Rubber Vulcanizates Degradation and Stabilization

Abstract

Abstract Degradation of rubber vulcanizates in the presence and absence of air as well as in presence of ozone is reviewed in this paper. The paper also outlines the means to overcome this undesirable phenomenon. Under anaerobic aging conditions, which is termed as reversion, the vulcanizates are exposed to elevated temperature in the absence of oxygen. The consequence of this process is reflected in a decline in physical properties and performance characteristics. These changes are directly related to modifications of the original crosslink structure. Decomposition reactions tend to predominate and thus leading to a reduction in crosslink density and physical properties as observed during extended cure or when using higher curing temperatures. The decrease in network density is common when vulcanizates are subject to an anaerobic aging process. However, in the presence of oxygen, the network density is increased with the main chain modifications playing a vital role. Over the years the rubber industry has developed several compounding approaches to address the changes in crosslink structure during thermal aging. This paper gives a review of these compounding approaches. As with many formulation changes in rubber compounding, there is a compromise that must be made when attempting to improve one performance characteristic. For example, improving the thermal stability of vulcanized natural rubber compounds by reducing the sulfur content of the crosslink through the use of the more efficient vulcanization systems will reduce dynamic performance properties such as fatigue resistance. The challenge is to define a way to improve thermal stability while maintaining dynamic performance characteristics. In the second part, the protection against aerobic ageing as well as in ozone environment is reviewed. The anti-degradant effects are summarized and means to counteract are outlined. The most commonly used antidegradants are N-isopropyl-N′-phenyl-p-phenylenediamine (IPPD) and N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD). Although conventional antidegradants such as IPPD and 6PPD are still the most widely used antidegradants in rubber, there is a trend and demand for longer-lasting and non-staining products. The relatively low molecular weight (MW) antioxidants have undergone an evolutionary change towards higher molecular weight products with the objective to achieve permanence in the rubber polymer, without loss of antioxidant activity. In the last two decades, several approaches have been evaluated in order to achieve this objective: attachment of hydrocarbon chains to conventional antioxidants in order to increase the MW and compatibility with the rubber matrix; oligomeric or polymeric antioxidants; and polymer bound or covulcanizable antioxidants. The disadvantage of polymer bound antioxidants was tackled by grafting antioxidants onto low MW polysiloxanes, which are compatible with many polymers. New developments on antiozonants have focused on non-staining and slow migrating products, which last longer in rubber compounds. Several new types of non-staining antiozonants have been developed, but none of them appeared to be as efficient as the chemically substituted p-phenylenediamines. The most prevalent approach to achieve non-staining ozone protection of rubber compounds is to use an inherently ozone-resistant, saturated backbone polymer in blends with a diene rubber. The disadvantage of this approach however, is the complicated mixing procedure needed to ensure that the required small polymer domain size is obtained