Pliolite-Rubber Mixtures

Cyclized rubbers have attracted the attention of chemists for a long time. When pure, all of these derivatives of rubber consist of a hydrocarbon or a mixture of hydrocarbons (C5H8)x, and all have less unsaturation than does the parent substance. Their physical properties, however, vary from rubbery to hard shellaclike types, and their formation is attributed to internal cyclic formation, since there is a change in the unsaturation but no change in the composition when compared to the original rubber. Several different methods have been utilized for the preparation of a cyclized rubber. Harries obtained a white inelastic solid by treating a rubber hydrogen halide with an organic base. When rubber is heated under conditions which preclude complete breakdown, a diminution in the number of double bonds is effected, together with increase in density. Cyclization by pyrogenic decomposition was accomplished as early as 1838 by Himly The heat-cyclized rubber obtained by Staudinger and Geiger was a white powder, the solutions of which were of low viscosity. The effect of heat on rubber in solution as investigated by Staudinger and Bondy showed that, up to 150° C, the number of double bonds remained the same, but at higher temperatures cyclization occurred. As early as 1781 Leonhardi referred to a tough elastic product obtained by treating rubber with sulfuric acid. In 1851 Macintosh obtained a patent for a process in which the extruded or moulded articles of gutta-percha were subjected to superficial hardening by immersion in concentrated sulfuric acid. However, the sulfuric- or sulfonic-acid-cyclized rubber of today is based upon the investigations of Fisher and coworkers. These materials, known as Thermoprenes, were shown to be cyclic isomers of rubber hydrocarbon, in agreement with the observations of Kirchhof and Staudinger. Rubber isomers from halides of amphoteric metals, such as stannic and stannous chlorides, aluminum chloride, boron fluoride, and chlorostannic acid, embrace some of the most valuable chemical derivatives of rubber. A comprehensive study of these reactions was made by Bruson, Sebrell, and Calvert. The properties of amphoteric metal halide derivatives of rubber which render them specially valuable as moulding materials were described by Thies and Clifford and by Jones and Winkelmann. Numerous other methods for isomerizing rubber include the silent electric discharge, heating in the presence of surface-active substances, phosphorus oxychloride, and hydrogen fluoride.