JP7650752B2 - Conductive rubber composition for sensing - Google Patents

Description

The present invention relates to a conductive rubber composition for sensing.

A rubber actuator (so-called McKibben type) is known that has a rubber tube that expands and contracts using a working fluid and a sleeve (braided reinforcement structure) that covers the outer surface of the rubber tube. For rubber products that repeatedly deform, such as the rubber tube of the rubber actuator, it is known that the amount of deformation of the rubber member can be measured from the electrical resistance value of the rubber member (Patent Document 1 below).

JP 2008-037906 A

According to the technology described in the above-mentioned Patent Document 1, the amount of deformation of a rubber member can be determined by measuring the electrical resistance value of the rubber member. However, it is not possible to detect whether or not a crack has occurred in the rubber member. For rubber actuators, which are filled with oil or other liquids, if a crack occurs in the tube rubber, it may lead to the tube bursting or the rubber actuator becoming unusable, so it is important to detect cracks in the tube rubber at an early stage.
The inventors have studied the rubber actuator and determined that when the rubber actuator expands and contracts in one cycle and the electrical resistance of the tube rubber is monitored in real time during the expansion and contraction, the waveform of the electrical resistance changes in one cycle before the crack occurs, in the early stage of the crack, and in the advanced state due to the strain caused by the crack (FIG. 1). However, with a typical conductive rubber, it is difficult to capture the change in the electrical resistance waveform caused by the strain caused by the crack, and it is difficult to confirm the early stage of the crack occurrence only by monitoring the electrical resistance. In contrast, by using a conductive rubber composition for sensing that has good responsiveness to the change in the electrical resistance value with respect to the strain, it is possible to accurately detect the occurrence of the crack.

Therefore, the present invention aims to solve the problems of the conventional technology and provide a conductive rubber composition for sensing that has good responsiveness to changes in electrical resistance value in response to strain and can accurately detect the occurrence of cracks.

The gist of the present invention, which solves the above problems, is as follows:

The conductive rubber composition for sensing of the present invention contains a rubber component and carbon black,
The rubber component contains 80% by mass or more of acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass,
The carbon black has a cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of 110 m ² /g or more;
The rubber composition is characterized in that the amount of the carbon black is 50 parts by mass or more based on 100 parts by mass of the rubber component.
The conductive rubber composition for sensing of the present invention has good responsiveness in terms of change in electrical resistance value to strain.

In the conductive rubber composition for sensing of the present invention, it is preferable that the rubber component consists of only acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45 mass %. In this case, the oil resistance of the conductive rubber composition for sensing is further improved.

In the conductive rubber composition for sensing of the present invention, it is preferable that the carbon black has a compressed dibutyl phthalate (24M4DBP) absorption of 90 mL/100 g or more. In this case, the responsiveness of the change in electrical resistance value to strain is further improved.

In a preferred embodiment of the conductive rubber composition for sensing of the present invention, the carbon black content is 55 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the rubber component. In this case, it becomes possible to detect changes in electrical resistance value due to strain without delay and with high sensitivity.

The present invention provides a conductive rubber composition for sensing that has good responsiveness to changes in electrical resistance with respect to strain and can accurately detect the occurrence of cracks.

1 is a graph showing an electrical resistance waveform for one cycle for an example rubber actuator. FIG. 2 is a side view of an example of a rubber actuator. FIG. 2 is a partially exploded perspective view of an example of a rubber actuator. FIG. 2 is a partial cross-sectional view of an example of a tube rubber. 11 is a graph showing an example of change in electrical resistance value when there is responsiveness and when there is no responsiveness.

The conductive rubber composition for sensing of the present invention will be described in detail below with reference to an embodiment.

<Conductive rubber composition for sensing>
The conductive rubber composition for sensing of the present invention includes a rubber component and carbon black. Here, the conductive rubber composition for sensing of the present invention is characterized in that the rubber component includes 80 mass % or more of acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45 mass %, the carbon black has a cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of 110 m ² /g or more, and the content of the carbon black is 50 mass parts or more relative to 100 mass parts of the rubber component.

