Über zusammenhange zwischen dem vernetzungsgrad und der degradation von naturkautschuk

Abstract

Oxidation reactions, as followed by infrared spectrometry, may serve as a measure for judging the relative extent of oxidative degradation and crosslinking of two independent parallel reactions subsequent upon the primary destruction. Such information can be derived from a kinetic analysis of the processes taking place in the rubber as a result of the absorption of energy, using the expression w = (k/b) (1 − e−bη) (1 − η). Since in this equation k is proportional to the rate of chain branching (which leads predominantly to oxidation), and b is proportional to the rate of recombination of the radicals (leading to crosslinking), the total degree of crosslinking will increase with increasing values of b/k. This assumption has been verified experimentally for the thermal and photochemical ageing of rubber, both in the absence of additives and in the presence of sulfur, of vulcanizing mixtures, and of copper as a catalyst of oxidation. At increased temperatures and pressures (130°C., 3 atm.) in the absence of vulcanizing additives and in a nitrogen atmosphere, radicals of long life are formed; this leads to an increased rate of the subsequent oxidative degradation. The number of active centers formed, and hence the final extent of oxidation, is a function of the time of exposure to the increased temperature.