Amaterial is defined as a substance with

Emerging Wood-Based Composites To get a true picture of what the future holds in the way of wood based composites, it is essential to consider a much broader spectrum of combinations. Amaterial is defined as a substance with consistent, uniform, continuous, predictable and reproducible properties. An engineering material is defined simply as any material used in construction. Wood has been used as an engineering material because it is economical, low in processingenergy, renewable and strong. But in some schools of thought wood is not considered a material because it does not meet the initial definition above. This is true for solid wood, but not necessarily true for composites made from wood. A wood composite is a reconstituted product made from a combination of wood and one or more substances, using some kind of a mastic to hold the components together. The best known wood composites are plywood, particleboard, fiberboard and laminated veneer lumber (LVL). For the most part, materials and composites can be grouped into two basic types: price-driven (costs dictate markets) and performance-driven(properties dictate markets). A few materials and composites might be both types, but usually these two types are very distinct. In general, the wood industry has produced the price-driventype. Even though the wood industry is growing, it has lost some market share to competing materials such as steel, aluminum, plastic and glass. And unless something is done to improve the competitive performance of wood composites, future opportunity for growth will be limited, Materials and composites used today range from high-performanceand high-cost to low-performance, low-cost. For example, the aerospace industry is very concerned about high performance from materials and composites. This results in high cost and low production.the military is also concerned about performance,but its requirements are not as strict as those for aerospace. Thus, the production rate of military-type materials and composites is higher than that of aerospace materials, and the cost is lower. In contrast, the wood industry depends on a high production rate and low cost. As a result, wood composites have even lower performance standards. New developments in wood chemistry and the application of materials science can enable the wood industry to develop high performance composites. The chemistry of a wood cell wall can be modified to 156 TheWoodBook. 92: 156-160; 1994. produce a material that has consistent, predictable and uniform properties. Properties such as dimensional instability, biodegradability,flammability and degradation caused by ultraviolet light, acids and bases can be altered to produce value-added, propertyenhanced composites. Obstacles To Overcome Even though wood is one of the most common building materials, it is not included in many university materials science courses. Classes tend to focus mainly on metals and, to a lesser extent, ceramics, glass and plastics, depending on the background of the professor. Wood simply isn t perceived as being consistent, uniform and predictable in the same sense as these other building materials. Public acceptance of wood composites also has been slow. This reluctance has been due primarily to early developmental and marketing problems (e.g. lack of consumer education about proper product applications, insufficient testing for particular applications, etc.). For instance, particleboard developed a bad reputation when it was first introduced,mainly because of failures in fasteners and swelling caused by moisture. This led to public skepticism. The markets that were developed for wood composites were all price driven, with few claims for high performance. While markets for composites today are still largely price driven, these products now offer high performance. For example, the superior performance of expensive furniture is a result of composite cores. And there is gradual market displacement as new composites are proven and accepted. For example, oriented strandboard has largely displaced plywood in the sheathing market. Some industry experts predict that composite lumber (LVL, Parallam and others) will gradually displace wider sizes of solid wood dimension lumber, due to price and shrinking log diameters. The perception that wood is not a material as defined above is easy to understand. Wood is anisotropic (different properties in all three growing directions of a tree); it may contain sapwood, heartwood, latewood,

