JPH0744101B2 - Water-based magnetic fluid composition and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a water-based magnetic fluid composition obtained by stably dispersing ferromagnetic fine particles in water as a dispersion medium through a dispersant, and a method for producing the same. Regarding, especially utilizing the hydrolysis reaction,
By chemically bonding a dispersant to the surface of ferromagnetic fine particles, a very stable water-based magnetic fluid composition and an efficient production method thereof are provided.

[Conventional technology]

As a conventional magnetic fluid using water as a dispersion medium, Japanese Patent Publication No.
There is one described in Japanese Patent No. 40069. In the aqueous phase, a monomolecular adsorption layer of a surfactant (first surfactant) containing unsaturated fatty acid or its salt as a main component is first formed on the surface of the ferromagnetic oxide fine powder obtained by the wet method. After letting
Further, an anionic or nonionic surface active agent (second surface active agent) is oriented and adsorbed on the second layer to stably disperse the ferromagnetic fine particles in water, thereby forming a water-based magnetic fluid. I will get it.

[Problems to be Solved by the Invention]

However, the conventional magnetic fluid using water as the dispersion medium has various problems as described below.

The adsorption of the second surfactant is physical adsorption, which has a very weak adsorption force, and can be easily desorbed (based on the van der Waals force due to the interaction of the lipophilic groups with the first surfactant). It is easy to cause this, and it is difficult to maintain a stable dispersion of the ferromagnetic fine particles for a long period of time.

Since the solubility of the second surfactant in water changes due to the temperature change, the equilibrium between the excess amount of the second surfactant present in the bulk and the adsorbed amount in the second layer is large in temperature. Dependent. That is, the second surfactant may be desorbed due to the change in temperature, and the dispersion stability of the ferromagnetic fine particles may be deteriorated.

It is difficult to handle because the ferrofluid itself easily foams due to the second surfactant that relies excessively on water.

By mixing a small amount of oil or polar solvent into the magnetic fluid,
Desorption of the surfactant adsorbed on the second layer and further the surfactant adsorbed on the first layer occurs. As a result, the dispersed ferromagnetic fine particles may be precipitated or the magnetic fluid itself may be emulsified.

The present invention has been made by paying attention to such a conventional problem, and a coupling agent having at least one hydrolyzable group together with a hydrophilic group is used as a surfactant. By chemically coupling the coupling agent to the surface of the ferromagnetic fine particles through the reaction product with water, the surface of the ferromagnetic fine particles, which is originally hydrophilic, is coated with a coupling agent that is not easily removable, The object of the present invention is to solve the above-mentioned conventional problems by more stably dispersing in water.

[Means for Solving the Problems]

The water-based magnetic fluid composition of the present invention which achieves the above object is a water-based magnetic fluid composition containing water as a dispersion medium and ferromagnetic fine particles as a dispersoid, and at least one hydrophilic group.
A coupling agent having one hydrolyzable group is chemically bonded to the surface of the ferromagnetic fine particles via a reaction product of the hydrolyzable group.

The hydrolyzable group of the coupling agent is preferably an alkoxy group.

Further, in the method for producing a water-based magnetic fluid of the present invention, water and a coupling agent having at least one hydrolyzable group in addition to a hydrophilic group are added to ferromagnetic fine particles to form a hydrolyzable group. Hydrolyzing, by chemically bonding the coupling agent to the surface of the ferromagnetic fine particles via the reaction product, to obtain an intermediate medium in which ferromagnetic fine particles are uniformly dispersed in an aqueous dispersion medium, The step of separating ferromagnetic fine particles having poor dispersibility in the intermediate medium, and thereafter evaporating and separating the double product of the hydrolysis reaction and at least part of water is included.

