Patent Description:
Transducers are known, such as actuators and sensors that perform conversion between mechanical energy and electric energy, or speakers and microphones that perform conversion between acoustic energy and electric energy. To form highly flexible, compact, and lightweight transducers, polymer materials such as dielectric elastomers are useful. For example, an actuator can be formed with a pair of electrodes arranged on both front and back surfaces of a dielectric layer of a dielectric elastomer. A capacitance-type sensor can also be formed with electrodes with a dielectric layer interposed therebetween.

In the actuators and sensors of this type, it is desirable that the electrodes are expandable and contractible in accordance with deformation of the dielectric layer. To form flexible electrodes, conductive materials have been developed that are formed by blending conductive carbon black or metal powder in a binder such as an elastomer. For example, Patent Document <NUM> discloses an electrode obtained by blending particular carbon nanotubes having a three dimensional shape and carbon black in an elastomer. Patent Document <NUM> discloses an electrode obtained by blending carbon nanotubes in a base rubber.

The conductivity of the conventional electrodes, however, is not considered to be sufficient. In particular, an increase in electric resistance during expansion is large. A thin film-like electrode is usually formed from a conductive coating in which a conductive material is dispersed in a polymer solution containing an elastomer component dissolved therein. For example, when conductive carbon black having a structure is used as a conductive material, a wet disperser, a jet mill, an ultrasonic disperser, or the like using media such as glass beads is required to be used in order to uniformly disperse the conductive carbon black into the polymer solution. When such a device is used, however, primary particles are broken to decrease the crystallinity, or the aspect ratio becomes smaller. This results in reduction in conductivity of the electrodes formed.

Carbon nanotubes having various structures with different diameters are known. For example, large-diameter carbon nanotubes having a diameter of about <NUM> can be easily dispersed in a polymer solution using, for example, a triple-roll mill. In the case where large-diameter carbon nanotubes are used, the viscosity is less likely to increase when a conductive coating is prepared. However, when large-diameter carbon nanotubes alone are blended in an elastomer, the number of contact points between carbon nanotubes is small. For this reason, entanglement of the carbon nanotubes with each other cannot be maintained during expansion, so that the electric resistance increases. In contrast, in the case where small-diameter carbon nanotubes having a diameter of less than <NUM> are used, the viscosity increase during dispersion in a polymer solution is large in accordance with the increased surface area. It is therefore difficult to prepare a conductive coating when small-diameter carbon nanotubes are used.

The present invention was made in view of the foregoing situation and aims to provide a conductive composition that has a high conductivity and from which a coating can be formed easily. The present invention also aims to provide a conductive film that has a high conductivity and in which electric resistance is less likely to increase even during expansion.

As described above, when a large-diameter fibrous carbon material is blended as a conductive material in an elastomer, it is difficult to ensure the conductivity during expansion because the number of contact points of the conductive material is small. In this respect, in the conductive composition used in the present invention, a conductive carbon black having a structure is blended in addition to a relatively large-diameter fibrous carbon material having a fiber diameter of not less than <NUM>. Thus, when a conductive film is formed, the conductive carbon black fills the gaps of the fibrous carbon material. The bridging of the gaps of the fibrous carbon material by the conductive carbon black can suppress an increase in electric resistance during expansion. The effects of the present invention will be described below using a schematic diagram.

<FIG> schematically shows changes in volume resistivity of conductive materials with respect to elongation. As shown in <FIG>, when a conductive carbon black alone (CB alone) is blended in the elastomer, a change in volume resistivity due to expansion is small but the initial volume resistivity is relatively large. That is, the conductivity is low in the conductive material in which the conductive carbon black alone is blended in the elastomer. When a large-diameter fibrous carbon material alone is blended in the elastomer, the volume resistivity in a natural state (before expansion) is smaller compared to the case where conductive carbon black is blended. However, the volume resistivity sharply increases with the expansion. By contrast, the conductive film formed from the conductive composition used in the present invention exhibits a high conductivity in a natural state because of the large-diameter fibrous carbon material, and an increase in volume resistivity is moderated during expansion by the bridging effect of the conductive carbon black. For comparison, when a small-diameter fibrous carbon material alone is blended in the elastomer, the volume resistivity in a natural state is small and the change in volume resistivity due to expansion is also small. However, it is difficult to make a coating with the small-diameter fibrous carbon material because its dispersion in a polymer solution increases the viscosity. For this reason, it is impossible to form a thin film-like conductive film by screen printing, for example.

