JPH0877546A - Magnetic recording medium and manufacturing method thereof - Google Patents

Abstract

(57) [Abstract] [Purpose] In a magnetic recording medium in which a magnetic layer containing magnetic powder, an abrasive and a resin binder is formed on at least one main surface of a non-magnetic support, durability is improved and head wear is reduced. Balance both. [Constitution] The average particle diameter (d1) of the abrasive primary particles is 0.3
The ratio (d2 / δ) of the average particle diameter (d2) of the secondary particles of the abrasive material protruding from the surface of the magnetic layer to the magnetic layer thickness (δ) is 0.4 or more. Is also large and smaller than 1.4. for that reason,
In preparing the magnetic coating material, the abrasive is dispersed separately from the magnetic powder, and then the maximum dispersed particle diameter is set so that (d2 / δ) is within the above range and filtered to prepare an abrasive masterbatch. .

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium having a high recording density such as a magnetic tape floppy disk used in the audio field, video field, computer field and the like and a method for manufacturing the same. . More specifically, the present invention relates to a magnetic recording medium having excellent durability and less likely to cause head wear, and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, magnetic recording media have been used as recording media for recording a large amount of information in a wide range of fields such as audio and video computers. One of such recording media is a coating type magnetic recording medium, for example, γ-F on a non-magnetic support such as a polyester film.
The e ₂ O _3, CrO _2, magnetic powder such as Co-γ-Fe ₂ O _3, iron powder, there are those comprising this by coating together with a resin binder and various additives is applied, inexpensive mass production in It is widely used because of its excellent reliability. Examples of the additives used when the magnetic paint is made into a paint include abrasives, lubricants and antistatic agents.

The abrasive gradually abrades the surface of the magnetic head when the medium is running, lowers the dynamic friction coefficient of the magnetic surface of the medium, and the shavings of the magnetic layer and minute foreign matter are caught in the gap of the magnetic head to cause clogging. It prevents you from waking up. As the abrasive having such a function, for example, inorganic fine particles having high hardness such as chromium oxide (Cr ₂ O ₃ ) and alumina (Al ₂ O ₃ ) are used.

The lubricant is added for the purpose of lowering the friction coefficient of the medium and improving the running property. The antistatic agent is added so that the medium is not charged due to friction during traveling, and prevents the medium from adsorbing dust and the like floating in the air.

In recent years, the amount of information to be recorded has increased exponentially, and along with this, there has been a demand for higher recording density and improved reliability of magnetic recording media. In order to increase the recording density, it is important to reduce the spacing between the magnetic head and the magnetic recording medium as much as possible in order not to reduce the reproduction output. Therefore, the higher the recording density of the magnetic recording medium, the higher the smoothness of the medium surface is desired.

By the way, high density magnetic recording media such as DAT, high image quality VTR tapes, data storage tapes and large capacity FDs are required to have higher density recording than conventional ones, and the surface of the medium is smooth. In addition to improving the properties, it has become necessary to highly disperse the ultrafine magnetic powder and further increase the filling rate of the magnetic powder in the magnetic layer. However, as the magnetic powder particles used become smaller in size, it is not possible to mix them together with other additives such as an abrasive in the resin binder in a state where each particle is dispersed. It's getting harder.

When dispersing the magnetic paint,
For example, media dispersers such as sand mills and ball mills are commonly used. The sand mill has a structure in which the contents are dispersed by the impact force and shearing force of a plurality of rotating disks mounted in a cylindrical container and a dispersion medium such as glass beads and zirconia beads contained in the container. .

[0008]

As described above, the higher the recording density of a magnetic recording medium, the more smooth the surface of the medium is desired to be improved. However, on the other hand, as the smoothness of the magnetic recording medium surface improves, the friction coefficient between the magnetic head and the medium increases. As a result, the magnetic layer is likely to be scraped or fallen off due to contact with the head, or the magnetic layer is likely to adhere to the head, which tends to impair the running property and durability.

