JP2002075724A - Method of manufacturing magnetic fluid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a magnetic fluid which is superior in stability at a high temperature, when magnetic fine particles are dispersed in a low vapor-pressure base oil by the use of oil-soluble sulfonic acid surface-active agent. SOLUTION: Magnetic fine particles such as ferrite fine particles are coated with unsaturated higher fatty acid whose surface coverage is about 20 to 70% or saturated higher fatty acid, whose surface coverage is about 25 to 55%. Oil-soluble surface-active agent is added to a dispersed system, composed of an organic solvent as a dispersion medium and the magnetic fine particles as the dispersed phase, and the surface-active agent is made to adsorb matter by the use of ultrasonic waves. After the adsorbed matter is recovered, the magnetic fine particles are dispersed into low vapor pressure base oil for the manufacture of a magnetic fluid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing a magnetic fluid. More specifically, the present invention relates to a method for producing a magnetic fluid having excellent stability under high temperature conditions.

[0002]

2. Description of the Related Art Conventionally, there has been known a method for preparing a magnetic fluid in which an unsaturated higher fatty acid is adsorbed on fine particles of ferrite represented by magnetite to coat the surface thereof, and the resultant is dispersed in oil (see, for example, Japanese Patent Application Laid-Open No. H11-157556). JP-B-53-17118, etc.). The magnetic fluid obtained by such a method has a problem that it deteriorates and gels over a long period of use due to the remaining salt, water, mixing of a small amount of water from the outside, and the like.
In recent years, magnetic fluids have also been widely used, and are often used under high temperature conditions. Therefore, development of a magnetic fluid exhibiting excellent stability even under such a high temperature environment is desired.

A method for preparing a magnetic fluid from such a viewpoint is disclosed in JP-A-8-113794. In this method, magnetic particles such as magnetite and Mn-Zn ferrite are coated with higher fatty acids and dispersed in an oil-based solvent to obtain a magnetic fluid.At this time, a sulfonic acid-based surfactant or animal wax is mixed. By doing so, the long-term stability of the magnetic fluid under high temperature and high humidity conditions is improved.

By the way, when the use under high temperature conditions is taken into consideration, it is natural that the oil solvent used and the volatility of the added surfactant itself also become a problem. When a solvent that evaporates easily is used, the concentration of fine particles in the magnetic fluid increases over a long period of use, resulting in loss of fluidity. Also, when using a surfactant that evaporates easily,
It is easily imagined that the dispersibility of the fine particles tends to decrease.

[0005] From such a point, it is indispensable to use a high molecular weight solvent or a surfactant, but a sulfonic acid-based surfactant R (SO ₃ ) nM [M: metal atom, n: 1 to As 2], those having a high molecular weight, for example, those having 20 or more carbon atoms in the R chain are available as commercial products, but they are generally commercially available after being diluted with an oil such as mineral oil. . That is, when a magnetic fluid is prepared using a commercially available high molecular weight sulfonic acid-based surfactant, mixing of mineral oil or the like is inevitable, which causes evaporation.

In order to disperse magnetic fine particles in higher alkylnaphthalene, which is a representative of high molecular weight oils, the molecular weight of the sulfonic acid-based surfactant needs to be about 350 or more, which is commonly used. R chain has 1 carbon
In the case of 2, dodecylbenzenesulfonic acid (MW: 326) or its sodium salt (MW: 348), there is a problem that dispersion of magnetic fine particles in higher alkyl naphthalene oil cannot be achieved.

The higher alkyl naphthalene is also
Since the vapor pressure of this high molecular weight oil at 20 ° C. is 1 × 10 ⁻⁵ Pa or less, it is superior in low evaporability as compared with other hydrocarbon-based oils, and has a molecular weight of 350 or more. Various proposals have been made for favorably dispersing magnetic fine particles.

For example, a method of preparing a magnetic fluid using a nonionic surfactant having an HLB of 9.5 or less (Japanese Patent Laid-Open No. 56-1528)
No. 80) has been proposed, but the nonionic surfactant used in this method is liable to be mixed with water because it has a polyoxyethylene chain, and is considered to lack long-term stability at high temperatures. . Similarly, in the method described in JP-A-8-113794, the addition of an alkyl phosphoric acid having a polyoxyethylene chain as a dispersing aid is considered to be effective, and in this case, water is likely to be mixed. Results. Further, Japanese Patent Application Laid-Open No. 58-89802 describes that a magnetic fluid is prepared using magnetic fine particles to which two types of higher fatty acids are adsorbed, and an unsaturated higher fatty acid is used as one of the types. Therefore, there is a concern that stability under high temperature conditions is lacking.

