EP4311872A1

EP4311872A1 - Wet strength improved fibrous non-woven fabrics, in particular paper, uses of these wet strength fabrics as well as production methods of the same

Info

Publication number: EP4311872A1
Application number: EP22187283.1A
Authority: EP
Inventors: Mathias Stanzel; Nicole Rath; Markus BIESALSKI; Annette ANDRIEU-BRUNSEN
Original assignee: Technische Universitaet Darmstadt
Current assignee: Technische Universitaet Darmstadt
Priority date: 2022-07-27
Filing date: 2022-07-27
Publication date: 2024-01-31
Also published as: WO2024023105A1

Abstract

Novel environmentally friendly, wet strength improved fibrous non-woven fabrics, preferably paper, impregnated with a mixture comprising a silicon component, further comprising water and/or an alcohol and/or a surfactant and a catalyst. Also encompassed are methods for production and uses of such wet strength improved fibrous non-woven fabrics.

Description

The present invention relates to environmentally friendly, wet strength improved fibrous non-woven fabrics, preferably 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.

Technical background of the invention

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.
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.
The pulp for paper production is produced by chemical or mechanical processes either from virgin fibres or by processing waste paper.
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.
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.
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.
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 waste paper recycling. For example, the incidence of insoluble particles in the normal dissolving process is steadily in-creasing.
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.
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.
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.
The present invention solves the aforementioned environmental problems by providing more environmentally friendly wet strength improved fibrous non-woven fabrics, preferably paper, impregnated with wet strength agents based on silicon components, such as silica (SiO₂) and aminosilica (SiO₂-NH₂)).

Description of the invention

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, preferably of about 2 wt.-% or more, more preferably of about 5 wt.-% or more, most preferably of about 10 wt-% or more and up to about 30 wt.-% or less, preferably up to about 25 wt-% or less, most preferably up to about 20 wt-% or less; in one aspect between 1 wt.-% to 30 wt.-%, preferably between 2 wt.-% to 25 wt.-%, most preferably between 5 wt.-% to 20 wt.-%.
In one aspect, 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.
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, preferably a ratio of less than 1.5:1, more preferably a ratio of less than 1.25:1, most preferably 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.
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. Preferably the reduction of the Cobb₆₀-value about 10% or less, more preferably about 5% or less, most preferably 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.
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.
In a further aspect the specific surface area determined by BET is about ± 10%, more preferably about ± 5%, most preferably 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.
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 aspect the swelling capacity of the wet strength improved fibrous non-woven fabric after silica-impregnation is reduced by about 20% or less, more preferably reduced by about 10% or less, even more preferably reduced by about 5% or less, even more preferably reduced by about 2% or less, most preferably reduced by about 1% or less as compared to the untreated fibrous non-woven fabric.
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 aspect the fibrous non-woven fabric is paper.
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 aspect 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, preferably about 3 times or more, more preferably about 5 times or more, most preferably about 10 times or more higher as compared to the untreated fibrous non-woven fabric. In another aspect the wet tensile strength is up to about 50 times or less, preferably up to about 35 times or less, more preferably up to about 30 times or less, most preferably up to about 25 times or less higher as compared to the untreated fibrous non-woven fabric. In one aspect of the present invention the wet tensile strength is between at least 2 times and up to 30 times, preferably between at least 2.5 times and up to 25 times, more preferably between at least 3 times and up to 20 times, most preferably between at least 4 times and up to 15 times higher as compared to the untreated fibrous non-woven fabric.
In one aspect, 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, preferably between 2 and 40 N/15 mm, more preferably between 3 and 35 N/15 mm, most preferably between 4 and 30 N/15 mm.
In some aspects, 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.
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 equation.
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°, more preferably less than 90°, even more preferably less than 75°, most preferably less than 60° on at least both major sides, preferably on all sides of the wet strength improved fibrous non-woven fabric.
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.
In one aspect 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 (HCI) may be used.

