JP2799205B2 - Magnetic recording media - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a special copolymer mainly composed of vinyl chloride, which exhibits excellent performance as a binder for a ferromagnetic fine powder. And an improved magnetic recording medium using the same.

[Prior Art] A magnetic recording medium such as a magnetic tape is generally produced by providing a coating film composed of a magnetic powder and a binder (synthetic resin) on the surface of a support such as a polyester film. In recent years, high signal density and high reproduction output in short-wavelength recording have been required due to high performance of video tapes and audio tapes. To cope with these trends, magnetic powders are more finely divided than before, and because of their very large magnetic moment, the particles tend to agglomerate with each other, resulting in uniform dispersion in the binder resin. Are becoming more difficult than ever.

Also, when dispersing the ferromagnetic fine powder, a method of applying a high shearing force to disperse the powder is used. In this case, a vinyl chloride binder is used due to the viscosity increase of the paint and the temperature rise, and the temperature rise when using a magnetic recording tape. There is a drawback that the thermal decomposition of the resin is induced to deteriorate the magnetic powder due to the hydrogen chloride gas, the durability of the magnetic recording medium is reduced, and the magnetic head is corroded.

As a means of preventing this thermal decomposition, a method of adding a stabilizer for vinyl chloride resin has been well known. However, since these are low molecular weight compounds, if they are present in a large amount in a magnetic recording medium, they will bleed from the magnetic recording medium. In order to cause the phenomenon and to reduce the durability and cause the magnetic head to be stained, the amount of its use is naturally limited, and its effect is also limited.

With respect to such technical problems, extensive studies have been made from the viewpoint of improving the affinity for magnetic powder as a binder resin. For example, as disclosed in JP-A-57-128711, vinyl chloride- Carboxylic acid vinyl ester-unsaturated carboxylic acid and unsaturated carboxylic acid anhydride-based copolymers have been proposed, but they are useful for various magnetic powders, including ultra-fine Co-containing γ-Fe ₂ O ₃ which is important as a magnetic powder. The dispersibility is poor, and the reactivity with the urethane resin is poor, so that when a magnetic paint using the same is applied to a substrate, the coating film peels off, making the magnetic recording medium impractical. It also has insufficient thermal stability.

Further, JP-A-61-117729 and JP-A-61-243934 propose a vinyl chloride copolymer in which a hydroxyl group, a carboxyl group, etc. are introduced in the molecular structure. Co-containing γ-Fe ₂ O ₃ , dispersibility in metal powder is not sufficient, and because of its high solution viscosity, a large amount of organic solvent is required at the time of magnetic paint preparation, and furthermore, thermal stability is poor, There is a disadvantage that the desorbed hydrogen chloride corrodes the magnetic head. In addition, JP-A-61-
The copolymers disclosed in JP-A-199220 and JP-A-62-132226 show improvement in dispersibility in magnetic powder, but are insufficient with respect to thermal stability.

[Problems to be Solved by the Invention] Accordingly, the present invention makes it possible to produce a magnetic paint having excellent ultrafine Co-containing γ-Fe ₂ O ₃ , excellent dispersibility and thermal stability in metal powder, and low viscosity. It has been made to develop a novel binder resin and to provide a high-performance magnetic recording medium.

[Means for Solving the Problems] The present invention relates to a magnetic recording medium which solves such problems, and comprises the following structural units (a) vinyl chloride unit, (b) )formula (Wherein R ¹ represents a hydrogen atom or a methyl group, and R ² represents a monovalent hydrocarbon group) (hereinafter referred to as an ester unit). A vinyl monomer unit which is a valent organic group (hereinafter simply referred to as a hydroxyl group-containing unit); (d) a vinyl monomer unit having an epoxy group (hereinafter simply referred to as an epoxy group-containing unit); formula (Where n is an integer of 1 to 8) (hereinafter simply referred to as a carboxyl group-containing unit) represented by the following formula: Wherein R ¹ is a hydrogen atom or a methyl group, and R ³ is a compound of the formula C _p
R 2 is a divalent group represented by H _2p (OC _p H _2p ) _q (where p = 2 to 4, q = 0 to 10), and R ⁴ is a hydrogen atom or an alkyl group having 1 to 6 carbon atoms. The magnetic recording medium is formed by forming a magnetic layer in which a ferromagnetic fine powder is dispersed in a copolymer comprising a monomer unit represented by the following formula (hereinafter simply referred to as a phosphate unit).

