Challenges in Biomass–Thermoplastic Composites

Wood and other biomass resources have been blended with thermoplastic such as polyethylene, polypropylene, polylactic acid and polyvinyl chloride to form wood plastic composites (WPC). WPCs have seen a large growth in the United States in recent years mainly in the residential decking market with the removal of CCA treated wood decking from residential markets. While there are many successes to report in WPCs, there are still some issues that need to be addressed before this technology will reach its full potential. This technology involves two different types of materials; one hygroscopic (biomass) and one hydrophobic (plastic) so there are issues of phase separation and compatiblization. This technology also involves two very different industries; wood and/or agricultural industries and plastic industries. Processing the two phases presents issues centered on maximizing mixing while minimizing damage to the biomass furnish. Melt flow index, processing temperatures, static electricity and density are also issues for the plastic industry which is used to high flowing high temperature processing conditions necessary when they either extrude plastics neat or use inorganic fillers such as glass, calcium carbonate or talc. While the biomass in the composite swells and shrinks due to moisture, the plastic phase swells and shrinks due to temperature. Freezing and thawing cycles also cause problems as well as the effects of microorganism attack and ultraviolet radiation. Dyeing, refinishing, color stability and fasteners are also concerns. Solid extrusion, profile extrusion, co-extrusion, injection molding and thermal molding are all being used to make different WPC products in different parts of the world.