The CTAB adsorption specific surface area of the carbon black is an index of the particle size of the carbon black. The larger the CTAB adsorption specific surface area, the smaller the particle size of the carbon black, and the lower the electrical resistance value of the rubber composition. Also, the higher the content of the carbon black, the lower the electrical resistance value of the rubber composition. In the present invention, in the case where 80 mass % or more of the rubber component is acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45 mass %, if the CTAB adsorption specific surface area of the carbon black is 110 m ² /g or more and the content of the carbon black is 50 parts by mass or more relative to 100 parts by mass of the rubber component, the electrical resistance value of the rubber composition is sufficiently low, and further, the electrical resistance value changes reproducibly when strain is applied, and the electrical resistance value returns reversibly when the strain is removed, and it is considered that the responsiveness of the change in the electrical resistance value to the strain is improved.

As described above, the conductive rubber composition for sensing of the present invention has good responsiveness in terms of changes in electrical resistance value to strain. Therefore, by combining the conductive rubber composition for sensing of the present invention with AI learning using electrical resistance value as input, it becomes possible to detect the occurrence of cracks (AI sensing) from the dynamic behavior of changes in electrical resistance value.
In addition, the conductive rubber composition for sensing of the present invention is an acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass, of which 80% by mass or more of the rubber component is high in oil resistance. Therefore, the conductive rubber composition for sensing of the present invention has high oil resistance and is suitable for applications requiring oil resistance, such as tube rubber for rubber actuators.

(Rubber component)
The conductive rubber composition for sensing of the present invention contains a rubber component, and the rubber component provides rubber elasticity to the composition, enabling the rubber composition to stretch.
The rubber component of the conductive rubber composition for sensing of the present invention contains 80% by mass or more, preferably 90% by mass or more, of acrylonitrile-butadiene rubber (NBR) having an acrylonitrile unit content of more than 45% by mass. In addition, the rubber component is preferably made only of acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass. Since acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass has excellent oil resistance, the conductive rubber composition for sensing of the present invention in which 80% by mass or more of the rubber component is acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass has high oil resistance and is suitable for applications requiring oil resistance, such as tube rubber for rubber actuators. In addition, when the rubber component is made only of acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass, the oil resistance is further improved.

The content of the acrylonitrile unit in the acrylonitrile-butadiene rubber (NBR) is more than 45% by mass, and if it exceeds 45% by mass, it is preferable from the viewpoint of oil resistance. Also, the content of the acrylonitrile unit in the acrylonitrile-butadiene rubber (NBR) is preferably 50% by mass or less from the viewpoint of fatigue resistance and crack extension resistance.
The content of acrylonitrile units (bound acrylonitrile amount) of the acrylonitrile-butadiene rubber (NBR) can be measured by the Kjeldahl method in accordance with JIS K6384.

The rubber component may further contain other rubber components. As such other rubber components, various rubber components that impart rubber elasticity to the composition can be used. Examples of such other rubber components include acrylonitrile-butadiene rubber (NBR), natural rubber (NR), synthetic isoprene rubber (IR), butadiene rubber (BR), styrene-butadiene rubber (SBR), chloroprene rubber (CR), etc., each of which has an acrylonitrile unit content of 45 mass% or less. In addition, the rubber component used may be one type or two or more types.

(Carbon Black)
The conductive rubber composition for sensing of the present invention contains carbon black, and the carbon black provides electrical conductivity to the composition, thereby enabling sensing of the rubber composition.

The carbon black has a cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of 110 ^m2 /g or more. When the CTAB adsorption specific surface area of the carbon black is 110 ^m2 /g or more, the particle size of the carbon black is sufficiently small, the electrical resistance value is sufficiently low, and the change in the electrical resistance value is easily detected, improving the responsiveness.
The cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of the carbon black is more preferably 120 m ² /g or more from the viewpoint of making it easier to detect changes in electrical resistance value, and is preferably 160 m ² /g or less from the viewpoint of workability.
In this specification, the cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of carbon black is a value measured in accordance with JIS K6217-3, and is the external surface area of carbon black excluding micropores, expressed as the specific surface area when CTAB (cetyltrimethylammonium bromide) is adsorbed onto the carbon black.

