ubstituted monovalent hydrocarbon group having 1 to 20 carbon atoms, Y is a monovalent group selected from the class consisting of alkoxy groups, acyloxy groups and alkenyloxy groups having 1 to 6 carbon atoms and a hydroxy group and the suffixes a and b are each a positive integer of 1, 2 or 3 with the proviso that a+b=4; (c) from 1 to 40 parts by weight of a finely divided silica powder; and (d)