VARIABLE CONDUCTANCE MATERIALS This invention relates to variable conductance materials and in particular to composites comprising an electrically conductive filler material dispersed in a carrier of a plastics material. The invention is particularly concerned with such materials that exhibit a change in electrical conductance when stressed, e. g. electrically or mechanically, for example by pressing, stretching or twisting.

In our WO 98 33193 and WO 99 38173 we describe such materials in the form of rubber-like sheets or granules.

These materials comprised a particulate conductive metal or metal oxide, particularly in the form of small particles having a"spiky"or dendritic shape, dispersed in for example a silicone elastomer. These compositions were made by mixing the particles with an uncured carrier material under low shear conditions followed by heating as necessary to effect curing of the elastomer. However, such materials, once cured, are generally not capable of being further shaped or moulded, i. e. deformed into a shape which is retained when not subjected to mechanical stress.

We have realised that it would be desirable to provide such materials which can be so moulded, e. g. by melt extrusion, compression moulding and the like.

To this end, the use of a carrier of a thermoplastics material is required. However we have found that the compounding and subsequent shaping techniques, such as melt extrusion, required for processing such materials can result in the application of such an amount of shearing that the variable electrical conductance properties are significantly diminished.

We have now found that compositions based on thermoplastic carriers can be made that exhibit the desired variable conductance properties if the electrically conductive filler material has a relatively high bulk density.

Accordingly the present invention-provides a variable conductance material that exhibits a first level of electrical conductance when quiescent and a higher level of conductance when subjected to change of stress applied by stretching or compression or electric field, comprising a thermoplastic carrier composition having dispersed therein finely divided particles of at least one electrically conductive metal or metal oxide filler having a tapped bulk density that is between 25% and 50%, preferably 30 to 40%, of its solid density.

The problems become particularly serious at high filler contents, especially where the composition contains at least 80%, particularly above 83%, by weight of the filler. Thus we have found that as the loading of

filler increases, the amount of energy required to achieve adequate mixing with the thermoplastic carrier composition becomes such that significant breakage of the lower bulk density fillers occurs. As a result there is a deterioration in the electrical conductance properties of the product, such that a greater degree of compression is required to effect increase in conductance to a desired level. Since processing of thermoplastic materials to form a suitable carrier requires more energy input than when producing materials such as filled elastomers, e. g. which can be produced by mixing a liquid resin with the filler followed by curing the elastomer, the problems of filler particle breakage when producing materials based upon thermoplastic carriers are significantly greater with thermoplastic carriers than with compositions made using a curable liquid elastomer.

The thermoplastic carrier component comprises a thermoplastic polymeric material and may be a blend of two or more polymeric materials, at least one of which is thermoplastic, for example a thermoplastic polymer and a thermosetting polymer, which may be an elastomer.

Specific carrier components include polyolefins (preferably branched such as low density polyethylene); polyurethanes, plastic siloxanes and polymer blends having elastomeric properties, for example thermoplastic elastomers such as blends of polypropylene and ethylene- propylene-diene copolymers. Water soluble thermoplastic

materials such as polyvinyl alcohol may be used. The carrier material is preferably resilient. The thermoplastic carrier component may be resilient enough to recover fully alone or a resilient operating member such as a spring may be provided to assist recovery. For reversing mechanical stress the variable conductance material may be fabricated into a form, such as a foam or woven or non-woven textile, having interstices. If the interstices form an open cell network, a mobile fluid, e. g. air may be forced into the interstices from an external source to assist recovery. If the interstices are in a closed cell form, recovery may result from gas, e. g. air trapped in the cells during their formation, expanding by application of heat and/or expanding from a compressed state occasioned by the application of mechanical compression forces applied to the variable conductance material.

The composition may contain additives commonly used in plastics technology, such as other fillers, pigments, foaming agents and plasticisers, and modifiers which accelerate the return of the composition to its quiescent shape.

The conductive filler has a secondary structure, including protuberances such as spikes and inter-particle filaments and dendrites. These give rise to voids in the starting filler powder. During compounding, these voids

become infilled with the carrier material and become set in close proximity in the composition. Thus during compounding with the carrier material, the latter forms a coating on the secondary structure: when stressed, particles come into such proximity that electron quantum tunneling conduction can take place through the coating on the secondary structure between adjacent particles even though there is no contact between these particles.

The filler particles generally may have two types of secondary structure, namely: (a) filamentary and/or dendritic, preventing close packing of particles and lowering bulk density; and (b) spiky surface on the particles (which may also be linked together in filaments or dendrites).

Type (a) structure leads to relatively long polymer paths between the filler particles, such that only very high stress can set up electron flow and thus conductance. Type (b) structures consist of the spikes and the spaces between them; the spikes provide much of the conductance because the distance between the spikes of one particle and those of its neighbour are relatively short and the electrical intensity at their points is high. The type (b) structure is substantially maintained during the mixing of carrier with filler powder. It may

include some dendritic and/or filamentary texture, which permits participation in the'small-between-large' interaction, though less easily visualised than when particles are spherical. Powders which have largely only type (a) structure are generally less suitable as the filler since this type (a) structure tends to be destroyed during compounding and processing of thermoplastic materials, e. g. by melt extrusion or moulding. For this reason, suitable filler materials are those having a bulk density more than 25% of the solid density. Lower density materials tend to have too great a proportion of the type (a) structure.

