AT525436B1 - TRANSPARENT PAPER - Google Patents

Abstract

Transparent paper consisting of a coated or chemically treated kraft paper, wherein the kraft paper consists of a bleached low-consistency and optionally high-consistency pulp ground with a freeness of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR, of which at least 30% preferably at least 35%, particularly preferably at least 40% and at most 60% of softwood pulp and at least 35%, preferably at least 40%, particularly preferably at least 45% of hardwood pulp and optionally other fibrous materials, 0.5 to 2.5 % cationic starch, process auxiliaries, 0.1 to 0.4% sizing agent and optionally up to 4% fillers and wherein the kraft paper is formed with other substances selected from PEG, glycerol, polyvinyl alcohol or mixtures of two or more thereof and optionally starch and optionally is mechanically surface treated and methods of making tracing paper and forming a kraft pulp.

Description

Description

The present invention relates to a tracing paper consisting of a coated or chemically treated kraft paper and to a method for producing tracing paper in which a kraft paper is subjected to chemical surface treatment and/or coating surface treatment.

Transparent papers or translucent papers have been produced for many years to serve, for example, as viewing windows in envelopes or boxes, food packaging, for a wide variety of graphic applications such as tracing paper, photocopy paper or the like. In addition to special types of transparent paper, such as glassine paper, which is only available with a low grammage of up to around 50 g/m? are producible and, moreover, only have a very limited strength, there is also a large number of papers in which a conventional paper is made translucent by means of special processes, which processes include both physical methods and chemical processes or mixtures thereof. Particularly when chemicals, particularly synthetic resins such as acrylics, polyesters and/or urethane resins, are used in such processes with which the paper is to be rendered translucent, it is usually necessary, in addition to coating the paper with the chemicals per se, to render the paper translucent to crosslink or harden the material to be made after its application by applying heat or UV rays or other crosslinking methods. Such a method, with which a transparent area can be formed in paper, is described, for example, in US Pat and thereby a transparent area is formed in the paper.

A similar method is described in US 6,902,770, in which a transparentizing material, after heating the same to facilitate and accelerate the penetration of the material inside the paper, is applied to a surface of the paper, whereby a transparent area can be formed in the paper.

Furthermore, oils, waxes or the like, in particular silicone oils or resins, can be used as additives to paper in a known manner in order to be able to achieve a certain transparency of the paper. Materials of this type, which are intended to make the paper translucent, are usually ground extremely finely together with the cellulose fibers and grease-resistant and slightly transparent papers are produced from this mixture.

[0005] Finally, there are also methods in which only individual areas of a paper can be made translucent or transparent. Such a method, in which a translucent area can be formed directly in the paper to be finished, is described, for example, in WO 2008/091523, in which that area of the paper that is to be made translucent is perforated by means of laser beams in order to form a network of micro-small, closely spaced holes to form a transparent area of the paper.

All of these methods are either very complex, can only be produced using large amounts of energy or use not inconsiderable amounts of chemicals, which makes these papers only very conditionally usable, especially if, for ecological reasons, a completely recyclable packaging material is to be provided . Also, some of them may not be used or only to a very limited extent in the food sector or in the area of sensitive substances, such as substances sensitive to solvents.

Another problem with such tracing papers or areas of paper that have been made transparent is that they are often brittle and, above all, have only low strength, which is particularly the case in applications in which types of paper with high

Strengths are required, such as packaging for products with a heavier weight or irregular surfaces with edges or corners and the like the strength of the classic transparent papers was not sufficient even for such a use.

Up until now, it has not been possible to make papers with high strength sufficiently translucent using conventional methods without reducing their strength to such an extent, either by perforating by means of a laser or by extremely heavy grinding and/or extremely heavy calendering of the cellulose fibers used, that such methods for the Production of transparent paper for large-scale technical use, in particular for the packaging of products that have irregular or sharp-edged surfaces and/or are heavier, such as electronic components, tokens, building materials or food such as rice, nuts or the like. are not suitable. Packaging papers for such objects or materials are usually made from kraft paper, which has a particularly high strength but is not transparent even when thin papers with a low basis weight are used.

