Magnetorheological fluid, method for manufacturing magnetorheological fluid, and magnetorheological fluid device

Abstract

Provided is a magnetorheological fluid which contains magnetic particles, a carrier fluid, an organic zinc compound, melamine (iso)cyanurate, a thioether-based compound, and fumed silica, and which has a shear viscosity of 10.0 Pa∙s or more as measured at a measurement temperature of 25°C and a shear rate of 0.1 s-1.

Description

Magnetorheological fluid, its manufacturing method, and magnetorheological fluid device

[0001] The present invention relates to a magnetorheological fluid, a method for producing the same, and a magnetorheological fluid device.

[0002] Magnetorheological fluids, also known as MR fluids (Magneto Rheological Fluids), are functional materials that have been expected to be useful in a variety of applications in recent years (see, for example, Patent Document 1).

[0003] WO2023 / 008359A1

[0004] A magnetorheological fluid (MR) fluid contains magnetic particles and a carrier fluid and exhibits a rheological change when a magnetic field is applied. A magnetorheological fluid exhibits a higher yield shear stress (hereinafter also referred to as "shear stress") when a magnetic field is applied than when no magnetic field is applied. This is because the magnetic particles form clusters when a magnetic field is applied, which reduces the fluidity of the magnetorheological fluid. By utilizing this property, the operation of various devices can be controlled by applying a magnetic field. Devices that use magnetorheological fluids (called "magneto-rheological fluid devices" or "MR fluid devices") are broadly divided into rotary types such as brakes and direct-acting types such as linear dampers. As an example, we will explain the operation control of a brake using a magnetorheological fluid. Such a brake is called an MR (Magneto-Rheological) brake. The general structure of an MR brake is as follows: Inside the brake, a disk that can rotate in conjunction with an external shaft and a coil for generating a magnetic field are arranged, and a magnetorheological fluid is further enclosed. When a magnetic field is applied to the magnetorheological fluid by generating a magnetic field from a coil, clusters of magnetic particles are formed in the magnetorheological fluid perpendicular to the direction of disk rotation. When the disk is rotated with the clusters formed, the clusters are cut by the disk, and the shear stress generated at this time is exerted as a braking torque.

[0005] Magnetorheological fluid devices are required to maintain stable device performance even after repeated use. To achieve this, it is necessary for the magnetorheological fluid to exhibit little performance degradation (i.e., excellent durability) and little increase in viscosity even after repeated application of a magnetic field. In particular, in rotational magnetorheological fluid devices, the application of a magnetic field usually applies shear stress to the entire magnetorheological fluid filled inside the device, so higher durability is required compared to direct-acting magnetorheological fluid devices, which are often used in valve mode.

[0006] Furthermore, if the magnetic field torque fluctuation before and after the magnetorheological fluid is left stationary is large, it will affect the drag torque when the magnetorheological fluid device is restarted. Therefore, it is also desirable for the magnetorheological fluid to have small magnetic field torque fluctuation before and after being left stationary.

[0007] In view of the above, one aspect of the present invention aims to provide a magnetorheological fluid that is excellent in durability, exhibits little increase in viscosity even when a magnetic field is repeatedly applied, and exhibits little torque fluctuation in the absence of a magnetic field before and after being left standing.

[0008] One aspect of the present invention is as follows: [1] A magnetorheological fluid comprising: magnetic particles; a carrier fluid; an organozinc compound; melamine (iso)cyanurate; a thioether-based compound; and fumed silica; and the magnetorheological fluid is measured at a shear rate of 0.1 s at a measurement temperature of 25°C. －１A magnetorheological fluid having a shear viscosity of 10.0 Pa·sec (Pascal·sec) or more, as measured by [2]. [2] The magnetorheological fluid according to [1], further comprising a phenolic compound. [3] The magnetorheological fluid according to [2], wherein the phenolic compound is a hindered phenolic compound. [4] The magnetorheological fluid according to any one of [1] to [3], wherein the content of the thioether compound is 0.3% by mass or more and 3.0% by mass or less, relative to the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica. [5] The magnetorheological fluid according to any one of [1] to [4], wherein the content of the thioether compound is 1.0% by mass or more and 3.0% by mass or less, relative to the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica. [6] The magnetorheological fluid according to any one of [1] to [5], wherein the content of the fumed silica is 0.5% by volume or more and 3.0% by volume or less, relative to the volume of the magnetic particles. [7] The magnetorheological fluid according to any one of [1] to [6], wherein the thioether compound is ditridecylthiodipropionate. [8] The magnetorheological fluid according to any one of [1] to [7], wherein the organozinc compound is zinc dialkyldithiophosphate. [9] The magnetorheological fluid according to any one of [1] to [7], wherein the organozinc compound is zinc dialkyldithiophosphate.

[10] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[11] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[12] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[13] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[14] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[15] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[16] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[17] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[18] The magnetorheological fluid according to any one of [1] to [6], wherein the organozinc compound is zinc dialkyldithiophosphate.

[19] The magnetorheological fluid according to any one of [1] to [7], wherein the organozinc compound is zinc dialkyldithiophosphate.

[20] The magnetorheological fluid according to any one of [1] to [7], wherein the organozinc compound is －１

[10] The magnetorheological fluid according to any one of [1] to [8], wherein the shear viscosity of the magnetorheological fluid measured at a shear rate of 1000 Pa sec or less at a measurement temperature of 25°C is 0.45 Pa sec or less.

[11] The magnetorheological fluid according to any one of [1] to [8], further comprising a hindered phenol compound, wherein the content of the thioether compound is 1.0 mass% or more and 3.0 mass% or less with respect to the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica, and the content of the fumed silica is 0.5 volume% or more and 3.0 volume% or less with respect to the volume of the magnetic particles, wherein the thioether compound is ditridecylthiodipropionate, and the organozinc compound is zinc dialkyldithiophosphate, and wherein the shear viscosity of the magnetorheological fluid measured at a shear rate of 1000 Pa sec or less at a measurement temperature of 25°C is 0.45 Pa sec or less. －１The magnetorheological fluid according to [1], wherein the shear viscosity of the magnetorheological fluid measured by the method described above is 0.45 Pa·s or less.

[11] A method for producing a magnetorheological fluid according to any of [1] to

[10] , comprising resonant acoustic mixing of a mixture containing at least magnetic particles, fumed silica, and a carrier fluid, wherein the total content of the magnetic particles and the fumed silica in the mixture is 50 volume % or more and 70 volume % or less, based on the total volume of the mixture.

[12] The production method according to

[11] , further comprising adding a carrier fluid to the mixture after the resonant acoustic mixing, and then performing resonant acoustic mixing.

[13] A magnetorheological fluid device comprising the magnetorheological fluid according to any of [1] to

[10] .

[0009] According to one aspect of the present invention, there are provided a magnetorheological fluid having excellent durability, exhibiting only a small increase in viscosity even when a magnetic field is repeatedly applied, and exhibiting only a small change in torque in the absence of a magnetic field before and after being left stationary, a method for producing the same, and a magnetorheological fluid device containing the magnetorheological fluid.

[0010] [Magnetorheological Fluid] One aspect of the present invention relates to a magnetorheological fluid comprising magnetic particles, a carrier fluid, an organozinc compound, melamine (iso)cyanurate, a thioether-based compound, and fumed silica. The magnetorheological fluid is measured at a temperature of 25° C. and a shear rate of 0.1 s －１ Shear viscosity measured at 0.1 s (hereinafter referred to as "shear viscosity (0.1 s") －１ ) is 10.0 Pa·sec or more.

[0011] The present inventors speculate as follows about the magnetorheological fluid. The fact that the magnetorheological fluid contains an organic zinc compound, melamine (iso)cyanurate, and a thioether-based compound may contribute to the fact that the magnetorheological fluid can exhibit excellent durability. Furthermore, the fact that the magnetorheological fluid contains melamine (iso)cyanurate and a thioether-based compound may contribute to the fact that the magnetorheological fluid can suppress an increase in viscosity even when a magnetic field is repeatedly applied. Furthermore, the fact that the magnetorheological fluid can suppress a torque variation without a magnetic field before and after being left standing may contribute to the fact that the shear viscosity (0.1 s －１ In this regard, the inclusion of fumed silica in the magnetorheological fluid and the preparation of the magnetorheological fluid as described below, for example, can contribute to the reduction of the shear viscosity (0.1 s －１ ) can be controlled within the above range. However, the present invention is not limited to the speculations described in this specification.

