GB2054277A - Pressure-sensitive electroconductive bodies - Google Patents

Abstract

A pressure-sensitive electroconductive elastic body comprises a matrix of an electrically insulating elastic material in which is dispersed metal powder having particular physical properties. Its bulk density should be from 1.8 to 6.0 g/cm<3> at a packing fraction of 0.20 to 0.65, its flowability should be from 15 to 50 seconds/50 g and its range of bulkiness factors should be at least 0.1. Such materials show good reproducibility of resistance charge on repeated compression.

Description

SPECIFICATION Pressu re-sensitive electroconductive bodies The present invention relates to pressure-sensitive electroconductive elastic bodies and, especially, to such bodies that are electrically insulating when in the ordinary state but which, upon compression, become electroconductive in the region under compression, generally to an extent corresponding to the compressive force.

Pressure-sensitive elastic bodies are widely used in various electronic instruments as components of, for instance, contact points and pressure-sensitive resistor elements. They have previously been made by shaping a suitable elastic, generally rubbery elastomer, composition filled with large amounts of electroconductive particles such as metal or carbon powder, e.g. carbon black or graphite powder. The known bodies may be classified into one of several classes according to the mechanism by which conductivity is exhibited in the region under compression.

In one class, when the body is sandwiched between two electrodes and compression is applied between these, the effective contacting area between the surfaces of the body and the electrodes gradually increases with increasing compression. This results in gradual decrease of the contact resistance and consequential increase in conductivity between the electrodes. Such materials are described in, for example, U.S. Patent No. 2,752,558 and Japanese Patent Publication 36-18879. Unfortunately the reproducibility of the compression-resistance relationship is rather poor after a large number of compression and pressure release cycles because of changes in the nature of the contact between the surface of the body and the electrodes.

Another class of pressure-sensitive electroconductive elastic bodies work on the principle that as the compressive force increases with consequential increase in the compressive strain in the body the contact area increases between the conductive particles dispersed within the matrix with consequential increase in electric conduction between the electrodes. Such materials are described in for example Japanese Patent Disclosure 46-6179 and Japanese Patent Publication 50-31945. Unfortunately these materials also suffer from the problem of poor stability or reproducibility in the changes in resistance and the compression-resistance relationship partly because the conditions of contact between the conductive particles within the matrix are rather susceptible to changes in ambient conditions such as temperature.Also poor reproducibility in the resistance change is due partly to boundary separation of the bonding between the particles and the matrix when the body is subjected to very large compressive strain.

Various proposals at overcoming such problems in this second class of materials have been made. For example the matrix may be impregnated with a small amount of an orga nopolysiloxane fluid (see Japanese Patent Disclosure 53-896), an artificial graphite powder formed of particles of ground pebble-like form having no sharp points or edges is used as the dispersed conductive particulate material (see Japanese Patent Disclosure 53-7993) or the material particles as the conductive particulate material are provided with a surface layer of a semi-conductive material (see Japa nese Patent Publication 52-5715) but the results are still not entirely satisfactory.

Attempts have also been made to obtain improved results by arranging for a specific distribution of the particulate material within the matrix. For example electroconductive particles which are also ferromagnetic may be brought into a localised arrangement in the matrix by application of an external magnetic field (see Japanese Patent Disclosure 53-3695) or the like particles may be arranged to be present with a denser distribution near the surfaces of the body than in the centre of the body (see Japanese Patent Disclosure 52-139989). Unfortunately it is rather difficult to make such bodies.

Although some known pressure-sensitive electroconductive elastic bodies can be used as switching elements producing on-off signals a general problem with such bodies is that they cannot generally be used satisfactorily as pressure-sensitive variable resistance elements. This is because such elements generally need to exhibit reliable and reproducible changes in resistance according to applied compressive force. The reason why the pressure-sensitive bodies generally cannot meet these requirements is primarily because of the relatively large compression-resistance coefficient that exists over a practical range of compressions when the resistance is varied from infinity to a sufficiently low value by repeated and increasing application of compressive force to the electrodes between which the body is sandwiched.

A pressure-sensitive electroconductive elastic body according to the invention comprises a matrix of an electrically insulating elastic material in which is dispersed from 60 to 280 parts by volume, per 100 parts by volume elastic material, of metal powder having a bulk density in the range from 1.8 to 6.0 g/cm3 at a packing fraction of 0.20 to 0.65 and having a flowability in the range of 1 5 to 50 seconds/50 g and in this body the range of the bulkiness factors of the metal powder particles is at least 0.1.

The bulk density of 1.8 to 6.0 g/cm3 at a packing fraction of 0.2 to 0.65 is measured according to JIS Z 2504. The bulk density is preferably from 2.0 to 5.0 g/cm3 at a pack ing fraction of 0.25 to 0.60.

The flowability is measured according to JIS Z 2502.

