JPH03280217A - Production of magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve dispersibility of ferromagnetic powder of small particle size in a coating material and to realize the production of an excellent high- density magnetic recording medium by kneading the ferromagnetic powder and a binder resin in a mixture soln. of water and an org. solvent, while removing water, and further adding the org. solvent thereto. CONSTITUTION:The magnetic recording medium is formed by coating a substrate with a coating material prepared by mixing a ferromagnetic powder with a binder resin. The coating material is prepared by kneading the ferromagnetic powder and a binder resin in a mixture of water and an org. solvent, while water is removed, and then adding the org. solvent to the kneaded material, and mixing. Then this coating material is applied on the substrate to obtain the magnetic recording medium. The number of hydrophilic groups in the binder resin is important, and for example, such a binder having 0.1 - 2.0mmol/g metal sulfonate groups is used. The mol.wt. of the metal sulfonate is not rigorously limited, but preferably 1,000 - 20,000.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a method for manufacturing a high recording density magnetic recording medium that is excellent in short wavelength recording.

(Prior Art) Conventionally, magnetic recording media have been obtained by coating gamma ferrite or iron powder together with a resin binder on a substrate such as a polyester film. In recent years, there has been a demand for higher density magnetic recording media, and attempts have been made to reduce the particle size of magnetic powder. On the other hand, magnetic powder suitable for this purpose has a particle size of 0.
It has been found that ultrafine hexagonal crystal powder such as barium ferrite with a particle diameter of 2 μm or less is suitable. In other words, this magnetic powder is a hexagonal plate-shaped single domain crystal, and in addition to having a very small particle size, it also has an axis of easy magnetization perpendicular to the plate surface, which provides a high-density magnetic recording medium. .

(Problem to be Solved by the Invention) When trying to obtain a high-density magnetic recording medium using these small particle-sized magnetic powders, the most important thing is to disperse the magnetic powders in a resin binder to a state close to that of primary particles. It is in. However, as the particle size of the magnetic powder used has become smaller, the technical difficulties have been increasing, and in order to solve this problem, a great deal of time and effort has been required for the dispersion operation. Furthermore, since a strong dispersion process is selected in order to achieve a good dispersion state, the incorporation of foreign matter into the paint has also become a problem.

On the other hand, if the dispersibility of the magnetic powder is insufficient, the smoothness of the coated surface may be impaired or the backing density in the magnetic layer may be reduced, resulting in a short wavelength recording/reproducing output that is not as expected from the particle size. There is a problem that it is not possible to obtain In addition, if the dispersibility is poor, there arises the problem of increased noise during recording and reproduction, which also makes short wavelength recording difficult. Furthermore, in coated magnetic recording media, after coating, the solvent is passed through a magnetic field before it is dry to orient the magnetic particles in the running (in-plane) direction or perpendicular direction of the substrate. In this orientation process, if magnetic powder agglomerates are present in the coating film, these agglomerates will rapidly grow into larger agglomerates in the orienting magnetic field, so there is a strong tendency for the coated surface to become rough. However, it has become difficult to obtain media with the high recording density expected.

Recently, particle size of 0.2 μm has been developed for high-density recording media.
The following materials coated with ultrafine barium ferrite are used. This ferromagnetic powder is a hexagonal plate-shaped crystal with an axis of easy magnetization perpendicular to the plate surface, so when it is applied to the substrate surface and then placed in a vertical magnetic field, the plate surface faces upward. A perpendicularly oriented medium can be produced by oriented in this manner, providing an ideal medium as a high-density magnetic recording medium. However, hexagonal barium ferrite powder usually has plate surfaces that overlap each other and tends to aggregate, and if dispersion is insufficient, these aggregate particles will aggregate in the orienting magnetic field, causing the coating surface to become rough and causing noise. increases.

In order to improve the dispersibility of hexagonal barium ferrite powder, attempts have been made to use many dispersants in combination, but in general, when too many dispersants are used, bleeding of these dispersants occurs on the surface of the medium after the coating film is formed. It may impair the running properties of the paint film,
It has problems such as blocking. Further, in order to improve the recording and reproducing output, it is desirable that the saturation magnetization of the magnetic recording layer is as large as possible, but in general, hexagonal barium ferrite powder has a low saturation magnetization Ms, which is disadvantageous in this respect. As a method to overcome this problem, attempts have been made to increase the packing density of hexagonal barium ferrite powder in the magnetic recording layer as much as possible. However, no measure has been found so far to maximize the filling rate while satisfying the durability of the coating film.

