. . . . . "r represented by the general formula CH2 ═CR2 --CO--OR3, in which R2 is a hydrogen atom or a methyl group and R3 is an alkyl group or an alkoxy-substituted alkyl group having 1 to 18 carbon atoms, alone or as a mixture of monomers (b-1) at least 70% by weight of which is an acrylic or methacrylic monomer (b) as defined above, and one or both of (b-2) up to 30% by weight of which is an ethyle" .