Oxidative polymer degradation

Jellinek et al. [312—314] discussed the role of diffusion in the kinetics of polymer oxidative degradation. It should be ascertained in the investigation of the oxidation of polymer films whether the observed oxidation rate is actually due to the chemical oxidation reaction and that it is not influenced by the relatively slow diffusion of oxygen into the film. When the polymer is very finely powdered, diffusion effects would be expected to be negligible. 

E. Chiellini, A. Corti, S. Dantone, R. Baciu, Oxo-biodegradable carbon backbone polymers— oxidative degradation of polyethylene under accelerated test conditions. Polym. Degrad. Stab. 91, 2739-2747 (2006)... 

Scheme 12.3 Bolland-Gee scheme [1] for free radical mechanism of polymer oxidative degradation valid to temperatures below 250 °C...

Fig. 12 General mechanism of polymer oxidative degradation. The data were taken from [OlGl, 08K1].

Chiellini, E., Corti, A., D Antone, S., and Baciu, R. (2006) Oxo-biodegradation carbon backbone polymers - oxidative degradation of polyethylene under accelerated test conditions. Polymer Degradation and Stability, 91(11) 2739-2747. 

In polymers, oxidative degradation is catalyzed strongly by traces of metals that can undergo redox reaction. Most prominent is copper, but also solubilized iron, cobalt, nickel, chromium and manganese may contribute to degradation of the polymers. Apart from polymers, metal deactivators are used widely in lubricating oils. 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Several stabilizers are useful in minimizing oxidative degradation during thermoplastic processing or in the bulk soHd. Phenothiazine, hindered phenohc antioxidants such as butylated hydroxytoluene, butylatedhydroxyanisole, and secondary aromatic amines in concentrations of 0.01—0.5% based on the weight of polymer, are effective. 

Ethylene—Propylene (Diene) Rubber. The age-resistant elastomers are based on polymer chains having a very low unsaturation, sufficient for sulfur vulcanization but low enough to reduce oxidative degradation. EPDM can be depicted by the following chain stmcture ... 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

The polymers also have excellent resistance to oxidative degradation, most chemicals other than strong bases and high-energy radiation. Exposure for 1500 hours to a radiation of about 10 rads at 175°C led to embrittlement but the sample retained form stability. 

Thermoplastic polyurethane elastomers have now been available for many years (and were described in the first edition of this book). The adipate polyester-based materials have outstanding abrasion and tear resistance as well as very good resistance to oils and oxidative degradation. The polyether-based materials are more noted for their resistance to hydrolysis and fungal attack. Rather specialised polymers based on polycaprolactone (Section 25.11) may be considered as premium grade materials with good all round properties. 

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