Magnetic fluid composition and motor

Abstract

Provided are a magnetic fluid composition and a motor with which a magnetic fluid is prevented from scattering even during high-speed rotation while maintaining the function as a fluid. This magnetic fluid composition contains a dispersion medium and magnetic particles each having at least a portion of the surface coated with a dispersant. The dispersion medium contains a base oil and an olefin-based polymer having a kinematic viscosity of 45-1000 mm2 / s at 100°C.

Description

Magnetic fluid composition and motor

[0001] The present invention relates to a magnetic fluid composition and a motor.

[0002] A magnetic fluid contains magnetic particles, a dispersant, and a dispersion medium, and is generally a colloidal liquid in which the magnetic particles are dispersed in the dispersion medium by the dispersant. The dispersibility of magnetic particles in a magnetic fluid is very good, and the fluid itself can be considered a homogeneous liquid that is magnetic, without solid-liquid separation such as the magnetic particles settling or separating due to gravity or a magnetic field. Although magnetic fluids can be manipulated by a magnetic field, they generally cannot maintain their magnetized state without a magnetic field.

[0003] One known application of magnetic fluids to mechanical devices is the use of magnetic fluids to control the temperature of permanent magnets and other components used in motors (see Patent Document 1). Fig. 1 shows a schematic diagram of a typical outer rotor magnet-type brushless DC motor (motor 10). Fig. 1 is a simplified schematic diagram used to explain how magnetic fluids are used to reduce thermal demagnetization due to heat generation in the motor by equalizing the heat generated by the motor's coils and permanent magnets, and does not show the actual detailed configuration of an outer rotor magnet-type brushless DC motor.

[0004] The motor 10 has a rotor 14 and a stator 15, with the rotor 14 using a permanent magnet 11 such as a neodymium magnet. The stator 15 is composed of an iron core 12 wound with a coil 13, which is typically made of copper wire. The motor 10 is driven by converting input electrical power into mechanical power, but losses inevitably occur during this process. These losses are primarily converted into heat, causing the motor 10 to generate heat. For this reason, it is necessary to ensure that the instantaneous temperature inside the motor 10 does not exceed the maximum allowable temperature of each component. In particular, neodymium-based permanent magnets 11 experience a significant decrease in magnetic force due to thermal demagnetization as the motor temperature increases, so it is necessary to control the temperature of the entire motor 10 to a temperature that does not degrade the performance of the permanent magnets.

[0005] To address this issue, a known technology is to fill the gap between the rotor and stator with magnetic fluid to prevent the gap between the stator and magnet in a motor from blocking the flow of heat.By filling the gap with magnetic fluid, the heat generated in the coils at the beginning of rotation and then in the magnets after stable rotation is transferred evenly throughout the motor through the magnetic fluid, controlling the temperature to a level that does not degrade the performance of the permanent magnets, etc.

[0006] Japanese Patent Application Publication No. 07-059317

[0007] This problem does not occur in motors that are always operating at a low rated speed, but motors used in bicycles, power tools, automobiles, etc., have high rotational speeds, so the magnetic fluid filled in the gap between the rotor and stator may be scattered by the centrifugal force generated by the motor's rotation. The centrifugal force generated by the high-speed rotation of such motors causes most of the magnetic fluid filled in the gap between the rotor and stator to scatter, with some of it becoming concentrated in areas with strong magnetic force, such as the corners and edges of the motor's magnets. This makes it impossible to fill the gap between the rotor and stator, making it difficult to control the deterioration of the performance of the permanent magnets inside the motor.

[0008] The present invention has been made in light of the above points, and aims to provide a magnetic fluid composition and a motor in which scattering is effectively suppressed even during high-speed rotation while maintaining the fluid function of the magnetic fluid.

[0009] In order to solve the above problems, the present invention is specified as follows: [1] [5] A magnetic fluid composition containing magnetic particles, at least a part of whose surface is coated with a dispersant, and a dispersion medium, wherein the dispersion medium is a mixture of a base oil and a viscoelastic polymer having a kinematic viscosity at 100°C of 45 to 1000 mm 2and an olefin-based polymer of 1000 to 10 ...

[0010] According to an embodiment of the present invention, it is possible to provide a magnetic fluid composition and a motor in which scattering is effectively suppressed even during high-speed rotation while maintaining the fluid function of the magnetic fluid.

[0011] 1 is a schematic diagram of an outer rotor magnet type brushless DC motor (motor 10).

[0012] Below, embodiments of the magnetic fluid composition and motor of the present invention will be described, but the present invention should not be construed as being limited thereto, and various changes, modifications, and improvements can be made based on the knowledge of those skilled in the art, as long as they do not deviate from the scope of the present invention.

