July 13, 2003/ In order to utilize wood-based particles and fibers effectively as fillers or reinforcements in thermoplastic composites, a fundamental understanding of the structural and chemical characteristics of wood is required. Wood is the secondary xylem of trees, shrubs, and woody lianas (vines). Although the physiological function of wood is similar among different groupings of trees, significant differences are found in the basic anatomical structure of the broadest groupings of gymnosperms (coniferous trees or "softwoods") and angiosperms (broad-leaved trees or "hardwoods"). In addition, there are anatomical differences in wood structure among the various species of trees. These structural differences may have effects on the use of these materials in composites. Wood cell walls comprise three major organic constituents, namely, cellulose, hemicelluloses, and lignin. In addition to these structural polymers, numerous other organic materials ("extractives") may be present within the wood. The chemical composition of wood varies between species. The basic characteristics of anatomy and structure combine to impart variations in permeability, bulk chemistry, and surface chemistry. Characterization of particle size and shape, as well as surface tension characteristics as indicators of wettability, becomes important as we try to understand how these biopolymeric materials will behave when introduced into synthetic polymer systems.

INTRODUCTION

Wood flour (wood finely ground to a powdery consistency) has been used as a filler in synthetic plastics, primarily thermosetting polymers (e.g., phenolics) for decades. The use of wood in thermoplastics is a relatively recent phenomenon spurred by improvements in processing technology, development of suitable chemical coupling agents (which serve to bond the chemically incompatible wood and plastic together) and economic factors (lower cost of wood flour relative to plastic resins). Advantages such as reductions in operating temperatures, cycle times, and mold shrinkage have also been instrumental in the growth of the fiber/ plastic composites industry. The importance and growing potential of wood/plastic materials were evidenced in 1991 by the advent of the International Conference on woodfiber/plastic composites, a forum on the science and technology for the processing and development of these materials. The seventh such conference was held in Madison, Wisconsin, May 2003.

The consulting firm of Kline & Company (Little Falls, New Jersey) recently conducted an extensive market survey regarding the fiber/ plastic composites industry (1, 2). The use of fillers by the plastics industry has grown steadily along with the growth in the production of major classes of plastic resins. In 1967, U.S. demand for fillers by the plastics industry was 525,000 tons; filler use had grown to 1,925,000 tons by 1998 (1). The projected use of fillers by the U.S. plastics industry in 2000 swelled to 5.5 billion pounds, of which 0.4 billion pounds (7%) was estimated to be bio- based fibers (2). Most bio-fiber plastic additives are derived from wood. However, other natural fibers, such as flax (marketed by Cargill Ltd. of Canada as Durafibre and Durafill) or wheat straw (Agroplastics, Inc. of Lawrence, Kansas) are finding their way into the fiber/plastic composites industry.

Although calcium carbonate constituted the major filler used by weight (66%), it accounted for only 32% ($140 million) of the total value of fillers used in 1998 ($435 million total). Other fillers, including natural fibers, command higher prices than calcium carbonate. Eckert (2) reported average per pound prices of commonly used plastics fillers as follows: fiberglass, $0.90, natural fibers other than wood, $0.20, wood fiber, $0.10, and calcium carbonate, $0.07.

Eckert also summarized major markets for natural fibers in plastic composites as follows, on a weight basis: Building products, 70%; other (including marine uses, infrastructure), 13%; industrial and consumer, 10%; and automotive, 7%. Although U.S. annual growth in plastics demand is forecast at approximately 4.5% per year for 1998-2005, substantially greater growth in the demand for natural fibers is expected. This includes a rate in excess of 50% per year for the period 2000-2005 in the building products area (2); a significant portion of this growth will be attributed to larger market share for fiber/plastic lumber in residential decking (3). A smaller, but significant subset of the building products market is also found in vinyl windows (4). Wood fiber, at weight loadings up to 70%, is used in vinyl or vinyl-clad wood window components. Andersen Windows, the leading manufacturer of windows in North America, pioneered the compounding of wood waste and vinyl extrusions. A representative but not exhaustive list of Anderson Corporation's patents on wood/vinyl materials includes Deaner el al., Advanced Polymer/Wood Composite Pellet Process, U.S. Patent Nos. 5,441,801 and 5,518,677; Deaner et al., Advanced Polymer Wood Composite, U.S. Patent Nos. 6,004,668 and 6,015,611; and Seethamraju et al., Advanced Compatible Polymer Wood Composite, U.S. Patent No. 6,210,792. Andersen developed their wood/ vinyl Fibrex(TM) material, containing 40% by weight wood, as a result of the need to recycle their co-mingled wood and vinyl waste stream. The Renewal by Andersen replacement windows is a product line based on this recycled material. Crane Plastics, Columbus, Ohio, also holds a number of patents for wood fiber/vinyl products.

