Evaluation of the Vulcanization Characteristics of Crude Rubber

Any vulcanization test of crude rubber which is based on a measurement of the strain modulus for only one time of vulcanization is insufficient as a source of information to the investigator of the rate of vulcanization and at the same time of the order of magnitude of the maximum modulus. It is proposed to complete this test by a second vulcanization for a different length of time and to correlate the complete vulcanization curves with the exponential functions of time. In this way it seems possible to derive two experimental values of the modulus: (1) A limiting modulus which characterizes the ultimate stiffness of the vulcanizate. (2) A vulcanization time constant which serves as an index of the rate of vulcanization of a rubber mixture. The more rapid the rate of vulcanization, the smaller is this constant. Application of this method to nine different lots of crude rubber proved that by this means it is possible to show well defined differences between various individual lots of crude rubber. Much more extensive studies will be necessary before it is known whether the method is applicable to the great majority of crude rubbers and whether it is adaptable in its present form to the formulation of a general specification for testing crude rubber. There is reason to believe that the use of the modulus value at 100 per cent elongation, already proposed by Gee, and his coworkers instead of 600 per cent elongation, would from this viewpoint offer numerous advantages.