EP3655575A1

EP3655575A1 - Asymmetrically silica-impregnated nonwoven fabrics and methods for producing said nonwovens and use thereof

Info

Publication number: EP3655575A1
Application number: EP19778972.0A
Authority: EP
Inventors: Maximilian Nau; Markus BIESALSKI; Nicole Herzog; Annette ANDRIEU-BRUNSEN
Original assignee: Technische Universitaet Darmstadt
Current assignee: Technische Universitaet Darmstadt
Priority date: 2018-10-01
Filing date: 2019-09-25
Publication date: 2020-05-27
Anticipated expiration: 2039-09-25
Also published as: WO2020069943A1; CN112840073B; EP3655575B1; CN112840073A; DE102018124255A1; US20220034041A1

Abstract

The invention relates to asymmetrically silica-impregnated nonwoven fabrics, to methods for producing said nonwovens and to uses of said nonwovens, especially in the field of packaging materials.

Description

Nonwoven fabrics with asymmetrical silica impregnation and process for producing the nonwovens and their uses

The present invention relates to nonwovens with asymmetrical silica impregnation and processes for their production and uses of the nonwovens, in particular in the field of packaging materials.

Modification of paper nonwovens to change the surface properties have been described in many ways. As a rule, composite materials are provided in which there is a modifying layer on the surface of the paper fleece. For example, composite materials made of paper and polyethylene are known. These composite materials are obtained by laminating the paper surface with polyethylene films. Layer composites are formed which have hydrophobic surface properties on one side or on both sides. An anisotropic distribution of the chemical impregnation within the paper fleece cannot be achieved with these techniques. In addition, such composite materials are difficult or impossible to recycle (keyword: microplastics). The relatively large amounts of material used for e.g. a coating is required.

Papers can also be hydrophobized with so-called sizing agents (for example alkylated ketene dimers (AKD)). In this regard, too, the setting of a chemical, anisotropic gradient has not yet been described, presumably because the molecules react statistically only with the functions on the fibers, but not with one another.

Another possibility for changing the surface properties of a paper fleece is in Dubois et al. (Langmuir, 2017, 33 (1), pp. 332-339). A hybrid material made of paper and SiO _{2 is} disclosed which has significantly more hydrophobic surface properties in comparison to unmodified paper. However, a Si0 ₂ gradient is not described. The materials disclosed therefore have neither surfaces with different water absorption properties nor a comparatively hydrophilic core. This is probably due to the fact that after the impregnation solution has been applied, the nonwovens are initially kept at low temperatures for a relatively long period of time, so that a polymeric impregnation which is uniformly distributed over the surface and the interior of the fleece can form before the solvent evaporates to a substantial extent.

An anisotropic distribution of the chemical impregnation, which can also be referred to as asymmetrical distribution, would, however, be associated with many advantages. Functional loading Stratification could be applied in much lower order weights with the same effect. In addition, the targeted adjustment of the material concentration would enable the creation of more complex structures (eg sandwich-like channels in paper) in one process step with minimal use of materials. The anisotropic chemical structure results in advantageous property profiles in the nonwovens, for example barrier effects. For example, a fleece could be obtained which cannot be wetted from the outside with fluids (for example water), but which could absorb (and deliver, pass on, and so on ...) the same fluid inside, as well as below described.

It is an object of the present invention to overcome the disadvantages of the prior art. In particular, nonwoven fabrics, for example paper nonwovens, are to be provided which are functionalized in such a way that they have chemically anisotropic properties in cross section. The nonwovens are said to be able to be produced by a process which allows the fibers, in particular paper fibers, to be given localized hydrophobic or hydrophilic properties. The process should be simple and also allow easy upscaling. In addition, in contrast to the polymer coatings known from the prior art, biocompatible materials are to be obtained. The distribution of Si0 ₂ in the form of a gradient enables one side of the nonwoven, in particular the paper, to absorb water and the other side to repel water, or that both surfaces repel water and water can only be absorbed inside the material. analogous to a chemical sandwich structure. In addition, the water absorption behavior can be adjusted by adjusting the Si0 ₂ amount.

The objects are solved by the subject matter of the claims. The objects are achieved in particular by a nonwoven fabric with asymmetrical silica impregnation, the nonwoven having two main surfaces, the weight fraction of SiO ₂ decreasing from at least one of the two main surfaces towards the inside of the nonwoven. The tasks are also solved by a method for producing a nonwoven fabric with asymmetrical silica impregnation, in particular a nonwoven fabric of the present invention, comprising the following steps: a) providing a nonwoven fabric, b) providing an impregnation solution, the Impregnation solution contains a silane component, c) impregnating the fleece with the impregnation solution, d) drying the fleece at temperatures in a range from 70 ° C. to 190 ° C., a period of at most 60 seconds between the completion of the impregnation according to step c) and the start of drying according to step d).

The method developed by the inventors preferably uses only a silane component (in particular tetraethylorthosilicate (TEOS), preferably prepolymerized) and a nonwoven fabric, in particular a paper nonwoven, to build up the anisotropic (asymmetrical) impregnation. The silane component can be inserted into the paper in a simple immersion step. Other impregnation processes are also possible. The nonwoven is preferably impregnated with the impregnation solution according to step c) of the method using an impregnation method which is selected from the group consisting of dip coating, spray coating (optionally also on both sides), size press, roller coating, blade coating and curtain coating . Dip coating and spray coating are particularly preferred. Dip coating is very particularly preferred. The impregnation solution is preferably distributed uniformly over the surface and the interior of the fleece.