When the metal powder has a bulk density smaller than 1.8 and a flowability in excess of 50 seconds/50 g, the body does not have superior performance over conventional metal powder-filled elastomer bodies. If the metal powder has a bulk density larger than 6.0 g/cm3 and a flowability smaller than 1 5 sec onds/50 g the body again does not have improved properties over conventional bodies.

Such a high bulk density, low flowability powder has a sphericity of particles close to 1.0 and a body containing these exhibits large compressive strain with a compressive force resulting in poor stability and reproducibility in the compression-resistance characteristics.

In this connection the more preferable range of the bulk density is from 2.0 to 5.0 g/cm3 at a packing fraction of 0.25 to 0.60 and the range of the bulkiness factors is at least 0.12.

In a modification of the invention the powder may be as defined but satisfactory results can be obtained even if the flowability is below 1 5 or above 50 seconds/50 g.

The configuration of the individual particles in the metal powder is important and preferably is not uniform. The particles may be of any shape, such as spheres, teardrops, rods, needles or flakes.

The non-uniformity of the particle configuration is such that the range of bulkiness factors is at least 0.1. The bulkiness factor is the factor which was first quoted by Hausner in 1 966 and is given by the equation A fs = - 1 b in which fB is the bulkiness factor, A is the area of a projection of a particle on a plane and 1 and b are each the length of the longer side and shorter side, respectively, of a rectangle having the smallest area among the rectangles circumscribed around the projection of the particle on the plane. This value can be readily determined from the microscopic photographs of the individual particles.

Preferably the particle size of the metal powder is sufficiently fine that substantially all of the metal powder passes through a 100 mesh or, preferably, 200 mesh Tyler standard screen so as that even a very thin body may be considered to have a uniform structure.

Metal powders meeting the above described requirements may be obtained by carefully selecting from commercially available metal powder products produced, for example, by the so-called atomizing method.

The metal powder may be formed of a wide variety of metals including nickel, stainless steel, copper and copper alloys, tungsten, titanium and titanium alloys, tin, solder alloys, zinc, bismuth, silver and silver alloys, aluminium and aluminium alloys. If desired the metal powder may be plated with a less corrosive metal or treated with a coupling agent prior to incorporation into the matrix.

The amount of powder dispersed in the matrix must be between 60 and 280 parts by volume per 1 00 parts by volume of the elastic material of the matrix. If the amount is below 60 parts by volume the metal powder generally will not give sufficient electroconductivity to the body while if the amount is above 280 parts by volume the body may become too brittle to be elastic.

The elastic material used for forming the matrix may be any of the materials conventionally used in pressure-sensitive electroconductive elastic bodies and are generally materials that are referred to as rubbery elastic or elastomeric materials, and exhibit such properties when, as is normal, they have been cured or vulcanised. Rubbery polymers suitable for use are exemplified by natural rubber, butyl rubber, isoprene rubber, acrylic rubber, nitrilebutadiene rubber, chlorinated polyethylene rubber, ethylene-propylene rubber, silicone rubber, fluorocarbon rubber and polyurethane rubber. Blends may be used. If required the polymer is incorporated admixed with a curing or vulcanising agent and optionally a curing accelerator.

Certain thermoplastic resins exhibiting rubber like elasticity to some extent at or below their melting or softening point can be used either alone or admixed with the aforementioned rubbery polymers. Suitable resins include polyurethane resins, polyester resins and polyamide resins. The elastic material being used to form the matrix may include additives conventional for rubber processing such as plasticisers, stabilisers, aging retarders, lubricants, colouring agents, foaming agents, body pigments, reinforcing fillers and coupling agents and the like.

Silicone rubbers are prefered as the matrix material owing to their excellent properties such as the heat resistance, electric properties, and inertness to chemicals. Various types of silicone rubbers which can be classified according to the mechanism of crosslink formation and the behaviour in curing, may be used satisfactorily.

The body comprising the matrix having dispersed therein the metal powder may be formed in conventional manner. Preferably the composition from which it is formed is plasticised by mastication prior to the incorporation of the metal powder to have a plasticity of not exceeding 500 or, preferably not exceeding 300 as measured with the Williams plastometer or a viscosity lower than 107 centistokes in order to obtain more uniform dispersion of the metal powder in the matrix. It is optional that small amounts of a carbon powder, e.g. carbon black or a graphite powder, are used in combination with the metal powder together with the other additive ingredients mentioned before.

The composition filled with the metal powder is then shaped into desired forms by compression moulding or other means with heating to effect curing into a rubbery elastomer body of, for example, a sheet or other desired shape.

When the body according to the invention is held sandwiched between two electrodes and compressed and released repeatedly, the changes in the resistance value between the electrodes have high reliability and good reproducibility. Further, the compression-resistance coefficient of the body is smaller than in the conventional ones within a wide range of compression.