In order to solve these problems and improve the durability of the magnetic recording medium, various resins added to the magnetic powder in the preparation of the magnetic paint, and various additives such as abrasives, lubricants and antistatic agents have been studied. And development, and examination of the addition method of these have been conducted so far.

Of these, the abrasives play an important role of ensuring the running durability of the magnetic recording medium, and therefore, they are often paid attention to in research.

When the abrasive is added to the magnetic paint, the abrasive having a relatively large particle size may be used with an emphasis on ensuring the running durability of the medium. In such a case, even if the addition amount is small, the abrasive particles are exposed as protrusions on the surface of the magnetic layer, so that the smoothness of the medium surface is impaired, and further, the magnetic head during traveling of the medium. There was a problem that it was worn excessively so as to be damaged.

Further, when trying to solve the above problems by using abrasive particles having a small particle diameter, it is usually necessary to increase the amount added to the medium in order to obtain the same polishing effect as in the past. there were. However, when the amount of the abrasive added increases, the filling rate of the magnetic powder in the magnetic layer relatively decreases, which causes a problem that the recording / reproducing output of the medium significantly decreases.

Until now, when adding an abrasive,
In order to achieve both improved durability of the magnetic recording medium and improved wear of the magnetic head, various studies have been conducted on the compounding aspect of the magnetic coating material, specifically the type, particle size, or addition amount of the abrasive. .

For example, in Japanese Patent Laid-Open No. Sho 60-93631, there are 20 or more abrasive agglomerates having a Mohs hardness of 7 or more per 100 (μm) ^{2 of the} magnetic layer, and the size of the abrasive agglomerates is that of the magnetic powder. A technique is disclosed in which the average particle size is not more than 1.5 times. JP-A-5-7389
No. 4, the average particle size of the abrasive is 1.0 to
A technique of using a mixed system of one having a particle size of 2.0 μm and one having an average particle size of less than 0.5 μm is disclosed.

Further, JP-A-2-89222 discloses the addition of alumina in which 90% or more of the particles have a particle size of less than 1 μm. Furthermore, Japanese Patent Laid-Open No. 5-2
Japanese Unexamined Patent Publication No. 0670 also discloses a technique of defining the ratio of the average particle diameter of the abrasive protruding to the surface of the magnetic layer and the average diameter of the protruding portion.

Further, studies have been so far made on abrasives from the aspect of manufacturing magnetic recording media. Since the abrasive particles have high hardness such as Mohs hardness of 6 or more, when the magnetic coating material composition is dispersed using a media disperser, the dispersion medium of the disperser, the rotating disk, or the inner wall surface of the container is There was a risk of scraping off and wearing the equipment. When the equipment is worn by the abrasive particles, the fine abrasion powder of the removed equipment mixes into the magnetic paint as a foreign substance and contaminates the magnetic paint. Therefore, when the dispersion process takes a long time or a large amount of abrasive is added, the amount of wear increases, and the amount of wear powder mixed in the magnetic coating also increases. As a result, the characteristics of the manufactured medium are adversely affected.

The media disperser is widely used in spite of such inconvenience because it has a strong dispersive power and high efficiency. However, in order to reduce the abrasion powder as much as possible, the residence time of the abrasive in the disperser is shortened as much as possible. , Or improvements have been made to the method for preparing magnetic paints. As an improvement of the method for preparing a magnetic paint, for example, a method in which an abrasive is separately dispersed with a magnetic powder to form a slurry, and these are added and mixed to prepare a magnetic paint (JP-A-5-20670). Japanese Patent Laid-Open No. 5-25066
No. 6) is disclosed.

As described above, most of the conventional techniques relating to abrasives are made by paying attention to one physical aspect of the abrasives used, such as the particle size, the amount added, and the protruding state of the medium surface. Is. They are partially effective in improving the durability of the magnetic recording medium and reducing head wear. However, the effect is not yet sufficient in order to satisfy various severe demands on the medium in recent years, such as high recording density, improvement in durability, and improvement in recording / reproducing characteristics.

