Oxidative Stress Relaxation Studies of Radiation Cured Vulcanizates, with and without Antioxidant

Turner has shown that irradiation of natural rubber results in the formation of carbon to carbon crosslinks by combination of allylic radicals; the same type of crosslinks are formed by heating rubber in presence of organic peroxides2. Since peroxide vulcanizates may be freed from excess vulcanizing ingredients and by products by extraction while radiation crosslinking may be effected without additive, it is expected that the two types of vulcanizate should age in an analogous manner. The present study was undertaken in order to see whether radiation vulcanizates did show the same aging behavior as peroxide vulcanizates according to measurements of oxidative stress relaxation, and, if so, to ascertain whether the efficiency of antioxidants of the dithiocarbamate type was affected by the irradiation and whether such materials had an adverse effect upon cure. It has been stated that radiation vulcanizates are resistant to aging even in the absence of antioxidant and their stability has been attributed to the presence of purely carbon-carbon crosslinks. However, it has recently been demonstrated that a carbon-carbon crosslink is not a sufficient condition for vulcanizate stability in the absence of antioxidant. Previous studies concerning the aging of radiation cures appear to have been made using materials containing carbon black, which is now known to act as an antioxidant; it is necessary, for work of fundamental significance, to use gum stocks based on very pure rubber.