Magnetorheological fluids and devices

Abstract

To provide a magnetic viscoelastic fluid that is excellent in the long-term dispersion stability of magnetic particles and in the fluidity of magnetic particles.SOLUTION: The magnetic viscoelastic fluid includes magnetic particles and a dispersion medium. The dispersion medium includes an organic compound which has at least two amide groups in one molecule. The ratio of the volume of the organic compound to the total volume of the magnetic viscoelastic fluid is in the range of 48% to 85%.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to magnetorheological fluids and devices. [Background technology]

[0002] A magnetorheological fluid (hereinafter, also referred to as "MR fluid") is a fluid in which magnetic particles such as iron are dispersed in a base oil such as silicone oil, which is a dispersion medium. When no magnetic field is applied to the MR fluid from the outside, the magnetic particles randomly float in the dispersion medium, whereas when a magnetic field is applied to the MR fluid from the outside, the magnetic particles form a number of clusters in which the magnetic particles are linked in a chain shape along the direction of the magnetic field. This allows the MR fluid to control the formation and release of clusters by applying and not applying a magnetic field, and when clusters are formed, the apparent viscosity of the MR fluid changes and the yield stress increases. The MR fluid can transmit and attenuate force and torque by applying a magnetic field. For this reason, applications of the MR fluid to dampers and the like are being considered (see, for example, Patent Document 1).

[0003] MR fluids usually use large magnetic particles with particle sizes of several μm to several tens of μm to generate a certain level of stress, so there is a problem that if the MR fluid is left alone, the magnetic particles will settle and separate from the dispersion medium. If an attempt is made to operate a device using MR fluid in a state in which the magnetic particles have settled and solidified, there is a risk that the device will not operate or that the device's mechanism will be damaged.

[0004] The present inventors have proposed in Patent Documents 1 and 2 a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and a large maximum change in yield stress under conditions of applied magnetic field. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6434672 [Patent Document 2] Patent No. 6682608 Summary of the Invention [Problem to be solved by the invention]

[0006] In addition to excellent long-term dispersion stability of magnetic particles, MR fluids are also required to have excellent fluidity. If the fluidity of an MR fluid is low, it may be difficult to fill the MR fluid into a machine or device that uses the MR fluid. Also, if the fluidity of an MR fluid is low, an excessively high de-excitation torque will result, and the advantageous feature of reverse mobility will be lost.

[0007] Fluidity generally means the property of flowing and moving unsteadily, but in the present invention, fluidity refers to the property of a fixed shape causing creep and becoming unable to maintain the initial shape for a certain period of time, causing it to flow.

[0008] Even if the magnetic particles have excellent long-term dispersion stability and do not separate from the dispersion medium, the fluidity of the MR fluid is low, and once the entire MR fluid has solidified, it becomes impossible to use the MR fluid in devices, etc. In addition, the solidified MR fluid must be stirred with strong force to restore the fluidity of the MR fluid.

[0009] SUMMARY OF THE PRESENT EMBODIMENTS An object of the present invention is to provide a magnetorheological fluid which has excellent long-term dispersion stability of magnetic particles and excellent flowability.

[0010] Another object of the present invention is to provide an apparatus which can be stably operated for a long period of time and has excellent mechanical reliability. [Means for solving the problem]

[0011] The magnetorheological fluid according to the present invention is a magnetorheological fluid including magnetic particles and a dispersion medium, the dispersion medium contains an organic compound having two or more amide groups in one molecule, The ratio of the volume of the organic compound to the total volume of the magnetorheological fluid is 48 to 85%, which provides excellent long-term dispersion stability of the magnetic particles and excellent fluidity of the magnetorheological fluid.

[0012] In one embodiment of the magnetorheological fluid according to the present invention, the organic compound is represented by the formula (1): [ka] having In the formula, L is a linking group of valence y, R is hydrogen or a methyl group; A is an alkylene group; x is a number from 1 to 30, y is a number greater than or equal to 2.

[0013] In one embodiment of the magnetorheological fluid according to the present invention, the linking group is a hydrocarbon group.

[0014] In one embodiment of the magnetorheological fluid according to the present invention, the L is an alkylene group.

[0015] In one embodiment of the magnetorheological fluid according to the present invention, the L is an alkylene group having 2 to 8 carbon atoms.