Further, the ferromagnetic fine particles have water and at least one hydrolyzable group in addition to the hydrophilic group, and the surface of the ferromagnetic fine particles is chemically bonded through the reaction product of the hydrolyzable group. A step of adding a coupling agent to be bonded, and immediately thereafter heating to a boiling point of water or higher to obtain ferromagnetic fine particles whose surface is coated with the coupling agent; and adding water to the ferromagnetic fine particles to form a mixture. And the step of separating the ferromagnetic fine particles having poor dispersibility in the mixture.

[Action]

The ferromagnetic fine particles, which are dispersoids, have -OH groups on their surfaces. On the other hand, in the coupling agent, the hydrolyzable group on one end side of the molecule is hydrolyzed in water to generate an -OH group. A dehydration condensation reaction is performed between the -OH group generated by this hydrolysis and the -OH group on the surface of the ferromagnetic fine particles. As a result, the coupling agent strongly bonds chemically to the particle surface and covers the particle surface with the hydrophilic group at the opposite end facing outward.
As a result, the ferromagnetic fine particles are extremely stably dispersed in water as a dispersion medium.

Hereinafter, the magnetic fluid composition of the present invention and the method for producing the same will be described in detail.

The dispersant for ferromagnetic fine particles of the present invention is a conventional surfactant containing a polar group acting as a hydrophilic group in the structure and a non-polar group acting as a lipophilic group (hydrophobic group), and is a fine particle. The binding mechanism and the mechanism of action are different.

That is, as the dispersant of the present invention, for example, a silane coupling agent represented by the general formula YRSiX _n is used. Where n in the formula
= 1 to 3 and X is a methoxy group (CH ₃ O-) or an ethoxy group (C ₂
H ₅ O-) hydrolyzable groups consisting of alkoxy group such as, R
Is a hydrocarbon chain of ethylenediamine or the like, and Y is a hydrophilic group composed of a carboxylic acid basic salt, a quaternary ammonium salt, an isothiouronium salt, or the like. However, the hydrophilic group may be any of anionic type, cationic type, and nonionic type, and the number thereof may be one or more.

As such a silane coupling agent, specifically,
For example, the following is preferable.

N-[(3-trimethoxysilyl) propyl] ethylenediamine triacetic acid sodium salt Trimethoxysilylpropyl isothiouronium chloride N-trimethoxysilylpropyl tri-N-butylammonium bromide In the silane coupling agent, the alkoxy group X is hydrolyzed by water in an aqueous solution, air in water, or water adsorbed on the surface of an inorganic material to generate a silanol group, and YRSi (O
H) _n . On the other hand, the ferromagnetic fine particles M have -O on their surface.
It has an H group (M-OH). Therefore, it is considered that a dehydration condensation reaction occurs between the two and they are chemically bonded by a metasiloxane bond (Si-OM).

The addition amount of the coupling agent as the dispersant of the present invention is optimally such that the surface of the ferromagnetic fine particles is completely covered with a monomolecular film. It can be calculated from the surface area of the ferromagnetic fine particles by the following formula.

Amount of coupling agent added = (weight of ferromagnetic fine particles × specific surface area) / minimum coating area of coupling agent Here, the size of the minimum coating area of the coupling agent is, for example, one molecule in the case of a silane coupling agent. The coated area per unit is 13Å ² , so per 1g is (13Å ² ×
6.02 × 10 ²³ ) / determined from the molecular weight. 6.02 × 10 ²³ here
Is the Avogadro number.

In the actual process, an appropriate amount of silane coupling agent is added to the ferromagnetic particles in consideration of the specific surface area of the ferromagnetic particles, the water content, the hydrolyzability of silane, and the difference in the film formation state. Then, the amount of addition is increased or decreased after checking the treatment state.

As the ferromagnetic fine particles of the present invention, those obtained as a colloidal aqueous suspension (slurry) by a well-known wet method may be used. Here, the wet method is a method in which an alkali is added to an acidic solution containing ferrous ions and ferric ions in a ratio of 1: 2, p
A magnetite colloid is obtained by aging at an appropriate temperature and at about H9 or higher. Further, it may be one obtained by a so-called wet pulverization method in which magnetite powder is pulverized by a ball mill in water or an organic solvent. further,
Alternatively, it may be obtained by a dry method.