[0012] The magnetorheological fluid will now be described in more detail.

[0013] <Organic Zinc Compound> The magnetorheological fluid contains an organic zinc compound. In the present invention and this specification, an "organic zinc compound" is an organic compound containing zinc (Zn). As described above, the inventors speculate that the organic zinc compound can contribute to improving the durability of the magnetorheological fluid. From this perspective, preferred organic zinc compounds include those that can function as extreme pressure agents or friction modifiers (also called FM agents).

[0014] A more preferred organic zinc compound is zinc dialkyldithiophosphate (ZnDTP). The zinc dialkyldithiophosphate may be a compound represented by the following general formula 1:

[0015]

[0016] In general formula 1, R １ ~R ４each independently represents a hydrocarbon group. The hydrocarbon group is preferably a hydrocarbon group having 1 to 20 carbon atoms. Specific examples of the hydrocarbon group include primary alkyl groups such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, and icosyl groups; secondary propyl, secondary butyl, secondary pentyl, secondary hexyl, secondary heptyl, and secondary octyl groups; Secondary alkyl groups such as secondary nonyl, secondary decyl, secondary undecyl, secondary dodecyl, secondary tridecyl, secondary tetradecyl, secondary pentadecyl, secondary hexadecyl, secondary heptadecyl, secondary octadecyl, secondary nonadecyl, and secondary icosyl groups; tertiary butyl, tertiary pentyl, tertiary hexyl, tertiary heptyl, tertiary octyl, tertiary nonyl, tertiary decyl, tertiary undecyl, tertiary dodecyl, tertiary tridecyl, and tertiary tetradecyl groups. tertiary alkyl groups such as a tertiary pentadecyl group, a tertiary hexadecyl group, a tertiary heptadecyl group, a tertiary octadecyl group, a tertiary nonadecyl group, and a tertiary icosyl group; a branched butyl group (such as an isobutyl group), a branched pentyl group (such as an isopentyl group), a branched hexyl group (such as an isohexyl group), a branched heptyl group (such as an isoheptyl group), a branched octyl group (such as an isooctyl group, a 2-ethylhexyl group), a branched nonyl group (such as an isononyl group), a branched decyl group (such as an isodecyl group), and a branched undecyl group. branched alkyl groups such as branched alkyl groups (such as an isoundecyl group), branched dodecyl groups (such as an isododecyl group), branched tridecyl groups (such as an isotridecyl group), branched tetradecyl groups (such as an isotetradecyl group), branched pentadecyl groups (such as an isopentadecyl group), branched hexadecyl groups (such as an isohexadecyl group), branched heptadecyl groups (such as an isoheptadecyl group), branched octadecyl groups (such as an isooctadecyl group), branched nonadecyl groups (such as an isononadecyl group), and branched icosyl groups (such as an isoicosyl group);Examples of aryl groups include a phenyl group, a toluyl group, a xylyl group, a cumenyl group, a mesityl group, a benzyl group, a phenethyl group, a styryl group, a cinnamyl group, a benzhydryl group, a trityl group, an ethylphenyl group, a propylphenyl group, a butylphenyl group, a pentylphenyl group, a hexylphenyl group, a heptylphenyl group, an octylphenyl group, a nonylphenyl group, a decylphenyl group, an undecylphenyl group, a dodecylphenyl group, a styrenated phenyl group, a p-cumylphenyl group, a phenylphenyl group, and a benzylphenyl group. １ ~R ４ may be the same or different.

[0017] As the organozinc compound, commercially available products can be used, and compounds prepared by known methods can also be used. Examples of commercially available products include the ADEKA Kiclub series manufactured by ADEKA Corporation. However, the present invention is not limited to these.

[0018] From the viewpoint of further improving the durability of the magnetorheological fluid, the content of the organozinc compound in the magnetorheological fluid is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more, relative to the mass of the magnetorheological fluid (i.e., the total mass of the magnetorheological fluid is taken as 100% by mass), and can be, for example, 5.0% by mass or less, 4.0% by mass or less, 3.0% by mass or less, 2.0% by mass or less, or 1.0% by mass or less.

[0019] The various contents described in the present invention and this specification are values ​​determined by known methods unless otherwise specified.

[0020] The presence of an organozinc compound in a magnetorheological fluid can be confirmed by known analytical methods, such as infrared spectroscopy, gas chromatography, and nuclear magnetic resonance (NMR). １３ C-NMR, ３１For example, specific analytical methods can be found in J. Japan Petrol. Inst., 26, (1), 50-56 (1983), JP-A-9-184832, etc.

[0021] <Melamine (iso)cyanurate> The magnetorheological fluid contains melamine (iso)cyanurate together with an organozinc compound. The inventors speculate that the combined use of an organozinc compound and melamine (iso)cyanurate can improve the durability of the magnetorheological fluid because melamine (iso)cyanurate does not or only little promotes the decomposition of the organozinc compound.

[0022] In the present invention and this specification, the term "melamine (iso)cyanurate" encompasses melamine cyanurate, melamine isocyanurate, and mixtures containing melamine cyanurate and melamine isocyanurate in any ratio. "Melamine cyanurate" is an organic salt of melamine and cyanuric acid, and "melamine isocyanurate" is an organic salt of melamine and isocyanuric acid. Isocyanuric acid is an isomer of cyanuric acid. Melamine (iso)cyanurate can function, for example, as a solid lubricant. Commercially available melamine (iso)cyanurate can be used, or a compound prepared by a known method can also be used. Examples of commercially available products include melamine (iso)cyanurate sold by Nissan Chemical Industries under the trade name "Melamine Cyanurate." However, the present invention is not limited to this.

[0023] From the viewpoint of further improving the durability of the magnetorheological fluid, the content of melamine (iso)cyanurate in the magnetorheological fluid is preferably 0.1% by mass or more, more preferably 0.2% by mass or more, relative to the mass of the magnetorheological fluid (i.e., the total mass of the magnetorheological fluid is taken as 100% by mass), and may be, for example, 1.0% by mass or less, 0.8% by mass or less, or 0.5% by mass or less.

[0024] <Fumed Silica> The magnetorheological fluid contains fumed silica. Fumed silica is also known as colloidal silica, synthetic silica, colloidal silicon dioxide, silica colloidalis anhydrica, or light anhydrous silicic acid. Fumed silica is produced by the combustion hydrolysis of silicon tetrachloride, and an aggregated particle structure is formed when newly formed molten particles of silicon dioxide become colloidal and form branched chains. In this way, multiple particles are connected, aggregated, and fused to form bulky aggregates. Fumed silica can function as a thixotropic agent, for example. Fumed silica can be used in a range of 200 ml. ２ The BET specific surface area of ​​fumed silica is preferably 1000 m / g or more. ２ / g or less, 800m ２ / g or less or 600m ２ / g or less. However, the BET specific surface area of ​​the fumed silica contained in the magnetorheological fluid is not limited to the range described herein. The BET specific surface area is the specific surface area determined by the BET method described in S. Brunauer, P. H. Emmet, and I. Teller, J. Am. Chemical Society, 60, 309 (1938).

[0025] The content of fumed silica in the magnetorheological fluid is determined based on the volume of the magnetic particles (i.e., the volume of the magnetic particles is taken as 100% by volume) by shear viscosity (0.1 sec －１ From the viewpoint of increasing the fumed silica content, the fumed silica content is preferably 0.5% by volume or more, more preferably 1.0% by volume or more, and even more preferably 2.0% by volume or more. The fumed silica content in the magnetorheological fluid may be, for example, 10.0% by volume or less, 7.0% by volume or less, or 5.0% by volume or less. As will be described in detail later, the fumed silica content is measured at a measurement temperature of 25°C and a shear rate of 1000 s －１ The shear viscosity of the magnetorheological fluid measured at 1000 s (hereinafter referred to as "shear viscosity (1000 s") －１A low shear viscosity (1000 s －１ From the viewpoint of reducing the porosity, the content of fumed silica in the magnetorheological fluid is preferably 4.0% by volume or less, and more preferably 3.0% by volume or less.