The present invention now aims to provide a transparent paper which has high strength, in particular strength properties corresponding essentially to kraft paper, and which has only biodegradable substances and no environmentally harmful chemicals in or on the paper.

To solve this problem, the transparent paper according to the invention is essentially characterized in that the kraft paper consists of a bleached, low-consistency and optionally high-consistency pulp that has been ground to a freeness of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR , which is at least 30%, preferably at least 35%, particularly preferably at least 40% and at most 60% softwood pulp and at least 35%, preferably at least 40%, particularly preferably at least 45% hardwood pulp and optionally other 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 that the kraft paper is formed with other substances selected from PEG (polyethylene glycol), glycerol, polyvinyl alcohol or mixtures of two or more of these and optionally starch, such as mixtures of PEG and starch, and optionally mechanically surface-treated. By forming the kraft paper from a bleached pulp, which pulp is formed from at least 30% softwood pulp and at least 35% hardwood pulp, it is possible to provide a paper whose mechanical strength is sufficiently high for use in the packaging sector and which, on the other hand, has sufficient formation and transparency. Due to the fact that small amounts of processing aids and 0.1 to 0.4% sizing agent and optionally other fillers are also added to the kraft paper, it is possible to provide a paper that is suitable for use in the food sector on the one hand and an optimal compromise between mechanical strength on the other and provide processability. The 0.5% to 2.5% cationic starch also contained in the paper further improves the strength of the paper, so that, surprisingly, the use of hardwood pulp in a kraft paper makes it possible to give the kraft paper transparency and high strength at the same time. With regard to the amounts of starch used, it is known to the person skilled in the art that bleached cellulose or bleached cellulose fibers only allow the use of small amounts of starch. On the other hand, pulps produced by kraft processes per se allow the use of relatively high amounts of starch. Surprisingly, despite the cationic starch contained in the kraft paper, together with the materials used for the surface treatment, it is possible to provide a translucent paper with both excellent surface and mechanical properties. This is particularly surprising since the starch as such has a rather disadvantageous or at most neutral effect in relation to the transparency of a paper and the selected combination of two different pulps, starch and surface

Surprisingly, chemical treatment agents allow the paper to be given sufficient strength and transparency at the same time. According to the invention, the procedure in this connection is such that the kraft paper is surface-treated with other substances or mixtures of other substances or mixtures of starch and other substances and optionally mechanically in addition to the cationic starch contained in the paper. Surprisingly, the surface treatment with the help of other substances or mixtures of other substances and optionally additional starch and an optional mechanical surface treatment, in particular pressing, not only further improves the strength and printability of the kraft paper, but also further improves the transparency of the paper .

Because the filler content is kept below 4% and preferably no fillers are contained in the paper, it is possible to further improve the transparency of the same.

A tracing paper according to the present invention is not understood to mean a completely clear and transparent paper, but a paper with an opacity as defined in ISO standard No.: 2471:2008. Accordingly, opacity is understood to mean the degree of turbidity of the translucent paper or tracing paper according to the invention.

In connection with the present application, mechanical surface treatment means pressing in the press section of a paper machine with one or more presses with the same or different pressure or at least one shoe press or combinations of one or more presses and one or two shoe presses in one understood in any order.

By, as corresponds to a development of the present invention, the paper is designed so that there are 20% to 30% other fibers selected from the group grass, hemp, jute, linen, cotton, unbleached fiber material, eucalyptus, waste paper , recycled paper, viscose fibers or the like, it is possible to provide a transparent paper which, on the one hand, has even greater strength and, on the other hand, has an extremely low CO »2 footprint of less than 450 kg CO »/t paper in production. The other fibrous materials can be added as colored fibrous materials, bleached or unbleached fibrous materials, in particular fibrous materials originating directly from a production or a recycling process. As this corresponds to a development of the present invention, the transparent paper is designed in such a way that only unbleached fiber material from softwood pulp, in particular 20 to 30% unbleached fiber material from softwood pulp, is contained as further fibers, it is possible to achieve a CO2 footprint that only has one CO»2 emissions of less than 350 kg CO»/t paper. At the same time, especially if only up to about 30% unbleached fiber material from softwood pulp is added as additional fibers, the strength of the paper can be further increased compared to a transparent paper consisting purely of bleached pulp(s) and in particular compared to conventionally produced transparent papers.