In this respect, the conductive composition used in the present invention includes a relatively large-diameter fibrous carbon material having a fiber diameter of not less than <NUM>. The relatively large-diameter fibrous carbon material can be easily dispersed into a polymer solution using a triple-roll mill, for example. In addition, the viscosity increase is small. The dispersion using a triple-roll mill does not exert excessive shear force or shock on the conductive carbon black, so that the structure is less likely to be broken, and reduction in crystallinity is also suppressed. As described above, the conductive composition used in the present invention can facilitate forming of a coating, and can achieve a conductive film having an excellent conductivity.

Examples of the method of applying the coating include printing processes such as screen printing, inkjet printing, flexographic printing, gravure printing, pad printing, metal mask printing, and lithography, and further include a dipping process, a spray process, and a bar coating process. The conductive composition used in the present invention formed into a coating has a relatively small viscosity. Therefore, it is suitable for printing processes. Printing processes can easily form a thin film or a large-area conductive film. Also, with the printing processes, separation between a section to be coated and a section not to be coated is easy. Thus, even a conductive film in the form of a thin line or a complicated shape can be formed easily.

(<NUM>) A conductive film according to the present invention is formed from the conductive composition used in the present invention. That is, the conductive film of the present invention contains an elastomer, a fibrous carbon material having a graphite structure with a fiber diameter of not less than <NUM>, and a conductive carbon black having a structure. As described above, in the conductive film of the present invention, the conductive carbon black bridges the gaps of the fibrous carbon material. The conductive film of the present invention therefore has a high conductivity, and the electric resistance is less likely to increase even during expansion. The conductive film according to the present invention is formed on a surface of a substrate made of an expandable and contractible elastomer. Moreover, the volume resistivity is not more than <NUM>Ω·cm when the conductive film is expanded in a uniaxial direction by <NUM>%.

(<NUM>) An electromagnetic shield according to the present invention is formed of the conductive film of the present invention having the constitution described in (<NUM>) above.

The electromagnetic shield of the present invention is flexible and has a high conductivity, and the electric resistance is less likely to increase even during expansion. Even for the use in an expandable/contractible member, the shield performance is less likely to be reduced. The electromagnetic shield of the present invention therefore has good durability.

(<NUM>) A transducer according to the present invention includes a dielectric layer made of a polymer, a plurality of electrodes arranged with the dielectric layer interposed therebetween, and a wire connected to each of the electrodes. Either or both of the electrodes and the wire are formed of the conductive film of the present invention having the constitution described in (<NUM>) above.

A transducer is a device that converts one form of energy into another form of energy. Transducers include actuators, sensors, power generation elements, and the like for performing conversion between mechanical energy and electrical energy, and speakers, microphones, and the like for performing conversion between acoustic energy and electrical energy. The electrodes and wires formed of the conductive film of the present invention are flexible and have a high conductivity, and the electric resistance is less likely to increase even during expansion. In the transducer of the present invention, therefore, the motion of the dielectric layer is less likely to be restricted by the electrodes and the wires. The electric resistance is less likely to increase over repeated expansion and contraction. In the transducer of the present invention, therefore, degradation in performance due to the electrodes and the wires is less likely to occur. The transducer of the present invention therefore has good durability.

(<NUM>) A flexible wiring board according to the present invention includes a substrate and a wire arranged on a surface of the substrate. At least part of the wire is formed of the conductive film of the present invention having the constitution described in (<NUM>) above.

The wire formed of the conductive film of the present invention is flexible and has a high conductivity, and the electric resistance is less likely to increase even during expansion. Thus, the performance of the flexible wiring board of the present invention is less likely to be degraded even when the substrate expands and contracts. The flexible wiring board of the present invention therefore has good durability.

The conductive composition used in the present invention includes an elastomer component, a fibrous carbon material having a graphite structure and a fiber diameter of not less than <NUM>, and a conductive carbon black having a structure. In this description, the elastomer includes cross-linked rubbers and thermoplastic elastomers. Examples of the elastomer component included in the conductive composition used in the present invention therefore include rubber polymers before cross-linkage and thermoplastic elastomers.