Therefore, in order to solve the above-mentioned drawbacks of the conventional magnetic recording medium and the manufacturing method thereof, the present invention is not limited to the one physical property such as the particle size and the addition amount of the abrasive, and the polishing is performed in the magnetic layer. The present invention was also made by paying attention to the state in which the material is dispersed, has good electromagnetic conversion characteristics, is less likely to cause head wear, and provides a magnetic recording medium with improved durability and a manufacturing method thereof. , And its purpose.

[0020]

[Means for Solving the Problems] Generally, the dispersed / aggregated state of particles is usually expressed by the expression of primary particles or secondary particles, and primary particles are dispersed particles that are in a dispersed state one by one. The secondary particles mean dispersed particles that are aggregated at various stages such as from soft aggregation to aggregation. The present invention, in pursuit of the ideal dispersion state of the abrasive particles in the resin binder for the above purpose, pay attention to not only the primary particles but also the state of the secondary particles,
Further, it is made paying attention to the relationship between the size of the secondary particles of the abrasive and the thickness of the magnetic layer.

That is, the magnetic recording medium of the present invention is a magnetic recording medium in which a magnetic layer formed by dispersing magnetic powder and an abrasive in a resin binder is formed on at least one main surface of a non-magnetic support. The average particle diameter (d1) of the primary particles of the abrasive is in the range of 0.3 to 0.8 .mu.m, and the average diameter (d2) of the secondary particles of the abrasive protruding to the surface of the magnetic layer.
The ratio (d2 / δ) to the magnetic layer thickness (δ) is 0.4
It is characterized by being larger than 1.4 and smaller than 1.4.

In the present invention, acicular fine particles such as Co-γ-Fe ₂ O ₃ and metal powder, or plate-like fine particles of hexagonal ferrite such as Ba ferrite can be used as the magnetic powder. Among them, Ba ferrite magnetic powder having an average particle diameter of about 0.03 to 0.08 μm is suitable for the purpose of the present invention and is preferable. In the magnetic recording medium of the present invention, the magnetic layer thickness (δ) is 1 to 3
The thickness is preferably in the range of μm, and when the thickness of the magnetic layer is in such a range, the effect of the present invention is remarkably obtained.

In the present invention, as the abrasive, for example, Cr ₂ O ₃ or α-Al ₂ O ₃ having an average particle size (d1) in the range of 0.3 to 0.8 μm in the state of primary particles is compared. Inorganic fine particles with high hardness are preferable, but α-
Al ₂ O ₃ is particularly preferable because it has low head wear.

Further, in the method for manufacturing a magnetic recording medium of the present invention, a magnetic coating material containing magnetic powder, an abrasive and a resin binder is applied on at least one main surface of a non-magnetic support to form a magnetic layer. In the method for producing a magnetic recording medium, an abrasive having an average particle diameter of 0.3 to 0.8 μm is dispersed in a resin binder and then filtered to obtain dispersed particles having a size exceeding a predetermined maximum dispersed particle diameter. A magnetic paint is prepared by kneading the abrasive masterbatch formed by removal and a magnetic powder masterbatch formed by dispersing magnetic powder in a resin binder,
Further, the maximum dispersed particle diameter is such that the ratio (d2 / δ) of the average particle diameter (d2) of the secondary particles of the abrasive protruding to the surface of the formed magnetic layer to the magnetic layer thickness (δ) is 0.4 or less. Is set to be larger and smaller than 1.4.

