GB322208A - Improvements in or relating to electric insulating materials - Google Patents

Abstract

322,208. Smith, W. S., Garnett, H. J., Dean, J. N., Channon, H. C., Gardner, W, and Wilson, H. F. Aug. 25, 1928. Divided on 307,390. Drawings to Specification. Compositions containing indiarubber and bitumen; vulcanizing.-A composition capable of being extruded at 70‹-120‹ C., particularly suitable for the insulation of submarine cables consists of an intimate admixture of 30-60 per cent of rubber and a high melting point bitumen of low ash and free carbon content. The bitumen should have a melting point above 100‹C., for example, gilsonite, glance pitch, manjak, grahamite. Other substances may be added; when these comprise guttapercha, balata &c., the rubber content may be reduced to 20 per cent or less. The rubber is mixed with the melted bitumen, the temperature being kept below that which would harm the rubber. Up to 10 per cent of a flux such as ceresin, candelilla or other wax may be added, to reduce the melting point, or the bitumen may be finely powdered, and mixing effected at a temperature below its melting point. When gutta percha or balata are added they are preferably purified as described in Specification 307,390, and may form 50-75 per cent of the composition. Hardeners as benzidine, softeners as stearic acid, and antioxidants as tannic acid may also be added. The composition may be vulcanized, preferably by heating to a temperature below the melting point of the bitumen, partially to vulcanize the rubber, and subsequently to a higher temperature to melt the bitumen and to complete the vulcanization. When the composition contains more than 40 per cent of rubber, it is desirable to extrude round the core before the second heating stage. The rubber may be previously treated as in Specification 320,370.