Release paper and method for the production thereof

Abstract

In a release paper consisting of a backing paper layer coated on at least one side with PVOH and a silicone coating on at least one side, in which the backing paper layer coated on at least one side with PVOH consists of bleached kraft paper, containing at least up to 30 wt. % softwood and hardwood with a total freeness between 30 °SR and 42 °SR, preferably between 33 °SR and 38 °SR, and processing aids, and also a release layer which is applied at least on one side to the PVOH-coated side of the backing paper layer and consists of 0.5 to 5 g / m2 of a silicone, and wherein the release paper coated at least on one side with PVOH and silicone has a density between 850 and 990 kg / m3, preferably between 900 and 970 kg / m3, and a method for producing a release paper.

Description

[0001] Separating paper and method for producing it

[0002] The present invention relates to a release paper comprising a carrier paper layer coated at least on one side with PVOH and a silicone coating on at least one side, and to a method for producing a release paper in which a carrier paper layer coated at least on one side with PVOH is provided with a silicone coating on at least one side.

[0003] Release papers are typically used to protect products or surfaces, or to create specially designed flat structures. A key characteristic of release papers is that they have at least one non-stick or anti-stick coated surface. Such a non-stick surface can be achieved by coating or impregnating a base paper with various polymers, the type of non-stick surface chosen depending on the intended application of the release paper.

[0004] Release liners are commonly used as backing material for self-adhesive labels or as release papers in technical applications such as adhesive tapes. These liners are often made from papers with a polymer coating of PE or PP, making them either impossible or extremely difficult to recycle, as the polymers are very hard to separate from the paper. Silicones are also used as release layers. While silicones themselves are not harmful to the environment, papers coated with them can usually only be recycled in specially equipped facilities, and often require significantly more energy and fiber cleaning.

[0005] KR 20230138197 A describes a recyclable release liner paper that has an aqueous-based coating and an environmentally friendly release layer on both sides of a backing paper. The environmentally friendly release layer is formed from a polymer such as polydimethylsiloxane, a water-soluble wax, or silicon dioxide. The aqueous-based coating can be an acrylic-based coating, ethylene vinyl acetate, or a mixture thereof. Such a release liner paper is characterized by its biodegradability in water. US 2023 / 0407145 A1 describes, in addition to special silicone compositions that can be used as release layers, papers in which, for example, a base paper, cardboard, or similar material is coated with other materials.A silicone composition as described above is applied as a separating layer to papers coated with polyolefins, for example by means of roller coating, spray coating or the like, and the coated paper or cardboard is then allowed to harden.

[0006] The JP 2006291383 A is a heat-resistant release paper which must have an impregnation with acrylic resin on a base paper layer before a silicone coating is applied.

[0007] The JP H07216327 A is a substrate for a release paper, which substrate must be based on a base paper consisting of a kraft pulp made from soft and hardwood fibers, each ground to a fineness of 470 ml, and must contain an aqueous solution of a polyvinyl alcohol-based resin, in particular fully or partially saponified polyvinyl alcohol in a specific concentration and with a degree of polymerization between 200 and 800.

[0008] Older, currently known release papers, due to the high degree of refining of the pulps underlying them, which form the backing paper layers, can only be produced using large amounts of energy and / or also employ considerable quantities of chemicals for the formation of the release layer and / or impregnations. This makes these coated papers unacceptable whenever, for ecological reasons, either an inexpensive and, if necessary, at least partially recyclable backing paper is required, or when recycling attempts fail to yield any paper components free from the plastic materials forming the release layer or the impregnations, since separating the plastic components from the paper components is either impossible or only possible using extremely complex processes.

[0009] Finally, so-called "multi-coated kraft papers" (PCK release papers) are also known. While these are theoretically recyclable using the established CEPI process, they have a polymer content of more than 20%, making any attempt to recover the paper component of these release papers economically unviable. Only a portion of the polymer content can be recovered from these papers. Therefore, from the perspective that today, for environmental reasons, almost exclusively papers with the lowest possible CO₂ footprint are desired, and that as many components of the paper as possible can be almost completely recycled or even fully recycled, they do not meet the demands of modern industry and the market.The present invention aims to provide a release paper based on a high-strength carrier paper which, compared to known release papers, has an improved COs footprint and which can also be processed and recycled without the need for complex and energy-intensive recycling processes.