In the present invention, the ratio (d2 / δ) of the secondary particle diameter (d2) of the abrasive particles to the magnetic layer thickness is preferably larger than 0.4 and smaller than 1.4. (D2 /
When δ) is 0.4 or less, the dispersibility of the abrasive particles is good, but the durability of the medium deteriorates. In order not to deteriorate the durability, it is necessary to increase the amount of abrasive added, so
Not preferred. On the other hand, when (d2 / δ) is 1.4 or more, the area where the abrasive secondary particles project to the surface of the magnetic layer becomes too large, and the surface property of the magnetic layer deteriorates. Further, it is not preferable because it is liable to cause cohesive failure due to contact with the magnetic head during traveling, which damages the medium and the magnetic head.

The magnetic powder masterbatch according to the present invention is
The material composition is the same as that of the usual magnetic paint except that it does not contain an abrasive, and can be prepared by the same procedure as that of the usual magnetic paint. Therefore, it goes without saying that various additives such as a dispersant, a lubricant, or an antistatic agent may be added in addition to the magnetic powder when preparing the magnetic powder masterbatch.

The abrasive masterbatch according to the present invention is
It can be prepared as follows. First, the abrasive,
A resin binder and a material composition such as an organic solvent are kneaded and dispersed using a pressure batch type or continuous type kneader,
Then, the particles are further dispersed using a media disperser made of a material having high hardness and high abrasion resistance such as ceramics. The obtained slurry contains, as dispersed particles, abrasive particles dispersed to a state close to primary particles and abrasive secondary particles in various aggregation stages. Therefore, the abrasive dispersion masterbatch according to the present invention is obtained by setting the maximum dispersed particle diameter and filtering the slurry so that (d2 / δ) when the magnetic layer is formed becomes a predetermined value. To be It should be noted that various additives such as a dispersant may also be added to this abrasive masterbatch, if necessary.

Further, in kneading the material composition of the abrasive masterbatch according to the present invention, the use of a continuous kneader makes it possible to easily obtain dispersed particles of a predetermined size.
Especially preferred.

The resin binder used to disperse the abrasive at the time of preparing the abrasive masterbatch may be the same as or different from the resin binder used to disperse the magnetic powder at the time of preparing the magnetic powder masterbatch. May be A polyurethane resin having a molecular weight of about 24,000 is particularly suitable for the abrasive masterbatch according to the present invention.

In the present invention, when α-Al ₂ O ₃ having an average particle diameter of 0.3 to 0.8 μm is used as an abrasive and a magnetic recording medium having a magnetic layer thickness (δ) of 1 to 3 μm is manufactured, Although it depends on the size of the abrasive particles, a masterbatch in which an agglomerate of approximately 3 to 4 abrasive particles is present as secondary particles is preferable.

The size and amount of secondary particles dispersed in the abrasive masterbatch vary depending on the particle size of the primary particles of the abrasive as well as the type of dispersing equipment, the length of dispersion time, and the amount of the abrasive during dispersion. It is also affected by the ratio between the amount and the resin binder, the molecular weight of the resin binder, the polar group, and the like. So by adjusting these,
It is possible to easily set a preferable maximum dispersed particle diameter that can bring (d2 / δ) to a desired value.

In the present invention, 0.4 <(d2 / δ) <
The magnitude of the secondary agglomeration of the abrasive particles shown by 1.4 is not likely to be destroyed even when the calender pressure is made higher than usual during the surface smoothing treatment in the production process of the medium. Therefore, according to the manufacturing method of the present invention, it becomes easy to obtain the excellent surface properties required for the high-density magnetic recording medium,
As a result, the electromagnetic conversion characteristics of the medium are also improved.

[0033]

The abrasive, which is indispensable for improving the durability of the medium, has been ideally dispersed up to the primary particles in the magnetic paint from the viewpoint of reducing head wear.
It was also considered that it is better to have as few secondary aggregates as possible. Therefore, in the past, studies were conducted in the direction of suppressing head wear as much as possible by making the average particle size of the abrasive as small as possible, and making up for the defects of the small particle size abrasives with poor durability improvement ability by increasing the addition amount. Has been.

