nt by weight of a methylpolysiloxane copolymer composed of trimethylsiloxane units and SiO2 units, said units being in such proportion that the ratio of methyl radicals to silicon atoms is from 1.0 to 2.5:1, and (3) 15 to 80 percent by weight of polysiloxane having the general formula R'n SiO.sub.(4-n)/2, where R' is a C1-4 alkyl or alkylene radical or a monocyclic aryl radical, and n having an av