[0026] <Thioether-Based Compound> The magnetorheological fluid contains a thioether-based compound. The thioether-based compound can function as, for example, an antioxidant. As described above, the thioether-based compound can contribute to the magnetorheological fluid exhibiting excellent durability. Furthermore, the thioether-based compound can contribute to the magnetorheological fluid being able to suppress an increase in viscosity even when a magnetic field is repeatedly applied. In the present invention and this specification, a "thioether-based compound" refers to a compound having a thioether bond (-S-), and is preferably an organic compound having a thioether bond. When the organozinc compound described above has a thioether bond (-S-), such an organozinc compound is not considered to fall under the "thioether-based compound" described herein.

[0027] Specific examples of the thioether compound include dilauryl thiodipropionate, ditridecyl thiodipropionate, distearyl thiodipropionate, pentaerythritol tetrakis(3-dodecylthiopropionate), and 4,4-thiobis(2-tertiary-butyl-5-methylphenol)bis-3-(dodecylthio)propionate.

[0028] As the thioether-based compound, commercially available products can be used, and those prepared by known methods can also be used. Examples of commercially available products that can be used include those sold as thioether-based antioxidants. Specific examples of commercially available products include the ADEKA Eco Royal AIN series manufactured by ADEKA Corporation. However, the present invention is not limited to these.

[0029] From the viewpoint of improving the durability of the magnetorheological fluid and suppressing an increase in viscosity when a magnetic field is repeatedly applied, the content of the thioether-based compound in the magnetorheological fluid is preferably 0.3% by mass or more, more preferably 0.5% by mass or more, and even more preferably 1.0% by mass or more, based on the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica (i.e., assuming this mass to be 100% by mass). The content of the thioether-based compound in the magnetorheological fluid can be, for example, 5.0% by mass or less or 4.0% by mass or less, and the shear viscosity (1000 s －１ From the viewpoint of reducing the amount of Cr, the amount is preferably 3.0 mass % or less.

[0030] <Magnetic Particles> The magnetorheological fluid contains magnetic particles. Examples of magnetic particles include particles of magnetic materials classified as paramagnetic, superparamagnetic, or ferromagnetic materials. Various magnetic particles commonly used in magnetorheological fluids can be used. Specific examples include metal particles containing at least one selected from iron, cobalt, and nickel, and metal compound particles containing at least one selected from iron nitride, iron carbide, carbonyl iron, ferrite, and magnetite. The metal particles can be particles containing at least one metal component selected from iron, cobalt, and nickel. Specifically, they can be particles containing a metal element selected from iron, cobalt, and nickel as the metal component, or particles containing an alloy containing at least one metal component selected from iron, cobalt, and nickel as a constituent component. The alloy-containing particles can contain metal components other than iron, cobalt, and nickel as constituent components of the alloy. In this case, it is preferable that at least one selected from iron, cobalt, and nickel is the main component. Furthermore, the metal compound particles preferably contain one or more components selected from iron nitride, iron carbide, carbonyl iron, ferrite, and magnetite as their main component. Here, the term "main component" refers to the component that has the largest mass ratio among the components constituting the magnetic particles. The magnetic particles may be composed solely of the main component.

[0031] Among the above magnetic particles, magnetic particles containing iron are preferred, and metal compound particles generally referred to as carbonyl iron are more preferred. Carbonyl iron is typically produced by thermal decomposition of iron pentacarbonyl. For example, commercially available carbonyl iron powder may be used as the magnetic particles to prepare the magnetorheological fluid. Examples of commercially available products include carbonyl iron powder available from Jiangsu Tianyi Ultrafine Metal Powder Co., Ltd. and carbonyl iron powder available from BASF. However, the magnetic particles are not limited to these, and commercially available magnetic particles or magnetic particles prepared by known methods can also be used.

[0032] The magnetic particles contained in the magnetorheological fluid may be magnetic particles whose surfaces are coated with a silane coupling agent or the like, or may be magnetic particles whose surfaces are not coated.

[0033] From the viewpoint of providing a magnetorheological fluid that can effectively exhibit the property of changing rheology upon application of a magnetic field, the average particle size of the magnetic particles is preferably 0.05 to 50 μm, more preferably 0.05 to 40 μm, and even more preferably 0.1 to 30 μm. The average particle size of the magnetic particles is the average particle size measured by a laser diffraction scattering method in accordance with JIS Z8825:2013.

[0034] In the present invention and this specification, the content of magnetic particles in a magnetorheological fluid is a value calculated assuming that the total volume of the magnetorheological fluid is 100% by volume. The magnetorheological fluid may contain only one type of magnetic particle or two or more types of magnetic particles. When two or more types of magnetic particles are contained, the content refers to the total content of the two or more types of magnetic particles. This also applies to the contents of various components in the present invention and this specification. From the viewpoint of increasing the output of a magnetorheological fluid device, the magnetorheological fluid preferably contains magnetic particles at a content of 20% by volume or more relative to the volume of the magnetorheological fluid, more preferably 25% by volume or more, and even more preferably 30% by volume or more. On the other hand, a low content of magnetic particles in the magnetorheological fluid can contribute to a low viscosity of the magnetorheological fluid. From this perspective, the content of magnetic particles in the magnetorheological fluid is preferably 50% by volume or less, more preferably 48% by volume or less, even more preferably 45% by volume or less, and even more preferably 43% by volume or less.

[0035] The magnetic particle content (unit: volume %) in the magnetorheological fluid described in this invention and this specification is a value determined by the following method. A magnetorheological fluid having a volume Vtotal and a mass Wtotal is allowed to stand naturally or is centrifuged (including ultracentrifugation) to separate it into a solid component and a liquid component. The volume of the separated solid component is Vm, and the volume of the separated liquid component is Vl. The mass of the separated solid component is Wm, and the mass of the separated liquid component is Wl. Vtotal, Vm, and Vl are determined by known volume measurement methods. Wtotal, Wm, and Wl are determined by known mass measurement methods. The solid component includes magnetic particles. Even if adsorbed components are adsorbed to the magnetic particles separated as solid components, the influence of the adsorbed components on the measured values ​​of volume, mass, and true density described below is usually negligible and can be ignored. Therefore, Vm is considered to be the volume of the magnetic particles, and Wm is considered to be the mass of the magnetic particles. The relationship of the following formula (1) holds between Vm, Vl, and Vtotal. The relationship of the following formula (2) holds between Wm, Wl, and Wtotal. Vm + Vl = Vtotal (1) Wm + Wl = Wtotal (2) Dividing the above formula (1) by Vtotal gives the following formula (3): Vm / Vtotal + Vl / Vtotal = 1 (3) Here, if Cm = Vm / Vtotal, the following formula (4) is obtained from the above formula (3): Vl / Vtotal = (1 - Cm) (4) Furthermore, let the true density of the magnetorheological fluid be dtotal, the true density of the separated solid component be dm, and the true density of the separated liquid component be dl. The following relationship holds: Wm (unit: kg) = Vm (unit: m ３ ) x dm (unit: kg / m ３ ) Wl (unit: kg) = Vl (unit: m ３ ) x dl (unit: kg / m ３ ) Wtotal (unit: kg) = Vtotal (unit: m ３ ) × dtotal (unit: kg / m ３) holds, and therefore, from these relationships, the above formula (2) can be rewritten as the following formula (2A): Vm×dm+Vl×dl=Vtotal×dtotal (2A) Dividing the above formula (2A) by Vtotal gives the following formula (5): (Vm / Vtotal)×dm+(Vl / Vtotal)×dl=dtotal (5) The above formula (6) can be obtained from the above formula (5) and formula (4): Cm×dm+(1−Cm)×dl=dtotal (6) Rearranging the above formula (6) with respect to Cm gives the following formula (7): Cm=(dtotal−dl) / (dm−dl) (7) Cm can be calculated from the above formula (7) and the various true densities mentioned above. The true density dm of the solid content can be determined by known methods such as liquid-phase displacement and gas-phase displacement. To determine the content of magnetic particles described in the Examples section below, the true density dm of the solid content was determined by gas-phase displacement in accordance with JIS Z8807:2012. The true density dtotal of the magnetorheological fluid and the true density dl of the liquid component can be determined by the pycnometer method in accordance with JIS K5600-2-4:2014. To determine the content of magnetic particles described in the Examples section below, the true density dtotal of the magnetorheological fluid and the true density dl of the liquid component were determined by the same method. Using the various true density values ​​thus determined, "Cm" is calculated using the above formula (7). The content of magnetic particles in the magnetorheological fluid (unit: volume %) is calculated as "calculated Cm x 100."

