JPS61144723A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve the dispersibility of pulverous magnetic particles by incorporating a prescribed ratio each of org. diisocyanate, polyol and polyurethane resin into a coating layer provided on a non-magnetic substrate. CONSTITUTION:The magnetic recording medium provided with the coating thermoplastic polyurethane resin with which the org. diisocyanate, 500-8,000mol. wt. long-chain polyol and <500mol.wt. chain extender are brought into reaction is incorporated into the coating layer. At least 50wt% of the long-chain polyol is composed of the polyester polyol copolymerized with 1-15mol% trifunctional groups with the total carboxylic acid component and/or total glycol component. The stable productivity as well as the dispersibility of the pulverous magnetic particles are thus improved and the magnetic recording medium constituted by coating the polyurethane resin having excellent curing property on the non- magnetic substrate is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to magnetic recording media such as magnetic tapes and magnetic disks. More specifically, the present invention relates to a magnetic recording medium using a thermoplastic polyurethane resin having excellent properties as a resin component of coating layers such as a magnetic layer, an anchor coat layer, and a back coat layer provided on a nonmagnetic support.

(Prior art) Magnetic tape, which is a general-purpose magnetic recording medium, is made by dispersing acicular magnetic particles with a long axis of 1 μm or less in a binder solution together with appropriate additives (dispersants, lubricants, antistatic agents, etc.). Magnetic paint is made by applying it to polyethylene terephthalate film. Conventionally used binders for magnetic particles include vinyl chloride/vinyl acetate copolymer, vinyl chloride/vinyl acetate/vinyl alcohol copolymer, vinyl chloride/vinylidene chloride copolymer, polyurethane resin, polyester resin, Acrylonitrile-butane diene copolymer, nitrocellulose, cellulose acetate-butyrate, synthetic resin, acrylic resin, etc. are used. Among these resins, polyurethane resin has properties such as excellent toughness and abrasion resistance that cannot be obtained with other resins due to intermolecular hydrogen bonding caused by urethane bonds, but it is It cannot be said that it is sufficient for applications that require high performance.

Thermoplastic polyurethane resin has traditionally been used as a binder for magnetic recording media such as magnetic tape. It is being However, this thermoplastic polyurethane resin has insufficient properties such as heat resistance, solvent resistance, and abrasion resistance. It is known to increase the concentration of urethane groups in order to improve these properties, but in this case, it becomes insoluble in the general-purpose organic solvents mentioned above, and is insoluble in high-boiling point solvents such as dimethylformamide, N-methylpyrrolidone, dimethylsulfoxide, etc.
It is soluble only in cyclic ethers such as tetrahydro7rane and dioxane. These high boiling point, highly polar solvents cause serious problems such as swelling, bending or dissolution of the non-magnetic support, and poor drying properties resulting in reduced workability.

In order to eliminate the drawbacks observed in conventional thermoplastic polyurethane resins, a so-called two-component method is known in which a curing reaction is carried out using a polyisocyanate compound for hydroxyl-terminated polyurethane resins and a polyol compound for isocyanate-terminated polyurethane resins. However, when these terminal groups are used as crosslinking points, sufficient improvement in heat resistance and solvent resistance cannot be observed because the crosslinking density is low. Further, crosslinking by an amaphanate reaction, which is a reaction between a urethane group and an isocyanate group, has the disadvantage of requiring high temperatures. Conventional magnetic recording media also use a method of curing a hydroxyl-terminated thermoplastic polyurethane resin used as a binder with a polyisocyanate compound in order to improve the properties such as heat resistance and durability of the magnetic layer, but this method is not sufficient. The current situation is that this is not possible.

Various improvements have been made to thermoplastic polyurethane resins in order to improve the crosslinking density of cured products obtained by the two-component method. For example, it is known to use raw materials for polyurethane resins to form branched polyurethane resins. However, although the properties of the cured product of this branched polyurethane resin improve depending on the amount of the trifunctional or higher functional polyol component used, in this case, the tendency of gelation during production of the branched polyurethane resin increases. Furthermore, when this polyurethane resin is used as a binder for a magnetic layer, it has a major drawback as a binder in that the dispersibility of magnetic particles becomes extremely poor compared to a polyurethane resin that is not branched.

(Problems to be Solved by the Invention) Branched polyurethane using a polyfunctional polyol as a chain extender improves the properties of the cured product, but on the other hand, the dispersibility of magnetic fine particles is extremely poor compared to polyurethane without branching. Become. Moreover, during manufacturing, there are concerns such as gelation.

The present invention aims to provide a magnetic recording medium that has excellent stable productivity, further improves the dispersibility of magnetic fine particles, and coats a polyurethane resin with excellent curability on a non-magnetic support. .

