According to the invention compounds which come into particular consideration are those in which the residue Z carries a carboxyl and/or sulfonyl group but also those in which one of the residues R1 or R2 denotes a lower alkyl group such as for example a methyl, ethyl, propyl, butyl, pentyl, an ethylenyl or propylenyl group or a C 10 to C 18 alkyl group.