[0010] To solve this problem, the release paper according to the invention is essentially characterized in that the carrier paper layer, coated at least on one side with PVOH, consists of bleached kraft paper, which comprises at least 30 wt.% of softwood and hardwood with a total fineness between 30 °SR and 42 °SR, preferably between 33 °SR and 38 °SR, as well as processing aids, and of a release layer applied at least on one side to the PVOH-coated side of the carrier paper layer, which consists of 0.5 to 5 g / m² 2a silicone and that the release paper, coated at least on one side with PVOH and silicone, has a density between 850 and 990 kg / m³ 3 , preferably between 900 and 970 kg / m³ aThe invention features a backing paper layer made of bleached kraft paper, consisting of at least 30% by weight of both softwood and hardwood, with a total refining grade between 30°SR and 38°SR. This results in a backing paper layer with good mechanical properties, sufficient compactness and strength, and, moreover, the ability to produce it with relatively low energy consumption due to the refining grade of the pulp contained within, which falls within a medium range of the Schopper-Riegler scale. Furthermore, by including common processing aids in the backing paper layer, such as starch, sizing agents selected from ASA and AKD, or fillers like calcium carbonate, a sack kraft paper or any other packaging paper or semi-transparent paper can be used as the backing paper layer.By further coating such a carrier paper layer with PVOH on at least one side, a carrier paper is provided whose surface pores are filled with PVOH and which has a particularly smooth and homogeneous surface, which can subsequently be coated with extremely small amounts of 0.5 to 5 g / m². 2 A silicone coating can be used as a release layer, as there is no longer a risk of the silicone penetrating the surface pores of the backing paper. These small amounts of silicone are advantageous from both an economic and ecological perspective. They enable cost-effective production of the release paper, reduce the CO2 footprint compared to conventional release papers, and surprisingly result in a release paper that exhibits both good mechanical and peel properties and is also fully recyclable.

[0011] The recyclability of the release papers is tested using the CEPI Recyclability Test Method, Version 2, October 2022 (Harmonized European laboratory test method to generate parameters enabling the assessment of the recyclability of paper and board products in standard paper and board recycling mills - short title: CEPI Recyclability Laboratory' Test Method, Version 2, October 2022, 15 pages, https: / / www.cepi.org). The release paper according to the invention contains no polymers and / or oligomers on or in the backing paper layer other than polyvinyl alcohol and silicone.

[0012] By continuing to use the release paper consisting of the PVOH-coated backing paper layer and the silicone release layer, a density between 850 and 990 kg / m³ 3This material not only produces a paper with exceptionally high stiffness, but also a very durable, compact release paper with excellent mechanical properties. It is suitable for a wide variety of applications, such as peel-off strips, which can be used as a replacement for glassine-based release papers, and as release papers in capacitors, batteries, and similar devices. At the same time, compared to conventional release papers that contain up to 20% polymer, particularly silicone, this type of release paper has a particularly low silicone content, namely a maximum of 12.5% ​​by weight at 5 g / m². 2 on a finished release paper with a basis weight of 40 g / m² 2 are applied. In practice, however, the high quantities of silicone are only used on paper with basis weights above 80 g / m². 2applied so that the average silicone content of the paper is a maximum of 6.5 wt.%.

[0013] Furthermore, since the backing paper can be produced with very low grammages due to the stiffness of the paper, a considerable saving of material is achieved, which makes the backing paper particularly advantageous from an ecological and economic point of view, also due to the reduced use of the natural material wood compared to conventional products.

[0014] The recyclability of the release paper is surprisingly achieved by the fact that, when such a release paper is immersed in a polar solvent, preferably water, the water can penetrate laterally between the PVOH-coated backing paper layer and the silicone layer by dissolving the water-soluble PVOH layer and thus delaminating the release paper. The water-insoluble silicone layer, acting as the release layer, can be recovered from the resulting suspension of water and backing paper components. This suspension can then be recycled, for example, into papermaking, thus enabling complete recycling of the entire release paper.

[0015] The overall grind rating is understood to be the average grind rating of all pulps used, such as softwood, hardwood, and any other fibrous components like recycled wood or reject material. To determine it, the grind ratings of the individual components are taken, multiplied by their percentage in the pulp, the resulting values ​​are added together, and then divided by 100, as expressed by the following formula.