[0036] <Carrier Fluid> In the present invention and this specification, the term "carrier fluid" refers to a liquid fluid. The liquid fluid refers to a fluid that is liquid in an environment at an ambient temperature of 25°C. As the carrier fluid, various liquid fluids that are generally used for magnetorheological fluids, such as hydrocarbon-based fluids, can be used.

[0037] In the present invention and this specification, the term "hydrocarbon-based fluid" refers to a liquid fluid containing a hydrocarbon-based compound (i.e., a compound having carbon and hydrogen atoms as constituent atoms). Specific examples of hydrocarbon-based fluids include polyalphaolefin-based fluids and ester-based fluids. The magnetorheological fluid may contain either a polyalphaolefin-based fluid or an ester-based fluid as a carrier fluid, or may contain a mixture of a polyalphaolefin-based fluid and an ester-based fluid.

[0038] In the present invention and this specification, the term "polyalphaolefin-based fluid" refers to a liquid fluid containing an α-olefin polymer. Here, the polymer includes homopolymers and copolymers. Polyalphaolefins are generally called "PAO" (poly-alpha-olefin). As the polyalphaolefin-based fluid, commercially available products or liquid fluids prepared by known methods can be used. An example of a commercially available product is polyalphaolefin-based fluid sold by Chevron Philipps Chemical Company. However, the present invention is not limited to this.

[0039] In the present invention and this specification, the term "ester-based fluid" refers to a liquid fluid containing an organic compound having an ester bond. A preferred form of ester-based fluid is a polyol ester-based fluid. A "polyol ester-based fluid" is a liquid fluid containing a polyol compound having an ester bond. A polyol compound having an ester bond is also called a "polyol ester." Examples of polyol esters include neopentyl polyol esters obtained by the esterification reaction of neopentyl alcohols such as trimethylolpropane, neopentyl glycol, and pentaerythritol with fatty acids. Commercially available ester-based fluids and polyol ester-based fluids can be liquid fluids prepared by known methods. Commercially available polyol ester-based fluids include, but are not limited to, commercial products from NYCO.

[0040] In a mixture of a polyalphaolefin fluid and an ester-based fluid, the content of the polyalphaolefin fluid can be, for example, 70% by mass or more, 80% by mass or more, or 90% by mass or more, and can be, for example, 99% by mass or less or 95% by mass or less, based on 100% by mass of the total mass of the polyalphaolefin fluid and the ester-based fluid. In a mixture of a polyalphaolefin fluid and an ester-based fluid, the content of the ester-based fluid can be, for example, 5% by mass or more or 10% by mass or more, and can be, for example, 30% by mass or less, 25% by mass or less, 20% by mass or less, or 15% by mass or less, based on 100% by mass of the total mass of the polyalphaolefin fluid and the ester-based fluid.

[0041] The content of the carrier fluid in the magnetorheological fluid is preferably 5% by mass or more, more preferably 10% by mass or more, relative to the mass of the magnetorheological fluid (i.e., the total mass of the magnetorheological fluid is taken as 100% by mass), from the viewpoint of reducing the viscosity of the magnetorheological fluid. Furthermore, from the viewpoint of increasing the output of the magnetorheological fluid device, the content of the carrier fluid is preferably 30% by mass or less, more preferably 20% by mass or less.

[0042] <Other Components> The magnetorheological fluid may contain only the above-mentioned components, or may contain one or more other components in addition to the above-mentioned components. The various components that may be contained in the magnetorheological fluid will be described below.

[0043] (Phenol-based Compound) The magnetorheological fluid may contain a phenol-based compound. The phenol-based compound may function, for example, as an antioxidant. If the organozinc compound described above is a compound having a phenol derivative structure, such an organozinc compound is not considered to be a "phenol-based compound" as described herein. The phenol-based compound may contribute to further improving the durability of the magnetorheological fluid.

[0044] In the present invention and this specification, the term "phenolic compound" includes phenol and its derivatives. Examples of phenolic compounds include 2,6-di-tert(tertiary)-butylphenol (hereinafter, "tert-butyl" will be abbreviated as "t-butyl"), 2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol, 2,4-dimethyl-6-t-butylphenol, 4,4'-methylenebis(2,6-di-t-butylphenol), 4,4'-bis(2,6-di-t-butylphenol), 4,4'-bis(2-methyl-6-t-butylphenol), 2,2 ... Bis(4-methyl-6-t-butylphenol), 2,2'-methylenebis(4-ethyl-6-t-butylphenol), 4,4'-butylidenebis(3-methyl-6-t-butylphenol), 4,4'-isopropylidenebis(2,6-di-t-butylphenol), 2,2'-methylenebis(4-methyl-6-cyclohexylphenol), 2,2'-methylenebis(4-methyl-6-nonylphenol), 2,2'-isobutylidenebis(4,6-dimethylphenol), 2,6-bis(2'-hydroxy-3'-t- butyl-5'-methylbenzyl)-4-methylphenol, 3-t-butyl-4-hydroxyanisole, 2-t-butyl-4-hydroxyanisole, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionate stearyl, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionate oleyl, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionate dodecyl, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionate decyl, 3-(4-hydroxy-3,5 3-(4-hydroxy-3,5-di-t-butylphenyl)propionic acid octyl, tetrakis{3-(4-hydroxy-3,5-di-t-butylphenyl)propionyloxymethyl}methane, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionic acid glycerin monoester, ester of 3-(4-hydroxy-3,5-di-t-butylphenyl)propionic acid and glycerin monooleyl ether, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionic acid butylene glycol diester, 3-(4-hydroxy-3,5-di-t-butylphenyl)propionic acid thiodiglycol diester, 4,4'-thiobis(3-methyl-6-t-butylphenol), 4,4'-thiobis(2-methyl-6-t-butylphenol), 2,2'-thiobis(4-methyl-6-t-butylphenol), 2,6-di-t-butyl-α-dimethylamino-p-cresol, 4,6-bis(octylthiomethyl)-o(ortho)-cresol, 4,6-bis(dodecylthiomethyl)-o-cresol, 2,6-di-t-butyl-4-(N,N'-dimethylaminomethylphenyl)propionic acid thiodiglycol diester ol), bis(3,5-di-t-butyl-4-hydroxybenzyl) sulfide, tris{(3,5-di-t-butyl-4-hydroxyphenyl)propionyl-oxyethyl}isocyanurate, tris(3,5-di-t-butyl-4-hydroxyphenyl)isocyanurate, 1,3,5-tris(3,5-di-t-butyl-4-hydroxybenzyl)isocyanurate, bis{2-methyl-4-(3-n-alkylthiopropionyloxy)-5-t-butylphenyl}sulfide, 1,3,5-tris(4-t-butyl-3-hydroxyethyl)isocyanurate bis(2,6-dimethylbenzyl)isocyanurate, tetraphthaloyl-di(2,6-dimethyl-4-t-butyl-3-hydroxybenzyl sulfide), 6-(4-hydroxy-3,5-di-t-butylanilino)-2,4-bis(octylthio)-1,3,5-triazine, 2,2'-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], tridecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, pentaerythrityl-tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate -butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, heptyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, octyl-3-(3-methyl-5-t-butyl-4-hydroxyphenyl)propionate, nonyl-3-(3-methyl-5-t-butyl-4-hydroxyphenyl)propionate, hexamethylenebis[3-(3,3,5-bis(1,1-dimethyl-ethyl)-4-hydroxy]benzenepropionic acid, alkyl esters having 7 to 9 carbon atoms in the side chain, 2,4,8-tetraoxaspiro[5,5]undecane-3,9-diylbis(2-methylpropane-2,1-diyl)bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 3,5-di-t-butyl-4-hydroxybenzyl phosphate diester, bis(3-methyl-4-hydroxy-5-t-butylbenzyl)sulfide, 3,9-bis[1,1-dimethyl-2-{β-(3-t-butyl-4-hydroxy-5-methylphenyl)propionyl ester] phenolic compounds such as]-2,4,8,10-tetraoxaspiro[5,5]undecane, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,1-bis(2-methyl-4-hydroxy-5-t-butylphenyl)butane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, 2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)mesitylene, 3,5-di-t-butyl-4-hydroxybenzyl alkyl ester, and bis{3,3'-bis-(4'-hydroxy-3'-t-butylphenyl)butyric acid}glycol ester.

