GB2338713A - Electrically conductive polymeric compositions - Google Patents

Abstract

There is provided a composition, suitable for use as an electrical switching material, which comprises a polymerisable monomer and a conductive particulate material. The particulate material, which may be nickel, may have an average particle size of at least 70Ám, may be prepared by gas atomisation, and may comprise substantially spherical particles. The polymer is a silicone. A switching sensor can be formed by depositing electrically conductive electrodes 12,14 on an insulative backing 10, and then coating the backing 10 and electrodes 12,14 with the polymer composition. When the resulting device is subjected to stresses, the resistance of the composition changes sharply, providing a switching effect.

Description

2338713 A COMPOSITION The present invention relates to a composition.

Specifically, the present invention relates to a composition comprising a polymerisable monomer and a conductive particulate material. The present invention also relates to a switching sensor, such as a pressure sensor or a temperature sensor, including such a composition.

The preparation of compositions and devices comprising the addition of a conducting material to a flexible polymer is known from the prior art. For example, US-A-3719913 describes a viscous strain gauge comprising such a system. The viscous strain gauge may exhibit a change in resistance during conditions of high elongation.

EP-Al-0443073 describes a diaphragm type pressure sensor which comprises a conducting rubber member.

The systems described above rely on a change in resistance to provide some form of calibrated output which is proportional to the applied stress.

US-A-4028276 describes a pressure sensitive composition which may be used in an elastic resistor. The resistor may be used in, for example, keyboard switches or vehicle crash sensors. However, as appreciated by USA-4028276 the resistors thereof do not exhibit a reliable performance over their lifetime. As discussed on column 9, line 34 of US-A-4028276 "devices made with piezoresistive compositions of [US-A-4028276] will, after repeated use, show a tendency to require a higher pressure to attain a given low resistance, or they will show a higher resistance at a given pressure".

The prior art is problematic in that compositions are not provided which may be used in switches, for example in automated computer based control systems, and which also satisfy the demands of industries, such as the automotive and

2 aeronautical industries, which require a composition which can perform a large number of operations whilst maintaining reliable performance over its lifetime.

The present invention seeks to overcome the problems of the prior art.

Accordina to a first aspect of the present invention there is provided a 0 composition comprising a polymerisable monomer and a conductive particulate material, wherein the particulate material has an average particle size of at least 70 gm.

The present invention is advantageous in that the composition exhibits a sharp switch between an on state and an off state. Furthermore, the composition is advantageous in that it exhibits consistent resistance changes after a high number of operations.

By "a sharp switch between an on state and an off state" is meant the maximum resistance/pressure gradient (RPG) as defined by RPG= resis tan cel - resis tan ce2 1 pressurel. - pressure2 1 wherein resistivity 1 is the resistivity at pressure 1 and resistivity 2 is the resistivity at pressure 2, has a magnitude of at least 1 X 107 L2 kg,-1cm'_. More 8 2 preferably, the RPG has a magnitude of at least 1 x 10!l kg-1 CM. Yet more preferably, the RPG has a magnitude of at least 1 x 109 Q kg- 1CM 2 The polymerisable material may be selected to ensure that the matrix thereof is chemically and/or electrically stable. Moreover, the polymerisable material may be selected to ensure that the matrix thereof does not degrade over time, particularly when subjected to repeated application of forces and/or to aggressive environments.

1 j Preferably, the polymerisable monomer is selected from any cross linking material capable of forming an elastomeric polymer, including HTV silicone rubber base and RTV silicone rubber base, such as Silastic 3112 c RTV (Dow Conlin.-), Silastic 9161 @ RTV (Dow Corning), RTV21 (GE Silicones), mixtures and derivatives thereof.

Preferably, the composition does not contain and/or require the presence of any corrosive or potentially corrosive compounds, e.g. organic acids, acetic acid, 1 ammonia or similar compounds. Preferably, no corrosive or potentially corrosive compounds are formed during the polymerisation of the monomer or during the treatment thereof.

The conductive particulate material may be chosen to have good corrosion resistance and/or which is free from surface oxidation and/or which is resistant to mechanical damage and/or is available in powder form.

