1 SYNTHESIS OF WOOD CORE SHELL STRUCTURES: APPROACH 1.
3.1.1 Initial synthesis and characterization of modified isocyanate derivatives
To assess the reaction conditions required to modify the isocyanates alcohols of
various chain lengths (C2 (ethanol), C8 (octanol), C12 (dodecanol) and C18 (octadecanol))
were reacted with MDI to form MDI-alcohol adducts. MDI (1g, 4 mmol) was dissolved
in toluene (200 mL) and reacted with an equimolar quantity of the desired alcohol (40
mmol). The mixture was reacted for 24 hours in a 100 mL round bottom flask under
reflux (110oC). Scaled up reactions (400 mmol MDI + 400 mmol alcohol + 2% dibutyl
tinlaurate catalyst + toluene (500mL) were used for to prepare the MDI-alcohol adducts
for subsequent wood modification experiments. To assess the reaction products, dibutyl
amine (DBA) derivatives of the isocyanate were prepared in order to characterize by a
combination of thin layer chromatography (TLC), and high performance liquid
chromatography (HPLC) in conjunction with mass spectrometry (MS). The MDI-alcohol
reaction mixture was evaporated to dryness and then reacted with 4 g of DBA in
acetonitrile (20 mL), which was added slowly and allowed to react for 3 hours at 70oC.
The DBA reaction mixture was evaporated to dryness and analyzed by a combination of
TLC, HPLC and MS (as described below).
3.1.2 Thin layer chromatography analysis (TLC)
Reaction mixture samples of MDI-alcohol adducts were spotted on reverse phase
26
TLC plates (RP-18 F 254 S, Merck) along with a MDI standard. The plates were developed
using a mixture of acetonitrile:water (70:30) and the compounds detected by UV
irradiation.
3.1.3 High performance liquid chromatography (HPLC) and mass spectrometry
Reaction products of MDI-alcohol reactions were quantitatively determined by HPLC as
their DBS derivatives (36). HPLC was performed using a reverse phase column (Waters,
Symmetry C18, 4.56 x 150 mm) on a Waters Breeze HPLC system with UV-VIS
detection at 240 nm. The compounds were separated by gradient elution (1 mL/min) of
acetonitrile (ACN) and water (70% ACN:30%H2O) for 5 mins and then linearly ramped
to 100% ACN over 20 mins and then returned to the original solvent ratio over a 5 mins
gradient). Typically 3 peaks were observed in the HPLC chromatograms for the DBA
derivatives of the alcohol-MDI reaction mixture. The eluted peaks were collected from
the HPLC system in vials, concentrated to dryness for subsequent MS analysis to confirm
the identity of the reaction products. The collected HPLC peaks of the DBA derivatives
of the alcohol-MDI reaction mixture were analyzed by electrospray ionization by mass
spectrometery (triple quadrapole, Micromass Quattro).
3.1.4 Wood fiber preparation
Maple wood flour (nominally 100 mesh) was obtained from American Wood Fibers. The
wood flour (500 g batches) was refined in a ball mill (4 L porcelain ball mill jar together
with 1” porcelain balls) for seven days at 83 rpm. The ball milled wood flour was
screened (50 g batches, 10 mins) into discrete fractions using 8-inch diameter test sieves
(100, 200, 300 and 400 mesh). The sieves were arranged in ascending order of there pore
size so that the smallest particles sieved will be collected in the pan below 400 mesh. Due
to the high yield of the 300 mesh wood fraction this material was used for subsequent
chemical wood modifications. The 300 mesh wood fiber (100 g batches) was extracted
with toluene prior to chemical modification. Particle size analysis was performed on each
27
fraction by light scattering on a Particle Sizing System, Inc. Model 770 Accusizer. A
water background count is subtracted from the sample run and the results were expressed
on a percent volume basis.
3.1.5 Wood fiber modification
Scaled up reactions to form the various MDI-alcohol adducts as described in
section 3.1.2 were used for wood modification experiments. Toluene extracted 300 mesh
wood flour (100 g) was reacted with the MDI-alcohol reaction mixture under reflux
(110oC) for 48 hours (Figures 3.1 and 3.2). The reaction mixture was filtered warm to
remove excess reagents and solvent and the modified wood fiber was further washed
three times with warm toluene (500mL), vacuum dried, and the gravimetric yield
determined.
Wood
fiber
-OH
OCN-R-NH-CO-O-R’
R’ = C 2, C 8, C 12, C 18
Wood
fiber
-O-CO-NH-R-NH-CO-O-R’
Figure 3.1 Reaction sequences of wood with an alcohol modified isocyanate
28
Figure 3.2 Experimental setup of for wood modification
3.1.6 Fourier transform infrared spectroscopy
The chemically modified wood flour was characterized by FTIR spectroscopy
(ThermoNicolet Avatar 370 FTIR spectrometer using an attenuated total reflectance
probe (4cm-1 resolution, 64 scans, 500 to 4000 cm-1 spectral range). Omnisec software
package was used to plot and analyze the resulting spectra.
3.1.7 Compounding/torque rheometery
High-density polyethylene (HDPE, Equistar Petrothene Type LB01000, powder)
was used as the matrix for compounding wood. WPCs were compounded (60 g total
weight batches) at three wood fiber loadings of 10, 30 and 50% using a mixer/torque
rheometer (Haake Polylab system with a 60 mL Roller rotor 600 mixing head). The
polymer and wood fiber were thoroughly mixed prior to compounding (1630 C, 35 rpm
and 600s run time). Torque and temperature measurements were recorded as a function
time during the compounding process. The compounded material was collected for
subsequent molding into test specimens and characterization.
29
3.1.8 Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) was performed on either a Mettler Toledo
DSC (Model FP82HT) or a TA 2920 DSC instrument to determine the extent of HDPE
crystallinity of the composite samples. Samples (5 mg, in duplicate) were equilibrated at
40°C for 2 minutes then ramped to 180°C at a heating rate of 10°C/min and held
isothermally for 3 minutes, cooled to 70°C at -10°C/min and held isothermally for 3
minutes and the temperature cycle repeated as described. Results were analyzed using
Mettler-Toledo FP99A Software. HDPE crystallinity was calculated from the ratio of the
melting enthalpy (110-1420C) of the sample to the melting enthalpy of 100% crystalline
HDPE with an enthalpy of 293 J/g. In addition, the plastic content was taken into account
when calculating % crystallinity.
3.1.9 Mechanical property evaluation
The compounded material was injection molded (Dynisco LMM mixer molder) into
tensile dog bone specimens (according to ASTM 1708-02a), flexural samples (60 x 9.2 x
3.15mm), and 2 mm x 25 mm diameter discs. The samples were processed at 180°C, 50
rpm and with a mold temperature 120° C. The tensile and flexural specimens were tested
on a universal testing machine (Instron 5500R testing machine). Data was recorded and
analyzed using the Bluehill software package. Tensile testing was performed according to
ASTM 1708-02a using self-tightening test grips and a crosshead speed of 1mm/ minute.
