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
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
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.
Additives are Key in High-Tech Wood Plastic Composites
June 1, 2009