Magnetorheological fluid and mechanical device

Abstract

Provided is a magnetorheological fluid and a mechanical device, which have good resistance when excited and excellent dispersion stability of magnetic particles. The magnetorheological fluid contains magnetic particles, base oil, and a polyether-modified silicone, and the value obtained by subtracting the solubility parameter (SP value) of the base oil from the HLB value of the polyether-modified silicone is 2.5 or greater.

Description

TECHNICAL FIELD

[0001] The present application relates to a magneto rheological fluid and a mechanical device. In particular, the present application relates to a magneto rheological fluid and a mechanical device for controlling frictional force acting between objects in various mechanical devices such as dampers of brakes, clutches, anti-vibration devices, vibration-damping devices, and the like. BACKGROUND

[0002] A magneto rheological (MR) fluid is a fluid in which metal particles that can be magnetized, i.e., magnetic particles, are dispersed in a dispersion medium. In the case of a magneto rheological fluid, when no magnetic field is applied, the magnetic particles are suspended in the dispersion medium in a random manner, and the magneto rheological fluid functions as a fluid. On the other hand, when a magnetic field is applied, the magnetic particles form many clusters, and the internal stress increases.

[0003] The magneto rheological fluid functions as a rigid body by the increase in the internal stress described above, and shows resistance to shear flow and pressure flow. Due to such characteristics, the magneto rheological fluid is used for controlling frictional force acting between objects in various mechanical devices such as dampers of brakes, clutches, anti-vibration devices, vibration-damping devices, and the like.

[0004] Therefore, it is preferable that the resistance to shear flow and pressure flow (hereinafter also referred to as "resistance at the time of excitation") of the magneto rheological fluid be large when the magneto rheological fluid is subjected to the action of a magnetic field (at the time of excitation). Note that the resistance at the time of excitation is evaluated by measuring a torque value, viscosity, or shear stress, or the like. In the present specification, the resistance at the time of excitation is evaluated by measuring the viscosity at the time of excitation.

[0005] In order to increase the resistance at the time of excitation, although the content of the magnetic particles in the magneto rheological fluid can be increased, the magnetic particles are easily precipitated due to the large difference in specific gravity between the magnetic particles and the dispersion medium in the magneto rheological fluid. Furthermore, due to the cohesive force of the magnetic particles, a hard slurry-like precipitate is easily generated.

[0006] In Patent Literature 1, as a magneto rheological fluid, a magneto rheological fluid containing a prescribed amount of magnetic particles, a clay mineral-based dispersion stabilizer, and a surfactant in a carrier fluid is proposed.

[0007] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: Japanese Patent Application Laid-Open No. 2002-121578 SUMMARY

[0008] PROBLEMS TO BE SOLVED BY THE INVENTION However, the magnetorheological fluid described in Patent Document 1 has a small resistance at the time of excitation, and is not satisfactory. Also, it is not sufficient in terms of dispersion stability of the magnetic particles. Particularly for a damper for buildings and the like, since it is in a long-time stationary state, it is required to have excellent dispersion stability of the magnetic particles.

[0009] Furthermore, the magnetorheological fluid contains additives such as a dispersant, a lubricity improver, in addition to the magnetic particles and the base oil, but in order to secure these additives, it is difficult to increase the amount of the magnetic particles, and as a result, it is difficult to increase the resistance at the time of excitation.

[0010] The present application is completed in view of the above, and aims to provide a magnetorheological fluid and a mechanical device in which the resistance at the time of excitation is good, and the dispersion stability of the magnetic particles is also excellent.

[0011] Solution to the problem The present application is completed in view of the above, and aims to provide a magnetorheological fluid and a mechanical device in which the resistance at the time of excitation is good, and the dispersion stability of the magnetic particles is also excellent.

[0012] [1] A magnetorheological fluid containing: magnetic particles, a base oil, and a polyether-modified silicone, wherein a value obtained by subtracting a solubility parameter of the base oil from an HLB value of the polyether-modified silicone is 2.5 or more.