The carbon black preferably has a compressed dibutyl phthalate (24M4DBP) absorption of 90 mL/100 g or more. The 24M4DBP absorption of carbon black is a physical property related to the structure of carbon black, and the larger the 24M4DBP absorption, the lower the electrical resistance of the rubber composition. The 24M4DBP absorption of carbon black is also an index of high strain loss and is related to the amount of friction between carbon blacks at high strain. At high strain, the greater the amount of friction between carbon blacks, the greater the change in electrical resistance due to expansion and contraction (high strain) during use, and the better the responsiveness of the change in electrical resistance to strain. When the 24M4DBP absorption of carbon black is 90 mL/100 g or more, the electrical resistance of the rubber composition is sufficiently low, and the responsiveness of the change in electrical resistance to strain is further improved.
The compressed dibutyl phthalate (24M4DBP) absorption amount of the carbon black is more preferably 100 mL/100 g or more from the viewpoint of making it easier to detect changes in electrical resistance value, and is preferably 130 mL/100 g or less from the viewpoint of suppressing variations due to kneading.
In this specification, the compressed dibutyl phthalate (24M4DBP) absorption of carbon black is a value obtained by measuring the DBP (dibutyl phthalate) absorption after repeatedly applying a pressure of 24,000 psi four times in accordance with ISO 6894. The 24M4DBP absorption is used to determine the DBP absorption based on the non-destructive true structural form (primary structure) while excluding the DBP absorption due to the deformable and destructive structural form (secondary structure) caused by so-called van der Waals forces, and is an index for evaluating the skeletal structure of carbon black mainly composed of the primary structure.

The content of the carbon black is 50 parts by mass or more, preferably 55 parts by mass or more and 75 parts by mass or less, and more preferably 60 parts by mass or more and 70 parts by mass or less, relative to 100 parts by mass of the rubber component. When the content of the carbon black is less than 50 parts by mass relative to 100 parts by mass of the rubber component, the responsiveness of the change in the electrical resistance value to the strain is insufficient. On the other hand, when the content of the carbon black is 55 parts by mass or more relative to 100 parts by mass of the rubber component, the electrical resistance value of the rubber composition becomes sufficiently low (electricity flows sufficiently), and the change in the electrical resistance value becomes easier to detect. In addition, when the content of the carbon black is 75 parts by mass or less relative to 100 parts by mass of the rubber component, the electrical resistance value of the rubber composition becomes sufficiently high (electricity does not flow too much), and the change in the electrical resistance value becomes easier to detect, and the hardness of the rubber composition becomes low, and for example, the hardness becomes suitable for application to the tube rubber of a rubber actuator.
If the electrical resistance is sufficiently high, electricity does not flow too much, so that the electrical resistance changes when the rubber is strained, and the change occurs immediately without delay, making it suitable for use in detecting cracks. If the electrical resistance is sufficiently low, electricity flows sufficiently and is conductive, making it suitable for use in detecting cracks. If the carbon black content is in the range of 55 parts by mass or more and 75 parts by mass or less with respect to 100 parts by mass of the rubber component, it becomes possible to detect the change in electrical resistance due to strain without delay and with high sensitivity.

The carbon black is not particularly limited as long as it satisfies the above conditions, and examples thereof include GPF, FEF, HAF, ISAF, and SAF grade carbon black. These carbon blacks may be used alone or in combination of two or more types.

(Carbon Nanotubes)
The conductive rubber composition for sensing of the present invention may further contain carbon nanotubes, which have the effect of improving the electrical conductivity of the composition.

When the conductive rubber composition for sensing of the present invention contains carbon nanotubes, the content of the carbon nanotubes is preferably less than 2 parts by mass, and more preferably less than 1 part by mass, per 100 parts by mass of the rubber component. When the content of the carbon nanotubes is less than 2 parts by mass per 100 parts by mass of the rubber component, the electrical resistance value of the rubber composition becomes sufficiently high (electricity does not flow too much), making it easier to detect changes in the electrical resistance value.