The present invention has been made as a result of intensive studies on the durability of the magnetic recording medium and the role of the abrasive against the wear of the magnetic head, without being bound by the above-mentioned research directions. It has been found that the presence of abrasive secondary particles in the magnetic layer, which has been disadvantageous for head wear, works to reduce head wear depending on the state of secondary particle formation. It was

The abrasive masterbatch according to the present invention is
The abrasive is previously dispersed in a resin binder, and the abrasive particles are contained therein as a mixture of primary particles and secondary particles. The primary particles in this mixture act to reinforce the formed magnetic layer and improve the durability of the medium. On the other hand, since the secondary particles have the resin binder adsorbed on the surface thereof, the polishing effect thereof is milder than the size thereof.

Therefore, by appropriately selecting the size of the secondary particles contained in the abrasive masterbatch, the secondary particles protruding on the surface of the formed magnetic layer are sufficient for preventing the clogging of the gap of the magnetic head. Moreover, it is possible to exhibit an ideal polishing effect that it is not so severe as to wear the magnetic head itself.

In the present invention, the preferred secondary particle size contained in the abrasive masterbatch is such that the ratio of the secondary particle diameter (d2) to the magnetic layer thickness (d2 / δ) is larger than 0.4. The above effect is exhibited by selecting the value in a range smaller than 4.

Therefore, according to the magnetic recording medium and the method of manufacturing the same of the present invention, it is possible to improve the durability of the magnetic recording medium and reduce the wear of the magnetic head at the same time.

[0039]

EXAMPLES The present invention will be described in detail below with reference to examples.

Example 1 In producing the magnetic recording medium of the present invention by the method of the present invention, the materials having the following compositions were kneaded by two continuous kneaders and then redispersed by a media disperser using zirconia beads. Then, a basic abrasive masterbatch was prepared.

<Basic composition of abrasive masterbatch> α-Al ₂ O ₃ (average particle size 0.7 μm) 90 parts by weight Polyurethane resin (molecular weight 24000) 10 parts by weight Methyl ethyl ketone 50 parts by weight Cyclohexanone 50 parts by weight Next, this polishing The material base masterbatch was filtered by gradually changing the filter mesh roughness from 10 μm to 5 μm, and then to 2 μm to prepare several additive abrasive masterbatches.

Next, a magnetic powder masterbatch material having the following composition was dispersed in a sand mill for 2 hours.

[0043] Then, one section of the abrasive masterbatch for addition prepared in advance was added so that the abrasive content was contained in an amount equivalent to 5 parts by weight with respect to the above composition, and redispersed for 0.5 hour.
After filtering the obtained coating material, 3 parts by weight of an isocyanate-based curing agent was added, and the mixture was stirred so as to be uniform to obtain a magnetic coating material according to the present invention.

The above magnetic coating material was applied onto a PET film having a thickness of 75 μm using a gravure coater so that the thickness (δ) of the dried magnetic layer coating film would be 2 μm. After the coating of the magnetic layer was dried, the surface was smoothed by a calendar device (calendar conditions?). Then, 24 in a constant temperature bath at 40 ° C
The magnetic layer coating film formed was left to stand for a time to cure. Then, a magnetic layer coating film was formed on the back surface of the film in the same process. Then, the coating film was cured by standing still in a constant temperature bath at 60 ° C. for 3 days to obtain a magnetic recording medium original fabric. Then, this original fabric was punched out into a disk shape having a diameter of 3.5 inches to obtain a disk-shaped magnetic recording medium which is Example 1 of the present invention.

Next, various physical properties and characteristics of the obtained medium were examined. As a result, the thickness (δ) of the magnetic layer was 2μ.
m, the primary particle size (d1) of the abrasive was 0.7 .mu.m, and the secondary particle size (d2) was 1.4 .mu.m. Therefore, it was calculated that (d2 / δ) = 0.7. The secondary particle diameter (d2) and the magnetic layer thickness (δ) were measured in the range of 20,000 to 1
It was performed using a transmission electron microscope with a magnification of 0,000 times.

