Grease-repelling paper

Abstract

In a grease-repelling paper comprising a base paper layer as well as, on at least one side of the base paper layer, a cover layer consisting substantially of a natural or synthetic, grease-repelling polymer as well as, optionally, further ingredients, such as solvents or waxes, the cover layer contains, in addition to the cover layer material selected from the natural or synthetic, grease-repelling polymer, such as a polyacrylate, an acrylic / styrene copolymer or a modified and / or derivatised starch, between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight of surface starch as well as, optionally, at least one further constituent, such as a polyvalent alcohol selected from sugar alcohols such as glycerol, sorbitol, polyethylene glycol and / or a substance selected from natural aluminas, aluminium oxide, kaolin or talc. The invention also relates to a method for producing such a paper.

Description

[0001] Greaseproof paper

[0002] The present invention relates to a grease-repellent paper comprising a base paper layer and, on at least one side of the base paper layer, a cover layer consisting essentially of a natural or synthetic, grease-repellent polymer and optionally further ingredients such as solvents or waxes, as well as to a process for producing a grease-repellent paper in which a base layer of at least one-sided coating is surface-coated with a cover layer consisting essentially of a natural or synthetic, grease-repellent polymer and optionally further ingredients such as solvents or waxes.

[0003] Greaseproof papers or greaseproof cardboard are used for a wide variety of purposes, such as in the food industry, for packaging fatty products like cheese or ready-made meals like French fries, etc., in packaging that is able to repel the high fat content rather than directly absorbing it. This prevents grease stains from forming on the paper used for packaging, and prevents users handling the packaging from coming into direct contact with the grease. The same applies to greaseproof cardboard, which is surface-treated in the same way as greaseproof papers to impart grease-repellent properties.Such cardboard boxes are frequently used in the packaging of machine parts provided with lubricating grease, for example, but they can also be used in the food sector, for example to provide dimensionally stable packaging for foods containing fat or foods prepared in fat. In order to make a paper or cardboard surface grease-repellent, a surface coating must be applied to such paper or cardboard. In the past, this grease-repellent coating was often formed from fluorine-containing chemicals or mixtures containing them. An example of grease-repellent papers having a surface coating with such fluorine-containing chemicals is described, for example, in EP 3 301 220 A1, according to which, in addition to an anionic fluorine-containing chemical, a starch component or starch mixture is also contained in the surface coating material.The disadvantages of such fluorine-containing surface coating materials are, on the one hand, that they are no longer approved for food contact, or recommendations exist for their non-use in the food sector. On the other hand, for environmental reasons, regulations always exist that require the replacement of fluorine-containing chemicals with other suitable substances. Recently, surface coating materials have been introduced onto the market that do not contain any fluorine-containing chemicals, such as mixtures of polyacrylates orPolymethacrylates and waxes, but also isocyanates, organic polysiloxanes and melamine resins, from which substances grease-repellent surface coatings can be made, which, however, due to their high proportion of organic chemicals, can cause problems during application and particularly during storage in that they must generally be applied at relatively high temperatures, require long drying times and often remain sticky, so that they are not only difficult to process but often form either stiff and thus brittle surfaces or permanently sticky surfaces when dried, they can also foam during application and thus tend to form inhomogeneous coatings and other problems of this kind.

[0004] For example, WO 2016 / 000830 describes a fluorine-free, water-repellent composition that contains polyacrylates as well as other substances such as blocked isocyanates, polysiloxanes, melamine resins, and the like. However, similar compositions are also available on the market with fluorine content and are therefore not permitted for use in the food sector.

[0005] Finally, it has recently become apparent that compositions containing derivatized or heat-treated starches and other substances such as waxes are particularly suitable as barrier compositions that can prevent the migration of fats through the cellulosic substrates.

[0006] WO 2019 / 189229 describes paper barrier materials in which multiple cover layers are applied to the paper to form either a grease and / or a gas barrier. This document shows that a coating containing a small amount of starch is overlaid by a cover layer of polymers.

[0007] WO 2018 / 200790 A1 describes a grease-resistant paper made of unbleached kraft paper, in which a top layer is applied to at least one base paper layer. The coating materials for the base paper essentially comprise polymers.

