WET STRENGTH IMPROVED FIBROUS NON-WOVEN FABRICS, IN PARTICULAR PAPER, USES OF THESE WET STRENGTH FABRICS AS WELL AS PRODUCTION METHODS OF THE SAME

FIELD OF THE INVENTION

[0001]The present invention relates to environmentally friendly, wet strength improved fibrous non-woven fabrics, optionally paper, impregnated with a mixture comprising a silicon component, further comprising water and/or an alcohol and/or a surfactant and a catalyst (e.g., acidic or basic). Also encompassed are methods for production and uses of such wet strength improved fibrous non-woven fabrics.

BACKGROUND OF THE INVENTION

[0002]Large quantities of paper are consumed in Germany every year. In 2019 consumption was 227 kilogrammes per capita. This corresponds to a total consumption of 18.9 million tonnes in Germany alone.

[0003]Paper is essentially a sheet or web of fibrous non-woven fabric (usually cellulose fibres) to which chemicals are added to improve its properties and quality. In addition to fibres and chemicals, the production of pulp and paper requires large amounts of process water and a lot of energy in the form of steam and electricity. Therefore, the main environmental issues related to the pulp and paper industry concern water and air emissions and energy consumption. Waste is also an ever-increasing source of environmental pollution.

[0004]The pulp for paper production is produced by chemical or mechanical processes either from virgin fibres or by processing wastepaper.

[0005]Untreated paper becomes mechanically unstable when it becomes damp or wet. Due to the splitting of the hydrogen bonds when water enters, the fibre fleece loses its internal cohesion.

[0006]In order to maintain mechanical strength—albeit limited—even when wet, wet strength agents (e.g., polyamidoamine epichlorohydrin (PAAE)) are added to the paper during production. Tear-resistant kitchen crepe is probably the best-known paper in this class, but cardboard, map paper, tea bags or security paper for banknotes also contain large amounts of wet strength agents.

[0007]Wet strength agents known in the prior art are water-soluble polymers in the processing state that react with themselves and/or the paper fibres. In the process, water-insoluble cross-links form between the fibres, which stabilise the fibre web.

[0008]The state of the art describes industrially established wet strength agents based on melamine and urea-formaldehyde resins and polyamidoamine-epichlorohydrin (PAAE). These commercially used wet strength agents are built up from toxic starting substances and are based on petrochemical materials, which at least partially are still present in the final product. Therefore, there are particular health concerns in paper production, and in wastewater and paper disposal, the environmental input of these polymers (or their precursors) cannot be satisfactorily avoided and represents a considerable environmental burden. The chemical linkage also prevents successful recycling, so the increasing use of wet strength agents in the sanitary paper sector has far-reaching consequences for wastepaper recycling. For example, the incidence of insoluble particles in the normal dissolving process is steadily in-creasing.

[0009]If conventional wet strength agents (similar to bitumen adhesives) are chemically broken down, the fibre degrades uncharacteristically quickly. The recovered paper quality thus decreases more quickly than in normal recycling processes.

[0010]Wet strength agents must not be confused with sizing chemicals (for example AKD), as the chemo-physical action process is different. For example, a wet-strength, unglued paper is still highly capillary, and a superglued paper can only be defibrillated after a long time in contact with water.

[0011]Another disadvantage of existing wet strength agents is that some develop their full wet strength only after a certain storage time, which of course is unprofitable.

SUMMARY OF THE INVENTION

[0012]The present invention solves the aforementioned environmental problems by providing more environmentally friendly wet strength improved fibrous non-woven fabrics, optionally paper, impregnated with wet strength agents based on silicon components, such as silica (SiO₂) and aminosilica (SiO₂—NH₂)).

[0013]In a first aspect, the present invention relates to a wet strength improved fibrous non-woven fabric with an air-dry silica-content of about 0.05 wt.-% or more, 0.1 wt.-% or more, 0.5 wt.-% or more, 1 wt.-% or more, or of about 2 wt.-% or more, or of about 5 wt.-% or more, or of about 10 wt.-% or more; up to about 30 wt.-% or less, or up to about 25 wt.-% or less, or up to about 20 wt.-% or less; in one embodiment between 1 wt.-% to 30 wt.-%, or between 2 wt.-% to 25 wt.-%, or between 5 wt.-% to 20 wt.-%.

[0014]In one embodiment, a low silica content of between 0.05 to 5 wt.-% is preferred since it improves the economics of the process. As is explained hereinunder, the silica content is related to the wet-strength and the composition of the impregnation solution.

[0015]Wherein the silica is distributed homogeneously in the wet strength fibrous non-woven fabric in so far that the silica-content on one side of the wet strength fibrous non-woven fabric and the silica-content on another side of the wet strength fibrous non-woven fabric is a ratio of less than 2:1, or a ratio of less than 1.5:1, or a ratio of less than 1.25:1, or a ratio of about 1:1. Such a distribution can be measured for example by IR-spectroscopy of each surface and calculating the silica content and the ratio between the silica-contents of each surface, respectively.

[0016]And wherein the Cobb₆₀-value of the wet strength improved fibrous non-woven fabric after silica-impregnation is about 20% or less reduced as compared to the untreated fibrous non-woven fabric. In one embodiment, the reduction of the Cobb₆₀-value about 10% or less, or about 5% or less, or about the same as compared to the untreated fibrous non-woven fabric. The “Cobb₆₀-value” is defined and measured according to DIN EN ISO 535 (2014-06-00) and describes the amount of water (in gram) which is absorbed by 1 m² of material after 60 seconds.

[0017]The term “untreated fibrous non-woven fabric” refers to the same fibrous non-woven fabric but not impregnated with any of the impregnation solutions disclosed in this application and/or otherwise silica treated.

[0018]In a further embodiment the specific surface area determined by BET is about ±10%, or about ±5%, or about ±2% as compared to the untreated fibrous non-woven fabric. The “B.E.T.” or “BET” (Brunauer-Emmett-Teller) theory aims to explain the physical adsorption of gas molecules on a solid surface and serves as the basis for an important analysis technique for the measurement of the specific surface area of materials. It is defined and measured according to DIN ISO 9277. The inventive wet strength improved fibrous non-woven fabric shows a similar surface and pore distribution and size as untreated fibrous non-woven fabric, thereby having the beneficial effect that water retention and/or permeability remain the same.

[0019]Without being bound to theory it seems that the swelling capacity of the fibres in the fibrous non-woven fabric is slightly reduced by the silica treatment and this effect adds to the improved wet strength. The swelling in water is one of the important fundamental properties of fibres in the fibrous non-woven fabrics and can be measured by the desiccation rate. Experimentally, this method is exceedingly simple—the fibre samples, fully swollen and containing excess water, are dried very slowly and weighed periodically. It is shown that for a time the moisture is lost at a nearly constant rate. Eventually, however, the drying rate changes and thereafter steadily diminishes. It is believed that this change occurs only after all nonswelling water external to the fibre has evaporated. The moisture regain at this point of transition in the desiccation rate is considered to be indicative of the maximum water-holding capacity of the cellulose—i.e., the swelling capacity. Thus, in one embodiment the swelling capacity of the wet strength improved fibrous non-woven fabric after silica-impregnation is reduced by about 20% or less, or reduced by about 10% or less, or reduced by about 5% or less, or reduced by about 2% or less, or reduced by about 1% or less as compared to the untreated fibrous non-woven fabric.