The applicant has identified that compression damage to a conductive particulate material reduces the operational life of a variable resistance composition. The composition of the present invention is advantageous in that by selecting the surface morphology of the conductive particulate material the consistency and/or repeatability of measurements of the composition is increased.

Preferably, the conductive particulate material is selected from metals, silicon, carbon including carbon black and graphite, mixtures, alloys and derivatives thereof, in particulate form. More preferably, the conductive particulate material is selected from the group of metals comprising aluminium, arsenic, beryllium, bismuth, cadmium, chromium, cobalt, copper, gold, hafnium, indium, iridium, iron, lead, magnesium, manganese, molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium, samarium, silver, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, mixtures and derivatives thereof., in particulate form. Yet more preferably, the conductive particulate material is nickel in particulate form.

4 Preferably, the conductive particulate material is a metal prepared by gas atomisation.

Accordina, to a second aspect of the present invention there is provided a composition comprising a polymerisable monomer and a conductive particulate material, wherein the particulate material is prepared by gas atomisation.

Metallic particles have been prepared according to the prior art by a number of methods. Typically, the water atomised carbonyl process is used. This process provides irregular shaped particles. In particular, this process provides elongate shaped particles.

The provision of a conductive particulate material prepared by gas atomisation advantageous in that the resistivity of the composition and the change of resistivity on application of a force to the composition remains consistent over a large number of applications of force i.e. operations. For example, the resistivity and change thereof may remain consistent for at least 10,000 operations. In further preferred examples, the resistivity and change thereof may remain consistent for at least 20, 000 operations, more preferably at least 30,000 operations. yet more preferably at least 50,000 operations, yet more preferably at least 60, 000 operations Without being bound by theory it is believed that this advantage is achieved because irregular shaped particles of conductive material may become damaged after a relatively low number of operations. For example, irregular shaped particles of conductive material may become damaged after a few hundred operations Furthermore, irregular shaped particles of conductive material may also damage the matrix of the polymerised monomer after a relatively small number of operations, for example after a few hundred operations.

Preferably, the conductive particulate material comprises substantially spherical particles.

Accordina to a third aspect of the present invention there is provided a W composition comprising a polymerisable monomer and a conductive particulate material, wherein the particulate material comprises substantially spherical particles.

Preferably, the conductive partiCUlate material comprises no greater than 60 volume % of the composition; more preferably, no greater than 50 volume % of the composition; more preferably, no greater than 40 volume % of the composition; yet more preferably, no greater than 30 volume % of the composition.

When the conductive particulate material is nickel, the nickel may comprise no greater than 90 wt% of the composition, or may comprise no greater than 80 wt% of the composition.

If the conductive particulate material contains greater than 60 volume % and/or 90 wt% there is a tendency for the composition to granulate. Such granulation prevents the mixing of the composition. Furthermore, in a preferred embodiment of the present invention the composition is introduced into a mould and polymerised to form a solid block. If a granulated composition is treated in this manner, the solid block may crumble.

Preferably, the conductive particulate material is substantially homogeneously dispersed in the composition of the present invention.

Advantageously, a substantially homogenous composition as described above exhibits throughout the composition a substantially consistent resistivity and 1 change thereof on application of a force. Moreover, in a homogenous composition as described above the change of resistance of the composition on 6 application of a slight force is less than in compositions containing an uneven distribution of conductive material particles. Compositions containing an uneven distribution of particles cannot be used in sensor applications because of their over-sensitivity.

Preferably, homogeneity may be achieved by, for example, control of the Z viscosity of the composition and/or control of the introduction of the conductive particulate material to the composition.

Preferably, the particulate material has an average particle size of at least 10Ogm, More preferably, the particulate material has an average particle size of at least 120pim. More preferably, the particulate material has an average particle size of from 120 to 170tm.

Preferably, the composition embodying the present invention further comprises an additional component selected from dispersants, catalysts, fillers, corrosion inhibitors, antioxidants, mixtures and derivatives thereof.

