Stress Relaxation of Some Rubber and Synthetic Rubber Vulcanizates in Compression

Abstract

Abstract Vulcanizates of rubber and synthetic rubber find wide and useful applications under conditions of compressive stress in gaskets, seals, suspension systems for vibration isolation, and other uses. In some of these applications the rubber is subjected to a load, either static or dynamic, or a combination of both. In other cases the compression strain is constant, and the resulting stress is utilized as a seal for fluids or gases. It is with this second application of rubber in compression that this work is concerned. In the use of rubber under compression as a seal the service life is dependent on maintaining a value of stress above a minimum necessary for the seal to be effective. It can readily be seen, then, that some method of evaluating materials with respect to service life is highly desirable. Compression set tests, method A or B, of the American Society for Testing Materials, depending on the service conditions, have been widely used for some time in evaluating rubber materials to be used in compression. This test, however, does not give an answer which is directly applicable as to units, and the test time ordinarily used is too short. Test results are reported as percentages secured from the ratio of the unrecovered portion of the amount compressed to the amount the sample is compressed. The unrecovered portion of the amount the sample is compressed is measured after a prescribed interval at room temperature after removing the stress from the sample. A better measure of the property in question, the stress as a function of time, would be to measure this property directly.