JPH079692B2 - Magnetic recording medium - Google Patents

Description

Description: TECHNICAL FIELD The present invention provides a magnetic recording medium having excellent electromagnetic conversion characteristics and good running durability.

[Background of the Invention]

At present, magnetic recording media are widely used in the fields of audio, video, magnetic disk, etc., and accordingly, the amount of information recorded on magnetic recording media is increasing year by year. However, there is an increasing demand for higher recording density.

As one of the conditions required for a magnetic recording medium for high density recording, high coercive force and short wavelength recording have been proposed both theoretically and substantially. Along with that, for the magnetic layer, smaller magnetic particles are used, and a method such as more mirror surface is adopted.On the other hand, the base is also in the direction of mirror surface with less unevenness, and physical properties such as running property and durability are improved. Has a negative impact. A back coat is considered as one of the improvement methods, and various proposals have been made for the composition of the binder inorganic filler, the lubricant, etc. (for example, Japanese Patent Publication No. 57-29).
No. 769), running durability (scraping of the back coat layer), adhesion, electromagnetic conversion characteristics, etc. are not well-balanced and satisfactory.

When higher performance is demanded as in recent years, it is desirable to make the back coat layer as mirror-like as possible in terms of electromagnetic conversion characteristics. In addition to the mirror surface of the magnetic layer, friction in the back coat layer is being further reduced, and since it is used in various environments, it is necessary to improve the durability and the adhesion by dropping out. As measures against these, as a method of reducing friction, use of a polymer meter binder, reduction of rubber components, and the like are considered. However, the high molecular weight binder has a problem from the viewpoint of dispersibility of the pigment, and the reduction of the rubber-based component causes a problem from the viewpoint of adhesiveness.

[Object of the Invention]

An object of the present invention is to provide a magnetic recording medium having a back coat layer having a small friction coefficient and excellent durability.
Since the back coat layer has a small coefficient of friction, it can be mirror-finished and a magnetic recording medium having excellent electromagnetic conversion characteristics can be obtained.

[Outline of Invention]

The magnetic recording medium of the present invention comprises a magnetic recording layer provided on a non-magnetic substrate and a back coat layer provided on the opposite side of the non-magnetic substrate. As a curing agent for the curable binder in the back coat layer, a cyclic poly It is characterized by using an isocyanate.

Polyisocyanates conventionally used as a curing agent for curing a binder containing a polyurethane prepolymer and other resins having a reactive group such as active hydrogen are all linear or branched. Since a cyclic polyisocyanate is used in the present invention, it is possible to obtain a binder resin structure having a tougher three-dimensional structure than that obtained by a conventional curing agent, and durability is increased, while the friction coefficient is reduced, and the rubber-based bonding is increased accordingly. By increasing the amount of the agent, the back coat layer can be made to have a mirror surface, and the electromagnetic conversion characteristics can be improved.

[Specific Description of the Invention]

The binder used in the back coat layer of the present invention is a resin having an active hydrogen group such as a hydroxyl group, and particularly a rubber type resin such as a urethane prepolymer. Resins that can coexist include vinyl chloride-vinyl acetate (containing several% of vinyl alcohol portion) copolymer. More rubber-based binders improve the adhesion to the supporting substrate but increase the coefficient of friction. However, according to the present invention, a back coat layer having a sufficiently low friction coefficient can be provided even if the amount of the rubber-based binder is large. Vinyl chloride-vinyl acetate copolymer improves durability and reduces friction coefficient. However, if the amount is too large, the adhesiveness decreases, and the durability deteriorates. Similarly, the surface roughness increases, and when the magnetic layer on the surface side is wound in contact with the magnetic layer, the magnetic layer is adversely affected and electromagnetic conversion characteristics are deteriorated. Therefore, 20% by weight or more of rubber binder,
When used in a proportion of preferably 40 to 90% by weight, durability,
A back coat layer having a good balance in all aspects of surface roughness, friction coefficient and electromagnetic conversion characteristics can be obtained.

The curing agent used in the present invention is a cyclic polyisocyanate, which is the following hexamethylene diisocyanate cyclic trimer or tolylene diisocyanate cyclic trimer.

Etc. can be used. Examples of such polyisocyanates include Coronate EH and Coronate 2030 (manufactured by Nippon Polyurethane Industry Co., Ltd.). The cyclic polyisocyanate acts as a curing agent for the binder of the back coat and acts with the hydroxyl groups of the polyurethane prepolymer (polymer polyol) and pinyl chloride-vinyl acetate copolymer to carry out the curing reaction. The cyclic polyisocyanate may be used in an amount of about 5 to 30% by weight based on the amount of the binder compounded.

