The behaviour of antioxidants in polyolefins under conditions of UV exposure

• Main Author: Chakraborty, Khirud B.
• Published: Aston University 1977
• Subjects: 547.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.451019

The effects of antioxidants and stabilizers on the oxidative degradat" ion of polyolefins Clow density polyethylene [LOPE] and polypropylene [pP] have been studied after subjecting to prior high temperature processing treatments. The changes in the both chemical and physical properties of unstabilized polymers occurring during processing were found to be strongly dependent on the amount of oxygen present in the mixer. Subsequent thermal and photo-oxidation showed very similar characteristics and the chromophore primarily responsible for: both thermo and photooxidative degradation of unstabilized polymers was found to be hydroperoxide formed during processing. Removal of hydroperoxide by heat treatment in an inert atmosphere although increasing ketonic carbonyl concentration, markedly decreased the rate of photo-oxidatiol1, introducing an induction period similar to that of an unprocessed sample. It was concluded that hydroperoxides are the most important initiators in normally processed polymers during the early stages of photo-oxidation. Antioxidants such as metal dithiocarfuamates which act by destroying peroxides into non-radical products were found to be efficient melt stabilizers for polyolefins and effective UV stabilizers during the initial photo-oxidation stage, whilst a phenolic antioxidant, n'octadecyl-3-(3', 5'-di-terbutyl 4'hydroxyphenol) propionate (Irganox lO76) retarded photo-oxidation rate in the later stages. A typical 'UV absorber' 2-hydroxy-4-octyloxy-benzophenone (HOBP) has a minor thermal antioxidant action but retarded photo-oxidation at all stages. A substituted piperidine derivative, Bis [2.2.6.6-tetramethylpiperidinyl-4] sebacate (Tinuvin 770) behaved as an pro-oxidant during thermal oxidation of polyolefins but was an e,ffeet1ve" stabilizer against UV light. The UV absorber, HOBP synergised effectively with both peroxide decomposing antioxidants (metal dithiocarbamates) and a chain-breaking antioxidant (Irganox 1076) during photo-oxidation of the polymers studied whereas the combined effect was additive during thermal oxidation. By contrast, the peroxide decomposers and chain-breaking antioxidant (Irganox l076) which were effective synergists during thermal oxidation of LDPE were antagonistic during photo-oxidation. The mechanisms of these processes are discussed.