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, preferably at least 43 °SR, particularly preferably at least 46 °SR, which is formed to an extent of at least 30%, preferably to an extent of at least 35%, particularly preferably to an extent of at least 40% and at most 60% of softwood pulp and to an extent of at least 35%, preferably to an extent of at least 40%, particularly preferably to an extent of at least 45% 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 optionally mechanically surface-treated with further substances selected from PEG, glycerol, polyvinyl alcohol or mixtures of two or more thereof, and optionally starch, as well as methods for producing transparent paper and forming a kraft pulp.

Description

[0001] TRANSPARENT PAPER

[0002] The present invention relates to a transparent paper consisting of a coated or chemically treated kraft paper and to a method for producing transparent paper in which a kraft paper is subjected to a chemical surface treatment and / or coating surface treatment.

[0003] Transparent papers or translucent papers have been produced for many years to serve as viewing windows in envelopes or boxes, food packaging, for various graphic applications such as tracing paper, photocopy paper, etc. In addition to special types of transparent papers, such as glassine papers, which are only available with low grammages of up to about 50 g / m 2can be produced and, moreover, have only very limited strength, there are also a large number of papers in which conventional paper is made translucent using special processes, which processes include both physical methods and chemical processes or mixtures thereof. In particular, when chemicals, particularly synthetic resins such as acrylic resins, polyester and / or urethane resins, are used in such processes to make the paper translucent, it is usually necessary, in addition to coating the paper with the chemicals per se, to crosslink or cure the translucent material after its application by using heat or UV rays or other crosslinking processes.Such a method for forming a transparent region in paper is described, for example, in US 5,418,205, in which a cellulosic substrate is impregnated with a polymerizable, transparentizing material, the impregnated region being subsequently cured or crosslinked by radiation, thereby forming a transparent region in the paper.

[0004] A similar process is described in US 6,902,770, in which a transparentizing material is applied to a surface of the paper after heating it to facilitate and accelerate the penetration of the material into the interior of the paper, whereby a transparent region can be formed in the paper.

[0005] Furthermore, oils, waxes, or the like, especially silicone oils or resins, can be used as additives to paper in a known manner to achieve a certain degree of transparency. Such materials, which are intended to make the paper translucent, are usually ground extremely finely together with the cellulose fibers, and this mixture is used to produce grease-resistant and slightly transparent papers. Finally, there are also processes in which only individual areas of a paper can be made translucent or transparent.One such process, in which a translucent surface can be formed directly in the finished paper, is described, for example, in WO 2008 / 091523. In this process, the area of ​​the paper to be made translucent is perforated using laser beams to create a transparent area of ​​the paper by forming a network of microscopic, closely spaced holes. All of these processes are either very complex, can only be produced using large amounts of energy, or use significant amounts of chemicals. This makes these papers of very limited use, especially when a fully recyclable packaging material is to be provided for ecological reasons.Some of them may not be used, or may only be used to a very limited extent, in the food sector or in the area of ​​sensitive substances, such as solvent-sensitive substances.

[0006] A further problem with such transparent 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 that require high-strength paper types, such as packaging for products with a higher weight or irregular surfaces with edges or corners, etc. Until now, only papers could be used in which the transparent window, usually made of plastic, had to be inserted separately, since the strength of the classic transparent papers was not sufficient even for such an application.

[0007] Until now, high-strength papers could not be made sufficiently translucent using conventional processes without reducing their strength so significantly, either through laser perforation or extremely intensive grinding and / or calendering of the pulp fibers used. Such methods are not suitable for the production of transparent papers for large-scale use, particularly for the packaging of products with irregular or sharp-edged surfaces and / or a high weight, such as electronic components, game pieces, building materials, or even foodstuffs such as rice, nuts, or the like. Packaging papers for such items or materials are usually made from kraft paper, which has particularly high strength but is not transparent even when using thin papers with a low basis weight.The present invention now aims to provide a transparent paper which has high strength, in particular strength properties substantially equivalent to kraft paper, and which contains only biodegradable substances and no environmentally harmful chemicals in or on the paper.

