Magnetic viscous fluid and mechanical device

Abstract

Provided are: a magnetic viscous fluid having suitable resistance during excitation and excellent dispersion stability of magnetic particles; and a mechanical device. The magnetic viscous fluid contains magnetic particles, a base oil, and a polyether-modified silicone. 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 more.

Description

Magnetorheological fluids and mechanical devices

[0001] The present invention relates to a magnetorheological fluid and a mechanical device, and more particularly to a magnetorheological fluid and a mechanical device used to control frictional forces acting between objects in various mechanical devices such as brakes, clutches, vibration isolators, and dampers for vibration suppression devices.

[0002] A magnetorheological (MR) fluid is a fluid in which magnetic particles, such as magnetizable metal particles, are dispersed in a dispersion medium. In a magnetorheological fluid, the magnetic particles are randomly suspended in the dispersion medium when no magnetic field is applied, and the fluid functions as a fluid. On the other hand, when a magnetic field is applied, the magnetic particles form numerous clusters, increasing viscosity and internal stress.

[0003] The increased internal stress causes magnetorheological fluids to function like rigid bodies, resisting shear and pressure flows. Because of these properties, magnetorheological fluids are used to control the frictional forces acting between objects in various mechanical devices, such as brakes, clutches, vibration isolation devices, and dampers for vibration control devices.

[0004] For this reason, it is preferable that the resistance to shear flow and pressure flow when a magnetic field is applied to the magnetorheological fluid (hereinafter also referred to as "resistance during excitation") is large. The resistance during excitation is evaluated by measuring the torque value, viscosity, shear stress, etc. In this specification, the resistance during excitation is evaluated by measuring the viscosity during excitation.

[0005] Increasing the magnetic particle content in the magnetorheological fluid can increase the drag force during excitation, but the large difference in specific gravity between the magnetic particles and the dispersion medium in the magnetorheological fluid makes the magnetic particles prone to settling, which then tends to produce a hard, slurry-like precipitate due to the cohesive force of the magnetic particles.

[0006] Patent Document 1 proposes a magnetorheological fluid in which magnetic particles, a clay mineral-based dispersion stabilizer, and a surfactant are contained in a carrier fluid in predetermined amounts.

[0007] Japanese Patent Application Laid-Open No. 2002-121578

[0008] However, the magnetorheological fluid described in Patent Document 1 has a small resistance force when magnetized, which is unsatisfactory. Furthermore, the dispersion stability of the magnetic particles is also insufficient. In particular, for construction dampers and the like, which are left stationary for long periods of time, excellent dispersion stability of the magnetic particles is required.

[0009] In addition to magnetic particles and base oil, magnetorheological fluids also contain additives such as dispersants and lubricity improvers, but in order to ensure the availability of these additives, it is difficult to increase the amount of magnetic particles, which makes it difficult to improve the drag force when excited.

[0010] The present invention has been made in view of the above points, and has as its object to provide a magnetorheological fluid and a mechanical device which have good resistance when magnetized and excellent dispersion stability of magnetic particles.

[0011] In order to solve the above problems, the present invention is specified as follows: [1] to [5]. [1] A magnetorheological fluid containing magnetic particles, a base oil, and a polyether-modified silicone, wherein 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 more. [2] The magnetorheological fluid according to [1], wherein the content of the magnetic particles is 75 to 90 mass% with respect to the total amount of the magnetorheological fluid. [3] The magnetorheological fluid according to [1] or [2], wherein the base oil is an α-olefin, polyα-olefin, or mineral oil. [4] The magnetorheological fluid according to any of [1] to [3], wherein the polyether-modified silicone is a side-chain polyether-modified silicone. [5] A mechanical device using the magnetorheological fluid according to any of [1] to [4].

[0012] According to the embodiments of the present invention, it is possible to provide a magnetorheological fluid and a mechanical device that exhibit good drag force when magnetized and also exhibits excellent dispersion stability of magnetic particles.

[0013] Below, we will explain embodiments of the magnetorheological fluid and mechanical device of the present invention, but the present invention should not be construed as being limited to these, and various changes, modifications, and improvements can be made based on the knowledge of those skilled in the art as long as they do not deviate from the scope of the present invention.

