Method for producing a long-life fibrous substrate and fibrous substrate

Abstract

A method (100) for increasing the longevity of a fiber substrate (10), for example for banknotes, is disclosed. The method (100) comprises providing (110) an untreated fiber substrate (10), impregnating (120) the untreated fiber substrate (10) with at least one soft polymer (21) having a relatively low glass transition temperature (Tg) to obtain an impregnated fiber substrate (10), drying (130) the impregnated fiber substrate (10), and coating (140) the impregnated and dried fiber substrate (10) with at least one hard polymer (41) having a higher glass transition temperature (Tg) relative to the soft polymer (21) to obtain a circulation-stable fiber substrate (10).A fiber substrate (10) is further disclosed, comprising a substrate core (11) having a core impregnation with a soft polymer (12) having a relatively low glass transition temperature (Tg) and having a surface coating (12) comprising a hard polymer (41) having a higher glass transition temperature (Tg).

Description

Field of the invention

[0001] The present invention relates to a method for producing a durable, in particular circulation-stable, fiber substrate and a corresponding fiber substrate. Background of the invention

[0002] In paper production, particularly in the production of banknote substrates, fiber substrates are used which consist primarily of cotton. For special substrates, the cotton can also be replaced or supplemented by other fiber materials, such as flax or cellulose. This process begins by producing a thick stock in a pulper, which is then diluted. The thin stock thus obtained is then fed, for example, into a cylinder mold machine or, more generally, a wire section, where it has a stock consistency of approximately 1%. Other additives, such as fillers (e.g., titanium dioxide), can also be added. The raw substrate produced in the cylinder mold machine is then pressed and dried (e.g., using steam-heated rollers), whereby the actual paper is formed in a known manner. This raw paper can then be further refined and put to use.

[0003] For applications with high stress, durable fiber substrates are necessary. Banknotes and other documents, in particular, are subjected to particularly severe stresses resulting from their frequent circulation, including frequent bending. Such stresses can lead to a type of "fibering" of the fiber substrate. The fiber fibrils generally interact with each other via hydrogen bonds. Under stress, these bonds and thus the fibrils break, causing the substrate to lose its strength. The term "fibering" is used in this context. Furthermore, such documents with frequent circulation are subject to severe stress due to dirt absorption into the fiber substrate, which reduces the lifespan of the documents.

[0004] To strengthen fiber substrates, various chemical additives can be added to the paper pulp before or in the cylinder molding machine, such as wet strength agents such as EPI (PAAE resin, i.e., polyamidoamine epichlorohydrin resin). Dry strength agents such as CMC (carboxymethylcellulose) can also be added, which is used, for example, in combination with EPI. Although these common paper strength agents provide a certain basic strength, this is often insufficient for heavy-duty use during circulation. The main reason for this is the paper core's insufficient z-strength (i.e., strength perpendicular to the plane of the paper), which leads to a significant increase in paper thickness during circulation. The result is a so-called "limpness" of the banknotes. In more recent developments, further additives (styrene-butadiene resins, acrylates, polyurethanes) are therefore added to the paper pulp (in the forming section or before) to increase z-strength.This is described, for example, in EP 4 010 529 A1.

[0005] Furthermore, surface coatings are known that are intended to prevent soiling of the surface of banknotes, for example. Impregnations can also improve the paper pulp. In the majority of fiber substrates for banknotes, such surface coatings consist of polyvinyl alcohol (PVOH) as the main component. With an application rate of typically 2-3 g / m², and also due to its good water solubility, PVOH polymers offer little protection under harsh circulation conditions.

[0006] Good retention, i.e., bonding to the fiber during the sheet formation process, is necessary for the additives used in the paper pulp. Depending on the dosage and dosing sequence, retention can be low, which is exacerbated by the fact that additional additives must be bonded to the fiber. Furthermore, the absolute proportion of reinforcing additives that can be incorporated into the fiber substrate is low due to the process. Disturbances in charge balance, sheet formation, and watermark formation can result from higher addition levels. Therefore, the use of additional strength-enhancing additives in the paper pulp besides CMC and EPI is not recommended.

[0007] The impregnations and coatings applied to the paper surface are essentially created in two ways. Either the substrate is treated with impregnations (e.g., PUs) in the size bath during the papermaking process instead of with PVOH, which does strengthen the paper pulp but offers little protection against contamination. Impregnations in the size bath and in the size press penetrate primarily into the paper core and provide only limited surface protection.

[0008] Coatings as described above are applied using a coating process and, due to the process, are primarily located on the surface rather than in the paper pulp. Therefore, while they offer good protection against contamination, they have no effect on paper reinforcement (z-strength). Description

[0009] Accordingly, it is an object of the present invention to provide a fiber substrate, for example for banknotes, which offers a longer circulation time by simultaneously increasing the z-strength and increasing the dirt repellency.

[0010] This object is achieved by the subject matter of the independent claims. Exemplary embodiments emerge from the dependent claims and the following description.