Sets of five specimens were tested until failure. Strain was measured using an
extensometer (Epsilon 3442). Flexural tests were performed according to ASTM
Standard D 790-02b with a crosshead speed of 1.31 mm/min and were conducted until
failure or 10% strain which ever came first. The results were analyzed statistically using
the SPSS soft ware package (11.5 for Windows) for significance for modification by
different alcohol and loading using multivariate analysis models. A comparative t-test
was used to compare the differences between two the approaches employed to modify
wood surface in this study.
30
3.2 SYNTHESIS OF WOOD CORE SHELL STRUCTURES: APPROACH 2 and 3.
This section is concerned with the development of wood core shell structures
using a different approach than described in section 3.1. The wood fiber (surface) is
reacted with a diisocyanate (MDI and TDI) to form a reactive layer around/on the fiber
that can readily react with either an alcohol or amine (of varying chain length). WPC was
then be prepared from the modified wood fibers and evaluated.
3.2.1 Wood-isocyanates (MDI) modified with alcohols (approach 2)
For this set of modifications, 100 g extractives free wood fiber (300 mesh) was
reacted with MDI (50 g) in toluene (500 mL) for 48 hours at 1100 C in a 2L reaction
kettle with constant stirring. The system was cooled and excess isocyanate dissolved in
toluene was removed by decanting. In separate experiments, each of the following
alcohols (ethanol, octanol, dodecanol, or octadecanol, 19.5 g) were added to the reaction
mixture together with additional toluene to cover the fiber, dibutyltinlaurate (DTL)
catalyst (1 g, 20% based on MDI) and the reaction continued for 24 hour at 110o C. The
modified wood flour was recovered by filtration to remove excess reagents and solvents,
and then the wet fiber washed three times with warm toluene (500mL). The fiber was air
dried in a fume hood then vacuum dried for 24 hours at 750 C and the yield was recorded.
3.2.2 Wood-isocynates (MDI, TDI, ODI) modified with amine (approach 3).
For this set of modifications, 100 g extractives free wood fiber (300 mesh) was
reacted with MDI, TDI or ODI (50 g) in toluene (500 mL) together with dibutyltinlaurate
catalyst (1g) for 48 hours at 1100 C in a 2 L reaction kettle with constant stirring. The
system was cooled and excess isocyanate and solvent were removed by decanting.
Octadecylisocyanate (ODI) is an aliphatic monofunctional isocyanate and was only
reacted with the extractives free wood and was not further reacted with ODA.
Octadecylamine (ODA) was added to the reaction mixture of MDI or TDI modified wood
31
fiber together with additional toluene to cover the fiber, dibutyltinlaurate (DTL) catalyst
(1 g) and the reaction continued for 48 hours at 110oC. The modified wood fiber (Fig.
4.1) was recovered by filtration to remove excess reagents and solvents and wet fiber
washed three times with warm toluene (500 mL). The fiber was air dried in a fume hood
then vacuum dried for 24 hrs at 750 C and the yield recorded.
NCO
CH2
N C (O )N H -R
N C (O )N H -R
+
R -N H 2
CH2
A m in e
NCO
M e t h y l d i is o c y n a t e
(M D I)
+
CH2
R -O H
Alc o h ol ( W o o d)
N C (O )O R
NCO
I s o c y a n t e - A m in e a d d u c t
I s o c y a n a t e - A m in e - W o o d
Figure 3.3. Reaction sequence in modification of wood surface by isocyanate and
attachment of long chain amine
Details on the methods (FTIR spectroscopy, WPC compounding/torque rheometry,
mechanical testing, DSC analysis) are described in Chapter 4.1.
32
CHAPTER-4
4.0 RESULTS AND DISCUSSION
4.1 Modification of wood fiber and characterization
4.1.1 Synthesis of core shell wood fiber structures (approach 1)
To prepare alkyl substituted modified wood fibers, it was first necessary to
synthesize a reactive alkyl derivative that can readily attach to the wood fiber. The alkyl
derivative chosen was an alcohol-modified diisocyanate (MDI), which could be prepared
with various alcohols of chain lengths (C2, C8, C12 and C18). The reaction between
MDI and an alcohol (in a 1:1 molar ratio) will potentially result in three compounds in
the reaction mixture, namely unreacted MDI, monosubstituted alcohol-MDI adduct (RO(CO)N-MDI) and the disubstituted alcohol-MDI adduct (RO-(CO)N-MDI-N-(CO)OR). The main product expected statistically would be the monosubstituted MDI-alcohol
adduct. To confirm the identities of the reaction products the isocyanate groups needs to
be derivatized in order to analyze them by chromatographic techniques such as TLC and
HPLC (36). This is because isocyanates can react readily with water or the stationary
phase (chromatography plates or columns) and thus not be analyzed. Derivatization of the
available isocyanate functionality was achieved with dibutyl amine (DBA) to form stable
compounds (Figure 4.1). Reverse phase TLC was used to qualitatively assess the
reaction results showed all three compounds after developing and exposure to UV
radiation. The alcohol modified MDI reaction products, after derivatization were: DBA2MDI-DBA2 (unreacted MDI); R-O(CO)N-MDI-DBA2 (monosubstituted adduct); and
(RO-(CO)N-MDI-N-(CO)-OR) (disubstituted adduct). Separation of the compounds is by
a combination of molecular size and lipophilicity. The movement of DBA-MDI-DBA
was the slowest due to its higher molecular weight (Figure 4.2).
33
Figure 4.1. Combinations of products obtained when MDI reacted with DBA and alcohol
Figure 4.2. Thin layer chromatogram of DBA derivatized ethanol-MDI reaction mixture
(top spot, MDI-Et2; middle spot, Et-MDI-DBA; bottom spot, MDI-DBA2).
34
4.1.2 High performance liquid chromatography (HPLC)
The combination of products obtained when MDI was reacted with DBA and
alcohol was separated out by HPLC. The peak obtained represents mono and
disubstituted adducts. Typically, there were three peaks, which represent the reaction
products (Figures 4.3 and 4.4). The peak with highest intensity is speculated to be the
product of interest (monosubstituted alcohol-MDI adduct). The elution time varied
according to molecular mass and hence for high molecular weight alcohol modification
the elution was towards the end of the separation run. Mass spectrometry was used to
characterize the products.
Figure 4.3. HPLC chromatogram of the DBA derivatives of octanol-MDI
reaction products.
35
.
0.80
AU
0.60
0.40
0.20
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00 16.00
Minutes
18.00
20.00
22.00
24.00
26.00
28.00
30.00
Figure 4.4. HPLC chromatogram of the DBA derivatives of octadecanol-MDI reaction
products.