During the subsequent drying step, the silane component produces a silica impregnation in the form of polymeric SiO ₂ , which can also be referred to as a silicate component. This is based not only on drying, but on a chemical reaction taking place at the same time. The distribution of the amount of polymeric silicate component is preferably controlled by the drying process, which particularly preferably includes the control of the condensation reaction of the silane component (in particular TEOS) taking place parallel to the drying. In contrast to papers known from the prior art, the location of the impregnation is preferably controlled by the self-diffusion and reactivity of the silane component, which in turn can be easily adjusted via the drying conditions (air humidity, temperature, pressure). It is therefore not necessary to produce a laminate in multiple steps with many additives.

The process enables the saving of process steps and the amount of material introduced and thus energy and materials based on fossil raw materials. In addition, the new materials obtainable with the process can be replaced in a variety of ways, for example for hydrophobization in the packaging and food sector.

The present invention relates to a nonwoven fabric with asymmetrical silica impregnation, where the nonwoven has two main surfaces, the weight fraction of Si0 ₂ decreasing from at least one of the two main surfaces towards the inside of the nonwoven. The nonwoven of the present invention is a nonwoven. The nonwoven is preferably selected from the group consisting of paper nonwovens, textile nonwovens and plastic nonwovens. The nonwoven is particularly preferably a paper nonwoven.

The impregnated nonwoven preferably comprises S1O2 in a proportion of 0.1 to 10% by weight, more preferably 0.2 to 7.5% by weight, further preferably 0.5 to 5% by weight. The impregnated nonwoven preferably consists of the fiber component (in particular paper) and the impregnation component (SiC> 2). The impregnated nonwoven preferably comprises the fiber component in a proportion of 90 to 99.9% by weight, more preferably 92.5 to 99.8% by weight, more preferably 95% to 99.5% % By weight.

The impregnated nonwoven fabric of the invention preferably consists of the fiber component (in particular special paper) and the impregnation component (S1O2). The fleece can contain further components, but preferably in a proportion of at most up to 50% by weight, for example 0 to 30% by weight, more preferably up to 25% by weight, further preferably up to 10 % By weight, more preferably up to 5% by weight, more preferably up to 2% by weight, more preferably up to 1% by weight, more preferably less than 0.5% by weight. These further components can in particular be inorganic and / or organic fillers.

The proportion of the fiber component and impregnation component in the nonwoven of the present invention is preferably at least 50% by weight, more preferably at least 75% by weight, more preferably at least 90% by weight, further preferably at least 95% by weight, further preferably at least 98% by weight, more preferably at least 99% by weight. The impregnated nonwoven fabric of the invention preferably consists of the fiber component and the impregnation component.

The nonwoven of the present invention has an asymmetrical (anisotropic) silica impregnation. The terms "asymmetrical" and "anisotropic" are used synonymously in the present description. The silica impregnation is in the form of polymeric Si0 ₂ , which can also be referred to as a silicate component. The silica impregnation is asymmetrical, ie anisotropic. This means that the proportion of Si0 _{2 is} not distributed homogeneously over the fleece, as explained in more detail below.

The nonwoven of the invention has two major surfaces. In other words, the length and width of the nonwoven, or in the case of nonwovens with a round base, the diameter of the nonwoven, are many times greater than the thickness of the nonwoven. The ratio of length and width or diameter of the fleece to the thickness of the fleece is preferably at least 5, more preferably at least 10, more preferably at least 20. The shape of the fleece can therefore also be as leaf-like, foil-like, plate-like or disc-like. Depending on the orientation of the fleece, the two main surfaces can also be referred to as the top and bottom or as the front and back of the fleece.

In particularly preferred embodiments, the weight fraction of SiO ₂ decreases from at least one of the two main surfaces towards the interior of the fleece. The proportion by weight of Si0 ₂ is therefore higher on at least one of the two main surfaces than the proportion by weight of Si0 ₂ below the corresponding main surface. In other words, there is an Si0 ₂ gradient. The formation of such a Si0 ₂ gradient brings diverse advantages with it compared to materials with SiO ₂ present essentially uniformly distributed over the thickness of the fleece. For example, inner channels and / or different wetting properties of the surfaces can be obtained. In addition, a low use of materials is made possible. The weight fraction of SiO ₂ on at least one of the two main surfaces is preferably at least 1.1 times as high, more preferably at least twice as high, more preferably at least three times as high, more preferably at least four times as high, more preferably at least five times as high, more preferably at least six times as high, more preferably at least seven times as high, more preferably at least eight times as high, more preferably at least nine times as high, further preferably at least ten times as high as the weight fraction of SiO ₂ in the middle of the fleece. The center of the fleece designates the positions in the interior of the fleece which are equally far away from the two main surfaces in the shortest possible connection, and which are therefore in the middle of the fleece in relation to the thickness of the fleece.

Particularly preferably, the SiO _{2 is} not only in a gradient distribution in relation to one main surface, but also in relation to the other main surface. However, the gradient does not have to be designed in the same way from both main surfaces to the interior of the fleece.