In addition to magnetite, it is possible to use ferromagnetic fine particles such as manganese ferrite, nickel ferrite, cobalt ferrite, composite ferrite of these and zinc, barium ferrite, or ferromagnetic metal fine particles such as iron or cobalt.

The content of the ferromagnetic fine particles is not only in the range of 1 to 20% by volume ratio that has been generally used in the past, but it can be further increased by producing via an intermediate medium using water as a solvent. It is also possible to adjust the concentration.

In the process of the present invention, first, water and the above-mentioned coupling agent are added to ferromagnetic fine particles to hydrolyze the hydrolyzable group, and the coupling agent is added to the ferromagnetic fine particles via the reaction product. Chemically bond to the surface of to obtain an intermediate medium. Then, the particles with poor dispersibility in this intermediate medium are centrifuged,
Thereafter, water in the intermediate medium and alcohol as a hydrolysis product are removed under reduced pressure and once dried, water as a dispersion medium is added to this to obtain a water-based magnetic fluid having a predetermined concentration. Alternatively, the centrifugally separated intermediate medium may be heated under reduced pressure to evaporate the alcohol as a hydrolysis product and a part of water to obtain a water-based magnetic fluid concentrated to a predetermined concentration.

Alternatively, before the intermediate medium is subjected to a centrifuge, it is heated under reduced pressure to be dried once, and then water is added to the dried particles to form a dispersion solution, which is then subjected to a centrifuge to remove particles having poor dispersibility. It may be separated and removed. In order to obtain a water-based magnetic fluid having a higher concentration, it is also possible to separate and remove particles having poor dispersibility as described above, once again dry it, and then add predetermined water to obtain a high concentration, Alternatively, the water-based magnetic fluid having a high concentration may be obtained by heating and concentrating without drying.

Thus, a water-based magnetic fluid is obtained, in which the coupling agent is bonded to the surface of the ferromagnetic fine particles by a metasiloxane bond with the alkoxy group and the hydrophilic group is coated outward in a single layer.

Examples of the present invention will be described below.

[Example 1] First, 6N NaOH _aq was added to an aqueous solution 1 containing 0.3 mol of ferrous sulfate and 0.3 mol of ferric sulfate, respectively, until the pH reached 11 or more, and then the solution was heated at 60 ° C for 30 minutes. By aging, a magnetite colloidal slurry liquid was obtained. Then, the electrolyte is removed from the slurry by washing with water at room temperature. The above is
This is a step of producing a magnetite colloid by a wet method. The magnetite colloidal solution thus obtained was centrifuged to recover only magnetite, which was dried under reduced pressure at 80 ° C. for 3 hours.

To 5.0 g of the dried magnetite fine particles, as a dispersant, N-[(3-trimethoxysilyl) propyl] ethylenediamineacetic acid sodium salt 3.0 as a silane coupling agent was used.
g and 12.0 g of water were added, and the mixture was pulverized and dispersed for 4 hours using a ball mill. In this way, an intermediate medium was obtained in which the magnetite fine particles whose surface was coated with the silane coupling agent were stably dispersed in water.

This intermediate medium is put into a centrifugal separator and treated for 30 minutes under a centrifugal force of 8000 G, and particles having a relatively large particle size and poor dispersibility among magnetite dispersed particles are sedimented and removed. Next, the supernatant liquid in which the magnetite fine particles remaining without settling are suspended is transferred to a rotary evaporator, and 50
The temperature is kept at ℃ and the methanol and water produced by hydrolysis of the silane coupling agent are removed by evaporation.