[0020]The wet strength improved fibrous non-woven fabric of the present invention may be made of any non-woven fabric made from staple fibres (short) and long fibres (continuous long), bonded together by chemical, mechanical, heat or solvent treatment. The term “non-woven fabric” is used hereinunder to denote fabrics, such as felt, which are neither woven nor knitted. Some non-woven materials lack sufficient strength unless densified or reinforced by a backing. In one important embodiment the fibrous non-woven fabric is paper.

[0021]The wet strength improved fibrous non-woven fabric of the present invention shows an excellent wet tensile strength according to DIN ISO 3781. Thus, in one embodiment the wet tensile strength according to DIN ISO 3781 of the wet strength improved fibrous non-woven fabric according to the invention is about 2 times or more, or about 3 times or more, or about 5 times or more, or about 10 times or more higher as compared to the untreated fibrous non-woven fabric. In another embodiment the wet tensile strength is up to about 50 times or less, or to about 35 times or less, or up to about 30 times or less, or up to about 25 times or less higher as compared to the untreated fibrous non-woven fabric. In one embodiment of the present invention the wet tensile strength is between at least 2 times and up to 30 times, or between at least 2.5 times and up to 25 times, or between at least 3 times and up to 20 times, or between at least 4 times and up to 15 times higher as compared to the untreated fibrous non-woven fabric.

[0022]In one embodiment, the absolute wet strength improved fibrous non-woven fabric of the present invention according to DIN ISO 3781 (1994-10) is between 1 and 50 N/15 mm, or between 2 and 40 N/15 mm, or between 3 and 35 N/15 mm, or between 4 and 30 N/15 mm.

[0023]In some embodiments, the wet strength improved fibrous non-woven fabric is still wettable and able to absorb water. This is important, since many applications of wet strength improved non-woven fabrics are used to absorb water, e.g., kitchen towels (e.g. made of paper), tissue paper and or handkerchiefs or the like, or still need to be permeable for water, such as for example tea-bags or coffee filters.

[0024]The wettability can be quantified by the “contact angle” which is conventionally measured where a liquid-vapor interface meets a solid surface (e.g., water drop on the fibrous non-woven fabric in air). It quantifies the wettability of a solid surface by a liquid via the Young's equation.

[0025]Thus, the contact angle of the surface of the wet strength improved fibrous non-woven fabric according to the present invention is less than 100°, or less than 90°, or less than 75°, or less than 60° on at least both major sides, or on all sides of the wet strength improved fibrous non-woven fabric.

[0026]In a further aspect, the present invention relates to a wet strength improving impregnating solution comprising a silicon component for increasing the wet tensile strength of fibrous non-woven fabric resulting in an environmentally friendly and non-toxic final product, in particular paper and board, wherein the impregnation solution further comprises an acidic catalyst, water and/or an alcohol, i.e. a type of organic compound that carries at least one hydroxyl functional group (—OH) bound to a saturated carbon atom (for example ethanol, 1-propanol, 2-propanol, 2-methylpropan-1-ol, 2-methylbutan-2-ol and/or methanol) and follows the general formula C_nH_2n+1OH, and/or a surfactant. It has been surprisingly found that some basic catalysts in some embodiments may lead to nanoparticles instead of a silica-coating and, thus, these embodiments cannot be used as wet-strength improving impregnation. Thus, such mixtures are excluded from this application. However, by checking for the absence of nanoparticles, the skilled person is able to identify basic catalysts, which do form coatings and are therefore suitable in the impregnation solutions of this application.

[0027]In one embodiment the wet strength improving impregnating solution comprises tetraethyl orthosilicate (TEOS) as “silicon component” for increasing the wet tensile strength of fibrous non-woven fabric, in particular paper, wherein the impregnation solution further comprises water, ethanol (EtOH) as alcohol and/or SDS (in case of an anionic surfactant) or CTAB (in case of a cationic surfactant) as a surfactant and, preferentially takes place under acidic or basic conditions. In case of an “acid catalyst” hydrochloric acid (HCl) may be used.

[0028]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0029]In one embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0030]Thus, in another embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0031]In this respect it was surprisingly found that cationic surfactants improve the wet-strength capacity of the fibrous non-woven fabric significantly more than anionic surfactants. Thus, although anionic surfactants selected from the group consisting of alkyl carboxylates, alkyl benzene sulfonates, secondary alkyl sulfonates, fatty alcohol sulfates (e.g sodium lauryl sulfate, SDS), alkyl ether sulfates, sulfoacetates and taurides may also be used in some embodiments, in an preferred embodiment cationic surfactants selected from the group consisting of tetraalkylammonium salts of the general formula R₄N⁺ which differ in their alkylic residue (e.g. stearyl-, palmityl-, methyl-, benzyl-, butyl-), for example DSDMAC (distearyldimethylammonium-chloride) and/or esterquats; benzalkoniumchloride, benzethoniumchloride, cetylalkoniumchloride, cetylpyridiniumchloride, cetyltrimethylammoniumbromide (cetrimonium bromide) and/or dequaliniumchloride.

[0032]In one embodiment emulsifiers may be added selected from the list of Ascorbyl palmitate (E 304), Lecithins (E 322), Sodium phosphate (E 339), Potassium phosphate (E 340), Propylene glycol alginate (E 405), Salts of edible fatty acids (E 405), Mono-and diglycerides of fatty acids (E 471), Acetic acid esters of mono-and diglycerides of fatty acids (E 472 a), Lactic acid esters of mono-and diglycerides of fatty acids (E 472 b), Citric acid esters of mono-and diglycerides of fatty acids (E 472 c), Tartaric acid esters of mono-and diglycerides of fatty acids (E 472 d), Mono-and diacetyl tartaric esters of mono-and diglycerides of fatty acids (E 472 e), Mixed tartaric and acetic esters of mono-and diglycerides of fatty acids (E 472 f), Sugar esters of fatty acids (E 473), Sugar glycerides (E 474), Polyglycerol esters of fatty acids (E 475), Polyglycerol polyricinoleate (E 476), Propylene glycol esters of edible fatty acids (E 477), (non-ionic): Polysorbate 80 (E433) and polysorbate 60 (E435), polysorbate 65 (E 436), polysorbate 40 (E 434), polysorbate 20 (E 432), Pectin (E 440), Carrageenan (E 407), and Gum arabic (E414), as well as any combination thereof.

[0033]In yet another embodiment further natural emulsifiers may be added selected from the list of Beeswax, Glycerin, Sorbitan derivatives (generally sorbitan esters), and/or Non-ionic emulsifiers, such as Ceteareth (polyoxyethylene ether) or PEG (polyethylene glycol) derivatives, as well as combinations thereof.

