Conductive thermoplastic elastomer composition, conductive sheet therewith, and method for producing conductive sheet

Abstract

To provide a flexible conductive thermoplastic elastomer composition excellent in conductivity, a conductive sheet therewith, and a method for producing the conductive sheet.SOLUTION: A conductive thermoplastic elastomer composition comprises a thermoplastic elastomer and a conductive material. The thermoplastic elastomer has a styrenic thermoplastic elastomer having a melt mass flow rate (MFR) of 4 g / 10 min or more at 230°C and a load of 2.16 kg, and a Shore A durometer hardness of less than 60. The conductive material contains branched carbon nanotubes, and the content of the branched carbon nanotubes is between 5 pts.mass and 10 pts.mass with respect to 100 pts.mass of the thermoplastic elastomer. When calculating the Iδ index, which indicates the degree of agglomeration of the conductive material in the conductive thermoplastic elastomer composition, the rate of change of the Iδ index in natural conditions and in damp heat conditions is 65% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to conductive thermoplastic elastomer compositions for use in electrodes of flexible sensors and the like. [Background technology]

[0002] As the Internet of Things (IoT) advances, there is a growing demand for wearable sensors that measure respiratory status, heart rate, and other parameters in the fields of nursing care, health management, and training. Furthermore, automobiles and other vehicles are equipped with various sensors, such as steering sensors and seating sensors, to detect the status of their occupants. These sensors often use flexible materials, such as elastomers, to improve their ability to follow the subject's movements and reduce discomfort.

[0003] For example, Patent Document 1 discloses a flexible sensor comprising an electrode made of a thermoplastic elastomer having a predetermined melt viscosity and a DBP absorption capacity of 300 cm 3 and a conductive material containing carbon black of 100 g or more, and a sheet-like flexible electrode having a thickness of 50 μm or more and 500 μm or less. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 172425 [Patent Document 2] Japanese Patent Publication No. 2022-20433 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-198425 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the electrical conductivity of conventional electrode materials is still insufficient, and studies are needed to prevent the decrease in electrical conductivity when stretched. Furthermore, for use in flexible sensors, the materials need to be formed into thin sheets, and efficient manufacturing is required for practical use.

[0006] The present disclosure has been made in consideration of the above-described circumstances, and aims to provide a conductive thermoplastic elastomer composition that is flexible and has excellent conductivity, a conductive sheet using the same, and a method for manufacturing the conductive sheet. [Means for solving the problem]

[0007] (1) In order to solve the above-mentioned problems, the present disclosure provides a conductive thermoplastic elastomer composition comprising a thermoplastic elastomer and a conductive material, wherein the thermoplastic elastomer comprises a styrene-based thermoplastic elastomer having a melt mass-flow rate (MFR) of 4 g / 10 min or more at 230°C under a load of 2.16 kg and a Type A durometer hardness of less than 60, and the conductive material comprises branched carbon nanotubes, the content of which is 5 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the thermoplastic elastomer, and wherein when the Iδ index, which indicates the degree of aggregation of the conductive material, is determined for the conductive thermoplastic elastomer composition in its natural state and in a humid and heat state after being left to stand for 100 hours in a humid and heat environment at a temperature of 90°C and a relative humidity of 95%, the rate of change in the Iδ index in the humid and heat state calculated by the following formula (I) is 65% or less: Change rate of Iδ index in moist and hot conditions (%) = (Iδ1-Iδ0) / Iδ0 × 100 (I) [Iδ0: Iδ index in natural conditions, Iδ1: Iδ index in moist and hot conditions]

[0008] The conductive thermoplastic elastomer composition of the present disclosure is flexible because it uses a thermoplastic elastomer as the base material. The thermoplastic elastomer used is a styrene-based thermoplastic elastomer having at least a predetermined MFR and hardness. The styrene-based thermoplastic elastomer has a high affinity for carbon materials such as branched carbon nanotubes due to π-π interactions caused by the aromatic ring. Therefore, it is believed that the conductive thermoplastic elastomer has a higher effect of enhancing conductivity than, for example, an olefin-based thermoplastic elastomer.

[0009] MFR is an index indicating the fluidity of a polymer. In the conductive thermoplastic elastomer composition of the present disclosure, the MFR of the styrene-based thermoplastic elastomer is limited to a predetermined value or higher to improve fluidity, thereby suppressing an increase in viscosity of the kneaded product (elastomer composition) during the manufacturing process. Therefore, the conductive thermoplastic elastomer composition of the present disclosure allows for the continuous production of thin sheet materials with uniform thickness by extrusion molding or the like. Furthermore, suppressing an increase in viscosity of the kneaded product suppresses the destruction of branched carbon nanotubes during kneading, which is effective in improving conductivity. Furthermore, the Type A durometer hardness of the styrene-based thermoplastic elastomer is less than 60, thereby enhancing the flexibility of the conductive thermoplastic elastomer composition.

[0010] The conductive thermoplastic elastomer composition of the present disclosure uses at least branched carbon nanotubes as the conductive material. Branched carbon nanotubes refer to carbon nanotubes with a branched structure. Because branched carbon nanotubes have a three-dimensionally extending shape, carbon nanotubes tend to come into contact with each other and easily form conductive paths. Furthermore, the conductive paths are less likely to be broken even when stretched. Therefore, compared to when non-branched linear carbon nanotubes are used, sufficient conductivity is achieved even with a small content, and the decrease in conductivity during stretching is small. As a result, a conductive thermoplastic elastomer composition that is flexible and has excellent conductivity can be realized.

