Polymer cross-linking antioxidants effect

Polypropylene differs from polyethylene in its chemical reactivity because of the presence of tertiary carbon atoms occurring alternately on the chain backbone. Of particular significance is the susceptibility of the polymer to oxidation at elevated temperatures. Some estimate of the difference between the two polymers can be obtained from Figure 1J.7, which compares- the rates of oxygen uptake of eaeh polymer at 93°C. Substantial improvements can be made by the inclusion of antioxidants and such additives are used in all commercial compounds. Whereas polyethylene cross-links on oxidation, polypropylene degrades to form lower molecular weight products. Similar effects are noted... 

The effect of antioxidants such as hindered phenohcs, secondary amine, and thioester on the radiation cross-linking efficiency of LDPE has been reported [260]. Amount of cross-linking at a given dose decreases with aU the antioxidants, the thioester being the most effective. IR absorption spectroscopy has been used to demonstrate dose-rate dependence of trani -vinylene unsaturation in irradiated Marlex 50 PE [261]. When the irradiated polymer is stored in vacuum a decrease is observed in trani-vinylene absorbance over a period of several weeks. After high dose-rate irradiation the decay is preceded by an initial increase. These phenomena have been ascribed to the reaction of trapped radicals. 

Introduction of Cl into a thermoplastic composition changes the kinetics of its thermal destruction processes. This is expressed in variations of the thermal resistance temperatures (Tt) and destruction initiation (Td) recorded by DTA. Tt and Td for the PE-based compositions shift to the region of lower temperatures by, correspondingly 1-50°C and 5-20°C. The activation energy (Ta) and macromolecular destruction found by DTA also diminish. Compositions based on HOPE show a reduction of Ta from 266 to 100-150kJ/mole, and the more efficient the antioxidizing effect of Cl towards HDPE the lower the Eg,. This is probably because the lower amount of oxide compounds in the polymer volume retards cross-linking of macromolecules and results in reduction. 

Despite the promising results presented by other antioxidants, the main oxidation stabilizers of choice are sterically hindered phenols, among which Vitamin E and Irganox (1010, 1035, 1076) stand out. These antioxidants have been shown to be more effective in radiative processes [80, 81]. Also, advances on the mechanisms of stabilization in polymer processing have shown that vitamin E is more efficient compared to other commercial additives [82,83]. Tocopherol compounds were proposed as stabilizers for polyolefins in the 1980s [84] and currently, they are the best alternative for the oxidative stabilization of highly cross-linked UHMWPEs. 

In contrast to the small amounts of antioxidant in raw rubber, compounded rubber often contains several percentages of protective material. It seems contradictory at first that antioxidants can continue to be very effective even after the polymer system has undergone a high-tanperature, oxidative cross-linking. A case in point is poly(vinyl ethyl ether) CToss-Unked by dicumyl peroxide ... 

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