Transparent paper

Abstract

Transparent paper consisting of a coated or chemically treated kraft paper, wherein the kraft paper is formed from a bleached, low-consistency and possibly high-consistency pulp beaten with a beating degree of at least 38° SR, which is formed to an extent of at least 30%, and at most 60% of softwood pulp and to an extent of at least 35%, of hardwood pulp, and optionally further fibrous materials, 0.5 to 2.5% cationic starch, processing aids, 0.1 to 0.4% sizing agent and optionally up to 4% fillers, and wherein the kraft paper is mechanically surface-treated with further substances selected from PEG, glycerol, polyvinyl alcohol or mixtures of two or more thereof, and starch, as well as methods for producing transparent paper and forming a kraft pulp.

1 . A transparent paper consisting of a coated or chemically treated kraft paper, wherein the kraft paper is made of a bleached, low-consistency and optionally high-consistency pulp beaten with a beating degree of at least 38° SR, which is made up of at least 30%, and at most 60% softwood pulp and at least 35%, hardwood pulp and other fibre materials, 0.5 to 2.5% cationic starch, processing aids, 0.1 to 0.4% sizing agent and up to 4% fillers and the kraft paper is mixed with other substances selected from PEG, glycerol, polyvinyl alcohol or mixtures of two or more thereof and starch and is mechanically surface treated. 
     
     
         2 . The transparent paper according to  claim 1 , wherein it contains 20% to 30% of further fibre materials selected from the group consisting of grass, hemp, jute, linen, cotton, unbleached fibre material, eucalyptus, waste paper, recycled paper or viscose fibres. 
     
     
         3 . The transparent paper according to  claim 2 , wherein it contains as further fibre materials exclusively unbleached fibre material from softwood pulp. 
     
     
         4 . The transparent paper according to  claim 1 , wherein it contains up to 3 g/m 2 , surface starch. 
     
     
         5 . The transparent paper according to  claim 1 , wherein a sheet is surface-coated with a PEG surface treatment agent, and the surface-coated sheet has a basis weight between 57 g/m 2  and 150 g/m 2 . 
     
     
         6 . The transparent paper according to  claim 1 , wherein it is additionally calendered. 
     
     
         7 . A method for producing transparent paper in which a kraft paper is subjected to a mechanical or chemical surface treatment by a paper machine comprising the following steps:
 a) forming the kraft pulp containing at least 30%, to a maximum of 60% bleached softwood pulp and at least 35%, bleached hardwood pulp and further fibre materials,   b) subjecting the kraft pulp to low-consistency beating or a high-consistency beating until a beating degree of at least 38° SR, until at least 46° SR is achieved,   c) adding cationic starch, 0.1 to 0.4% sizing agent and up to 4.0% fillers,   d) sheet formation on a screen of a paper machine,   e) pressing and drying to a dry content of between 30% and 80%,   f) and coating the formed sheet at least once with up to 3 g/m 2  surface starch,   g) calendering the formed sheet,   h) treating its surfaces with a surface treatment agent other than starch, and   i) drying the formed transparent paper, with the proviso that if at least one of steps f) or g) is applied in addition to step h), steps f), g) and h) are applied once or multiple times in any order.   
     
     
         8 . The method according to  claim 7 , wherein to produce a formed sheet with a basis weight between 50 and 120 g/m 2  the paper machine is operated at a speed of >650 to 1000 m/min. 
     
     
         9 . The method according to  claim 8 , wherein the formed sheet is subjected to a calendering step with a linear load between 40 and 500 kN/m. 
     
     
         10 . The method according to  claim 7 , wherein a surface treatment with PEG, with 7 to 30 g/m 2 , is carried out before or after calendering, and that a surface-coated sheet with a basis weight between 57 g/m 2  and 150 g/m g/m 2  is formed. 
     
     
         11 . The method according to  claim 7 , wherein the step of surface treatment with a surface treatment agent other than starch is carried out either in the paper machine or in a separate coating unit. 
     
     
         12 . The method according to  claim 7 , wherein the step of surface treatment with a surface treatment agent other than starch takes place simultaneously with the coating with surface starch. 
     
     
         13 . The method according to  claim 7 , wherein the method is carried out with a maximum steam input per hour in the range between 30 and 40 t/h.