[0008] To achieve this object, the transparent paper according to the invention is essentially characterized in that the kraft paper is made from a bleached, low-consistency and optionally high-consistency pulp milled to a freeness of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR, which pulp is milled to at least

[0009] 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 further fiber 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 surface-treated with further substances selected from PEG (polyethylene glycol), glycerin, polyvinyl alcohol or mixtures of two or more thereof and optionally starch, such as mixtures of PEG and starch and optionally mechanically.By forming the kraft paper from a bleached pulp, which pulp is made up of 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. By adding small amounts of processing aids and 0.1 to 0.4% sizing agent and, if necessary, other fillers to the kraft paper, it is possible to provide a paper which, on the one hand, is suitable for use in the food sector and, on the other hand, offers an optimal compromise between mechanical strength and processability.The 0.5% to 2.5% cationic starch contained in the paper further improves the strength of the paper, so that, surprisingly, the use of hardwood pulp in kraft paper makes it possible to impart transparency while maintaining high strength. Regarding the amount of starch used, it is known to those skilled in the art that bleached pulps or bleached cellulose fibers allow the use of only small amounts of starch. On the other hand, pulps produced using 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 produce a translucent paper with both excellent surface and mechanical properties.This is particularly surprising since starch as such has a rather detrimental, or at most neutral, effect on the transparency of a paper, and the selected combination of two different pulps, starch, and surface treatment agents surprisingly allows the paper to be given sufficient strength and transparency simultaneously. According to the invention, in this context, the kraft paper is treated with other substances or mixtures of other substances, or mixtures of starch and other substances, in addition to the cationic starch contained in the paper, and optionally mechanically surface-treated.By treating the surface with additional substances or mixtures of additional substances and, if necessary, additional starch and an optional mechanical surface treatment, in particular pressing, it is surprisingly possible not only to further improve the strength and printability of the kraft paper, but also to further improve the transparency of the paper.

[0010] By keeping the filler content below 4% and preferably not containing any fillers at all in the paper, the transparency of the paper can be further improved.

[0011] A tracing paper according to the present invention is not understood to mean a completely clear and transparent paper, but rather a paper with an opacity or haze as defined in ISO Standard No. 2471:2008. Accordingly, opacity is understood to mean the degree of haze of the translucent paper or tracing paper according to the invention.

[0012] In the context of the present application, mechanical surface treatment is understood to mean pressing in the press section of a paper machine with one or more presses with the same or different pressing pressure or at least one shoe press or combinations of one or more presses and one or two shoe presses in any desired sequence.

[0013] By forming the paper in such a way that it contains 20% to 30% additional fibers selected from the group consisting of grass, hemp, jute, linen, cotton, unbleached fibers, eucalyptus, waste paper, recycled paper, viscose fibers, or the like, as is the case with a further development of the present invention, it is possible to provide a transparent paper that, on the one hand, has even greater strength and, on the other hand, has an extremely low CO2 footprint of less than 450 kg CO2 / t of paper during production. The additional fibers can be added as dyed fibers, bleached or unbleached fibers, in particular fibers originating directly from a production or recycling process.By designing the tracing paper in such a way that, as corresponds to a further development of the present invention, it is possible to achieve a CO2 footprint with CO2 emissions of less than 350 kg CO2 / t of paper, as additional fibers exclusively comprised of unbleached fiber material from softwood pulp, in particular 20 to 30% unbleached fiber material from softwood pulp, as additional fibers, as follows: At the same time, the strength of the paper can be further increased compared to tracing paper consisting purely of bleached pulp(s), and in particular compared to conventionally produced tracing papers, particularly if only up to approximately 30% unbleached fiber material from softwood pulp is added as additional fibers.

[0014] As a further development of the invention, the transparent paper is further developed in such a way that it is coated with a chemical surface coating agent other than starch, such as PEG, in particular 7 to 30 g / m 2 PEG surface coated and that the surface coated sheet has a basis weight between 57 g / m 2 and 160 g / m 2 It is possible to produce surprisingly strong yet translucent papers which, due to their excellent tear resistance, are particularly suitable for use as packaging in the food sector or for objects with irregular or sharp-edged surfaces.