[0013] [2] The magnetorheological fluid according to [1], wherein a content ratio of the magnetic particles is 75 to 90 mass% with respect to the total amount of the magnetorheological fluid.

[0014] [3] The magnetorheological fluid according to [1] or [2], wherein the base oil is an alpha olefin, a polyalphaolefin, or a mineral oil.

[0015] [4] The magnetorheological fluid according to any one of [1] to [3], wherein the polyether-modified silicone is a side chain type polyether-modified silicone.

[0016] [5] A mechanical device using the magnetorheological fluid according to any one of [1] to [4].

[0017] Effects of the Invention According to the embodiment of the present application, it is possible to provide a magnetorheological fluid and a mechanical device in which the resistance at the time of excitation is good, and the dispersion stability of the magnetic particles is also excellent. DETAILED DESCRIPTION

[0018] Hereinafter, the embodiment of the magnetorheological fluid and the mechanical device of the present application will be described, but the present application is not limited to this explanation, and various changes, modifications, improvements can be made based on the knowledge of those skilled in the art without departing from the scope of the present application.

[0019] Note that in the present specification, "~" indicating a numerical range means a range including the numerical values recited as the upper limit value and the lower limit value, respectively. Further, in a numerical range in which only the upper limit value is recited with a unit, it means that the lower limit value is also the same unit as the upper limit value.

[0020] In the numerical ranges recited in stages in the present specification, the upper limit value or the lower limit value recited in a certain numerical range can be replaced with the upper limit value or the lower limit value of another numerical range recited in stages.

[0021] Further, in the numerical ranges recited in the present specification, the upper limit value or the lower limit value recited in a certain numerical range can be replaced with the value shown in the examples.

[0022] In the present specification, the content ratio or the content of each component in the composition, in the case where a plurality of substances corresponding to each component is present in the composition, means the content ratio or the content of the total of the plurality of substances present in the composition, unless otherwise specified.

[0023] (Magnetorheological fluid) The magnetorheological fluid of the present embodiment contains a magnetic particle, a base oil, and a polyether-modified silicone, and has the following configuration: the value obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone is 2.5 or greater. With such a configuration, the magnetorheological fluid of the present embodiment has good resistance at the time of excitation, and can have excellent dispersion stability of the magnetic particles. Hereinafter, each component contained in the magnetorheological fluid of the present embodiment will be described.

[0024] 1. Magnetic particle The magnetic particle contained in the magnetorheological fluid of the present embodiment can be selected according to the target magnetic permeability. For example, there can be mentioned: magnetite, carbonyl iron, γ-iron oxide, manganese ferrite, cobalt ferrite, or a composite ferrite thereof with zinc, nickel, barium ferrite, and the like, a strong magnetic oxide; a strong magnetic metal such as iron, cobalt, and rare earth; a nitride metal; Sendust (registered trademark), Permalloy (registered trademark), Supermalloy (registered trademark), and the like, various alloys, and the like. Among these, carbonyl iron is preferable from the viewpoint of a soft magnetic material having a small coercive force and a large magnetic permeability. Carbonyl iron is a high-purity metal particle manufactured by thermal decomposition of pentacarbonyl iron (Fe(CO)5).

[0025] Note that the magnetic particle can be used alone or in combination with two or more kinds.

[0026] In the magnetorheological fluid of this embodiment, when a magnetic field is applied from the outside, the dispersed magnetic particles orient themselves along the direction of the magnetic field and form chain-like clusters, thereby increasing viscosity and changing its flow characteristics and yield stress. The average particle size of the magnetic particles is determined in a manner that demonstrates this behavior. Specifically, it is preferably in the range of 0.1 to 100 μm. The lower limit of the average particle size is more preferably 1 μm or more, and even more preferably 4 μm or more. The upper limit of the average particle size is more preferably 80 μm or less, even more preferably 60 μm or less, even more preferably 50 μm or less, and even more preferably 40 μm or less. From the above viewpoint, it is more preferably 1 to 80 μm, more preferably 4 μm to 60 μm, and even more preferably 4 to 50 μm. The shape of the magnetic particles is preferably spherical or substantially spherical to facilitate dispersion.

