Page 324 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.4 CHAPTER 5

5.1.1.1 Free-Radical Chain-Breakers. These are most commonly and most widely
used and are therefore referred to as primary antioxidants. They react with a decomposing
free radical and convert it into a stable product,
.
R + AH → RH + A .
.
The antioxidant radical A is a relatively inactive species, stabilized by aromatic reso-
nance, and disposes of the excess energy harmlessly. Two major classes of compounds are
used as free-radical chain-breakers: hindered phenols and diaryl amines.
5.1.1.1.1 Hindered Phenols. The aromatic hydroxyl group in phenols
ArO:H

is very effective for terminating free-radical chain-reactions. In simple phenols, it tends to
form quinoid structures, which cause discoloration. Substitution of the ortho and para po-
sitions on the benzene ring prevents the formation of quinoid structures and produces non-
discoloring antioxidants (Fig. 5.1). The most common and widely used is named di-t-

FIGURE 5.1 Antioxidants.

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