The reason for this advantageous performance is not well understood but is presumably as follows. When the new body is compressed between two electrodes, the contacting areas between the electrode surfaces and the elastomer body as well as the contacting areas between the metal particles embedded in the elastomer matrix increase with the increase of the compressive force resulting in the decrease of the overall electric resistance between the electrodes. In this case, the matrix is less susceptible to compressive deformation because of the specific configuration of the metal particles. As a result the compression-resistance coefficient is remarkably decreased.Further, the reduced compressive deformation of the matrix is beneficial in preventing the boundary separation between the matrix and the surface of the metal particles, contributing to the improvement of the reproducibility in the compression-resistance relationship even under repeated application and releasing of the compressive force.

The pressure-sensitive body of the invention is very useful as a pressure-sensitive resistor element used in, for example, current and voltage regulators, computers, calculators, electronic musical instruments, cash registers and telephones as well as signal input end instruments for microcomputers pressure sensors, transducers, signalling devices, various measuring instruments, control system in automobiles and end instruments in information systems.

The following are examples to illustrate the present invention.

Several kinds of pressure-sensitive electroconductive elastic bodies each in a form of a sheet of 1 mm thickness were prepared with various kinds and amounts of conductive particulate materials dispersed in the matrix of a silicone rubber composition composed of, in each case 100 parts by volume of a silicone rubber composition (KE 951 U, a product by Shin-Etsu Chemical Co., Japan) and 4 parts by volume of a curing agent (Kayabutyl ST, a product by Kayaku Nulley Co.).The kinds and amounts of the conductive particulate materials for the samples A to G were as follows: Sample A: 85 parts by volume of a stainless steel powder passing through a 200 mesh screen and having a bulk density of 2.0 g/cm3 and a flowability of 32 seconds/50 g, in which the range of the bulkiness factors of the individual particles exceeded 0.5, a mixture of particles having irregular, tear droplike, needle-like and spherical configurations.

Sample B: 1 30 parts by volume of a powder of a solder alloy (1 :1 by weight of lead and tin) passing through a 1 50 mesh screen and having a bulk density of 5.0 g/cm3 and a flowability of 1 8 seconds/50 9, in which the range of the bulkiness factors of the individual particles exceeded 0.3, a mixture of particles having irregular, tear drop-like, needle-like and spherical configurations.

Sample C: 98 parts by volume of the same metal powder as in sample B.

Sample D: 110 parts by volume of the same metal powder as in sample A.

Sample E: 85 parts by volume of an electrolytic nickel powder passing through a 200 mesh screen and having a bulk density of 3.8 g/cm3 and a flowability of 40 seconds/50 g, in which the range of the bulkiness factors of the individual particles was about 0.08.

Sample F: 1 2 parts by volume of acetylene black. In this case, dicumyl peroxide was used as the curing agent instead of Kayabutyl ST.

Sample G: 1 8.6 parts by volume of acetylene black. In this case, dicumyl peroxide was used as the curing agent instead of Kayabutyl ST.

Each of the sample sheets was subjected to post-curing at 200"C for 1 hour and then the conductivity behaviour thereof was examined in the manner as described below.

The sample sheet was cut into strips of 5 mm wide and the electric resistance of the strip between two positions 1 5 cm apart was determined by use of a circuit tester to calculate the volume resistivity in ohm.cm.

The measurement with these strip samples indicated that the volume resistivity of the samples A to D was infinitely large within the sensitivity of the circuit tester and these samples exhibited measurable electric resistance in the portions under compression. On the other hand, samples E, F and G had volume resistivities of 0.5, 100 and 1 8 ohm.cm, respectively, with little decrease of the resistivity in the regions under compression.

Further, a disc of 1.6 mm diameter obtained by punching the sample sheet was sandwiched with two gold-plated copper electrodes with varied compressive force therebetween to determine the change of the electric resistance between the electrodes to give the results of the relationship of compressive force vs. electric resistance as shown. The accompanying drawing consists of a single Figure, Fig. 1, which is a graph of the compressive force vs. the electric resistance of the aforementioned bodies A to G.

Claims (4)

1. A pressure-sensitive electroconductive elastic body comprising a matrix of an electrically insulating elastic material in which is dispersed from 60 to 280 parts by volume, per 100 parts per volume of elastic material, of metal powder having a bulk density in the range from 1.8 to 6.0 g/cm3 at a packing fraction of 0.20 to 0.65 and having a flowability in the range from 1 5 to 50 seconds/50 g and in which the range of the bulkiness factors of the metal powder particles is at least 0.1.

2. A body according to claim 1 in which the metal particles have a bulk density of from 2.0 to 5.0 g/cm3 at a packing fraction of 0.25 to 0.60 and the range of bulkiness factors is at least 0.12.

3. A body according to claim 1 in which substantially all the metal powder passes through a 200 mesh Tyler standard screen

4. A body according to claim 1 substantially as herein described.