[0016] (A x X) + (B x Y) = total grinding degree,

[0017] 100 where A is the first pulp, B is the second pulp, X is the percentage of the first pulp, and Y is the percentage of the second pulp. The formula can be supplemented by adding any other fiber components that may be present and their percentages.

[0018] In the case of the release paper in question, the side coated with PVOH is also additionally coated with silicone. A coating of either PVOH or silicone on two different sides of the paper is not intended.

[0019] In a further development of the invention, the release paper is designed such that the backing paper layer has a hardwood content of between 30 and 70 wt.% with a fineness of refining between 28 °SR and 40 °SR, a softwood content of between 30 and 50 wt.% with a fineness of refining between 32 °SR and 45 °SR, and furthermore process aids such as 0.5 to 2.5 wt.% cationic starch, 0.1 to 0.8 wt.% bulk sizing agent and optionally up to 3 wt.% fillers, energy consumption during production can be further optimized, since the pulp forming the backing paper layer is easily dewaterable due to the fineness of refining in the medium range, the energy consumption for both refining and dewatering is relatively low, so that the overall energy consumption is significantly reduced compared to release papers based on highly refined glassine papers.

[0020] Such pulps with a medium grind size are easily and quickly dewatered, so that the energy required for dewatering and drying is significantly lower than for comparable glassine papers. Furthermore, they result in a dense and homogeneous substrate paper with good mechanical properties, so that, for example, the basis weight is low, i.e., below 130 g / m². 2 , preferably below 100 g / m² 2 or greater than 40 g / m² 2This can be maintained, which also reduces overall energy consumption. Furthermore, such a homogeneous paper requires less coating material for subsequent PVOH coating and is therefore not only economical to produce, but, in particular, the CO₂ footprint is significantly improved due to the small amount of PVOH required. Finally, due to the medium refining level of the cellulose fibers, such a carrier paper is more energy-efficient to produce than, for example, conventional glassine paper, which has been used to date as a carrier layer for release papers. State-of-the-art transparent papers typically require very high refining levels, which necessitates high energy consumption for both refining and subsequent dewatering and drying, rendering such transparent papers obsolete as carrier papers.It is all the more surprising that the papers used as carrier papers according to the present invention are at least semi-transparent, so that when using the carrier papers according to the invention, all the advantages that the use of classic transparent papers brought with them can also be achieved.

[0021] By selecting bulk sizing agents from ASA, AKD, cationic starch, and mixtures thereof, as is standard practice in advanced paper production, it is possible to manufacture a paper that contains as few processing aids as possible, and in particular, a high starch content in the finished release paper. The overall starch content in the paper also allows for a uniform density, resulting in surprisingly homogeneous properties for both the base and release paper across the entire width of the paper machine. At the same time, this paper is significantly more energy-efficient to produce than papers manufactured using state-of-the-art technology.In a further development of the invention, the release paper is designed such that the backing paper layer is a backing paper calendered in a hard nip calender with a line load between 60 and 200 kN / m, preferably 80 to 120 kN / m, preferably calendered on both sides. This results in a particularly smooth surface of the backing paper layer, so that extremely small amounts of PVOH are sufficient to homogenize the surface structure when the backing paper layer is subsequently coated with PVOH. This measure not only further improves the smoothness of the release paper but, in particular, minimizes the use of coating agents, which would otherwise have to be removed from the paper surface during recycling.At the same time, surprisingly, such a calendered and PVOH-coated carrier paper exhibits a reduced, but still clearly recognizable, transparency compared to glass-sine papers, so that a release paper based on such a carrier paper can be used in exactly the same way as release papers based on, for example, glass-sine papers.

[0022] According to a further development of the invention, the backing paper is designed such that it is coated on both sides with polyvinyl alcohol. Such a double-coated paper has identical properties on both surfaces, is particularly smooth, and exhibits consistently good mechanical stability, especially significantly improved stability compared to conventionally used Giassine papers as backing layers. At the same time, such a backing paper has the good fiber properties of a low-refining paper, which is advantageous when recycling paper, since excessively refined paper exhibits disadvantages during recycling, such as reduced impact resistance and requires a higher energy input compared to low-refining papers.Furthermore, with such a relatively finely milled paper, cross-linking of the fibers with glyoxal can be omitted, which is why it is not only more economical to produce, but in particular more water-soluble and therefore more recyclable than paper cross-linked with glyoxal.