[0016] In one embodiment of the magnetorheological fluid according to the present invention, the magnetic particles have an average particle size of 1 to 50 μm.

[0017] In one embodiment of the magnetorheological fluid according to the present invention, the magnetorheological fluid does not contain resin particles.

[0018] The device according to the present invention is a device selected from the group consisting of robots, brakes, clutches, dampers, shock absorbers, vibration control devices, haptics, force tactile presentation devices, medical devices, welfare devices, and suction devices, and uses any of the above magnetorheological fluids, thereby achieving long-term stable operation and excellent mechanical reliability. Effect of the Invention

[0019] According to the present invention, it is possible to provide a magnetorheological fluid having excellent long-term dispersion stability of magnetic particles and excellent fluidity, and also to provide a device having long-term stable operation and excellent mechanical reliability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, embodiments of the present invention will be described. These descriptions are intended to be illustrative of the present invention and are not intended to limit the present invention in any way.

[0021] In the present invention, two or more embodiments can be combined in any manner.

[0022] In the present invention, the term "solid content" is a concept that encompasses solid content, non-volatile content, and active ingredients.

[0023] In this specification, unless otherwise specified, numerical ranges are intended to include the upper and lower limits of the range. For example, 48-85% means a range of 48% or more and 85% or less.

[0024] In the present invention, the average particle size means the median value of the particle size distribution. The average particle size is measured using a laser diffraction particle size distribution measuring device or a scanning electron microscope (SEM).

[0025] (Magnetic viscoelastic fluid) The magnetorheological fluid according to the present invention is a magnetorheological fluid including magnetic particles and a dispersion medium, the dispersion medium contains an organic compound having two or more amide groups in one molecule, The ratio of the volume of the organic compound to the total volume of the magnetorheological fluid is 48 to 85%.

[0026] Each component contained in the magnetorheological fluid of the present invention will now be described.

[0027] ·Magnetic particles The magnetic particles may be any magnetic particles whose yield stress changes under a magnetic field, and known magnetic particles may be used. The magnetic particles may be used alone or in combination of two or more types of magnetic particles having different materials or average particle sizes.

[0028] Examples of materials for magnetic particles include iron, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low carbon steel, nickel, cobalt; iron alloys such as aluminum-containing iron alloys, silicon-containing iron alloys, cobalt-containing iron alloys, nickel-containing iron alloys, vanadium-containing iron alloys, molybdenum-containing iron alloys, chromium-containing iron alloys, tungsten-containing iron alloys, manganese-containing iron alloys, and copper-containing iron alloys, as well as oxides of the above materials; paramagnetic, superparamagnetic, and ferromagnetic compound particles made of gadolinium and organic derivatives of gadolinium.

[0029] The magnetic particles are preferably carbonyl iron.

[0030] The average particle size of the magnetic particles is, for example, 1 to 50 μm. In one embodiment, the average particle size of the magnetic particles is 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, 5 μm or more, 6 μm or more, 7 μm or more, 8 μm or more, 9 μm or more, 10 μm or more, 15 μm or more, 20 μm or more, 25 μm or more, 30 μm or more, 35 μm or more, 40 μm or more, or 45 μm or more. In another embodiment, the average particle size of the magnetic particles is 50 μm or less, 45 μm or less, 40 μm or less, 35 μm or less, 30 μm or less, 25 μm or less, 20 μm or less, 15 μm or less, 10 μm or less, 9 μm or less, 8 μm or less, 7 μm or less, 6 μm or less, 5 μm or less, 4 μm or less, 3 μm or less, or 2 μm or less.

[0031] In addition to the above magnetic particles (large magnetic particles having an average particle size of 1 μm or more), small magnetic particles having a smaller average particle size may or may not be used.

[0032] Examples of materials for small magnetic particles include iron, ferrite (ferrates of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), etc.), and magnetite (ferrates of Fe(II)). Examples of ferrite include Mn ferrite, Mn-Zn ferrite, Mn-Mg ferrite, Mn-Mg-Sr ferrite, and Mg ferrite; the above ferrites containing alkali metals, alkaline earth metals, and light metals. Other examples include the materials mentioned above.

[0033] The small magnetic particles are preferably one or more types selected from the group consisting of magnetite and ferrite.

[0034] In one embodiment, the small magnetic particles are Mn-Mg-Sr ferrite.

