Can we consider WPC dry-blend to be a composite? We can't. Since the material structure is yet not an integrated whole. One should apply a high volume process, such as extrusion, injection or compression molding to form a structure, during which wood flour (fibers) are incorporated into the polymer matrix in the presence of chemical additives. If we miss to put additives, wood flour can still be incorporated into the molten polymer however such a WPC product would rather be an experimental than a commercial one.
Depending on the production process, WPC appears either in the form of pellets (WPC granules), a feedstock for WPC profile extrusion or injection, or as a finished WPC product (WPC lumber/timber, injection molded or pressed item).
Principal WPC components in brief
Natural fibersWood-plastic composite (WPC) term is commonly used as a definition for almost all natural (ligno)cellulosic fiber filled plastic composites, whether they contain wood or any other fiber such as rice hulls, kenaf, flax, wheat or cotton stalks and so on. Whatever natural fiber is used as a filler, it typically appears in the form of pulverized (40-120 mesh) powder or, less often, as long fibers. Wood powder is often called wood flour.
Matrix polymersThe majority of commercial composite wood lumber manufactured today is based on three major industrial grade polymers - polyethylene (PE), polypropylene (PP) or polyvinyl chloride (PVC). There are basically two reasons. First, wood requires processing temperatures lower than 200C, and these polymers melt under lower temperatures, thus preventing wood from degrading / burning when processed. Second, market price for the polymers in question is comparatively low, especially if they are used in recycled form.
Additives for WPCWPC additives can be divided into two major groups - functional additives and processing aids. The group names are self-explanatory. To put it simple, the former is in charge of WPC's physical & mechanical properties such as strength, flexibility, UV-resistance, fire resistance, weathering resistance an so on. The latter, being in charge of smoother processing, includes lubricants, compatibilizers, wetting agents, plasticizers (for PVC) etc. As a matter of practice, same additives can perform both functions.
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