Biodeterioration of rubbers

The standard agar plate technique was found to be a good method for screening the presence of leachable antimicrobial additives in rubber compounds. However, this technique restricts the growth of microorganisms to the perimeter of the samples and is less satisfactory as a long term test. In a liquid medium, there should not be any problem for the microorganisms to gain access to the inorganic and organic nutrients as well as oxygen. Many experimental trials were carried out in liquid media under dynamic and static conditions with Streptomyces lipmanii NCIMB 9841 as test organism. This led to the development of the modified batch processes. The modified tests allowed experiments to be performed over a long period of time without concern of accumulation of toxins from the leachable antimicrobial additives in the rubber samples and the metabolites from the utilisation of carbon sources by the microorganisms. The test procedure developed proved to be very reproducible and repeatable. The use of neutral chemically defined media with no added carbon source, the absence of strong oxidation catalysts coupled with the use of proper control samples defined the least vigorous conditions for biodeterioration of rubber compounds. On the other hand, the degradation of the rubber compounds under field and semi-field tests is likely to be a combined effect of biodeterioration, hydrolytic degradation, leaching and oxidation of samples. Five strains of fungi and four strains of actinomycetes were tested against five carbon black-filled seal compounds. The types of rubbers used were based on natural, styrene/butadiene, peroxide-cured EPDM, sulphur-cured EPDM and acrylonitrile /butadiene rubbers. Among all the species tested, Nocardia was found to be the most aggressive group of microorganisms in affecting the rubber samples. The strains NCIMB 12811 and 12814 had activity towards vulcanised gum NR samples, whilst Nocardia asteriodes NCIMB 12082 was very capable of utilizing rubber additives. It was found that besides the physical dimensions, types and formulations of rubber compounds; test conditions such as temperature, agitation speed, pH; amount of rubber sample used per unit volume of test medium, and renewal or non-renewal of test media also played an important role in affecting the activities of microorganisms towards rubber compounds.