The Chemistry of Natural Rubber

Roberts (J. Chem. Soc., 1938, 215, 219) dissolved air-dried films from fresh latex in acetone-carbon tetrachloride and coagulated the solution by the addition of further acetone; the supernatant liquid contained an ingredient which was soluble in ligroin, contained sulfur, might be elastic, and was termed by Roberts, caoutchol. Roberts stated that removal of this caoutchol ingredient caused marked changes in the properties of the remaining rubber, “characterised by its lack of tensile strength and the remarkably low viscosity of its solutions.” This rubber was dried in air for 24 hours, dissolved in benzene, centrifuged, and emulsified with water and the benzene removed by steam distillation. The rubber remaining was tacky and feebly elastic after drying in a vacuum, and was termed caoutchene. Roberts developed the theory that the elasticity of crude rubber is due to the presence of caoutchol and that, without caoutchol, rubber has very poor elasticity and low viscosity in solution. Insufficient attention has been paid to the role of oxygen in the purifications described. Roberts states: “The process has been carefully examined, particularly with respect to the possibility of oxidative or other changes in the constituents. No evidence of such changes has been obtained.” Yet the coagulated rubber, after removal of the caoutchol but prior to steam distillation, was milled in air, and dried in air for 24 hours, during which time considerable oxidation could have occurred.