Electrically Conductive Non-Woven Fabric

Abstract

The invention concerns an electrically conductive non-woven fabric which fabric comprises or consists of a three-dimensional network of non-woven non-electrically conductive synthetic nanofibers and electrically conductive metal nanowires distributed therein, wherein the synthetic nanofibers comprise or consist of fibers having a diameter in the range of 10 nm to 2000 nm and a maximal length of 6 mm, wherein the electrically conductive metal nanowires comprise or consist of strands having a diameter in the range of 10 nm to 800 nm and a length in the range of 1 m to 500 m, wherein the electrically conductive metal nanowires occupy between 0.5% by volume to 5% by volume of said fabric, wherein the electrically conductive metal nanowires and the synthetic nanofibers are homogenously distributed within the electrically conductive non-woven fabric.

Claims

1. An electrically conductive non-woven fabric which fabric comprises or consists of a three-dimensional network of non-woven non-electrically conductive synthetic nanofibers and electrically conductive metal nanowires distributed therein, wherein the synthetic nanofibers comprise or consist of fibers having a diameter in the range of 10 nm to 2000 nm and a maximal length of 6 mm, wherein the electrically conductive metal nanowires comprise or consist of strands having a diameter in the range of 10 nm to 800 nm and a length in the range of 1 m to 500 m, wherein the electrically conductive metal nanowires occupy between 0.5% by volume to 5% by volume of said fabric, wherein the electrically conductive metal nanowires and the synthetic nanofibers are homogenously distributed within the electrically conductive non-woven fabric.

2. Electrically conductive non-woven fabric according to claim 1, wherein said synthetic nanofibers comprise or consist of fibers having a diameter in the range of 50 nm to 1350 nm, in particular in the range of 50 nm to 800 nm, and/or a length in the range of 0.5 mm to 0.15 mm.

3. Electrically conductive non-woven fabric according to claim 1, wherein said synthetic nanofibers comprise or consist of at least one of a polyimide, a polyamide, a polyester, polyacrylonitrile (PAN), a polyacrylonitrile comprising copolymer, polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polysulfone, poly(acrylonitrile/styrene/butadiene) copolymer (ABS), polycarbonate, polyamideimide, polyesterimide, polyurethane, polyguanidine, polybiguanidines, chitosan, silk, recombinant silk, collagene, cross-linked polyamide carboxylic acid, polyamide carboxylic acid, polyvinyl alcohol, polydiallyldimethylammonium chloride, polyvinylpyrrolidone, polystyrene (PS), polymethylmethacrylate (PMMA), a polycationic polymer, a polyanionic polymer, polycaprolactone, in particular poly(epsilon-caprolactone) (PCL), polylactic acid (PLA), poly-L-lactic acid (PLLA), and poly acrylic acid.

4. Electrically conductive non-woven fabric according to claim 3, wherein said synthetic nanofibers comprise or consist of polyacrylonitrile and/or polycaprolactone, in particular poly(epsilon-caprolactone).

5. Electrically conductive non-woven fabric according to claim 1, wherein said electrically conductive metal nanowires comprise at least 80% by weight, in particular at least 90% by weight, of silver, copper, gold, nickel, iron, cobalt, rhodium, rhenium, iridium, osmium, bismuth, platinum and/or palladium.

6. Electrically conductive non-woven fabric according to claim 1, wherein said electrically conductive metal nanowires comprise or consist of strands having a ratio of length to diameter of at least 80, in particular a ratio of length to diameter in the range of 120 to 700, in particular a ratio of length to diameter in the range of 120 to 500.

7. Electrically conductive non-woven fabric according to claim 1, wherein said electrically conductive metal nanowires occupy a volume in the range of 0.3% by volume to 5% by volume, in particular in the range of 0.36% by volume to 3.5% by volume, in particular in the range of 0.5% by volume to 1.8% by volume, of said fabric.

8. Electrically conductive non-woven fabric according to claim 1, wherein said fabric has a density below 0.1 g/m.sup.3 or a density in the range of 0.15 g/cm.sup.3 to 0.35 g/cm.sup.3, a surface weight in the range of 8 g/m.sup.2 to 20 g/m.sup.2 and/or a porosity in the range of 70% to 99.5%.

