|Wood Composites are made from any fibrous or particulate wood material that is bonded together either using natural bonding (i.e., no resin) or using a thermoset resin or a thermoplastic or inorganic binder. This product mix ranges from fiberboard to laminated beams. Composites are used for a number of structural and nonstructural applications in product lines ranging from panels for interior covering purposes to panels for exterior uses and in furniture and architectural trim materials in many different types of buildings. Lignocellulosic fibers and particles other than wood (e.g., straw) can many times be readily substituted for wood to produce other biocomposites with similar engineering properties in types of wood composites discussed below.
Wood composite materials (and other biocomposites) can be engineered to meet a range of specific properties. When wood materials and processing variables are properly selected, the result can provide high performance and reliable service. With solid wood, properties are determined at the cellular level, and properties can be highly variable for pieces of solid wood both within and between wood species. With composite wood materials, properties are determined at the fiber, particle, flake, or veneer level, and properties are less variable. A key determinant of composite properties is the type of woody element used (Figure 1) (See attached file at bottom of page for figure). These elements are available in a great variety of sizes and shapes and can be used alone or in combination. Wood and biocomposites fall into three general categories: engineered wood composites, wood–inorganic composites, and wood–plastic composites.
Engineered Wood Composites
Engineered wood composites use a thermoset or heat-curing resin binder and can be grouped into three sub-categories based on the physical configuration of the wood element used to make the products: laminated, particle- or flake-based, and fiber-based composites. Within limits, the manufacturing processes are variants of that shown for oriented strandboard (OSB) in Figure 2 (See attached file at bottom of page for figure). The performance of composites can be tailored to the end-use application of the product by varying the physical configuration of the wood material, adjusting the density of the composites, varying the resin type and amount, and incorporating additives to increase water, decay, or fire resistance.
Commonly used thermoset resin binder systems include phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, and isocyanate (diphenylmethane di-isocyanate, or MDI). These adhesives have been chosen based upon their suitability for bonding bio-based materials; the selection of one from this group is based on desired composite strength, durability requirements, and cost.
Laminated composites consist of wood veneers bonded with a resin binder and fabricated with either parallel- or cross-banded veneers. When laminae are laid parallel, the resulting product has higher performance properties parallel to grain and is often used as a lumber substitute. When cross-banded, the composite product is moderately strong but has higher dimensional stability, which is critical when used as a panel product such as plywood.
Particle-, flake-, strand- or fiberboard composites are normally classified by density and element size. Each is made with a dry woody element, except for fiberboard, which can be made by either dry or wet processes. Dry processes are used to make boards with high density (hardboard) and medium density (medium-density fiberboard, or MDF). Wet processes are used to make both high-density hardboard and low-density insulation board. Wet-process hardboards differ from dry-process fiberboards in several ways. First, water is used as the distribution medium for forming the fibers into a mat. As such, this technology is really an extension of paper manufacturing technology. Second, some wet-process boards are made without additional binders.
Interest has increased in combining wood and other raw materials, such as plastics, gypsum, and concrete, in composite products. These composites provide enormous opportunities to provide particular benefits and match product performance to end-use requirements:
Lower material costs by combining a lower cost material (acting as a filler or extender) with an expensive material
Products that use recycled materials and are recyclable themselves
Products that exhibit specific properties that are superior to those of the component materials alone (e.g., increased strength-to-weight ratio, improved abrasion resistance)
Many building materials can be made by combining wood fiber with inorganic binders, including panel products, siding, roofing tiles, and precast building members. Inorganic-bonded wood composites contain between 10% and 70% by weight wood particles or fibers and conversely 90% to 30% inorganic binder. All inorganic-bonded composites are very resistant to deterioration, particularly by insects, vermin, and fire. The downside of inorganic composites has traditionally been their lower strength-to-weight ratio and longer processing times.
Wood fiber–thermoplastic composites combine wood with thermoplastics. They soften when heated and harden when cooled. Thermoplastics used with wood must melt or soften at or below the degradation point of the wood component, normally 200°C to 220°C (392°F to 428°F) but be rigid at normal use temperatures (<65°C, <150°F). These thermoplastics usually include polypropylene, polystyrene, vinyls, and low- and high-density polyethylenes.
Wood flour is used as a filler in thermoplastic composites and offers little reinforcement. Commercial wood flour is often processed from post-industrial materials such as planer shavings, chips, and sawdust. Several grades are available depending upon wood species and particle size. Wood fibers, although more difficult to process than wood flour, can lead to superior composite properties and act more as reinforcement than as filler. A wide variety of wood fibers are available from both virgin and recycled resources.
Wood–plastic composites may be made in two ways. In the first, the wood fiber is a reinforcing agent or filler in a continuous thermoplastic matrix. In the second, the thermoplastic is a binder to the majority wood component. The presence or absence of a continuous thermoplastic matrix may also determine the processability of the composite material. In general, if the matrix is continuous, conventional thermoplastic processing equipment may be used to process composites; however, if the matrix is not continuous, other processes may be required.
References and Further Reading
Maloney, T.M. 1993. Modern particleboard and dry-process fiberboard manufacturing. San Francisco, CA: Miller Freeman Publications.
Marra, G. 1979. Overview of wood as material. Journal of Educational Modules for Materials Science and Engineering 1(4):699–710.
McKay, M. 1997. Plywood. In: Smulski, S., ed. Engineered wood products—A guide for specifiers, designers and users. Madison, WI: PFS Research Foundation.
O’Halloran, M.R.; Youngquist, J.A. 1984. An overview of structural panels and structural composite products. In: Rafik, Y. Itani; Faherty, Keith F., eds. Structural wood research. State-of-the-art and research needs. Proceedings, American Society of Civil Engineers; 1983 October 5–6; Milwaukee, WI. New York, NY: American Society of Civil Engineers:133–147.
Rowell, R.M.; Young, R.A.; Rowell, J.K., eds. 1997. Paper and composites from agro-based resources. Boca Raton, FL: CRC Lewis Publishers.
Suchsland, O.; Woodson, G.E. 1986. Fiberboard manufacturing practices in the United States, Agric. Handb. 640. Washington, DC: U. S. Department of Agriculture.
USDA Forest Service. 1999. Wood Handbook: Wood as an Engineering Material. Gen. Tech. Rep. FPL–GTR–113. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
Youngquist, J.A. 1987. Wood-based panels, their properties and uses—A review. In: Proceedings, Technical consultation on wood-based panel. Expert Consultation, Food and Agriculture Organization of the United Nations; 1987 September 28–October 1; Rome, Italy: 116–124.
Youngquist, J,A. 1995. Unlikely partners? The marriage of wood and nonwood materials. Forest Products Journal. 45(10): 25–30.
Youngquist, J.A.; English, B.E.; Spelter, H.; Chow, P. 1993a. Agriculture fibers in composition panels, In: Maloney, Thomas M., ed. Proceedings, 27th international particleboard/composite materials symposium; 1993 March 30–April 1; Pullman, WA. Pullman, WA: Washington State University: 133–152.
Youngquist, J.A.; Myers, G.E.; Muehl, J.M. [and others]. 1993b. Composites from recycled wood and plastics. Final Rep., U.S. Environmental Protection Agency, Project IAG DW12934608–2. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
Youngquist, J.A.; English, B.E.; Scharmer, R.C. [and others]. 1994. Literature review on use of non-wood plants fibers for building materials and panels. Gen. Tech. Rep. FPL–GTR–80. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.
J.E. Winandy and Ken Skog are scientists with the USDA Forest Service, Forest Products Laboratory, Madison, Wisconsin