Shelf Age Stiffening in High Diene Elastomers

High-diene elastomers like natural rubber and polybutadiene continue to stiffen over time during storage at ambient conditions. As bonded rubber-to-metal products become more complex and part performance requirements become more demanding, age-stiffening, both during storage and service, poses an increasing problem. Investigations were undertaken to determine the cause of this age-stiffening over time.  The project continued over the span of more than six years and determined that shelf age stiffening occurs in virtually all natural rubber and polybutadiene compounds. An increase in the elastic modulus G’ (±10% shear strain, 10 Hz) of 8% to 12% in six months is not out of the ordinary for a typical NR/BR blend. Natural rubber generally shelf age-stiffens less than polybutadiene even though polybutadiene is more resistant to high temperature aging than natural rubber. The only way to study shelf aging is to age parts at room temperature or slightly elevated temperatures. Using higher temperatures and shorter times appears to change the mechanism.  Age-stiffening occurs regardless of the choice of cure system, antidegradant system and filler system. The specific causes and mechanisms involved remain undetermined.