Stress-Temperature Relations in a Pure-Gum Vulcanizate of Natural Rubber

Abstract

Stress-temperature relations at constant elongation have been studied in a pure-gum vulcanizate of natural rubber. Such studies yield information useful for calculations involved in the theory of its elastic behavior, and furnish practical data regarding its tensile properties at different temperatures. The crystallization of the rubber which occurs on stretching is a factor of considerable importance in the explanation of the results obtained. The compound contained 100 parts by weight of smoked sheet rubber, 2 parts of sulfur, 1 part of zinc oxide, and 0.5 part of zinc dibutyl dithiocarbamate (``Butyl zimate''), and it was vulcanized 40 minutes at 115°C. The stress-temperature studies were made after the specimens were stretched and allowed to relax at a constant temperature of 25°C or 70°C for about two hours. The change of stress with time at the end of two hours was small enough to be without influence on later results, except when the flow was excessive or when the stretched rubber continued to crystallize, as it did at elongations between 150 percent and 500 percent at 25°C. At elongations of 150 percent and less, no evidences whatever of crystallization were found during the time of the experiments. At elongations of 500 percent and more, the crystallization which occurred during stretching and in the first few minutes thereafter was so complete that the stress soon became independent of time. At 70°C both the upper and lower limits of the range of elongations in which continued crystallization occurred were somewhat higher. Values of the stress after relaxation have been used to plot a stress-strain curve. The stress-temperature relations obtained by lowering the temperature to about −20°C were always linear and reproducible on successive runs of increasing and decreasing temperature, except when the flow was excessive or when the stretched rubber continued to crystallize. An additional period of relaxation was required if the rubber was heated above the temperature at which the first relaxation occurred. The slopes of the stress-temperature lines and the values of the stress at any temperature depend upon the temperature at which relaxation was carried out. The stress intercepts at 0°K are small compared with the total stress for elongations up to 150 percent. At the higher elongations the intercepts take on increasingly large negative values, finally becoming approximately equal in magnitude to the total stress.