[0011] During the development of a conductive thermoplastic elastomer composition using branched carbon nanotubes, the inventors discovered that placing the conductive thermoplastic elastomer composition in a high-temperature, high-humidity, and hot environment could result in a decrease in conductivity. After extensive research into this issue, the inventors discovered that in a hot and humid environment, branched carbon nanotubes aggregate, disrupting the conductive paths. They then found that the aggregation of branched carbon nanotubes can be suppressed by limiting the content of branched carbon nanotubes. Based on this finding, the content of branched carbon nanotubes in the conductive thermoplastic elastomer composition of the present disclosure is specified to be 5 to 10 parts by mass per 100 parts by mass of the thermoplastic elastomer.

[0012] In the conductive thermoplastic elastomer composition of the present disclosure, the degree of aggregation of conductive materials, including branched carbon nanotubes, is indicated by the Iδ index. The Iδ index is also known as the "Morishita Iδ index" after its creator, and is widely known as an index for analyzing the distribution patterns of living organisms. The Iδ index is calculated by dividing the survey area into several sections and counting the number of individuals (number of conductive materials) in each section using the following formula (A). The lower the dispersibility of the conductive material and the more likely it is to aggregate, the larger the Iδ index.

number

[0013] In the conductive thermoplastic elastomer composition of the present disclosure, the rate of change of the Iδ index in a humid heat state relative to the Iδ index in a natural state is 65% or less. In this specification, the natural state refers to the state after being left to stand at room temperature for 24 hours after production. The humid heat state refers to the state after being left to stand in a humid heat environment at a temperature of 90°C and a relative humidity of 95% for 100 hours. The smaller the rate of change of the Iδ index in a humid heat state, the less likely the conductive material will aggregate in the humid heat environment, and the less likely the conductive path will be disrupted. Thus, the conductive thermoplastic elastomer composition of the present disclosure, in which the rate of change of the Iδ index in a humid heat state is 65% or less, is less likely to lose conductivity even when left in a humid heat environment.

[0014] Incidentally, Patent Document 2 describes a composition containing an elastomer component (styrene-ethylene-butylene-styrene block copolymer and maleic anhydride-modified ethylene-vinyl acetate copolymer), paraffin oil, and branched multi-walled carbon nanotubes. In the composition described in Patent Document 2, paraffin oil is used as a lubricating component to disperse the branched multi-walled carbon nanotubes. However, as shown in Table 1 of the same document, the volume resistivity of the composition is high and the conductivity is insufficient. Furthermore, no study has been conducted on the change in conductivity in a humid and hot environment, nor on molding processability such as extrusion.

[0015] Furthermore, Patent Document 3 describes a flexible electrode having a thermoplastic elastomer and a carbon nanotube made of carbon fibers, the carbon fibers extending three-dimensionally from a central portion. However, Patent Document 3 also describes that the electrical resistance at 150% elongation is 10 3 The electrical conductivity is only described as less than Ω·cm, which is not sufficient. Furthermore, no consideration has been given to changes in electrical conductivity in a humid and hot environment. Furthermore, the molding method described is simply to apply the composition to a substrate and dry it, and molding processability such as extrusion has not been considered.

[0016] (2) In the above configuration, the content of the styrene-based thermoplastic elastomer may be 50 parts by mass or more, where the total amount of the thermoplastic elastomer is 100 parts by mass. This configuration makes it easy to increase electrical conductivity. In addition, since the softening point of the styrene-based thermoplastic elastomer is relatively high, when the elastomer is molded into sheets, the sheets are prevented from sticking to each other, making the elastomer easy to handle.

[0017] (3) In any of the above configurations, the thermoplastic elastomer may include an olefin-based thermoplastic elastomer. With this configuration, when the sheet is formed into a sheet, sticking of the sheets to each other is further suppressed, improving handleability.

[0018] (4) In any of the above configurations, the volume resistivity in the natural state may be 1 Ω cm or less. The conductive thermoplastic elastomer composition of this configuration has high conductivity and is therefore suitable for use as an electrode for a sensor.

[0019] (5) In any of the above configurations, the rate of change in volume resistivity in the humid and hot state calculated by the following formula (II) may be 100% or less. With this configuration, the conductivity is less likely to decrease even when left in a humid and hot environment. Change in volume resistivity under humid and hot conditions (%) = (R1 - R0) / R0 × 100 (II) [R0: volume resistivity (Ω·cm) in the natural state, R1: volume resistivity (Ω·cm) in the humid and hot state after leaving the sample in a humid and hot environment at a temperature of 90°C and a relative humidity of 95% for 100 hours]

[0020] (6) The conductive sheet of the present disclosure is obtained by molding the conductive thermoplastic elastomer composition having any of the above configurations into a sheet. The conductive sheet of the present disclosure is flexible and has excellent conductivity. The conductive sheet of the present disclosure is resistant to a decrease in conductivity even when stretched, and is also resistant to a decrease in conductivity even when left in a humid and hot environment. Therefore, the conductive sheet of the present disclosure is suitable for sheet-like electrodes that constitute flexible sensors, etc.

[0021] (7) In the configuration of (6) above, the thickness of the conductive sheet may be 50 μm or more and 500 μm or less. With this configuration, the thickness is relatively thin, so that the conductive sheet has excellent flexibility, and in the case of a flexible sensor, the subject is unlikely to feel uncomfortable when touching it.