The present invention provides a magnetic paint that enhances the dispersion of the above-mentioned small-particle ferromagnetic powder into a resin binder, thereby solving problems in surface properties, magnetic field orientation characteristics, and running durability, and providing excellent high-density magnetic recording. The purpose is to provide a method for providing media.

In addition, the present invention provides a magnetic coating material that improves the dispersion of small-particle hexagonal ferromagnetic powder, which is particularly difficult to disperse, into a resin binder, thereby solving the above-mentioned problems and providing excellent high-density magnetic recording. The purpose is to provide a method for providing media.

The present invention further provides a method of providing highly dispersed magnetic paints that is much easier than conventional dispersion processes.

[Structure of the Invention] (Means for Solving the Problems) A magnetic recording medium of the present invention is a magnetic recording medium in which a ferromagnetic powder is mixed with a resin binder and the mixture is coated on a substrate. The water is removed while kneading the powder in a mixed system of water and an organic solvent, and an organic solvent is further added to the resulting kneaded product and mixed to produce a paint.
This method of manufacturing a magnetic recording medium is characterized in that it includes the steps of applying this onto the substrate.

The ferromagnetic powder used in the present invention includes γ ferrite powder,
Co-γ ferrite powder, metal powder mainly composed of iron, and hexagonal ferrite powder are used. As hexagonal ferrite powder, M type (Magnetoplumbite
tip) to W-type holegonal system, Ba ferrite, St
Examples include ferrite, lead ferrite, Ca ferrite, solid solutions thereof, and ion-substituted products represented by the following formulas.

MaO・n (Fe Mb) 203 (
In the formula, 1-x x Ma represents any one element of Ba, St, Ca, or Pb, and Nb represents Co, Zn, Ni, or Cu.

Mg, Mn, In, Ti, Sn,
Ge, Zr.

It represents at least two elements selected from the group of Hf, V, Nb, Sb, Ta, Cr, Mo, and W, one of which is Nb. n represents a number from 5.4 to 6.0. ) More specifically, as the hexagonal ferrite powder used in the present invention, a part of the Fe atoms, which are the constituent elements of these -axial hexagonal ferrite crystals, are combined with a divalent metal and 5
Those substituted with Nb, which is a valent metal, or those substituted with Sn atoms in the range of 0.05 to 0.5 per chemical formula are suitable, and the amount of substitution is such that the coercive force is 600 to 20.
The amount is set to 000e.

Among the substituent elements, divalent metals mainly act to reduce the coercive force of hexagonal ferrite powder to an appropriate range, Nb, a pentavalent metal, acts to increase saturation magnetization, and Sn, a tetravalent metal, acts to increase the saturation magnetization. acts to reduce changes in the temperature characteristics of coercive force.

In the hexagonal ferrite used in the present invention, the appropriate amount of substitution of divalent metal (Mll) and pentavalent metal (MV) (7) varies depending on the combination of Mll and MV, but M
The amount of substitution per chemical formula of ll is approximately 0.5 to 1.
There are 2 pieces.

Looking at the relationship between the amounts of substitution of these substituent elements, for example with respect to magnetobrambite Ba ferrite, the chemical formula of the substituent is BaFe 12- (x+y(+Z)
)MIIxMVy(MIVz)019. where x+ y+ z are Mll, M+ and MIV
It is the amount of substitution per chemical formula of an element. Mll, MY and MIV are each divalent.

Since they are pentavalent and tetravalent, and the substituted Fe atom is trivalent, the relationship y − (x−Z)/2 holds true when valence compensation is considered. That is, the amount of substitution of MV is uniquely determined from the amount of substitution of Mll and the amount of substitution of MIV.

In the magnetic powder of the present invention, when Sn is used as the MIV element, the appropriate range of the substitution amount is 0.05 to 0.5 per chemical formula of hexagonal ferrite.

Note that Ti having the same valence may be used in place of Sn described above.

