JPH04344319A - Binder for magnetic recording medium and magnetic coating material compounded therewith - Google Patents

Abstract

PURPOSE:To improve surface smoothness, mechanical strength, wet heat resistance, and resilience at a low temp. by incorporating a polyurethane resin formed by bringing a polyhydroxy compd. for at least a part of which specific compds. are used and a polyisocyanate compd. into a binder. CONSTITUTION:The binder for the magnetic recording medium contg. the polyurethane resin formed by bringing the polyhydroxy compd. having >=2 pieces of active hydrogen atoms in the molecule and the polyisocyanate compd. into reaction is formed. The constitutional unit of at least a part of this polyhydroxy compd. consists of at least two kinds of the constitutional units selected from the constitutional units (I) to (IV) expressed by formula I to IV. The contents thereof are 90/10 to 20/80 molar ratios of [(I)+(III)]/[(II)+(III)] and 50/50 to 0/100 molar ratios of [(III)+(TV)]/[(I)+(II)]. The respective constitutional units are polyester carbonate polyols forming ester bonds or carbonate bonds with each other.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a binder for magnetic recording media containing a specific polyester carbonate polyurethane resin and a magnetic paint containing the same. The binder of the present invention provides a magnetic coating film with excellent surface smoothness, mechanical strength, heat and humidity resistance, and flexibility at low temperatures. Therefore, the binder and magnetic coating material of the present invention are useful for producing magnetic recording media with good electrical properties.

0002

[Prior Art] Magnetic recording media such as audio tapes, video tapes, floppy disks, and magnetic cards are made by applying a magnetic paint consisting of magnetic powder, a binder, an organic solvent, and various additives to a non-magnetic support such as a polyester film. It is obtained by applying it on the body and drying it. Thermoplastic polyurethane resins and the like are used as binders for magnetic recording media.

[0003] Such thermoplastic polyurethane resins contain chain extenders having two active hydrogen atoms such as long-chain polyols having hydroxyl groups at both ends of the molecule, organic diisocyanates, and relatively low molecular weight diamines and diols. Synthesized by reaction. As such long-chain polyols having hydroxyl groups at both ends of the molecule, polyester polyols, polyether polyols, or polycarbonate polyols are mainly used.

Typical examples of such polyester polyols include polyester polyols synthesized from adipic acid and linear diols such as ethylene glycol, 1,4-butanediol, and 1,6-hexanediol. Typical examples of polyether polyols include ring-opened polymers or copolymers of ethylene oxide and propylene oxide, and polytetramethylene glycol, which is a ring-opened polymer of tetrahydrofuran. Typical examples of polycarbonate polyols include:
Examples include polycarbonate polyols containing 1,6-hexanediol as a diol component.

[0005] In order to provide magnetic recording media with excellent electrical properties such as high reproduction output and high S/N ratio, it is necessary to uniformly disperse magnetic particles in the binder of the magnetic coating film, and also to disperse the magnetic particles on the surface of the magnetic coating film. It is necessary that the surface has a high degree of smoothness. However, urethane resins using long-chain polyols as described above generally do not provide sufficient surface smoothness of magnetic coatings. Furthermore, polyurethane resins using the above-mentioned polyester polyols have low heat and humidity resistance, and polyurethane resins using polyether polyols have ether bonds, so they are prone to oxidative deterioration and have low heat resistance. On the other hand, polyurethane resins using the above polycarbonate polyols have a problem in that they have insufficient flexibility at low temperatures.

[0006] Furthermore, in JP-A-61-157516 and JP-A-63-193913, a polyurethane resin using polycaprolactone polyol or polymethylvalerolactone polyol as a long-chain polyol component has mechanical strength, It has been proposed as a binder resin for magnetic recording media with excellent heat and humidity resistance, low-temperature properties, and coating surface smoothness.

