An electrically conductive ink

Abstract

 An electrically conductive ink, and film-type tracks made therefrom comprise a dendritic metal and carbon in a mixture which affords good electrical conductivity. Such inks and tracks are particularly adaptable for use in heated automotive mirrors.

Description

[0001] The present invention relates to an electrically conductive ink comprising a binder and an electrically conductive constituent, in particular to a conductive polymer, and to a thick film electrically resistive track made from the same.

[0002] Conductive polymer inks are known in the art. Japanese Patent Application No. 60-218689 laid open under No. 62-7495 (Nihon Shashin Insatsu) discloses examples of an electrically conductive ink in which the binder is a resin chosen from a group termed 'heat -resisting engineering plastics' and in which the electrically conductive powder is silver. This publication further dicloses that tests have shown that gold, silver, copper, nickel and carbon are satisfactory as the electrically conductive powder used in the ink and accordingly claims an electrically conductive ink characterized by the fact that the electrically conductive powder is at least one of the powders chosen from the group consisting of gold, silver, copper, nickel and carbon.

[0003] As disclosed by N. Nazarenko and C.N. Lazaridis 'Polymer Thick Film Conductors and Dielectrics for Membrane Switches and Flexible Circuitry' (Publication reprinted from the Proceedings of the 1982 ISHM Symposium), it is known that products having a low resistance can be made from silver-based conductive inks.

[0004] However, in view of the cost of silver, there is a need for conductive inks containing less silver. Figure 1 which is taken from that publication, shows the variation of resistivity with proportion by weight of blends of silver and a second constituent. Curve 2 is for a silver/graphite blend system; Curve 4 is for a blend system containing a silver-based polymeric ink and a non-conductive aluminium based polymeric ink. As can be seen from the Figure, the resistivity of the ink increases with the proportion of the second constituent.

[0005] It is an object of the present invention to provide a low resistivity electrically conductive ink suitable for producing a film-type heating track.

[0006] According to the present invention there is provided an electrically conductive ink comprising a binder and an electrically conductive constituent, wherein the electrically conductive constituent includes a dendritic metal. Thus, by incorporating a dendritic metal into the electrically conductive constituent, an ink of particularly low resistivity, as compared with the prior art is achieved.

[0007] Preferably, the electrically conductive constituent comprises a mixture of dendritic metal and carbon. The inventors have found that an ink of low resitivity can be achieved by a mixture of a dendritic metal and carbon, which is a contradistinction to the teachings of the prior art, as shown in Figure 1.

[0008] Preferably, the dendritic metal consists of nickel. Electrically conductive inks provided in accordance with the present invention are particularly suitable for the manufacture of heating elements for large areas such as door mirrors in motor vehicles. Heating elements so produced compare favourably in both cost and heating effect with prior art heating elements used for such applications.

[0009] An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawings of which:

Figure 1 shows variation in resitivity with relative proportions of constituents in prior art inks;

and figure 2 shows variation in resistivity with relative proportions of nickel and carbon in an ink provided in accordance with the present invention.

[0010] It has been found that a method of manufacturing the electrically conductive which employs a powdered dendritic metal produces an ink with high electrical conductivity as compared to the prior art.

[0011] A dendritic metal is one in which the surface of the metal appears spiky, and in powdered form, the granules are generally of equiaxial shape.

[0012] A method of making an electrically conductive ink provided in accordance with the present invention is described below.

a) A stock solution of the binder is formed by dissolving 20 g of poly (vinyl butyral) granules (Butvar B76) in l00g of N-methyl-2-pyrrolidone. This is mixed using a high speed stirrer for 1 hour and then allowed to settle until the air disperses.

b) 55 ml of carbon black (type 40220) is baked for 1 hour at 125^oC to remove any moisture. This is used with an equal volume of dendritic metal powder, nickel has been found to be particularly suitable for obtaining a high-conductivity ink, untilan homogenous blend is obtained. This forms the electrically conductive constituent.

c) A screen printing medium is produced by combining 50 g of Blythe medium type 63/182 with 5g of unaccelerated unsaturated polyester in styrene (manufactured by Scott Bader).

[0013] 50g of the binder and 100 g of the conductive constituent are added to 55g of the screen printing medium. The mixture is then passed through a triple roll mill until the rheological properties suitable for screen printing known to those skilled in the art are obtained. The resulting ink is screen printed in the desired pattern onto a substrate and cured at a temperature of 80^oC.

[0014] The ink exhibits good adhesion to a variety of types of substrates, including alumina, polyester, glass and painted surfaces. The resistivity of tracks made from this ink was in the range of from 1.5 to 2.0 ohms per square.

Claims

1. An electrically conductive ink comprising a binder and an electrically conductive constituent, wherein the electrically conductive constituent includes a dendritic metal.

2. An electrically conductive ink according to claim 1 wherein the electrically conductive constituent comprises a mixture of a dendritic metal and carbon.

3. An electrically conductive ink according to claim 1 or claim 2 wherein the dendritic metal consists of nickel.

4. An electrically conductive ink according to Claim 3 wherein the electrically conductive constituent includes nickel as a percentage by volume in the range from 30% to 85%.

5. An electrically conductive ink according to Claim 4 wherein the electrically conductive constituent includes nickel as a percentage by volume in the range of from 33% to 80%.

6. An electrically conductive ink according to Claim 5 wherein the electrically conductive constituent includes nickel as a percentage by volume in the range from 36% to 67%.

7. An electrically conductive ink according to claim 6 wherein the electrically conductive constituent constists of nickel and carbon in equal proportions by volume.

8. An electrically conductive ink according to any one of the proceding claims.

9. An electrically conductive ink according to claim 8 wherein the polymer comprises a thermoplastic resin.

10. An electrically conductive ink according to claim 8 wherein the binder comprises polyvinyl butyral.

11. A film-type track for use as a heating element constructed from an electrically conductive ink according to any one of the preceding claims.

Drawing