[0035] The small magnetic particles have an average particle size of, for example, 20 to 300 nm. From the viewpoints of yield stress and suppression of sedimentation, the small magnetic particles preferably have an average particle size of 40 to 200 nm.

[0036] When the magnetic particles include small magnetic particles, the proportion of small magnetic particles is, for example, less than 5% by weight, relative to the total weight of the magnetic particles.

[0037] In one embodiment of the magnetorheological fluid according to the present invention, the magnetic particles have an average particle size of 1 to 50 μm, that is, in this embodiment, the magnetic particles do not include small magnetic particles.

[0038] In the magnetorheological fluid of the present invention, the magnetic particles may be surface-treated with a silane coupling agent or the like to enhance dispersibility, or may not be surface-treated. In the magnetorheological fluid of the present invention, by using the resin particles described below, excellent long-term dispersion stability of the magnetic particles can be obtained without surface treatment of the magnetic particles.

[0039] In one embodiment, the magnetic particles are not surface treated, such as with a silane coupling agent, to enhance dispersibility.

[0040] The amount of magnetic particles in the magnetorheological fluid of the present invention is not particularly limited as long as the volume ratio of the organic compound having two or more amide groups in one molecule described later is 48 to 85% relative to the total volume of the magnetorheological fluid. The volume ratio of the magnetic particles in the magnetorheological fluid of the present invention is, for example, 52% or less relative to the total volume of the magnetorheological fluid. In one embodiment, the volume ratio of the magnetic particles in the magnetorheological fluid of the present invention is 52% or less, 50% or less, 40% or less, 30% or less, 20% or less, 10% or less, or 5% or less relative to the total volume of the magnetorheological fluid. In another embodiment, the volume ratio of the magnetic particles in the magnetorheological fluid of the present invention is 5% or more, 10% or more, 15% or more, 20% or more, 30% or more, 40% or more, or 50% or more relative to the total volume of the magnetorheological fluid.

[0041] ·Dispersion medium The dispersion medium of the magnetorheological fluid of the present invention contains an organic compound having two or more amide groups (-CON(R)-) in one molecule (hereinafter, simply referred to as "dispersion medium compound"), and the ratio of the volume of the dispersion medium compound to the total volume of the magnetorheological fluid is 48 to 85%.

[0042] Without wishing to be bound by theory, it is speculated that the reason why the use of a dispersion medium containing such a dispersion medium compound in the magnetic viscoelastic fluid of the present invention results in excellent long-term dispersion stability of the magnetic particles and excellent fluidity of the magnetic viscoelastic fluid is because, for example, when one molecule of the dispersion medium compound has two or more amide groups, the amide groups make it easier for the dispersion medium compound to be adsorbed to the magnetic particle surface than when there is only one amide group, thereby enhancing the dispersibility of the magnetic particles.

[0043] The carrier compound may be nonionic, cationic, or anionic, hi one embodiment, the carrier compound is nonionic.

[0044] For example, the carrier compound may have the formula (1): [ka] In formula (1), L is a linking group having a valence of y; R is hydrogen or a methyl group; A is an alkylene group; x is a number from 1 to 30; and y is a number of 2 or more.

[0045] Examples of the linking group of L include linear, branched, or cyclic organic groups. Examples of the organic group include hydrocarbons such as aliphatic hydrocarbons and aromatic hydrocarbons. The aliphatic hydrocarbons may be saturated aliphatic hydrocarbons or unsaturated aliphatic hydrocarbons having one or more unsaturated bonds.

[0046] In one embodiment, the linking group is a hydrocarbon group.

[0047] In one embodiment, L is a saturated or unsaturated alkylene group. In another embodiment, L is a saturated or unsaturated alkylene group having 2, 3, 4, 5, 6, 7, or 8 carbons. In yet another embodiment, L is a saturated or unsaturated alkylene group having 4 carbons.

[0048] The valence y, i.e., the number of amide groups, is a number equal to or greater than 2, for example, 2, 3, 4, or 5.

[0049] R is hydrogen or a methyl group. When R is a methyl group, (-CON(CH 3 It is presumed that when R is hydrogen (-CONH-) rather than when R is hydrogen (-CONH-), the electron-withdrawing ability of the amide group is greater, and the amide group enhances the adsorption of the dispersant compound to the magnetic particles.

