Photooxidation of polyolefins

Both thermooxidation and photooxidation of polyolefins can be prevented by using the same antioxidants as those employed for the stabilization of polypropylene, ie, alkylated phenols, polyphenols, thioesters, and organic phosphites in the amount of 0.2—0.5% (22,25). 

Opinions differ as to the nature of the species primarily responsible for photooxidation of polyolefins 2 47> Many workers put the main responsibility for photodegra-... 

To understand these reactions, the so-called Bolland and Gee reaction scheme17-18 and its subsequent developments has been applied to explain the chain reaction characteristics of both thermal and photooxidation of polyolefins. The scheme (Scheme 2.1) has been found to be a useful model for many other polymers comprising significant aliphatic character, such as aliphatic polyamides and polyesters and certain polyvinyls including poly(vinyl chloride) (PVC). 

Cracks also form in the final stages of thermal and photooxidation of polyolefins. Like those in polycarbonates (22), their growth seons to be associated with repeated temperature and humidity cycles. Otherwise moisture either alone or in combination with microorganisms has no significant effect on the service life of polyolefins. 

The polymers obtained by polymerization in the presehce of metal catalysts contain metal residues which cannot be removed so readily. It is also well known that transition metal ions act as sensitizers for the photooxidation of polyolefins (29). Kujirai et al. (30) found that photodegradation of polypropylene depends on the oxygen concentration and on the residues of the polymerization catalyst, and they concluded that oxidative photodegradation is sensitized by the initiator metal residues (ash). Very recently Scott (31) used transition metal ions as sensitizers to develop photodegradable polymers. 

Gugumus, F. "Mechanisms of Photooxidation of Polyolefins." Die Angewandte Makromolekulare Chemie 176/177(1990) 27-42. 

According to l69 2,2 -methylenebis(4-methyl-6-tert-butyl-phenol) XXXVII acts as an quencher in the Norrish cleavage of ketones, which is one of important processes accompanying the photooxidation of polyolefins. Some products of the transformation of antioxidant XXXVII probably also take part in this reaction, similarly as it is in quenching of l(>2 by quinone methionide compounds of the type XLII or XLVII103). 

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]. 

Formation and location of nitroxides, the key HAS-developed transformation products, was monitored using electron spin resonance imaging (ESRl) technique tliat confirms the heterogeneous character of photooxidation of polyolefins. 

Under atmospheric conditions, the photooxidation of polyolefins is promoted by the presence of many oxidizing agents contained in air ozone, SO2, NC>2, H2O2, free radicals, etc. Ozone enters from the upper layers of the atmosphere or is formed as a result of the photooxidation of SO2 and NO2 the formation of free radicals is possible in the photolysis of water... 

Transition metal ions by themselves, for example in the form of stearates or acetyl acetonates (I), have also been widely used to catalyse the photooxidation of polyolefins. Whilst they are very effective in this respect, they again cause degradation during processing and storage unless used with a metal restraining agent [32, 33] (see below). 

Mechanisms of Photooxidation of Polyolefins Prediction of Lifetime in Weathering Conditions, In Polymer Durability, American Chemical Society Publication, 1996... 

In this review of the mechanisms now most commonly accepted as occurring in the photooxidation of polyolefins, we all especially consider the studies made on polyethylene and polsqMropylene, on some liquid hydrocarbons, models of the former, and on oxygen-containing model compounds (hydroperoxides (4), ketones (5), etc.) which supply much of the information we have concerning the photodiemical reactions of polymers. 

Several investigations concerning the fundamental processes that occur in the photooxidation of polyolefins have been reported (7—P). 

To conclude, the transition metal residues of the pcljmerization catalysts act as sensitizers in the photooxidation of polyolefins and of polymers in general according to a mechanism that should involve light absorption and the production of free radicals throu a photoexcited d tion transfer from the anion to the cation. After rapidly reaching thermal equilibrium, tte free radicals should start the oxidation of the sutetrate following the pathways discusred in Section I. 

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