It is preferred, however, to use compounds wherein R is a radical containing up to 6 carbon atoms, some examples of which are N-cyclohexylaminomethyl -ethoxy-dimethylsilane, N-cyclohexylaminomethyl) -diethoxy-methylsilane, N-phenyl-(aminomethyl)-triethoxysilane, N-isobutylaminomethyl) -triisopropoxysilane, N-cyclohexyl- (aminomethyl) -triethoxysilane, N-cyclohexylaminomethyl) -diethoxy-tert.-butyl