JPH03142709A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium having excellent durability and high reliability by incorporating polyvinyl chloride resin comprising vinyl chloride-acrylic copolymers of 400 - 700 average polymn. degree into a binder. CONSTITUTION:The binder contains polyvinyl chloride resin comprising vinyl chloride-acrylic copolymers of 400 - 700 average polymn. degree comprising hydroxyl group-contg. vinyl monomers and sulfonic group-contg. vinyl monomers as the structural unit. Polyvinyl chloride resin comprising vinyl chloride-acrylic copolymers of 400 - 700 average polymn. degree has high Young's modulus, yield point and yield elongation. Therefore, by using this polymer as the binder, ferromagnetic powder particles are firmly bonded in the magnetic layer, which prevents dropout of the powder even when a magnetic head is brought into contact with the medium by a specified load. Thereby, the obtd. medium has excellent durability and high reliability.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magnetic recording medium used for, for example, a magnetic disk cartridge, a magnetic tape, or a magnetic card, and in particular, the present invention relates to a magnetic recording medium that is mainly composed of ferromagnetic metal powder and contains at least non-magnetic metal powder. The present invention relates to the structure of a magnetic layer containing a binder and a lubricant.

[Conventional technology] In recent years, efforts have been made to increase the recording density of magnetic recording media.

For example, a magnetic layer containing fine particles of a ferromagnetic metal such as iron, cobalt-nickel, or cobalt-phosphorus is used.

[Problem that the invention attempts to solve]

When a conventional magnetic recording medium of this type is used in a recording/reproducing device, there is a problem that ferromagnetic metal powder falls off from the magnetic layer due to sliding contact with the magnetic head, reducing the running performance of the magnetic recording medium. be. Magnetic disk cartridges that use especially thin and flexible magnetic disks are usually 5.
This is a very important issue because the magnetic head holds both sides of the rotating magnetic disk under a predetermined pressure, so the durability of the magnetic disk is required to be high.

As a result of various studies on the problem of decreased durability of magnetic recording media, the present inventors found that the BET surface area of the ferromagnetic metal powder used in the magnetic recording medium and the binder that binds the ferromagnetic metal powder to each other have been determined. It turned out that it had a big impact.

That is, in a magnetic recording medium having a recording density of 30 kbpi or more, the above-mentioned ferromagnetic metal powder having a coercive force of 1300 [Oa1 or more is generally used as the magnetic material. By the way, the conventionally used ferromagnetic metal powder (with a coercive force of 1300
The average BET surface area of [Oe] is about 50~
55 m2/g (the average BET surface area of metal oxide magnetic powder is approximately 20 m''/g), and is in the state of ultrafine particles with an extremely small particle size compared to metal oxide magnetic powder. Also, the magnetic powder in the magnetic layer The content of metal oxide magnetic powder is approximately 26
~28% by volume, whereas ferromagnetic metal powder has approximately 30% by volume.
~32% by volume, resulting in a highly filled state with a high content rate.

On the other hand, various binders have been studied, such as vinyl chloride-vinyl acetate-vinyl alcohol copolymer and vinyl chloride-vinyl acetate-vinyl alcohol-maleic acid copolymer, but those with a relatively low degree of polymerization When used, the adsorption ability to the magnetic powder surface is low and the molecular weight is also small, so the elastic strength is low.

As mentioned above, the large average BET surface area of ferromagnetic metal powder means that the surface area per magnetic particle is small. The bonding area (bonding force) between the powder and binder is small, and as mentioned above,
The binder itself also has poor adsorption ability and elastic strength to the magnetic powder surface.As a result, while the magnetic layer is in sliding contact with the magnetic layer, the 1 ferromagnetic metal falls off from the magnetic layer, causing a magnetic recording medium. It was discovered that the running performance of the vehicle was worsening.

Further, in order to increase the durability of the magnetic layer, reinforcing powder made of an abrasive such as aluminum oxide is added to the magnetic layer. However, if the content of reinforcing powder is increased in order to obtain excellent durability, the amount of reinforcing powder exposed on the surface of the magnetic layer increases accordingly.

