rtly or wholly replaced with substituent atoms or groups such as halogen atoms, e.g. chlorine atoms, and amino groups.The diorganopolysiloxane as the component (a) should have such a degree of polymerization that the viscosity thereof is in the range from 100 to 1,000,000 centistokes or, preferably, in the range from 1000 to 50,000 centistokes at 25.degree.