composites is higher than that of aerospace materials and the coat is lower. In Contrast, the 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 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 . 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 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 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 , lignin, and small amounts of extractives their materials and composites, and this results in high and ash. Cellulose is a linear 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 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 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 alcohol; that is, coumaryl, to give off volatile, flammable gases. The hemicellulose coniferyl, and syringyl alcohols. Lignin is highly and cellulose polymers are degraded by heat much before branched and not crystalline, and its structure and the lignin is [ 4 ]. The lignin component contributes to chemical composition is a function of its source. Lignin char formation, and the charred layer helps insulate the is associated with the hemicelluloses and plays a role in lignocellulosic from further thermal degradation. the natural decay resistance of the lignocellulosic substance. MODIFICATION OF PROPERTIES The remaining part of the chemical composition of lignmcellulosics consists of water, organic soluble extractives, and inorganic materials. The extractive Although solid wood may never be accepted by the materials are mainly made up of cyclic hydrocarbons. academic community as a true engineering material because These vary in structure dapending on the source, and they of its inconaiatent properties, lignocellulosic play a major role in natural decay and insect resistance composites conceivably can be made to conform to the and combustion properties of lignocellulosics. The definition of a material. The process of taking wood inorganic port ion (ash content) of lignocellulosics can apart and converting it into fiber removes knots, splits, vary from a few percent to over 15 parcent depending on checks, cracks, twists,and bends from the wood. The the source. mixture of sapwood, heartwood, springwood, and summerwood The strongest component of the lignocellulosic results in a consistent furnish. resource is the cellulose polymer. It is an order of If the fiber is reconstituted using a high magnitude stronger than the lignocellulosic fiber, which, performance adhesive, very strong and uniform in turn, is almmst another order of magnitude stronger lignocellulosic composites can be produced. However, than the intact plant wall. The fiber is thus the properties such as dimensional instability, flammability, smallest unit that can be used to produce high-yield biodegradability, and degradation caused by acids, bases, lignocellulosic composites. Using isolated cellulose and ultraviolet radiation are still present in the requires a loss of over 50 percent of the cell wall composite, which will restrict its use for many substance. applications. If the properties of the composite do not Host lignocellulosics have similar properties even meet the end-use performance requirements, then the though they may differ in chemical composition, fiber question is what must be changed, improved, and size, and matrix morphology. redesigned so that the composite does meet end-use The cell wall polymers and their matrix make up the expectations? Although such changes are regularly made cell wall and in general are responsible for the physical in the textile, plastic, glass, and metal industries, and chemical properties of lignocellulosics. they are rarely made in the lignocellulosic industry. Because the properties of lignocellulosics result PROPERTIES OF LIGNOCELLULOSIC COMPOSITES from the chemistry of the cell wall components, the basic properties of a lignocellulosic can be changed by The properties of lignocellulosic composites can be modifying the basic chemistry of the cell wall polymers. modified to be consistent, predictable, reproducible, Chemical modification technology can greatly improve the continuous, and uniform. Before we discuss how properties of lignocellulosics [ 5 - 8 ]. Dimensional lignocellulosic properties can be modified, it is stability can be greatly improved by bulking the important to describe how the components of lignocellulosic cell wall either with simple bonded lignocellulosic composites interact. chemicals or by impregnation with water-soluble polymers Lignocellulosics change dimensions with changing [ 6 ]. For example, acetylation of the cell wall polymers moisture content because the cell wall polymers contain using acetic anhydride produces a lignocellulosic hydroxyl and other oxygen-containing groups that attract composite with a dimensional stability of 90 to 95 moisture through hydrogen bonding [ 1 , 2 ]. The percent as compared to that of a control composite hemicelluloses are mainly responsible for moisture [ 7 , 8 ]. Many different types of lignocellulosic fibers sorption, but the accessible cellulose , noncrystalline have been acetylated. When acetyl content resulting from cellulose, lignin, and surface of crystalline cellulose the acetylation of many different types of fibers is also play major roles. Moisture swells the cell wall, plotted as a function of reaction time, all data points and the lignocellulosic expands until the cell wall is fit a common curve [ 9 ]. A maximum weight percent gain of saturated with water. Beyond this saturation point, about 20 percent was reached in 2 h of reaction time, and moisture exists as free water in the void structure and an additional 2 h increased the weight gain only by about does not contribute to further expansion. This process 2 to 3 percent. Without a strong catalyst, acetylation is reversible, and the lignocellulosic shrinks as it using acetic anhydride alone levels off at approximately loses moisture. 20 weight percent gain for the softwoods, hardwoods, Lignocellulosics are degraded biologically because grasses, and water plants. This means that this simple organisms recognize the carbohydrate polymers (mainly the procedure can be used to impart dimensional stability to hemicelluloses) in the cell wall and have very specific many different types of mixed lignocellulosic fibers systems capable of hydrolyzing these polymers into without the need for separation before reaction because digestible units. Biodegradation of the high molecular all the fibers react at the same rate. weight cellulose weakens the lignocellulosic cell wall Sorption of moisture is mainly due to hydrogen because crystalline cellulose is primarily responsible bonding of water molecules to the hydroxyl groups in the for the strength of the lignocellulosic [ 3 ]. Strength is cell wall polymers. Replacing some hydroxyl groups on 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.

