gen atom, an alkyl group of 1 to 5 carbon atoms or an alkylol group of 1 to 5 carbon atoms, and X is a halogen atom or a recurring unit derived from a nitrogen-containing acrylic monomer having, in its molecule, at least one CH2 ???CR1 --CO-- group, where R1 is a hydrogen atom, a methyl group or an ethyl group, and at least two --N+ R2 3 .X- groups, where each R2 is independently a hydrogen atom,