Method for coating a substrate

A method for coating a substrate comprising:

• i) applying an aqueous radiation-curable coating composition to said substrate;
• ii) thermally drying the applied aqueous radiation-curable coating composition;
• iii) irradiating the thermally dried composition with UV light having a wavelength≤300 nm under inert gas;
• iv) irradiating the thermally dried composition with UV light having a wavelength≥250 nm or with E-beam,
• wherein the aqueous radiation-curable coating composition comprises:
• A) from 15 to 60 wt % (% by weight) of a polyurethane, wherein said polyurethane is obtained by a process comprising the following steps:
• I) a prepolymer is obtained by reacting:
• a) from 20 to 60 wt % of at least one aliphatic or cycloaliphatic diisocyanate;
• b) from 0.5 to 10 wt % of least one diol having at least one carboxylic or carboxylate group;
• c) from 10 to 60 wt % of at least one diol with molecular weight between 500 and 5,000 g/mol;
• d) from 0 to 40 wt % of at least one ethylenically unsaturated compound containing one reactive group capable to react with an isocyanate;
• e) from 0 to 10 wt % of at least one diol or at least one polyol different from diols b) and c);
• f) from 0 to 20 wt % of an aliphatic or cycloaliphatic polyisocyanate having average functionality above 2,
• wherein the percentage amounts of a), b), c), d), e) and f) are referred to the sum of a)+b)+c)+d)+e)+f);
• II) the prepolymer obtained in step I) is reacted with a capping agent, said capping agent being a secondary amine or a monoalcohol, in such proportions that the molar ratio between the amino groups or hydroxy groups of the capping agent and the isocyanate groups (NCO) of the prepolymer is between 0.3 and 1;
• III) the capped prepolymer obtained in step II) is dispersed in water;
• B) from 1 to 35 wt % of at least one unsaturated reactive diluent.