4.1.3 Mass spectrometery
The collected HPLC peaks from the DBA derivatized isocyanates were
characterized by electrospray-MS. Electrospray-MS is a technique which generates
pseudomolecular ions (such as M+H+ and M+Na+) for each compound present in the
sample. For the DBA derivatized ethanol-MDI reaction mixture afforded 3 compounds
and their MS were assigned based on their molecular weight: (i) the monosubstituted
ethanol-MDI adduct gave ions at m/z = 426 and 448 which corresponds to the
pseudomolecular ions M + H+ and M + Na+, respectively (Figure 4.5); (ii) the
disubstituted ethanol-MDI adduct gave ions at m/z = 319 and 341 which corresponds to
the pseudomolecular ions M + H+ and M + Na+, respectively; and (iii) the DBA
derivatized MDI product gave ions at m/z = 509 and 531 which corresponds to the
pseudomolecular ions M + H+ and M + Na+, respectively. A similar approach of HPLC
separation followed by electrospray-MS was performed on the other alcohol modified
MDIs and the results are summarized in Table 4.1. An electrospray mass spectrum of the
DBA derivative of the monosubstituted dodecanol-MDI adduct is shown in Figure 4.6.
(Electrospray MS of the other DBA derivatized alcohol-MDI compounds are shown in
the Appendix). These data shows that the desired alkyl modified isocyanate (alcoholMDI adduct) was formed and thus could be used to modify the wood fiber surface
(Figure 3.1). The disubstituted alcohol-MDI adduct will not react with wood and can
easily be removed by toluene solvent washing.
36
Figure 4.5. Electrospray-MS spectrum of the DBA derivatized monosubstituted-ethanol
MDI adduct.
Figure 4.6. Electrospray-MS spectrum of the DBA derivatized monosubstituted
dodecanol-MDI adduct.
37
Table 4.1. Summary of the positive ion electrospray-mass spectral ions
(pseudomolecular) of the mono and di-substituted alcohol-MDI adducts.
Alcohol
(R)
DBA-MDI-R
R-MDI-R
(monosubstituted)
(disubstituted)
M (m/z)
M-H+
M-Na+
M
M-H+
M-Na+
(m/z)
(m/z)
319
341
445
467
599
621
767
789
(m/z)
(m/z)
C2H5OH
425
426
448
C25H35N3O3
C8H17OH
509
C17H22N2O4
510
532
C31H47N3O3
C12H25OH
565
649
444
C29H46N2O4
566
588
C35H55N3O3
C18H37OH
318
598
C37H62N2O4
650
C41H67N3O3
672
766
C49H86N2O4
4.1.4 Wood fiber modification
Commercial maple wood flour was refined into small particles by ball milling and
subsequently screened to obtain the major fraction (300 mesh, 50% yield) (Table 4.2).
The 300 mesh fraction was analyzed for particle size distribution using a light scattering
instrument the volume weighted average particle size was of 33 µm and the frequency
weighted average particle size was 6 µm.
Prior to fiber modification, the screened wood flour was extracted with toluene to
remove any wood extractives. The extractive free wood fiber was then reacted with the
alcohol modified MDI adduct. It is expected that the isocyanate group will react with
hydroxyl groups on the wood particle to form a urethane linkage. The modified fibers
38
were further extracted with toluene to remove any excess reagents and then vacuum
dried. The wood fiber weight gains for the octadecanol (C18), dodecanol (C12), octanol
(C8), and ethanol (C2) -MDI adduct reacted wood fiber were 63, 29, 51, and 37%,
respectively. From these results, it was found that octadecanol and octanol showed
greater weight gains. The lower weight gains for other alcohol may have arisen due to
loss of wood fiber during the reaction workup, which involved several solvent washes
and filtration steps.
Table 4.2. Yields of sieved ball milled maple wood flour and particle size analysis.
Mesh size fraction
Weight (%)
Volume weighted
Frequency weight
average particle size
ed average particle
(µm)
size (µm)
>100
0.8
110
3.7
100-200
5.2
109
3.9
200-300
49.8
33
6.6
300-400
22.2
29
6.9
<400
19.3
25
7.3
4.1.5 Fourier transform infrared spectroscopy
FTIR spectroscopy is widely used to elucidate chemical structures and functional
groups of woody and pulp samples (39,40) and therefore is a suitable technique to
characterize the isocyanate modified wood particles. The presence of the isocyanate
modified wood fiber was supported by the observation of the carbonyl groups of urethane
linkage (1735 cm-1), urea groups (1640 cm-1), aliphatic -CH2- bands (2920 and 2850 cm1
), amide bands (1550 and 1600 cm-1), NH stretching band (3340 cm-1) and the
disappearance of the isocyanate band (N=C=O, 2270cm-1) by FTIR spectroscopy. Figure
4.7 shows the FTIR spectrum of the ethanol-MDI-adduct modified wood fiber and
extractives free wood fiber.
39
The signals attributable to the urethane functionality were more prominent in
ethanol-MDI and dodecanol-MDI modified wood fibers. The signals attributable for the
urethane linkage were less prominent in the octanol-MDI and octadecanol-MDI modified
wood fibers. FTIR spectroscopy proved to be a rapid and useful tool to confirm the
chemical linkage between the isocyanate and the wood fiber and thus supports that wood
was chemically modified. These results suggest that a urethane bond was formed between
wood hydroxyl groups and isocyanate functional entities.
94
93
92
91
82
81
79
78
816.91
1103.75
77
76
4000
3500
3000
2500
2000
717.89
1163.73
1074.51
1533.66
80
1235.90
1510.33
83
1500
1055.11
84
750.26
1469.35
1702.41
85
1310.72
86
1412.79
87
2918.99
%Transmittance
88
1593.55
89
2850.91
3324.65
90
1000
500
Wavenumbers (cm-1)
14 21.39
%Transmittance
88
86
89 7.0 7
12 33.76
90
1371.15
33 82.60
92
15 04.57
1735.91
94
15 93.46
96
84
82
80
78
76
72
70
4000
3500
30 0 0
25 0 0
20 00
1 5 00
10 33.47
74
1 0 00
500
W a ve n u m b e r s ( c m - 1 )
Figure 4.7. FTIR spectra of the ethanol MDI-adduct modified wood fiber (top) and
extractives free maple fiber (bottom).
40
4.1.6 Torque rheometry
Torque rheometry was conducted to determine the compoundability of the
modified wood fiber via approach 1, with HDPE. Figure 4.8 shows the torque curves of
ethanol-MDI and dodecanol-MDI modified wood fibers as with an unmodified wood
fiber control. Ethanol-MDI modified wood fiber required a maximum torque of 10Nm as
compared to unmodified “control” wood HDPE blend at 40 Nm. Octadecanol-MDI,
dodecanol-MDI and octanol-MDI modified wood fibers required a maximum torque
between 25 and 30 Nm. The torque required to compound the modified wood fibers was
substantially lower than unmodified wood fiber controls. Furthermore, the steady state
torque values for the WPC made from control, octadecanol-MDI, and dodecanol-MDI
modified wood fibers were approximately 10 Nm as compared to WPC made with
octanol-MDI and ethanol-MDI fibers at 7 and 5 Nm, respectively.