The average particle size of these hexagonal ferrite powders is 0.01~
A range of 0.2 μm is desirable. If the particle size is less than 0.01 μm, the magnetization and coercive force will decrease, resulting in a decrease in the reproduction output of the magnetic recording medium.On the other hand, if it exceeds 0.2 μm, not only will the improvement in short wavelength recording and reproduction output be small, but also the This is because noise during reproduction becomes significantly large during density recording. In addition,
If the coercive force is less than 6,000e, the recording signal will not remain sufficiently in the recording medium, and if it exceeds 30,000e, it will be difficult to write the signal with a normal recording/reproducing head, so both are undesirable.

The resin binder used in the present invention includes vinyl chloride-vinyl acetate copolymers, polyester resins, and polyethers having hydrophilic groups such as carboxyl groups, phosphoric acid groups, sulfone groups, or metal bases thereof, or amino groups. Resins, polyurethane resins, polyacrylic resins, etc. are suitable, and those with sulfonic acid metal bases are particularly suitable as the resin binder of the present invention. The reason for this is probably due to the ability of this resin to dissolve in water or emulsify it. These hydrophilic groups do not need to exist alone in the resin molecule, and even if a plurality of types of magnetic groups coexist in the same molecule, the object of the present invention will not be impaired at all. The number of these hydrophilic groups in the resin binder is important and for the purposes of the present invention is at least 0.01m+++
ol/g to 4.0 mmol/g, preferably 0.01 mmol/g to 2.0 mguol/g. Although the molecular weight Mw of the resin binder does not require much strictness, it is usually 1,000 to eo, ooo.

Preferably, it is in the range of 1,000 to 20,000.

Among these polar groups, the sulfonic acid metal base is introduced as follows. First, when the resin binder containing a sulfonic acid metal base is a resin obtained by vinyl polymerization, it is usually obtained by copolymerizing a vinyl monomer containing these polar groups and a normal vinyl monomer. In addition, when the resin binder containing the above polar group is a polyester resin or polyurethane resin, the polybasic acid or polyhydric alcohol which is a constituent component of these and the polybasic acid or polyhydric alcohol into which the above polar group has been introduced are combined. mix alcohol,
Obtained by carrying out a condensation reaction. Monomers or polybasic acids or polyhydric alcohols used in the production of resins containing sulfonic acid metal bases include, for example, nylsulfonic acid, sodium vinylbenzenesulfonate,
-acrylamido-2-methylpropanesulfonic acid and vinyl monomers containing these metal bases, and the like.

Typical vinyl resin monomers copolymerized with vinyl monomers having these sulfonic acid metal groups include vinyl chloride, vinyl alcohol, maleic anhydride, vinyl acetate, various acrylate monomers vinyldene chloride, vinyl acetal, vinyl butyral, and acrylic acid. Examples include various monomers such as ester, acrylonitrile, and styrene. In addition, common polyhydric alcohols copolymerized with vinyl monomers having polar groups such as these sulfone groups include 1,4-butanediol, 1,6-hexamethylene diol, cyclohexanediol, ethylene glycol, diethylene glycol, and Ethylene glycol, propylene glycol, glycerin, neopentyl alcohol, etc., and porous basic acids such as terephthalic acid, isophthalic acid, adipic acid, sebacic acid, oxalic acid, succinic acid, geltaric acid, pyromellinic acid, speric acid, and azelaic acid. Examples include acids.

The molecular weight of these resins is preferably 1,000 to eoooo, preferably 1,000 to 20,000.

Low molecular weight hydrophilic group-containing resin molecules can penetrate between ferromagnetic powders in an aggregated state or hexagonal ferrite powders that overlap each other; Strong adsorption to the surface of magnetic powder is considered to be effective in improving dispersibility.

Among the low molecular weight sulfonic acid metal base-containing resins of the present invention obtained as described above, resins containing polyester chains are particularly excellent, and in particular, resins containing aliphatic vinyl monomers having 4 to 18 carbon atoms and polyester chains are particularly preferred. A polyester resin into which a basic acid or polyhydric alcohol has been introduced is suitable. Furthermore, these polyester resins may contain aromatic layer diisocyanates such as toluene diisocyanate or 1,4-tetramethylene diisocyanate,
By using a urethane resin combined with an aliphatic diisocyanate such as hexamethylene diisocyanate or isophorone diisocyanate, it is possible to further improve dispersibility and durability.

