JP3365424B2 - Manufacturing method of magnetic fluid - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic fluid. More specifically, the present invention relates to a magnetic fluid whose viscosity has been significantly reduced. [0002] A magnetic fluid in which surfactant-adsorbed ferrite fine particles are dispersed in a low vapor pressure base oil using a phosphate ester-based dispersant has been previously proposed by the present applicant.
(Japanese Unexamined Patent Publication (Kokai) No. 1-231933), the obtained magnetic fluid has improved saturation magnetization. In general, a magnetic fluid has a high electric resistance. For example, when the magnetic fluid is used in a seal mechanism of a magnetic disk device or the like, a separate grounding mechanism is required to remove static electricity accumulated in the device. It is necessary to provide. Therefore, a conductive magnetic fluid capable of preventing electrification by providing conductivity to the magnetic fluid itself without installing a grounding mechanism or the like has been proposed. [0004] Conventional conductive magnetic fluids are prepared by adding a cationic surfactant, an amphoteric surfactant, graphite and the like. However, these methods are still unsatisfactory in terms of dispersion stability and viscosity. [0005] The magnetic fluid proposed by the present applicant uses a relatively conductive low-vapor-pressure base oil, and therefore has sufficient antistatic performance, but is still improved in terms of viscosity. It turned out that room was left. Therefore, the present inventor has previously described a magnetic fluid in which surfactant-adsorbed ferrite fine particles are dispersed in a low vapor pressure base oil using a phosphate ester-based dispersant, and the viscosity of the magnetic fluid is greatly reduced. Surfactants adsorbed ferrite fine particles using an alkali metal salt or an ammonium salt of a phosphate ester having a mono- or di-oxyalkylene substituent to form a phosphate triester-based fine particle. A magnetic fluid dispersed in a vapor pressure base oil has been proposed (Japanese Patent Application No. 4-102,076). Although the newly proposed magnetic fluid achieves its intended purpose, it has been studied in more detail, and it has been found that a higher fatty acid salt is used as a surfactant to be adsorbed on ferrite fine particles. It was found that a portion of the higher fatty acid salt that had been completely adsorbed to the ferrite fine particles was desorbed, and the desorbed higher fatty acid salt promoted the deterioration of the performance of the magnetic fluid. SUMMARY OF THE INVENTION An object of the present invention is to use a phosphate triester based on a higher fatty acid salt-adsorbed ferrite fine particle by using a phosphate ester having a mono- or di-oxyalkylene substituent. It is an object of the present invention to provide a method for producing a magnetic fluid which is more excellent in terms of performance by eliminating a higher fatty acid salt which has been eliminated in advance when producing a magnetic fluid by dispersing it in a low vapor pressure base oil. [0009] The object of the present invention is as follows.
In producing the above magnetic fluid, the higher fatty acid salt-adsorbed ferrite fine particles are previously subjected to aliphatic hydrocarbon, aromatic hydrocarbon,
After washing with an organic solvent that is a hydrogenated hydrocarbon or ether, a coagulation solvent compatible with the organic solvent is added to form a coagulation solvent.
The ferrite fine particles separated from the organic solvent are subjected to a dispersion treatment. As the ferrite fine particles, those produced by an arbitrary method are used, but those produced by a coprecipitation method which is advantageous in terms of purity, control of particle size, and above all, productivity are preferred. Used in Generally, magnetic fine particles have a strong hydrophilicity,
As it is, it aggregates in oil and cannot form a magnetic fluid. Therefore, it is necessary to impart a lipophilic property by adsorbing a surfactant or the like on the surface of the magnetic fine particles. As the surfactant adsorbed on the ferrite fine particles, the following higher fatty acid salts are used. These adsorbed higher fatty acid salts such as sodium oleate, potassium oleate, ammonium oleate, sodium stearate, sodium erucate, sodium myristate and sodium behenate are used for inorganic acids used for agglomeration of fine particles. It is also adsorbed in the form of the free acid. In the present invention, first, these higher fatty acid salts are adsorbed and have a particle size of about 50 to 300 °, preferably about 70 to 300 °.