The carbon nanotube (CNT) is a structure made of carbon atoms with a diameter of about several nm to several tens of nm, and has an extremely fine tube-like structure.
The carbon nanotubes may be single-walled or multi-walled nanotubes.
The carbon nanotubes preferably have a length of 0.1 μm to 30 μm, and more preferably 0.1 μm to 10 μm.
The carbon nanotubes preferably have a diameter of 10 nm to 300 nm, and more preferably have a diameter of 100 nm to 250 nm.

The carbon nanotubes can be synthesized by plasma CVD (chemical vapor deposition), thermal CVD, surface decomposition, flow gas phase synthesis, arc discharge, etc., and commercially available products can also be used. Examples of commercially available carbon nanotubes that can be used include carbon nanotubes manufactured by KUMHO, vapor grown carbon fiber VGCF (registered trademark) manufactured by Showa Denko KK, and carbon nanotubes manufactured by Materials Technologies Research (MTR) of the United States.

<Method for producing conductive rubber composition for sensing>
The rubber composition of the present invention can be produced, for example, by using a Banbury mixer, a roll or the like to compound the rubber component with carbon black, and optionally further compound carbon nanotubes, and then kneading the mixture, followed by heating and extrusion, etc.

In addition to the above-mentioned components, the conductive rubber composition for sensing may contain softeners, stearic acid, antioxidants, zinc oxide (zinc oxide), vulcanization accelerators, vulcanizing agents, etc., which may be appropriately selected and blended within a range that does not impair the purpose of the present invention. Commercially available products can be used as these blending agents.

<Applications of the conductive rubber composition for sensing>
The conductive rubber composition for sensing of the present invention described above has good responsiveness of the change in electrical resistance value to strain, and therefore can be used for sensing (detecting, detecting, sensing) the amount of strain. The conductive rubber composition for sensing can be used for various rubber products, and can be particularly suitably used for the tube rubber of a rubber actuator. In addition to rubber actuators, the conductive rubber composition can be appropriately used for any rubber member that generates strain during use, such as tires, anti-vibration rubber, vibration-isolating rubber, conveyor belts, crawlers, gloves, etc., and it goes without saying that the uses are not limited to the above examples and can be widely applied.

(Rubber actuator)
A rubber actuator of a preferred embodiment using the conductive rubber composition for sensing of the present invention is shown in Fig. 2. The rubber actuator 10 shown in Fig. 2 includes an actuator body 100 including a cylindrical tube rubber 110 that expands and contracts due to fluid pressure, and a sleeve 120 that is a cylindrical structure in which cords oriented in a predetermined direction are woven and covers the outer circumferential surface of the tube rubber 110, and the above-mentioned conductive rubber composition for sensing of the present invention is used for the tube rubber 110.

Figure 2 is a side view of the rubber actuator 10 according to this embodiment. As shown in Figure 2, the rubber actuator 10 comprises an actuator body 100, a sealing mechanism 200, and a sealing mechanism 300. Furthermore, a connecting portion 20 is provided on each end of the rubber actuator 10. Furthermore, a resistance value measuring device 30 is connected to each connecting portion 20.

The actuator body 100 is composed of a tube rubber 110 and a sleeve 120. A working fluid flows into the actuator body 100 through a fitting 400 and a through hole 410. Here, the rubber actuator 10 is operated by fluid pressure and may be pneumatic or hydraulic. When a liquid is used as the working fluid, examples of the liquid include oil and water. When the rubber actuator is hydraulic, the working fluid may be hydraulic oil that has been used in hydraulic drive systems.

When the working fluid flows into the tube rubber 110, the actuator body 100 contracts in the axial direction D _AX of the actuator body 100 and expands in the radial direction D _R. When the working fluid flows out of the tube rubber 110, the actuator body 100 expands in the axial direction D _AX of the actuator body 100 and contracts in the radial direction D _R. This change in shape of the actuator body 100 allows the rubber actuator 10 to function as an actuator.
Furthermore, such a rubber actuator 10 is a so-called McKibben type, and can be used not only as an artificial muscle, but also as a robot limb (upper limb, lower limb, etc.) that requires higher performance (contractile force). Members constituting the limb are connected to the connecting part 20. In this embodiment, the resistance value measuring device 30 is connected to the connecting part 20, but the connection point of the resistance value measuring device 30 is not limited thereto, and may be directly connected to both ends of the tube rubber 110, for example.

