Theory of Synergism

The introduction of the sulfide R2S into the pol3rmer in sufficient concentration can make the denominator in the right-hand portion of equation (26) positive even if 6 1, which makes it possible for a critical antioxidant concentration to be realized. The correctness of this conclusion was confirmed by the experiments described in [71]. 

If the antioxidant 2,6-di-tert-octyl-4-methylphenol is introduced into polypropylene in various concentrations, then at 200°C and pQ =300mmI the induction period increases linearly from 8 to 65 min, i. e., no critical antioxidant concentration is observed. If, in addition to the indicated phenol, didecyl sulfide is introduced into the polymer, a critical concentration will be distinctly observed, and will decrease with increasing sulfide concentration, in accord with the formula of [34] (Fig. 21). 

Didecyl sulfide itself is a relatively ineffective antioxidant, and its introduction into polypropylene produces an extremely slight increase in the induction period. However, the introduction of 2,6-di-tert-octyl-4- 

Experiments have shown that the introduction of didecyl sulfide into polypropylene sharply reduces the rate of consumption of 2,6-di-tert-octyl-4-methylphenol (Fig. 23). The rate of consumption of the antioxidant di-tert-buiyl-p-cresylmethane in polypropylene is analogously reduced, but to a lesser degree, if didecyl sulfide is introduced into the polymer, as can be seen from the curves presented in Fig. 24. These facts are in full agreement with the theory developed in 

It is precisely by decomposition of the hydroperoxide and deceleration of the antioxidant consumption that the effect of mutual intensification of the effectiveness, observed in mixtures of 2,6-di-tert-octyl-4-methylphenol with didecyl sulfide is explained. Each of these inhibitors of oxidation, taken individually, only negligibly lengthens the induction period. In a mixture, on the other hand, they are capable of lengtheniag the induction period in the oxidation of polypropylene to 450 min at 200°C and an oxygen pressure of 300 mm Hg (Fig. 25). 

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