Dr. Mike Wolcott of the Wood Materials and Engineering Laboratory at Washington State University has, for the past three years, directed an interdisciplinary and inter-institutional research program for the development of HDPE- and PVC-wood composite materials for use in waterfront structures. This program was sponsored by the U.S. Navy. The materials development component of the Navy project focused on evaluation and improvement of existing wood-plastic composite technologies as well as developing novel systems appropriate to the production of pier structural components (5). Reinforcement of wood-plastic composites (WPCs) with carbon fibers was examined, but problems were encountered with PVC prepregs because of thermal degradation. The material structure studies revealed a large degree of processing-induced damage in the wood particles in PVC formulations as evidenced by reduced particle size. Co-extrusion of PVC WPC formulations with caps was successful. However, formulations were restricted to light color compounds. The PVC formulations were found to be viable for use in an industrial deckboard.

Natural fiber use in automotive fiber/plastic applications is projected to increase by 15% per year during 2000-2005 (2). To date, most natural fiber/plastic materials in the automotive arena have been HDPE or PP blends. While there is substantial growth in this area in North America, Europe appears to lead the way in the use of fiber/plastic materials for automobiles.

These examples serve to illustrate the growing levels of interest in wood/plastic materials from both research and commercial standpoints. In order to make effective use of wood in these systems, it is helpful to have some basic understanding of wood's chemical and structural characteristics. The goal of this paper is to introduce the fundamental nature of wood, with a view toward application as a filler or reinforcement in synthetic polymer matrices.

WOOD: A CELLULAR, BIOPOLYMERIC MATERIAL

Wood is a cellular material, based on a biopolymer framework that is hygroscopic, viscoelastic, and anisotropic. These key physical attributes are outcomes of the fundamental anatomical and chemical structure of the material.

A. Anatomy

1. Macroscopic Characteristics of Secondary Xylem

In botanical terms, wood is the secondary xylem of trees, shrubs, or woody lianas (vines), wherein xylem is a tissue that has the primary physiological function of transport of water and solutes from roots to stems and leaves. Wood cells are produced by division of the vascular cambium, a secondary meristem associated with thickening (diameter) growth, as opposed to the primary meristem(s) in a plant that are associated with elongation or height growth. Within the cross section of a tree trunk grown in temperate climates, one can distinguish annual or growth rings, due to the seasonal variations in the types of cells produced by the cambium.

Other zonal differences may also be macroscopically observed on tree cross sections, notably the contrast between sapwood and heartwood (Fig. 1). Sapwood, the outermost zone of wood nearest the bark, consists of a few to several growth rings in which living cells store and transport food and dead cells which transport water. The central heartwood zone, in which all cells are dead, does not transport water. Nevertheless, both heartwood and sapwood contain substantial amounts of water. Freshly cut trees may contain from 30 to over 200 percent water on an oven-dry weight of wood basis. Heartwood is often distinguishable from sapwood due to a darker color resulting from the deposition of "extractives" (water- or solvent-soluble organic substances) in the heartwood. These extractives lend distinctive color characteristics to various species of wood, for example, the rich brown of walnut or the yellowish hue of pine. E\xtractives can also impart characteristic odor or other attributes, such as natural decay resistance. These extractives also play a large role in the surface chemistry of wood, as described later in this paper.