As will become clear later, when a cyclic polyisocyanate is used as a curing agent, the coefficient of friction and durability are greatly improved, so even if the amount of rubber-based binder is increased and the degree of mirror finish is increased, running Properties are not impaired, and the mirror conversion reduces adverse effects on the magnetic layer and improves electromagnetic conversion characteristics.

For the back coat layer, it is possible to use conventionally known antistatic agents and dispersants such as carbon black, metal oxide powders such as TiO, Fe ₂ O ₃ and ZnO, and carbon salts such as calcium carbonate.

Regarding the surface roughness of the back coat layer, it is conventionally 0.05 to 0.
Although a material having a size of about 30 μm is often used, in the present invention,
Even with a considerably large mirror surface of about 0.01 to 0.20 μm, the use of cyclic polyisocyanate reduces friction, so that running performance and durability do not decline. The preferred surface roughness range is 0.
02 to 0.15 μm. Although it is necessary to use a soft binder for the mirror finishing, not only the mirror workability is increased because a large amount of the rubber binder can be used in the present invention, but also the dropout is reduced because the adhesiveness is improved, Has little impact on durability.

For the magnetic layer, γ-Fe ₂ O ₃ , CrO ₂ , cobalt adsorption γ
-Fe ₂ O _{3 and the} like are used, but other ferromagnetic alloy powders that have been conventionally proposed for high-density recording are used.In particular, cobalt adsorption type γ-Fe ₂ O ₃ and 40 m ² / BET method are used. A dispersion of a ferromagnetic alloy powder having a specific surface area of g or more in a resin binder may be used. The coercive force of the magnetic layer is 1000
It is preferably Oe or more.

Examples will be described below.

The following magnetic layers were used in Examples 1 to 8 and Comparative Examples 1 to 3 below.

Magnetic layer Cobalt adsorbed γ-Fe ₂ O ₃ (long axis 0.4μ, short axis 0.05μHc650O
e) 120 parts carbon black (for antistatic Mitsubishi Carbon Black MA
-600) 5 parts α-Al ₂ O ₃ powder (0.5 μ powder) 2 parts Dispersant (soybean oil lecithin) 3 parts Solvent MEK / toluene 50/50 100 parts The above composition was mixed in a ball mill for 3 hours, The acicular magnetic iron oxide is well wetted with the dispersant. Next, vinyl chloride vinyl acetate copolymer (VAGH manufactured by Union Carbide) 15 parts Thermoplastic urethane resin (Nitporan 2304 manufactured by Nippon Polyurethane Co.) 15 parts Lubricants (higher fatty acids) 3 parts Solvent (MEK / triene: 50/50) ) Mix and dissolve 200 parts of the mixture thoroughly. This is placed in a ball mill that has been previously treated with magnetic powder and dispersed again for 36 hours. After dispersion, 5 parts of an isocyanate compound (Desmodur L manufactured by Bayer Co., Ltd.) capable of cross-linking and reacting with a functional group mainly composed of a hydroxyl group in the binder is added. This is coated on a 14μ polyester film and the magnetic particles are oriented through a magnetic field. After drying this, the surface is smoothed at 60 ° C.
Promotes the crosslinking reaction of isocyanate in 47 hours. This is a magnetic layer.

Further, in Examples 9 to 10 and Comparative Examples 4 and 5, the following magnetic layers were used.

Ferromagnetic alloy powder 100 Polymerized part Oleic acid (dispersant) 2 parts by weight Lubricant 1.2 parts by weight Myristic acid 0.7 parts by weight Butyl stearate 0.5 parts by weight Polyuntan resin 15 parts by weight Vinyl chloride / vinyl acetate copolymer 10 parts by weight MEK 110 parts by weight Part MIBK 66 parts by weight A mixture of cyclohexanenon 44 parts by weight was dispersed by a sand grinder mill. After the dispersion, 10 parts by weight of an isocyanate compound (Desmodur manufactured by Bayer Co.) capable of cross-linking and reacting with a functional group mainly composed of a hydroxyl group in the binder was added. This is coated on a 14-micron-thick polyester film and the magnetic particles are oriented through a magnetic field. After drying this, the surface was smoothed and the crosslinking reaction with isocyanate was promoted at 60 ° C. for 47 hours. This is a magnetic layer. The BET specific surface area of the used ferromagnetic alloy powder was 55 m ² / g and the coercive force was 1450 Oe. The BET specific surface area is preferably 40 m ² / g or more and the coercive force is 1000 Oe or more.