The conductive filler is in the form of a finely divided powder. Preferably substantially all the particles have a size in the range 1 to 100 um.

The volume ratio of filler powder to polymer in the composition preferably permits formation of a continuous coating on the conductor particles. Typically it is at least 0.05 : 1, especially at least 0.1 : 1 and may be much higher, for example 10: 1 or more. Above a certain ratio the composition tends to form granules rather than a continuous matrix. The composition may contain a plurality of polymer components, at an aggregate content at least sufficient to give the stress-sensitive conductance.

The composition preferably exhibits conductance

increasing as a function of the magnitude of the applied stress. Especially it exhibits conductance by electrons passing through the thin polymer coating on the conductive filler particles and separating each from its neighbour. However not all the conductance need be through the polymer coating: for example metal powder substantially free of voids or unwetted by the polymer, and thus affording percolative conductance, may be present and may constitute part of either family of particles, preferably smaller ones. The composition may also contain weakly conductive filler materials such as finely divided carbon. This enables the composition to have some conductance even when unstressed.

The conductive filler may be a conductive metal, or mixture of metals, selected from titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminium, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, antimony, bismuth, scandium and metals of the lanthanide and actinide series. Alternatively the conductive filler may be a conductive reduced oxide of one or more of the above listed metals, especially titanium.

The conductive filler may be present as a powder or as a coating on a suitable carrier material itself

having a powder, grain, fibrous or other shaped form. The conductive filler as introduced may carry a surface layer, e. g. of oxide, provided it does not interfere in the preparation of the composition.

Preferred conductive fillers are not more oxidisable than iron. The most preferred filler is particulate nickel and/or copper, e. g. dendritic copper.

In the case of nickel particles (solid density 8.9 g/cm3), the filler should have a tapped bulk density above about 2.2 g/cm3. A particularly suitable particulate nickel is"Nickel Powder type 123"which is available from INCO Speciality Powder Products, London GB-SW1H OXB. The manufacturer's literature indicates that this material has an equiaxial, spiked irregular surface, a BET surface area typically about 0.4 m2/g and a fine particle size with over about 80% of the particles having a size in the range range 4.7 to 53 microns (Microtrac). Although the manufacturer's literature indicates that the material has a bulk density of 1.6-2. 6 g/cm3 that may not be a tapped bulk density measurement (where the container in which the bulk density is determined is tapped to ensure settling of the powder).

Measurement of the tapped bulk density of a sample of Nickel 123 powder gave a value of 3.16 g/cm3, i. e. about 35% of the solid density of nickel.

The compositions may be made by mixing the conductive filler powder with the carrier material in powder form and then melting and shaping the mixture, e. g. by extrusion. Such shaping typically comprises processing at above the glass-transition temperature, possibly at a sintering temperature or, preferably, above the melting temperature point, of the carrier material. The resulting shape may be already usable, but for greater uniformity it is preferred to extrude the composition followed by comminution and then using the comminuted product as a feed for hot-shaping, e. g. by extrusion, blow moulding, compression moulding or injection moulding. A melt processing process is preferred. These techniques give a simple route to repetition manufacture of stress-responsive electrically conductive articles.

The compounding and any subsequent processing is preferably operated under low shear conditions to minimise destruction of the secondary structure of the conductive filler particles. It has been found that a carefully controlled, screw extruding machine is particularly suitable for extrusion processing on a continuous basis with inherent low-shear mixing. While single screw machines have been used for polymer compounding and forming for many years, twin screw machines have been produced which have very fine control on the operating functions. Some versions of these machines have a co-rotating screw mixer and a series of

separate feed channels and temperature controlled regions along the length of the mixing barrel. An example is ThermoPrismTX range of machines.

These machines accept the basic ingredients of the mixture as separate inputs and mix them together in a controlled regime of force and temperature and under vacuum if required. The mixing is a continuous process and the resultant mix can be passed straight from the end of the mixing screws to a forming or comminuting process.

Suitable input hoppers and handling devices allow the inputs of powders, granules, liquids and gums to the mixer as the start components.

A second preferred process comprises shaping of the composition in contact with a support. In an important process the composition is delivered, e. g. by extrusion coating, onto the surface of metal or a metallised or non-metallised polymer film or web, especially textile.

If delivery is to be onto polymer film, such film may be from stock or freshly formed, as in co-extrusion'.

To limit shear during processing, mixing and/or shaping can be in presence of organic liquid. Such liquid may be volatile enough for removal in hot processing, or may be or include plasticiser required in the final shaped article. Mixing with polymer intermediates is convenient if the polymer of the composition is to be polyurethane.

