The Plastic Properties of Several Types of Unvulcanized Rubber Stocks at High Rates of Shear

Abstract

An extrusion plastometer operating at rates of shear comparable with those existing in rubber tubing machines (10 to 1000 sec.⁻¹) is described. The relation between efflux rate and pressure at constant temperature for various types of rubber stocks was determined. For highly compounded rubber stocks, such as tread stocks, the efflux rate vs. pressure curves attain linearity at low rates of shear. Both the slope of the curves and the extrapolated pressure intercept vary rapidly with the temperature, indicating that both the mobility and the yield stress are functions of temperature for highly compounded stocks. For lightly compounded stocks and for crude rubber, the curves are of the power function type at the lower rates of shear, but appear to attain linearity at the very high rates. These linear portions of the curves, for a given stock at different temperatures, are approximately parallel. Elastic recovery was determined as a function of rate of efflux. The slope of the recovery vs. efflux rate curves decreases with increasing rates of efflux. The relation between efflux rate at constant temperature and pressure, and time of milling, is approximately linear. The extrusion plastometer is shown to be more sensitive to overmilling than is the Williams plastometer. The partial failure of the compression‐type plastometer to correlate with the factory extrusion machine is explained on the basis of the much lower rates of shear employed in the compression‐type instrument than those existing in the extrusion machine.