Conductive fibres

Abstract

A method for making a fiber electrically conductive comprises the steps of: (a) providing a fiber having a negative electric charge at the surface of the fiber, (b) applying to the fiber a substance (such as a polyelectrolyte) which provides a layer of said substance on the fiber and changes the electric charge at the surface of the fiber from negative to positive, wherein said substance is not chitosan, and (c) making the surface of the fiber electrically conductive with a metal, wherein the metal of step (c) is provided in the form of metal ions and wherein a reducing agent (for example) is employed to reduce the metal ions to elemental metal. Fabrics formed from conductive fibers are also provided.

Claims

1. A method for making a fibre electrically conductive, comprising the steps of: (a) providing a fibre having a negative electric charge at the surface of the fibre, (b) applying to the fibre a substance (referred to below as a linker) which provides a layer of said substance on the fibre and changes the electric charge at the surface of the fibre from negative to positive, wherein said substance is not chitosan, wherein said substance is a cationic polyelectrolyte, (c) making the surface of the fibre electrically conductive with a metal, wherein the metal of step (c) is provided in the form of metal cations and wherein the metal cations are reduced to elemental metal; and (d) sintering the metal.

2. A method as claimed in claim 1, wherein a reducing agent is employed to reduce the metal ions to the elemental metal and wherein the reducing agent is applied to the surface of the fibre first and the metal ions are applied to the surface of the fibre second.

3. A method as claimed in claim 1, wherein a solution of metal ions, a reducing agent and said substance are combined and then the combination is applied to the fibre.

4. A method as claimed in claim 3, wherein the solution of metal ions and the reducing agent are combined first, and then said substance is added.

5. A method as claimed in claim 1, additionally comprising after step (a) the step of (a1) treating the fibre with an alkali or acidic solution in order to increase the negative electric charge at the surface of the fibre.

6. A method as claimed in claim 5, wherein step (a1) comprises treating the fibre with a sodium hydroxide aqueous solution of concentration less than 3.0 mol/dm.sup.3.

7. A method as claimed in claim 6, wherein the sodium hydroxide aqueous solution has a concentration of about 1 wt %.

8. A method as claimed in claim 1, wherein the substance of step (b) is selected from the group consisting of protamine sulfate, polybrene, poly(L-lysine), poly(allylamine hydrochloride), poly(ethylene glycol-co-dimethylaminoethyl methacrylate), poly(ethyleneimine), polyacrylamide, poly(acrylamide-co-diallyldimethylammoniumchloride), diallyldimethylammonium chloride, poly (diallyldimethylammonium chloride), poly[bis(2-chloroethyl) ether-alt-1,3-bis[3-(dimethylamino)propyl]urea] quaternized, polyquaternium-7, or any combination thereof.

9. A method as claimed in claim 8, wherein the substance of step (b) is an about 0.2 wt % aqueous solution of poly(diallyldimethylammonium chloride).

10. A method as claimed in claim 1, wherein said metal after reduction is in the form of metal particles having an average diameter less than 50 nm.

11. A method as claimed in claim 1, wherein sodium borohydride is employed to reduce the metal ions to the elemental metal.

12. A method as claimed in claim 1, wherein the metal ions are provided in the form of metal nitrate.

13. A method as claimed in claim 1, wherein the sintering step (d) takes place at a temperature from 50 to 70 C.

14. A method as claimed in claim 1, comprising the additional step of applying a further layer of metal to the metal on the fibre.

15. A method as claimed in claim 14, wherein said further layer of metal is applied by means of electroless plating.

16. A method as claimed in claim 15, wherein the metal in the further layer is different to the metal of step (c).

17. The method as claimed in claim 1, wherein sintering the metal particles comprises allowing the metal particles to self-sinter.