C2
C8
C12
C18
control
Torque (Nm)
40
30
20
10
0
0
1
2
3
Time (min)
4
5
6
Figure 4.8. Torque rheometery data for compounding WPC made from modified wood
using approach-1.
41
4.1.7 Differential Scanning calorimetry (DSC) results
To help explain why the properties of the WPC made from modified wood fiber
(in some instances improved) as compared to the control fiber, DSC was used to
investigate the extent of HDPE crystallization in these WPC systems (Table 4.3). Two
heating and cooling cycles were performed on the molded samples at 30% wood content.
The melting enthalpy (from the 2nd melt) was used to determine the extent of
crystallization (43) (Table 4.3). The crystallinity of HDPE in the WPC made from
modified wood fibers were higher (37-44%) than the unmodified control fiber (25%),
unfortunately no obvious trend was observed. The crystallization temperature of HDPE in
the WPC was shown to increase from 117.3 to 119.6o C with an increase in alkyl chain
length (C2 to C18) of the modified wood fiber. These data suggests that the
crystallization of HDPE was induced, at a slightly higher temperature, by improving the
nucleation at interfacial bond with the wood fiber by having an extended hydrophobic
shell (44).
Table 4.3. DSC results for crystallinity and crystallization temperature for WPC made
from modified wood fiber (30% loading) using approach-1.
Crystallization
Wood fiber
Heat of Enthalpy
HDPE crystallinity
temperature
sample
(J/g) 110-142°C
(%)
(oC)
Control
51.8
25.3
119.1
C2
78.9
38.5
117.3
C8
77.0
37.5
118.9
C12
91.0
44.4
118.8
C18
80.4
39.2
119.6
42
4.2.1 Synthesis of core shell wood fiber structures (approach 2)
A simpler approach was devised to make wood core shell structures. This approach “2”
first activates the wood fiber surface by reaction with the diisocyanate, MDI, which was
then reacted with an alcohol to form the wood core-shell structures, as shown below
(Figure 4.9). The wood fiber weight gains for ethanol, octanol, dodecanol and
octadecanol modified MDI activated wood fiber were 40, 53, 33, and 43%, respectively.
The octanol and octadecanol modification of the MDI-wood fibers gave maximum
weight gain. The loss of wood fiber during workout is not accounted which may be
relatively significant. The weight gain of wood fiber suggests a chemical modification
has occurred.
Wood
fiber
-O-CO-NH-R-NH-CO-O-R’
R’OH R’ = C2H5, C8H17, C12H25, C18H37
Wood
fiber
OCN-R-NCO
-OH
Wood
fiber
-O-CO-NH-R-NCO
Figure 4.9. Reaction scheme for preparing wood core-shell structures by approach 2.
4.2.2 Fourier Transform Infra red Spectroscopy (FTIR)
The modified wood fibers synthesized by approach 2 were characterized by FTIR
spectroscopy. Figure 4.10 shows the FTIR spectra of ethanol modified MDI-wood fiber
and extractives free wood fiber. It was generally observed that urea groups (1640 cm-1)
were present in the spectrum of all the modified wood fiber samples, which was a direct
evidence of wood modified by isocynates. -CH2- bands (2920 and 2850 cm-1) were
prominent which represent the chain length of the alcohol. The urea groups supports that
43
the reaction of wood OH or alcohol OH with an isocyanate group. In addition, a NH
stretching band was observed at (3340 cm-1) in the spectra and conversely observed the
disappearance of the isocyanate band at (N=C=O) 2270cm-1.
96
95
94
84
85 0.76
72 0.7 6
85
16 97.38
86
81 6.4 3
87
13 10.26
88
14 67.23
2920.91
89
29 52.57
%Transmittance
90
14 12.95
91
15 94.46
3313.36
92
1648.51
28 52.21
93
15 34.12
79
78
4000
3500
30 00
25 00
20 00
15 00
10 18.83
769.61
80
1057.24
81
12 31.33
15 11.40
82
11 08.17
83
10 00
500
Wa ven umb ers ( cm- 1)
14 21.39
%Transmittance
88
86
89 7.0 7
12 33. 76
90
1371. 15
33 82. 60
92
15 04.57
1735.91
94
15 93.46
96
84
82
80
78
76
72
70
4000
3500
30 0 0
20 00
25 0 0
1 5 00
10 33. 47
74
1 0 00
500
W a ve n u m b e r s ( c m - 1 )
Figure 4.10. FTIR spectra of the ethanol MDI-adduct modified wood fiber by approach-2
(top) and extractives free maple fiber (bottom).
44
4.2.3 Compounding/torque rheometery
The Haake torque rheometer was used to compound the wood fiber in the HDPE
matrix and the torque required for compounding was measured for each of the modified
wood fiber WPC systems. The maximum torque required to compound the C2, C8, and
C18 chemically modified wood fibers (23-29 Nm) was lower than the unmodified wood
fiber controls (42 Nm) and the C12 modified wood fibers (Figure 4.11). The maximum
torque values were comparable to those from the modified wood fibers using approach 1
(Figure 4.8). The steady state torque values for all the compounded WPC unmodified and
modified wood fibers were similar at 12-14 Nm. These data generally show that
modification of the wood fiber decreases the load required for blending wood fiber with
HDPE and this may be attributable to improved dispersion.
C2
C8
C12
C18
control
Torque (Nm)
40
30
20
10
0
0
1
2
3
Time (min)
4
5
6
Figure 4.11. Torque rheometery data for compounding WPC made from modified wood
using approach-2.
45
4.2.4. Differential scanning calorimetry (DSC)
DSC is a tool to estimate the extent of HDPE crystallization in WPC made from
modified wood fiber. Two heating and cooling cycles were performed on the molded
samples at 30% wood content. The heat of enthalpy was higher for the WPC made with
modified wood fibers when compared to the control (43) (Table 4.4). The crystallinity of
HDPE in the WPC made from modified wood fibers were higher (31-37%) than the
unmodified control fiber (25%), unfortunately no obvious trend was observed. The
results showed that as the alcohol chain length increased the degree of crystallinity
increased, except for the dodecanol modified fibers. These data suggests that the
crystallization of HDPE was induced by improving the nucleation at interfacial bond with
the wood fiber by having an extended hydrophobic shell (44). The crystallization
temperature of HDPE in the WPC made from the modified wood fiber between 116 to
119o C.
Table 4.4. DSC results for crystallinity and crystallization temperature for WPC made
from modified wood fiber (30% loading) using approach-2.
Heat of enthalpy
HDPE crystallinity
Crystallization
Wood fiber sample
(J/g)
(%)
temperature (oC)
Control
51.8
25
119.1
C2
64.7
32
118.9
C8
67.2
33
116.4
C12
62.7
31
118.6
C18
75.2
37
119.3
46
4.3.1 Synthesis of core shell wood fiber structures (approach 3)
An alternative approach was devised (to those already described, 1 and 2) to make
wood core shell structures. This approach “3” first activates the wood fiber surface by
reaction with the diisocyanate, like in approach 2, but instead of reacting the isocyanate
cover wood fiber with an alcohol, it is reacted with an long chain amine (octadecylamine
(ODA) to form the wood core-shell structures, as shown below (Figure 4.12). The
advantage of the amine over the alcohol is its higher reactivity with an isocyanate (41). In
this section three different isocyanates were evaluated, MDI, TDI and
octadecylisocyanate (ODI) to activate the wood fiber and the resultant weight gains for
the wood core shell structures were 60, 62 and 46 %, respectively.
