JP5798915B2 - Magnetic particle manufacturing method and magnetic recording medium manufacturing method - Google Patents

Description

  The present invention relates to a magnetic particle, a method for producing the same, and use thereof, and more particularly, to a magnetic particle having a surface modified so as to improve dispersibility in an organic solvent-based magnetic paint, a method for producing the same, and a method for producing the same. It is about use.

  In recent years, means for transmitting information at a high speed have remarkably developed, and it has become possible to transfer images and data having enormous information. Along with the improvement of this data transfer technique, recording and reproducing devices and recording media for recording, reproducing and storing information are required to have higher density recording. In order to obtain good electromagnetic conversion characteristics in the high-density recording area, use fine magnetic particles, highly disperse fine magnetic materials, and increase the smoothness of the magnetic layer surface to reduce spacing loss. Is known to be effective.

As a means for enhancing the dispersibility of the magnetic material, for example, as described in Patent Document 1, a method of incorporating a polar group such as SO ₃ Na group into the binder, as described in Patent Document 2 A method of using a dispersant as an additive when preparing a coating solution for forming a magnetic layer is widely used.

Japanese Patent Laid-Open No. 2003-132931 JP-A-1-232530

  As described in Patent Document 1, introduction of a polar group into a binder is an effective means for improving dispersibility. However, if the amount of polar group introduced into the binder becomes excessive, conversely Dispersibility may be reduced. On the other hand, the dispersant as described in Patent Document 2 improves the dispersibility of the magnetic particles in the coating solution for forming the magnetic layer by adsorbing on the surface of the magnetic particles and increasing the affinity between the magnetic particles and the binder or solvent. To do. However, even if fine magnetic particles having a small primary particle diameter are used, if the fine magnetic particles are strongly aggregated and added to the coating liquid for forming the magnetic layer as coarse aggregates, a strong dispersion load is applied to the magnetic material. Unless the agglomeration state of the body is crushed, the dispersant only covers the surface of the agglomerate, and as a result, the magnetic substance exists as coarse agglomerates in the formed magnetic layer, resulting in high surface smoothness. It cannot be realized. On the other hand, it may be possible to strengthen the dispersion conditions in order to break up the aggregates in which the fine magnetic particles are strongly aggregated, but this will lengthen the dispersion process, and the dispersion media may be worn due to wear. This is not preferable because it may cause a decrease in magnetic properties due to contamination by foreign matter or destruction of the magnetic material.

  Therefore, an object of the present invention is to provide means for easily forming a magnetic layer in which magnetic particles are present in a highly dispersed state.

While the inventors have repeatedly studied to achieve the above object, organic solvents are mainly used for the coating liquid for forming the magnetic layer in the production of the coating type magnetic recording medium, whereas the magnetic layer is magnetic. It was noted that the particles can be present in a highly dispersed state by the repulsive force between the positive charges because the particles are positively charged by adsorbing protons in the acidic aqueous solvent. Therefore, if a compound that can act as a dispersant in an organic solvent-based coating solution is deposited on the surface of the magnetic particles in an acidic aqueous solvent before the coating solution is prepared (if the surface is modified), coarse aggregation is caused. It is possible to obtain magnetic particles coated with a dispersant in the form of fine magnetic particles rather than aggregates. By using the magnetic particles thus obtained, it is possible to obtain a coating solution for forming a magnetic layer capable of forming a magnetic layer in which fine magnetic particles are present in a highly dispersed state.
Based on the above points, the inventors of the present invention have made further studies and have completed the present invention.

That is, the above object has been achieved by the following means.
[1] A method for producing surface-modified magnetic particles,
An aromatic compound having a polar group selected from the group consisting of a hydroxyl group and its alkali metal base is added to an aqueous magnetic liquid in which magnetic particles are dispersed in an acidic aqueous solvent (provided that a plurality of aromatic rings are connected via a linking group). In the case of the aromatic compound bonded to each other, the polar group is contained only in one of the two rings to be linked.), And then the magnetic particles are collected from the aqueous magnetic liquid. The production method according to claim 1, wherein surface-modified magnetic particles are obtained by adhering.
[2] The production method according to [1], wherein the compound is an aromatic compound having one or two polar groups.
[3] The production method according to [1] or [2], wherein the aromatic ring contained in the aromatic compound is a benzene ring or a naphthalene ring.
[4] The method according to any one of [1] to [3], wherein the magnetic particles are hexagonal ferrite magnetic particles.
[5] Magnetic particles obtained by the production method according to any one of [1] to [4].
[6] The magnetic particle according to [5], which is used as a magnetic powder for magnetic recording.
[7] A magnetic paint comprising the magnetic particles according to [5] or [6] and an organic solvent.
[8] The magnetic paint according to [7], further comprising a binder.
[9] The magnetic paint according to [7] or [8], which is used for forming a magnetic layer of a magnetic recording medium.
[10] A magnetic coating is prepared by dispersing the magnetic particles according to [ 5 ] or [ 6 ] together with an organic solvent and a binder, and a magnetic layer is formed using the prepared magnetic coating. A method for manufacturing a magnetic recording medium.
[11] A magnetic recording medium obtained by the method according to [10].