[0011] Disclosed herein is a method for increasing the longevity of a fiber substrate and a corresponding fiber substrate. The fiber substrate is preferably produced or obtained using the described method. Therefore, the descriptions of the method or the structural features of the fiber substrate achieved thereby also apply to the fiber substrate itself, and vice versa. In particular, any feature described with respect to the method can be embodied in the fiber substrate, and vice versa.

[0012] According to a first aspect, a method for increasing the longevity of a fiber substrate is disclosed. The method comprises providing an untreated fiber substrate, impregnating the untreated fiber substrate with at least one soft polymer having a relatively low glass transition temperature to obtain an impregnated fiber substrate, drying the impregnated fiber substrate to obtain an impregnated and dried fiber substrate, and coating the impregnated and dried fiber substrate with at least one hard polymer having a higher glass transition temperature relative to the soft polymer to obtain a circulation-stable fiber substrate.

[0013] With so-called "durable substrates," the surface of banknotes is impregnated and / or coated to provide better protection for the substrate. The PVOH surface sizing is replaced with polyurethanes (PUs), for example. Furthermore, a coating based on acrylates, for example, can be applied in an additional step.

[0014] The essential idea of the invention is that at least one "soft" polymer is introduced / impregnated into the untreated fiber substrate as an additive, and a relatively "hard" polymer is also provided as a surface coating. The at least one soft polymer is not only present on one surface of the fiber substrate, but penetrates deep into the fiber substrate and permeates it, while the surface coating is essentially present only on the surface of the fiber substrate.

[0015] Added soft polymers to the fiber substrate provide a good reinforcement of the z-strength due to their properties, thus preventing the fiber substrate from fraying under heavy loads during circulation. The introduction / impregnation of the soft polymers takes place primarily after the drying and cooling of the untreated fiber substrate from the cylinder screen. The corresponding additives therefore do not have to be added during or prior to the forming section; instead, the untreated fiber substrate from the cylinder screen is post-treated accordingly.

[0016] The hard polymers, which are intended to protect the surface from dirt absorption, are then applied in a downstream coating process, for example, a curtain coating or film pressing process, or any other suitable process. A downstream coating process refers to a coating process that takes place downstream of the impregnation, i.e., preferably also in the paper machine itself, but downstream of an impregnation station in the paper machine. However, it is also conceivable to carry out the coating process in a different machine.

[0017] The hardness, i.e. whether a polymer is soft or hard, refers in this disclosure to a glass transition temperature of the respective polymer or additive. In general, the glass transition temperature (T g ) describes the temperature at which an amorphous polymer changes from a hard / glass-like state to a soft / leather-like state, or vice versa. The glass transition temperature therefore describes the temperature below which the polymer is rigid. As the temperature rises, the polymer goes through a transition state in which the macromolecular chains can slide against each other, and the polymer becomes soft. The glass transition temperature is therefore linked to the mechanical properties of a polymer. These include, for example, tensile strength, impact strength, elastic modulus and operating temperature range. The glass transition temperature is therefore directly related to the strength of a material.Polymers with a lower glass transition temperature are softer than polymers with a higher glass transition temperature.

[0018] Having said these general explanations, the procedure of the disclosed method is described below.

[0019] In a first step, an untreated fiber substrate is provided. An untreated fiber substrate refers in particular to a fiber substrate to which strengthening additives (such as CMC and EPI) may already have been added, but not yet to soft and hard polymers according to the present disclosure, as described herein. The untreated fiber substrate can, for example, be a fiber substrate from a wire section of a paper machine that has already been dried and cooled. However, it is also conceivable that the untreated fiber substrate was pre-produced, for example, and is removed from a warehouse and then fed to a corresponding post-treatment machine that carries out the subsequent steps. Preferably, however, all steps of the process take place inline in a single paper machine.

[0020] In a second step, the untreated fiber substrate is impregnated with at least one soft polymer with a relatively low glass transition temperature. It should be noted that the at least one soft polymer can comprise either a single soft polymer or a mixture of any number of different soft polymers. It is important that the individual soft polymer or the mixture of several soft polymers has the desired glass transition temperature. The impregnation can take place in any suitable manner, in particular at a corresponding station in a paper machine, which allows the fiber substrate to be permeated with the soft polymer. The at least one soft polymer is preferably present in an aqueous dispersion such that the untreated fiber substrate can be drawn through the corresponding aqueous suspension, whereby the soft polymer penetrates the fiber substrate.Preferably, the impregnation takes place in an immersion bath, as described below with reference to one embodiment. During impregnation, the untreated fiber substrate can, for example, be drawn through an immersion bath containing the corresponding soft polymer resin instead of a polyvinyl alcohol (PVOH) immersion bath (as in the prior art).

[0021] In general, soft polymers, i.e., polymers with a comparatively low glass transition temperature, have an adhesive-like character. For this reason, while soft polymers produce good paper strength, they also often cause a so-called "tack" on the paper and are therefore not suitable for roll winding because the paper layers stick together. Furthermore, soft polymers also exhibit poor dirt repellency.