9. Method for producing the electrically conductive non-woven fabric according to claim 1 comprising or consisting of the following steps: a) providing the non-electrically conductive synthetic nanofibers, b) providing the electrically conductive metal nanowires, c) homogenously dispersing the non-electrically conductive synthetic nanofibers and the electrically conductive metal nanowires in a liquid, d) separating the liquid from the dispersion resulting from step c).

10. Method for producing the electrically conductive non-woven fabric according to claim 1 comprising or consisting of the following steps: a) providing a dispersion of the non-electrically conductive synthetic nanofibers in a liquid, b) separating the liquid from the dispersion thus resulting in a non-woven fabric of the synthetic nanofibers and optionally soaking the non-woven fabric with a hydrophilic liquid optionally followed by washing of the non-woven fabric, c) providing a dispersion of the electrically conductive metal nanowires, d) soaking the non-woven fabric of step b) with the dispersion of step c) optionally followed by washing of the non-woven fabric and e) drying of the non-woven fabric.

11. Method according to claim 9, wherein the liquid is removed by evaporation, soaking, suction, filtration and/or freeze drying.

12. Method according to claim 9, wherein the resulting non-woven fabric is heated to a temperature allowing the synthetic nanofibers to stick together, in particular to a temperature of up to 350 C.

13. Method according to claim 9, wherein the liquid is a mixture of at least two of ethanol, isopropanol and water.

14. Method according to claim 9, wherein the non-electrically conductive synthetic nanofibers are produced by electrospinning and subsequent cutting or braking of the resultant fibers.

15. Use of the electrically conductive non-woven fabric according to claim 1 for conducting electricity, for producing heat by conducting electricity through the non-woven fabric, for thermal insulation and/or for providing a pressure sensitive sensor, a filter, an electrode, a catalyst, a heating device, a biodegradable heating device, an electromagnetic shielding, a wrapping preventing electrostatic discharges, clothes preventing electrostatic discharges, or a membrane.

16. Method according to claim 10, wherein the liquid is removed by evaporation, soaking, suction, filtration and/or freeze drying.

17. Method according to claim 10, wherein the resulting non-woven fabric is heated to a temperature allowing the synthetic nanofibers to stick together, in particular to a temperature of up to 350 C.

18. Method according to claim 10, wherein the liquid is a mixture of at least two of ethanol, isopropanol and water.

19. Method according to claim 10, wherein the non-electrically conductive synthetic nanofibers are produced by electrospinning and subsequent cutting or braking of the resultant fibers.

Description

[0031] Embodiments of the invention:

[0032] FIG. 1 shows a scanning electron microscope (SEM) image of silver nanowires,

[0033] FIG. 2 shows the distribution of the length of the silver nanowires,

[0034] FIG. 3 shows the distribution of the diameters of the silver nanowires,

[0035] FIG. 4 shows schematically the preparation of an electrically conductive non-woven fabric comprising PAN nanofibers, PCL nanofibers and conductive silver nanowires,

[0036] FIG. 5 shows an SEM/Backscattered Electron (BSE) image of a non-woven fabric membrane comprising PAN and PCL nanofibers and silver nanowires,

[0037] FIG. 6 shows an SEM image of a sponge-like non-woven fabric comprising polyimide nanofibers and silver nanowires,

[0038] FIG. 7 shows a diagram of the electrical conductivity in dependency of the silver content of a PAN and PCL fibers containing non-woven membrane,

[0039] FIG. 8 shows a diagram of the thermal conductivity in dependency of the silver content of a PAN and PCL fibers containing non-woven membrane,

[0040] FIG. 9 shows a diagram of the temperature in dependency of time and silver content of a PAN and PCL fibers containing non-woven membrane after a short heating by conduction of electricity, and

[0041] FIG. 10 shows a diagram of the electrical conductivity in dependency of compression of a sponge-like non-woven fabric comprising polyimide nanofibers and silver nanowires.