18. The method according to claim 1, wherein the resulting fibre comprises a linker layer and a metal layer, wherein the linker layer is between the surface of the fibre and the metal layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A number of preferred embodiments of the invention will now be described, with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a series of scanning electron microscope (SEM) images (3 reduction) of cotton fabric treated by means of a method according to the invention;

(3) FIG. 2 shows a series of SEM images (3 reduction) of cotton fabric treated by means of a method according to the invention except with the mercerisation step being omitted;

(4) FIG. 3 shows a series of SEM images (3 reduction) of cotton fabric treated by means of a method according to the invention except that the step of applying a cationic polyelectrolyte has been omitted;

(5) FIG. 4 is a schematic diagram showing the steps of a preferred method in accordance with the present invention;

(6) FIG. 5 shows an SEM image (and an enlarged version) of another cotton fabric treated by means of a method according to the invention;

(7) FIG. 6 shows an SEM image (and an enlarged version) of another cotton fabric treated by means of a method according to the invention;

(8) FIG. 7 is a photograph showing (in use) a conductive track made by means of a method according to the invention;

(9) FIG. 8A shows SEM images of cotton fibres immersed in nanosilver solution (not in accordance with the invention); and

(10) FIG. 8B is a photograph of the cotton fabric of FIG. 8A after eletroless plating with copper.

EXAMPLE 1

(11) Cotton fabric was treated by means of the following four stage process:

(12) 1. Mercerization

(13) 2. Surface modification

(14) 3. Fabrication and deposition and sintering of silver nano-particles

(15) 4. Secondary conductive layer build-up

(16) A schematic diagram illustrating this Example is shown in FIG. 4.

(17) 1. Mercerization

(18) Cotton fabric was treated with 1 wt %. aqueous NaOH solution at room temperature for 30 minutes followed by rinsing with distilled water.

(19) 2. Surface Modification

(20) The sample was dried and then its fibres were coated with a 0.198 wt % aqueous solution of poly-diallyldimethylammonium chloride (PDADMAC). This was made up by taking 1 g of a 20 wt % solution of PDADMAC and mixing that with 100 ml of water, so that the resulting solution was 0.2 g of PDADMAC in 101 ml water, i.e. 0.198 wt %.

(21) After thoroughly wetting the fabric with the solution, the fabric was dried at 59 C. in the oven for 5 minutes in order to evaporate any remaining water molecules. It should be noted that the fabric can be dried naturally at room temperature prior to nanoparticle deposition.

(22) 3. Fabrication and Deposition

(23) An aqueous solution of 0.025 M (0.43 g/100 ml water) of silver nitrate was prepared. Next, the cotton fabric (1.5 g with surface area of 64 mm.sup.2) was wetted with 0.1 ml of 1.6110.sup.4 M (6.1 mg/98 ml water) of an aqueous solution of NaBH.sub.4. Then 10 l of the silver nitrate solution was added to the fabric.

(24) The colour of the fabric immediately changed from white to brownish colour, which is an indication of the formation of nanosilver particles. The size of the nanoparticles was verified by dynamic light scattering (DLS) to be approximately 20 nm. The textile was dried at 59 C. and then another reduction step was carried out in order to add another layer of nanoparticles to the fabric. The cotton fibres were fully covered with silver nanoparticles after three consecutive reductions.

(25) 4. Secondary Conductive Layer Buildup

(26) A conductive silver sheath with a thickness less than 100 nm was established on the fibres, and then electroless metal plating was employed to thicken the conductive layer. Specifically, copper electroless plating was carried out at a temperature of 46 C. for 25 minutes. With a copper thickness of approximately 1.25 microns, the resistivity was 0.1 /sq.

(27) A series of SEMs of the resulting treated cotton fibres are shown in FIG. 1.

(28) As a control, an identical cotton fabric is subjected to the method but with the mercerization step being omitted. During the subsequent process steps, the fibres were not fully covered with nano-silver particles (see FIG. 2). The mercerization process creates many more negative sites on the fibres, and without this step the opportunity for bonding in the subsequent steps is reduced.

(29) A further control is carried out by subjecting an identical cotton fabric to the method but without using any PDADMAC.

(30) PDADMAC plays an important role as a binder between the silver particles and textile resulting in a more uniform compact coating (FIG. 1). By contrast, the textile prepared with the mercerisation step but without PDADMAC had a random irregular morphology (FIG. 3).