[0022] (8) In the configuration of (6) or (7) above, when the conductive sheet is stretched by 20% in one direction along the surface, the rate of change in the volume resistivity in the stretched state calculated by the following formula (III) may be 10% or less. This configuration is suitable for sheet-like electrodes such as flexible sensors that expand and contract in response to the movements of a subject, because the decrease in conductivity during stretching is small. Change in volume resistivity in the stretched state (%) = (R2 - R0) / R0 × 100 (III) [R0: Volume resistivity in the natural state (Ω·cm), R2: Volume resistivity after 20% elongation (Ω·cm)]

[0023] (9) In any of the configurations (6) to (8) above, the conductive sheet may be configured to be used as an electrode of a capacitance sensor. This configuration can improve the ability to follow the subject's movements and improve detection sensitivity in a wearable sensor, steering sensor, seating sensor, etc., while also reducing discomfort to the subject.

[0024] (10) The method for producing a conductive sheet according to the present disclosure is a preferred embodiment of the method for producing a conductive sheet having any of the configurations (6) to (9) above, and is characterized by comprising: an elastomer composition production step of producing an elastomer composition containing the thermoplastic elastomer and the conductive material; a molding step of extruding the elastomer composition into a sheet; and a heat treatment step of heat treating the obtained sheet by holding it at a temperature of 150°C or higher and 230°C or lower for 2 minutes or longer and 6 minutes or shorter.

[0025] According to the method for producing a conductive sheet of the present disclosure, first, in the elastomer composition production step, an elastomer composition is produced, which includes a thermoplastic elastomer having a styrene-based thermoplastic elastomer with an MFR of 4 g / 10 min or more at 230°C and a load of 2.16 kg and a Type A durometer hardness of less than 60, and a conductive material having branched carbon nanotubes. As described above, the produced elastomer composition contains a styrene-based thermoplastic elastomer with an MFR of 4 g / 10 min or more, and therefore exhibits relatively high fluidity and a low melt viscosity. Therefore, in the subsequent molding step, the elastomer composition is extruded to produce a thin sheet material with a uniform thickness. Furthermore, extrusion molding can increase the production speed and enable continuous production, thereby improving productivity. Furthermore, by performing a heat treatment step after molding, the branched carbon nanotubes are less likely to aggregate, even when the resulting conductive sheet is left in a humid and hot environment, and a decrease in conductivity can be suppressed. [Effects of the Invention]

[0026] The conductive thermoplastic elastomer composition of the present disclosure is flexible and has excellent conductivity, and its conductivity is not likely to decrease even when left in a humid and hot environment. The conductive sheet of the present disclosure is flexible and has excellent conductivity. In addition, its conductivity is not likely to decrease even when stretched, and its conductivity is not likely to decrease even when left in a humid and hot environment. According to the manufacturing method of the present disclosure, the conductive sheet can be manufactured with high productivity. DETAILED DESCRIPTION OF THE INVENTION

[0027] The conductive thermoplastic elastomer composition of the present disclosure, the conductive sheet using the same, and the method for producing the conductive sheet are described in detail below. The conductive thermoplastic elastomer composition of the present disclosure, the conductive sheet using the same, and the method for producing the conductive sheet are not limited to the following embodiments, and can be embodied in various forms including modifications and improvements that can be made by those skilled in the art, without departing from the gist of the present disclosure.

[0028] <Conductive Thermoplastic Elastomer Composition> The conductive thermoplastic elastomer composition of the present disclosure comprises a thermoplastic elastomer and a conductive material.

[0029] [Thermoplastic elastomer] The thermoplastic elastomer includes a styrene-based thermoplastic elastomer having an MFR of 4 g / 10 min or more at 230° C. and a load of 2.16 kg, and a type A durometer hardness of less than 60.

[0030] Styrenic thermoplastic elastomers are block or random copolymers having polymer blocks (hard segments) made of polystyrene and polymer blocks (soft segments) made of polyolefin. Examples include styrene-butadiene-styrene block copolymer (SBS), styrene-ethylene-butylene-styrene block copolymer (SEBS), styrene-ethylene-propylene-styrene block copolymer (SEPS), and styrene-ethylene-ethylene-propylene-styrene block copolymer (SEEPS).

[0031] In styrene-based thermoplastic elastomers, the higher the styrene content, the better the electrical conductivity due to the π-π interactions caused by the aromatic rings mentioned above. However, as the styrene content increases, the elastomer becomes harder and its moldability decreases. In addition, when the elastomer is molded into sheets and then wound up and stored in roll form, the sheets tend to stick together. For this reason, taking into account electrical conductivity, moldability, and handleability, the styrene content is preferably 35% by mass or less. If moldability is more important, the styrene content should be less than 30% by mass, or even less than 25%. The styrene content is the mass ratio of styrene-derived structural units in the entire copolymer.

[0032] The thermoplastic elastomer may be a styrene-based thermoplastic elastomer having an MFR of 4 g / 10 min or more and a Type A durometer hardness of less than 60, or may contain a styrene-based thermoplastic elastomer or an olefin-based thermoplastic elastomer that do not satisfy these conditions. In either case, one or more types of thermoplastic elastomer may be used. Olefin-based thermoplastic elastomers have excellent mold releasability. Therefore, when both a styrene-based thermoplastic elastomer and an olefin-based thermoplastic elastomer are used, adhesion between sheets is suppressed when the elastomer is molded into a sheet, improving handleability. Furthermore, from the viewpoint of achieving the desired flexibility and suppressing an increase in the viscosity of the kneaded product during the manufacturing process, the content of the styrene-based thermoplastic elastomer having an MFR of 4 g / 10 min or more and a Type A durometer hardness of less than 60 is preferably 50 parts by mass or more, based on 100 parts by mass of the total thermoplastic elastomer. 75 parts by mass or more is even more preferable.

