Fatigue Behaviour of Carbon Black Filled Rubber Vulcanizates at High Temperature and under Swollen Conditions

Abstract

Fatigue behaviour of gum and carbon black filled natural rubber and styrene butadiene rubber vulcanizates has been investigated at constant strain and constant stress at 75° and 125 °C. Samples swollen in paraffin oil have also been examined. For all the vulcanizates, the cut growth increases with number of cycle. NR vulcanizates show rapid cut growth with increase in filler loading whereas cut growth is slowed for SBR vulcanizates up to 50 phr loading. At 50 phr loading of different types of carbon black (N220, N375, N550 and N660) NR samples display increase in cut growth with an increase in sur face area of carbon black. An opposite trend is shown for SBR sample. The criti cal number of cycles at which the cut growth becomes rapid varies with the nature and loading of filler and temperature. At constant stress however, the cut growth becomes slower with increase in loading. All the above results could be explained with the help of strain energy density and number of flaw sites. Cut growth rate increases with tearing energy for both NR and SBR at 75°C and 125 ° C and is higher at a particular tearing energy for filled NR vulcanizates (up to 50 phr) as compared to gum. SBR vulcanizates show an op posite trend. Cut growth could not be followed for paraffin oil swollen samples. These fail catastrophically. The fatigue life of these swollen samples could also be explained with the help of strain energy density. Fracture surface morphol ogy as revealed by Scanning Electron Microscopy (SEM)/Optical Microscopy of the samples is not much different from the surface generated during failure at room temperature. The swollen samples show large cracks and roughness; the gum samples show smoother surface.