[0050] A is a saturated or unsaturated alkylene group, and examples of such groups include alkylene groups having 2, 3, 4, 5, or 6 carbon atoms. The alkylene group may be linear, branched, or cyclic.

[0051] x is a number from 1 to 30. From the viewpoint of increasing the hydrophobicity of the dispersion medium compound by increasing the number of repetitions of the AO group and thereby further increasing the dispersion stability of the magnetic particles, x is preferably 4 or more. In one embodiment, x is 1 or more, 2 or more, 3 or more, 4 or more, 6 or more, 8 or more, 10 or more, 12 or more, 14 or more, 16 or more, 18 or more, or 20 or more. In another embodiment, x is 30 or less, 20 or less, 18 or less, 16 or less, 14 or less, 12 or less, 10 or less, 8 or less, 6 or less, 4 or less, 3 or less, or 2 or less.

[0052] In one embodiment, the carrier compound of formula (1) is of formula (2): H-(OA)-NRCO-L-CONR-(AO)H (i.e., y=2 in formula (1)). In another embodiment, the carrier compound of formula (1) is of formula (2): H-(OA)-NHCO-L-CONH-(AO)H (i.e., y=2 in formula (1)).

[0053] The dispersion medium compound may be used alone or in combination of two or more kinds.

[0054] The ratio of the dispersion medium compound to the total mass of the dispersion medium is, for example, 50% by mass or more. In one embodiment, the ratio of the dispersion medium compound to the total mass of the dispersion medium is 50% by mass or more, 60% by mass or more, 65% by mass or more, 70% by mass or more, 75% by mass or more, 80% by mass or more, 85% by mass or more, 90% by mass or more, 95% by mass or more, or 100% by mass. In another embodiment, the ratio of the dispersion medium compound to the total mass of the dispersion medium is 100% by mass or less, 95% by mass or less, 90% by mass or less, 85% by mass or less, 80% by mass or less, 75% by mass or less, 70% by mass or less, 65% by mass or less, or 60% by mass or less.

[0055] The dispersion medium of the magnetorheological fluid of the present invention may or may not contain a dispersion medium other than a dispersion medium compound, such as the dispersion medium described in Patent Document 1 or 2, or a dispersion medium such as silicone oil, fluorine oil or paraffin.

[0056] The volume ratio of the dispersion medium compound to the total volume of the magnetorheological fluid of the present invention is 48 to 85%. When the volume ratio of the dispersion medium compound is 48% or more, the fluidity of the magnetorheological fluid is increased. Furthermore, when the volume ratio of the dispersion medium compound is 85% or less, the long-term dispersion stability of the magnetic particles is excellent. In one embodiment, the volume ratio of the dispersion medium compound to the total volume of the magnetorheological fluid is 48% or more, 50% or more, 55% or more, 60% or more, 65% or more, 70% or more, 75% or more, or 80% or more. In another embodiment, the volume ratio of the dispersion medium compound to the total volume of the magnetorheological fluid is 85% or less, 80% or less, 75% or less, 70% or less, 65% or less, 60% or less, 55% or less, or 50% or less.

[0057] Other additives In addition to the above-mentioned components, the magnetorheological fluid according to the present invention may or may not contain other additives such as dispersion aids, resin particles, rheology control agents, metal detergent-dispersants, ashless detergent-dispersants, oiliness agents, antiwear agents, extreme pressure agents, rust inhibitors, friction modifiers, solid lubricants, antioxidants, metal deactivators, antifoaming agents, colorants, viscosity index improvers, pour point depressants, etc. Each of these other additives may be used alone or in combination of two or more.

[0058] Dispersion aid The dispersion aid may be a commonly used non-aqueous or aqueous wetting dispersant. The functional group of the dispersion aid that contributes to magnetic material adsorption may be any of acidic, basic, and salt, but basic or salt is preferred because it has little interaction with magnetic particles of some materials. The molecular weight of the dispersion aid may be selected from low molecular weight to high molecular weight, taking into account the combination with the dispersion medium. The dispersion aid may be used alone or in combination of two or more types.

[0059] When a dispersion aid is used, its amount may be adjusted appropriately. For example, the proportion of the dispersion aid is 0.5 to 10% by mass with respect to the total mass of the magnetic particles, the dispersion medium, and the dispersion aid.

[0060] In one embodiment, the magnetorheological fluid does not include a dispersing agent, hi another embodiment, the magnetorheological fluid includes a dispersing agent.