[0013] In this specification, the term "to" representing a numerical range indicates a range that includes the numerical values ​​recited as the upper and lower limits. Furthermore, when a unit is recited for only the upper limit of a numerical range, this means that the lower limit is also expressed in the same unit as the upper limit. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples. In this specification, when multiple substances corresponding to each component are present in the composition, the content or amount of each component in the composition refers to the total content or amount of the multiple substances present in the composition, unless otherwise specified.

[0014] (Magnetic fluid composition) The magnetic fluid composition according to this embodiment is a magnetic fluid composition containing magnetic particles at least partly coated with a dispersant and a dispersion medium, wherein the dispersion medium is a mixture of a base oil and a viscous fluid having a kinematic viscosity at 100°C of 45 to 1000 mm 2 and an olefin polymer of 1 / s. With this configuration, the magnetic fluid composition according to this embodiment maintains the fluid function of the magnetic fluid while effectively suppressing scattering even during high-speed rotation. The reason for this will be explained below.

[0015] In the magnetic fluid composition, the magnetic particles are dispersed in a dispersion medium. As described above, the magnetic fluid filled in a predetermined portion of a motor or the like is unlikely to cause problems when rotated at a low speed of less than 3500 rpm, but when rotated at a high speed of 3500 rpm or more, there is a risk of the magnetic fluid scattering due to centrifugal force. In contrast, the magnetic fluid composition according to this embodiment comprises magnetic particles at least part of whose surface is coated with a dispersant, a base oil, and a magnetic fluid having a kinematic viscosity at 100°C of 45 to 1000 mm. 2 The kinematic viscosity at 100°C is 45 to 1000 mm / s. 2 The olefin polymer of 1 / s acts as a tackifier in the magnetic fluid composition, and effectively prevents the magnetic fluid composition from scattering even when rotated at a high speed of 3500 rpm or more.

[0016] In addition, the kinematic viscosity at 100°C is 45 to 1000 mm 2 By including an olefin polymer of 1 / s, it is possible to suppress an increase in the sliding resistance of the magnetic fluid while improving the viscosity of the magnetic fluid toward metals such as magnets. Furthermore, it is possible to maintain the functionality of the fluid without deteriorating the compatibility between the magnetic particles and the base oil.

[0017] The magnetic fluid composition according to this embodiment contains magnetic particles at least partially coated with a dispersant and a dispersion medium, and the dispersion medium contains a base oil and a predetermined olefin polymer. Hereinafter, in this specification, a fluid containing magnetic particles coated with a dispersant and a base oil may be referred to as a magnetic fluid. Below, each component contained in the magnetic fluid composition will be described.

[0018] 1. Magnetic Fluid The magnetic fluid that can be used in the magnetic fluid composition of this embodiment contains magnetic particles at least part of the surface of which is coated with a dispersant, and a base oil.

[0019] The magnetic fluid used in the magnetic fluid composition of this embodiment may be prepared as needed, or a commercially available product may be used. A commercially available product in which the solvent is replaced with a base oil may also be used.

[0020] When preparing a magnetic fluid, preparation methods include a method of subdividing macroscopic magnetic particles to colloidal size and a method of obtaining magnetic fine particles by condensing atoms or ions. Methods of subdividing magnetic particles include pulverization and spark erosion. Methods of condensing atoms or ions include chemical coprecipitation (wet method), thermal decomposition of metal carbonyl, and vacuum deposition. Among these, chemical coprecipitation is preferred as a method for preparing a magnetic fluid because of its excellent productivity. For example, a method for preparing a magnetic fluid by chemical coprecipitation includes adding sodium oleate to a magnetite aqueous slurry prepared from an aqueous ferrous sulfate solution and an aqueous ferric sulfate solution, adsorbing the sodium oleate to the surface of the magnetite particles to obtain particles, washing the resulting particles with water, drying, and dispersing them in a base oil.

[0021] 2. Magnetic Particles Examples of magnetic particles contained in the magnetic fluid composition or magnetic fluid include ferromagnetic oxides, ferromagnetic metals, and metal nitrides. Examples of ferromagnetic oxides include magnetite (Fe3O4), γ-iron oxide, manganese ferrite, cobalt ferrite, or composite ferrites of these with zinc or nickel, and barium ferrite. Examples of ferromagnetic metals include iron, cobalt, and rare earth elements. Among these, magnetite is preferred as the magnetic particle from the viewpoint of mass production. The magnetic particles used in this embodiment are not particularly limited as long as they have an average particle size within a range that exhibits superparamagnetism, i.e., a critical particle size or smaller. For example, in the case of magnetite particles and γ-iron oxide particles, the average particle size is preferably 50 nm or smaller, and particularly preferably in the range of 10 nm to 40 nm. One type of magnetic particle may be used alone, or two or more types may be used in combination. The shape of the magnetic particles is preferably spherical or nearly spherical, as this facilitates dispersion.

