Corrosive Action of Sulfur against Metallic Copper during Vulcanization

Abstract

Abstract The use of copper wire electroplated with tin for insulated electric wires was originated by Siemens eighty-five years ago (through Saito, J. Soc. Chem. Ind. Japan, 18, 248 (1915)), and its merits lie in the fact that it prevents a lowering of the electrical conductivity and mechanical strength from corrosion of copper by the sulfur in the rubber, and also prevents the rubber from deteriorating abnormally fast by contact with the copper. The manufacture of rubber for coating wire has improved rapidly since the appearance of organic accelerators and combined accelerating and vulcanizing agents, such as the polynitrobenzenes and benzoyl peroxide of Ostromislensky, the selenium of Boggs and tetramethylthiuram tetrasulfide and tetramethylthiuram disulfide. Recently, it was reported (India Rubber World, 87, 38 (1933)) that rubber compounded with 2 per cent of 1,3,5-trinitrobenzene as vulcanizer does not corrode wire conductors and does not deteriorate abnormally fast when the rubber mixture is covered directly on the wire and vulcanized. The prediction was also made that tin-coated copper wire will disappear entirely from the cable world. As a matter of fact, marine cables have for a long time been covered with an insulator consisting chiefly of gutta-percha or balata hydrocarbon, in direct contact with the copper conductor, and have given good service. The para-gutta insulation of marine cables, recently developed by the Western Electric Company, U. S. A., is composed partly of rubber, with no tin coating on the copper conductor. Nevertheless, the principal factor which decides the “electrical life” of rubber submerged in water is, according to Curtis (J. Res. Natl. Bur. Standards, 5, 539 (1930)) the deterioration of the rubber by copper salts, and Asano (J. Soc. Chem. Ind. Japan, 24, 336 (1921)) reported that smoked sheet became so sticky that it required considerable strength to separate after it had been held between two copper plates for six months. The status of the electric insulating wire industry today is such that a thorough investigation of the tin coating of copper wire and the methods of tin coating in connection with the deterioration of rubber by the copper wire is desirable. The investigation described in the present paper was carried on with this object, and it deals with experiments on the corrosion of metallic copper by sulfur during vulcanization, and shows the formation of copper sulfides as corrosion products.