ADVANTAGE OF THE INVENTION According to this invention, the magnetic recording medium which can exhibit the outstanding electromagnetic conversion characteristic with which the fine particle magnetic body was disperse | distributed highly can be provided.
Furthermore, since the dispersion of the fine magnetic particles becomes easy, the dispersion load of the magnetic particles in the production process can be greatly reduced.

The present invention relates to a method for producing surface-modified magnetic particles. The method for producing magnetic particles of the present invention comprises an aromatic compound having a polar group selected from the group consisting of a hydroxyl group and an alkali metal base thereof in an aqueous magnetic liquid in which magnetic particles are dispersed in an acidic aqueous solvent (hereinafter, (Also referred to as “surface modifier.” However, in the case of an aromatic compound in which a plurality of aromatic rings are bonded via a linking group, only one of the two rings to be linked contains the polar group.) And then collecting the magnetic particles from the aqueous magnetic liquid to obtain magnetic particles whose surface has been modified by the deposition of the compound. Moreover, according to this invention, the magnetic particle obtained by the said manufacturing method is also provided. The magnetic particles of the present invention are highly dispersible in a coating solution for forming a magnetic layer of a magnetic recording medium, and are suitable as magnetic powder for magnetic recording.
In a magnetic recording medium, the presence of magnetic particles in the magnetic layer in a fine particle state leads to an improvement in electromagnetic conversion characteristics. Therefore, in order to improve electromagnetic conversion characteristics, it is preferable that fine magnetic particles are present in the magnetic layer in a highly dispersed state. For this purpose, the fine magnetic particles should be highly dispersed in the magnetic layer forming coating solution. Here, the magnetic particles obtained by the production method of the present invention are surface-modified by adhering the surface modifier described in detail below to the surface, and the organic solvent is obtained by the surface modification. It can exist in a highly dispersed state in the magnetic paint of the system. That is, according to the present invention, magnetic particles capable of forming a magnetic layer in which fine magnetic particles are highly dispersed can be obtained. In addition, as shown in the examples described later, by improving the dispersibility by the presence or absence of the following surface modification treatment, the surface of the magnetic particle is modified by the surface modifier adhering to the surface of the magnetic particle. Can be confirmed. In addition, it is known that when the dispersion conditions are strengthened to improve dispersibility, a phenomenon that dispersed particles re-aggregate (this is called overdispersion) occurs. It is also possible to suppress the occurrence of overdispersion by modifying.
Hereinafter, the method for producing magnetic particles of the present invention will be described in more detail.

Preparation of aqueous magnetic liquid The aqueous magnetic liquid refers to a liquid in which the magnetic particles are dispersed and not precipitated and aggregated to such an extent that separation between the solid phase and the liquid phase is not visually confirmed. In the first embodiment, it can be obtained by adding or mixing an aqueous solvent, magnetic particles (hereinafter also referred to as “raw material magnetic particles”) and an acid component simultaneously or sequentially. The raw material magnetic particles preferably have an average primary particle size of 35 nm or less from the viewpoint of being suitable for forming a magnetic recording medium for high density recording. Here, the average primary particle size of the magnetic particles is a value measured by the following method.
Magnetic particles are photographed at a photographing magnification of 100,000 using a Hitachi transmission electron microscope H-9000, and printed on photographic paper so that the total magnification is 500,000 times to obtain a particle photograph. The target magnetic particle is selected from the particle photograph, the outline of the particle is traced with a digitizer, and the particle size is measured with the image analysis software KS-400 manufactured by Carl Zeiss. The size of 500 primary particles is measured. Primary particles refer to independent particles without agglomeration. The arithmetic average value of the particle size measured by the above method is defined as the average primary particle size of the magnetic particles.
The average primary particle size is preferably 10 nm or more from the viewpoint of obtaining stable magnetization without thermal fluctuation. From the viewpoint of achieving both magnetization stability and high-density recording, the average particle size is preferably in the range of 10 to 35 nm, and more preferably in the range of 20 to 35 nm. However, even if the raw magnetic particles are fine particles, if they are present as aggregates in the magnetic layer, the aggregates behave like a single coarse particle, making it difficult to improve electromagnetic conversion characteristics. Therefore, in the present invention, in order to be able to form a magnetic layer in which magnetic particles are highly dispersed, treatment for modifying the surface of magnetic particles with a surface modifier in an aqueous solvent (surface modification treatment) as described later. )I do. This process will be described later.

In the present invention, the size of particles or powders (hereinafter referred to as “particle size”) such as magnetic particles is as follows. (1) The shape of the particles is needle-like, spindle-like, columnar (however, the height is the bottom surface). In the case of larger than the maximum major axis, etc., it is represented by the length of the major axis constituting the particle, that is, the major axis length, and (2) the shape of the particle is plate or columnar (however, the thickness or height is the plate surface) Or smaller than the maximum major axis of the bottom surface) (3) The shape of the particle is spherical, polyhedral, unspecified, etc., and the particle is composed of the shape. If the major axis cannot be specified, it is represented by the equivalent circle diameter. The equivalent circle diameter is a value obtained by a circle projection method.
Further, the average particle size of the particles is an arithmetic average of the particle sizes, and is obtained by carrying out the measurement as described above for 500 primary particles.

