New Reactions for the Vulcanization of Natural Rubber

Abstract

Abstract One objective of a new crosslinking system for NR is a vulcanizate possessing good strength and fatigue properties and a high resistance to reversion; a second is a vulcanizate with reversibly thermolabile crosslinks. The reaction of amine-chloroboranes with NR was expected to give carbon-boron-carbon crosslinks possessing some measure of lability at high temperatures. In practice, the proportion of covalent crosslinks formed is very low and this is attributed to intramolecular additions and cyclizations. The majority of the effective crosslinks are stress-labile at ambient temperatures and the vulcanizates, therefore, have extremely high rates of creep and stress-relaxation. The nature of these crosslinks is still unknown. In all other aspects the physical properties of the vulcanizates are acceptable, reversion and aging resistance being particularly good. Surprisingly, the presence of the stress-labile crosslinks either in large preponderance or in various proportions with covalent crosslinks introduced by the compatible vulcanizing agents, dicumyl peroxide or TMTD-zinc oxide, did not impart the expected, very high tensile strengths. The amine-chloroborane vulcanizates are resistant to ozone cracking because of the rapid relaxation of surface strains. The second novel vulcanization system examined utilizes the attachment of hydroxyl or amino groups to the rubber chain by the thermal reaction of the appropriately substituted nitrosobenzene. Crosslinking is induced by means of difunctional isocyanates. This can be achieved in a one-step process, mixing and vulcanization being carried out under conventional conditions. A preferred variation, which overcomes the handling problems of free nitroso compounds, makes use of novel urethanes formed by the prereaction of p-nitrosophenol and diisocyanates. The efficiency of crosslinking and physical properties of the vulcanizates are much improved by the use of extra diisocyanate and by small additions of zinc dimethyldithiocarbamate which appears to act catalytically. Gum and black-filled vulcanizates have excellent reversion and aging resistance and, even before serious attempts at optimization, physical properties similar to those of a good sulfur vulcanizate.