[0034]In yet another embodiment further emulsifiers may be added selected from the list of Styrene-maleic anhydride copolymer (SMA), Styrene-butadiene latex (SBR), Polyvinyl acetate (PVA), Carboxymethylcellulose (CMC), and Wax emulsions, as well as combinations thereof.

[0035]In yet another embodiment further emulsifiers may be added selected from the list of Modified starch as emulsifier/stabilizer, such as Starch sodium octenyl succinate (E 1450), Monostarch phosphate (E 1410), Distarch phosphate (E 1412), Phosphated distarch phosphate (E 1413), Acetylated distarch phosphate (E 1414), and Acetylated starch (E 1420), as well as any combination thereof.

[0036]In yet another embodiment further emulsifiers may be added selected from the list of Sodium hydroxy-octadecane sulfonate, Sodium, potassium and ammonium salts of hydroxy fatty acids of chain length C12-C20 and their sulfation or acetylation products, Alkyl sulfates, also as triethanolamine salts, alkyl (C10-C20) sulfonates, Alkyl (C10-C20)-arylsulfonates, Dimethyldialkyl (C8-C18)-ammonium chloride, not more than 0.005%, based on the Dispersion film, Acyl, alkyl, oleyl and alkylaryloxethylates and their sulfation products, sulfosuccinic acid 4-esters with polyethylene glycol nonylphenyl ether (di-sodium salt), Lignosulfonic acid and its calcium, magnesium, sodium and ammonium salts (in total, not more than 0.04 mg/dm²), Dodecylated diphenyl ether-disulfonic acid sodium, Copolymers of ethylene oxide and propylene oxide with a minimum content of 10%, Ethylene oxide, Lecithin (not more than 1 mg/dm²), disodium 4-[1-methyl-2-[(1-oxo-9-octadecenyl)amino]ethyl]-2-sulfosuccinate (not more than 0.8 mg/dm²), Sulfosuccinic acid 4-esters with polyethylene glycol ethers of monohydric aliphatic alcohols of chain length C10-C12 (di-sodium salt) (the transfer to food or food simulants shall not exceed 2 mg/kg), Sulphosuccinic acid bis-cyclohexyl ester (sodium salt) (the transfer to foodstuffs or Food simulants shall not exceed 5 mg/kg), Alkali salts of sulfosuccinic acid esters with aliphatic saturated monohydric alcohols of chain length C4-C20 (the transfer to food or food simulants shall not exceed 5 mg/kg).

[0037]The advantageous effect of the cationic surfactant may be used in two different improved embodiments: Either in order to improve the wet-strength of the impregnated fibrous non-woven fabric as compared to mixtures comprising anionic surfactants or to reduce the amount of silicon component, such as TEOS, needed for the impregnation solution, thereby reducing expenses.

[0038]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0039]In a one embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0040]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

[0041]In another embodiment the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

[0042]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

[0043]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

[0044]Thus, in a further embodiment, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

[0045]Further compositions can be found in the example section.

[0046]In yet another embodiment the present invention also pertains to a fibrous non-woven fabric obtained by impregnating the fibrous non-woven fabric with one or more of the mixtures disclosed hereinunder.

[0047]In certain preferred embodiments, the impregnating solution consists of the silicon component. In other words, the portion of the silicon component in the impregnating solution is 100% by weight. Thus, in particularly, the impregnating solution may be pure silane. In other preferred embodiments, the impregnating solution contains in addition to the silicon component still at least one further component, for example a solvent component such as for example an alcohol or water and/or an acid component. In further embodiments a surfactant, for example SDS may be also present in the solution.

[0048]In one embodiment, the portion of the silicon component in the impregnating solution is in a range of 5% by weight to 100% by weight, or 10% by weight to 99% by weight, or 20% by weight to 98% by weight, or 40% by weight to 97% by weight, or 60% by weight to 96% by weight, or 80% by weight to 95% by weight.

[0049]In one embodiment, the silicon component is selected from the group consisting of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate, polydimethoxysiloxane, 1,2-bis(triethoxysilyl)ethane, silicon tetraacetate, (3-Aminopropyl)triethoxysilane, (3-Aminopropyl)trimethoxysilane, N-[3-(Trimethoxysilyl)propyl]ethylenediamine, [3-(2-Aminoethylamino)propyl]trimethoxysilane, N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane, 3-(2-Aminoethylamino) propyldimethoxymethylsilane, Bis[3-(trimethoxysilyl)propyl]amine and mixtures of two or more thereof. Particularly preferable is the component TEOS. TEOS is a common basic chemical which is not expensive and readily available. In one embodiment, the component is pre-condensed (i.e., still liquid). The term “pre-condensed” means that only oligomers have already been formed, but that the material is not yet completely polymerized.

[0050]In one embodiment, the impregnating solution contains solvents in a portion which is in a range of 0 to 98% by weight, or of 0.1 to 50% by weight, or of 0.2 to 20% by weight, or of 0.5 to 10% by weight, still or of 1 to 5% by weight. In one embodiment, the solvent is selected from the group consisting of water, ethanol and mixtures of two or more thereof. In one embodiment, the solvent is water.

[0051]In one embodiment, the impregnating solution contains water in a portion which is in a range of 0 to 20% by weight, or of 0.5 to 10% by weight, or of 1 to 5% by weight.

[0052]In one embodiment, the impregnating solution contains an acid, for example HCl (37 wt. %) in a portion of 0.001 to 0.2% by weight, or of 0.005 to 0.1% by weight, or of 0.01 to 0.05% by weight.

[0053]In one embodiment, the impregnating solution according to the present invention consists of at least 95% by weight, or at least 98% by weight, or at least 99% by weight, or at least 99.9% by weight, or at least 99.99% by weight of ethanol, water, silicon component and HCl. It is a special advantage of the method according to the present invention that no further components are required in the impregnating solution. In one embodiment, the impregnating solution even consists of at least 95% by weight, or at least 98% by weight, or at least 99% by weight, or at least 99.9% by weight, or at least 99.99% by weight of water, silicon component and HCl.

[0054]In one embodiment, the impregnated fibrous non-woven fabric comprises a silicon component (e.g., silica or aminosilica) in a portion of 0.1 to 30% by weight, or of 0.5 to 10% by weight, or of 1 to 5% by weight, or of 1.5 to 7.5% by weight. In one embodiment, the impregnated fibrous non-woven fabric consists of the fibre component (in particularly paper) and the impregnating component (SiO₂).

[0055]In one embodiment, the impregnated fibrous non-woven fabric comprises the fibre component in a portion of 90 to 99.9% by weight, or of 92.5 to 99.8% by weight, or of 95% by weight to 99.5% by weight.

[0056]In one embodiment, the impregnated fibrous non-woven fabric of the invention consists of the fibre component (e.g., paper) and the impregnating component (e.g. SiO₂). The fibrous non-woven fabric may contain further components, or however in a portion of at most up to 50% by weight, such as for example 0 to 30% by weight, or up to 25% by weight, or up to 10% by weight, or up to 5% by weight, or up to 2% by weight, or up to 1% by weight, or less than 0.5% by weight. These further components may in particularly be inorganic and/or organic fillers.

