JP2523844B2 - Magnetic recording media - Google Patents

Description

The present invention relates to a magnetic recording medium capable of high-density recording having excellent short-wavelength recording characteristics.

(Prior Art) A coating type magnetic recording medium is coated on a substrate such as a polyester film with magnetic powder such as γ-Fe ₂ O ₃ , CrO ₂ , Co-γ-Fe ₂ O ₃ or iron powder together with a resin binder. It is obtained by doing.

In recent years, magnetic recording media have been required to have higher recording densities, and as one means for solving them, attempts have been made to reduce the particle size of magnetic powder. On the other hand, as magnetic powder for high recording density magnetic recording media, part of Fe is C
The particle size is 0.2 μm, with coercive force of 200 Oe to 2000 Oe that can be recorded / reproduced by a normal recording / reproducing head by replacing with o, Ti, etc.
It has been found that the following ultrafine hexagonal ferrite powders such as Ba-ferrite are suitable.

That is, since ultrafine hexagonal ferrite powder such as Ba-ferrite has a hexagonal plate-like crystal structure and has an easy axis of magnetization in the direction perpendicular to the plate surface, it provides a high-density perpendicular magnetic recording medium. .

(Problems to be Solved by the Invention) In a magnetic recording medium obtained by using such magnetic powder, in order to obtain excellent recording characteristics for recording a short wavelength signal, the surface roughness of the magnetic recording layer is Make it extremely small,
The saturation magnetization (Ms) needs to be as large as possible.

However, it is usually used for this purpose,
For example, since ultra-fine grain hexagonal ferrite powder such as Ba-ferrite originally has a small saturation magnetization, there is a limit from the viewpoint of sufficiently extending short wavelength characteristics.

As one method of increasing the saturation magnetization of the magnetization recording layer, a method of reducing the ratio of the resin binder is known. When increasing the saturation magnetization by this method, magnetic powder,
There is a problem that even if the amount is reduced below a certain value, for example, 14 parts by weight or less, with respect to 100 parts by weight, the saturation magnetization hardly increases further and only the surface roughness increases.

In addition, after coating the base material with a coating mainly composed of hexagonal ferrite powder and a resin binder, it is placed in a magnetic field perpendicular to the base to orient the easy axis of magnetization of the hexagonal ferrite powder in the vertical direction. A method for improving short wavelength characteristics by further increasing the vertical alignment rate has also been attempted. However, this method has a problem that the surface of the coating film becomes rough due to the magnetic field, and the desired increase in short wavelength output cannot be obtained.

The present invention has been made in order to address the above-mentioned problems of the prior art, and an object thereof is to provide a magnetic recording medium capable of high-density recording with increased saturation magnetization and improved short-wavelength recording characteristics. I am trying.

[Structure of the Invention] (Means for Solving the Problems) A magnetic recording medium of the present invention has an average particle size of 0.01 μm to 0.2 μm in which a part of Fe is replaced with Nb on a substrate.
In a magnetic recording medium having a magnetic recording layer formed by coating the hexagonal ferrite magnetic powder with a resin binder and a curing agent, the total amount of the resin binder and the curing agent contained in the magnetic recording layer is 100 parts by weight of the magnetic powder. 15 parts by weight or less, at least 20% or more of the resin binder contains at least one polar group selected from a sulfonic group, a phosphoric acid group, these metal bases and an amino group, and has a glass transition point of −. The resin binder is in the range of 20 ° C. to −60 ° C., and the filling factor of the magnetic recording layer is 0.93 or more and less than 1.

The hexagonal ferrite powder used in the present invention is, for example, M type (Magnetoplumbite typ) represented by the following general formula.
e), W-type hexagonal Ba ferrite, Sr ferrite, P
B of b ferrite, Ca ferrite, or Ba ferrite
Examples thereof include those obtained by substituting a part of a with other Sr, Pb, or Ca, or ion-substituted products thereof.

General formula: MaO · n (Fe _1-x Mb _x ) ₂ O ₃ (In the formula, Ma is at least 1 selected from Ba, Sr, Ca and Pb.
Indicates the element of the species. Mb is Co, Zn, Ni, Cu, Mg, Mn, In, T
At least two elements selected from the group consisting of i, Sn, Ge, Zr, Hf, V, Nb, Sb, Ta, Cr, Mo and W are shown. However, one of Mb is Nb. Further, n represents a number of 5.4 to 6.0. ) More specifically, as the hexagonal ferrite powder used in the present invention, a part of Fe atoms, which are the constituent elements of these uniaxially anisotropic hexagonal ferrite crystals, is Nb which is a pentavalent metal, As a valence match, for example, it is substituted with a divalent metal, and further, 0.05 to 0.5 per chemical formula.
Those substituted with Sn atoms in the range of 10 are suitable, and the amount of substitution is such that the coercive force is 200 Oe to 2000 Oe.