For this reason, in magnetic disk cartridges, the cleaning sheet placed on the inner surface of the cartridge case wears down significantly as the magnetic disk rotates, and the abrasion particles generated thereby adhere to the disk surface and drop. This will cause you to go out.

Furthermore, various lubricants such as higher fatty acids, higher fatty acid esters, liquid paraffin, squalane, and fluororesins have been studied to improve the lubrication performance on the surface of the magnetic layer.

However, many of the lubricants mentioned above become cloudy and semi-solid at temperatures as low as 0°C.If a magnetic disk cartridge using such lubricants is used at low temperatures, the cartridge case may deteriorate. Due to the influence of the semi-solid lubricant, abrasion powder from the cleaning sheet stuck inside the magnetic disk strongly adheres to the surface of the magnetic disk, causing dropouts.

Furthermore, lubricants with low viscosity have technical problems, such as the fact that they are hardly expected to be effective in improving the durability of magnetic recording media under severe conditions.

An object of the present invention is to eliminate the drawbacks of the prior art and thereby provide a highly durable and highly reliable magnetic recording medium.

[Means to solve the problem]

In order to achieve the above-mentioned object, the present invention mainly includes ferromagnetic metal powder on a substrate made of a non-magnetic material.

The object is a magnetic recording medium provided with a magnetic layer containing at least a nonmagnetic binder and a lubricant.

The binder contains a polyvinyl chloride resin made of a vinyl chloride-acrylic copolymer having a hydroxyl group-containing vinyl monomer and a sulfone group-containing vinyl monomer as constituent units and having an average degree of polymerization regulated within a range of 400 to 700. It is characterized by the presence of

[Effect]

As mentioned above, the present invention uses vinyl chloride which has a hydroxyl group-containing vinyl monomer and a sulfone group-containing vinyl monomer as constituent units and has an average degree of polymerization regulated within the range of 400 to 700.
A polyvinyl chloride resin made of an acrylic copolymer has higher Young's modulus, stress at yield point, and elongation at yield point than conventional resins.

Therefore, if this is used as a binder, the binding force of the ferromagnetic metal powder in the magnetic layer will be strong, so even if the magnetic head contacts it under a certain load, there will be almost no dropout of the ferromagnetic metal powder, and it will exhibit excellent durability. do.

[Embodiments of the invention]

Figures 1 to 6 are for explaining a magnetic disk cartridge according to an embodiment of the present invention. Figure 1 is an exploded perspective view of the disk cartridge, and Figure 2 is a cleaning sheet affixed to the upper case. 3 is a plan view showing the state in which the cleaning sheet is attached to the lower case; FIG. 4 is an enlarged sectional view of the vicinity of the magnetic head insertion slot showing the usage state of this magnetic disk cartridge; FIG. 5 is an enlarged sectional view of the vicinity of the elastic piece of this magnetic disk cartridge, and FIG. 6 is an enlarged sectional view of the cleaning sheet.

The magnetic disk cartridge includes a cartridge case 1, a magnetic disk 2 rotatably housed in the cartridge case 1, and a shutter 3 slidably supported by the cartridge case 1.
It is mainly composed of.

The cartridge case 1 is composed of an upper case la and a lower case 1b, which are injection molded from a hard synthetic resin such as ABS resin.

Approximately in the center of the lower case 1b is an opening 4 for inserting the rotational drive shaft.
is formed, and a rectangular head insertion opening 5 is provided near it, and a head insertion opening 5 is similarly provided in the upper case la. As shown in FIG. 1, in the vicinity of the front surfaces of the upper case 1a and the lower case 1b, recesses 6 which are slightly lowered to restrict the sliding range of the shutter 3 are formed. The magnetic head insertion port 5 is opened at .

As shown in FIG. 3, counterbore-shaped recesses 7, 7 are formed on both left and right sides of the magnetic head insertion opening 5 on the inner surface of the upper case 1a. The vertical width W1 of the recess 7 is approximately equal to the longitudinal width (vertical width) W2 of the magnetic head insertion port 5, and the horizontal width W3 of the recess 7 is the width (horizontal width) in the direction orthogonal to the longitudinal direction of the magnetic head insertion port 5. It is restricted to 0.3 times or more of W4, preferably in the range of 0.5 to 1.5 times.

