Method for producing durable fibrous substrate and fibrous substrate

Abstract

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

Description

Field of the Invention

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

[0002] In papermaking, especially in the manufacture of banknote substrates, fiber substrates consisting mostly of cotton are used. In the case of special substrates, cotton can also be replaced or supplemented by other fiber materials, such as linen or cellulose. First, a viscous material is produced in a pulper and then diluted. The dilute material thus obtained is then introduced, for example, into a round sieve or generally into the wire section, where the material consistency is about 1%. Other additives, such as fillers (e.g., titanium dioxide), can also be added here. The raw substrate produced in the round sieve is then pressed and dried (e.g., by steam-heated rollers), where the actual paper is formed in a known manner. This raw paper can then be further improved and put into use.

[0003] In applications with high loads, a strong fiber substrate is required. Banknotes and other documents are particularly strongly stressed here, which is caused by their frequent circulation and, in addition, frequent buckling loads. This load leads to a "fibrillation" of the fiber substrate. Fiber fibrils are usually held together by hydrogen bonds. When stressed, these bonds and the fibrils loosen accordingly, resulting in the substrate losing strength. Here, the term "fibrillation" is used. In addition, such frequently circulated documents are strongly burdened by dirt absorbed in the fiber substrate, which shortens the service life of the documents.

[0004] Therefore, in order to strengthen the fiber substrate, various chemical additives, such as wet strength agents, such as EPI (PAAE-resin, i.e., polyaminoamine-epichlorohydrin-resin), can be added to the paper stock before or during the round sieve. In addition, dry strength agents, such as CMC (carboxymethyl cellulose), can be added, which are used, for example, in combination with EPI. Although these common paper strengthening agents provide a certain basic strength, this is usually insufficient for the high stresses in circulation. For this reason, first of all, the z-strength (i.e., the strength perpendicular to the plane of the paper) of the paper core is too low, which causes the paper thickness to increase significantly during circulation. The result is the so-called "slumping" of banknotes. Therefore, in more recent developments, additional additives (styrene-butadiene-resin, acrylate, polyurethane) are added to the paper stock (in the wire section or beforehand) to increase the z-strength. This is described, for example, in the patent document EP 4 010 529 A1.

[0005] In addition, surface coatings are known, which are supposed to prevent, for example, the contamination of the surface of banknotes. In addition, impregnation can improve the paper stock. In most fiber substrates used for banknotes, this surface coating consists mainly of polyvinyl alcohol (PVOH). The PVOH polymer is usually present in a total amount of 2 - 3 g / m 2is provided in an amount that, due to its good water solubility, provides little protection under adverse circulation conditions.

[0006] Additives used in the furnish require good retention, i.e., binding to the fibers during sheet formation. Depending on the metered amount and the order of metered addition, this retention can be low, which is exacerbated by the fact that additional additives must be bound to the fibers. In addition, process reasons result in a relatively small absolute amount of strengthening additives that can be introduced into the fiber substrate. Higher addition amounts can lead to interference with the charge balance (Ladungshaushalte), sheet formation, and watermark formation. Therefore, it is not recommended to use strengthening additives other than CMC and EPI in the furnish.

[0007] Impregnations and coatings used in the paper surface are basically produced in two ways. One is during the papermaking machine process, where the substrate is treated with an impregnating agent (e.g., PU´s) in a sizing bath instead of with PVOH. This, while resulting in strengthening of the furnish, hardly prevents contaminants. The impregnating agent mainly penetrates into the paper core in the sizing bath and in the size press and only provides limited protection to the surface.

[0008] The coatings as described above are applied in the coating method and, due to the process, are mainly located on the surface and less in the paper mass (Papiermasse). Thus, although they provide good protection against contamination, they have no effect on the strengthening (z-strength) of the paper. Summary of the Invention

[0009] Therefore, the technical problem to be solved by the present invention is to provide a fiber substrate, for example for banknotes, which provides an extended circulation duration by simultaneously strengthening the z-strength and improving the stain resistance.

[0010] This technical problem is solved by the content of the independent claims. Exemplary embodiments result from the dependent claims and the following description.

[0011] Disclosed herein is a method for increasing the durability of a fiber substrate and a corresponding fiber substrate. The fiber substrate is preferably manufactured or obtained by the method. Therefore, the description of the structural features of the method or the fiber substrate achieved thereby also applies 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 durability (or lifespan) of a fibrous substrate is disclosed. The method includes providing an untreated fibrous substrate; impregnating the untreated fibrous substrate with at least one soft polymer having a relatively low glass transition temperature to obtain an impregnated fibrous substrate; drying the impregnated fibrous substrate to obtain an impregnated and dried fibrous substrate; and coating the impregnated and dried fibrous substrate with at least one hard polymer having a higher glass transition temperature relative to the soft polymer to obtain a circulation-stable fibrous substrate.

