JPH05135354A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To provide a multilayered coated type magnetic recording medium excellent in recording and reproducing property in a wide range from a long wavelength range to a short wavelength range. CONSTITUTION:An underside magnetic layer containing magnetic powder having relatively low coercive force suitable for long wavelength recording, is provided on a nonmagnetic base body as the underside magnetic layer, an upperside magnetic layer containing a magnetic powder suitable for recording in short wavelength and having small grain diameter and high coercive force is provided on it. And kinds and volume of the magnetic powder and resin binder contained in the upperside magnetic layer are properly selected, and, at the same time, much lubricant is contained in upperside magnetic layer at the stable condition. As a result, as enough stable lubricant effect under various environments for usage is shown, surface and interface of both magnetic layers of upperside and underside become smooth even when upperside magnetic layer is made thin. Accordingly, an excellent magnetic recording medium providing thin and smooth upperside magnetic layer improved in dispersing ability and filling ratio of magnetic powder and keeping stable traveling property and durability even at a wide range environment for usage can be obtained.

Description

Detailed Description of the Invention

[Object of the Invention]

[0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high recording density magnetic recording medium having excellent recording characteristics in a wide wavelength range from a short wavelength range to a long wavelength range.

[0003]

2. Description of the Related Art Conventionally, a coating type magnetic recording medium has been obtained by coating a substrate such as a polyester film with magnetic powder such as γ-ferrite or iron powder together with a resin binder. This resin binder is added to ensure the dispersibility of the magnetic powder and the running durability of the medium. Normally, the addition amount is required to be at least 10 parts by weight or more per 100 parts by weight of the magnetic powder. ..

By the way, as one of the methods for meeting the recent demand for higher recording density for magnetic recording media, it has been attempted to reduce the particle size of the magnetic powder and increase its coercive force (Hc). .. As described above, ultrafine hexagonal powder such as metal powder or barium ferrite having a particle diameter of 0.3 μm or less is currently suitable as the magnetic powder for the magnetic recording medium which has been made finer to cope with higher recording density. because,
A medium coated with these magnetic powders at a high filling rate is
This is because, in addition to the fine magnetic powder, it is less susceptible to the diamagnetic field effect, which enables high-density recording.

However, in general, a magnetic layer using a magnetic powder having a small particle size and high coercive force has a large recording / reproducing output in the short wavelength region, but has a drawback that the recording / reproducing output in the long wavelength region decreases. ing. Therefore, in order to eliminate this drawback, a lower magnetic layer using a magnetic powder having a relatively low coercive force as a long-wavelength recording magnetic powder is provided on a substrate, and a small particle size, high coercive force short wavelength recording is provided thereon. A two-layer type coated magnetic recording medium has been proposed which is provided with an upper magnetic layer using magnetic powder for use and is adapted to a wide wavelength range.

In the case of such a two-layer type coating medium, it is desirable to set the thickness of the upper magnetic layer in the submicron region in order to fully exhibit its characteristics. For that purpose, not only the surface roughness of the upper magnetic layer should be made as small as possible to enhance the smoothness of the surface, but also the surface roughness of the lower magnetic layer should be made as small as possible. It is also necessary to prevent the property from affecting the surface property of the upper magnetic layer.

Further, in order to improve the recording / reproducing output of the medium, in the magnetic paint constituting the upper magnetic layer, the magnetic powder particles are dispersed in the resin binder in a state close to the primary particles, and the magnetic powder particles are dispersed. It is required to increase the filling rate of the resin binder into the resin binder as much as possible.

[0008]

However, in such a conventional two-layer type coating medium, since the surface property of the lower magnetic layer generally affects the surface property of the upper magnetic layer, the coating thickness of the upper magnetic layer is Was difficult to maintain in the submicron range. Furthermore, the disturbance generated at the interface between the lower magnetic layer and the upper magnetic layer causes a problem of increasing noise during recording and reproduction. Such a problem is particularly remarkable when the lower magnetic layer and the upper magnetic layer are simultaneously coated. For example, when an upper magnetic layer having a coating thickness of 0.5 μm or less is formed by simultaneous coating, part of the lower magnetic layer projects into the upper magnetic layer. Simultaneous coating here means that, after applying the lower layer coating film, the upper layer is coated before the organic solvent of the coating film evaporates and does not finish drying. Even if time passes, this is regarded as simultaneous.

The surface property of the interface between the upper magnetic layer and the upper and lower magnetic layers as described above is also dependent on various organic and inorganic additives such as carbon black used with a resin binder, an abrasive or a lubricant. It is greatly affected by the type and quantity of.

