Examples of suitable crosslinking agents are compounds containing, in the molecule, n polymerisable radicals, particularly those of the formula CH2 ???CH--, CH2 ???CH--CH-- or CH2 ???CH(R5)--CO-- or CH2 ???CH(R5)--CO--O--, n having the meaning already mentioned, normally 2, 3 or 4 and preferably 2, and R5 denoting hydrogen or C1 -C4 -alkyl, in particular hydrogen or methyl.