[0022] The content of magnetic particles is preferably in the range of 25 to 70 mass % relative to the total mass of the magnetic fluid composition according to this embodiment, and more preferably 28 to 60 mass %. The content of magnetic particles is calculated from the mass of magnetic particles having a dispersant attached to at least a portion of their surface. By setting the content of magnetic particles in the range of 25 to 70 mass % relative to the total mass of the magnetic fluid composition according to this embodiment, the dispersibility of the magnetic particles can be maintained, thereby maintaining the functionality of the fluid and allowing the composition to function as a magnetic material when a magnetic field is applied.

[0023] 3. Dispersant A dispersant is added to improve the dispersibility of magnetic particles in the dispersion medium. Known surfactants, polymer dispersants, etc. can be used as appropriate as the dispersant. Among these, surfactants are preferred as the dispersant from the viewpoint of dispersibility and the performance of the resulting magnetic powder. By including the magnetic particles and dispersant in the magnetic fluid composition, at least a portion of the dispersant adheres to the magnetic particles, and at least a portion of the surface of the magnetic particles is coated with the dispersant, preferably the surfactant. Therefore, the hydrophilic groups of the surfactant are adsorbed toward the surface of the magnetic particles, and the hydrophobic groups of the surfactant are oriented toward the dispersion medium, resulting in stable dispersion of the magnetic particles in the dispersion medium. Examples of surfactants used as dispersants in this embodiment include anionic surfactants, which are hydrocarbon compounds having polar groups such as carboxyl groups, hydroxyl groups, and sulfonic acid groups, such as oleic acid or its salts, petroleum sulfonic acid or its salts, synthetic sulfonic acid or its salts, eicosylnaphthalenesulfonic acid or its salts, polybutenesuccinic acid or its salts, and erucic acid or its salts; nonionic surfactants such as polyoxyethylene nonylphenyl ether; and amphoteric surfactants having both cationic and anionic moieties in their molecular structure, such as alkyldiaminoethylglycine. Among these, sodium salt of oleic acid (hereinafter sometimes referred to as sodium oleate) is preferred as a dispersant from the standpoint of low cost and easy availability. Furthermore, to further improve the dispersibility of magnetic particles in a dispersion medium, it is most preferable that the dispersant be attached to the entire surface of the magnetic particles.

[0024] The dispersant may be used alone or in combination of two or more types. The total content of the dispersant in the magnetic fluid composition is not particularly limited as long as it is an amount that can prevent aggregation of the magnetic particles, and may be selected appropriately depending on the intended use. The content of the dispersant can be, for example, 3 to 30 mass % relative to the total mass of the magnetic particles, and preferably 5 to 20 mass %.

[0025] In the magnetic fluid composition, the dispersant is adsorbed onto the magnetic particles, resulting in at least a portion of the magnetic particle surface being coated with the dispersant. From the viewpoint of preventing aggregation of the magnetic particles, it is preferable that the dispersant has a median diameter of about 1 to 5 nm and more preferably about 2 to 3 nm. When the magnetic particles are magnetite or γ-iron oxide, taking into account the preferred particle diameters of the magnetic particles described above, the average particle diameter of the dispersant-coated magnetic particles is preferably an average primary particle diameter of 55 nm or less, more preferably 11 to 45 nm. In this specification, unless otherwise specified, the average primary particle diameter of the magnetic particles refers to the average particle diameter of the dispersant-coated magnetic particles. In this specification, the average primary particle diameter of the magnetic particles is a value measured by dynamic light scattering using a nano Partica SZ-100 series nanoparticle analyzer manufactured by Horiba, Ltd.

[0026] 4. Dispersion Medium The dispersion medium contained in the magnetic fluid composition according to this embodiment is a mixture of a base oil and a viscous fluid having a kinematic viscosity at 100°C of 45 to 1000 mm 2 and an olefin polymer of formula (I) / s. The dispersion medium may contain components other than the base oil and the olefin polymer (other dispersion medium components), and there are no particular restrictions on the other dispersion medium components as long as they are liquid at room temperature (25°C) and can disperse magnetic particles. By containing a dispersion medium, the dispersibility of magnetic particles in the magnetic fluid is very good, and solid-liquid separation, such as precipitation or separation of the magnetic particles due to gravity, magnetic fields, etc., does not occur, and the magnetic fluid composition fluid itself can be considered a homogeneous liquid having magnetism.