  As the raw material magnetic particles that can be used in the first embodiment, various ferromagnetic particles such as hexagonal ferrite magnetic particles and ferromagnetic metal magnetic particles that are generally used in the magnetic layer of a magnetic recording medium can be used.

  The aqueous solvent is a solvent mainly composed of water, and water or water and a water-soluble organic solvent such as methanol, ethanol, acetone, N, N-dimethylformamide, N, N-dimethylacetamide, tetrahydrofuran and the like. Can be mentioned.

  The acid component may be an inorganic acid or an organic acid. Examples of the inorganic acid that can be used include hydrochloric acid, nitric acid, and the like. Examples of the organic acid include formic acid and acetic acid. Of these, acetic acid is preferably used from the viewpoint of ease of handling, and hydrochloric acid, which is a strong acid, is preferably used from the viewpoint that pH adjustment is possible with a small amount. By sufficiently charging the surface of the magnetic particles, the magnetic particles can be highly dispersed in the aqueous magnetic liquid. From this point, the amount of acid used is an amount that can adjust the pH of the aqueous magnetic liquid to 5 or less. Preferably, the amount can be adjusted to 4 or less. Moreover, since the magnetic particle surface may melt | dissolve on strongly acidic conditions, it is preferable that the said pH shall be 3 or more.

  The aqueous magnetic liquid can be obtained by adding and mixing an aqueous solvent, raw material magnetic particles, and an acid component simultaneously or sequentially, and preferably stirring. In addition, in order to enhance the magnetic properties of magnetic particles, the step of dispersing the magnetic particles in an acidic aqueous solvent (dispersing in an acidic aqueous solvent and collecting the solid content by solid-liquid separation) is performed multiple times. Is preferred. This is because the foreign matter causing the deterioration of the magnetic properties can be removed from the particle surface by the treatment with the acid. This point will be further described. For example, glass components that could not be removed by acid treatment may remain in hexagonal ferrite magnetic particles obtained by a glass crystallization method described later. Such a residual glass component may degrade the magnetic properties of the magnetic particles, but can be dissolved and removed from the particle surface by repeating the dispersion treatment in an acidic aqueous solvent. It is known that the electrolyte concentration (ion concentration) affects the dispersion / aggregation state in a water slurry containing charged particles. When the electrolyte concentration is low, the electric double layer spreads around the charged particles, and a repulsive force acts so as to avoid the overlapping of the electric double layers, and the charged particles are dispersed and do not settle easily. On the other hand, when a large amount of ionic components are present in the water slurry, the ion balance in the system is lost, so that dispersion stabilization by the electric double layer is hindered. As a result, the magnetic particles settle in a relatively short time. Therefore, the more foreign matter dissolved and ionized by dispersion treatment in an acidic aqueous solvent, the more ionic components in the system, so the dispersion stability of the magnetic particles decreases.The less foreign matter, the more stable and stable the magnetic particles are. It will not settle easily. From this point, repeating the dispersion treatment in an acidic aqueous solvent so that the magnetic particles do not settle easily is a preferable measure for removing foreign substances from the surface of the magnetic particles and improving the magnetic properties of the magnetic particles. In order to coat the surface of the highly dispersed magnetic particles with the surface modifier, it is preferable to repeat the dispersion treatment in an acidic aqueous solvent.

  On the other hand, a glass crystallization method is known as a method for producing hexagonal ferrite magnetic particles widely used as magnetic powder for magnetic recording media. General glass crystallization methods are as follows: (1) Dissolving raw material mixture containing glass forming component and hexagonal ferrite forming component → (2) Rapid solidification → (3) Heat treatment of solidified product (hexagonal ferrite magnetic particles are precipitated) ) → (4) Acid treatment of the solidified product (dissolving and removing the glass component) → (5) Washing with an aqueous solvent is included. Since the step (4) is usually performed in an acidic aqueous solvent, an aqueous magnetic liquid containing magnetic particles (hexagonal ferrite magnetic particles) is necessarily obtained. The aqueous magnetic liquid obtained here is normally in a state where the hexagonal ferrite magnetic particles are not settled and dispersed as they are. In the present invention, by utilizing this dispersed state, the surface modification treatment described later can be performed on the aqueous magnetic liquid obtained in the acid treatment step (4). That is, in the present invention, the aqueous magnetic liquid is obtained in the step of obtaining hexagonal ferrite magnetic particles by the glass crystallization method, specifically, in the acid treatment step of the glass crystallization method (hereinafter referred to as “second method”). Also referred to as "embodiment"). The above-described steps of the glass crystallization method in the second aspect are known techniques described in JP 2010-282671 A, JP 2010-235411 A, JP 2010-080608 A, and the like. Can be applied.

