Polyolefin degradation mechanisms

Previous investigations have shown that polyisobutene and poly(isobutene-co-iso-prene), i. e. butyl rubber, are unable to cross-link in the presence of free radicals, since extensive chain scissions occur and thus low molecular weight products are formed The degradation mechanism proposed by Loan involves, in the case of polyisobutene, the H abstraction from methyl groups followed by chain scission. Apparently, the formation of secondary alkyl radicals, which are believed to be responsible of polyolefin radical curing is prevented for steric reasons by the presence of two adjacent dimethyl substituted carbon atoms and hence j3 scission reactions prevail. 

Py-GC-MS can also be used to identify the polymer type in a plastic (Learner, 2001). The principle of the technique is based on the use of heat to volatilize and break down macromolecules into smaller components capable of being analysed using GC-MS (Table 5.6). Degradation mechanisms for polymers under pyrolysis are free radical processes initiated by bond dissociation due to the heat. The specific pathway followed by a particular polymer is dependant on the strength of the polymer bonds and the structure of the polymer chain. Pathways may be described as random scission, unzipping and side group elimination. Polyolefins such as polyethylene and polypropylene follow the random scission pathway and break into pieces of the original molecule to form... 

Polyolefins are susceptible to several degradative mechanisms, out of which thermo-oxidative and thermomechanical are dominant. The PO stability depends on macromolecular configuration and it follows the order HOPE > LLDPE > LDPE > i-PP. Chemiluminescence, FT-IR, mechanical properties, and Ihermogravimetty have been used for detecting and quantifying degradations (Cran 2004). 

The knowledge of the degradation mechanisms of polyolefins has grown over the past few years owing to the scientific work of several industrial research groups... 

Scheme 15.1. General oxidative degradation mechanism for polyolefins (PH designates the polymer, P a macroradical, and X a radical of unspecified nature).

A good deal is now known about the kinetics of abiotic peroxidation and stabilisation of carbon-chain polymers and it is possible in principle to extrapolate to the time for ultimate oxidation from laboratory experiments. As already indicated, the key determinant of the time to bioassimilation is the antioxidant and if this is chosen to optimize the service life, bioassimilation can also be achieved as in the case of wood, straw, twigs, etc. It seems that straw is a particularly appropriate model for the biodegradation of the polyolefins since, like the polyolefins, it fully bioassimilated in biologically active soil over a period of about ten years. The most important conclusion from recent work is that nature does not depend on just one degradation mechanism. Abiotically initiated peroxidation is just as important, at least initially as biooxidation. 

Some other degradation mechanisms other than simple hydrolysis are presented for biodegradable polymers. Such mechanisms include oxidative cleavage by a radical mechanism. Oxidative degradation is the main mechanism for non-hydrolyzable polymers, such as polyolefins, natural rubber, lignins, and polyurethanes. For many polymers, hydrolysis and oxidation occur simultaneously in the environment. 

Degradation of polyolefins such as polyethylene, polypropylene, polybutylene, and polybutadiene promoted by metals and other oxidants occurs via an oxidation and a photo-oxidative mechanism, the two being difficult to separate in environmental degradation. The general mechanism common to all these reactions is that shown in equation 9. The reactant radical may be produced by any suitable mechanism from the interaction of air or oxygen with polyolefins (42) to form peroxides, which are subsequentiy decomposed by ultraviolet radiation. These reaction intermediates abstract more hydrogen atoms from the polymer backbone, which is ultimately converted into a polymer with ketone functionahties and degraded by the Norrish mechanisms (eq. 

However, Pacansky and his coworkers77 studied the degradation of poly(2-methyl-l-pentene sulfone) by electron beams and from infrared studies of the products suggest another mechanism. They claim that S02 was exclusively produced at low doses with no concomitant formation of the olefin. The residual polymer was considered to be essentially pure poly(2-methyl-l-pentene) and this polyolefin underwent depolymerization after further irradiation. However, the high yield of S02 requires the assumption of a chain reaction and it is difficult to think of a chain reaction which will form S02 and no olefin. 

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