JP7660893B2 - Magnetorheological fluids and devices - Google Patents

Description

The present invention relates to a magnetorheological fluid and a device.

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, while when a magnetic field is applied to the MR fluid from the outside, the magnetic particles form numerous clusters in which they 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. When a magnetic field is applied to the MR fluid, it can transmit or attenuate force and torque. For this reason, applications of the MR fluid to dampers and the like are being considered (see, for example, Patent Document 1).

MR fluids usually use large magnetic particles with particle sizes ranging from several μm to several tens of μm in order to generate a certain level of stress, and 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 that uses MR fluid when 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.

In Patent Documents 1 and 2, the present inventors have proposed a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and a large maximum change in yield stress under magnetic field application conditions.

Patent No. 6434672 Patent No. 6682608

In addition to having 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 machine or device in which the MR fluid is used with the MR fluid. Furthermore, if the fluidity of an MR fluid is low, an excessively high de-excitation torque will result, and the advantageous characteristic of reverse mobility will be lost.

Fluidity generally refers to the property of flowing and moving without being constant, but in this invention, fluidity refers to the property of a fixed shape creeping and no longer being able to maintain its initial shape for a certain period of time, causing it to flow.

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, it is necessary to strongly stir the solidified MR fluid to restore the fluidity of the MR fluid.

The present invention aims to provide a magnetic viscoelastic fluid that has excellent long-term dispersion stability of magnetic particles and excellent fluidity.

Another object of the present invention is to provide a device that has stable operation over a long period of time and excellent mechanical reliability.

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.

を有し、
式中、Ｌは、価数ｙの連結基であり、
Ｒは、水素またはメチル基であり、
Ａは、アルキレン基であり、
ｘは、１～３０の数であり、
ｙは、２以上の数である。 In one embodiment of the magnetorheological fluid according to the present invention, the organic compound is represented by formula (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;
y is a number of 2 or greater.

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

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

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

In one embodiment of the magnetorheological fluid according to the present invention, the average particle size of the magnetic particles is 1 to 50 μm.

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

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-mentioned magnetorheological fluids. This provides long-term stable operation and excellent mechanical reliability.

The present invention can provide a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and excellent fluidity. The present invention can also provide a device that has long-term stable operation and excellent mechanical reliability.

The following describes embodiments of the present invention. These descriptions are intended to be illustrative of the present invention and are not intended to limit the present invention in any way.

In the present invention, two or more embodiments may be combined in any manner.

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

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

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), etc.

(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%.

The components contained in the magnetorheological fluid of the present invention are described below.

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

Materials for magnetic particles include, for example, 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.

Carbonyl iron is preferred as magnetic particles.

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.

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

Examples of materials for small magnetic particles include iron, ferrite (ferric acid salts of Mn(II), Co(II), Ni(II), Cu(II), Zn(II), etc.), and magnetite (ferric acid salts 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.

As small magnetic particles, one or more types selected from the group consisting of magnetite and ferrite are preferred.

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

The average particle size of the small magnetic particles is, for example, 20 to 300 nm. From the viewpoint of yield stress and suppression of sedimentation, the average particle size of the small magnetic particles is preferably 40 to 200 nm.

When the magnetic particles include small magnetic particles, the proportion of small magnetic particles is, for example, less than 5 mass% relative to the total mass of the magnetic particles.

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

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.

In one embodiment, the magnetic particles are not surface-treated with a silane coupling agent or the like to enhance dispersibility.

The amount of magnetic particles in the magnetic viscoelastic 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% of the total volume of the magnetic viscoelastic fluid. The volume ratio of the magnetic particles in the magnetic viscoelastic fluid of the present invention is, for example, 52% or less of the total volume of the magnetic viscoelastic fluid. In one embodiment, the volume ratio of the magnetic particles in the magnetic viscoelastic 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 of the total volume of the magnetic viscoelastic fluid. In another embodiment, the volume ratio of the magnetic particles in the magnetic viscoelastic 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 of the total volume of the magnetic viscoelastic fluid.

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, sometimes 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%.

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 magnetorheological fluid of the present invention results in excellent long-term dispersion stability of the magnetic particles and excellent fluidity of the magnetorheological fluid is that, for example, when one molecule of the dispersion medium compound has two or more amide groups, the dispersion medium compound is more easily adsorbed to the magnetic particle surface by the amide groups than when there is only one amide group, thereby enhancing the dispersibility of the magnetic particles.

The dispersion medium compound may be nonionic, cationic, or anionic. In one embodiment, the dispersion medium compound is nonionic.

を有し、式（１）中、Ｌは、価数ｙの連結基であり；Ｒは、水素またはメチル基であり；Ａは、アルキレン基であり；ｘは、１～３０の数であり；ｙは、２以上の数である。 For example, the dispersion medium compound may be represented by the formula (1):

wherein 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 equal to or greater than 2.

The linking group of L may be, for example, a linear, branched, or cyclic organic group. The organic group may be, for example, a hydrocarbon such as an aliphatic hydrocarbon or an aromatic hydrocarbon. The aliphatic hydrocarbon may be a saturated aliphatic hydrocarbon or an unsaturated aliphatic hydrocarbon having one or more unsaturated bonds.

In one embodiment, the linking group is a hydrocarbon group.

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 carbon atoms. In yet another embodiment, L is a saturated or unsaturated alkylene group having 4 carbon atoms.

The valency y, i.e., the number of amide groups, is 2 or more, for example, 2, 3, 4, or 5.