[0013] In the case of a so-called "durable substrate", the surface of the banknote is provided with an impregnating agent and / or a coating, which provides better protection for the substrate. Here, the PVOH sizing agent is replaced, for example, by polyurethanes (PU's). In addition, an acrylate-based coating can be applied in an additional step.

[0014] The basic idea of the present invention is to introduce / impregnate at least one "soft" polymer as an additive into the untreated fibrous substrate, and in addition, to provide a relatively "hard" polymer as a surface coating. At least one soft polymer is present here not only on the surface of the fibrous substrate, but also penetrates deep into and through the fibrous substrate, while the surface coating is present essentially only on the surface of the fibrous substrate.

[0015] Due to its properties, the soft polymer added to the fibrous substrate provides good reinforcement of the z-strength and thus prevents the fibers of the fibrous substrate from loosening under the high stresses during circulation. The introduction / impregnation of the soft polymer is carried out here especially after drying and cooling the untreated fibrous substrate from the round sieve. The corresponding additive does not have to be added in the forming section or before the forming section, but rather the untreated fibrous substrate from the round sieve is post-treated accordingly.

[0016] The hard polymer that should protect the surface from absorbing dirt is then applied in a downstream coating process, for example, in a curtain coating or film pressing method, or in any other suitable method. The downstream coating process here refers to a coating process carried out downstream of the impregnation, i.e., especially preferably also in the paper machine itself, but after the impregnation station in the paper machine. However, it is also conceivable to carry out the coating process in another machine.

[0017] Hardness, i.e., whether the polymer is soft or hard, is related here in the present disclosure to the glass transition temperature of the corresponding polymer or additive. Generally, the glass transition temperature (T g)Describes the temperature at which an amorphous polymer transitions from a hard / glass state to a soft / leathery state, and vice versa. Thus, the glass transition temperature indicates that the polymer is rigid at this temperature. When the temperature rises, the polymer undergoes a transition state where the macromolecular chains can slide past each other and the polymer softens. Therefore, the glass transition temperature is related to the mechanical properties of the polymer. For example, this includes tensile strength, impact strength, modulus of elasticity, and the operating temperature range. Thus, the glass transition temperature is directly related to the strength of the material. Polymers with a lower glass transition temperature are softer than polymers with a higher glass transition temperature.

[0018] After these general explanations, the method process of the present disclosure will be described below.

[0019] In a first step, an untreated fiber substrate is provided. Here, the untreated fiber substrate especially refers to a fiber substrate that, although reinforcing additives (such as CMC and EPI) can already be added, the soft and hard polymers according to the present disclosure have not been added as described herein. The untreated fiber substrate can be, for example, a fiber substrate from the wire section of a paper machine that has been dried and cooled. However, it is also conceivable that the untreated fiber substrate has been pre-produced and taken out of a warehouse and then supplied to a corresponding post-treatment machine that performs subsequent steps. Preferably, all steps of the method are carried out online in a single paper machine.

[0020] In a second step, the untreated fiber substrate is impregnated with at least one soft polymer having a relatively low glass transition temperature. It should be noted here that the at least one soft polymer can include only a single soft polymer or can include a mixture composed of any number of different soft polymers. Importantly, the individual soft polymer or the mixture of multiple soft polymers has the desired glass transition temperature. The impregnation can be carried out in any suitable manner, especially at a corresponding station in the paper machine that enables the penetration of the soft polymer into the fiber substrate. The at least one soft polymer preferably exists as an aqueous dispersion so that the untreated fiber substrate can be pulled through the corresponding aqueous suspension, where the soft polymer penetrates into the fiber substrate. The impregnation is preferably carried out in a dipping bath, as will be further described with reference to the examples below. In the impregnation, the untreated fiber substrate can, for example, be pulled through a dipping bath with the corresponding soft polymer resin instead of through a polyvinyl alcohol (PVOH) dipping bath (as in the prior art).

[0021] Generally, soft polymers, i.e., polymers with a relatively low glass transition temperature, have adhesive-like properties. For this reason, although soft polymers produce good paper strength, they often cause so-called "tack" on the paper and therefore cannot be wound into a roll because the paper layers will stick together. In addition, soft polymers also exhibit poor stain resistance.