ODI
Wood
fiber
OCN-R-NCO
-OH
Wood
fiber
-O-CO-NH-C18H37
Wood
fiber
-O-CO-NH-R-NCO
ODA
Wood
fiber
-O-CO-NH-R-NH-CO-NH-C18H37
Figure 4.12. Reaction scheme for preparing wood core-shell structures by approach 3.
47
4.3.2 Fourier transform infra red spectroscopy
FTIR spectroscopy was employed to characterize the modified wood fibers and
several characteristics bands were observed, namely: (i) highly prominent aliphatic (CH2-) bands (2920 and 2850 cm-1) associated with ODI and ODA, (ii) carbonyl groups
assigned to urethane and urea linkages were observed at 1720 cm-1 and 1640 cm-1,
respectively and (iii) the disappearance of the isocyanate band at (N=C=O) 2270cm-1 in
the spectrum. These data supports that wood fiber was chemically modified with the
expected functionality (Figure 4.13).
Figure 4.13. FTIR spectrum of TDI- Octadecyl amine modified wood fiber (approach 3).
4.3.3 Torque rheometry
Toque rheology data showed that various wood-isocyanate modified fibers showed
considerable reduction in the torque required to compound the WPC as compared to the
unmodified wood fiber control (Figure 4.14). The maximum torque required to
compound the WPC with the MDI-ODA, TDI-ODA, ODI modified fibers and control
were 27, 26, 11 and 41 Nm, respectively. The maximum torque values were comparable
to those from the modified wood fibers using approaches 1 and 2 (Figures 4.8 and 4.12).
48
The steady state torque values for compounding WPC with unmodified fiber and MDIODA and TDI-ODA modified wood fibers were similar at 8-12 Nm, while the ODI
modified wood fiber had a low steady state torque value of 5 Nm. These data generally
show that modification of the wood fiber decreases the torque required for compounding
wood fiber with HDPE and this may be attributable to improved dispersion.
TDI
ODI
MDI
Control
Torque(Nm)
40
30
20
10
0
0
1
2
3
Time Min
4
5
6
Figure 4.14. Torque rheometery data for compounding WPC made from modified wood
using approach-3.
4.3.4 Differential scanning calorimetry (DSC)
As a tool to gain insight on HDPE, DSC was again employed to measure the
extent of HDPE crystallization as well as the onsets of crystallization and melt (Table
4.6). It was observed that the WPC made from the MDI-ODA and ODI modified wood
fiber had a lower observed crystallization temperature than the WPC made from control
and TDI-ODA modified fibers. HDPE crystallinity was higher for the WPC made from
MDI-ODA and TDI-ODA modified fibers samples when compared to the control.
49
However, the crystallinity was lowest for the WPC made from ODI modified wood fibers
(Table 4.5). There was no obvious trend between crystallinity content and Tc for the
WPC.
Table 4.5. Summary of DSC results for WPC (30% loading) made from the amine
modified wood fibers.
Crystallization
Wood fiber samples Heat of Enthalpy (J/g)
Crystallinity (%)
temperature (oC)
Control
51.8
25
119.1
MDI-ODA
66.5
32
116.6
TDI-ODA
72.3
35
119.6
ODI
42.2
21
117.8
The three different chemical approaches used for modifying wood fiber for use in
WPC was statistically compared by t-test, the results showed that there was no significant
difference for the approaches followed in terms of tensile strength and tensile modulus
(Appendix Table-4). Flexural strength and modulus was also compared for the three
approaches to show there is no significant difference between both approaches (Appendix
Table-8).
50
CHAPTER 5
CONCLUSIONS
WPCs are experiencing a growing market demand; hence it is logical to study
ways to enhance the performance attributes of WPCs. The application of various wood
modification techniques and their effect has to be typified. This will augment the industry
with better understanding of modification and consequently delivering better products.
The major findings from this research have been summarized as follows based on the
objectives stated in the beginning.
Ball milling proved to be an efficient way to disintegrate large sized wood flour to
smaller wood particle fragments. Subsequent screening afforded a 200-300 mesh fraction
(50%), with a volume weighted average of 25µ and frequency weighted average of 6 µm.
However, it was not possible using the ball milling procedure and screening to generate
sub-µm sized wood particle for making nano-wood fiber composites.
Three approaches were assessed for the generation of wood core-shell type
structures for use in WPC. The first approach was to modify MDI, a diisocyanate, with a
fatty alcohol (ethanol, octanol, dodecanol, and octadecanol) to form the alcohol-MDI
adduct which could then be reacted to the wood surface. This approach proved to be time
consuming, in a 2-stage operation, in preparing wood-core shell structures with weight
gains of between 30 and 60 %. The second approach was devised as an easy “one-pot”
synthesis in which the wood fiber was reacted with MDI to activate the surface, which
could then be reacted with a fatty alcohol to form a wood core-shell structure and resulted
in weights gains between 30 and 50%. FTIR spectroscopy confirmed that similar wood
modifications occurred for both approaches. In order to improve the yields obtained by
approach 2 and quicker reaction times, a modified scheme, approach 3, was trailed in
which MDI or TDI was used to activate the wood surface and then could be reacted with
51
a fatty amine to form wood core-shell structures and resulted in weight gains of 60%.
FTIR spectroscopy confirmed that the desired wood modification occurred.
The modified wood fibers (from all 3 approaches) were readily compounded
(lower maximum and steady state torque values) into HDPE relative to unmodified wood
fiber controls. It is speculated that modifying the wood surface with alkyl chains made
the wood fibers more disperable/miscible within the HDPE matrix and therefore required
less mechanical energy to compound. Thermal analysis data showed that the WPC made
from modified wood fibers had a higher level of HDPE crystallization than WPC made
from unmodified fiber controls. These results suggest that nucleation at interfacial region
between HDPE and the wood fiber was improved by wood surface modification (44).
The mechanical properties of WPC made from chemically engineered wood fibers gave
variable results and show potential for improved performance.
The outcome of this preliminary study has shown that wood fiber modification for
use in WPC has potential benefits in improved processability, such as production rate.
Detailed data analysis has identified some limitations of this study. Therefore several
areas of future research activities to address these gaps are: (i) increase the number of
sample replicates for mechanical properties in order to reduce the coefficient of variation,
(ii) assess the melt flow characteristic of the compounded WPC material by either melt
flow rate and/or parallel plate rheometry, (iii) determine the surface energies of modified
wood fibers and control fibers to assess the extent of surface modification, and (iv)
further chemical characterization of the modified wood fibers by solid state 13C CP-MAS
NMR spectroscopy and X-ray photo-electron spectroscopy.