For example, the surface property of the medium is improved and smoothed by increasing the amount of lubricant added. However, even if the added amount is increased, it is difficult to obtain a lubricant that satisfies the condition that the running durability of the medium is maintained for a long time under a wide range of use environments, and it has been tried to add various lubricants so far. However, at present, a medium having both surface smoothness and running durability has not been obtained yet.

Furthermore, as the magnetic powder particles used in the upper magnetic layer are made smaller, it becomes very difficult to mix them with the resin binder in a state close to the primary particles. Similarly, it is becoming more difficult to reduce the amount of resin binder as much as possible to increase the filling rate of the magnetic powder and reduce the surface roughness. As described above, with the decrease in the particle size of the magnetic powder particles, the technical difficulty for improving the recording / reproducing output of the magnetic recording medium is currently doubled.

That is, at present, it cannot be said that the dispersion technique of ultrafine magnetic powder for the upper magnetic layer has been sufficiently established in response to the demand for higher recording density, and the smoothness of the coating film surface and the upper magnetic layer are not achieved. The improvement of the packing density of the magnetic powder is not sufficient. As a result, the short wavelength recording / reproducing output as expected from the particle size of the magnetic powder in the upper magnetic layer has not been obtained yet. Furthermore, if the dispersion of the magnetic powder used in the upper magnetic layer coating is poor, there is a problem that noise during recording and reproduction increases, which also contributes to making short-wavelength recording difficult. ..

Therefore, the present invention has been made to solve the above-mentioned conventional problems, and in a high recording density magnetic recording medium having a two-layer structure, the dispersibility and filling rate of the magnetic powder in the upper magnetic layer are increased. An object of the present invention is to provide an excellent high recording density magnetic recording medium which has improved surface smoothness and exhibits stable running durability for a long time even in a wide range of use environments.

[Constitution of the Invention]

[0015]

The magnetic recording medium of the present invention comprises a lower magnetic layer containing magnetic powder for long wavelength recording and a resin binder, and magnetic powder for short wavelength recording and a resin binder on a non-magnetic substrate. A magnetic recording medium formed by forming an upper magnetic layer containing, the amount of the resin binder contained in the upper magnetic layer is in the range of 2 to 12 parts by weight with respect to 100 parts by weight of the magnetic powder, and the upper magnetic layer. It is characterized in that the amount of the lubricant contained in is in a range of at least a half amount and not more than 5 times the amount of the resin binder.

Examples of the lubricant that can be used in the present invention include fatty acids or fatty acid esters having 12 or more carbon atoms, silicone oil, aliphatic modified silicone oil, fluorinated silicone oil, fluorinated hydrocarbon oil and the like. These can be used alone or as a mixture of several kinds.

When the amount of the lubricant added is less than half the amount of the resin binder, the desired smoothing effect of the coating film cannot be achieved, while when it exceeds 5 times the amount of the resin binder, the amount of the magnetic powder It is not preferable because the filling rate decreases and the recording / reproducing output of the medium decreases.

In the present invention, the upper magnetic layer is thin as described later, the amount of resin binder added is small, and the coating film structure has fine gaps between the magnetic powder particles of the upper magnetic layer. The magnetic layer can also be used as a lubricant replenishing layer. In this case, a large amount of lubricant may be added to the lower magnetic layer to form a lubricant replenishing layer, or
A multi-layer structure in which an intermediate layer containing a large amount of lubricant is provided between the upper magnetic layer containing a large amount of lubricant and the lower magnetic layer containing a conventional amount of lubricant may be used.

In the present invention, the resin binder is defined as the total amount of the resin binder added during dispersion and the curing agent added prior to coating. The amount of resin binder is
The amount is preferably 2 to 12 parts by weight, more preferably 3 to 8 parts by weight, based on 100 parts by weight of the magnetic powder.
When the amount of the resin binder is less than 2 parts by weight, it is difficult to sufficiently disperse the magnetic powder, the desired recording / reproducing output cannot be obtained, and the required coating film strength cannot be obtained, which is not preferable. On the other hand, when the amount of the resin binder exceeds 12 parts by weight, the filling rate of the magnetic powder is lowered and the desired recording / reproducing output cannot be obtained, which is not preferable.

The thickness of each layer in the magnetic recording medium of the present invention is 1 to 3 μm for the lower magnetic layer and 0.05 to 0.5 for the upper magnetic layer.
It is desirable to be constructed in the μm range. When the thickness of the lower magnetic layer is less than 1 μm, it becomes difficult to secure the characteristics as a long wavelength recording layer. When the thickness of the lower magnetic layer is thicker than 3 μm, it becomes difficult to secure the surface property and the coating film strength. On the other hand, if the thickness of the upper magnetic layer is less than 0.05 μm, the desired function in adhesive strength and short wavelength recording / reproducing characteristics will be deteriorated, which is not preferable. The upper limit of the film thickness of the upper magnetic layer is set from the viewpoints of a decrease in surface property and a space loss effect during long wavelength recording.