1. SYNTHESIS OF ELECTRICALLY CONDUCTIVE SILVER NANOWIRES (AGNW)

[0042] The AgNW were synthesized using the polyol process according to S. M. Bergin, Y.-H. Chen, A. R. Rathmell, P. Charbonneau, Z.-Y. Li, B. J. Wiley, Nanoscale 2012, 4, 1996-2004. In detail, 160 mL ethylene glycol (EG) were added to a 500 mL schlenk flask, stirred at 500 rpm and preheated in an oil bath at 130 C. for 1 hour. 0.2 mL of 0.1985 g NaCl in 10 mL EG, 0.1 mL of 0.054 g FeCl.sub.3 in 10 mL EG, 20.76 mL of 1.05 g polyvinylpyrolidone K30 in 25 mL EG and 20.76 mL of 1.05 g AgNO.sub.3 in 25 mL EG were given to the schlenk flask. The reaction took place at 130 C. for 6 h. Afterwards the reaction solution was cooled down to room temperature and acetone was added until silver nanowires flocculated. This purification process with acetone was repeated three times. During the last purification process the silver nanowires were centrifuged at 1000 rpm for 10 minutes. The obtained silver nanowires were dispersed in water to receive a silver nanowire dispersion having a silver concentration of 183 mg/mL.

[0043] FIG. 1 shows a SEM image of the silver nanowires obtained by the above method. FIG. 2 shows a length distribution of these nanowires. It shows that the arithmetic average of length is 14.24 m6.27 m. FIG. 3 shows the diameter distribution of the silver nanowires. It shows that the arithmetic average of diameters is 76 nm28 nm. Length and diameter can be influenced by the parameters and conditions of the reaction for generating the silver nanowires.

2. PREPARATION OF POLYACRYLONITRILE (PAN) NANOFIBERS

[0044] PAN nanofibers were obtained by electrospinning of a solution of 10% by weight PAN (Mw=150000 g/mol) in dimethylformamide (DMF) using a needle having a diameter of 0.9 mm at a voltage of +25 kV and 1 kV, at 25 C., at a relative humidity of 35% and at a temperature of 20 C. The PAN nanofibers were collected on an aluminum foil at a distance of 15 cm to the needle.

3. PREPARATION OF POLY(EPSILON-CAPROLACTONE) (PCL) NANOFIBERS

[0045] PCL nanofibers were obtained by electrospinning of a solution of 15% by weight PCL (Mw=150000 g/mol) in a mixture of 70% by weight of tetrahydrofuran (THF) and 30% by weight of DMF using a needle having a diameter of 0.9 mm at a voltage of +14 kV and 1 kV, at 25 C., at a relative humidity of 35% and at a temperature of 20 C. The PCL nanofibers were collected on an aluminum foil in a distance of 15 cm to the needle.

4. PREPARATION OF A DISPERSION OF SHORT PAN AND PCL NANOFIBERS

[0046] Dispersions of short PAN and PCL nanofibers were obtained by cutting of 1 g of electrospun PAN or PCL nanofiber mats in a solution of 700 mL 2-propanol and 300 mL demineralized water at 18 C. in a blender with cutting unit (Robot Coupe Blixer 4, Rudolf Lange GmbH & Co.KG) for 2 minutes at 3500 rpm. The diameter of the electrospun PAN and PCL fibres, respectively ranged from 30782 nm and 714593 nm according to SEM images.

5. PREPARATION OF A CONDUCTIVE NON-WOVEN FABRIC MEMBRANE COMPRISING PAN AND PCL NANOFIBERS AND SILVER NANOWIRES

[0047] The preparation of the electrically conductive non-woven fabric comprising PAN nanofibers, PCL nanofibers and conductive silver nanowires is schematically shown in FIG. 4.