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to disperse magnetic fine particles in a low-vapor-pressure base oil using an oil-soluble sulfonic acid-based surfactant and to have excellent stability under high-temperature conditions. An object of the present invention is to provide a method for manufacturing a magnetic fluid.

[0010]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
Unsaturated higher fatty acids having a surface coverage of about 20 to 70% or about 25
An oil-soluble surfactant is added to an organic solvent dispersion of magnetic fine particles coated with ~ 55% of a saturated higher fatty acid, the mixture is adsorbed using ultrasonic waves, and the adsorbed material is recovered. This is achieved by producing a magnetic fluid by dispersion.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Ferrite fine particles are generally used as magnetic fine particles. As the ferrite fine particles, those manufactured by an arbitrary method are used, but the purity, the control of the particle size, and above all, the particle size of about 50 manufactured by the coprecipitation method which is advantageous in terms of productivity.
~ 300Å, preferably about 70-120Å magnetite, Mn-Z
n Ferrite or the like is generally used, and by adding an aqueous solution of a water-soluble higher fatty acid salt to an aqueous dispersion thereof, the water-soluble higher fatty acid salt is adsorbed thereon.

The water-soluble higher fatty acid salt has a carbon number of 10
The above unsaturated or saturated higher fatty acids, for example, sodium, potassium or ammonium salts such as oleic acid, erucic acid, lauric acid, myristic acid, palmitic acid, stearic acid and behenic acid are used. After adding these water-soluble higher fatty acid salt aqueous solution to the aqueous dispersion of ferrite fine particles, the mixture is subjected to an adsorption treatment at room temperature to about 90 ° C. for about 0.5 to 2 hours under stirring conditions, and then separated by filtration to coat the higher fatty acid salt. Form ferrite fine particles.

The magnetic fine particles having the higher fatty acid salt adsorbed thereon are:
Because it has a hydrophobic surface, it can be dispersed in an organic solvent. Examples of the organic solvent include aliphatic hydrocarbons such as n-hexane, n-octane, isooctane, n-decane, n-dodecane, and n-hexadecane; aromatic hydrocarbons such as toluene, xylene, and dodecylbenzene; chloroform; Halogenated hydrocarbons such as carbon chloride and chlorobenzene, and ethers such as tetrahydrofuran are used.

The magnetic fine particles prepared in an aqueous solution have high hydrophilicity and cannot be dispersed in an organic solvent and aggregate. However, by adsorbing a water-soluble surfactant, the fine particles become hydrophobic. Then, it can be dispersed in various solvents as exemplified above, and can be treated with an oil-soluble surfactant.

In the case where the oil-soluble surfactant is treated with the water-soluble surfactant-coated magnetic fine particles in an organic solvent, the oil-soluble surfactant penetrates into the existing water-soluble surfactant adsorbing layer and adheres to the surface of the magnetic fine particles. Otherwise, the oil-soluble surfactant will not be effectively adsorbed on the surface of the magnetic fine particles. However, oil-soluble sulfonic acid-based surfactants (Ca salts, Ba salts), which are thought to improve the stability of magnetic fluids at high temperatures, are of two-chain type, especially those with a high molecular weight of about 700 or more. Due to the structure, it is difficult for the water-soluble surfactant to penetrate into the adsorption layer, and it is very difficult to increase the amount of adsorption to the magnetic fine particles.

According to the present invention, a divalent metal salt of an oil-soluble sulfonic acid-based surfactant having a large structure is obtained by reducing the density of the adsorption layer formed by the higher fatty acid salt to a relatively low density state and performing ultrasonic treatment. Even if there is, it is possible to effectively adsorb it on the magnetic fine particles.

