In: Emerging technologies for materials and chemicals from biomass: Proceedings of symposium; 1990 August 26-31; Washington, DC. Washington, DC: American Chemical Society; 1992. Chap. 2. ACS symposium series 476. Chapter 2 Opportunities for Lignocellulosic Materials and Composites Roger M. Rowell Forest Products Laboratory and Forestry Department, U.S. Department of Agriculture Forest Service, University of Wisconsin, Madison, WI 53705 High performance lignocellulosic composites with uniform densities, durability in adverse environments, and high strength can be produced by using fiber technology, bonding agents, and fiber modification to overcome dimensional instability, biodegradability, flammability, and degradation caused by ultraviolet light, acids, and bases. Products with complex shapes can also be produced using flexible fiber mats, which can be made by nonwoven needling or thermoplastic fiber melt matrix technologies. The wide distribution, renewability, and recyclability of lignocellulosics can expand the market for low-cost materials and high performance composites. Research is being done to combine lignocellulosics with materials such as glass, metals, plastics, inorganic, and synthetic fibers to produce new materials that are tailored for end-use requirements. Lignocellulosic substances contain both cellulose and lignin. Sources of lignocellulosics include wood, agricultural residues, water plants, grasses, and other plant substances. In general, lignocellulosics have been included in the term biomass, but this term has broader implications than that denoted by lignocellulosics. Biomass also includes living substances such as animal tissue and bones. Lignocellulosic materials have also been called photomass because they are a result of photosynthesis. A material is usually 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 and other lignocellolosics have been used as engineering materials because they are economical, low in processing energy, renewable, and strong. In some schools of thought, This chapter not subject to U.S. copyright Published 1992 American Chemical Society
2. ROWELL Lignocellulosic Materials and Composites 13 however, lignocellulosics are not considered materials because they do not have consistent, predictable, reproducible, continuous, and uniform properties. This is true for solid lignocellulosics such as wood, but it is not necessarily true for composites made from lignocellulosics. A lignocellulosic composite is a reconstituted product made from a combination of one or more substances using some kind of a bonding agent to hold the components together. The best known lignocellulosic composites are plywood, particleboard, fiberboard, and laminated lumber. For the most part, materials and composites can be grouped into two basic types: price-driven, for which costs dictate the markets, ad performancedriven, for which properties dictate the markets. A few materials and composites might be of both types, but usually these two types are very distinct. In general, the wood industry has produced the price-driven type. Even though the wood industry is growing, it has lost some market share to competing materials such as steel, aluminum, plastic, and glass. Unless something is done to improve the competitive performance of lignocellulosics, future opportunities for growth will be limited. Performance-driven materials and composites in the wood industry range from high performane-high cost, to low performance-low cost. Figure 1 shows three examples of industrial uses for materials and composites. The aerospace industry is very concerned about high performance in their materials and composites, and this results in high cost and low production. The military is also concerned about performance, but their requirements are not as strict as those of the aerospace industry. Thus, the rate of production 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 materials and composites have low performance. Lignocellulosic materials can provide the wood industry with the opportunity to develop high performance materials and composites. The chemistry of lignocellulosic components can be modified to produce a material that has consistent, predictable, and uniform properties. As will be discussed later, properties such as dimensional instability, biodegradability, flammability, and degradation caused by ultraviolet light, acids, and bases can be altered to produce value-added, property-enhanced composites. Even though wood is one of the most common materials used for construction, information on wood and other lignocellulosics is not included in many university courses in materials science. Wood and other lignocellulosics are perceived as not being consistent, uniform, and predictable in comparison to other materials. Most materials science classes deal mainly with metals and, to a lesser extent, with ceramics, glass, and plastics, depending on the background of the professor. The general public is also reluctant to accept lignocellulosic composites. In the early days of wood composites, products such as particleboard developed a bad reputation, mainly because of failures in fasteners and swelling under moist conditions. This led to public skepticism about the use of wood composites. The markets that were developed for wood composites were all price-driven, with few claims for high performance. This is still true, for
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2. ROWELL Lignocellulosic Materials and Compsites 15 the most part, although such wood composite products do have high performance. For example, the high performance of expensive furniture is a result of composite cores. The perception that wood is not a material as defined earlier 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, 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 lignocellulosic fiber can be very uniform, reproducible, and consistent, and it is very close to being a true material. The perception that lignocellulosics are the only substance with property shortcomings is not entirely accurate. Table I shows the properties of several commonly used engineering materials. Wood and other lignocellulosics swell as a result of moisture, but metals, plastics, and glass also swell, as a result of increases in temperature. Lignocellulosics are not the only substances that decay. Metals oxidize and concrete deteriorates as a result of moisture, ph changes, and microbial action. Lignocellulosics and plastic burn, but metal and glass melt and flow at high temperatures. Lignocellulosics are excellent insulating substances; the insulating capacity of other materials ranges from poor to good. Furthermore, the strength-to-weight ratio is very high for lignocellulosic fibers when compared to that for almost every other fiber. Given these properties, lignocellulosics compare favorably to other products. Composition of Lignocellulosics To understand how wood and other lignocellulosics can become materials in the materials science sense, it is important to understand the properties of the components of the cell wall and their contributions to the composite. Wood and other lignocellulosics are three-dimensional, naturally occurring, polymeric composites primarily made up of cellulose, hemicelluloses, lignin, and small amounts of extratives and ash (Figure 2). 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 percent 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 only partially accessible to water and other solvents 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-methylglucuronic acid. The hemicelluloses are