Cross section of the trunk of a small oak tree. Water- conducting sapwood (indicated by short arrow) surrounds the somewhat darker-colored, centrally located heartwood (long arrow). The more- or-less concentric circular pattern throughout results from annual growth of the tree.

2. Anatomical Distinctions

In addition to species-dependent variations in extractives, woods vary in their anatomical structure. Trees are spermatophytes, that is, seed-bearing plants, and these may be further divided into gymnosperms (naked seeds, i.e., seeds borne in a woody cone) and angiosperms (seeds covered by a fleshy structure or fruit). The gymnosperm trees are conifers, or trees that bear cones which contain their seeds, and such trees generally have needle-like, non- deciduous (i.e., evergreen) leaves. In contrast, angiosperms (specifically, dicotyledenous trees) are those trees that are generally broad-leaved and deciduous. Owing to an anomaly of nomenclature, the gymnosperms are commonly referred to as "softwood," whereas the angiosperms are called "hardwoods." The anomaly arises in that the term often has no relationship to the actual "hardness" of the wood per se; for example, both oak and balsa wood are (botanically) hardwoods! Such seemingly nonsensical terms are often discovered when one works with wood, perhaps arising simply from humanity's long history of the common use of wood. Nevertheless, the fact remains that there are fundamental anatomical distinctions between the broad classifications of "softwoods" and "hardwoods."

a. Softwoods. The wood of "softwood" (coniferous trees, e.g., pine, spruce, etc.) is composed mainly of one type of cell (typically constituting over 90% of the wood, by volume), namely the longitudinal tracheid (Fig. 2). These tracheids (or "fibers" in the terms of the pulp and paper trade) are elongated, somewhat thick- walled, hollow cells that taper to rounded or pointed ends. Softwood tracheids are typically 3-4 mm long and have average diameters of 30 to 40 micrometers (i.e., the 1/d ratio is approximately 100:1). These dimensions vary with species (ranging from 1.2 to over 5 mm in length and from 15 to 80 [mu]m in diameter for North American softwoods) and with age of the tree and location of the cell within the tree.

The walls of tracheids are peppered with small openings, known as "pits." In the living tree, water, solutes, and gasses pass through these openings. Single fibers may be obtained from wood by chemical pulping means, in which the lignin, and some or perhaps most of the hemicellulose is removed. For use in plastics, wood is commonly ground into fine flour. Thus, the material blended with the plastic is not individual fibers, but particulates consisting of "fiber bundles" or larger aggregations of the constituent cells of the wood. Softwoods also contain small food storage cells (parenchyma) and other specialized cells that vary with species. Most of the parenchyma cells are organized into aggregate structures known as "rays." These rays form ribbon-like radial conduits for flow of food and storage material in the living tree. In utilization, they create very fine particulates.

Cross section of the wood of Ponderosa pine (softwood). The honeycomb-like appearance is due to the predominance of longitudinal tracheids. Vertical lines here represent rays (conductive and food storage cells); horizontal lines are the growth ring boundaries. The large diameter circles at the growth ring boundaries are resin canals. Bar represents one millimeter .....

b. Hardwoods. Hardwood structure has a somewhat greater complexity than that of softwoods, in that hardwoods have a greater variety of cell types and arrangements (Fig. 3). Hardwoods contain specialized water-conducting cells known as vessel elements. These vessel elements (also termed "pores" are typically large in diameter (>100 [mu]m; range, approximately 20-300 [mu]m for North American commercial species), short (generally well below 1 mm in length), and are stacked end-to-end to form longer water conducting conduits or vessels. The arrangement of the pores, as viewed on a cross- sectional (end-grain) surface, varies with species. Pore arrangement generally falls into the broad categories of ring-porous, semi-ring porous, or diffuse-porous, the former if the largest diameter pores within a given growth ring form a distinct ring, the latter if the pore diameter and distribution is relatively uniform within a growth ring.