Preferably, the composition further comprises a charge dispersant. The term "charge dispersant" means a material which allows charge within the composition to be evenly dispersed throughout. In other words the term "charge dispersant" means a material which prevents the build up of static charge in the composition. Such a charge may result from repeated application of forces to the composition.

Preferably, the charge dispersant is present in the composition in an amount of no greater than 2 wt%. Preferably, the charge dispersant is present in an amount of Z no greater than 1 wt%. More preferably, the charge dispersant is present in an amount of no greater than 0.5 wt%. Still more preferably, the charge dispersant C1 is present in an amount of from 0.2 to 0.5 wt%.

Preferably, the composition of the present invention is polymerised.

7 Preferably, the polymerised monomer is an elastomer.

Preferably, the polymerisation is initiated by addition of a catalyst and/or by an increase in temperature.

Preferably, the catalyst is an organic, inorganic or organometallic compound selected from dibutyltin dilaurate, zirconium silicate, ethyl silicate, stannous octoate (tin octoate), tetraethylorthosilicate, mixtures and derivatives thereof.

The catalyst may be in the form of a liquid, a paste, or a solution, preferably in an organic solvent.

The amount of catalyst may be determined so as to control the rate of polymerisation and therefore to control the rate of change of viscosity of the composition.

The temperature may be controlled so as to control the rate of polymerisation and therefore to control the rate of change of viscosity of the composition.

Preferably, the composition may comprise a filler selected from silica.

Fillers may be incorporated to increase the strength of the polymerised composition. Particularly, fillers may be incorporated to increase the tear resistance of the polymerised composition and/or to increase the viscosity of the composition.

Preferably, the composition of the present invention is incorporated into a switching sensor.

According to a fourth aspect of the present invention there is provided a switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer; and ii) a conductive 8 particulate material; wherein the particulate material has an average particle size of at least 70im.

According to a fifth aspect of the present invention there is provided a switching z: sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer; and ii) a conductive particulate material; wherein the particulate material comprises substantially spherical particles.

According to a sixth aspect of the present invention there is provided a switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer; and ii) a conductive particulate material; wherein the particulate material is prepared by gas atomisation.

According to a seventh aspect of the present invention there is provided a switching sensor comprising at least two discrete conductive electrodes each of 1 which is in contact with a composition which comprises a polymer and a conductive particulate material, the electrodes being disposed on an electrically insulative backing.

In one embodiment of this aspect of the invention, the polymerised composition covers the insulative backing (which may be a polyimide backing) and the electrodes. The electrodes are preferably disposed in a serpentine configuration, such as a mutually interdigitated configuration.

The preferred features of the composition of the present invention as described above equally apply to the composition in the sensor embodying the present invention.

Preferably, the conductive electrodes are formed from a metal. Preferably, the metal is selected from copper, silver, gold, mixtures and alloys thereof. More 9 preferably, the metal is copper. Yet more preferably, the conductive electrodes are formed by electrodeposition of copper.

The applicant has identified that the structure of the electrode material is important in the development of the electrode/composition bond and therefore in the operation of a sensor employing such a composition. The applicant has found that sensors comprising conductive electrodes formed by electrodeposition (ED) of a metal, bond very strongly to composition.

Without being bound by theory it is believed that the surface of an electrodeposited metal onto which the composition may be applied is slightly rough. The main characteristic of ED metal is its grain structure which is vertical because of the electrolytic process. This means the metal is less ductile than other forms of metal, such as rolled annealed (RA) metal. However, the combination of the vertical grain structure and the rough surface makes it possible for a strong bond to be formed between ED metal and the composition of the present invention. This bond may be enhanced by using a suitable coupling agent.

In a preferred embodiment the electrodes are mounted on a non-conducting substrate. Preferably, the substrate is selected from electrically insulating materials including polyimide, Kaptoii (Du Pont de Numours), polyester, Mylar15 (Du Pont de Numours), Teflon, random fibre aramid, NomexO (Du Pont de Numours), epoxy/glass, epoxy and all woven glass laminate, mixtures and derivatives thereof. More preferably, the substrate is selected from polyimide, epoxy and all woven glass laminate.