The elastomer preferably has a glass transition temperature (Tg) being room temperatures or lower, in view of having rubber-like elasticity at room temperatures. When Tg is lower, the crystallinity decreases. The elastomer becomes more expandable and contractible accordingly. For example, an elastomer having Tg of not more than -<NUM>, more preferably not more than -<NUM>, is more flexible and thus suitable. As the elastomer, one kind thereof may be used singly, or a mixture of two or more kinds thereof may be used.

The elastomer is preferably a cross-linked rubber because the restorability is good when deformation is repeated. Another preferred example is a material, such as a thermoplastic elastomer, that has a microphase separation structure of a hard segment and a soft segment and forms a pseudo cross-link. Examples of a material having a cross-linkable functional group include urethane rubber, acrylic rubber, silicone rubber, butyl rubber, butadiene rubber, ethylene oxide-epichlorohydrin copolymer, nitrile rubber, chloroprene rubber, natural rubber, isoprene rubber, styrene-butadiene rubber, ethylene-propylene-diene copolymer (EPDM), silicone rubber, and polyester rubber. Among those, acrylic rubber has a low crystallinity and a weak intermolecular force and thus has a Tg lower than those of the other rubbers. Thus, it is flexible and well expandable, and is suitable for electrodes of actuators, sensors, and the like.

The fibrous carbon material is carbon nanotubes having a graphite structure and a fiber diameter of not less than <NUM>. In particular, carbon nanotubes of which the intensity ratio (G/D ratio) of a peak (G band) appearing in the vicinity of <NUM>-<NUM> to a peak (D band) appearing in the vicinity of <NUM>-<NUM> of Raman spectrum is not less than <NUM> are used in view of high crystallinity and high conductivity. Examples of the carbon nanotubes include those with a bamboo-like structure, such as cup-stacked carbon nanotubes having a structure in which cup-shaped carbon networks open at the bottom are stacked. A large fiber diameter facilitates dispersion into a polymer solution. For this reason, a material having a fiber diameter of not less than <NUM>, more preferably not less than <NUM>, is preferred. With a larger fiber diameter, however, the number of fibrous carbon material per unit mass is reduced when a conductive film is formed. Conductive paths are thus less likely to be formed. The fiber diameter is therefore preferably not more than <NUM>. As the fibrous carbon material, one kind thereof may be used singly, or a mixture of two or more kinds thereof may be used.

The conductive carbon black is present in the gaps of the dispersed fibrous carbon material and serves the function of coupling the fibrous carbon material with each other to keep the conductivity. If the gaps of the fibrous carbon material dispersed in a polymer solution are small, the viscosity is likely to increase. Since the conductive carbon black is present in the gaps of the fibrous carbon material, the conductive carbon black also serves the function of widening the gaps of the fibrous carbon material to restrain an increase in viscosity. For example, the primary particle diameter of the conductive carbon black is preferably not more than <NUM>. In addition, a material having a large structure is preferred.

The conductive composition used in the present invention may contain an organic solvent and an additive such as a cross-linking agent, a cross-linking accelerator, a cross-linking aid, a dispersant, a plasticizer, a processing aid, an antioxidant, a softener, and a colorant. The cross-linking agent, cross-linking accelerator, cross-linking aid, and the like may be selected as appropriate depending on, for example, the kind of the elastomer.

The addition of a dispersant can suppress excessive aggregation of the conductive carbon black. Thus, a viscosity increase due to aggregation of the conductive carbon black and an increase in pseudoplasticity can be suppressed. A compound having a high affinity for the fibrous carbon material and the conductive carbon black may be used as the dispersant. Preferred examples thereof include: polymer compounds having a substituent such as an amino group and a carboxy group or a substituent having a π conjugated system such as a phenyl group, pyrene, and porphyrin derivatives; phosphates; amine salts; polyethers; polyesters; and polyurethanes. A polymer compound having a high affinity for the elastomer may be used. Examples thereof include polymers of acrylic esters, methacrylic acid esters, acrylamide, and the like. Examples of polymer compounds having a high affinity for all of the fibrous carbon material, the conductive carbon black, and the elastomer include polyurethanes, polyamines, copolymers thereof, and polyamides.

