GB2253856A

GB2253856A - Conductive ink compositions

Info

Publication number: GB2253856A
Application number: GB9206080A
Authority: GB
Inventors: Takehide Okami
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1991-03-20
Filing date: 1992-03-20
Publication date: 1992-09-23
Anticipated expiration: 2012-03-20
Also published as: GB2253856B; JPH0715072B2; GB9206080D0; JPH04292670A; GB2253856B8

Abstract

A curable composition containing an organopolysiloxane, a crosslinking agent, and conductive carbon particles is blended with a silazane compound of formula (1): <IMAGE> wherein R<1>, R<2>, and R<3> are independently substituted or unsubstituted monovalent hydrocarbon groups, obtaining a conductive ink composition which tightly adheres to insulators, especially insulating silicone rubber. The conductive ink composition is applied to an insulating silicone rubber substrate by a printing technique, resulting in a contact member useful as business machine keyboards.

Description

CONDUCTIVE INK COMPOSITIONS, METHODS OF MAKING THEM, AND USE THEREOF IN CONTACT MEMBERS This invention relates to conductive ink compositions, methods of making them, and to contact members formed therefrom e.g. for use in keyboards.

These may be e.g. for business machines including calculators, computers, and phones. In this specification the term "conductive" means "electroconductive".

Silicone elastomer compositions can be made conductive by mixing them with metal particles such as silver and nickel or conduc- tive particles of carbon black or the like in various forms. On the other hand, addition curing type adhesive compositions based on organopolysiloxanes having an alkenyl group (e.g., vinyl) attached to a silicon atom are well known in the art although they are not satisfactory in adhesion and workability, especially with respect to viscosity.

Contact members for use in keyboards of business machines including electronic calcula- tors, computers, and phones are conventionally manufactured by embedding cured forms of conductive silicone rubber containing conductive particles (such as carbon black) in insulating members of plastics or silicone rubber or by attaching cured forms of conductive silicone rubber to the insulating members.

In either method, conductive silicone rubber compositions are previously cured into cured forms, insulating members are provided with contact sites as by punching, and the cured forms are embedded in or affixed to the insu- lating members at the contact sites. This is undesirable in the case of keyboards having a number of contacts or special articles having contacts of a complex shape, for example, a horseshoe shape or a centrally perforated rectangular shape, because molding is very difficult and the percentage of rejected parts is increased.

In contrast to the prior art method of embedding or attaching cured forms of conductive silicone rubber in or to insulating members, Japanese Patent Publication (JP-B) No. 34982/ 1986 proposes to integrate non-conductive silicone rubber with conductive silicone rubber having carbon black blended therein. This integration method involves first curing a first layer of a conductive silicone rubber composi- tion having carbon black blended therein, forming a second layer of a carbon black-free, non-conductive, addition curing type silicone rubber composition on the first layer, and curing the second layer to the cured (first) layer into an integral body.The carbon black containing conductive silicone rubber composi- tion and the carbon black - free non - conductive silicone rubber composition can be joined together only by the sequence of first curing the former and thereafter curing the latter to the cured layer. It is difficult to achieve integration of two compositions if the carbon black - free non - conductive silicone rubber composition is first cured and thereafter, the carbon black-containing conductive silicone rubber composition is formed thereon and cured thereto.

It was also proposed to form contact portions on keyboards using conductive ink compositions instead of the conductive silicone rubber compositions. The use of conductive ink compositions can solve the above-mentioned problems, but have some problems. Nonelastomeric conductive ink compositions tend to form contact portions which are liable to cracking. Elastomeric conductive ink compositions include non-silicone and silicone types of elastomeric conductive ink composi tions, among which the non-silicone elastomeric conductive ink compositions do not adhere to insulating substrates of cured silicone rubber, or are poor in adhesion thereto.The silicone elastomeric conductive ink compositions are good in adhesion and bonding to insulating substrates of silicone rubber, but tend to form an insulating layer of organosilicone on the surface which excludes their use in contact portions where high conductivity is required.

