A composition comprising an aqueous polymer dispersion, characterised in that a) the composition contains 0.001 to 0.5 mol of a photo-initiator, which causes a cross-linking reaction on irradiation with energy-rich light and b) the polymer dispersed in the dispersion has 0.0001 to 1 mol of keto or aldehyde groups, whereby the mol amounts relate to 100 g of the dispersed polymer.