The addition of a plasticizer and a softener can improve workability of the elastomer and further improve flexibility. Any plasticizer that has good compatibility with the elastomer can be used. Examples of the plasticizer that can be used include organic acid derivatives such as known phthalate diesters, phosphoric acid derivatives such as tricresyl phosphate, adipic acid diesters, chlorinated paraffins, and polyether esters. Plant-based softeners and mineral softeners can be used as the softener. Examples of the plant-based softeners include stearic acid, lauric acid, ricinoleic acid, palmitic acid, cottonseed oil, soybean oil, castor oil, palm oil, pine tar oil, tall oil, and factice. Examples of the mineral softeners include paraffin-based, naphthene-based, and aroma-based oils.

The conductive composition used in the present invention can be prepared, for example, by kneading a mixture of the elastomer component, the fibrous carbon material, the conductive carbon black, and an additive blended if necessary, with a pressure kneading machine such as a kneader or a Banbury mixer, a twin roll, or the like. Alternatively, the conductive composition can be prepared by adding the fibrous carbon material, the conductive carbon black, and an additive blended if necessary, to a polymer solution containing the elastomer component dissolved in an organic solvent, and mixing them, for example, with a triple-roll mill (conductive coating). In the latter, when the viscosity is measured with a B-type viscometer with an H7 rotor under the conditions of a temperature of <NUM> and a rotation speed of <NUM> rpm, with a solid content concentration of not less than <NUM>% by mass, the viscosity of the conductive composition used in the present invention is preferably not more than <NUM> Pa·s. Thus, a conductive film can be formed easily from the conductive composition used in the present invention by a printing process.

Among the printing processes, screen printing or metal mask printing are preferred. With screen printing and metal mask printing, plate-making is inexpensive, and large-area conductive films in various shapes can be formed easily. For example, it is possible to form a conductive film having an area as large as <NUM><NUM> or more per printing pattern. In addition, the film thickness can be easily controlled, and therefore a conductive film as thick as <NUM> or more can be formed easily, for example. A thick conductive film has a smaller electric resistance, thereby improving the performance of devices.

The conductive film of the present invention is formed from the conductive composition used in the present invention. For example, the conductive film can be formed by press-forming the conductive composition prepared by kneading. Alternatively, the conductive film can be formed by applying the conductive composition prepared in a coating state (conductive coating) on a substrate and drying by heating. A variety of known methods can be employed as the method of application. In particular, screen printing is preferred as previously described.

The thickness of the conductive film of the present invention can be determined as appropriate depending on applications. For example, when the conductive film of the present invention is used as electrodes and/or wiring of actuators, sensors, or the like, the thickness may be set to not less than <NUM> and not more than <NUM>. In the conductive film of the present invention, the electric resistance is less likely to increase even during expansion. For example, in consideration of the use in a part subjected to repeated expansion and contraction, such as electrodes and wiring of flexible transducers, the volume resistivity is not more than <NUM>Ω·cm when the conductive film of the present invention is expanded in a uniaxial direction by <NUM>%.

The conductive film of the present invention is formed on a surface of a substrate made of an expandable and contractible elastomer. Examples of the substrates include expandable and contractible elastomer sheets. Examples of the elastomer include acrylic rubber, EPDM, nitrile rubber, urethane rubber, butyl rubber, silicone rubber, chloroprene rubber, ethylene-vinyl acetate copolymer, and thermoplastic elastomers (olefinic, styrenic, polyester-based, acrylic, urethane-based, and polyvinyl chloride-based). When the conductive film of the present invention is formed on a surface of an expandable and contractible substrate, the flexibility is high, and the effect of being less likely to increase electric resistance during expansion is enhanced. For example, a substrate whose elongation at break measured in accordance with JIS K6251: <NUM> is <NUM>% or more is preferred.

If the adhesiveness of the conductive film on the substrate is insufficient, the conductive film may be separated from the substrate over repeated expansion and contraction. In addition, dielectric breakdown may be caused by, for example, voids generated between the conductive film and the substrate. The conductive film is therefore required to be bonded to the substrate reliably. For example, a polymer having a cross-linkable functional group is used as the elastomer component in the conductive composition for forming the conductive film, and a polymer having a functional group is used as the substrate. The functional group of the substrate is allowed to react during cross-linking of the elastomer component, so that the conductive film and the substrate can be bonded through chemical bonding. Thus, the adhesiveness of the conductive film with the substrate can be improved. For example, a functional group can be provided by performing surface treatment on the substrate. The surface treatment is performed by corona discharge, irradiation of plasma, laser, ultraviolet rays, etc, primer coating, or other means.