[0061] Resin particles The resin particles are particles made of resin and contribute to the dispersion stability of the magnetic particles. The resin particles may be used alone or in combination of two or more kinds.

[0062] The material of the resin particles is not particularly limited, and may be appropriately selected from known resins. Examples of the material of the resin particles include acrylic resin, styrene resin, polyolefin resin, polyester resin, melamine resin, benzoguanamine resin, polyacrylonitrile resin, polyamide resin, polycarbonate resin, phenol resin, urea resin, fluororesin, cellulose resin, polyurethane resin, vinyl chloride resin, polyether resin, silicone resin, and alkyd resin.

[0063] The acrylic resin may be a homopolymer or copolymer of a monomer such as acrylic acid, methacrylic acid, an acrylic acid ester, or a methacrylic acid ester, or a copolymer of one or more of these monomers with a monomer such as ethylene or styrene (e.g., an ethylene-acrylic acid copolymer, a styrene-acrylic acid copolymer, etc.), or a modified product of these polymers or copolymers (e.g., a fluorine-substituted acrylic resin).

[0064] The resin particles may be core-shell particles, in which the core and shell are each made of one or more of the materials mentioned above.

[0065] The resin particles may be, for example, non-crosslinked resin particles in which polymers in the resin are not crosslinked, or crosslinked resin particles in which polymers in the resin are crosslinked. Crosslinked resin particles are preferable because they have better heat resistance and solvent resistance than non-crosslinked resin particles, and are less likely to melt due to heat caused by sliding of a device to which the magnetorheological fluid is applied or due to the dispersion medium itself, and a more stable magnetic field generation stress can be obtained.

[0066] The crosslinked resin particles are not particularly limited, and known crosslinked resin particles can be used. Examples of the crosslinked resin particles include resin particles described in Patent Document 2; organic resin particles (A) described in International Publication No. 2015 / 152187; crosslinked resin particles described in JP-A-2010-024289; crosslinked resin particles such as resin particles having a crosslinked structure obtained mainly from an ethylenically unsaturated monomer described in JP-A-2009-235351, resin particles made of an internally crosslinked urethane resin, and crosslinked resin particles made of an internally crosslinked melamine resin; crosslinked resin particles of carboxyl group-containing acrylic resin described in JP-A-2009-114392; and acrylic resin fine particles having an internal crosslinked structure described in JP-A-6-025567.

[0067] In one embodiment, the resin particles are crosslinked resin particles. In another embodiment, the resin particles are one or more selected from the group consisting of acrylic resin, polyurethane resin, melamine resin, and modified products thereof. In yet another embodiment, the resin particles are one or more crosslinked resin particles selected from the group consisting of acrylic resin, polyurethane resin, melamine resin, and modified products thereof.

[0068] In one embodiment, the acrylic resin is at least one selected from the group consisting of an acrylic acid ester polymer, a methacrylic acid ester polymer, a styrene-acrylic acid copolymer, a styrene-methacrylic acid copolymer, an ethylene-acrylic acid copolymer, an ethylene-methacrylic acid copolymer, and modified versions thereof.

[0069] The weight average molecular weight (Mw) of the resin particles is not particularly limited and can be adjusted appropriately. In one embodiment, the Mw of the resin particles is 1,000 or more, 5,000 or more, 6,000 or more, 10,000 or more, 50,000 or more, 100,000 or more, 500,000 or more, or 1,000,000 or more. In another embodiment, the Mw of the resin particles is 10,000,000 or less, 5,000,000 or less, 3,000,000 or less, 1,000,000 or less, 500,000 or less, 300,000 or less, 100,000 or less, or 50,000 or less.

[0070] The average particle diameter of the resin particles is, for example, 20 to 1500 nm. In one embodiment, the average particle diameter of the resin particles is 50 nm or more, 80 nm or more, 100 nm or more, 150 nm or more, 200 nm or more, 300 nm or more, 400 nm or more, 500 nm or more, 600 nm or more, 700 nm or more, 800 nm or more, 900 nm or more, 1000 nm or more, 1100 nm or more, 1200 nm or more, 1300 nm or more, or 1400 nm or more. In another embodiment, the average particle diameter of the resin particles is 1400 nm or less, 1300 nm or less, 1200 nm or less, 1100 nm or less, 1000 nm or less, 900 nm or less, 800 nm or less, 700 nm or less, 600 nm or less, 500 nm or less, 400 nm or less, 300 nm or less, 200 nm or less, or 150 nm or less.