The present inventors have studied a binder resin for magnetic recording media comprising a copolymer of a monomer containing a vinyl chloride ester unit and a hydroxyl group-containing vinyl monomer, and further specified from among carboxyl group-containing monomers. Is used as a component for the copolymerization of a specific amount of a dibasic acid, and a phosphate-containing monomer is combined with the half-ester to form Co-containing γ-Fe ₂ O of ultrafine particles.
_{3) The} dispersibility of metal powder is remarkably improved, and these copolymer resins have extremely good solubility in various organic solvents used as a coating medium. It was found that the properties could be greatly improved.

The present inventors have further found that by using an epoxy group-containing monomer as a component for copolymerization, it is possible to improve the thermal stability for a long time without causing a bleeding phenomenon of a stabilizer for vinyl chloride resin, and completed the present invention. It was made.

Further, the binder resin used in the present invention has an advantage that it has sufficient compatibility with a polyurethane resin or the like which is appropriately used in combination, and is easy to react and crosslink with an isocyanate group-containing urethane polymer or the like. Therefore, by using this as a binder, a high-performance magnetic recording medium with improved long-term durability can be obtained.

 Hereinafter, the present invention will be described in detail.

The copolymer as a binder resin used in the present invention,
It is characterized by being composed of the units (a) to (f) described above, and in particular, (a) the unit 60 to
95% by weight, (b) unit 0 to 15% by weight, (c) unit 5 to 15
Weight%, (d) unit 0.1-8%, (e) unit 0.1-10
%, And (f) a unit of 0.01 to 3% by weight, preferably having an average degree of polymerization of 200 to 500.

(A) If the amount of the vinyl chloride unit as the unit is too small, the physical strength decreases, while if it is too large, the solubility decreases, which is disadvantageous in use. A particularly preferred range is 65 to
88% by weight.

The ester unit (b) is used to improve the solubility, flexibility and the like of the copolymer.
As the monomer corresponding to the unit, Is mentioned. R ¹ in the formula is a hydrogen atom or a methyl group, R ²
Is a monovalent hydrocarbon group.

Desirable vinyl esters corresponding to the above formula [I] include vinyl acetate, vinyl propionate, vinyl versatate (manufactured by Shell Chemical Company), vinyl laurate, vinyl stearate, vinyl benzoate and the like.
Examples of desirable (meth) acrylates corresponding to the formula [II] include ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, propyl methacrylate and the like.

As the ester unit (b), any of the above-mentioned formulas [I] to [II] may be used alone or in combination of two or more thereof, but the polymerization ratio in the copolymer is high. If it is too much, the resin becomes soft and the durability of the magnetic recording medium (such as a magnetic tape) decreases, so that the range is 0 to 15% by weight as described above. In particular, the content is desirably in the range of 3 to 7% by weight.

Examples of the monomer corresponding to the hydroxyl group-containing unit (c) include the following.

If the weight ratio of the hydroxyl group-containing units introduced by these monomers in the copolymer is too small, not only does the dispersibility of the ferromagnetic powder deteriorate, but also the compatibility with the appropriately used polyurethane resin and the like is reduced. On the other hand, if the amount is too large, the viscosity increases greatly when the isocyanate prepolymer is blended, which causes a problem in pot life, lowers the physical strength and decreases the durability of the magnetic recording medium (such as a magnetic tape). As described above, the content is preferably in the range of 5 to 15% by weight, particularly preferably in the range of 7 to 10% by weight.

Examples of the monomer corresponding to the epoxy group-containing unit (d) include glycidyl ethers of unsaturated alcohols such as allyl glycidyl ether and methallyl glycidyl ether, glycidyl acrylate, glycidyl methacrylate, methyl glycidyl itaconate, and glycidyl ethyl. Glycidyl esters of unsaturated acids such as malate and the like can be mentioned. The proportion by weight of these in the copolymer is preferably in the range of 0.1 to 8% by weight, particularly preferably in the range of 1 to 5% by weight. If the amount is too small, the dehydrochlorination property upon heating is remarkable. If it is too much, the resin will not be soluble in various organic solvents used as a coating medium.