The ferrite fine particles of up to 120 ° are sufficiently dried and removed of water serving as an inhibitory factor at the time of dispersion in oil, and then immersed in an organic solvent to dissolve and remove the higher fatty acid salt to be eliminated in the organic solvent. Since the ferrite fine particles subjected to the organic solvent washing are agglomerated by drying or the like, it is difficult to remove the higher fatty acid salt taken in the fine particles. Then it is very efficient. In order to dissolve ferrite fine particles, it differs slightly depending on the type of higher fatty acid salt adsorbed,
Generally, organic solvents having relatively small polarity are preferred. Examples of such organic solvents include n-heptane and n-heptane.
Aliphatic hydrocarbons such as hexane, n-octane, isooctane, n-decane, 1-decene, cyclohexane, cyclooctane and methylcyclohexane, benzene, toluene,
Aromatic hydrocarbons such as xylene, mesitylene, ethylbenzene, methylnaphthalene, and tetralin; halogens such as chloroform, carbon tetrachloride, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, bromoform, monochlorobenzene, and dichlorobenzene Hydrocarbons, diethyl ether, di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran, ethers such as dioxane and the like are used in an amount of about 5 to 100 times, preferably about 10 to 30 times the amount of the fine particles. Can be The washing treatment using these organic solvents is as follows.
It is performed under agitation and, if necessary, is irradiated with ultrasonic waves, so that it can be dissolved in a short time. If so, it becomes difficult to separate only the fine particles, so that it is necessary to add a solvent that aggregates the fine particles. The flocculating solvent must be compatible with the peptizing solvent used previously, and here, alcohols such as methanol, ethanol, n-propanol and isopropanol, ketones such as acetone, methyl ethyl ketone and cyclohexanone, and methyl acetate And esters such as ethyl acetate and butyl acetate are used in an amount of about 0.5 to 5 times the amount of the peptizing solvent. After the aggregation by adding such an aggregation solvent, the ferrite fine particles are separated from the organic solvent. At this time, decantation or centrifugation using a magnetic field is preferably used. The separated fine particles are dried at room temperature to about 100 ° C. under normal pressure or reduced pressure. FIG. 1 is a graph showing the relationship between the number of washings and the amount of higher fatty acid salt adsorbed (evaporation loss when the adsorbed fine particles are heated to 500 ° C. in air) in Examples described later. As is clear from this result, a decrease in the amount of adsorption was observed in the initial washing, and the amount of adsorption became substantially constant after the sixth time. Further, the reduction rate was about 20% by weight. The ferrite fine particles washed in this manner, particularly the ferrite fine particles which did not show a decrease in the adsorption amount, are subjected to a dispersion treatment by adding a low vapor pressure base oil and a dispersant thereto. . As a low vapor pressure base oil, 0.1 m at 25 ° C.
