Patent ID: 7258815

Claim:
A method for brightening an aqueous coating slip comprising a latex binder and at least one synthetic co-binder differing from the latex binder, comprising treating the aqueous coating slip with an optical brightener composition comprising an optical brightener consisting of at least one optical brightener of the formula (IIa): wherein M denotes H or one equivalent of a cation selected from the group consisting of Li, Na, K, Ca, Mg, ammonium and ammonium which is mono-, di-, tri- or tetrasubstituted by the radicals C 1 -C 4 -alkyl or C 2 -C 4 -hydroxyalkyl, -and water, wherein the optical brightener composition is carrier-free: wherein the at least one latex binder consists of a latex binder based on styrene/butadiene, a latex binder based on styrene/acrylate, or a latex binder based on vinyl acetate; wherein the at least one synthetic co-binder consists of carboxymethylcellulose, hydroxyalkylcellulose, polyvinyl alcohol, synthetic acrylate-based thickener, or a combination thereof; and wherein the aqueous coating slips exhibits improved brightening as compared to a coating slip containing (i) at least one latex binder and (ii) a natural binder.