Surface Modification Treatment In the present invention, the following surface modifier is added to the aqueous magnetic liquid obtained by the first or second aspect. In the aqueous magnetic liquid, since the magnetic particles can exist in a highly dispersed state due to the repulsive force between the positive charges as described above, the surface modification is not performed on the surface of the magnetic particles in the state of the fine particle magnetic body instead of the coarse aggregate. It becomes possible to deposit a quality material (surface modification). Although the surface is coated with a surface modifier, the repulsive force due to the positive charge between the magnetic particles may be reduced and the magnetic particles may be aggregated. However, the aggregate formed here is easily dissolved in an organic solvent. It can be crushed. This is because the surface of the magnetic particles is coated with a surface modifier that can achieve improved dispersibility in an organic solvent.

The compound used as the surface modifier in the present invention is an aromatic compound having a polar group selected from the group consisting of a hydroxyl group and its alkali metal base. Here, the alkali metal base of the hydroxyl group (—OH) is a monovalent substituent represented by —OM (M represents an alkali metal atom). However, among aromatic compounds in which a plurality of aromatic rings are bonded via a linking group, it is difficult to achieve improved dispersibility when the polar group is included in both of the two rings to be linked. . Therefore, as an aromatic compound in which a plurality of aromatic rings are bonded via a linking group, a compound containing the polar group only in one of the two rings to be linked is used.
The above compound can achieve improvement in the dispersibility of the magnetic particles in an organic solvent or in an organic solvent containing a binder (that is, in a coating solution for forming a magnetic recording medium) because of the adsorption functional group. The present inventors speculate that the presence of the polar group allows the surface to be adsorbed on the surface of the magnetic particle and the surface of the magnetic particle can be modified so that the organic solvent and / or the binder can easily spread. .
Hereinafter, the compound will be described in more detail.

  The aromatic ring contained in the aromatic compound having a polar group may be an aromatic hydrocarbon ring, an aromatic heterocyclic ring, or a condensed ring. From the viewpoint of availability, an aromatic hydrocarbon ring is preferable. Examples of the aromatic ring contained in the aromatic compound include a benzene ring and a naphthalene ring. Two or more aromatic rings may be bonded via a divalent linking group such as an alkylene group. However, in this case, as described above, those containing the polar group only in one of the two rings to be connected are used.

In the aromatic compound, the polar group may be directly substituted with an aromatic group or may be substituted via a divalent linking group such as an alkylene group. Further, the aromatic group may or may not have a substituent other than the polar group. Specific examples of the substituent include an alkyl group (for example, an alkyl group having 1 to 6 carbon atoms), an alkoxyl group (for example, an alkoxyl group having 1 to 6 carbon atoms), a halogen atom (for example, a fluorine atom, a chlorine atom, a bromine atom), A cyano group, an amino group, a nitro group, an acyl group, etc. can be mentioned. Hereinafter, unless otherwise specified, for the “substituent” in the present invention, the description of the substituent can be referred to.
In the present invention, the “carbon number” of a group having a substituent means the carbon number of a portion not containing the substituent. Further, in the present invention, “to” indicates a range including numerical values described before and after that as a minimum value and a maximum value, respectively.

  As described above, the surface modifier can be adsorbed on the surface of the magnetic particles and modified on the surface by containing the polar group. As the proportion of the portion other than the polar group in the molecule increases, the number of molecules of the surface modifier adsorbed on the surface of the magnetic particles decreases. It is preferable that the proportion of the portion other than the polar group is not large. In this respect, the surface modifier preferably has a molecular number of the portion excluding the polar group of 400 or less, for example, about 200 or less. Moreover, it is preferable from the point of the surface modification effect that the number of molecules of the part except the said polar group is 50 or more.

  Any of the surface modifiers described above can be synthesized by a known method, or can be obtained as a commercial product. The addition amount of the surface modifier to the aqueous magnetic liquid in which the magnetic particles are dispersed in the acidic aqueous solvent is 0.1 to 10 parts by mass with respect to 100 parts by mass of the magnetic particles, and the total mass of the aqueous magnetic liquid is 100 parts by mass. From 0.0001 to 5 parts by mass, it is preferable to obtain magnetic particles that can easily be in a highly dispersed state in an organic solvent. The surface modifier used here may be used alone or in combination of two or more. In addition, it is preferable to perform a stirring process when the surface modifier is added to the aqueous magnetic liquid or after the addition in order to uniformly deposit the surface modifier on the magnetic particle surfaces.

  By adding a surface modifier and performing a stirring treatment as necessary, the surface modifier can be deposited on the surface of the magnetic particles. When the magnetic particles aggregate and settle due to the surface modification in this way, the surface-modified magnetic particles can be taken out of the aqueous solvent by removing the supernatant. Even if the magnetic particles taken out are agglomerated, the agglomerated state can be easily crushed in an organic solvent because the surface of each of the magnetic particles constituting the agglomerate is coated with a surface modifier. is there. Therefore, a highly dispersed state can be realized in an organic solvent-based magnetic coating without increasing the dispersion load. Alternatively, when the magnetic particles are dispersed in the aqueous magnetic liquid even after the surface modification treatment, the surface-modified magnetic particles are taken out from the aqueous solvent by a known solid-liquid separation means such as centrifugation or filtration. be able to. The magnetic particles taken out can be obtained as a dry powder by subjecting them to washing by decantation or the like in an aqueous solution adjusted to a slightly acidic to neutral pH by adding a base as necessary. In the second aspect, the step of obtaining the magnetic particles after the surface modification treatment can be performed in the same manner as or in accordance with the washing step in the ordinary glass crystallization method. In both the first and second embodiments, the surface-modified magnetic particles can be obtained in this way.

