EP0210002A1 - Pressure responsive electrically conductive materials - Google Patents

Abstract

A pressure sensitive electrically conductive material comprises a cured non-conductive matrix of flexible elastomeric material. The matrix contains filler particles, each of which comprises a substantially spherical core of electrically non-conductive material, preferably glass, having a coating of electrically conductive material preferably silver, thereon.

Description

• This invention relates to pressure sensitive electrically conductive materials. Many composite materials have now been proposed based on the mixing of electrically conductive particles into an electrically insulating elastomer which is subsequently shaped and cured. The resultant product is electrically non-conductive, but is rendered conductive when the material is deformed.
• Pressure sensitive resistors of this form have failed to win wide acceptance, despite the numerous attempts that have been made to produce them in a form satisfactory to industry. The most common drawbacks have been the lack of uniform electrical characteristics and inadequate mechanical strength and durability. It is generally required that a switch element utilising such material should be capable of at least one million switching operations, but wear on the conductive particles limits known materials to significantly fewer operating cycles. Existing particles also exhibit resistance hysteresis which is generally too high to be acceptable. Known materials are also very expensive, for example selling prices of the order of $1000/m² for sheet 1/2 mm thick are not uncommon.
• The importance of shape of the conductive particle in reducing hysteresis and improving wear characteristics appears first to have been recognised in U.S. patent No.3806471, which indicates that the best particles are somewhat regular in shape, that is, of spherical, generally rounded or granular characteristic. G.B. patent No.1561189 also recognises the importance of shape, and teaches that the conductive particles should be generally rounded particles of artificial graphite having a Wadell roundness degree of at least 0.4. However, none of the prior art documents teach how an elastomeric material can be loaded with particles which are consistently substantially spherical at a cost which renders the resulting composition economically acceptable, and such is the objective of the present invention.
• According to the invention a pressure sensitive electrically conductive material comprises a non-conductive matrix of flexible elastomeric material, the matrix containing filler particles each of which comprises a substantially spherical core of electrically non-conductive material having a coating of electrically conductive material thereon.
• The invention stems from the realisation that it is not generally possible economically to form homogeneous electrically conductive materials into small, substantially spherical particles. However, materials are available which can be so formed and such materials are suitable for the deposition thereon of an electrically conductive coating. The preferred core material is glass, and a number of types of small spherical glass particles are commercially available. These may be solid such as ballotini, or hollow. Alternative core materials that may be used are certain thermosetting polymeric compositions and certain metal compounds. The core material will desirably be such as to be capable of resisting deformation at the maximum applied load to which the material is to be subjected; thus, solid rather than hollow core particles are preferred although the latter may be suitable for some applications. Preferably all particles should have a surface to volume ratio that is less than 3.7/r where r is the radius of a sphere of the same volume. Desirably at least 50% of the particles should be of the minimum surface to volume ratio of 3/r, i.e. be truly spherical.
• Glass particles are capable of receiving and retaining a coating of any one of a number of electrically conductive materials. Examples thereof are metals such as silver, :opper, cobalt, nickel, brass, iron, chromium, titanium, platinum, gold, aluminium, zinc and their alloys; electrically conductive metal compounds; natural or artificial graphite; and electrically conductive polymeric naterials. Coating thickness may be selected as required. rhe size of the coated particles will generally be up to 300 microns, although it is more usually preferred that no particle has a size of less than 1 micron, and that no particle has a size in excess of 200 microns in order to avoid, respectively, problems of oxidation and release from the matrix. More preferably, the particles used in the material of the invention have a size distribution lying within the range of 5 to 105 microns. Larger particles are found to give better conductivity than smaller particles at equal particle loadings by weight of matrix material, and will thus provide a wider spread of resistance values between the material in its uncompressed and fully compressed states.
• Preferably the coated filler particles make up between 20% and 70% of the volume of the material, and more desirably between 35% and 50% of the volume of the material. Alternatively or additionally the particles preferably are present in from 50 to 200 parts per hundred parts by weight of matrix material (phr), and more desirably in from 80 to 140 phr. Below the preferred lower limits, it is found that unacceptably high compression may need to be applied to the material to cause the required drop in resistance, while above the upper limit the material in its state of rest may be found to be too conductive due to contact betweeen the conductive particles.
• The elastomeric matrix may be formed from any suitable polymeric material or blend thereof as long as it is electrically insulating and exhibits the required properties. Representative of suitable elastomers are silicone rubbers, whether of the condensation reaction, addition reaction or vinyl group-containing type, rubbery condensation polymers such as polyurethane rubber obtained by reaction of polyisocyanates with polyalkylene glycols, ethylene propylene-non-conjugated diene rubbers, natural rubber, synthetic polyisoprene rubber, styrene butadiene rubber, nitrile-butadiene rubber,halogenated hydrocarbon rubbers such as elastomeric chloroprene rubber, fluoroolefin rubber, chlorosulfonated polyethylene, thermoplastic elastomers such as ethylene-vinyl acetate copolymers, and plasticizer containing thermoplastic resins.
• Other materials such as plasticising agents, stabilizers, pigments, colouring agents and extending oils may be incorporated into the matrix composition. Such composition may contain fillers such as silica, silicates, kaolin, mica, talc, carbonates or alumina. Generally speaking, the matrix material should be compounded so that it can resist a high-intensity electric field, has good electrically insulating properties and the mechanical properties appropriate to the end use. In some cases these properties include low permanent set and high elongation at break. In other fields it may be advantageous for the matrix to be of cellular material, and any suitable blowing agent or other expanding system may then be compounded with the elastomer.
• The coated filler materials may be mixed with the elastomeric matrix material in any suitable manner. Mixing is facilitated if the matrix material is in liquid form, however, it is possible to effect mixing into a solid elastomer. The aim should be to obtain a reasonably uniform dispersion of the filler particles throughout the matrix. After mixing, a cross-linking system is added to the mixture which is then cured to any required shape. The cured material may be de-gassed if necessary.For many uses a room temperature vulcanising material is used, for ease in compounding and casting and for better control of particle distribution. When materials with better mechanical properties are required, however, high temperature vulcanising materials may be used. Alternatively, the properties of room temperature vulcanising materials may be improved by appropriate compounding ingredients.
• It is particularly preferred to use a castable material such as silicone or a polyurethane rubber, which can readily be compounded to give the required properties, and can be vulcanised at room temperature.
• A particularly preferred material comprises silver-coated ballotini loaded into a silicone rubber matrix. To give some indication of the economic advantages, such materials may be produced at a cost approximately one tenth of certain of the currently existing products.
• Most usually the material will be cured in the form of a thin flat sheet, which may then be cut into individual elements of required size. Preferred sheet thicknesses are from 0.25 to 3 mm, more preferably from 0.35 to lmm. It is important that any given element be of substantially uniform thickness within a close tolerance, eg. 1_%. Elements moulded from identical compositions and under identical conditions but to different thicknesses are found to have widely different electrical characteristics.
• The invention will now be described in more detail with reference to the following examples thereof, given in conjunction with the accompanying drawings in which Figure 1 and 3 are graphs of resistance against compression; and Figures 2 and 4 are graphs of resistance against load.
Example 1.
• A batch of material was made up as follows:-