52
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microspheres by precipitation polymerization. Macromolecules., 33:4354-4360.
56
47. Kalinina, O and Kumacheva, E. (1999) A “core-shell” approach to producing 3D
polymer nanocomposites. Macromolecules., 32: 4122-4129.
48. Bucknall, CB. (2001) Blends containing core-shell impact modifiers Part1:
Structure and tensile deformation mechanisms. Pure Appl. Chem., 73(6):897-912.
49. Kozlowski, M and Bucknall, CB. (2001) Blends containing core-shell impact
modifiers Part2: Melt rheology of rubber-toughened plastics. Pure Appl. Chem.,
73(6):913-926.
57
APPENDIX
14 21. 39
%Transmittance
88
86
89 7.0 7
1371. 15
90
12 33. 76
33 82. 60
92
15 93.46
1735.91
94
15 04.57
96
84
82
80
78
76
72
70
4 0 00
3500
30 0 0
25 0 0
2 0 00
15 00
10 33.47
74
1 0 00
Wa ven u m b e rs ( cm - 1)
Figure A-1. FTIR spectra of 300 mesh extractives free maple wood flour.
Figure A-2. Electrospray-MS spectra for Octadecanol-NCO-DBA deriavtive
500
58
Figure A-3. Electrospray-MS spectrum for Octanol-NCO-DBA deriavtive
Figure A-4. Tensile stress-strain curves for injection molded WPC (10% composite,
Octanol-MDI modified wood + HDPE) sample.
59
WPC Mechanical properties
Approach-1: Tensile and flexural testing
Tensile WPC specimens were injection molded from the torque rheometer compounded
samples .WPC tensile strength was compared between samples made from the modified
wood fiber and unmodified wood fiber controls. (Table A-1 and Figure A-5). Statistical
analysis was performed to analyze the effect of alcohol modification and loading. It was
found that there was no significant difference in tensile strength properties due to
different alcohols used to modify wood when compared with control. There was
significant difference in WPC tensile strength when 50% loading was compared to 30and
10% loading.
There was no statistical significance in tensile strength of WPC made from the
various wood fibers, between the effect of 10 and 30% loading. For tensile modulus the
effect of carbon chain was significant between WPC made from control, octadecanoland octanol-MDI modified wood fibers. There was also a significant difference when
WPC made from ethanol-MDI modified wood fibers was compared with octanol-MDI,
octadecanol-MDI, modified wood fibers. WPC made with octadecanol-MDI modified
wood fibers was also significantly different from dodecanol-MDI modified wood fibers.
There was a significant difference in tensile modulus between WPC samples made at 10,
30, and 50% wood loading (Table A-A)
60
30
25
20
Tensile
15
Strength (Mpa)
10
5
0
c2
10% Wood loading
c8
c12
30% Wood loading
c18
Control
50% Wood loading
Figure A-5. Tensile strength for various alcohols–wood modifications in approach-1.
Generally, an increase in wood content resulted in an increase in WPC tensile modulus
for all samples and especially notable at the 50% loading for the ethanol-MDI and
dodecanol-MDI modified fibers (Figure A-6).
2.5
2
Tensile
Modulus
(Gpa)
1.5
1
0.5
0
c2
10% Wood loading
c8
c12
30% Wood loading
c18
Control
50% Wood loading
Figure A-6. Tensile modulus for modified wood with different chain length alcohols in
approach –1
61
Table A-A. Tensile strength and tensile modulus for WPC formulation by approach–1.
Modified wood
10% wood
30% wood
50% wood
Tensile strength (MPa) (Variance)
Control
23.0 (3.65)
22.3 (6.1)
17.3 (11.0)
C2
24.0 (7.91)
23.0 (12.2)
23.7 (27.5)
C8
24.0 (10.20)
26.5 (16.8)
19.2 (3.9)
C12
23.4 (5.76)
21.1 (8.2)
19.0 (8.2)
C18
25.7 (3.91)
25.0 (3.7)
18.1 (24.0)
Tensile modulus (GPa) (Variance)
Control
1.5 (0.02)
1.7 (0.05)
1.9 (0.03)
C2
1.4 (0.03)
1.9 (0.06)
2.1 (0.02)
C8
1.3 (0.08)
1.6 (0.03)
1.5 (0.04)
C12
1.4 (0.02)
1.5(0.04)
2.0 (0.08)
C18
1.4(0.01)
1.2(0.03)
1.5 (0.13)
Modified wood
10% wood
30% wood
50% wood
Flexural strength (MPa) (Variance)
Control
28.3 (7.01)
33.5 (3.8)
35.3 (3.9)
C2
24.0 (29.86)
31.1 (15.0)
30.6 (8.9)
C8
27.2 (26.28)
35.2 (22.7)
36.0 (6.6)
C12
32.5 (6.02)
29.2 (7.7)
34.0 (9.2)
C18
33.5 (19.04)
32.1 (8.7)
34.1 (9.1)
Flexural modulus (GPa) (Variance)
Control
1.1 (0.05)
1.4 (0.03)
2.1 (0.04)
C2
1.0 (0.03)
1.9 (0.07)
2.1 (0.13)
C8
0.9 (0.03)
1.2 (0.13)
1.9 (0.26)
C12
1.1 (0.04)
1.3 (0.07)
1.9 (0.13)
C18
1.1 (0.04)
1.2 (0.04)
2.1 (0.14)
Table A-B. (above) Flexural strength and modulus for WPC formulation by approach-1.
The flexural properties of the WPC made from MDI-alcohol adducts (approach 1)
and controls are given in Table A-B. Statistical analysis was performed to analyze the
62
effect of alcohol modification and loading. It was observed that there was no significant
difference in flexural strength between the WPC made from the various MDI-alcohol
modified wood fiber at a given wood loading. WPC flexural strength was significantly
different between samples made from 50% and 10 % wood loading. There was no
significance between 30 and 50% loading on flexural strength. For flexural modulus
results for the WPCs, there was no significant difference observed between fiber
modifications. However, there were significantly differences in WPC flexural modulus as
a function of wood loading (Appendix, Table-5).
Approach- 2: Tensile and flexural testing
The tensile properties of the WPC made from the modified alcohol MDI-wood
fiber and control (approach 2) are given in Table A.C and shown in Figs A.8 and A.10.
The effect of different long chain alcohol used to modify wood for WPC on tensile
strength and modulus was statistically analyzed. There was significant difference in the
WPC tensile strength when ethanol modified fibers was compared with octanol and
dodecanol modified fibers. There was no difference in tensile strength between other
formulations. WPC made at 50% wood loading was significantly different than WPC at
wood loadings of 10 and 30%. There was no difference between tensile strength results
of WPC at 10 and 30% wood loadings.
