Page 327 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.7

ers (iron particularly), or from corrosion of process equipment, or as adjacent surfaces on
final products (insulation on copper wire), they may aggravate the attack of atmospheric
oxygen and the resulting degradation of the polymer. One way to remove the metal from
the system is to tie it up in an inactive complex, in which form it is no longer able to cata-
lyze the oxidation reaction. These complexing agents are usually organo nitrogen com-
pounds or polyols. They are not used alone but are added as synergists to a system that
already contains primary antioxidants.
5.1.1.4 Acid Scavengers. Oxidation of polymers produces organic acids. Chlorine and
bromine, from catalyst residues and flame-retardants, produce stronger acids. These can
cause hydrolysis of polymers and corrosion of process equipment. Therefore, it is fairly
common practice to add acid scavengers to neutralize them. These are mildly alkaline sub-
stances such as calcium and zinc stearates, hydrotalcite, hydrocalumite, and zinc oxide.

5.1.1.5 Use in Commercial Plastics. LDPE is usually stabilized by 0.005 to 0.05 per-
cent BHT. DLTDP and nonylphenyl phosphite may be added as well. For wire and cable
insulation, metal deactivator is also needed.
LLDPE and HDPE use higher-molecular-weight phenols and higher concentrations.
Cross-linked polyethylene, containing carbon black, permits use of thiodiphenols and dia-
ryl amines, since their discoloration is masked by the carbon black. For wire and cable,
hydrazides and triazines are common metal deactivators to protect against copper catalysis
of oxidation.
Polypropylene contains less-stable tertiary hydrogens and processes at higher tempera-
tures, so it requires higher concentrations (0.25 to 1.0 percent) of higher-molecular-weight
phenols and more vigorous use of aliphatic sulfides and aromatic phosphites. Poly-1-
butene is similar.
ABS contains 10 to 30 percent of butadiene rubber, whose C=C bonds are very sensi-
tive to oxidation, producing embrittlement and discoloration. Triaryl phosphites are used
as primary antioxidants, in concentrations up to 2.5 percent, producing excellent stabiliza-
tion.
“Crystal” polystyrene is resistant to oxidation, but most “polystyrene” is actually im-
pact styrene containing 2 to 10 percent of butadiene rubber. Like ABS, it requires similar
stabilization, but lower concentrations are sufficient.
Acetal resins are sensitive to oxidation and are generally stabilized by high-molecular-
weight phenols. Polyesters and polyurethanes are commonly stabilized by phosphites.
Polyamides are stabilized by phosphites and also (surprisingly) by copper and manganese
salts, presumably through complex formation with the amide groups themselves.

5.1.1.6 Market Analysis
See Table 5.6 for an analysis of worldwide consumption of antioxidants.

5.1.2 Antiozonants
The C=C in most rubber molecules, and in many high-impact plastics, is very sensitive to
traces of natural and man-made ozone in the atmosphere. Ozone adds to the double bonds,
forming ozonides that break down into various oxidized species, causing severe embrittle-
ment. This requires vigorous protection to give products with useful lifetimes. Two types
of additives are used: physical and chemical.

5.1.2.1 Physical Antiozonants. Saturated waxes are added during rubber compounding.
Being immiscible, they migrate to the surface (bloom), forming a barrier coating that

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