Today’s composites made with natural fibers are high-tech materials with outstanding properties. Offering utmost versatility and high quality, they are continuously opening up new, demanding application fields.

A relatively new type of natural fiber composite is Wood Plastic Composites (WPC) with up to 90% wood fiber or wood flour content. In most cases the plastic matrix is polypropylene (PP) or polyethylene (PE). Polyvinylchloride (PVC) is also used. The material combines the advantages of the renewable resource, wood, with the benefits of a plastic that can be tailored as needed. WPCs are more rigid and have a lower thermal expansion coefficient than pure plastic, while the attractive look and pleasant haptic of the wood component remains unaltered. WPCs are also significantly more resistant against exterior influences than wood and do not splinter. WPCs are equally superior to wood in terms of shapability. The composite can be shaped by way of extrusion, injection-molding, or compression molding to almost all types of parts: from small, complicated, or hollow parts and profiles to large three-dimensional parts.

However, combining wood and plastic is not enough to produce WPCs with suitable properties. Even though they do not make up more than a few percent of the total weight, additives are indispensable. Only additives give WPCs sufficient stiffness (rigidity) and good stability against light and heat. Another crucial contributor to WPC quality are flame retardants. Without them, the use of WPCs in many areas would not be possible.

Inner Quality Counts

The most important additives in WPCs are coupling agents. Wood and plastics differ, especially in their polarity. In order to produce a sufficiently sturdy material, suitable coupling agents, such as polyolefins grafted with maleic anhydride, must be added. They significantly improve the mechanical WPC key parameters of tensile stress and bending load. Two opposite forces are at work here. The polar functional groups of the coupling agent react amongst others with the OH groups of the wood and form a genuine chemical bond. But the non-polar areas of the coupling agent have strong affinity to the non-polar polyolefin chains.

The “designer waxes” of the Licocene® series from Clariant are excellent coupling agents. They are produced by metallocene catalysis, which makes it possible to perfectly tailor their properties to the individual application. For polypropylene-based WPCs, the recommended coupling agent is Licocene PP MA 7452 TP, while for polyethylene-based composites, Licocene PE MA 4351 TP is highly effective.

Flame retardants are an entirely different, but no less important, additive for WPCs. Although flames spread rather slowly in WPCs because of their high wood content, the material is highly combustible once it catches fire. With the addition of flame retardants to WPCs, the safety standards in construction and transportation (e.g. DIN 4102 B2) can be met. An excellent fit for WPCs are flame retardants from Clariant’s Exolit AP series. They feature top effectiveness, have minimal secondary effects, and can also be recommended for ecological and toxicological reasons – the products are non-halogenated, do not release corrosive gases in case of a fire, and have comparatively low smoke density.

Looks Are Equally Important

Mainly in outdoor applications, WPC parts are exposed to wind and weather, especially sunlight. To protect the attractive look of wood and the pleasant, smooth haptic of WPCs against UV-induced fading and brittling, it is necessary to add light stabilizers. Polypropylene-based WPCs can be effectively protected with additional coatings that contain light stabilizers. If the WPC consists of wood and polyethylene, different HALS (Hindered Amine Light Stabilizer) can be used. Light stabilizers such as Hostavin® N 321, or heat stabilizers such as Hostanox® M 101 prevent or delay the decomposition of the plastic matrix due to sunlight and heat.

WPCs also have a lot to offer in terms of color design. Their original medium brown color can be changed with high-performance organic pigments (for example PV Fast®) to any medium to dark color. Depending on individual taste the color concentration can be adjusted so that the wood structure is still visible or the color is fully opaque. Pigments from Clariant are used to produce WPC with brilliant colors that remain light- and weather-resistant over a long time and also do not bleed.

Only high-performance additives transform WPCs into a true high-tech material for many applications, including sophisticated and demanding ones. Architects and builders increasingly decide in favor of WPC parts such as porch flooring, railings, or various profiles. The furniture industry and automotive suppliers have also discovered WPC, using them for chairs and shelf elements as well as for interior components of passenger cars. The potential applications for WPCs are far from being exhausted, and it will be interesting to see which other applications WPC enter in the future.