JPS61188729A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium which is excellent in both of electromagnetic conversion characteristic and durability by having a magnetic layer formed by dispersing and binding magnetic powder having a specific length or below of average long axis diameter in the cured and coated film of a specific radiation curing resin. CONSTITUTION:The radiation curing resin to be used has a polar group in the molecule and has at least two polymerizable carbon-carbon double bonds. Satisfactory crosslinking hardenability is not obtainable if said bonds are <2. Particularly the resin having <=8,000, more preferably <=5,000 average mol. wt. is required to be selectively used. The magnetic powder having <=0.5mum, and usually 0.05-0.5mum average long axis diameter is used as the magnetic powder to be dispersed and bound into the binder. The easy dealing with the tendency toward the higher density recording is made possible by using such finely pulverous magnetic powder.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to magnetic recording media such as magnetic tapes and magnetic disks.

[Conventional technology]

A magnetic recording medium such as a magnetic tape has a structure in which a magnetic layer formed by dispersing and bonding magnetic powder in a binder is provided on a base such as a polyester film.

The electromagnetic conversion characteristics and durability of this medium are mainly determined by the dispersibility of the magnetic powder in the magnetic layer and the wear resistance of the magnetic layer itself, so a binder that gives good results in these characteristics is selected and used. From this point of view, relatively high molecular weight resins with an average molecular weight of 20,000 or more, such as vinyl chloride-vinyl acetate resins, butyral resins, cellulose resins, etc., are generally used as the binder. Ta.

[Problem that the invention seeks to solve]

However, in recent years, with the demand for higher density recording in magnetic recording media, instead of the conventional magnetic powder with an average major axis diameter of 0.6/a or more, magnetic powders with an average major axis diameter of 0.5/a or less are being used. Magnetic powders with extremely small particle sizes have come to be used.

When a magnetic layer is formed by applying the conventional binder to such a magnetic powder with a very small particle size, the dispersibility of the magnetic powder is extremely reduced, and the electromagnetic conversion characteristics are greatly impaired. There was a problem with putting it away.

Therefore, the present invention not only improves the dispersibility of the magnetic powder in the form of fine particles as described above, but also achieves good results in terms of wear resistance of the magnetic layer, thereby improving electromagnetic conversion characteristics and durability. The purpose of this invention is to obtain a magnetic recording medium that is both excellent in both.

[Means for Solving the Problems] As a result of intensive studies to achieve the above object, the inventors found that the conventional binder for dispersing and binding magnetic powder with a very small particle size was used. The use of a resin with a low molecular weight rather than a resin with a high molecular weight, such as the However, it has been found that this drawback can be overcome by applying a magnetic paint containing the above-mentioned resin to a base and then crosslinking and curing it.

Therefore, as a result of further investigation based on the above knowledge, the following are low molecular weight resins that have crosslinking curability.
Specific thermoplastic resins such as cellulose resins, which can be relatively low in molecular weight, are cross-linked and cured with polyisocyanate compounds, etc., thermosetting resins such as epoxy resins, and There are known radiation-curable resins that have polymerizable carbon-carbon double bonds such as acryloyl groups and can be cross-linked and cured by radiation such as ultraviolet rays or electron beams. It has been found that this material is extremely suitable for improving the properties and wear resistance of the magnetic layer.

In other words, with materials that inherently have thermoplastic properties such as cellulose resins, it is possible to improve the dispersibility of the magnetic powder by lowering the molecular weight, but on the other hand, the abrasion resistance of the magnetic layer is lowered by materials such as polyisocyanate compounds. Even crosslinking and curing does not provide sufficient improvement. In addition, in thermosetting resin,
The pot life of magnetic paint is short because it usually contains a hardening agent (
(In addition, since it is generally difficult to completely crosslink and cure, unreacted and uncured resin remains in the magnetic layer and adheres to the magnetic head that rotates at high speed, reducing the durability of the magnetic recording medium.) There is a problem of loss of sexuality.

On the other hand, radiation-curable resins do not have any of the above-mentioned problems found in cellulose-based resins and thermosetting resins, and in particular, there is a risk that the pot life of magnetic paints may be shortened or curing may be insufficient. By using a specific resin with a defined molecular weight, etc., it is possible to significantly improve the dispersibility of the magnetic powder and also satisfy the abrasion resistance of the magnetic layer, resulting in improved electromagnetic conversion characteristics. It has been found that it is possible to manufacture a magnetic recording medium that has excellent properties and durability.