Hardwoods also contain fibers. This cell type is somewhat analogous to the tracheids found in softwoods. However, fibers are smaller (representative diameters of 10-20 [mu]m, length 1-2 mm) and have less conspicuous pitting than softwood tracheids. Fibers provide mechanical strength to the wood. Parenchyma in hardwoods are arranged into rays that have greater variation in the shapes of the cells and the width and height of the rays. In addition, the amount and arrangement of longitudinal parenchyma (i.e., parenchyma distributed in vertical strands in a standing tree) is more varied than in softwoods, again, a species-dependent phenomenon.

B. Cell Wall Structure/Chemistry

1. Fundamental Building Blocks

Through the process of photosynthesis, trees convert carbon dioxide and water into simple sugars, the fundamental building blocks from which the cell walls of fibers, vessels, parenchyma, or tracheids are constructed. A single type of six-carbon sugar, glucose, is utilized to produce cellulose, the major structural polymer of the cell wall. Both five- and six-carbon sugars are used to synthesize another class of carbohydrate-based structural polymers, the hemicelluloses. Some glucose is shunted to a biosynthetic pathway that converts it into aromatic p-coumaryl alcohol, coniferyl alcohol, or synapyl alcohol. These phenylpropane precursor monomers enzymatically polymerize to form lignin.

Fig. 3. Cross section of the wood of sugar maple (hardwood). The large circular-to-elliptical structures are the water-conducting vessel elements. The smaller diameter cells are predominantly fibers. Vertical lines are rays: horizontal lines are growth ring boundaries. Bar represents one millimeter ....

2. Major Organic Constituents of Wood

Wood comprises primarily three structural components: Cellulose (45-50% by weight), hemicelluloses (20-25%), and lignin (20-30%). Cellulose is a long, straight chain homopolymer (d.p. 5,000-10,000) consisting of anhydro d-glucopyranose linked via [beta] 1,4 glycosidic bonds. Hemicelluloses have a lower degree of polymerization (150-200) and may be relatively straight or branched. These consist variously of five- and six- carbon sugars. Although the type and amount of hemicellulose in wood varies with species, most hardwoods have a predominance of glucuronoxylan, consisting of a linear backbone of xylopyranose with a 4-O-methylglucuronic acid residue on approximately 10% of the xylan rings. Softwoods primarily contain galactoglucomannan consisting of [beta]-D-mannopyranose, [beta]-D-glucopyranose, and [alpha]-D-galactopyranose. (6). Cellulose and hemicelluloses contain free hydroxyl groups that lend wood its inherent hygroscopicity. Lignin is a large, amorphous polymer consisting of varying ratios of the phenyl propane precursors linked mainly (> 2/3) by ether bonds and the rest by C-C bonds (7).

3. Cell Wall Ultrastructure

a) Microfibril structure. Cellulose chains are aggregated through hydrogen bonding and van der Waals forces into linear microfibrils that are approximately 3.5 x 10 nm in cross-sectional dimension and of indeterminate length. The microfibrils have both crystalline and non-crystalline regions. The linear cellulose molecules and the supramolecular microfibrils have a dominant influence on the overall behavior of wood as a material. Although various theories or models exist, it is generally thought that hemicelluloses are interspersed between adjacent microfibrils. The entire aggregation is cemented together with lignin, again in ways that are not fully elucidated, but some models suggest that lignin is deposited in a lamellar fashion (8), whereas others suggest a radial deposition of lignin across the cell wall (9).

b) Cell wall layering and resulting anisotropy.

The cell wall of tracheids and fibers is composed of microfibrils arranged in distinctive layers. The outermost cell wall layer, the primary wall, is a loose aggregation of randomly oriented microfibrils (wall is < 0.1 [mu]m thick). Most of the cell wall volume is in the secondary wall, generally identified with three distinctive layers. The first, or S^sub 1^, is a thin (approx. 0.1 [mu]m thick) layer with a "microfibril angle" or mfa (i.e., orientation of microfibrils with respect to the cell's longitudinal axis) of 50-70[degrees]. The S^sub 2^ layer, generally at least six times thicker than the S^sub 1^ and possibly several times thicker (0.6 to several [mu]m), has a mfa of 10-30[degrees]. As the thickest wall layer, the S^sub 2^ exerts a dominant influence on the overall behavior of the cell wall. The anisotropy (specifically, orthotropic nature) of wood is largely due to the arrangement of the microfibrils within the S^sub 2^ layer of the cell wall. The innermost layer, the S^sub 3^, is typically 0.1 [mu]m thick and has a mfa of 60-90[degrees]. A lignin-rich layer known as the middle lamella bonds adjacent tracheids or fibers to one another.