(Means for Solving the Problems) That is, the present invention provides a magnetic recording medium in which a coating layer is provided on a non-magnetic support, in which the coating layer is made of organic diisocyanate (A), a long chain polyol with a molecular weight of 500 to 8000. ω
) and a thermoplastic polyurethane resin reacted with a chain extender (C) having a molecular weight of less than 500, and at least 50% of the long chain polyol ω) contains a thermoplastic polyurethane resin reacted with a chain extender (C) having a molecular weight of less than 500. 1 trifunctional component
The magnetic recording medium is characterized in that it is a polyester polyol copolymerized with ~15 mol%.

The coating layer in the present invention specifically includes a magnetic layer, a back coat layer, an anchor foot layer, and a top coat layer provided on the magnetic layer.

The organic diisocyanate (T) used in the production of the thermofluorescent polyurethane resin used in the present invention includes 2
．． 4-) lylene diisocyanate, 2,6-lylene diisocyanate, p-phenylene diisocyanate,
Diphenylmethane diisocyanate, m-phenylene diisocyanate, hexamethylene diisocyanate, tetramethylene diisocyanate, 3,3'-dimethoxy-4,4'-biphenylene diisocyanate, 2,4-
Naphthalene diisocyanate, 3,3'-dimethyl-4
, 4'-biphenylene diisocyanate, 4,4'-diphenylene diisocyanate), 4,4'-diisocyanate diphenyl ether, 1,5-naphthalene diisocyanate, p-xylylene diisocyanate, m-xylylene diisocyanate ), 1,3-diisocyanatemethylcyclohexane, 1,4-diisocyanatemethylcyclohexane, 4,4'-diisocyanatedicyclohexane, 4,4'-diisocyanatecyclohexylmethane, inphorone diisocyanate, and the like.

Furthermore, the long chain polyol used in the production of the thermoplastic polyurethane resin used in the present invention has a molecular weight of 5.
It ranges from 00 to 5ooo, and includes polyester polyols, polyether diols, polycarbonate diols, and the like.

The trifunctional components used in the production of polyester polyols that characterize the present invention include trimellitic acid and its esters, glycerin, trimethylolpropane, trimethylolethane, trimethylolhebutane, 1,3.6- Hexandriol, 1,2.
Examples include 6-hexandriol and derivatives thereof, but trimellitic acid is particularly effective in preventing gelation and dispersing properties.

The copolymerization ratio of the trifunctional component is 1 to 15 mol%, preferably 3 to 15 mol% of the total carboxylic acid component and/or total glycol component.
It is in the range of 10 mol%. If necessary, this polyester polyol can be mixed with a non-copolymerized polyester diol of trifunctional components or other long chain diols, but the proportion should be 50% by weight or more to avoid reactivity with the curing agent. It is necessary from this point of view.

The carboxylic acid components of the polyester polyol obtained by copolymerizing these trifunctional components include terephthalic acid, isophthalic acid, orthophthalic acid, and 1.5-Natsuta/'
Aromatic dicarboxylic acids such as wI, p-oxybenzoic acid,
Aromatic oxycarboxylic acids such as p-(human tetraxyethoxy)benzoic acid, succinic acid, adipic acid, azelaic acid,
Examples include aliphatic dicarboxylic acids such as sebacic acid and dodecanedicarboxylic acid. Especially terephthalic acid,
Isophthalic acid, adipic acid and sebacic acid are preferred.

In addition, as the glyfur component of polyester polyol, ethylene glyfur, propylene glycol, 1.3
-Propanediol, 1,4-butanediol, 1,5
-bentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol, 2,2゜4-trimethyl-1,3-bentanediol, cyclohexane dimetatool, ethylene oxide adduct of bisphenol A, and Examples include propylene oxide adducts, ethylene oxide adducts and propylene oxide adducts of hydrogenated bisphenol A, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol. Raw materials for polyester polyols other than those mentioned above include dicarboxylic acids containing sulfonic acid metal bases such as 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, and sodium sulfoterephthalic acid, and the following general formula [
Examples include phosphorus-containing dicarboxylic acids represented by III) and aliphatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid, and dodecanedioic acid.

(In the formula, R1 is a monovalent ester-forming functional group.

R2 and R3 are the same or different groups, and include a monovalent hydrocarbon group having 1 to 10 carbon atoms, a halogen atom, and 1
selected from the group consisting of valent ester-forming functional groups. Humans exhibit divalent or trivalent organic residues. Further, nl represents 1 or 2, and R2 and R3 each represent an integer of 0 to 4. ) (In the formula, R4 hydrogen atom, a hydrocarbon group having 1 to 12 carbon atoms that may contain an ester-forming functional group, or M2O
. R5 is a divalent or trivalent hydrocarbon group having 1 to 6 carbon atoms. R6 is a hydrogen atom, a hydrocarbon group having 1 to 12 carbon atoms that may contain a hydroxyl group, or M3゜M, , M, ,
M is an alkali metal atom. m is 0 or 1° t is an integer of 1 or 2 0) Polyester polyol containing a sulfonic acid metal base, a polyester obtained using a phosphorus-containing compound represented by the general formula CI) or CII) as at least one component of the raw material When polyol is used as a component of raw materials for thermoplastic polyurethane resins, it is effective in greatly improving the drawback of low dispersibility of inorganic pigments and fillers found in conventional thermoplastic polyurethane resins. When used as a binder for a magnetic layer in a coated magnetic recording medium, the dispersibility of magnetic particles is improved, and the properties caused by the dispersibility of magnetic particles, such as the electromagnetic conversion properties of the magnetic recording medium, are greatly improved. .