Of the substituting elements, the divalent metal mainly acts to reduce the coercive force of the hexagonal ferrite powder to an appropriate range.
Nb of the valent metal acts to increase the saturation magnetization, and Sn of the tetravalent metal acts to reduce the change in the temperature characteristic of the coercive force.

As the divalent metal, an element such as Co, Zn, Ni, Mn, Cu, or Mg which is relatively well substituted for the Fe atom in the ferrite is selected.

In the hexagonal ferrite used in the present invention, the proper substitution amount of the divalent metal (M ^II ) and the pentavalent metal (M ^V ) depends on the combination of M ^II and M ^V , but one chemical formula of M ^II The substitution amount per hit is about 0.5 to 1.2.

The relationship between the substitution amounts of these substitution elements is, for example, M-type Ba.
As for ferrite, the chemical formula of its substitution product is represented by BaFe _{12- (x + y (+ z))} M ^II _x M ^V _y (M ^IV _z ) 0 ₁₉ . Where x, y and z are ferrites 1 of M ^II , M ^V and M ^IV elements
The amount of substitution per chemical formula. M ^II , M ^V and M ^IV elements are
Since they are respectively divalent, pentavalent, and tetravalent, and the substituted Fe atoms are trivalent, the relationship of y = (x−z) / 2 is established in consideration of valence compensation. That is, the substitution amount of M ^V is uniquely determined from the substitution amounts of M ^II and M ^IV .

In the magnetic powder of the present invention, when using Sn as M ^IV element, a proper range of the amount of substitution is from 0.05 to 0.5 or the range of per formula of the hexagonal ferrite. Note that Ti having the same valence may be used instead of Sn.

Also, the average particle size of hexagonal ferrite powder is 0.01 μm to 0.
The range limited to 2 μm is that if it is less than 0.01 μm, the magnetization and coercive force decrease, and the reproduction output of the magnetic recording medium decreases, while if it exceeds 0.2 μm, the coercive force decreases and high density recording This is because the noise during reproduction becomes remarkable.
If the coercive force is less than 200 Oe, the recording signal on the recording medium does not remain sufficiently. If the coercive force exceeds 2000 Oe, it becomes difficult to write a signal with a normal recording / reproducing head.

The resin binder containing at least one polar group selected from the sulfone group, the phosphoric acid group, these metal bases and amino groups used in the present invention usually has the polar group in its molecular skeleton, The one having a molecular weight of about 5,000 to 100,000, which is contained in the range of 0.5 to 10 per molecule, is used.

The resin binder containing the polar group is usually obtained by copolymerizing a monomer containing these groups and a normal resin monomer. For example, when the resin binder containing a polar group is a resin by vinyl polymerization, it is obtained by copolymerizing a vinyl monomer containing a polar group and a normal vinyl monomer, and the resin binder containing a polar group is a polyester resin or In the case of a polyurethane resin, it can be obtained by mixing the polybasic acid or polyhydric alcohol which is these constituents with the polyvalent basic acid or polyhydric alcohol into which the polar group has been introduced, and conducting a condensation reaction.

Examples of monomers having these groups, polybasic acids or polyhydric alcohols used for the production of resins containing sulfonic groups and their metal bases include vinyl sulfonic acid, sodium vinylbenzene sulfonate, and 2-acrylamido-2. Vinyl monomers containing methylpropane sulfonic acid and these metal bases, and And the like.

Also, as a monomer used for producing a resin having a phosphate group and its metal base, or an amino group,
A vinyl monomer having each polar group, a polyvalent basic acid or polyhydric alcohol having each polar group, or another reactive monomer is used.

As a usual resin monomer copolymerized with a monomer having a polar group such as a sulfone group or a phosphoric acid group,
For example, vinyl chloride, vinyl alcohol, maleic anhydride, vinyl acetate, various acrylate monomers,
Examples thereof include various monomers such as vinylidene chloride, vinyl acetal, vinyl butyral, acrylic acid ester, acrylonitrile and styrene.

Further, these copolymerization ratios are set so that, when the molecular weight is 5,000 to 100,000, a polar group such as a sulfone group is introduced in a range of 0.5 to 10 per molecule as described above.