By, as corresponds to a further development of the invention, the transparent paper is developed in such a way that it is coated with a chemical surface coating agent that differs from strength, such as PEG, in particular 7 to 30 g/m? PEG is surface coated and that the surface coated sheet has a basis weight between 57 g/m? and 160 g/m*, it is possible to provide surprisingly strong and nevertheless translucent papers which, due to their excellent resistance to tear propagation, are suitable for use as packaging in the food sector or for objects with irregular or even sharp-edged surfaces.

By additionally calendering the tracing paper, as corresponds to a development of the invention, it is possible on the one hand to reduce the thickness of the paper, to reduce its surface roughness and (air) permeability, which on the one hand improves the printability of the same and on the other hand, surprisingly, the transparency of the paper even further

improved.

The present invention further aims to provide a method for producing the tracing paper with which it is possible to provide a tracing paper having excellent mechanical properties.

To solve this problem, the method according to the invention is essentially characterized in that it has the following steps:

a) formation of a kraft pulp containing at least 30%, preferably at least 35%, particularly preferably 40% to at most 60% bleached softwood pulp and at least 35%, preferably at least 40%, particularly preferably at least 45% bleached hardwood pulp and optionally other fibers,

b) subjecting the pulp to low-consistency beating and optionally high-consistency beating until a degree of beating of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR is achieved,

c) addition of processing aids, in particular from 0.5 to 2.5% cationic starch, 0.1 to 0.4% sizing agent and optionally up to 4.0% fillers,

d) sheet formation on a wire of a paper machine, e) pressing and drying to a solids content between 30% and 80%,

f) and optionally at least one coating of the sheet formed with up to 3 g / m? surface thickness,

9) optionally calendering the sheet,

h) surface treatment with a surface treatment agent other than starch, and

i) drying of the transparent paper,

with the proviso that when in addition to step h) at least one of steps f) or g) is applied, steps f), g) and h) are applied in any order, one or more times.

By conducting the process in such a way that a kraft pulp is produced which contains both bleached softwood pulp and bleached hardwood pulp, it is possible to lay the basis for a paper which on the one hand is translucent or transparent and on the other hand has excellent mechanical properties . By subjecting such a pulp to low-consistency refining and, if necessary, high-consistency refining as a next step, until a freeness of at least 38 °SR is reached, the mechanical properties of the paper are further improved on the one hand and a freeness is selected at which rapid dewatering is ensured on the other hand so that the energy costs for the subsequent drying do not become excessively high and at the same time the machine speed can be kept at values over 700 m/min. Moreover, with such a process management, the water retention capacity of the pulp as such is not excessively increased by the presence of relatively small amounts of fines, so that overall good dewatering of the pulp can be achieved. Furthermore, with such a combination of a kraft pulp containing both softwood and hardwood and the subsequent moderate beating to a freeness of at least 38 °SR, a starting material can surprisingly be provided which, by means of the following moderate treatment steps, in particular a surface treatment with one of starch various surface treatment agents, and if necessary calendering, results in a kraft paper which, both due to the refining and the subsequent treatment, has essentially no fiber-air-fiber interfaces and is therefore transparent on the one hand and has high strength and very good tear propagation resistance on the other. By combining the use of bleached softwood pulp and hardwood pulp, both of which are produced using the kraft process, it is surprisingly possible to