Other long-chain diols used in combination with polyester polyols include, in addition to polyester diols composed of these components, lactone-based polyester diols obtained by ring-opening polymerization of lactones such as l-caprolactone.

Examples of polyether diols include polyalkylene glycols such as polyethylene glycol, polypropylene glycol, and polytetramethylene glycol.

The polycarbonate diol has the general formula H(-0-
It is a long-chain diol represented by R-OCO+n ROH, and R includes residues such as diethylene glycol, 1,6-hexanediol, and bisphenol.

The long chain polyol @) has a molecular weight of 500 to 8,000. If the molecular weight is less than 500, the urethane group concentration will increase, and the flexibility and solvent solubility of the resin will decrease.

Moreover, when the molecular weight exceeds 8,000, the urethane group concentration decreases, and the toughness, abrasion resistance, etc. characteristic of polyurethane resins decrease.

In the production of the thermoplastic polyurethane resin used in the present invention, the compound used to increase the urethane group concentration and improve toughness, abrasion resistance, etc. has two groups that are reactive with isocyanate groups. , and the molecular weight is 5
Chain extender (C) less than 00, such as ethylene glycol, 1,3-propylene glycol, 1,4-tetramethylene glycol, 1,6-hexanediol,
Cyclohexane dimetatool, xylylene glycol,
Linear glycols such as ethylene oxide adducts of bisphenol A, propylene glycol, neopentyl glycol, 1.2-butanediol, 1.3-butanediol, 2,2.4-trimethyl-1,3-bentanediol, Branched glycols such as propylene oxide adducts of bisphenol A, neopentyl hydroxypivalate, stearic acid monoglyceride, amino alcohols such as monoethanolamine, N-methylethanolamine, diamines such as ethylenediamine, hexamethylenediamine, isophoronediamine, piperazine, etc. Water etc. can be given.

Examples of compounds that are difunctional with respect to incyanates other than those mentioned above and have a molecular weight of less than 500 include phosphorus-containing compounds represented by the following general formula [■].

(In the formula, R1 and R4 represent a -valent hydrocarbon group,
May be the same or different. R35R4 is a divalent alkylene group, oxyalkylene group or polyoxyalkylene group with or without a substituent, and may be the same or different. X and Y represent a hydroxyl group when R3 and R4 are alkylene groups, and hydrogen when RB and R4 are oxyalkylene groups or polyoxyal. ) As a raw material for thermoplastic polyurethane resin, the general formula (II
When the phosphorus-containing compound shown in I) is used, it is effective in improving the low dispersibility of inorganic pigments and fillers, which is a drawback of conventional polyurethane resins, and it is effective in improving the bonding of the magnetic layer in coated magnetic recording media. When used as an agent, the dispersibility of the magnetic particles is improved, and the properties caused by the dispersibility of the magnetic particles, such as the electromagnetic conversion properties of the magnetic recording medium, are greatly improved.

When producing the thermoplastic polyurethane resin used in the present invention, the usage ratio of each raw material used is such that the number of equivalents of isocyanate groups in the organic diisocyanate prisoner is equal to the number of equivalents of hydroxyl groups in a1 long chain g +) all ω), and 1 molecule. When the number of equivalents of active hydrogen of the compound C] with molecules less than 11,500 containing two active hydrogen atoms is represented by C, the following intermediate pressure is obtained when expressed as an equivalent ratio.

b + cm≧1 (b+c)/a determines the molecular weight of the obtained thermoplastic polyurethane resin. This value is determined according to the required performance of the thermoplastic polyurethane resin, but if it is less than 1, the hydroxyl group in the side chain may react with the isocyanate group, resulting in gelation, which is not preferable.

The polyaddition reaction of thermoplastic polyurethane resins is carried out by a one-shot method in which all components are reacted simultaneously.First, a long chain diol is reacted under the condition of excess incyanate, and the resulting incyanate group-terminated prepolymer is reacted with a chain extender. Furthermore, there is a prepolymer method in which the polymer is made into a polymer. In the case of the thermoplastic polyurethane resin of the present invention, f! Can be built. The reaction conditions include a method in which the raw materials are in a molten state, a method in which the raw materials are dissolved in a solution, and the like.

