Relationships between the Structures of Natural Rubber Vulcanizates and Their Thermal and Oxidative Aging

Abstract

Abstract Accelerated sulfur NR vulcanizate networks are known to contain poly-, di- and monosulfidic crosslinks, sulfur-containing main chain modifications including cyclic sulfides and pendent accelerator groups, and extra-network material, primarily vulcanization residues. Both oxidative and non-oxidative aging characteristics of the vulcanizate can be improved by use of an efficient or EV curing system. These systems utilize either a high accelerator to sulfur ratio or an accelerator in conjunction with a sulfur release agent to insert predominantly mono- and disulfidic crosslinks, and produce a minimum of main chain modifications. The resulting vulcanizates have excellent aging properties, but fatigue life is poor in comparison to that of a conventional vulcanizate. With a compromise between the conventional and EV curing systems, namely use of intermediate accelerator to sulfur ratios or mixtures of sulfur and sulfur donor, vulcanizates with good fatigue properties and improved aging characteristics can be obtained. Since the most obvious difference between EV and conventional vulcanizate networks is the per cent of monosulfidic crosslinks, it has been assumed that monosulfide crosslinks are responsible for the poor fatigue resistance of EV systems. Recently, it was found that a zinc dimethyldithiocarbamate (ZDC) EV system gave an NR gum vulcanizate having fatigue resistance equal to that of a conventional sulfenamide accelerated compound. The main features of the network are a high percentage of monosulfide crosslinks and extensive main chain modification. In this paper, the aging and fatigue behavior of black filled substituted phenylenediamine protected NR ZDC accelerated systems is examined. The ZDC EV vulcanizate has imaged fatigue resistance equal to that of a conventional sulfenamide accelerated vulcanizate, but oxidative aging is very poor. By decreasing the ZDC/sulfur ratio, a vulcanizate structure equivalent in every measurable way to that of a conventional sulfenamide accelerated system is obtained. Unaged fatigue properties of the two compounds are similar. However, oxidative aging resistance of the ZDC compound is again very poor, and fatigue life of an air aged specimen is virtually zero. It is concluded that fatigue life is not necessarily a function of the crosslink type distribution. Accelerator residues and possibly main chain modifications play an important role insofar as oxidative and non-oxidative aging characteristics are concerned.