earlywood, juvenile wood, reaction wood, knots, cracks, splits, and checks. And it may be bent, twisted or bowed. These defects occur in solid wood but they need not exist in wood composites. The smaller the size of the components in the composite furnish, the more uniform the properties of the composite. For example. chipboard is less uniform than flakeboard, which is less uniform than particleboard, which is less uniform than fiberboard. However, fiberboard made from wood fiber can be very uniform, reproducible and consistent, and it is very close to being a true material. The perception that wood is the only substance with property shortcomings is not entirely accurate. Wood swells as a result of moisture, but metals, plastics and glass also swell, as a result of increases in temperature. Wood is not the only substance that rots. Metals oxidize, which is a form of rotting, and concrete rots as a result of moisture, ph changes and microorganisms. Wood and plastic burn, but metal and glass melt and flow at high temperatures. Wood is an excellent insulating substance; the insulating capacity of these other materials ranges from good to poor. Strength to weight ratio is very high for wood fibers as compared to that for almost every other fiber. Given these properties, wood fiber compares favorably to other products. Future Of Wood Composites Wood should be used in the future to produce a wide spectrum of products ranging from very inexpensive, low performance composites to expensive, high performance composites. The wide distribution, renewability, and recyclability of wood can expand the market for low-cost composites. Fiber technology, high performance adhesives and fiber modificationcan be used to manufacturewood composites with uniform densities, durability in adverse environments and high strength. Products having complex shapes can also be produced using flexible fiber mats, which can be made by nonwoven needling or thermoplastic fiber melt matrix technologies. The mats can be pressed into any desired shape and size. Figure 1 shows a wood fiber combined with a binder fiber to produce a flexible mat. The binder fiber can be a synthetic or natural fiber. A high performance adhesive can be sprayed on the fiber before mat formation, added as a powder during mat formation, or be included in the binder fiber system. Figure 2 shows a complex shape that was produced in a press using fiber mat technology. Within certain limits, any size, shape, thickness, and density is possible. With fiber mat technology, a complex part can be made directly from a wood fiber blend. Present technology requires the formation of flat sheets prior to the shaping of complex parts. Many different types of lignocellulosicfibers can be Figure 1. Lignocellulosic fiber mixed with a synthetic fiber to form a nonwoven fiber mat. Figure 2. Fiber mats can be molded into any desired thickness, density, size and shape. used to make composites. Wood fiber is currently used as the major source of fiber, but other sources can work just as well. Problems such as availability, collection, handling, storage, separation and uniformity will need to be considered and solved. Some of these are technical problems whereas others are a matter of economics. Currently available sources of fiber can be blended with wood fiber or used separately. For example, large quantities of sugar cane bagasse fiber are available from sugar refineries, rice hulls from rice processing plants, and sunflower seed hulls from oil extraction plants. All of this technology can be applied to recycled wood fiber as well as virgin fiber, which can be derived from many sources. Agricultural residues, all types of paper, yard waste, industrial fiber residues, residential fiber waste and many other forms of waste lignocellulosic fiber base can be used to make composites. Recycled plastics can also be used as binders in both cost-effective and value-added composites. Thermoplasticscan be used to make composites for nonstructural parts, and thermosetting resins can be used for structural materials. More research will be done to combine wood with other materials such as glass, metal, inorganics, plastic and synthetic fibers to produce new materials to meet end-use requirements.the objective will be to combine two or more materials in such a way that a synergism between the materials results in a composite that is much better than the individual components. TheWoodBook 157

Wood/glass composites can be made using the glass as a surface material or combined as a fiber with wood fiber. Composites of this type can have a very high stiffness to weight ratio. Metal films can be overlayed onto wood composites to produce a durable surface coating, or metal fibers can be combined with fiber in a matrix configuration in the same way metal fibers are added to rubber to produce wear-resistant aircraft tires. A metal matrix offers excellent temperature resistance and improved strength properties, and the ductility of the metal lends toughness to the resulting composite. Another application for metal matrix composites is in the cooler parts of the skin of ultra-high-speed aircraft. Technology also exists for making molded products using perforated metal plates embedded in a phenolic-coated wood fiber mat. which is then pressed into various shapes. Wood fibers can be combined in an inorganic matrix. Such composites are dimensionally and thermally stable, and they can be used as substitutes for asbestos composites. Wood can also be combined with plastic in several ways. In one case, thermoplastics are mixed with wood fiber and the plastic material melts, but each component remains as a distinct separate phase. One example of this technology is reinforced thermoplastic composites, which are lighter in weight, have improved acoustical, impact, and heat reformability properties, and cost less than comparable products made from plastic alone. These advantages make it possible to explore new processing techniques, new applications and new markets in such areas as packaging, furniture. housing and automobiles. Another combination of wood and plastic is wood/plastic alloys. At present, the plastics industry commonly alloys one polymer to another. (A polymer is a compound of high molecular weight derived either by adding many smaller molecules-e.g. polyethylene-or by condensation of many smaller molecules, eliminating water, alcohol etc.