[0008] For example, EP 3 825 368 A1 describes a coating composition containing starch and a synthetic latex polymer for forming a grease barrier layer. However, latex polymers suffer from the disadvantage of usually remaining sticky even after processing, so that the application of such a grease barrier layer can only be carried out on one side of the paper. Even in this case, the sticking together of several paper layers, particularly on a drum, cannot be avoided due to the direct contact of at least two paper layers.

[0009] The present invention now aims to provide paper with grease-repellent properties in which the disadvantages of the prior art, in particular sticking together of the paper layers during or after production and during storage, can be avoided, and furthermore to be able to provide an elastic, flexible, in particular durable barrier layer which is absolutely fluorine-free and can therefore be used in the food sector.

[0010] To achieve this object, the present invention is essentially characterized in that the cover layer contains a polyacrylate, an acrylic / styrene copolymer or a modified and / or derivatized starch as a natural or synthetic polymer, that the cover layer contains, in addition to the cover layer material, between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight of surface starch and optionally at least one further component, such as a polyhydric alcohol selected from sugar alcohols such as glycerol, sorbitol, polyethylene glycol and / or a substance selected from natural clay, aluminum oxide, kaolin or talc.By containing the top layer, in addition to the top layer material selected from a natural and synthetic grease-repellent polymer and optionally further ingredients, between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight of surface starch and optionally a further component such as a polyhydric alcohol selected from sugar alcohols such as glycerol, sorbitol and / or polyethylene glycol or a substance selected from natural clay, aluminum oxide, kaolin or talc, it is surprisingly possible to eliminate the disadvantages of the prior art, in particular the formation of a sticky surface coating that is difficult to dry, and to provide a grease-repellent surface coating which is easy to apply, dries quickly and is not sticky or remains sticky, i.e. has no tendency to block. Due to this surprising reduction in the tendency to stick orBy blocking the formed grease-repellent layer, such a grease-repellent paper can be applied on conventional coating systems, i.e. either on the paper machine itself or offline, without the absolute need to increase the drying capacity of the paper machine or the coating unit or, in particular, to extend the drying distance. In this way, significant energy savings can be achieved by applying a coating material containing, in addition to commercially available and common cover layer materials, between 15 and 50 parts by weight, preferably between 20 and 30 parts by weight of surface thickness. Surprisingly, such a grease-repellent cover layer has a KIT value according to TAPPI T 559 cm-12 between 3 and 8 on the flat sheet and a KiT value according to TAPPI T 559 cm-12 on edges orFolds, where the fold was produced with a forward and immediately subsequent reverse rolling movement with the metal roller used for the Cobb test according to ISO 535:2014, between 2 and 6, which is essentially the same as a KIT value according to TAPPI T 559 cm-12 of papers coated only with a conventional and commercially available natural or synthetic, grease-repellent polymer.

[0011] By containing a polyacrylate, an acrylic / styrene copolymer or a modified and / or derivatized starch as a natural or synthetic polymer in the top layer of the grease-repellent paper, it is possible, on the one hand, to keep the grease barrier of the coating in the range of medium grease resistance, i.e. in the range of a KIT value according to TAPPI T 559 between 3 and 8, and on the other hand, by adding surface starch in an amount of between 15 and 50 wt.%, to provide grease-repellent surface coatings that are inexpensive to produce and to avoid problems that are associated with grease-repellent surface coatings according to the prior art based essentially exclusively on natural or synthetic polymers, such as polyacrylates, acrylic / styrene copolymers and optionally waxes, i.e.In particular, to avoid a long drying time for the grease-repellent surface coating and thus to ensure significant energy savings in the formation of the surface coating.