As the resin binder for the lower magnetic layer and the upper magnetic layer usable in the present invention, a hydroxyl group, a carboxyl group, a phosphoric acid group, a sulfone group or their metal bases, or an amino group, an alkylamino group, or ammonium. Group, vinyl chloride vinyl acetate copolymer having a polar group such as alkylammonium group, polyester resin, polyether resin, polyurethane resin, polyacrylic resin, etc. are suitable, and those having a metal sulfonate group are particularly suitable for the present invention. Suitable as a resin binder. The reason is probably that these resins are well adsorbed on the magnetic powder and help dispersion. It is not necessary for these polar groups to exist alone in the resin molecule, and even if a plurality of types of polar groups coexist in the same molecule, the object of the present invention is not impaired at all. The number of these polar groups in the resin binder is important and is at least in the range of 0.01 mmol / g to 4.0 mmol / g, more preferably for the purposes of the present invention.
The range is 0.05 mmol / g to 2.0 mmol / g.

In the present invention, the resin binder preferably has a molecular weight of 1,000 to 60,000. However, it is necessary to increase or decrease the molecular weight of the resin binder of each layer depending on the purpose. For example, in the upper magnetic layer, the molecular weight of the resin binder is
As (Mw), those relatively small within the above range are used. If the molecular weight is less than 1,000, the effect as a binder is small, and if it exceeds 20,000, the required amount of the resin binder increases and it becomes difficult to achieve a desired high filling rate. That is, the molecular weight of the resin binder for the upper magnetic layer is 1,0
The range of 00 to 20,000 is desirable. Further, in general, a resin binder having a high molecular weight of 10.000 or more is used for the lower magnetic layer for the purpose of improving durability.

By the way, among the polar groups of the resin binder according to the present invention, the metal sulfonate base is introduced as follows. First, when the resin binder containing a metal sulfonate group is a resin obtained by vinyl polymerization, it is usually obtained by copolymerizing a vinyl monomer containing these polar groups with an ordinary vinyl monomer. Further, when the resin binder containing the polar group is a polyester resin or a polyurethane resin, a polyvalent basic acid or polyhydric alcohol which is a constituent of these and a polyvalent basic acid or a polyvalent acid in which the polar group is introduced. It is obtained by mixing alcohols and conducting a condensation reaction. Examples of monomers, polybasic acids or polyhydric alcohols used in the production of a resin containing a metal sulfonate base include, for example, vinyl sulfonic acid, vinyl benzene sulfonic acid, and 2-acrylamide.
2-Methylpropanesulfonic acid metal salt, or a compound represented by the following chemical formula, (M is an alkali metal, ph is an aromatic ring) and the like.

Usable vinyl resin monomers copolymerized with vinyl monomers having polar groups such as metal sulfonate groups are vinyl chloride, vinyl alcohol, maleic anhydride, vinyl acetate, various acrylate monomers, vinylidene chloride, Examples thereof include various monomers such as vinyl acetal, vinyl butyral, acrylic acid ester, acrylonitrile, and styrene.

The usual polyhydric alcohols copolymerized with polyvalent basic acids having polar groups such as metal sulfonates are 1,4-butanediol, 1,6-hexamethylenediol and cyclohexanediol. , Ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, glycerin, neopentyl alcohol and the like. As the polybasic acid copolymerized with a polyhydric alcohol having a polar group such as a metal sulfonate base, terephthalic acid, isophthalic acid, adipic acid, sebacic acid, oxalic acid, succinic acid, glutaric acid, pyromellitic acid, Examples thereof include suberic acid and azelaic acid.

Among the resins according to the present invention containing the metal sulfonate group obtained as described above, the polyester polyurethane resin is particularly excellent, and particularly has an aliphatic chain having 4 to 18 carbon atoms in the resin skeleton. A polyester polyurethane resin in which a polybasic acid or a polyhydric alcohol is introduced is suitable. Furthermore, these polyester polyurethane-based resins are aromatic diisocyanates such as tolylene diisocyanate or fats such as 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, and isophorone diisocyanate. It is obtained by reacting a group diisocyanate with a chain extender such as ethylene glycol, and can further improve the dispersibility and durability of the polyester resin. Various resin binders may be used together for the purpose of improving the mechanical strength and running property 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 thereof include vinyl acetate resins, vinyl alcohol resins, and mixtures or copolymers thereof. The compounding amount of these combined resin binders is appropriately set within 80% by weight with respect to the total resin binder.