  Furthermore, this invention relates to the magnetic coating material containing the magnetic particle and organic solvent of this invention. As described above, the magnetic layer forming coating liquid for magnetic recording media is required to have highly dispersed magnetic particles in order to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics. The magnetic particles of the invention are highly dispersible in organic solvents. Therefore, the magnetic coating material of the present invention can be suitably used for forming a magnetic layer of a magnetic recording medium. In addition, the magnetic coating material of this invention may contain the binder (resin component) with the magnetic particle and organic solvent of this invention. Details thereof will be described later.

  Next, a method for manufacturing the magnetic recording medium of the present invention will be described.

The method for producing a magnetic recording medium of the present invention includes producing a magnetic paint by dispersing the magnetic particles of the present invention together with an organic solvent and a binder, and forming a magnetic layer using the produced magnetic paint. Moreover, according to this invention, the magnetic recording medium obtained by the said method is also provided.
The method for producing magnetic particles of the present invention is carried out in an aqueous solvent. However, if a magnetic layer is formed using a magnetic paint containing an aqueous solvent, the formed magnetic layer becomes hydrophilic, and thus there are adverse effects such as plasticization due to moisture absorption. There is a concern that this will occur. On the other hand, in the method for producing a magnetic recording medium of the present invention, an organic solvent-based magnetic paint is used to form a magnetic layer. Here, the organic solvent refers to a non-aqueous organic solvent, but it is allowed to leave a trace amount of water in the magnetic coating material to such an extent that no adverse effects such as plasticization occur.

  Examples of the organic solvent used for preparing the magnetic coating material include organic solvents generally used for producing a coating type magnetic recording medium. Specifically, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, isophorone, tetrahydrofuran, etc., methanol, ethanol, propanol, butanol, isobutyl alcohol, isopropyl alcohol, methylcyclohexanol, etc. at an arbitrary ratio Alcohols, methyl acetate, butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, glycol acetates such as glycol acetate, glycol dimethyl ether, glycol monoethyl ether, dioxane and other glycol ethers, benzene, toluene, xylene, cresol, chlorobenzene Aromatic hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin Chlorinated hydrocarbons such as dichlorobenzene, N, N- dimethylformamide, may be used hexane. Among these, it is preferable to use an organic solvent containing a ketone (ketone-based organic solvent) from the viewpoint of the solubility of the binder usually used in magnetic recording media and the adsorption of the binder to the surface of the magnetic particles.

  The organic solvent is not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, by-products, decomposition products, oxides, and moisture in addition to the main components. These impurities are preferably 30% by mass or less, more preferably 10% by mass or less. In order to improve the dispersibility, it is preferable that the polarity is somewhat strong, and it is preferable that 50% by mass or more of the solvent having a dielectric constant of 15 or more is included in the solvent composition. Moreover, it is preferable that a solubility parameter is 8-11.

  The magnetic particles subjected to the dispersion treatment with the organic solvent were surface-modified with the surface modifier compound as described above, and were temporarily aggregated in a dry powder state. However, the aggregate can be crushed by a normal dispersion process performed for forming the magnetic layer, and a magnetic coating material containing highly dispersed magnetic particles can be easily obtained without applying a high dispersion load. it can.

  Hereinafter, specific embodiments of the method for producing a magnetic recording medium of the present invention will be described.

Magnetic layer The magnetic layer in the present invention is a layer containing the magnetic particles subjected to the surface treatment and a binder. Binders used in magnetic coatings to form magnetic layers include polyurethane resins, polyester resins, polyamide resins, vinyl chloride resins, acrylic resins copolymerized with styrene, acrylonitrile, methyl methacrylate, etc., nitro A single resin or a mixture of a plurality of resins may be used from a cellulose-based resin such as cellulose, an epoxy resin, a phenoxy resin, a polyvinyl alkylal resin such as polyvinyl acetal, and polyvinyl butyral. Among these, preferred are polyurethane resins, acrylic resins, cellulose resins, and vinyl chloride resins. These resins can also be used as a binder in the nonmagnetic layer described later. JP, 2010-24113, A paragraphs [0029]-[0031] can be referred to for the above binder. It is also possible to use a polyisocyanate curing agent together with the resin.

  Additives can be added to the magnetic layer as necessary. Examples of the additive include an abrasive, a lubricant, a dispersant / dispersion aid, an antifungal agent, an antistatic agent, an antioxidant, and carbon black. As the additive, commercially available products can be appropriately selected and used according to desired properties. Use of the surface modifier as an additive is effective for further improvement of dispersibility. This is because the dispersibility can be further enhanced by adhering the surface modifying agent exhibiting a dispersibility improving action to magnetic particles highly dispersed in an organic solvent.