[0027] The content of the dispersion medium is preferably in the range of 30 to 75% by mass with respect to the total amount of the magnetic fluid composition according to this embodiment. By making the content of the dispersion medium 30% by mass or more, it is possible to disperse the magnetic particles and improve fluidity. By making the content of the dispersion medium 75% by mass or less, it is possible to function as a magnetic material when a magnetic field is applied. The content of the dispersion medium is more preferably in the range of 40 to 70% by mass with respect to the total amount of the magnetic fluid composition according to this embodiment, and even more preferably in the range of 42 to 68% by mass.

[0028] 4-1. Base Oil The base oil contained in the magnetic fluid composition according to this embodiment is liquid at room temperature (25°C), and is preferably a mineral oil or synthetic oil that has traditionally been used as a dispersion medium for magnetic particles, and has a boiling point of 100°C or higher. One type of base oil may be used alone, or two or more types may be used in combination. The base oil used in the magnetic fluid composition according to this embodiment may be either a mineral oil or a synthetic oil, or a combination of a mineral oil and a synthetic oil. The use of a base oil in the magnetic fluid composition according to this embodiment can improve lubricity.

[0029] Examples of synthetic oils include α-olefins, low-viscosity polyα-olefins (hereinafter sometimes referred to as low-viscosity PAOs), normal paraffins, hydrocarbon-based solvents such as halogenated hydrocarbons, glycol-based solvents, and silicone-based solvents. In this specification, low-viscosity polyα-olefins refer to oils having a kinematic viscosity of 20 mm at 100°C. 2 / s.

[0030] Examples of mineral oils include paraffin-based mineral oils, intermediate-based mineral oils, and naphthene-based mineral oils obtained by conventional refining methods such as solvent refining and hydrorefining, and waxes (gas-to-liquid waxes) produced by the Fischer-Tropsch process.

[0031] Examples of α-olefins include 1-hexene, 1-octene, 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, and 1-octadecene. Among these, α-olefins having 10 to 14 carbon atoms, such as 1-decene and 1-dodecene, are preferred. Polyα-olefins are homopolymers or copolymers of these α-olefins. Homopolymers or copolymers of α-olefins having 10 to 14 carbon atoms, such as 1-decene and 1-dodecene, are preferred. The α-olefins and polyα-olefins may be used alone or in combination of two or more.

[0032] Examples of glycol-based solvents include polyethylene glycol, polypropylene glycol, polybutylene glycol, ethylene oxide-propylene oxide copolymer, propylene oxide-butylene oxide copolymer, and derivatives thereof. The glycol-based solvents may be used alone or in combination of two or more.

[0033] The kinematic viscosity of the base oil at 100°C is 20.0 mm 2 / s or less, and 1 to 18.0 mm 2 It is more preferable that the kinematic viscosity of the base oil at 100°C is in the range of 20.0 mm / s. 2 / s or less is more preferable in that it makes it easier to disperse the magnetic particles. Note that the kinematic viscosity is the kinematic viscosity measured in accordance with JIS K2283:2000 kinematic viscosity testing method.

[0034] The pour point of the base oil is preferably −10° C. or lower, more preferably −20° C. or lower, particularly preferably −30° C. or lower, and most preferably −50° C. or lower. A pour point of −10° C. or lower is more preferable in terms of excellent low-temperature fluidity. The pour point is measured in accordance with JIS K2269:1987.

[0035] The content of the base oil is preferably 25 to 50 mass % relative to the total mass of the magnetic fluid composition according to this embodiment, and more preferably 28 to 40 mass % or more. By making the content of the base oil 25 mass % or more, it is possible to impart lubricity to the magnetic fluid composition. By making the content of the base oil 50 mass % or less, it is possible to impart viscosity to the magnetic fluid composition.

[0036] 4-2. Prescribed Olefin Polymer The olefin polymer contained in the magnetic fluid composition according to this embodiment has a kinematic viscosity at 100°C of 45 to 1000 mm 2 The olefin polymers may be used alone or in combination of two or more.

[0037] The kinematic viscosity of the olefin polymer at 100°C is 45 mm 2 When the kinematic viscosity of the olefin polymer at 100°C is 1000 mm / s or more, good adhesion can be imparted to the magnetic fluid composition, and scattering of the magnetic fluid composition during high-speed rotation of 3500 rpm or more can be effectively suppressed. 2 If the kinematic viscosity at 100°C is 60 to 800 mm / s or less, the function as a fluid can be maintained. 2 / s, and 80 to 400 mm 2 / s is more preferable, and 80 to 200 mm 2 It is particularly preferred that the formula is / s.