mHg or less, preferably a phosphate triester type having a vapor pressure of 0.01 mmHg or less, such as tributyl phosphate, tri-2-ethylhexyl phosphate, tricresyl phosphate, etc., the magnetic fluid finally obtained Ferrite fine particles in the dispersion concentration is about 10-50% by weight
It is used in such a ratio that When dispersing the ferrite fine particles in the phosphoric acid triester-based low vapor pressure base oil having excellent conductivity, simply dispersing the higher fatty acid salt on the surface of the ferrite fine particles has poor dispersibility. A dispersant is used to stably disperse it. As the dispersant, a phosphoric ester containing a mono- or di-oxyalkylene group represented by the following general formula or a mixture thereof _R: alkyl group of C 5 _{-C 24,} alkyl phenyl group having an alkyl group of the phenyl group C ₅ ~C ₁₀ n: 2 or 3 m: 4 to 20 integer of used. In practice, commercially available products are used as they are. The dispersion treatment with the addition of the low vapor pressure base oil and the dispersant is performed by a conventional method using a homogenizer, an ultrasonic wave, a ball mill, or the like, and the ferrite fine particles are stably contained in the low vapor pressure base oil. A dispersed magnetic fluid is formed therein. In dispersing the higher fatty acid salt-adsorbed ferrite fine particles into a phosphate triester-based low vapor pressure base oil using a phosphate ester-based dispersant, the higher fatty acid salt-adsorbed ferrite fine particles are preliminarily dispersed. When used after being washed with an organic solvent, not only can the viscosity of the obtained magnetic fluid be significantly reduced, but also its life performance can be greatly improved. As a result, when a magnetic fluid is used for a magnetic seal or the like, the problems such as the high starting torque and the scattering of the fluid, which have conventionally been problems, are released. In addition, when used in a sealing mechanism of a magnetic disk device or the like, the problem of viscosity being a problem and saturation magnetization being unable to be increased is eliminated. Next, the present invention will be described by way of examples. Example 1 A 6N NaOH aqueous solution having a pH of 11.1 was added to 1850 ml of an aqueous solution in which 184 g of FeCl ₂ .4H ₂ O and 500 g of FeCl ₃ .6H ₂ O were dissolved while stirring.
After the addition, the mixture was aged at 80 ° C. for 30 minutes and cooled. After repeating the washing of decanting the obtained magnetite fine particles with demineralized water several times, 70 g of sodium oleate and water were added to make the total liquid volume 4000 ml, and the mixture was heated at 90 ° C. to 3 ml.
Stirred for 0 minutes. After cooling, the pH was adjusted to 6.0 by adding 1N HCl, and the aggregated magnetite fine particles were decanted several times, washed and dried. 6.0 g of the sodium oleate-adsorbed magnetite fine particles were added to 100 ml of toluene, and the mixture was homogenized.
The mixture was stirred for 1 hour (10000 rpm) using a (Nippon Seiki Seisakusho Excel homogenizer DX type), and then subjected to ultrasonic irradiation for 30 minutes. Thereafter, 100 ml of acetone was added to this toluene solution to aggregate the fine particles and centrifuged (12000 G) to remove the supernatant. When such operations were repeated six times, the oleic acid adsorption amount was reduced from 19.4% by weight to 15.6% by weight. To 3.0 g of the oleic acid-adsorbed magnetite microparticles thus prepared, 6.2 g of tricresyl phosphate, 0.4 g of a phosphate ester-based surfactant (RP710, a product of Toho Chemical Co . ; R = phenyl group ) and toluene After adding 15.0 g and stirring with a homogenizer (Excel homogenizer DX type) for 1 hour, ultrasonic waves were irradiated for 2 hours. The toluene was distilled off, and the precipitate was removed by centrifugation (12000 G) to obtain a magnetic fluid. This has a saturation magnetization (16 KOe) of t210G and a conductivity of 1 × 1.
0 ^-6 s / m and the viscosity was 95cps (25 ℃).
The standing life at 120 ° C. was 800 hours. COMPARATIVE EXAMPLE In the examples, when the sodium oleate-adsorbed magnetite fine particles were used without washing, the saturation magnetization
A magnetic fluid having 5G, conductivity of 5 × 10 ⁻⁷ s / m, and viscosity of 180 cps (25 ° C.) was obtained. The standing life at 120 ° C. was 350 hours.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the relationship between the number of washings and the adsorption amount of higher fatty acid salts in Examples.

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Claims (1)

(57) [Claims 1] Higher fatty acid salt-adsorbed ferrite fine particles are treated with a phosphate ester having a mono- or di-oxyalkylene substituent to form a phosphate triester-based low vapor pressure group. When dispersing in oil, the higher fatty acid salt-adsorbed ferrite fine particles are previously subjected to aliphatic hydrocarbon, aromatic hydrocarbon.
After washing with an organic solvent such as hydrogen, halogenated hydrocarbon or ether, a coagulating solvent compatible with the organic solvent is added.
Ferrite particles separated from the organic solvent
A method for producing a magnetic fluid, comprising subjecting particles to a dispersion treatment.