EXAMPLE 2

(31) In order to make the second sample of conductive fabric the steps of Example 1 above were repeated except that the fabric was coated with an aqueous solution of poly(acrylamide-co-diallyldimethylammoniumchloride) (PAADADMAC) instead of PDADMAC.

(32) The polymeric solution was made up by taking 1 g of a 10 wt % solution of PAADADMAC and mixing that with 100 ml of water.

(33) SEM images (FIG. 5) showed full coverage of fibres within the fabric. Copper Electroless plating was carried out at a temperature of 46 C. for 25 minutes. This resulted in a copper thickness of approximately 1.25 microns with a resistivity of 0.2 /sq.

EXAMPLE 3

(34) Example 1 was repeated but with a different cationic polyelectrolyte, namely poly(allylamine hydrochloride) (PAAHC) with a molecular weight of 58000, purchased from Sigma Aldrich.

(35) The polymeric solution was made up by making 1 wt % aqueous solution of PAAHC.

(36) SEM images (FIG. 6) showed coverage of fibres within the fabric. Copper Electroless plating was carried out at a temperature of 46 C. for 25 minutes. This resulted in a copper thickness of approximately 1.25 microns, the resistivity was 0.2 /sq.

EXAMPLE 4

(37) An experiment was conducted to investigate the addition of polyelectrolyte to the nanoparticle solution prior to the deposition of nanoparticles on the fabric. The following steps were carried out:

(38) (a) 1 ml of a solution of 0.025 M of silver nitrate was added to 100 ml of 1.6110.sup.4 M NaBH.sub.4 solution. The colour immediately changed to a yellow toned colour, due to the formation of silver nanoparticles.

(39) (b) An aqueous solution of poly-diallyldimethylammonium chloride (PDADMAC) was made up by taking 1 g of a 20 wt % solution of PDADMAC and mixing that with 100 ml of water, so that the resulting solution was 0.2 g of PDADMAC in 101 ml water, i.e. 0.198 wt %.
(c) 0.1 ml of the PDADMAC solution prepared in step (b) was added to the silver nanoparticle solution. The colour changed from yellow to a red toned colour. The solution was then centrifuged at 3500 rpm for 100 minutes. The effluent was discharged and the precipitation was used for coating the fabric after which the textile was dried at 60 C. prior to copper electroless plating. This method enabled a very fine conductive track to be created. One such track has been shown in FIG. 7.

(40) It should be noted that when the polyelectrolyte was added to the nanoparticle solution, the zeta potential of the solution changed from negative to positive (+36). Also, after centrifuging, the particles were not agglomerated and did not form large silver metal particles. Without wishing to be constrained by theory, it is thought that this is due to the bonding between the nanosilver particles and the charge groups on the polymer chain. Therefore, when positioned on the fabric it was adsorbed by the fabric evenly.

COMPARATIVE EXAMPLE 5

(41) An experiment was conducted to investigate the effectiveness of nanosilver particles solution for making the conductive fabric. The fibres within the fabric were coated with the PDADMAC linker. The nanosilver solution was prepared approximately 2 hours prior to the application. It was observed that the fabric was not coated with nanosilver particles even after eight consecutive deposition of nanosilver particle solution. SEM images of cotton fibres immersed in nanosilver solution are shown in FIG. 8A.

(42) In addition, when copper electroless plating was carried out at a temperature of 46 C. for more than 3 hours, the fabric was not covered with copper (see FIG. 8B). This not surprising as there was poor nanosilver coverage.

COMPARATIVE EXAMPLE 6

(43) In order to make a sample of conductive fabric the same steps as those previously mentioned (see Example 1) were used. However, the only change made to the process was that the fabric was coated with an aqueous solution of 1% w chitosan (purchased from Sigma Aldrich).

(44) It should be noted that Chitosan does not dissolve in water; therefore an aqueous solution of (1 gram of chitosan in 98 ml of deionised water and 2 ml of acetic acid) was made.

(45) The resultant conductive fabric has lost its stretchablity and the surface was quite grainy and rough. The resistance was equal to the resistance of the fabric of Example 1.

(46) Since the texture of the textile had been altered so much, it was decided to explore using a diluted chitosan solution. However a similar effect was observed when chitosan solution was diluted to 0.1% w.