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Polyethylene free-radical degradation

Free radicals are also produced by chain scission during deformation of polyethylene and FT-IR has been used to follow this process 237). The polyethylene samples were unaxially drawn and the resultant spectra corrected for orientation. An increase in the vinyl and methyl end groups created by decay of the free radicals occured in going from draw ratios of 5 to 20 44). A similar study involving deformation was made of polystyrene 246) and a comparison demonstrated between the results of thermal and mechanical degradation 24S. ... [Pg.135]

Oxidative degradation of polyethylene (PE) and polypropylene (PP) can occur at all stages of their lifecycle (polymerisation, storage, processing, fabrication and in-service). The auto-oxidation process of polyolefins is best described by the classical free-radical-initiated chain reaction outlined in Scheme 1 [1]. Impurities initially present in the polymers during polymerisation or melt processing, exert profound effects on the behaviour of the final polymer article in service. [Pg.122]

It is important to recognize that polypropylene, which is the major constituent of TPO, is a typical degrading-type polymer in the radiation chemistry of polymers, i.e., once a free radical is formed on a polymer chain, the free radical unzips the chain rather than cross-links. CASING effect was first found with polyethylene [24], which is a typical cross-linking-type polymer. The same CASING effect, however, could not be anticipated with the treatment of the degrading-type polymers because the degradation of substrate polymer enhances the extent of weak boundary layer. [Pg.639]

In polyethylene, the tertiary carbon atom, which dominated the chemistry of the oxidative degradation of PP, is present only at branch points. This suggests that there may be a difference among LDPE, LLDPE and HDPE in terms of the expected rates of oxidation. This is complicated further by the presence of catalyst residues from the Ziegler-Natta polymerization of HDPE that may be potential free-radical initiators. The polymers also have differences in degree of crystallinity, but these should not impinge on the melt properties at other than low temperatures at which residual structure may prevail in the melt. Also of significance is residual unsaturation such as in-chain tra s-vinylene and vinylidene as well as terminal vinyl, which are defects in the idealized PE strucmre. [Pg.145]

As isotactic polystyrene has a similar crystal structure to that of polyethylene, the concept of the intramolecular cydization may be applied to the degradation of the former to account for the formation of the products of degradation. Let the three-unit one turn radical form the cyclic compound (VIII) by the interaction of the Cn and CK atoms. The cyclic compound (VIII) may lose hydrogens to other free radicals to form 1,3,5-triphenyl-benzene (IX). [Pg.58]

Wojtczak have found that the photosensitized degradation of polyethylene glycols decreases in the order triethylene glycol > polyethylene glycol 400 mol. wt. > polyethylene glycol 4000 mol. wt. Sastre and Gonzalez have shown that bromoalkanes are powerful sensitizers for the photo-oxidation of polystyrene, and Rabek and Ranby have found that polynuclear aromatics are photosensitizers for polybutadiene. Aromatic carbonyls have been shown to induce free-radical formation in cellulosic materials. [Pg.546]

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... [Pg.138]

Polypropylene is more susceptible to melt degradation than polyethylene, because of the presence of more reactive tertiary hydrogen atoms (attached to the carbon atom that is bonded to three other C atoms). At a temperature of 270 °C in injection moulding, tertiary alkyl free radicals R are generated thermally. If oxygen is present, a rapid reaction (R + 02- ROO ) produces a peroxide radical, which reacts further to form hydroperoxides (ROO -h RH ROOH-h R ). When the dissolved oxygen is used up, there is a greater chance of the chain scission reaction... [Pg.293]

The main commerdally useftil methods for aosslinking use agents whose sole purpose is to promote free-radical processes that essentially initiate polyethylene degradation. This ultimately aeates bonds direcdy between carbon atoms on adjacent polymer chains. Some crosslinking agents, such as silanes, integrate themselves between polymers to form intermediate "bridging" links that connect one chain to another. [Pg.216]


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See also in sourсe #XX -- [ Pg.41 ]




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