wise to introduce relatively high bulk starch amounts of 0.5 to 2.5% into the pulp without adversely affecting the transparency of the paper. This is because, as is known to a person skilled in the art, comparatively high amounts of starch can be introduced into a kraft pulp, but the presence of bleached cellulose fibers is rather disadvantageous for such higher amounts of starch, since the starch is then discharged into the white water in the course of the process becomes. Surprisingly, the mixture of two different celluloses selected in the present process makes it possible to introduce large amounts of starch into the pulp, in order to subsequently provide a transparent paper with surprisingly good mechanical properties. After pressing at least once the formed sheet with up to 3 g/m? Surface thickness is coated and optionally also calendered, it is possible to provide a transparent paper which, in addition to the good mechanical properties of kraft paper, also has excellent printability and surface smoothness, with a Bendtsen roughness on the smooth side of between 100 and 150 ml/min. Such a paper has a tensile strength according to ISO 1924-3:2005 in machine direction between 6.0 and 8.5 kN/m and in cross direction between 2.7 and 4.5 kN/m, especially a tear propagation resistance according to ISO 1974: 2012 in the machine direction of at least 400 mN and in the transverse direction of at least 600 mN and a Gurley air permeability according to ISO 5636-5:2013 of >95 s. The achievable opacity according to ISO 2471:2008 is in the range of <80%, preferably <70 %.

This is all the more surprising as the tracing papers or glassine papers known in the art are not or hardly mechanically resilient, have almost no tear propagation resistance and can therefore not be used for packaging purposes, especially for sharp-edged objects, since they tear or break immediately would.

According to a further development of the invention, the method is performed in such a way that for the production of a sheet with a basis weight between 50 g/m? and 120gsm? the paper machine is operated at a speed of > 650 m/min to 1000 m/min, preferably about 850 m/min. By running the process in such a way that sheets with a basis weight between 50 and 120 g/m? are formed, despite the relatively high freeness and the content of fines in the mass, good dewatering can be guaranteed and thus a relatively high machine speed can be maintained, whereby high production quantities can be provided with low energy consumption at the same time. The weight per unit area between 50 and 120 g/m* given in this context is to be understood as the weight per unit area of a base paper to which a coating with a surface treatment agent other than starch is subsequently applied.

By performing the calendering of the sheet with a line load between 40 and 500 kN/m, it is possible not only to further improve the surface properties of the paper, but surprisingly also to further increase the transparency of the paper without anything else in the process an excessive amount of energy must be expended. The energy consumption in the calendering step can be kept particularly low, in particular because of the low temperatures of about 80° C. to about 130° C. at which the calendering is carried out. By additionally applying a surface treatment with a chemical surface treatment agent such as PEG (polyethylene glycol) before or after calendering, in particular in an amount of 7 to 30 g/m? is carried out, and a surface coated sheet with a basis weight between 57 g/m? and 150gsm? is formed, it is possible to further increase the transparency of the kraft paper, so that goods contained in a packaging made of a kraft paper can be clearly and unambiguously recognized and checked. For example, packaging that has viewing windows with such kraft paper can also be designed in such a way that texts underneath can be clearly read or packaged goods can be seen through the viewing window. By providing a surface coated paper with basis weights of 57 g/m? and 150gsm? Due to its excellent mechanical properties, it can also be used for packaging heavy or irregularly shaped goods.

By conducting the process such that the additional surface treatment step

with a surface treatment agent other than starch is carried out either in the paper machine itself or in a separate unit, it is possible to provide a particularly flexible process which can either be carried out directly on a conventional paper machine or a separate additional surface treatment unit is used. In particular, if a separate surface treatment unit is used, the process can be carried out on a conventional paper machine at speeds > 650 m/min without the additional surface treatment step and a transparent paper can still be produced which is sufficiently translucent and, in particular, has excellent mechanical strengths. By providing a separate surface treatment unit that applies a chemical surface treatment agent such as PEG, the opacity of the kraft paper can be further reduced to below 70% without degrading the mechanical properties of the paper.