[0048] 20 mL of the PAN nanofiber dispersion and 10 mL of the PCL nanofiber dispersion were mixed with the dispersion of the silver nanowires, wherein each of the dispersions was obtained as described above. The mixture was shaken and the contained liquid was sucked through a round 325 mesh stainless steel grid having a diameter of 60 mm. The grid was pressed on a glass frit to achieve a homogeneous flow through the stainless steel grid. The obtained fiber mat was dried at air at room temperature and detached from the steel grid after drying. Afterwards it was pressed between two glass plates at a temperature of 75 C. for 15 seconds to achieve a cross-linking of the synthetic nanofibers and therewith a high stability composite membrane. The picture in the center at the bottom of FIG. 4 shows two button cells connected via this membrane and via a light emitting diode. The shining of the diode demonstrates that the electric circuit is closed by the membrane. Furthermore, the inventors found that conductivity is obviously not or nearly not affected by bending of the electrically conductive non-woven fabric according to the invention. Since conductivity is nearly independent of the bending angle, operation of the light emitting diode by the bended non-woven fabric shown in FIG. 4 was possible. FIG. 5 shows an SEM/BSE image of such membrane containing 0.45% by volume of AgNW. The thin silver nanowires and the thicker synthetic nanofibers are clearly visible.

[0049] Results obtained with different fractions of AgNW in the non-wovens according to the invention are given in the following table 1:

TABLE-US-00001 TABLE 1 Fraction of PAN PCL AgNW AgNW in Electrical Experiment dispersion.sup.1 dispersion.sup.2 dispersion.sup.3 non-woven.sup.4 Density Porosity Resistance conductivity No. (mL) (mL) (L) (vol %) (g/cm.sup.3) (%) (m) (S/m) 1 20 10 0 0.00 0.245 79.5 333 143 0.003 0.004 2 20 10 32 0.16 0.248 86.4 566 532 0.002 0.002 3 20 10 64 0.27 0.253 88.7 148 110 0.007 0.009 4 20 10 96 0.36 0.258 90.0 3.28 10.sup.4 1.18 10.sup.3 3047 845 5 20 10 128 0.45 0.259 91.0 7.96 10.sup.5 2.40 10.sup.4 12570 4170 6 20 10 160 0.65 0.295 91.1 4.42 10.sup.5 2.62 10.sup.4 22640 3010 7 20 10 192 0.63 0.263 92.6 2.73 10.sup.5 3.32 10.sup.4 36670 3810 8 20 10 224 0.78 0.297 92.1 1.97 10.sup.5 2.02 10.sup.4 50720 4950 9 20 10 256 0.96 0.284 93.0 1.52 10.sup.5 9.71 10.sup.5 66000 7200 10 20 10 288 0.92 0.302 93.5 1.46 10.sup.5 1.39 10.sup.5 68530 10300 11 20 10 320 1.07 0.307 93.3 1.07 10.sup.5 6.94 10.sup.5 93610 14400 12 20 10 690 2.30 0.487 91.6 4.56 10.sup.6 4.18 10.sup.6 219519 19729 13 20 10 690 2.25 0.414 93.6 2.46 10.sup.6 2.33 10.sup.6 406811 23167 14 20 10 835 2.79 0.441 94.0 1.79 10.sup.6 1.67 10.sup.6 557929 41027 15 20 10 1003 3.35 0.467 94.1 1.32 10.sup.6 9.37 10.sup.7 756375 310993 .sup.1Concentration of PAN dispersion = 1.00 g/L .sup.2Concentration of PCL dispersion = 1.00 g/L .sup.3Concentration of AgNW dispersion = 183 g/L .sup.4Determined gravimetrically by thermogravimetric analysis (TGA)

[0050] Table 1 shows that electrical conductivity increased from 0.27 vol % AgNW to 0.36 vol % AgNW by about seven orders of magnitude. Furthermore, the table shows that a metal-like electrical conductivity of about 750000 S/m could be achieved with the relative low content of 3.35 vol % AgNW. Furthermore, the table shows that porosity increased with increasing amount of AgNW.