[0007]

[Problems to be Solved by the Invention] As mentioned above, the drawbacks of many polyurethane resin binders for magnetic recording media are those described in the above-mentioned Japanese Patent Application Laid-Open No. 157516/1982 and No. 193913/1983. This can be improved to some extent by using polyurethane resins. However, in order to provide magnetic recording media with even more excellent electrical properties, binders with improved performance are required.

One of the objects of the present invention is to provide a high-performance magnetic recording medium capable of forming a magnetic coating film having excellent surface smoothness, mechanical strength, heat-and-moisture resistance, and flexibility at low temperatures. An object of the present invention is to provide a polyurethane resin binder. Another object of the present invention is to provide a high-performance magnetic coating material that can form a magnetic coating film having such excellent properties.

[0009]

[Means for Solving the Problems] According to the present invention, one of the above objects can be obtained by reacting a polyhydroxy compound having two or more active hydrogen atoms in the molecule with a polyisocyanate compound. In the binder for magnetic recording media containing a polyurethane resin, at least a part of the polyhydroxy compound has a structural unit (I) whose main structural unit is represented by the following chemical formula 5, a structural unit (I) represented by the following chemical formula 6, etc.
I), at least two kinds of structural units selected from the group consisting of the structural unit (III) shown in the following chemical formula 7 and the structural unit (IV) shown in the following chemical formula 8, and the structural unit is [
The molar ratio of (I) + (III)] / [(II) + (IV)] becomes 90/10 to 20/80, and [(III
) + (IV)] / [(I) + (II)] molar ratio is 50
Provided is a binder for a magnetic recording medium, characterized in that it is a polyester carbonate polyol containing a polyester carbonate polyol in a ratio of /50 to 0/100, and each structural unit is bonded to each other by forming an ester bond or a carbonate bond. This is achieved by Further, the other objects mentioned above are achieved by providing a magnetic coating material containing the binder for magnetic recording media.

0010

[C5]

[0011]

[C6]

(In formula 6, R represents a divalent organic group)

0
013]

[C7]

(In chemical formula 7, p represents an integer from 6 to 10)

[
0015

[Chemical formula 8]

(In formula 8, R and p are as defined above)

The polyhydroxy compound includes a specific polyester carbonate polyol having an average of two or more active hydrogen atoms in one molecule. The polyester carbonate polyol has both carbonate bonds and ester bonds in the molecule, and 3-methyl-1,5-pentanediol alone or 50 mol% or more of 3-methyl-1,5-pentanediol as the main monomer diol component. A polymer polyol containing a diol mixture consisting of pentanediol and 50 mol% or less of a linear alkylene glycol having 6 to 10 carbon atoms, and having an average of 2 or more alcoholic hydroxyl groups at the molecular chain end. . The linear alkylene glycol includes 1,6-hexanediol,
Examples include 1,7-heptanediol, 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol, but 1,9- Nonanediol is preferred.

The polyester carbonate polyol used in the present invention has structural units (I), structural units (II),
At least two structural units selected from the group consisting of structural unit (III) and structural unit (IV), [(I) +
(III)]/[(II)+(IV)] molar ratio is 90
/10 to 20/80, and [(III)
+(IV)]/[(I)+(II)] molar ratio is 50/
It is contained in a proportion within the range of 50 to 0/100. When the molar ratio of [(I)+(III)]/[(II)+(IV)] exceeds (90/10), the low-temperature flexibility of the resulting binder decreases; ), the moisture and heat resistance of the resulting binder decreases. Also, [(III)+(IV)]/[(I)+(
II)] exceeds (50/50), the flexibility of the binder at low temperatures decreases.

