0 to 10; each of R6, R7, R8 and R9, which may be the same or different, is a linear or ramified alkyl group having 1 to 8 carbon atoms or an aralkyl group having 7 to 18 carbon atoms, and B is hydrogen, a linear or ramified alkyl group having 1 to 8 carbon atoms, a cycloalkyl, aryl or alkyl-aryl group having 4 to 18 carbon atoms, a phosphite group ##STR10## or a phosphate group ##STR11## in which