Polyolefins photooxidation

Thermal, Thermooxidative, and Photooxidative Degradation. Polymers of a-olefins have at least one tertiary C-H bond in each monomer unit of polymer chains. As a result, these polymers are susceptible to both thermal and thermooxidative degradation. Reactivity in degradation reactions is especially significant in the case of polyolefins with branched alkyl side groups. For example, thermal decomposition of... 

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). 

A further effect of sterically hindered amines that could contribute to polymer stabilization has been described by Allan / McKellar31i-lft-41. They reported a conversion of a,3 to 3,Y unsaturated ketones in polyolefins and assumed that this played a role in the photooxidation of the polymers. The conversion... 

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). 

The photooxidation mechanisms of polyolefins continues to attract much interest and controversy. A comparison of the photooxidation rates of linear low density polyethylene with low density polyethylene indicates that catalyst levels in the former have little, if... 

Polyolefins - An Arrhenius model has been developed for lifetime prediction of the light stability of polypropylene . Photooxidation processes in blends of polypropylene with poly(butylene terephthalate) (PBT) are complicated by the thermal sensitivity of the polypropylene and the screening effect of the terephthalate ester . This effect is shown in Scheme 1. 

Polyurethanes and Rubbers - The photooxidation of polyether-polyurethanes exhibits sensitivity due to the ether segments. Formates were the primary products of photorearrangement. The addition of styrene-butadiene copolymers to polyolefins significantly enhances their susceptibility to photooxidation via the butadiene component. Horizontal attenuated FTIR spectroscopy has been found useful for detecting the products of photooxidation of rubbers ... 

Reactions of ozone with polyolefins at room temperature produce oxidized surface layers that act as jAiysical barriers to further attack. Bie layers are too thin (affect mechanical properties but the products are capable of initiating thermal and photooxidations. 

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. 

A number of reviews of topical interest have appeared with degradable plastics making a come back. These include the role of plastics in the environment " and agricultural film , ethylene carbon monoxide copolymers and their analysis. Several other topic reviews of interest relate to the role of titanium dioxide pigments 22 laser ablation and photolysis, degradation and stabilisation of polyolefins , conformation defects in poly(vinyl chloride)and poly(ethylene terephthalate) photooxidation . 

The time to embrittlement of polyolefins during photooxidation is not directly related to initiation carbonyl concentration (9-11). This is exemplified for HDPE in Figure 3 and a similar relationship exists for LDPE and... 

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. 

The contribution made to these studies by my co-workers is gratefully acknowledged. In particular I wish to thank J. Vyvoda who carried out the study of PVC, A. Scott for the work on the photodegradation of HIPS, K. B. Chakraborty for his studies of the photooxidation and stabilization of the polyolefins and M. Humphrey for his studies of model compound autoxidation. 

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