[0057]In one embodiment, the portion of the fibre component and the impregnating component in the fibrous non-woven fabric of the present invention is at least 50% by weight, or at least 75% by weight, or at least 90% by weight, or at least 95% by weight, or at least 98% by weight, or at least 99% by weight. In one embodiment, the impregnated fibrous non-woven fabric of the invention consists of the fibre component and the impregnating component.

[0079]In yet another embodiment the present invention pertains to any silica coated fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with any of the wet strength improving impregnation solutions mentioned hereinunder.

[0080]In one aspect a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 12-20 mL TEOS (1 eq.), 10-45 μL HCl (37 wt. %), 80-88 ml of water (ad 100 mL total liquid), and 0.25-3 g sodium dodecyl sulfate (SDS).

[0081]In yet another embodiment a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 10-20 mL TEOS (1 eq.), 15-40 μL HCl (37 wt. %), 70-85 ml of water, and 0.5-2 g sodium dodecyl sulfate (SDS).

[0082]In yet another embodiment a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 15.2 mL TEOS (1 eq.), 28 μL HCl (37 wt. %), 84.8 mL of water, and 1g sodium dodecyl sulfate (SDS).

[0083]In yet another embodiment a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 0.5-3 mol-% tetraethyl orthosilicate (TEOS): 20-60 mol-% EtOH: 5-30 mol-% H₂O: 0.001-0.2 mol-% HCl (37 wt. %).

[0084]In yet another embodiment a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 1-2 mol-% tetraethyl orthosilicate (TEOS): 30-50 mol-% EtOH: 10-20 mol-% H₂O: 0.01-0.05 mol-% HCl (37 wt. %).

[0085]In yet another embodiment a fibrous non-woven fabric, optionally a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising mixing 1 mol-% tetraethyl orthosilicate (TEOS): 40 mol-% EtOH: 10 mol-% H₂O: 0.02 mol-% HCl (37 wt. %),

[0086]The present invention also relates to a method for the production of a fibrous non-woven fabric with homogeneous silica impregnation, in particularly a fibrous non-woven fabric of the present invention such as described above. The invention also relates to a fibrous non-woven fabric with a homogeneous silica impregnation which can be obtained or has been obtained by the method.

[0087]In one embodiment the impregnation solution may be added already to the bath comprising the fibres of the fibrous non-woven fabric before the fibrous non-woven fabric itself is produced and dried. The advantage of this process is an evenly distribution of the impregnation solution on all fibres and a simplified process. However, one disadvantage may be the need of higher volumes of impregnation solution, thus, impregnation solutions with cationic surfactant are preferred.

[0093]In another embodiment the fibrous non-woven fabric is impregnated later during the drying process.

[0111]The advantage of the described product and process according to the invention compared to the state of the art is an environmentally friendly, non-toxic final product (i.e., only comprising SiO₂), which also allows the technical use of the wet-strength improved fibrous non-woven fabric according to the invention in the food industry. Furthermore, the impregnation can be carried out at temperatures below 150° C. (between 4-40° C.; or ambient temperature at 20° C.) and atmospheric pressure, which leads to significant energy savings. Furthermore, since the impregnation works so fast, the paper can be further treated rather quickly in the production line. Further, since a homogeneous distribution is preferred, a “zick-zack” arrangement of contact elements and/or rolls may be used to dry the paper equally from both sides.

[0112]According to step a) of the method according to the present invention, a fibrous non-woven fabric is provided. In one embodiment, the fibrous non-woven fabric is selected from the group consisting of non-woven paper fabrics, non-woven textile fabrics and non-woven plastic fabrics. In one embodiment, the fibrous non-woven fabric is a non-woven paper fabric. In one embodiment, the provided non-woven paper fabric has a grammage of 10-1000 g/m². In one embodiment between 10-20 g/m², or 12-15 g/m² (for example in case of paper for tea-bags). In other embodiments the grammage is between 250-1050 g/m², or 300-1000 g/m² (for example in case of wood pulp boards). In further embodiments the grammage is 65 to 120 g/m², or of 70 to 100 g/m², or of 75 to 90 g/m².

[0113]The fibrous non-woven fabric can be a commercially available fibrous non-woven fabric. In particularly, the non-woven paper fabric can be a commercially available non-woven paper fabric. In an alternative, the step of providing the fibrous non-woven fabric, in particularly the non-woven paper fabric, can also contain the step of the production of the fibrous non-woven fabric, in particularly the non-woven paper fabric. The production of a non-woven paper fabric is optionally conducted with the Rapid-Köthen method, in one embodiment in a sheet forming Rapid-Köthen plant, in one embodiment according to DIN 54358 and/or ISO 5269/2 (ISO5269-2:2004(E), “Pulps—Preparation of Laboratory Sheets for Physical Testing—Part 2: Rapid Köthen Method, 2004”).

[0114]In some embodiments, in the production of the fibrous non-woven fabric, in particularly the non-woven paper fabric, no further additives or fillers are used. In other embodiments, additives such as conservation agents, bleaching agents, thickeners, etc. may be used.

[0115]According to step b) of the method according to the present invention, an impregnating solution which contains a silicon component is provided. In the present description the terms “impregnating solution” and “impregnation solution” are used in a synonymous sense. The impregnating solution may be a single-component one, thus may consist of one single component. In such a case the impregnating solution may in particularly also be referred to as “impregnating fluid” or “impregnation fluid”.

[0116]After the addition of the single components, the impregnating solution is optionally stirred for a period of time of 6 to 48 hours, or of 12 to 36 hours, or of 18 to 30 hours, before the impregnating of the fibrous non-woven fabric, in particularly the non-woven paper fabric, is conducted with the impregnating solution according to step c) of the method according to the present invention. In embodiments of the invention in which the impregnating solution consists of silicon component (in particularly TEOS), in one embodiment, no such stirring is conducted.

[0117]The step c) of the impregnating of the fibrous non-woven fabric, in particularly the non-woven paper fabric, with the impregnating solution is optionally conducted at a relative air humidity in a range of 10% to 95%, or of 30% to 70%, or 40% to 60%, or 45% to 55% and/or at a temperature in a range of 15° C. to 30° C., or 20° C. to 25° C.

[0118]In one embodiment, the impregnating of the fibrous non-woven fabric with the impregnating solution is realized by exposing the fibrous non-woven fabric to the impregnating solution, thus in other words, by bringing the fibrous non-woven fabric into contact with the impregnating solution. For the design of this step of contacting the fibrous non-woven fabric with the impregnating solution there are many possibilities. In one embodiment, the impregnating of the fibrous non-woven fabric with the impregnating solution according to step c) of the method is realized with an impregnating method which is selected from the group consisting of film coating, kiss coating, immersion coating, spray coating, size press, roller coating, blade coating and curtain coating. Immersion coating and spray coating are particularly preferred. Especially preferred is immersion coating. In one embodiment, the impregnating solution is uniformly distributed across the surface and the interior of the fibrous non-woven fabric.