Preferably, the substrate has thickness of from 0.01 to 10 nun. More preferably, the substrate has thickness of from 0.03 to 5 mm. More preferably, the substrate has thickness of from 0.1 to 1 mm. Yet more preferably, the substrate has thickness of approximately 0.075 mm.

The electrodes may be configured such that the composition is sandwiched between the electrodes. In another aspect the electrodes may be configured such that the electrodes form a plane with a track of the composition running between the electrodes. This latter aspect may be obtained by providing the electrodes with a gap therebetween and applying the composition on at least one surface thereof thereby allowing the composition to fill the gap.

Preferably, the electrodes are spaced apart from each other at a distance of from 0.01 to 100 min. More preferably, the electrodes are spaced apart from each other at a distance of from 0. 1 to 10 mm. Yet more preferably, the electrodes are spaced apart from each other at a distance of from 0.5 to 2 mm.

The electrode and track structure is selected to have a geometry which allows optimum sensitivity to be achieved for a change of resistance against loading. The electrode structure may consist of overlapping discs, inter-digitated electrode structures, parallel electrodes, spirals, petals or similar structures. The electrode structure should be arranged to provide a sensing circuit with a required impedance, and result in a switching operation which produces an appropriate resistivity or voltage change.

Preferably, the device embodying the present invention further comprises a coupling agent disposed between at least one of the electrodes and the composition. Preferably, the coupling agent is selected from 3arninopropyl triethoxysilane, 3-mercaptopropyl trimethoxysilane, Dow CorningO 1200 primer, Dow Corning'-'I 1201 primer, SS4004 silicone primer (GE Silicones).

In this re-ard, according to an eighth aspect of the present invention there is provided a method of preparing a sensor comprising at least two discrete electrodes. the method comprising the steps of (i) providing an electrode material; (ii) contacting the electrode material with a coupling agent; and (iii) contactina a sensor material with at least a portion of the electrode material treated with the coupling agent.

11 In the above aspect, the sensor material is preferably a composition in accordance with the present invention.

Preferably, the sensor is mounted on material to which a force may be applied, such as on a vehicle body panel.

The switching sensor of the present invention may be used to sense changes in 1 forces, in particular to sense pressure. The switching sensor of the present invention may also be used to sense temperature. In particular, the device of the present invention may be used to sense change in temperature and/or absolute temperature.

Without being bound by theory it is believed that after polymerisation the particles of the conductive particulate material of the present invention are spaced within the polymer such that when a force is applied to the polymer -the conductive particles contact each other. A conductive path may then be formed from a point on one discrete electrode to a point on another discrete electrode, or part' of the way. Thus, the resistance between those two points drops, preferably considerably. The electrodes may be linked to a suitable circuit and the change in resistance may be identified. Preferably, a signal of the change may be provided, or another circuit activated, for example to deploy a vehicle air bag when a crash sensor is activated. In an alternative scenario, the change of resistance may be recorded.

The sensor should be formed or mounted in such a way that the forces which are to be measured may be easily applied to the sensor. For example, one may wish to measure compression, extension or torsional forces. Compression may be readily measured with a sensor comprising parallel electrodes. Extension may be readily measured with a sensor comprising spiral electrodes. Torsion may be readily measured with a sensor comprising inter-digitated electrodes.

12 Turning to the temperature sensor embodying the present invention, without being bound by theory, it is believed, as described above, that after polymerisation the particles of the conductive particulate material are spaced within the polymer. Thus, when the temperature of the polymer changes the conductive particles within the polymer may expand or contract and contact or separate from each other, respectively. A conductive path from a point on one discrete electrode to a point on another discrete electrode, or part of the way, may then be formed or broken. Thus, the resistance between those two points drops or rises, preferably considerably. The electrodes may be linked to a suitable circuit and the change in resistance may be identified.

The temperature at which the resistance switches as described above is known as the Curie point. The present invention is advantageous in that the change of resistance at the Curle point is sufficiently sharp to allow for tile device to operate as a temperature sensor.