[0033] In this specification, the MFR of a thermoplastic elastomer is a value measured using a melt flow rate tester "Melt Indexer P-111H" manufactured by Toyo Seiki Seisakusho Co., Ltd. The Type A durometer hardness of a thermoplastic elastomer is a value measured using a hardness tester "ASKER P1-A" manufactured by Kobunshi Keiki Co., Ltd. (compliant with JIS K6253-3:2012). The hardness is measured using two 2 mm thick test pieces stacked on top of each other, and the value measured 15 seconds after the indenter comes into contact with the test piece is used. When using a commercially available product, the catalog value may be used.

[0034] [Conductive material] The conductive material includes branched carbon nanotubes. The carbon nanotubes may be single-walled carbon nanotubes, in which a single layer of graphene is rolled into a cylindrical shape, or multi-walled carbon nanotubes, in which multiple cylindrical graphenes are stacked coaxially. The branching structure is not particularly limited, and examples include a structure in which a single main chain branches into multiple chains (e.g., Y-shaped branching), or a structure in which multiple sub-chains extend from a single main chain. The size of the branched carbon nanotubes is not particularly limited, and for example, the average outer diameter of the main chain can be approximately 5 nm to 50 nm, and the average length of the main chain can be approximately 0.1 μm to 100 μm. The structure and size of the branched carbon nanotubes can be determined, for example, by observation with a transmission electron microscope (TEM). The average outer diameter and average length of the main chain can be the average values ​​of 20 branched carbon nanotubes in a TEM image.

[0035] The content of branched carbon nanotubes is 5 to 10 parts by mass per 100 parts by mass of thermoplastic elastomer. If the content is less than 5 parts by mass, the conductive paths are not sufficiently formed, and the effect of increasing conductivity is small. Furthermore, the branched carbon nanotubes tend to aggregate in a humid and hot environment, which may reduce conductivity. Conversely, if the content exceeds 10 parts by mass, not only does flexibility decrease, but the viscosity of the kneaded product during the manufacturing process increases, reducing moldability.

[0036] As the conductive material, materials other than branched carbon nanotubes may be added as long as the effects of the present disclosure are not impaired. Examples of materials that can be added include carbon materials such as conductive carbon black, carbon fiber, and graphite. From the perspective of achieving desired flexibility, conductivity, and moldability, the total content of the conductive material is preferably 5 to 15 parts by mass per 100 parts by mass of the thermoplastic elastomer.

[0037] [Other ingredients] The conductive thermoplastic elastomer composition of the present disclosure may contain lubricants, resins, plasticizers, reinforcing materials, antioxidants, colorants, etc. in addition to the thermoplastic elastomer and conductive material. However, oil components such as process oil may bleed when used in a sensor or the like, potentially affecting the detection accuracy of adjacent components or the sensor. For this reason, it is desirable that the conductive thermoplastic elastomer composition of the present disclosure does not contain oil components. If an oil component is included, it is desirable that the amount of the oil component be 10 parts by mass or less, based on 100 parts by mass of the thermoplastic elastomer.

[0038] The incorporation of a lubricant reduces shear stress even when a load is applied to the elastomer composition during press processing or other processes during the manufacturing process, making the branched carbon nanotubes less susceptible to breakage. It is desirable to use one or more lubricants selected from fatty acids and fatty acid compounds. Among these, bisstearic acid amide is preferred because it tends to stabilize electrical resistance when used as an electrode.

[0039] When the conductive thermoplastic elastomer composition of the present disclosure contains a resin, the elastomer and resin portions form a sea-island structure. The island-side resin portion acts as a cushion, preventing sheets from sticking together when molded into sheets, improving handleability. Furthermore, the thickness of the composition is less susceptible to change even under load. A relatively hard resin is desirable. For example, a resin with a Type A durometer hardness of 80 or more is desirable. Suitable resins include polyethylene, polypropylene, polyamide, polyethylene terephthalate, and polycarbonate. One of these resins may be used alone, or two or more may be used in combination. To maximize the effectiveness of the resin, the resin content should be 1 part by mass or more, 3 parts by mass or more, or 5 parts by mass or more per 100 parts by mass of the thermoplastic elastomer. On the other hand, excessive resin content may result in hardness and loss of flexibility. Therefore, considering flexibility, the resin content should be 20 parts by mass or less, or 10 parts by mass or less per 100 parts by mass of the thermoplastic elastomer.