[0022] To avoid this drawback and additionally to achieve good stain resistance, according to the method according to the invention, the fibrous substrate is additionally coated with at least one hard polymer, which hard polymer then is at least substantially only on the surface and does not penetrate into the fibrous substrate itself. Different from the soft polymer, this hard polymer provides good stain resistance and can be wound well. Therefore, it is advantageous to apply the two systems successively online (i.e., impregnating with a soft polymer and coating with a hard polymer), and to perform sufficient intermediate drying.

[0023] Therefore, the dried impregnated fibrous substrate is first dried. The drying can be carried out in any suitable manner, but is carried out substantially online after impregnation (especially because the impregnated fibrous substrate cannot be wound into a roll, as described above). The drying can be carried out, for example, in an infrared dryer (IR dryer), followed by cylinder drying, as further explained below with reference to the examples.

[0024] After the impregnated fibrous substrate is dried, it is coated with at least one additional, but this time harder, polymer (i.e., a polymer having a higher glass transition temperature compared to at least one soft polymer used during impregnation). The coating can be carried out in any suitable manner, for example, in a roll coating station downstream of the dryer in a paper machine (which can be designed, for example, as a curtain coating station or a film pressing station).

[0025] Basically, there is a large design space in the selection of the polymer. However, it must be noted that the polymer has corresponding physical properties, i.e., especially corresponding glass transition temperatures to each other. When selecting the polymer, for example, a part can also come from renewable raw materials in the polymer. For example, the polyol component can be made from polymers from renewable raw materials such as sugar cane or vegetable oil. For example, depending on the share of the polyol component in the polymer, the biomass share can be up to more than 50% here.

[0026] By the said method, it is possible to use the soft polymer as an impregnating agent, for example, in a controlled manner in a dipping bath / sizing bath. Compared with the soft polymer that may be absorbed when used in the furnish, i.e., for example, in the forming section or before the forming section, this can especially achieve the absorption of a larger amount of soft polymer. In addition, the interference of charge balance during sheet formation, which occurs when adding the corresponding additives (i.e., especially soft polymers) to the furnish, is avoided. In addition, performing the method online after the forming section enables the use of very soft and thus very strength-increasing additives for the fibrous substrate. In addition, compared with applying additives in the furnish, or compared with only impregnating without subsequent coating, the hard coating provides better protection against contamination.

[0027] According to one embodiment, the untreated fibrous substrate is impregnated in a dipping bath.

[0028] For example, the untreated fibrous substrate can come from the forming section (e.g., a cylinder sieve), and after the first drying section, advantageously after cooling (e.g., by means of a suitable cooling roller), it is pulled through a dipping bath / glue bath by means of corresponding conveying rollers, in which an aqueous dispersion with at least one soft polymer is located. Here, the untreated fibrous substrate is completely covered with the aqueous dispersion on both sides, and the aqueous dispersion with at least one soft polymer penetrates into the fibrous substrate through the surface of the fibrous substrate. Thus, the dipping bath with polyvinyl alcohol (usually in an aqueous solution; used for sealing the surface to prevent, for example, additives or fillers such as titanium dioxide from dusting off during subsequent printing processes) commonly used in the prior art is replaced by a dipping bath with an aqueous dispersion containing at least one soft polymer. The impregnation according to the invention accomplishes the same task as the polyvinyl alcohol dipping bath, but additionally leads to a good increase in strength and thus to a good increase in the running stability of the fibrous substrate, as described above.

[0029] According to another embodiment, the drying of the impregnated fibrous substrate is carried out in a dryer device.

[0030] The dryer device can be, for example, an infrared dryer with subsequent cylinder drying. In the infrared dryer (IR dryer), contactless drying is carried out in the manner of a suspension dryer. Then, contact drying of the paper substrate is carried out in the subsequent cylinder dryer. For this purpose, the paper substrate is first guided through the IR dryer on a conveyor belt and is dried here by infrared radiation under the action of heat. Subsequently, the substrate is dried by contact in the cylinder dryer. However, these dryer devices are only exemplary, and it should be recognized that any suitable dryer device can be used.

[0031] According to another embodiment, the coating of the impregnated and dried fibrous substrate is carried out in a roll coater.

[0032] Such a roll coater can include, for example, at least two rollers, preferably four rollers, wherein the impregnated and dried fibrous substrate is guided through two of them under pressure. The roller (engraving roller) that abuts against the fibrous substrate is used to apply at least one hard polymer to the impregnated and dried fibrous substrate. The engraving roller (Klischeewalzen) is preferably smooth and provided with rubber. If only two rollers are used (i.e., only the engraving roller), it passes through a reservoir with at least one hard polymer, preferably also in the form of an aqueous dispersion, at a position away from the fibrous substrate. Then, at least one hard polymer is transferred from the roller (engraving roller) that abuts against the fibrous substrate to the passing fibrous substrate.