By applying an amount of 7 to 30 g / m? of a non-starch surfactant such as PEG (polyethylene glycol), the basis weight of the finished paper further increases depending on the amount of the non-starch chemical surfactant applied. By choosing the amount of chemical surface treatment agent different from starch applied and by choosing the surface treatment agent used, it is surprisingly possible in combination with the base paper produced to produce transparent or translucent paper in a wide range of basis weights, which with the methods used to date, in particular glassine papers or glassine paper was not possible. Such transparent papers produced according to the prior art could only be used with basis weights of up to about 50 g/m? are produced, they have as starting material almost 100% resin wood pulp which is extremely highly refined and subsequently subjected to extreme pressing to achieve transparency. All of these process steps are very energy-intensive, since on the one hand it is very difficult to dewater the sheet due to the high degree of grinding and on the other hand the very high pressing pressure or pressure in a calender means that a lot of energy has to be used for the pressing itself on the one hand and the paper as a result massively less mechanically resilient. This can be avoided in connection with the present invention, a moderate degree of beating in the range of 38 °SR is used, at least 30% softwood pulp fibers are used, which contribute to the extremely good mechanical strength of the paper and at least 35% hardwood pulp is used, with With these small amounts, it is nevertheless possible to form a transparent paper by subsequent pressing and surface coating. Such moderately ground pulps can be easily drained, sheet formation is easy and by applying only moderate pressures during pressing and, if necessary, calendering, the energy consumption for sheet formation is kept low and finally it is possible with the combination of all these measures and the coating of the base paper produced with a surprisingly, using chemical surface treatment agents different from starch, to produce a transparent paper which, in addition to its transparency, has excellent mechanical properties and, in particular, a very high resistance to tear propagation, which could not be achieved with known transparent papers such as cellophane or glassine papers.

[0035] A particularly elegant and in particular efficient method is achieved if the step of surface treatment with a surface treatment agent other than starch takes place at the same time as the coating with surface starch. In this case, both the surface thickness and the additional surface treatment agent can be applied to the paper by, for example, application units that are separate from one another, and a high-quality, highly translucent paper with excellent mechanical properties can thus be obtained.

By continuing to conduct the process in such a way that the drying unit of the paper machine is operated with a maximum use of steam per hour in the range of 30 to 40 t/h, it is possible to quickly and reliably provide a dried translucent kraft paper which has excellent mechanical properties exhibits and at the same time in comparison

compared to conventional translucent papers, it has a specific CO» emission in the range of less than 450 kg CO»/t paper.

[0037] The invention is explained in more detail below using exemplary embodiments.

EXAMPLE 1: Production of a transparent paper with a grammage of 99 g/m? PROCESS DESCRIPTION:

A pulp consisting of 42% bleached softwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 254 kWh/t, and 58% bleached hardwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 98 kWh/t, was used . A freeness of 40 °SR is achieved for both pulps. After beating, the two pulps are mixed according to the specified proportions. The additives are added in the approach flow of the paper machine. The pH value in the white water was 8.1, cationic starch, with a degree of cationization DS of 0.05, in an amount of 11 kg/t paper dry (atro means absolutely dry) was metered in and the sizing agent was alkenylsuccinic anhydride in an amount of 2.3 kg/t paper dry used. 3% calcium carbonate was used as a filler. The consistency of the pulp at the headbox was 0.1%. Dewatering took place on a wire section with a top former and in a press section with four nips, the line pressure in the three nips being 48 kN/m, 95 kN/m and 115 kN/m and in the shoe press nip 600 kN/m. The paper was pre-dried and coated in a coating unit at 1 g/m? Surface thickness treated. The paper was then further dried and dried to a final moisture content of 6%. The speed at the pope was 859 m/min.

The paper thus obtained with a grammage of 78 g / m? was applied in a separate coating unit in two passes with a total coating of 21 g/m? provided with PEG.

