Studies of the oxidative degradation of butyl rubber in tyre inner tube
Tyre inner tubes made of butyl rubber have been found to have poor heat resistance when used in the environment of Iraq. Research investigations were undertaken to establish the causes of the problem: these consisted of examining the individual contributions of oxidative scission processes in both rubber hydrocarbon and the various processing oils. The principal method used to monitor the degradation processes was ageing in hot air using the reduction in strength for the rubber and change in chemical composition of the processing oils. This preliminary study established that the processing oil made in Iraq was found to be of different composition from the common European rubber processing oils and contributed considerably to the poor life of the butyl inner tubes. Analysis of the oil from Iraq identified the presence of trace amounts of copper (3 ppm) known, from the literature, to adversely affect the resistance of many rubbers to elevated temperatures and, as found in this work, to also degrade butyl rubber. Replacement of the butyl rubber by chlorobutyl rubber overcame the problem of inner tube degradation and an antioxidant system based on a combination of an acetone/diphenylamine (ADPA) plus mercaptobenzimidazole (MBI) with MgO was found effective in minimising chlorobutyl inner tube degradation. However, unexpectedly, the research also established that trace quantities of copper were useful as stabilisers in enhancing the heat ageing properties of chlorobutyl rubber and the addition of a particular copper salt (copper sulphatel in small proportions (3 ppm) as an anti-degradant was found beneficial. Another method studied to improve the resistance of halogenated butyl rubber to heat induced oxidative degradation tried to use ZnO as the curative, as recommended by the manufacturers of chlorobutyl rubber; however it was found that with this technique higher degradation was obtained in chlorobutyl than with the standard ZnO/ sulphurless curing system (i.e. ZnO/TMTDin proportions 5:1 phr). It was concluded from these studies that: 1. Reducing the oil level in butyl inner tubes from 25 phr to 15 phr enhances heat ageing properties of the rubber. Factory trials confirmed this finding. 2. Adding a copper inhibitor (ZDC) in the proportions 1-2 phr in butyl inner tubes was found successful in minimising oxidative degradation. 3. The existence of elemental copper in a chlorobutyl inner tube rubber formulation in the proportion 3 ppm improved the rubber properties at elevated temperatures.