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.

Claims

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.sup.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.sup.2 and 150 g/m.sup.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.sup.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.sup.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.sup.2, is carried out before or after calendering, and that a surface-coated sheet with a basis weight between 57 g/m.sup.2 and 150 g/m g/m.sup.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.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0018] To achieve this object, the method according to the invention is substantially characterized in that it comprises the following steps: [0019] 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 other fibrous materials, [0020] b) subjecting the pulp to low-consistency beating and optionally to 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, [0021] 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, [0022] d) forming a sheet on a screen of a paper machine, [0023] e) pressing and drying to a dry content of between 30% and 80%, [0024] f) and optionally coating the sheet formed at least once with up to 3 g/m.sup.2 surface starch, g) optionally calendering the sheet, [0025] h) surface treatment with a surface treatment agent other than starch, and [0026] i) drying the transparent paper, provided that if at least one of steps f) or g) is applied in addition to step h), the steps f), g) and h) may be applied in any order, once or several times.

[0027] By carrying out 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 foundation for a paper which is on the one hand translucent or transparent and on the other hand has excellent mechanical properties. By subjecting such a pulp to low-consistency beating as a next step and, if necessary, also to high-consistency beating until a degree of beating of at least 38 SR is achieved, on the one hand the mechanical properties of the paper are further improved and, on the other hand, a degree of beating is selected at which rapid dewatering can be ensured and thus the energy costs for the subsequent drying do not become excessively high and at the same time the machine speed is kept at values above 700 m/min. Moreover, with such a process control, 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 a 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 degree of beating of at least 38 SR, a starting material can surprisingly be provided 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 beating and the subsequent treatment, has substantially no fibre-air-fibre interfaces and thus is 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 mass 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 introduced into a kraft pulp, the presence of bleached pulp fibres is rather disadvantageous for such higher amounts of starch, since the starch is then carried out into the white water in the course of the process. Surprisingly, due to the mixture of two different pulps selected in the present process it is possible to introduce high amounts of starch into the mass, in order to thus subsequently provide a transparent paper with surprisingly good mechanical properties. Since after pressing at least once the sheet formed is coated with up to 3 g/m.sup.2 surface starch 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 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 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%.

[0028] This is all the more surprising as the transparent papers known in technology or glassine papers are not or barely mechanically resilient, have almost no tear resistance and therefore cannot be used for packaging purposes, especially for sharp-edged objects, since they would tear or break immediately.

[0029] According to a further development of the invention, the method is carried out in such a way that, in order to produce a sheet with a basis weight between 50 g/m.sup.2 and 120 g/m.sup.2, the paper machine is operated at a speed of >650 m/min to 1000 m/min, preferably about 850 m/min. By carrying out the process in such a way that sheets with a basis weight of between 50 and 120 g/m.sup.2 are formed, good dewatering can be ensured despite the relatively high degree of beating and the content of fines in the mass and thus a relatively high machine speed can be maintained, with the result that high production quantities can be provided with low energy consumption. The basis weight between 50 and 120 g/m.sup.2 mentioned in this context is to be understood as the basis weight of a base paper onto which a coating with a surface treatment agent other than starch is subsequently applied.

[0030] By calendering the sheet with a linear 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 having to use excessive energy elsewhere in the process. The energy consumption in the calendering step can be kept particularly low due to the low temperatures of about 80 C. to about 130 C. at which the calendering is carried out. By additionally carrying out a surface treatment with a chemical surface treatment agent such as PEG (polyethylene glycol), in particular in an amount of 7 to 30 g/m.sup.2 before or after calendering, and forming a surface-coated sheet with a basis weight between 57 g/m.sup.2 and 150 g/m.sup.2, it is possible to increase the transparency of the kraft paper even further, so that goods contained in a packaging made of kraft paper can be clearly and unambiguously identified and checked. For example, packaging that has a viewing window made of such kraft paper can be configured in such a manner that the text 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.sup.2 and 150 g/m.sup.2, this can also be used for the packaging of heavy or irregularly shaped goods due to its excellent mechanical properties.

[0031] By carrying out the method in such a manner 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 without the additional surface treatment step on a conventional paper machine at speeds >650 m/min and nevertheless a transparent paper can be produced which is sufficiently translucent and in particular has excellent mechanical strengths. By providing a separate surface treatment unit which applies a chemical surface treatment agent, such as PEG, the opacity of the kraft paper can be further reduced to below 70% without the mechanical properties of the paper being impaired.