[0033] However, preferably, an additional transfer roller (so-called dipping roller) is provided between each engraving roller in contact with the fiber substrate and the reservoir, which receives at least one polymer from the reservoir and transfers it to the engraving roller in direct contact with the fiber substrate. This particularly enables better control of the application amount. The engraving roller in direct contact with the impregnated and dried fiber substrate is provided with rubber and is smooth, while the dipping roller is provided with an engraved pattern that "dips" a determined amount of the formulation from the reservoir and transfers it to the engraving roller in contact with the fiber substrate. Then, the application amount of the formulation can be controlled by the volume of the engraved pattern and the relative speed of the dipping roller with respect to the engraving roller. Thus, the roller coater is a flexographic printing (Flexodruck) device that is arranged online after the dryer and the fiber substrate is printed on the entire surface with at least one hard polymer, i.e., coated.

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

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

[0036] These temperature ranges of the glass transition temperature have proven to be particularly preferred for increasing the flow stability of the fiber substrate, i.e., especially for increasing the z-strength.

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

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

[0039] These temperature ranges of the glass transition temperature have shown to be particularly preferred for increasing the stain resistance while maintaining the flow stability provided by at least one soft polymer.

[0040] According to another embodiment, the at least one soft polymer includes at least one of the following polymers: soft polyacrylate, styrene-butadiene, carboxylated styrene-butadiene, polyacrylamide, soft polyacrylic acid, soft polyacrylamide, polystyrene, and polyethylene.

[0041] These soft polymers have proven to be particularly effective in increasing the z-strength. In addition, the at least one soft polymer can contain any combination of the listed soft polymers, as long as the corresponding formulation generally has the desired physical properties, especially the desired glass transition temperature.

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

[0043] These hard polymers have proven to be particularly resistant to absorption of contamination. In addition, the at least one hard polymer may comprise any combination of the listed hard polymers, provided that the corresponding formulation has the desired physical properties overall, in particular the desired glass transition temperature.

[0044] According to another embodiment, the at least one soft polymer comprises at least one of the following polymers: polyurethane; carboxylated anionic polyurethane, from the following classes: polyester polyurethane, polyether polyurethane and polycarbonate polyurethane. The relatively low glass transition temperature is associated with a corresponding elongation that provides the corresponding properties.

[0045] In the case of polyurethane, the glass transition temperature is generally not defined, but rather the elongation (or elongation rate) is defined. Thus, in the case of polyurethane, a relatively low glass transition temperature refers to the corresponding elongation that provides the same or at least very similar properties as the relatively low glass transition temperature of a comparative polymer (such as the other soft polymers described herein).

[0046] The elongation of a polymer in general describes the tensile behavior of the polymer, where tensile is a form of polymer deformation. Deformation is in turn the change in shape that a polymer undergoes under stress. Under tensile stress, the polymer deforms and elongates by stretching. This is understood as elongation. Elongation can be given, for example, as a percentage elongation, which is the percentage change in the length of a polymer sample after stretching relative to the original length of the polymer sample.

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

[0048] The fungicidal and bactericidal additives provide effective protection of the fibrous substrate against the growth of particularly the main families of microbacteria and fungi.

[0049] The virucidal additive prevents the accommodation of viruses (or their survival on the fibrous substrate), and thereby prevents the transmission of viruses when the fibrous substrate is passed from one hand to another. Such a virucidal additive can be effective, for example, against human pathogen viruses such as retroviruses, cytomegaloviruses, rotaviruses, paramyxoviruses, polioviruses, hantaviruses, coxsackieviruses, encephalomyocarditis viruses, picomaviruses (including rhinoviruses), DNA or RNA viruses, especially flaviviruses, human immunodeficiency viruses, influenza viruses, smallpox viruses, yellow fever viruses, hepatitis C viruses, herpesviruses, Epstein - Barr viruses, varicella - zoster viruses, rubella viruses, simian virus 40 or SV40 or even coronaviruses.

[0050] An antifungal agent is an antibacterial substance that combats diseases caused by fungi.

