Initiation step photo-oxidative degradation

The literature describes the mechanism of photo-oxidative degradation in isotactic and atactic polypropylene well (Figure 5.25). However, the initiation step - the formation of primary radicals - has not been precisely explained. Photo-oxidative degradation in polypropylene, like thermal-oxidative degradation, also leads to the formation of hydrogen peroxides, ketones, esters, and acids. Figure 5.26 [653]. Simultaneously, molecular weight decreases. Table 5.2. 

The formed hydroperoxide and ketone groups may further participate in the initiation step of photodegradation however it has been suggested that they do not play an important role in this process [559]. On other side it has shown that a-chloroketones are mainly involved in the photo-oxidative degradation of poly(vinyl chloride) [1641]. 

In many types of degradation the interference of oxidative attack, which has an accelerating role, happens [56, 62-65], i.e., auto-oxidation. However, in oxidative degradation, the most commonly known phenomena are photo-oxidation and thermal oxidation T < 300°C), the effects of which are discoloration, hardening, and surface cracking or flaking. These two phenomena are mainly differentiated by the initiation step, since other aspects are similar. 

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