When solvents are added to the silicone elastomeric conductive ink compositions to reduce their viscosity to an acceptable level for application, the dispersion of conductive particles (such as carbon black, Ni powder or Ag coat beads) becomes non-uniform if they are not tightly bound by the binder or silicone oil, resulting in non-uniformity of conductivity.

There exists a need for a silicone elastomeric conductive ink composition, preferably effectively usable in the manufacture of contact members for use in business machine keyboards.

In particular it would be preferable to be able to provide a conductive ink compo sition which has good adhesion and bond to various insulating substrates, especially insulating silicone rubber, can effectively form conductive patterns on the substrates without restriction in curing sequence, and is thus suitable for the manufacture of contact members for use in keyboards or the like. It would also be preferable to be able to provide a contact member prepared using the conductive ink composition.

The inventors have developed new compositions by blending a curable composition containing an organopolysiloxane, a crosslinking agent, and conductive carbon particles, with a silazane compound of the general formula (1):

wherein R1, R2, and R3 are independently sub stituted or unsubstituted monovalent hydrocarbon groups. They have been able to prepare such conductive inks, i.e. curable compositions containing an organopolysiloxane, a crosslinking agent, and conductive carbon particles in admixture with a silazane compound of formula (1), with good adhesion and bonding to various insulating sub - strates, especially insulating silicone rubber.

When conductive carbon particles are surface treated with the silazane compound of formula (1), the binding force between the carbon particles and the organopolysiloxane is increased, little drop of viscosity occurs, stable dispersion is maintained after solvent dilution, and adequate thixotropy is imparted.

They found that resulting cured products are wear resistant. No silicone insulating layer formed on the surface of the cured products.

The new compositions have been found to be applicable to substrates in various ways, for example, by applying the composition to a pre cured layer of insulating silicone rubber and then curing it, or by applying the composition to an uncured layer of insulating silicone rubber and heat curing them at the same time.

In this way, a conductive silicone rubber layer may be joined with an insulating silicone rubber layer to form an integral body. Alternatively, the composition is applicable by screen printing techniques to readily form a pattern of contact portions as found in keyboards having a complex conductor pattern.

Therefore, the present invention provides a conductive ink composition comprising a curable composition containing an organopoly- siloxane, a crosslinking agent, and conductive carbon particles, and a silazane compound of formula (1). Also, preparation methods and use thereof.

Also contemplated herein is a contact member in which a conductive layer is formed on an insulating substrate from the conductive ink composition.

A conductive ink composition of the present invention is obtainable by adding a specific silazane compound to a curable composition containing an organopolysiloxane, a crosslinking agent therefor, and conductive carbon particles. The curable composition containing an organopolysiloxane and a cross linking agent is not limited in curing system, and therefore, organopolysiloxane and cross linking agent components may be selected in accordance with well - known organopolysiloxane crosslinking systems. Particularly when a conductive ink composition of this type is printed on an insulator e.g. by coating and then cured to form a contact member, the crosslinking system based on organic peroxides is undesirable because the surface might remain tacky.Then, the preferred curable compositions are conden- sation reaction or addition reaction type organopolysiloxane compositions. As opposed to the condensation reaction type compositions which are difficult to apply by coating methods like screen printing because of premature formation of a surface skin and thus limit their application to dripping and similar methods, the addition curing type compositions are free of such disadvantages. For this reason, the addition curing type organopolysiloxane compo sitions are best suited for the manufacture of contact members such as keyboards.

The addition curing type organopoly siloxane compositions may be composed of conventional well-known components. Preferred are compositions containing an organopoly- siloxane having at least two alkenyl groups in a molecule and a viscosity of 100 to 200,000 centistokes (cs) at 250C as the organopoly siloxane, an organohydrogenpolysiloxane having at least two hydrogen atoms attached to silicon atoms in a molecule as the crosslinking agent, and a platinum catalyst.

The addition curing type organopoly- siloxane compositions are described in more detail. The base is an organopolysiloxane having at least two alkenyl groups in a molecule and a viscosity of 100 to 200,000 cs at 25 C.