[0045] As the phenolic compound, commercially available products can be used, or those prepared by known methods can also be used. As the phenolic compound, hindered phenolic compounds are preferred. In the present invention and this specification, the term "hindered phenolic compound" refers to a compound having a substituent at the ortho position of the phenolic hydroxy group. Examples of the ortho-position substituent include alkyl groups, alkoxy groups, amino groups, and halogen atoms. Among these, alkyl groups such as methyl groups, ethyl groups, n-propyl groups, isopropyl groups, n-butyl groups, secondary butyl groups, isobutyl groups, and tertiary butyl groups are preferred, with isopropyl groups, secondary butyl groups, isobutyl groups, and tertiary butyl groups being more preferred, and tertiary butyl groups being even more preferred. Furthermore, it is preferred that both ortho positions relative to the phenolic hydroxy group are substituted with a substituent. Hindered phenolic compounds are generally commercially available as hindered phenolic antioxidants. Specific examples of commercially available phenolic compounds include the Adeka STAB AO series and the Adeka Eco Royal AIN series manufactured by ADEKA Corporation. However, the present invention is not limited to these.

[0046] From the viewpoint of further improving the durability of the magnetorheological fluid, the content of the phenolic compound in the magnetorheological fluid is preferably 0.1% by mass or more, and more preferably 0.2% by mass or more, relative to the mass of the magnetorheological fluid (i.e., the total mass of the magnetorheological fluid is taken as 100% by mass), and the content of the phenolic compound in the magnetorheological fluid can be, for example, 1.0% by mass or less, 0.8% by mass or less, or 0.5% by mass or less.

[0047] (Copper Carboxylate) In one embodiment, the magnetorheological fluid may contain one or more copper carboxylates. The inclusion of a copper carboxylate is preferred from the viewpoint of further improving the durability of the magnetorheological fluid.

[0048] Copper carboxylate is a compound represented by the following general formula 4: In the structure represented by general formula 4, the bond between the copper atom and the oxygen atom can be a covalent bond or an ionic bond.

[0049]

[0050] When the bond is an ionic bond, the compound represented by general formula 4 can also be represented by the following general formula 4-1.

[0051]

[0052] In general formula 4 and general formula 4-1, R ４１ and R ４２each independently represents a hydrocarbon group, preferably a hydrocarbon group having 1 to 24 carbon atoms. Examples of such hydrocarbon groups include the following: methyl group, ethyl group, n-(normal-)propyl group, isopropyl group, n-butyl group, isobutyl group, sec-(secondary-)butyl group, t-butyl group, linear or branched pentyl group, linear or branched hexyl group, linear or branched heptyl group, linear or branched octyl group, linear or branched nonyl group, linear or branched decyl group, linear or branched undecyl group, linear or branched dodecyl group, linear or branched butyl group, is an alkyl group having 1 to 24 carbon atoms, such as a branched tridecyl group, a linear or branched tetradecyl group, a linear or branched pentadecyl group, a linear or branched hexadecyl group, a linear or branched heptadecyl group, a linear or branched octadecyl group, a linear or branched nonadecyl group, a linear or branched icosyl group, a linear or branched heneicosyl group, a linear or branched docosyl group, a linear or branched tricosyl group, or a linear or branched tetracosyl group; A linear or branched butenyl group, a linear or branched pentenyl group, a linear or branched hexenyl group, a linear or branched heptenyl group, a linear or branched octenyl group, a linear or branched nonenyl group, a linear or branched decenyl group, a linear or branched undecenyl group, a linear or branched dodecenyl group, a linear or branched tridecenyl group, a linear or branched tetradecenyl group, a linear or branched alkenyl groups having 4 to 24 carbon atoms such as a pentadecenyl group, a linear or branched hexadecenyl group, a linear or branched heptadecenyl group, a linear or branched octadecenyl group, a linear or branched nonadecenyl group, a linear or branched icosenyl group, a linear or branched henicosenyl group, a linear or branched docosenyl group, a linear or branched tricosenyl group, or a linear or branched tetracosenyl group; cycloalkyl groups having 5 to 7 carbon atoms such as a cyclopentyl group, a cyclohexyl group, or a cycloheptyl group;Alkylcycloalkyl groups having 6 to 24 carbon atoms, such as methylcyclopentyl, dimethylcyclopentyl (including all structural isomers), methylethylcyclopentyl (including all structural isomers), diethylcyclopentyl (including all structural isomers), methylcyclohexyl, dimethylcyclohexyl (including all structural isomers), methylethylcyclohexyl (including all structural isomers), diethylcyclohexyl (including all structural isomers), methylcycloheptyl, dimethylcycloheptyl (including all structural isomers), methylethylcycloheptyl (including all structural isomers), and diethylcycloheptyl (including all structural isomers); saturated hydrocarbon group moieties having 6 to 24 carbon atoms, excluding the carboxy group, of naphthenic acid (a general term for saturated carboxylic acids having a naphthenic nucleus) having 7 to 25 carbon atoms, which is the main component of petroleum acid; aryl groups, such as phenyl and naphthyl; alkylaryl groups having 7 to 18 carbon atoms, such as tolyl (including all structural isomers), xylyl (including all structural isomers), ethylphenyl (including all structural isomers), linear or branched propylphenyl (including all structural isomers), linear or branched butylphenyl (including all structural isomers), linear or branched pentylphenyl (including all structural isomers), linear or branched hexylphenyl (including all structural isomers), linear or branched heptylphenyl (including all structural isomers), linear or branched octylphenyl (including all structural isomers), linear or branched nonylphenyl (including all structural isomers), linear or branched decylphenyl (including all structural isomers), linear or branched undecylphenyl (including all structural isomers), and linear or branched dodecylphenyl (including all structural isomers); Arylalkyl groups having 7 to 12 carbon atoms, such as a benzyl group, a phenylethyl group, a phenylpropyl group (including an isomer of a propyl group), a phenylbutyl group (including an isomer of a butyl group), a phenylpentyl group (including an isomer of a pentyl group), and a phenylhexyl group (including an isomer of a hexyl group);etc. Typical structures of the above-mentioned "saturated hydrocarbon group moiety having 6 to 24 carbon atoms excluding the carboxy group of naphthenic acid (a general term for saturated carboxylic acids having a naphthenic nucleus) having 7 to 25 carbon atoms, which is the main component of petroleum acid" include (alkyl)cyclopentylalkyl groups having 6 to 24 carbon atoms, represented by the following general formula 5, and (alkyl)cyclohexylalkyl groups having 7 to 24 carbon atoms, represented by the following general formula 6. The term "(alkyl)cyclopentylalkyl group" is used to encompass both alkylcyclopentylalkyl groups and cyclopentylalkyl groups. The term "(alkyl)cyclohexylalkyl group" is used to encompass both alkylcyclohexylalkyl groups and cyclohexylalkyl groups.