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2. ROWELL Lignocellulosic Materials and Composites 17
18 MATERIALS AND CHEMICALS FROM BIOMASS not crystalline, but are highly branched with a much lower degree of polymerization than cellulose. Hemicelluloses vary in structure and sugar composition depending on the source. Lignins are composed of nine carbon units derived from substituted cinnamyl alcohol; that is, coumaryl, coniferyl, and syringyl alcohols. Lignins are highly branched, not crystalline, and their structure and chemical composition is a function of their source. Lignins are associated with the hemicelluloses and plays a role in the natural decay resistance of the lignocellulosic substance. The remaining mass of lignocellulosics consists of water, organic soluble extratives, and inorganic materials. The extractive materials are primarily composed of cyclic hydrocarbons. These vary in structure depending on the source and play a major role in natural decay and insect resistance and combustion properties of lignocellulosics. The inorganic portion (ash content ) of lignocellulosics can vary from a few percent to over 15 percent, depending on the source. The strongest component of the lignocellulosic resource is the cellulose polymer. It is an order of magnitude stronger than the lignocellulosic 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 percent of the cell wall substance. Most lignocellulosics have similar properties even though they may differ in chemical composition and matrix morphology. Table II shows the chemical composition and Table III the dimensions of some common lignocellulosic fibers.
2. ROWELL Lignocellulosic Materials and Composites 19 Table II. Chemical Composition of Some Common Fibers The cell wall polymers and their matrix make up the cell wall and in general are responsible for the physical and chemical properties of lignocellulosics. Properties of Lignocellulosic Composites The properties of lignocellulosic composites can be modified to be consistent, predictable, reproducible, continuous, and uniform. Before we discuss how lignocellulosic properties can be modified, it is important to describe how the components of lignocellulosic composites interact.
20 MATERIALS AND CHEMICALS FROM BIOMASS Interaction of Chemical Components. Figure 3 shows how the components of lignocellulosics interact in various processes. Lignocellulosics change dimensions with changing moisture content because the cell wall polymers contain hydroxyl and other oxygen-containing groups that attract moisture through hydrogen bonding (1,2). 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 lignocellulosic 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 lignocellulosic shrinks as it loses moisture. Lignocellulosics are 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 lignocellulosic cell wall because crystalline cellulose is primarily responsible for the strength of the lignocellulosic (3). 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. Lignocellulosics exposed outdoors undergo photochemical degradation caused by ultraviolet light. This degradation takes place primarily in the lignin component, which is responsible for the characteristic color changes (4). The lignin acts as an adhesive in lignocellulosic 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
2. ROWELL Lignocellulosic Materials and Composites 21 wood surface to become rough and can account for a significant loss in surface fibers. Lignocellulosics burn 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 (4). The lignin component contributes to char formation, and the charred layer helps insulate the lignocellulosic 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, lignocellulosic 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 composite. If the fiber is reconstituted using a high performance adhesive, very uniform lignocellulosic 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-use expectations? Although such changes are regularly made in the textile, plastic, glass, and metal industries, they are rarely made in the lignocellulosic industry. Because the properties of lignocellulosics result from the chemistry of the cell wall components, the basic properties of a lignocellulosic can be changed by modifying the basic chemistry of the cell wall polymers. Chemical modification technology can greatly improve the properties of lignocellulosics (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 lignocellulosic composite with a dimensional stability of 90 to 95 percent as compared to that of a control composite (7,8). Many different types of lignocellulosic fibers have been acetylated. When acetyl content resulting from the acetylation of many different types of fibers is plotted as a function of reaction time, all data points fit a common curve (Figure 4) (9). A maximum weight percent gain of about 20 percent was reached in 2 h of reaction time, and an additional 2 h increased the weight gain only by about 2 to 3 percent. Without a strong catalyst, acetylation using acetic anhydride alone levels off at approximately 20 weight percent gain for the softwoods, hardwoods, grasses, and water plants. This means that this simple procedure can be used to impart dimensional stability to many different types of mixed lignocellulosic fibers without the need for separation before reaction because all the fibers react at the same rate. Sorption of moisture is mainly due to hydrogen bonding of water molecuies to the hydroxyl groups in the cell wall polymers. Replacing some
22 MATERIALS AND CHEMICALS FROM BIOMASS Figure 4. Rate of acetylation of various lignocellulosic fibers. o, southern yellow pine; q aspen; A, bamboo; 0, bagasse;, jute; +, pennywort; v, water hyacinth.