As described above, in order to increase the adsorption amount of the two-chain high molecular weight oil-soluble sulfonic acid surfactant, it is necessary to reduce the adsorption amount of the fatty acid salt on the magnetic fine particles.
At this time, the lower the amount of adsorption, the better, but a certain amount of adsorption is required to make the surface of the magnetic fine particles hydrophobic and disperse them in an organic solvent. When the amount of fatty acid salt adsorbed is high, the number of sites of adsorption of the two-chain high molecular weight sulfonic acid-based surfactant is reduced.

A typical unsaturated higher fatty acid salt, sodium oleate, or a saturated higher fatty acid salt, sodium stearate, will be described as an example. Using this, it is adsorbed on Fe ₃ O ₄ fine particles having a primary particle diameter of 9 nm. In this case, the minimum adsorbable amount that can be dispersed in the organic solvent is about 20 to 25% or about 25 to 30%, respectively, in terms of surface coverage. Below this, the dispersibility in the organic solvent deteriorates, the proportion of particles that do not disperse increases, and the yield deteriorates significantly. However, only a low yield does not mean that a good magnetic fluid cannot be obtained at all. Also, when the amount of adsorption is increased, the dispersibility in the organic solvent becomes very good, but the amount of adsorption of the two-chain high molecular weight oil-soluble sulfonic acid-based surfactant cannot be increased, and the magnetic fluid cannot be used. Although it can be formed, it lacks stability under high temperature conditions.To ensure this stability, the adsorption amount (surface coverage) of sodium oleate or sodium stearate is about 70% or less, respectively. Or less than about 55%. Unsaturated higher fatty acid salts can take a larger upper limit of the surface coverage because the alkyl chain is bent at the double bond,
It is considered that the fatty acids can take a relatively sparse adsorption state.

Here, the surface coverage can be easily calculated by the following method. That is, the magnetic fine particles on which sodium oleate or sodium stearate is adsorbed are analyzed, and the number N of adsorbed magnetic particles per magnetic fine particle is calculated from the amount of the adsorbed organic component. The surface area S of one magnetic fine particle is calculated as S = πd ² (nm ² ) from the average primary particle diameter d of the magnetic fine particles observed by a TEM or the like. The effective area of sodium oleate or sodium stearate is 0.3 nm ²
Or so it has been found to be 0.2 nm ^2, surface coverage
A is A = N × 0.3 / S or A = N × 0.2 / S.

The surface coverage is about 20-70%, preferably about 35
Various methods may be used to prepare ~ 70% (for unsaturated higher fatty acids) or about 25-55%, preferably about 35-55% (for saturated higher fatty acids) higher fatty acid-coated magnetic microparticles. Can be. As described above, the higher fatty acid salt aqueous solution is added to and adsorbed to the magnetic fine particles prepared by the coprecipitation method. At this time, it is possible to obtain a desired amount of adsorption by changing the amount of addition. is there. Also, first 100%
Alternatively, after preparing higher fatty acid-coated magnetic fine particles having a surface coverage higher than that, by washing with an organic solvent, surplus or weakly adsorbed fatty acid salts can be washed off and desorbed, so that a desired surface can be obtained. Washing may be repeated until the coverage is reached.

Next, an oil-soluble sulfonic acid-based surfactant is subjected to an adsorption treatment on the ferrite particles coated with a higher fatty acid having such a surface coverage by using ultrasonic waves. The addition amount of the oil-soluble sulfonic acid-based surfactant at this time is about 0.1 to 10 times the weight of the magnetic fine particles, and preferably about 0.1 to 10 times.
0.5 to 5 times the amount. The ultrasonic treatment is performed for about 0.5 to 10 hours using an ultrasonic homogenizer, an ultrasonic bath or the like. At this time, pulverization treatment such as heating and stirring or a ball mill can be performed or used together.

After the ultrasonic treatment, the adsorbed material is collected by a method such as filtration, centrifugation, decantation and the like. By this recovery treatment, the diluent oil contained in the commercially available oil-soluble sulfonic acid-based surfactant together with the organic solvent is also removed. The residue is washed with acetone, methanol, a mixed solvent of acetone-toluene or the like in order to remove heat denatured substances and impurities.