The divalent organic group R in chemical formula 6 representing the structural unit (II) and chemical formula 8 representing the structural unit (IV) is a compound obtained by removing two carboxyl groups from the molecule of an ordinary dicarboxylic acid monomer. Typical examples thereof include alkylene groups such as trimethylene group, tetramethylene group, pentamethylene group, hexamethylene group, heptamethylene group, octamethylene group, and dodecamethylene group;
- Arylene groups such as phenylene group, m-phenylene group, and p-phenylene group; divalent carbonized saturated aliphatic, saturated alicyclic, or aromatic carbon atoms having 3 to 12 carbon atoms such as cycloalkylene groups such as cyclohexylene group; Examples include hydrogen groups. From the viewpoint of providing a binder with excellent flexibility at low temperatures, R is preferably a tetramethylene group, a heptamethylene group, or an octamethylene group. Note that the organic group R contained in the polyester carbonate polyol may be two or more types of groups.

As mentioned above, the monomer diol component in the polyester carbonate polyol used in the present invention is mainly 3-methyl-1,5-pentanediol or a mixed diol of 3-methyl-1,5-pentanediol and a linear alkylene glycol having 6 to 10 carbon atoms. However, other monomer diol components may be included as long as they are in small amounts that do not substantially impair the effects of the present invention. Such optional monomer diol components include ethylene glycol, 1,4-butanediol, neopentyl glycol,
Examples include 2-methyl-1,8-octanediol. Further, the polyester carbonate polyol may contain a small amount of a trihydric or higher polyhydric alcohol component such as trimethylolpropane, pentaerythritol, and decanetriol. By containing the polyhydric alcohol component having a valence of 3 or more, it becomes possible to make the polyester carbonate polyol contain an average number of more than 2 alcoholic hydroxyl groups at the molecular ends.

[0021] The number average molecular weight of the polyester carbonate polyol used in the present invention is preferably 25.
It is 0 to 10,000, more preferably 250 to 5,000. When the number average molecular weight is 250 or more, the binder has particularly good flexibility at low temperatures. In addition, when the number average molecular weight is 10,000 or less, especially 5,000 or less,
The mechanical strength of the binder is particularly high.

The polyester carbonate polyol used in the present invention can be produced according to a known method for producing polyester carbonate polyols. for example,
the desired diol, such as 3-methyl-1,5-pentanediol, and the formula

HOOC-R-COOH

(wherein R is as defined above)

[
A desired dicarboxylic acid represented by the above formula or an ester-forming derivative thereof such as a methyl ester, and a desired carbonate compound such as a dialkyl carbonate such as dimethyl carbonate; a diaryl carbonate such as diphenyl carbonate; or an alkylene carbonate such as ethylene carbonate; The desired polyester carbonate polyol can be obtained by subjecting it to a transesterification reaction or a transesterification reaction accompanied by an esterification reaction, if desired, together with a trihydric or higher polyhydric alcohol. In addition, the desired polyester carbonate polyol can be produced by transesterifying the corresponding polyester polyol and polycarbonate polyol with a carbonate compound, or by transesterifying or transesterifying the corresponding polyester polyol and polycarbonate polyol with a diol and dicarboxylic acid or an ester-forming derivative thereof. It is also possible to obtain

In the present invention, it is necessary to use the polyester carbonate polyol as the polyhydroxy compound having two or more active hydrogen atoms in the molecule, but other polyhydroxy compounds may be used in combination.

Other polyhydroxy compounds having two or more active hydrogen atoms in the molecule are usually
Compounds having a molecular weight of 50 to 10,000 are used, and known polyhydroxy compounds generally used for polyurethane production, such as low-molecular glycols, low-molecular triols, low-molecular tetraols, polyether glycols, and polyester glycols. One or more of the following can be used.

Examples of low molecular weight glycols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, and 1,5- Pentanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, 2-methyl-1,8-octanediol, 1,4-bis(2-hydroxyethoxy)benzene,
Bis(β-hydroxyethyl) terephthalate, xylylene glycol, N-alkyldiethanolamine, bisphenol A, etc. are used. Furthermore, glycols having a carboxyl group such as dimethylolpropionic acid,
It is also possible to use a mixture of metal salt type sulfonic acid group-containing glycols such as a reaction product of sodium sulfoisophthalate and ethylene glycol.