Typically, the Curie point of the sensor of the present invention ranges from -400C to 1100C. The Curie point of the sensor may be varied by a number of ways. In particular, the Curie point may be varied by:

i) selecting the amounts and properties of the constituents of the composition. For example, the particle size and/or distribution of particle size of the conductive particulate matter may be varied. Alternatively, the concentration and/or distribution of the conductive particulate matter may be varied; this variation will effect the mean distance between particles and thus effect the amount of contraction or expansion required to make or break a conductive path.

ii) applying pressure to the sensor. This pressure may be fixed, for example by mounting the sensor in a compressed state in block of material, or may be variable. for example by mounting the sensor between two movable surfaces. By applying pressure to the sensor the distance between the particles of the conductive particulate matter may be varied and accordingly the degree of contraction or expansion required to form or break a conductive pathway will be modified.

13 The invention will now be described, by way of example only, with reference to the accompanying drawings in which:- Figure 1 is a graph of the resistance of a composition against cycles of application of a force; Figure 2 is a graph of the resistance of a composition in accordance with an embodiment of the present invention against cycles of application of a force; Figure 3 is a graph of the resistance of a composition in accordance with an embodiment of the present invention against cycles of application of a force; Figure 4 is a graph of the resistance of a composition in accordance with an embodiment of the present invention against applied pressure; Figure 5 is a graph of the resistance of a composition in accordance with an embodiment of the present invention against temperature; and Figure 6 is a plan view of a sensor according to an embodiment of the present invention.

In the development of pressure sensitive elastomeric conducting materials C1 suitable for use in electrical switching applications, for example when mounted on a flexible or rigid conducting substrate to form a switching sensor, the following requirements were set as provisional parameters for a practical device:

(I) a ratio between the minimum, operating resistance and "on" resistance of at least 103A, for example a minimum operating resistance range of 106 ohms, with an "on" resistance below 100 ohms; (11) ability to perform over a wide temperature range; 14 (III) ability to perform over a wide humidity range; (IV) consistent, stable operation for more than 60,000 operations over full environmental specification.

A wide variety of systems have been considered for the polymer matrix, the high temperature characteristics of RTV and HTV silicone polymers offering good prospects. In selecting an appropriate system, it is important that the matrix is chemically and electrically stable, and not likely to degrade with time. It is also important that the composition or polymerised composition contains no corrosive substances, in particular by-products from the polymerisation process. For this reason, acetic acid cured systems have not been a first choice, although it is possible that acceptable results could be obtained. Initially, activities were restricted to systems without complex additives such as the lubricants and charge dispersants mentioned in earlier references, which could give rise to variable electrochemical effects.

In the initial experiments, a number of failure mechanisms were identified, mechanical damage to the matrix being one of these. The relationship between the hardness of the cured system, the sensor operating force, the sensor thickness and the physical properties of the conductive particulate material determines the likelihood of permanent damage resulting in premature failure. For example, given a sensor thickness of lmm, and an applied force of 4 to 6 Bar, a suitable system has been found to be Dow Coming Silastic 3112, having a Shore A hardness of 60.

As to the conducting medium, the main requirements of a practical system are:

(1) low electrical resistivity; (11) -ood corrosion resistance and freedom from surface oxidation; (Ill) resistance to mechanical damage; (IV) availability in powder form; and (V) substantially spherical surface morphology.

A wide range of metallic elements, compounds and alloys are potentially suitable, but as an example of a convenient, readily available, cost effective material, nickel is presently preferred as largely satisfying the above requirements.

During experimentation using nickel, it has become clear that compression damage to the conducting particles is a further potential failure mechanism, causing spurious resistance characteristics and premature failure of the sensor.

Metallic powders are available in a variety of forms, the most common being those produced by the water atomised carbonyl process which can provide irregular shaped or elongate forms of powder. A less common form is produced by the gas atomisation process, producing regular spherical shaped particles.