The tracing paper produced in this way had the following properties:

ramming

Thickness ISO 534:2011 - 114 Tensile strength ISO 1924-3:2005 MD 6.3 Tensile strength ISO 1924-3:2005 CD 2.4 Elongation at break ISO 1924-3:2005 % MD 2.27 Elongation at break ISO 1924-3:2005 % CD 5.64 Tensile work ISO 1924-3:2005 J/m? MD 100 tensile work ISO 1924-3:2005 J/m? CD 113 Burst Strength ISO 2758:2014 kPa - 236 Tear Resistance ISO 1974:2012 mN MD 683 Tear Resistance mN CD 788 Air Permeability ISO 5636-5:2013 s 9830 Gurley

Bendtsen roughness ISO 8791-2:2013 ml/min rough side 850 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side 770 Opacity ISO 2471:2008 % 67.2

EXAMPLE 2: Production of a calendered transparent paper with a grammage of 95 g/m?

PROCESS DESCRIPTION:

A pulp consisting of 42% bleached softwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 254 kWh/ton, and 58% bleached hardwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 98 kWh/ton, was used . A freeness of 40 °SR is achieved for both pulps. After beating, the two pulps are mixed according to the specified proportions. The additives are added in the approach flow of the paper machine. The pH value in the white water was 8.2, cationic starch, with a degree of cationization DS of 0.05, in an amount of 11 kg/ton paper dry was metered in and the sizing agent was alkenylsuccinic anhydride in an amount of 2.3 kg/ton Paper atro used. The filler contained 3% in the form of calcium carbonate. The consistency of the pulp at the headbox was 0.1%. Dewatering took place on a wire section with a top former and in a press section with four nips, the line pressure in the three nips being 48 kN/m, 95 kN/m and 115 kN/m and in the shoe press nip 600 kN/m. The paper was pre-dried and coated in a coating unit at 1 g/m? Surface thickness treated. The paper was then dried further, treated in a calender with a line load of 60 kN/m and dried to a final moisture content of 6%. The speed at the pope was 859 m/min.

The paper thus obtained with a grammage of 78 g / m? was applied in a separate coating unit in one operation with a total coating of 17 g/m? provided with PEG.

The tracing paper produced in this way had the following properties:

Grammage ISO 536:2019 g/m? 95 Thickness ISO 534:2011 Um - 93 Tensile strength ISO 1924-3:2005 kN/m MD 6.4 Tensile strength ISO 1924-3:2005 kN/m CD 2.4 Elongation at break ISO 1924-3:2005 % MD 2.25 Elongation at break ISO 1924-3:2005 % CD 5.14 Tensile work ISO 1924-3:2005 J/m? MD 101 Tensile work ISO 1924-3:2005 J/m? CD 103 Burst Strength ISO 2758:2014 kPa - 220 Tear Resistance ISO 1974:2012 mN MD 655 Tear Resistance mN CD 815 Air Permeability ISO 5636-5:2013 s 14300 Gurley

Bendtsen roughness ISO 8791-2:2013 ml/min rough side 226 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side 153 Opacity ISO 2471:2008 % 63.1

Comparing Examples 1 and 2 with each other, it becomes clear that by calendering the base paper, an opacity that is about 4% (in absolute value) lower is achieved, and that at a 4 g/m? reduced use of PEG for surface treatment. In both versions

example, the grammage of the base paper was 78 g/m®.

Furthermore, calendering in the specific case results in a 21% lower thickness, 83% lower roughness and 42% lower air permeability in the base paper, which also has a favorable effect on achieving a desired transparency.

EXAMPLE 3: Production of a calendered transparent paper with a grammage of 61 g/m?