The sealing mechanisms 200 and 300 seal both ends of the actuator body 100 in the axial direction D _AX . Specifically, the sealing mechanism 200 includes a sealing member 210 and a crimping member 230. The sealing member 210 seals the end of the actuator body 100 in the axial direction D _AX . The crimping member 230 crimps the actuator body 100 together with the sealing member 210. An indentation 231, which is a mark caused by crimping the crimping member 230 with a jig, is formed on the outer circumferential surface of the crimping member 230.

The difference between the sealing mechanism 200 and the sealing mechanism 300 is that the roles of the fittings 400 and 500 (and the passage holes 410 and 510) are different.
The fitting 400 provided on the sealing mechanism 200 protrudes so that a drive pressure source for the rubber actuator 10, specifically a hose (pipe) connected to a compressor for the working fluid, can be attached. The working fluid that flows in through the fitting 400 passes through the through hole 410 and flows into the inside of the actuator body 100, specifically, into the inside of the tube rubber 110.
On the other hand, the fitting 500 provided on the sealing mechanism 300 protrudes so that it can be used as a gas vent when injecting the working fluid into the rubber actuator 10. When the working fluid is injected into the rubber actuator 10 at the beginning of the operation of the rubber actuator 10, the gas originally present inside the rubber actuator 10 is discharged from the fitting 500 through the passage hole 510.

3 is a partially exploded perspective view of the rubber actuator 10. As shown in FIG. 3, the rubber actuator 10 includes an actuator body 100 and a sealing mechanism 200.
As described above, the actuator body 100 is composed of the tube rubber 110 and the sleeve 120 .

The tube rubber 110 is a cylindrical body that expands and contracts due to fluid pressure. The tube rubber 110 repeatedly contracts and expands due to the working fluid, and the above-mentioned conductive rubber composition for sensing of the present invention is applied to it.

The sleeve 120 is cylindrical and covers the outer circumferential surface of the tube rubber 110. The sleeve 120 is a structure in which cords oriented in a specific direction are woven, and the oriented cords cross to form a repeated diamond shape. By having such a shape, the sleeve 120 undergoes pantograph deformation, and follows the contraction and expansion of the tube rubber 110 while regulating it.

3, the sealing mechanism 200 seals the end portion in the axial direction _DAX of the actuator body 100. The sealing mechanism 200 is composed of a sealing member 210, a locking ring 220, and a crimping member 230.

The sealing member 210 has a body portion 211 and a flange portion 212. The sealing member 210 is preferably made of a metal such as stainless steel, but is not limited to such a metal and may be made of a hard plastic material or the like.

The body portion 211 is tubular, and has a through hole 215 through which the working fluid passes. The through hole 215 communicates with the through hole 410 (see FIG. 2). The tube rubber 110 is inserted into the body portion 211.

The flange portion 212 is connected to the body portion 211 and is located closer to the end of the actuator 10 in the axial direction _DAX than the body portion 211. The flange portion 212 has a larger outer diameter in the radial direction D _R than the body portion 211. The flange portion 212 engages the tube rubber 110 and the engaging ring 220 inserted into the body portion 211.

An uneven portion 213 is formed on the outer circumferential surface of the body portion 211. The uneven portion 213 contributes to preventing slippage of the tube rubber 110 inserted into the body portion 211. It is preferable that the uneven portion 213 is formed with three or more convex portions.
Further, a small diameter portion 214 having an outer diameter smaller than that of the body portion 211 is formed at a position closer to the flange portion 212 of the body portion 211 .

