Wood Plastic Composites (WPC) belong to a group of natural fiber-reinforced compounds in which wood fibers (or wood flour) are used as reinforcing fillers in amounts generally ranging from 50 to 80%. A number of other natural fibers can be used for comparable applications, for example, hemp, flax, sisal, and coconut fibers. However, wood fibers are used most frequently on a volume basis. Polymeric materials such as polyethylene (PE), polypropylene (PP) and PVC are used as binders.

Advantages

WPC products are generally more expensive to produce than conventional wood materials, but have a number of advantages. Alongside much simpler forming and the variety of diversified shapes, they are more durable due to their better weather resistance. Because of their high wood content, WPC are significantly more attractive, both on a cost and technical basis, than products made of pure plastic. They are frequently used as a replacement for tropical timber on account of their longevity, thereby contributing to the preservation of ecologically important tropical forests.

Applications

Wood plastic composite materials are used wherver wood has traditionally been employed. First and foremost, WPC is employed as a flooring material (decking) mainly outdoors, but also indoors. WPC is also used as railing, privacy fencing, exterior wall cladding, furniture and much more.

Manufacture

WPC profiles, which are produced in a single or two-stage extrusion process, are the most commonly manufactured form of WPC.

Single-stage Extrusion: This is a so-called direct extrusion process. The mixing of the wood fibers with the plastic granules (compounding) and the extrusion of the profile are performed in one step. Usually conical, counter-rotating double screw extruders are employed that are able to both thoroughly mix the wood and plastic and generate the pressure required for extruding the specified profile.

Two-stage Extrusion: In this method, compounding and extrusion of the profile occur in two separate processes. In this case, the compound can be prepared using different types of extruders. Most often parallel, co-rotating double screw extruders are used. Planetary gear extruders for the particularly gentle preparation of highly filled compounds and heating/cooling mixer combinations are also used. Other mixing methods are pellet mills and special grinding methods in which the wood and the plastic are ground at the melting point range of the plastic. The profile is then usually produced using single-screw extruders.

If a WPC is manufactured according to the two-stage extrusion compounding process described above, it must not necessarily be extruded, but can also be further processed using other methods. Injection molding is mentioned here in particular because it is a technique that allows the manufacture of complex shapes. Other possibilities include rotational molding and the plate press method. In principle, WPC compounds can be processed by almost all conventional techniques used in plastics engineering. The limiting factor is usually the flow properties of the WPC, which are primarily determined by the wood content.