Use of Recycled materials in WPC's
Use of Recycled materials in WPC’s
In 1994, about 190 million metric tonnes of municipal solid waste was generated in the United States (Falk 1997). Waste wood, waste paper, and waste plastics are major components of municipal solid waste (MSW) and offer great opportunities as recycled ingredients in wood fiber-plastic composites (Youngquist 1992). Recycled sources of both wood and plastic are commonly used in WPC’s (Table 2). Both post-consumer and post-industrial materials are used. Finding an inexpensive, yet suitable source, with consistent and sufficient performance can present challenges but the broad use of recycled materials in WPC’s demonstrates its feasibility. Big companies see the use of their own waste by-products as a shrewd business decision that happens to spare landfills from tons of material.
Roughly one-half of all industrial materials used in the United States are wood-based (Falk 1997). Wood flour is the most common wood filler used in WPCs. It is typically a post-industrial source consisting of wood shavings, chips, and sawdust produced by secondary wood product manufacturers. Post-consumer sources can include wood pallets, building construction waste, and old newspapers. Currently, waste wood fiber-recycling is a large part of the wood pallet industry with an estimated 44% rate of reclamation (Bush et al 1995). Many commercial WPC products currently use recycled wood fiber (Table 2). For example, Trex decking uses about 50% reclaimed wood fiber (Table 1) and CorrectDeck uses 60 percent oak and pine sawdust primarily from shavings from wood flooring manufacturers (Table 1). Other fillers besides wood are also used in plastic composites. Nexwood used approximately ~9.1 million kg of rice hulls in 2002 (Principia 2003).
Recycled Plastics in WPCs
Combining cellulose fibers with post-consumer waste plastic is more economical than using fiber for composites with thermoset phenolic resins (Hettinga 1997). Power (2004) reviewed the use and development of WPC lumber in the U.S. residential construction market and he cited several examples of various commercial WPC manufactures using recycled polymers, recycled fiber or both. In 2002, Trex, the largest supplier of wood-plastic composite lumber, purchased on average over 227,000 kg of plastic scrap each day (Principia 2002). CorrectDeck uses about half virgin plastic and the other half (20 percent) is obtained from recycled grocery bags and used pallet wrap (Powers 2004). Anderson developed engineered WPC materials for its Renewal line of windows from a wood-PVC composite material made from wood and PVC material partially reclaimed from its wood window plants. Boise has recently developed and introduced its new HomePlate siding made with 50 percent recycled polyethylene and 50 percent reprocessed “urban” wood fiber. In total, the wood-plastic industry in North America consumed an estimated 204 million kg of plastic in 2001, of which more than 95% of it was recycled (Principia 2002). Of all the polyolefins reclaimed from the post-consumer waste stream, 38% ended up in wood-plastic composites. (Principia 2002). Because a large portion of the plastic waste stream is already commonly used in WPCs, it is becoming more difficult to secure recycled raw material sources. Many commercial WPC manufacturers use a combination of recycled wood-based materials and recycled plastics.
Potential for use of other recycled sources
Wood flour or fiber bundles (often generically, but mistakenly referred to as “wood fiber”) are often used in WPC’s in the US. The wood flour is clean, consistent, inexpensive, free flowing, and is readily available as a waste byproduct from secondary manufacturing operations. However, this approach does not efficiently use the potential strength of the individual wood fiber. Possibilities exist for using recycled or “waste” wood fibers as reinforcements in WPC’s rather than the commonly used fillers provided processing and handling issues could be overcome and that the fibers are well bonded to the plastic matrix.
For example, Youngquist et al (1993) found that using fiber from old newspapers as reinforcing fiber provided measurable property advantages over wood flour. Also, these recycled newsprint WPC systems could themselves be recycled (re-extruded and injection molded) numerous times with little or no apparent loss in mechanical properties. Stark (1 999) found that reinforcing polypropylene with wood fiber derived from recycled hardwood and softwood pallets provided improved bending and tensile strength over the use of wood flour. Stark (1999) also found that the use of maleated PP as a coupling agent further enhanced the mechanical properties of these recycled wood fiber-PP composites. Other recycled sources that are being investigated include waste paper mill sludge and commingled plastic-wood fiber sources such as plastic coated paper milk and juice box containers. Ultimately, additional research is needed on WPC’s made from recycled wood fiber and plastics to improve properties and processing and thereby increase the number of potential applications. To do this we must learn to improve melt-blending processes to achieve better fiber dispersion with minimal fiber breakage, improve the bonding between the wood fiber and plastic matrix especially as this is related to more variable recycled materials, and improve the mechanical performance of these WPC versus impact energy and creep. Improving the resistance of WPC’s (both virgin and recycled systems) to moisture, to biodegradation, and to fire are important properties that also need to be addressed. Such investigations continue at FPL and worldwide.