[0053]

[0054] (In general formula 5, R ５１ , R ５２ , R ５３ and R ５４ each independently represents a hydrogen atom, a methyl group, or an ethyl group, and n5 represents an integer ranging from 1 to 18. * represents the bonding position to the adjacent structure.

[0055]

[0056] (In general formula 6, R ６１ , R ６２ , R ６３ , R ６４ and R ６５ each independently represents a hydrogen atom, a methyl group, or an ethyl group, and n6 represents an integer ranging from 1 to 18. * represents the bonding position to the adjacent structure.

[0057] Among the compounds encompassed by general formula 4, R ４１ and R ４２However, copper carboxylates each independently representing an alkyl group having 8 to 24 carbon atoms, an alkenyl group having 8 to 24 carbon atoms, an alkylcycloalkyl group having 8 to 24 carbon atoms, or a saturated hydrocarbon group having 8 to 24 carbon atoms excluding the carboxy group of naphthenic acid having 9 to 25 carbon atoms are preferred from the viewpoint of further improving the durability of the magnetorheological fluid. More preferred examples of copper carboxylates include copper 2-ethylhexanoate, copper n-dodecanoate (copper laurate), copper isododecanoate, copper n-octadecanoate (copper stearate), copper oleate, copper naphthenate having 9 to 25 carbon atoms, and mixtures thereof.

[0058] From the viewpoint of further improving the durability of the magnetorheological fluid, the content of the copper carboxylate in the magnetorheological fluid is preferably 0.1% by mass or more relative to the mass of the magnetorheological fluid (i.e., the total mass of the magnetorheological fluid is taken as 100% by mass), and the content of the copper carboxylate in the magnetorheological fluid can be, for example, 1.0% by mass or less, 0.8% by mass or less, or 0.5% by mass or less.

[0059] Incidentally, WO 2023 / 008359 A1 (Patent Document 1) discloses that an organic compound having a siloxane bond (Si—O—Si) can be used as a dispersant to enhance the dispersibility of magnetic particles in a magnetorheological fluid. In contrast, in one embodiment, the magnetorheological fluid described above can be a magnetorheological fluid that does not contain an organic compound having a siloxane bond. Here, "does not contain an organic compound having a siloxane bond" refers to the absence of an organic compound having a siloxane bond as an ingredient in preparing the magnetorheological fluid, although the unintentional inclusion of an organic compound having a siloxane bond as an impurity is acceptable. In the magnetorheological fluid described above, the content of the organic compound having a siloxane bond relative to the mass of the magnetorheological fluid (i.e., assuming the total mass of the magnetorheological fluid to be 100% by mass) can be, for example, 0% by mass or more and 0.1% by mass or less, and can even be 0% by mass.

[0060] <Shear viscosity> (shear viscosity (0.1 sec) －１ )) Shear viscosity of the above magnetorheological fluid (0.1 sec －１) is 10.0 Pa·sec or more. This makes it possible to suppress the increase in viscosity even when a magnetic field is repeatedly applied. From the viewpoint of further suppressing the increase in viscosity, the shear viscosity (0.1 sec －１ ) is preferably 12.0 Pa·s or more, more preferably 15.0 Pa·s or more, 18.0 Pa·s or more, and 20.0 Pa·s or more in that order. －１ ) can be, for example, 200.0 Pa·s or less, 150.0 Pa·s or less, 100.0 Pa·s or less, or 50.0 Pa·s or less. From the viewpoint of suppressing the increase in viscosity, the shear viscosity (0.1 s －１ The higher the shear viscosity (0.1 sec), the better, so it may exceed the values ​​exemplified here. －１ ) can be controlled within the above range by including fumed silica in the magnetorheological fluid and by preparing the magnetorheological fluid, for example, as described below.

[0061] (shear viscosity (1000 sec －１ )) From the viewpoint of reducing the torque of the magnetorheological fluid device when there is no magnetic field, the shear viscosity (1000 s －１ The inventors believe that it is preferable that the shear rate γ and shear viscosity η are low. Details are as follows. In magnetorheological fluid devices, disk-shaped or cylindrical rotors are often used. The torque when there is no magnetic field can be calculated from the rotor radius r, rotor surface area S, and stress σ. The stress σ is a value that depends on the physical properties of the magnetorheological fluid, and can be calculated from the shear rate γ and shear viscosity η using the following formula: σ (Pa) = η (Pa·sec) γ (sec －１ In magnetorheological fluid devices, the gap between the rotor and the magnetic pole is about several hundred micrometers, and the shear rate is several hundred to several thousand seconds. －１ Magnetorheological fluids usually have thixotropic properties, and the shear viscosity changes depending on the shear rate. －１ ) is a shear rate of 1000 s －１ Since this is a measurement value at 1000 s, it can be said that this value corresponds to the stress that can occur in a magnetorheological fluid device. －１) has a low viscosity, which leads to a reduction in the stress σ, and as a result, can contribute to a reduction in torque when no magnetic field is present.

[0062] The shear viscosity of the magnetorheological fluid (1000 s －１ The shear viscosity (1000 s) of the magnetorheological fluid can be, for example, 1.00 Pa·s or less or 0.80 Pa·s or less, and from the viewpoint of reducing the torque of the magnetorheological fluid device when there is no magnetic field, it is preferably 0.45 Pa·s or less. －１ ) can be, for example, 0.10 Pa·s or more, or 0.15 Pa·s or more. From the viewpoint of reducing the torque of the magnetorheological fluid device when there is no magnetic field, the shear viscosity (1000 s －１ ) is preferably low, so the shear viscosity (1000 s －１ ) may be lower than the values ​​exemplified above. －１ Specific examples of the means for controlling the shear viscosity (1000 s －１ Therefore, the use of melamine (iso)cyanurate as a component capable of functioning as a solid lubricant tends to increase the shear viscosity (1000 s －１ ) can contribute to lowering the

[0063] In the present invention and this specification, the shear viscosity of a magnetorheological fluid is measured by the following method. A coaxial double-cylinder rotational viscometer with a constant outer cylinder speed is used as the viscometer. A specific example of a usable viscometer is the coaxial double-cylinder rheometer ONRH-1 type viscometer manufactured by Ohnai Giken Co., Ltd. For example, an Ohnai Giken G2B-145 can be used as the inner cylinder and an Ohnai Giken Small-175 can be used as the outer cylinder. The measurement temperature is controlled using a temperature control means or the like provided in the viscometer, and measurement is started after confirming that the reading of the thermometer (e.g., a platinum thermometer) installed in the inner cylinder is within the range of 25°C ± 1.0°C. In the viscometer, a shear rate of 0.0464 s －１ The viscosity was measured at a shear rate of 0.0464 s－１ 1000 seconds from －１ The shear rate is increased in 13 steps, and the viscosity is measured while maintaining the increased shear rate after each step. This process is repeated twice in total. －１ 10 to １／３ (= 2.1527...) multiplied n times. The final stage (n = 13) accelerated shear rate was 1000 s －１ is 0.0464 seconds －１ 10 to １／３ (= 2.1527...) multiplied 13 times. Viscosity measurement was performed at a shear rate of 0.0464 s －１ , and at the shear rate after each of the 13 stages of acceleration. Therefore, in the above two repetitions, the total number of measurement points in each run is 14. In the viscosity measurement at each of the 14 shear rates, the viscosity is measured when it reaches a steady state. The steady state refers to a state in which the viscosity value displayed on the viscometer reaches a constant value. In the present invention and this specification, the shear rate after acceleration in the final stage (n=13) of the second acceleration is 1000 s . －１ The viscosity measured at a shear rate of 1000 s －１ Shear viscosity measured at 1000 s －１ In the present invention and this specification, the shear rate after the first stage (n=1) of the second speed increase is set to 0.1 s －１ The viscosity measured at a shear rate of 1000 s －１ Shear viscosity measured at 0.1 s －１ )

[0064] In addition to the components described above, the magnetorheological fluid may contain one or more of various components known as additives that can be used in magnetorheological fluids, at any content.