Such a resin binder having a hydrophilic group is mixed with water. In this process, water is important because the hydrophilic groups of the resin are adsorbed on the surface of the magnetic powder, and because it penetrates between the particles of the aggregated magnetic powder and introduces the hydrophilic group-bearing resin molecules between the particles. play a role.

In order to further promote the penetration of water and the hydrophilic group-bearing resin molecules between the particles, a surfactant, which is usually used as a penetrant, can also be used in combination. Suitable penetrants include aerosol OT, sodium alkylbenzene sulfonate, sodium lauryl sulfate, Necal BX, butyl oleate sulfide, nonylphenyl ethylene oxide adduct or its phosphoric acid ester, lecithin, and the like.

When mixing the hydrophilic group-containing resin binder and magnetic powder in water, various resin binders can be used in combination for the purpose of improving the mechanical strength, runnability, etc. of the coating film. Resins that can be used in combination include polyurethane resin, polyester resin, polycarbonate resin, polyacrylic resin, polyamide resin, epoxy resin, phenol resin, polyether resin, phenoxy resin, melamine resin, vinyl butyral resin, furan resin, vinyl chloride resin, Examples include vinyl acetate resin, vinyl alcohol resin, and mixtures or copolymers thereof.

The blending amount of these combined resin binders is appropriately set within 80% by weight based on the total resin binder.

In paints using the above resin binders, polyamine or polyisocyanate is added in order to increase the mechanical strength and durability of the paint film when it is coated on a support together with ferromagnetic powder. curing agent is added.

Furthermore, a lubricant, a dispersant, an abrasive, or a conductivity imparting agent such as carbon black may be added to the components of the magnetic coating film, if desired.

As the lubricant, a fatty acid or fatty acid alkyl ester type, a silicone type, a fluorinated hydrocarbon type, or a mixture or compound thereof can be used. Also,
As the dispersant, anionic surfactants, cationic surfactants, and nonionic surfactants can be used.
Cationic surfactants having a phosphate group are particularly effective. Furthermore, a surface treatment agent such as a silane coupling agent or a titanium coupling agent may be used if desired. Furthermore, TiO2, Cr2O3, Al
Inorganic powders having a Mohs hardness of 5 or higher, such as 2O3, SiC, and ZrO2, are suitable.

It is desirable that the amounts of the various additives mentioned above be as small as possible.For example, lubricants and dispersants are each 5 parts by weight or less, abrasives are 6 parts by weight or less, carbon Black is selected in a range of 3 parts by weight or less.

The magnetic recording medium of the present invention is produced by kneading ferromagnetic powder and a resin binder in a mixed system of water and an organic solvent. In this crosstalk process, the resin binder is adsorbed on the surface of the ferromagnetic powder, and as a result, the ferromagnetic powder moves into the organic solvent and separates water. The separated water is removed by decanting or heating under reduced pressure. The ferromagnetic powder used in the above process may be a dry powder, but it is more desirable if it is already an aqueous slurry, such as barium ferrite powder washed out in water by a glass crystallization method. This is because such a slurry does not have a history of agglomeration during the step of drying the powder.

Through the above steps, the content of water in the kneaded product finally obtained is 0.5 to 4.0 parts by weight per 100 parts by weight of ferromagnetic powder.
The amount is adjusted to be within the range of 0 parts by weight. This adjustment can be easily carried out by adjusting the degree of water removal by heating under reduced pressure.

If desired, a resin binder, an organic solvent and various additives are further added to the obtained kneaded material, and the mixture is thoroughly dispersed using a ball mill or a sand grinder to form a paint. If necessary, a curing agent such as a polyisocyanate compound is added to this coating material, and the coating material is coated onto a substrate, followed by orientation treatment and drying treatment to prepare a coating medium. The characteristics of this medium can be further improved by smoothing it with a calender.

Note that, if necessary, a conductive primer layer may be formed on the substrate, and a magnetic recording layer may be formed on this primer layer. The magnetic recording medium thus obtained has a squareness ratio of 0.7 or more and has excellent magnetic recording and reproducing characteristics.