In the paint using the above resin binder,
In order to increase the mechanical strength of the coating film and increase its durability, a polyamine or polyisocyanate-based curing agent is usually added when it is coated on a substrate together with the ferromagnetic powder.

In the above-mentioned constitution of the present invention, as the magnetic powder used in the lower magnetic layer, one having a coercive force (Hc) of 300 to 1,000 Oe and a saturation magnetization (Ms) of 70 emu / cc or more is suitable. .. As the magnetic powder having such characteristics, for example, a metal powder containing iron as a main component, ferrosoferric oxide powder, γ-ferrite powder, Co-modified γ-ferrite powder, chromium dioxide powder, iron nitride powder, etc. should be used. You can And the particle size is
0.3 to 1.0 μm is suitable.

The magnetic powder usable for the upper magnetic layer is
There is a metal powder mainly composed of iron or a hexagonal ferrite powder. Examples of the metal powder mainly composed of iron include F
e-Al metal powder, Fe-Ni metal powder, Fe-Al-P metal powder, F
e-Ni-Si-Al metal powder, Fe-Si-Al-Mn metal powder, Fe-Mn-Zn
Metal powder, Fe-Co-Ni metal powder, Fe-Co-Ni-Cr metal powder,
Fe-Co-Ni-P metal powder and the like can be mentioned.

Hexagonal ferrite powder is particularly suitable for this purpose, and is M-type (Magnetoplumbitetype) or W-type hexagonal Ba ferrite, Sr ferrite, lead ferrite, Ca ferrite, a solid solution thereof, or the following formula. An ion-substituted compound represented by can be used.

Ma O.n (Fe _1-x Mb _x ) ₂ O ₃ (In the formula, Ma represents any one element of Ba, Sr, Ca and Pb, and Mb represents Co, Zn, Ni and Cu. , Mg, Mn, In, Ti, Sn, Ge, Zr, Hf, V, N
It represents at least two elements selected from the group consisting of b, 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 Fe atoms, which is a constituent element of these uniaxial hexagonal ferrite crystals, is composed of a divalent metal and a pentavalent metal. Substituted with a certain Nb, or
Further, it is suitable to substitute Sn atom in the range of 0.05 to 0.5 per chemical formula, and the substitution amount is 500 to 500.
The amount is 3,000 Oe.

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

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. The substitution amount per chemical formula of ^II is about 0.5 to
It is 1.2.

Looking at the relationship of the substitution amounts of these substitution elements, for example, in magnetoplumbite type Ba ferrite, the chemical formula of the substitution body is Ba Fe _{12- (X + Y (+ Z))} M ^II _x M ^V It is represented by _y (M ^IV _z ) O ₁₉ . Where x, y, and z are M ^II , M ^V and M ^IV
It is the substitution amount per chemical formula of the element. Since M ^II , M ^V and M ^IV are divalent, pentavalent and tetravalent, respectively, and the substituted Fe atom is trivalent, considering the valence compensation, the relation of y = (xz) / 2 Holds. That is, the substitution amount of M ^V is uniquely determined from the substitution amounts of M ^II and M ^IV .

When Sn is used as the M ^IV element, the appropriate range of the substitution amount is 0.05 to 0.5 per one chemical formula of hexagonal ferrite.

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

In most of the metal powders containing iron as a main component used in the upper magnetic layer in the present invention, they have a needle-like crystal form, and the grain size is represented by the length of the major axis.
The desirable average particle size of the metal powder used in the present invention is 0.05 to
It is in the range of 0.3 μm.

The hexagonal ferrite powder is a hexagonal plate-shaped single crystal, and its grain size is represented by the length of the diagonal line of the hexagonal plate surface. The desirable average particle size of the hexagonal ferrite powder used in the present invention is in the range of 0.01 to 0.1 μm. If the particle diameter of these magnetic powders is less than 0.01 μm, the magnetization and coercive force are reduced and the recording / reproducing output of the magnetic recording medium is reduced, which is not preferable. On the contrary, if the particle size exceeds 0.1 μm, not only the improvement effect of the short wavelength recording / reproducing output becomes small but also the noise at the time of recording / reproducing remarkably increases.
After all it is not preferable.

The coercive force of the metal powder and the hexagonal ferrite powder according to the present invention is preferably in the range of 500 to 3,000 Oe. If the coercive force is less than 500 Oe, the recording signal on the recording medium does not remain sufficiently,
If e is exceeded, it becomes difficult to write a signal by a normal recording / reproducing head, and thus neither is preferable.

Further, between the lower magnetic layer and the upper magnetic layer, for example, an intermediate layer for the purpose of improving the adhesiveness of both layers and various characteristics is provided to form a film structure of three or more layers. Good.