[0038] The content of the olefin polymer is preferably 10 to 50% by mass, and more preferably 20 to 40% by mass, relative to the total mass of the magnetic fluid composition according to this embodiment. When the content of the olefin polymer is 10% by mass or more relative to the total mass of the magnetic fluid composition according to this embodiment, better adhesion can be imparted to the magnetic fluid composition, and scattering of the magnetic fluid composition during high-speed rotation of 3,500 rpm or more can be more effectively suppressed. When the content of the olefin polymer is 50% by mass or less relative to the total mass of the magnetic fluid composition according to this embodiment, the function as a fluid can be maintained.

[0039] The magnetic fluid composition according to this embodiment has a kinematic viscosity of 45 mm at 100°C. 2 The olefin polymer having a kinematic viscosity of 45 mm / s or more at 100°C is 2 / s or more (hereinafter referred to as high viscosity PAO), kinematic viscosity at 100 ° C. is 45 mm 2 Polybutene having a kinematic viscosity of 45 mm / s or more at 100°C 2 Examples of high-viscosity PAOs include ethylene-propylene copolymers with a viscosity of 1 / s or more. Among these, polybutene functions as a particularly preferred tackifier in magnetic fluid compositions. As the high-viscosity PAO, poly-α-olefins (mPAOs) obtained by polymerizing the α-olefins using a metallocene catalyst are preferred.

[0040] Kinematic viscosity at 100°C is 45 mm 2 Examples of polybutenes having a molecular weight of 1 / s or more include copolymers of isobutylene and 1-butene, which are olefins having four carbon atoms, and preferably have a mass average molecular weight (Mw) of 1,000 to 4,000. Furthermore, those having a mass average molecular weight (Mw) of 1,200 to 3,000 are more preferred. Polymers consisting of 100% by mass of isobutylene or 100% by mass of n-butene may also be used as polybutene. While polybutene may be a polymer consisting only of linear butene (1-butene), polymers containing branched butene (isobutene) are more preferred because they can impart better adhesion to the magnetic fluid composition.

[0041] The blending ratio of the base oil to the predetermined olefin polymer is preferably 30:70 to 50:50, more preferably 35:65 to 45:55, by mass ratio.

[0042] 5. Compatibilizer Although a compatibilizer is not an essential component of the magnetic fluid composition according to this embodiment, it is preferable to add it because it can improve the effect of suppressing aggregation of magnetic particles due to the addition of the olefin polymer. The compatibilizer preferably contains one or more selected from esters, alkylnaphthalenes, and alkylbenzenes. One type of compatibilizer may be used alone, or two or more types may be used in combination.

[0043] (Esters) Examples of the esters include polybasic acid esters obtained by reacting polybasic acids (e.g., succinic acid, phthalic acid, alkylsuccinic acid, alkenylsuccinic acid, azelaic acid, maleic acid, suberic acid, sebacic acid, fumaric acid, adipic acid, linoleic acid dimer, etc.) with various alcohols (e.g., butyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, dodecyl alcohol, ethylene glycol, diethylene glycol monoether, propylene glycol, etc.); polyol esters obtained by reacting monocarboxylic acids having 5 to 18 carbon atoms with polyols (e.g., neopentyl glycol, trimethylolpropane, pentaerythritol, dipentaerythritol, tripentaerythritol, etc.); and polyoxyalkylene glycol esters. Furthermore, the esters preferably have a molecular weight of 200 or more, more preferably 250 or more, and even more preferably 300 or more.

[0044] (Alkylnaphthalene, alkylbenzene) The alkylnaphthalene and alkylbenzene preferably have an alkyl chain length of 6 to 24 carbon atoms, more preferably 10 to 20 carbon atoms, and particularly preferably 12 to 18 carbon atoms. These can be produced by the Friedel-Crafts alkylation reaction of benzene or naphthalene with an olefin. The olefin used here may be linear or branched.

[0045] The content of the compatibilizer is preferably 2% by mass or more, more preferably 3 to 20% by mass, and even more preferably 3 to 15% by mass, relative to the total mass of the magnetic fluid composition according to this embodiment.

[0046] 6. Other Components In addition to the components described above, the magnetic fluid composition according to this embodiment may further contain various other components depending on the purpose, as long as the effects of this embodiment are not impaired. Examples of other components include viscosity index improvers, anti-wear agents, surfactants, viscosity modifiers, settling inhibitors, pour point depressants, extreme pressure agents, rust inhibitors, antioxidants, corrosion inhibitors, metal deactivators, antifoaming agents, and inorganic fillers.

[0047] Examples of anti-wear agents include sulfur-based compounds such as sulfides, sulfoxides, sulfones, and thiophosphinates; halogen-based compounds such as chlorinated hydrocarbons; organometallic compounds such as molybdenum dithiophosphate (MoDTP) and molybdenum dithiocarbamate (MoDTC); and phosphate esters. Examples of phosphate esters include phosphate triesters such as trioctyl phosphate and tricresyl phosphate, monooctyl phosphate, and dioctyl phosphate. One type of anti-wear agent may be used alone, or two or more types may be used in combination.