[0032] By applying an amount of 7 to 30 g/m.sup.2 of a surface treatment agent other than starch, such as PEG (polyethylene glycol), the basis weight of the paper in the finished state increases further depending on the amount of the chemical surface treatment agent other than starch applied. By selecting the amount of chemical surface treatment agent other than starch that is applied and by selecting the surface treatment agent used, in combination with the base paper produced, it is surprisingly possible to produce transparent or translucent paper in a wide range of basis weights, which was not possible with the processes used up to now, in particular Pergamin 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.sup.2; they had as their starting material almost 100% resin wood pulp, which is beaten to an extremely high degree and subsequently subjected to extreme pressing in order to achieve transparency. All of these process steps are very energy-intensive, since on the one hand the high degree of beating makes it very difficult to dewater the sheet and on the other hand the very high pressing pressure or pressure in a calender on the one hand requires a large amount of energy to be used for the pressing itself and furthermore, as a result the paper becomes much 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 fibres 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 beaten pulps are easy to dewater, sheet formation is simple and by applying only moderate pressures during pressing and, if necessary, calendering, the energy required for sheet formation is kept low and finally, by combining all of these measures and coating the base paper produced 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.

[0033] A particularly elegant and particularly efficient process control is achieved if the step of surface treatment with a surface treatment agent other than starch is carried out simultaneously with the coating with surface starch. Both the surface starch and the additional surface treatment agent can be applied to the paper by, for example, separate application units. and thus a high-quality, highly translucent paper with excellent mechanical properties is obtained.

[0034] By further guiding 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, in comparison to conventional translucent papers, has a specific CO.sub.2 emission in the range of less than 450 kg CO.sub.2/t of paper.

[0035] The invention is further explained below using exemplary embodiments.

Example 1: Production of a Transparent Paper with a Basis Weight of 99 g/m.SUP.2

Process Description:

[0036] A pulp consisting of 42% bleached softwood pulp, which is subjected to low-consistency beating with a beating capacity of 254 kWh/t, and 58% bleached hardwood pulp, which is subjected to low-consistency beating with a beating capacity of 98 kWh/t, was used. For both pulps a degree of beating of 40 SR is achieved. After beating, the two pulps are mixed according to the specified proportions. The auxiliary materials 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 in an amount of 11 kg/t of paper atro (atro means absolutely dry) and alkenyl succinic anhydrides were used as sizing agent in an amount of 2.3 kg/t of paper atro. 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, where the linear pressure in the three nips was 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 treated in a coating unit with 1 g/m.sup.2 surface starch. The paper was then further dried to a final moisture content of 6%. The speed at the Pope was 859 m/min.

[0037] The resulting paper with a basis weight of 78 g/m.sup.2 was coated with a total coating of 21 g/m.sup.2 PEG in two passes in a separate coating unit.

[0038] The transparent paper produced in this way had the following properties:

TABLE-US-00001 Paper property Standard Unit Direction Result Basis weight ISO 536: 2019 g/m.sup.2 99 Thickness ISO 534: 2011 m 114 Tensile strength ISO 1924-3: 2005 kN/m MD 6.3 Tensile strength ISO 1924-3: 2005 kN/m CD 2.4 Elongation at break ISO 1924-3: 2005 % MD 2.27 Elongation at break ISO 1924-3: 2005 % CD 5.64 Tensile energy ISO 1924-3: 2005 J/m.sup.2 MD 100 absorption Tensile energy ISO 1924-3: 2005 J/m.sup.2 CD 113 absorption Bursting strength ISO 2758: 2014 kPa 236 Tear resistance ISO 1974: 2012 mN MD 683 Tear resistance mN CD 788 Gurley air ISO 5636-5: 2013 s 9830 permeability 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 Basis Weight of 95 g/m.SUP.2

Process Description:

[0039] A pulp consisting of 42% bleached softwood pulp, which is subjected to low-consistency beating with a beating capacity of 254 kWh/ton, and 58% bleached hardwood pulp, which is subjected to low-consistency beating with a beating capacity of 98 kWh/ton, was used. For both pulps a beating degree of 40 SR is achieved. After beating, the two pulps are mixed according to the specified proportions. The auxiliary materials are added in the approach flow system 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 was added in an amount of 11 kg/ton of dry paper and alkenyl succinic anhydrides were used as sizing agents in an amount of 2.3 kg/ton of dry paper. 3% calcium carbonate was present as filler. The consistency of the pulp at the headbox was 0.1%. Dewatering was carried out on a wire section with a top former and in a press section with four nips, with the linear 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 treated in a coating unit with 1 g/m.sup.2 surface starch. The paper was then further dried, treated in a calender with a linear load of 60 kN/m and dried to a final moisture content of 6%. The speed at the Pope was 859 m/min.

[0040] The resulting paper with a basis weight of 78 g/m.sup.2 was coated with a total coating of 17 g/m.sup.2 PEG in a separate coating unit in one operation.