-e.g. nylon.) In this process, two or more polymers interface with each other, resulting in a homogeneous material. Wood/plastic alloys are possible through grafting and compatibility research. Combining wood with other materials provides a strategy for producing advanced composites that take advantage of the enhanced properties of all types of materials. It allows the scientist to design materials based on end-use requirements within the framework of cost, availability, renewability, recyclability. energy use, and environmental considerations. Composition Of Wood To understand how wood can become a material in the materials science sense, it is important to understand the properties of the components of the wood cell wall and their contributions to the composite. Wood is a three-dimensional, naturally occurring, polymeric composite primarily made up of cellulose, hemicelluloses, lignin, and small amounts of extractives and ash. Cellulose is a linear polymer of D-glucopyranose sugar units (actually the dimer, cellobiose) linked in a beta configuration. The average cellulose chain has a degree of polymerization of about 9,000 to 10,000 units. There are several types of cellulose in the cell wall. Approximately 65% of the cellulose is highly oriented crystalline and not accessible to water or other solvents (1). The remaining cellulose, composed of less oriented chains, is partly accessible to water and other solvents and partly not accessible as a result of its association with hemicellulose and lignin. None of the cellulose is in direct contact with the lignin in the cell wall. The hemicelluloses are a group of polysaccharide polymers containing the pentose sugars D-xylose and L-arabinose and the hexose sugars D-glucose, D-galactose, D-mannose, and 4-0 methylglycuronic acid. The hemicelluloses are not crystalline but are highly branched with a much lower degree of polymerization than cellulose. Hemicelluloses vary in structure and sugar types depending on the source. Lignin is composed of nine carbon units derived from substituted cinnamyl alcohol; that is, coumaryl, coniferyl, and syringyl alcohols. Lignin is highly branched and not crystalline, and its structure and chemical composition is a function of its source. Lignin is associated with the hemicelluloses and plays a role in the natural decay resistance of the wood substance. The remaining part of the chemical composition of wood consists of water, organic soluble extractives and inorganic materials. The extractive 158 TheWoodBook

materials are mainly made up of cyclic hydrocarbons. These vary in structure depending on the source, and they play a major role in natural decay and insect resistance and combustion properties of wood. The inorganic portion (ash content) of wood can vary from a few percent to over 15% depending on the source. The strongest component of wood is the cellulose polymer. It is an order of magnitude stronger than the wood fiber, which, in turn, is almost another order of magnitude stronger than the intact plant wall. The fiber is thus the smallest unit that can be used to produce high-yield lignocellulosic composites. Using isolated cellulose requires a loss of over 50% of the cell wall substance. The cell wall polymers and their matrix make up the cell wall and in general are responsible for the physical and chemical properties of wood. Properties Of Wood Composites Interaction of Chemical Components. Wood changes dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding. The hemicelluloses are mainly responsible for moisture sorption, but the accessible cellulose, noncrystalline cellulose, lignin, and surface of crystalline cellulose also play major roles. Moisture swells the cell wall, and the wood expands until the cell wall is saturated with water. Beyond this saturation point, moisture exists as free water in the void structure and does not contribute to further expansion. This process is reversible, and the wood shrinks as it loses moisture. Wood is degraded biologically because organisms recognize the carbohydrate polymers (mainly the hemicelluloses) in the cell wall and have very specific enzyme systems capable of hydrolyzing these polymers into digestible units. Biodegradation of the high molecular weight cellulose weakens the wood cell wall because crystalline cellulose is primarily responsible for the strength of the wood. Strength is lost as the cellulose polymer undergoes degradation through oxidation, hydrolysis and dehydration reactions. The same types of reactions take place in the presence of acids and bases. Wood exposed outdoors undergoes photochemical degradation caused by ultraviolet light. This degradation takes place primarily in the lignin component, which is responsible for the characteristic color changes. The lignin acts as an adhesive in wood cell walls, holding the cellulose fibers together. The surface becomes richer