[0048] Examples of viscosity modifiers include castor oil, hydrogenated castor oil, fatty acid amides, beeswax, carnauba wax, benzylidene sorbitol, metal soaps, polyethylene oxide, sulfate ester-based anionic surfactants, (meth)acrylic acid esters, polyisobutylene, polyalkylstyrene, etc. One type of viscosity modifier may be used alone, or two or more types may be used in combination.

[0049] Examples of inorganic fillers include alumina, calcium carbonate, silica, and boron nitride. In this specification, the term "inorganic filler" also includes the magnetically responsive composite material defined in Japanese Patent No. 7240436. One type of inorganic filler may be used alone, or two or more types may be used in combination. The shape of the inorganic filler is preferably spherical.

[0050] <Remaining Amount of Magnetic Fluid Composition> The remaining amount of the magnetic fluid composition according to this embodiment is evaluated by preparing a typical outer rotor magnet brushless DC motor (product name: AM30-1450KV, radius: approximately 16 mm, manufactured by T-MOTOR Corporation) as shown in Figure 1, filling the gap between the rotor and stator of the motor with the magnetic fluid composition, rotating the motor at a rotational speed of 11,000 rpm for 10 seconds, and then rotating the motor at a rotational speed of 11,000 rpm for an additional 10 minutes, and subtracting the weight of the motor before filling with the magnetic fluid composition from the weight of the motor after that. The evaluation result is preferably 0.02 g or more, and more preferably 0.025 g or more.

[0051] <Oil Separability of Magnetic Fluid Composition Upon Magnetic Excitation> The oil separation of the magnetic fluid composition according to this embodiment before excitation is evaluated by visually observing whether or not oil separates while holding a magnet having a magnetic flux density of 1500 mT in contact with a beaker containing the magnetic fluid composition. The evaluation is preferably performed when no separation occurs for 1 minute, more preferably when no separation occurs for 3 minutes, and even more preferably when no separation occurs for 5 minutes.

[0052] (Method of Manufacturing Magnetic Fluid Composition) As a method of manufacturing the magnetic fluid composition according to this embodiment, for example, a magnetic fluid containing magnetic particles at least partially coated with a dispersant and a base oil is added to a magnetic fluid having a kinematic viscosity at 100° C. of 45 to 1000 mm 2 The magnetic fluid composition can be prepared by measuring predetermined amounts of the olefin polymer (I) and / or (II) and other components, such as a compatibilizer, which may be added as needed, into a container, and thoroughly stirring and mixing them. In the production of the magnetic fluid composition, the mixture may be heated or cooled as necessary.

[0053] (Motor Using Magnetic Fluid Composition) The magnetic fluid composition according to this embodiment can be used by filling the gap between the rotor and stator of a motor. It is particularly suitable for motors used at high speeds of 3500 rpm or more. There are no limitations on the type (DC motor, AC motor, permanent magnet field type, electromagnet field type, etc.) or application (for bicycles, automobiles, etc.) of the motor, as long as it has a rotor and stator as shown in FIG. 1 .

[0054] Examples of the present invention are given below, but these examples are provided for a better understanding of the present invention and its advantages, and are not intended to limit the invention.

[0055] Example 1 A magnetite water slurry was produced by a known chemical coprecipitation method. 3N (normal) HClaq (hydrochloric acid aqueous solution) was added to this magnetite water slurry to adjust its pH to 3, and then sodium oleate was added as a dispersant and stirred at 60°C for 30 minutes. This process allowed the dispersant to be adsorbed onto the surface of the magnetite fine particles. Next, the mixture was left to stand, causing the magnetite fine particles in the liquid to coagulate and settle, and the supernatant was discarded. Next, fresh water was added, stirred, and then left to stand, and the supernatant was discarded. This water washing was repeated several times to remove the electrolytes in the aqueous solution, and then the mixture was filtered, dehydrated, and dried to obtain powdered magnetite fine particles whose surfaces were coated with sodium oleate as a dispersant.

[0056] Next, hexane, a low-boiling organic solvent, was added to the powdered magnetite microparticles and the mixture was thoroughly shaken to obtain an intermediate medium in which the magnetite microparticles were dispersed in hexane. Next, methanol, a low-boiling polar organic solvent, was added to the slurry, and the particles were once coagulated and precipitated, and the supernatant was discarded. This removed excess dispersant other than the dispersant monomolecularly adsorbed on the magnetite microparticles. The precipitated magnetite microparticles were then dispersed again in hexane to obtain an intermediate medium containing dispersed magnetite particles.