PROCESS DESCRIPTION:

A pulp consisting of 55% bleached softwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 276 kWh/t, and 45% bleached hardwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 124 kWh/t, was used . A freeness of 44 °SR is achieved for both pulps. After beating, the two pulps are mixed according to the specified proportions. The additives are added in the approach flow of the paper machine. The pH value in the white water was 8.1, cationic starch with a degree of cationization DS of 0.05 was added in an amount of 17 kg/t paper atro and the sizing agent was alkenylsuccinic anhydride in an amount of 2.6 kg/t paper atro used. No filler was used. The consistency of the pulp at the headbox was 0.1%. Dewatering took place on a wire section with a top former and in a press section with four nips, the line pressure in the three nips being 48 kN/m, 95 kN/m and 115 kN/m and in the shoe press nip 620 kN/m. The paper was pre-dried and coated in a coating unit at 2 g/m? Surface thickness treated.

The paper was then dried further, treated in a calender with a line load of 120 kN/m and dried to a final moisture content of 7%. The speed at the pope was 795 m/min.

The paper obtained in this way had a grammage of 52 g/m? and was coated in a separate coating unit with a coating of 9 g/m? (mixture of 90% PEG/10% glycerin).

The tracing paper produced in this way had the following properties:

Grammage ISO 536:2019 g/m? 61 Thickness ISO 534:2011 um 69 Tensile strength ISO 1924-3:2005 kN/m MD 4.3 Tensile strength ISO 1924-3:2005 kN/m CD 1.6 Elongation at break ISO 1924-3:2005 % MD 2.12 Elongation at break ISO 1924-3:2005 % CD 5.21 Tensile work ISO 1924-3:2005 J/m? MD 83 Tensile work ISO 1924-3:2005 J/m? CD 72 bursting strength ISO 2758:2014 kPa - 187 tear resistance |ISO 1974:2012 mN MD 490 tear resistance mN CD 571

Air permeability ISO 5636-5:2013 s 17250 Gurley

Bendtsen roughness ISO 8791-2:2013 ml/min rough side 141 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side 117 Opacity ISO 2471:2008 % 49.3

EXAMPLE 4: Production of a calendered transparent paper with a grammage of 135 g/m?

PROCESS DESCRIPTION:

A pulp consisting of 35% bleached softwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 271 kWh/t, and 40% bleached hardwood pulp, which is subjected to low-consistency grinding with a grinding capacity of 98 kWh/t, and 25 % unbleached softwood pulp subjected to low-consistency beating with a beating capacity of 271 kWh/t was used. A freeness of 38 °SR is thus achieved for both pulps. The two softwood pulps were previously mixed according to the proportions and ground together. The final mixture of all pulps was carried out after grinding according to the specified proportions. The additives are added in the approach flow of the paper machine. The pH value in the white water was 8.1, cationic starch, with a degree of cationization DS of 0.05, was metered in in an amount of 20 kg/ton paper atro and the sizing agent was alkenylsuccinic anhydride in an amount of 2.0 kg/ton Paper atro used. No fillers were used. The consistency of the pulp at the headbox was 0.1%. Dewatering took place on a wire section with a top former and in a press section with four nips, the line pressure in the three nips being 50 kN/m, 100 kN/m and 120 kN/m and in the shoe press nip 600 kN/m. The paper was pre-dried and coated in a coating unit at 1.1 g/m? Surface thickness treated. The paper was then dried further, treated in a calender with a line load of 200 kN/m and dried to a final moisture content of 7%. The speed at the pope was 720 m/min.

The paper thus obtained with a grammage of 110 g / m? was applied in a separate coating unit in two passes with a total coating of 25 g/m? provided with PEG.

The tracing paper produced in this way had the following properties:

Grammage ISO 536:2019 g/m? 135 Thickness ISO 534:2011 µm - 119 Tensile strength ISO 1924-3:2005 kN/m MD 10.2 Tensile strength ISO 1924-3:2005 kN/m CD 5.2

Elongation at break ISO 1924-3:2005 % MD 2.15 Elongation at break ISO 1924-3:2005 % CD 5.32 Tensile work at break ISO 1924-3:2005 |J/m? MD 152 Tensile work ISO 1924-3:2005 | y/m? CD 147 Burst Strength ISO 2758:2014 kPa - 412

Tear resistance ISO 1974:2012 mN MD 1170 Tear resistance mN CD 1310 Air permeability ISO 5636-5:2013 s 28740 Gurley

