Polymeric Composites with Different Constituents - Introduction to Polymeric Composites with Rice Hulls - page 70

and reinforcement plus the types of additives used decide the nature of the properties
of the final product. WPC are designed and formulated to meet end-application
requirements. The general theory is that for high-end applications it is best to keep
the wood flour content 70% whereas, for low-end applications, this content could
be much higher.
WPC are more rigid than natural lumber and have a lower thermal expansion
coefficient than polymers. They are available in very attractive finishes, much like
high-end wood. Unlike wood, WPC do not splinter easily and can be produced in
many shapes and sizes. These composites can be produced by standard extrusion,
injection moulding or compression moulding to almost any shape in solid or hollow
designs, and large three-dimensional profiles are also possible. WPC producers are
constantly on the lookout for improving: retention of physical properties, colour
retention, as well as resistance to mould, mildew and water penetration, particularly
related to outdoor applications. Constant development is taking place on improving
the resistance to weather conditions because most productions of WPC are for outdoor
applications such as decking, fencing, and benches.
To achieve good-quality WPC, additives are essential and indispensable, even
though they are used in small quantities in a formulation. These additives provide
homogenous mixing which results in sufficient stiffness, rigidity and good stability
against light and heat. WPC are much stronger than natural lumber and are less prone
to warping. Critical contributors to the quality of these composite products are flame
retardants because without them their applications will be limited due to building-
trade regulations. Flames may spread slowly in WPC but they can become highly
combustible due to wood content. However, safety standards for construction and
transportation can be met easily with the use of correct fire-retardant additives. Most
flame-retardants do not have major adverse effects on mechanical properties while
providing ecological and environmental benefits. Non-halogenated flame-retardant
additives do no release corrosive gases in the case of a fire and will emit only very
low levels of smoke.
6.1.2 Technology of Wood Plastic Composites - The Basics
Wood is not the only natural fibre used in polymer composites but is the most
common. However, it is expected that the use of rice hulls with emerging technology
will take precedence in the future because rice hulls have much better physical and
chemical properties.
WPC were first produced and marketed as superior weather-resistant material as
compared with natural wood. WPC have rapidly expanded into many other end uses.