[0040] [Iδ index] When the Iδ index, which indicates the degree of aggregation of the conductive material, is determined for a conductive thermoplastic elastomer composition in its natural state and in a humid and hot state after being left to stand for 100 hours in a humid and hot environment at a temperature of 90°C and a relative humidity of 95%, the rate of change in the Iδ index in the humid and hot state, calculated by the following formula (I), is 65% or less. Change rate of Iδ index in moist and hot conditions (%) = (Iδ1-Iδ0) / Iδ0 × 100 (I) [Iδ0: Iδ index in natural conditions, Iδ1: Iδ index in moist and hot conditions]

[0041] The method for calculating the Iδ index for a conductive thermoplastic elastomer composition is described below: (1) A thin sample is taken from the conductive thermoplastic elastomer composition and observed using a scanning electron microscope (SEM) at a magnification that allows the conductive material to be recognized, and a grayscale image (SEM image) of an area of ​​11.25 μm long x 15 μm wide is obtained. (2) The SEM image is binarized. The threshold for binarization is determined according to the brightness of the image, for example, a threshold between 120 and 180. In the binarized image, conductive materials are shown in white and other materials are shown in black. (3) After the binarization process, remove extremely small white areas from the image to remove noise. (4) The image after the binarization process is divided into 100 or more square sections, and the Iδ index is calculated using the above formula (A) based on the number of conductive materials in each square section.

[0042] [Volume resistivity] From the viewpoint of having suitable conductivity for use as a sensor electrode, the volume resistivity of the conductive thermoplastic elastomer composition of the present disclosure is preferably 1 Ω·cm or less, and more preferably 0.6 Ω·cm or less. Furthermore, from the viewpoint of preventing a decrease in conductivity even when left in a humid and hot environment, the rate of change in volume resistivity in a humid and hot state, calculated by the following formula (II), is preferably 100% or less, and more preferably 90% or less. Change in volume resistivity under humid and hot conditions (%) = (R1 - R0) / R0 × 100 (II) [R0: volume resistivity (Ω·cm) in the natural state, R1: volume resistivity (Ω·cm) in the humid and hot state after leaving the sample in a humid and hot environment at a temperature of 90°C and a relative humidity of 95% for 100 hours] The volume resistivity of the conductive thermoplastic elastomer composition can be measured, for example, by preparing a dumbbell-shaped No. 6 test piece (thickness: 150 μm) specified in JIS K6251:2023, attaching copper foil to a certain width portion of the test piece, and connecting both longitudinal ends of the copper foil to a resistance measuring device.

[0043] <Conductive sheet> The conductive sheet of the present disclosure is formed by molding the conductive thermoplastic elastomer composition of the present disclosure into a sheet shape. The conductive sheet of the present disclosure may have a uniform planar shape or a mesh shape with multiple openings. In consideration of moldability and durability, the thickness of the conductive sheet is desirably 50 μm or more, and preferably 100 μm or more. When used as a sheet electrode for a flexible sensor, the conductive sheet has a thickness of desirably 500 μm or less, and more preferably 300 μm or less, from the viewpoints of excellent flexibility and being less likely to cause discomfort when touched by a subject.

[0044] As explained above as a suitable volume resistivity for the conductive thermoplastic elastomer composition of the present disclosure, the volume resistivity of the conductive sheet of the present disclosure is preferably 1 Ω·cm or less, and more preferably 0.6 Ω·cm or less. Furthermore, from the viewpoint of minimizing the decrease in conductivity upon elongation and improving detection accuracy when used as a sheet-like electrode for a flexible sensor or the like, when the conductive sheet is elongated by 20% in one direction along the surface, the rate of change in volume resistivity in the elongated state, calculated by the following formula (III), is preferably 10% or less. Change in volume resistivity in the stretched state (%) = (R2 - R0) / R0 × 100 (III) [R0: Volume resistivity in the natural state (Ω·cm), R2: Volume resistivity after 20% elongation (Ω·cm)]

[0045] From the viewpoint of flexibility, the elongation at break (Eb) of the conductive sheet of the present disclosure is desirably 80% or more, and more desirably 100% or more. The elongation at break can be determined by conducting a tensile test as defined in JIS K6251:2023 using a dumbbell-shaped No. 5 test piece (thickness: 150 μm) as defined in the same JIS. The Type A durometer hardness of the conductive sheet of the present disclosure is desirably 95 or less, and more desirably 92 or less. The Type A durometer hardness can be measured using a hardness tester "ASKER P1-A" manufactured by Kobunshi Keiki Co., Ltd., in the same manner as in the method for measuring the Type A durometer hardness of thermoplastic elastomers described above, using two stacked 2 mm thick test pieces, and the value measured 15 seconds after the indenter comes into contact with the test pieces is taken.

[0046] The conductive sheet of the present disclosure is suitable for use as an electrode for, for example, a piezoelectric sensor, a capacitance sensor, or the like. For example, in the former case, the conductive sheet of the present disclosure may be disposed between a piezoelectric layer containing elastomer and piezoelectric particles. In the latter case, the conductive sheet of the present disclosure may be disposed between an insulating layer containing elastomer. The capacitance sensor may be a sensor that detects capacitance between two electrodes disposed with an insulating layer between them, one of which is a detection electrode and the other is a shield electrode that shields noise from the detection electrode, and the sensor may detect capacitance generated between the detection electrode and the object to be detected. The conductive sheet of the present disclosure can improve the ability to track the subject's movements in wearable sensors, steering sensors, seating sensors, and the like, thereby improving detection sensitivity and reducing discomfort to the subject.

[0047] <Method of manufacturing conductive sheets> The method for producing the conductive sheet of the present disclosure is not particularly limited as long as it can be formed into a sheet from the conductive thermoplastic elastomer composition of the present disclosure. For example, the conductive sheet can be produced by press molding, extrusion molding, injection molding, or the like of an elastomer composition containing a predetermined thermoplastic elastomer and a conductive material having branched carbon nanotubes. Below, as one embodiment of the method for producing the conductive sheet, a production method using extrusion molding suitable for mass production is described. This production method includes an elastomer composition production step, a molding step, and a heat treatment step.

