Optical brightener for aqueous high yield pulp and process for making brightened paper

Abstract

Paper is brightened by addition of a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt in an amount greater than 0.04% but no more than 5% by weight in wet pulp. The amount of THP salt added is sufficient to enhance the brightness of paper made from fibers in the wood pulp to a desired brightness level. The THP salt is commercially available and previously used as a fire retardant and a biocide. Though particularly useful in the production of brightened cellulose fibers of high yield pulp, with the benefit of generally improving opacity, the process may be used to treat all wet pulp which is to be brightened.

1 . A process for enhancing the brightness of an aqueous dispersion of wood pulp comprising contacting wet pulp with a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt in an amount greater than 0.04% by weight of the wet pulp, sufficient to enhance the brightness of paper made from the pulp to a desired brightness level, but no more than 5%, the phosphonium compound being represented by the structural formula:  
       
         
           
           
               
               
           
         
       
       wherein R represents alkyl having from 1 to 4 carbon atoms; and, 
 X represents a suitable and desirable anion selected from the group consisting of HCO 3   −1 , CO 3   −2 , PO 4   −3 , HPO 4   −1 , HPO 4   −2 , OH −1 , Cl −1 , Br −1 , HSO 3   −1 , SO 3   −2 , HSO 4   −1 , SO 4   −2 , CH 3 CO 2   −1  and CH 3 SO 4   −1 .  
 
     
     
         2 . The process of  claim 1  wherein the THP salt is present in an amount in the range from about 1% to 2.5% by weight of the wet pulp.  
     
     
         3 . The process of  claim 1  wherein the pH of the wet pulp is in the range from neutral to 12, and treated wet pulp provides fibers having a brightness at least 0.5 point higher than a brightness measured for fibers of the same pulp without being contacted with the THP.  
     
     
         4 . The process of  claim 1 , including feeding the aqueous dispersion of wood pulp to a papermachine, and forming paper without loss of opacity relative to paper made with the same wood pulp untreated with the THP salt.  
     
     
         5 . The process of  claim 3  wherein X −  is SO 4   −2  and R is selected from the group consisting of methyl (—CH 2  ) and ethyl (—C 2 H 4 ).  
     
     
         6 . The process of  claim 1 , including feeding the aqueous dispersion of wood pulp as wet pulp to a papermachine, and forming paper having at least the same opacity as that of paper made with the same wood pulp untreated with the THP salt.  
     
     
         7 . The process of  claim 3  including heating the wet pulp to a temperature in the range from 30° C. to 90° C., and wherein the pH is in the range 4 to 9.  
     
     
         8 . A process for brightening deinked wood pulp in a wet pulp of recycle paper comprising, 
 (i) treating the wet pulp with at least enough of a tetrakis(hydroxyalkyl) phosphonium (“THP”) salt represented by the formula                          wherein R represents alkyl having from 1 to 4 carbon atoms; and, 
 X represents a suitable and desirable anion selected from the group consisting of HCO 3   −1 , CO 3   −2 , PO 4   −3 , HPO 4   −1 , HPO 4   −2 , OH −1 , Cl −1 , Br −1 , HSO 3   −1 , SO 3   −2 , HSO 4   −1 , SO 4   −2 , CH 3 CO 2   −1  and CH 3 SO 4   −1 ;  
   to result in a concentration of from 0.05% to 5% THP in the wet pulp;    (ii) maintaining the pulp in contact with the THP for from about 10 to 90 minutes to provide sufficient time for the fibers in the pulp to sorb the THP;    (iii) forming THP-coated fibers; feeding wet pulp containing from about 0.1 to 15% by weight of the THP-coated fibers, to a paper machine, to form a web of fibers of wet wood pulp;    (iv) removing water from the wet wood pulp;    (v) drying the resulting web of paper; and,    (vi) measuring a brightness of the web of paper which brightness is at least 0.5 point higher than a brightness measured for paper of the same pulp without being contacted with the THP.    
     
     
         9 . The process of  claim 8  wherein the THP salt is present in an amount in the range from about 1% to 2.5% by weight of the wet pulp.  
     
     
         10 . The process of  claim 8  wherein the pH of the wet pulp is in the range from neutral to 12, and treated wet pulp provides fibers having a brightness at least 0.5 point higher than a brightness measured for the same pulp without being contacted with the THP.  
     
     
         11 . The process of  claim 8 , including feeding the aqueous dispersion of wood pulp to a papermachine, and forming paper without loss of opacity relative to paper made with the same wood pulp untreated with the THP salt.  
     
     
         12 . The process of  claim 10  wherein X −  is SO 4   −2  and R is selected from the group consisting of methyl (—CH 2  ) and ethyl (—C 2 H 4 ).  
     
     
         13 . The process of  claim 8 , including feeding the aqueous dispersion of wood pulp as wet pulp to a papermachine, and forming paper having at least the same opacity as that of paper made with the same wood pulp untreated with the THP salt.  
     
     
         14 . The process of  claim 12  including heating the wet pulp to a temperature in the range from 30° C. to 90° C., and wherein the pH is in the range 4 to 9.  
     
     
         15 . The process of  claim 13 , wherein the opacity of paper formed from treated fibers of wood pulp is in the range from 1 to 10% higher than the opacity of paper form from fibers of the same wood pulp except that it is untreated with the THP salt.