Common Polymers and Additives used for Processing - Introduction to Polymeric Composites with Rice Hulls - page 52

(e.g., bromine and chlorine) or phosphorous reduce the possibility of ignition within
a polymer component or, once ignited, slowing down the spread of flames and
possibly self-extinguishing of the flames. Due to the importance of their safe uses,
many flame-retardants are under constant review and some are even banned. Many
flame-retardants are used with synergists such as antimony oxide, iron oxide, zinc
borate, zinc phosphate and zinc stearate.
4.7.7 Ultraviolet Stabilisers
UV action on plastics causes discolouration and degradation of the polymer material,
which is due to the action of light and oxygen. In the post-production of polymeric
composite lumber this could pose a problem, especially for light-coloured finishes
if they are used for outdoor applications, whereas dark-coloured finishes show very
minimal effects or not at all, even if they are used outdoors. Most of the damage
occurs because of the action of UV rays in sunlight, which causes deterioration
of appearance due to micro-crack formations and, consequently, a reduction in
mechanical performance, as well as chemical resistance, among others. To reduce
this photo-oxidation, special chemicals called 'light-stabilisers' or 'UV-stabilisers' are
available. The protection offered by these UV-stabilisers involves blocking, absorption
and dissipation of the harmful radiation so that it does not turn into heat.
Sterically hindered amine light-stabilisers give very good results. Organic nickel
compounds are also used often. Carbon black and other pigments are also often used
to protect plastics from UV rays.
4.7.8 Antistatic Agents
Polymers have such low electrical conductivity that they can easily build-up electric
charges. The rate of static charges generated at the surface of a component can be
decreased by reducing the amount of surface contact, whereas the rate of decay of
charges is increased through surface conductivity. Hence, a good antistatic agent
should be an ionisable additive that allows the charge to migrate to the surface,
while also creating bridges to the atmosphere through moisture in the surroundings.
In general, antistatic agents are applied externally to the polymer surface by spraying
or dip-coating after being dissolved in a suitable solvent carrier. The advantage of
externally applied antistatic agents is that only very small amounts of these agents are
needed, and that the effects of the additives are immediate. A disadvantage would be
that externally applied additives wear off with time due to friction, moisture and other
external factors. Internally applied agents can be blended directly into the polymer