Sketch of the layered cell wall structure of a softwood longitudinal tracheid showing approximate orientation of the cellulose microfibrils within each layer. The cell wall consists of the primary (P) and secondary wall. The secondary wall has three sub- layers, designated S^sub 1^, S^sub 2^, and S^sub 3^. In atypical softwood, the cell wall thickness would vary from a few to several micrometers. Adjacent tracheids are joined together by the middle lamella (m.L). (Sketch by D. Stokke.)

4. Extractives and Inorganic Ash

Extractives, also known as extraneous materials, secondary components, non-skeletal polymers, or non-framework polymers, are compounds that are extractable with neutral organic solvents (e.g., ethanol, acetone, benzene, dichlorobenzene) and/or water. In general, the extractives are any compound that does not belong to the classes of cellulose, hemicelluloses, or lignin. Extractives are found in the heartwood and vary in composition and content by wood species. These relatively mobile materials have a profound influence on the surface chemistry of wood. The total extractives content of temperate woods ranges from 4-10 percent by weight. Tropical woods may contain over 20 percent extractives. Inorganic ash ranges from 0.2 to 0.5 percent by weight in domestic softwoods and from 0.1 to 1.4 percent in domestic hardwoods. Bark and some tropical hardwoods may contain considerably more ash. Silica content is generally negligible in domestic timber species, but may be as high as 9 percent in some tropical woods (6).

WOOD CHARACTERISTICS THAT INFLUENCE ITS INTERACTION WITH OTHER MATERIALS

A. Surface Properties

Methods used to measure material surface properties include contact angle analysis, inverse gas chromatography (IGC), infrared (IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Contact angle analysis and IGC provide information on surface thermodynamics and work of adhesion. IR spectroscopy provides information on surface functional group composition while XPS probes surface elemental composition. XPS can also be used to depth- profile functional groups on material surfaces up to 10 nm in depth. Depth profiling is accomplished through angle dependent XPS analysis and this technique has been used successfully on wood surfaces (10). The surface properties of wood can be viewed from the standpoint of both organic and physical chemistry. Organic functional groups present on the wood surface contribute significantly to the chemical interactions that occur between the wood and polyvinyl chloride. In addition, functional groups influence the intrinsic adhesion (wettability) and acid-base interactions that occur between the wood and the PVC. The surface tension of wood comprises dispersive (non polar) and acid-base (polar) components. The dispersive component of surface tension constitutes up to 90% of the wood surface, and as the wood surface ages or is exposed to heat, the ratio of dispersive to acid-base component increases, indicating a loss in the acid- base character of the surface. Both contact angle and IGC analysis indicate that wood has higher non-polar surface energy than PVC (11). Wood and PVC also have stronger affinity for acidic liquids and probes, which suggests that the surfaces have a predominantly basic character (11). However, other studies have shown that PVC has an acidic character (12, 13). This discrepancy in the literature concerning surface acid base characterization indicates a need for further research. The polar component of surface tension in PVC can contribute some secondary chemical bonds (hydrogen bonding) with the wood surface polar components.

B. Surface Properties of Wood Fibers and Particles

The surface properties of wood are highly dependent on the method of preparation for a given wood element. The interaction of wood with PVC is usually in the form of fibers, and particles. Both chemical and mechanical processes are used to produce wood fibers. Wood fibers produced by thermomechanical means usually display lignin-rich surfaces while fibers produced by chemical means display carbohydrate-rich surfaces. The surface tension of chemically produced fibers is often higher than the mechanically produced fibers. Along with a higher surface tension, the functional groups on chemically produced fibers are capable of stronger secondary interactions with PVC. Wood particles are produced by both knife (sawing and planing) and hammer milling. The machining processes for producing wood particles also result in the particle surfaces having a lower surface energy (14).

