JP2006286890A - Magnetic viscous fluid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic viscous fluid which is superior in re-dispersion of magnetic grains and keeps up a stable state for a long time. <P>SOLUTION: The magnetic viscous fluid contains Neuburg siliceous earth, a dispersant and magnetic grains. The Neuburg siliceous earth content is preferably 0.05-10 mass% in 100 mass% of the magnetic viscous fluid. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

The present invention relates to a magnetorheological fluid.

Conventionally known are liquid compositions whose fluid properties change in response to a magnetic field, which are called magnetorheological fluids, magnetic fluids, or magnetorheological materials (Non-patent Documents 1 and 2 to 6). This magnetorheological fluid is a thickened or gelled material in which the contained magnetic particles (average particle size: several nanometers to several tens of micrometers) are aligned by an externally applied magnetic field to form chain clusters. The flow characteristics and yield stress change significantly. For example, bearings, sealing materials, centering devices, speakers, clutches, brakes, dampers, shock absorbers, engine mounts, lifting function members, building vibration control Conventionally, it has been applied to devices and the like.

In these applications, the magnetorheological fluid can be operated again not only during the steady operation of the device, but also when the device is stopped for a long time and the magnetic particles in the magnetorheological fluid have settled and aggregated. When started, the magnetic particles need to be uniformly dispersed instantaneously in the dispersion medium to exhibit stable characteristics, and thus high redispersibility of the magnetic particles is required.

Normally, the specific gravity of the magnetic particles that make up the magnetorheological fluid is significantly larger than the specific gravity of the dispersion medium, so that the magnetic particles in the magnetorheological fluid settle over time and form a hard slurry based on the cohesive force of the magnetic particles. To produce a precipitate. The slurry-like magnetic particle precipitate gradually becomes densely packed with time and becomes very hard, and it becomes difficult to uniformly disperse it again in the dispersion medium. In order to disperse the magnetic particles again in the dispersion medium from such a state, it is necessary to apply a physically large force over a long period of time.

In order to prevent such a phenomenon, a substance such as a low molecular weight and / or high molecular weight dispersant or a surfactant is generally blended in the magnetorheological fluid. However, even when substances such as these dispersants and surfactants are blended, it is difficult to prevent sedimentation / aggregation of magnetic particles and formation of a hard slurry over a long period of time. In addition, substances such as dispersants and surfactants are decomposed and altered by the effects of heat generation and mechanical stimulation during device operation, and catalytic action of magnetic particles, etc., preventing the formation of hard slurry precipitates There is a problem that the performance is lowered, and in some cases, these substances are polymerized, so that the produced slurry is changed to a harder state and re-dispersion is completely impossible.

On the other hand, as another method for preventing the precipitation of magnetic particles, a method of adding a swellable clay mineral such as bentonite or montmorillonite as a thickener or thixotropic agent is known.
Patent Documents 6 to 8 contain carrier fluid and magnetically responsive particles, and further improve redispersibility or precipitation resistance by containing organoclay that has been hydrophobized bentonite as a thickener or thixotropic agent. A magnetorheological material is disclosed.

However, only containing bentonite hydrophobized organic clay does not sufficiently prevent the formation of slurry-like precipitates due to precipitation of magnetic particles in hydrophobic dispersion media such as oils and organic solvents. It was difficult to maintain the stable performance of viscous fluid for a long time. Further, the redispersibility of the slurry-like precipitate was not sufficient.

US Pat. No. 2,661,596 U.S. Pat. No. 3,006,656 U.S. Pat. No. 4,604,229 JP 51-13995 JP 51-44579 A US Pat. No. 6,203,717 US Pat. No. 6,547,986 US Patent Application Publication No. 2004/84651 AIEE Transactions, “Magnetic Fluid Properties”, February 1955, p. 149-152 (JD Coolidge Jr. and RW Hallberg paper 55-170)

An object of the present invention is to provide a magnetorheological fluid that is excellent in redispersibility of magnetic particles and that can maintain stable performance for a long period of time.

The present invention is a magnetorheological fluid comprising Neuburg silica, a dispersion medium and magnetic particles.
The blending amount of the Neuburg silica is preferably 0.05 to 10% by mass in 100% by mass of the magnetorheological fluid.
Hereinafter, the present invention will be described in detail.