The electromagnetic shield according to the present invention is formed of the conductive film of the present invention. The electromagnetic shield serves the functions of blocking leakage of electromagnetic waves generated in electronic equipment to the outside and intrusion of external electromagnetic waves to the inside. For example, to provide the electromagnetic shield on the inner surface of the casing of electronic equipment, the conductive composition used in the present invention prepared in a liquid state may be applied on the inner surface of the casing of electronic equipment and dried.

The transducer of the present invention includes a dielectric layer made of a polymer, a plurality of electrodes arranged with the dielectric layer interposed therebetween, and a wire connected to each of the electrodes. The transducer of the present invention may have a stack structure in which the dielectric layer and the electrode are alternately stacked.

The dielectric layer is formed of a polymer, that is, a resin or an elastomer. Elastomers are preferred because they are expandable and contractible. In particular, an elastomer having a high dielectric constant is desirable in terms of increasing the amount of displacement and the force produced. Specifically, preferred is an elastomer having a dielectric constant (<NUM>) of <NUM> or more at room temperatures, more preferably <NUM> or more. Examples of the elastomer that may be used include elastomers having a polar functional group such as an ester group, a carboxyl group, a hydroxy group, a halogen group, an amide group, a sulfone group, a urethane group, and a nitrile group, or elastomers to which a polar low-molecular-weight compound having these polar functional groups. Examples of preferable elastomers include silicone rubber, acrylonitrile-butadiene rubber (NBR), hydrogenated acrylonitrile-butadiene rubber (H-NBR), EPDM, acrylic rubber, urethane rubber, epichlorohydrin rubber, chlorosulfonated polyethylene, and chlorinated polyethylene. The wording "made of a polymer" means that the base material of the dielectric layer is a resin or an elastomer. Thus, a component other than the elastomer or resin component, such as an additive, may be contained.

The thickness of the dielectric layer may be determined as appropriate depending on, for example, applications of the transducer. For example, in the case of an actuator, the thickness of the dielectric layer is desirably small in light of size reduction, low potential driving, larger displacement, and the like. In this case, also in consideration of a dielectric breakdown property and the like, the thickness of the dielectric layer is desirably not less than <NUM> and not more than <NUM> (<NUM>). The thickness of not less than <NUM> and not more than <NUM> is more preferred.

Either or both of the electrodes and the wire are formed of the conductive film of the present invention. The configuration and production process of the conductive film of the present invention are as described above. A description thereof is omitted here. Preferred modes of the conductive film of the present invention are also applied to the electrodes and wires of the transducer of the present invention. An embodiment of an actuator will be described below as an example of the transducer of the present invention.

<FIG> is a schematic sectional view of the actuator of the present embodiment. The voltage-off state is shown in (a), and the voltage-on state is shown in (b).

As shown in <FIG>, the actuator <NUM> includes a dielectric layer <NUM>, electrodes 11a, 11b, and wires 12a, 12b. The dielectric layer <NUM> is made of silicone rubber. The electrode 11a is arranged so as to cover almost the entire top surface of the dielectric layer <NUM>. Similarly, the electrode 11b is arranged so as to cover almost the entire bottom surface of the dielectric layer <NUM>. The electrodes 11a and 11b are connected to a power source <NUM> through the wires 12a and 12b, respectively. The electrodes 11a and 11b are formed of a conductive film formed by screen-printing the conductive composition used in the present invention.

To switch the off state to the on state, voltage is applied between a pair of electrodes 11a and 11b. With the application of voltage, the thickness of the dielectric layer <NUM> decreases, and the dielectric layer <NUM> expands accordingly parallel to the plane of the electrodes 11a and 11b, as shown by white arrows in <FIG>. The actuator <NUM> thereby outputs a drive force in the up/down direction and the left-right direction in the figure.