R is hydrogen or a methyl group. It is presumed that when R is hydrogen (-CONH-), the electron-withdrawing ability of the amide group is higher than when R is a methyl group (-CON(CH ₃ )-), and therefore the amide group is more likely to adsorb the dispersion medium compound to the magnetic particles.

A is a saturated or unsaturated alkylene group, for example an alkylene group having 2, 3, 4, 5 or 6 carbon atoms. The alkylene group may be linear, branched or cyclic.

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.

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

The dispersion medium compound may be used alone or in combination of two or more.

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.

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 of Patent Documents 1 or 2, or a dispersion medium such as silicone oil, fluorine oil, or paraffin.

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.

Other Additives 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. In addition to the above-mentioned components, each of these other additives may be used alone or in combination of two or more.

Dispersion Aids The dispersion aids may be non-aqueous or aqueous wetting dispersants that are commonly used. The functional groups of the dispersion aids that contribute to magnetic material adsorption may be acidic, basic, or salt-based, but basic or salt-based functional groups are preferred because they have less 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 aids may be used alone or in combination of two or more types.

When a dispersion aid is used, its amount can be adjusted as appropriate. For example, the proportion of the dispersion aid is 0.5 to 10% by mass relative to the total mass of the magnetic particles, the dispersion medium, and the dispersion aid.

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

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.

The material of the resin particles is not particularly limited, and any known resin may be appropriately selected and used. 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.

The acrylic resin may be a homopolymer or copolymer of monomers such as acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters, or a copolymer of one or more of these monomers with monomers such as ethylene or styrene (e.g., ethylene-acrylic acid copolymer, styrene-acrylic acid copolymer, etc.), or a modified form of these polymers or copolymers (e.g., fluorine-substituted acrylic resin).

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

The resin particles may be, for example, non-crosslinked resin particles in which the polymers in the resin are not crosslinked, or crosslinked resin particles in which the 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, are less likely to melt due to heat caused by sliding in 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.

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 made of a resin 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 a 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.

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.

In one embodiment, the acrylic resin is at least one selected from the group consisting of acrylic acid ester polymers, methacrylic acid ester polymers, styrene-acrylic acid copolymers, styrene-methacrylic acid copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, and modified versions thereof.

The weight average molecular weight (Mw) of the resin particles is not particularly limited and can be adjusted as appropriate. 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.

The average particle size of the resin particles is, for example, 20 to 1500 nm. In one embodiment, the average particle size 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 size 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.

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

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.

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

- Method for 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.

The applications of the magnetorheological fluid according to the present invention are not particularly limited, and it can be used for the applications of known MR fluids. Examples of applications of magnetorheological fluids include robots, brakes, clutches, dampers, shock absorbers, vibration control devices, haptics, force-tactile presentation devices, medical equipment, welfare equipment, and suction devices.

(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.

The present invention will be described in more detail below with reference to examples. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention in any way.

The materials used in the examples are as follows:
Magnetic particles: carbonyl iron powder, product name "CIP-SQ" manufactured by BASF, average particle size 4.5 μm, density 7.87 (g/cc)
Dispersion medium adipic acid diamide: manufactured by 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: manufactured by Aoki Oil Industries Co., Ltd., trade 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: manufactured by Aoki Oil Industries Co., Ltd., trade 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: manufactured by Aoki Oil Industries Co., Ltd., product name "Brownon O-15", one amide group, polyoxyethylene adduct of oleic acid amide, polyoxyethylene repeat number 7, density 0.92 (g/cc)
Succinic diester: manufactured by Aoki Yushi Kogyo Co., Ltd., trade name "SAES PO-4", formula: H(OC ₃ H ₆ ) ₄ OCOC 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)

The apparatus used in the examples is as follows:
Three-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)

(Examples 1 to 11 and Comparative Examples 1 to 4)
Magnetic particles, a dispersion medium, a comparative dispersion medium, and resin particles were prepared in the formulations (mass%) shown in Table 1. A container was filled with the 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 five times using a three-roll disperser under the narrowest opening conditions to prepare a magnetorheological fluid or a comparative MR fluid.

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:

(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 were 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 passed.
Criteria A: No separation is observed B: Separation width is 5 mm or less C: Separation width is more than 5 mm and less than 10 mm D: Separation width is more than 10 mm

(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 the comparative MR fluid flowed and became flat. Fail: The magnetorheological fluid or the comparative MR fluid maintained its original shape.

(Maximum change in yield stress)
A rheometer was filled with 0.6 mL of the magnetorheological fluid or the comparative MR fluid, and a vibration analysis was carried out at 10% strain and 10 Hz frequency under the condition of a magnetic field of 1 T (tesla) magnetic flux density. The shear yield stress (Pa) generated under the magnetic field 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 exceeds 7,000 Pa to 20,000 Pa or less. C: Shear yield stress is 7,000 Pa or less.

As shown in Table 1, the present invention provides a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and excellent fluidity.

The present invention can provide a magnetorheological fluid that has excellent long-term dispersion stability of magnetic particles and excellent fluidity. The present invention can also provide a device that has long-term stable operation and excellent mechanical reliability.

Claims (8)

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%. The organic compound has the formula (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. The magnetorheological fluid according to claim 2, wherein the linking group is a hydrocarbon group. The magnetorheological fluid according to claim 2, wherein L is an alkylene group. The magnetorheological fluid according to claim 4, wherein L is an alkylene group having 2 to 8 carbon atoms. The magnetic viscoelastic fluid according to any one of claims 1 to 4, wherein the average particle size of the magnetic particles is 1 to 50 μm. The magnetorheological fluid according to any one of claims 1 to 6, wherein the magnetorheological fluid does not contain resin particles. 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.