The characteristics were evaluated with respect to the cycle durability of the medium and the wear of the magnetic head. For the medium of this example, the cycle durability was 18 million passes and the magnetic head abrasion was 2.3%.

In the durability evaluation, the disc-shaped medium mounted on the jacket was cycled for 3 hours in a 24-hour cycle.
The temperature and humidity environmental conditions that change according to
Running with a double-sided head disk drive of 350 rpm for inch, reproduction output is 3 against initial writing output
The time point of 0% reduction was taken as the end point, and the degree of durability was expressed by the number of passes up to that time point. The three different temperature and humidity environments are: temperature 5 ° C, relative humidity 10%, temperature 30
C. relative humidity 80%, and temperature 60.degree. C. relative humidity 10%.

To evaluate head wear, the initial output of the medium is measured, the output is measured again after 10 million passes, the output reduction fraction from the initial output is calculated, and this reduction fraction expresses the degree of head wear. Regarded as a thing. The heads to be evaluated are average values of two heads for one type of medium.

The thickness of the magnetic layer after drying, the primary particle diameter and the secondary particle diameter of the abrasive were changed so that Examples 2 to 5 (d2 / δ) were different from Example 1 within the scope of the present invention. A magnetic recording medium was manufactured in the same manner as in Example 1 except for the above. Then, this was evaluated in the same manner as in Example 1. The physical property values of these examples are shown in Table 1 together with Example 1.
It is shown in. Similarly, the evaluation results are shown in Table 2.

Comparative Examples 1 to 5 In preparing the abrasive basic masterbatch, only the media disperser using zirconia beads was used to measure the thickness of the dried magnetic layer, the primary particle diameter of the abrasive, or the secondary particle diameter. A magnetic recording medium was manufactured in the same manner as in Example 1 except that (d2 / δ) was changed so that it was out of the range of the present invention. And this was evaluated similarly. The physical property values of these comparative examples are also shown in Table 1 together with the examples. Similarly, the evaluation results are shown in Table 2.

[0051]

[Table 1]

[Table 2] As is clear from Table 1 and Table 2, the magnetic recording medium obtained by the manufacturing method of the present invention has excellent durability and hardly wears the head.

[0052]

As described above, according to the present invention,
(EN) A magnetic recording medium having good electromagnetic characteristics and less likely to cause head wear and having improved durability, and a method for manufacturing the same are easily provided.

[0053]

Claims (3)

[Claims] 1. A non-magnetic support on at least one main surface,
In a magnetic recording medium in which a magnetic layer containing magnetic powder, an abrasive and a resin binder is formed, the average particle diameter (d1) of the abrasive primary particles is 0.3 to 0.
The ratio (d2 / δ) of the average particle diameter (d2) of the secondary particles of the abrasive in the range of 8 μm and protruding to the surface of the magnetic layer to the thickness (δ) of the magnetic layer is larger than 0.4 and is 1 A magnetic recording medium characterized by being smaller than 0.4. 2. The magnetic recording medium according to claim 1, wherein the magnetic powder is hexagonal ferrite magnetic powder. 3. A method for producing a magnetic recording medium, wherein a magnetic coating material containing magnetic powder, an abrasive and a resin binder is applied onto at least one main surface of a non-magnetic support to form a magnetic layer. An abrasive masterbatch obtained by dispersing an abrasive having an average particle diameter of 0.3 to 0.8 μm in a resin binder and then filtering to remove dispersed particles having a size exceeding a predetermined maximum dispersed particle diameter; A magnetic coating material is prepared by kneading a magnetic powder masterbatch obtained by dispersing powder in a resin binder, and the maximum dispersed particle diameter is an abrasive secondary particle protruding onto the surface of the formed magnetic layer. A method of manufacturing a magnetic recording medium, characterized in that a ratio (d2 / δ) of an average particle diameter (d2) to a magnetic layer thickness (δ) is set to be larger than 0.4 and smaller than 1.4.