[0051] The volume percentage of AgNW in the non-wovens was calculated using following equations (S1) and (S2):

[00001]$\begin{array}{cc}\mathrm{vol}.Math..Math.\%.Math..Math.\left(\mathrm{Ag}\right)=\frac{V_{\mathrm{Ag}}}{V_{\mathrm{conductive}.Math..Math.\mathrm{non}.Math.\text{-}.Math.\mathrm{woven}}}=\frac{\frac{m_{\mathrm{Ag}}}{{}_{\mathrm{Ag}}}}{.Math..Math.r^2.Math.h}& \left(S.Math..Math.1\right)\\ m_{\mathrm{Ag}}=\left(\mathrm{wt}.Math..Math.{\%}_{\mathrm{sample}}-\mathrm{wt}.Math..Math.{\%}_{\mathrm{blank}}\right)m_{\mathrm{conductive}.Math..Math.\mathrm{non}.Math.\text{-}.Math.\mathrm{woven}}& \left(S.Math..Math.2\right)\end{array}$

[0052] The values are based on the percent by weight of the recess of the conductive non-woven of the original conductive non-woven (wt %.sub.sample) and the percent by weight of the recess of a non-conductive non-woven of the original non-conductive non-woven (wt % blank), which non-conductive non-woven is identical to the conductive non-woven except that is does not contain AgNW and which recesses were obtained by thermogravimetric analysis.

[0053] In the equations .sub.Ag is the density of silver (10.5 g/cm.sup.3), V is volume, m is mass, r is the radius of the non-woven (2.75 cm) and h is the thickness of the non-woven (approximately 53 m). Determined weight percent of AgNW ranged from 1.8 to 77.15 wt %.

[0054] Electrical conductivities of the PAN/PCL/AgNW non-wovens were calculated according to following equations (S3)-(S5).

[00002]$\begin{array}{cc}{R}_{\mathrm{sh}}=0.1526{10}^0.Math.\frac{1000.Math..Math.\mathrm{mm}}{m}=0.1526{10}^0.Math.\frac{.Math..Math.m}{m}& \left(S.Math..Math.3\right)\\ =R_{\mathrm{sh}}l.Math..Math.=0.526{10}^0.Math.\frac{1000.Math..Math.\mathrm{mm}}{m}0.056.Math..Math.\mathrm{mm}=8.55.Math.\frac{.Math..Math.{\mathrm{mm}}^2}{m}& \left(S.Math..Math.4\right)\end{array}$

[0055] where is the resistivity, R.sub.sh is the sheet resistance, l is the thickness of the non-woven, where is the electrical conductivity. Resistivity measurements (Four point measurements) were performed using a Keithley 2420 High-Current Source Meter (Keithley Instruments GmbH, Landsberger Str. 65, 82110 Germering, Germany) coupled with a Signatone SYS-301 (Signatone Corporation, 393 Tomkins Ct# J, Gilroy, Calif. 95020, USA). The resistivity was measured ten times for each sample.

[00003]$\begin{array}{cc}=\frac{1}{}=\frac{1}{8.55.Math.\frac{.Math..Math.{\mathrm{mm}}^2}{m}}=0.116959.Math.\frac{m}{.Math..Math.{\mathrm{mm}}^2}=116959.Math.\frac{s}{m}& \left(S.Math..Math.5\right)\end{array}$

[0056] When testing silver nanowires having an average length of 42.0 m instead of an average length of 14.2 m as the nanowires used for the non-wovens of above table 1, the inventors found that electrical conductivity was almost doubled for the same fraction of about 2.30 vol % AgNW in the non-woven fabric according to the invention.

6. PREPARATION OF A SPONGE-LIKE NON-WOVEN FABRIC

[0057] A mixture of 50% by volume of dioxane and 50% by volume of water was prepared. A polyimide non-woven fabric obtained by electrospinning was cut into pieces, immersed in a part of the dioxane-water-mixture and frozen in liquid nitrogen. To the rest of the dioxane-water-mixture liquid nitrogen was added. The resulting mixture was homogenized in a homogenizer until a paste developed. Frozen pieces of the polyimide non-woven fabric were added to this paste and homogenized for 2 minutes under addition of a small amount of polyamide carboxylic acid dissolved in dimethyl sulfoxide (DMSO). The resulting dispersion was dried in a freeze dryer. Subsequently, it was slowly heated up to 300 C. and kept under this temperature for 1 hour under vacuum in a vacuum furnace. The resulting non-woven fabric had a sponge-like structure. It was cut into a piece of 1.4 cm1.6 cm2.0 cm by means of a razor blade.