The alkenyl groups include vinyl, allyl, methacryl, and hexenyl groups, for example.

Organic groups other than the alkenyl group, for example, monovalent hydrocarbon groups such as methyl, ethyl, propyl, trifluoropropyl, and phenyl groups may be included.

Illustrative examples of the organopoly- siloxane are given below.

In the formulae, p is equal to 2 or 3, s, u and w are positive integers, and t, v and x are 0 or positive integers.

The organopolysiloxanes may be used alone or in admixture of two or more. They may have a partially branched structure.

The organohydrogenpolysiloxane serving as the crosslinking agent for the alkenyl containing organopolysiloxane should have at least two hydrogen atoms attached to silicon atoms in a molecule. The organohydrogen polysiloxanes may have a linear, branched or cyclic structure and be used alone or in admixture of two or more. Examples are given below.

In the formulae, b, c, d, e, f, g and i are 0 or positive integers, and h is an integer of at least 2.

In the formulae, R; is hydrogen, a mono valent hydrocarbon group or triorganosiloxy group.

The organohydrogenpolysiloxane is used in a sufficient amount to provide 2 to 30, preferably 5 to 20 hydrogen atoms attached to silicon atoms per alkenyl group in the alkenyl containing organopolysiloxane.

The platinum catalyst added to the organopolysiloxane components (including both base and curing agent) is an addition reaction catalyst serving as a curing promoter. The catalyst is platinum or a platinum compound, for example, platinum black, solid platinum on supports of alumina or silica, chloroplatinic acid, alcohol-modified chloroplatinic acid, complexes of chloroplatinic acid with olefins, and complexes of platinum with vinylsiloxanes, although the catalyst is not limited thereto.

The catalysts are added in any desired form. In the case of solid catalysts, pre ferably they are finely divided for improving dispersion, or supports having a smaller particle size and a larger specific surface area are used. The chloroplatinic acid and chloro- platinic acid - olefin complexes are used after dissolving them in solvents such as alcohols, ketones, ethers and hydrocarbons.

The amount of the catalyst used is a catalytic amount to provide a desired curing rate. From an economical standpoint or for obtaining acceptable cured products, the catalyst is used in an amount to provide about 0.1 to 100 ppm of platinum for those catalysts compatible with the organopolysiloxane components, for example, alcohol- or vinyl siloxane - modified chloroplatinic acid and about 20 to 500 ppm of platinum for solid catalysts, for example, platinum black, both based on the total of the organopolysiloxane components.

For imparting conductivity to the ink composition, conductive carbon particles are added to the curing composition constituting the conductive ink composition.

The conductive carbon particles may be selected from known ones conventionally used in conduc tive rubber compositions. Exemplary are conductive furnace black (CF) such as Continex CF (manufactured by Continental Carbon Co.) and Vulcan C (Cabot Corp.); super conducting furnace black (SCF) such as Continex SCF (Continental Carbon Co.) and Vulcan SC (Cabot Corp.); extra conducting furnace black (XCF) such as Asahi HS-500 (Asahi Carbon K.K.) and Vulcan XC-72 (Cabot Corp.); conducting channel black (CC) such as Corax L (Degussa Co); acetylene black such as Denka Acetylene Black and Denka Black HS-100 (Denki Kagaku K.K.); furnace black and channel black heat treated at elevated temper atures of about 1500 C; and modified furnace black such as Ketjen Black EC (Akzo Co.).

Conductive carbon particles having a relatively low sulfur content, especially of the acetylene black type are preferably used.

Otherwise, a substantial content of sulfur can poison the platinum catalyst of a conductive ink composition, especially an addition curing type organopolysiloxane composition during its long- term storage, detracting from the curing ability of the composition. The specific surface area of conductive carbon particles is not critical although particles having a specific surface area of about 2 to 1,000 m2/g are preferably used.