C. Influence of Extractives on Particle Properties

Although extractives constitute a small weight percentage of the chemical composition of wood, extractives dominate the surface chemistry of wood. The primary reason for this is that extractives tend to concentrate at the wood surface when the wood is dried. As water is volatilized from the wood during drying, many extractives move to the surface of the wood and those that are not volatilized with the water will remain on the surface. The chemical structure of extractives that aggregate at the surface of wood is readily changed by environmental influences such as heat, light and oxygen. The changes in chemical structure of extractives as a result of most environmental exposure lead to a lower surface tension, less polar character, and subsequently impaired interaction with polymers. Woods with high extractive contents are known to be difficult to bond with conventional wood adhesives, but the impact of wood extractives on adhesion interactions with thermoplastics such as PVC has received little attention. However, much effort has focused on the use of coupling agents and treatments for wood in wood/polymer composites (15). It should be noted that certain wood species (Cedars, Black Locust) contain extractives that can contribute to the biological resistance of PVC-wood plastic composites.

D. Mechanisms of Adhesion

There are four primary mechanisms or theories of adhesion: 1) mechanical interlocking, 2) diffusion, 3) adsorption and surface reaction, and 4) electrostatic. For the interaction of wood with high molecular weight polymers such as PVC, 1) and 3) are most important. In mechanical interlocking, the thermoplastic acts as an anchor in the cellular or porous structure of the wood particle. When functional groups contained in the PVC polymer are attracted to functional groups on the wood surface, the polymer will be adsorbed on the wood surface. The chemical bonding between the PVC will be secondary, and secondary chemical bonds include hydrogen bonding, acid-base interactions, and Lifshitz van der Waals interactions. The diffusion theory relies on low molecular weight polymer molecules diffusing into the wood cell wall and forming an interphase between the wood and the polymer. This is unlikely to occur with high molecular weight thermoplastics. Electrostatic interactions are also unlikely to be germane to adhesion in extruded wood plastic composites. Following the interaction of a thermoplastic with a wood particle surface, several steps occur in the formation of an adhesive bond. When the polymer becomes fluid, it will wet the wood particle surfaces. Following wetting, the polymer will spread over the particle surfaces by the mixing occurring in the extruder. After spreading, the polymer will penetrate into the porous structure of the wood particles. Upon cooling, the adhesive bond strength between the PVC and wood particles will be realized.

E. Structure of Fiber/Plastic Composites

Commercial formulations of PVC for extrusion contain resin, stabilizers, wetting agents, and lubricants for ease of processing. The PVC additives also appear to have the added benefit of improved interaction with the wood particles during extrusion, as evidenced by Fig. 5. The PVC compound penetrates into the wood cell lumen. The filling of voids in the wood cell structure by the PVC can help impart better physical and mechanical properties to the composite.

APPLICATIONS/ PROCESSING IMPLICATIONS

A. Hygroscopicity

An important physical characteristic of wood affecting its interaction with thermoplastic polymers in composites is its hygroscopicity. Dry wood particles at ambient temperature and relative humidity contain from 5% to 12% moisture content (oven dry weight basis). The moisture content will vary from summer to winter depending on the particular climatic conditions. More detailed information on wood moisture relations can be found in the Wood Handbook (16). Wood particles used in wood plastic composites need to be dried to O to 2% moisture content ("bone dry") to process adequately with thermoplastic polymers. Wood can be dried using horizontal rotating and horizontal fixed dryers. The fuels used to dry wood include natural gas, oil and wood fuel. Certain types of wood extrusion systems are designed to dry the wood in the screw barrel as will be mentioned later. It should be noted that dry wood particles are an explosion hazard.

Scanning electron micrograph of PVC-Ponderosa pine composite. (Micrograph courtesy of Dr. Michael Wolcott, Washington State University.)