In the present invention, the filling factor of the magnetic recording layer is set to be in the range of 0.93 or more and less than 1. The filling rate of the magnetic recording layer can be controlled by adjusting the degree of dispersion of the magnetic powder during the production of the magnetic coating material, or by adjusting the strength of the calendar treatment after forming the magnetic layer coating film.

The resin binder containing a polar group in the present invention thus obtained is used with its glass transition point set in the range of -20 to -60 ° C. By thus setting the temperature of the glass transition point to be low, it becomes possible to further improve the filling rate of the magnetic powder in the magnetic layer.

In this way, the gas transition point of the resin binder is changed from -20 ℃ to -60 ℃.
The temperature can be controlled within the range of ° C by introducing an appropriate amount of an aliphatic vinyl monomer having 4 to 18 carbon atoms, a polybasic acid, or a polyhydric alcohol into the resin skeleton. In the case of polyurethane-based resins, 1,4 as an isocyanate that bonds with the aliphatic polyester polyol
-By using an aliphatic diisocyanate such as tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, and isophorone diisocyanate, the glass transition point can be significantly lowered.

These resin binders having polar groups may be used alone, but may also be used in combination with other resin binders having no polar groups such as sulfone groups to improve the mechanical strength of the coating film and the running property. It is possible.

Examples of other resin binders that can be used in combination with the resin binder having such a polar group include, for example, polyurethane resin, polyester resin, polycarbonate resin, polyacrylic resin, polyamide resin, epoxy resin, phenol resin, polyether resin, phenoxy resin, Examples thereof include melamine resin, vinyl butyral resin, furan resin, vinyl chloride resin, vinyl acetate resin, vinyl alcohol resin, and mixtures or copolymers thereof.

The blending amount of other resin binder is appropriately set within 80% by weight with respect to the total resin binder. A hardener of polyamine or polyisocyanate type is usually added to these resin binders in order to increase the mechanical strength and durability of the coating film when it is coated on a support together with the ferromagnetic powder. It In the present invention, the total content of the resin binder and such a curing agent is 15 parts by weight or less with respect to 100 parts by weight of the magnetic powder, which is an amount sufficient to maintain the strength of the coating film.

If desired, 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.

As the lubricant, fatty acid or fatty acid alkyl ester type, silicone type, fluorinated hydrocarbon type, or a mixture or compound thereof can be used. In addition, as the dispersant, an anionic surfactant, a cationic surfactant, and a nonionic surfactant can be used. In particular, a surfactant having a phosphate group, a silane coupling agent, and a titanium cup. Ring agents are effective. further,
Inorganic powders having a Mohs hardness of 5 or more, such as TiO ₂ , Cr ₂ O ₃ , Al ₂ O ₃ , SiC, and ZrO ₂ , are suitable as the abrasive.

It is desirable that the amount of these additives to be blended is as small as possible. For example, 4 parts by weight or less of a lubricant and a dispersant are added to 100 parts by weight of hexagonal ferrite powder.
The range of 6 parts by weight or less of the abrasive and the range of 3 parts by weight or less of the carbon black are suitable.

The magnetic recording medium of the present invention is a magnetic coating prepared by thoroughly mixing hexagonal ferrite powder, a resin binder, and various additives as necessary with a solvent, and further adding a curing agent such as a polyisocyanate compound if necessary. Is prepared, the magnetic coating material is applied onto a substrate, subjected to an orientation treatment and a drying treatment, and the resulting coating film is subjected to a smoothing treatment with a calendar. If necessary, a conductive primer layer may be formed on the substrate, and the magnetic recording layer may be formed on the primer layer. The magnetic recording medium thus obtained is provided with an excellent magnetic recording medium having a squareness ratio of 0.5 or more.

(Operation) In the magnetic recording medium of the present invention, hexagonal ferrite powder having a high saturation magnetization and at least a part of Fe substituted with Nb is used as the magnetic powder, and a resin containing a resin containing a magnetic group such as a sulfone group is used as the resin binder. Since it is used as
The dispersibility and dispersion stability of the hexagonal ferrite powder containing Nb in the magnetic layer are significantly improved, the packing ratio of the magnetic powder is improved, and the characteristics of the hexagonal ferrite powder containing Nb can be fully utilized. At the same time, the surface property of the coating film is improved, and the roughness of the coating film surface during orientation is significantly reduced.

Further, as a resin binder, the glass transition point is from -20 ° C to-
By using the one having a temperature of 60 ° C., for example, the surface property of the coating film and the packing density of the magnetic powder during the calendar treatment are further improved.

As a result, the magnetic recording medium of the present invention exhibits excellent recording / reproducing output in the short wavelength region.

