Polypropylene hydroperoxide photolysis

During each photodegradation (in vacuum or air) small amounts of several gaseous and liquid low molecular weight compounds are formed and they can only be detected by chromatographic or mass spectrometry methods (cf. section 10.6). Table 2.1 shows, for example, products formed from polypropylene hydroperoxide photolysis. 

Upon photolysis of polypropylene hydroperoxide (PP—OOH) a major absorption at 1726 and 1718 cm has been observed in the IR spectrum, which is attributed to the carbonyl groups. Sometimes the macroradical having free radical site reacts with a neighboring newly born hydroperoxide causing the formation of a macroalkoxy radical [116]. 

Hydroperoxide photolysis by sunlight in the presence of air is considered to be the major source of free radicals and backbone scission during the photodegradation of polypropylene [37]. 

Table 2.1 Photolysis products from polypropylene hydroperoxides [382]...

Photolysis (and/or thermolysis) of polypropylene hydroperoxides (3.10) yields polypropylene oxy radicals (PO ), which undergo ) -scission reaction [61, 365, 372, 723, 1858] ... 

The major volatile product in the photolysis of polypropylene hydroperoxide is water, which is formed as a result of hydrogen abstraction by hydroxyl (HO ) radicals ... 

Hydroperoxides are much more efficient than ketones for initiating photooxidation of ethylene-propylene copolymers [19]. This fact was confirmed by the results from photolysis of low-molecular model compounds and isotactic polypropylene [20]. 

Thus the formation of free radicals in the photolysis of polypropylene at —196° C can be reasonably explained by the mechanism which involves photolysis of oxidative groups such as hydroperoxides and carbonyl groups, although Kujirai et al. (50) proposed that ash residues may be responsible for light absorption. 

Finally, it can be concluded that the chemical effects of ultraviolet irradiation of polypropylene, as well as polyethylene, are due to the photolysis of impurities such as hydroperoxides and ketonic groups. 

In spite of the numerous studies reported on photooxidation of polyolefins, the detailed mechanism of the complete process remains unresolved. The relative contribution by species involved in photoinitiation, the origins of the oxidative scission reaction, and the role played by morphology in the case of photoreactions in solid state are not completely understood. Primary initiator species in polyethylenes [123] and polypropylenes [124] are believed to be mainly ketones and hydroperoxides. During early oxidation hydroperoxides are the dominant initiator, particularly in polypropylene, and can be photolyzed by wavelengths in solar radiation [125]. Macro-oxy radicals from photolysis of polyethylene hydroperoxides undergo rapid conversion to nonradical oxy products as evidenced by ESR studies [126]. Some of the products formed are ketones susceptible to Norrish I and II reactions leading to chain scission [127,128]. Norrish II reactions predominate under ambient conditions [129]. Concurrent with chain scission, crosslinking, for instance via alkoxy macroradical combination [126], can take place with consequent gel formation [130,131]. 

A similar photo-degradation process is believed to take place in polypropylene [541]. The formation of free-radicals is ascribed to presence of oxidized molecules that form during processing. The oxidation products are carbonyl compounds and hydroperoxides [542]. The photolysis of the carbonyl derivatives is as follows [541] ... 

Polyethylene (PE) is inherently less sensitive to oxidative attack than PP, but stabilization of PE is also mandatory for outdoor use. The stability varies with the type of polyethylene and manufacturing process. Linear low-density polyethylene (LLDPE) (1-octene comonomer) is significantly less sensitive to photooxidation than low-density polyethylene (LDPE) with comparable density and molecular weight [20, 21]. Generally, LDPE is less susceptible to photooxidation than high-density polyethylene (HDPE). The most fundamental difference between polyethylene homopolymers and polypropylene is the behavior of hydroperoxides toward photolysis. On photooxidation, hydroperoxides accumulate in PP, but decrease rapidly on UV exposure of PE. In copolymers of polyethylene with vinyl acetate, the stabihty depends on the content of vinyl acetate. The higher the content, the more the copolymers act like polyvinyl acetate, which is more susceptible to photooxidative degradation than polyethylene. 

The photolysis of hydroperoxide groups under solar irradiation is a slow process. The average lifetime of an —OOH group in 10/im polypropylene film under constant UV irradiation is 24h, equivalent to roughly 4-5 days of solar radiation [374, 387]. 

The photooxidation of polypropylene involves the initiation of free radical chain reaction by the photolysis of hydroperoxides, producing peroxyl radicals as well as alkoxy radicals. In the last decades many studies dealing with the types of stabilizers used to delay the polypropylene photodegradation were reported as well as several studied on the experimental methods for assessing their photostability effectiveness [73-78]. 

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