In certain preferred embodiments, the weight fraction of Si0 ₂ decreases from one of the two main surfaces towards the interior of the fleece, while the weight fraction of Si0 _{2 increases} from the other of the two main surfaces towards the interior of the fleece. Such fleeces preferably differ in terms of their properties on the two main surfaces. In particular, it is advantageous for certain applications if one main surface is significantly more hydrophobic than the other main surface. In such embodiments, a fleece with a hydrophobic main surface and a hydrophilic main surface is particularly preferred. The weight fraction of SiO ₂ on one of the two main surfaces is preferably at least 1.1 times as high, more preferably at least two. times as high, more preferably at least three times as high, more preferably at least four times as high, more preferably at least five times as high, more preferably at least six times as high, more preferably at least seven times as high, more preferably at least eight times as high, more preferably at least nine times as high, more preferably at least ten times as high as the weight fraction of S1O2 in the middle of the fleece. The proportion by weight of S1O2 on the other of the two main surfaces is preferably at most 0.9 times, more preferably at most half, more preferably at most one third, more preferably at most one quarter, more preferably at most one fifth, more preferably at most one Sixth, more preferably at most one seventh, more preferably at most one eighth, more preferably at most one ninth, further preferably at most one tenth of the weight fraction of S1O2 in the middle of the fleece. The weight fraction of S1O2 on one of the two main surfaces is preferably at least 1.2 times, more preferably at least 4 times, more preferably at least 10 times, more preferably at least 20 times, further preferably at least 50 times , more preferably at least 100 times the weight fraction of SiO ₂ on the other of the two main surfaces.

In other preferred embodiments, the weight fraction of SiO ₂ decreases from both main surfaces towards the inside of the fleece. Such fleeces preferably do not differ, or do not differ significantly, with regard to their properties on the two main surfaces. In particular, it is advantageous for certain applications if both main surfaces are hydrophobic. In such embodiments, a fleece with two hydrophobic main surfaces is particularly preferred. Less material is required compared to a uniform impregnation across the entire thickness of the fleece. The proportion by weight of SiO ₂ on both main surfaces is preferably at least 1.1 times as high, more preferably at least 1.2 times as high, more preferably at least 1.5 times as high as the proportion by weight of SiO ₂ in the middle of the fleece. The ratio of the proportion by weight of S1O2 on one main surface to the proportion by weight of S1O2 on the other main surface is in a range from 0.95: 1 to 1.05: 1, more preferably from 0.98: 1 to 1.02 : 1, further preferred from 0.99: 1 to 1, 01: 1.

The relative Si0 ₂ distribution in the nonwovens is preferably analyzed with the help of confocal laser scanning microscopy (CLSM, English: "confocal laser scanning microscopy") on cross sections of embedded samples. In combination with absolute Si0 ₂ quantities per fleece, which are preferably determined with the help of thermogravimetric analysis (TGA), this enables a quantitative statement about the material quantities per volume increment.

The nonwovens of the invention preferably have a high degree of flexibility. The present invention also relates to a method for producing a nonwoven fabric with asymmetrical silica impregnation, in particular a nonwoven fabric of the present invention as described above. The invention also relates to a nonwoven fabric with asymmetrical silica impregnation that can be obtained or obtained with the method. The method comprises the following steps: a) providing a nonwoven fabric, b) providing an impregnating solution, the impregnating solution containing a silane component, c) impregnating the nonwoven fabric with the impregnating solution, d) drying the nonwoven fabric at temperatures in a range from 70 ° C to 190 ° C, with a period of at most 60 seconds between the completion of the impregnation according to step c) and the start of drying according to step d).

According to step a) of the method according to the invention, a nonwoven fabric is provided. The nonwoven is preferably selected from the group consisting of paper nonwovens, textile nonwovens and plastic nonwovens. The nonwoven is particularly preferably a paper nonwoven. The paper fleece provided preferably has a grammage of 65 to 120 g / m ² , more preferably 70 to 100 g / m ² , more preferably 75 to 90 g / m ² .

The nonwoven can be a commercially available nonwoven. In particular, the paper fleece can be a commercially available paper fleece. Alternatively, the step of providing the nonwoven, in particular the nonwoven, can also include the step of producing the nonwoven, in particular the nonwoven. A paper fleece is preferably produced using the Rapid-Köthen process, particularly preferably in a Rapid-Köthen sheet-forming system, very particularly preferably in accordance with 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 ").

No further additives or fillers are preferably used in the production of the nonwoven fabric, in particular the paper nonwoven.

According to step b) of the method according to the invention, an impregnation solution is provided which contains a silane component. In the present description, the terms “impregnation solution” and “impregnation solution” are used synonymously. The impregnation solution can be one component, that is, it can consist of a single component. In such a case, the impregnation solution can in particular also be referred to as “impregnation fluid” or “impregnation fluid”.

In certain preferred embodiments, the impregnation solution consists of the silane component. In other words, the proportion of the silane component in the impregnation solution is 100% by weight. In particular, the impregnation solution can therefore be pure silane. In other preferred embodiments, the impregnation solution contains, in addition to the silane component, at least one further component, for example a solvent component and / or an acid component.

The proportion of the silane component in the impregnation solution is preferably in a range from 5% by weight to 100% by weight, more preferably 10% by weight to 99% by weight, further preferably 20% by weight to 98 % By weight, more preferably 40% by weight to 97% by weight, more preferably 60% by weight to 96% by weight, more preferably 80% by weight to 95% by weight. The extent of the water-repellent surface properties of the nonwovens can be specifically adjusted via the proportion of the silane component. Higher proportions of the silane component are associated with more hydrophobic surface properties.