The present invention is a magnetorheological fluid comprising Neuburg silica, a dispersion medium and magnetic particles. For this reason, it is excellent in the redispersibility of a magnetic particle, the production | generation of a slurry-like precipitate can be prevented, and the stable performance can be maintained for a long period of time. Since the magnetorheological fluid of the present invention contains Neuburg silica, re-dispersion of magnetic particles than conventional swellable clays such as bentonite and montmorillonite, or those containing hydrophobic organic clay treated with them. It has excellent properties.

The above-mentioned Neuburg Siliceous Earth is a silica produced specifically in the Neuburg region in the south of Germany. It is a mineral, and the quartz and the kaolinite are in a loosely bonded state to each other. Since polar groups such as oxygen and hydroxyl groups exist on the surface of kaolinite constituting the Neuburg silica, the Neuburg silica can be physically adsorbed on the surface of the magnetic particles. Furthermore, because Neuburg silica has a bulky structure in which quartz penetrates between plate-shaped kaolinites, the magnetic particles adsorbing Neuburg silica cannot be aggregated densely after settling, It can be easily dispersed in the dispersion medium again by applying a slight mechanical stimulus.

Such a phenomenon is not observed when kaolinite is used alone or when swellable clay minerals such as bentonite and montmorillonite are used. When these substances are used, hydrophobicity such as oils and organic solvents is not possible. Magnetic particles are easily precipitated in a porous dispersion medium.
When kaolinite and swellable viscosity minerals are treated with organic treatment, they swell in a hydrophobic dispersion medium and interact with magnetic particles. Since it is not shown, it is extremely difficult to completely prevent the formation of a hard slurry-like precipitate based on the sedimentation of the magnetic particles.

The Neuburgh silica is easily swollen in the hydrophobic dispersion medium, not only when it is organically treated but also when it is used without any organic treatment. And since the polar group which the plate-shaped kaolinite surface mentioned above has can also give a certain thixotropic property to a dispersion medium, the effect of reducing the sedimentation rate of a magnetic particle can be expressed.

In addition, Neuburg silica is an inorganic material, so it is more resistant to mechanical and thermal stimuli associated with the operation of applying a magnetorheological fluid to the device than organic dispersants and surfactants. It is highly durable and can exhibit stable performance over a long period of time.

Furthermore, since the magnetorheological fluid generally has a relatively high viscosity, when it is applied to a device or the like, an operation (degassing) of removing air mixed in the magnetorheological fluid is required. Since Neuburg silica has a structure with more voids than general clay minerals, a magnetorheological fluid containing this can be easily degassed.

The Neuburg silica in the magnetorheological fluid of the present invention preferably contains 15 to 30% by mass of kaolinite and 60 to 80% by mass of quartz. By using Neuburg silica within the above range, excellent redispersibility can be obtained, and generation of slurry-like precipitates can be suppressed.
The kaolinite has the following formula (I):
Al ₂ Si ₂ O ₅ (OH) ₄ (I)
The quartz is a tectosilicate mineral made of SiO ₂ .

Moreover, in the chemical composition of the Neuburg silica, the SiO ₂ content is preferably 75 to 91% by mass, and more preferably 78 to 82% by mass. In the above chemical composition, it is preferable that _Al 2 _{O 3} minutes is 5 to 15 wt%, more preferably 12 to 14 wt%.

The average particle diameter of the Neuburg silica is preferably 1.0 to 10 μm. By being in the above range, excellent redispersibility can be obtained, and the formation of slurry-like precipitates can be suppressed. More preferably, it is 1.5-2.0 micrometers. The average particle diameter (average aggregate particle diameter) is the median particle diameter (50% volume particle diameter). In this specification, the average particle diameter is a value (D ₅₀ ) obtained using a commercially available particle size distribution measuring apparatus such as CAPA-700 manufactured by Horiba, Ltd.

The Neuburg silica preferably has a true specific gravity of 2.5 to 3.0. By being in the above range, excellent redispersibility can be obtained, and the formation of slurry-like precipitates can be suppressed. More preferably, it is 2.5-2.8. The true specific gravity can be determined by a pycnometer method (liquid phase replacement method).

It is preferable that the Neuburg silica bead has a bulk density of 0.2 to 0.4 g / cm ³ . By being in the above range, excellent redispersibility can be obtained, and the formation of slurry-like precipitates can be suppressed. More preferably, it is 0.25 to 0.30 g / cm ³ . The bulk density can be measured by a known method.