[0119]According to embodiments of the invention, the impregnating in step c) of the invention is realized by immersing the fibrous non-woven fabric into the impregnating solution. In one embodiment, the fibrous non-woven fabric, in particularly the non-woven paper fabric, is completely immersed into the impregnating solution. In one embodiment, the immersion is conducted such that the fibrous non-woven fabric, in particularly the non-woven paper fabric, is substantially vertically oriented. A vertical orientation means, in other words, that both main surfaces of the fibrous non-woven fabric are arranged such that surface vectors which are orthogonal with respect to the main surfaces are substantially horizontally oriented. In one embodiment, the surface vectors of both main surfaces with the vector of the immersion direction each form an angle of at least 70° and at most 110°, or of at least 80° and at most 100°, or of at least 85° and at most 95°.

[0120]In one embodiment, the removal of the fibrous non-woven fabric from the impregnating solution is conducted at a time point 0.1 to 10 seconds, or 1 to 5 seconds after the completion of the immersion of the fibrous non-woven fabric into the impregnating solution. The minimum and maximum time of contact with the impregnation solution depends also on the production process: In case of impregnation by dipping processes up to 30 seconds, up to a minute, up to 5 minutes are still acceptable, whereas in case of size-press systems the contact is much shorter between 0.1 second and 5 seconds. However, the contact should not be significantly below 0.05 second and not much more than 30 minutes.

[0121]In one embodiment, the removal of the fibrous non-woven fabric from the impregnating solution is conducted such that the fibrous non-woven fabric is substantially vertically oriented. A vertical orientation means, in other words, that both main surfaces of the fibrous non-woven fabric are arranged such that surface vectors which are orthogonal with respect to the main surfaces are substantially horizontally oriented. In one embodiment, the surface vectors of both main surfaces with the vector of the removal direction each form an angle of at least 70° and at most 110°, or of at least 80° and at most 100°, or of at least 85° and at most 95°.

[0122]According to step d) of the method according to the present invention, the drying of the fibrous non-woven fabric is conducted at temperatures in a range of 70° C. to 190° C. In one embodiment, the drying of the fibrous non-woven fabric is conducted at temperatures in a range of 80° C. to 180° C., or of 90° C. to 170° C., or of 100° C. to 160° C., or of 110° C. to 150° C., or of 120° C. to 140° C., or of 125° C. to 135° C. In one embodiment, the drying of the fibrous non-woven fabric according to step d) is conducted, until the residual moisture content of the fibrous non-woven fabric is in a range of 3% by weight to 7% by weight. In one embodiment, the residual moisture content is determined by means of gravimetric analysis, in particularly according to DIN EN 20287. At higher temperatures, the pulp of the paper may suffer and show discolouration.

[0123]Between the completion of the impregnating according to step c) and the begin of the drying according to step d) there is a period of time of at most 60 seconds, or at most 45 seconds, or at most 30 seconds, or at most 20 seconds, or at most 10 seconds, or at most 5 seconds, or at most 2 seconds, or at most 1 second. In one embodiment, the drying according to step d) of the method according to the present invention is conducted immediately, in other words directly, after the impregnating of the fibrous non-woven fabric with the impregnating solution according to step c) of the method.

[0124]In one embodiment, the impregnating according to step c) is finished, when the fibrous non-woven fabric is no longer exposed to the impregnating solution, or in other words, when the fibrous non-woven fabric is no longer brought into contact with the impregnating solution. In the case of immersion coating, for example, the impregnating according to step c) is optionally finished, when the fibrous non-woven fabric again has completely been removed from, for example pulled out of, the impregnating solution. In the case of spray coating, for example, the impregnating according to step c) is optionally finished, when the impregnating solution is no longer sprayed onto the non-woven fabric.

[0125]The drying according to step d) optionally begins, when the fibrous non-woven fabric enters an environment which is intended for removing humidity and/or condensation products, such as for example an oven.

[0126]Presumably, with the short period of time between the completion of the impregnating and the beginning of the drying it is achieved that the silicon component of the impregnating solution has still not been converted in a substantial extent into the silicate component of the silica impregnation, when the drying is realized at the increased drying temperatures. This allows the targeted adjustment of the homogeneity of the silica impregnation. Because the silicon component migrates at the increased drying temperatures depending on the drying conditions, in particularly the environmental pressures, through the non-woven fabric, as long as it has not been converted into a silica impregnation with a polymerization degree which is too high.

[0127]In embodiments in which the impregnating solution does not contain a solvent, the migration of the silicon component through the fibrous non-woven fabric at the increased drying temperatures can in particularly be influenced by an adjustment of the environmental pressures during the drying. In embodiments in which the impregnating solution contains solvent(s), the migration of the silicon component can in particularly also be influenced by the evaporation of the silicon component and/or the solvent at the increased drying temperatures, because the silicon component together with the solvent migrates through the fibrous non-woven fabric.

[0128]When the polymerization further progresses, then the mobility in the fibrous non-woven fabric is further reduced. Thus, when after the impregnating of the fibrous non-woven fabric with the impregnating solution there is a longer period of time, until the drying at the increased temperatures is started, thus when the period of time between the steps c) and d) of the method is relatively long, then this results in a substantial polymerization, before a relevant migration of the silicon component through the fibrous non-woven fabric has taken place. This results in the fact that fibrous non-woven fabrics with a homogenous distribution of the silica impregnation are obtained, because the distribution of the already polymerized coating can no longer be influenced by the drying conditions at the increased drying temperatures.

[0129]Contrary to this, the method of the present invention is designed such that between the completion of the impregnating according to step c) and the beginning of the drying according to step d) a period of time of at most 60 seconds is provided. Therefore, at the begin of the drying a substantial poly-condensation has still not taken place so that the silicon component which is present in the impregnating solution migrates through the fibrous non-woven fabric, and therefore via the migration the distribution of the silica impregnation can be influenced in a targeted manner.

[0130]In one embodiment, the drying is achieved with the help of a dryer. In one embodiment, the dryer is selected from the group consisting of hot air/convective dryers, ovens, drum dryers (contact drying) and IR dryers. For example, it can be dried in an oven, or in a drying oven (in some embodiments a vacuum oven) or in a muffle furnace. In one embodiment, the oven/drying device is preheated, in particularly to the drying temperature so that after the impregnating according to step c) it is possible to start the drying according to step d) very quickly. However, in one embodiment, the dryer is a hot air dryer.

[0131]According to the present invention, the properties of the obtained fibrous non-woven fabrics cannot only be influenced by the above-mentioned method steps, but in particularly also by the prevailing environmental pressure during the drying. Surprisingly, it has been found that the distribution of the content of SiO₂ in the fibrous non-woven fabrics can be controlled in a targeted manner by the pressure conditions during the drying. Fibrous non-woven fabrics with relatively high contents of SiO₂ at both main surfaces can in particularly be obtained with low pressures.

