JPH04373102A - Ferromagnetic metallic fine particle for magnetic recording medium - Google Patents

Abstract

PURPOSE:To enhance the oxidation resistance and to prevent aging of the title ferromagnetic metallic fine particle. CONSTITUTION:The surface of the title ferromagnetic metallic fine particles is processed using multiple organic acids in different acidities so that the organic acids may efficiently react to a hydroxyl groups produced by the chemically adsorbed water containing said metallic fine particles on the surface of an oxide film. At this time, the multiple organic acids in different acidities are to be divided into those contained in the three regions, i.e., the groups No.1, 2 and 3 respectively having the pi charge of hetero atoms of +0.1 to +0.2, +0.2 to +0.4 and +0.4 to +0.7.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to fine ferromagnetic metal particles used as magnetic powder for coated magnetic recording media.

0002

[Prior Art] Generally, magnetic recording media such as magnetic tape are
It is manufactured by applying a magnetic paint consisting of magnetic powder and binder resin onto a non-magnetic support and drying it. recent years,
In the field of magnetic recording, particularly in video tape recorders and the like, higher recording densities are required in order to achieve higher image quality. With this increase in density, iron or iron-based metal materials have come to be used in place of iron oxide materials that have been conventionally used as magnetic powders in magnetic recording media and the like. Recently, in order to meet these demands, products with extremely fine particles have become available, and by using them in the magnetic powder of magnetic recording media, it is possible to achieve higher recording densities and higher performance. Excellent electromagnetic conversion characteristics in the frequency band have been achieved.

[0003] These iron or ferromagnetic metal fine particles composed of iron are iron oxide, iron oxyhydroxide, or Co
, Ni, Mn, Cu, Zn, Ti, V, and other metals other than iron, such as iron oxide or iron oxyhydroxide, are reduced with hydrogen gas. These ferromagnetic metal particles are
It has better magnetic recording properties than conventional iron oxide-based ferromagnetic particles.

However, the above-mentioned ferromagnetic metal fine particles have a high surface activity and are easily oxidized in the atmosphere, which may cause ignition in some cases. These properties tend to become stronger as magnetic powders become finer and finer as the noise of magnetic recording media becomes lower. For this reason, when the above-mentioned ferromagnetic metal fine particles are used as magnetic powder for a magnetic recording medium, it is necessary to use polyester or After being coated onto a non-magnetic support such as a film to form a sheet, oxidation progresses mainly due to the influence of oxygen, certain gases, moisture, etc. during storage under specified conditions such as atmosphere, temperature, humidity, etc. As a result, magnetic properties such as saturation magnetization deteriorate over time, resulting in storage stability problems.

[0005] To solve this problem, in order to stabilize the surface of ferromagnetic metal fine particles, an oxide film is generally formed on the surface of the particles to passivate it using a liquid phase method or a gas phase method. methods have been taken. In addition, methods have been taken in which ferromagnetic metal fine particles are coated with certain metal elements, surfactants, organic substances such as resins, and the like.

[0006]

[Problems to be Solved by the Invention] However, with the method of forming a passive oxide film or the method of covering with a certain metal element or organic substance, the oxidation of ferromagnetic metal fine particles is suppressed, and the magnetic recording characteristics deteriorate over time. It is difficult to say that it is necessarily sufficient to prevent this. Furthermore, depending on the treatment method, surface treatment itself may cause deterioration of magnetic recording properties or may cause a decrease in dispersibility when made into a paint.

The present invention was proposed in view of the above-mentioned circumstances, and provides magnetic recording with excellent oxidation resistance and less deterioration over time by treating the surface of ferromagnetic metal fine particles for magnetic recording media. The purpose of the present invention is to provide ferromagnetic metal fine particles for media.

[0008]

[Means for Solving the Problem] As a result of intensive research to achieve the above object, the present inventors treated the surface of ferromagnetic metal fine particles for magnetic recording media with a plurality of organic acids having different acidities. It was discovered that remarkable storage stability could be achieved by this, and the present invention was completed.

The ferromagnetic metal fine particles for magnetic recording media in the present invention include ferromagnetic metal materials such as Fe, Co, and Ni, as well as Fe-Co, Fe-Ni, Fe-Co-Ni, and Co.
-Ni, Fe-Mn-Zn, Fe-Ni-Zn, Fe-
Co-Ni-Cr, Fe-Co-Ni-P, Fe-Co
-B, Fe-Co-Cr-B, Fe-Co-V, etc.
, Co, and Ni as the main components.Al, Si, Ti, Cr, Mn, and Cu are also used to improve these various properties.
, Zn, Mg, P, or other elements may be added. Although the specific surface area of these ferromagnetic metal fine particles is arbitrary, it is most effective when applied to particles with a specific surface area of 25 m2/g or more, particularly 30 m2/g or more.

