Peroxide-initiated chemical modification of polyolefins: In search of a latent antioxidant

Abstract

The ability of piperidine-based compounds to confer oxidative stability to polyolefin thermosets without compromising the yields of peroxide-initiated crosslinking and monomer grafting is demonstrated. Unlike phenolic, nitroxyl and phosphite antioxidants that lower the concentration of macroradical intermediates that support polyolefin modifications, additives based upon 2,2,6,6-tetramethylpiperidine (TEMPH) are shown to have little to no effect on the extent of LLDPE crosslinking or the conversion of vinyltriethoxysilane (VTEOS) to grafted hydrocarbon adducts. Notwithstanding this lack of interference in radical-mediated polymer modification, this class of hindered light stabilizer (HAS) compounds are shown to limit the extent of radical oxidation of linear low-density polyethylene (LLDPE) in an accelerated aging test. The origins of this paradox are discussed in terms of the current state of knowledge regarding HAS activation. The latent antioxidant concept is extended to an alternate approach, wherein 4-acryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (AOTEMPO) is used as an alkyl radical scavenger bearing an oligomerizable functional group. When added at a fraction of the peroxide loading used to produce an LLDPE thermoset, ATEMPO is shown to provide a predictable induction delay without impacting ultimate crosslink density, and produces polymer-bound alkoxyamine functionality that stabilizes the product against oxidation.