in which R is a C7-21 alkyl or alkenyl group, R' is a hydrogen atom or a C1-18 alkyl, alkenyl or acyl group, AO represents C3 H6 O or C4 H8 O groups or a mixture of C2 H4 O, C3 H6 O and C4 H8 O groups, m is an integer of 1 to 100 and n is an integer of 1 to 100.