Generally, the acidity of an organic acid is often expressed by the degree of dissociation of its protons as an acid. However, this degree of acid dissociation increases uncertain factors such as problems with ionic strength, problems when changing the solvent, and multistage acid dissociation constants associated with sequential dissociation of polyfunctional acids. Therefore, in the present invention, the π charge determined by the simple LCAO method is employed as a parameter representing the acidity of the aromatic organic acid. This molecular orbital method is generally called the Huckel approximation, and we have already determined that the anticorrosion performance of the rust inhibitor based on this molecular orbital calculation is based on the π charge of the heteroatom that directly bonds with the metal ion in the oxide layer on the surface of the ferromagnetic metal fine particle. It has been found that there is a close correlation between

[0011] In the surface oxidation layer of ferromagnetic metal fine particles, surface hydroxyl groups derived from chemically adsorbed water are generally present, and it is known that chemisorbed water that has been adsorbed to six-coordinated metal ions has a strong basicity. There is. Organic acids are chemically adsorbed onto this basic hydroxyl group through a dehydration reaction. This indicates that the 3690 cm -1 O
This is also clear from the fact that -H stretching vibration disappears. Therefore, when pyrocatechol is used as the organic acid, the reaction between the organic acid and the oxide film on the surface of the ferromagnetic metal particles is expressed by the following equation (1).

0012

[Chemical formula 1]

[0013] Here, when the functional group of the organic acid that effectively exhibits antirust ability is an aromatic hydroxyl group, it is desirable that the organic acid be difunctional and exist in the ortho position. This means that, as seen in the case of catechol shown above, the distance between the two oxygen atoms in catechol is approximately equal to the distance between the iron atoms in the oxide film lattice of the ferromagnetic metal particles, and the valence state is reasonable. This originates from the chemical adsorption reaction that occurs in the form of When the functional group is an aromatic carboxyl group, for example, when it has a naphthalene skeleton, it is an ortho-type difunctional compound such as 2,3-naphthalic acid, or a cross-linked carbon compound such as 1,8-naphthalic acid. Atoms may be sandwiched between them. This is because the carboxyl group has a more flexible structure than the hydroxyl group.

The above-mentioned 2,3-naphthalic acid and 1,8-naphthalic acid have the structural formula shown in the following chemical formula 2.

[0015]

[Case 2]

As already reported, the relationship between the π charge on a heteroatom, which is a parameter of the acidity of an organic acid, and its rust-preventing ability makes it possible to effectively design the molecules of a rust-preventing agent. (For example, see Japanese Patent Application No. 2-217226.) The acidity parameter referred to in the present invention follows exactly the same method. In other words, among the above acid functional groups such as hydroxyl groups and carboxyl groups, the π charge on the heteroatom to which the proton that dissociates as an acid is bonded is calculated by simple LCA.
It is calculated by the molecular orbital method of the O method (π electron approximation), and this π charge is used as an index of the acidity of the organic acid used as a treatment agent. This π charge is the value obtained by subtracting the diagonal term (charger density on the atom) of the bond order matrix obtained in molecular orbital calculation from the number of electrons that the heteroatom contributes to the π-electron conjugated system. Generally, the larger the positive value, the stronger the acidity. In this way, if the above-mentioned π charge is used as an index of the acidity of an organic acid, it becomes possible to express the acidity of divalent or higher organic acids such as catechol with one parameter,
The complexity of using multiple acid dissociation constants due to sequential dissociation is eliminated. Therefore, it becomes possible to easily evaluate the correlation between the acidity of the organic acid and the oxidation resistance of the ferromagnetic metal fine particles surface-treated with the organic acid.

[0017] In the case where there are a plurality of heteroatoms to which acid-dissociable protons are directly bonded, such as in divalent or higher-valent organic acids, the one with greater acidity is used as the index. This is determined from the fact that even in organic acids with equivalent acidity, the first dissociation is generally larger than the second dissociation. The Coulomb and resonance integral values used in the calculation are disclosed in, for example, Japanese Patent Application No. 2-217226.

FIG. 1 shows the relationship between the π charge determined by the simple π electron approximation LCAO molecular orbital method and the rate of decrease Δσs in the saturation magnetization σs of ferromagnetic metal fine particles two weeks after surface treatment with an organic acid. The rate of decrease in saturation magnetization is shown in (2) below.
It was evaluated using the formula.