[0015] By additionally calendering the transparent paper, as is the case with a further development of the invention, it is possible, on the one hand, to reduce the thickness of the paper and to lower its surface roughness and (air) permeability, which, on the one hand, improves its printability and, on the other hand, surprisingly further improves the transparency of the paper.

[0016] The present invention further aims to provide a process for producing the transparent paper, with which it is possible to provide a transparent paper having excellent mechanical properties.

[0017] To achieve this object, the process according to the invention is essentially characterized in that it comprises the following steps: a) forming 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 further fibers, b) subjecting the pulp to low-consistency refining and optionally to high-consistency refining until a freeness of at least 38 °SR, preferably at least 43 °SR, particularly preferably at least 46 °SR is achieved, c) adding 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 dry content between 30% and 80%,f) and optionally coating the formed sheet at least once with up to 3 g / m, 2 Surface starch, g) optionally calendering the sheet, h) surface treating with a surface treatment agent other than starch, and i) drying the tracing paper, provided that if 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, once or several times.

[0018] By conducting the process in such a way that a kraft pulp is produced that contains both bleached softwood pulp and bleached hardwood pulp, it is possible to create the basis for a paper that is, on the one hand, translucent or transparent and, on the other hand, exhibits 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 achieved, the mechanical properties of the paper are further improved, and on the other hand, a freeness is selected that ensures rapid dewatering, thus preventing excessive energy costs for subsequent drying, while simultaneously maintaining machine speeds above 700 m / min.Furthermore, with such a process, 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 refining to a freeness of at least 38 °SR, it is surprisingly possible to provide a starting material which, by means of the subsequent moderate treatment steps, in particular a surface treatment with a surface treatment agent other than starch, and optionally calendering, results in a kraft paper which, due to both the refining and the subsequent treatment, has essentially no fiber-air-fiber interfaces and is thus transparent on the one hand and has high strength and very good tear resistance on the other.By combining the use of bleached softwood pulp and hardwood pulp, both produced using the kraft process, it is surprisingly possible to incorporate relatively high amounts of starch 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, although relatively high amounts of starch can be incorporated into a kraft pulp, the presence of bleached pulp fibers is rather disadvantageous for such higher starch amounts, as the starch is then carried out into the white water during the process. Surprisingly, the mixture of two different pulps selected in the present process makes it possible to incorporate high amounts of starch into the pulp, thus subsequently producing a transparent paper with surprisingly good mechanical properties.By pressing the formed sheet at least once with up to 3 g / m. 2 surface thickness and, if necessary, calendered, it is possible to provide a transparent paper which, in addition to the good mechanical properties of kraft paper, also has excellent

[0019] Printability and surface smoothness, with a Bendtsen roughness on the smooth side between 100 and 150 ml / min. Such a paper has a tensile strength according to ISO 1924-3:2005 in the machine direction between 6.0 and 8.5 kN / m and in the cross direction between 2.7 and 4.5 kN / m, in particular a tear strength according to ISO 1974:2012 in the machine direction of at least 400 mN and in the cross direction of at least 600 mN, as well as a

[0020] Air permeability according to Gurley 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%.

[0021] This is all the more surprising as the transparent papers or glassine papers known in technology are not or hardly mechanically resilient, have almost no tear resistance and therefore cannot be used for packaging purposes, especially for sharp-edged objects, as they would immediately tear or break.

[0022] According to a further development of the invention, the process is carried out in such a way that in order to produce a sheet with a basis weight between 50 g / m 2 and 120 g / m 2 the paper machine is operated at a speed of > 650 m / min to 1000 m / min, preferably about 850 m / min. By conducting the process in such a way that sheets with a basis weight between 50 and 120 g / m 2Despite the relatively high degree of freeness and the content of fines in the pulp, good dewatering can be ensured and a relatively high machine speed can be maintained, enabling high production volumes with low energy consumption. The basis weight between 50 and 120 g / m² mentioned in this context 2 is to be understood as the surface wiping of a base paper, onto which a coating with a surface treatment agent other than starch is subsequently applied.

