Patent Application
20250154721
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.
Transparent paper or translucent paper has 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 can only be produced with low basis weights of up to about 50 g/m2 and, moreover, only have very limited strength, there are 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, especially synthetic resins such as acrylic resins, polyester and/or urethane resins are used in such processes which are intended to make the paper translucent, in addition to coating the paper with the chemicals per se, it is usually necessary to crosslink or cure the transparentizing material after it has been applied by applying heat or UV rays or other crosslinking processes. Such a method, by means of which a transparent region can be formed in paper, is described, for example, in U.S. Pat. No. 5,418,205, in which a cellulosic substrate for impregnation is impregnated with a polymerizable, transparentizing material, the impregnated region being subsequently hardened or crosslinked by radiation and a transparent region in the paper is thus formed.
A similar method is described in U.S. Pat. No. 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.
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 achieve a certain transparency of the paper. Such materials, which are intended to make the paper translucent, are usually ground extremely finely together with the cellulose fibres and grease-resistant and slightly transparent papers are produced from this mixture.
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 surface can be formed directly in the paper to be finished, is described, for example, in WO 2008/091523, in which the area of the paper that is to be made translucent is perforated by means of laser beams in order to achieve a transparent area of the paper by forming a network of micro-small, closely spaced holes.
All of these methods are either very complex, can only be produced using large amounts of energy or use considerable amounts of chemicals, which makes these papers only of very limited use, especially if 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.
Another problem with such transparent papers or areas of a paper that have been made transparent are that they are frequently brittle and, above all, have only low strength, which is particularly the case for applications where high-strength paper types are required, such as in packaging for products with a higher weight or irregular surfaces with edges or corners and the like where up to now, only papers could be used in which the transparent window, usually consisting of plastic, had to be inserted separately, since the strength of the classic transparent papers was not sufficient even for such an application.
Up to now, it has not been possible to make high-strength papers sufficiently translucent using conventional methods without reducing their strength so much, either by perforating them with a laser or by extremely strong beating and/or extremely strong calendering of the pulp fibres used, that such methods are not suitable for the production of transparent papers for large-scale use, in particular for the packaging of products that have irregular or sharp-edged surfaces and/or have a greater 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 of 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 a high strength, in particular strength properties substantially corresponding to kraft paper, and which comprises only biodegradable substances and no environmentally harmful chemicals in or on the paper.
To achieve this object, the transparent paper according to the invention is essentially characterized in that the kraft paper is formed from a bleached, low-consistency and optionally high-consistency pulp beaten to a degree of beating 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 at least 35%, particularly preferably at least 40% and at most 60% from softwood pulp and to an extent of at least 35%, preferably at least 40%, particularly preferably at least 45% from 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 optionally mechanically surface-treated with further substances selected from PEG (polyethylene glycol), glycerol, polyvinyl alcohol or mixtures of two or more thereof and optionally starch, such as mixtures of PEG and starch. By producing 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 or translucency. By adding furthermore 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 is, on the one hand, suitable for use in the food sector and, on the other hand, to provide an optimal compromise between mechanical resilience and processability. The 0.5% to 2.5% cationic starch contained in the paper further improves the strength of the paper, so that surprisingly, by using hardwood pulp in a kraft paper it is possible to give it transparency whilst at the same time maintaining high strength. With regard to the amounts of starch used, it is known to the person skilled in the art that bleached pulp or bleached pulp fibres allow the use of only 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 the starch as such has a rather detrimental or at most neutral effect in relation to a 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 at the same time. According to the invention, in this context the procedure is such that in addition to the cationic starch contained in the paper, the kraft paper is optionally mechanically surface-treated with other substances or mixtures of other substances or mixtures of starch and other substances. As a result of the surface treatment with the aid of 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.
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.
A transparent paper according to the present invention is not understood to mean a completely clear and transparent paper, but a paper with an opacity or turbidity as defined in ISO standard No.: 2471:2008. Accordingly, opacity is understood as the degree of turbidity of the translucent paper or transparent paper according to the invention.
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 order.
Since, as corresponds to a further development of the present invention, the paper is formed in such a manner that it contains 20% to 30% of additional fibrous materials selected from the group of grass, hemp, jute, linen, cotton, unbleached fibrous material, eucalyptus, waste paper, recycled paper, viscose fibres or the like, it is possible to provide a transparent paper which, on the one hand, has an even greater strength and, on the other hand, has an extremely low CO2 footprint of less than 450 kg CO2/t of paper in production. The additional fibrous materials can be added as coloured fibrous materials, bleached or unbleached fibrous materials, in particular fibrous materials originating directly from a production or recycling process. Since, as corresponds to a further development of the present invention, the transparent paper is configured in such a manner that the additional fibres only contain unbleached fibrous material from softwood pulp, in particular 20 to 30% unbleached fibrous material from softwood pulp, it is possible to achieve a CO2 footprint which only has a CO2 emission of less than 350 kg CO2/t paper. At the same time, 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, particularly if only up to about 30% unbleached fibrous material from softwood pulp is added as additional fibrous materials.
Since, as corresponds to a further development of the invention, the transparent paper is further developed in such a manner that it is surface-coated with a chemical surface coating agent other than starch, such as PEG, in particular 7 to 30 g/m2 PEG, and the surface-coated sheet has a basis weight between 57 g/m2 and 160 g/m2, it is possible to provide surprisingly strong and nevertheless translucent papers which, in particular due to their excellent tear resistance, are suitable for use as packaging in the food sector or for objects with irregular or sharp-edged surfaces.
Since, as corresponds to a further development of the invention, the transparent paper is additionally calendered, 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 improves the transparency of the paper even further.
The present invention further aims to provide a method for producing the transparent paper, by means of which it is possible to provide a transparent paper having excellent mechanical properties.
To achieve this object, the method according to the invention is substantially characterized in that it comprises the following steps:
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/m2 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%.
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.
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/m2 and 120 g/m2, 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/m2 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/m2 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.
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/m2 before or after calendering, and forming a surface-coated sheet with a basis weight between 57 g/m2 and 150 g/m2, 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/m2 and 150 g/m2, this can also be used for the packaging of heavy or irregularly shaped goods due to its excellent mechanical properties.
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.
By applying an amount of 7 to 30 g/m2 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/m2; 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.
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.
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 CO2 emission in the range of less than 450 kg CO2/t of paper.
The invention is further explained below using exemplary embodiments.
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/m2 surface starch. The paper was then further dried to a final moisture content of 6%. The speed at the Pope was 859 m/min.
The resulting paper with a basis weight of 78 g/m2 was coated with a total coating of 21 g/m2 PEG in two passes in a separate coating unit.
The transparent paper produced in this way had the following properties:
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/m2 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.
The resulting paper with a basis weight of 78 g/m2 was coated with a total coating of 17 g/m2 PEG in a separate coating unit in one operation.
The transparent paper produced in this way had the following properties:
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/m2 lower use of PEG for surface treatment. In both exemplary embodiments, the basis weight of the base paper was 78 g/m2.
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.
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/m2 surface starch.
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.
The paper thus obtained had a basis weight of 52 g/m2 and was provided with a coating of 9 g/m2 (mixture 90% PEG/10% glycerol) in a separate coating unit.
The transparent paper produced in this way had the following properties:
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/m2 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.
The resulting paper with a basis weight of 110 g/m2 was coated with a total coating of 25 g/m2 PEG in two passes in a separate coating unit.
The transparent paper produced in this way had the following properties:
| Number | Date | Country | Kind |
|---|---|---|---|
| A 50090/2022 | Feb 2022 | AT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/AT2023/060033 | 2/6/2023 | WO |