[0023] To produce a particularly stable and simultaneously recyclable release paper, the release paper according to the invention is designed such that the carrier paper layer coated on both sides with PVOH is coated on at least one side with 0.7 to 3 g / m² 2 , preferably 1 to 2 g / m² 2 Silicone, preferably double-sided with 0.7 to 3 g / m² on each side. 2 , preferably 1 to 2 g / m² 2It is coated with silicone. Such a release paper, coated with two different coating materials, has a higher silicone content compared to conventional release papers, especially Giassine papers, which typically contain very small amounts of silicone relative to their total weight, at basis weights between 40 and 150 g / m². 2Maximum amounts of silicone of approximately 12 wt.% are applied. Preferably, these amounts are significantly below 10 wt.%, and preferably even below 7 wt.%, relative to the basis weight. Despite the comparatively higher silicone content, such papers are more economical to produce because the significantly reduced degree of refining compared to glassine papers leads to substantial energy savings during the lamination, dewatering, and drying processes. Therefore, the slightly higher silicone content in the coating is negligible for economic reasons. Furthermore, their properties as release papers are not impaired compared to release papers coated with lower amounts of silicone.This is particularly important because the application of the first PVOH layer closes the pores in the paper and smooths the paper surface, so that the amount of silicone required for the release layer is of minor importance for the paper's properties. With a silicone coating according to the invention, it is possible, on the one hand, to provide a paper with good mechanical properties that can be recycled by simply soaking it in water. During recycling, the PVOH layer formed on the surface of the backing paper dissolves, and the water-insoluble silicone layer floats to the surface separately from the paper layer, which swells in the water, and can then be separated.

[0024] Such a recyclable release paper has the immense advantage that, for environmental reasons, it can be separated into its individual components. The paper component can be recycled, and the silicone component can potentially be reprocessed. This significantly reduces the overall energy and material consumption during production and recycling compared to conventional release papers, even when reprocessing the paper itself. The recyclability of such a release paper can be determined using the aforementioned CEPI Recyclability Laboratory Test procedure for standard recycling equipment.

[0025] As is known to those skilled in the art, applying silicone to paper increases the density of the coated paper, since the silicone, like PVOH applied directly to the backing paper, penetrates or can penetrate the honeycomb spaces of the paper and thus increases the overall density of the paper. According to the invention, the PVOH-coated backing paper is provided to have a density of no more than 980 kg / m³. 2 , preferably between 830 kg / m³ 2 and 980 kg / m² 2Such papers are sufficiently dense to meet the requirements of a release paper, particularly when peeling from a surface, and also possess sufficiently good mechanical properties so that the release paper does not tear during peeling, even though the density selected according to the invention is significantly lower than, for example, that of conventional glassine papers. In this case, the lower density is compensated for by the better mechanical properties of the fibers used, especially the lower milling grades compared to glassine papers, so that the paper, while having a relatively low density overall, simultaneously exhibits good mechanical properties and, due to the lower energy consumption during production (namely, milling, dewatering, and drying), has a significantly improved CO2 footprint than conventional glassine papers used as backing papers for release papers.Furthermore, due to the double coating of the release paper with PVOH and silicone, the amount of silicone can be optimized, since the previously applied PVOH closes the surface pores of the paper, and only relatively small amounts of silicone are required. This is achieved, as in a further development of the invention, by designing the release paper such that the backing paper layer coated with PVOH on at least one side has a basis weight according to ISO 536 between 40 and 130 g / m². 2 , preferably 55 to 110 g / m² 2Due to its properties, the release paper according to the invention can be used in a wide variety of applications. For example, it can be used as a peel-off strip on stationery such as envelopes, glue rolls, or the like, in which case a relatively low basis weight is sufficient. However, it can also be used, for example, in the area of ​​industrial products such as feed bags, as a process liner for fiber composites, for laminating fiber drums, or the like, in which case it has proven advantageous to provide the release paper with a higher basis weight.

