Properties of Hard Rubber. VII. Measurement of Plastic Flow as Affected by Temperature

Abstract

Abstract The merits of the Martens apparatus as an instrument for measuring plastic flow of solid insulating materials at various temperatures are discussed and the features of a new torsion apparatus which is designed to overcome certain objections to the Martens apparatus are explained. Details of the construction of the new apparatus and of its method of operation are given. Deflection-time and deflection-temperature curves determined with the torsion apparatus for a typical hard rubber are given and the form of these curves is discussed. The “yield temperature” is defined from the characteristic shape of the deflection-temperature curve. It is shown that deflection is proportional to stress, and that the magnitude of the stress used and the dimensions of the test-piece have no influence on the general shape of these curves. An empirical formula connecting deflections, stress and test-piece dimensions is given. The influence of degree of vulcanization on the plastic flow of an unloaded (rubber-sulfur) hard rubber at various temperatures is described. “Yield temperature” increases with time of vulcanization up to some limiting value. Recovery of shape of the test-piece after removal of torsional stress is briefly discussed. Comparative tests on several hard rubbers were made with the Martens apparatus and the torsion apparatus, both used at a series of steady temperatures. The same type of deflection-temperature curve was obtained, but the. “yield temperature” as slightly higher with the Martens apparatus than with the torsion apparatus. The torsion apparatus provides more complete data and is more convenient to operate than the Martens apparatus.