IMPROVED CONDUCTIVE POLYMER

Abstract

A polymer composition comprises at least one substantially non-conductive polymer binder and at least first and second electrically conductive fillers, wherein the first electrically conductive filler is comprised of particles having a void bearing structure, and the second electrically conductive filler is comprised of particles which are substantially spherical in shape.

Claims

1. A polymer composition comprising at least one substantially non-conductive elastomeric polymer binder and at least first and second electrically conductive fillers, wherein the conductivity of the polymer composition in an unstressed state is related to the conductivity of the elastomeric non-conductive polymer binder and in a stressed state to the conductivity resulting from the presence of the at least first and second electrically conductive fillers in the composition, wherein the first electrically conductive filler is comprised of particles having a void bearing structure, and the second electrically conductive filler is comprised of particles which are substantially spherical in shape.

2. A polymer composition according to claim 1, wherein the second electrically conductive filler is comprised of magnetite.

3. A polymer composition according to claim 2, wherein the magnetite is synthetic magnetite and/or naturally occurring magnetite processed such that the particles are substantially spherical in shape.

4. A polymer composition according to claim 1, wherein the particle size of the magnetite is five micron or less.

5. A polymer composition according to claim 4, wherein the particle size of the magnetite is less than one micron.

6. A polymer composition according to claim 5, wherein the particle size of the magnetite is between 0.1 and 0.3 micron.

7. A polymer composition according to claim 6, wherein the particle size of the magnetite is around 0.2 micron.

8. A polymer composition according to claim 1, wherein the first electrically conductive filler is spiky nickel.

9. A polymer composition according to claim 8, wherein the particle size of the spiky nickel is around 3 micron.

10. A polymer composition according to claim 8, wherein the form of spiky nickel is nickel 123 or nickel 255.

11. A polymer composition according to claim 1, wherein the polymer binder is one of: polyurethane, polyvinyl acetate, polyvinyl chloride, an acrylic polymer, natural rubber and silicone.

12. A force sensor comprising a polymer composition comprising at least one substantially non-conductive elastomeric polymer binder and at least first and second electrically conductive fillers, wherein the conductivity of the polymer composition in an unstressed state is related to the conductivity of the elastomeric non-conductive polymer binder and in a stressed state to the conductivity resulting from the presence of the at least first and second electrically conductive fillers in the composition, wherein the first electrically conductive filler is comprised of particles having a void bearing structure, and the second electrically conductive filler is comprised of particles which are substantially spherical in shape and electrical connections thereto.

13. A force sensor according to claim 12, wherein the second electrically conductive filler is comprised of magnetite.

14. A force sensor according to claim 12, wherein the magnetite is synthetic magnetite and/or naturally occurring magnetite processed such that the particles are substantially spherical in shape.

15. A force sensor according to claim 12, wherein the particle size of the magnetite is five micron or less.

16. A force sensor according to claim 12, wherein the particle size of the magnetite is less than one micron.

17. A force sensor according to claim 16, wherein the particle size of the magnetite is between 0.1 and 0.3 micron.

18. A force sensor according to claim 17, wherein the particle size of the magnetite is around 0.2 micron.

19. A force sensor according to claim 12, wherein the first electrically conductive filler is spiky nickel.

20. A force sensor according to claim 19, wherein the particle size of the spiky nickel is around 3 micron.

21. A force sensor according to claim 19, wherein the form of spiky nickel is nickel 123 or nickel 255.

22. A force sensor according to claim 12, wherein the polymer binder is one of: polyurethane, polyvinyl acetate, polyvinyl chloride, an acrylic polymer, natural rubber and silicone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In the Drawings, which are by way of example:

[0036] FIG. 1 is a graph showing the change in resistance with changing force for each of seven different examples (see section entitled, Examples below) of the material of the invention; and

[0037] FIG. 2 is a schematic representation of the test electrode used to collect the data represented in the graphs of FIG. 1.

DETAILED DESCRIPTION OF ILLUSTRATIVE EXAMPLES

[0038] In each of the Examples Vale Nickel 123 was mixed with Bayferrox 4330 synthetic magnetite in a Witcobond 781 polyurethane binder. The minimum amount of the composite to form a spreadable coating was applied to fine organza net to form a 40 mm40 mm sample having a thickness of 0.16 mm. The test electrode used in the Examples is illustrated in FIG. 2 and comprised an upper electrode 1 in the form of a 5 mm diameter gold plated rod conductor, a lower electrode 2 in the form of a silver plated metal disc 10 mm in diameter and the sample 3. The upper and lower electrodes are brought into contact with the sample 3, and a potential difference was applied through the electrodes 1, 2 to the sample. An increasing force was applied to the sample 3 by the electrodes 1, 2. The applied force was measured. The change in current was measured and the changing resistance calculated

Example 1

[0039] 10 g Nickel 123 were mixed with 10 g synthetic magnetite and 16.0 g of polyurethane binder.