[0058] 2 mL of a 200 mg/L polyethyleneimine (PEI) solution was diluted with 18 mL deionized water. The sponge-like fabric was immersed in the PEI solution and subsequently washed ten times with deionized water for removing excess PEI. Afterwards, the sponge-like fabric was immersed in the above described dispersion of silver nanowires on a shaker over night and subsequently washed with deionized water and dried at 70 C. The content of silver by weight was about 50%. FIG. 6 shows an SEM image of the resulting sponge-like structure. The thin silver nanowires are clearly visible on thicker polyimide nanofibers.

7. DETERMINATION OF ELECTRIC CONDUCTIVITY OF THE CONDUCTIVE NON-WOVEN FABRIC MEMBRANE COMPRISING PAN AND PCL NANOFIBERS AND SILVER NANOWIRES

[0059] Different non-woven fabrics having a thickness of 50 m to 60 m were prepared with silver nanowires having arithmetic average lengths of 14.2 m and 42.0 m. The arithmetic average of the ratio of length to diameter of the silver nanowires having arithmetic average length of 14.2 m was about 190. The silver content by volume of the different non-woven fabrics comprising these nanowires ranged from 0 to 2.3% by volume. The arithmetic average of the ratio of length to diameter of the silver nanowires having an average length of 42.0 m was about 550 and the silver content by volume of the different non-woven fabrics comprising these nanowires ranged from 2.2 to 3.35 percent by volume. As can be seen from FIG. 7 a low content of silver did not result in a high electric conductivity of the membrane probably due to a low number of connections between the silver nanowires. Such a non-woven fabric can be used as an antistatic membrane. However, there is a drastic increase of the electrical conductivity at a silver content of between 0.27% to 0.36% by volume. An electric conductivity of up to 95000 S/m was reached. Such a conductivity is sufficient to supply typical electric consumers. Furthermore, FIG. 7 shows an increased conductivity for non-woven fabrics having identical content of AgNW but longer silver nanowires.

8. DETERMINATION OF THERMAL CONDUCTIVITY OF THE CONDUCTIVE NON-WOVEN FABRIC MEMBRANE COMPRISING PAN AND PCL NANOFIBERS AND SILVER NANOWIRES

[0060] The non-wovens provided for the previous experiment were also used to determine thermal conductivity in dependency of the content of silver by volume. The result is shown in FIG. 8. This shows that the thermal conductivity increases with silver content. However, non-wovens having a silver content that allows good electrical conductivity still have a thermal conductivity of a usual non-woven fabric containing no silver.

[0061] FIG. 9 shows the temperature of non-woven fabrics having different contents of silver by volume when a current of 1.1 V is applied for 20 seconds such that electricity is conducted through the non-woven. FIG. 9 shows that the heating of the fabrics occurs very quickly and that cooling down also occurs very quickly after current has been switched off. Within 20 seconds the non-woven fabric with 1.07 vol % AgNW heats up to about 80 C. for 15 to 20 seconds and cools down immediately when the voltage is off.

9. ELECTRIC CONDUCTIVITY IN DEPENDENCY OF COMPRESSION

[0062] The sponge-like silver nanowires containing polyimide non-woven described above is cut to a block of 0.8 cm0.9 cm1.2 cm. It was used for examination of dependency of electric conductivity from compression. For measuring conductivity electrodes were positioned on opposite sides of the block. The result is shown in FIG. 10. A compression by 50% means that the length of one side is reduced to the half of its original length. FIG. 10 shows that a compression by about 50% results in a jump of conductivity. It also shows that a mechanical relaxation after the compression reduces conductivity to its original value.

[0063] When the non-woven is compressed such that electric conductivity is high the non-woven shows the typical heat emission when electricity is conducted through the non-woven. At the same time the non-woven shows a thermal conductivity that is typical for thermal insulators. A sponge-like non-woven can be used as a pressure sensor or for another electric element that shows conductivity in dependency of compression.