[0022] To avoid these disadvantages and also achieve good dirt repellency, the fiber substrate is additionally coated with at least one hard polymer according to the inventive method, which is then at least essentially only present on the surface and does not penetrate the fiber substrate itself. Unlike soft polymers, such hard polymers provide good dirt repellency and are easy to wind. Therefore, a sequential inline application of both systems (i.e., impregnation with the soft polymers and coating with the hard polymers) with sufficient intermediate drying is effective.

[0023] Thus, the impregnated fiber substrate is first dried. Drying can be performed in any suitable manner, but generally occurs inline after impregnation (particularly since, as explained, the impregnated fiber substrate cannot be wound into rolls). Drying can be performed, for example, in an infrared dryer (IR dryer) with subsequent cylinder drying, as explained below with reference to one embodiment.

[0024] After drying the impregnated fiber substrate, it is coated with at least one additional, this time harder, polymer (i.e., a polymer with a higher glass transition temperature than the at least one soft polymer used during impregnation). Coating can be carried out in any suitable manner, for example, in a roll coating station of the paper machine downstream of the dryer (which can be configured, for example, as a curtain coating station or film press station).

[0025] There is generally considerable scope for design when selecting polymers. However, it is important to ensure that the polymers have the appropriate physical properties, particularly the corresponding glass transition temperatures. When selecting polymers, a portion of the polymer can also come from renewable raw materials. For example, the polyol component of polymers can be made from renewable raw materials such as sugar cane or vegetable oils. The biomass content can be up to over 50%, depending on the proportion of the polyol component in the polymer.

[0026] The process described enables the controlled use of the soft polymer as an impregnation, e.g. in an immersion bath / size bath. This enables the absorption of larger quantities of the soft polymer than would be possible if it were added to the fiber mass, e.g. in or before the wire section. In addition, disturbances in the charge balance during sheet formation, which would occur if the corresponding additives (e.g. in particular the soft polymers) were added to the fiber mass, are avoided. Carrying out the process inline after the wire section also enables the use of very soft and therefore good strength-enhancing additives for the fiber substrate. The hard coating also offers better protection against contamination compared to applying the additives in the fiber mass or compared to just impregnation without subsequent coating.

[0027] According to one embodiment, the impregnation of the untreated fiber substrate takes place in an immersion bath.

[0028] For example, the untreated fiber substrate can come from the wire section (e.g. a cylinder wire) and, after a first drying section, advantageously after cooling down (e.g. with suitable cooling rollers), be pulled via corresponding transport rollers through an immersion bath / glue bath in which there is an aqueous dispersion with the at least one soft polymer. The untreated fiber substrate is completely covered on both sides by the aqueous dispersion and the aqueous dispersion with the at least one soft polymer is absorbed into the fiber substrate via the surfaces of the fiber substrate. This means that an immersion bath with polyvinyl alcohol (usually in aqueous solution) is used, which is commonly used in the prior art; it is used to seal the surface, for example to prevent additives or adhesives from being dusted off in subsequent printing processes.fillers such as titanium dioxide) is replaced by an immersion bath containing an aqueous dispersion comprising the at least one soft polymer. This impregnation according to the invention fulfills the same functions as an immersion bath of polyvinyl alcohol, but additionally leads to a significant increase in the strength and thus the circulation stability of the fiber substrate, as described above.

[0029] According to a further embodiment, the drying of the impregnated fiber substrate takes place in a drying device.

[0030] The drying device can, for example, be an infrared dryer with subsequent cylinder drying. In the infrared dryer (IR dryer), contactless drying takes place in the manner of a flotation dryer. In the subsequent cylinder dryer, contact drying of the paper substrate then takes place. The paper substrate is first guided through the IR dryer on a conveyor belt and dried under the influence of heat via infrared radiation. The substrate is then further dried in the cylinder dryer with contact. However, these drying devices are only examples, and it should be recognized that any suitable drying device can be used.

[0031] According to a further embodiment, the coating of the impregnated and dried fiber substrate takes place in a roller coater.

[0032] Such a roller coater can, for example, comprise at least two rollers, preferably four rollers, wherein the impregnated and dried fiber substrate is guided through two of the rollers under pressure. The rollers resting against the fiber substrate (plate rollers) serve to apply the at least one hard polymer to the impregnated and dried fiber substrate. The plate rollers are preferably smooth and rubberized. If only two rollers (i.e. only the plate rollers) are used, they run through a reservoir containing the at least one hard polymer, preferably also in the form of an aqueous dispersion, at a location remote from the fiber substrate. The at least one hard polymer is then transferred from the rollers resting against the fiber substrate (plate rollers) to the passing fiber substrate.

[0033] Preferably, however, a further transfer roller (a so-called scoop roller) is provided between each of the plate rollers adjacent to the fiber substrate and the reservoir. This transfer roller receives the at least one polymer from the reservoir and transfers it to the plate rollers directly adjacent to the fiber substrate. This allows, in particular, better control of the application quantity. The plate rollers directly adjacent to the impregnated and dried fiber substrate are rubberized and smooth, while the scoop rollers are provided with an engraving that "scoops" a defined amount of the formulation from the reservoir and transfers it to the plate rollers adjacent to the fiber substrate. The application quantity of formulation can then be controlled via the volume of the engraving and the relative speeds of the scoop rollers to the plate rollers.The roller coater is therefore a type of flexographic printing device which is arranged inline after the dryer and prints, i.e. coats, the entire surface of the fibre substrate with at least one hard polymer.