Trimethylolpropane, glycerin, etc. are used as low molecular weight triols. Furthermore, pentaerythritol and the like are used as low molecular weight tetraols.

Examples of polyether glycols include ring-opening polymerization products or copolymers of cyclic ethers such as ethylene oxide and tetrahydrofuran.

Examples of polyester glycols include polyester glycols obtained by dehydration condensation reaction from the above-mentioned low-molecular-weight glycols and dibasic acids, and polyester glycols obtained by ring-opening polymerization of various lactones in the presence of the above-mentioned low-molecular-weight glycols. can be mentioned.

In the present invention, the content of the polyester carbonate polyol is preferably at least 50% by weight based on the total amount of the polyhydroxy compound having two or more active hydrogen atoms. When the amount is 50% by weight or more, a binder having particularly good heat and humidity resistance, physical properties at low temperatures, and dispersibility of magnetic powder can be obtained.

Suitable polyisocyanate compounds used in the present invention to produce a polyurethane resin by reacting with a polyhydroxy compound having two or more active hydrogen atoms in the molecule include those known in the art. Mention may be made of aliphatic, cycloaliphatic or aromatic polyisocyanates. Specifically, 4,4'-diphenylmethane diisocyanate, p-phenylene diisocyanate, tolylene diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate. , 4,4'-dicyclohexylmethane diisocyanate; and triisocyanates produced by adding 3 moles of diisocyanate to 1 mole of trihydric alcohol such as trimethylolpropane and glycerin. .

Among these polyisocyanate compounds, diisocyanate compounds having an aromatic ring, especially 4,
When 4'-diphenylmethane diisocyanate is used, the resulting polyurethane resin has particularly excellent durability of the magnetic coating.

[0035] Furthermore, when a diisocyanate compound having a cyclohexyl group among polyisocyanate compounds, especially isophorone diisocyanate, is used, the resulting polyurethane resin has excellent dispersibility of magnetic powder and smoothness of the surface of the magnetic coating film. It has particularly good properties.

In the present invention, when the polyhydroxy compound and the polyisocyanate compound are reacted, other compounds having at least two hydrogen atoms capable of reacting with the isocyanate may be used in addition to the polyhydroxy compound. good. Examples of active hydrogen atom-containing compounds other than such polyhydroxy compounds include diamines such as ethylenediamine, propylene diamine, isophorone diamine, hydrazine, 4,4'-diaminodiphenylmethane, 4,4'-diaminodicyclohexylmethane, and xylylene diamine. Examples include low-molecular compounds having two or more amino groups or imino groups.

[0037] In order to produce a polyurethane resin from the polyhydroxy compound and polyisocyanate compound, it is possible to adopt a conventionally known urethanization reaction technique, taking into account the target degree of polymerization of the polyurethane resin, the type of raw materials to be used, etc. can. For example, using a solvent as necessary and, if necessary, using a urethanization catalyst, the polyhydroxy compound is stoichiometrically mixed with high molecular weight polyols such as polyester carbonate polyols in the temperature range of 10°C to 150°C. is reacted with an excess of the polyisocyanate compound to produce a prepolymer having an isocyanate group at the end, and then low-molecular polyols such as low-molecular glycols among the polyhydroxy compounds and, if desired, other A method for obtaining a polyurethane resin containing a hydroxyl group at the end by reacting an active hydrogen atom-containing compound, or using a stoichiometrically excessive amount of hydroxyl groups to react with a polyhydroxy compound, a polyisocyanate compound, and optionally an active compound other than a polyhydroxy compound. Examples include a method of simultaneously reacting a hydrogen atom-containing compound to obtain a polyurethane resin having terminal hydroxyl groups.

Solvents that can be used in the production of polyurethane resins in the present invention usually include ketones such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone; esters such as ethyl acetate and butyl acetate; aromatic solvents such as toluene and benzene. Hydrocarbon; a solvent inert to the isocyanate group, such as a cyclic ether such as tetrahydrofuran, is used.

