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parameters of the profiles: width, thickness, and production rate. Some producers
may opt to use sophisticated extrusion systems that can eliminate the pelletising stage
and extrude profiles direct from the compounding stage in one fluent and continuous
process. Composite resins in the form of pellets are used by injection moulders and
compression moulders, and these practices are gaining popularity and momentum,
especially because these resins are cheaper.
The technology of producing polymeric composites shows the importance of the
planning and designing of suitable matrices, reinforcement and additives to produce
practical and cost-effective products to suit pre-determined end applications. The first
important factor is that the compatibility of the polymer matrix and the reinforcement
materials. The matrix may contain virgin material or a combination of virgin and
recycled material, but the moisture content of the reinforcing fibre must be as low as
possible (preferably ≈1%) and the smaller the particle size, the better the chance of
achieving a homogenous mix. For compounding, additives must be selected carefully.
Coupling agents are essential for achieving polarity. Typically, polyolefins grafted
with maleic anhydride act as coupling agents to significantly improve mechanical
properties. The polar functional groups of a coupling agent reacts among others, with
the hydroxyl (OH) groups of the wood to form a genuine chemical bond, whereas the
non-polar areas of the coupling agent have strong affinity to the non-polar polyolefin
chains. Coupling agents improve adhesion between polymer resins and fibres, which
reduces the rate of moisture absorption and increases strength properties such as
impact, tensile and flexural strength. Coupling agents also help in the dispersion of
fibres, which may allow increased wood content. Most WPC tend to be heavy so,
depending on the end application, producers may opt to include a foaming agent in
the formula to lighten the end product. In this case, coupling agents aid in foaming
with better dispersion and improve melt strength at the extruder die exit.
Lubricants are used widely in the production of WPC to improve extrusion rates.
Developers are coming out with excellent lubricants that have better compatibility
with other additives and great processing improvements. The demand is for more
colour variety, especially dark wood colours, so a wider range of pigment colours is
being made available by the producers. Most WPC are used for outdoor applications,
so light stabilisers are required to maintain the original colour. Light stabilisers are
also essential to aid weather resistance, counter UV rays, and to arrest brittleness
due to weathering. Biocides are needed to be added to control mould and mildew,
which result in surface staining or discolouration and affect long-term durability. The
addition of antimicrobials will protect the wood in WPC against decay and fungi,
as well as preventing surface mould growth and slowing down the effects of UV
degradation. Foaming of WPC is becoming increasingly important because it offers
benefits such as weight reduction, cost reduction and increases stiffness. Foaming
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