[0051] Although the disclosed fibrous substrate provides good protection against the transfer of corresponding microorganisms even without such an additive due to the hard polymer coating and the resulting high resistance to the accommodation of dirt, the addition of corresponding antifungal, virucidal and bactericidal additives and antifungal agents (collectively referred to herein as biocides) can provide additional and stronger protection, especially in applications with high circulation, such as in the case of using the fibrous substrate for banknotes. Such additives also prevent the risk of cross - contamination with pathogens such as bacteria, molds and viruses. It should be noted that other biocides can also be added as additives. However, care must always be taken here that the biocides do not harm human health and environmental compatibility.

[0052] The corresponding biocide can be present, for example, in an amount of 0.05% to 0.8% based on the mass of the circulation - stable fibrous substrate.

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

[0054] Based on the mass of the circulation - stable fibrous substrate, the share of fibers is preferably between 75% and 85%, more preferably between 77% and 83%, and most preferably between 79% and 82%.

[0055] Based on the mass of the circulation - stable fibrous substrate, the share of at least one soft polymer is preferably between 5% and 10%, more preferably between 5% and 8%, and more preferably between 5% and 7%. Most preferably, the share of at least one soft polymer is 6%.

[0056] A mass meter based on a fiber substrate with stable circulation, the share of at least one hard polymer is preferably between 3% and 7%, more preferably between 4% and 7%, and even more preferably between 4% and 6%. Most preferably, the share of at least one hard polymer is 5%.

[0057] These formulations, and in particular the respective shares of at least one soft polymer and at least one hard polymer herein, have proven to be particularly effective in general in improving circulation stability, that is, increasing the z-strength while increasing the antifouling property, and also providing good windability (i.e., a good reduction in "stickiness", as described above).

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

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

[0060] The fiber substrate can particularly include all the structural features generated by the above methods in any combination.

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

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

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

[0064] According to another aspect, a fiber substrate with stable circulation according to one of the above embodiments is disclosed. The fiber substrate with stable circulation is manufactured according to one of the embodiments of the above method.

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

[0066] According to another aspect, the above-described fibrous substrate is disclosed for use as a substrate for banknotes, securities, visas or identification documents.

[0067] In particular, due to frequent handling, banknotes have a particularly high circulation load and thus particularly benefit from the use of the fibrous substrate described herein or a fibrous substrate produced by the method described herein. In principle, however, the disclosed circulation-stable fibrous substrate can be used in all applications where paper materials are used. Description of the Drawings

[0068] Figure 1 A highly schematic view of a part of a paper machine for carrying out the disclosed method for increasing the durability of a fibrous substrate is shown.

[0069] Figure 2 A flow chart of a method for increasing the durability of a fibrous substrate is shown, which method can be carried out, for example, by Figure 1 a part of a paper machine.

[0070] Figure 3 A cross-section of a circulation-stable fibrous substrate is schematically shown, which fibrous substrate can be obtained, for example, by using a part of a paper machine according to Figure 1 when carrying out the method according to Figure 2 herein. Detailed Description

[0071] The illustrations in the figures are schematic and not to scale. If the same reference numerals are used in different figures in the following description of the drawings, they denote the same or similar elements. However, the same or similar elements can also be denoted by different reference numerals.

[0072] Figure 1 A highly schematic view of a part of a paper machine is shown, which paper machine can be used to carry out the method 100 disclosed herein, which method is described below with reference to Figure 2 herein.

[0073] The part of the paper machine includes a bath 20 filled with a soft polymer 21 having a relatively low first glass transition temperature T g,1 . The soft polymer 21 is present in the bath 20 in the form of an aqueous dispersion. It should be noted further that the soft polymer 21 does not necessarily have to consist of only a single soft polymer 21, but can also be, for example, a mixture of different soft polymers 21, provided that the corresponding desired physical properties are present in general, i.e., in particular, ensuring the presence of the corresponding glass transition temperature T g,1 .

[0074] The bath 20 is arranged downstream of an arrangement of cooling rollers 50, which arrangement receives and cools the untreated fibrous substrate 13 from the wire section of the paper machine having a subsequent drying section (e.g., a cylinder sieve with a drying section; both not shown).

[0075] Downstream of the dip bath 20, a calender roll 22 is arranged, which is used to press at least one soft polymer 21 absorbed into the fiber substrate on the one hand, and at the same time "extrude" the excess formulation on the substrate.

[0076] A drying device 30 is arranged downstream of the calender roll 22. Here, the drying device is in the form of a suspension dryer 30 (such as an IR dryer), which is configured to dry the impregnated fiber substrate 14 from the dip bath 20 with IR radiation (for example, non-contact). However, it should be noted that basically all other suitable (especially non-contact) drying devices 30 can also be used. In addition, the dryer device 30 can also include, for example, contact drying (such as cylinder drying), which is carried out after non-contact drying.

