Kraft paper

Abstract

The invention relates to a kraft paper with an elongation at break in the machine direction of at least 7.5% and an elongation at break in the transverse direction of at least 7.5%, wherein the elongation at break in the machine direction and the elongation at break in the transverse direction differ by less than 2.5%, in particular by less than 1.5%, and the bending stiffness in the machine direction and the bending stiffness in the transverse direction equal at least 50 mN. The invention additionally relates to a packaging material, to a method for producing a packaging material, and to a method for producing a kraft paper.

1 . A kraft paper with an elongation at break in the machine direction A MD  of at least 7.5% and an elongation at break in the transverse direction A CD  of at least 7.5%, and the bending stiffness in the machine direction S b(MD)  and the bending stiffness in the transverse direction S b(CD)  equal at least 50 mN, wherein the bending stiffness is determined according to ISO 2493-1:2010, characterised in that the elongation at break in the machine direction and the elongation at break in the transverse direction differ by less than 2.5%, in particular by less than 1.5%, wherein the elongation at break is determined according to ISO 1924-3:2005. 
     
     
         2 . The kraft paper according to  claim 1 , wherein the bending stiffness index in the machine direction is between 60 and 90 Nm 6  kg −3 , and in that the bending stiffness index in the transverse direction is between 50 and 80 Nm 6  kg −3 . 
     
     
         3 . The kraft paper according to  claim 1 , wherein the elongation at break in the machine direction A MD  and/or the elongation at break in the transverse direction A CD  is between 7.5% and 13%, preferably between 8% and 12%, more preferably between 9% and 11%. 
     
     
         4 . The kraft paper according to  claim 1 , wherein it has a grammage of at least 90 g/m 2 , in particular between 95 g/m 2  and 140 g/m 2 , wherein the grammage is determined according to ISO 536:2019. 
     
     
         5 . The kraft paper according to  claim 1 , wherein it has an air resistance of at least 16 s Gurley, wherein the air resistance is determined according to ISO 5636-5:2013. 
     
     
         6 . The kraft paper according to  claim 1 , wherein the kraft paper includes kraft pulp formed entirely from fresh fibre, in particular entirely from fresh fibre from softwood, and/or in that the kraft paper includes kraft pulp which has a kappa number between 30 and 50, in particular between 38 and 47, wherein the kappa number is determined according to ISO 302:2015. 
     
     
         7 . The kraft paper according to  claim 1 , wherein the kraft paper includes cationic bulk starch in a content of between 1.2 wt. % and 1.6 wt. % relative to the dry weight of the kraft paper, and/or in that the kraft paper includes a sizing agent, in particular at least one alkenyl succinic anhydride, in a content of between 0.06 wt. % and 0.09 wt.-% relative to the dry weight of the kraft paper. 
     
     
         8 . The kraft paper according to  claim 1 , wherein the kraft paper is filler-free. 
     
     
         9 . The kraft paper according to  claim 1 , wherein the kraft paper has a residual moisture content of between 6 wt. % and 11 wt. %, relative to the total weight of the kraft paper. 
     
     
         10 . A packaging material comprising or consisting of kraft paper according to  claim 1 , wherein the kraft paper has a nub structure produced by forming, in particular deep drawing, embossing or pressing, the kraft paper. 
     
     
         11 . A method for producing a packaging material comprising or consisting of kraft paper according to  claim 1 , wherein a nub structure is produced in the kraft paper by forming, in particular by deep drawing, embossing or pressing, the kraft paper. 
     
     
         12 . The method for producing kraft paper, in particular kraft paper according to  claim 1 , comprising the following steps:
 a. milling of kraft pulp,   b. optionally mixing the milled kraft pulp with one or more additives,   c. applying the milled kraft pulp, optionally mixed with one or more additives, to the wire of a paper machine and pressing to obtain a paper web,   d. pre-drying of the paper web obtained in step (c),   e. creping of the pre-dried paper web obtained in step (d), in particular in a Clupak unit,   f. post-drying of the creped paper web obtained in step (e),   wherein step (a) comprises high consistency milling, wherein the stock consistency in the high consistency milling is between 30% and 35%, and   wherein the percentage speed difference of the paper web in steps (d) and (f) is between 6% and 10%, in particular between 7.5% and 8.5%, wherein the speed of the paper web in step (d) is higher than in step (f),   wherein the ratio between stock consistency during high consistency milling and percentage speed difference is set such that the elongation at break in the machine direction and the elongation at break in the transverse direction of the kraft paper differ by less than 2.5%, in particular by less than 1.5%, wherein the elongation at break is determined according to ISO 1924-3:2005, and that the post-drying in step (f) is carried out by convection drying, in particular at a temperature of at least 170° C.   
     
     
         13 . The method according to  claim 12 , wherein the milling capacity during high consistency milling is between 150 kWh/t and 320 kWh/t, in particular between 220 kWh/t and 270 kWh/t. 
     
     
         14 . The Method according to  claim 12 , wherein step (a) comprises, following the high consistency milling, a low consistency milling in which the stock consistency is between 4% and 6%. 
     
     
         15 . The method according to  claim 14 , wherein the milling capacity during low consistency milling is between 90 kWh/t and 160 kWh/t, in particular between 110 kWh/t and 140 kWh/t. 
     
     
         16 . The method according to  claim 12 , wherein the pH value in step (b) is set to 6.2 to 7.2, in particular 6.6 to 7.0. 
     
     
         17 . The method according to  claim 12 , wherein the pre-drying in step (d) is carried out by contact drying.