Degradation mechanisms polyolefins

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. 

PBS (Figure 30) is an alternating copolymer of sulfur dioxide and 1-butene. It undergoes efficient main chain scission upon exposure to electron beam radiation to produce, as major scission products, sulfur dioxide and the olefin monomer. Exposure results first in scission of the main chain carbon-sulfur bond, followed by depolymerization of the radical (and cationic) fragments to an extent that is temperature dependent and results in evolution of the volatile monomers species. The mechanism of the radiochemical degradation of polyolefin sulfones has been the subject of detailed studies by O Donnell et. al. (.41). 

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. 

As described previously, thermooxidative degradation of polyolefins proceeds by a typical free-radical chain mechanism in which hydroperoxides are key intermediates because of their thermally-induced hemolytic decomposition to free radicals, which in turn initiate new oxidation chains. However, since the monomolecular hemolytic decomposition of hydroperoxides into free radicals require relatively high activation energies, this process becomes effective only at temperatures in the range of 120°C and higher. 

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... 

Hindered amine stabilizers (HAS) are the most common class of the curative additives and their application is the state-of-the-art in photoprotection of carbon-chain polymers, polyolefins in particular. HAS shape future polymer development, promote their consumption in new areas and expand material performance by increasing its lifetime. Application of HAS is based on a long-term effective development and is connected with commercial benefits for polymers. An optimized technical application of HAS required explanation of their chemistry and activity mechanisms in different phases of the oxidative degradation of polyolefins [14-17]. 

Polymers may be attacked by molecular oxygen, ozone, or by indigenous free radicals in the polymer. Thermal-oxidative degradation of polyolefins in air is autocatalytic, i.e., the rate is slow at first but gradually accelerates to a constant value. According to the three-step mechanism outlined below, the RO2 peroxy radicals formed (Step 1) are sufficiently reactive to attack some primary CH bonds of the chain R H (Step 2). The peroxy radical RO2 is thus reformed (Step 3) and can attack another CH bond. This chain reaction continues until termination occurs (Step 4) [1-11]. 

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). 

Although this scheme explains, at least qualitatively, the degradation of polyolefins whatever external forces (heat, mechanical stress, ultraviolet radiation, etc.) act on the pol5uner, some of the reactions are more... 

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).

Researchers [2, 40] have investigated the catalytic degradation of polyolefins, using TGA, as a potential method for screening catalysts and have found that the presence of a catalyst led to a decrease in the apparent activation energy. There are different steps in the carbonium ion reaction mechanism, such as H-transfer, chain/] scission. 

Sterically hindered amines (HALS) play an important role as stabilizers against the light-induced degradation of polyolefins " the detailed mechanism of their action is still being discussed. Therefore, the method of triplet-sensitized... 

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. 

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