[0014] In this specification, the term "to" representing a numerical range indicates a range that includes the numerical values ​​recited as the upper and lower limits. Furthermore, when a unit is recited for only the upper limit of a numerical range, this means that the lower limit is also expressed in the same unit as the upper limit. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples. In this specification, when multiple substances corresponding to each component are present in the composition, the content or amount of each component in the composition refers to the total content or amount of the multiple substances present in the composition, unless otherwise specified.

[0015] (Magnetic Rheological Fluid) The magnetic rheological fluid according to this embodiment contains magnetic particles, a base oil, and a polyether-modified silicone, and has a configuration in which 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 more. With this configuration, the magnetic rheological fluid according to this embodiment has good drag force when excited and can have excellent dispersion stability of the magnetic particles. Below, each component contained in the magnetic rheological fluid according to this embodiment will be described.

[0016] 1. Magnetic Particles The magnetic particles contained in the magnetorheological fluid according to the present embodiment can be selected depending on the desired magnetic permeability. Examples include ferromagnetic oxides such as magnetite, carbonyl iron, gamma iron oxide, manganese ferrite, cobalt ferrite, or composite ferrites of these with zinc or nickel, or barium ferrite; ferromagnetic metals such as iron, cobalt, and rare earth elements; metal nitrides; and various alloys such as Sendust (registered trademark), Permalloy (registered trademark), and Supermalloy (registered trademark). Among these, carbonyl iron is preferred because it is a soft magnetic material with low coercive force and high magnetic permeability. Carbonyl iron is a high-purity metal particle produced by the thermal decomposition of pentacarbonyl iron (Fe(CO)5). Note that one type of magnetic particle may be used alone, or two or more types may be used in combination. In the magnetorheological fluid according to the present embodiment, when an external magnetic field is applied, the dispersed magnetic particles align in the direction of the magnetic field to form chain-like clusters, thereby increasing viscosity and changing the flow characteristics and yield stress. The average particle size of the magnetic particles is determined so as to exhibit this behavior. Specifically, it is preferably in the range of 0.1 to 100 μm. The lower limit of the average particle size of the magnetic particles is more preferably 1 μm or more, and even more preferably 4 μm or more. The upper limit of the average particle size of the magnetic particles 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 perspective, the average particle size is more preferably 1 to 80 μm, even 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 nearly spherical, as this facilitates dispersion. The average particle size of the magnetic particles is the average primary particle size measured using a laser diffraction / scattering particle size distribution analyzer.

[0017] The content of magnetic particles is preferably in the range of 75 to 90% by mass relative to the total amount of the magnetorheological fluid according to this embodiment. By setting the content of magnetic particles in the range of 75 to 90% by mass relative to the total amount of the magnetorheological fluid according to this embodiment, the necessary drag force can be obtained when a magnetic field is applied, and the dispersibility of the magnetic particles can be maintained, so the fluid also functions as a fluid. The lower limit of the content of the magnetic particles is more preferably 80% by mass or more. The upper limit of the content of the magnetic particles is more preferably 88% by mass or less, and even more preferably 85% by mass or less. From the above-mentioned viewpoints, the content is more preferably 80 to 88% by mass, and even more preferably 80 to 85% by mass.

[0018] 2. Base Oil The magnetorheological fluid according to this embodiment contains a base oil. The base oil is liquid at room temperature (25°C) and can be appropriately selected from mineral oils and synthetic oils conventionally used as dispersion media for magnetic particles, as long as it satisfies 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 described below is 2.5 or greater. Among these, α-olefins, polyα-olefins, or mineral oils are preferred because they are easily adjusted to a relationship that the value obtained by subtracting the solubility parameter (SP value) of the base oil from the HLB value of the ether-modified silicone is 2.5 or greater. One type of base oil may be used alone, or two or more types may be used in combination. The base oil used in the magnetorheological fluid according to this embodiment may be either a mineral oil or a synthetic oil, or a combination of a mineral oil and a synthetic oil. The number in parentheses for each base oil indicates the SP value ((cal / cm)) of the base oil. 3 ) 1 / 2 ) is shown.