[0041]

In the present invention, since the lower magnetic layer having a low coercive force and high saturation magnetic susceptibility and the upper magnetic layer containing a high coercive force and a small particle size magnetic powder at a high packing rate are provided, long wavelength recording and short wavelength A medium excellent for both recording is obtained.

In the upper magnetic layer according to the present invention, a resin binder containing a polar group having high adsorptivity such as a sulfonate group and having a molecular weight limited to a predetermined range is selected. Is well adsorbed on the magnetic powder to help disperse the magnetic powder. Therefore, even if the addition amount of the resin binder to 100 parts by weight of the metal powder or the hexagonal ferrite powder is 2 to 10 parts by weight, which is significantly smaller than that in the case of the magnetic layer of an ordinary magnetic recording medium, The magnetic powder can be sufficiently dispersed.

It has been found that when a magnetic powder having a small particle size is used and the resin content is limited as described above, a larger amount of lubricant can be added as compared with the conventional case.

Since the lubricant added in a large amount is retained in the gap between the magnetic powders, which was originally filled with the resin binder, it does not spill on the surface of the coating film due to environmental changes such as temperature. Further, at the time of recording / reproducing, the head is pressed and an appropriate amount thereof is constantly supplied to the surface of the medium. Therefore,
A stable lubricant addition effect is exhibited over a long period of time.

[0045]

EXAMPLES The details of the present invention will be described below based on examples. Prior to the examples, a general description will be given to the present invention.

In the magnetic coating material for the lower magnetic layer used in the magnetic recording medium of the present invention, magnetic powder, a resin binder and an organic solvent are mixed, and then the magnetic powder is further dispersed by using a ball mill, a sand grinder or the like. Made by. Examples of the organic solvent include toluene, xylene, cyclohexanone, methyl ethyl ketone, methyl isobutyl ketone, and nitropropane, and these can be used alone or in admixture of several kinds. At this time, a polyamine- or polyisocyanate-based curing agent can be added to the coating material in order to increase the mechanical strength and durability of the coating film. As described above, in the present invention, the resin binder is defined as the total amount of the resin binder added at the time of dispersion and this curing agent added prior to coating. In addition to the curing agent, if desired, a dispersant, an abrasive, a conductive agent, and the like described later can be added.

As the dispersant for dispersing the magnetic powder of the lower magnetic layer or the upper magnetic layer in the resin binder,
An anionic surfactant, a cationic surfactant, or a nonionic surfactant can be used. Among these, noniphenol ester-based phosphoric acid and lecithin are particularly effective as anionic surfactants. In addition, in the kneading step of the magnetic powder and the resin binder, in addition to such a dispersant, a silane coupling agent or a titanium coupling agent may be added to make the dispersion uniform.

For the upper magnetic layer or the lower magnetic layer according to the present invention, for example, Ti 0 ₂ , α-Fe ₂ 0 ₃ , Cr ₂
0 ₃ , α-Al ₂ O ₂ , SiO ₂ and SiC, Mohs hardness 5
Abrasives composed of the above inorganic powders can be used.

If desired, a conductive powder such as carbon black, titanium oxide or tin oxide may be added to the lower magnetic layer.

As the non-magnetic substrate according to the present invention, a polyester film, a polycarbonate film, a polyimide film, a polysulfonate film or the like can be used. Then, it is desirable that a back layer containing a conductive powder such as carbon black is formed on the surface of this substrate surface on the side not coated with the magnetic paint. This back layer plays a very important role in preventing the generation of static electricity on the surface of the substrate, improving the running property, and preventing blocking with the magnetic layer. Therefore, this back layer is usually made to have a surface resistance of 10 ⁶ ohms or less. Further, the surface property of the back layer is also very important, and the surface roughness measured by a stylus-type surface roughness meter may be prepared so that the 10-point average roughness Rz is 0.05 μm or less. desirable.

Furthermore, in the magnetic recording medium of the present invention,
In order to enhance the adhesive strength of the magnetic layers of the upper magnetic layer and the lower magnetic layer and thereby enhance the durability of the medium, plasma treatment of the base surface or the base layer containing carbon black coated on the base surface is remarkable. It is valid. Such an underlayer, a lower magnetic layer, and an upper magnetic layer can also be produced by one-time coating with a three-layer coating slot die coater.

In preparing the coating material for the upper magnetic layer, magnetic powder such as metal powder for the upper magnetic layer or hexagonal ferrite powder is mixed and dispersed in the resin binder, the organic solvent, and the above-mentioned various additive mixtures. .. What is important in this mixing / dispersing step is to uniformly disperse the magnetic powder in the resin binder, which enables the magnetic powder to be highly filled in the magnetic layer. Further, as the dispersion becomes more uniform, the recording / reproducing noise is reduced and the surface property of the magnetic layer is enhanced, so that the short wavelength output is increased.