[0041] The transparent paper produced in this way had the following properties:

TABLE-US-00002 Paper property Standard Unit Direction Result Basis weight ISO 536: 2019 g/m.sup.2 95 Thickness ISO 534: 2011 m 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 energy ISO 1924-3: 2005 J/m.sup.2 MD 101 absorption Tensile energy ISO 1924-3: 2005 J/m.sup.2 CD 103 absorption Bursting strength ISO 2758: 2014 kPa 220 Tear resistance ISO 1974: 2012 mN MD 655 Tear resistance mN CD 815 Gurley air ISO 5636-5: 2013 s 14300 permeability Bendtsen roughness ISO 8791-2: 2013 ml/min Rough 226 side Bendtsen roughness ISO 8791-2: 2013 ml/min Smooth 153 side Opacity ISO 2471: 2008 % 63.1

[0042] Comparing examples 1 and 2, it becomes clear that by calendering the base paper, an opacity lower by about 4% (absolute value) is achieved, and this with a 4 g/m.sup.2 lower use of PEG for surface treatment. In both exemplary embodiments, the basis weight of the base paper was 78 g/m.sup.2.

[0043] Furthermore, calendering in this specific case results in a 21% lower thickness, an 83% lower 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 Basis Weight of 61 g/m.SUP.2

Process Description:

[0044] A pulp consisting of 55% bleached softwood pulp, which is subjected to low-consistency beating with a beating capacity of 276 kWh/t, and 45% bleached hardwood pulp, which is subjected to low-consistency beating with a beating capacity of 124 kWh/t, was used. A beating degree of 44 SR is achieved for both pulps. After beating, the two pulps are mixed according to the specified proportions. The auxiliary materials 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 in an amount of 17 kg/t of dry paper and alkenyl succinic anhydrides were used as sizing agent in an amount of 2.6 kg/t of dry paper. No filler was used. The consistency of the pulp at the headbox was 0.1%. Dewatering was carried out on a wire section with a top former and in a press section with four nips, where the linear pressure in the three nips was 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 2 g/m.sup.2 surface starch.

[0045] The paper was then further dried, treated in a calender with a linear load of 120 kN/m and dried to a final moisture content of 7%. The speed at the Pope was 795 m/min.

[0046] The paper thus obtained had a basis weight of 52 g/m.sup.2 and was provided with a coating of 9 g/m.sup.2 (mixture 90% PEG/10% glycerol) in a separate coating unit.

[0047] The transparent paper produced in this way had the following properties:

TABLE-US-00003 Paper property Standard Unit Direction Result Basis weight ISO 536: 2019 g/m.sup.2 61 Thickness ISO 534: 2011 m 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 energy ISO 1924-3: 2005 J/m.sup.2 MD 83 absorption Tensile energy ISO 1924-3: 2005 J/m.sup.2 CD 72 absorption Bursting strength ISO 2758: 2014 kPa 187 Tear resistance ISO 1974: 2012 mN MD 490 Tear resistance mN CD 571 Gurley air ISO 5636-5: 2013 s 17250 permeability Bendtsen roughness ISO 8791-2: 2013 ml/min Rough 141 side Bendtsen roughness ISO 8791-2: 2013 ml/min Smooth 117 side Opacity ISO 2471: 2008 % 49.3

Example 4: Production of a Calendered Transparent Paper with a Basis Weight of 135 g/m.SUP.2

Process Description:

[0048] A pulp consisting of 35% bleached softwood pulp, which is subjected to low-consistency beating with a beating capacity of 271 kWh/t, as well as 40% bleached hardwood pulp, which is subjected to low-consistency beating with a beating capacity of 98 kWh/t, and 25% unbleached softwood pulp, which is subjected to low-consistency beating with a beating capacity of 271 kWh/t, was used. A beating degree of 38 SR is thus achieved for both pulps. The two softwood pulps were mixed in advance according to their proportions and beaten together. The final mixing of all pulps was carried out after beating according to the specified proportions. The auxiliary materials 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 in an amount of 20 kg/t of dry paper and alkenyl succinic anhydride was used as sizing agent in an amount 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, where the linear pressure in the three nips was 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 treated in a coating unit with 1.1 g/m.sup.2 surface starch. The paper was then further dried in a calender with a linear load of 200 kN/m and dried to a final moisture content of 7%. The speed at the Pope was 720 m/min.

[0049] The resulting paper with a basis weight of 110 g/m.sup.2 was coated with a total coating of 25 g/m.sup.2 PEG in two passes in a separate coating unit.

[0050] The transparent paper produced in this way had the following properties:

TABLE-US-00004 Paper property Standard Unit Direction Result Basis weight ISO 536: 2019 g/m.sup.2 135 Thickness ISO 534: 2011 m 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 energy ISO 1924-3: 2005 J/m.sup.2 MD 152 absorption Tensile energy ISO 1924-3: 2005 J/m.sup.2 CD 147 absorption Bursting strength ISO 2758: 2014 kPa 412 Tear resistance ISO 1974: 2012 mN MD 1170 Tear resistance mN CD 1310 Gurley air ISO 5636-5: 2013 s 28740 permeability Bendtsen roughness ISO 8791-2: 2013 ml/min Rough 157 side Bendtsen roughness ISO 8791-2: 2013 ml/min Smooth 135 side Opacity ISO 2471: 2008 % 51.2