[0034] According to a further embodiment, the relatively low glass transition temperature of the at least one soft polymer is in a range between -45 °C and 0 °C.

[0035] Preferably, the glass transition temperature of the at least one soft polymer is in a range between -45 °C and -20 °C, more preferably in a range between -40 °C and -20 °C, and most preferably in a range between -40 °C and -25 °C.

[0036] These temperature ranges of the glass transition temperature have been found to be particularly preferred for increasing the circulation stability of the fiber substrate, ie in particular for increasing the z-strength.

[0037] According to a further embodiment, the higher glass transition temperature of the at least one hard polymer is in a range between -15 °C and 20 °C.

[0038] Preferably, the glass transition temperature of the at least one hard polymer is in a range between -15 °C and 10 °C, more preferably in a range between -10 °C and 10 °C, and most preferably in a range between -10 °C and 0 °C.

[0039] These glass transition temperature ranges have been found to be particularly preferred for increasing soil repellency while maintaining the circulation stability provided by the at least one soft polymer.

[0040] According to a further embodiment, the at least one soft polymer comprises at least one of the following polymers: soft polyacrylates, styrene butadienes, carboxylated styrene butadienes, polyacrylamides, soft polyacrylics, soft polyacrylamides, polystyrenes, and polyethylenes.

[0041] These soft polymers have proven particularly effective in increasing z-strength. The at least one soft polymer may also contain any combination of the listed soft polymers, as long as the corresponding formulation exhibits the desired physical properties, especially the desired glass transition temperature.

[0042] According to a further embodiment, the at least one hard polymer comprises at least one polyacrylate.

[0043] These hard polymers have proven particularly resistant to dirt absorption. The at least one hard polymer can also contain any combination of the listed hard polymers, as long as the corresponding formulation exhibits the desired physical properties, especially the desired glass transition temperature.

[0044] According to a further embodiment, the at least one soft polymer comprises at least one of the following polymers: polyurethanes; and carboxylated anionic polyurethanes from the class of polyester polyurethanes, polyether polyurethanes, and polycarbonate polyurethanes. The relatively low glass transition temperature relates to a corresponding elongation, which provides corresponding properties.

[0045] For polyurethanes, it's not a glass transition temperature that's usually defined, but rather an elongation. In polyurethanes, the relatively low glass transition temperature refers to a corresponding elongation that provides the same or at least very similar properties as a relatively low glass transition temperature of a comparative polymer (such as the other soft polymers described herein).

[0046] In general, the elongation of a polymer describes its stretching behavior, with elongation being a form of deformation of the polymer. Deformation, in turn, is a change in shape that the polymer undergoes under stress. Under tensile stress, the polymer deforms by stretching and becomes longer. This is referred to as elongation. Elongation can be expressed, for example, as percentage elongation, i.e., the percentage change in the length of a polymer sample after stretching compared to the original length of the polymer sample.

[0047] According to a further embodiment, at least one of a fungicidal additive, a virucidal additive, a bactericidal additive, and an antimycotic is added to the at least one soft polymer and / or the at least one hard polymer.

[0048] Fungicidal and bactericidal additives provide effective protection of the fiber substrate against the growth of germs, especially the most important microscopic families of bacteria and fungi.

[0049] Virucidal additives prevent the uptake of viruses (or their survival on the fiber substrate) and thus their transmission when the fiber substrate is passed from hand to hand. Such virucidal additives can be effective against human pathogenic viruses such as retroviruses, cytomegaloviruses, rotaviruses, paramyxoviruses, polioviruses, hantaviruses, coxsackieviruses, encephalomyocarditis virus, picornaviruses (including rhinoviruses), DNA or RNA viruses, especially Flaviviridae, AIDS virus, influenza viruses, smallpox virus, yellow fever virus, hepatitis C virus, herpes viruses, Epstein-Barr virus, varicella zoster virus, rubella virus, simian virus 40 or SV40, or even coronaviruses.

[0050] An antifungal is an antimicrobial substance that works against diseases caused by fungi.

[0051] Although the disclosed fiber substrate already offers good protection against the transmission of corresponding microorganisms without such additives due to the coating with the hard polymer and the resulting high resistance to dirt absorption, the addition of appropriate fungicidal, virucidal, and bactericidal additives, as well as antifungals (collectively referred to herein as biocides), can provide additional, stronger protection, particularly for applications with high circulation, such as the fiber substrate for banknotes. Such additives also prevent the risk of cross-contamination with pathogenic germs such as bacteria, mold, and viruses. It should be noted that other biocides can also be added as additives. However, attention must always be paid to the safety of the biocides for humans and their environmental compatibility.