A large number of linearly extending protrusions 8 are provided on the upstream side of the magnetic head insertion opening 5 in the disk rotation direction in the upper case la, and two of the protrusions 8 on both sides are slightly higher than the protrusions 8 on the inner side.

An arcuate regulating rib 9 for regulating the storage position of the magnetic disk 2 is protruded from the inner surface of the upper case 1a, a part of which passes through the end of the recess 7, and the regulating rib 9 is connected to the reinforcing body around the recess 7. It is also useful.

A C-shaped cleaning sheet 10 is inserted inside the regulating rib 9. An opening 11 provided at a position corresponding to the magnetic head insertion opening 5 of the cleaning sheet 10
The width W5 of the magnetic head insertion port 5 is designed to be slightly larger than the width W4 of the magnetic head insertion port 5. By placing the cleaning sheet 10 on the inner surface of the upper case 1a, the recess 7 and the protrusion 8 are also covered, and the peripheral portion of the cleaning sheet 10 is ultrasonically welded to the upper case la. As mentioned above, since the recess 7 of the upper case 1a has a sufficient width, the opening 1 in the cleaning sheet 10 can be opened as shown in FIG.
1 can be ultrasonically welded 12 within the recess 7 .

As shown in FIGS. 3 and 5, a protruding support portion 13 and a protruding strip 14 for preventing sticking are provided on the inner surface of the lower case 1b at a position substantially facing the protrusion 8 of the upper case 1a. It is provided. As shown in FIGS. 3 and 5, a base portion 16 of an elastic piece 15 formed by bending a plastic sheet is attached to the inner surface of the lower case lb and near the support portion 13 by an appropriate means such as adhesive or heat fusion. Fixed. The free end 17 of the elastic piece 15 is supported by the support portion 13 to maintain its inclined state.

Also on the inner surface of the lower case 1b is 9! Control ribs 9 are provided protrudingly.

A cleaning sheet 10 is arranged inside it. The width W5 of the opening 11 provided in the cleaning sheet 10 at a position corresponding to the magnetic head insertion opening 5 is designed to be slightly larger than the width W4 of the magnetic head insertion opening 5. As shown in FIG. 3, by placing the cleaning sheet 10 inside the lower case 1b, the elastic piece 15 is covered.

The peripheral part of the cleaning sheet 10 is ultrasonically welded 12 to the lower case 1b, but the vicinity of the elastic piece 15 is ultrasonically welded 12.
2 is omitted.

When a magnetic disk cartridge is assembled by combining the upper case 1a and the lower case 1b, the cleaning sheet 10 is partially lifted from the lower case 1b by the elastic piece 15 as shown in FIG. The lift is slightly pushed down by the raised projection 8.

Cleaning sheet 10.1 with magnetic disk 2 on top and bottom
It is elastically clamped by 0. and magnetic disk 2
As the disc rotates, the cleaning sheet 10 cleans the disc surface.

FIG. 4 is a diagram showing the state in which the magnetic disk cartridge is used, and 18 in the figure is a magnetic head.

The cleaning sheet 10 has three parts as shown in FIG.
It has a layered structure. That is, the disk-side nonwoven fabric layer 19 facing the magnetic disk 2 and the cartridge case l
A case side nonwoven fabric layer 20 facing the disc side nonwoven fabric layer 20 and an intermediate nonwoven fabric layer 21 for connecting the disk side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20 are provided.

The disc-side nonwoven fabric layer 19 and the case-side nonwoven fabric layer 20 are both made of rayon fibers alone, and are made so that there is no distinction between the front and back sides.The rayon fibers include viscose rayon, ammonia rayon, and acetate. Rayon etc. are used. The tensile strength of rayon fiber (stable) is approximately 2.5 to 3.1 g/D,
The elongation rate is about 16 to 22%, the elongation modulus (at 3% elongation) is about 55 to 80%, and the specific gravity is about 1.50 to 1.52.

The intermediate nonwoven fabric layer 21 is composed of a mixed fiber layer of rayon fibers and polyamide fibers. The mixing ratio of rayon fiber and polyamide fiber (rayon/polyamide) is approximately 1
Selected appropriately in the i range of /9 to 9/1, especially 3/
A range of 7 to 7/3 is suitable.

