Page 334 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.14 CHAPTER 5

gano-nickel compounds are often useful as quenchers. Carbon black probably functions
partly as a quencher.
5.1.4.4 Hindered-Amine Light Stabilizers (HALS). Most UV degradation is actually
photooxidation—UV-accelerated free-radical attack by atmospheric oxygen. The most re-
cent and most popular way of stabilizing polymers against it is by addition of hindered
amines to interfere with the free-radical chain reaction.
.
R NH + O → R NO
2
2
2
This nitroxide radical reacts with a degrading polymer radical R´ .
.
.
R NO + R´ → R NOR´
2 2
This reacts with another degrading polymer radical R´´ .
. .
R NOR´ + R´´ → R NO + R´R´´
2
2
This produces stable polymer R´R´´ and regenerates the nitroxide radical to continue its
work (Tables 5.11 and 5.12).
Since UV absorbers and HALS operate by different mechanisms, combined use of the
two types of stabilizers offers beneficial synergism (Table 5.12).
5.1.4.5 Market Volume. Table 5.15 provides market volume information for some lead-
ing stabilizers.
TABLE 5.15 Leading UV Stabilizers
HALS 46 percent
Benzotriazoles 27 percent
Benzophenones 20 percent
Others 7 percent
Total 24,800 tons

Use in polymers
Polypropylene 45 percent
Polyethylene 29 percent
PVC 9 percent
Engineering plastics 7 percent
Styrenics 5 percent
Others 5 percent

5.1.4.6 Prodegradants. When plastics accumulate in solid waste, it might be desirable
to accelerate their UV degradation. This has been accomplished semicommercially by in-
corporating enough C=O groups to absorb UV energy and initiate photodegradation pro-
cesses. It has also been demonstrated experimentally by adding transition metal
compounds such as ferrous laurate to catalyze photooxidation of the polymer (Table 5.16).

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