[0077] Downstream of the drying device 30 (especially downstream within the same paper machine), a coating device 40 in the form of the roll coater 40 introduced in detail above is arranged. The coating device is configured to provide a coating composed of at least one hard polymer 41 on the impregnated and dried fiber substrate 15. The hard polymer has a lower first glass transition temperature T g,1 ) and a higher second glass transition temperature T g,2 compared to at least one soft polymer 21 in the dip bath 20. Here, it should also 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, especially the corresponding glass transition temperature T g,2 .

[0078] The following refers to Figure 2 the flowchart of Figure 1 and further refers to

[0079] to illustrate a method 100 for improving the durability (i.e., especially the circulation stability) of the fiber substrate 10. Figure 1 The method 100 starts with providing 110 an untreated fiber substrate 13. The untreated fiber substrate 13 is a fiber substrate that has not been added with the corresponding additives according to the present disclosure, especially not impregnated with the soft polymer 21 and not coated with the hard polymer 41. However, dry strengthening agents (such as CMC) and wet strengthening agents (such as EPI) may already be added to the untreated fiber substrate 13. Therefore, "untreated" means not treated with the soft and hard polymers according to the present invention. The untreated fiber substrate 13 is, for example, a fiber substrate from the wire section of a printing machine, such as from a round sieve. The untreated fiber substrate 13 basically only includes the components of a normal paper substrate, as described above. The untreated fiber substrate 13 can be, for example, a fiber substrate 13 from

[0080] In the next step, the untreated fiber substrate 13 is impregnated 120 with at least one soft polymer 21, for example in Figure 1 the immersion bath 20 in such a way that the untreated fiber substrate 13 is drawn through the immersion bath 20, for example by means of corresponding conveying rollers. The untreated fiber substrate 13 is hereby completely covered in the immersion bath 20 with at least one soft polymer 21 (which is present, for example, in the form of an aqueous dispersion) as described in reference Figure 1 such that both the upper and lower surfaces of the untreated fiber substrate 13 come into contact with at least one soft polymer 21. The untreated fiber substrate 13 hereby absorbs at least one soft polymer 21 by surface absorption, and this soft polymer thereby penetrates into the untreated fiber substrate 13. The impregnation 120 preferably also includes pressing in the absorbed at least one soft polymer 21, for example with the Figure 1 calender roller 22. Thereby, at the same time, the excess formulation is "squeezed out" from the fiber substrate.

[0081] The at least one soft polymer 21 can include, for example, at least one of the following polymers: soft polyacrylate, styrene-butadiene, carboxylated styrene-butadiene, and polyacrylamide. 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 the 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 the range between -45 °C and -20 °C, more preferably in the range between -40 °C and -20 °C, and most preferably in the range between -40 °C and -25 °C.

[0082] Polymers with these properties have proven to be particularly suitable for increasing the z-strength of the fiber substrate.

[0083] After the impregnation 120 of the untreated fiber substrate 13, in the next method step, drying 130 of the now impregnated fiber substrate 14 discharged from the immersion bath 20 (and the calender roller 22) is carried out. For this purpose, the impregnated fiber substrate 14 is guided through a drying device 30, for example Figure 1 the IR suspension dryer 30. The impregnated fiber substrate 12 is hereby dried accordingly. At the end of the suspension dryer 30, the impregnated and dried fiber substrate 15 is discharged from the drying device 30.

[0084] In the next step, the impregnated and dried fiber substrate 15 is then coated 140 with at least one hard polymer 41, i.e., with a polymer 41 having a higher glass transition temperature T g,2 than the soft polymer 21. When generally referring to soft and hard polymers and low and high glass transition temperatures here, these terms should be understood relative to each other.

[0085] The at least one hard polymer 41 may include, for example, at least one of the following polymers: polyacrylate; polyurethane and carboxylated polyurethane. Preferably, the at least one hard polymer 41 is also present in carboxylated form. Compared with the glass transition temperature T g,1 of the soft polymer 21, the glass transition temperature T g,2 of the at least one hard polymer 41 is in the range between -15 °C and 20 °C. Preferably, the glass transition temperature T g,2 of the at least one hard polymer 41 is in the range between -15 °C and 10 °C, more preferably in the range between -10 °C and 10 °C, and most preferably in the range between -10 °C and 0 °C.

[0086] Polymers with these properties have proven to be particularly suitable for improving the stain resistance of the fibrous substrate.

