composites is higher than that of aerospace materials and the coat is lower. In Contrast, the wood industry depends on a high production rate and low cost. As a HIGH PERFORMANCE COMPOSITES MADE FROM CHEMICALLY MODIFIED result, wood composites have lower performance as WOOD AND OTHER LIGNOCELLULOSIC FIBERS compared to, aerospace and military compoistes. Wood fiber can provide the wood industry with the opportunity to develop high performance materials and ROGER M. ROWELL composites. The chemistry of wood and other lignocellulosic components can be modified to preduce a USDA Forest Service, Forest Products Laboratory and material that has consistent, predictable, and uniform Department of Forestry, University of Wisconsin, Madison, propertied. As will be discussed later, propertied such WI, USA as dimensional instability, biodegradability, flammability, and degradation caused by ultraviolet light, acids, and bases can be altered to produce value-added, property-enhanced composites. ABSTRACT Even though wood is one of the most common 'materials” used for building, information on wood and High performance wood and other lignocellulosic other lignocellulosics is not included in many university composites with uniform densities, durability in adverse courses in materials science. Wood and other environments, and high strength can be produced by using lignocellulosics are perceived as not being consistent, fiber technology, high performance adhesives, and fiber uniform, and predictable in comparison to other reedification to overcome dimensional instability, materials. Moat materials science classes deal mainly biodegradability, flammability, and degradation caused by with metals and, to a lesser extent, with ceramics, ultraviolet light. acids, and base. Products with glass, and plastics, depending on the background of the complex shapes can also be produced using flexible fiber professor. The general public is also reluctant tO mete, which can be made by nonwoven needling or accept wood composites. In the early days of wood thermoplastic fiber melt matrix technologies. The wide composites, products such as particleboard developed a distribution, renewability, and recyclability of bad reputation, mainly because of failures in fasteners lignocellulosics can expand the market for low-coet and swelling under moist condition. This led to public cnmposites and high performance materials. Reeeerch ie skepticism about the use of wood composites. The markets being done to combine lignocelluloaica with meteriale that were developed for wood composites ware all price euch ae glaes, metals , plaatice, inorganic, and driven, with few claims for high performance. This is symthetic fibers to produce new composite chat are still true for the meet part, although some wood tailored for end-use requirements. composite products do have rather good performance. For example, the performance of expensive furniture ie a INTRODUCTION result of composite cores. The perception that wood is not a material as A lignocellulosic is any substance that contains both defined earlier is easy tO understand. Wood is cellulose and lignin. Lignocellulosics include wood, anisotropic (different properties in all three growing agricultural residues. water plants, grasses, and other directions of a tree); it may contain sapwood, heartwood, plant substances. In general, lignocellulosics have been latewood, earlywood, juvenile wood, reaction wood, knots, included in the term biomass, but this term has broader cracks, splits, and checks; and it may be bent, twisted, implications than that denoted by lignocelullosics. or bowed. These defects occur in solid wood but they Biomass includes living subsstances such as animal tissue need not exist in wood composites. The smaller the size and bones. Lignocellulosics have also been called of the components in the composite furnish, the more photomass because they are a result of photosynthesis. uniform the properties of the composite. For example, A material is defined as a substance with chipboard is less uniform than flakeboard, which is less consistent, uniform, continuous, predictable, and uniform than particleboard, which is less uniform than reproducible properties. An engineering material is fiberboard. Fiberboard made from wood fiber can be very defined simply as any material used in construction. uniform, reproducible, and consistent, and it is very A composite is a reconstituted product made from a close to being a true "material". combination of one or more substances using some kind of The perception that lignocellulosics are the only a mastic to hold the components together. The best known substance with property shortcomings is not entirely wood composites are plywood, particleboard, fiberboard, accurate. Wood and other lignocellulosics swell as a and laminated lumber. Wood has been used as an result of moisture, but metals, plastics, and glass also “engineering material” because it is economical, low in swell. as a result of increases in temperature. processing energy, renewable, and strong. In some Lignocellulosics are not the only substance that rots. echoole of thought, however, wend is not considered a Metals oxidize, which is a form of rotting, and concrete material because it does not have consistent, rots as a result of moisture, pH changes, and predictable, reproducible, continuous, and uniform microorganisms. Lignocellulosics and plastics burn, but properties. This is true for solid wood, but it is not metal and glass melt and flow at high temperature. necessarily true for composites made from wood. Lignocellulosics are excellent insulating substances; the For the most part, materials and composites can be insulating capacity of other materials ranges from poor grouped into two basic types: price-driven, for which to good. Strength to weight ratio is very high for coats dictate the markets, and performance-driven, for lignocellulosic fibers as compared to that for almost which properties dictate the markets. A few materials every other fiber. Given these properties, and composites might be of both types, but usually these lignocellulosics compare favorably to other products. two types are very distinct. In general, the wood industry has produced the price-driven type of COMPOSITION OF LIGNOCELLULOSICS composites. Even though the wood industry is growing, it has lost some market share to competing materials such as To understand how wood and other lignocellulosics can steel, aluminum, plastic, and glass. Unless something is become materials in the materials science sense, it is done to improve the competitive performance of wood and important to understand the propertied of the components ocher lignocellulosics, future opportunity for growth of the lignocellulosic cell wall and their contributions will be limited. tO the composite. Materials and composites used by todays industry Wood and other lignocellulosics are range from high performance-high cost to low three-dimensional, naturally occurring, polymeric performance-low cost. For example, the aerospace composites primarily made up of cellulose, industry is very concerned about high performance in hemicelluloses, lignin, and small amounts of extractives their materials and composites, and this results in high and ash. Cellulose is a linear polymer of cost and low rate of prediction. The military is also D-glucopyranose sugar units (actually the dimer, concerned about performance, but their requirements are cellobiose) linked in a beta configuration. The average not as strict as those of the aerospace industry. Thus, cellulose chain has a degree of polymerization of about the rate of production of military-type materials and 9,000 to 10,000 units. There are several types of 6th ISWPC/341 cellulose in the cell wall. Approximately 65 percent of Lignocellulosics exposed outdoors undergo the cellulose is highly oriented, crystalline, and not photochemical degradation caused by ultraviolet light. accessible to water or other solvents [ 1 ]. The remaining This degradation takes place primarily in the lignin cellulose, composed of less oriented chains, is partly component, which is responsible for charactacteristic accessible to water and other solvents and partly not color changes [ 4 ]. The lignin acts as an adhesive in accessible as a result of its association with lignocellulosic cell walls, holding the cellulose fibers hemicellulose and lignin. None of the cellulose is in together. The surface becomes richer in cellulose direct contact with the lignin in the cell wall. content as the lignin degrades. In comparison to lignin, The hemicelluloess are a group of polysaccharide cellulose is much less susceptible to ultraviolet light polymers containing the pentose sugars D-xylose and degradation. After the lignin has been degraded, the L-arabinose and the hexoee sugars D-glucoee, D-galac tose, poorly bonded carbohydrate-rich fibers erode easily from D-mannose, and 4-methylglycuronic acid. The the surface, which exposes new lignin to further hemicelluloses are not crystalline but are highly degradative reactions. In time, this “weathering” branched with a much lower degree of polymerization than process causes the wood surface to become rough and can cellulose. Hemicelluloses vary in structure and sugar account for a significant loss in surface fibers. types depending on the source. Lignocellulosics burn because the cell wall polymers Lignin is composed of nine carbon unite derived from undergo pyrolysis reactions with increasing temperature substituted cinnamyl