. . . "The quaternary ammonium salt according to claim 1, wherein R5 and R6 each represents an alkyl or hydroxyalkyl group having 1 or 2 carbon atoms, R7 and R8 each represents an alkylene group having 1 or 2 carbon atoms, Q is a group represented by the formula: ##STR174## or the formula: ##STR175## wherein R14' represents a hydrogen atom or a methyl group, R9 and R10 each represents an alkyl or alkenyl" . .