Additive Effects on Melt-State Modification of Commodity Polyolefins

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Twigg, Christopher
Crosslinking , Antioxidant , Hindered Amine , Peroxide , Nitroxyl , Isoprene , Polyolefin , Oxidation , Butyl Rubber
In this work, melt-state peroxide mediated crosslinking of butyl rubber (IIR) and linear low density polyethylene (LLDPE) was used to determine additive effects, as well as the effect of higher isoprene content on the extent of crosslinking. Grafting of vinyltriethoxysilane (VTEOS) to polymer and hydrocarbon substrate was used to determine the effects of unsaturation and additives on radical intermediates. Grafting of VTEOS to butyl rubber (melt-state) was sensitive to isoprene content. Higher isoprene improved the rate of combination relative to chain scission because of resonance stabilization of allylic macroradical intermediates. Graft yields for IIR were lower than those of poly(isobutylene) homopolymer, because unsaturation dramatically increased the relative rate of degradative chain transfer. Higher reactivity of high isoprene rubber was employed to graft acrylated radical traps that combine with polymer macroradicals and subsequently oligomerize. This occurred in appreciable quantities, giving a modest crosslinked network that was less prone to stress relaxation, compared to poly(isobutylene). The effects of several antioxidants on peroxide crosslinking of LLDPE and grafting of VTEOS to cyclohexane were assessed. All but the hindered amine 2,2,6,6-tetramethylpiperidine (TEMPH) were found to affect peroxide cures and graft addition. TEMPH survived grafting reactions, as well as DCP-only crosslinking reactions in cyclohexane in the absence of oxygen, while 1,2,2,6,6-pentamethylpiperidine (TEMPMe) did not. High molecular weight hindered amine suppressed oxidative degradation in LLDPE films for 16 days, quantitatively as well as BHT (industry standard). Thus, hindered amine 2,2,6,6-tetramethylpiperidine (TEMPH) was selected as an ideal latent antioxidant, and this was explained by the requirement of oxidative activation to form a reactive nitroxyl for radical scavenging. Activation mechanisms were reviewed, the most likely being the formation of a tetramethylpiperinyl radical intermediate before oxidation to generate nitroxyl.
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