(Function) In the magnetic recording layer of the magnetic recording medium of the present invention, the ferromagnetic powder is dispersed in the organic solvent through a step of kneading the ferromagnetic powder and the resin binder in water and an organic solvent. It increases the adsorption of resin binder to the powder surface, significantly improves the dispersion of ultrafine ferromagnetic powder in paint, and significantly improves surface properties, magnetic field orientation, and running durability. In addition, we have provided a magnetic recording medium that significantly improves the dispersion of hexagonal ferromagnetic powder, which is particularly difficult to disperse, into the resin inder. The present invention provides a manufacturing method for obtaining an excellent high-voice magnetic recording medium.

(Example) Next, an example of the present invention will be described. All amounts of resin binder and additives shown in the examples below are based on 100% solids.

Example I Co, Ti, Nb substituted Ba-ferrite powder 1100 parts by weight Low molecular weight polyurethane resin Book 2 8 parts by weight Cyclohexanone 15 parts by weight Gafaq RE-610 3 1 part by weight*INb substitution amount y -0,45 (Average particle size: 0.05 μm.

Plate ratio: particle size/thickness - 4/1. Hc-10000e)
Washed out in water by glass crystallization method (slurry NY 70%). The above parts by weight indicate the parts by weight of the net magnetic powder.

Book 2 - Aliphatic polyurethane resin having 8O3Na groups, molecular weight: 20,000, number of 18O3Na groups per molecule: 1
(Cyclohexanone solution NY 30%).

Parts by weight refer to parts by weight of net resin.

*3 Phosphate ester surfactant of nonyphenol-ethyleneoxide adduct, trade name Toho Chemical Demagnetizing powder slurry, polyurethane resin, cyclohexanone, and Gafatsu are added to it, kneaded in a vacuum kneader, and the separated water is mixed. It was removed by decanting. The mixture is further mixed with water until the water content reaches 2 parts by weight per 100 parts by weight of magnetic powder.
Dehydration was performed by heating under reduced pressure until the weight part was reduced.

After adding and mixing the following additives to the obtained kneaded product,
Further, the paint was obtained by passing it through a sand grinder.

Toluene/cyclohexanone 180 parts by weight
1/2 mixed solution Alumina 3 parts by weight Stearic acid 2 parts by weight To the paint obtained above, 3 parts by weight of Coronate polyisocyanate (trade name: manufactured by Nippon Polyurethane Co., Ltd.) was added and coated on a polyester film using a reverse coater. Then, it was dried while passing under a magnetic field (strength +4 KOe) perpendicular to the surface of the substrate.

The resulting coating film was cured at 40° C. for 3 days and then cut into 1/2 inch widths to produce magnetic tapes.

Example 2 The polyurethane resin of Example 1 was added to 4 parts by weight, and the polyvinyl acetate copolymer resin had a molecular weight of 10,000. A magnetic paint was prepared in the same manner as in Example 1, except that 4 parts by weight was added, and a magnetic tape was produced using this.

Example 3 Low molecular weight sulfonic acid metal base-containing polyurethane resin (
Molecular weight 2,000. Number of sulfone groups per molecule: 1, 30% cyclohexanone solution) 6 parts by weight (resin net weight parts), cyclohexanone 20 parts by weight, Co, Ti, Nb
, water slurry of Sn-substituted barium ferrite powder (particle size 0
．． 045μm1 plate ratio: particle size/thickness -4/1. Hcl,
3000e) 100 parts by weight (net weight part of magnetic powder) and 2 parts by weight of Gafac R8710 (phosphoric acid ester of noniphenyl ethylene oxide adduct, trade name: Toho Chemical Co., Ltd.) were kneaded to remove water and MEK, and to obtain a dry powder. Obtained.

The obtained dry powder was mixed with 3 parts by weight of myristic acid, 4 parts by weight of alumina, and 150 parts by weight of a 1/1 mixed solvent of MEK/toluene and passed through a sand grinder to prepare a paint. After adding 5 parts by weight of isophorone polyisocyanate (Mytic NY-218) to the obtained paint, Example 1
A tape was produced in exactly the same manner.