According to the present embodiment, the electrodes 11a and 11b are flexible and have expandability and contractibility. For this reason, the electrodes 11a and 11b are less likely to restrict the motion of the dielectric layer <NUM>. A large force and amount of displacement therefore can be obtained with the actuator <NUM>. The electrodes 11a and 11b have a high conductivity. In addition, the electric resistance is less likely to increase over repeated expansion and contraction. For this reason, degradation in performance due to the electrodes 11a and 11b is less likely to occur in the actuator <NUM>. The actuator <NUM> therefore has good durability.

The flexible wiring board of the present invention includes a substrate and a wire arranged on a surface of the substrate. Examples of the substrate include expandable and contractible elastomer sheets listed above as preferred examples of the substrate on which the conductive film of the present invention is formed.

At least part of the wire is formed of the conductive film of the present invention. The configuration and production process of the conductive film of the present invention are as described above. A description thereof is omitted here. Preferred modes of the conductive film of the present invention are also applied to the flexible wiring board of the present invention. An embodiment of the flexible wiring board of the present invention will be described below.

First, the configuration of the flexible wiring board of the present embodiment is described. <FIG> is a top perspective view of the flexible wiring board of the present embodiment. In <FIG>, the electrodes and wires on the back side are shown by thin lines. As shown in <FIG>, a flexible wiring board <NUM> includes a substrate <NUM>, front electrodes 01X to 16X, back electrodes 01Y to 16Y, front wires 01x to 16x, back wires 01y to 16y, a front wire connector <NUM>, and a back wire connector <NUM>.

The substrate <NUM> is made of urethane rubber and shaped like a sheet. A total of <NUM> front electrodes 01X to 16X are arranged on the top surface of the substrate <NUM>. The front electrodes 01X to 16X are each shaped like a strip. The front electrodes 01X to 16X each extend in the X direction (the left-right direction). The front electrodes 01X to 16X are arranged so as to be spaced apart at particular intervals in the Y direction (the front-rear direction) and approximately parallel to each other. Similarly, a total of <NUM> back electrodes 01Y to 16Y are arranged on the bottom surface of the substrate <NUM>. The back electrodes 01Y to 16Y are each shaped like a strip. The back electrodes 01Y to 16Y each extend in the Y direction. The back electrodes 01Y to 16Y are arranged so as to be spaced apart at particular intervals in the X direction and approximately parallel to each other. As shown by the hatching in <FIG>, the intersections (overlapping portions) of the front electrodes 01X to 16X and the back electrodes 01Y to 16Y, with the substrate <NUM> interposed therebetween, form a detector for detecting a load or the like.

A total of <NUM> front wires 01x to 16x are arranged on the top surface of the substrate <NUM>. The front wires 01x to 16x are each shaped like a line. The front wires 01x to 16x are formed of a conductive film formed by screen-printing the conductive composition used in the present invention. The front wire connector <NUM> is arranged on the rear-left corner of the substrate <NUM>. The front wires 01x to 16x connect the left ends of the front electrodes 01X to 16X, respectively, to the front wire connector <NUM>. The top surface of the substrate <NUM>, the front electrodes 01X to 16X, and the front wires 01x to 16x are covered with a front cover film (not shown) from above.

A total of <NUM> back wires 01y to 16y are arranged on the bottom surface of the substrate <NUM>. The back wires 01y to 16y are each shaped like a line. The back wires 01y to 16y are formed of a conductive film formed by screen-printing the conductive composition used in the present invention. The back wire connector <NUM> is arranged on the front-left corner of the substrate <NUM>. The back wires 01y to 16y connect the front ends of the back electrodes 01Y to 16Y, respectively, to the back electrode connector <NUM>. The bottom surface of the substrate <NUM>, the back electrodes 01Y to 16Y, and the back wires 01y to 16y are covered with a back cover film (not shown) from below.

The front wire connector <NUM> and the back wire connector <NUM> are each electrically connected with an operation unit (not shown). The operation unit receives impedance at the detector from the front wires 01x to 16x and the back wires 01y to 16y. Based on this, a surface pressure distribution is measured.

The operation effects of the flexible wiring board <NUM> of the present embodiment will now be described. According to the present embodiment, the front wires 01x to 16x and the back wires 01y to 16y are each flexible and have expandability and contractibility, and thus, can be deformed in accordance with deformation of the substrate <NUM>. The front wires 01x to 16x and the back wires 01y to 16y each have a high conductivity, and the electric resistance is less likely to increase even during expansion. The performance of the flexible wiring board <NUM> is therefore less likely to be degraded even when the substrate <NUM> expands and contracts. The flexible wiring board <NUM> therefore has good durability. As described above, the flexible wiring board <NUM> is suitable for connecting an expandable/contractible element to an electrical circuit.

