Chemical additives seem practically "invisible" (except mineral fillers and pigments, if added) in the composite structure. They provide for integration of polymer and wood flour (powder) while facilitating optimal processing conditions.
In addition to wood fiber and plastic, WPCs can also contain other ligno-cellulosic and/or inorganic filler materials. WPCs are a subset of a larger category of materials called natural fiber plastic composites (NFPCs), which may contain no cellulose-based fiber fillers such as pulp fibers, peanut hulls, bamboo, straw, digestate, etc.
Wood-plastic composites are still new materials relative to the long history of natural lumber as a building material. The most widespread use of WPCs in North America is in outdoor deck floors, but it is also used for railings, fences, landscaping timbers, cladding and siding, park benches, molding and trim, window and door frames, and indoor furniture. Wood-plastic composites were first introduced into the decking market in the early 1990s. Manufacturers[who?] claim that wood-plastic composite is more environmentally friendly and requires less maintenance than the alternatives of solid wood treated with preservatives or solid wood of rot-resistant species. These materials can be molded with or without simulated wood grain details.
Wood Plastic Composites (WPCs) are produced by thoroughly mixing ground wood particles and heated thermoplastic resin. The most common method of production is to extrude the material into the desired shape, though injection molding is also used. WPCs may be produced from either virgin or recycled thermoplastics including HDPE, LDPE, PVC, PP, ABS, PS, and PLA. Polyethylene based WPCs are by far the most common. Additives such as colorants, coupling agents, UV stabilizers, blowing agents, foaming agents, and lubricants help tailor the end product to the target area of application. Extruded WPCs are formed into both solid and hollow profiles. A large variety of injection molded parts are also produced, from automotive door panels to cell phone covers.
In some manufacturing facilities, the constituents are combined and processed in a pelletizing extruder, which produces pellets of the new material. The pellets are then re-melted and formed into the final shape. Other manufacturers complete the finished part in a single step of mixing and extrusion. 
Advantages and disadvantages
WPCs do not corrode and are highly resistant to rot, decay, and Marine Borer attack, though they do absorb water into the wood fibers embedded within the material. They have good workability and can be shaped using conventional woodworking tools. WPCs are often considered a sustainable material because they can be made using recycled plastics and the waste products of the wood industry. Although these materials continue the lifespan of used and discarded materials, and have their own considerable half life; the polymers and adhesives added make wood-plastic composite difficult to recycle again after use. They can however be recycled easily in a new wood-plastic composite, much like concrete. One advantage over wood is the ability of the material to be molded to meet almost any desired shape. A WPC member can be bent and fixed to form strong arching curves. Another major selling point of these materials is their lack of need for paint. They are manufactured in a variety of colors, but are widely available in grays and earth tones. Despite up to 70 percent cellulose content (although 50/50 is more common), the mechanical behavior of WPCs is most similar to neat polymers. Neat polymers are polymerized without added solvents. This means that WPCs have a lower strength and stiffness than wood, and they experience time and temperature-dependent behavior. The wood particles are susceptible to fungal attack, though not as much so as solid wood, and the polymer component is vulnerable to UV degradation. It is possible that the strength and stiffness may be reduced by moisture absorption and freeze-thaw cycling, though testing is still being conducted in this area. Some WPC formulations are also sensitive to staining from a variety of agents.
As time goes on, product defects have become well known. The industry leader (Trex Company, Inc.) has been involved in numerous lawsuits with home owners and installers in regards to mildew, discoloration and increased weakness in the fiber of the materials. As of mid 2008, Trex has replaced over 37,000 decks in regards to the issues previously stated and had to change their 'maintenance-free' claims to 'low-maintenance' explaining that decking would require cleaning multiple times every year to remove mildew spots.
Once more, in 2013, Trex notified consumers in 16 western states who may have purchased defective Trex decking product suffering from surface flaking to contact the company for replacement materials. Trex has previously announced that a small percentage of decking boards manufactured at its Fernley, Nevada plant between 2002 and 2007 suffered from surface flaking due to a manufacturing problem.
The next generation of composite decking includes a durable plastic cap that grants added protection. A PVC cap surrounding the wood plastic composite material helps reduce staining, fading and scratching that may occur with 1st generation composite decking. Capped composites cost considerably less than pure plastic decking as the core is still wood plastic composite materials. Although PVC capping is one type of cap, there are many different types of caps that are coming out on the market. They will also have the same properties of reducing staining, fading, and scratching.
The types of plastics normally used in WPC formulations have higher fire hazard properties than wood alone, as plastic has a higher chemical heat content and can melt. The inclusion of plastic as a portion of the composite results in the potential for higher fire hazards in WPCs as compared with wood. Some code officials are becoming increasingly concerned with the fire performance of WPCs.
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