As a method for obtaining a uniform dispersion, a method in which the above mixture is kneaded in advance and the kneaded product is further dispersed by a ball mill or a sand grinder is effective. When the hexagonal ferrite powder is produced by the glass crystallization method and obtained in a slurry state washed out in water, the resin binder and the additive may be directly added to the slurry and kneading may be performed. This kneading method is usually called a flushing method, and the resin binder molecule or the dispersant molecule can be adsorbed on the powder surface without being affected by the agglomeration action in the drying step of the magnetic powder, so that it is extremely uniform. Dispersion can be easily achieved.

The kneaded product obtained by the above-mentioned flushing method is dehydrated under reduced pressure while adding an organic solvent, or taken out as a dry powder, then added with an organic solvent and transferred to a dispersing machine such as a sand grinder, and further dispersed uniformly. Let

Next, the magnetic coating material which has undergone the dispersing step is passed through a filter for filtration, and then a curing agent such as polyisocyanate is added thereto, and then a reverse coater, a gravure coater,
It is applied onto the substrate by an ordinary application method such as an applicator coater or a slot die coater.

Among these, the slot die coater is suitable for the purpose of the present invention, and the lower magnetic layer and the upper magnetic layer can be coated simultaneously. Simultaneous here means that, as described above, after coating the lower layer coating, the upper layer is coated before the organic solvent of the coating evaporates and dries, and each layer is coated. This is regarded as simultaneous even if some time has passed during the application. Therefore, the slot dies for simultaneous coating do not necessarily have to be combined into one.

The multi-layer coating film formed on the substrate through the above-mentioned coating and coating steps is introduced into the longitudinal or vertical orientation magnetic field of the substrate surface before the organic solvent in the film evaporates. The contained magnetic powder is oriented so that its easy axis of magnetization is oriented in the desired orientation magnetic field direction.

The vertical orientation is carried out by arranging the N-pole iron core and the S-pole iron core vertically and passing them through the surface of the substrate in a direction perpendicular to the magnetic field generated between the iron cores. The longitudinal orientation is performed by passing the base between the same poles of the facing iron cores or passing the base through the center of the solenoid (air core coil). It is desirable that the organic solvent remaining in the coating film is removed in the magnetic field during the orientation process and that the coating film be in a dry state when it leaves the magnetic field. Examples of means for removing the organic solvent in the magnetic field include a hot plate and a vapor suction device, or a hot air blowing and exhaust device.

After the coating film is subjected to a desired orientation treatment through the steps described above, the surface is smoothed by a calender and further cured, and then slit to a desired width to obtain the magnetic recording medium of the present invention. can get. In the magnetic recording medium of the present invention, the state of the cut surface in the slit step is important, and when the slitting means is inappropriate, cracks occur on the surface of the magnetic layer in the vicinity of the cut portion and the recorded and reproduced image is disturbed. May occur or running durability may deteriorate. Such problems can be solved by slitting with a laser beam.

Specific examples of the present invention will be described below.

Example 1 First, as a <coating material for lower magnetic layer>, the following material composition was used: Co-γ ferrite powder (Hc 650 Oe, number average particle size 0.5 μm, specific surface area d = 40 m ² / g) 100 parts by weight carbon black (average particle size 0.02 μm, specific surface area d = 200 m ² / g) 5 parts by weight Sulfonate-containing urethane resin (Mw = 30,000, Na sulfonate content 0.3 mmol / g) 8 parts by weight Parts Vinyl chloride resin (Mw = 20,000, carboxyl group content 0.4 mmol / g) 4 parts by weight Soybean oil lecithin 2 parts by weight Stearic acid 2 parts by weight Butyl stearate 2 parts by weight Cyclohexanone / methyl ethyl ketone (1/1) mixed solvent 30 Parts by weight: Weigh in the kneader.

After kneading for about 30 minutes, 150 parts by weight of the mixed solvent as described above was added and diluted with a dissolver.
Further dispersed with a sand grinder. The paint obtained is
After passing through a filter with a pore size of 0.6 μm,
Coronate L for 100 parts by weight of Co-γ ferrite powder
(Brand name: Tolylene diisocyanate-based curing agent 50% diluted) 1.0 part by weight was added and mixed to obtain a lower magnetic layer coating material.