[0019] Examples of synthetic oils include hydrocarbon solvents such as α-olefins, polyα-olefins, isoparaffins, normal paraffins, and halogenated hydrocarbons, ester solvents, ether solvents, glycol solvents, and silicone solvents.

[0020] As the mineral oil, any of those generally used in the field of lubricating oils, such as naphthenic mineral oil, paraffinic mineral oil, liquid paraffin, and hydrodewaxed oil, can be used.

[0021] 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 having 8 to 14 carbon atoms such as 1-octene, 1-decene, and 1-dodecene are preferred. Polyα-olefins are polymers of such α-olefins. It is more preferred that the polyα-olefins are polymers of α-olefins having 8 to 14 carbon atoms such as 1-octene, 1-decene, and 1-dodecene. The α-olefins and polyα-olefins may be used alone or in combination of two or more.

[0022] Examples of ester-based solvents include monoesters, polyol esters, dibasic acid esters (diesters), polyoxyalkylene glycol esters, etc. Of these, monoesters are preferably monoesters having 12 to 30 carbon atoms, such as 2-ethylhexyl laurate, 2-ethylhexyl palmitate, and n-butyl stearate. Polyol esters refer to esters of polyhydric alcohols (polyols) and linear or branched, saturated or unsaturated fatty acids. Examples of polyol esters include hindered esters. Ester-based solvents may be used alone or in combination.

[0023] Examples of ether-based solvents include polyvinyl ethers, polyphenyl ethers, and perfluoroethers. Examples of polyvinyl ethers include a homopolymer of methyl vinyl ether (8.7) and a homopolymer of ethyl vinyl ether (8.6). One type of ether-based solvent may be used alone, or two or more types may be used in combination.

[0024] Examples of glycol-based solvents include polyethylene glycol (9.4), polypropylene glycol (8.7), polybutylene glycol (8.6), ethylene oxide-propylene oxide copolymer, propylene oxide-butylene oxide copolymer, and derivatives thereof. The glycol-based solvents may be used alone or in combination of two or more.

[0025] The kinematic viscosity of the base oil at 40°C is 50.0 mm 2 / s or less, and 5.0 to 40.0 mm 2 It is more preferable that the kinematic viscosity of the base oil at 40°C is in the range of 50.0 mm / s. 2 / s or less is more preferable in that it makes it easier to disperse the magnetic particles. Note that the kinematic viscosity is the kinematic viscosity measured in accordance with JIS K2283:2000 kinematic viscosity testing method.

[0026] The base oil content is preferably in the range of 10 to 25 mass % of the total amount of the magnetorheological fluid according to this embodiment, and more preferably in the range of 12 to 20 mass %. By making the base oil content 10 mass % or more, it is possible to disperse magnetic particles and improve fluidity. By making the base oil content 25 mass % or less, it is possible to improve magnetic properties during excitation.

[0027] The solubility parameter (SP value) of the base oil can be calculated according to the method proposed by Fedors et al. (see Polymer Engineering and Science, 14, 147-154 (1974)). That is, it can be calculated based on the following formula: SP value δ=(ΣΔe / ΣΔv) 1 / 2 (In the above formula, Δe is the vaporization energy of each atom or atomic group at 25°C, and Δv is the molar volume of each atom or atomic group at the same temperature.)

[0028] 3. Polyether-Modified Silicone The magnetorheological fluid according to this embodiment contains polyether-modified silicone. A magnetorheological fluid containing polyether-modified silicone is paste-like before magnetization and has high thixotropy. Therefore, even when the magnetic particle content is increased, the dispersion of the magnetic particles is stable, improving the drag force during magnetization. The content of polyether-modified silicone contained in the magnetorheological fluid according to this embodiment is preferably 0.1 to 3 mass% relative to the total amount of the magnetorheological fluid. A polyether-modified silicone content of 0.1 mass% or more provides a greater drag force during magnetization. Furthermore, a polyether-modified silicone content of 3 mass% or less ensures thixotropy and suppresses an increase in viscosity. The content of polyether-modified silicone contained in the magnetorheological fluid according to this embodiment is more preferably 0.3 to 2.5 mass%, and even more preferably 0.5 to 1.5 mass%, relative to the total amount of the magnetorheological fluid.