An advantage of using a thermoplastic material as the carrier component is that the composition readily adheres to metal and adhesion affords electrical contact. Thus the device offers a solution to the problem that otherwise an electrode must be applied with sufficient pressure for electrical contact but insufficient pressure to increase conductance significantly. Adhesion is preferably direct between the composition and the electrode, but an extraneous adhesive may be used provided it does not cover the whole area of adhesion.

Alternatively an intermediate layer, conductive when quiescent and/or when stressed, and adherent to the electrode and the composition, may be present. Adhesion of composition layer or intermediate layer to the electrode is conveniently effected by hot-melt application.

Shaped articles made with the compositions of the present invention may be used for a variety of purposes, for example as variable resistance devices and switches.

Examples of uses are disclosed in aforementioned WO 98 33193 and WO 99 38173.

The invention is illustrated by the following examples.

Example 1 4 parts by weight of nickel powder INCO grade 123 were stirred gently into 1 part by weight of a molten hot-melt adhesive grade polymer mixture (glue-sticks' BOSTIKTM) at 140°C. [This corresponds to 8.4 parts by volume of the metal powder per part by volume of the molten polymer, and gives a compounded composition containing about 30% by volume nickel. ] A sample of the resulting composition was formed by hot compression moulding at 80-100°C into a sheet 1.18 mm thick.

Example 2 4 parts by weight of nickel powder INCO grade 123 were gently dry mixed with 1 part by weight of a coarse powder (approx. 2 mm size) Santoprene, (a thermoplastic blend of about 40 parts of polypropylene and 60 parts of an ethylene-propylene-diene copolymer). The resulting mixture was fed to a screw extruder heated to 185°C and thereby formed into film 2 mm thick. The film was allowed to cool, then chopped into 1.5-2 mm pellets and stored for use as moulding feed. A sample of the pellets was fed to an injection moulder heated to 185°C and thereby formed into a sheet 1 mm thick.

Circular test pieces 5.5 mm in diameter were cut from the sheets of Examples 1 and 2 and placed on a flat metal plate acting as lower electrode and a metal plate acting

as upper electrode was placed on the test piece. The electrodes were connected via a constant current 10 volt DC supply and a 20M ohm high-impedance buffer amplifier to a Picoscope ADC 100 signal processor and recorder. The assembly was subjected to increasing pressure in a Lloyd Instruments LRX load tester. The effect of applied pressure on resistivity is shown in Tables 1 and 2.

Table 1-Ni 123/LDPE Compression % Force, N R, ohms Log (10) R 0.0 0.0 5.2E+12 12.7 18.0 14.3 1. OE+07 7.0 13.1 26.3 5. OE+05 5.7 17.1 36.2 3. OE+04 4.5 21.4 47.4 2. OE+03 3.3 28.3 64.4 1. OE+02 2.0 Table 2-Ni 123/polypropylene/EDPM blend Compression % Force, N R, ohms log (10) R 0.0 0.0 2.9E+12 12.5 4.6 2.1 1. OE+07 7.0 8.3 5.0 5. 0E+05 5.7 10.1 8.3 3. 0E+04 4.5 15.2 12.9 2. OE+03 3.3 191.17. 5 2. OE+02 2.3 32.2 33.4 1. OE+01 1.0 It is evident that the resistivity can be varied over a wide range of levels and that the variation is

proportional to the applied pressure. These runs were repeatable, showing good elastic recovery.

Example 3 In order to illustrate the difference between high and low bulk density nickel powders, compositions were made by mixing in one case, Nickel powder Grade 123 (tapped bulk density 3.16 g/cm3) as described above, and in another case INCO grade 287 in varying amounts with a liquid silicone rubber (Silastic T-4 supplied by Dow Corning) followed by curing the silicone to form an silicone rubber. Although the manufacture's literature indicates that the bulk density of Grade 287 is 0.75- 0.95 g/cm3, the tapped bulk density of a sample was measured and found to be 1.58 g/cm3, i. e. about 18% of the solid density of nickel. The manufacturer's literature indicates that nickel 287 is a filamentary powder with a chain-like network of spiky beads of average 2.5-3. 5 um in cross-section with the chains may be 15-20 um or more in length. 75% of the powder has a size in the range 4.7 to 53.0 um microns (by Microtrac light-scattering method), and the BET surface area is typically 0. 6m2/g.

Cured samples, in the form of circular discs of diameter 5.6 mm and thickness 1.5 mm, were placed between the electrodes of a resistance measuring device. In the uncompressed state the samples had a very high resistance

and, on compression, the electrical resistance decreased.

In the following Table 3, the degree of compression requird to decrease the resistance to 104 ohm is quoted for compositions of the differing nickel contents.

Table 3 Nickel 287 Nickel 123 % filler by Compression Compression 80 27- 81-42 82 20- 83 19 34 85 32- 86-23 88-19 By the term compression, we mean the change in thickness divided by the original thickness.

It is seen that the compression required decreased steadily for the higher bulk density Nickel 123 as the proportion of filler increased, whereas that required for the lower bulk density Nickel 287 decreased, and then increased as the filler content increased, indicating a deterioration of the electrical properties at high filler contents. This is believed to result from significant breakage at high filler loadings of the lower bulk density nickel particles during mixing with the resin.