[0132]In one embodiment, the pressure during the drying according to step d) is in a range of 0.1 kPa to 500 kPa, or of 0.2 kPa to 200 kPa. In certain preferred embodiments, the pressure during the drying according to step d) is in a range of 0.1 kPa to 30 kPa, or of 0.2 kPa to 20 kPa, or of 0.5 kPa to 10 kPa, or of 1 kPa to 5 kPa. Such embodiments are in particularly suitable for the production of such fibrous non-woven fabrics, where the content of SiO₂ at both main surfaces is relatively high and, however, in the center of the fibrous non-woven fabric is relatively low (so-called sandwich structure). On the other hand, in other preferred embodiments, the pressure during the drying according to step d) is in a range of >30 kPa to 500 kPa, or of 50 kPa to 200 kPa, or of 60 kPa to 150 kPa, or of 70 kPa to 130 kPa, or of 80 kPa to 120 kPa, or of 90 kPa to 110 kPa. Such embodiments are in particularly suitable for the production of such fibrous non-woven fabrics, where the content of SiO₂ at one of both main surfaces is considerably higher than at the other of both main surfaces, while the content of SiO₂ in the center of the fibrous non-woven fabric is lower than at the one of both main surfaces, however higher than at the other of both main surfaces. This is in particularly true, when impregnating solutions with low or medium portions of silicon component are used, in particularly with a portion of silicon component in a range of 0.1% by mol to 3.5% by mol, or of 0.2% by mol to 3% by mol, or of 0.5% by mol to 2.5% by mol, or of 0.9% by mol to 2.2% by mol.

[0133]In one embodiment, after the drying, the fibrous non-woven fabrics are cooled to a temperature of 15° C. to 30° C., or of 20° C. to 25° C.

[0134]In one embodiment, the method consists of the mentioned steps. It is a special advantage of the method according to the present invention that the method gets by with very few steps.

[0135]The present invention also includes papers with increased wet strength produced according to said processes, according to steps a) to d).

[0136]The present invention also relates to the use of a fibrous non-woven fabric of the present invention, in particularly as packaging material.

[0137]Preferred uses as packaging material comprise the use as freezing paper, the use for products which come into contact with foodstuffs, such as in particularly (paper) cups and/or (paper) straws, as well as the use as packaging for materials which are protected from liquid, but nevertheless should allow diffusion of moisture. In particularly, the fibrous non-woven fabrics of the present invention can be used for plastic-free straws and/or pasteboard cups. In view of possible regulations for the reduction of plastic wastes it can be assumed that there will be an increased demand for plastic-free products. Fibrous non-woven fabrics for the mentioned uses, in particularly for the use in drinking straws and cups, often are also referred to as specialty papers.

[0138]Further examples of suitable materials which could be improved in their wet strength are tea bags, kitchen paper, cardboard, map paper, security paper for bank notes, coffee or tea filters, wrapping paper, toilet paper, paper towels, cooking paper, paper labels, display boards, promotional poster, corrugated boards, paper bags, and/or cloths, and the like.

[0139]The present invention also relates to the use of a fibrous non-woven fabric of the present invention as specialty paper for the use at increased temperatures, in particularly the use of a fibrous non-woven fabric of the present invention as baking paper.

[0140]The present invention also includes the use of the wet strength agents described for the manufacture of a paper product having increased relative wet strength of greater than 2%, greater than 5%, or greater than 10%, or greater than 15% as measured by a tensile elongation test according to ISO 1924-2.

[0141]In further embodiments, papers are measured by tensile elongation strength according to ISO 1924-2 and achieve relative wet strengths above 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30% and up to 31%, 32%, 33%, 34%, 35%, 40%, 50%.

[0142]The method for determining properties under tensile stress, as specified in this part of ISO 1924-2, is one of the most commonly used methods. It is similar to the method specified in ISO 1924-3. In this part of ISO 1924 (ISO 1924-2) a constant strain rate of 20 mm/min is applied, whereas in ISO 1924-3 the constant strain rate is 100 mm/min.

[0143]Since the results of the tensile test depend on the constant strain rate, tests according to this part of ISO 1924 and ISO 1924-3 do not give the same results. The dependence on the speed can vary due to the paper grade and is therefore different for the width-related breaking force, the breaking elongation, the work absorption capacity and the modulus of elasticity.

[0144]The International Standard was prepared by ISO/TC 6 “Paper, board and pulp” in cooperation with CEN/TC 172 “Fibrous materials, paper and board”, whose secretariat is held by DIN. The Standards Committee for Materials Testing (NMP) played a major role in the preparation of the standard. The national mirror committee is the joint committee NA 062-04-26 GA “Physical-technological test methods for paper and board”. For the rest, we refer to the specifications in the corresponding DIN standard.

[0145]The fibrous non-woven fabric of the present invention is a fibrous non-woven fabric. In one embodiment, the fibrous non-woven fabric is selected from the group consisting of non-woven paper fabrics and non-woven textile fabrics. In one embodiment, the fibrous non-woven fabric is a non-woven paper fabric.

[0146]In one embodiment, the fibrous non-woven fabrics of the invention have high flexibility.

