Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 53

matrix. These also work as lubricating agents (lowering surface friction and reducing
generation of static charges) or as conductors (by creating a water-absorbing layer
on the surface of the polymer and creating a conduction path to the atmosphere).
Many antistatic agents are available but one may consider using fatty acid esters,
ethoxylated alkylamines or diethanolamides , which are non-ionic antistatic agents.
For more polar polymers such as PVC and styrenics, anionic antistatic agents such
as alkylsulpfonates or alkylphosphates can be used.
4.7.9 Antimicrobial Agents
Bacterial growth is a very high priority concern, especially for polymeric composites
used for outdoor applications, due to wet and damp conditions. This growth leads
to many hazards for composite products and must be dealt with effectively. For
polymeric composite materials used for indoor applications, this may not be a real
problem but, if used in highly air-conditioned atmospheres, microbial growth is
possible. Antimicrobial agents (also known as 'fungicides') are used to slow down
or prevent the growth and propagation of bacteria.
Plasticised PVC, LDPE and polyesters are particularly susceptible to microbial attack.
PVC and polyurethanes (PU) are by far the main plastics that require fungicides to
be incorporated. PVC itself is resistant to microbial attack, but the plasticisers used
serve as nutrients for fungal growth. Antimicrobial agents are also known as 'bio-
stabilisers' and, because of their prices and efficacy, over half of antimicrobial agents
are arsenic-based agents. However, due to ecological and toxicity concerns, other
non-arsenic-based agents are being used widely.
4.7.10 Blowing Agents
In the processing of plastic resins, the task of a blowing or foaming agent is to
produce cellular polymers (also called 'expanded plastics'). The cells can be closed
completely (e.g., rigid foams) or open cells (e.g., flexible PU foams). These agents
can be incorporated during polymerisation, as in EPS, or added to the polymer
mix during processing. In the present context, polymeric composites in which the
reinforcement content in the polymer matrix with biomasses is high result in heavy end
products (e.g., extruded lumber) and this property is good for outdoor applications,
where they have to bear loads and withstand the environment. This property gives
the composite products a longer life and lessens maintenance that may be required.
However, for polymeric composites for indoor uses, lighter products will be preferred
for easy handling, fabrication and lighter load-bearing. This is where blowing agents
or foaming agents will be useful.