Effect of Temperature on Stretched Rubber Bands

Abstract

Specimens of each of two vulcanized rubber compounds of known composition were subjected to various constant gravity loads in a controlled temperature chamber for several months. The lengths of the stretched bands were observed from time to time while the temperature was altered in gradual steps within a range of about thirty degrees centigrade centering around room temperature. A projection method for observing small changes in length was devised for the experiment. The contraction of stretched rubber with rise in temperature, first observed about the middle of the eighteenth century, was verified, as also was its positive elongation when unstretched or when but slightly stretched. The present experiment shows that the expansion or contraction for each specimen was constant when measured in cm per degree temperature change, but not when measured in percent of the prevailing length of the band. That is, the thermal elongation was most simply expressed when considered independent of the after effect or drift. For molded bands originally 1 mm in thickness, 4 mm in width, and 10 cm in mean circumference, and with loads ranging from 30 to 836 grams producing elongations up to about 700 percent, the rates of expansion were found to lie between plus 0.0037 and minus 0.0550 cm per degree. The relation between the rate of thermal elongation, dl/dθ, and the stretching load, w, was found to be linear within the limits of the experiment, and the constants c and d in the empirical equation, dl/dθ=cw+dhave been evaluated, the former being negative in sign. The critical tensions for which there appeared no thermal change in length were 63 and 97 grams, respectively, for the particular shapes and compositions used. These tensions are equivalent to critical stresses of 7.72 and 11.9×105 dynes/cm2 (based on the cross section of the unstretched band) or of 11.2 and 17.2 lb./in.2, respectively, for the two compounds. Extrapolation by placing w =0 does not seem justifiable because when the resulting rate is divided by one-half the circumference of the bands the magnitude of the coefficients of linear expansion turn out to be several times the generally accepted values based on volumetric determinations.