What are wood-plastic composites (WPCs)?
WPCs, also referred to as Natural Fiber Polymer Composites, are a mixture of wood, thermoplastic resins, and other additives. Wood can be used in various forms, however, it is commonly used in the form of wood flour (fine particles). Some of the thermoplastic resins include low and high density polyethylenes (LDPE, HDPE), polypropylene (PP), and polyvinylchloride (PVC). Either recycled or virgin plastic materials could be used to produce WPCs. In general, polyethylene based WPCs are more thermally stable and ducticle in nature; and, polypropylene based WPCs have higher stiffness and tend to be more brittle in nature.
Wood-plastic composite formulations also include additives, such as lubricants, inorganic fillers, colorants, UV stabilizers, biocides and fire retardants. Lubricants aid in processing WPCs, inorganic fillers help in improving the properties and biocides improve decay resistance.
WPCs are either extruded, injection molded, or compression molded. Extrusion is the most commonly used processing technology for manufacturing wood-plastic products. WPCs are ideally suited for exterior use. Current applications for WPCs include decking, railing, interior auto parts, cladding and window frames.
A report published by the Pacific Northwest Pollution Prevention Resource Center states the following regarding WPCs:
"An alternative source of building materials is waste plastic, which can be used to manufacture plastic lumber and wood-plastic composites that resist insect attacks and moisture without chemical preservatives.
King County has experimented with plastic playground structures, picnic tables, bus shelters, and bridge timbers. The Sleeping Lady resort in central Washington installed wood-plastic composite decking.
The Navy is working with Washington State University's Composite Materials and Engineering Center on developing a wood-plastic composite for pier construction. The composite is manufactured with wood and recycled plastic from milk jugs. The goal is to produce a material that can stand up to the marine environment without the use of hazardous preservatives such as pentachlorophenol."
The main drivers for WPC acceptance are:
- Improved resource utilization
- Improved performance/appearance attributes over existing products
- Stricter regulations on the use of chemicals in building materials (such as phasing out of CCA treated lumber for residential use)
- Gaining acceptability among builders
- Life-cycle cost benefits
Attributes of Wood-Plastic Composites
- Recycled materials
- Low maintenance
- More thermally stable than plastics
- More dimensional stable than wood
- Low moisture absorption
- Decay and insect resistant
- Engineered profiles
- Lower variability
- No splinters, cracks
- Good machinability
- Accepts fasteners
- High initial costs
- Lower stiffness than wood
- Thermal expansion