The monomer corresponding to the carboxyl group-containing unit (e) is a half-ester of a polymerizable unsaturated dibasic acid, such as a monoalkyl maleate or a monoalkyl fumarate (both are methyl as an alkyl group). ,
Ethyl, propyl, butyl, hexyl, octyl, etc.)
Is mentioned.

The proportion by weight of these in the copolymer is preferably in the range of 0.1 to 10% by weight, particularly preferably in the range of 0.5 to 5% by weight. If the amount is too small, the dispersibility decreases. Aggregates.

(F) Monomers corresponding to the phosphate unit, which is a unit, include mono [(meth) acryloyloxyethyl]
Acid phosphate, [where (meth) acryloyloxyethyl represents acryloyloxyethyl or methacryloyloxyethyl, and the same applies hereinafter. Mono [(meth) acryloyloxypropyl] acid phosphate, mono [(meth) acryloyloxybutyl] acid phosphate, mono [(meth) acryloyloxyethoxyethyl] acid phosphate, mono [(meth) acryloyloxypolyoxyethylene glycol] Acid phosphate, mono [(meth) acryloyloxypropoxyoxypropyl] acid phosphate, mono [(meth) acryloyloxypolyoxypropylene glycol] acid phosphate, mono [(meth) acryloyloxypolyoxybutylene glycol] acid phosphate, (meth) Acryloyloxyethyl methyl acid phosphate,
(Meth) acryloyloxyethyl butyl acid phosphate, (meth) acryloyloxypropylethyl acid phosphate, (meth) acryloyloxyethoxyethyl methyl acid phosphate, (meth) acryloyloxy polyoxyethylene glycol butyl acid phosphate, (meth) acryloyloxy poly Oxypropylene glycol ethyl acid phosphate is exemplified.

The polymerization ratio of these in the copolymer is preferably in the range of 0.01 to 3% by weight, particularly preferably in the range of 0.1 to 2% by weight. If the amount is too small, the magnetic powder (particularly, the BET specific surface area is 35 m ² / g) The above dispersibility is insufficient, and too much dispersibility deteriorates the solubility in a solvent, which is not preferable.

If the average degree of polymerization of the copolymer composed of such unit components is too low, the physical strength of the magnetic layer decreases, and the durability of the magnetic tape or the like also decreases. If the concentration is too high, the solution viscosity at a predetermined concentration becomes high and the workability deteriorates remarkably, so that the average polymerization degree is preferably in the range of 200 to 500.

In addition, this copolymer can be produced by a general suspension polymerization method, emulsion polymerization method, solution polymerization method, bulk polymerization method, or the like.

When the above copolymer is used as a binder resin, other resins may be used in equal amounts or less as necessary, and the resins that can be used in combination include polyurethane resins, nitrocellulose, epoxy resins, and polyamides. Examples include resins, phenolic resins, and various polymers such as polymers or copolymers of acrylates, methacrylates, styrene, acrylonitrile, butadiene, ethylene, propylene, vinylidene chloride, acrylamide, vinyl ethers and the like. Among these, polyurethane resin and nitrocellulose are particularly preferable.

In addition to this, it is desirable to use a polyisocyanate-based curing agent in combination. Examples of the curing agent include tolylene diisocyanate, diphenylmethane diisocyanate,
Bifunctional isocyanates such as hexane diisocyanate,
Examples thereof include trifunctional isocyanates such as Coronate L (trade name, manufactured by Nippon Polyurethane Industry) and Dismodur L (trade name, manufactured by Bayer AG), and urethane prepolymers containing isocyanate groups at both ends.

The ferromagnetic powder used in the present invention includes γ-Fe ₂ O ₃ ,
Fe ₃ O ₄ and those adsorbed or doped with cobalt ions, or CrO ₂ , and further, Fe, Co, Fe-
Other conventionally known magnetic powders such as acicular fine particle material containing Co or optionally Ni or the like are exemplified. The mixing ratio of the ferromagnetic powder and the binder resin is desirably 8 to 30 parts by weight of the binder resin per 100 parts by weight of the ferromagnetic powder.

In order to uniformly disperse the ferromagnetic powder and the binder resin, conventionally used lubricants, abrasives,
The addition of antistatic agents, dispersing aids, rust inhibitors, etc., and the use of various other organic solvents such as methyl ethyl ketone, methyl isobutyl ketone, and toluene as the coating medium may be the same as in the past. There are no restrictions.

