Aqueous solutions of optical brighteners

Abstract

The instant invention relates to storage stable aqueous solutions of stilbene optical brighteners with polyvinyl alcohols which can be directly used by the papermaker in that they may be diluted with water and/or be metered directly into a coating composition, to provide coated papers of a particularly high whiteness.

1 . An aqueous optical brightener solution consisting essentially of 
       (a) at least one optical brightener of formula (1) 
       
         
           
           
               
               
           
         
       
       wherein
 M is hydrogen, an alkali metal atom, ammonium or a cation derived from an amine, 
 N is 1 or 2, and 
 X is a natural or unnatural amino acid from which a hydrogen atom of the amino group has been removed; 
 
       (b) a polyvinyl alcohol having a degree of hydrolysis greater than 75% and a Brookfield viscosity of 2-40 mPa·s; and 
       (c) water. 
     
     
         2 . An aqueous optical brightener solution according to  claim 1  wherein
 M is hydrogen or sodium, 
 n is 1 or 2, 
 X is aspartic acid, glutamic acid or iminoacetic acid, and 
 
       wherein the polyvinyl alcohol has a degree of hydrolysis of greater than or equal to 80% and a Brookfield viscosity of 2-20 mPa·s 
     
     
         3 . An aqueous optical brightener solution according to  claim 1  wherein the weight ratio of the polyvinyl alcohol to the optical brightener is from 0.01:1 to 1.5:1 and wherein the concentration of the optical brightener is from 6 to 60% by weight. 
     
     
         4 . An aqueous optical brightener solution according to  claim 1  wherein the weight ratio of the polyvinyl alcohol to the optical brightener is from 0.03:1 to 1:1 and wherein the concentration of the optical brightener is from 10 to 50% by weight. 
     
     
         5 . A paper coated with a coating composition comprising an aqueous optical brightener solution according to  claim 1 . 
     
     
         6 . A process for the production of a coated paper comprising the step of coating a coating composition comprising a solution according to  claim 1  onto the paper after sheet formation. 
     
     
         7 . A process according to  claim 6  wherein the coating composition comprises from 10 to 70% by weight of one or more white pigments. 
     
     
         8 . A process according to  claim 7  wherein the coating composition contains a primary binder based on synthetic latex, wherein the primary binder is selected from the group consisting of: styrene-butadiene, vinyl acetate, styrene acrylic, vinyl acrylic and ethylene vinyl acetate polymer and optionally a secondary binder selected from the group consisting of starch, carboxymethylcellulose, casein, soy polymers, and polyvinyl alcohol. 
     
     
         9 . A process according to  claim 8  wherein the primary binder is used in an amount from 5 to 25% by weight of the one or more white pigments, wherein the secondary binder is used in an amount from 0.1 to 10% by weight of the one or more white pigments and wherein the optical brightener of formula (1) is used in an amount from 0.01 to 1% by weight of the one or more white pigments. 
     
     
         10 . The aqueous optical brightener solution according to  claim 1 , further comprising up to 10% by weight of salt, wherein the salt is a by-product from the production of the optical brightener. 
     
     
         11 . The aqueous optical brightener solution according to  claim 1 , further comprising one or more antifreezes, biocides, or complexing agents. 
     
     
         12 . A process according to  claim 9  wherein the optical brightener of formula (1) is used in an amount from 0.05 to 0.5% by weight of the one or more white pigments. 
     
     
         13 . The aqueous optical brightener solution according to  claim 1 , further comprising one or more organic by-products formed during the preparation of the optical brightener.