[0065] The magnetorheological fluid can be applied to magnetorheological fluid devices, as will be described further below.

[0066] <Method for Producing a Magnetorheological Fluid> The magnetorheological fluid can be produced by mixing the various components described above in any order or simultaneously. A preferred production method includes subjecting a mixture containing the various components described above to resonant acoustic mixing.

[0067] "Resonance acoustic mixing" refers to mixing using acoustic resonance energy. Preferably, a mixture containing particles is sealed in a container, and low-frequency vibrations of around 60 Hz (Hertz) are applied to vibrate the container up and down with high acceleration force. The natural vibration frequency (e.g., about 60 Hz) of the particles in the container is used to transmit high energy to the particles through resonance, and the components in the container are mixed by particle-to-particle collisions and / or particle-to-container collisions. A mixer for performing resonance acoustic mixing is generally called a resonance acoustic mixer. A resonance acoustic mixer is usually referred to as a RAM (Resonance Acoustic Mixer). For example, a low-frequency resonance acoustic mixer LabRAM II manufactured by Resodyn can be used as the resonance acoustic mixer. Regarding processing conditions, for example, a frequency of around 60 Hz, a gravitational acceleration of about 50 to 100 G, and a mixing processing time of about 1 to 60 minutes can be used. However, the above processing conditions are merely examples, and the mixing conditions may be set depending on the types of components used to produce the magnetorheological fluid, the mixing ratio, the processing amount, etc. Regarding the unit of gravitational acceleration, G, 1 G = 9.80665 m / s ２ is.

[0068] Shear viscosity (0.1 sec －１ In order to produce a magnetorheological fluid having a viscosity of 10.0 Pa·sec or more, it is preferable to carry out at least one of the following (1) and (2), and more preferably to carry out both:

[0069] (1) Resonant acoustic mixing is performed multiple times. The number of times resonant acoustic mixing is performed is, for example, two times, and can be three or more times. The number of times resonant acoustic mixing is performed can be, for example, five or fewer times, or four or fewer times. However, resonant acoustic mixing can be performed more times.

[0070] (2) A mixture containing at least magnetic particles, fumed silica, and a carrier fluid, in which the total content of the magnetic particles and fumed silica (hereinafter also referred to as "volume fraction") is 50% by volume or more and 70% by volume or less relative to the total volume of the mixture, is subjected to resonant acoustic mixing.

[0071] Regarding (2) above, the mixture can contain one or more of the various components described above in addition to the magnetic particles, fumed silica, and carrier fluid. It is preferable to further add a carrier fluid to the mixture after this resonant acoustic mixing, and then perform further resonant acoustic mixing. The volume fraction is preferably 55% by volume or more, and more preferably 60% by volume or more.

[0072] After the mixing, post-treatment may be carried out as needed by a known method, such as stirring, dispersion using ultrasound or a sand mill, or filtration.

[0073] [Magnetorheological Fluid Device] One aspect of the present invention relates to a magnetorheological fluid device containing the magnetorheological fluid described above.

[0074] Specific examples of magnetorheological fluid devices include brakes, clutches, dampers, shock absorbers, and the like. These can be used in automobiles, various vehicles, architectural structures, radio-controlled devices, home appliances, and the like. Specific examples of magnetorheological fluid devices also include various devices used in the health and welfare fields, such as prosthetic limbs, prosthetic hands, and training equipment. The magnetorheological fluid device includes the magnetorheological fluid described in detail above. Because the magnetorheological fluid exhibits excellent durability, it is suitable for application to rotary magnetorheological fluid devices, which require even higher durability. Furthermore, because magnetorheological fluids with excellent durability are also preferred in direct-acting magnetorheological fluid devices, the magnetorheological fluid is also suitable for application to direct-acting magnetorheological fluid devices.

[0075] The magnetorheological fluid device may be any device in which the magnetorheological fluid is contained in a portion of the device where the magnetorheological fluid is to be introduced, and known techniques relating to magnetorheological fluid devices can be applied to the details of the device configuration and the like.

[0076] The present invention will be described below based on examples. However, the present invention is not limited to the embodiments shown in the examples. In the following, "parts" refers to parts by mass.

[0077] Example 1 Formulation of Magnetorheological Fluid Magnetic particles: 100.00 parts Carbonyl iron powder Rz manufactured by Jiangsu Tianyi Ultrafine Metal Powder Co., Ltd. Carrier fluid: 19.50 parts (Breakdown of carrier fluid) Polyalphaolefin-based fluid (Synfluid PAO 4 cSt manufactured by Chevron Philipps Chemical Co.): 17.55 parts Polyol ester-based fluid (NYCOBASE 7300 manufactured by NYCO Corporation): 1.95 parts Melamine (iso) cyanurate: 0.19 parts Melamine cyanurate MC-6000 manufactured by Nissan Chemical Co., Ltd. Fumed silica: 0.85 parts Cab-O-SilEH-5 manufactured by Cabot Corporation Organic zinc compound: 0.63 parts ADEKA Kiclube Z-112 (zinc dialkyldithiophosphate (ZnDTP) represented by general formula 1) Phenol compound: 0.21 parts ADEKA STAB AO-50 (hindered phenol compound) Thioether compound: 0.13 parts ADEKA ECO ROYAL AIN-700 (ditridecylthiodipropionate) Copper oleate: 0.10 parts Fujifilm Wako Pure Chemical Industries, Ltd. Copper (II) oleate

[0078] <Preparation of Magnetorheological Fluid> The carrier fluid was premixed so that the mass ratio of the polyalphaolefin-based fluid to the polyol ester-based fluid was 9:1. The carrier fluid obtained by this mixing is hereinafter referred to as the "mixed carrier fluid." All of the above components, except for the mixed carrier fluid, were added to a 120 ml sealed polypropylene container so that the amount of magnetic particles was 200 g. The amount of mixed carrier fluid was then adjusted and added so that the volumetric content (volume fraction) calculated by dividing the mass of the magnetic particles and fumed silica by their true specific gravities was the value listed in Table 1. This container was placed in a Resodyn low-frequency resonant acoustic mixer LabRAM II, and resonant acoustic mixing (first RAM) was performed for 2 minutes under mixing conditions of a frequency of 60 Hz and a gravitational acceleration of 65 G. To the composition obtained by this mixing, the amount of mixed carrier fluid added was calculated by subtracting the amount of mixed carrier fluid added above from the amount of mixed carrier fluid that would be added to achieve 200 g of magnetic particles. This composition was again placed in a Resodyn low-frequency resonant acoustic mixer, LabRAM II, and resonant acoustic mixing was performed for 2 minutes under mixing conditions of a frequency of 60 Hz and a gravitational acceleration of 65 G (second RAM). The liquid after the second resonant acoustic mixing was filtered using a nylon mesh with an opening size of 100 μm. In this way, the magnetorheological fluid of Example 1 was prepared. The contents of the various components described above in the magnetorheological fluid prepared in this way are shown in Table 1. In Table 1, the unit column, for example, "volume % / MR fluid volume" indicates the content relative to the total volume of the magnetorheological fluid. Similarly, in other descriptions, the description to the right of " / " indicates the standard of content.

[0079] Examples 2 to 9 The magnetorheological fluids of Examples 2 to 9 were prepared by the method described for Example 1, except that the items shown in Table 1 were changed as shown in Table 1.