The Neuburg silica is preferably 9 to 18 m ² / g in specific surface area. By being in the above range, excellent redispersibility can be obtained, and the formation of slurry-like precipitates can be suppressed. More preferably, it is 11-15 m < ² > / g. The specific surface area is a value measured by a known BET method.

The Neuburg silica clay preferably has an oil absorption of 35 to 60 g / 100 g. By being in the above range, excellent redispersibility can be obtained, and the formation of slurry-like precipitates can be suppressed. More preferably, it is 50-55g / 100g. The oil absorption can be measured by a known method.

Further, the Neuburgk silica may be obtained by organically treating the surface. By carrying out the organic treatment, it is possible to further swell in the hydrophobic dispersion medium and further impart thixotropic properties to the dispersion medium.
As a method of organically treating the surface of the Neuburg silica, a known method may be used. For example, an organic material is used to replace a non-organic surface cation with an organic surface cation by an ion exchange reaction. A method can be mentioned. Suitable organic modifiers for the organic treatment include amines, carboxylic acids, phosphonium, sulfonium salts, benzyl or other organic substances. Examples of the amine include quaternary amines or aromatic amines. Examples of the quaternary amine include dimethyl distearyl ammonium chloride, dimethyl stearyl benzyl ammonium chloride, trimethyl stearyl ammonium chloride, trimethyl benzyl ammonium chloride, Myristyl diethylmethyl ammonium chloride, myristyl trimethyl ammonium chloride, cetyl trimethyl ammonium chloride, behenyl diethyl methyl ammonium chloride, benzyl diethyl methyl ammonium chloride, dipalmityl propyl ethyl ammonium methyl sulfate, hexyl trimethyl ammonium chloride, octyl trimethyl ammonium chloride, dioctyl dimethyl ammonium Examples include muchloride, trioctylmethylammonium chloride, decyltrimethylammonium chloride, decylisononyldimethylammonium chloride, didecyldimethylammonium chloride, lauryltrimethylammonium chloride, trilaurylmethylammonium chloride, and the aromatic amine includes, for example, Examples include aniline, toluidine, benzylamine, diphenylamine, triphenylamine and the like.
Examples of the organic modifier include aminosilane, epoxy silane, mercaptosilane, vinyl silane, hydrophobic silane, and tetrasulfane.

As commercial products of the above-mentioned Neuburg silica clay, Siltintin Z86, N82, N85, N87, V85; Sillikoloid P87; Can be mentioned.

The amount of Neuburg silica is preferably 0.05 to 10% by mass in 100% by mass of the magnetorheological fluid. If it is less than 0.05% by mass, sufficient redispersibility cannot be imparted to the magnetic particles, and a hard slurry-like precipitate that is difficult to redisperse in the dispersion medium because the magnetic particles settle early. There is a risk of generating. If it exceeds 10% by mass, the sedimentation of the magnetic particles can be suppressed, but the viscosity of the magnetorheological fluid becomes too high, making it difficult to apply to the device and failing to exhibit sufficient performance. More preferably, it is 0.1-5 mass%.

The magnetorheological fluid of the present invention contains a dispersion medium.
The dispersion medium is not particularly limited as long as it is a substance capable of dispersing Neuburg silica and magnetic particles. For example, mineral oils composed of organic solvents such as toluene, xylene, hexane, and petroleum hydrocarbons. Synthetic oils such as alkylbenzene, polyphenyl ether, alkylphenyl ether, polybutene, silicone oil and fluorine oil; animal oils such as fish oil, pork oil and cow oil; soybean oil, rapeseed oil, corn oil, palm oil, palm Vegetable oils such as oil, cottonseed oil, sunflower oil, castor oil; water and glycol derivatives; ions represented by ethylmethylimidazolium salt, 1-butyl-3-methylimidazolium salt, 1-methylpyrazolium salt, etc. Liquids (room temperature molten salts) and the like. These can be used alone or in admixture of two or more.

The magnetorheological fluid contains magnetic particles.
The magnetic particle is not particularly limited as long as it is a substance having magnetism. For example, iron, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low carbon steel, nickel, cobalt, aluminum-containing iron alloy, silicon-containing iron alloy , 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, copper-containing iron alloys, etc., gadolinium, gadolinium organic derivatives Paramagnetic, superparamagnetic, or ferromagnetic compound particles made of, and particles made of a mixture thereof. These magnetic particles may be used alone or in combination of two or more.