The locking ring 220 locks the sleeve 120. Specifically, the sleeve 120 is folded back outward in the radial direction D _R via the locking ring 220.
The outer diameter of the locking ring 220 is larger than the outer diameter of the body portion 211. The locking ring 220 locks the sleeve 120 at the position of the small diameter portion 214 of the body portion 211. In other words, the locking ring 220 locks the sleeve 120 at a position that is on the outer side of the body portion 211 in the radial direction D _R and adjacent to the flange portion 212.
In this embodiment, the locking ring 220 is divided into two parts so that it can be locked to the small diameter portion 214 that is smaller than the body portion 211. Note that the locking ring 220 is not limited to being divided into two parts, and may be divided into more parts, or some of the divided parts may be connected to each other so as to be rotatable.
The locking ring 220 can be made of the same metal or hard plastic material as the sealing member 210 .

The crimping member 230 crimps the actuator body 100 together with the sealing member 210. The crimping member 230 may be made of a metal such as an aluminum alloy, brass, or iron. When the crimping member 230 is crimped by a crimping jig, an indentation 231 as shown in FIG. 2 is formed on the crimping member 230.

FIG. 4 is a partial cross-sectional view of one embodiment of the tube rubber 110.
The tube rubber 110 shown in FIG. 4 has a two-layer structure made up of an inner layer rubber 111 located on the inner surface side of the tube rubber, and an outer layer rubber 112 located on the outer surface side of the tube rubber 110, adjacent to the outer side of the inner layer rubber 111 in the radial direction D _R.

The conductive rubber composition for sensing of the present invention described above has excellent oil resistance and can therefore be suitably applied to the inner rubber layer 111 that comes into contact with the working fluid. If the oil resistance of the inner rubber layer 111 is insufficient, the working fluid may penetrate between the inner rubber layer 111 and the outer rubber layer 112, which may reduce the durability of the tube rubber 110. However, by applying the conductive rubber composition for sensing of the present invention described above to the inner rubber layer 111, cracks in the inner rubber layer 111 can be detected early, and the infiltration of the working fluid can be prevented. By replacing the tube rubber 110 at the appropriate time, the rubber actuator can be prevented from stopping functioning.

On the other hand, since the outer rubber layer 112 comes into contact with the sleeve 120, it is preferable to use a rubber composition that has excellent crack resistance, abrasion resistance, sliding properties, etc., and can withstand the load from the sleeve 120 side.

The tube rubber 110 is not limited to a two-layer structure, but may have a one-layer structure or a laminated structure of three or more layers. When the tube rubber 110 has a laminated structure of three or more layers, it is preferable to apply the above-mentioned conductive rubber composition for sensing of the present invention to the innermost layer.

As described above, the tube rubber 110 repeatedly contracts and expands due to the working fluid, and the conductive rubber composition for sensing of the present invention is applied to the tube rubber 110.
Here, as the contraction rate of the actuator body 100 increases, the electrical resistance value of the actuator body 100 (tube rubber 110) decreases. This is because, when a fluid flows into the actuator body 100 and the actuator body 100 contracts in the axial direction D _AX , the tube rubber 110 expands in the radial direction D _R within a predetermined range restricted by the sleeve 120, and the thickness of the tube rubber 110 becomes thinner, narrowing the distance between the carbon black particles contained in the tube rubber 110.
Specifically, when the actuator body 100 contracts, the tube rubber 110 expands, reducing the thickness of the tube rubber 110. As a result, the dimension (distance) of the carbon black in the thickness direction becomes narrower, and the carbon black comes closer to each other.
That is, when the contraction rate of the actuator body 100 increases and the length decreases, the distance between the carbon black particles decreases, so that the conductivity of the tube rubber 110 increases, in other words, the electrical resistance value of the tube rubber 110 decreases.

In addition, cracks may occur in the tube rubber 110 due to repeated contraction and expansion by the working fluid, but as shown in FIG. 1, the pattern of change in electrical resistance is different at each stage: no cracks, early cracking, and cracks. Here, by analyzing the dynamic behavior of the change in electrical resistance caused by distortion due to the occurrence of cracks in the tube rubber 110 using statistical methods or having AI learn the dynamic behavior of the change in electrical resistance, it becomes possible to detect the occurrence of cracks in the tube rubber 110 from the dynamic behavior of the change in electrical resistance.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

<Preparation of Rubber Composition>
A rubber composition having the composition shown in Table 1 was prepared by kneading in a conventional manner. The type, physical properties and compounding amount of the carbon black used are shown in Table 2.

