Polymeric Composites with Rice Hulls for Extrusion - Introduction to Polymeric Composites with Rice Hulls - page 120

may opt for extruding the ready-made pelletised resins. The advantage of the former
is that a producer can vary the end properties desired using different formulations
whereas, in the case of the latter, a producer will have to go with the already set
properties of pelletised resins. Some of the important basic additives such as coupling
agents, lubricants, colourants, chemical foaming agents (CFA) and biocides that are
essential are presented below.
10.2.1 Coupling Agents
Coupling agents bond rice hull fibres to the resin matrix. They boost the flexural
strength and stiffness (usually referred to as the modulus of rupture and modulus of
elasticity, respectively) which are terms used in the lumber industry. Coupling agents
also improve dimensional stability, impact resistance and fibre dispersion, while
reducing creep. Added strength is important in structural applications, especially in
the building construction industry. In outdoor applications such as decking, coupling
agents are used mainly to reduce water absorption, which tends to swell the surface
and could lead to stresses that cause cracking.
Coupling agents are especially valuable in polyethylene (PE)-based composites because
they overcome the possible incompatibility between the polar chemistry of wood
and non-polar resin matrix. Chemically modified grafted polyolefins are available.
However, newer developments include chemically modified polyolefins that are not
made by grafting, long-chain chlorinated paraffin, and reactive agents suitable for
polyvinyl chloride (PVC) composites. Some useful and effective coupling agents are:
Polybond3029MP [(high-density polyethylene (HDPE)], Fusabond MB-226D (PE),
MD-353D [polypropylene (PP)] and Epolene G-2608 (PE).
10.2.2 Lubricants
Lubricants facilitate easier mixing at the time of compounding, increase throughput
and improve the extruded surface finish. PCRH composites can use standard lubricants
for PE and PVC, such as ethylene bistearamide (EBS), zinc stearate, paraffin waxes
and oxidised PE. EBS with zinc stearate is commonly used in HDPE composites.
However, there are new alternatives because metal stearates are known to decouple
maleic anhydride groups of a maleated coupling agent, thus cancelling the effectiveness
of the lubricant and coupling agent.
Polymer composites use nearly as twice as much lubricant as standard plastics. For
a HDPE composite with a typical 50-60% fibre content, the lubricant level can be
4-5%, whereas a similar fibre-PP composite uses ≈1-2%. Some of the common