[0052] Corresponding biocides can, for example, be present in amounts of 0.05% to 0.8% based on the mass of the circulation-stable fiber substrate.

[0053] According to a further embodiment, the circulation-stable fiber substrate comprises, based on a mass of the circulation-stable fiber substrate: between 70% and 90% of fibers, between 4% and 12% of the at least one soft polymer, and between 3% and 8% of the at least one hard polymer.

[0054] The proportion of fibers relative to the mass of the circulation-stable fiber substrate is preferably between 75% and 85%, more preferably between 77% and 83%, and most preferably between 79% and 82%.

[0055] The proportion of the at least one soft polymer relative to the mass of the circulation-stable fiber substrate is preferably between 5% and 10%, more preferably between 5% and 8%, and even more preferably between 5% and 7%. Most preferably, the proportion of the at least one soft polymer is 6%.

[0056] The proportion of the at least one hard polymer relative to the mass of the circulation-stable fiber substrate is preferably between 3% and 7%, more preferably between 4% and 7%, and even more preferably between 4% and 6%. Most preferably, the proportion of the at least one hard polymer is 5%.

[0057] These formulations, and in particular the corresponding proportions of the at least one soft polymer and the at least one hard polymer, have proven in combination to be particularly effective in increasing the overall circulation stability, ie in increasing the z-strength while simultaneously increasing the soil repellency and also provide good windability (ie a good reduction of "tack", as described above).

[0058] According to a further embodiment, the method further comprises, after coating the fiber substrate, printing the circulation-stable fiber substrate with at least one suitable printing process, such as offset printing, intaglio printing, and any other suitable printing process.

[0059] According to another aspect, a circulation-stable fiber substrate is disclosed. The fiber substrate comprises a substrate core with a core impregnation and a surface coating. The core impregnation comprises at least one soft polymer with a relatively low glass transition temperature. The surface coating comprises at least one hard polymer having a higher glass transition temperature relative to the soft polymer.

[0060] In particular, the fiber substrate may contain all structural features resulting from the process described above in any combination.

[0061] According to one embodiment, the relatively low glass transition temperature of the at least one soft polymer is in a range between -45°C and 0°C. The higher glass transition temperature of the at least one hard polymer is in a range between -15°C and 20°C.

[0062] Preferably, the glass transition temperature of the at least one soft polymer is in a range between -45 °C and -20 °C, more preferably in a range between -40 °C and -20 °C, and most preferably in a range between -40 °C and -25 °C.

[0063] Preferably, the glass transition temperature of the at least one hard polymer is in a range between -15 °C and 10 °C, more preferably in a range between -10 °C and 10 °C, and most preferably in a range between -10 °C and 0 °C.

[0064] According to a further aspect, a rotationally stable fiber substrate according to one of the above embodiments is disclosed. The rotationally stable fiber substrate is manufactured according to one of the embodiments of the method described above.

[0065] In particular, the circulation-stable fiber substrate has all the advantages described with reference to the process.

[0066] According to a further aspect, the use of a previously described fiber substrate as a substrate for banknotes, for securities, for visas, or for identity documents is disclosed.

[0067] Banknotes, in particular, are subject to particularly high circulation stress due to their frequent circulation and therefore particularly benefit from the use of the fiber substrate described herein or a fiber substrate produced using the method described herein. In principle, however, the disclosed circulation-stable fiber substrate can be used for all applications in which paper-like materials are used. Brief description of the drawings

[0068] Fig. 1 shows a highly schematic representation of a section of a paper machine for carrying out the disclosed method for increasing the longevity of a fiber substrate. Fig. 2shows a flow chart of a process for increasing the durability of a fiber substrate, which for example is connected to the section of the paper machine from Fig. 1 can be carried out. Fig. 3 shows schematically a cross-section of a rotationally stable fiber substrate, which is produced, for example, using the section of a paper machine according to Fig. 1 when carrying out the procedure according to Fig. 2 is available. Detailed description

[0069] The representations in the figures are schematic and not to scale. Where the same reference symbols are used in different figures in the following description, they refer to identical or similar elements. Identical or similar elements may also be designated by different reference symbols.

[0070] Fig. 1shows a highly schematic representation of a section of a paper machine which is used to carry out the method 100 disclosed herein, which is described further below with reference to Fig. 2 described can be used.

[0071] The section of the paper machine comprises an immersion bath 20 filled with a soft polymer 21 having a first, low, glass transition temperature T g,1 . The soft polymer 21 is present in the form of an aqueous dispersion in the immersion bath 20. It should also be noted that the soft polymer 21 does not necessarily have to comprise only a single soft polymer 21, but can also be, for example, a mixture of different soft polymers 21, as long as the corresponding desired physical properties are present overall, ie, in particular, it is ensured that the corresponding glass transition temperature T g,1 is present.

[0072] The immersion bath 20 is arranged downstream of an arrangement of cooling rollers 50, which receives and cools an untreated fiber substrate 13 from a wire section with a subsequent drying section (e.g. a cylinder wire with a drying section; both not shown) of the paper machine.