*1 NBR: Acrylonitrile-butadiene rubber, acrylonitrile unit content = 48% by mass, manufactured by JSR Corporation, product name "N215SL"
*2 Carbon black: The type, properties and amount of carbon black used are shown in Table 2. *3 Ester oil: Manufactured by Hallstar Company, product name "TP-95"
*4 Hydrogenated fatty acid: New Japan Chemical Co., Ltd., product name "Stearic acid 50S"
*5 Antioxidant: 2,2,4-trimethyl-1,2-dihydroquinoline polymer, manufactured by Seiko Chemical Co., Ltd., trade name "Nonflex RD"
*6 Zinc oxide: Manufactured by Hakusui Tech Co., Ltd., product name "Zinc oxide type 2 granulated product"
*7 Vulcanization accelerator A: N-cyclohexyl-2-benzothiazolyl sulfenamide, manufactured by Sanshin Chemical Industry Co., Ltd., trade name "Suncerer CM-G"
*8 Vulcanization accelerator B: Tetrakis (2-ethylhexyl) thiuram disulfide, manufactured by Ouchi Shinko Chemical Industry Co., Ltd., trade name "Noccela TOT"
*9 Sulfur: Manufactured by Hosoi Chemical Industry Co., Ltd., product name "HK200-5", shown in Table 1 as the amount of sulfur content

<Tests on vulcanized rubber>
A rubber sheet having a thickness of 1 mm was prepared from the obtained rubber composition and vulcanized at 155° C. for 45 minutes to prepare a vulcanized rubber sheet. A rectangular test piece having a size of 4.7 mm×40 mm was prepared from the vulcanized rubber sheet. The test piece was evaluated for the presence or absence of strain response by the following method.

(1) Strain Response Test The prepared test specimen was stretched 10 times by 70% at room temperature with a chuck distance of 20 mm using a tensile tester (K18003) manufactured by System One Co., Ltd. (after repeating the cycle of 0% → 70% stretch → 0% 10 times), and then dynamic electrical measurement was performed at a tensile speed of 6 mm/sec. The electrical resistance value was measured by sampling at DC 10 V and 200 Hz. The electrical resistance value varies reproducibly depending on the strain, and the ratio of the signal value (S), which is the difference between the minimum electrical resistance value and the maximum electrical resistance value in the region of 10% stretch → 70% stretch → 10% stretch, to the standard deviation (n) of the electrical resistance value in the region of 0% to 10% stretch, was evaluated according to the following criteria.
◯: S/n ratio of 9 or more, strain responsiveness present ×: S/n ratio of less than 9, strain no responsiveness An example of the strain dependency of electrical resistance value is shown in Figure 5. As shown in Figure 5, rubber with a sufficiently large S/n ratio has a large change in electrical resistance value due to strain and has strain responsiveness, whereas rubber with a small S/n ratio has a small change in electrical resistance value due to strain and has no strain responsiveness.
The results of the above tests are shown in Table 2.

<Test with rubber actuator>
The rubber compositions of Example 7 and Comparative Example 8 were applied to the inner layer rubber of a tube rubber having a two-layer structure of an inner layer rubber and an outer layer rubber as shown in FIG. 4 to prepare a cylindrical tube rubber.

Two 2200 dtex aramid fibers were used as the raw yarn, and an aramid fiber cord with a diameter of 0.7 mm was produced by first twisting 12 times/10 cm and then top twisting 12 times/10 cm. A mesh-like sleeve was prepared by weaving 64 of the aramid fiber cords. This sleeve was a mesh-like tubular body in which 64 aramid fiber cords were observed on the circumference in cross section. Specifically, this sleeve was a mesh-like tubular body in which 32 aramid fiber cords arranged in parallel and spirally at equal intervals were alternately woven with another 32 aramid fiber cords arranged in parallel and spirally at equal intervals, intersecting the 32 aramid fiber cords at an angle, and the angle of each cord with respect to the axial direction of the sleeve was 25 degrees.

