Method of Producing Kraft Paper and Kraft Paper

Abstract

There is provided a method of producing a kraft paper having a density measured according to ISO 534:2011 of 630-870 kg/m 3 and a strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of 1.0-8.9%, comprising a step of calendering a paper web at a dry content of 55-79%, wherein the line load of the calendering step is 8-90 kN/m, such as 10-70 kN/m, such as 12-50 kN/m, such as 15-40 kN/m. Further a method of producing a porous sack paper as well as a new kraft paper qualities are also provided.

1 . A method of producing a kraft paper having a density measured according to ISO 534:2011 of 630-870 kg/m 3  and a strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of 1.0-8.9%, comprising a step of calendering a paper web at a dry content of 55-79%, wherein the line load of the calendering step is 8-90 kN/m. 
     
     
         2 . The method according to  claim 1 , wherein the density measured according to ISO 534:2011 of the kraft paper is 690-850 kg/m 3 . 
     
     
         3 . The method according to  claim 1 , wherein the strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of the kraft paper is 1.0-2.9% and the bending resistance index in the MD of the kraft paper measured according to ISO 2493-1:2010 is 190-250 Nm6/kg 3 . 
     
     
         4 . The method according to  claim 1 , wherein the paper web is compacted in a Clupak unit, the strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of the kraft paper is 3.0-8.9%, and the bending resistance index measured according to ISO 2493-1:2010 in the MD of the kraft paper is 90-120 Nm 6 /kg 3 . 
     
     
         5 . A method of producing a sack paper having a Gurley value according to ISO 5636-5:2013 of 2-15 s, comprising the steps of compacting a paper web in a Clupak unit and calendering the paper web at a dry content of 55-79%, wherein the line load of the calendering step is 8-90 kN/m. 
     
     
         6 . The method according to  claim 4 , wherein the paper web is compacted in the Clupak unit before the step of calendering. 
     
     
         7 . The method according to  claim 4 , wherein the dry content in the calendering step is 55-75%. 
     
     
         8 . The method according to  claim 4 , wherein the step of calendering is carried out in a drying section of a paper machine, in which drying section the paper web is dried before and after the calendering step. 
     
     
         9 . The method according to  claim 4 , wherein a soft nip calender is used for the calendering step. 
     
     
         10 . The method according to  claim 4 , wherein the Bendtsen roughness measured according to ISO 8791-2 of at least one side of the kraft or sack paper is 300-700 ml/min. 
     
     
         11 . A single-layer kraft paper having:
 a density measured according to ISO 534:2011 of 720-850 kg/m 3 ;   a strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of 1.0-2.9%; and   a bending resistance index measured according to ISO 2493-1:2010 in the MD of 190-250 Nm 6 /kg 3 ,   wherein at least one side of the kraft paper has a Bendtsen roughness measured according to ISO 8791-2 of 300-700 ml/min.   
     
     
         12 . The kraft paper according to  claim 11 , wherein the density measured according to ISO 534:2011 is 730-830 kg/m 3 . 
     
     
         13 . The kraft paper according to  claim 11 , wherein the grammage measured according to ISO 536:2012 is 65-100g/m 2 . 
     
     
         14 . A single-layer kraft paper having:
 a density measured according to ISO 534:2011 of 735-835 kg/m 3 ;   a strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of 3.0-4.5%; and   a bending resistance index measured according to ISO 2493-1:2010 in the MD of 118-158 Nm 6 /kg 3 ,   wherein at least one side of the kraft paper has a Bendtsen roughness measured according to ISO 8791-2 of 250-700 ml/min.   
     
     
         15 . The kraft paper according to  claim 14 , wherein the grammage measured according to ISO 536:2012 is 50-140 g/m 2 . 
     
     
         16 . The method according to  claim 2 , wherein the strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of the kraft paper is 1.0-2.9% and the bending resistance index in the MD of the kraft paper measured according to ISO 2493-1:2010 is 190-250 Nm 6 /kg 3 . 
     
     
         17 . The method according to  claim 2 , wherein the paper web is compacted in a Clupak unit, the strain at break measured according to SS-ISO 1924-3:2011 in the machine direction (MD) of the kraft paper is 3.0-8.9%, and the bending resistance index measured according to ISO 2493-1:2010 in the MD of the kraft paper is 90-120 Nm 6 /kg 3 . 
     
     
         18 . The method according to  claim 1 , wherein the dry content in the calendering step is 55-75%. 
     
     
         19 . The method according to  claim 1 , wherein the step of calendering is carried out in a drying section of a paper machine, in which drying section the paper web is dried before and after the calendering step. 
     
     
         20 . The method according to  claim 1 , wherein a soft nip calender is used for the calendering step. 
     
     
         21 . The method according to  claim 1 , wherein a soft nip calender is used for the calendering step. 
     
     
         22 . The method according to  claim 1 , wherein the Bendtsen roughness measured according to ISO 8791-2 of at least one side of the kraft or sack paper is 300-700 ml/min.