[0019]

[Math 1]

[0020] Here, in order to make the decrease in saturation magnetization noticeable for two weeks, the surface-treated ferromagnetic metal fine particles were placed at a humidity of 90% and a temperature of 60°C. The vertical axis of the correlation between the π charge on the heteroatom and Δσs shown in FIG. 1, that is, the deterioration rate of saturation magnetization, depends on the micropores of the surface oxide layer of the ferromagnetic metal fine particles. That is, there is a tendency that metal fine particles having a larger specific surface area have a larger reduction rate. However, the correlation curve itself holds true for almost all ferromagnetic metal particles.

FIG. 1 shows that the aging deterioration of saturation magnetization due to oxidation of ferromagnetic metal fine particles depends on the π charge on the heteroatom of the organic acid, and as this value increases, Although the rate of decrease in saturation magnetization over time decreases, once a certain value is exceeded, the rate of deterioration in saturation magnetization tends to increase. Therefore, by regulating the acidity of the organic acid represented by the π charge, it is possible to suppress the rate of decrease in saturation magnetization and improve the oxidation resistance of the ferromagnetic metal fine particles. When the correlation shown in FIG. 1 was confirmed using various ferromagnetic metal fine particles, it was found that the optimum value of π charge, which is generally an index of acidity, is about +0.3.

As mentioned above, an oxide film is generally formed on the surface of ferromagnetic metal fine particles for stabilization.
However, this oxide film formation is accompanied by volume expansion, and the oxide film has a markedly irregular structure, unlike a well-ordered oxide lattice. Therefore, it is easily inferred that the hydroxyl groups derived from chemically adsorbed water present on the oxide film surface of the ferromagnetic metal fine particles should also have a distribution in acidity and basicity due to this structural asymmetry.

In view of the above points, in the present invention, the surface of the ferromagnetic metal fine particles as described above is treated with a plurality of organic acids having different acidities, and the chemically adsorbed water on the oxide film surface of the ferromagnetic metal fine particles is treated. It was discovered that the derived hydroxyl groups can be efficiently reacted with an organic acid, thereby achieving even more effective oxidation resistance. That is, the π charge of the heteroatom in FIG. 1 is set to +0.1 to +0.2, +0.
The organic acid is divided into three regions: 2 to +0.4 and +0.4 to +0.7, and the organic acid is divided into those included in these three regions according to the value of the π charge on its heteroatom. π charge is +0
．． 1 to +0.2 organic acids in the first group, +0.2 to +
The values of 0.4 are classified as the second group, and the values of +0.4 to +0.7 are classified as the third group. Regarding the organic acids contained in each group, please refer to Japanese Patent Application No. 2-2172.
It is disclosed in the specification of No. 26. To name a few representative ones, the first group includes orthodithiol, catechol, 2,3-dihydroxynaphthalene, etc., and the second group includes 2,3- and 1,8-naphthalene dicarboxylic acids. , many catechols and nitro derivatives of 2,3-dihydroxynaphthalene, and the third group includes phthalimide, phthalhydrazide, and derivatives of these compounds into which an amino group or a methoxy group is introduced.

[0024] As a method for surface-treating the ferromagnetic metal fine particles with the above organic acid, for example, a method of immersing the ferromagnetic metal fine particles in a treatment liquid dissolved in an organic solvent can be mentioned. In this case, the solvent for the organic acid is not particularly limited, but any of water, alcohol solvents such as ethanol, ketone solvents such as acetone, and aromatic solvents such as toluene can be used.

The amount of these three groups of organic acids deposited on the metal fine particles is desirably 0.03 to 30 parts by weight per 100 parts by weight of the ferromagnetic metal fine particles.
．． More preferably, it is 1 to 10 parts by weight. Even if the organic acid is present in excess beyond the above range, the effect remains unchanged and the excess amount is wasted. Furthermore, if too much of the coating is applied, there is a possibility that the physical properties of the magnetic coating film of the magnetic recording medium will be adversely affected. Conversely, if it falls below the above range, that is, 0
．． If the amount is less than 0.3 parts by weight, the effect will be insufficient and sufficient stability over time will not be obtained.

The metal fine particles for magnetic recording media of the present invention can be made into a magnetic paint together with a binder, an organic solvent, and various additives, and a magnetic recording medium can be produced by coating this magnetic paint on a non-magnetic support. be done. In this case, the binder, organic solvent, and various additives used in ordinary magnetic recording media can all be used, and the compounding ratio and the like are set in accordance with those for ordinary magnetic recording media.

