Aging of a Black-Filled Natural Rubber Vulcanizate at Fixed Strain: Part II. Tear Resistance

In service, rubber goods are often deformed at elevated temperature.  We previously reported that aging of a black-filled, natural rubber vulcanizate (inefficient sulfur cure) held at fixed strain gives anisotropic tensile properties.  We now report on the tearing of edge-cut tensile specimens of this vulcanizate that had been aged in nitrogen at 90^oC or 110^oC, with and without pre-strain prior to testing.

Tensile strips that had been aged without pre-strain became “super-blunted” by developing longitudinal cracks from the cut tips.  “Perpendicular” specimens, extended perpendicular to the pre-stretch direction, contained a razor cut of length c that was parallel to the pre-strain direction.  These developed cracks that were either “super-blunted” or exhibited “forward” crack splitting.  “Parallel” specimens, extended parallel to the pre-stretch direction, contained a razor cut that was perpendicular to the pre-strain direction.  Strength of these was less than that of “perpendicular” specimens and ones from a normal, isotropic network.   Some “parallel” specimens had a critical cut size, c_cr, above which strength decreased sharply.  When c < c_cr, cracks became “super-blunted”, while specimens with c > c_cr usually showed simple lateral growth of a single crack.  There was some evidence that normal, uncut parallel specimens have increased ability to strain-crystallize; however, when a cut is present, parallel samples are quite weak.  This suggests that limited chain extensibility is the controlling factor.