After the washing operation is performed about 1 to 5 times, preferably, low boiling hydrocarbons such as toluene and n-decane (boiling point of 2
(Less than 00 ° C), centrifugal separation method, magnetic sedimentation method, magnetic sedimentation method-decantation method, removing the precipitate by a separate separation method, and then subjected to dispersion treatment by adding a low vapor pressure base oil there,
A magnetic fluid is prepared by distilling off low-boiling hydrocarbons.

As the base oil, an aromatic hydrocarbon represented by toluene or the like is used, but a low vapor pressure base oil is generally used. As the low vapor pressure base oil, a liquid having a vapor pressure of 0.1 mmHg or less at 25 ° C., preferably 0.01 mmHg or less, for example, a natural oil such as white oil (liquid paraffin), mineral oil, spindle oil, or a synthetic oil Lubricating oils containing so-called lubricating additives such as higher alkylbenzenes, higher alkylnaphthalenes, polybutenes (molecular weight: about 300 to 2,000), and antioxidants, antiwear agents, oil agents, detergent dispersants and the like are used.

[0025]

According to the method of the present invention, a magnetic fluid which is stable even under high temperature conditions can be produced. The obtained magnetic fluid does not contain mineral oil while using an oil-soluble sulfonic acid-based surfactant. Since it is a surfactant that hardly mixes moisture, it also ensures long-term stability.

[0026]

Next, the present invention will be described with reference to examples.

Example 1 A 4.5N aqueous solution of sodium hydroxide was added to 500 ml of an aqueous solution in which 8.95 g of FeCl ₂ .4H ₂ O and 24.33 g of FeCl ₃ .6H ₂ O were dissolved.
Was added to prepare Fe ₃ O ₄ fine particles. In this aqueous dispersion, an aqueous solution 600 dissolved 13.7 g of sodium oleate
Then, the mixture was adsorbed at 80 ° C. for 1 hour with stirring, and separated by filtration.

The oleic acid-coated Fe thus obtained
After dispersing ₃ O ₄ fine particles in 500 ml of toluene, acetone was added thereto to aggregate the fine particles, and the operation of discarding the supernatant by decantation was repeated.
A 200% toluene dispersion of 67% sodium oleate-coated fine particles (content of Fe ₃ O ₄ fine particles: 8 g) was obtained.

In addition, an oil-soluble sulfonic acid surfactant
(Sulhole Ca45) 20 g was added, and an ultrasonic homogenizer (Bran
It was set on a Sonifier 450D manufactured by Son Co., Ltd., and subjected to adsorption treatment at a power of 35 W for 2 hours, and then filtered. Then acetone-toluene
After washing with a mixed solvent several times (volume ratio: 3: 1), the precipitate was once dispersed in toluene and centrifuged (5000 G, 30 minutes) to remove a precipitate. Thereto, 8 g of higher alkyl naphthalene (diffusion pump oil S) was added and dispersed, and then toluene was distilled off to obtain a magnetic fluid.

Example 2 In Example 1, the same amount of a toluene dispersion of sodium oleate-coated fine particles having a surface coverage of 27% was used in the same amount. Ba30N was used in the same amount.

Example 3 In Example 1, the same amount of a toluene dispersion of sodium erucate-coated fine particles having a surface coverage of 52% was used.

Comparative Example 1 In Example 1, the same amount of a toluene dispersion of sodium oleate-coated fine particles having a surface coverage of 79% was used.

Comparative Example 2 In Example 1, the same amount of a toluene dispersion of sodium erucate-coated fine particles having a surface coverage of 76% was used.

Comparative Example 3 In Example 1, the same amount of a dispersion in which sodium oleate-coated fine particles having a surface coverage of 18% were suspended in toluene was used in the same amount. It settled and the saturation magnetization could not be increased (saturation magnetization 30G).

With respect to the magnetic fluids obtained in Examples 1 to 3 and Comparative Examples 1 and 2, the saturation magnetization was unified to 300 G and the viscosity was
(25 ° C) and heat resistance evaluation (holding the magnetic fluid in an oven at 150 ° C, the time until gelation is 300 hours or more ◎, 100-300 hours △, 100 hours or less X).

