Nitroxyls for scorch suppression,cure control,and functionalization in free-radical crosslinking of polyethylene

This paper describes the use of 2,2,6,6-tetramethylpiperidinyl-oxy (TEMPO) derivatives for scorch suppression, cure control, and functionalization in peroxide crosslinking of polyethylene. When 4-hydroxy 2,2,6,6-tetramethylpiperidin-1-oxyl was used for scorch suppression, there was often a loss in ultimate degree of crosslinking. In contrast, with bis(1-oxyl-2,2,6,6-tetramethylpiperidine-4-yl)sebacate, both scorch suppression and ultimate degree of crosslinking were enhanced. A model study in hexadecane showed that TEMPO radicals terminate with carbon-centered radicals formed as a consequence of peroxide homolysis and propagation steps. This termination occurs preferentially over peroxide-initiated crosslinking and results in TEMPO-grafted polymer. In addition to polymer radical formation, several additional reaction pathways are available following thermal homolysis of the peroxide, including unimolecular disassociation of the peroxy radical to yield a methyl radical and a ketone, and proton extraction from one of several substrates by the peroxy radical to yield an alkyl radical and an alcohol. This study reveals that the reaction rate is limited by the rate of peroxide homolysis, and proceeds to statistical products with little or no preference for any specific species. The implication is that choice of peroxide is a dominant controlling factor over whether the TEMPO derivatives are ultimately grafted to the polymer or are bound to small alkyl radicals. POLYM. ENG. SCI., 47:50–61, 2007. © 2006 Society of Plastics Engineers