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

Compound	wt.-%
silicon component, such as tetraethyl	3.5 - 16
orthosilicate (TEOS)
surfactant, (for example sodium lauryl sulfate or cetrimonium bromide)	0.2 - 1.5
H₂O	80 - 95 (ad 100%)
acid catalyst (e.g. HCI)	0.005- 0.05

In a more preferred aspect, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

Compound	wt.-%
silicon component, such as tetraethyl orthosilicate (TEOS)	7 - 16
surfactant, (for example sodium lauryl sulfate or cetrimonium bromide)	0.5 - 1.5
H₂O	80 - 95 (ad 100%)
acid catalyst (e.g. HCI)	0.015- 0.04

Thus, in a most preferred aspect, the present invention also pertains to a wet-strength improving impregnation mixture (option II) comprising the following compounds in weight-%:

Compound	wt.-%
silicon component, such as tetraethyl orthosilicate (TEOS)	about 14.3
surfactant, (for example sodium lauryl sulfate or cetrimonium bromide)	about 1.0
H₂O	about 85 (ad 100%)
acid catalyst (e.g. HCI)	about 0.03

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, cetylalkoniumchlo-ride, cetylpyridiniumchloride, cetyltrimethylammoniumbromide (cetrimonium bromide) and/or dequaliniumchloride.
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.

Compound	wt.-%
silicon component, such as tetraethyl orthosilicate (TEOS)	1.75 - 8
cationic surfactant (e.g. cetrimonium bromide)	0.2 - 1.5
H₂O	80 - 95 (ad 100%)
acid catalyst (e.g. HCI)	0.005- 0.05

Compound	wt.-%
silicon component, such as tetraethyl orthosilicate (TEOS)	3.5-8
cationic surfactant (e.g. cetrimonium bromide)	0.5 - 1.5
H₂O	80 - 95 (ad 100%)
acid catalyst (e.g. HCI)	0.015- 0.04

Compound	wt.-%
silicon component, such as tetraethyl orthosilicate (TEOS)	about 7
cationic surfactant (e.g. cetrimonium bromide)	about 1.0
H₂O	about 85 (ad 100%)
acid catalyst (e.g. HCI)	about 0.03

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

Compound wt.-%

silicon component, such as tetraethyl orthosilicate (TEOS) 2 - 20

H₂O 6 - 10

alcohol (e.g. EtOH) 75 - 90

acid catalyst (e.g. HCI) 0.001 - 0.05
Thus, in a most preferred aspect, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

Compound wt.-%

silicon component, such as tetraethyl 5-18

orthosilicate (TEOS)

H₂O 6 - 10

alcohol (e.g. EtOH) 75 - 90

acid catalyst (e.g. HCI) 0.01 - 0.04
Thus, in a more preferred aspect, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

Compound wt.-%

silicon component, such as tetraethyl orthosilicate (TEOS) about 9.3

H₂O about 8

alcohol (e.g. EtOH) about 82.6

acid catalyst (e.g. HCI) about 0.03
Thus, in a more preferred aspect, the present invention also pertains to a wet-strength improving impregnation mixture (option I) comprising the following compounds in weight-%:

Compound wt.-%

silicon component, such as tetraethyl orthosilicate (TEOS) about 17.1

H₂O about 7.4

alcohol (e.g. EtOH) about 75.5

acid catalyst (e.g. HCI) about 0.03
Further compositions can be found in the example section.
In yet another aspect 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.
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.
Preferably, the portion of the silicon component in the impregnating solution is in a range of 5% by weight to 100% by weight, further preferably 10% by weight to 99% by weight, further preferably 20% by weight to 98% by weight, further preferably 40% by weight to 97% by weight, further preferably 60% by weight to 96% by weight, further preferably 80% by weight to 95% by weight.
Preferably, 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. Preferably, 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.
Preferably, the impregnating solution contains solvents in a portion which is in a range of 0 to 98% by weight, further preferably of 0.1 to 50% by weight, further preferably of 0.2 to 20% by weight, further preferably of 0.5 to 10% by weight, still further preferably of 1 to 5% by weight. Preferably, the solvent is selected from the group consisting of water, ethanol and mixtures of two or more thereof. Particularly preferably, the solvent is water.
Preferably, the impregnating solution contains water in a portion which is in a range of 0 to 20% by weight, further preferably of 0.5 to 10% by weight, further preferably of 1 to 5% by weight.
Preferably, the impregnating solution contains an acid, for example HCI (37 wt%) in a portion of 0.001 to 0.2% by weight, further preferably of 0.005 to 0.1% by weight, further preferably of 0.01 to 0.05% by weight.
Preferably, the impregnating solution according to the present invention consists of at least 95% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight, further preferably at least 99.9% by weight, further preferably 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. Particularly preferably, the impregnating solution even consists of at least 95% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight, further preferably at least 99.9% by weight, further preferably at least 99.99% by weight of water, silicon component and HCl.
Preferably, 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, further preferably of 0.5 to 10% by weight, further preferably of 1 to 5% by weight, further preferably of 1.5 to 7.5% by weight. Preferably, the impregnated fibrous non-woven fabric consists of the fibre component (in particularly paper) and the impregnating component (SiO₂).
Preferably, the impregnated fibrous non-woven fabric comprises the fibre component in a portion of 90 to 99.9% by weight, further preferably of 92.5 to 99.8% by weight, further preferably of 95% by weight to 99.5% by weight.
Preferably, 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, preferably however in a portion of at most up to 50% by weight, such as for example 0 to 30% by weight, further preferably up to 25% by weight, further preferably up to 10% by weight, further preferably up to 5% by weight, further preferably up to 2% by weight, further preferably up to 1% by weight, further preferably less than 0.5% by weight. These further components may in particularly be inorganic and/or organic fillers.
Preferably, 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, further preferably at least 75% by weight, further preferably at least 90% by weight, further preferably at least 95% by weight, further preferably at least 98% by weight, further preferably at least 99% by weight. Preferably, the impregnated fibrous non-woven fabric of the invention consists of the fibre component and the impregnating component.
In one aspect the present invention pertains to a process for producing the inventive wet strength improving impregnation solution, comprising the steps of

a. mixing between 10-20 mL TEOS (1 eq.) with between 20-40 µL HCI (37 wt%); and
b. stirring the mixture for between 12-48 h at between 18-25 °C; and
c. adding water to a total liquid of 100 mL and between 0.5 - 3 g sodium dodecyl sulfate (SDS); and
d. stirring the mixture for between 0.5 - 3 h at between 18-25 °C;
e. thereby obtaining the wet strength improving impregnation solution in form of an emulsion;
f. optionally, adding at least one further excipient selected from the group consisting of a thickener (e.g. carboxymethyl-cellulose), a biocide, and a conservation agent, and any combination thereof.

In one other aspect the present invention pertains to method for producing the wet strength improving impregnation solution, comprising the steps of

a. mixing 15.2 mL TEOS (1 eq.) with 28 µL HCI (37 wt%); and
b. stirring the mixture for 24 h at 20 °C; and
c. adding 84.8 mL of water and 1 g sodium dodecyl sulfate (SDS); and
d. stirring the mixture for 1 h at 20 °C;
e. thereby obtaining the wet strength improving impregnation solution in form of an emulsion.

In yet another aspect the present invention pertains to method for producing the wet strength improving mixture, comprising the steps of

a. mixing 0.5-5 mol-% tetraethyl orthosilicate (TEOS) : 20-50 mol-% EtOH : 5-20 mol-% H₂O : 0.005-0.1 mol-% HCI (37 wt%),
b. stirring for 24 h at room temperature,
c. thereby obtaining the wet strength improving impregnation solution in form of a coating sol.

a. mixing 1 mol-% tetraethyl orthosilicate (TEOS) : 40 mol-% EtOH : 10 mol-% H₂O : 0.02 mol-% HCI (37 wt%),
b. stirring for 24 h at room temperature,
c. thereby obtaining the wet strength improving impregnation solution in form of a coating sol.