[0026] The present invention further aims to provide a process for producing a release paper that is as completely recyclable as possible, with the greatest possible energy efficiency. To achieve this objective, the process according to the invention is essentially characterized by the following steps: a) forming both a softwood pulp and a hardwood pulp; b) subjecting the pulps to low-consistency grinding, preferably separate low-consistency grinding in a refiner or grinding device; c) mixing the low-consistency ground pulps such that a pulp is formed which consists of at least 30 wt.% softwood and hardwood with a total degree of refining between 30 °SR and 42 °SR, preferably between 33 °SR and 38 °SR, and adding process aids, such as 0.5 to 2.5 wt.-% cationic starch, 0.1 to 0.8 wt.% bulk sizing agent and optionally up to 3 wt.% fillers; d) dewatering the formed mixed pulp on a wire of a paper machine and in a press section of the paper machine with at least 3 press rolls, optionally using steam or vacuum; e) coating at least one side of the formed sheet with PVOH and optionally discharging the sheet from the paper machine; f) calendering the formed sheet with a calender, preferably a hardnip calender; g) optionally printing the formed sheet; h) coating at least one side of the PVOH-coated side of the sheet with a silicone; and i) curing the silicone and optionally drying the sheet, wherein a release paper with a density between 850 and 990 kg / m² is obtained. 3 , preferably between 900 and 970 kg / m³ 3is formed. With this type of process, it is possible to produce a pulp with a medium degree of refining, so that relatively low energy is required for pulp milling. The energy consumption in the production of the release paper can be kept even lower by dewatering such pulp on the wire section of the paper machine, since the pulp is only refined to the point where the mechanical properties of the resulting paper are excellent, but not so finely refined that dewatering would be difficult or require more energy and time. The same applies to the subsequent pressing and drying of a sheet produced in this way, which can also be done without high energy consumption.During pressing in the paper machine, a formed sheet is further dewatered, with the lowest possible pressures and / or temperatures being used to minimize the energy required for pressing and drying. Coating at least one side of a formed sheet with polyvinyl alcohol (PVOH) smooths out surface irregularities, fills the pores with PVOH, and thus produces a very smooth paper on at least one side without the need for special measures such as pressing the base paper with a high line load. Further smoothing of such a paper is achieved according to the inventive method by calendering the paper after coating with PVOH. This forces the PVOH densely into the paper fibers, resulting in a particularly homogeneous surface that is essentially pore-free.

[0027] Finally, by applying a silicone layer to the PVOH-coated surface, the paper according to the invention can be used as release paper, wherein such release paper is characterized in that it can be completely recycled; in particular, the silicone layer can be completely separated from the paper layer. This can optionally be subjected to further processing or recycling. For example, the catalyst contained in the silicone, usually platinum, can be recovered.

[0028] According to a further development of the invention, the process is carried out such that a mixed pulp is produced, comprising a hardwood content of between 30 and 70 wt.% with a fineness of refining between 30 °SR and 40 °SR and a softwood content of between 30 and 50 wt.% with a fineness of refining between 32 °SR and 45 °SR. With the selected mixture of hardwood and softwood, it is possible to optimally adjust the strength values ​​as well as the surface quality of a substrate paper produced from this mixture to the respective requirements, such as printability, tear resistance, etc. This is achieved by pressing the release paper in the press section of the paper machine with a plurality of press rolls, which apply different line loads, such as...With a coefficient of friction of less than 120 kN / m and a maximum of 800 kN / m, very good dewatering of the carrier paper is achieved, and at the same time, such a process makes it possible to keep the energy expenditure for dewatering the carrier paper as low as possible, so that overall the COa footprint of the produced release paper is significantly better than that of conventional glassine-based papers.