Polyamide fiber is a fiber whose raw material monomer is a polycondensate of adipic acid and hexamethylene diamine. The tensile strength of polyamide fibers is approximately 4.5 to 7.5. Elongation rate is approximately 25-60%, elongation modulus (at 3% elongation) is approximately 95-1
00%, and the specific gravity is approximately 1.14.

The cleaning sheet 10 can be made of rayon fiber alone, but as in this embodiment, an intermediate nonwoven fabric layer 21 made of a nonwoven fabric containing thermoplastic fibers is provided between the disk side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20. By providing this, it is possible to achieve good thermal fusion with the cartridge case 1.

For the intermediate nonwoven fabric layer 21, in addition to the mixture of rayon fibers and polyamide fibers as described above, various thermoplastic fibers such as polyethylene terephthalate, polyethylene, polypropylene, polystyrene, polyvinyl chloride, and acrylic resin can be suitably used. . In particular, since rayon fibers are used in the disc side nonwoven fabric layer 19 and the case side nonwoven fabric layer 20 as described above, the intermediate nonwoven fabric layer 21 was made to contain rayon fibers in consideration of compatibility with both [19, 20]. It is preferable to consist of a mixed fiber layer.

The amount of date on this cleaning sheet 10 is not particularly limited, but is suitably about 20 to 50 g/rrr. Note that the basis weight here is a value obtained by measuring the weight of five 50×50 cm samples and taking the average value thereof.

The thickness of the cleaning sheet 10 is 150 to 300 μm
is appropriate. The specific physical properties of the cleaning sheet 10 according to the example are as follows.

As mentioned above, the cleaning sheet LO has a three-layer structure, but when actually assembling a magnetic disk cartridge using this cleaning sheet 1, all of the fibers that come into sliding contact with the magnetic disk 2 may not be rayon fibers.
Thermoplastic fibers (polyamide fibers) in the intermediate nonwoven fabric layer 21
A very small portion of the cleaning sheet 10 may come into contact with the magnetic disk 2 due to intertwining of the fibers. However, even in such a case, the proportion of thermoplastic fibers in sliding contact with the magnetic disk 2 is about 10% or less, and substantially 100% of the fibers in contact with the magnetic disk 2 are rayon fibers.

Next, the composition of the magnetic layer will be explained. The magnetic layer.

It consists of ferromagnetic metal powder, binder, lubricant, reinforcing powder, etc.

As the ferromagnetic metal powder, metal iron powder consisting essentially of α-Fe is used. Here, the metallic iron powder that is defined as consisting essentially of α-Fe includes, in addition to Fe, for example, Ni and C.
r, Mn, S tt w, Mo.

This means that it also includes solid solutions in which a small amount of metal elements such as CO are dissolved. α-F used in this example
e The physical properties of the metal powder are shown in Table 2 below.

The binder mentioned above is a polyvinyl chloride system consisting of a vinyl chloride-acrylic copolymer whose constituent units are a hydroxyl group-containing vinyl monomer and a sulfone group-containing vinyl monomer and whose average degree of polymerization is regulated within the range of 400 to 700. A mixture of resin and polyurethane resin is used.

The lubricant is a fatty acid ester having a liquid state at 0°C, which is the lower limit of the operating temperature range of magnetic recording media such as magnetic disk cartridges, that is, a melting point of 0°C or less, and a viscosity of 30 cp or more at 20°C, and the following: Those having the general structural formula are preferred.

-Shell structural formula/2 In the formula, Rz and R2 are each a linear or branched saturated alkyl group having 1 to 15 carbon atoms.

R:I is a linear or branched saturated or unsaturated alkyl group having 17 carbon atoms.