in cellulose content as the lignin degrades. In comparison to lignin, cellulose is much less susceptible to ultraviolet light degradation. After the lignin has been degraded, the poorly bonded carbohydrate-rich fibers erode easily from the surface, which exposes new lignin to further degradative reactions. In time, this weathering process causes the wood surface to become rough and can account for a significant loss in surfacefibers. Wood burns because the cell wall polymers undergo pyrolysis reactions with increasing temperature to give off volatile, flammable gases. The hemicellulose and cellulose polymers are degraded by heat much before the lignin is. The lignin component contributes to char formation, and the charred layer helps insulate the wood from further thermal degradation. Modification of Properties. Although solid wood may never be accepted by the academic community as a true engineering material because of its inconsistent properties, wood fiber composites conceivably can be made to conform to the definition of a material. The process of taking wood apart and converting it into fiber removes knots, splits, checks, cracks, twists, and bends from the wood. The mixture of sapwood, heartwood, springwood, and summerwood results in a consistent furnish. If the fiber is reconstituted using a high performance adhesive, very uniform wood fiber composites can be produced. However, properties such as dimensional instability, flammability, biodegradability, and degradation caused by acids, bases, and ultraviolet radiation are still present in the composite, which will restrict its use for many applications. If the properties of the composite do not meet the end-use performance requirements, then the question is what must be changed, improved, and redesigned so that the composite does meet end- TheWoodBook 159

use expectations? Although such changes are regularly made in the textile, plastic, glass, and metal industries, they are rarely made in the wood industry. Because the properties of wood result from the chemistry of the cell wall components, the basic properties of wood can be changed by modifying the basic chemistry of the cell wall polymers. Chemical modification technology can greatly improve the properties of wood (5-8). Dimensional stability can be greatly improved by bulking the cell wall either with simple bonded chemicals or by impregnation with water-soluble polymers (6). For example, acetylation of the cell wall polymers using acetic anhydride produces a wood composite with a dimensional stability of 90 95% as compared to that of a control composite (7,8). Sorption of moisture is mainly due to hydrogen bonding of water molecules to the hydroxyl groups in the cell wall polymers. Replacing some hydroxyl groups on the cell wall polymers with acetyl groups reduces the hydroscopicity of the wood material. Sorption of moisture is not completely stopped because the acetate group is larger than the water molecule so not all hygroscopic hydrogen-bonding sites are covered. Biological resistance can be improved by several methods. Bonding chemicals to the cell wall polymers increases resistance as a result of lowering the EMC point below that needed for microorganism attack and changing the conformation and configuration requirements of the enzyme-substrate reactions (3). Acetylation has also been shown to be an effective way of slowing or stopping the biological degradation of wood composites. Toxic chemicals can also be added to wood to stop biological attack. This is the basis of operation for the wood preservation industry(4). Resistance to ultraviolet light radiation can be improved by bonding chemicals to the cell wall polymers, which reduces lignin degradation, or by adding polymers to the cell matrix, which helps hold the degraded fiber structure together so water leaching of the undegraded carbohydrate polymers cannot occur (4). The fire performance of wood composites can be greatly improved by bonding fire retardants to the cell wall (4). Soluble inorganic salts or polymers containing nitrogen and phosphorus can also be used. These chemicals are the basis of the fire retardant wood treating industry. The strength properties of a wood composite can be greatly improved in several ways (9). Wood composites can be impregnated with a monomer and polymerized in situ or impregnated with a preformed polymer. In most cases, the polymer does not enter the cell wall and is located in the cell lumen. Mechanical properties can be greatly enhanced by using this technology (4). For example, composites impregnated with methyl methacrylate and polymerized to weight gain levels of 60-100% show increases (compared to untreated controls) in density of 60% to 150%, compression strength of 60-250%, and tangential hardness of 120 400%. Static bending tests show increases in modulus of elasticity of 25%, modulus of rupture of 80%, fiber stress at proportional limit of 80%, work to proportional limit of 150%, and work to maximum load of 80%, and, at the same time, a decrease in permeability of from 200% to 1200%. Source: RogerM. Rowell, Research Chemist, USDA Forest Service-ForestProducts Laboratory, Madison, WI and Forestry Dept., UniversityofWisconsin-Madison. LITERATURE CITED 160 TheWoodBook Printedon recycled paper