The mechanism of action of crosslinking additives in polyethylene vulcanisation: an EPR investigation
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The nature and kinetics of the free radicals intermediates in the peroxide induced thermal vulcanisation of low density polyethylene and the mechanism of action of two crosslinking co-agents with the structure of alkenyl aromatic hydrocarbons have been elucidated by EPR spectroscopy. At low peroxide concentration, the crosslinking mechanism in the pure low density polyethylene (LDPE) matrix is based, as expected, on the H abstraction from the chains and the coupling of the resulting macroalkyl radicals. The latter reaction takes place in competition with the disproportionation causing the formation of chain unsaturations that accumulate with the reaction time. At relatively large (>2%) peroxide concentrations, such unsaturations reach high enough levels to scavenge all the initiating and macroalkyl radicals giving macroallyl radicals as the dominant intermediates and determining changes in the structure of the crosslinks during the course of the reaction. At the concentration used, the activities of the co-agents 1,3-diisopropenylbenzene and 2,4-diphenyl-4-methyl-1-pentene are sufficient to scavenge all the initiating and macroalkyl radicals by double bond addition; this mechanism, when involving macroalkyl radicals, leads to the attachment of saturated and unsaturated pendants on the LDPE chains respectively. The unsaturated pendants are thought to be responsible for the crosslinking promotion effect by the 1,3 diisopropenylbenzene coagent. The lack of the second unsaturation causes 2,4-diphenyl-4-methyl-1-pentene to act essentially as a mild antiscorch agent.
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