The present invention will be described more specifically with examples.

First, <NUM> parts by mass of acrylic rubber polymer ("Nipol (registered trademark) AR42" manufactured by ZEON CORPORATION) and <NUM> parts by mass of isophoronediamine as a cross-linking agent were mixed with a roll kneader to prepare a mixture. The mixture was then dissolved in ethylene glycol monobutyl ether acetate as a solvent to prepare a polymer solution. Subsequently, <NUM> parts by mass of carbon nanotubes ("VGCF (registered trademark)" manufactured by SHOWA DENKO K. , fiber diameter <NUM>, length <NUM>, G/D ratio = <NUM>), <NUM> parts by mass of a conductive carbon black ("KetjenBlack EC300J" manufactured by Ketjen Black International Company), and <NUM> parts by mass of a high-molecular compound having an amino group ("BYK-<NUM>" manufactured by BYK Japan KK) as a dispersant were added to the prepared polymer solution, and kneaded with a triple-roll mill to prepare a conductive composition (hereinafter called "conductive coating" as appropriate).

The viscosity of the conductive coating was measured with a B-type viscometer with an H7 rotor under the conditions of a temperature of <NUM> and a rotation speed of <NUM> rpm. The resulting viscosity of the conductive coating was <NUM> Pa·s. The mass of a nonvolatile component in the subsequent drying of the applied film was measured to calculate the solid content concentration of the conductive coating. The resulting solid content concentration of the conductive coating was <NUM>% by mass.

The conductive coating was applied on a surface of a PET substrate by a bar coating process. The coating was thereafter dried by heating at <NUM> for one hour, while a cross-linking reaction proceeded. A conductive film of <NUM> in thickness was thus produced.

The freeze fracture face of the produced conductive film was observed with a scanning electron microscope (SEM). <FIG> is an SEM image of the freeze fracture face of the conductive film of Example <NUM>. As shown in <FIG>, the fibrous carbon nanotubes are dispersed in the conductive film. The conductive carbon black in a structure state is contained so as to fill the gaps of the dispersed carbon nanotubes.

A conductive coating was prepared with the kind of carbon nanotubes changed. Specifically, <NUM> parts by mass of carbon nanotubes ("VGCF-S"' manufactured by SHOWA DENKO K. , fiber diameter <NUM>, length <NUM>, G/D ratio = <NUM>), <NUM> parts by mass of the conductive carbon black (described above), and <NUM> parts by mass of the dispersant (described above) were added to the polymer solution prepared in Example <NUM>, and kneaded with a triple-roll mill to prepare a conductive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

A conductive coating was prepared with the kind of elastomer changed. First, <NUM> parts by mass of silicon rubber polymer ("X40-<NUM>-<NUM>" manufactured by Shin-Etsu Chemical Co. ) was dissolved in xylene as a solvent to prepare a polymer solution. Next, <NUM> parts by mass of the same carbon nanotubes as in Example <NUM>, <NUM> parts by mass of the conductive carbon black (describe above), and <NUM> parts by mass of the dispersant (described above) were added to the prepared polymer solution, and kneaded with a triple-roll mill to prepare a conducive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

A conductive coating was prepared with the kind of carbon nanotubes changed. Specifically, <NUM> parts by mass of cup-stacked carbon nanotubes ("Carber (registered trademark) 24HHT" manufactured by GSI Creos Corporation, fiber diameter (outer diameter) <NUM>, length <NUM>, G/D ratio = <NUM>), <NUM> parts by mass of the conductive carbon black (described above), and <NUM> parts by mass of the dispersant (described above) were added to the polymer solution prepared in Example <NUM>, and kneaded with a triple-roll mill to prepare a conductive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

A conductive coating was prepared with the kind of carbon nanotubes changed. Specifically, <NUM> parts by mass of carbon nanotubes ("MWNT-<NUM>" manufactured by Hodogaya Chemical Co. , fiber diameter <NUM>, length <NUM>, G/D ratio = <NUM>), <NUM> parts by mass of the conductive carbon black (described above), and <NUM> parts by mass of the dispersant (described above) were added to the polymer solution prepared in Example <NUM>, and kneaded with a triple-roll mill to prepare a conductive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