(Example) Next, the Example of this invention is described.

The amounts of resin binders shown in the following examples are all 100
% Solid content.

Example 1 Co, Ti, Nb substitution type 100 parts by weight Ba-ferrite powder * 1 Sulfonated vinyl chloride resin * 2 6 parts by weight Polyurethane resin TA-107 * 3 6 〃 Toluene / cyclohexanone 180 〃 = 1/2 mixture Solution Lecithin 3〃 Alumina (average particle size 0.3μm) 3〃 Stearic acid 2〃 * 1: Nb substitution amount y = 0.45 (average particle size = 0.05μm, plate ratio =
4/1, Hc = 600 Oe) * 2: Molecular weight = 20,000, glass transition point = 68 ° C., a sulfonated vinyl chloride-vinyl acetate copolymer resin having one sodium sulfonate group per molecule.

* 3: Product name, Nippon Polyurethane Company, glass transition point =-
20 ° C.

After the above materials were mixed, they were dispersed for another 2 hours with a sand grinder. 3 parts by weight of Coronate L (polyisocyanate, trade name: manufactured by Nippon Polyurethane Co., Ltd.) was added to the obtained magnetic coating material, and then coated on a polyester film with a reverse coater.

The obtained coating film was cured at 40 ° C. for 3 days and then cut into 1/2 inch width to prepare a magnetic tape, which was used for measurement.

Example 2 In place of the polyurethane resin TA-107 of Example 1, a phosphoric acid group-containing polyurethane resin (molecular weight = 40,000, phosphoric acid group number per molecule = 5, glass transition point = -30 ° C) was blended. A magnetic coating material was prepared in the same manner as in Example 1 except that the magnetic tape was prepared.

Example 3 After the magnetic coating material was applied onto a polyester film in Example 2, the coating film surface was dried while passing through a magnetic field in a direction perpendicular to the film surface, while the coating film surface was still undried.
A magnetic tape was produced in the same manner as.

Example 4 Carbon black 10 parts by weight (Ketjenblack EC) Sulfonated polyurethane resin * 4 10 〃 Toluene / cyclohexanone 80 〃 = 1/2 mixed solution * 4: Molecular weight = 20,000, glass transition point = -20 ° C, 1 Number of sulfone groups per molecule = 1.0.

The above raw materials were mixed and further dispersed by a sand grinder. Coronate L3 is added to 100 parts by weight of the obtained paint.
After adding the weight part and apply | coating to a polyester film, cure treatment was given and the polyester film which has a conductive primer layer was produced.

On the film having the conductive primer layer, the magnetic coating material prepared under the same conditions as in Example 3 was applied according to the same procedure to prepare a magnetic tape.

Comparative Example 1 Same as Example 1 except that Co, Ti substitution type Ba-ferrite powder (average particle size = 0.05 μm, plate ratio = 4/1, Hc = 600 Oe) containing no Nb was used. To produce a magnetic tape.

Comparative Example 2 The vinyl chloride-vinyl acetate-vinyl alcohol copolymer resin / VAGH was used as the sulfonated vinyl acetate resin in Example 1.
A magnetic tape was produced in the same manner as in Example 1 except that (Molecular weight: 20,000) was used.

The surface properties of the tapes obtained in the above Examples and Comparative Examples, the filling factor of the magnetic recording layer, the saturation magnetization, the vertical orientation ratio, and the Y signal output when the tape was recorded / reproduced at a relative speed of 5.8 m and a frequency of 7 MHz were measured. The measurement results are shown in the following table. The filling rate of the magnetic recording layer was determined as follows. First,
The magnetic layer thickness of the obtained magnetic tape was measured, and the theoretical saturation magnetization Mso per unit area of these magnetic tapes was measured.
Was calculated. On the other hand, the saturation magnetization per unit area of these magnetic tapes was measured with a vibration type magnetometer, and this was designated as Ms. From the above, the filling factor of the magnetic recording layer in the magnetic tape was defined as Ms / Mso.

Example 5 Co, Ti, Nb substitution type 100 parts by weight Ba-ferrite powder * 1 Sulfonated polyurethane resin * 5 10 parts by weight Toluene / cyclohexanone 180 〃 = 1/2 mixed solution Lecithin 2 〃 Alumina (average particle size 0.3 µm) 3〃 Stearic acid 2〃 * 1: Nb substitution amount y = 0.45 (average particle size = 0.05μm, plate ratio =
4/1, Hc = 600Oe) * 5: Aliphatic polyurethane resin with 1 sodium sulfonate group per molecule (sulfone group amount = 0.014 mmol / g),
Molecular weight = 15,000, glass transition point = -25 ° C.