[0027]

[Action] 2,2 is a typical compound that forms complexes with metals.
When the surface of ferromagnetic metal particles is treated with '-bipyridyl or 9,10-phenanthroline, the retention rate of saturation magnetization is almost the same as in the untreated case, although a large amount of these compounds are adsorbed to the surface of the metal particles. The results are about the same, and the oxidation resistance effect does not appear at all. On the other hand, it has been confirmed that the organic acids used in the present invention having acidity within a certain range generate water in the treatment reaction. This is because the adsorption reaction between the treatment agent and the surface of the ferromagnetic metal fine particles is a dehydration type, and the heteroatom of the organic acid as the treatment agent is directly bonded to the surface of the ferromagnetic metal fine particle (e.g. iron). It suggests that there is. This matter has been described above.

Therefore, in surface treatment with an organic acid having acidity within a certain range, oxidation resistance is not obtained due to its water repellency, but rather due to the Fe-OH present on the surface of the ferromagnetic metal particles and the treatment agent. It is thought that the organic acids form strong ionic bonds, and the potential of this bond formation affects the internal structure and contributes to improving oxidation resistance.

That is, when the surface of ferromagnetic metal fine particles is treated with an organic acid having acidity within a certain range, a dehydration reaction occurs between the organic acid and the surface of the ferromagnetic metal fine particles, and the organic acid is applied to the surface of the ferromagnetic metal fine particles. A film is formed. This organic acid coating prevents an oxidation reaction on the surface of the ferromagnetic metal particles that occurs during storage of the ferromagnetic metal particles, preparation of a magnetic paint containing the ferromagnetic metal particles, storage of a magnetic recording medium, and the like. In addition, the organic acids used in the present invention are a mixture of acids having different acidities, and react effectively with hydroxyl groups derived from chemisorbed water having different basicities due to structural irregularities on the surface of the ferromagnetic metal fine particles. Formation of an organic acid film on the surface of the ferromagnetic metal fine particles progresses more effectively, thereby realizing high oxidation resistance.

[0030]

EXAMPLES Next, the present invention will be explained with reference to Examples, but it goes without saying that the present invention is not limited to these Examples. Table 1 below shows the names of the compounds of the first to third groups used in the examples and the compounds used in the comparative examples. As mentioned above, the total amount of these compounds is 2.5 x 10-3m
ol, but in all Examples, the first group of compounds was 2
0%, the second group compound is 70%, and the third group compound is 10%.

[0031]

[Table 1]

Add 2. ferromagnetic metal fine particles to 50 ml of this solution.
5g was added and left to stand for 2 hours. Thereafter, the mixture was filtered using a membrane filter and washed repeatedly with methanol to remove physically adsorbed organic acids. The obtained ferromagnetic metal fine particles were vacuum dried for 8 hours while being kept at 30° C. to obtain a treated powder. The coercive force Hc and saturation magnetization σs of the obtained treated powder were measured immediately after drying. Then humidity 90
%, after being left for 2 weeks in a constant temperature and humidity chamber maintained at a temperature of 60° C., the same measurement was performed again to evaluate the amount of decrease over time. As a comparative example, ferromagnetic metal fine particles that were not subjected to surface treatment but were washed with methanol and then dried were also examined for changes in magnetic properties over time using the method described above. Here, Δσs and ΔHc are used as indicators representing the decrease in saturation magnetization and coercive force during the two-week storage period. Note that these Δσs,
ΔHc is expressed by the following equations (3) and (4). The results are shown in Table 2.

[0033]

[Math 2]

[0034]

[Table 2]

[0035]

[Effects of the Invention] As is clear from the examples, the treatment significantly reduces the aging deterioration of saturation magnetization compared to untreated magnetic powder, and at the same time, the coercive force is kept stable. Therefore, according to the present invention, the ferromagnetic metal magnetic powder for magnetic recording media has significantly improved stability of magnetic properties over time and storage stability.

[Brief explanation of the drawing]

[Figure 1] π charge on heteroatoms of organic acids and after surface treatment 2
Decrease rate Δ of saturation magnetization σs of ferromagnetic metal fine particles after a week
FIG. 3 is a characteristic diagram showing the relationship between σs.

Claims (1)

[Claims] 1. Ferromagnetic metal fine particles for use in magnetic recording media, the surfaces of which are treated with a plurality of organic acids having different acidities.