Example 2

[0040] 10 g Nickel 123 were mixed with 6 g synthetic magnetite and 10.0 g of polyurethane binder.

Example 3

[0041] 10 g Nickel 123 were mixed with 5 g synthetic magnetite and 8.5 g of polyurethane binder.

Example 4

[0042] 10 g Nickel 123 were mixed with 4 g synthetic magnetite and 7.0 g of polyurethane binder.

Example 5

[0043] 10 g Nickel 123 were mixed with 2 g synthetic magnetite and 5.5 g of polyurethane binder.

Example 6

[0044] 10 g Nickel 123 were mixed with 0.6 g synthetic magnetite and 4.0 g of polyurethane binder.

Example 7

[0045] 10 g Nickel 123 were mixed with 0.2 g synthetic magnetite and 3.5 g of polyurethane binder.

[0046] Table 1 and the graphs of FIG. 1 illustrate the effect on resistance of changing the force applied to the composites of Examples 1 to 7 when a potential difference of 5 volts is applied thereto.

[0047] As can be seen, Example 7 is most sensitive to changes in applied force, that is the resistance of the material of Example 7 changes the most per unit of force applied.

[0048] For each example where no force is applied the start resistance is substantially infinite, that is the material of each example behaves as an insulator. For the materials of each of the Examples 1 to 7, the response to the application of an increasing force is a significant reduction in resistance.

[0049] The curve for each of the Examples 1 to 7 exhibits a rapid decrease in resistance upon the initial application of force, followed by a more steady decrease in resistance as the applied force continues to increase.

[0050] It is believed that the initial rapid decrease in resistance of the composite material is due to field assisted tunnelling from the spikes of the spiky nickel particles, and that the steady decrease in resistance is due to standard tunnelling and hopping conduction associated with the synthetic magnetite particles. This is borne out by the results which show that where the composite includes the greater proportion of synthetic magnetite the more rapid the change in resistance upon application of a force occurs for a smaller range of resistance, after which the rate of change of resistance for an increasing force decreases.

[0051] In the region of the curve where the relationship between applied force and resistance is not as marked it is believed that percolative conduction predominates, percolative conduction including standard tunnelling conduction and hopping conduction.

[0052] The two distinct characteristics exhibited by the material of the invention make it suitable for different uses. In the part of the curve where resistance changes very rapidly the material may function as a switch, whereas in the part of the curve where the change in resistance is much slower the material may be used for sensing. It is conceivable that the material could be used to provide a switched sensor, that is the material may sense a parameter up to a threshold and when the threshold is exceeded switch an electric circuit on or off.

[0053] It has been found that the noise associated with signals passing through the material of the invention as compared to the material of GB2450587 is much reduced, in both parts of the curve, although noise is greater in part of the region of the curve where change in resistance is more rapid.

Noise and Phases of Conduction

[0054] The materials of the invention all exhibit a similar noise profile which is understood to relate to the conduction mechanism. Each Example exhibits the same three phases of operation and each phase of operation has a distinctive type of conduction.

Phases of Conduction

[0055] Phase 1this phase occurs during the initial contact with the surface of the material. In this phase the contact force is very low, there is very low conduction (hence low current density) and very high resistance. The mode of conduction is understood to be standard tunnelling and hopping conduction.

[0056] Phase 2As the force on the material increases the material operates in the second phase. In phase 2 there is a very rapid, non-linear drop in resistance (and hence an increase in current density) as the pressure increases. In phase 2 the mode of conduction is predominantly field enhanced tunnelling. The increasing force brings the particles sufficiently close together and the current density increasing sufficiently for the spikes of the spiky nickel to exhibit field enhanced tunnelling. The noise level increases as the material switches from phase 1 to phase 2 and continues to increase until a peak is reached.

[0057] Phase 3The pressure on the material is increased still further. The increased pressure causes the conductive particles to come closer together, so close in fact that the mechanism of standard tunnelling and hopping conduction once again predominates. Noise levels fall dramatically as the mode of conduction changes from field enhanced tunnelling to standard tunnelling and hopping conduction.

[0058] The noise profile for Examples 1 and 7 was assessed by viewing a trace on an oscilloscope that was connected to the test electrode as the force on the sample was increased.

[0059] For both Examples 1 and 7, phase 1 conduction commenced above 10.sup.9 ohms.

[0060] For Example 1, the change over from conduction phase 1 to conduction phase 2 occurs at about 10.sup.9 ohms. The change over from phase 2 conduction to phase 3 conduction occurs at about 10.sup.8 ohms.

[0061] For Example 7, the change over from conduction phase 1 to conduction phase 2 occurs at about 10.sup.3 ohms. The change over from phase 2 conduction to phase 3 conduction occurs at about 10.sup.1 ohms.