[0029] Furthermore, since the magnetorheological fluid according to this embodiment contains polyether-modified silicone, which has an extremely high dispersion stability effect for magnetic particles as described above, the content of magnetic particles can be increased, thereby further improving the drag force during magnetization. Furthermore, when the magnetorheological fluid is used in a device, liquid phase separation of the magnetorheological fluid is suppressed, and the seepage of the base oil can be suppressed. For example, while the magnetic particles have traditionally been contained up to about 80% by mass to maintain the fluidity of the magnetorheological fluid, the magnetic particles can now be contained up to about 87% by mass, thereby better suppressing the seepage of the base oil.

[0030] Furthermore, since the magnetorheological fluid according to this embodiment contains polyether-modified silicone, which has an extremely high effect of stabilizing the dispersion of magnetic particles, the magnetic particles are less likely to settle even when highly packed, and the dispersion stability of the magnetic particles is maintained for a long period of time. Therefore, the magnetorheological fluid according to this embodiment has good drag force when excited and can maintain the dispersion stability of the magnetic particles for a long period of time. Therefore, the magnetorheological fluid according to this embodiment is ideal for applications such as architectural dampers that remain stationary for long periods of time.

[0031] The polyether-modified silicone contained in the magnetorheological fluid according to this embodiment may be, for example, a polyalkylsiloxane having a polyether structure. The polyether structure is preferably present in a side chain of the molecular chain. Hereinafter, a polyalkylsiloxane having a polyether structure in a side chain of the molecular chain may be referred to as a side-chain polyether-modified silicone or simply as a "side-chain type." The number of carbon atoms in the alkyl group of the polyalkylsiloxane is preferably 1 to 3.

[0032] The HLB value (hydrophilic-lipophilic balance value) of the polyether-modified silicone is preferably in the range of 10 to 16, and more preferably in the range of 10.5 to 15. The HLB value is defined by the following formula (Griffin method): HLB value = 20 x (total formula weight of hydrophilic moieties / molecular weight).

[0033] From the viewpoint of easily achieving a balance between hydrophilicity and hydrophobicity, it is preferable that the polyether structure contains a polyoxyalkylene structure (a repeating structure of oxyalkylene units).

[0034] In the polyoxyalkylene structure, the number of repeating oxyalkylene groups (oxyalkylene units) is appropriately selected so that the HLB value of the modified silicone falls within the above range, but is preferably 2 to 20, more preferably 2 to 16, and particularly preferably 2 to 10.

[0035] In the polyoxyalkylene structure, the alkylene preferably has 1 to 6 carbon atoms, more preferably 2 to 6 carbon atoms, even more preferably 2 to 4 carbon atoms, and most preferably 2 to 3 carbon atoms.

[0036] The terminal of the polyoxyalkylene structure is preferably a hydroxyl group, and the terminal of the polyoxyalkylene structure may be capped with an alkyl group such as a methyl group, as long as the effect of this embodiment is not impaired.

[0037] The ratio of the total number of oxyethylene units to the total number of oxyalkylene units in the polyether-modified silicone (EO ratio) is, for example, 40% or more, and preferably 50% or more. The EO ratio is 100% or less. Therefore, 40 to 100% is preferred, and 50 to 100% is more preferred. The total number of oxyalkylene units refers to the number of moles of oxyalkylene units contained in one molecule of the polyether-modified silicone. The total number of oxyethylene units refers to the number of moles of oxyethylene units contained in one molecule of the polyether-modified silicone. A polyether-modified silicone containing only oxyethylene units as oxyalkylene units (EO ratio = 100%) may also be used.

[0038] Examples of polyether-modified silicones having the above EO ratios include TSF4440 (HLB: 14) and TSF4452 (HLB: 11, number of moles of oxyethylene units / number of moles of oxypropylene units (=EO / PO) = 50 / 50 (EO ratio 50%)) manufactured by Momentive Performance Materials, Inc. All of these polyether-modified silicones are side chain types.