Bendtsen roughness ISO 8791-2:2013 ml/min rough side 157 Bendtsen roughness ISO 8791-2:2013 ml/min smooth side 135 Opacity ISO 2471:2008 % 51.2

Claims (13)

patent claims 1. Transparent paper consisting of a coated or chemically treated kraft paper, characterized in that the kraft paper consists of a bleached, low-consistency and optionally high-consistency pulp ground with a freeness of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR, at least 30%, preferably at least 35%, particularly preferably at least 40% and at most 60% of softwood pulp and at least 35%, preferably at least 40%, particularly preferably at least 45% of hardwood pulp and optionally other fibrous materials, 0 5 to 2.5% cationic starch, processing aids, 0.1 to 0.4% sizing agent and optionally up to 4% fillers is formed and that the kraft paper with other substances selected from PEG, glycerol, polyvinyl alcohol or mixtures of two or more thereof and optionally starch and optionally mechanically surface-treated. 2. Transparent paper according to claim 1, characterized in that it contains 20% to 30% other fibers selected from the group consisting of grass, hemp, jute, linen, cotton, unbleached fiber material, eucalyptus, waste paper, recycled paper and viscose fibers. 3. Transparent paper according to claim 2, characterized in that it contains exclusively unbleached fiber material made from softwood pulp, in particular 20 to 30% unbleached fiber material made from softwood pulp, as further fibers. 4. Transparent paper according to claim 1, 2 or 3, characterized in that it contains up to 3 g/m², preferably up to 2.2 g/m², particularly preferably up to 1.8 g/m². contains surface strength. 5. Transparent paper according to one of Claims 1 to 4, characterized in that it is coated with a surface treatment agent other than starch, such as PEG, in particular 7 to 30 g/m? PEG is surface coated and that the surface coated sheet has a basis weight between 57 g/m* and 150 g/m? having. 6. Transparent paper according to any one of claims 1 to 5, characterized in that 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, such as coating, characterized in that it has the following steps: a) formation of a kraft pulp containing at least 30%, preferably at least 35%, particularly preferably 40% to at most 60% bleached softwood pulp and at least 35%, preferably at least 40%, particularly preferably at least 45% bleached hardwood pulp and optionally other fibers, b) subjecting the pulp to low-consistency beating and optionally high-consistency beating until a degree of beating of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR, is achieved, c) adding process auxiliaries, in particular from 0.5 to 2.5% cationic starch, 0.1 to 0.4% sizing agent and optionally up to 4.0% fillers, d) sheet formation on a wire of a paper machine, e) pressing and drying to a dry content between 30% and 80%, f) and optionally at least one coating of the formed sheet with up to 3 g/m? surface thickness, g) optionally calendering the sheet, h) surfacing with a non-starch surface treatment agent, and ij) drying of the tracing paper, with the proviso that if in addition to step h) at least one of steps f) or g) is applied, steps f), g) and h) in one any order, applied once or multiple times. 8. The method according to claim 7, characterized in that to produce a sheet with a basis weight between 50 and 120 g / m? the paper machine is operated at a speed of >650 to 1000 m/min, preferably about 850 m/min. 9. A method according to claim 7 or 8, characterized in that the sheet formed is subjected to a calendering step with a line load of between 40 and 500 kN/m, preferably 50 to 300 kN/m. 10. The method according to claim 7, 8 or 9, characterized in that before or after the calendering, a surface treatment with PEG, in particular 7 to 30 g / m? is carried out and that a surface coated sheet with a basis weight between 57 g/m? and 150gsm? is trained. 11. The method according to any one of claims 7 to 10, characterized in that 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 any one of claims 7 to 11, characterized in that the step of surface treatment with a non-starch surface treatment agent occurs simultaneously with the coating with surface starch. 13. The method according to any one of claims 7 to 12, characterized in that it is carried out with a maximum use of steam per hour in the range between 30 and 40 t/h. No drawings for this