lower amounts of boron nitride were used 80 grams 25 30 microns boron nitride after mixing according to procedure of example 1 the mixture had a working time of less than 2 0 hours molded specimens according to procedure of example 1 had good physical properties before and after aging at 260 u00b0 c for 500 hours these materials showed improvements in thermal conductivity and lower weight loss on aging the base silicone elastomer prepared as described in example 1 is mixed with the following ingredients as in example 1 ______________________________________component amount______________________________________1 base material 500 00 grams2 boron nitride particle diameter 25 30 microns 125 00 grams3 silicone fluid 200 100 000 centi stokes at 25 u00b0 c 65 00 grams4 silastic j curing agent 60 00 grams5 cure modifier e 1990 76 1 00 grams______________________________________ after mixing the ingredients the mix had a working time of about 3 hours molded specimens according to procedure of example 1 exhibit good physical properties and improvements in thermal conductivity example 6 is repeated with the exception that a larger amount of silicone fluid 200 is used 90 0 grams silicone fluid 200 the mixture has longer working time and was moldable the base material rtv silicone base described in example 1 is mixed with the following ingredients according to procedure of example 1 ______________________________________component amount______________________________________1 base material 400 00 grams2 boron nitride particle diameter 48 90 microns 50 00 grams3 boron nitride particle diameter 25 30 microns 50 00 grams4 silicone fluid 200 100 00 centi stokes at 25 u00b0 c 40 00 grams5 silastic j curing agent 40 00 grams6 cure modifier e 1990 76 0 80 grams______________________________________ after mixing the above components the mixture has a working time of over 4 hours molded specimens according to procedure.