In the above formula, R11 represents an alkyl or hydroxyalkyl group having 1 to 4 carbon atoms; A1, A2 and A3 may be either the same or different from one another and each represents a hydrogen atom or a group represented by the formula: R12 CO-- (wherein R12 represents a linear or branched alkyl or alkenyl group having 7 to 35 carbon atoms) and at least one of A1, A2 and A3 is a group represented