B. Thermal Performance in Processing

In many instances, dried wood particles still contain limited amounts of moisture, and this moisture will be released in the barrel of the extruder. Venting of the screw barrel is important to remove this remaining moisture so that it doesn't negatively affect the mixing of the extrusion components. Along with the moisture, volatile wood extractives will be released from the wood, and the extractives contribute to the wood odor emitted during wood plastic extrusion processing. Examples of volatile wood extractives include the terpenes emitted from pine, and acetic acid (vinegar odor) emitted from certain hardwoods such as oak. Dry wood particles are limited in thermal processing to a temperature of about 225[degrees]C before thermal decomposition occurs. Thermal decomposition of wood is usually detected by a "burning wood" odor and discoloration (darkening) of the resulting composite. Most of the common thermoplastics (polyethylene, polypropylene, polystyrene and PVC) are easily processed with wood because the melt temperatures a\re below the thermal decomposition temperature of wood.

C. Melt Blending Techniques

Wood particles have a bulk density ranging from 10 to 2O lbs./ cubic foot depending on particle size and distribution, considerably lower than inorganic fillers. Proper mixing of wood, polymers and additives is important to manufacturing a consistent composite product. There are four primary process schemes for mixing wood fiber-plastic composites (17): 1) pre-dry the wood and pre-mix with the polymer and/or additives, 2) pre-dry and split feed the material into an extruder, 3) add the wood first, followed by drying in the extruder and introduce the polymer in a melted state, and 4) add the wood first, followed by drying and a split feed for polymer and additives.

In process 1, a weighed amount of dried wood is mixed with the polymer and or additives in a powdered form. Mixing is usually accomplished using a ribbon blender or similar mixing device. The powdered wood-polymer mixture can then be fed into an extruder using a crammer feeding device. In process 2, polymer and additives are gravimetrically fed into a compounding twin screw, and the polymer mixture is melted and blended. Down the screw barrel, the dried wood is gravimetrically fed into the melt stream. This provides good mixing of the wood and polymer. In process 3, wood (6-8% MC) is fed into the extruder and dried. Molten polymer and additives are introduced to the dry wood by means of a single or twin-screw extruder. This process also provides excellent mixing of the wood and polymer. In process 4, wood (6-8% MC) is fed into the extruder and dried, and the polymer and additives are introduced by a side stuffer in the dry state. Wood particle loading limits are around 70 weight percent. Most manufacturers are making products that contain 50 to 60 weight percent wood loadings.

D. Influence (of Wood) on Material Properties

A great deal of information has been generated on the influence of wood on the material properties of wood plastic composites. A good source for information is the proceedings from International Conference on Wood-fiber-Plastic Composites, which is held every two years in Madison, Wisconsin. Wood influences both the physical and mechanical properties of thermoplastic polymers. English and Falk (18) provide a good general overview of factors influencing the properties of wood plastic composites.

Physical Properties

Wood filled plastics have a weight advantage compared to inorganic fillers. The wood cell wall has a specific gravity of 1.5, while common inorganic fillers like glass and talc are 2.5 and 2.9, respectively. This provides a greater strength-to-weight ratio in composites of similar composition percentages. Wood also provides a less abrasive composite compared to inorganic filled plastics. Wood plastic composites may absorb more water than neat thermoplastics. However, if the wood is properly encapsulated during processing, then the wood plastic composite will absorb little moisture (below 5%). The addition of wood to thermoplastics is particularly helpful in reducing thermal linear expansion of the resulting composite. Thermal creep is reduced as a function of increased wood content in wood plastic composites.

Mechanical Properties

One of the primary benefits of adding wood particles to thermoplastics is the increases in stiffness (modulus of elasticity) and strength (modulus of rupture) of the resulting composites. Depending on the wood loading, the wood plastic composite stiffness can be increased 3 to 4 times while the strength can be increased two-fold. Impact resistance of thermoplastic composites is reduced depending on the type, amount, and treatment of wood fiber added to the composite (19).

