Page 325 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.5

butyl-p-cresol (TBC) or butylated hydroxy toluene (BHT). This is perfectly satisfactory in
polymers that can be processed at relatively low temperatures, such as LDPE. In higher-
temperature processing, it is too volatile and can be lost too rapidly to be useful. Then, we
resort to polyphenols of higher molecular weight and therefore lower volatility. Di- and
tetra-phenols are commonly used for such higher-temperature processing, giving much
better permanence (Tables 5.2 through 5.5).

TABLE 5.2 HDPE Stabilization: Days in 125°C Oven to
Reach Embrittlement

Hindered phenol
Hindered phenol alone + 0.25 percent DLTDP
0 10 days 10 days

0.025 percent 80 170
0.05 140 250
0.1 190 310

TABLE 5.3 Polypropylene Stabilization:
Days in 135°C Oven to Reach Embrittlement
Hindered phenol, % Days

0 5
0.03 70
0.09 120
0.15 150
0.2 160

0.4 200
0.5 220

5.1.1.1.2 Diaryl Amines. The most active free-radical chain-breakers are diaryl
amines. These generally discolor badly due to formation of quinoid structures, so they are
not often used in plastics. In rubber, where the many C=C bonds create a great need for
stabilization, and where carbon black generally masks any discoloration, diaryl amines are
almost universally used. In plastics containing carbon black, they may also be used very
effectively.

5.1.1.2 Nonradical Peroxide-Decomposers. Free-radical chain breakers alone may not
completely prevent the formation of peroxides. A second additive is then included to pro-
vide a second mechanism for protection. This is an additive to decompose the peroxide
that did form, but decompose it by a nonradical process, so that it does not kick off the

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