This invention was completed based on the above knowledge, and its gist is a base, a polymerizable carbon-carbon compound having a polar group in the molecule provided on the base,
It has few carbon double bonds (both have two and the average molecular weight is 8,
The magnetic recording medium has a magnetic layer formed by dispersing and bonding magnetic powder having an average major axis diameter of 0.5 μm or less in a cured coating film of a radiation-curable resin having a particle diameter of 0.000 μm or less.

[Structure and operation of the invention] The radiation-curable resin used in this invention has a polar group in the molecule and has a small number of polymerizable carbon-carbon double bonds (
The above polar groups include hydroxyl groups, carboxyl groups, sulfonic acid groups, phosphoric acid groups, or salts thereof such as alkali metal salts, which contribute to improving the dispersibility of the magnetic powder. The above-mentioned polymerizable carbon-carbon double bond is typically the carbon-carbon double bond contained in the (meth)acryloyl group, but other carbon-carbon double bonds in the polybutadiene molecule are mentioned. Groups other than (meth)acryloyl groups such as (meth)acryloyl groups may also be used.

The content of the above polar groups varies considerably depending on the type of polar groups. For example, for a hydroxyl group or its salt, 0.2 to 2 milliequivalents/y1 for a carboxyl group or its salt, 0.02 to
0.2 milliequivalent/? , for a sulfonic acid group or its salt, it is 0.02 to 0.2 mm IJ equivalent/9°, and for a phosphoric acid group or its salt, it is 0.02 to 0.2 mm IJ equivalent/y1 (both per y of resin). is desirable. Further, the number of polymerizable carbon-carbon double bonds per molecule must be at least 2, usually 3 to 6, and if there are less than 2, sufficient crosslinking and curability cannot be obtained.

Such radiation-curable resins have (meth)acryloyl groups as polymerizable carbon-carbon double bonds, for example, have ester bonds, ether bonds, urethane bonds, etc. in the molecular main chain, and Examples include so-called acrylic oligomers obtained by reacting (meth)acrylic acid with base polymers having the aforementioned polar groups, or base polymers made of epoxy resin, polybutadiene, or the like having the aforementioned polar groups.

Moreover, as a substance having a polymerizable carbon-carbon double bond other than a (meth)acryloyl group, polybutadiene having the above-mentioned polar group can be mentioned as a typical example.

Some of the known radiation-curable resins include
Although some have a relatively high molecular weight, in this invention, the average molecular weight is 8,000 or less, preferably 5.
00α or less must be selected and used. If the average molecular weight exceeds s, ooo, it becomes difficult to highly improve the dispersibility of the fine particulate magnetic powder. on the other hand,
If the molecular weight is too low, good results may not be obtained in terms of improving dispersibility and good results may not be obtained in terms of wear resistance of the magnetic layer, so generally the average molecular weight is 500 or more. , especially preferably 1.000 or more.

In this invention, a polymerizable monomer that acts as a reactive diluent for this resin may be used in combination with the above-mentioned radiation-curable resin, if necessary.

This polymerizable monomer has at least one polymerizable carbon-carbon double bond in its molecule, and has a molecular weight of s, oo, like the curable resin.

Below, it is usually about 300 to 2,000. This polymerizable monomer is essentially different from the radiation-curable resin in that it does not have a polar group in its molecule, and for this reason, this heptamer alone cannot produce the magnetic powder that is the object of the present invention. A high degree of dispersibility improvement effect cannot be obtained.

Specific examples of the above polymerizable monomers include (meth)acrylate compounds such as alkyl esters of (meth)acrylic acid, polyesters of polyhydric alcohols and (meth)acrylic acid, and mono- or polyallyl ester compounds. Can be mentioned. These polymerizable monomers are usually 70% by weight or less, preferably 50% by weight or less, based on the total amount with the radiation-curable resin.
It is preferable to use less than % by weight; if the amount used is too large, good results cannot be obtained in terms of the dispersibility of the magnetic powder and the abrasion resistance of the magnetic layer.

The binder component used in this invention essentially consists of the above-mentioned radiation-curable resin and a polymerizable monomer that can be used in combination with it, but it is used for the purpose of modifying various properties such as softness and strength of the magnetic coating film. Conventionally, relatively high molecular weight resins that have been widely used in the past, such as vinyl chloride-vinyl acetate resins, butyral resins, cellulose resins, and other known resins are usually used within a range that does not impede the effects of the present invention. It may be used in combination with the above-mentioned radiation curable resin within the range of 50% by weight or less of the entire binder.