Example (back coat layer) Carbon black RAVEN1000 (Colombian carbon 28
μm) 100 parts (Nitsuporan 5033) Dispersant (soybean oil lecithin) 3 parts Solvent (MEK / toluene / 50/50) 500 parts The ratio of vinyl chloride vinyl acetate copolymer to urethane prepolymer is shown in Table 1. After the above composition was dispersed in a ball mill for 36 hours, a cyclic isocyanate compound Coronate EH or 2030 (manufactured by Nippon Polyurethane Co., Ltd.) capable of reacting with a functional group mainly composed of a hydroxyl group in a binder and cross-linking was used.
Add some. This is applied to the opposite side of the above magnetic layer to 0.5 to 2.0 μm and dried. This is surface-treated, and the crosslinking reaction of the isocyanate is promoted at 60 ° C. for 48 hours. 1 /
It was cut into 2 ″ to obtain a video tape. The magnetic tape having a back coat layer obtained by curing with Coronate EH is shown in Table 1.
Examples 1 to 5 shown in Table 1 and the magnetic tapes having a back coat layer cured with Coronate 2030 are Examples 6 to 8 shown in Table 1. For comparison, back coat layers cured with a conventional chain polyisocyanate (Coronate L) and a branched chain polyisocyanate (Coronate HL) were similarly prepared (Comparative Examples 1 to 3).

The results of measuring various properties of the above tape are shown in Table 1, and the relationship among the friction coefficient, the binder component ratio and the curing agent is shown in FIG. In FIG. 1, the case of 100 parts of urethane prepolymer in the system of Examples 1 to 5 is also shown.

As a result, when a cyclic isocyanate (Coronate EH, Coronate 2030) is used, the friction coefficient is considerably reduced, so that the rubber component is increased, the surface processability is improved, and the magnetic recording medium is sufficiently durable even if it is mirror-finished. It was found that In Example 1, if the urethane component is decreased, the workability and the surface property deteriorate. Further, although the friction coefficient is lowered, it is considered that the durability is rather lowered due to the poor adhesion.

The measurement of each characteristic was performed as follows.

1) Surface roughness: Cut-off 0.17 mm, obtained by a 20-point average method from a chart obtained using Talystep (manufactured by TAYLOR-HOBSON),
A needle pressure of 0.1 × 2.5 μ was used. Those that were found to be non-uniform from the measured data were picked up and excluded from the uniform data section.

2) C-S / N In Comparative Examples 1 to 3, the deck is a relative value when Matsushita NV-8300 (trade name) is used and TDKRef is set to 0. For Examples 9 and 10 and Comparative Examples 4 and 5, the tape was cut to a width of 8 mm, and an 8 mm video deck camcorder 2400 (trade name of Eastman Kodak Company) was used to hold an 8 mm round-table conference type A.
It is a relative value when measured using Ref.

3) A load (T ₂ ) when a load was applied (40 g) to a cylinder with a friction coefficient of 40 mmφ and the inside of the back coat surface was rotated (40 g) was read with a tension analyzer.

4) Durability With a back coat inside a 20 mmφ cylinder, a load of 150 g was applied and the product was rotated at 180 rpm for 20 minutes, then evaluated according to the degree of scratches on the back coat surface.

[Brief description of drawings]

FIG. 1 is a plot of the friction coefficient of each curing agent when the ratio of vinyl chloride / vinyl acetate copolymer and urethane prepolymer was changed.

Claims (4)

[Claims] 1. A magnetic recording medium having a magnetic recording layer provided on a non-magnetic substrate and a back coat layer provided on the opposite surface thereof, wherein a vinyl chloride vinyl chloride copolymer as a curable binder in the back coat layer and Urethane prepolymer,
A magnetic recording medium characterized in that a cyclic isocyanate is used as the curing agent component. 2. The magnetic recording medium according to claim 1, wherein the surface roughness of the back coat layer is 0.01 to 0.20 micron. 3. The magnetic recording medium according to claim 1, wherein an amount of urethane prepolymer in an amount exceeding 20% is a curable binder component in the back coat layer. 4. A magnetic recording layer comprising a ferromagnetic alloy powder having a specific surface area of 40 m ² / g or more by a BET method dispersed in a resin binder, and the magnetic layer has a coercive force of 1000 Oe or more. The magnetic recording medium according to any one of claims 1 to 3, which is characterized.