ed alkyl group having 9 to 22 carbon atoms, an alkyl group having a cycloalkyl moiety and having 9 to 22 carbon atoms in total, or a linear or branched alkenyl group having 9 to 22 carbon atoms, two of X1, X2 and X3 are hydrogen atoms, and one of X1, X2 and X3 is a group represented by the formula: RCO-, wherein R is as defined above; ##STR2## wherein X4, X5 and X6 each represents a hydrogen atom