Non-magnetic layer Next, detailed contents regarding the non-magnetic layer will be described. In the present invention, a nonmagnetic layer containing a nonmagnetic powder and a binder can be formed between the nonmagnetic support and the magnetic layer. The nonmagnetic powder that can be used in the nonmagnetic layer may be an inorganic substance or an organic substance. Carbon black or the like can also be used. Examples of the inorganic substance include metals, metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, and metal sulfides. These nonmagnetic powders are available as commercial products, and can also be produced by a known method. Details thereof can be referred to paragraphs [0036] to [0039] of JP2010-24113A.

  As the binder, lubricant, dispersant, additive, solvent, dispersion method and the like of the nonmagnetic layer, those of the magnetic layer can be applied. In particular, known techniques relating to the magnetic layer can be applied to the amount of binder, type, additive, and amount of dispersant added, and type. Further, carbon black or organic powder can be added to the nonmagnetic layer. For example, paragraphs [0040] to [0042] of JP 2010-24113 A can be referred to.

Nonmagnetic Support The magnetic coating material prepared by the above method is applied on the nonmagnetic support directly on the nonmagnetic layer or through another layer such as a nonmagnetic layer. Thereby, a magnetic recording medium having a magnetic layer on a nonmagnetic support via another layer such as a nonmagnetic layer as required can be obtained.

Examples of the nonmagnetic support include known ones such as biaxially stretched polyethylene terephthalate, polyethylene naphthalate, polyamide, polyamideimide, and aromatic polyamide. Among these, polyethylene terephthalate, polyethylene naphthalate, and polyamide are preferable.
These supports may be subjected in advance to corona discharge, plasma treatment, easy adhesion treatment, heat treatment and the like. The surface roughness of the nonmagnetic support that can be used in the present invention is preferably a center average roughness Ra of 3 to 10 nm at a cutoff value of 0.25 mm.

Layer Configuration As for the thickness configuration of the magnetic recording medium obtained by the present invention, the thickness of the nonmagnetic support is preferably 3 to 80 μm. The thickness of the magnetic layer is optimized depending on the saturation magnetization amount, head gap length, and recording signal band of the magnetic head to be used, but is generally 0.01 to 0.15 μm, preferably 0.02 to 0.02. The thickness is 0.12 μm, more preferably 0.03 to 0.10 μm. There may be at least one magnetic layer, and the magnetic layer may be separated into two or more layers having different magnetic characteristics, and a configuration related to a known multilayer magnetic layer can be applied.

  The thickness of the nonmagnetic layer is, for example, 0.1 to 3.0 μm, preferably 0.3 to 2.0 μm, and more preferably 0.5 to 1.5 μm. Note that the nonmagnetic layer of the magnetic recording medium of the present invention exhibits its effect if it is substantially nonmagnetic. For example, even if it contains a small amount of magnetic material as an impurity or intentionally, This shows the effect of the invention and can be regarded as substantially the same configuration as the magnetic recording medium in the present invention. Note that “substantially the same” means that the residual magnetic flux density of the nonmagnetic layer is 10 mT or less or the coercive force is 7.96 kA / m (100 Oe) or less, and preferably has no residual magnetic flux density and coercive force. Means.

Backcoat layer In the present invention, a backcoat layer can be provided on the surface of the nonmagnetic support opposite to the surface having the magnetic layer. The back coat layer preferably contains carbon black and inorganic powder. For the binder and various additives for forming the backcoat layer, the formulation of the magnetic layer and the nonmagnetic layer can be applied. The thickness of the back coat layer is preferably 0.9 μm or less, and more preferably 0.1 to 0.7 μm.

Manufacturing process The coating liquid (magnetic coating material) for forming the magnetic layer is the same as the coating liquid for forming a magnetic layer except that the surface-modified magnetic particles obtained by the magnetic particle manufacturing method of the present invention are used. It can be produced by the same method as the preparation method.
The process for producing a coating solution for forming a magnetic layer, a nonmagnetic layer or a backcoat layer usually comprises at least a kneading process, a dispersing process, and a mixing process provided before and after these processes as necessary. Each process may be divided into two or more stages. All the raw materials such as magnetic particles, nonmagnetic powder, binder, carbon black, abrasive, antistatic agent, lubricant, and solvent used in the present invention may be added at the beginning or during any step. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion. In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. Further, glass beads or other beads can be used to disperse the magnetic layer coating solution, nonmagnetic layer coating solution or backcoat layer coating solution. As such a dispersion medium, zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are suitable. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser. For details of the method of manufacturing the magnetic recording medium, reference can be made, for example, to paragraphs [0051] to [0057] of JP 2010-24113 A.

  According to the present invention, a magnetic layer in which magnetic particles are highly dispersed can be formed. Thus, according to the present invention, it is possible to provide a magnetic recording medium for high-density recording that can exhibit excellent electromagnetic conversion characteristics.

