ASYMMETRICALLY SILICA-IMPREGNATED NONWOVEN FABRICS AND METHODS FOR PRODUCING SAID NONWOVENS AND USE THEREOF

Abstract

The present invention relates to fibrous non-woven fabrics with asymmetric silica impregnation and methods for their production as well as uses of the non-woven fabrics, in particularly in the field of packaging materials.

Claims

1. A fibrous non-woven fabric with asymmetric silica impregnation, wherein the non-woven fabric comprises two main surfaces, wherein the portion by weight of SiO.sub.2 starting from at least one of both main surfaces decreases towards the interior of the non-woven fabric.

2. The fibrous non-woven fabric according to claim 1, wherein the fibrous non-woven fabric is a non-woven paper fabric.

3. The fibrous non-woven fabric according to claim 1, wherein the portion by weight of SiO.sub.2 at least one of both main surfaces is at least 1.1 times as high as the portion by weight of SiO.sub.2 in the center of the non-woven fabric.

4. The fibrous non-woven fabric according to claim 1, wherein the portion by weight of SiO.sub.2 at both main surfaces is at least 1.1 times as high as the portion by weight of SiO.sub.2 in the center of the non-woven fabric.

5. The fibrous non-woven fabric according to claim 1, wherein the ratio of the portion by weight of SiO.sub.2 at the one main surface to the portion by weight of SiO.sub.2 at the other main surface is in a range of 0.95:1 to 1.05:1.

6. The fibrous non-woven fabric according to claim 1, wherein the portion by weight of SiO.sub.2 at one of both main surfaces is at least 1.1 times as high as the portion by weight of SiO.sub.2 in the center of the non-woven fabric, and wherein the portion by weight of SiO.sub.2 at the other of both main surfaces is at most the 0.9-fold of the portion by weight of SiO.sub.2 in the center of the non-woven fabric.

7. The fibrous non-woven fabric according to claim 1, wherein the portion by weight of SiO.sub.2 at one of both main surfaces is at least the 1.2-fold of the portion by weight of SiO.sub.2 at the other of both main surfaces.

8. A method for the production of a fibrous non-woven fabric according to claim 1, comprising the following steps: a) providing of a fibrous non-woven fabric, b) providing of an impregnating solution, wherein the impregnating solution contains a silane component, c) impregnating of the fibrous non-woven fabric with the impregnating solution, d) drying of the non-woven fabric at temperatures in a range of 70° C. to 190° 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).

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

10. The method according to claim 8, wherein the silane component is selected from the group consisting of tetraethyl orthosilicate (TEOS), tetramethyl orthosilicate, polydimethoxysiloxane, 1,2-bis(triethoxysilyl)ethane, tetramethyl orthosilicate (TMOS), silicon tetraacetate and mixtures of two or more thereof.

11. The method according to claim 8, wherein the pressure during the drying according to step d) is in a range of 0.1 kPa to 30 kPa.

12. The method according to claim 8, wherein the pressure during the drying according to step d) is in a range of >30 kPa to 500 kPa.

13-17. (canceled)

Description

DESCRIPTION OF THE FIGURES

[0071] FIG. 1 shows schematically the evaporation from the non-woven fabrics (1) during drying under normal pressure (FIG. 1A) or in vacuum (FIG. 1B). Under normal pressure there was a high evaporation rate (2) at the upper side (illustrated by the longer arrows) and a low evaporation rate (3) at the lower side of the non-woven fabric (1) (illustrated by the shorter arrows). In vacuum at the upper and lower sides similarly a mean evaporation rate (4) results.

[0072] FIG. 2 shows schematically the fluorescence intensity (y-axis) of rhodamine B across the thickness of the non-woven fabrics (x-axis). The distribution is schematically outlined in FIG. 2A for non-woven fabrics which have been dried under normal pressure and in FIG. 2B for non-woven fabrics which have been dried in vacuum. The position of both main surfaces (“main sides”) is given for orientation. The outline shows an exemplary fluorescence distribution in the case of fibrous non-woven fabrics with an asymmetric silica impregnation which is designed such that the portion by weight of SiO.sub.2 at the one main surface is higher than in the center of the non-woven fabric, while the portion by weight of SiO.sub.2 at the other main surface is lower than in the center of the non-woven fabric (FIG. 2A), and in the case of fibrous non-woven fabrics with an asymmetric silica impregnation which is designed such that the portion by weight of SiO.sub.2 at both main surfaces is nearly the same, namely higher than in the center of the non-woven fabric (FIG. 2B).