[0057] This intermediate medium was centrifuged for 30 minutes under a centrifugal force of 8000 G to settle and remove relatively large particles with poor dispersibility among the magnetite dispersion particles. Next, the supernatant liquid in which the remaining magnetite fine particles that did not settle were dispersed was transferred to a rotary evaporator and kept at 90°C to evaporate and remove the low-boiling organic solvent component, i.e., hexane, to obtain lipophilic magnetite fine particles (average particle size: 15 nm) whose surfaces were coated by monomolecular adsorption of the dispersant sodium oleate.

[0058] 6.5 g of the magnetite particles were collected and redispersed in hexane, and then a low viscosity PAO (kinematic viscosity at 100°C: 1.7 mm 2 6.5 g of a hydroxypropyl cellulose copolymer (polypropylene glycol ester, pour point -70°C) was mixed.

[0059] The mixture was then transferred to a rotary evaporator and kept at 90°C to evaporate and remove the low-boiling organic solvent component, i.e., hexane. As a result, the magnetite particles were dispersed in the low-viscosity PAO. This was then centrifuged under a centrifugal force of 8000G for 30 minutes to remove undispersed solid matter, yielding a magnetic fluid. Next, polybutene A (mass average molecular weight: 1600, kinematic viscosity at 100°C: 85 mm 2 / s) 8 g and hexadecylnaphthalene (kinematic viscosity at 100 ° C: 4.7 mm 2 0.66 g of hexadecylnaphthalene (a monoalkylated naphthalene in which the alkyl group is hexadecene, kinematic viscosity at 100°C: 4.7 mmHg) was added and stirred thoroughly at a liquid temperature of 100°C to obtain a mixture of polybutene A and hexadecylnaphthalene (a monoalkylated naphthalene in which the alkyl group is hexadecene, kinematic viscosity at 100°C: 4.7 mmHg). 2 / s) was dissolved in the magnetic fluid and allowed to cool to room temperature to obtain a magnetic fluid composition.

[0060] Example 2 Polybutene A was replaced with polybutene B (mass average molecular weight: 2500, kinematic viscosity at 100°C: 270 mm 2 A magnetic fluid composition was obtained in the same manner as in Example 1, except that the composition was changed to (I).

[0061] Example 3 Polybutene A was replaced with polybutene C (mass average molecular weight: 3700, kinematic viscosity at 100°C: 670 mm 2 A magnetic fluid composition was obtained in the same manner as in Example 1, except that the composition was changed to (I).

[0062] Comparative Example 1: Polybutene A was replaced with polybutene D (mass average molecular weight: 550, kinematic viscosity at 100°C: 4.7 mm 2 A magnetic fluid composition was obtained in the same manner as in Example 1, except that the composition was changed to (I).

[0063] Comparative Example 2 A magnetic fluid composition was obtained in the same manner as in Example 1, except that polybutene A was changed to a butadiene polymer (trade name: Kuraplane (registered trademark) LBR-305, manufactured by Kuraray Co., Ltd.).

[0064] Comparative Example 3 A magnetic fluid composition was obtained in the same manner as in Example 1, except that polybutene A was changed to a maleic anhydride adduct of isoprene polymer (trade name: Kurapren (registered trademark) LIR-403, manufactured by Kuraray Co., Ltd.).

[0065] Comparative Example 4 A magnetic fluid composition was obtained in the same manner as in Example 1, except that polybutene A was changed to an isoprene polymer with maleic anhydride monomethyl ester adduct (product name: Kurapren (registered trademark) LIR-410, manufactured by Kuraray Co., Ltd.).

[0066] Example 4 Polybutene A was converted into a high viscosity PAO (a poly-α-olefin based on 1-decene and polymerized using a metallocene catalyst, kinematic viscosity at 100°C: 135 mm 2 A magnetic fluid composition was obtained in the same manner as in Example 1, except that the composition was changed to (I).

[0067] Example 5 A magnetic fluid composition was obtained in the same manner as in Example 4, except that 8 g of high-viscosity PAO was changed to 6 g of high-viscosity PAO and 2 g of polybutene C.

[0068] Example 6 A magnetic fluid composition was obtained in the same manner as in Example 4, except that 8 g of high-viscosity PAO was changed to 4 g of high-viscosity PAO and 4 g of polybutene C.

[0069] Example 7 A magnetic fluid composition was obtained in the same manner as in Example 4, except that 8 g of high-viscosity PAO was changed to 4 g of high-viscosity PAO and 4 g of polybutene B. Tables 1 and 2 show the blending ratios (mass ratios) of the raw materials in Examples 1 to 7 and Comparative Examples 1 to 4.