[0027] It should be noted that the average particle size of the magnetic particles is the average primary particle size measured using a laser diffraction / scattering particle size distribution measuring device.

[0028] The content of magnetic particles relative to the total amount of magnetorheological fluid in this embodiment is preferably in the range of 75 to 90% by mass. By setting the content of magnetic particles relative to the total amount of magnetorheological fluid in this embodiment to a range of 75 to 90% by mass, the required resistance can be obtained when a magnetic field is applied, and the dispersion of magnetic particles can be maintained, thus also functioning as a fluid. The lower limit of the content of magnetic particles is more preferably 80% by mass or more. The upper limit of the content of magnetic particles is more preferably 88% by mass or less, and even more preferably 85% by mass or less. From the above viewpoint, 80 to 88% by mass is more preferred, and 80 to 85% by mass is even more preferred.

[0029] 2. Base oil The magnetorheological fluid of this embodiment includes a base oil. The base oil is appropriately selected from any substance among mineral oils and synthetic oils conventionally used as dispersion media for magnetic particles, provided that the following relationship is satisfied: the value obtained by subtracting the solubility parameter (SP value) of the base oil from the HLB value of the polyether-modified silicone (described later) is 2.5 or higher, and it is liquid at room temperature (25°C). Alpha-olefins, polyalphaolefins, or mineral oils are preferred because they are easily adjusted to satisfy the relationship that the value obtained by subtracting the solubility parameter (SP value) of the base oil from the HLB value of the polyether-modified silicone is 2.5 or higher. The base oil can be used alone or in combination with two or more. As the base oil used in the magnetorheological fluid of this embodiment, either mineral oil or synthetic oil can be used alone, or a combination of mineral oil and synthetic oil can be used. It should be noted that the numbers in parentheses for each base oil represent the SP value (cal / cm³) of each base oil. 3 ) 1 / 2 ).

[0030] Synthetic oils include, for example, hydrocarbon solvents such as α-olefins, polyα-olefins, isoparaffins, n-paraffins, and halogenated hydrocarbons; ester solvents; ether solvents; glycol solvents; and organosilicon solvents.

[0031] As a mineral oil, for example, any substance used in the field of general lubricants, such as cycloalkane mineral oils, alkane mineral oils, liquid paraffin, hydrodewaxed oils, etc., can be used.

[0032] Examples of α-olefins include 1-hexene (7.5), 1-octene (7.6), 1-decene (7.8), 1-dodecene (7.9), 1-tetradecene, 1-hexadecene, and 1-octadecene. Among these, α-olefins with 8 to 14 carbon atoms, such as 1-octene, 1-decene, and 1-dodecene, are preferred.

[0033] Polyalphaolefin is a polymer of the alpha olefin. More preferably, polyalphaolefin is a polymer of alpha olefins with 8 to 14 carbon atoms, such as 1-octene, 1-decene, and 1-dodecene. Alpha olefin and polyalphaolefin can be used alone or in combination of two or more.

[0034] Examples of ester-based solvents include monoesters, polyol esters, diesters (esters), and polyoxyalkylene glycol esters. Among these, monoesters with 12 to 30 carbon atoms are preferred, such as 2-ethylhexyl laurate, 2-ethylhexyl palmitate, and n-butyl stearate. Polyol esters are esters formed from polyols and straight-chain or branched saturated or unsaturated fatty acids. Hindered esters are examples of polyol esters. Ester-based solvents can be used alone or in combination of two or more.

[0035] Examples of ether-based solvents include polyethylene ethers, polyphenylene ethers, and perfluoroethers. Examples of polyethylene ethers include homopolymers of methyl vinyl ether (8.7) and homopolymers of ethyl vinyl ether (8.6). Ether-based solvents can be used alone or in combination with two or more.

[0036] Examples of glycol-based solvents include polyethylene glycol (9.4), polypropylene glycol (8.7), polybutane glycol (8.6), or ethylene oxide-propylene oxide copolymers, propylene oxide-butane oxide copolymers, and their derivatives. A single glycol-based solvent can be used alone, or two or more can be used in combination.