EXAMPLES

[0073] 1. Paper Production

[0074] For the production of a non-woven paper fabric fibrous eucalyptus-sulfate material (“curl”: 16.2%; fibrillation degree: 1.3%; fines: 15.2%) was used. The fibrous material was ground in a Voith LR 40 laboratory refiner. It was ground with an effective specific energy of 16 kWh/t (750,000 rotations). From the fibrous eucalyptus-sulfate material non-woven paper fabrics having a grammage of 80±0.9 g/m.sup.2 were prepared using a sheet forming Rapid-Köthen plant according to DIN 54358 and ISO 5269/2 (IS05269-2:2004(E), “Pulps—Preparation of Laboratory Sheets for Physical Testing—Part 2: Rapid Köthen Method, 2004”). No additives or fillers were used.

[0075] 2. Production of the SiO.sub.2-Paper Hybrid Materials

[0076] Three different immersion solutions were provided, which in particularly differed with respect to the content of TEOS in the immersion solution and which in the following are referred to as low-concentrated, medium-concentrated and high-concentrated solution, respectively. The solutions contained TEOS, ethanol (EtOH), water (H.sub.2O) and HCl in the following molar ratios:

TABLE-US-00001 1 TEOS:80 EtOH:20 H.sub.2O:0.04 HCl (low-concentrated solution) 1 TEOS:40 EtOH:10 H.sub.2O:0.02 HCl (medium-concentrated solution) 1 TEOS:20 EtOH:5 H.sub.2O:0.01 HCl (high-concentrated solution)

[0077] These solutions were stirred for 24 hours and then they were used for the production of the SiO.sub.2-paper hybrid materials. Eucalyptus-sulfate non-woven paper fabrics according to example 1 with a length of 8 cm and a width of 1 cm were immersed into the immersion solution at 50% relative air humidity and a temperature of 23° C. and were removed from the immersion solution with a velocity of 2 mm/s. Then, the non-woven fabrics were dried in horizontal orientation at a temperature of 130° C. for 2 hours, either in a vacuum oven or in a muffle furnace. Subsequently, the non-woven fabrics were cooled to room temperature.

[0078] 3. Investigation of the Non-Woven Fabrics Made of SiO.sub.2-Paper Hybrid Material

[0079] The non-woven fabrics obtained according to example 2 were used for different experiments for testing the properties of the non-woven fabrics.

[0080] a) Contact Angle

[0081] Contact angle measurements were conducted with the model TBU90E of DataPhysics Instruments GmbH and the SCA software. All samples were measured at five positions and the mean value and the standard deviation were calculated. For static contact angle measurements a drop volume of 2 μl was used (application rate: 1 μl/s). The results of the contact angle measurements with water are shown in the following table 1.

TABLE-US-00002 TABLE 1 low silica medium silica high silica concentration concentration concentration environmental upper side hydrophilic hydrophobic hydrophobic pressure lower side hydrophilic hydrophilic hydrophobic vacuum upper side hydrophilic hydrophilic hydrophobic lower side hydrophilic hydrophilic hydrophobic

[0082] In the case of the non-woven fabrics which were dried in the vacuum oven independently of the used TEOS solution no differences between the surface properties of the upper and lower sides were determined. The non-woven fabrics which were dried under environmental pressure in a muffle furnace also have not shown differences between the upper and lower sides, when the non-woven fabrics were obtained by a treatment with the low-concentrated TEOS solution or the high-concentrated TEOS solution. The low-concentrated TEOS solution resulted in non-woven fabrics with a hydrophilic wetting behavior at the upper and lower sides, while the high-concentrated TEOS solution resulted in a hydrophobic wetting behavior at the upper and lower sides. However, surprisingly, in the case of non-woven fabrics which were obtained by a treatment with the medium-concentrated TEOS solution, after drying under normal pressure, a different wetting behavior at the upper and lower sides has been shown. The upper side showed a hydrophobic wetting behavior and the lower side showed a hydrophilic wetting behavior. Thus, the non-woven fabric showed a kind of amphiphilic behavior or “Janus” behavior.

[0083] b) Thermogravimetric Analysis (TGA)

[0084] Thermogravimetric analysis was conducted with a TGA 1 (Mettler-Toledo). The samples were heated from 25° C. to 600° C. with a rate of 10° C./min under a constant air current of 30 ml/min. With these measurements it is possible to determine the content of SiO.sub.2, because up to temperatures of 1700° C. SiO.sub.2 is stable.

[0085] The results of the thermogravimetric analysis are summarized in the following table 2.

TABLE-US-00003 TABLE 1 Sample Weight loss (TGA) Portion of SiO.sub.2 non-woven paper fabric of  95.6% — example 1 non-woven fabric of example  95.0% 0.60% 2 (low-concentrated solution) non-woven fabric of example 93.67% 1.93% 2 (medium-concentrated solution) non-woven fabric of example 91.57% 4.03% 2 (high-concentrated solution)

[0086] Thus, the portion of SiO.sub.2 in the SiO.sub.2-paper hybrid materials is about 0.6% by weight for the non-woven fabrics which were obtained by a treatment with the low-concentrated TEOS solution, and about 4% by weight for the non-woven fabrics which were obtained by a treatment with the high-concentrated TEOS solution.