[0048] [Elastomer composition manufacturing process] This process produces an elastomer composition containing a predetermined thermoplastic elastomer and a conductive material containing branched carbon nanotubes. In this process, a thermoplastic elastomer containing a styrene-based thermoplastic elastomer having an MFR of 4 g / 10 min or more at 230°C and a load of 2.16 kg and a Type A durometer hardness of less than 60 is mixed with a conductive material containing branched carbon nanotubes, and optionally with a lubricant, to produce the elastomer composition. The amount of branched carbon nanotubes is 5 to 10 parts by mass per 100 parts by mass of the thermoplastic elastomer. Mixing can be performed using commonly used equipment such as a Banbury mixer, kneader, twin-screw mixer, or twin-screw extruder. The mixing temperature should be between 180°C and 220°C, taking into account the softening point of the thermoplastic elastomer, for example. Considering moldability, the melt viscosity of the elastomer composition at 200°C should be 5,000 Pa·s or less.

[0049] [Molding process] This step involves extruding the elastomer composition produced in the previous step into a sheet. A commonly used extrusion molding machine, such as a T-die extruder, may be used. The molding temperature should be, for example, between 180°C and 220°C, taking into account the softening point of the thermoplastic elastomer.

[0050] [Heat treatment process] This step involves heat-treating the sheet obtained in the previous step by holding it at a temperature of 150°C or higher and 230°C or lower for 2 minutes or longer and 6 minutes or shorter. By carrying out the heat treatment, even if the manufactured conductive sheet is left in a humid and hot environment, the branched carbon nanotubes are less likely to aggregate, and a decrease in conductivity can be suppressed. The heat treatment can be performed simply by heating in an oven or the like, or while applying pressure using a heat press or the like. If the heat treatment is performed without applying pressure, it is desirable to add a pressurizing step in which the sheet is pressed after this step. If pressure is applied, it is recommended to apply pressure at a pressure of 0.4 MPa or higher and 2.5 MPa or lower. Pressurizing the sheet makes it possible to make the thickness thinner and more uniform. [Example]

[0051] Next, the present disclosure will be described more specifically with reference to examples. <Manufacturing of conductive sheets> First, predetermined raw materials were blended in the amounts shown in Tables 1 to 3 below and kneaded at a temperature of 200°C in a compounding twin-screw extruder ("TEX (registered trademark) 25αIII" manufactured by The Japan Steel Works, Ltd.) to produce a pellet-shaped elastomer composition. Next, the elastomer composition was formed into a sheet by two processing methods, (A) and (B), to obtain a conductive sheet.

[0052] (A) Press processing The elastomer composition was placed in a press molding machine (a 150-ton press manufactured by Sanyu Industries Co., Ltd.) equipped with a 150 μm-thick SUS spacer, and the temperature was adjusted to 200°C for 5 minutes, followed by heat pressing under a load of 50 tons. In this way, a conductive sheet having a thickness of 150 μm was produced.

[0053] (B) T-die extrusion processing The elastomer composition was T-die extruded using a single-screw extruder (UT-25, manufactured by Plastics Engineering Research Institute Co., Ltd.) at a temperature of 200°C and a rotation speed of 100 rpm to produce a sheet with a width of 150 mm and a thickness of 500 μm (molding process). The produced sheet was hot-pressed to a thickness of 150 μm (heat treatment process). The hot-pressing was carried out as follows: First, the produced sheet was cut into a square shape measuring 50 mm in length and 50 mm in width, and sandwiched between release films (Cerapeel (registered trademark), manufactured by Toray Advanced Film Co., Ltd.) on both sides in the thickness direction. This was placed in the same press molding machine as (A), and after regulating the temperature at 200°C for 5 minutes, it was hot-pressed under a load of 50 tons (2.0 MPa). The sheet was then cooled, and the release films were removed.

[0054] The conductive sheets of Examples 1 to 5 are included in the concept of the conductive sheet in the present disclosure. Details of the raw materials used are as follows.

[0055] [Thermoplastic elastomer] (1) Styrenic thermoplastic elastomer Styrene-based elastomer 1: "Tuftec (registered trademark) H1221" manufactured by Asahi Kasei Corporation, styrene content 30 mass%, MFR (230°C, load 2.16 kg) 4.5 g / 10 min. Styrene-based elastomer 2: Kraton (registered trademark) MD1648 manufactured by Kraton Polymer Japan, styrene content 20 mass%, MFR (230°C, load 2.16 kg) 220 g / 10 min. Styrene-based elastomer 3: Kraton (registered trademark) G1645 manufactured by Kraton Polymer Japan, styrene content 11.5 to 13.5 mass%, MFR (230°C, load 2.16 kg) 3.5 g / 10 min. The MFR values ​​of the styrene elastomers 1 to 3 are catalog values.

[0056] (2) Olefin-based thermoplastic elastomer Olefin-based elastomer 1: "Engage (registered trademark) 8401" manufactured by Dow Chemical Company. Olefin-based elastomer 2: "Engage 8402" manufactured by the same company. Olefin-based elastomer 3: "Engage 8407" manufactured by the same company. The MFR (190°C, load 2.16 kg) of olefin-based elastomers 1 to 3 is 30 g / 10 min in all catalog values.

[0057] [Conductive material] (1) Branched carbon nanotubes: Cabot Corporation's "ATHLOS 200." (2) Conductive carbon black: Ketjenblack (registered trademark) EC600JD manufactured by Lion Specialty Chemicals Co., Ltd. (3) Unbranched carbon nanotubes: multi-walled carbon nanotubes.