The silane component is preferably selected from the group consisting of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate, polydimethoxysiloxane, 1,2-bis (triethoxysilyt) ethane, tetramethyl orthosilicate (TMOS), silicon tetraacetate and mixtures of two or more thereof. The silane component TEOS is particularly preferred. TEOS is a common basic chemical that is cheap and readily available. The silane component is preferably prepolymerized. The term "prepolymerized" means that only oligomers have already been formed and the material has not yet been polymerized.

The impregnation solution preferably contains solvents in a proportion which is in a range from 0 to 98% by weight, more preferably from 0.1 to 50% by weight, more preferably from 0.2 to 20% by weight, more preferably from 0.5 to 10% by weight, even more preferably from 1 to 5% by weight. The solvent is preferably selected from the group consisting of water, ethanol and mixtures of two or more thereof. The solvent is particularly preferably water.

The impregnation solution preferably contains water in a proportion which is in a range from 0 to 20% by weight, more preferably from 0.5 to 10% by weight, more preferably from 1 to 5% by weight. The impregnation solution preferably contains HCl in a proportion of 0.001 to 0.2% by weight, more preferably 0.005 to 0.1% by weight, more preferably 0.01 to 0.05% by weight.

The impregnation solution according to the invention preferably consists of at least 95% by weight, more preferably at least 98% by weight, more 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, silane component and HCl. It is a particular advantage of the method according to the invention that no further components are required in the impregnation solution. The impregnation solution is particularly preferably composed of at least 95% by weight, more preferably at least 98% by weight, more preferably at least 99% by weight, more preferably at least 99.9% by weight, more preferably at least 99% 99% by weight of water, silane component and HCl.

After the addition of the individual components, the impregnation solution is preferably stirred for a period of 6 to 48 hours, more preferably from 12 to 36 hours, more preferably from 18 to 30 hours, before the impregnation of the nonwoven fabric, in particular the paper nonwoven, with the impregnation solution according to step c) of the method according to the invention it follows. In embodiments of the invention in which the impregnation solution consists of a silane component (in particular TEOS), preferably no such stirring takes place.

Step c) of impregnating the nonwoven fabric, in particular the paper nonwoven, with the impregnation solution is preferably carried out at a relative atmospheric humidity in a range from 10% to 95%, more preferably from 30% to 70%, further preferably 40% to 60% , more preferably 45% to 55% and / or at a temperature in a range from 15 ° C to 30 ° C, more preferably 20 ° C to 25 ° C.

The nonwoven fabric is preferably impregnated with the impregnating solution by exposing the nonwoven to the impregnating solution, in other words by bringing the nonwoven into contact with the impregnating solution. There are various possibilities for designing this contacting of the fleece with the impregnation solution. The nonwoven fabric is preferably impregnated with the impregnation solution according to step c) of the process using an impregnation process which is selected from the group consisting of dip coating, spray coating (optionally also on both sides), size press, roller coating, blade coating and curtain coating. Dip coating and spray coating are particularly preferred. Dip coating is very particularly preferred. The impregnation solution is preferably distributed uniformly over the surface and the interior of the fleece. According to embodiments of the invention, the impregnation in step c) of the invention is carried out by immersing the fleece in the impregnation solution. In such embodiments, the nonwoven fabric, in particular the paper nonwoven, is preferably completely immersed in the impregnation solution. The immersion is preferably carried out in such a way that the nonwoven, in particular the nonwoven, is oriented essentially vertically. In their words, a vertical orientation means that the two main surfaces of the fleece are arranged in such a way that surface vectors perpendicular to the main surfaces are oriented essentially horizontally. The surface vectors of the two main surfaces preferably each form an angle of at least 70 ° and at most 110 °, more preferably of at least 80 ° and at most 100 °, more preferably of at least 85 ° and at most 95 °, with the vector of the immersion direction.

The fleece is preferably removed from the impregnation solution at a point in time which is one to ten seconds, more preferably two to five seconds after the immersion of the fleece in the impregnation solution has ended.

The fleece is preferably removed from the impregnation solution in such a way that the fleece is oriented essentially vertically. In other words, a vertical orientation means that the two main surfaces of the fleece are arranged in such a way that surface vectors perpendicular to the main surfaces are oriented essentially horizontally. The surface vectors of the two main surfaces preferably form an angle of at least 70 ° and at most 110 °, more preferably of at least 80 ° and at most 100 °, more preferably of at least 85 ° and at most 95 °, with the vector of the distance direction.

According to step d) of the method according to the invention, the fleece dries at temperatures in a range from 70 ° C. to 190 ° C. The fleece dries at temperatures in a range from 80 ° C. to 180 ° C., more preferably from 90 ° C. to 170 ° C., more preferably from 100 ° C. to 160 ° C., more preferably from 110 ° C. C to 150 ° C, more preferably from 120 ° C to 140 ° C, more preferably from 125 ° C to 135 ° C. The fleece is preferably dried in accordance with step d) until the residual moisture of the fleece is in a range from 3% by weight to 7% by weight. The residual moisture is preferably determined by means of gravimetric analysis, in particular according to DIN EN 20287.

