Page 323 - Handbook of Plastics Technologies
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PLASTICS ADDITIVES
PLASTICS ADDITIVES 5.3
More commonly, however, we use a small amount of additive in a fairly stable polymer
to make it more stable and useful. Individual polymers have individual stability problems,
for which we use different types of additives. It is a mistake to think of stabilizers in gen-
eral producing stability in general. We can distinguish five major types of stabilizers to
solve five types of stability problems:
1. Antioxidants are used to protect against combined attack of atmospheric oxygen and
heat during processing and during use above room temperature.
2. Antiozonants are used to protect the C=C double bonds in rubber against attack by at-
mospheric ozone.
3. PVC heat stabilizers are used to protect polyvinyl chloride against thermal dehydro-
chlorination during processing and during use above room temperature. Similar addi-
tives are sometimes useful in other halogenated systems as well.
4. Ultraviolet stabilizers are used to protect polymers in outdoor use against the high-
energy ultraviolet radiation from the sun, and occasionally indoors when exposed to
fluorescent lights.
5. Biostabilizers are used to protect polymers against biological attack, primarily by mi-
croorganisms.
5.1.1 Antioxidants
The attack of atmospheric oxygen on polymers is a free-radical reaction accelerated by
heat, particularly during processing, and also during long-term use above room tempera-
ture. Typical autoxidation reactions such as
. .
R:H → R + H
.
R + O → RO 2 .
2
.
.
RO + R´H → RO R´ + H or RO H + R´ .
2
2
2
.
H + O → HO 2 .
2
. . .
HO + RH → HO R + H or HO H + R
2
2
2
proceed rapidly forming peroxides and hydroperoxides. These are unstable and decom-
pose into free radicals
.
.
RO:OR → RO + OR
.
.
RO:OH → RO + OH
which decompose further forming carbonyl, carboxyl, cross-linking, cleavage, dispropor-
tionation, and/or depolymerization products. Change in molecular weight affects most
properties. Change in unsaturation causes discoloration. Change in polarity ruins electrical
resistance. And oxidized species are more susceptible to biological attack.
Antioxidants are additives to prevent or at least delay the attack of oxygen. There are
three main types, as described in the following sections.
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