[0029] By applying a suction pressure in the range of -300 to -600 mbar, preferably about -500 mbar, to the paper web in the shoe press, further improvement in dewatering is achieved. At the same time, the application of relatively high drying temperatures can be avoided, resulting in at least equally good dewatering of the paper with significantly lower energy consumption than with release papers according to the prior art. To achieve a particularly smooth surface, as is the case in a further development of the invention, the PVOH-coated carrier paper is further calendered with a line load between 80 and 200 kN / m², the calendering preferably being carried out on both sides, in which case completely smooth carrier paper can be produced on both sides.With this method, the PVOH coating is pressed into the pores and surface irregularities of the substrate paper, so that no open pores or irregularities remain on the paper surface. Since the method according to the invention further provides that a silicone coating is applied only after the substrate paper is finished, in particular to the PVOH-coated surface of the substrate paper and any calendered surface of the substrate paper, relatively large quantities of silicone coating material can subsequently be used, as these can be recovered almost quantitatively during paper recycling. This makes the method significantly better than conventional methods from both an economic and ecological perspective.The inventive method can be produced on a conventional paper machine at speeds between 700 and 950 m / min, producing paper with basis weights between 40 and 130 g / m². 2 such speed wedges are within the absolutely average range of a paper machine and thus allow the machine to be operated extremely economically without the need for high-performance units for the production of the base paper.

[0030] The release paper according to the invention can thus be produced on a conventional paper machine using bleached pulps, particularly those with low basis weights and medium density, with relatively low overall energy consumption for production. Furthermore, due to the PVOH layer provided according to the invention between the backing paper layer and the silicone release layer, it is possible to provide a release paper that is fully recyclable, particularly because, during recycling, for example by treatment with water, the water penetrates laterally between the layers of the release paper and the silicone, dissolves the polyvinyl alcohol, and thus separates the silicone layer from the paper layer.Surprisingly, the inclusion of the PVOH layer prevents the fibers from clumping due to the silicone coating. This results in a significantly higher yield of recovered fibers during subsequent recycling, as they can be cleanly separated from the silicone coating and thus reused. Consequently, this approach also significantly reduces the rejection rate of fibers that cannot be immediately recycled.

[0031] Such a process is very economical on the one hand, and on the other hand, both the production and recycling of this paper require very little energy compared to conventional release papers based on glassine papers, so that overall the CO2 footprint of such a release paper is significantly improved compared to conventional products.

[0032] The invention is explained in more detail below with reference to exemplary embodiments shown in the examples.

[0033] Example 1: Method for producing two carrier papers as a basis for the release papers according to the invention

[0034] Process description:

[0035] A pulp consisting of 42% softwood primary pulp was subjected to low-consistency refining, yielding a softwood pulp fineness of 40 °SR. Furthermore, 58% hardwood primary pulp was used, which was subjected to low-consistency refining until a hardwood pulp fineness of 35 °SR was achieved. The pulps were blended, resulting in a pulp mixture with an overall fineness of 37.1 °SR. In the constant section of the paper machine, the pH was adjusted to 8.1, and additives were added as follows: cationic starch was added in an amount :Alkenyl succinic anhydride (ASA) was added at a rate of 2.5 kg / t of dry paper to the pulp, and calcium carbonate was added as a bulk sizing agent at a rate of 23 kg / t. The two different types of pulp blended easily into a homogeneous mixture. The starch, bulk sizing agent, and fillers were added to the blended pulp. The consistency of the pulp at the headbox was adjusted to 0.3% ± 0.01%. Dewatering took place on the wire section of the paper machine with a top and bottom former at a wire speed of 850 m / min, and in a press section with three press rollers and a shoe press, with the line pressure in the three nips being 48 kN / m, 95 kN / m and 115 kN / m and in the nip of the shoe press 700 kN / m.

[0036] Despite the use of softwood and hardwood fibers, a kraft paper with homogeneous properties and a uniform surface was obtained. The kraft paper was then pre-dried and coated on both sides with 1 g / m² in a coating unit. 2 PVOH coated. The PVOH-coated paper was further smoothed in a hardnip calender with a line load of 110 kN / m and 85°C.

[0037] The resulting carrier paper had a basis weight of 79 g / cm² according to ISO 536:2019. 2 , a density of 920 g / cm³ 3 The PVOH-coated carrier paper was subsequently printed with a logo on both sides and then coated with a second layer of 3 g / m² using a smoothing roller. 2 coated with silicone.