[0012] Surface starch refers to a starch without any special functionality, i.e. without any derivatization and without any additives. Conventional surface starches are selected from potato starch, corn starch, wheat starch, rice starch, sweet potato starch or cassava starch as well as mixtures of two of these. The surface starch can be a native starch or, if necessary, partially pre-degraded. The surface starches do not have any grease-repellent properties, i.e. they have no barrier effect against fats and / or oils and they are used in the paper industry, for example, as a carrier medium for sizing agents. When surface starch is used in this way on a paper surface, it is possible to increase the concentration of the sizing agent on the paper surface and thus make the paper hydrophobic but by no means grease-repellent.The surface starch used is therefore usually a starch that is as inexpensive to produce as possible and as untreated, i.e., non-derivatized, as far as possible. The term "surface coating" or "grease-repellent surface coating" according to the present invention always means a surface coating or grease-repellent surface coating that is absolutely free of any fluorine content. In this context, a natural polymer is understood to mean, in particular, a modified starch, namely either a derivatized starch or a starch to which further substances have been added that modify its properties. The properties of the starch derivatives relevant to the respective desired application are determined either by the choice of substituents or, on the other hand, by the molecular composition of the starch derivatives.This is achieved in a known manner by the fact that starch derivatization often involves a molecular degradation of the macromolecules, so that, depending on the desired application, a greater or lesser degradation of the macromolecules is sought in order to obtain a starch derivative with specific properties. The commercially available fat-repellent natural polymers used in connection with the present invention together with surface starch are often such derivatized starches that have been made fat-repellent through a special treatment or a corresponding additive. Despite the fact that their starting material was also a starch corresponding to the surface starch, they are not comparable with it in terms of their properties, molecular weight, composition, or even production costs.Well-known fat-repellent starch products available on the market include Perfectafilm B3085 (Perfectafilm is a trademark of the Cooperative AVEBE UA), which is a modified starch, Stabiiys EVO 280 (Stabilys is a trademark of Roquette Freres, Societe Anonyme), which is a thermally treated starch, and Amyloseal (Amyloseal is a trade name of Cargill Deutschland GmbH), a copolymer of starch and a vinyl acetate / ethylene copolymer.

[0013] Finally, a crosslinking compound, such as PAE (polyaminoamide epichlorohydrin resins), can be added to the grease-repellent surface coating to maintain the high grease-repellent properties of the paper and also to further improve its wet strength. Crosslinking compounds react with the surface starch via an esterification reaction when used in an amount of approximately 5% by weight of the starch. This not only reduces the weight of grease-repellent coating to be applied, but also increases the viscosity of the coating solution. This promotes curling of the paper between its production and its actual use, thus facilitating the winding of the grease-repellent paper onto parent rolls or even smaller rolls.Synthetic polymers that form a grease-repellent surface on paper or cardboard are understood here to mean exclusively fluorine-free chemicals or polymers. These are preferably acrylic-based polymers, such as Cartaseai SCR liq Ml 000, Cartaseai VWF and Cartaseal VWAF (Cartaseai is a trademark of Archroma IP GmbH, Germany), acrylic / styrene copolymers, such as Joncryl HPB 1701 (Jöncryl is a trademark of BASF Corporation), CHT Coat 2293 or CHT Coat 250 (CHT is a trademark of CHT Germany GmbH). In accordance with a further development of the invention, the grease-repellent paper is characterized in that the base paper is a paper selected from kraft paper or NSSC-.

[0014] By using paper comprising both softwood and hardwood pulp fibers, wherein in particular between 5 and 80% of the total weight of the pulp fibers, more preferably between 5 and 50% of the total weight of the pulp fibers, even more preferably between 5 and 30% of the total weight of the pulp fibers are hardwood pulp fibers, it is possible to provide a grease-repellent paper which has a particularly tear-resistant and stable base paper, so that packaging made therefrom can not only be made grease-repellent, but in particular is sufficiently stable and tear-resistant to be able to package even heavier objects, for example hinges, machine parts or the like, which have to be greased, without there being any risk of the base paper or the grease-repellent paper tearing.At the same time, due to the absence of additives that are questionable for use in the food sector, such a base paper can be used to produce grease-resistant papers or even cardboard that are suitable for packaging food. If, as is the case with a further development of the invention, unbleached kraft paper is used as the base paper, it is possible to provide a base paper that is free from harmful chemicals, particularly those that are questionable for use in the food sector, and thus, like the grease-resistant surface coating, can be used unconditionally in the food sector. Such unbleached base papers preferably have a kappa value according to ISO 302:2015 between 38 and 60. Furthermore, such unbleached kraft paper as the base paper is preferably made from more than 90% primary pulp fibers in order to prevent any contamination by substances that are not permitted or prohibited in the food sector.to reliably prevent disruptive contaminants. As is the case with a further development of the invention, by designing the grease-repellent paper in such a way that the cover layer contains up to 15 parts by weight, in particular up to 10 parts by weight of the total weight of the cover layer, of glycerin, where the parts by weight of glycerin relate to the total weight of the surface coating, it is possible to prevent the grease-resistant surface coating from drying out and thus to avoid the formation of a brittle or fragile coating whose grease-repellent properties would no longer be guaranteed, for example, in the area of ​​creases, cracks, or abraded or chipped areas. In particular, the addition of glycerin makes it possible to provide better processability and better foldability of a grease-repellent paper according to the present invention.The grease barrier properties of the paper are not affected per se by the addition of glycerin, sugar alcohols or polyethylene glycols, but merely ensure that the coating does not dry out too much and thus become brittle.