During compression of compositions containing different forms of nickel, it has become clear that damage to the irregular shaped particles occurs typically after a few hundred operations resulting in spurious resistance readings, without subsequent recovery. An example of this is shown in Figure 1, which shows a graph of resistance of the composition against cycles of application of a force. As can be seen from Figure 1, initial readings of below 50 ohms quickly deteriorate, after less than 250 operations, to variable results including some of a few thousand ohms which would not be acceptable in a practical system. The results shown in Figure 1 were obtained by repeated actuations of the sample at 10 kg /CM2.

By comparison, results using spherical powder in the range 75 to 180 microns have shown a considerable improvement in life performance, with stable 16 resistance at constant pressure above 8,000 operations, as shown in Figure 2 and above 60,000 operations, as shown in Figure 3, which show graphs of the resistance of a spherical nickel/rubber composition against cycles of application of a force.

An important requirement of a switching sensor is that there should be a well defined change of state from "on" to "off", it being considered that a change of 10 6 ohms is more than adequate. Figure 4 shows a graph of a typical operation curve for the material tested in Figure 2. The graph of Figure 4 shows the resistance of the composition against applied pressure. It will be seen that the composition provides a steep and distinctive change of resistance., from about 4x 107 ohms to almost zero ohms.

The temperature ranee for satisfactory switching performance is mainly limited by the PTC (positive temperature coefficient of resistance) effect. Temperature Resistance curves were plotted for both unloaded and compressed specimens, the Curie point changing from AWC to HO'C, confirming a practical operating range of > 1OWC.

In view of the fact that the Curie point change in resistance of the composition is significantly and predictably sharp, the device may be adapted to operate as a temperature sensor, such as a thermostat, adjustable over a range of some 1OWC by the application of pressure. Figure 5 shows an example of an uncompressed temperature / resistance curve. It should be noted that, with application of pressure, theswitching point would be shifted from the position shown for the uncompressed state. Thus, if the device is to be used as a temperature sensor, some means needs to be provided for applying pressure to the device, and if this is made adjustable such that a range of pressures may be applied, the resulting temperature sensor would be operable over a range of temperatures.

In any practical form of switching (and/or temperature) sensor, it is likely that the 1 desiRn of the electrode system will need to be tailored to each individual application. Nevertheless, the basic requirements will remain largely similar. Thus, the electrode configuration should be such as to provide maximum sensitivity to the applied stress, while preferably forming an integral part of the 1 sensor, such that minimal degradation of the connection occurs during mechanical stress or exposure to the environment, throughout the expected lifetime of the Z> sensor.

An example of a flexible contact system, which when coated with polymer composite will respond to pressure, flex and twist stresses, is shown in Figure 6. This particular design is formed from a 0.05mm polyimide backing 10, with a conductine track of 0.035mm copper overlay. The conducting track forms a first electrode 12 and a second electrode 14, which are preferably disposed in a manner to minimise the contact spacing therebetween while maximising the potential inter-contact area. This can be achieved by disposing the electrodes 12,14 in a serpentine configuration such as in the interdigitated layout shown in Figure 6. The electrodes 12,14 terminate at an edge of the sensor in respective contact portions 16,18 for connection to wiring or the like. When the backing 10 and electrodes 12,14 have been coated with the polymer composite, then a current path will exist between the electrodes via the polymer composite, and the resistance of the current path will depend on the stresses applied to the device. If the characteristics of the resistance change are similar to those shown in Figure 4, the resulting. device will be suitable for use as a switching sensor.

Another important requirement in the development of a practical sensor system is achieving a strong hermetic chemical bond which will not degrade over time, nor in use. In the preferred arrangement, this has been achieved by the use of a Dow Corninas 1200 coupling agent used as a primer on the copper surface prior to the laying down of the polymer composite. Tests have shown that the resulting bond C1 15 will survive 1,000 hours immersion in water without any sign of contact degradation.

Modifications of the present invention will be apparent to those skilled in the art.

18

Claims (29)

1. A composition comprising i) a polymerisable monomer; and ii) a conductive particulate material; wherein the particulate material has an average particle size of at least 70pim.