[0073] Downstream of the dipping bath 20 are size press rollers 22, which serve to press the absorbed at least one soft polymer 21 into the fiber substrate and simultaneously to "squeeze out" excess formulation on the substrate.

[0074] Downstream of the size press rollers 22, in turn, is a drying device 30, here in the form of a flotation dryer 30 (for example, an IR dryer), which is configured to carry out (for example, contactless) drying of the impregnated fiber substrate 14 from the immersion bath 20 using IR radiation. However, it should be noted that, in principle, any other suitable (in particular contactless) drying device 30 can also be used. Furthermore, the drying device 30 can, for example, also comprise contact-based drying (such as cylinder drying), which takes place subsequent to the contactless drying.

[0075] Downstream of the drying device 30 is a coating device 40, here in the form of a roll coater 40 as described in detail above (in particular downstream within the same paper machine), which is configured to provide the impregnated and dried fiber substrate 15 with a coating of at least one hard polymer 41 having a second, higher (relative to the first, low glass transition temperature T g,1 of the at least one soft polymer 21 in the dip bath 20) glass transition temperature T g,2 . Here too, it should be noted that the hard polymer 41 can be a single hard polymer 41 or any suitable mixture of any number of hard polymers 41, as long as the corresponding formulation has the corresponding desired physical properties, in particular the corresponding glass transition temperature T g,2 .

[0076] In the following, with reference to the flowchart of the Fig. 2and with further reference to Fig. 1 a method 100 for increasing the longevity (ie, in particular, the circulation stability) of a fiber substrate 10 is described.

[0077] The method 100 begins with providing 110 an untreated fiber substrate 13. The untreated fiber substrate 13 is a fiber substrate to which no corresponding additives according to the present disclosure have yet been added, in particular no impregnation with a soft polymer 21 and no coating with a hard polymer 41. However, dry strength agents (such as CMC) and wet strength agents (such as EPI) may already have been added to the untreated fiber substrate 13. Accordingly, "untreated" refers to untreated with the soft and hard polymers according to the invention. The untreated fiber substrate 13 is, for example, a fiber substrate from a wire section of the printing press, such as a cylinder wire. The untreated fiber substrate 13 essentially comprises only components of a regular paper substrate, as described hereinabove.The untreated fiber substrate 13 can, for example, be the fiber substrate 13 from the cooling rollers 50 of the . Fig. 1 be.

[0078] In a next step, the untreated fiber substrate 13 is impregnated 120 with at least one soft polymer 21, for example in the immersion bath 20 of the Fig. 1 takes place by pulling the untreated fiber substrate 13 through the dipping bath 20, e.g. over corresponding transport rollers. The untreated fiber substrate 13 is completely immersed in the dipping bath 20 by the at least one soft polymer 21 (which, as described with reference to Fig. 1described, for example in the form of an aqueous dispersion), so that both the upper surface and the lower surface of the untreated fiber substrate 13 come into contact with the at least one soft polymer 21. The untreated fiber substrate 13 absorbs the at least one soft polymer 21 via the surfaces, which thus penetrates into the untreated fiber substrate 13. The impregnation 120 also preferably comprises pressing in the absorbed at least one soft polymer 21, for example with the size press rollers 22 of the Fig. 1 . This also simultaneously squeezes out excess formulation from the fiber substrate.

[0079] The at least one soft polymer 21 can, for example, comprise at least one of the following polymers: soft polyacrylates, styrene butadienes, carboxylated styrene butadienes, and polyacrylamides. Preferably, the at least one soft polymer 21 is in carboxylated form. The glass transition temperature T g,1 of the at least one soft polymer 21 is in a range between -45°C and 0°C. Preferably, the glass transition temperature T g,1 of the at least one soft polymer 21 is in a range between -45°C and -20°C, more preferably in a range between -40°C and -20°C, and most preferably in a range between -40°C and -25°C.

[0080] These polymers with these properties have proven to be particularly suitable for increasing the z-strength of the fiber substrate.

[0081] After impregnation 120 of the untreated fiber substrate 13, the next process step involves drying 130 the now impregnated fiber substrate 14 emerging from the dip bath 20 (and the size press rollers 22). For this purpose, the impregnated fiber substrate 14 is passed through a drying device 30, such as the IR flotation dryer 30 of Fig. 1 The impregnated fiber substrate 12 is dried accordingly. At the end of the flotation dryer 30, an impregnated and dried fiber substrate 15 exits the drying device 30.

[0082] In a next step, the impregnated and dried fiber substrate 15 is then coated 140 with at least one hard polymer 41, i.e., a polymer 41 that has a higher glass transition temperature T g,2 than the soft polymer 21. In general, when reference is made herein to soft and hard polymers and to low and high glass transition temperatures, these terms are to be understood as relative to one another.

[0083] The at least one hard polymer 41 can, for example, comprise at least one of the following polymers: polyacrylates; polyurethanes, and carboxylated polyurethanes. Preferably, the at least one hard polymer 41 is also in carboxylated form. The glass transition temperature T g,2 of the at least one hard polymer 41, which is higher than the glass transition temperature T g,1 of the soft polymer 21, lies in a range between -15°C and 20°C. The glass transition temperature T g,2 of the at least one hard polymer 41 is preferably in a range between -15°C and 10°C, more preferably in a range between -10°C and 10°C, and most preferably in a range between -10°C and 0°C.