  
• 1. Ambersil Silcoset 105 RTV (room temperature vulcanising, with curing agent 'A' as supplied).
• 2. Obtained from Potters Industries Inc., New Jersey, (U.S.A.) under designation 2429S. The spheres have a density of 2.5 gm/cc, a typical particle size range of 53 to 105 microns, a silver concentration by weight of 4% and a minimum percent of rounds of 85%, ie. at least 85% of the particles had a minimum surface to volume ratio of 3/r.
• After mixing, the curing agent was added and the mixture was poured into 2 mm deep moulds and allowed to cure at room temperature.
• The cured material contained 37% by volume (120 phr) of the silver coated glass spheres.
Examples 2 and 3
• Example 1 was repeated with loadings of 61% and 64% by weight respectively of the silver coated glass spheres, to give volume percentages of 40.6% and 43.5% respectively.
• One centimetre square pieces were cut from the cured sheets of all three examples, each piece was placed between metal foil electrodes and resistance was monitored as the rubber was compressed. The results are shown in Figure 1 and 2 for the three examples. It will be seen in each case that from an initial resistance in excess of 4 Megohms at zero load, the resistance is reduced with increasing compression or load to very low values. In each instance, when the applied pressure was removed the resistance of the material reverted to its former value. The results indicated in the Figures were found to be reproducible consistently and reliably over large numbers of operations.
Example 4
• A batch of material was made up as follows:-
• 
• The batch was allowed to harden at room temperature. Performance of the material was similar to that already described, although higher forces were necessary to reduce the resistance due to the material hardness being greater.
Example 5
• Batches of material were made up from Ambersil Silcoset RTV and silver coated solid glass spheres 2429S as aforesaid, the material being formed into sheets of different thicknesses and cured. From batch to batch there were variations in concentration of the glass spheres and of the thickness of the silver coating on the spheres. In run M the spheres as used were designated 2429S GE RTV 910, supplied by Potters Industries Inc. These are the same silver-coated glass particles 2429S, but coated with a bonding system for room temperature vulcanising silicone rubbers. One centimetre square pieces were cut from the sheets and resistance was monitored to find the load (known as the trigger load) and the compression where resistance drops very sharply to a low valve. The results are given below.