[0037] The preferred kinematic viscosity of the base oil at 40°C is 50.0 mm. 2 / s or less, more preferably 5.0 to 40.0 mm 2 Within the range of / s. This is achieved by setting the kinematic viscosity of the base oil at 40°C to 50.0 mm.2 For speeds below / s, it is more preferable in terms of easily dispersing magnetic particles.

[0038] It should be noted that the kinematic viscosity is determined according to the JIS K2283:2000 kinematic viscosity test method.

[0039] The base oil content relative to the total amount of magnetorheological fluid in this embodiment is preferably in the range of 10 to 25% by mass, more preferably in the range of 12 to 20% by mass. By setting the base oil content to 10% by mass or more, magnetic particles can be dispersed and fluidity can be improved. By setting the base oil content to 25% by mass or less, the magnetic properties during excitation can be improved.

[0040] The solubility parameter (SP value) of the base oil can be calculated according to the method proposed by Fedors et al. "Refer to Polymer Engineering and Science, 14, 147-154 (1974)".

[0041] That is, it can be calculated based on the following formula.

[0042] SP value δ=(Σ△e / Σ△v) 1 / 2 (In the above formula, Δe is the evaporation energy of each atom or group of atoms at 25℃, and Δv is the molar volume of each atom or group of atoms at the same temperature.) 3. Polyether-modified organosilicon The magnetorheological fluid of this embodiment contains polyether-modified organosilicon. The magnetorheological fluid containing polyether-modified organosilicon is in a paste-like state before excitation and exhibits high thixotropy. Therefore, even with an increased content of magnetic particles, the dispersion stability of the magnetic particles remains stable, which can improve the resistance during excitation. The content of polyether-modified organosilicon in the magnetorheological fluid of this embodiment is preferably 0.1 to 3% by mass relative to the total amount of magnetorheological fluid. When the content of polyether-modified organosilicon is 0.1% by mass or more, a greater resistance during excitation can be obtained. Furthermore, when the content of polyether-modified organosilicon is 3% by mass or less, thixotropy can be ensured, and the increase in viscosity can be suppressed. The content of polyether-modified organosilicon in the magnetorheological fluid of this embodiment is more preferably 0.3 to 2.5% by mass relative to the total amount of magnetorheological fluid, and even more preferably 0.5 to 1.5% by mass.

[0043] Furthermore, since the magnetorheological fluid of this embodiment contains polyether-modified organosilicon, which has a significant effect on the dispersion stability of magnetic particles, as described above, the content of magnetic particles can be further increased, thereby further improving the resistance during excitation. In addition, when the magnetorheological fluid is used in devices, liquid-phase separation of the magnetorheological fluid can be suppressed, and base oil seepage can be inhibited. For example, where conventional methods limit the content of magnetic particles to approximately 80% by mass to maintain the fluidity of the magnetorheological fluid, the content can be increased to approximately 87% by mass, thus better suppressing base oil seepage.

[0044] Furthermore, since the magnetorheological fluid of this embodiment contains polyether-modified organosilicon, which has an extremely high effect on the dispersion stability of magnetic particles, as described above, even when highly packed with magnetic particles, the magnetic particles are difficult to settle, and the dispersion stability of the magnetic particles can be maintained for a long time. Therefore, the magnetorheological fluid of this embodiment not only has good resistance during excitation but also can maintain the dispersion stability of magnetic particles for a long period of time. Therefore, the magnetorheological fluid of this embodiment is most suitable for applications such as building dampers that are in a static state for a long time.

[0045] Examples of polyether-modified organosilicones included in the magnetorheological fluid of this embodiment include polyalkylsiloxanes having a polyether structure. This polyether structure is preferably present on the side chains of the molecular chain. Hereinafter, polyalkylsiloxanes having a polyether structure on the side chains of the molecular chain are sometimes referred to as side-chain type polyether-modified organosilicones, or simply "side-chain type". The number of carbon atoms in the alkyl group of this polyalkylsiloxane is preferably 1 to 3.

