at least one monomer of the alkyl (meth)acrylate type comprising an alkyl radical having between 1 and 15, preferably between 1 and 10, carbon atoms; and optionally other monomers, such as styrene derivatives, vinyl ethers, (meth)acrylic acids or other polyunsaturated derivatives, it being possible to use these other monomers in contents of up to 50% by weight relative to the combined total of the