JAPAN FIRM DEVELOPS ULTRA HEAT-RESISTING RUBBER Shin-Etsu Chemical Co Ltd said it had developed an ultra thermal resistance rubber which can be used at 250 centigrade continuously without losing its properties. Shin-Etsu put potential demand for the product at two tonnes a month mainly for microwave ovens and as a sealant. But increasing use of the product in parts attached to car engines will raise demand and the company will boost output to 10 tonnes by March 1988 from two now, he said.