A Simplified Hot Tensile Test for GR-S

Abstract

Abstract In September, 1942, the hot-iron test developed by E. Valden, of these laboratories, was demonstrated as a simple rapid method of evaluating the heat embrittlement of GR-S. The test-specimen, a T-50 test-piece, may be cut from the remnants of test sheets cured for stress-strain determinations. for GR-S treads a convenient set of conditions is 200 per cent elongation and an iron temperature of 400° F. The iron is fastened to the test-block at a sufficient height to assure positive contact of the specimen with the iron, and the screws in the spacer blocks are set to give a 200 per cent elongation. The iron, a “patch heater”, for spotting or dry-mounting photo prints, has a built-in thermostat. The time in seconds between contact of the specimen with the iron and its failure due to heat is recorded. The relative behavior of various elastomers with 27.8 volumes of carbon is illustrated in Figure 1. The excellence of Hevea at both optimum and prolonged overcure is apparent. The hot-iron test does not serve to pick best cure. It does provide an effective warning against overcure and a means of recognizing compounding changes which benefit heat stability, as, for example, the use of high carbon high softener ratios.