In yet another aspect the present invention pertains to any silica coated fibrous non-woven fabric, preferably a paper sheet, obtained by coating the paper with any of the wet strength improving impregnation solutions mentioned hereinunder.
In one aspect a fibrous non-woven fabric, preferably 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 HCI (37 wt%), 80-88 mL of water (ad 100 mL total liquid), and 0.25-3 g sodium dodecyl sulfate (SDS).
In yet another aspect a fibrous non-woven fabric, preferably 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 HCI (37 wt%), 70-85 mL of water, and 0.5-2 g sodium dodecyl sulfate (SDS).
In yet another aspect a fibrous non-woven fabric, preferably a paper sheet, obtained by coating the paper with a wet strength improving impregnation solution comprising 15.2 mL TEOS (1 eq.), 28 µL HCI (37 wt%), 84.8 mL of water, and 1g sodium dodecyl sulfate (SDS).
In yet another aspect a fibrous non-woven fabric, preferably 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-% HCI (37 wt%).
In yet another aspect a fibrous non-woven fabric, preferably 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-% HCI (37 wt%).
In yet another aspect a fibrous non-woven fabric, preferably 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-% HCI (37 wt%),
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.
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.
The method comprises then the following steps:

a) obtaining an impregnating solution, wherein the wet-strength improving impregnating solution contains a silicon component,
b) adding the impregnating solution in a bath comprising also the fibres of the fibrous non-woven fabric;
c) extracting the fibrous non-woven fabric from the bath comprising the fibres of the fibrous non-woven fabric,
d) drying of the fibrous non-woven fabric at temperatures in a range of 4 °C to 250 °C.

In another embodiment the fibrous non-woven fabric is impregnated later during the drying process.
The method comprises the following steps:

a) providing of a fibrous non-woven fabric,
b) obtaining an impregnating solution, wherein the wet-strength improving impregnating solution contains a silicon component,
c) impregnating of the fibrous non-woven fabric with the wet-strength improving impregnating solution,
d) drying of the fibrous non-woven fabric at temperatures in a range of 4 °C to 250 °C,
wherein there is a period of time of at most 60 seconds between the completion of the impregnating according to step c) and the begin of the drying according to step d).

In one aspect the method for producing the silica coated paper sheet comprises the steps of

a. providing of a fibrous non-woven fabric, preferably paper;
b. obtaining the wet-strength improving impregnating solution as explained hereinunder;
c. applying a coating mixture to at least one side, preferably evenly to both sides of the paper with a size press with a speed of between 1 to 15 m/min at a roll pressure of between 0.25 - 3.0 bar;
d. heating the paper from between 18-25 °C to between 60-200 °C with a heating rate of between 5-15 K/min under constant air flow of between 15-50 mL/min;
e. thereby obtaining a wet-strength improved fibrous non-woven fabric, preferably paper sheet.

In one aspect, the method for producing the silica coated paper sheet comprises the steps of

a. providing of a fibrous non-woven fabric, preferably paper;
b. obtaining the wet-strength improving impregnating solution as explained hereinunder;
c. applying a coating mixture to at least one side, preferably evenly to both sides of the paper with a size press with a speed of about 5 m/min at a roll pressure of between 0.5 - 1.0 bar;
d. heating the paper from between 18-25 °C to between 60-200 °C, optionally with a heating rate of between 5-15 K/min under constant air flow of between 15-50 mL/min;
e. thereby obtaining a wet-strength improved fibrous non-woven fabric, preferably paper sheet.