  Hereinafter, the present invention will be described more specifically with reference to examples. It should be noted that the components, ratios, operations, order, and the like shown here can be changed without departing from the spirit of the present invention, and should not be limited to the following examples. “Parts” and “%” described below represent “parts by mass” and “% by mass”.

[Preparation Example 1]
To 1.0 part of the barium ferrite magnetic particles shown below, 3.0 parts of water, 0.24 part of 30% acetic acid water and 13 parts of 0.1 mmφ zirconia beads (manufactured by Nikkato Co., Ltd.) were added and dispersed for 6 hours by a bead mill. An aqueous magnetic liquid in which ferrite magnetic particles are dispersed was prepared. The particle diameter (dispersed particle diameter) of barium ferrite magnetic particles in the prepared aqueous magnetic liquid was measured and found to be 30 nm, and the barium ferrite magnetic particles can be dispersed in an acidic aqueous solution close to the primary particle diameter. Was confirmed.
Barium ferrite magnetic particles Composition excluding oxygen (molar ratio): Ba / Fe / Co / Zn = 1/9 / 0.2 / 1
Hc: 176 kA / m (2200 Oe), average plate diameter (primary particle diameter): 25 nm, average plate ratio: 3
BET specific surface area: 65 m ² / g
σs: 49 A · m ² / kg (49 emu / g)
pH: 7

(Measurement method of dispersed particle size)
The aqueous magnetic liquid obtained in Preparation Example 1 was diluted with water to a solid content concentration of 0.2% by mass.
The average particle size in the diluent measured using a dynamic light scattering particle size distribution analyzer LB-500 manufactured by HORRIBA was used as the dispersed particle size.

[Example 1]
(1) Preparation of surface-modified magnetic particles An aqueous solution comprising 0.20 parts of 4-tert-butylphenol, 3.9 parts of water, and 0.05 parts of sodium hydroxide as a surface modifier was prepared in Preparation Example 1. Added to 42 parts of the aqueous magnetic liquid. The obtained mixture aqueous solution was centrifuged under conditions of 100,000 rotation × 100 minutes (600,000 G) using a Himac ultracentrifuge CS150GXL to obtain a solid 1.
The solid 1 was dried at 140 ° C./36 hours and then pulverized in a mortar to obtain the magnetic body 1. It was confirmed that the surface modifier was adsorbed on the magnetic body 1 by the method described later.

(2) Preparation of magnetic coating material 1.0 part of the magnetic material 1 obtained above was suspended in a solution comprising 0.14 part of polyester polyurethane, 1.6 parts of methyl ethyl ketone (2-butanone) and 1.1 parts of cyclohexanone. It was. 10 parts of 0.1 mmφ zirconia beads were added and dispersed by ultrasonic irradiation for 10 minutes to obtain a magnetic coating material 1. The particle diameter (dispersed particle diameter) of barium ferrite magnetic particles in the magnetic coating 1 obtained was 52 nm as measured by the following method. In addition, the dispersed particle size could be further reduced by extending the dispersion time.

(Measurement method of dispersed particle size)
The magnetic coating material 1 was diluted with a mixed solution containing cyclohexanone and methyl ethyl ketone in a volume ratio of cyclohexanone 6.0: methyl ethyl ketone 9.0 to a solid content concentration of 0.2% by mass (the solid content is defined as magnetic particles and Represents the total mass of the polyurethane resin).
The average particle size in the diluent measured using a dynamic light scattering particle size distribution analyzer LB-500 manufactured by HORRIBA was used as the dispersed particle size. A smaller dispersed particle size means that the magnetic particles do not aggregate and the dispersibility is better.

(Confirmation of surface modifier coating)
5 mg of the magnetic material 1 was weighed and heated from 30 ° C. to 500 ° C. at a temperature rising rate of 10 ° C./1 minute using a differential thermal analyzer (TG / DTA 6300 manufactured by Seiko Instruments Inc.). The mass reduction was confirmed in the temperature range from 250 ° C. to 500 ° C. where the surface modifier used in this example volatilizes, confirming that the surface modifier was adsorbed on the surface of the magnetic particles. It was.

[Example 2]
(1) Preparation of surface-modified magnetic particles An aqueous solution added to the aqueous magnetic liquid prepared in Preparation Example 1 was prepared by using 0.15 parts of catechol, 2.8 parts of water, 0.05% sodium hydroxide as a surface modifier. A magnetic body 2 was produced in the same manner as in Example 1 except that the aqueous solution was composed of parts. When differential thermal analysis was performed in the same manner as in Example 1, a mass reduction was confirmed from 250 ° C. to 500 ° C., which is the temperature range in which the surface modifier used in this example volatilizes. It was confirmed that the surface modifier was adsorbed on the particle surface.

(2) Preparation of magnetic paint Magnetic paint 2 was produced in the same manner as in Example 1 except that the magnetic material used was changed to 1.0 part of magnetic material 2 obtained above. It was 55 nm when the dispersed particle diameter in the obtained magnetic coating material was measured by the same method as Example 1. When the dispersion time was extended to 22 minutes, the dispersed particle size was 41 nm.

