Effect of holding time on high strain hysteresis loss of carbon black filled rubber vulcanizates

Hysteresis loss has been measured at constant stress and constant strain, at various holding times under tensile deformation of natural rubber (NR) and styrene-butadiene rubber (SBR) vulcanizates filled with various loadings of carbon black filler. The effects of temperatures (25 °C to 150 °C), strain rates (3.78×10-5 sec-1 to 210×10-3 sec-1) and strain levels (20% to 300%) have been studied. Hysteresis loss and hysteresis loss ratio increase with an increase in strain rate, filler loading, strain level and holding time. It decreases with an increase of temperature. However, higher hysteresis loss and hysteresis loss ratio are observed at constant stress than at constant strain. NR and SBR vulcanizates show similar behavior. Evidence has been produced for the existence of a distinct relaxation process that occurs within first 120 seconds of holding time at room temperature. This process becomes less important as the strain or the temperature is increased. However, at high temperature another distinct relaxation process has been observed. The activation energy has been found to be 66.3 kJ/mole for the rates at the higher holding time, while it has been found to be 17.3 kJ/mole for the rates at the lower holding time using the data of hysteresis loss at first cycle of 40 phr black filled NR vulcanizates.