Abstract
It has been demonstrated that the tendency of natural rubber to crystallize at low temperatures can be considerably reduced by the reaction of natural rubber with relatively small quantities of a thiol acid of the RCOSH type. Most thiol acids react already at room temperature with natural rubber. The reaction is very easy to carry out. A simple admixture with dry natural rubber, e.g., on a mixing mill or in an internal mixer, is sufficient. The reaction can also be carried out in solution or in latex. If amounts of thiol acids are used which do not exceed 1 mole %, pure gum vulcanizates can be obtained which show a strongly reduced tendency to crystallize and which nevertheless show a tensile strength of more than 150 kg/cm2. This may be explained by the fact that these rubbers still show appreciable crystallization upon stretching to elongation approaching the elongation at break, as indicated by the x-ray diffraction patterns of the stretched vulcanizates. If the thiol acid rubber vulcanizates are reinforced with carbon blacks, the tensile and tear strengths are much improved. Vulcanizates with very satisfactory room temperature properties are thus obtained, whilst the improved resistance to crystallization is retained, The abrasion resistance and the aging resistance are at least as good as those of comparable vulcanizates of unmodified natural rubber. If low temperature plasticizers are mixed into thiol acid rubbers, the glass point of the vulcanizates can be strongly reduced. TR 10 values as low as −80° C in pure gum vulcanizates and −69° C in carbon-loaded vulcanizates have been obtained. These rubbers, moreover, did not show any crystallization at low temperatures.
The glass point of elastin.