Example 4 The low molecular weight sulfonic acid metal base-containing polyurethane resin in Example 1 was prepared using a low molecular weight sulfonic acid metal base-containing salt-vinyl acetate copolymer resin (vinyl chloride monomer 83, vinyl acetate monomer 12.vinyl sulfonic acid monomer 5). , molecular weight 5.000) and 30 parts by weight of cyclohexanone, a tape was produced in exactly the same manner as in Example 1, except that cyclohexanone was used in an amount of 30 parts by weight.

Example 5 The Co, Ti, Nb-substituted Ba-ferrite powder slurry in Example 1 was mixed with CO-adhered rougherite powder having an average particle size of 0.3 μm, Hc8500e, and specific surface area SSA.
A tape was prepared in the same manner as in Example 1, except that the mixture was changed to 100 parts by weight and 30 parts by weight of water.

Example 6 The Co, Ti, Nb-substituted Ba-ferrite powder slurry in Example 1 was mixed with Fe-Ni metal powder (average particle size 0).
．． 2 μm + saturation magnetic charge 220 etnu/g.

Hc 15000e, SSA 50m+2/g
) A tape was produced in the same manner as in Example 1 except that the mixture was changed to 100 parts by weight and 40 parts by weight of water.

Comparative Example 1 The low molecular weight sulfonated polyurethane resin and Ba-ferrite dry powder in Example 1 were mixed and dispersed together with other additives in a toluene/cyclohexane-1/2 mixed solution without being mixed in water. A tape was produced in exactly the same manner as in Example 1.

Comparative Examples 2 and 3 A tape was produced in exactly the same manner as in Example 1, except that the CO-γ powder and metal powder in Examples 5 and 6 were not mixed with the low molecular weight sulfonated polyurethane resin in water.

The surface properties of the tapes obtained in the above Examples and Comparative Examples,
The following table shows the results of measuring the saturation magnetization, the perpendicular orientation rate after demagnetizing field correction, and the output when recording and reproducing this tape at a relative speed of 11.8 m% and a frequency of 18 MHz.

(Left below) Note: SQl? * indicates in-plane (3 and no mark indicate vertical cold).

[Effects of the Invention] As is clear from the above examples, the magnetic recording medium of the present invention not only has large saturation magnetization and a high perpendicular orientation rate, but also has problems that have conventionally been encountered in this type of media. As a result of significantly reducing surface roughness, short wavelength recording and reproducing characteristics are improved, providing an excellent high-density magnetic recording medium. Also,
In the magnetic recording medium according to the present invention, excellent dispersibility and surface smoothness can be obtained without using a dispersant.

Claims (7)

[Claims] (1) In manufacturing a magnetic recording medium in which a paint obtained by mixing ferromagnetic powder with a resin binder is coated on a substrate, the resin binder and ferromagnetic powder are kneaded in a mixed system of water and an organic solvent. A method for manufacturing a magnetic recording medium, comprising the steps of: removing water, further adding and mixing an organic solvent to the obtained kneaded product to prepare a paint, and applying the paint onto the substrate. (2) The resin binder is 0.1 mmol/g to 2
．． 2. The method for producing a magnetic recording medium according to claim 1, wherein the magnetic recording medium contains 0 mmol/g of sulfonic acid metal base. (3) The resin binder is 0.1 mmol/g to 2
．． 2. The method for producing a magnetic recording medium according to claim 1, wherein the sulfonic acid metal base has a molecular weight of 0 mmol/g and a molecular weight of 1,000 to 20,000. (4) The method for producing a magnetic recording medium according to claim 1, wherein in the kneading step, a surfactant having a milky, dispersing or penetrating effect is added in addition to the resin binder. (5) The method for manufacturing a magnetic recording medium according to claim 1, wherein the ferromagnetic powder is a hexagonal Ba ferrite ferromagnetic powder. (6) The water content in the kneaded material is 0.0% relative to the ferromagnetic powder.
2. The method of manufacturing a magnetic recording medium according to claim 1, wherein water is removed to a content of 5 to 3.0 wt%. (7) The method for manufacturing a magnetic recording medium according to claim 1, wherein the ferromagnetic powder is a slurry-like hexagonal Ba ferrite ferromagnetic powder washed out in water by a glass crystallization method.