Catalysts that can be used in the polyurethane production reaction in the present invention include tin-based, iron-based, and tertiary amine-based catalysts, which are common urethane-forming reaction catalysts. Examples of tin-based catalysts include dibutyltin laurate, dioctyltin laurate, dibutyltin dioctoate,
Examples include stanas octoate. Examples of iron-based catalysts include iron acetylacetate and ferric chloride. Examples of tertiary amine catalysts include triethylamine and triethylenediamine.

The thermoplastic polyurethane resin thus obtained has a weight average molecular weight of 5,000 to 200,000.
It is preferable that it is, and it is more preferable that it is 8000-150000. Weight average molecular weight is 5000 or more,
In particular, when it is 8000 or more, a binder with particularly high mechanical strength can be obtained. Also, the weight average molecular weight is 2
When it is 00,000 or less, particularly 150,000 or less, the dispersibility of the magnetic powder of the resulting binder and the smoothness of the surface of the magnetic coating film will be particularly good.

The binder of the present invention may contain a low molecular weight polyisocyanate compound as a crosslinking agent in addition to the thermoplastic polyurethane resin. Such low molecular weight polyisocyanate compounds include, for example, an addition product of trimethylolpropane and tolylene diisocyanate at a molar ratio of 1:3 (Coronate L manufactured by Nippon Polyurethane Industries), and an addition product of trimethylolpropane and hexamethylene diisocyanate at a molar ratio of 1:3 (Coronate L manufactured by Nippon Polyurethane Industries). Examples include addition products with a molar ratio of 1:3 (Coronate HL manufactured by Nippon Polyurethane Industries). Other examples of crosslinking agents include Desmodur L and Desmodur N manufactured by Bayer.

These low molecular weight polyisocyanate compounds are used in an amount of 3 to 50% by weight based on the total amount of resin components in the binder. The binder of the present invention further includes vinyl chloride-vinyl acetate copolymers, nitrocellulose, epoxy resins,
Phenoxy resin, other thermoplastic polyurethane resins, etc. may be contained.

The magnetic paint of the present invention is prepared by mixing the binder, magnetic powder, and, if necessary, appropriate additives, and kneading the mixture with an appropriate organic solvent. As the magnetic powder, conventionally known magnetic powder for magnetic recording media, such as γ-iron oxide, alloy magnetic powder, chromium (IV) oxide, metal magnetic powder, etc., can be used. Various additives used in the preparation of magnetic paints include, for example, antistatic agents, carbon black, lubricants, nonmagnetic inorganic pigments, and the like.

The magnetic coating material of the present invention is applied to the surface of a non-magnetic support such as a film, sheet, tape, disk, etc. by a conventional method and then dried to produce a magnetic recording medium. As the material for the nonmagnetic support, polyester (for example, polyethylene terephthalate), holamide, polyolefin, cellulose derivative, nonmagnetic metal, paper, etc. are used.

As explained above, the binder of the present invention can be used to form a magnetic layer of a magnetic recording medium after preparing a magnetic coating containing the binder. Polyurethane resin for forming the so-called top coat layer (top layer of the magnetic layer), back coat layer (bottom layer of the base film), or undercoat layer (intermediate layer between the magnetic layer and the base film) of magnetic recording media. It is also possible to use it as a resin.

[0046]

[Examples] The present invention will be specifically explained below with reference to Examples, but the present invention is not limited to these Examples. Further, the weight average molecular weight of the polyurethane resin was calculated from a GPC (gel permeation chromatography) curve using standard polystyrene.