There is a period of at most 60 seconds, preferably at most 45 seconds, more preferably at most 30 seconds, more preferably at most 20 seconds, between the completion of the impregnation according to step c) and the beginning of the drying according to step d). more preferably at most 10 seconds, more preferably at most 5 seconds, more preferably at most 2 seconds, more preferably at most 1 second. Drying in step d) of the method according to the invention preferably takes place immediately, in other words immediately, after impregnation of the fleece with the impregnation solution according to step c) of the method.

The impregnation according to step c) is preferably completed when the nonwoven is no longer exposed to the impregnation solution, or in other words when the nonwoven is no longer brought into contact with the impregnation solution. In the case of dip coating, the impregnation according to step c) is preferably completed, for example, when the nonwoven fabric has been completely removed from the impregnation solution again, for example pulled out. In spray coating, the impregnation according to step c) is preferably completed, for example, when the fleece is no longer sprayed with the impregnation solution.

The drying according to step d) preferably begins when the fleece comes into an environment which is intended to remove moisture and / or condensation products, for example in an oven.

The short period between completion of the impregnation and the beginning of the drying probably means that the silane component of the impregnation solution has not yet been converted to a substantial extent into the silicate component of the silica impregnation if the drying at the increased Drying temperatures occur. This enables the targeted adjustment of the asymmetrical silica impregnation. This is because the silane component migrates through the fleece at the elevated drying temperatures depending on the drying conditions, in particular the ambient pressures, as long as it has not yet been converted into an overly polymerized silica impregnation.

In embodiments in which the impregnation solution contains no solvent, the migration of the silane component through the fleece at the elevated drying temperatures can be influenced, in particular, by adjusting the ambient pressures during drying. In embodiments in which the impregnation solution contains solvents, the migration of the silane component can in particular also be influenced by the evaporation of the silane component and / or the solvent at the elevated drying temperatures, since the silane component passes through with the solvent the fleece wanders.

The further the polymerization progresses, the lower the mobility in the fleece. So if after impregnating the fleece with the impregnation solution for a longer period if one waits until drying begins at the elevated temperatures, i.e. if the period between steps c) and d) of the process is relatively large, substantial polymerization takes place before there is a relevant migration of the silane component comes through the fleece. The consequence of this is that nonwovens with a homogeneous distribution of the silica impregnation are obtained, since the distribution of the already polymerized coating can no longer be influenced by the drying conditions at the increased drying temperatures.

In contrast to this, the method of the present invention provides that between the completion of the impregnation according to step c) and the start of drying according to step d) there is a period of at most 60 seconds. Extensive polymerization has therefore not yet taken place at the start of drying, so that the silane component present in the impregnation solution migrates through the fleece and the distribution of the silica impregnation can therefore be influenced in a targeted manner via the migration. If, for example, drying conditions are selected which result in increased migration of the silane component to one of the two main surfaces, for example as a result of reduced ambient pressure on one of the two main surfaces compared to the other main surface, nonwovens with asymmetrical silica impregnation can be obtained be formed such that the SiC weight fraction on one main surface is higher than in the middle of the fleece, while the SiO ₂ weight fraction on the other main surface is lower than in the middle of the fleece. If, on the other hand, drying conditions are selected which result in a comparable amount of migration to both main surfaces, for example due to negative pressure on both main surfaces, non-woven fabrics can be obtained with an asymmetrical silica impregnation which is designed such that the Si0 ₂ weight fraction on both main surfaces is approximately the same, namely higher than in the middle of the fleece, since the silane component migrates from the center towards the main surfaces and then there is an increased formation of the polymeric silica Impregnation is coming.

Drying is preferably carried out with the aid of a dryer. The dryer is preferably selected from the group consisting of hot air dryers, ovens, drum dryers and IR dryers. For example, drying can be carried out in an oven, preferably in a vacuum oven or in a muffle oven. The oven is preferably preheated, in particular to the drying temperature, so that after the impregnation in step c), drying in step d) can be started particularly quickly. However, the dryer is particularly preferably a hot air dryer. Hot air drying is particularly preferred. According to the invention, the properties of the nonwovens obtained can be influenced not only by the process steps described above, but in particular also by the ambient pressure prevailing during drying. It has surprisingly turned out that the distribution of the SiO ₂ content in the nonwovens can be controlled in a targeted manner via the pressure conditions during drying. Nonwovens with comparatively high Si0 ₂ contents on both main surfaces can be obtained in particular at low pressures. Higher pressures, on the other hand, promote differences between the SiO ₂ contents of the two main surfaces, so that nonwovens are obtained in which one of the main surfaces is significantly more hydrophobic than the other main surface.

The pressure during the drying according to step d) is preferably in a range from 0.1 kPa to 500 kPa, more preferably from 0.2 kPa to 200 kPa. In certain preferred embodiments, the pressure during the drying according to step d) is in a range from 0.1 kPa to 30 kPa, more preferably from 0.2 kPa to 20 kPa, more preferably from 0.5 kPa to 10 kPa, more preferably from 1 kPa to 5 kPa. Such embodiments are particularly suitable for the production of such nonwovens in which the SiC> 2 content on both main surfaces is comparatively high, but is comparatively low in the middle of the nonwoven (so-called sandwich structure). In other preferred embodiments, however, the pressure during the drying according to step d) is in a range from> 30 kPa to 500 kPa, more preferably from 50 kPa to 200 kPa, more preferably from 60 kPa to 150 kPa, more preferably from 70 kPa to 130 kPa, more preferably from 80 kPa to 120 kPa, more preferably from 90 kPa to 110 kPa. Such embodiments are particularly suitable for the production of those nonwovens in which the SiO ₂ content on one of the two main surfaces is significantly higher than on the other of the two main surfaces, while the content of S1O2 in the middle of the nonwoven is less than one of the two main surfaces, but higher than the other of the two main surfaces. This applies in particular when impregnation solutions with low or medium proportions of silane component are used, in particular with a proportion of silane component in a range from 0.1 mol% to 3.5 mol% preferably from 0.2 mol% to 3 mol%, more preferably from 0.5 mol% to 2.5 mol, more preferably from 0.9 mol% to 2.2 mol%.

