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Flame retardants: They are materials which deter or extinguish flame propagation,
dripping of flaming particles and smoke creation in case plastics catches fire. Help
prevent fire or its propagation by interrupting or hindering the combustion process.
Additives should be compatible, have properties of low toxicity, low smoke emission
and minimise toxic breakdown products in a fire situation. Also it should not
adversely affect physical properties, be effective at low addition rates and meet
various safety standards and government regulations.
Nucleating agents: They are used in polymer systems to increase the rate of
crystallization. These intentional contaminate acts as sites for crystalline formation
and help change the polymer's crystallization temperature, crystal spherulite size,
density, clarity, impact and tensile properties. Their addition into polymers also
yields benefits such as higher productivity, improved optical properties, elastic
modulus and can shorten cycle time by reducing set-up time in the mould. Alkaline
earth or aluminium salt of aromatic and aliphatic carboxylic acids are particularly
used in PP and other polyolefins having medium crystal growth rate.
Heat stabilizers: Heat stabilizers are used to prevent the thermal degradation of
resins during periods of exposure to elevated temperatures. They are used to
stabilize PVC, PVDC, PVC Blends and other polymers. Thermal degradation is
prevented not only during processing but also during the useful life of the finished
products. PVC essentially requires heat stabilizers for its processing as at higher
temperature dehydrochlorination causes degradation of PVC and release HCL.
Metal based stabilizers minimise the initial degradation and then react with HCL to
reduce its autocatalysis of additional dehydrochlorination. Lead tin and lead-
barium-cadium systems are powerful stabilizers.
UV additives: Most polymers are sensitive to solar radiation and mainly to the UV
region. UV radiation can stimulate molecules to an extent so as to break their
chemical bonds that can release highly reactive radicals, capable of destroying
other molecules causing product embitterment, discoloration, crazing and
disintegration. Thus UV additives are used to prevent harmful effects on polymer.
They protect polymers from the UV region mainly by two mechanisms:
UV Absorbers: Solutions which protect organic materials by absorbing the UV
radiation and convert the energy to heat thereby prevents the formation of free
radicals and thus damaging effect to materials.
UV Stabilizers: Additives which stabilize polymer materials against UV Radiation.
Quenchers - The deactivation of the excited state of polymer molecule is called
quenching. Organic nickel compounds are used exclusively as quenchers for
polyolefins.
Radical Scavengers - When UV absorbers and quenchers do not help and radicals
are actually formed, then radical scavengers are needed. Hindered-amine light
stabilizers (HALS) are most effective radical scavengers.
Lubricants: Lubricants are added to decrease internal and external friction in
molten polymer and facilitate its processing by improving flow properties. It also
enhances properties such as better colour or pigment dispersion, higher surface
lustre or clarity of moulded part, elimination of flow lines, stronger weld lines, faster
cycle and easier mould release. Lubricants are of two types –

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