As a support, polyester, polyolefin, cellulose acetate, synthetic resins such as polycarbonate, other non-magnetic metals, ceramics are used,
The form is used as a film, a tape, a sheet, a plate, or the like.

The coating means for forming the magnetic layer on the support may be a conventionally known method, and by appropriately performing a smoothing process such as a calendering process, the high-performance magnetic recording medium aimed at by the present invention is obtained. can get.

EXAMPLES Next, the present invention will be specifically described based on examples. In addition,
All parts in the examples are parts by weight, and all percentages are by weight.

Example 1 After nitrogen replacement in an autoclave equipped with a stirrer, 210 parts of methanol, 210 parts of ion-exchanged water, 119 parts of vinyl chloride,
8 parts of vinyl acetate, 2-hydroxypropyl acrylate
15 parts, allyl glycidyl ether 3 parts, maleic acid mono (2-ethylhexyl) ester 5 parts, mono (methacryloyloxyethyl) acid phosphate 1 part, di (2-ethylhexyl) peroxydicarbonate 5 parts as a polymerization initiator , And 1 part of methylcellulose, and the mixture was heated to 55 ° C. with stirring to start the reaction. Then, the following mixture (278 parts of vinyl chloride, 17 parts of vinyl acetate,
35 parts of hydroxypropyl acrylate, 7 parts of allyl glycidyl ether, 9 parts of mono (2-ethylhexyl) maleate, 3 parts of mono (methacryloyloxyethyl) acid phosphate, 100 parts of methanol
49 parts were continuously injected under pressure for 7 hours to cause a copolymerization reaction.

After the internal pressure of the autoclave reached 2 kg / cm ² after 12 hours, the residual pressure was released, the mixture was cooled, filtered, washed, drained, and dried to obtain 450 parts of a copolymer powder (polymer).

Example 2 A copolymer powder (polymer-) was obtained in the same manner as in Example 1 except that the kind and the charged amount of the monomer were variously changed as shown in Table 1.

Example 3 After purging with nitrogen in an autoclave equipped with a stirrer, 520 parts of ion-exchanged water, 197 parts of vinyl chloride, 26 parts of vinyl acetate, 10 parts of allyl glycidyl ether, and di (2-ethylhexyl) peroxydioxide as a polymerization initiator were used. Carbonate 5
And 1 part of methylcellulose, and stir with stirring.
After the reaction was started by raising the temperature to 50 ° C., the following mixture [198 parts of vinyl chloride, 2-hydroxypropyl acrylate 50
Part, maleic acid mono (2-ethylhexyl) ester 15
Parts, mono (methacryloyloxyethyl) acid phosphate, 4 parts], and 267 parts were continuously charged for 5 hours to carry out a copolymerization reaction.

After the internal pressure of the autoclave reached 2 kg / cm ² after 11 hours, the residual pressure was released, the mixture was cooled, filtered, washed, drained, and dried to obtain 420 parts of a copolymer powder (polymer).

Comparative Example 1 After nitrogen replacement in an autoclave equipped with a stirrer, 210 parts of methanol, 210 parts of ion-exchanged water, 124 parts of vinyl chloride,
8 parts of vinyl acetate, 2-hydroxypropyl acrylate
15 parts, 5 parts of maleic acid, 2 parts of mono (methacryloyloxyethyl) acid phosphate, 5 parts of di (2-ethylhexyl) peroxydicarbonate as a polymerization initiator, and 1 part of methylcellulose were charged to 55 ° C. while stirring. After the reaction was started by raising the temperature, the following mixture [vinyl chloride 283 parts, vinyl acetate 17 parts, 2-hydroxypropyl acrylate 35 parts, maleic acid 9 parts, mono (methacryloyloxyethyl) acid phosphate 3
Parts, 100 parts of methanol] and 447 parts were continuously injected for 7 hours to carry out a copolymerization reaction.

After the internal pressure of the autoclave became ² kg / cm ² after 12 hours, the residual pressure was released, and the mixture was cooled, filtered, washed, drained, and dried to obtain 440 parts of a copolymer powder (polymer).

Comparative Example 2 A copolymer powder (polymer-) was obtained in the same manner as in Comparative Example 1 except that the types and the charged amounts of the monomers were variously changed as shown in Table 2.

