Use of brighteners for the preparation of coating slips

1. 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.

2. The method of claim 1 , wherein the latex binder is a latex based on styrene/butadiene.

3. The method of claim 1 , wherein the synthetic co-binder is selected from the group consisting of carboxymethylcellulose, polyvinyl alcohol, and combinations thereof.

4. The method of claim 1 , wherein at least one latex binder based on styrene/butadiene is used as the latex binder and carboxymethylcellulose and/or polyvinyl alcohol is used as the synthetic co-binder.

5. The method of claim 1 , wherein the coating slip contains at least one white pigment.

6. The method of claim 1 , wherein the coating slip contains the latex binder in an amount of from about 3 to about 20% by weight and the co-binder in an amount of from about 0.1 to about 3% by weight, based in each case on the amount of white pigment.

7. The method of claim 1 , wherein the latex binder is in an amount ranging from about 5 to about 15% by weight, and the co-binder is in an amount ranging from about 0.5 to about 1.5% by weight, based on the amount of white pigment.

8. A coating slip comprising:

(a) at least one white pigment,

(b) at least one latex binder consisting of a latex binder based on styrene/butadiene, a latex binder based on styrene/acrylate, or a latex binder based on vinyl acetate,

(c) at least one synthetic co-binder differing therefrom, consisting of carboxymethylcellulose, hydroxyalkylcellulose, polyvinyl alcohol, synthetic acrylate-based thickener, or a combination thereof; and

(d) an optical brightener composition comprising

an optical brightener consisting of at least one 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.

9. The coating slip according to claim 8 , wherein the slip comprises:

(a) from 3 to 20% by weight of the latex binder;

(b) from 0.1 to 3% by weight of the co-binder;

(c) 0.025 to 1% by weight of the brightener of the formula (IIa), based in each case on the amount of white pigment.

10. The coating slip of claim 9 , wherein the latex binder is in an amount ranging from about 5 to about 15% by weight, and the co-binder is in an amount ranging from about 0.5 to about 1.5% by weight, based on the amount of white pigment.

11. A method for making a coated paper comprising applying to a paper substrate a coating slip comprising:

(a) at least one white pigment,

(b) at least one latex binder consisting of a latex binder based on styrene/butadiene, a latex binder based on styrene/acrylate or a latex binder based on vinyl acetate,

(c) at least one synthetic co-binder differing therefrom, consisting of carboxymethylcellulose, hydroxyalkylcellulose, polyvinyl alcohol, synthetic acrylate-based thickener, or a combination thereof, and

(d) an optical brightener composition comprising

an optical brightener consisting of at least one 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.

12. The method of claim 11 , wherein the coating slip comprises from about 3 to about 20% by weight, of the latex binder; from 0.1 to 3% by weight, of the co-binder; 0.025 to 1% by weight of the brightener of formula (IIa), based in each case on the amount of white pigment.

13. The method of claim 11 , wherein the latex binder is in an amount ranging from about 5 to about 15% by weight, and the co-binder is in an amount ranging from about 0.5 to about 1.5%.

14. A coated paper made from the method of claim 11 .

15. The method of claim 1 , wherein the coating slip further comprises a dispersant based on poly(acrylic acid).

16. The coating slip of claim 8 , further comprising a dispersant based on poly(acrylic acid).

17. The method of claim 11 , wherein the coating slip further comprises a dispersant based on poly(acrylic acid).

18. A coating slip comprising:

at least one white pigment;

a binder consisting of

a latex binder based on styrene/butadiene, a latex binder based on styrene/acrylate,

or a latex binder based on vinyl acetate, and

a synthetic co-binder consisting of carboxymethylcellulose,

hydroxyalkylcellulose, polyvinyl alcohol, synthetic acrylate-based thickener, or a combination thereof; and

an optical brightener composition comprising

an optical brightener consisting of at least one optical brightener of the formula (IIa)

wherein M is Na, and

water;

wherein the optical brightener composition is carrier-free.

19. The coating slip of claim 18 , further comprising a dispersant based on poly(acrylic acid).