The Introduction and Background of wood plastic composite extrusion

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The manufacture of wood filled polymer materials has been an industrial process for about 80 years. Most of the early industrial applications of wood filled polymers occurred in thermosetting resin systems such as phenol-formaldehyde resins. Industrial applications of lignocellulosic filled thermoplastics were reported in the late 1960s (Rowell et al. 2002). A great deal of scientific literature began to appear on this topic during the 1980s into the 1990s (Lu et al. 2000). Also, in the 1990s, the industrial production of wood filled thermoplastic polymers became more wide spread. Also, during this time conferences in North America covering the topic of wood fiber plastic composites began to be held on annual basis. The last was held in Toronto, Canada in May 2002.

One of the largest segments of the wood filled thermoplastic industry centers on the production of wood plastic composite (wpc) decking material as a replacement for preservative treated wood and the more expensive durable wood species such as redwood and teak. Some of the positive attributes of wood filled plastic composite lumber compared to wood include it’s durability, low maintenance, no painting, insect and decay resistance, and it will not warp, splinter, crack, is denser and thus holds screws better and can be manufactured to be resistant to ultraviolet light. This paper will address the manufacture of wood plastic decking by extrusion, and resulting properties of the wood plastic composite lumber. Topics to be covered include the materials used to manufacture wood plastic composites including thermoplastics, wood form and processing additives, the extrusion processes used to manufacture wpc lumber, and properties and products.

As an industry, the manufacture of wood plastic lumber decking is less than 15 years old. It has experienced an annual growth rate of 30% during the late 1990s (ASTM 1999). Various extrusion processes are used to manufacture wood plastic lumber and some manufacturers, to reduce weight and material costs have adopted the manufacture of hollow cross-section profiles. The materials used to produce wood plastic composite lumber include thermoplastic resins, most notably, low and high density polyethylene, polypropylene and polyvinyl chloride. The polymers utilized typically need to have melting temperature that is below the thermal decomposition of wood (~225 degrees C). These thermoplastics can come from both virgin and recycled sources. The wood used to manufacture these composites is mostly in the form of wood flour (typically 40 mesh or lower) or recycled paper fiber. Wood contents typically range from 20 to 60% in these composites. Additives to the composites will include processing aids (lubricants, antioxidants, acid scavengers), and property enhancers (biocides, coupling agents, inorganic fillers, fire retardants, uv stabilzers, colorants, etc).

Keywords: Extrusion, wood, thermoplastic, composite, extruder