[0023] By calendering 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 also, surprisingly, to further increase its transparency, without having to use excessive energy elsewhere in the process. The energy consumption in the calendering step can be kept particularly low, particularly due to the low temperatures of approximately 80 °C to approximately 130 °C at which the calendering is carried out. By additionally treating the surface with a chemical surface treatment agent such as PEG (polyethylene glycol), particularly in an amount of 7 to 30 g / m, before or after calendering, the resulting high-quality paper is ideally suited for the production of high-quality paper. 2 and a surface-coated sheet with a basis weight between 57 g / m 2 and 150 g / m 2is formed, the transparency of the kraft paper can be further increased, so that goods contained in packaging made of kraft paper can be clearly and unambiguously identified and inspected. Packaging with viewing windows made of this type of kraft paper can also be designed in such a way that underlying text can be clearly read or packaged goods can be seen through the viewing window. By providing a surface-coated paper with a basis weight of 57 g / m 2 and 150 g / m 2 Due to its excellent mechanical properties, it can also be used for packaging heavy or irregularly shaped goods.

[0024] By conducting the process in such a way 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 that can either be carried out directly on a conventional paper machine or using a separate, additional surface treatment unit. In particular, if a separate surface treatment unit is used, the process can be carried out without the additional surface treatment step on a conventional paper machine at speeds > 650 m / min and still produce a transparent paper that is sufficiently translucent and, in particular, exhibits excellent mechanical strength.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 impairing the paper's mechanical properties. By applying a quantity of 7 to 30 g / m. 2a surface treatment agent other than starch, such as PEG (polyethylene glycol), the basis weight of the finished paper increases further depending on the applied amount of the chemical surface treatment agent other than starch. By selecting the amount of the chemical surface treatment agent other than starch applied and by selecting the surface treatment agent used, in combination with the produced base paper, it is surprisingly possible to produce transparent or translucent paper in a wide range of basis weights, which was not possible with previously used processes, particularly glassine papers or glassine papers. Such transparent papers produced according to the state of the art could only be produced with basis weights of up to about 50 g / m 2They were produced from almost 100% resin wood pulp as their starting material, which is milled to an extremely high degree and then subjected to extreme pressing to achieve transparency. All of these process steps are very energy-intensive. Firstly, the high milling makes it very difficult to dewater the sheet. Secondly, the very high pressure in a calender requires a lot of energy for the pressing itself, which also makes the paper significantly less mechanically resilient.This is precisely what 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 an extremely good mechanical strength of the paper, and at least 35% hardwood pulps are used, whereby with these small amounts it is still possible to form a transparent paper by subsequent pressing and surface coating.Such moderately ground pulps are easy to dewater, sheet formation is simple and by applying only moderate pressures during pressing and, if necessary, calendering, the energy expenditure for sheet formation is kept low and finally, by combining all these measures and coating the produced base paper with a chemical surface treatment agent other than starch, it is surprisingly possible to produce a transparent paper which, in addition to its transparency, has excellent mechanical properties and, in particular, a very high tear resistance, which was not achievable with known transparent papers such as cellophane or glassine papers.

[0025] A particularly elegant and efficient process is achieved when the surface treatment step with a surface treatment agent other than starch is carried out simultaneously with the coating with surface starch. In this case, both the surface starch and the additional surface treatment agent can be applied to the paper using separate application units, for example, thus producing a high-quality, highly translucent paper with excellent mechanical properties.By continuing to manage the process in such a way that the drying unit of the paper machine is operated with a maximum steam input 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 and, at the same time, has a specific CO2 emission in the range of less than 450 kg CO2 / t of paper compared to conventional translucent papers.

[0026] The invention is further explained below using exemplary embodiments.

[0027] Example 1 : Production of a transparent paper with a grammage of 99 g / m 2 Process description:

[0028] A pulp consisting of 42% bleached softwood pulp, subjected to low-consistency refining with a refining capacity of 254 kWh / t, and 58% bleached hardwood pulp, subjected to low-consistency refining with a refining capacity of 98 kWh / t, was used. A freeness of 40 °SR was achieved for both pulps.