[0015] Substantially no change in the KIT values ​​according to TAPPI T 559 compared to grease-repellent surface coatings of papers made from conventional, commercially available, even fluorine-free, surface coating materials based on natural or synthetic grease-repellent polymers can be provided according to the invention by forming the top layer of such a grease-repellent paper from 50 to 80 parts by weight of the natural or synthetic polymer or a modified and / or derivatized starch, 15 to 50 parts by weight of surface starch, in particular 20 to 30 parts by weight and up to 15 parts by weight of glycerol.By adding 15 to 50 parts by weight, especially 20 to 30 parts by weight, of surface starch, it is possible to keep the KIT value essentially unchanged. It has been found that with the addition of surface starch in quantities of up to 25 parts by weight, no change in the KIT value, especially no reduction in the KIT value, is observable compared to a grease-repellent surface coating of a paper made of conventional, commercially available, natural or synthetic, fluorine-free polymers. For quantities of.

[0016] Surface thickness in the surface coating material which is above 25 wt.%, a slight decrease in the KIT value according to TAPPI T 559 can be observed. Surprisingly, however, with such a surface coating, it is not only possible to provide a grease-repellent paper with good grease-repellent properties, but in particular to avoid the disadvantages of the prior art, namely to provide a surface coating which does not tend to block and which is not sticky, so that paper can be wound onto rolls or spools without the layers sticking together, and in particular, it is possible to provide a paper which can be dried quickly and can therefore be produced either directly on the paper machine or externally, without the need for an extension of the drying section compared to a conventional

[0017] Surface coating equipment would be required. Grease-resistant papers according to the invention can be produced in a much wider grammage range, namely between 25 g / m 2 and 250 g / m 2 produced, with coatings in quantities of 2 to 6 g / m 2 and side of the paper, preferably between 3 and 5 g / m 2 The coating can be applied to either one or both sides, which is in contrast to prior art coatings, which can usually only be applied to one side due to their poor drying properties and tendency to be sticky.

[0018] The Gurley value according to ISO 5636-5:2013 of the base paper can be in a wide range, for example between 9 s and 150 s, preferably about 20 s to 50 s and the

[0019] Cobb value 60s according to ISO 535:2014 of the coating can usually be between 15 and 30 g / m 2 preferably 20 to 28 g / m 2 lay.

[0020] It is surprising that the physical parameters that otherwise significantly influence paper properties, such as Gurley (air permeability) and sizing level, smoothness (expressed as Bendtsen roughness), have a much smaller influence on the coating result when the coating according to the invention is used, which is why a significantly larger variety of base papers with different sizing levels and air permeabilities can be used when using the coating according to the invention.

[0021] Another object of the invention is to provide a process for producing a grease-repellent paper which can be used in a conventional paper machine or conventional coating device without requiring excessive energy expenditure for drying or an excessive drying time.