After the above materials were mixed, they were dispersed for another 2 hours with a sand grinder. Add 3 Coronate L to the obtained magnetic paint.
After adding parts by weight, apply it on a polyester film with a reverse coater, and apply a magnetic field (4 kO
e) After drying while passing underneath, the surface was smoothed by performing a calendar treatment.

The obtained coating film was cured at 40 ° C. for 3 days and then cut into 1/2 inch width to prepare a magnetic tape, which was used for measurement.

Example 6 Instead of the sulfonated polyurethane resin * 5 of Example 5, a phosphate group-containing aliphatic polyurethane resin (molecular weight = 1
In addition, the number of phosphoric acid groups per molecule = 2 (phosphoric acid group amount = 0.2 mmo
l / g), glass transition point = -30 ° C. A magnetic coating material was prepared under the same conditions except that (4) was used, and a magnetic tape was prepared in the same manner as in Example 5.

Example 7 Instead of the sulfonated polyurethane resin * 5 of Example 5, a sulfo group-containing polyester resin (molecular weight = 20,000,
Number of sulfone groups per molecule = 1 (Amount of sulfone groups = 0.05 mm
ol / g), glass transition point = −40 ° C.), a magnetic coating material was prepared under the same conditions, and a magnetic tape was prepared in the same manner as in Example 5.

Example 8 As a resin binder component in Example 5, 5 parts by weight of a sulfonated polyurethane resin * 5 and an amino group-containing vinyl chloride-vinyl acetate copolymer resin (molecular weight = 30,000, number of amino groups per molecule = 1 (amino A magnetic coating material was prepared under the same conditions except that a mixed system of 5 parts by weight (base amount = 0.33 mmol / g, glass transition point = 70 ° C.) was used, and a magnetic tape was prepared in the same manner as in Example 5.

Comparative Example 3 Same as Example 5 except that Co, Ti substitution type Ba-ferrite powder (average particle size = 0.05 μm, plate ratio = 4/1, Hc = 600 Oe) containing no Nb was used. To produce a magnetic tape.

Comparative Example 4 Instead of the sulfone group-containing polyester resin of Example 7, a polyester resin containing no sulfone group (molecular weight =
A magnetic coating material was prepared under the same conditions except that 20,000, glass transition point = 20 ° C.) was used, and a magnetic tape was prepared in the same manner as in Example 7.

Comparative Example 5 Instead of the amino group-containing vinyl chloride-vinyl acetate copolymer resin of Example 8, an amino group-free vinyl chloride-vinyl acetate copolymer resin (molecular weight = 20,000, glass transition point = 70) was used.
Magnetic coating was prepared under the same conditions except that (.degree. C.) was used, and a magnetic tape was prepared in the same manner as in Example 8.

Using the magnetic tapes obtained in the above Examples 5 to 8 and Comparative Examples 3 to 5, each characteristic was measured in the same manner as in Example 1. Table 2 shows the results.

As is clear from the results shown in Table 2, by adjusting the glass transition point of the resin binder containing a polar group in the range of -20 ° C to -60 ° C, the smoothness of the surface and the filling factor of the magnetic recording layer can be further improved. Is improved, the saturation magnetization is improved,
It can be seen that the recording / reproducing characteristics on the short wavelength side are further improved.

[Effects of the Invention] As is clear from the above examples, the magnetic recording medium of the present invention has a large saturation magnetization and is extremely excellent in short wavelength recording characteristics.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Minoru Yanagisawa, Minoru Komushi Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa, Toshiba Research Institute Co., Ltd. Address within Toshiba Research Institute Co., Ltd. (56) Reference JP-A-62-202321 (JP, A) JP-A-63-40302 (JP, A)

Claims (1)

(57) [Claims] 1. A magnetic recording layer formed by coating a hexagonal ferrite magnetic powder having an average particle size of 0.01 μm to 0.2 μm in which a part of Fe is replaced with Nb on a substrate together with a resin binder and a curing agent. In the magnetic recording medium, the total amount of the resin binder and the amount of the curing agent contained in the magnetic recording layer is 15 parts by weight or less with respect to 100 parts by weight of the magnetic powder,
At least 20% or more of the resin binder is a sulfo group,
It contains a phosphoric acid group, at least one polar group selected from these metal bases and amino groups, and has a glass transition point of -20.
A magnetic binder, which is a resin binder in the range of ℃ to -60 ℃, wherein the filling factor of the magnetic recording layer is 0.93 or more and less than 1.