Specific examples of the fatty acid ester include 2-hebutyl undecyl oleate, 2-octyl dodecyl oleate, 2-methylhexadecyl stearate, 2-hexyl dodecyl stearate, 2-octyl decyl stearate, 2- Examples include ethyl hexadecyl oleate and 2-hexyldodecyl oleate. Among these fatty acid esters, Rz in the above-mentioned -shell structural formula has 7 to 8 carbon atoms.
2 has a straight chain or branched alkyl group having 9 to 10 carbon atoms, for example, 2-hebutyl undecyl oleate (Rx has 7 carbon atoms and R2 has 9 carbon atoms), or 2-octyl dodecyl oleate (R1 The number of carbon atoms in R2 is 8, and the number of carbon atoms in R2 is 10). One type of fatty acid ester may be used, or two or more types may be used in combination.

It is also possible to use it in combination with other lubricants if necessary.

The lubricant (fatty acid ester) is either added to the magnetic paint in advance to form a magnetic layer, or it is toluene.

An appropriate amount of lubricant can be added to the magnetic layer by dissolving the lubricant in a suitable organic solvent such as n-hexane, coating the solution on the magnetic layer, and impregnating the lubricant into the magnetic layer.

In either method, the appropriate addition rate of the lubricant is 5 to 25% by weight based on the ferromagnetic metal powder. If the addition rate is less than the above lower limit, the lubricant will not be effective and the durability of the magnetic recording medium will not be sufficiently improved.On the other hand, if it is more than the above upper limit, the surface of the magnetic layer will become sticky and dust will form. or the magnetic recording medium may stick to the flat surface of the magnetic head.

The starting torque becomes high and uneven rotation occurs.

Also, examples of reinforcing powder include aluminum oxide.

Silicon carbide, silicon nitride, etc. are used, and this example shows an example using aluminum oxide.

The average particle size of the reinforcing powder is suitably 0.4 to 0.5 μm.

As will be described later, the present invention uses a ferromagnetic metal powder having a smaller average BET surface area (that is, a larger bonding force with the binder) than conventional powders.

The content of this reinforcing powder is 5 to 30% of the ferromagnetic metal powder.
It is advisable to regulate it within a range of % by weight. If the content of reinforcing powder is less than 5% by weight, sufficient durability of the magnetic layer cannot be obtained even if ferromagnetic metal powder with a small BET surface area is used. On the other hand, if it exceeds 30% by weight, it will cause wear of the cleaning sheet and other problems such as dropouts, so it is better to limit the content of the reinforcing powder to the above range.

The preferred composition ratio of the magnetic paint to be applied onto the base film is shown in Table 3 below.

The vinyl chloride-acrylic copolymer used as the binder in the table above has a hydroxyl group vinyl monomer and a sulfone group vinyl monomer as constituent units, and its molecular structure is as follows.

Note that R and R' in the formula represent carbon chains, and the average degree of polymerization of this copolymer is 570.

After sufficiently cross-dispersing the above-mentioned composition, it is coated on both sides of a base film made of polyethylene terephthalate having a thickness of 75 μm to a thickness of 2.5 μm to form a magnetic layer. This was then punched out into a disk shape with a diameter of 86 mm to obtain a magnetic disk.

The magnetic properties of this magnetic disk are shown in Table 4 below.

The relationship between the average BET surface area and wear resistance of the ferromagnetic metal powder (α-Fe) used in the following table will be explained.

The following Table 5 shows α-Fe with various average BET surface areas.
A magnetic disk cartridge is assembled using metal powder, and this is installed in a normal recording/playback device.

The magnetic disk is rotated at a speed of 36 Orpm.

While repeating temperature cycles from 0°C to 50°C, a magnetic head with a nearly rectangular sliding surface is brought into sliding contact with the surface of the magnetic disk under a load of 20g, causing running scratches on the surface of the magnetic disk. The number of rotations (confirmed with the naked eye) until
Durability was evaluated by counting the number of passes). The composition of the magnetic layer of each sample was the same as that in Table 3.

Table As is clear from this table, conventional magnetic disks using a-Fe metal powder with an average BET surface area of 5C1 to 55 m'/g suffer from running scratches during the severe running test described above. However, as in the present invention, the average BET surface area of α-Fe metal powder is 48 m”7
ferromagnetic metal powder and the binder have a very strong affinity and binding force, and as a result, the number of passes (400
1,000,000 times or more), indicating that the magnetic disk has excellent wear resistance.