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 (preferably 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.
According to step a) of the method according to the present invention, a fibrous non-woven fabric is provided. Preferably, 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. Particularly preferably, the fibrous non-woven fabric is a non-woven paper fabric. Preferably, the provided non-woven paper fabric has a grammage of 10 - 1000 g/m². In one aspect preferably between 10 - 20 g/m², more preferably 12 - 15 g/m² (for example in case of paper for tea-bags). In other aspects the grammage is preferably between 250 - 1050 g/m², more preferably 300 - 1000 g/m² (for example in case of wood pulp boards). In further aspects the grammage is preferably 65 to 120 g/m², further preferably of 70 to 100 g/m², further preferably of 75 to 90 g/m².
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 preferably conducted with the Rapid-Köthen method, particularly preferably in a sheet forming Rapid-Köthen plant, especially preferably 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").
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.
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".
After the addition of the single components, the impregnating solution is preferably stirred for a period of time of 6 to 48 hours, further preferably of 12 to 36 hours, further preferably 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), preferably, no such stirring is conducted.
The step c) of the impregnating of the fibrous non-woven fabric, in particularly the non-woven paper fabric, with the impregnating solution is preferably conducted at a relative air humidity in a range of 10% to 95%, further preferably of 30% to 70%, further preferably 40% to 60%, further preferably 45% to 55% and/or at a temperature in a range of 15 °C to 30 °C, further preferably 20 °C to 25 °C.
Preferably, 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. Preferably, 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. Preferably, the impregnating solution is uniformly distributed across the surface and the interior of the fibrous non-woven fabric.
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. Preferably, in such embodiments, the fibrous non-woven fabric, in particularly the non-woven paper fabric, is completely immersed into the impregnating solution. Preferably, 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. Preferably, 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°, further preferably of at least 80° and at most 100°, further preferably of at least 85° and at most 95°.
Preferably, the removal of the fibrous non-woven fabric from the impregnating solution is conducted at a time point 0.1 to 10 seconds, further preferably 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.
Preferably, 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. Preferably, 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°, further preferably of at least 80° and at most 100°, further preferably of at least 85° and at most 95°.
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. Preferably, the drying of the fibrous non-woven fabric is conducted at temperatures in a range of 80°C to 180°C, further preferably of 90°C to 170°C, further preferably of 100°C to 160°C, further preferably of 110°C to 150°C, further preferably of 120°C to 140°C, further preferably of 125°C to 135°C. Preferably, 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. Preferably, 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.
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, preferably at most 45 seconds, further preferably at most 30 seconds, further preferably at most 20 seconds, further preferably at most 10 seconds, further preferably at most 5 seconds, further preferably at most 2 seconds, further preferably at most 1 second. Preferably, 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.
Preferably, 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 preferably 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 preferably finished, when the impregnating solution is no longer sprayed onto the non-woven fabric.
The drying according to step d) preferably 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.
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.
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.
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.
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.
Preferably, the drying is achieved with the help of a dryer. Preferably, 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, preferably in a drying oven (in some aspects a vacuum oven) or in a muffle furnace. Preferably, 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, particularly preferably, the dryer is a hot air dryer. Hot air drying is particularly preferred.
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.
Preferably, the pressure during the drying according to step d) is in a range of 0.1 kPa to 500 kPa, further preferably 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, further preferably of 0.2 kPa to 20 kPa, further preferably of 0.5 kPa to 10 kPa, further preferably 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, further preferably of 50 kPa to 200 kPa, further preferably of 60 kPa to 150 kPa, further preferably of 70 kPa to 130 kPa, further preferably of 80 kPa to 120 kPa, further preferably 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, further preferably of 0.2% by mol to 3% by mol, further preferably of 0.5% by mol to 2.5% by mol, further preferably of 0.9% by mol to 2.2% by mol.
Preferably, after the drying, the fibrous non-woven fabrics are cooled to a temperature of 15°C to 30°C, further preferably of 20°C to 25°C.
Preferably, 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.
The present invention also includes papers with increased wet strength produced according to said processes, according to steps a) to d).
The present invention also relates to the use of a fibrous non-woven fabric of the present invention, in particularly as packaging material.
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.
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.
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.
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%, preferably greater than 10%, most preferably greater than 15% as measured by a tensile elongation test according to ISO 1924-2.
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%.
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.
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.
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.
The fibrous non-woven fabric of the present invention is a fibrous non-woven fabric. Preferably, the fibrous non-woven fabric is selected from the group consisting of non-woven paper fabrics and non-woven textile fabrics. Particularly preferably, the fibrous non-woven fabric is a non-woven paper fabric.
Preferably, the fibrous non-woven fabrics of the invention have high flexibility.
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 abaca (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 verge 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.
The term grammage or basis weight is to be used interchangeably here.
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.
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.
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.

