Suitable methacrylic acid esters are methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, pentyl methacrylate, hexyl methacrylate, heptyl methacrylate, octyl methacrylate, nonyl methacrylate or the methacrylic acid esters of the higher homologs of the above-mentioned alcohols containing up to about 44 carbon atoms, more particularly up to about 18 carbon atoms.