In this invention, the magnetic powder to be dispersed and bound in the binder has an average major axis diameter of 0.5μ or less, usually 0.05 to
Use 0.5p magnetic powder. Generally acicular average axial ratio (
For those with average major axis diameter / average minor axis diameter) 3 to 5, the specific surface area measured by nitrogen adsorption method is 40 m'/2 or more, usually 40 to 60
It is about rrl/y. By using such fine particulate magnetic powder, it is possible to easily meet the demands for high-density recording in magnetic recording media, which has been particularly required in recent years.

Such magnetic powders include T-Fe203, Fe50
Oxide-based magnetic powders such as iron oxide, cobalt-containing iron oxide, and chromium oxide are preferred, and metal magnetic powders such as Fe, Ni, Cr, or alloys thereof may also be used in some cases.

In this invention, first, a magnetic paint containing the above-mentioned binder, magnetic powder, and solvent is prepared. Such magnetic paints contain dispersants, lubricants, antistatic agents,
It goes without saying that known additives such as fillers, pigments, and surfactants may be added.

This magnetic paint is applied on a base such as a polyester film according to a conventional method, dried if necessary, and subjected to orientation treatment and mirror polishing treatment in the same way as conventional methods, and then irradiated with radiation. By crosslinking and curing the binder component, that is, the radiation-curable resin or the same and the polymerizable monomer, the magnetic recording medium of the present invention can be obtained.

Incidentally, as the above-mentioned radiation, an electron beam is preferable, but when performing this electron beam irradiation, the absorbed dose is 2 to 10 Mra.
It is better to make it d. Other applicable radiation includes ultraviolet rays, and when irradiating with ultraviolet rays, a high-pressure mercury lamp or the like may be used to provide an irradiation dose of 10 to 1,000 millijoules (mJ)/cd. In addition, in order to accelerate the curing speed with ultraviolet irradiation,
It is desirable to include an appropriate photopolymerization initiator in advance in the magnetic coating material.

The magnetic recording medium obtained in this manner has magnetic powder in the form of fine particles dispersed and bonded in a cured coating film in which a base and a binder that essentially includes the specific radiation-curable resin are three-dimensionally cross-linked and cured. The magnetic layer is characterized by excellent dispersibility of the magnetic powder and excellent abrasion resistance with the magnetic layer.

Thus, in the present invention, the specific radiation-curable resin has an average molecular weight of s, oo.

At present, it is not necessarily clear why the dispersibility of fine particulate magnetic powder improves when a resin with a low molecular weight as below is used. The inventors estimate that conventional magnetic powder with a relatively large particle size requires a correspondingly long length to surround each particle of magnetic powder and uniformly separate and bind each particle. However, in the case of magnetic powder with a very small particle size as applied to this invention, the polymer chain necessary to surround one particle is shorter than that of the conventional one. In other words, if the polymer chain becomes too long, the magnetic powder particles
It seems that it would be rather difficult to separate and coat each magnetic powder, and therefore, using a binder with a shorter polymer chain, that is, a lower molecular weight, would be more likely to contribute to improving the dispersibility of the magnetic powder. I don't think so.

〔Effect of the invention〕

As described above, in this invention, by using a specific radiation-curable resin with a low molecular weight as a binder for magnetic powder with a very small particle size, it is possible to improve the dispersibility of the magnetic powder and to make the resin To provide a magnetic recording medium, such as a magnetic tape, which has excellent electromagnetic properties and durability, and can be easily adapted to high-density recording, since the abrasion resistance of the magnetic layer can be satisfied by hardening the magnetic layer. I can do it.

〔Example〕

EXAMPLES Below, examples of the present invention will be described in more detail. In the following, parts shall mean parts by weight.

Example 1 Alumina (trade name AKP-48 manufactured by Sumitomo Chemical) 35 parts myristic acid 15 parts n-butyl stearate 1o part cyclohexanone 700 parts toluene
700 parts A magnetic paint was prepared by uniformly mixing each of the above components, and this was mixed in 14/'
It was coated on a 1-thick polyester film to a thickness of 5 tones. After mirror polishing, the absorbed dose is 8 Mrad at an acceleration voltage of 165 KV.
The magnetic coating film was cured by irradiation with an electron beam, and then slit into a width of 100% inch to produce a magnetic tape of the present invention.