342/6th ISWPC the cell wall polymers with acetyl groups reduces the hydroscopicity of the lignocellulosic material. The reduction in equilibrium moisture content (EMC) at any given percent relative humidity of acetylated fiber compared to unacetylated fiber as a function of the bonded acetyl content yields a straight line plot [9.1. Even though the points on the graph come from many different lignocellulosic fibers, they all fit a common line. A maximum reduction in EMC was achieved l t about 20 percent bonded acetyl. Because the acetate group is larger than the water molecule, not all hydroscopic hydrogen-bond ing l it es are covered. The f l ct that EMC reduction as a function of acetyl content is the same for many different lignocellulosic fibers indicates that this technology can be used to improve dimensiona1 stability of a diverse mixture of Figure 1. Lignocellulosic fiber mixed with a synthetic lignocellulosic fibers without separation. fiber to form a nonwoven fiber mat. Biological resistance can be improved by several methods. Bonding chemicals to the cell wall polymers The binder fiber can be a synthetic or natural fiber. A increases resistance as a result of lowering the EMC high performance adhesive can be sprayed on the fiber point below that needed for microorganism attack and before mat formation, added as a powder during met changing the conformation and configuration requirements formation, or be included in the binder fibar ayatem. of the enzyme-substrate reactions [ 3 ]. Acetylation has Figure 2 shows a complex shape that was produced in a also been shown to be an effective way of slowing or press using fiber mat technology. Within certain limits, stopping the biological degradation of lignocellulosic any size, shape, thickness, and density is possible. composites. Toxic chemicals can also be added to the With fiber met technology, a complex part can be made lignocellulosic to stop biological attack. This is the directly from a lignocellulosic fiber blend; the present basic of operation for the wood preservation industry technology requires the format ion of flat sheets prior to [ 4 ]. the shaping of complex parts. 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 uodegreded carbohydrate polymers cannot occur [ 4 ]. The fire performance of lignocellulosic composites can be greatly improved by bonding fire retardants to the lignocellulosic cell wall [ 4 ]. Soluble inorganic salts or polymers containing nitrogen and phosphorus can also be used. These chemica1s are the basis of the fire retardant wood treating industry. The strength propertied of a lignocellulosic composite can be greatly improved in several ways [ 10 ]. Lignocellulosic 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. Figure 2. Fiber mats can be molded into any desired Mechanical properties can be greatly enhanced by using thickness, density, size, and shape. this technology [ 4 ]. For example, composites impregnated with methyl me thacrylate l nd polymerized to weight gain Many different types of lignocellulosic fibers can leve1s of 60 to 100 percent show increases (compared to be used to make composites. Wood fiber is currently used untreated controls) in density of 60 to 150 percent, as the major source of fiber, but other sources can work compression strength of 60 to 250 percent, and tangential just as well. Problems such as availability, collection, hardness of 120 to 400 percent. static bending tests handling, storage, separation, and uniformity will need show increases in modulus of elasticity of 25 percent, to be considered and eolved. Some of these are technical problems whereas others are a matter of economics. modulus of rupture of 80 percent, fiber stress at Currently available sources of fiber can be blended with proportional limit of 80 percent, work to proportional wood fiber or used separately. For example, Large limit of 150 percent, and work to maximum load of 80 quantities of sugar cane begasse fiber are available from percent, and, at the same time, a decrease in sugar refineries, rice hulls from rice processing plants, permeability of 200 to 1,200 percent. and sunflower aced hulls from oil extract ion plants. All of this technology can be applied to recycled FUTURE OF LIGNOCELLULOSIC COMPOSITES lignocellulosic fiber as well as virgin fiber, which can be derived from many sources. Agricultural residues. all Lignocellulosics will be used in the future to produce a types of , yard waste, industrial fiber residues, wide spectrum of productS ranging from very inexpensive, residential fiber waste, and many other forms of waste low performance composites to expensive, high perfomance lignocellulosic fiber base can be used to make composites. The wide distribution, renewability, and composites. Recycled plastics can also be used as recyclability of lignocellulosics can expand the market binders in both cost-affective and value-added for low-cost composites. Fiber technology, high composites. Thermoplastics can be used to make performance adhesives, and fiber modification can be used composites for nonstructural parts, and thermosetting to manufacture lignocellulosic composites with uniform resins can be used for structural materials. densities, durability in adverse environment, and high Thermoplastic a can be converted to thermosetting strength. materials by oxidation and cross linking reactions so it Products having complex shapes can also be produced is possible to make structural products using recycled using flexible fiber mats, which can be made by nonwoven thermoplastics. needling or thermoplastic fiber melt matrix More research will be done to combine technologies. The mats can be pressed into any desired lignocellulosics with other materials such as glass, shape and size. Figure 1 shows a lignncellulosic fiber metal, inorganic, plastic, and synthetic fibers to combined with a binder fiber to produce l flexible mat. produce new materials to meet end-use requirements [ 10 ]. 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.