[0147]Thus, in one embodiment, the paper mentioned hereinunder is selected from the group consisting of eucalyptus paper and/or northern bleached softwood kraft (NBSK). In other embodiments cotton linters (80 g/m²) and/or abacá (13 g/m²). Other papers that can be used according to the invention include: Aburatorigami, Affiche paper, Albumin paper, Alfapaper, Amatl, APCO II/II (DIN 16519 T2), Watercolour paper (between 120 g/m² up to 850 g/m², for watercolour board from approx. 150 g/m²), copying paper, lining paper, baking paper, banana paper, banknote paper, baryta paper, bible printing paper (grammage between 25 and 60 g/m²; also known as thin printing paper), illustration paper, blueprint paper, bombyzine paper, letter paper, stamp paper, book paper, coloured paper, handmade paper, buttered paper or parchment substitute, China paper, Chinese rice paper, chromo paper, down printing paper, decalcifying paper, thick printing paper, document paper, double wax paper, printing paper, duplex paper, carbonless paper (also known as carbonless paper or copy paper, grammage 30-40 g/m²), Ice paper (ice board or alabaster paper), elephant skin, electrical insulating paper, ivory paper, ivory board (grammage from 240 to 320 g/m²), continuous printing paper for EDP printing (DIN 672), label paper, fabriano paper, Fine papers (all wood-free and rag-containing papers according to DIN), filter paper, felt paper, flock paper (velour paper), florpost (florpost paper), photo paper, coated paper, ribbed paper or vergé paper, glass-fibre paper, Gummed paper, rag paper (at least 10% rag or cotton, hemp or flax fibre), semi-chemical paper (at least 65% semi-chemical pulp of the total fibre pulp), hemp paper, hard paper (fibre composite of paper and a phenol-formaldehyde synthetic resin (phenoplast), hard post paper (also bank post paper), high-gloss paper (one-sided cast-coated but not calendered paper), Wood-free paper (not more than 5% by weight of lignified fibres), Wood-containing paper (not less than 5% by weight of lignified fibres), Hydrographic paper, Hygienic papers (especially tissue papers), Ingres paper (hand-made or cylinder mould hand-made paper, Japan paper, potassium iodide starch paper, chancery paper (grammage of 60 and 120 g/m²), carbon paper, Khoi paper, copying paper, corrosion protection paper (VCI paper, Volatile Corrosion Inhibitor), kraft liner, kraft paper (also: Packing paper) (grammage formerly around 130 g/m², today around 80 g/m²), Chalk paper, Crepe paper, Kitchen crepe, Art paper, Plastic fibre papers or plastic papers, Copper printing paper, Map paper, Leather paper, Light tracing paper, Blotting paper (also non-woven paper), Manila paper, Medical papers (ISO 11607, EN 868-6, EN 868-3, and ISO 10993-5), Metallised paper, Metal-laminated paper, Medium-fine papers, Mummy paper, Natural fibre papers (these include papers made from cotton, banana, sisal, mulberry fibres, rice, maize, wheat straw paper, kudzu paper, daphne paper (Lokta, Nepal, Himalayan paper) Daphne bholua and Daphne papyracea), Uncoated paper (uncoated paper, at most with a surface treatment or pigmentation of up to 5 g/m²), offset printing paper (also known as offset paper for short), oiled paper or waxed paper according to DIN 6730, parabaic, parchment substitute (also known as buttered paper), parchment paper, parchment substitute (also known as greaseproof paper), parchment, poster paper, plotter paper, post paper (grammage between 70 and 120 g/m²), quartz fibre paper (filter paper containing quartz fibres), swell paper, recycled paper (any paper containing more than 25% recovered paper before further refinement), rice paper, raw paper, saa paper (also known as siam paper), absorbent paper, SC paper, silhouette paper (or clay paper) (grammage 80-90 g/m²), sandpaper, writing paper, typewriter paper (also known as SM paper), DIN 6730), scratch paper, swell paper, tissue paper (25 g/m² grammage), carbonless copy paper, also NCR paper (no carbon required), security paper, silicone paper, tension paper, spider paper, spinning paper, stone paper, straw paper, synthetic paper, ropes (or packing ropes, lift ropes), tea bag paper (mainly made of abaca fibres), directory paper (approx. 35 g/m²), thermal paper, rotogravure paper, tracing paper, release paper, vellum paper (velin), velour paper, wasli paper, watermark paper, work printing paper, xuan paper, drawing paper (in different grammages from 60 g/m²), newsprint, cigarette paper and combinations thereof.

[0148]The term grammage or basis weight is to be used interchangeably here.

[0149]Wet-strength papers produced by the processes or wet-strength agents described are particularly suitable for applications that are naturally exposed to moisture but still require paper stability. Such papers are, for example, packaging papers and cartons, banknote papers, handkerchiefs, paper kitchen crepe and cleaning cloths, cigarette papers, sandwich papers, newsprint papers, filter papers for e.g., tea and coffee, etc.

[0150]In another aspect also uses of said wet-strength impregnation solution and fibrous non-woven fabrics as well as papers in/as products with improved wet tensile strength selected from the group comprising tea bags, kitchen paper, cardboard, map paper, security paper for bank notes, coffee or tea filters, wrapping paper, toilet paper, paper towels, cooking paper, paper labels, display boards, promotional poster, drinking straws, packaging papers, corrugated boards, paper bags, cloths.

[0151]In one aspect of the invention the wet-strength improved non-woven material (e.g. paper) can be particularly used for food contact materials due to their non-toxic nature. This is important for paper cups, especially when holding hot beverage or food (e.g., instant soup cups), microwave pots or other plates and/or cutlery which come in contact with acidic and or hot food or beverage, tea bags, coffee filters, paper straws, and the like. In those instances, an aminosilica-content of between 0.7-1% is preferred.

[0152]In one embodiment the mixture comprises the following compounds in weight-%:

[0153]The term “air dry” is a term well-known in the field for describing additives added to a porous material such as a fibrous non-woven fabric. When describing the portion of an additive in “air dry” porous material such as fibrous non-woven fabric, it refers to the state of the material with less than 10 wt. % of residual moisture (e.g., water). This state can be achieved by drying the fibrous non-woven fabric by drying at SATP-conditions (“Standard Ambient Temperature, Pressure”), i.e., T=298.15 K (25° C.); pressure (p)=101.300 Pa=1013 hPa=101.3 kPa=1,013 bar. “Oven dry” is defined as the state of porous material such as a fibrous non-woven fabric without any residual moisture, i.e., 0 wt. % residual moisture. This may be achieved for example by drying the material in a preheated oven at 160° C. for 15 minutes at pressure (p)=101.300 Pa but may also be achieved by other drying schemes which dries the material until no shift in weight can be measured.

[0154]Thus, “air dry” is the general term for a residual moisture in the porous material, such as a fibrous non-woven fabric, of less than 10%, e.g., between 0% and 10%, whereas “oven dry” is the specific case with a residual moisture of 0% in the porous material, such as a fibrous non-woven fabric.

BRIEF DESCRIPTION OF THE FIGURES

[0155]FIG. 1: Depicted is the schematic illustration of the bilateral coating process using a size press.

[0156]FIG. 2: Depicted is the thermogravimetric analysis of a) model paper sheets with various composition of long fibres (LF; NBSK) and short fibres (SF; eucalyptus), namely 20LF80SF (black line), 50LF50SF (dashed line), and 80LF20SF (dotted line), coated with silica via option I and b) coated with silica via option II.

[0157]FIG. 3: SEM micrographs of eucalyptus paper sheets are shown before (left) and after silica coating via option I (mid) and option II (right) at a magnification of 2000 respectively. Scale bar: 10 μm.

[0158]FIG. 4: a) Wet tensile strength of unmodified (black) and silica coated model paper sheets with various composition of long fibres (LF; NBSK) and short fibres (SF; eucalyptus) using coating option I (grey). b) Wet tensile strength of unmodified (black) and silica coated model paper sheets via coating option II (grey), and a reference that was proceeded without the silica source in the coating solution (white).

DETAILED DESCRIPTION OF THE INVENTION

EXAMPLES

Reagents

[0159]All chemicals and solvents were purchased from Merck Millipore and Thermo Fisher Scientific and used as received unless otherwise stated.

Example 1. Preparation of Paper Sheets

[0160]Lab-engineered paper sheets were prepared using bleached eucalyptus sulfate pulp and Northern bleached softwood kraft (NBSK; pine and spruce) pulp. The pulp was refined in a Voith LR® 40 laboratory refiner at 75000 revolutions. Lab-engineered paper sheets with a grammage of 80±1.6 g/m2 were prepared using a Rapid-Köthen® sheet former according to DIN 54358 and ISO 5269/2. No additives or fillers were used. Model paper sheets with different fibre ratios of long fibres (LF; NBSK) and short fibres (SF; Eucalyptus) of 100LF:0SF (Eucalyptus), 20LF:80SF, 50LF50SF, and 80LF20SF were prepared.