[0029] After refining, the two pulps are mixed according to the specified proportions. The auxiliaries are added in the approach flow system 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 at a rate of 11 kg / t of dry paper (dry means absolutely dry), and alkenylsuccinic anhydrides were used as a sizing agent at a rate of 2.3 kg / t of dry paper. 3% calcium carbonate was used as 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, with the line pressure in the three nips being 48 kN / m, 95 kN / m, and 115 kN / m, and in the shoe press nip being 600 kN / m. The paper was pre-dried and coated in a coating unit with 1 g / m 2The paper was then further dried to a final moisture content of 6%. The speed at the pope was 859 m / min.

[0030] The resulting paper with a grammage of 78 g / m 2 was applied in a separate coating unit in two passes with a total coating of 21 g / m 2 PEG provided.

[0031] The tracing paper produced in this way had the following properties:

[0032] Example 2: Production of a calendered tracing paper with a grammage of 95 g / m 2

[0033] Process description:

[0034] A pulp consisting of 42% bleached softwood pulp, which was subjected to low-consistency refining with a refining capacity of 254 kWh / t, and 58% bleached hardwood pulp, which was subjected to low-consistency refining with a refining capacity of 98 kWh / t, was used. A grinding degree of 40 °SR was achieved for both pulps. After refining, the two pulps were mixed according to the specified proportions. The auxiliaries were added in the approach flow system of the paper machine. The pH value of the white water was 8.2. Cationic starch, with a degree of cationization DS of 0.05, was added at a rate of 11 kg / t of dry paper, and alkenylsuccinic anhydrides were used as a sizing agent at a rate of 2.3 kg / t of dry paper. 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, with line pressures of 48 kN / m, 95 kN / m, and 115 kN / m in the three nips and 600 kN / m in the shoe press nip. The paper was pre-dried and coated with 1 g / m in a coating unit. 2 The paper was then further dried, 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 calender was 859 m / min.

[0035] The resulting paper with a grammage of 78 g / m 2 was applied in a separate coating unit in one operation with a total coating of 17 g / m 2 PEG. The resulting transparent paper had the following properties:

[0036] Comparing examples 1 and 2, it becomes clear that by calendering the base paper, an opacity lower by about 4% (in absolute value) is achieved, and this at a weight of 4 g / m 2 lower use of PEG for surface treatment. In both examples, the base paper weight was 78 g / m 2

[0037] Furthermore, calendering in this specific case results in a 21% reduction in thickness, an 83% reduction in roughness, and a 42% reduction in air permeability in the base paper, which also has a positive effect on achieving the desired transparency. Example 3: Production of a calendered transparent paper with a grammage of 61 g / m² 2 Process description:

[0038] A pulp consisting of 55% bleached softwood pulp, which is subjected to low-consistency refining with a refining capacity of 276 kWh / t, and 45% bleached hardwood pulp, which is subjected to low-consistency refining with a refining capacity of 124 kWh / t, was used. A refining degree of 44 °SR is achieved for both pulps.

[0039] After refining, the two pulps are mixed according to the specified proportions. The auxiliaries are metered in in the approach flow system of the paper machine. The pH value in the white water was 8.1, cationic starch with a cationization degree DS of 0.05 was added at a rate of 17 kg / t of dry paper, and alkenylsuccinic anhydrides were used as a sizing agent at a rate of 2.6 kg / t of dry paper. 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, with 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 treated in a coating unit with a surface thickness of 2 g / m®.

[0040] The paper was then further dried, 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 calender was 795 m / min.

