Change of volume of rubber on stretching: Effects of time, elongation, and temperature

Abstract

The change of volume on stretching was studied for four rubber compoundsj two were pure gum, and one each contained 10 percent by volume of carbon black and whiting, respectively. Samples were made in the form of narrow molded rings. Measurements of the change with time up to about 20 minutes were made with a water-filled dilatometerj for longer periods a mercury-filled dilatometer was used. The whiting compound increased in volume on stretching, presumably on account of the formation of vacuoles around the coarse particles of filler. With the other compounds the volume remained constant up to an elongation of 200 or 300 percent, above which it decreased by an amount which was greater the higher the elongation, the lower the temperature, and the longer the time the rubber was kept stretched. The decrease was an approximately linear function of the logarithm of the time from a few minutes after stretching, to about 4 weeks, the duration of the experiment. The volume thermal expansivity of the stretched rubber was greater than that of the unstr etched. Increased vulcanization of a given compound, in general, decreased the volume change on stretching. The results indicate that, in the absence of coarse fillers, Poisson's ratio for rubber is 0.5 or greater, the numerical value depending upon the composition, the temperature, the time after stretching, and other factors.