Page 117 - Handbook of Plastics Technologies

THERMOPLASTICS

THERMOPLASTICS 2.57

boxylic acids, paraffin oils, and certain alcohols and ketones for PVC. Low-molecular-
weight materials that do not affect the solid properties, but act to enhance flow in the melt
state, are termed internal lubricants. Internal lubricants for PVC include amine waxes,
montan wax ester derivatives, and long-chain esters. Polymeric flow promoters are also
examples of internal lubricants. They have solubility parameters similar to the thermoplas-
tic, but lower viscosity at processing temperatures. They have little effect on the mechani-
cal properties of the solid polymer. An example is the use of ethylene-vinyl acetate
copolymers with PVC.
Anti-aging additives are incorporated to improve the resistance of the formulation. Ex-
amples of aging include attack by oxygen, ozone, dehydrochlorination, and UV degrada-
tion. Aging often results in changes in the structure of the polymer chain such as cross-
linking, chain scission, addition of polar groups, or the addition of groups that cause dis-
coloration. Additives are used to help prevent these reactions. Antioxidants are added to
the polymer to stop the free-radical reactions that occur during oxidation. Antioxidants in-
clude compound such as phenols and amines. Phenols are often used because they have
less of a tendency to stain. 404 Peroxide decomposers are also added to improve the aging
properties of thermoplastics. These include mecaptans, sulfonic acids, and zinc dialkylth-
iophosphate. The presence of metal ions can act to increase the oxidation rate, even in the
presence of antioxidants. Metal deactivators are often added to prevent this from taking
place. Chelating agents are added to complex with the metal ion.
The absorption of ultraviolet light by a polymer may lead to the production of free rad-
icals. These radicals react with oxygen resulting in what is termed photodegradation. This
leads to the production of chemical groups that tend to absorb ultraviolet light, increasing
the amount photodegradation. To reduce this effect, UV stabilizers are added. One way to
accomplish UV stabilization is by the addition of UV absorbers such as benzophenones,
salicylates, and carbon black. 405 They act to dissipate the energy in a harmless fashion.
Quenching agents react with the activated polymer molecule. Nickel chelates and hindered
amines can be used as quenching agents. Peroxide decomposers may be used to aid in UV
stability.
In certain applications, flame resistance can be important. In this case, flame retarders
may be added. 406 They act by one of four possible mechanisms. They may act to chemi-
cally interfere with the propagation of flame, react or decompose to absorb heat, form a fire
resistant coating on the polymer, or produce gases that reduce the supply of air. Phosphates
are an important class of flame retarders. Tritolyl phosphate and trixylyl phosphate are of-
ten used in PVC. Halogenated compounds such as chlorinated paraffins may also be used.
Antimony oxide is often used in conjunction to obtain better results. Other flame retarders
include titanium dioxide, zinc oxide, zinc borate, and red phosphorus. As with other addi-
tives, the proper selection of a flame retarder will depend on the particular thermoplastic.
Colorants are added to produce color in the polymeric part. They are separated into
pigments and dyes. Pigments are insoluble in the polymer, while dyes are soluble in the
polymer. The particular color desired and the type of polymer will affect the selection of
the colorants.
407
Blowing agents are added to the polymer to produce a foam or cellular structure.
They may be chemical blowing agents that decompose at certain temperatures and release
a gas, or they may be low boiling liquids that become volatile at the processing tempera-
tures. Gases may be introduced into the polymer under pressure and expand when the
polymer is depressurized. Mechanical whipping and the incorporation of hollow glass
spheres can also be used to produce cellular materials.
Peroxides are often added to produce cross-linking in a system. Peroxides can be se-
lected to decompose at a particular temperature for the application. Peroxides can be used
to cross-link saturated polymers.

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