The analysis results of the polymer composition and the average degree of polymerization of the copolymer powder (polymer to) obtained above, and a sample were prepared using each polymer by the following method, and various physical properties (solution viscosity,
Thermal stability, dispersibility, and magnetic performance)
As shown in the table.

(1) Solution viscosity 40 parts of the copolymer resin was dissolved in 160 parts of a solvent (methyl ethyl ketone / methyl isobutyl ketone / toluene = 1/1/1), and the viscosity of the solution adjusted to 25 ° C. was measured using a B-type viscometer. Measured.

(2) Thermal stability 1 g of the copolymer resin was precisely weighed in a 15 cc test tube, and the opening was plugged with absorbent cotton sandwiched with Congo Red test paper.
The sample was placed in an oil bath at 0 ° C., and the time required for the Congo Red test paper to change its color due to the generated hydrochloric acid was measured.

(3) Dispersibility 10 parts of copolymer resin is put into a solvent composed of 30 parts of methyl ethyl ketone, 30 parts of methyl isobutyl ketone, and 30 parts of toluene.
After dissolving at 50 ° C. for 1 hour, 5 parts of polyurethane resin N-2304 (manufactured by Nippon Polyurethane Industry Co., Ltd.) and 40 parts of the following magnetic powders A to C were added while mixing with an Eiger mill, and kneaded for 1 hour. Further, 2 parts of Coronate L (manufactured by Nippon Polyurethane Co., Ltd.) was added and kneaded for 5 minutes to obtain a magnetic paint. This was applied on a 21 micron thick polyester film to a thickness of 6 μm, subjected to a magnetic field orientation treatment, and dried.

Magnetic powder A: 45.0 m ² / g Co-containing γ-Fe ₂ O ₃ magnetic powder B: 50.0 m ² / g Co-containing γ-Fe ₂ O ₃ magnetic powder C: 67.5 m ² / g metal powder The gloss (%) of the magnetic tape prepared as described above was measured.

The gloss was measured using a gloss meter (manufactured by Murakami Color Research Laboratories) and the reflectance at 60 degrees was compared with that of a standard glass plate. This numerical value represents the dispersibility of the magnetic powder in the magnetic coating film, and the larger the numerical value, the better the dispersibility.

(4) Magnetic Performance Using the magnetic tape prepared above, the squareness ratio was measured using a vibrating sample magnetometer (manufactured by Toei Kogyo).

[Effects of the Invention] The binder resin forming the magnetic recording medium of the present invention has excellent dispersibility in ultrafine Co-containing γ-Fe ₂ O ₃ and metal powder and solubility in various organic solvents used as a coating medium. The magnetic recording medium is extremely good, has a low viscosity, and exhibits excellent thermal stability, so that a high-performance magnetic recording medium having good durability and excellent stability over time can be obtained, and its practical value is great.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-1-292621 (JP, A) JP-A-62-181310 (JP, A) JP-A-64-39624 (JP, A) (58) Investigation Field (Int.Cl. ⁶ , DB name) G11B 5/702

Claims (2)

(57) [Claims] (1) The following structural units (a) a vinyl chloride unit, and (b) a formula (Wherein R ¹ represents a hydrogen atom or a methyl group, and R ² represents a monovalent hydrocarbon group), (c) a vinyl-based unit whose side chain is a hydroxyl-containing monovalent organic group. Monomer unit, (d) vinyl monomer unit having epoxy group, (e) formula (Where n is an integer of 1 to 8) and (f) a formula Wherein R ¹ is a hydrogen atom or a methyl group, and R ³ is a compound of the formula C _p
R 2 is a divalent group represented by H _2p (OC _p H _2p ) _q (where p = 2 to 4, q = 0 to 10), and R ⁴ is a hydrogen atom or an alkyl group having 1 to 6 carbon atoms. A magnetic recording medium comprising a magnetic layer in which a ferromagnetic fine powder is dispersed in a copolymer comprising monomer units represented by the following formula: 2. The copolymer comprises (a) 60 to 95% by weight of a unit, (b) 0 to 15% by weight of a unit, (c) 5 to 15% by weight of a unit, and (d) 0.1 to 8% by weight of a unit. 2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium comprises 0.1 to 10% by weight of (e) unit and 0.01 to 3% by weight of (f) unit and has an average degree of polymerization of 200 to 500.