Method for making a low density multi-ply paperboard with high internal bond strength

Abstract

Methods for improving the internal bond strength of paperboard with greater than 25 percent crosslinked fiber in at least one ply are described. In the methods, additives are added to the slurry in various combinations and order while maintaining the ionic demand of the slurry at less than zero. Paperboard with high ZDT, Scott Bond and Taber Stiffness is obtained.

1 . A method for forming at least one ply of a paperboard comprising the steps of:
 forming a slurry of cellulose fibers comprising crosslinked fibers;   adding mechanically refined fiber;   adding an anionic starch subsequent to adding said mechanically refined fiber;   adding a cationic fixative subsequent to adding said anionic starch;   wherein, after each addition step, the slurry ionic demand is less than zero;   depositing said slurry on a foraminous support;   forming a fibrous web layer by withdrawing liquid from said slurry;   drying said web to form a paperboard.   
     
     
         2 . The method of  claim 1  wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard. 
     
     
         3 . The method of  claim 1  wherein the total starch level is 50 to 120 lb/t. 
     
     
         4 . The method of  claim 1  wherein the mechanically refined fiber has a CSF of less than 125 CSF, a curl index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber. 
     
     
         5 . The method of  claim 1  wherein the cationic fixative has an anionic demand of greater than zero but less than 1 meq/g. 
     
     
         6 . The method of  claim 1  wherein the cationic fixative has an anionic demand of from 1 meq/g to 10 meq/g. 
     
     
         7 . The method of  claim 1 , wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers. 
     
     
         8 . The method of  claim 1 , wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers. 
     
     
         9 . A method for forming a paperboard comprising the steps of:
 forming a slurry of cellulose fibers comprising crosslinked fibers;   adding mechanically refined fiber,   adding a cationic fixative and mixing with said slurry;   adding an anionic starch subsequent to adding said cationic fixative;   wherein, after each addition step, the slurry ionic demand is less than zero;   depositing said slurry on a foraminous support;   forming a fibrous web layer by withdrawing liquid from said slurry;   drying said web to form a paperboard.   
     
     
         10 . The method of  claim 9  wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard. 
     
     
         11 . The method of  claim 9  wherein the total starch level is 50 to 120 lb/t. 
     
     
         12 . The method of  claim 9  wherein the mechanically refined fiber has a CSF of less than 125 CSF, a index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber. 
     
     
         13 . The method of  claim 9  wherein the cationic fixative has an anionic demand of greater than zero but less than 1 meq/g. 
     
     
         14 . The method of  claim 9  wherein the cationic fixative has an anionic demand of from 1 meq/g to about 10 meq/g. 
     
     
         15 . The method of  claim 9 , wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers. 
     
     
         16 . The method of  claim 9 , wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers. 
     
     
         17 . A method for forming at least one ply of a paperboard comprising the steps of:
 forming a slurry of cellulose fibers comprising crosslinked fibers;   adding mechanically refined fiber;   adding an anionic starch subsequent to adding said mechanically refined fiber;   adding a first cationic fixative subsequent to adding said anionic starch;   adding a second cationic fixative subsequent to adding said first cationic fixative;   wherein, after each addition step, the slurry ionic demand is less than zero;   depositing said slurry on a foraminous support;   forming a fibrous web layer by withdrawing liquid from said slurry;   drying said web to form a paperboard.   
     
     
         18 . The method of  claim 17  wherein said crosslinked fibers are present at a level from 25 to 80 percent of the total fiber weight in at least one ply of said paperboard. 
     
     
         19 . The method of  claim 17  wherein the total starch level is 50 to 120 lb/t. 
     
     
         20 . The method of  claim 17  wherein the mechanically refined fiber has a CSF of less than 125 CSF, a curl index of ⅓ or less of the unrefined fiber and a kink angle of ½ or less of the unrefined fiber. 
     
     
         21 . The method of  claim 17  wherein the first cationic fixative has an anionic demand of from 1 meq/g to 10 meq/g. 
     
     
         22 . The method of  claim 17  wherein the second cationic fixative has an anionic demand of greater than zero but less than 1 meq/g. 
     
     
         23 . The method of  claim 17 , wherein said paperboard is at least a two-ply board, said at least one ply containing said crosslinked fibers. 
     
     
         24 . The method of  claim 17 , wherein said paperboard is at least a three-ply board, said at least one ply containing said crosslinked fibers.