Production Example 1 (Production method of polyester carbonate diol) 3-Methyl-1,5-pentanediol 7 under nitrogen stream
Heating a mixture consisting of 16 parts by weight, 282 parts by weight of azelaic acid and 750 parts by weight of diphenyl carbonate,
Phenol and water produced from the reaction system were distilled off at 160°C. When the acid value became 0.3 mgKOH/g or less, vacuum was applied (2 to 10 mmHg) to proceed with condensation. As a result, a polyester carbonate diol (A) having a hydroxyl value of 116 and a number average molecular weight of 970 was obtained.

Production Examples 2 to 5 (Production of polyester carbonate diol) The number averages shown in Table 1 were carried out in the same manner as in Production Example 1, except that diol, dicarboxylic acid and diphenyl carbonate were each used in the predetermined proportions shown in Table 1. Polyester carbonate diols (BE) of molecular weight were obtained.

Production Example 6 (Production of polyester diol)
3-Methyl-1,5-pentanediol (MPD) 15
34 parts by weight of adipic acid and 1460 parts by weight of adipic acid (MPD/adipic acid molar ratio: 1.3/1) were subjected to an esterification reaction under normal pressure while passing nitrogen gas and distilling off the water produced at a temperature of about 220°C. It was attached to. The acid value of the produced polyester is 0
．． When the temperature became 3 mgKOH/g or less, the degree of vacuum was gradually increased using a vacuum pump to complete the reaction. In this way, a polyester diol (F) having a hydroxyl value of 118.1 and a number average molecular weight of 950 was obtained.

Production Example 7 (Production of polycarbonate diol) A mixture consisting of 1,392 parts by weight of 1,6-hexanediol and 2,140 parts by weight of diphenyl carbonate was heated under a nitrogen stream, and the phenol produced from the reaction system was distilled off at 200°C. did. The temperature was gradually raised to 210 to 220°C, and after most of the produced phenol was distilled off, vacuum was applied to completely distill off the remaining phenol. As a result, polycarbonate diol (G) having a number average molecular weight of 950 was obtained.

[0051]

[Table 1]

Production Example 8 (Production of polyurethane resin) 100 g of 4,4'-diphenylmethane diisocyanate and 552 g of methyl isobutyl ketone were placed in a four-necked flask equipped with a thermometer, a stirrer, a dropping funnel, a condenser with a drying tube, etc. and stirred to bring the internal temperature to 80°C. To this, the polyester carbonate diol A33
3 g was added dropwise from the dropping funnel, and after the completion of the addition, the reaction was further carried out under heating at 80° C. for 2 hours to synthesize a prepolymer having isocyanate groups at the terminals. Subsequently, 5.4 g of 1,4-butanediol were added at once to the reaction mixture. After the addition, the reaction was carried out under heating at 90° C. for 5 hours. The completion of the reaction was confirmed by an infrared absorption spectrum at 2250 cm.
This was confirmed by the disappearance of the absorption of the isocyanate group in ↑-1. The weight average molecular weight of the obtained polyurethane resin by GPC was 50,000. This is referred to as polyurethane resin H.

Production Examples 9 to 14 (Production of polyurethane resin) Polyurethane resins were produced in the same manner as Production Example 8, except that polyester carbonate diols B to E, polyester diol F, or polycarbonate diol G were used instead of polyester carbonate diol A. I~
N was produced. The weight average molecular weight of each resin obtained was 45,000 to 63,000 as shown in Table 2.

[0054]

[Table 2]

Example 1 (Production of magnetic paint and magnetic recording medium) 300 parts by weight of polyurethane resin H, 80 parts by weight of vinyl chloride-vinyl acetate-vinyl alcohol copolymer, 10 parts by weight of lecithin, 40 parts by weight of carbon black, Co- γ-
740 parts by weight of Fe↓2O↓3, methyl ethyl ketone 9
After kneading a mixture consisting of 00 parts by weight, 400 parts by weight of methyl isobutyl ketone, and 400 parts by weight of cyclohexanone in a sand grind mill for 6 hours, Coronate L (manufactured by Nippon Polyurethane Co., Ltd.; tolylene diisocyanate and trimethylolpropane) was added as a crosslinking agent. adduct) 36
Parts by weight were mixed and filtered to obtain a magnetic paint as a filtrate.