2. ROWELL Lignocellulosic Materials and Composites 23 hydroxyl groups on the cell wall polymers with acetyl groups reduces the hygroscopicity of the lignocellulosic material. The reduction in equilibrium moisture content (EMC) at 65 percent relative humidity of acetylated fiber compared to unacetylated fiber, as a function of the bonded acetyl content, yields a linear plot (Figure 5) (9). Even though the points shown in Figure 5 came from many different lignocellulosic fibers, they all fit a common line. A maximum reduction in EMC was achieved at about 20 percent bonded acetyl. Extrapolation of the plot to 100 percent reduction in EMC would occur at about 30 percent bonded acetyl. Because the acetate group is larger than the water molecule, not all hydroscopic hydrogen-bonding sites are covered. The fact 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 dimensional stability of a diverse mixture of lignocellulosic fibers without separation. 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 lignocellulosic composites. Toxic chemicals can also be added to the lignocellulosics to stop biological attack. This is the focus of the wood preservation industry (4). Resistance to ultraviolet light radiation can also 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 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 chemicals are the basis of the fire retardant wood treating industry. The strength properties 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. Mechanical properties can be greatly enhanced using this technology (4). For example, composites impregnated with methyl methacrylate and polymerized to weight gain levels of 60 to 100 percent show increases in density of 60 to 150 percent, compression strength of 60 to 250 percent, and tangential hardness of 120 to 400 percent compared to untreated controls. Static bending tests show increases in modulus of elasticity of 25 percent, modulus of rupture of 80 percent, fiber stress at proportional limit of 80 percent, work to proportional limit of 150 percent, and work to maximum load of 80 percent, and, at the same time, a decrease in permeability of 200 to 1,200 percent.
24 MATERIALS AND CHEMICALS FROM BIOMASS Future of Lignocellulosic Composites Lignocellulosics will 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 lignocellulosics can expand the market for low-cost composites. Fiber technology, high performance adhesives, and fiber modification can be used to manufacture lignocellulosic 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 textile nonwoven needling or thermoplastic fiber melt matrix technologies. The mats can be pressed into any desired shape and size. Figure 6 shows a lignocellulosic 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 7 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 lignocellulosic fiber blend; the present technology requires the formation of flat sheets prior to the shaping of complex parts. Many different types of lignocellulosic fibers can be 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 area 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 lignocellulosic 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. Thermoplastics can be used to make composites for nonstructural parts, and thermosetting resins can be used for structural materials. More research will be done to combine lignocellulosics with other materials such as glass, metal, inorganics, plastic, and synthetic fibers to 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. 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.
2. ROWELL Lignocellulosic Materials and Composites 25 Figure 5. Reduction in equilibrium moisture content as a function of bonded acetyl content for various acetylated lignocellulosic fibers. o, southern yellow pine; q aspen; A, bamboo; 0, bagasse;, jute; +, pennywort; v water hyacinth. Figure 6. Lignocellulosic fiber mixed with a synthetic fiber to form a nonwoven fiber mat.
26 MATERIALS AND CHEMICALS FROM BIOMASS Figure 7. Fiber mats can be molded into any desired thickness, density, size, and shape. 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 wearresistant 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 also be combined in an inorganic matrix. Such composites are dimensionally and thermally stable, and they can be used as substitutes for asbestos composites. Lignocellulosics can also be combined with plastic in several ways. In one case, thermoplastics are mixed with lignocellulosics 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 possible the exploration of new processing techniques, new applications, and new markets in such areas as packaging, furniture. housing, and automobiles.
2. ROWELL Lignocellulosic Materials and Composites 27 Another combination of lignocellulosics and plastic is lignocellulosicplastic alloys. At present, the plastic industry commonly alloys one polymer to another. In this process, two or more polymers interface with each other, resulting in a homogeneous material. Lignocellulosic-plastic alloys are possible through grafting and compatibilization research. Combining lignocellulosics 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. Literature Cited RECEIVED March 4, 1991 Printed on Recycled Paper