Supercalendered kraft paper comprising recycled fiber

Abstract

The invention relates to a supercalendered Kraft paper production for a release liner, which contains both non-recycled bleached chemical pulps and recycled pulp produced from release liner supercalendered Kraft paper. When highly specific raw material is used for recycling, the characteristics of the recycled pulp may be adjusted already upon recycling. The recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner may be used without further refining for manufacturing supercalendered Kraft paper. Excessive refining can thus be avoided and the compatibility of the recycled pulp may be optimized for supercalendered Kraft paper production. The non-recycled pulps may be refined less, as well. This leads to positive effects in supercalendered Kraft paper manufacturing process, such as improved dewatering and energy efficient drying. The produced paper demonstrates reduced shrinkage and improved dimensional stability, while maintaining sufficient quality for use as a substrate for a release liner. A supercalendered Kraft paper with improved sustainability is produced.

1 . A method for manufacturing supercalendered Kraft paper suitable for use as a substrate of a release liner, the method comprising
 mixing fibers from
 recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner, 
 non-recycled bleached chemical pulp produced from hardwood, and 
 non-recycled bleached chemical pulp produced from softwood, 
   
       such that a stock is obtained,
 forming a paper web of the stock on a paper machine, 
 reducing moisture content of the paper web in a press section, 
 drying the paper web in a drying section, thereby forming paper, and 
 calendering the paper, thereby forming supercalendered Kraft paper, 
 
       the supercalendered Kraft paper having
 a density equal to or higher than 1030 g/m 3 , when determined according to ISO 534, 
 a transparency equal to or higher than 36%, when determined according to ISO 2469, 
 a CIE whiteness, wherein
 L* is in the range of 92 to 98, 
 a* is in the range of −4 to +2, and 
 b* is in the range of +5 to +11, 
 
 when determined according to ISO 11475:2017(en), and comprising the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner in an amount equal to or higher than 5 wt. %, when determined as dry matter content according to SCAN-P 39:80 and wherein the pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner contains silicon in an amount in the range of 0.01 to 0.3 wt. %, determinable as dry matter content from a paper specimen which is combusted at 900° C. with an Energy Dispersive X-ray Spectroscopy, in accordance with Tappi standard T 413. 
 
     
     
         2 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner, prior to the mixing, has a Schopper-Riegler number equal to or higher than 25, when determined according to ISO 5267-1. 
     
     
         3 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner contains particles derived from the recycled pulp having a length less than 200 micrometers in an amount equal to or higher than 10%, of the total amount of fibers in the recycled pulp, when determined as length weighted average fiber length by automated optical analysis using unpolarized light according to ISO 16065-2:2014. 
     
     
         4 . The method according to  claim 1 , wherein fibers of the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner have an average fiber width of less than 25 micrometers, when determined by automated optical analysis using unpolarized light according to ISO 16065-2:2014. 
     
     
         5 . The method according to  claim 1 , wherein the non-recycled bleached chemical pulp produced from softwood, prior to the mixing, has a Schopper-Riegler number equal to or less than 50, when determined according to ISO 5267-1. 
     
     
         6 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner has a pH which is in the range of 6.0 to 9.1, when determined from aqueous pulp extracts by means of a pH meter according to the standard ISO 6588-2 (2020). 
     
     
         7 . The method according to  claim 1 , wherein the stock at a machine chest of a paper machine has a water retention value which is in the range of 1.5 to 1.9 g/g, determinable according to ISO 23714:2014 from a sample having a dry matter content of 1 gram. 
     
     
         8 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner has been prepared of supercalendered Kraft paper having a CIE whiteness, wherein
 L* is in the range of 92 to 98,   a* is in the range of −4 to +2, and   b* is in the range of +5 to +11,   
       when determined according to ISO 11475:2017(en). 
     
     
         9 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner has not been bleached. 
     
     
         10 . The method according to  claim 1 , the supercalendered Kraft paper comprising the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner in an amount equal to or higher than 10 wt. %, when determined as dry matter content according to SCAN-P 39:80. 
     
     
         11 . The method according to  claim 1 , the supercalendered Kraft paper comprising the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner in the range of 5 to 50 wt. %, when determined as dry matter content according to SCAN-P 39:80. 
     
     
         12 . The method according to  claim 1 , the supercalendered Kraft paper comprising non-recycled bleached chemical pulp produced from softwood in an amount equal to or higher than 10 wt. %, when determined as dry matter content according to SCAN-P 39:80. 
     
     
         13 . The method according to  claim 1 , comprising mixing fibers from recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner without further refining. 
     
     
         14 . The method according to  claim 1 , wherein the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner has not been bleached. 
     
     
         15 . A supercalendered Kraft paper suitable for use as a substrate of a release liner, the supercalendered Kraft paper comprising fibers from
 non-recycled bleached chemical pulp produced from hardwood,   non-recycled bleached chemical pulp produced from softwood, and   recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner,   
       the supercalendered Kraft paper having
 a density equal to or higher than 1.030 g/cm 3 , when determined according to ISO 534, 
 a transparency equal to or higher than 36%, when determined according to ISO 2469, 
 a CIE whiteness, wherein
 L* is in the range of 92 to 98, 
 a* is in the range of −4 to +2, and 
 b* is in the range of +5 to +11, 
 
 
       , when determined according to ISO 11475:2017(en), and
 comprising the recycled pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner in an amount equal to or higher than 5 wt. %, when determined as dry matter content according to SCAN-P 39:80 and wherein the pulp obtained from supercalendered Kraft paper that has been used as a substrate of a release liner contains silicon in an amount in the range of 0.01 to 0.3 wt. %, determinable as dry matter content from a paper specimen which is combusted at 900° C. with an Energy Dispersive X-ray Spectroscopy, in accordance with Tappi standard T 413. 
 
     
     
         16 . A release liner comprising a supercalendered Kraft paper according to  claim 15  and a release coating.