The amount of conductive carbon particles blended may be properly selected although they are preferably used in amounts of about 50 to 200 parts, more preferably about 80 to 150 parts by weight per 100 parts by weight of the organopolysiloxane base. If compositions contain less than 50 parts by weight of carbon on this basis, cured products tend to form a silicone insulating layer on the surface in the case of single side open curing systems (coating and screen printing methods), which exhibits an increased surface resistance, eventually failing to provide desired conductivity. More than 200 parts of carbon is undesirable because blending becomes difficult and cured products have practically unacceptable wear resistance due to a relatively low content of the binder or siloxane.

The conductive ink composition of the invention contains a silazane compound as an essential component, which we find can increase the binding force of conductive carbon particles with the siloxane components and reduce or stop viscosity drop in the composition. The silazane compound is of the general formula (1).

In formula (1), Rl, R2, and R3 are independently substituted or unsubstituted monovalent hydro- carbon groups, preferably monovalent hydrocarbon groups having 1 to 8 carbon atoms, for example, methyl, ethyl, vinyl, phenyl, and trifluoro- propyl groups. Where the conductive ink composition is of the addition curing type, it is preferred that vinyl and phenyl groups be contained as Rl, R2, and R3. The inclusion of vinyl and phenyl groups in the silazane compound of formula (1) imparts the binding force to conductive carbon particles with the siloxane components, thus contributing to improvements in stable dispersion of the conductive ink compo sition after solvent dilution and the wear resistance of cured products.

The silazane compounds of formula (1) are advantageous over other silazane compounds. As compared with silazane compounds of formula (2):

wherein Rs is a monovalent hydrocarbon group, for example, the silazane compounds of formula (1) have a higher boiling point and the - (R2SiR3) group is more reactive than the R35Si - group in formula (2). The silazane compounds of formula (1) allow for effective heat treatment when combined with conductive carbon particles. Then the use of silazane compounds of formula (1) can adjust the thixotropy of the conductive ink composition or screen printing.

Exemplary of the silazane compound of formula (1) are those of formulae (3) to (6) given below. The silazane compounds may be used alone or in admixture of two or more.

The amount of the silazane compound of formula (1) used is not particularly limited, but preferably selected in accordance with the amount and specific surface area of conductive carbon particles used. About 0.05 to 1 part, especially about 0.1 to 0.8 parts by weight of the silazane compound is used relative to 100 m2 of surface area of conductive carbon particles.

Use of excess silazane compound is uneconomical and often results in cured rubber having too high hardness or low reinforcement.

In addition to the above-mentioned components, the conductive ink composition of the invention preferably has blended therein an additional organopolysiloxane having at least one hydrogen atom attached to a silicon atom in a molecule and containing at least one of alkoxy and epoxy group because significant improvement in adhesion is expectable. This additional organopolysiloxane is effective for improving self-adhesion of the conductive ink composition to substrates.The additional organopoly - siloxane may be selected from known organopoly- siloxanes, for example, those having an alkoxy siloxy group as disclosed in JP - B 21026/1978, those having an epoxy containing hydrocarbon group as disclosed in JP - B 13508/1978, and those having an alkoxysiloxy group and an epoxy-containing hydrocarbon group as disclosed in JP-B 5219/1984. The following silicon compounds are exemplary of the additional organopolysiloxane.

If desired, the additional organopoly- siloxane may be used after some modifications, for example, by increasing its degree of polymerization. The additional Organopoly- siloxane is preferably used in an amount of about 0.5 to 20 parts, especially about 1 to 10 parts by weight per 100 parts by weight of the base organopolysiloxane.

For further improving adhesion, an adhesion modifier may be included in the composition Such adhesion modifier include triallyl isocyanurate, triallyl trimellitate, and siloxane-modified ones thereof and a mixture thereof. Some examples are given below.

The adhesion modifier is preferably blended in an amount of about 0.5 to 3 parts by weight per 100 parts by weight of the base organopolysiloxane although the amount is not critical.