[0038] In a test, it was examined whether release paper produced in this way is recyclable, specifically whether the non-recyclable silicone coating can be separated from the backing paper. For this purpose, unprinted test sheets (1. Test Sheets) were produced from paper coated with both PVOH and silicone and subjected to a CEPI Recyclability Laboratory Test Procedure, Version 2 - October 2022, for standard recycling plants without ink removal technology. For this, 50 g of oven-dried paper sheets were treated in 2 L of 40 °C water for 10 minutes at 30,000 rpm. The suspension was then diluted to 8 L and homogenized in a distributor for 5 minutes. After fractionation with a 5 mm plate, test sheets (2. Test Sheets) were formed from the non-foaming suspension. 0.1% of the solids contained in the suspension were retained as coarse residue on the sieve plate.In a further experiment, the percentage of residue was determined after identical treatment as described above, but after filtration through a sieve plate with 0.15 mm slots. Sample sheets were again formed from the fraction passing through the slots. 6.6% of the originally contained solids were retained on the sieve plate with 0.15 mm slots. This so-called fine residue contained, in addition to individual cellulose fibers, silicone and primarily printing ink particles.

[0039] From the fractions that passed through the sieve plates, new test sheets (3rd test sheets) were produced, and the three different types of test sheets were compared with each other with regard to their homogeneity. The 2nd test sheets were the least homogeneous, and the 3rd test sheets had the greatest degree of homogeneity.

[0040] For an initial assessment of recyclability, the PTS threshold values ​​(PTS-RH 021 :2012) were used. The total amount of residues was evaluated – in this case, it is 30.2%, thus falling into the middle category of PTS threshold values ​​of 20–50% total residue. This means the product is recyclable, but further product improvements should be made.

[0041] In a second experiment, the paper from Example 1 was produced with the modification that the first PVOH coating was changed. 3 g / nT of PVOH was applied to each side. No other changes were made to the paper. Its density was 940 g / cm³. 3Unprinted test sheets (1. Test sheets), test sheets from the pulp after passing through the 5 mm holes (2. Test sheets), and test plates from the pulp after passing through the 0.15 mm screens (3. Test sheets) were produced from this paper, which was coated with both PVOH and silicone. These were subjected to a "Cepi Recyclability Laboratory Test Procedure," Version 2 - October 2022, for standard recycling plants without ink removal technology. In this case, 5.0% of the solids contained in the suspension were retained as coarse residue on the screen plate, and 14.7% of the originally contained solids were retained on the screen plate with 0.15 mm slots. The PTS threshold values ​​(PTS-RH 021:2012) were again used to estimate recyclability. The total amount of residues in the second case was 19.7%.The second paper produced thus falls into the category of less than 20% total residue, which, according to PTS-RH 021:2012, constitutes recyclable paper. The paper according to the invention is therefore recyclable due to its double coating with PVOH and silicone. This can be explained by the fact that the PVOH coating is water-soluble. During recycling tests, water penetrates between the silicone layer and the paper, dissolving the PVOH. Consequently, when separated using a coarse filter (5 mm holes), essentially all of the still-present, non-water-soluble silicone, which exists as a sheet polymer, can be removed, and the paper itself can be recycled. In this way, the present release paper can be recycled with a surprisingly low energy expenditure. Simultaneously, the paper provides a release paper whose backing paper exhibits excellent mechanical properties, in particular high strength and easy peelability.The lower density and smoothness compared to conventional glassine papers can be compensated for by the PVOH layer, so that overall the energy expenditure in the production of the present release paper is significantly reduced compared to glassine-based release papers, while at the same time maintaining sufficiently good peelability and improved mechanical properties and strengths.

Claims

Patent claims 1. Release paper comprising a carrier paper layer coated at least on one side with PVOH and a silicone coating on at least one side, characterized in that the carrier paper layer coated at least on one side with PVOH is made of bleached kraft paper, which consists of at least 30 wt.% each of softwood and hardwood with a total fineness between 30 °SR and 42 °SR, preferably between 33 °SR and 38 °SR, and process aids, and a release layer applied at least on one side to the PVOH-coated side of the carrier paper layer, which consists of 0.5 to 5 g / m² 2 a silicone and that the release paper, coated at least on one side with PVOH and silicone, has a density between 850 and 990 kg / m³ 3 , preferably between 900 and 970 kg / m³ 3 2. Release paper according to claim 1, characterized in that the backing paper layer has a The product contains a hardwood content between 30 and 70 wt.% with a milling grade between 28 °SR and 40 °SR, a softwood content between 30 and 50 wt.% with a milling grade between 32 °SR and 45 °SR, and furthermore process aids such as 0.5 to 2.5 wt.% cationic starch, 0.1 to 0.8 wt.% mass sizing agent and optionally up to 3 wt.% fillers.