[0161]In one embodiment paper was made from mixing the paper fibre pulp suspension with the impregnation solution and drying the paper afterwards. For use in the fibre suspension, a wet strength was achieved of 1.58±0.1 N/15 mm for 20LF80SF with a silica content of 1.28% oven dry and 1.22% air dry.

Example 2. Silica Coating

Option I: Impregnating Sol

[0162]For silica coating of paper sheets, a coating sol (option I) with following compositions were prepared.

[0163]Very good results with respect to wet-strength were achieved with:

[0164]Also very good results with respect to wet-strength were achieved with:

[0165]Still good results with respect to wet-strength were achieved with:

[0166]Still mediocre results with respect to wet-strength were achieved with:

[0167]The components of the sol were mixed, stirred for 24 h at room temperature, and used for silica coating of paper sheets using a size press (FIG. 1) for bilateral coating with a speed of 5 m/min at a roll pressure of 0.5-1.0 bar. Freshly coated paper sheets were placed in a pre-heated oven at 160° C. for 5-15 min at atmospheric pressure before cooling down to ambient temperature.

Option II: impregnation Emulsion

[0168]For silica coating of paper sheets, a coating emulsion (option II) was prepared as follows.

[0169]Very good results with respect to wet-strength were achieved with:

[0170]Good results with respect to wet-strength were achieved with:

[0171]Mediocre results with respect to wet-strength were achieved with:

[0172]TEOS was mixed with HCl (37 wt %) and stirred for 24 h at room temperature. Then, water and sodium dodecyl sulfate (SDS) were added and the mixture is rigorously stirred for 1 h at room temperature, before applying the obtained emulsion for coating with a size press with a speed of 5 m/min at a roll pressure of 0.5-1.0 bar. Freshly coated paper sheets were placed in a pre-heated oven at 160° C. for 5-15 min at atmospheric pressure before cooling down to ambient temperature.

[0173]Surprisingly, when the paper was treated with a mixture according to option II comprising as surfactant the cationic surfactant cetrimonium bromide, the wet-strength of model paper sheets with different fibre ratios of long fibres (LFSF 20/80%; NBSK 100%) was 23±4 N/15 mm, i.e. much higher than with other impregnations.

[0174]Surprisingly it was found that TEOS can be brought into solution as emulsion without the use of ethanol. Thus, option II shows some advantages as compared to option I. For example, is EtOH (or alcohol in general) a security risk (alcohol fumes, explosivity) and it is more expensive than water. Thus, option II is the preferred mixture for impregnation.

Example 3. Thermogravimetric Analysis (TGA)

[0175]TGA was performed using a TGA 1 Mettler-Toledo®. The samples were heated from 25° C. to 600° C. with a heating rate of 10 K/min under constant air flow of 30 mL/min.

Example 4. Scanning Electron Microscopy (SEM)

[0176]SEM micrographs were recorded using a Zeiss EVO 10 SEM® operated at an acceleration voltage of 10 kV. Prior to measurements, the samples were sputter coated with a platinum/palladium layer of 10 nm.

Example 5. Determination of Wet Tensile Strength

[0177]Wet tensile strength was determined as an average of 10 samples according to DIN ISO 3781 with a Zwick Z1.0 with a 1 kN load cell using the software testXpert® II V3.71 from ZwickRoell GmbH & Co. Kg in a controlled environment with 23° C. and 50 rH %. Thereby, the paper samples were immersed in water for at least 1 h prior to measurements.

Example 6 . Determination of Silica Content

[0178]The silica content of silica coated paper sheets is determined via TGA. Thereby, the silica content can be determined from the plateau of the TGA curves at 550° C., since silica is stable up to 1700° C., whereas natural organic fibres are totally combusted. Comparing the values between 550° C. and 600° C. before and after silica coating allows to take the natural ash content of the substrates into account. Furthermore, the weight loss occurring till 150° C. can be assigned to physically adsorbed water and is considered in the calculation to determine the silica content with respect to oven-dry samples. TGA curves of model paper sheets coated with silica via option I and eucalyptus paper sheets coated with silica via option II are depicted in FIG. 2, whereas the calculated silica contents are illustrated in Table 1.

Example 7. Morphological Analysis

[0179]Eucalyptus paper sheets were investigated with respect to their morphology before and after silica coating using SEM (FIG. 3). Silica coated paper sheets coated with option I and option II do not show morphological differences on the micrometer scale compared to uncoated paper sheets. This can be explained due to the low silica amount as deduced from TGA.

Example 8. Determination of Wet Tensile Strength

[0180]The wet tensile strength of paper sheets before and after coating with silica was determined according to DIN ISO 3781. Thereby, paper samples of 15 mm width and approximately 120 mm length are immersed in water for 1 h before measurement. The determined values of wet tensile strength are depicted in FIG. 4. Comparing the wet tensile strength before and after silica coating, higher values are obtained due to silica coating for both option I and option II. Thereby, the wet tensile strength of model paper coated with silica via option I show higher values as compared to eucalyptus paper sheets coated with silica via option II. Furthermore, comparing the model paper sheets with different composition of long and short fibres, the wet tensile strength increases more with higher number of short fibres (eucalyptus).

Example 9: Wet Strength Features of Different Papers and Impregnations Mixtures

[0181]The wet strength was compared for different papers and different mixtures. As comparative paper (asymmetric impregnated with-type TEOS) paper was used, i.e., with a silica content on one side at least 2 times higher than on the other side.

[0182]In case of the impregnations with the mixtures according to options I and II a homogeneous silica-distribution was applied with a silica content on one side about 2 times or less higher than on the other side.

[0183]The silica content is depicted in example 6, table 1.

Example 10: Contact Angle Measurements

[0184]Contact angle measurements were performed with the model TBU90E from DataPhysics® Instruments GmbH and the SCA software. All samples were measured at five positions and the mean value and standard deviation were calculated. A drop volume of 2 μl was used for static contact angle measurements (application rate: 0.5 μl/s) (at 23° C. with 50% relative humidity).

Example 11: Comparison with Prior Art Impregnations

[0185]Paper from eucalyptus fibres with a grammage of 80 gsm were prepared in the laboratory. The paper sheets were coated using a size press and an ethanolic solution according to US 2022/034041 as well as an aqueous emulsion and ethanolic solution according to the disclosure.

[0186]The paper sheets were coated via size press and subsequently dried in an oven. For the preparation according to US '041, the coated paper sheets were placed in a preheated oven at 130° C. for 15 minutes. For the preparation of wet-strength paper due to silica coating according to the present disclosure, the coated paper sheets were placed in a preheated oven at 160° C. for 15 minutes.

[0187]The paper sheets were equilibrated at 50% relative humidity and 23° C. after coating. Then, Cobbeo values of the two main sides of the coated paper sheets were determined according to DIN EN ISO 535. Uncoated paper made from eucalyptus fibres with a grammage of 80 g/m² have an initial water absorption capacity of 310±37 g/m².