[0022] To achieve this object, the method according to the invention is essentially characterized in that the cover layer, which contains, in addition to the cover layer material selected from a natural or synthetic, grease-repellent polymer, such as a polyacrylate, an acrylic / styrene copolymer or a modified and / or derivatized starch, between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight of surface starch and optionally at least one further component, such as a polyhydric alcohol or alumina, is applied by means of a coating process selected from a one-sided coating process, such as a doctor blade coating process, a curtain coating process, a gravure roller coating process, a one- or two-sided coating process, such as a film press coating process or a two-sided coating process,such as a film or size press coating process and then subjected to at least one-stage drying by means of IR, roller, or hot air drying at temperatures between 65 and 95 °C, in particular 70 to 90 °C, with a drying power per meter of paper web width of 500 to 650 kW / m, preferably 530 to 620 kW / m, particularly preferably 500 to 600 kW / m. Surprisingly, with such a process, it is possible to apply the grease-repellent top layer to the base paper using a conventional coating process selected from single-sided coating processes, doctor blade coating processes, curtain coating processes, gravure roller coating processes, or even a double-sided coating process, such as a film or size press coating process,that adhesion of the formed grease-repellent coating to an uncoated paper side or another can be avoided. By subjecting the coating material, following application, to an at least one-stage drying process using infrared roller or hot-air drying at temperatures between 65 and 95 °C, in particular 70 to 90 °C, at a drying power per meter of paper web width of 500 to 650 kW / m, preferably 530 to 620 kW / m, particularly preferably 500 to 600 kW / m, or with a paper machine running at a speed between 500 and 1000 m / min, a drying power of 0.5 to 1.3 kW / min / m, 2 , preferably 0.53 to 1.24 kW.min / m 2 , especially 0.55 to 1.2 kW.min / m 2When subjected to this process, the grease-repellent coating dries quickly and completely without remaining sticky, so that this grease-repellent surface coating can surprisingly be produced using a conventional coating process with extremely reduced energy consumption—up to 60% less than for the formation of grease-repellent coatings according to the state of the art. Furthermore, any drying process can be used without fear of further polymerization or derivatization of the surface coating material.

[0023] According to a further development, the process according to the invention is carried out in such a way that the grease-repellent surface coating, in addition to a natural or synthetic, grease-repellent polymer, such as a polyacrylate, an acrylic / styrene copolymer, or a modified and / or derivatized starch, and the surface starch, additionally comprises up to 15 parts by weight, in particular up to 10 parts by weight of the total weight of the top layer, of glycerin by means of a barrier coating, and is subjected to drying in a roller dryer. By the presence of up to 15 parts by weight, in particular up to 10 parts by weight of the total weight of the top layer, of glycerin, it is possible to create a grease-repellent barrier layer or barrier coating in a drying process, such as in a roller dryer.To provide a surface layer that, surprisingly, does not dry out completely, yet also remains non-sticky and thus is not prone to cracks or tears, nor to adhesion of the layers to one another. Despite the addition of glycerin to the surface coating, papers of this type with a grease-repellent surface coating exhibit KIT values ​​according to TAPP! T 559 on a flat sheet in the average range between 3 and 8, without any reduction compared to grease-repellent coatings that do not contain glycerin. Instead of glycerin, other sugar alcohols or polyethylene glycols, for example, can be used.At the same time, to improve the surface smoothness of the grease-repellent paper and, in particular, to improve its sliding properties, other substances such as kaolin, talc, natural clays, or synthetic clays can be added to the surface coating material in amounts of up to 3 parts by weight of the total weight of the surface coating material. Surprisingly, it has been found that only natural clays, synthetic clays, aluminum oxide, kaolin, or talc, which preferably have a platelet-like structure, can be used. Substances with a spherical structure, such as calcium carbonate, are unsuitable as agents for maintaining surface smoothness in the context of the present invention.

[0024] The invention is explained in more detail below using exemplary embodiments.

[0025] For these examples, three different base papers, as defined in Table 1 below, were produced as base material and used in laboratory coating tests.

[0026] The following base paper qualities S11, S12 and S13 were used for coating tests with a Sumet® laboratory coating machine (Sumet® is a trademark of Frohnweiser Consulting GmbH, Germany).

[0027] Table'! : Raw papers

[0028] 1 Kraft is a base paper produced using the Kraft process:

[0029] 2 NSSC is a base paper produced using the Neutral Sulfite Semi-Chemical Process)

[0030] After coating, which was carried out as described in Table 2 below, the KIT values ​​of the various coated papers were determined according to the methods described in TAPPI T 559. The KIT values ​​shown in Table 2 according to TAPPI T 559 were achieved in the coating tests:

[0031] Table 2

[0032]

[0033]

[0034] ( 3 (Cartaseal is a trademark of Archroma IP GmbH, Germany;

[0035] 4 Strength in this context means a surface thickness)

[0036] The KIT values ​​according to TAPPI T 559 shown in Table 2 show that the addition of up to 35 parts by weight of surface starch to a commercially available material for forming a grease-repellent coating results in virtually no deterioration in the grease-repellent properties of the coated paper. At the same time, surprisingly, a product was obtained whose surface coating was neither sticky nor difficult to dry. Furthermore, the surface coating according to the invention makes it possible not to significantly increase the drying time for the formed grease-repellent surface coating, or even to reduce it in some cases, thus achieving equally significant energy savings in the formation of the surface coating.