For reinforcing cured products in strength, the composition of the invention may further contain an organopolysiloxane of resin structure containing SiO2, CH2=CH(R'2) - Sill,2, and R'3-SiO1,2 units wherein R' is a monovalent hydrocarbon group free of an unsaturated aliphatic group as disclosed in JP-B 26771/1963 and 9476/1970. This organopolysiloxane is preferably blended in an amount of up to about 30 parts by weight per 100 parts by weight of the base organopolysiloxane although the amount is not critical. Use of more than 30 parts of the further organopolysiloxane is uneconomical and results in cured rubber which is too hard and brittle.

Moreover, for controlling the curing rate of the composition, there may be blended organopolysiloxanes containing CH=CHR'SiO units wherein R' is as define above as disclosed in JP-B 10947/1973, acetylene compounds as disclosed in U.S.P. No. 3,445,420 , and ionic heavy metal compounds as disclosed in U.S.P. No.

3,532,649. Also, a non- functional organo siloxane may be blended for improving the thermal impact and flexibility of cured prod ucts.

Additionally, the composition of the invention may contain any of other additives, for example, heat resistance modifiers such as red iron oxide, black iron oxide, and cerium oxide; flame retardants such as benzotriazole, zinc carbonate, and manganese carbonate; added tion reaction control agents such as acetylene compounds; and foaming agents.

For usual application, a conductive ink composition of the invention is blended with a solvent for adjusting its viscosity and improv ing workability. The solvent used is a volatile organic compound where the composition is of the addition curing type. Almost all volatile organic compounds are useful exclusive of those poisoning the addition reaction, those inviting dehydrogenation, and those having an alkenyl group in a molecule and adversely affecting crosslinking balance.Examples of the volatile organic compound include aromatic hydrocarbons such as benzene, toluene, xylene and ethylene benzene; aliphatic hydrocarbons such as hexane and octane; esters such as methyl acetate, ethyl acetate, and isobutyl acetate; cyclic ethers such as furan and tetrahydrofuran; ethers such as 1-propyl ether, n-butyl ether, and anisole; cyclic siloxanes such as octamethylcyclotetra- siloxane and decamethylcyclopentasiloxane; linear siloxanes such as hexamethyldisiloxane, octamethyltrisiloxane, and decamethyltetra siloxane; chlorinated hydrocarbons such as carbon tetrachloride, perchloroethylene, and trichloroethylene; and fluorinated hydrocarbons such as octafluoropropane. These solvents may be used alone or in admixture of two or more.

Preferred among others are those solvents having a boiling point of 130 to 2000C, especially solvent systems containing at least 60% by weight of an aromatic solvent having such a boiling point. The amount of the solvent used may be appropriately selected although it preferably ranges from about 50 to 2,000 parts, especially from about 200 to 1,000 parts by weight per 100 parts by weight of the base organopolysiloxane.

It is generally not critical how the conductive ink composition is prepared. It may be prepared by mixing the above-mentioned components and diluting the mixture with a solvent if necessary. Preferably, the silazane compound of formula (1) is heat treated in a kneader along with the organopolysiloxane base and conductive carbon particles. A heating temperature of about 50 to 200 OC , especially about 100 to 1600C is preferred. Water may be added to the system for promoting decomposition.

Conductive ink compositions as described herein are useful for the manufacture of contact members as found in keyboards by applying and curing the composition to substrate tes of various dielectric materials. The dielectric material should preferably have a volume resistance of at least 1010 Q ~cm. It may be either resinous or elastomeric. Such electrically insulating resistive materials in resinous form include thermoplastic resins such as ABS, PET, acrylic polycarbonate, PBT, and PPS resins, and thermosetting resins such as epoxy and urethane resins, to name a few. Where the substrates are of hard resins, the conductive silicone rubber is applied to a thickness of about 0.5 to 3 mm.With a thickness of less than 0.5 mm, key touch is less pleasant, excess stresses occur at the interface between the substrate and the conductive silicone rubber, adversely affecting adhesion retentivity.

Coating to a thickness in excess of 3 mm is difficult in commercial practice and sometimes uneconomical. The elastomeric substrates include silicone rubbers and various synthetic rubbers although the manufacture of keyboards favors the use of silicone rubbers in view of electrical insulation and durability.

