Rheological Properties of Natural and Synthetic Rubbers

Abstract

Abstract One of the important factors determining the processing characteristics of a rubber or unvulcanized rubber stock is the “plasticity”. This term has been used by rubber technologists to define the rate at which a material can be made to flow under a set of conditions of stress, geometry, and temperature. Strictly speaking, plasticity is not a quantitative term but, because of past usage, it seems justifiable to continue to employ it as an inverse index of resistance to flow. Three principal types of instruments have been employed for measuring plasticity of rubberlike materials: parallel plate, rotating disk or cylinder, and extrusion plastometers. The parallel-plate type, like the rotary type plastometer, is designed to operate with no slippage between rubber and confining surfaces, and both operate at rather low rates of shear. The rotating disk plastometer, however, permits attainment of thixotropic equilibrium and simple calculation of the mean shearing stress at a given average rate of shear. All extrusion plastometers described in the literature have employed rather short, large bore extrusion tubes, which permit appreciable slippage between rubber and tube walls, and have operated at a constant driving pressure and temperature, the rate of efflux serving as the plasticity index. The existence of slippage, of course, eliminates the possibility of calculating absolute consistency (inverse of plasticity), but is an advantage in that factory processing conditions, where slippage is a factor, are simulated. High rates of shear of the order of magnitude of those existing in factory extruders may be readily attained with extrusion plastometers. It has been argued that high rates of shear in a plastometer are desirable from the standpoint of predicting processability, since the shear stress vs. rate of shear relationship for rubberlike materials departs radically from linearity.