2. A composition comprising i) a polymerisable monomer; and ii) a conductive particulate material; wherein the particulate material comprises substantially spherical particles.

3. A composition comprising i) a polymerisable monomer; and ii) a conductive particulate material; wherein the particulate material is prepared by gas atomisation.

4. A polymer comprising a composition according to any one of claims 1,2 or 3 when polymerised.

5. A switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer; and ii) a conductive particulate material; wherein the particulate, material has an average particle size of at least 70Lm.

6. A switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer., and 19 ii) a conductive particulate material; wherein the particulate material comprises substantially spherical particles.

7. A switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition comprising i) a polymer; and ii) a conductive particulate material; wherein the particulate material is prepared by gas atomisation.

8. A switching sensor according to claim 5, 6 or 7, wherein the composition is in the form of a sheet.

9. A switching sensor according to claim 8, wherein the sheet has a thickness of from 0.2 to 10 mm.

10. A composition, polymer or sensor according to any one of the preceding claims, wherein the conductive particulate material has an average particle size of greater than 100im.

11. A composition, polymer or sensor according to claim 10, wherein the conductive particulate material has an average particle size of greater than 120im.

12. A composition, polymer or sensor according to claim 11, wherein the conductive particulate material has an average particle size of from 120 to 170gm.

13. A composition, polymer or sensor according to any one of the preceding claims wherein the conductive particulate material is selected the group of metals comprising aluminium, arsenic, beryllium, bismuth, cadmium, chromium, cobalt, CI copper, gold, hafnium, indium, iridium, iron, lead, magnesium, manganese, 1 molybdenum, nickel, osmium, palladium, platinum, rhenium, rhodium, ruthenium, samarium, silver, tantalum, tin, titanium, tungsten, vanadium, zinc, zirconium, mixtures and derivatives thereof.

14. A composition, polymer or sensor according to claim 13, wherein the conductive particulate material is nickel.

15. A composition or sensor according to claim 1 or claim 4, wherein the conductive particulate material comprises substantially spherical particles.

16. A composition or sensor according to claim 1, 2, 5 or 6, wherein the particles in the conductive particulate material are prepared by gas atomisation.

17. A composition or sensor according to any one of the preceding claims wherein the polymerisable monomer is selected from RTV silicone rubber base, HTV silicone rubber base, Silastic 3112'5 RTV (Dow Corning), Silastic 91618 RTV (Dow Corning), RTV21 (GE Silicones), mixtures and derivatives thereof.

18. A composition or sensor according to any one of the preceding claims wherein the conductive particulate material comprises no greater than 60 volume% of the composition.

19. A composition or sensor according to any one of the preceding claims wherein the composition further comprises an additional component selected from dispersants, catalysts, fillers, corrosion inhibitors, antioxidants, mixtures and derivatives thereof.

20. A switching sensor according to any one of claims 5 to 19, wherein the electrodes are disposed on an electrically insulative backing.

21. A switching sensor comprising at least two discrete conductive electrodes each of which is in contact with a composition which comprises a polymer and a 21 conductive particulate material, the electrodes being disposed on an electrically insulative backing.

22. A switching sensor according to claim 20 or 21, wherein the insulative backing is a polyimide backing.

23. A switching sensor according to claim 20, 21 or 22, wherein the composition covers the insulative backing and the electrodes.

24. A switching sensor according to any one of claims 5 to 23, wherein the electrodes are disposed in a serpentine configuration.

25. A switching sensor according to claim 24, wherein the electrodes are mutually interdigitated.

26. A switching sensor according to any one of claims 5 to 25, when configured as a pressure or stress sensor.

27. A switching sensor according to any one of claims 5 to 25, when configured as a temperature sensor.

28. A method of preparing a sensor comprising at least two discrete electrodes, the method comprising the steps of (i) providing an electrode material; (ii) contactincr the electrode material with a coupling t, agent; and (iii) contacting a sensor material with at least a portion of the electrode material treated with the coupling agent.

29. A composition, sensor, or method of preparing a sensor, substantially as herein described with reference to any one or more of Figures 2 to 6 of the accompanying drawings.