[0070] <Evaluation of the amount of scattering of the magnetic fluid composition due to centrifugal force during initial rotation> The amount of scattering of the magnetic fluid composition according to this embodiment was evaluated by preparing a typical outer rotor magnet type brushless DC motor (product name: AM30-1450KV, radius: approximately 16 mm, manufactured by T-MOTOR) as shown in FIG. 1 , installing the motor on a flat surface of a cylindrical S45C fixed base with a diameter of 70 mm and a height of 70 mm, and installing a transparent PET cover around the motor's periphery. The magnetic fluid composition was then filled into the gap between the rotor and stator of the motor, and the motor was rotated at a rotation speed of 11,000 rpm for 10 seconds. The amount of scattering of the magnetic composition was evaluated by visually observing the state of adhesion of the magnetic composition to the cover installed around the periphery, according to the following criteria. The evaluation results are shown in Tables 1 and 2. A: Almost no scattering B: Some scattering C: Large amount of scattering

[0071] <Evaluation of Residual Amount of Magnetic Fluid Composition> The residual scattering rate of the magnetic fluid composition according to this embodiment was measured as follows. First, the weight (X) of the motor used in evaluating the amount of scattering caused by centrifugal force during the initial rotation of the magnetic fluid composition was measured. Next, in evaluating the amount of scattering caused by centrifugal force during the initial rotation of the magnetic fluid composition, the motor was rotated for 10 seconds, and then rotated at a rotation speed of 11,000 rpm for another 10 minutes, and the weight (Y) of the motor was measured. The value obtained by subtracting X from Y was taken as the remaining amount of magnetic fluid composition. The evaluation criteria were as follows. The evaluation results are shown in Tables 1 and 2. A: 0.025 g or more B: 0.02 g or more but less than 0.025 g C: Less than 0.02 g

[0072] <Evaluation of oil separation properties of magnetic fluid composition upon magnetic excitation> A magnet having a magnetic flux density of 1500 mT was held in contact with a 120 mL beaker containing the magnetic fluid composition, and visual observation was made to see if oil separated. The evaluation criteria were as follows. The evaluation results are shown in Tables 1 and 2. A: No separation within 1 minute B: Separation within less than 1 minute

[0073]

[0074]

[0075] The magnetic fluid compositions of Examples 1 to 7 are all magnetic fluid compositions containing magnetic particles at least partly coated with a dispersant and a dispersion medium, wherein the dispersion medium is a mixture of a base oil and a viscous fluid having a kinematic viscosity at 100°C of 45 to 1000 mm 2 and an olefin polymer having a molecular weight of 1 / s. For this reason, the magnetic fluid compositions of Examples 1 to 7 all maintained their fluid function, and showed almost no scattering even when the motor was rotated at a high speed of 11,000 rpm, resulting in favorable results.

[0076] The magnetic fluid composition of Comparative Example 1 is a tackifier having a kinematic viscosity of 45 to 1000 mm 2 The magnetic fluid composition of Comparative Example 2 did not contain a tackifier containing a maleic anhydride adduct of an isoprene polymer, and the amount of splashing increased due to the high rotation speed of the motor at 11,000 rpm, resulting in a poor remaining amount of magnetic fluid composition. The magnetic fluid composition of Comparative Example 3 contained a maleic anhydride adduct of an isoprene polymer as a tackifier, and in an evaluation of the remaining amount of the magnetic fluid composition, it was unable to fill the gap between the rotor and the stator, and its oil separation properties upon magnetic excitation were also poor. The magnetic fluid of Comparative Example 4 contained a maleic anhydride monomethyl ester adduct of an isoprene polymer as a tackifier, and in an evaluation of the remaining amount of the magnetic fluid composition, it was unable to fill the gap between the rotor and the stator, and its oil separation properties upon magnetic excitation were also poor.

[0077] 10 Motor 11 Permanent magnet 12 Iron core 13 Coil 14 Rotor 15 Stator

Claims

1. A magnetic fluid composition containing magnetic particles, at least a portion of whose surface is coated with a dispersant, and a dispersion medium, wherein the dispersion medium is a mixture of a base oil and a viscous fluid having a kinematic viscosity at 100°C of 45 to 1000 mm 2 and an olefin-based polymer of formula (I).

2. The magnetic fluid composition according to claim 1, wherein the content of the olefin polymer is 10 to 50 mass % based on the total mass of the magnetic fluid composition.

3. The magnetic fluid composition according to claim 1, wherein the olefin polymer is at least one selected from the group consisting of poly-α-olefin, polybutene, and ethylene-propylene copolymer.

4. The magnetic fluid composition of claim 1, further comprising a compatibilizer.

5. A motor using the magnetic fluid composition according to any one of claims 1 to 4.

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