As mentioned above, the average BET surface area of the ferromagnetic metal powder is 48 m
``If the amount is 7g or less, the binding force with the binder can be greatly increased, but the average BET surface area is 30m
”/g, medium noise becomes large and C/N
Therefore, it is preferable to limit the average BET surface area of the ferromagnetic metal powder to a range of 30 to 48 m''7 g.

Next, the average BET surface area is 41m” and 7g of ferromagnetic metal powder (
[alpha]-Fe powder) and (2) a vinyl chloride-acrylic copolymer with an average degree of polymerization of 570, using the hydroxyl group-containing vinyl monomer and sulfone group-containing vinyl monomer as constituent units as a binder (Example).

(2) As shown in the molecular structure below, vinyl chloride-vinyl acetate alcohol copolymer with only hydroxyl groups (average degree of polymerization 420 Comparative Example 1), (1) As shown in the molecular structure below, A magnetic disk cartridge is assembled using a vinyl chloride-acrylic copolymer valve fluoride having an epoxy group (average degree of polymerization: 300 Comparative Example 2). Table 6 below shows the results of a comparison between various properties of these binders and the durability of the assembled magnetic disk cartridge.

The durability in Table 1 was evaluated by counting the number of rotations (number of passes) until running marks appeared on the media surface while repeating the temperature atmosphere of 0°C and 50°C and 80% RH. It is.

As is clear from this table, vinyl chloride according to the example
The binder made of acrylic copolymer has particularly high yield point stress and yield point elongation rate compared to the binders of Comparative Examples 1 and 2, and therefore exhibits excellent performance as a binder, and as a result, it can withstand harsh conditions. Even under such conditions, the durability of the magnetic recording medium can be significantly improved.

The average degree of polymerization of the vinyl chloride-acrylic copolymer according to the present invention needs to be controlled within the range of 400 to 700 (in the above example, the average degree of polymerization was 570). Average degree of polymerization is 40
If it is less than 0, the adsorption ability to the magnetic powder surface will not be sufficient and the elastic strength will be low, which is undesirable.
If the average degree of polymerization exceeds 700, the solubility of the resin in a solvent will deteriorate. This is not preferable because the magnetic powder is not sufficiently fractionated in the crosstalk and the surface roughness of the coated film becomes large, leading to a decrease in output. For these reasons, the average degree of polymerization of the vinyl chloride-acrylic copolymer needs to be controlled within the range of 400 to 700.

In the present invention, the vinyl chloride-acrylic copolymer binder can be used alone, but it is better to use it in combination with a polyurethane resin.This polyurethane resin has abrasion resistance and appropriate flexibility. It is added to give the effect.

The addition ratio is 1 to 1 for the vinyl chloride-acrylic copolymer.
00 parts by weight, a range of 20 to 400 parts by weight is suitable. Less than 20 parts by weight.

The coating film may easily become brittle or the abrasion resistance may be reduced, and if it exceeds 400 parts by weight, the fraction may become poor, friction may become high, and deterioration may occur at high temperatures, which is not preferable.

As described in the above embodiment, when a polyurethane resin is added to the magnetic layer, the cleaning sheet (at least the surface layer that contacts the magnetic disk is made of non-thermoplastic fibers such as rayon fibers). Even if wear particles are generated from ), there is no affinity with the wear particles, so there is extremely little adhesion of wear particles to the magnetic disk.
Therefore, the occurrence of errors can be suppressed.

Although rayon fibers were used as the non-thermoplastic fibers in the above embodiments, similar effects can be obtained by using other cellulose fibers such as cotton fibers.

Next, examples of lubricants used in the magnetic layer will be explained. Specific examples of lubricants used in the present invention, such as 2-heptyl undecyl oleate (HpUO) and 2-octyl dodecyl oleate (O
Molecular structural formula, oxygen ratio, and melting point of 2-ethyl hesyl oleate (EHO) and oleyl oleate (00) of D o O) and comparative examples.

The surface tension and heating loss characteristics are shown in FIG. 7, and the viscosity characteristics at each temperature are shown in FIG. 8.

