The rapidly changing economic and environmental needs of society are putting ever increasing pressures on the forestry industry to do more with less. In practical terms, this means, for example, increasing conversion and efficient use of wood fiber resources, producing more fiber on a shrinking land base, using environmentally friendly processes and technologies, and remaining competitive in the global market place. Within the next decade, composites are expected to constitute the most prominent segment of the board industry. Competition in high volume markets has focused attention on low priced materials that offer a more favorable strength to weight ratio. Compared to other polymeric materials, wood plastic composite (WPC) has the lowest material cost. Wood plastic composites are an attractive alternative because their manufacturing process is highly automated and adaptable to various species and forms of raw materials.
Sometimes wood is not considered as an engineering material because it does not have consistent, predictable, reproducible, continuous, and uniform properties. This might be true for solid wood but is not necessarily true for composites made from wood (Rowell et al., 1993). Wood flour has been used as filler in synthetic plastics (primarily thermosetting polymers) for decades. The use of wood in thermoplastics is a relatively recent phenomenon spurred by improvements in processing technology, development of suitable chemical coupling agents and economic factors. Advantages such as reductions in operating temperatures, cycle times, and mold shrinkage have also been instrumental in the growth of the fiber/plastic composite industry. The importance and growing potential of wood plastic composites were evidenced in 1991 by the advent of the international conference on wood fiber-plastic composites, a forum on the science and technology for the processing and development of these materials.
The consulting firm of Kline&Company (Little Falls, New Jersey) recently conducted an extensive market survey regarding the fiber/plastic composite industry. The use of fillers by the plastic industry has grown steadily along with the growth in the production of major classes of plastic resins. In 1967, the U.S. demand for fillers by the plastic industry was 525,000 tons; filler use had grown to 1,925,000 tons by 1998 (Eckert 1999). The projected use of fillers by the U.S. plastic industry in 2000 swelled to 5.5 billion pounds, of which 0.4 billion pounds (7%) was estimated to be bio-based fibers (Eckert 2000). Most bio-fiber plastic additives are derived from wood. However, other natural fibers, such as flax or wheat straw are finding their way into the fiber/plastic industry. Although calcium carbonate constitutes the major filler used by weight (66%), it accounted for only 32% ($140 million) of the total value of fillers used in 1998 ($435 million total). Other fillers, including natural fibers, command higher prices than calcium carbonate. Eckert (2000) reported average per pound prices of commonly used plastic fillers as follows: fiberglass, $0.90, natural fibers other than wood, $0.20, wood fiber, $0.10, and calcium carbonate, $0.70. Eckert (2000) also summarized major markets for natural fibers in plastic composites as follows, on a weight basis: building products, 70%; other (including marine uses, infrastructure), 13%; industrial consumer, 10%; and automotive, 7%. Although the U.S. annual growth in plastic demand is forecast at approximately 4.5% per year for 1998-2005, substantially greater growth in the demand for natural fibers is expected. This includes a rate in excess of 50% per year for the period 2000-2005 in the building products area; a significant portion of this growth will be attributed to larger market share for fiber/plastic lumber in residual decking (Smith et al., 2001). A smaller, but significant subset of the building products market is also found in vinyl windows (Cannon, 1999). Wood fiber, at weight loading up to 70%, is used in vinyl or vinyl-clad wood window components. The wood materials and engineering laboratory at Washington State University has directed an interdisciplinary and interinstitutional research program for the development of HDPE (high density polyethylene)- and PVC-wood composite materials for use in waterfront structures. A major research and development effort is centered around waterfront applications for Navy facilities (Smith 2001). WPC is being investigated to replace treated timber currently used to support piers and absorb the shock of docking ships. The material development component of the Navy project is focusing on evaluation and improvement of existing wood-plastic composite technologies as well as developing novel systems appropriate to the production of pier structural components (Wolcott, 2000). Reinforcement of woodplastic composites with carbon fibers was examined, but problems were encountered with PVC prepregs because of thermal degradation. Material structural studies revealed a large degree of processing-induced damage in the wood particles in PVC formulations as evidenced by reduced particle size. Co-extrusion of PVC WPC formulations with caps was successful. However, formulations were restricted to light color compounds. The PVC formulations were found to be viable for use in industrial deckboards. Natural fiber use in automotive fiber/plastic applications has been increased by 15% per year during 2000-2005. To date, most natural fiber/plastic materials in the automotive area have been HDPE or PP blends. While there is substantial growth in this area in North America, Europe appears to lead the way in the use of wood-plastic composites for automobiles. These examples serve to illustrate the growing levels of interest in wood-plastic composites from both research and commercial standpoints.