[0080] Comparative Examples 1 to 8 The magnetorheological fluids of Comparative Examples 1 to 8 were prepared by the method described for Example 1, except that the items shown in Table 1 were changed as shown in Table 1. The dispersant used in Comparative Example 1 was KP578 (an organic compound having a siloxane bond) manufactured by Shin-Etsu Chemical Co., Ltd. The solid lubricant MoDTC (molybdenum dithiocarbamate) used in Comparative Example 6 was Sakuralube 600 (molybdenum dithiocarbamate) manufactured by ADEKA Corporation. The solid lubricant BN (boron nitride) used in Comparative Example 7 was AP-100S manufactured by Maruka Corporation. The magnetorheological fluids of the comparative examples indicated as "RAM 1 time" in Table 1 were prepared by the method described for Example 1, with the following exceptions. All of the above components were charged into a 120 ml polypropylene sealed container so that the amount of magnetic particles was 200 g, and resonance acoustic mixing was performed only once, as described for the first resonance acoustic mixing in Example 1. The volume fractions of the magnetic particles and fumed silica in this resonant acoustic mixing were the values ​​shown in Table 1. The liquid after the first resonant acoustic mixing was filtered using a nylon mesh with an opening size of 100 μm.

[0081] [Evaluation Method] <Shear Viscosity of Magnetorheological Fluid> A portion of each magnetorheological fluid of the Examples and Comparative Examples was used as a measurement sample. Using this sample, the shear viscosity was measured at a measurement temperature of 25° C. and a shear rate of 0.1 sec －１ Shear viscosity measured at a shear rate of 1000 s －１ The shear viscosity measured in was determined by the method described above. For the measurement, a coaxial double cylindrical rheometer ONRH-1 type viscometer manufactured by Ohna Giken Co., Ltd. was used as the viscometer, a G2B-145 manufactured by Ohna Giken Co., Ltd. was used as the inner cylinder, and a Small-175 manufactured by Ohna Giken Co., Ltd. was used as the outer cylinder. The measurement temperature was confirmed using a platinum thermometer installed in the inner cylinder.

[0082] <Time to Fail in Durability Test> The durability of each magnetorheological fluid in the Examples and Comparative Examples was evaluated using an MR (Magneto Rheological) characteristic evaluation device according to the following method. The magnetorheological fluid was placed in a groove provided in the coil portion of the MR characteristic evaluation device, a rotor with a gap of 0.05 mm was inserted into the gap, and the overflowing magnetorheological fluid was removed with a paper rag (Kimwipe, manufactured by Nippon Paper Crecia Co., Ltd.). The rotor rotation speed was set to 80 rpm (rotations per minute), and the coil current was controlled using proportional-integral-differential (PID) control so that the torque measured by a torque meter was 2 Nm. In the PID control, the control parameters of each PID are denoted as Kp, Ki, and Kd. The durability test was conducted with the Kp, Ki, and Kd parameters fixed, updating the current value every 0.7 seconds. When the upper limit of 2.08 Nm was reached for 42 seconds (when the upper limit was exceeded over 60 PID updates), the PID control was deemed to have been lost and a Fail signal was issued, halting the test. The durability test was limited to a maximum of 24 hours. If a Fail signal was detected during that time, the test time until the Fail signal was recorded and defined as the "time until the durability test failed." If no Fail signal was detected within the 24-hour limit, the "time until the durability test failed" was deemed to be "24 hours or more." A time until the durability test failed of 18 hours or more was deemed to have excellent durability.

[0083] <Viscosity increase during durability test> A durability test was carried out for 6 hours using the method described above. After the 6-hour durability test, the magnetorheological fluid was collected and the viscosity of the magnetorheological fluid before and after the durability test was compared. The viscosity was measured using a coaxial double cylindrical rheometer ONRH-1 type viscometer manufactured by Ohnai Giken Co., Ltd., with the parallel plate option. The distance between the plates was set to 0.48 mm, and the temperature was set at room temperature (23°C) and a shear rate of 100 s －１The shear viscosity of each sample was measured. In principle, the viscosity value changes depending on the amount of liquid, so an electronic balance was used to adjust the amount of liquid to within the range of 0.24±0.04 g. In Table 1, the column for "Viscosity increase during durability test" shows the calculated ratio of "shear viscosity after durability test / shear viscosity before durability test." If the ratio calculated in this way is less than 2.0, it can be determined that there is little increase in viscosity even when a magnetic field is repeatedly applied.

[0084] <No-magnetic field torque variation before and after standing> Using the MR characteristic evaluation device used in the durability test described above, torque (referred to as "torque before standing") was measured under the following conditions: gap spacing of 0.2 mm, no magnetic field, rotation speed of 80 rpm, and sampling interval of 1 ms. The torque was defined as the arithmetic average of the torque during the last 1 second of the 10-second measurement period. After standing for 15 hours, the torque (referred to as "torque after standing") was measured again under the above conditions, and the rate of change was calculated as "rate of change = (torque after standing / torque before standing) × 100". The calculated value is shown in the "No-magnetic field torque variation before and after standing" column in Table 1. If the rate of change calculated in this way is less than 120%, it can be determined that the no-magnetic field torque variation before and after standing is small.

[0085] The above results are shown in Table 1 (Table 1-1 to Table 1-4).

[0086]

[0087]

[0088]

[0089]

[0090] From the results shown in Table 1, it can be confirmed that the magnetorheological fluids of Examples 1 to 9 are magnetorheological fluids that have excellent durability, exhibit little increase in viscosity even when a magnetic field is repeatedly applied, and exhibit little fluctuation in torque without a magnetic field before and after being left standing.

[0091] One aspect of the present invention is useful in various technical fields in which magnetorheological fluids are used or in which their use is expected.

Claims

1. A magnetorheological fluid comprising: magnetic particles; a carrier fluid; an organozinc compound; melamine (iso)cyanurate; a thioether-based compound; and fumed silica; and a shear rate of 0.1 s at a measurement temperature of 25°C. －１ The shear viscosity of the magnetorheological fluid measured by is 10.0 Pa·sec or more.

2. The magnetorheological fluid of claim 1, further comprising a phenolic compound.

3. The magnetorheological fluid according to claim 2, wherein the phenolic compound is a hindered phenolic compound.

4. The magnetorheological fluid according to claim 1, wherein the content of the thioether compound is 0.3 mass % or more and 3.0 mass % or less relative to the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica.

5. The magnetorheological fluid according to claim 1, wherein the content of the thioether compound is 1.0 mass % or more and 3.0 mass % or less relative to the mass of the magnetorheological fluid excluding the mass of the magnetic particles and the mass of the fumed silica.

6. The magnetorheological fluid according to claim 1, wherein the content of said fumed silica is 0.5% by volume or more and 3.0% by volume or less relative to the volume of said magnetic particles.

7. The magnetorheological fluid according to claim 1, wherein the thioether compound is ditridecylthiodipropionate.

8. The magnetorheological fluid of claim 1, wherein the organozinc compound is a zinc dialkyldithiophosphate.

9. Measurement temperature: 25°C, shear rate: 1000 s －１ 2. The magnetorheological fluid of claim 1, wherein the shear viscosity of the magnetorheological fluid measured at 1000 kJ / s is 0.45 Pa·s or less.

10. A magnetorheological fluid further comprising a hindered phenol compound, wherein the content of the thioether compound is 1.0% by mass or more and 3.0% by mass or less relative to the mass of the magnetic particle excluding the mass of the magnetic particle and the mass of the fumed silica, and the content of the fumed silica is 0.5% by volume or more and 3.0% by volume or less relative to the volume of the magnetic particle, wherein the thioether compound is ditridecylthiodipropionate, and the organozinc compound is zinc dialkyldithiophosphate, and wherein the magnetorheological fluid is measured at a shear rate of 1000 seconds at a measurement temperature of 25°C. －１ 2. The magnetorheological fluid of claim 1, wherein the shear viscosity of the magnetorheological fluid measured at 1000 kJ / s is 0.45 Pa·s or less.

11. A method for producing a magnetorheological fluid according to any one of claims 1 to 10, comprising resonant acoustic mixing of a mixture containing at least magnetic particles, fumed silica, and a carrier fluid, wherein the combined content of the magnetic particles and the fumed silica in the mixture is 50% by volume or more and 70% by volume or less, based on the total volume of the mixture.

12. The method of claim 11, further comprising adding a carrier fluid to the resonant acoustically mixed mixture followed by resonant acoustic mixing.

13. A magnetorheological fluid device comprising the magnetorheological fluid according to any one of claims 1 to 10.

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