[0084] These polymers with these properties have proven to be particularly suitable for increasing the dirt repellency of the fiber substrate.

[0085] The coating 140 of the impregnated and dried fiber substrate 15 with the at least one hard polymer 41 can be carried out, for example, in the roll coater 40 of the Fig. 1 The roller coater 40 of the Fig. 1comprises, for example, two cliché rollers 42, 43 and two scoop rollers 44, 45. A first roller 42 (cliché roller 42) receives the at least one hard polymer 41 from a third roller 44 (scoop roller 44), and a second roller 43 (cliché roller 43) receives the at least one hard polymer 41 from a fourth roller 45 (scoop roller 45). The third roller 44 and the fourth roller 45 (i.e., the scoop rollers 44, 45), in turn, receive the at least one hard polymer 41 from a corresponding reservoir (not shown), which holds the at least one hard polymer 41, preferably likewise in the form of an aqueous dispersion. The first roller 42 and the second roller 43 (i.e. the cliché rollers 42, 43) are preferably rubberized and smooth, while the scoop rollers 44, 45 are provided with an engraving which "scoops" a defined amount of the formulation from the reservoir and transfers it to the cliché rollers 42, 43, which rest against the fiber substrate.The amount of formulation applied can then be controlled via the volume of the engraving and the relative speeds of the scoop rollers 44, 45 to the cliché rollers 42, 43. The roller coater 40 of the . Fig. 1 Accordingly, the coating of the impregnated and dried fiber substrate 15 is carried out in a type of flexographic printing process, which applies the at least one hard polymer 41 over the entire surface. However, it should be noted that the coating 140 of the surfaces of the impregnated and dried fiber substrate 15 can also be carried out in any other suitable manner. In addition, the roller coater 40 of the Fig. 1 can also be configured with a different number and configuration of rollers.

[0086] After coating 140 the impregnated and dried fiber substrate 15, the finished circulation-stable fiber substrate 10 is obtained.

[0087] While the at least one soft polymer 21 penetrates into the core of the fiber substrate 10 itself during impregnation, the at least one hard polymer 41 lies at least substantially only on the surfaces of the impregnated and dried fiber substrate 15 and thus at least does not significantly penetrate the core of the fiber substrate 10, even though a certain near-surface penetration can of course occur. The at least one soft polymer 21 leads to an increase in the z-strength and thus reduces the tendency of the fiber substrate 10 to fray, i.e., the tendency for the individual fibers of the fiber substrate 10 to separate from one another, thus leading to an increase in thickness and thus to a limpness of the fiber substrate 10.

[0088] Optionally, one or more biocides and antifungals may be added to both the at least one soft polymer 21 and the at least one hard polymer 41, as described hereinabove.

[0089] After coating 140, the rotationally stable fiber substrate 10 can optionally be printed 150, followed by the optional application of a final coating (e.g., a UV varnish) for further surface protection. Printing 150 takes place separately from the remaining process steps of process 100 in a different machine. Likewise, the application of the final coating takes place after printing 150 in a different machine. Thus, process 100 is interrupted during printing 150, and the application of the final coating 160 takes place thereafter.

[0090] Finally, Fig. 3 another exemplary circulation-stable fiber substrate 10, which, for example, is produced using the method 100 of Fig. 2 (using the paper machine part of the Fig. 1or with any other suitable device) or in any other suitable manner.

[0091] The circulation-stable fiber substrate 10 has a substrate core 11, which is permeated / impregnated with at least one soft polymer 21 (indicated by dots). The substrate core 11 without the at least one soft polymer 21 would, for example, correspond to the untreated fiber substrate 11 of the Fig. 1 On both surfaces of the substrate core 11 there is a surface coating 12 made of at least one hard polymer 41.

[0092] The properties of the soft polymer 21 and the hard polymer 41, as well as all other features and properties of the circulation-stable fiber substrate 10, may be similar to those described with reference to the method 200 of Fig. 2and the features and properties generally described in this disclosure. These features and properties are accordingly not repeated here.

[0093] The described method 200 and the described circulation-stable fiber substrate 10 enable a controlled use of a soft polymer as an impregnation, e.g., in an immersion bath / size bath. This enables, in particular, the absorption of larger quantities of the soft polymer than would be possible if it were added to the fiber mass, e.g., in or before the wire section. Furthermore, disturbances in the charge balance during sheet formation, which would occur if the corresponding additives (e.g., in particular the soft polymers) were added to the fiber mass, are avoided. Carrying out the process inline after the wire section also enables the use of very soft, and thus strength-enhancing, additives for the fiber substrate.The hard coating also provides better protection against contamination compared to applying the additives in the fiber mass or compared to impregnation alone, without subsequent coating. The hard coating also provides a remedy for the problem that fiber substrates impregnated only with soft polymers tend to stick together and thus cannot be wound into rolls. The disclosure thus also provides for the windability of the rotationally stable fiber substrate 10.