[0071] The average particle diameter of the resin particles may be within a range of 20 to 1500 nm, and two or more types of resin particles having different average particle diameters may be used in combination. For example, first resin particles having an average particle diameter of 20 to 200 nm may be used in combination with second resin particles having an average particle diameter of more than 200 nm and not more than 1500 nm.

[0072] When resin particles are blended, the mass ratio of the resin particles is, for example, 0.3 to 20% by mass with respect to the total mass of the magnetorheological fluid. In one embodiment, the ratio is 0.5% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 4% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 8% by mass or more, 9% by mass or more, 10% by mass or more, or 15% by mass or more. In another embodiment, the ratio is 19% by mass or less, 15% by mass or less, 10% by mass or less, 9% by mass or less, 8% by mass or less, 7% by mass or less, 6% by mass or less, 5% by mass or less, 4% by mass or less, 3% by mass or less, 2% by mass or less, or 1% by mass or less.

[0073] In one embodiment of the magnetorheological fluid according to the present invention, the magnetorheological fluid does not contain resin particles.

[0074] -Method of preparing magnetorheological fluid The method for preparing the magnetorheological fluid is not particularly limited, and the magnetic particles, the dispersion medium, and other additives as necessary can be mixed in any order to prepare the magnetic fluid. For example, the method for preparing the magnetorheological fluid includes preparing a dispersion medium and magnetic particles, adding the dispersion medium to the magnetic particles and stirring, and then optionally adding other additives such as a dispersion aid and stirring.

[0075] The applications of the magnetorheological fluid according to the present invention are not particularly limited, and the fluid can be used in applications for known MR fluids, such as robots, brakes, clutches, dampers, shock absorbers, vibration control devices, haptics, haptic devices, medical devices, welfare devices, and suction devices.

[0076] (Device) The device according to the present invention is a device selected from the group consisting of robots, brakes, clutches, dampers, shock absorbers, vibration control devices, haptics, force tactile presentation devices, medical devices, welfare devices, and suction devices, and uses any of the above magnetorheological fluids, thereby achieving long-term stable operation and excellent mechanical reliability. EXAMPLES

[0077] The present invention will be described in more detail below by way of examples. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention in any way.

[0078] The materials used in the examples are as follows: Magnetic particles: Carbonyl iron powder, BASF product name "CIP-SQ", average particle size 4.5 μm, density 7.87 (g / cc) ·Dispersion medium Adipic acid diamide: Aoki Oil Industries Co., Ltd., product name "AA PO-20", 2 amide groups, in formula (2), L is an alkylene group having 4 carbon atoms, A is an alkylene group having 3 carbon atoms, x is 20, and R is hydrogen. Density 0.92 (g / cc) Succinic acid diamide: Aoki Oil Industries Co., Ltd., product name "SA PO-20", 2 amide groups, in formula (2), L is an alkylene group having 2 carbon atoms, A is an alkylene group having 3 carbon atoms, x is 20, and R is hydrogen. Density 0.92 (g / cc) Succinic acid diamide: Aoki Oil Industries Co., Ltd., product name "SA PO-4", 2 amide groups, in formula (2), L is an alkylene group having 2 carbon atoms, A is an alkylene group having 3 carbon atoms, x is 4, and R is hydrogen. Density 0.93 (g / cc) ·Comparative dispersion medium Polyoxyethylene oleylamide: Aoki Oil Industries Co., Ltd., product name "Brownon O-15", 1 amide group, polyoxyethylene adduct of oleic acid amide, 7 repeating polyoxyethylene units, density 0.92 (g / cc) Succinic acid diester: Aoki Oil Industries Co., Ltd., product name "SAES PO-4", rational formula: H(OC 3 H 6 ) 4 O.C.O.C. 2 H 4 COO(C 3 H 6 O) 4 H, density 0.91 (g / cc) Resin particles: styrene-acrylic acid copolymer fine particles (crosslinked resin particles), product name "MG-451" manufactured by Nippon Paint Industrial Coatings Co., Ltd., average particle size 100 nm, density 1.01 (g / cc)