During the drying process, the nonwovens are preferably arranged essentially horizontally or oriented horizontally. In other words, a horizontal orientation means that the two main surfaces of the fleece are arranged in such a way that surface vectors perpendicular to the main surfaces are oriented essentially vertically.

After drying, the nonwovens are preferably cooled to a temperature of from 15 ° C. to 30 ° C., more preferably from 20 ° C. to 25 ° C. The method preferably consists of the specified steps. It is a particular advantage of the method according to the invention that the method requires very few steps.

The present invention also relates to the use of a nonwoven of the present invention, in particular as packaging material.

Preferred uses as packaging material include the use as frozen paper, the use for products that come into contact with food, such as (paper) cups and / or (paper) straws, and the use as packaging for mate- rialien that are protected from liquid, but should still exchange moisture. In particular, the nonwovens of the present invention can be used for plastic-free straws and / or paper cups. Against the background of possible regulations for the reduction of plastic waste, it can be assumed that there will be an increased demand for plastic-free products. Nonwovens for the uses mentioned, in particular for use in drinking straws and cups, are often also referred to as special papers.

Embodiments in which the SiCV content on one of the two main surfaces is significantly higher than on the other of the two main surfaces are particularly suitable for use as paper cups. The main surface with the higher Si0 ₂ content is suitable because of its hydrophobicity as the inside of the cup, because the hydrophobic properties of the surface prevent excessive entry of the liquid in the cup into the interior of the fleece. The more hydrophilic surface works well as the outer surface of the cup because it promotes printability.

Embodiments are particularly suitable for use as a straw in which the SiO ₂ content on both main surfaces is comparatively high, but is comparatively low in the middle of the fleece (so-called sandwich structure). The high Si0 ₂ content on the two main surfaces prevents excessive liquid from entering the interior of the fleece. The fact that the Si0 ₂ content in the middle of the fleece is comparatively low enables material to be saved.

The present invention also relates to the use of a nonwoven of the present invention as a membrane, in particular as a membrane for water / oil separation. Such a membrane can be used in particular for the spatial separation of mixtures consisting of a liquid, hydrophobic component (in particular oil) and water.

The present invention also relates to the use of a nonwoven of the present invention as special paper for use at elevated temperatures, in particular the use of a nonwoven of the present invention as baking paper. Description of the figures

FIG. 1 shows schematically the evaporation from the nonwovens (1) when drying under normal pressure (FIG. 1A) or in a vacuum (FIG. 1B). Under normal pressure there was a high evaporation rate (2) on the top (illustrated by the longer arrows) and a low evaporation rate (3) on the underside of the fleece (1) (illustrated by the shorter arrows). In the vacuum there is an average evaporation rate on the top and bottom (4).

FIG. 2 shows schematically the fluorescence intensity (y-axis) of rhodamine B over the thickness of the fleeces (x-axis). The distribution is schematically sketched for non-woven fabrics dried under normal pressure in FIG. 2A and for non-woven fabrics dried in a vacuum in FIG. 2B. The location of the two main surfaces ("main sides") is given for orientation. The sketch shows an exemplary fluorescence distribution in nonwoven fabrics with an asymmetrical silica impregnation, which is designed in such a way that the Si0 ₂ weight fraction on one main surface is higher than in the middle of the nonwoven, while the Si0 ₂ - The proportion by weight of the other main surface is less than that in the middle of the fleece (FIG. 2A) and in the case of non-woven fabrics with an asymmetrical silica impregnation which is designed such that the SiO ₂ proportion by weight on both main surfaces is approximately the same , namely higher than in the middle of the fleece (Figure 2B).

Examples

1. Paper making

Eucalyptus sulfate fiber ("curl": 16.2%; degree of fiberization: 1.3%; fine fraction: 15.2%) was used to produce a paper fleece. The fibrous material was ground in a Voith LR 40 laboratory refiner. It was milled with an effective specific energy of 16 kWh / t (750,000 revolutions). Paper fleeces with a grammage of 80 ± 0.9 g / m ² were made from the eucalyptus sulfate fibrous material using a Rapid-Köthen sheet formation system in accordance with DIN 54358 and ISO 5269/2 (ISO5269-2: 2004 (E), "Pulps - Preparation of Laboratory Sheets for Physical Testing - Part 2: Rapid Köthen Method, 2004"). No additives or fillers were used. 2. Production of the Si0 ₂ paper hybrid materials

Three different immersion solutions were provided, which differed in particular with regard to the TEOS content in the immersion solution and which are referred to below as a low-concentration, medium-concentration or highly-concentration solution. The solutions contained TEOS, ethanol (EtOH), water (H2O) and HCl in the following molar ratios:

• 1 TEOS: 80 EtOH: 20 H2O: 0.04 HCl (low-concentrated solution)

1 TEOS: 40 EtOH: 10 H ₂ 0: 0.02 HCl (medium-concentrated solution)

1 TEOS: 20 EtOH: 5 H ₂ 0: 0.01 HCl (highly concentrated solution)

These solutions were stirred for 24 hours and then used for the production of the Si0 ₂ paper hybrid materials. Eucalyptus sulfate paper webs according to Example 1 with a length of 8 cm and a width of 1 cm were immersed in the immersion solution at 50% relative atmospheric humidity and a temperature of 23 ° C. and removed from the immersion solution at a speed of 2 mm / s . The webs were then dried in a horizontal orientation at a temperature of 130 ° C for 2 hours either in a vacuum oven or in a muffle oven. The nonwovens were then cooled to room temperature.