Claims

Patent claims 1. Grease-resistant paper comprising a base paper layer and, on at least one side of the base paper layer, a top layer consisting essentially of a natural or synthetic grease-resistant polymer and optionally further ingredients such as solvents or waxes, characterized in that the top layer contains a polyacrylate, an acrylic / styrene copolymer, or a modified and / or derivatized starch as a natural or synthetic polymer, and that, in addition to the top layer material, the top layer contains between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight, surface thickness and optionally at least one further component, such as a polyhydric alcohol selected from sugar alcohols such as glycerol, sorbitol, polyethylene glycol, and / or a substance selected from natural alumina, aluminum oxide, kaolin, or talc. 2.Grease-resistant paper according to claim 1, characterized in that the base paper is a. Paper selected from kraft paper or NSSC paper comprising both softwood and hardwood pulp fibers, wherein in particular between 5 and 80% of the total weight of the pulp fibers, more preferably between 5 and 50% of the total weight of the pulp fibers, and more preferably between 5 and 30% of the total weight of the pulp fibers, are resinous wood pulp fibers.

3. Grease-resistant paper according to claim 1 or 2, characterized in that unbleached kraft paper is used as the base paper.

4. Grease-resistant paper according to any one of claims 1 to 3, characterized in that the top layer contains up to 15 parts by weight, in particular up to 10 parts by weight of the total weight of the top layer, of glycerin.

5. Grease-resistant paper according to one of claims 1 to 4, characterized in that the top layer is formed from 50 to 80 parts by weight of the natural or synthetic polymer or a modified and / or derivatized starch, 15 to 50 parts by weight surface thickness, in particular between 20 and 30 parts by weight surface thickness and up to 15 parts by weight glycerin.

6. Grease-resistant paper according to one of claims 1 to 5, characterized in that it has a KIT value according to TAPPI T 559 between 3 and 8, in particular between 4 and 7, on the flat sheet.

7. A process for producing a grease-repellent paper, in which a base layer of at least one-sided coating is surface-coated with a top layer material consisting essentially of a natural or synthetic grease-repellent polymer and optionally further ingredients such as solvents or waxes, characterized in that the top layer material, in addition to the top layer material selected from a natural or synthetic grease-repellent polymer, such as a polyacrylate, an acrylic / styrene copolymer or a modified and / or derivatized starch, has a surface thickness of between 15 and 50 parts by weight, in particular between 20 and 30 parts by weight, and optionally contains at least one further component, such as a polyhydric alcohol or alumina, by means of a coating process selected from one-sided coating processes, such as a doctor blade coating process.The coating is applied by a nap coating process or curtain coating, a engraved roller coating process, or a two-sided coating process such as a film or size press coating process, and is subsequently dried at least once by means of IR, roller, or hot air drying at temperatures between 65 and 95 °C, in particular 70 to 90 °C, with a drying capacity per meter of paper web width of 500 to 650 kW / m, preferably 530 to 620 kW / m, particularly preferably 500 to 600 kW / m, or, in the case of a paper machine running at a speed between 500 and 1000 m / min, a drying capacity of 0.5 to 1.3 kW / min / m. 2 preferably 0.53 to 1.24 kW / min / m 2 , in particular 0.55 to 1.2 kW / min / m 2 is subjected.

8. Method according to claim 7, characterized in that the grease-repellent surface coating, in addition to the natural or synthetic grease-repellent polymer, such as a polyacrylate, an acrylic / styrene copolymer or the modified and / or derivatized starch and the surface thickness, is additionally coated with up to 15 parts by weight, in particular up to 10 parts by weight of the total weight of the top layer, containing glycerin, by means of a barrier coating and is subjected to drying in a roller dryer.

Images