[0058] <Type A durometer hardness of thermoplastic elastomer> The Type A durometer hardness of the thermoplastic elastomer used was measured using a hardness tester ("ASKER P1-A" manufactured by Kobunshi Keiki Co., Ltd.) conforming to JIS K6253-3: 2012. The measurement was performed using two 2 mm thick test pieces stacked on top of each other, and the value measured 15 seconds after the indenter came into contact with the test pieces was used.

[0059] <Melt viscosity of elastomer composition> The melt viscosity of the elastomer composition was measured using a Capilograph (registered trademark) 1D PMD-C (manufactured by Toyo Seiki Seisakusho Co., Ltd.) (compliant with JIS K7199:1999) at a temperature of 200°C and a shear rate of 61 s -1 The measurement was carried out under the following conditions.

[0060] <Evaluation of conductive sheets> The produced conductive sheets were evaluated using the following measurement items and methods.

[0061] [Volume resistivity] (1) Natural state After production, the conductive sheet was left to stand at room temperature for 24 hours, and then the volume resistivity of the sheet in its natural state was measured using a resistance measuring device "KEITHLEY 2000" manufactured by Tektronix & Fluke Corp. Measurements were performed by preparing a dumbbell-shaped No. 6 test piece (thickness: 150 μm) specified in JIS K6251:2023, attaching copper foil to a certain width section, and connecting both longitudinal ends of the copper foil to the resistance measuring device.

[0062] (2) Humid heat conditions The volume resistivity of the conductive sheet in the humid and hot state after leaving it for 100 hours in a humid and hot environment at a temperature of 90°C and a relative humidity of 95% was measured in the same manner as in the case of the natural state in (1).

[0063] (3) 20% elongation A dumbbell-shaped No. 6 test piece (150 μm thick) as specified in JIS K6251:2023 was prepared from the conductive sheet in its natural state, and copper foil was attached to a certain width section. This test piece was stretched 20% in the longitudinal direction at a tensile speed of 100 mm / min using a Shimadzu Corporation universal testing machine, Autograph AGS-10kNX, and the volume resistivity in this state was measured in the same manner as for the natural state in (1).

[0064] [Iδ index] The Iδ index was calculated for both the natural and humid heat conditions using the following procedure. First, the cross section of the conductive sheet was observed using an SEM at 10,000x magnification, and a grayscale image (SEM image) of 960 pixels vertically and 1,280 pixels horizontally, corresponding to an area of ​​11.25 μm vertically and 15 μm horizontally, was obtained. Next, the SEM image was binarized using a threshold value of 150, and white areas with an area of ​​20 pixels or less were removed from the binarized image. In the binarized image, conductive material is shown in white, and the rest is shown in black. Next, the binarized image was divided into 10 x 10 square sections, and the number of conductive material particles in each section was counted in 10-pixel increments (100 sections) up to 100 pixels in length and width. The Iδ index for the natural condition and the Iδ index for the humid heat condition were then calculated using the aforementioned formula (A), and each Iδ index was substituted into the aforementioned formula (I) to calculate the rate of change in the Iδ index.

[0065] [Elongation at break] A tensile test was conducted using a dumbbell-shaped No. 5 test piece (thickness: 150 μm) specified in JIS K6251:2023 to determine the elongation at break. The tensile test was conducted using a Shimadzu Corporation universal testing machine, Autograph AGS-10kNX, at a tension speed of 100 mm / min.

[0066] [Type A durometer hardness] The Type A durometer hardness was measured using a hardness tester "ASKER P1-A" (compliant with JIS K6253-3:2012) manufactured by Kobunshi Keiki Co., Ltd. The hardness was measured by stacking two 2 mm thick test pieces, and the value measured 15 seconds after the indenter came into contact with the test pieces was used.

[0067] [Moldability] The moldability of sheets produced by T-die extrusion (extrusion molding) was evaluated. Sheets that could be continuously molded into good quality sheets were evaluated as having good moldability (indicated by a circle in the table below), and other sheets were evaluated as having poor moldability. "Good sheets could be continuously molded" means that a sheet of practical quality could be molded over a length of 1 m or more in the extrusion direction. Examples of other sheets include sheets that did not come out of the tip discharge section of the single-screw extruder, sheets that were torn at the edge, sheets with holes, or sheets with streaks or patterns on the surface.

[0068] Tables 1 to 3 show the raw materials, blending amounts, and evaluation results of the conductive sheets.

[0069] [Table 1]

[0070] [Table 2]

[0071] [Table 3]

[0072] As shown in Table 1, the conductive sheets of Examples 1 to 5 had a melt viscosity of 5000 Pa·s or less before molding. Therefore, good sheets could be produced using either extrusion or press molding. Furthermore, the volume resistivity was low at 0.6 Ω·cm or less, and the rate of change in Iδ index and volume resistivity after exposure to a humid and hot environment was 65% or less and 100% or less, respectively. This confirmed that the conductive sheets of Examples 1 to 5 had high conductivity and were resistant to degradation even when exposed to a humid and hot environment. Furthermore, the elongation at break was 80% or more, the Type A durometer hardness was 95 or less, and the rate of change in volume resistivity at 20% elongation was 10% or less. This confirmed that the conductive sheets of Examples 1 to 5 had high flexibility and were resistant to degradation in conductivity even when elongated.