6th ISWPC/343 Wood-glass composites can be made using the glass as a surface material or combined as a fiber with lignocellulosic fiber. Composites of this type can have a very high stiffness to weight ratio. Metal films can be overlayed onto lignocellulosic 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. Lignocellulosic fibers can be combined in an inorganic matrix. Such composites are dimensionally and Figure 4. Scanning electron microphrph of surface of thermally stable, and they can be used as substitutes for succinic anhydride modified fiberboard. asbestos composites. Lignocellulosics can also be combined with plastic fiber. All boards were pressed under identical in several ways. In one case, thermoplastics are mixed conditions. The SEM of the succinic anhydride modified with lignocellulosics and the plastic material melts, but fiberboard shows a more compact structure than the each component remains as a dietinct separate phase. One control and the fibers have flattened and are spreading example of this technology is reinforced thermoplastic out. This indicates that thermoplasticity has been composites, which are lighter in weight, have improved achieved. acoustical, impact, and heat reformability properties, Combining lignocellulosics with other materials and coat leas than comparable products made from plastic provides a strategy for producing advanced composites alone. These advantages make possible the exploration of that take advantage of the enhanced properties of all new processing techniques, new applications, and new types of materials. It allows the scientist to design markets in such areas as packaging, furniture, housing, materials based on end-use requirements within the and automobiles. framework of cost, availability, renewability, A second combination of lignocellulosics and recyclability, energy use, and environmental plastics it to use a compatibilized to make the considerations. hydrophore mix better with the hygrophil. The two materials remain as separate materials but if phase REFERENCES separation can be avoided, properties can be improved over those of either separate phase. Another combination of lignocellulosics and plastic 1. STAMM, A.J. Wood and Cellulose Science; The Ronald is lignocellulosic-plastic alloys. In this case, the Press Co.: New York, (1964) wood and plastic have become one material and it is not 2. ROWELL, R.M. and BANKS, W.B., Water Repellency and possible to separate them. They have become a Dimensional Stability of Wood Gen. Techn. Rep. homogeneous material. Lignocellulosic-plastic alloys are ; USDA Forest Service, Forest Products possible through fiber modification and grafting Laboratory, Madison, WI, 24 pp, (1985) research. 3. ROWELL, R.M. , ESENTHBR, C. R., YOUNGQUIST, J.A., If the lignocellulosic component is made NICHOLAS, D.D., NILSSON, T.; IMAMURA, Y., thermoplastic through grafting chemistry, it is possible KERNER-GANG, W., TRONG, L., and DEON, G. Wood to thermo-form the lignocellulosic without the use of a Modification in the Protection of Wood Composites. thermoplastic additive. Figure 3 shows SEM of the face In Proceedings: , of a high density board made from unmodified fiber using Honey Harbor, Ontario, Canada; Canadian Forestry no resins. Figure 4 shows the SEM of the face of an Service, 238-266, (1988) aspen fiberboard made from succinic anhydride modified 4. ROWELL, R.M. , Ed Advances in Chemistry Series No. 207; American Chemical Society, Washington, DC, (1985) 5. ROWELL, R.M. and KONKOL, P. Treatments that Enhance Physical Properties of Wood. Gen. Tech. Rep. Forest Products Laboratory, Madison, WI, 12 pp., (1987) 6. ROWELL, R.M. and YOUNGS, R.L. Dimensional Stabilization of Wood in Res. Note FPL-0243; Forest Products Laboratory, Madison, WI, 8 PP, (1981) 7. ROWELL, R.M. Chemical Modification of Wood: A Review. Commonwealth Forestry Bureau, Oxford, England, 6 (12) ,363-382, (1983) 8. ROWELL, R.M. Property Enhancement of Wood Composites. In Vigo, T. L.; Kinzig, B. J., Eds. , Advances in Chemistry Series, (American Chemical Society: Washington, DC, (in press) 9. ROWELL, R.M. and ROWELL, J.S. Moisture Sorption of Various Types of Acetylated Lignocellulosic Fibers .

Schuerch, C., Ed. Proceedings of the 10th Cellulose Conference, Syracuse, NY, May 1988. John Wiley and Sons, Inc.: New York, 343-353, (1989) 10. YOUNGQUIST, J .A. and ROWELL, R.M. Opportunities for Combining Wood with Nonwood Materials. In Figure 3. Scanning electron micrograph of surface of an Proceedings, aspen fiberboard. s Maloney, T. M., Ed., Pullman, WA, (1989) In: 6th International symposium on wood and pulping chemistry proceedings; 1991 April 30–May 4; Melbourne, Victoria, Australia. 344/6th ISWPC Melbourne, Victoria, Australia: Australian Pulp and Paper Industry Printed on recycled paper Technology Association; 1991:341-344. Vol. 1.