[0046] The HLB value (hydrophilic-lipophilic balance value) of polyether-modified silicone is preferably in the range of 10 to 16, and more preferably in the range of 10.5 to 15. It should be noted that the HLB value is defined by the following formula (Griffin method).

[0047] HLB value = 20 × (total formula weight of hydrophilic portion / molecular weight) From the perspective of easily achieving a balance between hydrophilicity and hydrophobicity, it is preferable to have a polyoxyalkylene structure (a repeating structure of oxyalkylene units) in the polyether structure.

[0048] In the polyoxyalkylene structure, the number of repetitions of oxyalkylene (oxyalkylene units) is appropriately selected such that the HLB value of the modified organosilicon is within the above-mentioned range, preferably 2 to 20, more preferably 2 to 16, and particularly preferably 2 to 10.

[0049] In the polyoxyalkylene structure, the number of carbon atoms in the alkylene group is preferably 1 to 6, more preferably 2 to 6, further preferably 2 to 4, and most preferably 2 to 3.

[0050] The terminal end of the polyoxyalkylene structure is preferably a hydroxyl group. However, without impairing the effects of this embodiment, the terminal end of the polyoxyalkylene structure may also be capped with an alkyl group such as methyl.

[0051] The proportion (EO ratio) of the total number of oxyethylene units among the total number of oxyalkylene units in the polyether-modified organosilicon is, for example, 40% or more, preferably 50% or more. The EO ratio is 100% or less. Therefore, 40% to 100% is preferred, and 50% to 100% is more preferred.

[0052] It should be noted that the total number of alkylene units refers to the molar number of alkylene units contained in one molecule of polyether-modified organosilicon. The total number of ethylene units refers to the molar number of ethylene units contained in one molecule of polyether-modified organosilicon. Polyether-modified organosilicon containing only ethylene units as alkylene units can also be used (EO ratio = 100%).

[0053] Examples of polyether-modified silicones with the aforementioned EO ratio include TSF4440 (HLB: 14) and TSF4452 (HLB: 11, molar number of ethylene oxide units / molar number of propylene oxide units (=EO / PO)=50 / 50 (EO ratio 50%)) manufactured by Momentive Performance Materials. These polyether-modified silicones are all side-chain type.

[0054] In the magnetorheological fluid of this embodiment, the value obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone is 2.5 or higher. Based on this configuration, the polyether-modified silicone is incompatible with the base oil, and intermolecular hydrogen bonds are formed between the terminal hydroxyl groups of the polyether portion in the polyether-modified silicone, forming a thixotropic gel. Therefore, it is speculated that the dispersion stability of the magnetic particles contained in the magnetorheological fluid is improved. It should be noted that side-chain type polyether-modified silicone is more preferred because it is easier to form a three-dimensional structure.

[0055] 4. Other ingredients Regarding the magnetorheological fluid of this embodiment, various other components may be used in addition to the aforementioned components, depending on the purpose, without compromising the effects of this embodiment.

[0056] Other components include, for example: wear-resistant agents, dispersants, surfactants, viscosity modifiers, flow improvers, sedimentation inhibitors, pour point depressants, extreme pressure agents, rust inhibitors, antioxidants, corrosion inhibitors, metal deactivators, defoamers, etc.

[0057] Examples of wear-resistant agents include: sulfur compounds such as sulfides, sulfoxides, sulfones, and thiophosphinates; halogen compounds such as chlorinated hydrocarbons; and organometallic compounds such as molybdenum dithiophosphate (MoDTP), molybdenum dithiocarbamate (MoDTC), and tricresyl phosphate.

[0058] Wear-resistant agents can be used alone or in combination with two or more.

[0059] Dispersants are added to improve the dispersibility of magnetic particles in base oils. Well-known examples include low-molecular-weight dispersants and high-molecular-weight dispersants. A single dispersant can be used, or two or more can be used in combination.

[0060] Examples of viscosity modifiers include: castor oil, hydrogenated castor oil, fatty acid amides, beeswax, carnauba wax, benzyl sorbitol, metal soaps, oxidized polyethylene, sulfate-based anionic surfactants, polyolefins, (meth)acrylates, polyisobutylene, ethylene-propylene copolymers, and polyalkyl styrene.

