A Review of Recent Developments in the Mechanisms of Antifatigue Agents

In spite of the remarkable advances that have been made in the engineering design of tires during the past two decades, the basic formulations used in vulcanization and protection during service have essentially remained unchanged. This is to a large extent due to two major factors: 1. The versatility of the traditional accelerated sulfur curing system which provides the necessary combination of resilience and strength with good resistance to cyclical stress. 2. The development of diarylamine antidegradants which confer a high level of thermal-oxidative and mechano-oxidative (fatigue) resistance to the rubber. Both of these developments have occurred slowly with small incremental improvements and, probably because of their success, relatively little fundamental work has been published which addresses the questions of why a polysulfide network is so resistant to fatigue and why N-sec-alkyl-N′-phenyl-p-phenylenediamines are so much more successful as antidegradants than other classes of antioxidant. It is becoming evident, however, that if tires are to withstand the increasingly demanding conditions to which they are subjected in service, much more attention must be paid to the material design as opposed to the structural design of tires. Nowhere is this more evident than in aircraft tires. Recent studies have shown that the tires of heavily laden wide-bodied aircraft reach temperatures in excess of 70°C at the relatively modest speed of 32 km/h (20 mph). Similarly, in heavy duty truck tires, temperatures over 100°C are not abnormal in the shoulder region. This leads to extensive restructurization of the fatigue resistant polysulfide network, particularly in the shoulder of the tire, to give a much weaker mono-disulfide structure. It is no coincidence then that failure normally occurs in this region. The practice of multiple retreading exacerbates the change in chemical decomposition of the rubber. It is well known to the rubber scientist that extending the vulcanization process also leads to restructurization of the rubber network from polysulfide to mono- and disulfide. This is shown typically for a HAF-black tire formulation at 140°C in Figure 1. Antidegradants have virtually no effect on anaerobic restructurization (see Figure 2), and indeed, the established antifatigue agent, IPPD (I), actually accelerates the loss of polysulfide crosslinks from the vulcanizate at 140°C. During fatiguing, on the other hand, IPPD effectively retards restructurization (see Figure 3), whereas a typical bisphenol, nonstaining antidegradant, II, has much less effect.