<Upper magnetic layer coating material> As a material, the following material composition: Co, Ti, Nb-substituted barium ferrite powder (Hc 1,000 Oe, number average particle diameter 0.05 μm, specific surface area d = 38 m ² / g) 100 parts by weight Sulfone group-containing urethane resin (Mw = 10,000, Na sulfonate content 0.5 mmol / g) 6 parts by weight Gafac RE-610 (Product name: Phosphate ester surfactant, Toho Chemical Co., Ltd.) 2 parts by weight Parts Stearic acid 3 parts by weight Alumina (average particle size 0.4 μm) 3.5 parts by weight Cyclohexanone / methyl ethyl ketone (1/1) mixed solvent 25 parts by weight: is weighed into a kneader.

After kneading for about 30 minutes, 170 parts by weight of the above mixed solvent was added, diluted with a dissolver, and further dispersed with a sand grinder. The obtained paint was passed through a filter with a pore size of 0.3 μm, and then Coronate L (trade name: Tolylene diisocyanate curing agent 50% dilution) was added to 100 parts by weight of barium ferrite powder in the paint.
0 part by weight and 1.0 part by weight of butyl stearate were added and mixed, and this was used as the upper magnetic layer coating material.

The conductive paint containing carbon black was applied to the back surface while supplying the coating materials for <lower magnetic layer> and <upper magnetic layer> obtained above to the respective slot die coaters arranged in this order. It was applied on the applied polyester film (layer thickness: 11 μm). The thickness of each coating film was controlled by the amount supplied to the die so that the lower magnetic layer was 2.5 μm and the upper magnetic layer was 0.5 μm. While the obtained two-layer coating film was not dried, it was passed through a solenoid generating a magnetic field strength of 6 kOe, and the organic solvent was volatilized and dried.

The coating film thus obtained was allowed to stand in a curing oven at 40 ° C. for 4 days and then slit into a tape having a width of 8 mm. With respect to this medium sample, surface roughness, recording / reproducing characteristics, S / N ratio, still durability, and jitter characteristics were evaluated. The evaluation result will be described later.

Example 2 Instead of the Co, Ti, Nb-substituted barium ferrite powder in <Coating for upper magnetic layer> of Example 1, Fe-Ni metal powder (Hc
1,500 Oe, number average particle size 0.15 μm, specific surface area d = 50 m ² /
A coating material was prepared in the same manner as in Example 1 except that g) was used and the amount of the sulfonic group-containing urethane resin was 8.5 parts by weight.

The paint obtained has a pore size of 0.3.
After passing through a μm filter, to 100 parts by weight of barium ferrite powder in the paint, 2.0 parts by weight of Coronate L (trade name: Tolylene diisocyanate curing agent 50% dilution), 1.5 parts by weight of myristic acid, and 1.0 part by weight of butyl stearate was added and mixed, and this was used as the coating material for the upper magnetic layer.

The coating material for the upper magnetic layer obtained as described above was used in Example 1
A two-layer coating type 8 mm width tape was prepared in the same manner as in Example 1 except that it was used in place of the <upper magnetic layer coating material> in Example 1 and was used for evaluation.

Example 3 6 parts by weight of the sulfo group-containing urethane resin in the <upper magnetic layer coating material> of Example 1 was replaced by 2 parts by weight, 3 parts by weight of stearic acid, 2 parts by weight of stearic acid and myristin were added. A two-layer coating type 8 mm wide tape was prepared in the same manner as in Example 1 except that 3 parts by weight of acid was used, and this was provided for evaluation.

Example 4 The Co, Ti, Nb-substituted barium ferrite powder in the <upper magnetic layer coating material> of Example 1 was replaced with a Co, Ti, Nb, Sn-substituted barium ferrite powder (Hc 500 Oe, number average particle size 0.045). μm, specific surface area d = 40 m ² / g), and the thickness of the upper magnetic layer is 0.3 μm.
A two-layer coated tape medium was prepared and evaluated in the same manner as in Example 1 except that m was 2 and the slit width was 1/2 inch.

Example 5 Two-layer coating type 8 mm width in the same manner as in Example 1 except that the blending amount of stearic acid in <Coating for lower magnetic layer> of Example 1 was changed to 5 parts by weight. A tape was made. Then, this was subjected to evaluation in the same manner as in Example 1.

Example 6 10 parts by weight of the same sulfonate group-containing urethane resin as used in <Coating for upper magnetic layer> of Example 1, 10 parts by weight of myristic acid-modified silicone lubricant (molecular weight about 1,000), and cyclohexanone A mixed solution containing 30 parts by weight of a mixed solvent of / methyl ethyl ketone (1/1) was prepared.

Using this mixed solution as a paint for the intermediate layer,
A two-layer coating type 8 mm wide tape was prepared in the same manner as in Example 1 except that the material was supplied in the middle of the <lower magnetic layer coating material> and the <upper magnetic layer coating material> of the three-layer coating slot die coater. It was made. However, at this time, the thickness of the intermediate layer was set to 0.2 μm. Then, this was subjected to evaluation in the same manner as in Example 1.