As is clear from FIGS. 7 and 8, the lubricant OO becomes cloudy at low temperatures (near 0 to 5°C) and becomes semi-solid with remarkable stickiness, and is made of thermoplastic fibers that easily generate wear particles. Since a cleaning sheet made by the manufacturer is used, abrasion powder adheres to the surface of the magnetic disk, causing errors and causing problems in terms of reliability. Furthermore, although the melting point of the lubricant EHO is -10° C. or lower, the durability of the magnetic disk is not good because the viscosity at each temperature is low.

Furthermore, the relationship between the surface roughness of the magnetic disk and the wear of the cleaning sheet will be explained.

In order to reduce the wear of the cleaning sheet as much as possible, it is also necessary to consider the surface roughness of the magnetic disk. When the surface roughness (Ra) of a conventional magnetic disk is relatively rough, for example, 0.020 to 0.025 pm, the cleaning sheet is inevitably prone to wear.

On the other hand, the surface roughness Ra of the magnetic disk is 0.01μ
It has been found that abrasion of the cleaning sheet can be minimized over a long period of time by setting the particle diameter to less than m, preferably less than o, oos μm.

The surface roughness Ra of the magnetic layer is as follows: stylus diameter: 2 μm, stylus load: 25 mg, cutoff: 0.08 mm. In the above embodiment, the case of a magnetic disk cartridge was explained, but the present invention is not limited to this. It can also be applied to other magnetic tapes, magnetic cards, etc.

〔Effect of the invention〕

By using the above-described binder, the present invention can provide a highly durable and highly reliable magnetic recording medium.

[Brief explanation of the drawing]

All figures are for explaining a magnetic disk cartridge according to an embodiment of the present invention, and FIG. 1 is an exploded perspective view of the magnetic disk cartridge. FIG. 2 is a bottom view showing the cleaning sheet attached to the upper case. FIG. 3 is a plan view showing a state in which a cleaning sheet is attached to the lower case. FIG. 4 is an enlarged sectional view of the vicinity of the magnetic head insertion port showing the usage state of this magnetic disk cartridge. FIG. 5 is an enlarged sectional view of the vicinity of the elastic piece of this magnetic disk cartridge. FIG. 6 is an enlarged sectional view of the cleaning sheet. FIG. 7 is a diagram showing the molecular structural formula and various characteristics of each lubricant, and FIG. 8 is a diagram showing the viscosity characteristics of each lubricant. 1...Cartridge case. 1a... Upper case, 1b... Lower case. 2...Magnetic disk, O...Cleaning sheet,

Claims (7)

[Claims] (1) A magnetic recording medium in which a magnetic layer mainly composed of ferromagnetic metal powder and containing at least a nonmagnetic binder and a lubricant is provided on a substrate made of a nonmagnetic material, wherein the binder is a hydroxyl group-containing vinyl The average degree of polymerization using monomer and sulfone group-containing vinyl monomer as constituent units is 4.
1. A magnetic recording medium comprising a polyvinyl chloride resin made of a vinyl chloride-acrylic copolymer whose molecular weight is regulated to be within the range of 00 to 700. (2) A magnetic recording medium according to claim (1), characterized in that the average BET surface area of the ferromagnetic metal powder is regulated within a range of 30 to 48 m^2/g. (3) In claim (1), the binder comprises:
A magnetic recording medium characterized in that it is a mixture of the vinyl chloride resin and the polyurethane resin. (4) A magnetic recording medium according to claim (1), wherein the lubricant is a fatty acid ester having the following general structural formula. General structural formula ▲ Numerical formula, chemical formula, table, etc. are available ▼ In the formula, R_1 and R_2 are each a linear or branched saturated alkyl group having 1 to 15 carbon atoms. R_3 is a linear or branched saturated or unsaturated alkyl group having 17 carbon atoms. (5) In the description of claim (1), the magnetic recording medium is a flexible magnetic disk, and the magnetic disk is rotatably housed in a cartridge case with a cleaning sheet pasted on the inner surface. Features of magnetic recording media. (6) In the magnetic disk according to claim (5), at least almost all of the disk-side surface layer of the cleaning sheet that is in direct contact with the magnetic layer of the magnetic disk is composed of non-thermoplastic fibers. recoding media. (7) A magnetic recording medium according to claim (6), wherein the non-thermoplastic fiber is a rayon fiber.