Among the above magnetic particles, carbonyl iron is preferable from the viewpoint of expressing a large stress even with a small magnetic field.
The magnetic particles may be those obtained by subjecting the surface of these magnetic particles to a dispersion treatment. By subjecting the surface to a dispersion treatment, the dispersibility of the magnetic particles is improved, and a magnetorheological fluid having excellent responsiveness can be obtained. Examples of the magnetic particles (surface-treated magnetic particles) whose surface has been subjected to a dispersion treatment include those obtained by treating the surface of the magnetic particles with a silane coupling agent.

Examples of the surface-treated magnetic particles include those obtained by treating the surface of magnetic particles with a silane coupling agent containing an epoxy group or an amino group. The silane coupling agent containing an epoxy group or amino group is not particularly limited as long as it is a silane coupling agent containing at least one epoxy group or amino group in one molecule, but is represented by the following formula (1). The compound to be used is preferably used.

X- (Y) -SiR _3-b L _b (1)
In the formula, X represents an epoxy group, a cyclic epoxy group or an amino group. Y represents (CH ₂ ) _k or a hydrocarbon group containing an ether bond, an ester bond or a ketone bond. k represents an integer of 1 to 4. R represents an alkyl group such as a methyl group, an ethyl group, a propyl group, or a butyl group. L represents an alkoxyl group such as a halogen atom, a hydroxyl group, a methoxyl group, an ethoxyl group, a propoxyl group, or a butoxyl group, or an acyloxyl group such as a formyl group, an acetoxyl group, a propionyloxyl group, or a butyryloxyl group. b represents an integer of 1 to 3.

As a method for treating the surface of the magnetic particles with a silane coupling agent containing an epoxy group or an amino group, for example, a solution in which the silane coupling agent containing the epoxy group or amino group is dissolved in a solvent such as alcohol In addition, the magnetic particles may be immersed, or the silane coupling agent solution may be sprayed onto the magnetic particles and then the solvent is volatilized. Furthermore, after volatilizing the solvent, heat treatment may be performed at 40 to 150 ° C. for 5 minutes to 24 hours. The surface-treated magnetic particles are much more excellent in dispersion stability than untreated magnetic particles.

The amount of the silane coupling agent containing the epoxy group or amino group can be appropriately adjusted depending on the specific surface area of the magnetic particles. For example, 0.05 to 10 parts per 100 parts by mass of the magnetic particles. It is preferable that it is a mass part.

The magnetic particles preferably have an average particle diameter (D ₅₀ ) of 0.01 to 100 μm. If it is less than 0.01 μm, since the particle size is too small, the rate of increase in the viscosity of the magnetorheological fluid at the time of applying a magnetic force becomes small, and there is a possibility that sufficient performance cannot be exhibited when applied to a device. When the thickness exceeds 100 μm, the magnetic particles tend to aggregate and settle, and there is a possibility that a magnetorheological fluid excellent in dispersion stability cannot be obtained. More preferably, it is 0.5-20 micrometers.

In the magnetorheological fluid, the content of the magnetic particles is preferably 10 to 95% by mass in 100% by mass of the magnetorheological fluid. If it is less than 10% by mass, the viscosity of the magnetorheological fluid does not sufficiently increase when magnetism is applied, and there is a possibility that sufficient performance cannot be exhibited when applied to a device. If it exceeds 95% by mass, the viscosity of the magnetorheological fluid will increase so much that it will not function as a fluid, and may not be able to exhibit sufficient performance when applied to a device. More preferably, it is 50-85 mass%.

The magnetorheological fluid according to the present invention includes a dispersant, a thixotropic agent, an oil improver, an extreme pressure additive, and a solid lubricant as additives within a range that does not impair the properties of the magnetorheological fluid, particularly redispersibility of magnetic particles. , Viscosity index improvers, pour point depressants, antioxidants, rust inhibitors, antifoaming agents and the like can be added alone or in combination.

Examples of the dispersant include fatty acids, fatty acid esters, sorbitan fatty acid esters, fatty acid amides, fatty acid amines, polyoxyethylene derivatives, polyoxypropylene derivatives, glycerin derivatives, castor oil derivatives, amino acid derivatives, silicon surfactants, polyacrylic Examples include acid soda, ammonium polyacrylate, and polyvinyl alcohol.