63
25
20
15
Tensile
strength (Mpa) 10
5
0
c2
c8
c12
c18
Control
Alcohols
10% Wood Loading
30% Wood Loading
50% Wood Loading
Figure A.8 Tensile strength graph for modification-2
A
B
Figure A.9 A-Instron testing machine and B- Injection molder
64
2
1.5
Tensile
modulus (Gpa)
1
0.5
0
c2
c8
c12
c18
Control
Alcohols
10% Wood Loading
30% Wood loading
50% Wood loading
Figure A.10 Tensile modulus for modification method-2
The results of flexural strength and flexural modulus are presented in Table A.D. There
was no statistically significant difference in flexural modulus of WPC made from the
alcohol modified wood fibers. It was established that all different wood loadings: 10, 30,
and 50 are significantly different from one another (Appendix Table-2).
Table A.C Tensile strength and tensile modulus of WPC formulation by approach-2
Modified wood
10% wood
30% wood
50% wood
Tensile strength (MPa) (Variance)
Control
23.0 (3.6)
22.3 (6.1)
17.3 (11.0)
C2
24.0 (5.2)
23.4 (6.6)
22.6 (19.7)
C8
23.2 (9.9)
19.9 (1.7)
13.4 (5.6)
C12
21.5 (1.4)
20.3 (2.1)
17.0 (22.5)
C18
20.1 (4.2)
21.4 (2.9)
20.6 (5.1)
Tensile modulus (GPa) (Variance)
Control
1.5 (0.02)
1.7 (0.05)
1.9 (0.03)
C2
1.3 (0.01)
1.9 (0.03)
1.8 (0.12)
C8
1.4 (0.01)
1.5 (0.01)
1.6 (0.18)
C12
1.3 (0.01)
1.4 (0.02)
1.8 (0.11)
C18
1.4 (0.01)
1.6 (0.13)
1.8 (0.01)
65
Table A.D Flexural strength and modulus for WPC formulated by approach-2
Modified wood
10% wood
30% wood
50% wood
Flexural strength (MPa) (Variance)
Control
28.3 (7.0)
33.5 (3.8)
35.3 (3.9)
C2
24.0(7.0)
25.0(16.6)
32.7(2.5)
C8
30.1(4.7)
29.0 (2.9)
36.0 (2.8)
C12
26.1 (6.7)
28.1 (5.0)
35.2 (7.8)
C18
31.3 (6.5)
29.1 (6.3)
32.1 (3.7)
Flexural modulus (GPa) (Variance)
Control
1.1 (0.05)
1.4 (0.03)
2.07 (0.04)
C2
0.97 (0.02)
1.3 (0.04)
1.8 (0.03)
C8
1.3 (0.02)
1.2 (0.03)
2.1 (0.02)
C12
1.1 (0.03)
1.4 (0.01)
2.0 (0.01)
C18
1.2 (0.02)
1.4 (0.01)
1.9( 0.01)
There was significant difference in flexural strength between MDI-wood-ethanol
modifications, MDI-wood-octanol modification and control. Flexural strength was also
significant when 50% loading was compared to 10 and 30% loading. It was statistically
shown that there was no significant difference in flexural modulus by the use of various
alcohols to modify wood. All different loading had significant difference in flexural
modulus (Appendix, Table-6).
Tensile and flexural testing of WPC prepared from amine modified wood fiber
Tensile properties of the WPC prepared from MDI-ODA, TDI-ODA and ODI
modified wood fibers are given in Table A.E and shown in Figures. A.11 and A.12. It
was observed that the WPC tensile strength was shown to decrease with an increase in
wood content for all the modified wood fiber and control samples. WPC made from the
MDI-ODA and ODI modified wood fiber gave a lower tensile strength values as
66
compared to the unmodified control fiber. However, WPC made from the TDI-ODA
modified wood fiber gave higher tensile strength properties as compared to the controls at
the three wood loadings. The WPC preparation with the lowest strength properties was
observed for the ODI modified wood fiber at a 50% wood loading. Statistical analysis of
the tensile strength data showed that the WPC made from TDI-ODA modified wood
fibers was significantly different than the to MDI-ODA and ODI wood fiber
modifications. There was no significant difference in WPC tensile strength between
samples made from control fiber, TDI-ODA modified fiber, MDI-ODA modified fiber
and ODI modified fiber. Fiber loading had a significant (negative) effect in tensile
strength for the WPC samples when going from 10 to 50% wood content. There was no
significant difference in WPC tensile strength from samples made at 10 and 30% wood
loadings.
The tensile modulus results for the WPC samples are given in Table A.F and
Figure A.12. There was no significant difference in tensile modulus of WPC made with
MDI-ODA, TDI-ODA and controls wood fiber. However, tensile modulus properties for
WPC made from ODI modified wood fiber was statistically different than from those
made from the control, MDI-ODA and TDI-ODA fiber. The effect of wood loading on
tensile modulus of WPC showed that (i) there was no significant difference in samples
made at 30 and 10% wood fiber loadings and (ii) there was a significant difference in
samples made at the 50% wood loading with those made from either 10 and 30% wood
loading (Appendix Table-3).The results show that as wood fiber content increases the
tensile strength is lowered.
67
30
25
20
Tensile
15
strength (Mpa)
10
5
0
MDI
TDI
ODI
Control
Isocynates
10% Wood Loading
30% Wood loading
50% Wood loading
Figure A.11 Tensile strength graph for modification by wood-isocyanate and amine
2
1.5
Tensile
modulus 1
(Gpa) 0.5
0
MDI
TDI
ODI
Control
Isocynates
10% Wood Loading
30% Wood Loading
50% Wood loading
Figure A.12 Tensile modulus graph for modification by wood-isocyanate and amine
68
Table A.E Tensile strength and tensile modulus of various isocyanate-amine modification
of wood
Modified wood
10% wood
30% wood
50% wood
Tensile strength (MPa) (Variance)
Control
23.0 (3.6)
22.3 (6.1)
17.3 (11.0)
MDI-ODA
20.0 (43.3)
17.6 (2.8)
14.9 (12.4)
TDI-ODA
25.2 (43.3)
22.6 (42.4)
19.2 (5.4)
ODI
21.9 (19.1)
16.7 (8.6)
13.6 (15.4)
Tensile modulus (GPa) (Variance)
Control
1.5 (0.02)
1.7 (0.05)
1.9 (0.03)
MDI-ODA
1.5 (0.11)
1.4 (0.03)
1.8 (0.05)
TDI-ODA
1.6 (0.03)
1.5 (0.04)
1.6 (0.03)
ODI
1.3 (0.01)
1.4 (0.02)
1.3 (0.01)
Table A.F. Flexural strength and flexural modulus for various isocyanate amine
modification of wood
Modified wood
10% wood
30% wood
50% wood
Flexural strength (MPa) (Variance)
Control
28.3 (7.0)
33.5 (3.8)
35.3 (3.1)
MDI-ODA
26.1 (8.5)
30.0 (20.7)
29.7 (3.1)
TDI-ODA
29.7 (6.3)
33.6 (5.9)
36.0 (18.6)
ODI
23.1 (10.2)
21.0 (12.3)
23.7 (12.0)
Flexural modulus (GPa) (Variance)
Control
1.1 (0.05)
1.4 (0.03)
2.1 (0.04)
MDI-ODA
1.3 (0.12)
1.5 (0.17)
1.8 (0.44)
TDI-ODA
1.5 (0.17)
1.4 (0.03)
1.7 (0.04)
ODI
1.2 (0.02)
1.3 (0.01)
1.4 (0.01)
The flexural properties of the WPC prepared from MDI-ODA, TDI-ODA and ODI
modified wood fibers are given in Table A.F. There were significant differences in WPC
flexural strength between the various isocyanates modified wood. It was shown that
MDI-ODA, TDI-ODA and ODI wood modifications for use in WPC were significantly
69
different in their flexural strength. There was no significant difference in flexural strength
from the WPC made from TDI-ODA and control fiber. The flexural strength of the WPC
made at 10% wood loading was significantly different in when compared to 30 and 50%
loading, however, there was no difference between 30 and 50% loading results. The
Flexural modulus of the WPC made from ODI modified wood fiber was significantly
different than the samples made from MDI-ODA, TDI-ODA and control fibers. As
expected, the flexural stiffness of WPC made at the higher 50% loading was significantly
different from 30 and 10% loading (Appendix, Table-A.F).