The conductive ink composition is applied onto a preform of silicone rubber where contacts are to be formed, by any desired printing method, typically screen printing method, thus forming a desired pattern of coating having a thickness of about 5 to 500 gm. Mere curing yields a contact member or keyboard. In one process, an ink composition based on a conduc- tive silicone rubber compound according to the present invention is applied and cured to a cured preform of carbon black - free insulating silicone rubber. Also after an insulating silicone rubber composition is pre - cured and post-cured for complete curing, a conductive ink composition of the invention may be applied thereto and heat cured.Since the conductive ink compositions as described can have good adhesion and bond to insulating silicone rubber compositions as substrates, an alternative process involves primary curing an insulating silicone rubber composition to form a substrate, applying a conductive ink composition of the invention to the substrate, and post-curing both the components at the same time to achieve simultaneous curing and bonding. By applying and curing a conductive ink composition embodying the invention to a substrate, there is obtained a silicone rubber product having a conductive silicone rubber layer integrated with an insulating silicone rubber layer. It will be understood that an insulating silicone rubber preform may be coated with a primer, if necessary, before a cured layer of a conductive ink composition of the invention is formed on the preform.

BENEFITS OBTAINABLE We find that conductive ink compositions embodying the invention have good adhesion and bond to various insulating substrates, especially insulating silicone rubber, little viscosity drop, and stable dispersion after solvent dilution, and cures to products having wear resistance. The composition is advantageously used in the manufacture of contact members such as keyboards. Since a conductive layer of the present composition can be print formed on an insulating silicone rubber preform to constitute a contact member, there is an advantage that contact members such as keyboards having a complex conductor pattern may be readily manufactured by a screen printing technique.

The composition may be either applied and cured to a cured preform of insulating silicone rubber or applied to a primarily cured preform of insulating silicone rubber and concurrently cured therewith. In either case, there is obtained a contact member having a conductive contact layer with good adhesion. Thus manufacturing efficiency is high enough.

EXAMPLE Examples of the present invention are given below by way of illustration and not by way of limitation. In the examples, all parts are by weight and the viscosity is at 25 OC .

Example 1 To 100 parts of a linear dimethylpoly siloxane blocked with a dimethylvinylsilyl group at either end and having a viscosity of 1,000 cs, 10 parts of a silazane of formula (7), and 3 parts of water was added 70 parts of acetylene black having a specific surface area of 60 m2/g (manufactured by Denki Kagaku K.K.). The mixture was heated and milled for 4 hours in a kneader at 150 to 160 C.

After milling, the mixture was cooled down to room temperature, and 0.12 parts of an octanol solution of chloroplatinic acid (containing 2% by weight of platinum), 0.30 parts of ethynyl cyclohexanol, and 1.8 parts of triallyl isocyanurate were added to the mixture and kneaded therewith. The mixture was milled by a triple roll mill. The mixture was added to and dissolved in a solution containing 600 parts of mineral spirit, 13.5 parts of a crosslinking agent of formula (8), and 6.0 parts of an adhesive aid of formula (9), obtaining conduc- tive ink composition I.

The resulting conductive ink composition I had a viscosity of 180 poise before curing. It was applied to a PET film by means of a bar coater, air dried for 30 minutes, and heat cured at 1200C for 60 minutes. The coated film was tested by "Method for measuring the volume resistivity of conductive rubber and plastics" prescribed by the Japan Rubber Associate Standards, 2301-1969. The volume resistivity was 1.5 Q ~cm at a thickness of 0.02 mm.

Using a screen printing technique, conductive ink composition I was applied to contact areas (diameter 3 mm) on a cured sheet of peroxide curing type silicone rubber KE 951 (Shin-Etsu Chemical Co., Ltd., JIS scale A hardness 50) to a thickness of 50 Am. After air drying for 30 minutes, the composition was baked and cured at 1500C for 60 minutes. The cured layer of conductive ink composition I tightly adhered to the silicone rubber sheet.