[0061] Viscosity modifiers can be used alone or in combination with two or more.

[0062] As a flowability improver, modified silicone oil can be cited as an example. It is obtained by modifying pure silicone oil with alkyl, aralkyl, higher fatty acid esters, amino, epoxy, carboxyl, or alcohol groups. It should be noted that polyether-modified silicone is not included in this flowability improver. A single flowability improver can be used, or two or more can be used in combination.

[0063] <Viscosity of magnetorheological fluids> The pre-excitation viscosity of the magnetorheological fluid in this embodiment is preferably in the range of 0.5 to 5.0 Pa·s at 25°C, and more preferably in the range of 0.8 to 4.5 Pa·s. It should be noted that the measurement conditions for the pre-excitation viscosity are as follows.

[0064] 3 ml of magnetorheological fluid was injected into the test plate of a TA Instruments DHR-2 rheometer equipped with a magnetic measurement option, and the viscosity (Pa·s) was measured at 25°C with a 100 μm gap and 20 revolutions.

[0065] The excitation viscosity of the magnetorheological fluid in this embodiment, when a magnetic field of 0.8T is applied at 25°C, is preferably 5000 Pa·s or more, more preferably 6000 Pa·s or more, even more preferably 8000 Pa·s or more, particularly preferably 9000 Pa·s or more, and most preferably 14000 Pa·s or more.

[0066] It should be noted that the viscosity during excitation is measured in the following manner. That is, the same measuring device as that used to measure the viscosity before excitation is used, and a 0.8T magnetic field is applied at 25°C with a 700μm gap and 20 revolutions to measure the viscosity.

[0067] (Methods for manufacturing magnetorheological fluids) The method for manufacturing the magnetorheological fluid in this embodiment is not particularly limited. For example, a method can be described as follows: magnetic particles, base oil, polyether-modified silicone, and other components to be added as desired are mixed in predetermined amounts using a homogenizer, bead mill, mechanical mixer, or other processing equipment capable of applying high shear forces. It should be noted that heating or cooling may be performed during the manufacturing of the magnetorheological fluid as needed.

[0068] (A mechanical device that uses magnetorheological fluid) The magnetorheological fluid of this embodiment is applicable to various mechanical devices such as brakes, clutches, vibration damping devices, and dampers used to control frictional forces acting between objects. In particular, the magnetorheological fluid of this embodiment is best suited for building dampers and the like, which are in a static state for extended periods, due to the excellent dispersion stability of its magnetic particles.

[0069] Example The following are embodiments of the invention, but these embodiments are provided to better understand the invention and its advantages and are not intended to limit the invention.

[0070] <Examples 1-7, Comparative Examples 1-8> The components shown in Table 1 were added to a beaker according to their stated mass ratios, and the mixture was stirred at 40 Hz for 20 minutes at room temperature using a Seiko Advance AD-MIX universal vibratory stirrer to produce a magnetorheological fluid. The raw materials for each component shown in Table 1 are listed below.