[0039] In the magnetorheological fluid according to this embodiment, the HLB value of the polyether-modified silicone minus the solubility parameter of the base oil is 2.5 or greater. With this configuration, the polyether-modified silicone and the base oil are not compatible with each other, and the terminal hydroxyl groups of the polyether moieties in the polyether-modified silicone form intermolecular hydrogen bonds to form a thixotropic gel. This is thought to improve the dispersion stability of the magnetic particles contained in the magnetorheological fluid. Side-chain polyether-modified silicones are more preferred because they are more likely to form a three-dimensional structure.

[0040] 4. Other Components In addition to the components described above, the magnetorheological fluid according to this embodiment may further contain various other components depending on the purpose, as long as the effects of this embodiment are not impaired. Examples of other components include antiwear agents, dispersants, surfactants, viscosity modifiers, flow improvers, settling inhibitors, pour point depressants, extreme pressure agents, rust inhibitors, antioxidants, corrosion inhibitors, metal deactivators, and antifoaming agents.

[0041] Examples of anti-wear agents include sulfur-based compounds such as sulfides, sulfoxides, sulfones, and thiophosphinates, halogen-based compounds such as chlorinated hydrocarbons, and organometallic compounds such as molybdenum dithiophosphate (MoDTP), molybdenum dithiocarbamate (MoDTC), and tricresyl phosphate. One type of anti-wear agent may be used alone, or two or more types may be used in combination.

[0042] The dispersant is added to improve the dispersibility of the magnetic particles in the base oil, and examples thereof include known low-molecular-weight dispersants and polymer-based dispersants. One type of dispersant may be used alone, or two or more types may be used in combination.

[0043] Examples of viscosity modifiers include castor oil, hydrogenated castor oil, fatty acid amides, beeswax, carnauba wax, benzylidene sorbitol, metal soaps, polyethylene oxide, sulfate ester-based anionic surfactants, polyolefins, (meth)acrylic acid esters, polyisobutylene, ethylene-propylene copolymers, polyalkylstyrenes, etc. One type of viscosity modifier may be used alone, or two or more types may be used in combination.

[0044] Examples of flow improvers include modified silicone oils obtained by modifying straight silicone oils with alkyl, aralkyl, higher fatty acid esters, amino, epoxy, carboxyl, alcohol, etc. Polyether-modified silicones are not included in the flow improvers. One type of flow improver may be used alone, or two or more types may be used in combination.

[0045] <Viscosity of Magnetorheological Fluid> The viscosity of the magnetorheological fluid according to this embodiment before magnetization 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. The conditions for measuring the viscosity before magnetization are as follows: 3 ml of the magnetorheological fluid is poured into the test plate of a TA Instruments DHR-2 rheometer equipped with a magnetic measurement option, and the viscosity (Pa·s) is measured at 25°C with a 100 μm gap and 20 rotations. The viscosity of the magnetorheological fluid according to this embodiment during magnetization is preferably 5000 Pa·s or more, more preferably 6000 Pa·s or more, even more preferably 8000 Pa·s or more, even more preferably 9000 Pa·s or more, and most preferably 14000 Pa·s or more, when a magnetic field of 0.8 T is applied at 25°C. The viscosity during magnetization is measured as follows. That is, using the same measuring device as that used to measure the viscosity before excitation, the viscosity is measured by applying a magnetic field of 0.8 T with a gap of 700 μm and 20 rotations in an atmosphere of 25° C.

[0046] (Method for producing magnetorheological fluid) The method for producing the magnetorheological fluid according to this embodiment is not particularly limited. For example, a method may be used in which predetermined amounts of magnetic particles, base oil, polyether-modified silicone, and other components added as needed are mixed in a processing machine that applies high shear force, such as a homogenizer, a bead mill, or a mechanical mixer. In producing the magnetorheological fluid, heating or cooling may be performed as necessary.

[0047] (Mechanical Device Using Magnetorheological Fluid) The magnetorheological fluid according to this embodiment can be applied to various mechanical devices such as brakes, clutches, vibration isolators, and dampers for vibration control devices, which are used to control frictional forces acting between objects. In particular, the magnetorheological fluid according to this embodiment, which has excellent dispersion stability of magnetic particles, is ideal for use in architectural dampers and the like that are left stationary for long periods of time.

[0048] The following examples of the present invention are provided for a better understanding of the present invention and its advantages, but are not intended to limit the invention.