The results obtained are shown in Table 1 below. Table 1 Example viscosity (mPa · S) heat resistance Example 1 400 ◎ 〃 2 600 ◎ 〃 3 800 ◎ Comparative Example 1 800 〇 〃 2 350 ×

Example 4 100 ml of a 4.5N sodium hydroxide aqueous solution was added to 500 ml of an aqueous solution in which 8.95 g of FeCl ₂ .4H ₂ O and 24.33 g of FeCl ₃ .6H ₂ O were dissolved.
Was added to prepare Fe ₃ O ₄ fine particles. An aqueous solution in which 13.7 g of sodium stearate was dissolved in this aqueous dispersion 6
Then, the mixture was adsorbed at 80 ° C. for 1 hour with stirring, and then separated by filtration.

The stearic acid coating thus obtained
After dispersing Fe ₃ O ₄ fine particles in 500 ml of chloroform,
Acetone was added thereto to aggregate the fine particles, and the operation of discarding the supernatant by decantation was repeated.A 200% chloroform dispersion of sodium stearate-coated fine particles having a surface coverage of 52% (content of Fe ₃ O ₄ fine particles: 8 g) was obtained. Obtained.

In addition, an oil-soluble sulfonic acid-based surfactant
(Sulhole Ca45) 20 g was added and an ultrasonic homogenizer (Soni
fier450D), and subjected to adsorption treatment at a power of 35W for 2 hours, and then separated. Next, acetone-toluene (volume ratio 3: 1)
After washing several times with a mixed solvent, the precipitate was once dispersed in toluene and centrifuged (5000 G, 30 minutes) to remove a precipitate. There, higher alkyl naphthalene (diffusion pump oil S) 8
After adding and dispersing g, toluene was distilled off to obtain a magnetic fluid.

Example 5 In Example 4, the same amount of a toluene dispersion of sodium stearate-coated fine particles having a surface coverage of 29% was used in the same amount, and Matsumura Petroleum Product Sulfol, a Ba salt, was used as an oil-soluble sulfonic acid-based surfactant. Ba30N was used in the same amount.

Example 6 In Example 4, the same amount of a toluene dispersion of sodium laurate-coated fine particles having a surface coverage of 40% was used.

Comparative Example 4 In Example 4, the same amount of a toluene dispersion of sodium stearate-coated fine particles having a surface coverage of 60% was used.

Comparative Example 5 In Example 4, the same amount of a toluene dispersion of sodium laurate-coated fine particles having a surface coverage of 76% was used.

Comparative Example 6 In Example 4, the same amount of a dispersion in which sodium stearate-coated fine particles having a surface coverage of 18% were suspended in toluene was used in the same amount. It settled and the saturation magnetization could not be increased (saturation magnetization 20G).

For the magnetic fluids obtained in Examples 4 to 6 and Comparative Examples 4 to 5, the saturation magnetization was unified to 300 G and the viscosity was unified.
(25 ° C) and heat resistance evaluation (holding the magnetic fluid in an oven at 150 ° C, the time until gelation is 300 hours or more ◎, 100-300 hours △, 100 hours or less X).

The results obtained are shown in Table 2 below. Table 2 Example Viscosity (mPa · S) Heat resistance Example 4 450 ◎ 〃 5 600 ◎ 〃 6 400 ◎ Comparative example 4 900 〃 450 5 450 ×

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Claims (4)

[Claims] An oil-soluble sulfonic acid-based surfactant is added to an organic solvent dispersion of unsaturated higher fatty acid-coated magnetic fine particles having a surface coverage of about 20 to 70%, and the mixture is adsorbed using ultrasonic waves. Recovering and dispersing in a low vapor pressure base oil. 2. An oil-soluble sulfonic acid-based surfactant is added to an organic solvent dispersion of magnetic particles coated with a saturated higher fatty acid having a surface coverage of about 25 to 55%, and the mixture is adsorbed using ultrasonic waves. A method for producing a magnetic fluid, comprising recovering and dispersing in a low vapor pressure base oil. 3. The method for producing a magnetic fluid according to claim 1, wherein the oil-soluble sulfonic acid-based surfactant is a divalent metal salt. 4. Prior to dispersion in a low-vapor-pressure base oil, the precipitate is removed by dispersing in a low-boiling-point hydrocarbon solvent, and after the dispersion in the low-vapor-pressure base oil, the low-boiling hydrocarbon solvent is removed. 3. The method for producing a magnetic fluid according to claim 1, wherein the magnetic fluid is distilled off.