[0087] Coating 140 the impregnated and dried fibrous substrate 15 with the at least one hard polymer 41 can be carried out, for example, in a Figure 1 roller coater 40. Figure 1 The roller coater 40 includes, for example, two engraved rollers 42, 43 and two doctor rollers 44, 45. Here, the first roller 42 (engraved roller 42) receives the at least one hard polymer 41 from the third roller 44 (doctor roller 44), and the second roller 43 (engraved roller 43) receives the at least one hard polymer 41 from the fourth roller 45 (doctor roller 45). The third roller 44 and the fourth roller 45 (i.e., doctor rollers 44, 45) in turn receive the at least one hard polymer 41 from corresponding reservoirs (not shown), and the reservoirs store the at least one hard polymer 41 preferably also in the form of an aqueous dispersion. The first roller 42 and the second roller 43 (i.e., engraved rollers 42, 43) are preferably provided with rubber and are smooth, while the doctor rollers 44, 45 are provided with engraved patterns, and the engraved patterns "scoop" a determined amount of the formulation from the reservoir and transfer it to the engraved rollers 42, 43 that are in contact with the fibrous substrate. Then, the application amount of the formulation can be controlled by the volume of the engraved pattern and the relative speed of the doctor rollers 44, 45 with respect to the engraved rollers 42, 43. Figure 1 The roller coater 40 thus performs the coating of the impregnated and dried fibrous substrate 15 by a flexographic printing method, which applies the at least one hard polymer 41 over the entire surface. It should be noted, however, that the coating 140 of the surface of the impregnated and dried fibrous substrate 15 can also be carried out in any other suitable manner. In addition Figure 1 the roller coater 40 can also be configured with different numbers and configurations of rollers.

[0088] After coating 140 the impregnated and dried fibrous substrate 15, the finished circulation-stable fibrous substrate 10 is obtained.

[0089] When at least one soft polymer 21 penetrates into the core itself of the fibrous substrate 10 during impregnation, at least one hard polymer 41 is located at least substantially only on the surface of the impregnated and dried fibrous substrate 15 and thus at least substantially does not penetrate into the core of the fibrous substrate 10. Of course, there will be a certain penetration near the surface. At least one soft polymer 21 causes an increase in the z-strength and thereby reduces the tendency of the fibrous substrate 10 to fluff, i.e., the tendency for the individual fibers of the fibrous substrate 10 to separate from each other and thereby cause an increase in thickness and thus cause the fibrous substrate 10 to slump.

[0090] Optionally, one or more biocides and fungicides can be added to both at least one soft polymer 21 and at least one hard polymer 41, as described above.

[0091] Optionally, printing 150 of the circulation-stable fibrous substrate 10 is carried out after coating 140, and then optionally a final lacquer layer (e.g., a UV lacquer) is applied to further protect the surface. Printing 150 is carried out in another machine separately from the remaining method steps of method 100. The application of the final lacquer layer is also carried out in another machine after printing 150. Method 100 is thus interrupted during printing 150, and then the application of the final lacquer layer 160 is carried out.

[0092] Finally Figure 3 Also shown is an exemplary circulation-stable fibrous substrate 10 produced, for example, by method 100 (in the case of using part or all of other suitable devices of a paper machine) or in all other suitable ways. Figure 2 of method 100 (in the case of using Figure 1 part or all of other suitable devices of a paper machine) or in all other suitable ways.

[0093] The circulation-stable fibrous substrate 10 has a substrate core 11 that is penetrated / impregnated (represented by dots) by at least one soft polymer 21. The substrate core without at least one soft polymer 21 would, for example, correspond to Figure 1 the untreated fibrous substrate 11. A surface coating 12 composed of at least one hard polymer 41 is located on both surfaces of the substrate core 11.

[0094] The properties of the soft polymer 21 and the hard polymer 41, as well as all other features and properties of the circulation-stable fibrous substrate 10, can correspond to the features and properties generally described in reference Figure 2 to method 200 and this disclosure. Therefore, these features and properties are not repeated here.

[0095] With the method 200 and the fiber substrate 10 with stable circulation described above, it is possible to use a soft polymer as an impregnating agent, for example, in a controlled manner in an immersion bath / sizing bath. Compared with the soft polymer that may be absorbed when used in a furnish, such as in a forming section or before the forming section, this can particularly achieve the absorption of a larger amount of the soft polymer. In addition, interference with the charge balance during sheet formation, which occurs when adding a corresponding additive (i.e., particularly a soft polymer) to the furnish, is avoided. In addition, performing the method online after the forming section enables the use of very soft and thus strength-increasing additives for the fiber substrate. In addition, compared with applying an additive in a furnish or compared with only impregnating without subsequent coating, the hard coating provides better protection against contamination. In addition, the hard coating provides a remedy for the problem that a fiber substrate impregnated only with a soft polymer sticks and thus cannot be wound into a roll. Therefore, the present disclosure also provides the windability of the fiber substrate 10 with stable circulation.