[0049] Examples 1 to 7, Comparative Examples 1 to 8 The components shown in Table 1 were placed in a beaker in the mass ratios shown therein and stirred at 40 Hz at room temperature for 20 minutes using a Seiko Advance Corporation universal vibration mixer AD-MIX to produce a magnetorheological fluid. The raw materials for the components shown in Table 1 are as follows: (A) Magnetic particles Carbonyl iron (spherical, average particle size D50 = 4.0 to 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 solvent [hindered ester, kinematic viscosity at 40 ° C. 16.0 mm 2 / s, SP value 9.1 (cal / cm 3 ) 1 / 2 ] Hydrocarbon solvent [polyαolefin, a mixture of 1-decene dimer and trimer, kinematic viscosity at 40°C 5.5 mm 2 / s, SP value 7.8 (cal / cm 3 ) 1 / 2(C) Dispersant Alkyl naphthalene (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 silicone (product name: TSF4470, manufactured by Momentive Performance Materials) Unmodified silicone (dimethyl silicone, product name: Element 14PDMS50J, manufactured by Momentive Performance Materials) Polybutene (product name: Polybutene-0N, manufactured by NOF Corporation) (E) Extreme pressure agent Zinc dithiophosphate (Zn-DTP) (product name: Kicrube Z-112, manufactured by ADEKA Corporation)

[0050] <Evaluation of viscosity before and during magnetization> 3 ml of the magnetorheological fluids of Examples 1 to 7 and Comparative Examples 1 to 8 were poured 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 20 revolutions with a 100 μm gap in an atmosphere of 25°C to measure the viscosity before magnetization. Using the same measuring device, a magnetic field of 0.8 T was applied at 20 revolutions with a 700 μm gap in an atmosphere of 25°C to measure the viscosity during magnetization. The above test conditions and test results are shown in Table 1.

[0051] <Evaluation of Dispersion Stability> The dispersion stability of the magnetorheological fluids of Examples 1 to 7 and Comparative Examples 1 to 8 was evaluated. The magnetorheological fluid was placed in a sample bottle, and the thicknesses of the magnetic particle-containing layer and the dispersion medium layer (supernatant layer) were measured after 1 day, 10 days, 20 days, 30 days, 60 days, and 90 days at 23°C. The evaluation value was the ratio of the thickness of the dispersion medium layer to the total thickness of the magnetic particle-containing layer and the dispersion medium layer (separation rate), expressed as a percentage. The results are shown in Table 1. The separation rate is preferably 5% or less, more preferably 3% or less, and particularly preferably 1% or less.

[0052]

[0053] The magnetorheological fluids of Examples 1 to 7 all contained magnetic particles, a base oil, and a 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 was 2.5 or greater. Therefore, the viscosity upon excitation was a high value of 5,000 Pa·s or greater. Among these, Examples 1 to 5 and 7 exhibited a higher viscosity upon excitation of 6,000 Pa·s or greater. Furthermore, Examples 1 to 3 and 7 exhibited an even higher viscosity upon excitation of 8,000 Pa·s or greater. Example 7 exhibited an extremely high viscosity upon excitation of 14,000 Pa·s or greater. Furthermore, all of the magnetorheological fluids of Examples 1 to 7 exhibited excellent dispersion stability even after 90 days had elapsed.

[0054] In the magnetorheological fluids of Comparative Examples 1 to 3, the value obtained by subtracting the solubility parameter of the base oil from the HLB value of the polyether-modified silicone was less than 2.5, and therefore the separation rate after one day was 30 to 40%, indicating poor dispersion stability.

[0055] The magnetorheological fluids of Comparative Examples 4 to 8 did not contain polyether-modified silicone, and therefore had poor dispersion stability, with separation rates of 10 to 35% after one day.

Claims

1. A magnetorheological fluid containing magnetic particles, a base oil, and a polyether-modified silicone, wherein 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 more.

2. The magnetorheological fluid according to claim 1, wherein the content of said magnetic particles is 75 to 90 mass % based on the total amount of said magnetorheological fluid.

3. The magnetorheological fluid of 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 silicone is a side-chain type polyether-modified silicone.

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

Images