Examples of the thixotropic agent include inorganic whiskers such as basic magnesium sulfate and calcium silicate; clay minerals such as kaolinite, ceviolite, hydrotalcite, zeolite, allophane, imogolite, carostarnite, pyrophyllite, and halloysite. And organic derivatives thereof; silica; talc and the like.
Examples of the oiliness improver include higher fatty acids, higher alcohols, aliphatic amines, aliphatic amides and esters.

Examples of the extreme pressure additive include olefin polysulfide, dibenzyl disulfide, alkyl phosphate ester, allyl phosphate ester, amine salt of phosphate ester, thiophosphate ester and its amine salt, naphthenate, chlorinated paraffin, etc. Can be mentioned.
Examples of the solid lubricant include graphite, fullerene, and carbon nanotube.

Examples of the washing dispersant include metal sulfinate, metal phosphonate, metal carboxylate, metal phenate, succinimide, succinate, benzylamine, and alkylphenolamines.

Examples of the viscosity index improver include polymethacrylate, polyacrylate, polyisobutylene, olefin copolymer, polyalkylstyrene, ethylene-propylene copolymer, hydrogenated styrene-diene copolymer, and the like. Can be mentioned.

Examples of the pour point depressant include low molecular weight polymethacrylic acid esters and polyacrylic acid esters, chlorinated paraffin-naphthalene condensates, chlorinated paraffin-phenol condensates, and polyalkylstyrenes.

Examples of the antioxidant include phenol derivatives, amines, benzotriazole, zinc phosphate derivatives, metal phenates, and organic nitrogen compounds.
Examples of the rust inhibitor include amine salts of metal soaps, succinic acid derivatives, metal sulfonate salts, oleic acid derivatives, alkylamines, and phosphate esters.
Examples of the antifoaming agent include silicon compounds, aliphatic alcohols, metal soaps, succinic acid derivatives, polyacrylic acid esters, and the like.
In the present invention, it is preferable to use propylene carbonate as the additive. Thereby, redispersibility and the suppression of the production | generation of a slurry-like precipitate can be improved.

The method for producing the magnetorheological fluid of the present invention may be a known method. For example, the magnetorheological fluid may be produced by kneading and dispersing the above-mentioned Neuburg silica, a dispersion medium, magnetic particles, and other additives. The kneading / dispersing procedure is not particularly limited, and even in a method of kneading / dispersing all the components of the measured magnetorheological fluid at one time, materials other than magnetic particles are pre-kneaded / dispersed in advance, A two-stage kneading / dispersing method in which magnetic particles are added and kneading / dispersing again may be used.
The kneading / dispersing means is not particularly limited, and examples thereof include a sand mill, a bead mill, a ball mill, a roll mill, a kneader, a planetary mixer, a high speed mixer, a universal mixer, and a homogenizer. Moreover, in order to improve kneading | mixing and dispersibility, it is also possible to use a heating apparatus, a supersonic bath, etc. together.

Since the present invention is a magnetorheological fluid characterized by containing Neuburg silica, a dispersion medium and magnetic particles, it is excellent in redispersibility of magnetic particles and can maintain stable performance for a long time. is there. Moreover, deaeration etc. can be performed easily and handling is also easy. For this reason, the magnetorheological fluid of the present invention can suitably control the generated stress by applying a magnetic force.

Since the magnetic viscous liquid of this invention consists of the said structure, it is excellent in the redispersibility of a magnetic particle, and can maintain the performance stable for a long period of time. For this reason, it can utilize suitably for stress control apparatuses, such as a damper, a shock absorber, a brake, and a clutch.

Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In the examples, “parts” and “%” mean “parts by mass” and “% by mass” unless otherwise specified.

Example 1
The constituent materials of the magnetorheological fluid were weighed so that the total amount was 430 g at the composition ratio shown in Table 1. Synthetic oil as a dispersion medium (benzyltoluene, NeOSK-OIL 1300 manufactured by Soken Chemical Co., Ltd.), Neuburg silica (Sillitin Z86 manufactured by HOFFMAN MINEAL, average particle size: D ₅₀ = 2.0 μm, bulk density: 0.25 g / cm ³ , True specific gravity: 2.6, specific surface area: 14 m ² / g, oil absorption: 50 g / 100 g, SiO ₂ min 82 mass%, Al ₂ O ₃ min 12 mass%) and propylene carbonate as additive by a homogenizer at 7600 rpm For 10 minutes. To this was added magnetic particles carbonyl iron powder (BASF carbonyl iron powder CM), and kneaded and dispersed in a ball mill for 1.5 hours using a 1/4 inch steel ball as a medium to produce a magnetorheological fluid.