[0073] In contrast, as shown in Tables 2 and 3, the conductive sheets of Comparative Examples 1 to 3, which do not contain branched carbon nanotubes, have low conductivity and are difficult to use alone as sensor electrodes. In particular, the conductive sheets of Comparative Examples 1 and 2 could not be produced by extrusion molding because the melt viscosity of the elastomer composition before molding was high. The conductive sheets of Comparative Examples 4 to 6, which contain a low content of branched carbon nanotubes, also have low conductivity and are difficult to use alone as sensor electrodes. In addition, after being left in a humid and hot environment, the rate of change in the Iδ index was large, and the conductivity decreased. The conductive sheet of Comparative Example 7 had a higher content of conductive carbon black than Comparative Example 6, so the conductivity was improved, but after being left in a humid and hot environment, the rate of change in the Iδ index was large, and the conductivity decreased.

[0074] The conductive sheet of Comparative Example 8 did not use a styrene-based thermoplastic elastomer and contained a low amount of branched carbon nanotubes. Therefore, the rate of change in Iδ index after exposure to a humid and hot environment was large, and the rate of change in volume resistivity at 20% elongation was also large. The conductive sheet of Comparative Example 9, which used a styrene-based thermoplastic elastomer with an MFR of 3.5 g / 10 min, had good conductivity and flexibility, but the melt viscosity of the elastomer composition before molding exceeded 5000 Pa·s, making it impossible to produce a sheet by extrusion molding. The conductive sheet of Comparative Example 10 contained a high amount of branched carbon nanotubes, so the melt viscosity of the elastomer composition before molding exceeded 5000 Pa·s, making it impossible to produce a sheet by extrusion molding. In addition, the elongation at break also decreased. In the conductive sheet of Reference Example 1, the content of branched carbon nanotubes was within the specified range, but the total amount with conductive carbon black exceeded 15 parts by mass. Therefore, although the conductivity and flexibility were good, the melt viscosity of the elastomer composition before molding exceeded 5000 Pa·s, making it impossible to produce a sheet by extrusion molding.

Claims

1. A conductive thermoplastic elastomer composition comprising a thermoplastic elastomer and a conductive material, The thermoplastic elastomer comprises a styrene-based thermoplastic elastomer having a melt mass flow rate (MFR) of 4 g / 10 min or more at 230°C and a load of 2.16 kg and a type A durometer hardness of less than 60; the conductive material contains branched carbon nanotubes, and the content of the branched carbon nanotubes is 5 parts by mass or more and 10 parts by mass or less per 100 parts by mass of the thermoplastic elastomer; 1. A conductive thermoplastic elastomer composition characterized in that, when an Iδ index indicating the degree of aggregation of the conductive material is calculated in each of the conductive thermoplastic elastomer composition's natural state and in a humid and heat state after being left to stand for 100 hours in a humid and heat environment at a temperature of 90°C and a relative humidity of 95%, the rate of change in the Iδ index in the humid and heat state calculated by the following formula (I) is 65% or less. Change rate (%) of Iδ index in humid and hot conditions = (Iδ １ -Iδ ０ ) / Iδ ０ ×100...(I) [Iδ ０ : Iδ index in natural state, Iδ １ : Iδ index in humid and hot conditions]

2. 2. The conductive thermoplastic elastomer composition according to claim 1, wherein the content of the styrene-based thermoplastic elastomer is 50 parts by mass or more, where the total amount of the thermoplastic elastomer is 100 parts by mass.

3. The conductive thermoplastic elastomer composition according to claim 1 , wherein the thermoplastic elastomer comprises an olefin-based thermoplastic elastomer.

4. 2. The conductive thermoplastic elastomer composition according to claim 1, wherein the volume resistivity in the natural state is 1 Ω·cm or less.

5. 2. The conductive thermoplastic elastomer composition according to claim 1, wherein the rate of change in volume resistivity under a wet heat condition calculated by the following formula (II) is 100% or less. Change rate of volume resistivity in a humid and hot state (%) = (R １ -R ０ ) / R ０ ×100...(II) [R ０ : volume resistivity in natural state (Ω cm), R １ : Volume resistivity (Ω cm) in a humid and hot state after being left standing in a humid and hot environment at a temperature of 90°C and a relative humidity of 95% for 100 hours]

6. A conductive sheet obtained by molding the conductive thermoplastic elastomer composition according to claim 1 into a sheet shape.

7. The conductive sheet according to claim 6, which has a thickness of 50 μm or more and 500 μm or less.

8. 7. The conductive sheet according to claim 6, wherein when the conductive sheet is stretched by 20% in one direction along the surface, the rate of change in volume resistivity in the stretched state calculated by the following formula (III) is 10% or less. Change in volume resistivity in elongated state (%) = (R ２ -R ０ ) / R ０ ×100...(III) [R ０ : volume resistivity in natural state (Ω cm), R ２ : Volume resistivity (Ω cm) when stretched 20%

9. The conductive sheet according to claim 6, which is used as an electrode of a capacitance sensor.

10. A method for producing a conductive sheet according to claim 6, an elastomer composition production step of producing an elastomer composition having the thermoplastic elastomer and the conductive material; a molding step of extruding the elastomer composition into a sheet; a heat treatment step of heat-treating the obtained sheet by holding it at a temperature of 150°C or higher and 230°C or lower for 2 minutes or longer and 6 minutes or shorter; A method for producing a conductive sheet, comprising:

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