Stabilizers
provide protection against degradation caused by heat, oxidation, and solar
radiation. Thus, when used in plastic compositions they may be classified as
heat (or thermal) stabilizers, antioxidants and UV light stabilizers.
It is the role of heat stabilizers to prevent the
polymer from degrading during the short period of high temperature (150℃ to 300℃) processing
and to protect the finished plastic article against slow aging over longer
periods at service temperatures.
Antioxidants inhibit or retard oxidative degradation
at normal or elevated temperatures during processing, storage or service. Most
polymers undergo some oxidation degradation, but hydrocarbon polymers are
specially susceptible.Antioxidants, therefore, are generally added in small
quantities.
Most plastics exhibit varying degrees of degradation
upon prolonged outdoor exposure. Polypropylene, poly (vinyl chloride),
polyethylene, polyesters, crstalline and high impact polystyrenes, and ABS are
particularyly sensitive. Other plastics, particularly poly (methyl
methacrylate) and the fluorocarbons, are much more resistant. To arrest or
retard polymer degradation caused by the ultraviolet portion of solar
radiation, plastic formulations contain UV absorbers. These are compounds such
as substituted benzophenones, benzotriazoles and acrylonitriles that
selectively absorb harmful radiation and convert it to heat energy.
Pigments such as titanium dioxide and zinc oxide are
also used to protect plastics against the harmful effect of ultraviolet
radiation.
They function by absorbing some UV radiation, but
their ability to reflect radiation (heat as well as light) accounts for much of
their effectiveness.In applications where colour is not a requirement, carbon
black, which absorb UV light, is widely used as a very effective stabilizer
(e.g. in black polyethylene).