(Example 2)
Silica colloid P87 (average particle diameter: D ₅₀ = 1.6 μm, bulk density: 0.25 g / cm ³ , true specific gravity: 2.6, specific surface area: 14 m ² / g, oil absorption: 40 g / as Neuburg silica clay manufactured by HOFFMAN MINEAL A magnetorheological fluid was prepared in the same manner as in Example 1 except that 100 g, 78 mass% for SiO _{2 and} 14 mass% for Al ₂ O ₃ ) were used.

(Example 3)
A magnetorheological fluid was produced in the same manner as in Example 2 except that the blending amount of Neuburg silica (Sillikloid P87 manufactured by HOFFMANN MINEAL) and propylene carbonate was reduced.

(Comparative Example 1)
The constituent materials of the magnetorheological fluid were weighed so that the total amount was 430 g at the composition ratio shown in Table 1. Add carbonyl iron powder (BASF carbonyl iron powder CM) as a magnetic particle to a synthetic oil (benzyltoluene, Soken Chemical's NeOSK-OIL 1300) as a dispersion medium, and use a 1/4 inch steel ball as a medium with a ball mill. Kneaded and dispersed for 5 hours to produce a magnetorheological fluid.

(Comparative Example 2)
The constituent materials of the magnetorheological fluid were weighed so that the total amount was 430 g at the composition ratio shown in Table 1. Synthetic oil as a dispersion medium (benzyltoluene, NeOSK-OIL 1300 manufactured by Soken Chemical), finely powdered kaolinite as a clay mineral, and propylene carbonate as an additive were stirred at 7600 rpm for 10 minutes with a homogenizer. To this was added magnetic particles carbonyl iron powder (BASF carbonyl iron powder CM), and kneaded and dispersed in a ball mill for 1.5 hours using a 1/4 inch steel ball as a medium to produce a magnetorheological fluid.

(Comparative Example 3)
A magnetorheological fluid was prepared in the same manner as in Comparative Example 2 except that finely powdered bentonite was used as the clay mineral.

(Comparative Example 4)
The constituent materials of the magnetorheological fluid were weighed so that the total amount was 430 g at the composition ratio shown in Table 1. Synthetic oil (benzyltoluene, NeOSK-OIL 1300 manufactured by Soken Chemical) and a nonionic dispersant (polyoxyethylene octylphenyl ether) as a dispersion medium were stirred with a homogenizer at 7600 rpm for 10 minutes. To this was added magnetic particles carbonyl iron powder (BASF carbonyl iron powder CM), and kneaded and dispersed in a ball mill for 1.5 hours using a 1/4 inch steel ball as a medium to produce a magnetorheological fluid.

(Evaluation of redispersibility)
The produced magnetorheological fluid is put into a self-made piston type damper tester (inner diameter: 30 mm, flow path width: 0.35 mm, capacity: 15 ml, made of steel), 15 ml, 19 hours under conditions of 3 Hz circumference and ± 2 mm amplitude. I was tired continuously.
After completion of the test, the magnetorheological fluid was removed from the testing machine, transferred to a 20 ml capacity glass bottle, sealed, and left for 30 days.
Thereafter, this was set on a rotary shaker (amplitude: 20 mm) and shaken at 150 rpm for 15 seconds. Immediately after shaking, the glass bottle was opened and inverted to allow the magnetorheological fluid to flow out, and the presence or absence of a slurry-like precipitate remaining in the glass bottle was visually confirmed. The results are shown in Table 1.

From Table 1, the magnetorheological fluids of the examples showed excellent redispersibility and showed long-term stability without the formation of slurry-like precipitates. The product formed and did not show long-term stability.

The magnetorheological fluid of the present invention can be suitably used for stress control devices such as dampers, shock absorbers, brakes and clutches.

Claims (2)

A magnetorheological fluid comprising Neuburg silica, a dispersion medium and magnetic particles. 2. The magnetorheological fluid according to claim 1, wherein the amount of Neuburg silica is 0.05 to 10% by mass in 100% by mass of the magnetorheological fluid.