[Example 3]
(1) Preparation of surface-modified magnetic particles An aqueous solution to be added to the aqueous magnetic liquid prepared in Preparation Example 1 was used as a surface modifier, 0.22 parts of 2,3-dihydroxynaphthalene, 4.1 parts of water, water A magnetic body 3 was produced in the same manner as in Example 1 except that the aqueous solution was 0.05 parts of sodium oxide. When differential thermal analysis was performed in the same manner as in Example 1, a mass reduction was confirmed from 250 ° C. to 500 ° C., which is the temperature range in which the surface modifier used in this example volatilizes. It was confirmed that the surface modifier was adsorbed on the particle surface.

(2) Preparation of magnetic paint Magnetic paint 3 was produced in the same manner as in Example 1 except that the magnetic material used was changed to 1.0 part of magnetic material 3 obtained above. It was 55 nm when the dispersed particle diameter in the obtained magnetic coating material was measured by the same method as Example 1. In addition, the dispersed particle size could be further reduced by extending the dispersion time.

[Comparative Example 1]
A comparative magnetic paint 1 was prepared in the same manner as in Example 1 except that the magnetic material used was changed from the magnetic material 1 to 1.0 part of barium ferrite magnetic particles used as the raw material magnetic material in Preparation Example 1. The dispersed particle size in the obtained magnetic coating material was measured by the same method as in Example 1, and found to be 78 nm. Further, when the dispersion time was extended from 10 minutes, the dispersion particle diameter became 60 nm after 20 minutes of dispersion time, and immediately after that, re-aggregation occurred due to overdispersion, and the dispersion particle diameter became 100 nm or more, and thereafter the dispersion particle diameter was 60 nm or less. No magnetic paint was obtained.
On the other hand, as described above, in the examples, the dispersed particle diameter was reduced as the long-term dispersion was performed, so that it was also confirmed that the surface modifier exhibited the effect of suppressing the occurrence of overdispersion.

  The raw material magnetic material used in Comparative Example 1 was subjected to differential thermal analysis in the same manner as in Example 1. As a result, no mass reduction was observed in the temperature range of 250 ° C. to 500 ° C. It was confirmed that the reduced mass was due to volatilization of the surface modifier adsorbed on the magnetic particles.

[Comparative Example 2]
(1) Preparation of surface-modified magnetic particles An aqueous solution to be added to the aqueous magnetic liquid prepared in Preparation Example 1 was prepared by adding 0.27 parts of 2,2′-dihydroxydiphenylmethane, 5.1 parts of water, 0. A comparative magnetic body 2 was produced in the same manner as in Example 1 except that the aqueous solution was changed to 05 parts. When differential thermal analysis was performed in the same manner as in Example 1, mass loss was confirmed in the temperature range from 250 ° C. to 500 ° C. where 2,2′-dihydroxydiphenylmethane volatilizes. It was confirmed that 2,2′-dihydroxydiphenylmethane was adsorbed.

(2) Preparation of magnetic paint Comparative magnetic paint 2 was produced in the same manner as in Example 1 except that the magnetic material used was changed to 1.0 part of the comparative magnetic material 2 obtained above. It was 65 nm when the dispersed particle diameter in the obtained magnetic coating material was measured by the same method as Example 1. When the dispersion time was extended to 20 minutes, the most agglomeration occurred due to overdispersion, and the dispersed particle diameter became 100 nm or more. Thereafter, a magnetic paint having a dispersed particle diameter of 65 nm or less was not obtained.

  Since the examples had smaller dispersed particle sizes at the same dispersion time than the comparative examples, according to the present invention, it is easy to apply a magnetic coating material containing magnetic particles in a highly dispersed state without applying a high dispersion load. It was confirmed that it can be obtained. By using the magnetic coating thus obtained, it is possible to form a magnetic layer in which magnetic particles are highly dispersed, thereby obtaining a magnetic recording medium having excellent electromagnetic conversion characteristics.

  The present invention is useful in the field of manufacturing high-density magnetic recording media such as backup tapes.

Claims (5)

A method for producing surface-modified magnetic particles comprising:
An aromatic compound having a polar group selected from the group consisting of a hydroxyl group and its alkali metal base is added to an aqueous magnetic liquid in which magnetic particles are dispersed in an acidic aqueous solvent (provided that a plurality of aromatic rings are connected via a linking group). In the case of the aromatic compound bonded to each other, the polar group is contained only in one of the two rings to be linked.), And then the magnetic particles are collected from the aqueous magnetic liquid. The production method according to claim 1, wherein surface-modified magnetic particles are obtained by adhering. The production method according to claim 1, wherein the compound is an aromatic compound having one or two polar groups. The production method according to claim 1 or 2, wherein the aromatic ring contained in the aromatic compound is a benzene ring or a naphthalene ring. The manufacturing method according to claim 1, wherein the magnetic particles are hexagonal ferrite magnetic particles. Producing magnetic particles by the production method according to any one of claims 1 to 4, and
The magnetic particles produced are dispersed together with an organic solvent and a binder to produce a magnetic paint, and a magnetic layer is formed using the produced magnetic paint .
Method of manufacturing a magnetic recording medium which comprises a.