Using the tube rubber and the mesh sleeve, a rubber actuator with the structure shown in Figures 2 and 3 was produced. The length between sealing mechanism 200 and sealing mechanism 300 was 250 mm. UF46 manufactured by Cosmo Super Epoch Co., Ltd. was used as the hydraulic oil for the tube rubber.

(2) Crack detection test Hydraulic oil was injected into the tube rubber to fully replace the air in the tube rubber with hydraulic oil. Hydraulic oil was injected so that the pressure of the hydraulic oil in the tube rubber alternated between 0 MPa and 5 MPa every 3 seconds, and the tube was repeatedly stretched until a crack appeared and propagated until the actuator could no longer function.
At this time, the electrical resistance value was monitored to attempt to detect the occurrence of cracks, and the occurrence of cracks was successfully detected using AI for the rubber actuator using the rubber sample of Example 7, which was evaluated as having "strain responsiveness," as the tube rubber. On the other hand, the occurrence of cracks could not be detected for the rubber actuator using the rubber sample of Comparative Example 8, which was evaluated as having "no strain responsiveness," as the tube rubber.

*10 CB1: CTAB adsorption specific surface area = 112 m ² /g, 24M4DBP absorption capacity = 100 mL/100 g, manufactured by Tokai Carbon Co., Ltd., product name "Seat 6"
*11 CB2: CTAB adsorption specific surface area = 123 m ² /g, 24M4DBP absorption capacity = 100 mL/100 g, manufactured by Asahi Carbon Co., Ltd., product name "Asahi #78"
*12 CB3: CTAB adsorption specific surface area = 83 m ² /g, 24M4DBP absorption capacity = 72 mL/100 g, manufactured by Asahi Carbon Co., Ltd., product name "Asahi #70L"
*13 CB4: CTAB adsorption specific surface area = 74 m ² /g, 24M4DBP absorption capacity = 90 mL/100 g, manufactured by Asahi Carbon Co., Ltd., product name "Asahi #70K"

From the results shown in Table 2, it can be seen that when 80 mass % or more of the rubber component is acrylonitrile-butadiene rubber with an acrylonitrile unit content of more than 45 mass %, if the CTAB adsorption specific surface area A of carbon black is 110 m ² /g or more and the carbon black content is 50 parts by mass or more per 100 parts by mass of the rubber component, the responsiveness of the change in electrical resistance value to strain is improved and the occurrence of cracks can be detected.

The conductive rubber composition for sensing of the present invention has good responsiveness in terms of changes in electrical resistance to strain. For example, when applied to the rubber tube of a rubber actuator, it can be used to detect the occurrence of cracks from changes in electrical resistance.

10: Rubber actuator, 20: Connection portion, 30: Resistance measuring device, 100: Actuator body, 110: Tube rubber, 111: Inner layer rubber, 112: Outer layer rubber, 120: Sleeve, 200: Sealing mechanism, 210: Sealing member, 211: Body portion, 212: Flange portion, 213: Concave and recessed portion, 214: Small diameter portion, 215: Passing hole, 220: Locking ring, 230: Crimping member, 231: Indentation, 300: Sealing mechanism, 400, 500: Fitting, 410, 510: Passing hole, D _AX : Axial direction, D _R : Radial direction

Claims (4)

The rubber composition includes a rubber component and carbon black,
The rubber component contains 80% by mass or more of acrylonitrile-butadiene rubber having an acrylonitrile unit content of more than 45% by mass,
The carbon black has a cetyltrimethylammonium bromide (CTAB) adsorption specific surface area of 110 m ² /g or more;
A conductive rubber composition for sensing, characterized in that the content of the carbon black is 50 parts by mass or more per 100 parts by mass of the rubber component. The conductive rubber composition for sensing according to claim 1, wherein the rubber component consists solely of acrylonitrile-butadiene rubber having a content of acrylonitrile units exceeding 45% by mass. The conductive rubber composition for sensing according to claim 1 or 2, wherein the carbon black has a compressed dibutyl phthalate (24M4DBP) absorption of 90 mL/100 g or more. The conductive rubber composition for sensing according to any one of claims 1 to 3, wherein the carbon black content is 55 parts by mass or more and 75 parts by mass or less per 100 parts by mass of the rubber component.

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