In a key touch test using the product as a contact key, no interfacial separation occurred after 500,000 cycles.

Example 2 To 100 parts of a linear dimethylpoly siloxane blocked with a dimethylvinylsilyl group at either end and having a viscosity of 5,000 cs, 0.8 parts of a silazane of formula (10), 12.0 parts of a silazane of formula (11), and 4 parts of water was added 100 parts of acetylene black having a specific surface area of 30 m2/g (manufactured by Denki Kagaku K.K.). The mixture was mixed in a planetary mixer, heated and blended for 1 hour at 160 Or , and blended at 1600C for a further 1 hour while passing nitrogen gas therethrough for removing by produced ammonia gas.

The powdery mixture was milled in a triple roll mill into a film, which was mixed with 700 parts of petroleum hydrocarbon solvent mixture HAWS (Shell Chemical K.K.) for dissolution. To the solution were added 0.1 parts of an octanol solution of chloroplatinic acid (containing 2% by weight of platinum), 0.2 parts of ethynyl cyclohexanol, and 1.0 part of triallyl tri mellitate. To the mixture were added 5.0 parts of a compound of formula (12) and 4.0 parts of a compound of formula (13). The mixture was uniformly blended, obtaining conductive ink composition II

The resulting conductive ink composition II had a viscosity of 5.4 poise before curing.

It was applied and cured to a PET film and then tested as in Example 1. The volume resistivity was 1.2 # ~cm.

Using a screen printing technique, con ductive ink composition II was applied to U - shaped contact areas (width 12 mm, length 5 mm, line width 2 mm) on a cured preform of addition curing type injection molding silicone rubber KE 1940-40 (Shin-Etsu Chemical Co., Ltd., JIS scale A hardness 40) to a thickness of 50 m.

After air drying for 30 minutes, the composition was heated and cured at 150 C for 60 minutes.

The cured layer of conductive ink composition II tightly adhered to the cured silicone rubber.

In a key touch test using the product as a contact key, neither cracking nor interfacial separation occurred after 100,000 cycles.

Although some preferred embodiments have been described, many modifications and varia- tions may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

CLAIMS:

1. A conductive ink composition comprising a curable composition containing an organopolysiloxane, a crosslinking agent therefor, and conductive carbon particles, and a silazane compound of the general formula (1)

wherein Rl, R2, and R3 are independently substi tuted or unsubstituted monovalent hydrocarbon groups.

2. The conductive ink composition of claim 1 wherein the curable composition is an addition curing type organopolysiloxane composition containing an organopolysiloxane having at least two alkenyl groups in a molecule and a viscosity of 100 to 200,000 cs at 25 C, an organohydrogenpolysiloxane having at least two hydrogen atoms attached to silicon atoms in a molecule as the crosslinking agent, and a platinum catalyst.

3. The conductive ink composition of claim 1 or 2 wherein the conductive carbon particles are blended in amounts of about 50 to 200 parts by weight of the organopolysiloxane and the silazane compound is blended in amounts of about 0.05 to 1 part by weight relative to 100 m2 of surface area of the conductive carbon particles.

4. The conductive ink composition of any one of claims 1 to 3 which further comprises an organopolysiloxane having at least one hydrogen atom attached to a silicon atom in a molecule and containing at least one of alkoxy and epoxy groups.

5. The conductive ink composition of any one of claims 1 to 4 which further comprises an adhesion modifier selected from trially iso cyanurate, triallyl trimellitate and siloxane- modified ones thereof.

6. A contact member comprising an insu- lating silicone rubber preform and a conductive layer print formed thereon from a conductive ink composition as set forth in any one of claims 1 to 5.

7. A contact member comprising an insu- lating silicone rubber preform and a conductive layer of conductive silicone rubber, which is prepared by applying a conductive ink composition as set forth in claim 2 in uncured state to the preform in cured state, and heat curing the composition.

8. A conductive composition, or method of making a conductive composition, substantially as described herein with reference to the Examples.

9. A contact member substantially as described herein with reference to the Examples.