[0087] c) Analysis of the SiO.sub.2 Distribution

[0088] The relative SiO.sub.2 distribution in the non-woven fabrics was analyzed with the help of confocal laser scanning microscopy (CLSM, English: “confocal laser scanning microscopy”) at cross sections of embedded samples. In combination with the absolute SiO.sub.2 distributions obtained in b) a quantitative statement about the amounts of material per volume increment can be made.

[0089] Aa) Production of the Non-Woven Fabrics

[0090] Non-woven fabrics were prepared such as described in the examples 1 and 2. But, prior to the treatment with the TEOS solution, for the production of the hybrid materials according to example 2 the dye Calcofluor White (CFW) was introduced into the non-woven fabrics in the follow manner:

[0091] Non-woven paper fabrics of example 1 were immersed into a CFW solution with 10 μM CFW in ethanol (absolutized) and subsequently dried at 40° C. in a vacuum oven for one hour. Later, this staining is used as reference, because CFW due to the high binding affinity to cellulose is homogenously distributed across the non-woven paper fabrics and does not migrate during the drying. The non-woven fabrics labeled such were treated with TEOS solutions and dried such as described in example 2, wherein the immersion solutions, in addition, contained 20 μM rhodamine B.

[0092] Bb) Production of the Cross Sections

[0093] Each sample was embedded into a mixture of 49.9875% by weight of Desmodur 3200, 49.9875% by weight of Albodur 956 VP and 0.025% by weight of TIB-KAT 318. This mixture is a commercial polyurethane system. The freshly embedded samples were subjected to several vacuum cycles at room temperature for removing remaining air bubbles. Subsequently, the resin was hardened at 80° C. for 18 hours. Then, samples with a thickness of 120 μm were cut. The cutting plane was chosen such that it is orthogonally oriented with respect to both main surfaces.

[0094] Cc) Confocal Laser Scanning Microscopy

[0095] The samples were placed between two round 25 mm microscope cover glasses using type F immersion liquid from Leica. The picture was made with a Leica TCS SP8.

[0096] An objective of the type “HC PL APO CS2 20×/10.75 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.

[0097] For each of the examined non-woven fabrics the data of the images of the different confocal planes were combined and a grey scale analysis was conducted for each series of pixels, that is, that the grey values of each series for each single column were added. From this the distribution through the non-woven fabric from one of the main sides to the other main side was determined.

[0098] dd) Results

[0099] In the present experimental setup CFW is used as reference value. CFW has high affinity to cellulose and therefore it is uniformly distributed across the whole thickness of the non-woven fabric. When the fluorescence value of CFW across the paper cross section shows considerable fluctuations, then this may be indicative of problems in the ray path (such as, e.g., entrapped air), because physically CFW is homogenously distributed on the paper. But in the case of very low thicknesses of SiO.sub.2 layers it may be that amino groups of CFW react with the polyurethane resin, whereby the fluorescence of CFW is deactivated. When, however, CFW is protected by SiO.sub.2, then there is no reaction with the resin so that the fluorescence is maintained. Therefore, the extent of the CFW fluorescence in addition to the reference is a measure for the content of SiO.sub.2 at a certain depth position within the non-woven fabric.

[0100] Rhodamine B (RhoB) is used as ratiometric marker for the portion of SiO.sub.2. The higher the RhoB fluorescence, the higher the portion of SiO.sub.2.

[0101] In the case of the non-woven fabrics which were dried at normal pressure a decrease of the RhoB fluorescence from the upper side of the non-woven fabric to the lower side of the non-woven fabric was detected. Thus, the content of SiO.sub.2 within the non-woven fabric decreases from the upper side to the lower side. This effect was observed independently of the portion of TEOS in the immersion solution. In the case of the lowest TEOS concentration, in addition, a tendency of the CFW fluorescence towards lower values was present, which shows that parts of the cellulose fibers were no longer masked.

[0102] In the case of the non-woven fabrics which were dried in the vacuum oven such a distribution of the portion of SiO.sub.2 cannot be observed, neither in the case of the non-woven fabrics which were treated with low-concentrated TEOS solution nor in the case of the non-woven fabrics which were treated with medium-concentrated TEOS solution or high-concentrated TEOS solution. Instead, a sandwich-like intensity distribution with high fluorescence at the upper and lower sides and low RhoB fluorescence in the interior of the non-woven fabric can be seen. In addition, a constant CFW fluorescence can be seen which suggests that during the drying in vacuum the whole surface of the non-woven fabric is provided with SiO.sub.2 at least such that no reaction takes place between CFW and resin. The described sandwich-like distribution of SiO.sub.2 arose independently of the portion of TEOS in the immersion solution. But the relative imbalance of the distribution between the surface of the non-woven fabric and the interior of the non-woven fabric was larger, when the portion of TEOS in the immersion solution was lower.

[0103] The results show that the hydrophobic properties correlate with the portion of SiO.sub.2.