[0071] (A) Magnetic particles • Carbonyl iron (spherical, average particle size D50 = 4.0–6.0 μm) (B) Base oil Mineral oil 1 [liquid paraffin, kinematic viscosity at 40°C 7.827 mm] 2 / s, SP value 7.9 (cal / cm) 3 ) 1 / 2 ] • Mineral oil 2 [liquid paraffin, kinematic viscosity at 40°C 28.65 mm] 2 / s, SP value 8.0 (cal / cm) 3 ) 1 / 2 ] • Ester solvents [hindered ester, kinematic viscosity at 40°C 16.0 mm] 2 / s, SP value 9.1 (cal / cm) 3 ) 1 / 2 ] • Hydrocarbon solvent [mixture of dimers and trimers of polyalphaolefin, 1-decene, kinematic viscosity at 40°C 5.5 mm] 2 / s, SP value 7.8 (cal / cm) 3 ) 1 / 2 ] (C) Dispersant ·alkylnaphthalene (D) Lubricity improver • Polyether-modified silicone 1 (side chain type, EO ratio = 100%, HLB = 14, product name: TSF4440, manufactured by Momentive Performance Materials) • Polyether-modified silicone 2 (side chain type, EO ratio = 50%, HLB = 11, product name: TSF4452, manufactured by Momentive Performance Materials) • Polyether-modified silicone 3 (side chain type, EO ratio = 0%, HLB = 1, product name: TSF4460, manufactured by Momentive Performance Materials) • Amino-modified organosilicon (product name: TSF4470, manufactured by Momentive Performance Materials) • Unmodified organosilicon (dimethylsilicon, product name: Element 14 PDMS50J, manufactured by Momentive Performance Materials) • Polybutene (product name: Polybutene-ON, manufactured by NIPPON OIL) (E) Extreme pressure agent • Zinc dithiophosphate (Zn-DTP) (Product name: KIKULUB Z-112, manufactured by ADEKA) <Evaluation of viscosity before and during excitation> 3 ml of the magnetorheological fluids from Examples 1-7 and Comparative Examples 1-8 were injected into the test plate of a TA Instruments DHR-2 rheometer equipped with a magnetic measurement option. The viscosity (Pa·s) was measured at 25°C with a 100 μm gap and 20 revolutions. The viscosity before excitation was also measured. Using the same measuring apparatus, the viscosity during excitation was measured at 25°C with a 0.8 T magnetic field applied at a 700 μm gap and 20 revolutions.

[0072] The above experimental conditions and results are shown in Table 1.

[0073] <Evaluation of Dispersion Stability> The dispersion stability of the magnetorheological fluids in Examples 1-7 and Comparative Examples 1-8 was evaluated.

[0074] Magnetorheological fluid was placed in a sample vial, and after 1, 10, 20, 30, 60, and 90 days at 23°C, the thicknesses of the magnetic particle layer and the dispersion medium layer (supernatant) were measured. The separation rate (a percentage of the thickness of the dispersion medium layer relative to the total thickness of the magnetic particle layer and the dispersion medium layer) was used as the evaluation value. The results are shown in Table 1. It should be noted that the separation rate is preferably 5% or less, more preferably 3% or less, and particularly preferably 1% or less. The magnetorheological fluids of Examples 1-7 are all magnetorheological fluids containing magnetic particles, base oil, and polyether-modified silicone, and the value obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone is 2.5 or higher. Therefore, the viscosity at excitation is high, exceeding 5000 Pa·s. Among them, the viscosity at excitation of Examples 1-5 and 7 is even higher, exceeding 6000 Pa·s. Furthermore, the viscosity at excitation of Examples 1-3 and 7 is even higher, exceeding 8000 Pa·s. The viscosity at excitation of Example 7 is extremely high, exceeding 14000 Pa·s. In addition, the magnetorheological fluids of Examples 1-7 all exhibit excellent dispersion stability even after 90 days.

[0075] For the magnetorheological fluids in Comparative Examples 1-3, the values ​​obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone were all less than 2.5. Therefore, regarding dispersion stability, the separation rate after 1 day was 30-40%, indicating poor dispersion stability.

[0076] The magnetorheological fluids in Comparative Examples 4-8, lacking polyether-modified organosilicon, exhibited poor dispersion stability, with a separation rate of 10-35% after 1 day.

Claims

1. A magnetorheological fluid, wherein, Contains: magnetic particles, base oil, and polyether-modified silicone. The value obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone is greater than 2.

5.

2. The magnetorheological fluid according to claim 1, wherein, The content of the magnetic particles is 75-90% by mass relative to the total amount of the magnetorheological fluid.

3. The magnetorheological fluid according to claim 1, wherein, The base oil is an alpha olefin, a polyalpha olefin, or a mineral oil.

4. The magnetorheological fluid according to claim 1, wherein, The polyether-modified organosilicon is a side-chain type polyether-modified organosilicon.

5. A mechanical device that uses the magnetorheological fluid as described in any one of claims 1 to 4.

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