[0094] The disclosed fiber substrate 10 and the corresponding method 100 are particularly advantageous for use with documents with high circulation and thus heavy use, such as banknotes, securities, visas, or identity documents.

[0095] Additionally, it should be noted that "comprising" or "having" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims are not to be considered as limitations. List of reference symbols

[0096] 10 Fiber substrate 11 Substrate core 12 Surface coating 13 Untreated fiber substrate 14 Impregnated fiber substrate 15 Impregnated and dried fiber substrate 20 Immersion bath 21 Soft polymer 22 Size press rolls 30 Drying device, flotation dryer 40 Roll coater 41 Hard polymer 42 First roll, cliché roll 43 Second roll, cliché roll 44 Third roll, scoop roll 45 Fourth roll, scoop roll 50 Cooling rolls 100Process 110Providing the untreated fiber substrate 120Impregnating the fiber substrate 130Drying the fiber substrate 140Coating the fiber substrate 150Printing the fiber substrate 160Applying a final coating (topcoat, UV coating)

Claims

1. A method (100) for increasing the longevity of a fiber substrate (10), the method (100) comprising: providing (110) an untreated fiber substrate (13); impregnating (120) the untreated fiber substrate (13) with at least one soft polymer (21) having a relatively low glass transition temperature (T g,1 ) to obtain an impregnated fiber substrate (14); drying (130) the impregnated fiber substrate (14) to obtain an impregnated and dried fiber substrate (15); and coating (140) the impregnated and dried fiber substrate (15) with at least one hard polymer (41) which, relative to the soft polymer (21), has a higher glass transition temperature (T g,2 ) in order to obtain a circulation-stable fiber substrate (10).

2. The method (100) according to claim 1, wherein the impregnation (120) of the untreated fiber substrate (13) takes place in an immersion bath (20).

3. The method (100) according to any one of the preceding claims, wherein the drying (130) of the impregnated fiber substrate (14) takes place in a drying device (30).

4. The method (100) according to any one of the preceding claims, wherein the coating (140) of the impregnated and dried fiber substrate (15) is carried out in a roller coater (40).

5. The method (100) according to any one of the preceding claims, wherein the relatively low glass transition temperature (T g,1 ) of the at least one soft polymer (21) is in a range between -45 °C and 0 °C.

6. The method (100) according to any one of the preceding claims, wherein the higher glass transition temperature (T g,2 ) of the at least one hard polymer (41) is in a range between -15 °C and 20 °C.

7. The method (100) of any preceding claim, wherein the at least one soft polymer (21) comprises at least one of the following polymers: soft polyacrylates; styrene butadienes, carboxylated styrene butadienes, polyacrylamides; soft polyacrylics; soft polyacrylamides; polystyrenes; and polyethylenes.

8. The method (100) according to any one of the preceding claims, wherein the at least one soft polymer (21) comprises at least one of the following polymers: polyurethanes; and carboxylated anionic polyurethanes from the class of polyester polyurethanes, polyether polyurethanes, and polycarbonate polyurethanes; wherein the relatively low glass transition temperature (T g,1 ) refers to a corresponding elongation which provides corresponding properties.

9. The method (100) according to any one of the preceding claims, wherein the at least one hard polymer (41) comprises at least one polyacrylate.

10. The method (100) according to any one of the preceding claims, wherein at least one of a fungicidal additive, a virucidal additive, a bactericidal additive, and an antimycotic is added to at least one of the at least one soft polymer (21) and / or the at least one hard polymer (41).

11. The method (100) according to any one of the preceding claims, wherein the rotationally stable fiber substrate (10) comprises, based on a mass of the rotationally stable fiber substrate (10): between 70% and 90% of fibers; between 4% and 12% of the at least one soft polymer (21); and between 3% and 8% of the at least one hard polymer (41).

12. A circulation-stable fiber substrate (10), comprising: a substrate core (11) with a core impregnation, and a surface coating (12); wherein the core impregnation comprises at least one soft polymer (21) having a relatively low glass transition temperature (T g,1); and wherein the surface coating (12) comprises at least one hard polymer (41) which, relative to the soft polymer (21), has a higher glass transition temperature (T g,2 ).

13. Fiber substrate (10) according to claim 12, wherein the relatively low glass transition temperature (T g,1 ) of the at least one soft polymer (21) is in a range between -45 °C and 0 °C; and wherein the higher glass transition temperature (T g,2 ) of the at least one hard polymer (41) is in a range between -15 °C and 20 °C.

14. A circulation-stable fiber substrate (10) according to any one of claims 12 or 13, wherein the circulation-stable fiber substrate was produced by a method (100) according to any one of claims 1 to 11.

15. Use of a fiber substrate (10) produced by a method (100) according to one of claims 1 to 11, or of a fiber substrate (10) according to one of claims 12 to 14, as a substrate for banknotes, for securities, for visas, or for identity documents.

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