SUMMARY

Wood may be considered a cellular composite material consisting mainly of cellulose, hemicelluloses, and lignin. These natural polymers impart the basic structure and behavior of wood, notably anisotropy, hygroscopicity, and thermoplastic softening behavior. Other compounds found in wood, namely extractives, further influence the surface chemistry of wood and thereby affect the wettabiliry and chemical interaction of wood particles and fibers with synthetic thermoplastics. Knowledge of the inherent structure, chemistry, and variability of wood raw materials provides a basis for better utilization of wood as a useful component of filled PVC blends.

REFERENCES

1. C. Eckert, in Proceedings of the Fifth International Conference on Woodfiber-Plastic Composites, Proceedings No. 7263, Forest Products Society, Madison, WI (1999).

2. C. Eckert, in Proceedings of the Conference on Progress in Woodfibre-Plaslic Composites Conference, University of Toronto (2000).

3. P. M. Smith, G. M. Carter, T. M. Smith, and M. P. Wolcott, 33rd International Particleboard/Composite Materials Symposium Proceedings. M. P. Wolcott, R. J. Tichy, and D. A. Bender, eds.. Washington State University, Pullman, WA (1999).

4. C. Cannon, in Proceedings of the Fifth International Conference on Woodfiber-Plastic Composites. Proceedings No. 7263, Forest Products Society, Madison, WI (1999).

5. M. P. Wolcott, in Executive Summary Materials Group, Engineered Wood Composites for Naval Waterfront Facilities. Project End Report, Office of Naval Research Contract N00014-97-C-0395 (2001).

6. R. C. Pettersen, in The Chemistry of Solid Wood. Advances in Chemistry Series 207, R. M. Rowell, ed., American Chemical Society, Washington, D.C. (1984).

7. E. Sjostrom, Wood Chemistry: Fundamentals and Applications, Academic Press, New York (1981).

8. D. Fengel and G. Wegener, Wood: Chemistry, Ultrastructure, Reactions, Walter de Gruyter, Berlin (1983).

9. M. J. Larsen, J. E. Winandy, and F. Green, III, Material und Organismen, 29(3), 197 (1995).

10. D. J. Gardner, M. P. Wolcott, L. Wilson, Y. Huang, and M. Carpenter, in Wood Adhesives 1995, Proceedings No. 7296, Forest Products Society, Madison, WI (1996).

11. M. E. P. Walinder and D. J. Gardner, in Sixth International Conference on Woodfiber-Plastic Composites, Proceedings No. 7251, Forest Products Society, Madison, WI (2002).

12. H. P. Schreiber, in The Interfacial Interactions in Polymeric Composites, Kluwer Academic Publishers, The Netherlands (1993).

13. L. M. Matuana, R. T. Woodhams, J. J. Balatinecz, and C. B. Park, Polymer Composites. 19, 446 (1998).

14. D. J. Gardner, W. T. Tze, and S. Q. Shi, in Advances in Lignocellulosics Characterization, D. S. Argyropoulos, ed., Tappi Press, Atlanta, Ga. (1999).

15. J. Z. Lu, Q. Wu, and H. S. McNabb, Jr., Wood and Fiber Science. 32, 88 (2000).

16. Wood Handbook: Wood as an Engineering Material Reprinted from Forest Products Laboratory General Technical Report FPL-GTR-113 with the consent of the USDA Forest Service, Forest Products Laboratory, FPS Catalogue No. 7269, Forest Products Society, Madison, WI (1999).

17. A. Machado and S. Kapp, in Proceedings ofWoodftber-Plastic Conference, Sponsored by Plastics Technology and Polymer Process Communications.

18. B. W. English and R. H. FaIk, in Proceedings of the Fifth International Conference on Woodfiher-Plastic Composites. Proceedings No. 7263, Forest Products Society, Madison, WI (1996).

19. K. Oksman and C. clemons, in Fourth International Conference on Woodflber-Plastlc Composites, Proceedings No. 7277, Forest Products Society, Madison, WI (1997).

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DOUGLAS D. STOKKE1

1Iowa State University Center for Crops Utilization Research 253 Bessey Hall Ames, Iowa 50011-1021

and

DOUGLAS J. GARDNER2

2University of Maine Advanced Engineered Wood Composites Center 5793 AEWC Building Orono, Maine 04469