Wood Plastic Composite Decking (also known as wood polymer composite, WPC or simply just composite) is a relatively new material in the UK. It is usually composed of wood from sawdust mixed with virgin or waste plastics – typically high density PE (polyethylene), PVC (polyvinyl chloride) or PP (polypropylene). The powder or fibres are mixed to a dough-like consistency and then extruded or moulded to the desired shape. Extrusion is the technique used for decking and fencing and related products.
Additives – such as colourants, coupling agents, stabilizers, blowing agents, reinforcing agents, foaming agents and lubricants – help to tailor the end product to the target area of application.
The overriding principle is to fully encapsulate the wood fibre particles with melted plastic to form a homogenous material so that the plastic encases the wood fibre and protects it from moisture ingress. However, plastic and wood cannot easily be mixed. Manufacture is a complex process, which requires careful handling of the materials in a series of stages – each with critical aspects to get right, such as particle size and distribution and removal of moisture. Any short cuts can lead to disastrous results.
There may also be a finishing stage because newly extruded materials may have an unappealing, shiny surface. If steel brushing is used then this actually breaks the surface and exposes unprotected wood fibre.
In addition, embossing can be used to create a wood grain effect; while capping adds an extra protective layer. Capping, however, should not be used to compensate for inferior quality boards.