Example 7 <Coating for upper magnetic layer> in Example 1 Alumina (average particle size 0.4 μm) in the composition having a median diameter of 0.25 μm 4
A two-layer coating type 8 mm tape was produced in the same manner as in Example 1 except that parts by weight were replaced. Then, this was subjected to evaluation in the same manner as in Example 1. Comparative Examples 1 and 2 Examples 1 and 2 respectively, except that 3 parts by weight of stearic acid in the <upper magnetic layer coating composition> in Example 1 and 2 were replaced with 2 parts by weight. 2 in the same way
A layer coating type 8 mm tape was prepared. Then, this was subjected to evaluation in the same manner as in Example 1.

Comparative Example 3 <Coating for Upper Magnetic Layer> in Example 3 The same as Example 1 except that 2 parts by weight of stearic acid and 3 parts by weight of myristic acid in the composition were replaced by 2 parts by weight of stearic acid. hand,
A two-layer coating type 8 mm wide tape was prepared and used for evaluation.

Comparative Example 4 <Upper magnetic layer coating material> In Example 1, the addition amount of the sulfo group-containing urethane resin in the composition was 1 part by weight, and the addition amount of Coronate L to the obtained coating material was 0.5 part by weight. A two-layer coating type 8 mm tape was produced in the same manner as in Example 1 except for the above. Then, this was subjected to evaluation in the same manner as in Example 1.

Comparative Example 5 <Upper magnetic layer coating material> In Example 1, the addition amount of the sulfo group-containing urethane resin in the composition was 12 parts by weight, and the addition amount of Coronate L to the obtained coating material was 3 parts by weight. A two-layer coating type 8 mm tape was produced in the same manner as in Example 1 except for the above. Then, this was subjected to evaluation in the same manner as in Example 1.

The surface roughness, recording / reproducing characteristics, and S / N of the above-mentioned 12 types of multilayer coating type tape samples thus obtained
The ratio, still durability, and jitter characteristics were evaluated. The evaluation results are shown in Table 1 below. The recording / reproducing characteristics and
When measuring the S / N ratio, S-VHS and high band 8
I used the mm deck.

[0080]

[Table 1]

[0081]

As described above, according to the present invention,
By appropriately selecting the type and amount of resin binder in the upper magnetic layer, the addition amount is reduced, so the dispersibility of the magnetic powder is increased and a large amount of lubricant is kept stable in the upper magnetic layer. It can be included. Since a sufficient lubricant effect is stably exhibited even under various use environments, even when the upper magnetic layer is thinned, the surface and the interface between the upper and lower magnetic layers can be made smooth.

Therefore, according to the present invention, an excellent magnetic property is provided which has a thin and smooth upper magnetic layer in which the dispersibility and filling rate of the magnetic powder are enhanced, and which ensures stable running durability even in a wide range of use environments. A recording medium is obtained.

[0083]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Murano No. 1 Komukai Toshiba-cho, Sachi-ku, Kawasaki-shi, Kanagawa Stock company Toshiba Research Institute

Claims (5)

[Claims] 1. A magnetic structure comprising a non-magnetic substrate, a lower magnetic layer containing magnetic powder for long wavelength recording and a resin binder, and an upper magnetic layer containing magnetic powder for short wavelength recording and a resin binder. In the recording medium, the amount of resin binder contained in the upper magnetic layer is in the range of 2 to 12 parts by weight with respect to 100 parts by weight of magnetic powder, and the amount of lubricant contained in the upper magnetic layer is the amount of resin binder. A magnetic recording medium, characterized in that the amount is at least a half or more and five times or less. 2. The upper magnetic layer has a thickness of 0.05 to 0.5 μm.
The magnetic recording medium according to claim 1, wherein the magnetic recording medium has a range of m 2. 3. The magnetic recording medium according to claim 1, wherein the resin binder contained in the upper magnetic layer is a resin binder having a metal sulfonate group. 4. The magnetic powder for long wavelength recording has a coercive force Hc 3
The magnetic recording medium according to claim 1, 2 or 3, wherein the magnetic powder is a magnetic powder having a magnetization of 00 to 1,000 Oe and a saturation magnetization of 70 emu / cc or more. 5. The magnetic powder for short wavelength recording has a coercive force Hc 5
Metal powder mainly composed of iron with 00-3,000 Oe, particle size 0.05-0.3 μm, or coercive force Hc 500-3,000 Oe, particle size 0.01
The magnetic recording medium according to claim 1, 2, 3 or 4, wherein the magnetic recording medium is hexagonal ferrite powder having a particle size of 0.1 μm.