Concentrated optical brightener solutions

Abstract

The instant invention relates to concentrated aqueous solutions of hexasulphonated stilbene optical brighteners which are storage-stable in the absence of solubilizing agents like urea. By removal of salts resulting from the preparation of the optical brightener one can obtain a concentration of up to 0.350 mol/kg without losing storage stability. The reduced water content enables coating compositions which require less drying energy and which show less water and binder migration into the paper.

1 . A storage-stable aqueous solution comprising at least one optical brightener of formula (1)  
       
         
           
           
               
               
           
         
       
       wherein 
 M is hydrogen, an alkali metal cation, ammonium, or ammonium which is mono-, di- or trisubstituted by a C 2 -C 3 -hydroxyalkyl radical, and  
 n is from 1 to 4,  
 wherein the amount of the at least one optical brightener is higher than 0.214 mol/kg and no solubilizing agent is contained in the solution.  
 
     
     
         2 . A storage-stable aqueous solution according to  claim 1  wherein 
 M is hydrogen or a sodium cation, and    n is 1 or 2.    
     
     
         3 . A storage-stable aqueous solution according to  claim 1  wherein the concentration of the at least one optical brightener is from 0.215 mol/kg to 0.350 mol/kg.  
     
     
         4 . A storage-stable aqueous solution according to  claim 3  wherein the concentration of the at least one optical brightener is from 0.250 mol/kg to 0.340 mol/kg.  
     
     
         5 . A storage-stable aqueous solution according to  claim 1  further comprising one or more compounds selected from the group consisting of inorganic salts, carriers, antifreezes, preservatives or complexing agents.  
     
     
         6 . A process for preparing a storage-stable aqueous solution according to  claim 1  comprising the steps of preparing the at least one optical brightener of formula (1) by reacting a cyanuric halide with 
 a) a diamine of formula (A)                          b) an amine of formula (B)                          c) an amine of formula (C)                           to form a reaction solution,     and wherein at least 50% by weight, of the alkali metal or amine salt generated as a by-product by the reactions between the diamine of formula a), the amine of formula b) and the amine of formula c) and the cyanuric halide is removed from the reaction solution.    
     
     
         7 . The process according to  claim 6  wherein the removal of the alkali metal or amine salt is done by ultrafiltration or membrane filtration of the reaction solution or by isolating the optical brightener and then redissolving the reaction solution.  
     
     
         8 . The process according to  claim 7  wherein the removal is done by membrane filtration.  
     
     
         9 . A process for brightening of paper or cellulosic substrate containing a white pigment comprising the step of adding the storage stable aqueous solution of the optical brightener according to  claim 1  in a concentration of 0.05 to 0.5% by weight of the white pigment to the paper or cellulosic substrate during production thereof.  
     
     
         10 . A process according to  claim 9  for brightening paper in a pigmented coating composition after sheet formation comprising the step of adding to the pigmented coating composition the storage-stable aqueous solution of  claim 1 .  
     
     
         11 . The process according to  claim 6  wherein at least 80% by weight, of the alkali metal or amine salt generated as a by-product by the reactions between the diamine of formula a), the amine of formula b) and the amine of formula c) and the cyanuric halide is removed from the reaction solution.  
     
     
         12 . A paper or cellulosic substrate produced by the process according to  claim 9 .  
     
     
         13 . A paper or cellulosic substrate produced by the process according to  claim 10 .  
     
     
         14 . A paper or cellulosic substrate brightened with the storage-stable aqueous solution according to  claim 1.