3. Examination of the nonwovens made of Si0 ₂ paper hybrid material

Various experiments were carried out with the nonwovens obtained according to Example 2 in order to test the properties of the nonwovens. a) Contact angle

Contact angle measurements were carried out with the TBU90E model from DataPhysics Instruments GmbH and the SCA software. All samples were measured at five positions and the average and standard deviation were calculated. A drop volume of 2 pl was used for static contact angle measurements (application rate: 1 mI / s). The results of the contact angle measurements with water are shown in Table 1 below. Table 1 low silica- medium silica- high silica-

Speak concentration concentration speak

Regardless of the TEOS solution used, no differences were found between the surface properties of the top and bottom of the nonwovens dried in the vacuum oven. The nonwovens that were dried under ambient pressure in a muffle furnace also showed no differences between the top and bottom when the nonwovens were obtained by treatment with the low-concentration TEOS solution or the high-concentration TEOS solution. The low-concentration TEOS solution resulted in nonwovens with a hydrophilic wetting behavior on the top and bottom, while the high-concentration TEOS solution led to hydrophobic wetting behavior on the top and bottom. Surprisingly, however, nonwovens obtained by treatment with the medium-concentrated TEOS solution showed a different wetting behavior on the top and bottom after drying under normal pressure. The top showed a hydrophobic wetting behavior and the bottom showed a hydrophilic wetting behavior. The fleece showed a kind of amphiphilic behavior or "Janus" behavior. b) Thermoqravimetric analysis (TGA)

Thermogravimetric analysis was carried out with a TGA 1 (Mettler-Toledo). The samples were heated from 25 ° C to 600 ° C at a rate of 10 ° C / min under a constant air flow of 30 ml / min. With these measurements it is possible to determine the content of S1O2, since S1O2 is stable up to temperatures of 1700 ° C.

The results of the thermogravimetric analysis are summarized in Table 2 below. Table 1

The proportion of Si0 ₂ in the Si0 ₂ paper hybrid materials is therefore about 0.6% by weight for the nonwovens obtained by treatment with the low-concentration TEOS solution and about 4% by weight for the Nonwovens obtained by treatment with the highly concentrated TEOS solution. c) Analysis of the Si0 ₂ distribution

The relative Si0 ₂ distribution in the nonwovens was analyzed with the help of confocal laser scanning microscopy (CLSM, English: “confocal laser scanning microscopy”) on cross sections of embedded samples. In combination with the absolute Si0 ₂ distributions obtained in b), a quantitative statement about the amounts of material per volume increment is obtained. aa) Production of the nonwovens

Nonwovens were made as described in Examples 1 and 2. However, before treatment with the TEOS solution for producing the hybrid materials according to Example 2, the dye Calcofluor White (CFW) was introduced into the nonwovens in the following manner:

Paper webs from Example 1 were immersed in a CFW solution containing 10 mM CFW in ethanol (absolute) and then dried at 40 ° C. in a vacuum oven for one hour. This staining will later serve as a reference, since CFW is homogeneously distributed over the paper nonwovens due to the high binding affinity for cellulose and does not migrate during drying. The nonwovens labeled in this way were treated with TEOS solutions and dried as described in Example 2, the immersion solutions additionally containing 20 mM Rhodamine B. hb) production of the cross sections

Each sample was embedded in a mixture of 49.9875% by weight Desmodur 3200, 49.9875% by weight Albodur 956 VP and 0.025% by weight TIB-KAT 318. The mixture is a commercial polyurethane system. The freshly embedded samples were subjected to several vacuum cycles at room temperature in order to remove remaining air bubbles. The resin was then cured at 80 ° C for 18 hours. Then samples with a thickness of 120 μm were cut. The section plane was chosen so that it is oriented orthogonally to both main surfaces. cc) Confocal laser scanning microscopy

The samples were placed between two 25 mm round microscope coverslips using Leica Type F immersion liquid. The images were taken on a Leica TCS SP8.

A lens of the type "HC PL APO CS2 20x / 0.75 IMM" in water immersion was used, CFW was excited with a 405 nm laser and detected at 415-557 nm. Rhodamine B was excited with a 552 nm laser and detected at 562-753 nm.