Table-1 Statistics for Tensile strength and tensile modulus Approach-1
Tensile strength
CARBON
N
Control
C18
LOADING
C12
50
Subset
15
Tensile modulus
15
25
212.9733
21.5067
19.4872
C8
30
15
25
23.2687
23.5740
C2
10
15
25
23.5300
24.2260
.265
1.000
.766
N
15
1
20.8667
Subset
Sig.
Sig.
CARBON
N
2
Subset
1
Control
15
C18
15
C12
15
C8
15
C2
15
2
3
1.7000
1.6100
1.6100
1.3767
1.4533
1.4533
1.8133
Sig.
.856
.250
.066
Subset
LOADING
50
N
1
25
30
25
10
25
Sig.
2
3
1.7860
1.5820
1.4040
1.000
1.000
1.000
70
Table-2 Statistics for Tensile strength and Flexural modulus Approach-2
Tensile strength
Subset
CARBON
Control
N
15
1
20.8667
2
20.8667
C18
15
20.6833
20.6833
C12
15
19.6000
C8
15
18.8587
C2
15
23.3280
Sig.
.387
.135
Subset
LOADING
50
N
1
18.1800
25
30
25
10
25
2
21.4720
22.3500
Sig.
1.000
.515
Tensile modulus
Subset
CARBON
Control
N
15
1
1.7000
C18
15
1.5100
C12
15
1.5187
C8
15
1.4967
C2
15
1.6653
Sig.
LOADING
.216
N
Subset
1
50
25
30
25
10
25
Sig.
2
3
1.7852
1.5808
1.3684
1.000
1.000
1.000
71
Table-3 Statistics for Tensile strength and Tensile modulus for amine modification
Tensile strength
Isocyanate
N
Subset
Control
15
1
20.8667
ODI
15
17.0800
TDI
15
MDI
15
2
20.8667
22.3300
17.4847
Sig.
.125
.825
Subset
LOADING
50
N
20
1
16.1310
2
30
20
19.6650
10
20
22.5250
Sig.
1.000
.112
Tensile modulus
Isocynates
N
Subset
1
2
1.7000
Control
15
ODI
15
TDI
15
1.5533
MDI
15
1.5631
Sig.
1.3187
1.000
.249
Subset
LOADING
50
N
1
2
1.6540
20
30
20
1.4925
10
20
1.4548
Sig.
.830
1.000
72
Table-4 T-Test for tensile properties between two modification approaches
Tensile Strength
Paired Samples Correlations
N
Pair 1
Correlation
Sig.
Tensile strength
4
.468
.532
Correlation
Sig.
Approach1&2
Tensile Modulus
Paired Samples Correlations
N
Pair 1
Flexural modulus
4
Approach1&2
.892
.108
73
Figure A.6 Characteristic flexural stress-strain curve for injection molded WF/HDPE
composite, ethanol modification with 30% loading.
74
Table-5 Statistics for Flexural strength and flexural modulus Approach-1
Flexural strength
Subset
CARBON
Control
N
15
1
32.3733
C18
15
33.2333
C12
15
31.9067
C8
15
32.8000
C2
15
29.6800
Sig.
.117
Subset
LOADING
50
N
1
2
34.0120
25
30
25
32.2160
10
25
29.7680
Sig.
32.2160
.055
.202
Flexural modulus
Subset
CARBON
Control
N
15
1
1.5233
C18
15
1.3667
C12
15
1.4600
C8
15
1.3667
C2
15
1.6867
Sig.
.078
Subset
LOADING
10
N
25
30
25
50
25
Sig.
1
1.0540
2
3
1.4380
1.9500
1.000
1.000
1.000
75
Table 6 Statistics for Flexural strength and flexural modulus Approach-2
Flexural strength
Subset
CARBON
Control
N
1
2
32.3733
15
C18
15
30.7900
30.7900
C12
15
29.8167
29.8167
C8
15
C2
15
Sig.
31.7300
28.2400
.093
.092
Subset
LOADING
10
N
25
1
28.5400
30
25
28.9500
50
25
Sig.
2
34.2800
.836
1.000
Flexural modulus
Subset
CARBON
Control
N
15
1
1.5233
C18
15
1.4533
C12
15
1.5000
C8
15
1.5367
C2
15
1.3567
Sig.
.089
Subset
LOADING
10
N
25
30
25
50
25
Sig.
1
1.1220
2
3
1.3180
1.9820
1.000
1.000
1.000
76
Table-7 Statistics for Flexural strength and flexural modulus for amine modification
Flexural strength
Isocyanates
N
Subset
1
Control
15
ODI
15
TDI
15
MDI
15
Sig.
2
3
32.3733
22.6233
33.1233
28.6167
1.000
1.000
Subset
LOAD
10
N
20
1
26.8275
2
30
20
29.5450
50
20
31.1800
Sig.
1.000
.250
Flexural modulus
Isocynates
N
Subset
1
2
1.5233
Control
15
ODI
15
TDI
15
1.5200
MDI
15
1.5500
Sig.
1.2967
1.000
.973
Subset
LOADF
10
N
20
1
1.2750
30
20
1.4075
50
20
Sig.
2
1.7350
.063
1.000
.929
77
Table 8 T-test for flexural properties between two approaches
Paired Samples Correlations
Pair 1
FM1LOADT &
FM2LOADT
N
Correlation
Sig.
3
.975
.142
Paired Samples Correlations
Pair 1
FMC1TT &
FMC2TT
N
Correlation
Sig.
5
-.714
.176
N
Correlation
Sig.
5
.812
.095
Paired Samples Correlations
Pair 1
FSC1TT &
FSC2TT
Paired Samples Correlations
Pair 1
FSLOAD1T &
FSLOAD2T
N
Correlation
Sig.
3
.853
.350