Description of the figures

Figure 1: Depicted is the schematic illustration of the bilateral coating process using a size press.
Figure 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.
Figure 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.
Figure 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).

Examples

Reagents

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

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.
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/15mm for 20LF80SF with a silica content of 1.28% oven dry and 1.22% air dry.

Example 2. Silica Coating

Option I: impregnating sol

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

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

Compound	molar amount	volume (50mL) / mL	mass / g	wt%
TEOS
	1	152.30	140.302356	17.07
H₂O	5	60.66	60.6620971	7.38
EtOH	20	786.47	620.527963	75.51
HCl	0.01	0.56	0.24554428	0.03

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

Compound	molar amount	volume (50mL) / mL	mass / g	wt%
TEOS
	1	82.43	75.9336755	9.33
H₂O	10	65.66	65.6624185	8.07
EtOH	40	851.30	671.677517	82.56
HCI	0.02	0.61	0.26578427	0.03

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

Compound	molar amount	volume (50mL) / mL	mass / g	wt%
TEOS
	1	43.01	39.617674	4.90
H₂O	20	68.52	68.5174864	8.47
EtOH	80	888.32	700.882731	86.63
HCl	0.01	0.16	0.06933521	0.009

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

Compound	molar amount	volume (50mL) / mL	mass / g	wt%
TEOS
	1	21.98	20.2466071	2.50
H₂O	40	70.03	70.0317049	8.66
EtOH	160	907.95	716.372055	88.54
HCl	0.005	0.04	0.01771687	0.002

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 (Figure 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 preheated oven at 160°C for 5-15 min at atmospheric pressure before cooling down to ambient temperature.

Option II: impregnation emulsion

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

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

Compound	V/ mL	m/g	n /mol	molar ratios	wt%
TEOS	15.2	14.288	0.06721183	1.00	14.29
H₂O	84.8	84.6304	4.69777408	69.90	84.67
HCl	0.028	0.03318	0.00033671	0.01	0.03
SDS	NA		1	0.00350208	0.05	1.00

Good results with respect to wet-strength were achieved with:

Compound	V/ mL	m/g	n /mol	molar ratios	wt%
TEOS	15.2	14.288	0.06721183	1.00	7.15
H₂O	184.8	184.4304	10.237602	152.32	92.33
HCl	0.028	0.03318	0.00033671	0.01	0.02
SDS	NA		1	0.00350208	0.05	0.50

Mediocre results with respect to wet-strength were achieved with:

Compound	V/ mL	m/g	n /mol	molar ratios	wt%
TEOS	15.2	14.288	0.06721183	1.00	3.58
H₂O	384.8	384.0304	21.3172578	317.17	96.16
HCl	0.028	0.03318	0.00033671	0.005	0.01
SDS	NA		1	0.00350208	0.05	0.25

TEOS was mixed with HCI (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 preheated oven at 160°C for 5-15 min at atmospheric pressure before cooling down to ambient temperature.
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.
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)

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)

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

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

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 Figure 2, whereas the calculated silica contents are illustrated in Table 1.

Table 1: Calculated silica contents derived from TGA.

Sample	Coating Option I		Coating Option II (SDS)		Coating Option II (cetrimonium bromide)
Sample	Silica content air-dry / %	Silica content oven-dry / %	Silica content air-dry / %	Silica content oven-dry / %	Silica content air-dry / %	Silica content oven-dry /%
20LF80SF	1.3	1.38	5.05	5.27	2.23	2.36
50LF50SF	1.61	1.72	4.77	4.98	-	-
80LF20SF	2.17	2.3	2.72	2.84	-	-

Example 7. Morphological analysis

Eucalyptus paper sheets were investigated with respect to their morphology before and after silica coating using SEM (Figure 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

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 Figure 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 amount of short fibres (eucalyptus).

Example 9: Wet strength features of different papers and impregnations mixtures

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.
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.

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

Table 2: Comparison of wet-strength.

