Application of a Strained Natural Rubber at High Temperatures

Robustness of the natural rubber (NR) crystal at high temperatures is important because vehicle tires are easily subjected to high temperatures due to friction. To understand the robustness of crystals in the strained NR, as its plausible application to high temperatures, changes in the following structural parameters as a function of temperature were examined in this study by conducting two-dimensional wide-angle X-ray diffraction measurements: crystal lattice constants (a, b, c, and β), unit cell (volume, thermal expansivity, and orientation factor), degree of crystallinity, and crystallite (size, volume, and number density in the strained NR specimen). As a result for a vulcanized NR specimen subjected to a constant strain of 6, thermal shrinkage of the c-axis length was found in the heating process, while both a- and b-axis lengths were found to increase with an increase in the temperature. Furthermore, the volume of the crystal lattice was found to increase with the temperature, while the orientation degree of the crystal lattice remained unchanged. The degree of crystallinity was found to gradually decrease with temperature from the beginning of heating. As for the crystallite, the size along the stacking direction of the (200), (120), and (201) planes decreased with temperature, while that of the (002) plane (i.e., along the c-axis direction) was found to increase. These results indicate that crystallites grow in the NR main chain direction, while they are subjected to melting in the other perpendicular directions upon heating. However, it was found that the crystallite volume and the number density of crystallites in the strained NR specimen continuously decreased with the increasing temperature. The thermo-reversibility of all the structural parameters was also examined experimentally