  

  Typical resistance-compression graphs and resistance-load graphs are shown in Figures 3 and 4 for runs B and D respectively.
• The results show that the trigger load increases as the concentration of conductive spheres in the matrix reduces, as the conductivity of the individual spheres reduces and as the thickness of the sheet increases. The compound and sheet thickness can thus readily be tailored for specific applications.
Example 6.
• In order to check reproducibility of the switching effect of the material between the off (high resistance) and on (low resistance) modes a number of tests were run. In the first of these a sample of material from run C of Example 5 was subject to repeated compressive loading and unloading, with the resistance being monitored during each cycle. The on/off switching effect was noted during each of 160,000 cycles, at the end of which the material was checked for physical wear. This was minimal, and there was no reason to suppose that the switching effect would not be obtained for many thousands more cycles.
• In a second test an 0.56 mm thick sheet was cast from a mixture of 100 parts by weight Ambersil Silcoset 105 RTV and 100 parts by weight of the aforesaid 2429S silver coated glass spheres. The cured sheet was placed between two copper foil electrodes and repeatedly cycled over 1 million times on a de Mattia flexometer used in a compression mode. The on/off switching effect was noted during each cycle . The test material showed minimal physical wear and exhibited no sign of failure.
• The properties were then examined and compared to those measured before the cycling test started.

  
Example 7
• 100 parts by weight of the aforesaid 2429S silver coated glass spheres were mill mixed together with 100 parts by weight of KE650 from Shin-etsu Chemical Company, Kagaku, Japan comprising silicone rubber with a peroxide curing agent. The mixture was sheeted to a thickness of 0.7 mm and the sheet was press cured for two hours at 180°C. When placed under test the sheet was found to have a trigger load of 255 kg/cm² at 54% compression.
Example 8
• 110 parts by weight of the aforesaid 2429S silver coated glass spheres were mill mixed together with 100 parts by weight of Keltan 778 from Wilfred Smith Ltd. of_Edgware, Middlesex, England, an EPDM rubber and 5 parts by weight of Retilox 40 from Montedison SpA, a peroxide curing agent. The mixture was sheeted to a thickness of 0.7 mm and cured in hot air at 160°C for 75 minutes. When tested, the sheet was found to have a trigger load of 4.5_Kg/cm² at 12% compression.
Example 9
• A batch of material was made up as follows:

  
• 1. Ambersil silcoset 105 RTV with curing agent A.
• 2. Obtained from Microfine Minerals and Chemicals Ltd., Derby, England under designation C-USPHERES 200. The particles have a density of 0.9 gm/cc and a typical particle size range of 50 to 300 microns. Only the particles with a size of 200 microns or less were used.Typical shell thickness 10% of diameter.
• After mixing, the cure system was added, and the mixture poured into an open mould and allowed to cure at room temperature.
• After curing, sheets of different thickness were placed between a lower copper electrode and an upper electrode of aspherical steel ball 5 mm in diameter. The load was applied through the ball and then resistance monitored, with the following results:-

  

Claims (12)

1. A pressure sensitive electrically conductive material comprising a cured non-conductive matrix of flexible elastomeric material, the matrix containing filler particles each of which comprises a substantially spherical core of electrically non-conductive material having a coating of electrically conductive material thereon.

2. A material according to claim 1 in which the core material is glass.

3. A material according to claim 1 in which the core material is solid.

4. A material according to claim 1 in which all particles have a surface to volume ratio that is less than 3.7/r where r is the radius of a sphere of the same volume.

5. A material according to claim 4 in which at least 50% of the particles are truly spherical.

6. A material according to claim 1 in which the conductive material is selected from silver, copper, cobalt, nickel, brass, iron, chromium, titanium, platinum, gold, aluminium, zinc and their alloys; electrically conductive metal compounds; natural or artificial graphite; and electrically conductive polymeric materials.

7. A material according to claim 1 in which no particle has a size of less than 1 micron, and no particle has a size in excess of 200 microns.

8. A material according to claim 1 in which the particles have a size distribution lying within the range of 5 to 105 microns.

9. A material according to claim 1 in which the coated filler particles make up between 20% and 70% of the volume of the material.

10. A material according to claim 1 in which the particles are present in from 50 to 200 parts per hundred parts by weight of matrix material.

11. A material according to claim 1 in which the matrix elastomer is selected from silicone rubber, polyurethane rubber and ethylene-propylene-non-conjugated diene rubber.

12. A material according to claim 1, that is in the form of a thin flat sheet having a thickness of from 0.25 to 3 mm.

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