[0041] The paper obtained in this way had a grammage of 52 g / m 2 and was coated in a separate coating unit with a coating of 9 g / m 2 (mixture 90% PEG / 10% glycerin). The resulting transparent paper had the following properties:

[0042] Example 4: Production of a calendered tracing paper with a grammage of 135 g / m 2 Process description:

[0043] A pulp consisting of 35% bleached softwood pulp, which was subjected to low-consistency refining with a refining capacity of 271 kWh / t, 40% bleached hardwood pulp, which was subjected to low-consistency refining with a refining capacity of 98 kWh / t, and 25% unbleached softwood pulp, which was subjected to low-consistency refining with a refining capacity of 271 kWh / t, was used. A freeness of 38 °SR was thus achieved for both pulps. The two softwood pulps were pre-blended according to the respective proportions and refined together. The final blending of all pulps took place after refining according to the specified proportions. The auxiliary materials are added in the approach flow system of the paper machine.The pH value of the white water was 8.1, cationic starch with a cationization degree DS of 0.05 was added at a rate of 20 kg / t of dry paper, and alkenyl succinic anhydride was used as a sizing agent at a rate of 2.0 kg / t of dry paper. 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, with 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 with 1.1 g / m. 2 The paper was then further dried, 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 calender was 720 m / min.

[0044] The resulting paper with a grammage of 110 g / m 2 was applied in a separate coating unit in two passes with a total coating of 25 g / m 2 PEG provided.

[0045] The tracing paper produced in this way had the following properties:

Claims

Patent claims:

1. Transparent paper consisting of a coated or chemically treated kraft paper, characterized in that the kraft paper is made from bleached, low-consistency and optionally high-consistency milled pulp with a fineness of at least 38 °SR, preferably at least 43 °SR, and particularly preferably at least 46 °SR, which consists of 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, as well as 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 that the kraft paper is formed with further substances selected from PEG, glycerin,Polyvinyl alcohol or mixtures of two or more thereof and, if applicable, starch, and, if applicable, mechanically surface-treated.

2. Transparent paper according to claim 1, characterized in that it contains 20% to 30% further 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 as further fibers exclusively unbleached fiber material from softwood pulp, in particular 20% to 30% unbleached fiber material from softwood pulp.

4. Transparent paper according to claim 1, 2 or 3, characterized in that it weighs up to 3 g / m² 2 , preferably up to 2.2 g / m³ 2 , particularly preferably up to 1.8 g / m³ 2 surface thickness is included.

5. Transparent paper according to one of claims 1 to 4, characterized in that it is treated with a surface treatment agent of different strength, such as PEG, in particular 7 to 30 g / m² 2 PEG surface-coated and that the surface-coated sheet has a surface weight between 57 g / m² 2 and 150 g / m² 2 exhibits.

6. Transparent paper according to one of claims 1 to 5, characterized in that it is additionally calendered.

7. A process 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 comprises the following steps: a) Forming 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, as well as optionally further fibrous materials; b) Subjecting the pulp to low-consistency refining and optionally high-consistency refining until a degree of refining of at least 38 °SR, preferably at least 43 °SR, and particularly preferably at least 46 °SR, is achieved; c) Adding processing aids, in particular 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 / m² 2 Surface thickness, g) optionally calendering the sheet, h) treating the surface with a surface treatment agent other than starch, and i) drying the tracing paper, provided that if at least one of steps f) or g) is applied in addition to step h), steps f), g) and h) are applied in any order, once or several times.

8. Method according to claim 7, characterized in that to produce a sheet with a basis weight between 50 and 120 g / m² 2 The paper machine is operated at a speed of > 650 to 1000 m / min, preferably about 850 m / min.

9. Method according to claim 7 or 8, characterized in that the formed sheet is subjected to a calendering step with a line load between 40 and 500 kN / m, preferably 50 to 300 kN / m.

10. Method according to claim 7, 8 or 9, characterized in that a surface treatment with PEG, in particular 7 to 30 g / m², is carried out before or after calendering. 2 is carried out, and that a surface-coated sheet with a basis weight between 57 g / m² 2 and 150 g / m² 2 is being trained.

11. Method according to one of claims 7 to 10, characterized in that the step of surface treatment with a surface treatment agent different from starch is carried out either in the paper machine or in a separate coating unit.

12. A method according to any one of claims 7 to 11, characterized in that the step of surface treatment with a surface treatment agent of different thickness is carried out simultaneously with the coating with surface thickness.

13. A method according to any one of claims 7 to 12, characterized in that it is carried out with a maximum steam input per hour in the range between 30 and 40 t / h.

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