Further, it was dried under reduced pressure at 80 ° C. to remove water by evaporation to obtain magnetite fine particles coated with a silane coupling agent. Next, 80 wt% water of the magnetite fine particles was added and redispersed in water. This is further centrifuged and 8000
It was processed for 30 minutes under G centrifugal force. By this centrifugation operation, the non-dispersed solid matter was removed, and the colloidal liquid above it was a very stable ferrofluid.

[Example 2] A magnetite colloid was produced by a wet method in the same manner as in Example 1 above, and further dried under reduced pressure. To 8.0 g of the dried magnetite particles, trimexysilylpropylisothiouronium chloride as a silane coupling agent 7.
6 g and 12.0 g of water were added, and the mixture was pulverized and dispersed for 4 hours using a ball mill. Thus, an intermediate medium was obtained in which the magnetite fine particles whose surface was coated with the silane coupling agent were stably dispersed in water.

Next, transfer this intermediate medium to a rotary evaporator,
Maintaining at 90 ° C, the methanol and water produced by hydrolysis of the silane coupling agent are removed by evaporation.

After that, 20 g of water was added and redispersed, and this was treated with a centrifuge for 30 minutes under the centrifugal force of 8000 G to settle the particles with relatively large particle size of the magnetite dispersed particles with poor dispersibility. Remove at least.

The magnetic fluid obtained was extremely stable.

〔The invention's effect〕

According to the present invention, in order to stably disperse a ferromagnetic material in a water-based dispersion medium, a dispersant has a hydrophilic group on one end side in its molecular structure and a hydrolyzable one on the other end side. A coupling agent having a group was used to chemically bond to the surface of the ferromagnetic fine particles via the reaction product of the hydrolyzable group of the coupling agent. Therefore, unlike the conventional one in which the dispersant is simply physically adsorbed on the surface of the ferromagnetic fine particles,
Even if the temperature is changed, the polar solvent is mixed, or a small amount of foreign matter such as oil is mixed, it is not easily desorbed, and the effect of maintaining a good dispersion state for a long period of time can be obtained.

Further, since the amount of the dispersant added is such an amount that a monomolecular layer is formed on the surface of the ferromagnetic fine particles, the magnetic fluid itself can be easily formed by the surfactant existing excessively in water as in the conventional case. The phenomenon of bubbling is prevented, and the effect of facilitating handling is obtained.

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Claims (4)

[Claims] 1. A water-based magnetic fluid composition containing water as a dispersion medium and ferromagnetic fine particles as a dispersoid, wherein a coupling agent having a hydrophilic group and at least one hydrolyzable group is hydrolyzed. A water-based magnetic fluid composition characterized in that it is chemically bonded to the surface of the above-mentioned ferromagnetic fine particles through a reaction product of a functional group. 2. The water-based magnetic fluid composition according to claim 1, wherein the hydrolyzable group of the coupling agent is an alkoxy group. 3. Ferromagnetic fine particles are hydrolyzed by adding water and a coupling agent having at least one hydrolyzable group in addition to a hydrophilic group to hydrolyze the hydrolyzable group. A step of obtaining an intermediate medium in which the ferromagnetic fine particles are uniformly dispersed in an aqueous dispersion medium by chemically bonding the coupling agent to the surface of the ferromagnetic fine particles via the method, and the dispersibility in the intermediate medium is poor. Separate the ferromagnetic particles, then
A method for producing a water-based magnetic fluid composition, which comprises a step of evaporating and separating the multiple product of the hydrolysis reaction and at least a part of water. 4. The ferromagnetic fine particles have water and at least one hydrolyzable group in addition to a hydrophilic group, and the surface of the ferromagnetic fine particles is formed through a reaction product of the hydrolyzable group. And a coupling agent which chemically bonds to, and immediately thereafter heated to a temperature above the boiling point of water to obtain ferromagnetic fine particles whose surface is coated with the coupling agent; and adding water to the ferromagnetic fine particles. A method for producing a water-based magnetic fluid composition, which comprises a step of forming a mixture and a step of separating ferromagnetic fine particles having poor dispersibility in the mixture.