The radiation curable resin used above has raw ingredients such as adipic acid, isophthalic acid, 1,6-hexanediol,
Average molecular weight consisting of trimethylolpropane, neopentyl glycol, hexamethylene diisocyanate and acrylic acid, which has an ester bond and a urethane bond in the molecular main chain, and has an acryloyl group as well as a hydroxyl group and a carboxyl group as polar groups in the molecule. 5,
000° The average number of acryloyl groups per molecule is 5
, hydroxyl value is 1 milliequivalent/y, carboxyl value is 0
．． 05mi! It is a resin with J equivalent/y. Moreover, the polymerizable monomer used above is composed of dipentaerythritol hexaacrylate.

Example 2 A radiation-curable resin was synthesized by changing the raw material components described in Example 1 and the polymerization conditions, and had an ester bond and a urethane bond in the main chain of the molecule, and a polar group as well as an acryloyl group in the molecule. As a resin having a hydroxyl group and a carboxyl group, an average molecular weight of s, ooo, an average number of acryloyl groups per molecule of 3, a hydroxyl value of 1 meq/y1, a carboxyl group value of 0.06 meq/2. A magnetic tape of the present invention was produced in exactly the same manner as in Example 1, except that the same amount as in Example 1 was used.

Example 3 A radiation-curable resin was synthesized by changing the raw material components described in Example 1 and the polymerization raw materials, and had an ester bond and a urethane bond in the main chain of the molecule, and a polar group as well as an acryloyl group in the molecule. The average molecular weight with sulfonic acid groups is 5,000, the number of acryloyl groups per molecule is 4 on average, and the sulfonic acid group value is 0.
．． 04mi! A magnetic tape of the present invention was produced in exactly the same manner as in Example 1, except that the same amount of J equivalent/2 resin as in Example 1 was used.

Comparative Example 1 In place of radiation-curable resin and polymerizable monomer, 75 parts of nitrocellulose H(2) with an average molecular weight of 25,000 (trade name Cell/Bath-1, manufactured by Asahi Kasei Corporation), 75 parts of nitrocellulose H(2) with an average molecular weight of 25,000 Using 50 parts of a polyurethane elastomer (trade name Bandex T-5250, manufactured by Dainippon Ink Co., Ltd.) and 25 parts of a low molecular weight trifunctional polyisocyanate (trade name Coronet L, manufactured by Nippon Polyurethane Co., Ltd.),
A magnetic paint was prepared in the same manner as in Example 1, except that 2,000 parts of a 1:1 mixed solvent of cyclohexanone and toluene was used as a solvent, and Example 1 was prepared using this paint.
A magnetic tape for comparison was produced in almost the same manner as described above.

Comparative Example 2 Nitrocellulose H (2) with an average molecular weight of 5,000 was used instead of nitrocellulose H (2) with an average molecular weight of 25,000.
(1/le) (Product name Seltsuba BTH manufactured by Asahi Kasei Corporation (
A magnetic tape for comparison was produced in the same manner as in Comparative Example 1, except that the same amount of ['/+a) ] was used.

The results of examining the magnetic properties, electromagnetic conversion properties, and durability of the magnetic tapes according to Examples 1 to 3 and Comparative Examples 1 to 2 above are as shown in the table below. Furthermore, the electromagnetic conversion characteristics are based on the video tape recorder (National NV-8).
200) was compared with Comparative Example 1 as the standard (Od
It is expressed as a relative value as B). In addition, the durability is
Repeated recording and playback of a 2-hour recording and playback length using the same video tape recorder as above, and the S/N ratio was 3dB.
We investigated the number of times the vehicle was driven before it decreased.

As is clear from the results in the table above, the magnetic tape of the present invention has excellent dispersibility of magnetic powder and wear resistance of the magnetic layer, so it can satisfy both electromagnetic conversion characteristics and durability. I understand.

Claims (1)

[Claims] (1) A base and a cured coating of a radiation-curable resin having an average molecular weight of 8,000 or less and having a polar group in the molecule and at least two polymerizable carbon-carbon double bonds provided on the base. 1. A magnetic recording medium comprising a magnetic layer in which magnetic powder having an average major axis diameter of 0.5 μm or less is dispersed and bonded.