3. Release paper according to claim 1 or 2, characterized in that the bulk sizing agent is selected from ASA, AKD, cationic starch and mixtures thereof.

4. Release paper according to claim 1, 2 or 3, characterized in that the carrier paper layer is a carrier paper calendered in a hard nip calender with a line load between 60 and 200 kN / m, preferably 80 to 120 kN / m, preferably a carrier paper calendered on both sides, 5. Release paper according to one of claims 1 to 4, characterized in that the carrier paper layer is coated on both sides with PVOH.

6. Release paper according to one of claims 1 to 5, characterized in that each PVOH-coated side of the carrier paper layer has a density of 0.5 to 10 g / m² 2 PVOH is present.

7. Release paper according to one of claims 1 to 6, characterized in that the carrier paper layer coated with PVOH is coated on at least one side with 0.7 to 3 g / m² 2 , preferably 1 to 2 g / m² 2 Silicone, preferably double-sided with 0.7 to 3 g / m² on each side. 2 , preferably 1 to 2 g / m² 2 It is coated with silicone.

8. Release paper according to any one of claims 1 to 7, characterized in that the PVOH-coated carrier paper has a density greater than 830 kg / m³ 3 and less than or equal to 980 kg / m² 3 exhibits.

9. Release paper according to any one of claims 1 to 8, characterized in that the carrier paper layer coated at least on one side with PVOH has a basis weight according to ISO 536 between 40 and 130 g / m². 2, preferably 55 to 110 g / m² 2 exhibits.

10. A process for producing a release paper, in which a carrier paper layer coated at least on one side with PVOH is provided at least on one side with a silicone coating, characterized in that it comprises the following steps: a) forming both a kraft pulp consisting of softwood and a kraft pulp consisting of hardwood; b) subjecting the pulps to low-consistency milling, preferably separate low-consistency milling in a refiner; c) mixing the low-consistency milled pulps such that a pulp is formed which consists of at least 30 wt.% softwood and hardwood with a total degree of refining between 30 °SR and 42 °SR, preferably between 33 °SR and 38 °SR, and adding process aids such as 0.5 to 2.5 wt.% cationic starch, 0.1 to 0.8 wt.% mass sizing agent and optionally up to 3 wt.%-% fillers; d) dewatering the formed mixed pulp on a wire of a paper machine and in a press section of the paper machine with at least 3 press rolls, optionally using steam and / or vacuum; e) coating at least one side of the formed sheet with PVOH and optionally discharging the sheet from the paper machine; f) calendering the formed sheet with a calender, preferably a hardnip calender; g) optionally printing the formed sheet; h) coating at least one side of the PVOH-coated side of the sheet with a silicone; and i) curing the silicone and optionally drying the sheet, wherein a release paper with a density between 850 and 990 kg / m². 3 , preferably between 900 and 970 kg / m³ 3 is formed.

11. Method according to claim 10, characterized in that a mixed pulp comprising a hardwood content of between 30 and 70 wt.% with a degree of grinding between 30 °SR and 40 °SR and has a softwood content between 30 and 50 wt.% with a grinding degree between 32 °SR and 45 °SR.

12. Method according to claim 10 or 11, characterized in that the formed sheet is dewatered in a press section of the paper machine with two press rollers with line loads of less than 120 kN / m and in a shoe press with a line load between 500 and 800 kN / m.

13. Method according to claim 12, characterized in that a suction pressure in the range between 350 and 600 mbar, preferably about 500 mbar, is exerted on the formed sheet by at least one of the press rollers, preferably by the shoe press roller.

14. Method according to one of claims 10 to 13, characterized in that the formed sheet is calendered with a line load between 60 and 200 kN / m, preferably between 80 and 120 kN / m, preferably on both sides.

15. Method according to one of claims 10 to 14, characterized in that the carrier paper layer is produced with a basis weight according to ISO 536:2011 between 40 and 130 g / m². 2 , preferably 55 to 110 g / m² 2 The paper machine is operated at a speed between 700 and 950 m / min.

16. Method according to one of claims 10 to 15, characterized in that steps g), h) and i) are carried out in a separate coating and optionally printing unit.