(Evaluation method) Specular gloss: This magnetic paint was applied to a polyethylene terephthalate film so that the film thickness after drying was 5 μm. After the solvent was evaporated in a hot air dryer, the surface was polished by calendering. Smoothed. Thereafter, heat treatment was performed at 60° C. for 24 hours to obtain a magnetic recording medium. The 60 degree specular gloss of the magnetic coating film of the obtained magnetic recording medium was measured. Specular gloss was measured using a digital variable angle gloss meter (manufactured by Suga Test Instruments Co., Ltd.).
(incident angle 60 degrees/light receiving angle 60 degrees).

Mechanical strength: A solution prepared by adding 11 parts by weight of Coronate L as a crosslinking agent to 100 parts by weight of polyurethane resin H was applied onto a sheet, left at room temperature for 30 minutes, and then dried at 80°C for 4 hours to obtain a The mechanical strength of the cured film with a thickness of 100 μm was measured, as well as the mechanical strength of the cured film after being treated for 4 weeks under moist heat conditions of 70°C and 95% relative humidity (using a Tabai constant temperature and humidity tester). . The measurement was carried out using an Autograph (manufactured by Shimadzu Corporation).
The length between the chucks of the test piece is 50 mm, the width is 10 mm,
The thickness was 100 μm.

Low-temperature properties: The dynamic viscoelasticity of the cured film was measured, and the peak temperature Tα of the loss modulus E'' was used as an index of low-temperature properties. The lower Tα, the better the flexibility at low temperatures.

[0059] The results of these evaluations are summarized in Table 3.

Examples 2 to 4 and Comparative Examples 1 to 3 Magnetic recording media and cured films were produced in the same manner as in Example 1 except that polyurethane resins I to N were used in place of polyurethane resin H. Table 3 also shows the surface gloss measurement results of the obtained magnetic recording medium and the evaluation results of the low temperature properties based on mechanical strength and dynamic viscoelasticity before and after the wet heat test of the cured film.

[0061]

[Table 3]

[0062]

[Effect of the invention] The binder for magnetic recording media of the present invention is
As is clear from the above examples, a coating film excellent in mechanical strength, heat and humidity resistance, low temperature properties and surface smoothness is provided. Further, the magnetic paint of the present invention effectively exhibits the excellent performance of the binder for magnetic recording media. Therefore, the binder and magnetic paint for magnetic recording media of the present invention provide magnetic recording media with good electrical properties.

Claims (1)

[Claims] 1. A binder for a magnetic recording medium comprising a polyurethane resin obtained by reacting a polyhydroxy compound having two or more active hydrogen atoms in the molecule with a polyisocyanate compound, wherein at least one of the polyhydroxy compounds is The main structural unit is a structural unit (I) whose main structural unit is represented by the following chemical formula 1, a structural unit (I) whose main structural unit is represented by the following chemical formula 2 (
II), at least two types of structural units selected from the group consisting of the structural unit (III) shown in the following chemical formula 3 and the structural unit (IV) shown in the following chemical formula 4, and the structural unit is [(I) + (III)]/[(II)+(IV)] becomes 90/10 to 20/80, and [(II
I) + (IV)] / [(I) + (II)] molar ratio is 5
A binder for magnetic recording media, characterized in that it is a polyester carbonate polyol containing a polyester carbonate polyol in a ratio of 0/50 to 0/100, and each structural unit is bonded to each other by forming an ester bond or a carbonate bond. [Chemical 1] [Chemical 2] (In Chemical 2, R represents a divalent organic group) [Chemical 3] (In Chemical 3, p represents an integer of 6 to 10) [Chemical 4] (In Chemical 4, p represents an integer of 6 to 10) , R and p are as defined above). 2. A magnetic paint comprising the binder for magnetic recording media according to claim 1.