For each of the nonwovens examined, the image data of the different confocal planes were combined and a gray value analysis was carried out for each row of pixels, which means that the gray values of each row were added for each individual column. From this, the distribution through the fleece from one of the main pages to the other main page was determined. dd) results

In this experimental setup, CFW serves as a reference value. CFW has a high affinity for cellulose and is therefore evenly distributed over the entire thickness of the fleece. If the CFW fluorescence value fluctuates significantly across the paper cross-section, this can indicate problems in the beam path (such as air pockets), since CFW is physically homogeneously distributed on the paper. However, with very low SiO ₂ layer thicknesses, it can happen that amino groups of CFW react with the polyurethane resin, whereby the fluorescence of CFW is deactivated. However, if CFW is protected by S1O2, there is no reaction with the resin, so that the fluorescence is retained. Therefore, the extent of CFW fluorescence in addition to the reference is a measure of the content of S1O2 at a certain depth position within the fleece. Rhodamine B (RhoB) serves as a ratiometric marker for the proportion of Si0 ₂ . The higher the RhoB fluorescence, the higher the proportion of Si0 ₂ .

In the case of the nonwovens which were dried at normal pressure, there was a decrease in the RhoB fluorescence from the upper side of the nonwoven to the underside of the nonwoven. The Si0 ₂ portion thus decreases within the fleece from the top to the bottom. This effect was observed regardless of the proportion of TEOS in the immersion solution. At the lowest TEOS concentration there was also a downward trend in CFW fluorescence, which shows that parts of the cellulose fibers were no longer masked.

In the nonwovens dried in the vacuum oven, such a distribution of the SiO ₂ content is not observed either in the nonwovens treated with low-concentration TEOS solution or in nonwovens with medium-concentrated TEOS solution or highly-concentrated TEOS solution were treated. Instead, there is a sandwich-like intensity distribution with high fluorescence on the top and bottom and low RhoB fluorescence inside the fleece. In addition, there is a constant CFW fluorescence, which indicates that when drying in a vacuum, the entire surface of the fleece is at least provided with SiO _{2 in} such a way that no reaction takes place between the CFW and the resin. The described sandwich-like distribution of Si0 ₂ was independent of the proportion of TEOS in the immersion solution. However, the lower the proportion of TEOS in the immersion solution, the greater the relative unequal distribution between the fleece surface and the interior of the fleece.

The results show that the hydrophobic properties correlate with the amount of Si0 ₂ .

Claims

Expectations

1. Nonwoven fabric with asymmetrical silica impregnation, the nonwoven having two main surfaces, the weight fraction of S1O2 decreasing from at least one of the two main surfaces towards the inside of the nonwoven.

2. Nonwoven fabric according to claim 1, wherein the nonwoven fabric is a paper nonwoven.

3. Nonwoven fabric according to at least one of claims 1 or 2, wherein the weight fraction of S1O2 on at least one of the two main surfaces is at least 1.1 times as high as the weight fraction of S1O2 in the middle of the fleece.

4. Nonwoven fabric according to at least one of the preceding claims, wherein the weight fraction of S1O2 on both main surfaces is at least 1.1 times as high as the weight fraction of S1O2 in the middle of the fleece.

5. Nonwoven fabric according to at least one of the preceding claims, wherein the ratio of the weight fraction of Si0 ₂ on one main surface to the weight fraction of Si0 ₂ on the other main surface is in a range from 0.95: 1 to 1.05: 1.

6. Nonwoven fabric according to at least one of claims 1 to 3, wherein the weight fraction of Si0 ₂ on one of the two main surfaces is at least 1.1 times as high as the weight fraction of Si0 ₂ in the middle of the fleece, and wherein the weight fraction of Si0 ₂ on the other of the two main surfaces is at most 0.9 times the weight fraction of Si0 ₂ in the middle of the fleece.

7. Nonwoven fabric according to at least one of claims 1 to 3 and 6, wherein the weight fraction of Si0 ₂ on one of the two main surfaces is at least 1.2 times the weight fraction of S1O2 on the other of the two main surfaces.

8. A method for producing a nonwoven fabric according to at least one of the preceding claims, comprising the following steps: a) providing a nonwoven fabric, b) providing an impregnation solution, the impregnation solution containing a silane component, c) impregnating the nonwoven fabric with the impregnation solution, d ) Drying the fleece at temperatures in a range from 70 ° C to 190 ° C, a period of at most 60 seconds between the completion of the impregnation according to step c) and the start of drying according to step d).

9. The method according to claim 8, wherein the impregnation solution consists of the silane component, so that the proportion of the silane component in the impregnation solution is 100% by weight.

10. The method according to at least one of claims 8 and 9, wherein the silane component is selected from the group consisting of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate, polydimethoxysiloxane, 1, 2-bis (triethoxysilyt) ethane, tetramethyl orthosilicate (TMOS), Silicon tetraacetate and mixtures of two or more thereof.

1 1. The method according to at least one of claims 8 to 10, wherein the pressure during the drying according to step d) is in a range from 0.1 kPa to 30 kPa.

12. The method according to at least one of claims 8 to 10, wherein the pressure during the drying according to step d) is in a range from> 30 kPa to 500 kPa.

13. Use of a nonwoven fabric according to at least one of claims 1 to 7 as packaging material, as a membrane and / or as special paper for use at elevated temperatures.

14. Use according to claim 13, wherein the use as packaging material, the use as frozen paper, the use for products which come into contact with food, such as in particular (paper) cups and / or (paper) straws, and the Use as packaging for materials that are protected from liquid, but still should exchange moisture.

15. Use according to claim 13 or 14, wherein the nonwoven fabric is used for plastic-free straws and / or paper cups.

16. Use according to claim 13, wherein the membrane is a membrane for water / oil separation.

17. Use according to claim 13, wherein the special paper for use at elevated temperatures is baking paper.