[0079] The apparatus used in the examples is as follows: 3-roll disperser: EXAKT product name "50I" Rheometer: Anton Paar's product name "MCR 302", equipped with a double-gap structure MRD cell (magnetic field measurement unit)

[0080] (Examples 1 to 11 and Comparative Examples 1 to 4) Magnetic particles, dispersion medium, comparative dispersion medium, and resin particles were prepared in the formulation (mass%) shown in Table 1. A container was filled with magnetic particles, and then either or both of the dispersion medium and the comparative dispersion medium were added and stirred briefly until homogenous. The mixture was then dispersed using a three-roll disperser, passing it through five times under the narrowest opening conditions, to prepare a magnetorheological fluid or a comparative MR fluid.

[0081] [Table 1]

[0082] For each of the prepared magnetorheological fluids or comparative MR fluids, the dispersion stability, fluidity, and maximum change in yield stress were measured as follows.

[0083] (dispersion stability) Immediately after preparing the magnetorheological fluid or comparative MR fluid, the magnetorheological fluid or comparative MR fluid was filled into a 30 mL sample tube up to the shoulder and allowed to stand and store at 25°C. For the magnetorheological fluid or comparative MR fluid 90 days after storage, the distance from the shoulder of the sample tube to the interface where the dispersion medium and the magnetic particles separated (separation width) was measured and the dispersion stability was evaluated according to the following criteria. The smaller the separation width, the better the dispersion stability. If the criteria A, B or C were met 90 days after storage, the sample was considered to have passed. ·standard A: No separation is observed. B: Separation width is 5mm or less C: Separation width is more than 5 mm and less than 10 mm D: Separation width exceeds 10 mm

[0084] (Liquidity) 2 cc of the immobilized magnetorheological fluid or comparative MR fluid was taken out from the container onto a flat plate and left to stand for 5 minutes at 25° C. The state of the magnetorheological fluid or comparative MR fluid was then observed and evaluated according to the following criteria. Pass: The magnetorheological fluid or comparative MR fluid flows and becomes flat. Fail: Magnetorheological fluid or comparative MR fluid maintained its original shape

[0085] (Maximum change in yield stress) A rheometer was filled with 0.6 mL of magnetorheological fluid or comparative MR fluid, and a 10% strain, 10 Hz frequency vibration analysis was performed under magnetic field application conditions of a magnetic flux density of 1 T (tesla). The shear yield stress (Pa) generated under the magnetic field application conditions was measured and evaluated according to the following criteria. The results are shown in Table 1. A: Shear yield stress exceeds 20,000 Pa B: Shear yield stress is over 7000 Pa to 20000 Pa or less C: Shear yield stress is 7000 Pa or less

[0086] As shown in Table 1, the present invention can provide a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and also has excellent flowability. [Industrial Applicability]

[0087] According to the present invention, it is possible to provide a magnetorheological fluid having excellent long-term dispersion stability of magnetic particles and excellent fluidity, and also to provide a device having long-term stable operation and excellent mechanical reliability.

Claims

1. A magnetorheological fluid comprising magnetic particles and a dispersion medium, the dispersion medium contains an organic compound having two or more amide groups in one molecule, A magnetorheological fluid, wherein the ratio of the volume of the organic compound to the total volume of the magnetorheological fluid is 48 to 85%.

2. The organic compound has the formula (1): 【Chemistry 1】 having In the formula, L is a linking group of valence y, R is hydrogen or a methyl group; A is an alkylene group; x is a number from 1 to 30; The magnetorheological fluid according to claim 1 , wherein y is a number equal to or greater than 2.

3. The magnetorheological fluid according to claim 2 , wherein the linking group is a hydrocarbon group.

4. The magnetorheological fluid according to claim 2 , wherein L is an alkylene group.

5. 5. The magnetorheological fluid according to claim 4, wherein L is an alkylene group having 2 to 8 carbon atoms.

6. 5. The magnetorheological fluid according to claim 1, wherein the magnetic particles have an average particle size of 1 to 50 μm.

7. The magnetorheological fluid according to any one of claims 1 to 6, which does not contain resin particles.

8. A device using the magnetorheological fluid according to any one of claims 1 to 7 in a device selected from the group consisting of a robot, a brake, a clutch, a damper, a shock absorber, a vibration control device, a haptic device, a force-tactile presentation device, a medical device, a welfare device, and an adsorption device.

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