Paper	Untreated	Comparative paper (asymmetric impregnated)	Option I (cf. example 2)	Option II w. anionic surfactant (SDS) (cf. example 2)	Option II w. cationic surfactant (cetrimonium bromide) (cf. example 2)
	wet strength in N/15 mm, determined according to DIN ISO 3781
20LF80SF	0.3	6.2 ± 2	10.2 ± 0.6	7.3 ± 1	24.3 ± 1.3
50LF50SF	0.6	4.1 ± 1	8.3 ± 0.5	12.8 ± 1.5	-
80LF20SF	1.0	3.5 ± 1	7.7 ± 0.4	7.6 ± 0.9	-

Example 10: Contact angle measurements

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).

Claims

Wet strength improved fibrous non-woven fabric with an air-dry silica-content of at least 0.1 wt.-%, 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, and wherein the Cobb₆₀-value is less than 20% reduced as compared to the untreated fibrous non-woven fabric.
The wet strength improved fibrous non-woven fabric according to claim 1, wherein the fibrous non-woven fabric is selected from the group consisting of paper and cotton fleece.
The wet strength improved fibrous non-woven fabric of any of claims 1 or 2, wherein the wet tensile strength according to DIN ISO 3781 of the fibrous non-woven fabric is between at least 5 times and up to 50 times higher than the wet tensile strength according to DIN ISO 3781 of a fibrous non-woven fabric without silica-content.
The wet strength improved fibrous non-woven fabric of any of the previous claims, wherein the wet tensile strength according to DIN ISO 3781 (1994-10) is between 1 and 50 N/15 mm.
The wet strength improved fibrous non-woven fabric of any of the previous claims, wherein the air-dry silica content is between at least 0.1 wt-% and up to 30 wt-%.
The wet strength improved fibrous non-woven fabric of any of the previous claims, wherein the contact angle on all surfaces of the wet strength improved fibrous non-woven fabric according to the present invention is less than 90°.
The wet strength improved fibrous non-woven fabric of any of the previous claims, wherein the silica coated paper has a water absorption capacity according to DIN EN ISO 535 (2014) is between 10 g/m² and 300 g/m².
The wet strength improved fibrous non-woven fabric of any of the previous claims, obtained by impregnating the fibrous non-woven fabric with a mixture comprising a silicon component, water, a surfactant and an acidic catalyst.
A mixture comprising the following compounds in weight-%: Compound wt.-% silicone compound (e.g. tetraethyl orthosilicate, (TEOS)) 3.5 - 15 surfactant 0.2 - 1.5 H₂O 80-95 catalyst (acidic or basic) 0.05 - 0.05
The mixture of claim 9, wherein the surfactant is a cationic surfactant.
A mixture comprising the following compounds in weight-%: Compound wt.-% silicone compound (e.g. tetraethyl orthosilicate, (TEOS)) 2 - 20 H₂O 6 - 10 alcohol (e.g. EtOH) 75 - 90 catalyst (acidic or basic) 0.01 - 0.05
The fibrous non-woven fabric according to any of the previous claims 1 - 8, obtained by impregnating the fibrous non-woven fabric with a mixture according to any of the previous claims 9 to 11.
A method for producing a wet-strength improved fibrous non-woven fabric, preferably paper, comprising the steps of:
a. Obtaining a fibrous non-woven fabric;

b. Applying an impregnation solution according to any of the previous claims 9 to 11 evenly to both sides of the paper with a size press with a speed of between 1 to 15 m/min at a roll pressure of between 0.25 - 3.0 bar;

c. Heating the paper from between 18-25°C to between 60-200°C with a heating rate of between 5-15 K/min under constant air flow of between 15-50 mL/min;

d. Thereby obtaining the wet-strength improved fibrous non-woven fabric, preferably paper.
The method according to claim 13, wherein instead of steps a. and b. the impregnation solution according to any of the previous claims 9 to 11 is mixed in a bath with fibres for the fibrous non-woven fabric before the fibrous non-woven fabric is extracted from said bath and the steps c. and d. are performed.
Use of the wet-strength improved fibrous non-woven fabric of any of claims 1 to 8 or using an impregnation solution according to any of the previous claims 9 to 11 on a fibrous non-woven fabric for producing an article 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.