[0096] The disclosed fiber substrate 10 or the corresponding method 100 is particularly advantageously used for documents with high circulation and thus high load, such as banknotes, securities, visas, or certification documents.

[0097] As a supplement, it should be noted that "comprising" or "having" does not exclude other elements or steps, and the indefinite article "a" does not exclude a plurality. In addition, it should be pointed out that the features or steps described with reference to one of the above embodiments can also be used in combination with other features or steps of the other above embodiments. The reference signs in the claims should not be regarded as limiting.

[0098] List of reference signs

[0099] 10 Fiber substrate

[0100] 11 Substrate core

[0101] 12 Surface coating

[0102] 13 Untreated fiber substrate

[0103] 14 Impregnated fiber substrate

[0104] 15 Impregnated and dried fiber substrate

[0105] 20 Immersion bath

[0106] 21 Soft polymer

[0107] 22 Sizing press roll

[0108] 30 Drying device, suspension dryer

[0109] 40 Roller coater

[0110] 41 Hard polymer

[0111] 42 First roller, engraved roller

[0112] 43 Second roller, engraving roller

[0113] 44 Third roller, dipping roller

[0114] 45 Fourth roller, dipping roller

[0115] 50 Cooling roller

[0116] 100 Method

[0117] 110 Provide an untreated fiber substrate

[0118] 120 Impregnate the fiber substrate

[0119] 130 Dry the fiber substrate

[0120] 140 Coat the fiber substrate

[0121] 150 Print the fiber substrate

[0122] 160 Apply the final paint layer (topcoat, UV paint)

Claims

1. A method (100) for increasing the durability of a fibrous substrate (10), the method (100) comprising: Providing (110) an untreated fibrous substrate (13); Impregnating (120) an 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 fibrous substrate (14) to obtain an impregnated and dried fibrous substrate (15); Coating (140) the impregnated and dried fiber substrate (15) with at least one hard polymer (41) having a higher glass transition temperature (T g,2 ) than the soft polymer (21) to obtain a fiber substrate (10) with stable flowability.

2. The method (100) according to claim 1, wherein, The impregnation (120) of the untreated fibrous substrate (13) is carried out in a dipping bath (20).

3. The method (100) according to one of the above claims, wherein, The drying (130) of the impregnated fibrous substrate (14) is carried out in a drying device (30).

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

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

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

7. The method (100) according to one of the preceding claims, wherein, The at least one soft polymer (21) comprises at least one of the following polymers: Soft polyacrylate; Styrene butadiene; Carboxylated styrene butadiene; Polyacrylamide; Soft polyacrylic acid; Soft polyacrylamide; Polystyrene; and Polyethylene.

8. The method (100) according to one of the preceding claims, wherein The at least one soft polymer (21) comprises at least one of the following polymers: Polyurethane; Carboxylated anionic polyurethane, from the categories: polyester polyurethane, polyether polyurethane and polycarbonate polyurethane; Among them, the relatively low glass transition temperature (T g,1 ) is related to the corresponding elongation that provides the corresponding properties.

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

10. The method (100) according to one of the preceding claims, wherein, Adding at least one of a fungicidal additive, a virucidal additive, a bactericidal additive and an antifungal agent 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 one of the above claims, wherein, Based on the mass of the circulation - stable fibrous substrate (10), the circulation - stable fibrous substrate (10) comprises: 70% to 90% fibers; 4% to 12% of the at least one soft polymer (21); and 3% to 8% of the at least one hard polymer (41).

12. A circulation - stable fibrous substrate (10), which comprises: A substrate core (11) having a core impregnation part; And A surface coating (12); Among them, the core impregnation part includes at least one soft polymer (21) having a relatively low glass transition temperature (T g,1 ); and Among them, the surface coating (12) includes at least one hard polymer (41) that has a higher glass transition temperature (T g,2 ) compared to the soft polymer (21).

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

14. The fiber substrate (10) with stable flow according to claim 12 or 13, wherein, The circulation - stable fibrous substrate is manufactured by the method (100) according to one of claims 1 to 11.

15. An application of a fibrous substrate (10), the fibrous substrate being manufactured by the method (100) according to one of claims 1 to 11 or being the fibrous substrate (10) according to one of claims 12 to 14, the fibrous substrate being used as a substrate for banknotes, securities, visas or documentary evidence.

Images