A conductive coating was prepared with the kind of carbon nanotubes changed. Specifically, <NUM> parts by mass of carbon nanotubes ("VGCF-X" manufactured by SHOWA DENKO K. , fiber diameter <NUM>, length <NUM>, G/D ratio = <NUM>), <NUM> parts by mass of the conductive carbon black (described above), and <NUM> parts by mass of the dispersant (described above) were added to the polymer solution prepared in Example <NUM>, and kneaded with a triple-roll mill to prepare a conductive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

A conductive coating was prepared with the kind of carbon nanotubes changed. Specifically, <NUM> parts by mass of the same carbon nanotubes as in Example <NUM>, <NUM> parts by mass of the same carbon nanotubes as in Comparative Example <NUM>, <NUM> parts by mass of the conductive carbon black (described above), and <NUM> parts by mass of the dispersant (described above) were added to the polymer solution prepared in Example <NUM>, and kneaded with a triple-roll mill to prepare a conductive coating. A conductive film was then produced in the same manner as in Example <NUM>. The solid content concentration of the conductive coating was calculated, and the viscosity of the conductive coating was measured in the same manner as in Example <NUM>. The solid content concentration was <NUM>% by mass, and the viscosity was <NUM> Pa·s.

Table <NUM> shows the kinds and blended amounts of raw materials used, and the solid content concentrations and viscosities of the conductive coatings. In Table <NUM>, the blended amount of raw material is a proportion by mass where the mass (solid content) of the conductive film is <NUM>% by mass.

The conductivity of the produced conductive films was evaluated as follows. First, the volume resistivity of the conductive film in a natural state (before expansion) was measured. The measurement of the volume resistivity was conducted in accordance with the parallel terminal electrode method of JIS K6271 (<NUM>). In measurement of the volume resistivity, a commercially available rubber sheet ("VHB (registered trademark) <NUM>" manufactured by Sumitomo <NUM> Limited) was used as an insulating resin support for supporting the conductive film (test piece). The conductive film was then expanded in a uniaxial direction at an expansion ratio of <NUM>%, and the volume resistivity was measured. The expansion ratio was calculated by the following equation (<NUM>). <MAT> [L<NUM>: distance between reference lines of the test piece, ΔL<NUM>: an increase due to expansion of the distance between reference lines of the test piece].

The measurement results of the volume resistivity are shown in Table <NUM> above. As shown in Table <NUM>, it was confirmed that all the conductive films of Examples <NUM> to <NUM> had a high conductivity and an increase in volume resistivity was small even when expanded. Specifically, the volume resistivities of the conductive films of Example <NUM> to <NUM> expanded in a uniaxial direction by <NUM>% were each <NUM>Ω·cm or less.

As for the conductive coatings of Examples <NUM> to <NUM>, the viscosity was not more than <NUM> Pa·s although the solid content concentration was not less than <NUM>% by mass. A conductive film can be formed easily by a printing process such as screen printing with the conductive coatings of Examples <NUM> to <NUM>, because the viscosity of the conductive coatings is small. By contrast, as for the conductive coatings of Comparative Examples <NUM> and <NUM>, in which small-diameter carbon nanotubes with a fiber diameter of less than <NUM> were blended, the viscosity greatly exceeded <NUM> Pa·s although the solid content concentration was less than <NUM>% by mass. Therefore, screen printing cannot be used for forming a conductive film.

Claim 1:
A conductive film characterized by comprising a conductive composition and being formed on a surface of a substrate made of an expandable and contractible elastomer, wherein
the conductive composition includes:
an elastomer component;
a fibrous carbon material having a graphite structure and a fiber diameter of not less than <NUM>; and
a conductive carbon black having a structure, and
the fibrous carbon material is carbon nanotubes of which an intensity ratio (G/D ratio) of a peak (G band) appearing in the vicinity of <NUM>-<NUM> to a peak (D band) appearing in the vicinity of <NUM>-<NUM> of Raman spectrum is not less than <NUM>; and
wherein the volume resistivity is not more than <NUM>Ω·cm when the conductive film is expanded in a uniaxial direction by <NUM>%.