Production of paper that is highly stretchable in the cross direction

Abstract

There is provided a method of producing a paper having a grammage according to ISO 536 of 50-250 g/m2, a Gurley value according to ISO 5636-5 of above 15 s and a stretchability according to ISO 1924-3 in the cross direction of at least 8%, said method comprising the steps of: a) providing a pulp, preferably sulphate pulp; b) subjecting the pulp to refining; c) diluting the pulp from step b) and discharging the diluted pulp at a discharge rate to a forming wire to form a paper web, wherein the speed of the forming wire is at least 7 m/min higher or at least 7 m/min lower than the discharge rate; d) pressing the paper web from step c); e) drying the paper web from step d), which drying comprises a step of compacting the paper web in a Clupak unit.

1 . Method of producing a paper having a grammage according to ISO 536 of 50-250 g/m 2 , a Gurley value according to ISO 5636-5 of above 15 s and a stretchability according to ISO 1924-3 in the cross direction of at least 8%, said method comprising the steps of:
 a) providing a pulp, preferably sulphate pulp;   b) subjecting the pulp to refining;   c) diluting the pulp from step b) and discharging the diluted pulp at a discharge rate to a forming wire to form a paper web, wherein the speed of the forming wire is at least 7 m/min higher or at least 7 m/min lower than the discharge rate;   d) pressing the paper web from step c);   e) drying the paper web from step d), which drying comprises a step of compacting the paper web in a Clupak unit.   
     
     
         2 . The method of  claim 1 , wherein the speed of the forming wire is 8-m/min higher or 8-25 m/min lower than the discharge rate. 
     
     
         3 . The method of  claim 1  or  2 , wherein the diluted pulp is discharged through a discharge gap of at least 40 mm, such as at least 50 mm, such as 50-70 mm. 
     
     
         4 . The method of any one of the preceding claims, wherein the paper has a stretchability according to ISO 1924-3 in the cross direction of at least 9%, such as at least 9.5%. 
     
     
         5 . The method of any one of the preceding claims, wherein the paper has a stretchability according to ISO 1924-3 in the machine direction of at least 9%, such as at least 10%, such as at least 11%. 
     
     
         6 . The method of any one of the preceding claims, wherein step b) comprises subjecting the pulp to high consistency (HC) refining. 
     
     
         7 . The method of  claim 6 , wherein the consistency of the pulp subjected to HC refining is at least 33%, such as at least 37%, such as at least 38%. 
     
     
         8 . The method of any one of the preceding claims, wherein step b) comprises subjecting the pulp to low consistency (LC) refining. 
     
     
         9 . The method of  claim 8 , wherein the energy supply in the LC refining is 20-200 kWh per ton paper, such as 30-200 kWh per ton paper, such as 40-200 kWh per ton paper. 
     
     
         10 . The method of any one of the preceding claims, wherein the paper web is compacted in the Clupak unit at a moisture content of 20-48%, such as 30-45%, such as 35-45%. 
     
     
         11 . The method of any one of the preceding claims, wherein the line load of the nip bar in the Clupak unit is at least 22 kN/m, such as at least 25 kN/m, such as at least 28 kN/m. 
     
     
         12 . The method of any one of the preceding claims, wherein the grammage according to ISO 536 of the paper is 60-220 g/m 2 , such as 80-200 g/m 2 . 
     
     
         13 . The method of any one of the preceding claims, wherein the Gurley value according to ISO 5636-5 of the paper is at least 20 s, preferably at least 25 s, more preferably at least 35 s. 
     
     
         14 . The method of any one of the preceding claims, wherein step e) comprises drying the paper web from the Clupak unit in a drying group and the speed of the paper web in the dryer group is 8-14% lower than the speed of the paper web entering the Clupak unit. 
     
     
         15 . The method of any one of the preceding claims, wherein the Bendtsen roughness according to ISO 8791-2 of at least one side of the paper is 1900 ml/min or lower, such as 1700 ml/min or lower, such as 1500 ml/min or lower. 
     
     
         16 . Method according to  claim 1 , wherein:
 step b comprises the substeps of
 b1) subjecting the pulp to high consistency (HC) refining at a consistency of 33-42% to the extent that the pulp obtains a Schopper-Riegler (SR) number measured according to ISO 5267-1 of 13-19 and 
 b2) subjecting the pulp from step b1) to low consistency (LC) refining at a consistency of 2-6% to the extent that the pulp obtains a Schopper-Riegler (SR) number measured according to ISO 5267-1 of 18-40; and 
   the speed of the forming wire is 7-30 m/min higher or 7-30 m/min lower than the discharge rate in step c).