As a reaction catalyst during the production of thermoplastic polyurethane resin,
Octylic acid octylate, dibutyltin dilaurate, triethylamine, etc. may also be used.

Further, ultraviolet absorbers, hydrolysis inhibitors, antioxidants, etc. may be added before, during or after the production of the thermoplastic polyurethane.

In addition to being a binder for the magnetic layer of a magnetic recording medium, the thermoplastic polyurethane resin used in the present invention may also be used as a binder for a back coat layer, a top coat agent for a magnetic layer, or an anchor coat agent for a magnetic layer. .

In the present invention, in addition to the thermoplastic polyurethane resin used in the present invention, other resins may be added for the purpose of adjusting flexibility, improving cold resistance, heat resistance, etc., and/or thermoplastic polyurethane resins may be added. It is desirable to mix a compound that reacts with the resin and crosslinks it. Other resins include vinyl chloride resin, polyester resin, cellulose resin, epoxy resin, phenoxy resin, polyvinyl butyral,
Examples include acrylonitrile-butadiene copolymer. On the other hand, compounds that crosslink with thermoplastic polyurethane resins include polyisocyanate compounds, epoxy resins, melamine resins, urea resins, etc. Among these, polyisocyanate compounds are particularly desirable.

The ferromagnetic particles used in the magnetic layer of the magnetic recording medium of the present invention include r-Fe2e3, fFe2O3 and Fe3.
O4 mixed crystal, CrO2, r-Fe doped with cobalt
zOa or Fe3O4, barium ferrite, and F
Examples include ferromagnetic alloy powders such as e-Co and Fe-Co-Ni.

Further, the back coat layer may contain carbon black such as channel carbon and 7 Arness carbon, or inorganic particles such as silica and talc.

The coating layer of the magnetic recording medium of the present invention may optionally contain a degradable agent such as tesiphthylphthalate, tri-7-enyl phosphate, dioctylsulfosodium succinate, t
- Lubricants and various antistatic agents such as butylphenol polyethylene ether, ethylnaphthalene sodium sulfonate, dilauryl succinate, zinc stearate, soybean oil lecithin, and silicone oil can also be added.

(Function) The magnetic recording medium using the polyurethane resin of the present invention has a coating layer with excellent heat resistance, durability, and abrasion resistance by reacting with polyisocyanate if necessary, and can be used as a binder for the magnetic layer. When used, the dispersibility of magnetic particles is excellent.

The polyurethane resin used in the present invention has a broader molecular weight distribution than that using a low molecular weight trifunctional branching agent. Therefore, the reactivity with the curing agent is reduced, and the resulting cured product has good heat resistance, durability, and abrasion resistance.

Further, the dispersibility of the magnetic particles of the polyurethane resin used in the present invention is superior to that using a low molecular weight trifunctional branching agent, but the reason is not clear.

(Example) Hereinafter, the present invention will be specifically explained with reference to Examples.

In the examples, parts simply indicate parts by weight.

In a reaction vessel equipped with a thermometer, a stirrer, and a cooling tube, 699 parts of dimethyl tere7thaleate, 699 parts of dimethyl isophthalate, 54N of trimellitic anhydride, 462M of ethylene glyfur, 666 parts of neopentyl glycol,
0.54 part of titanium tetrabutoxide was charged as a catalyst, and the temperature was gradually raised from 160°C to 230°C over about 2 hours to distill methanol out of the system. Next, the pressure inside the system was gradually reduced, and polycondensation was carried out for about 1 hour.

As a result of NMR analysis, the obtained polymer was found to be terephthalic acid/isophthalic acid/trimellitic acid/ethylene glycol/neopentyl glycol = 45/45/101501.
50 (molar ratio) and a hydroxyl value of 1280 equivalent f
It was a polyester with i/10'f and acid value of 10 equivalents/10'P.

Synthesis of polyurethane resin Synthesis Example 2 In a reaction vessel equipped with a thermometer, a stirrer, and a reflux type cooling tube, 100 parts of the long chain polyol of Synthesis Example 1 and 10 parts of neopentyl glycol were dissolved in 168 parts of toluene and 169 parts of methyl ethyl ketone. Then, 34 parts of 4.4'-diphenylmethane diincyanate and 0.03 part of dibutyltin diurarate as a catalyst were added, and the mixture was reacted at 70°C for 10 hours.

The resulting polyurethane skin oil (I) was a pale yellow, transparent solution with a solution viscosity of 6 voids/25° C. and a solid content concentration of 30%. The number average molecular weight of the polyurethane resin (I) was determined by gel permeation chromatography to be 14,000 in terms of polystyrene, and the hydroxyl value was 460 equivalents/10'.
? Met.

Synthesis Examples 3 to 7 Polyurethane resins (IF) to (■) were obtained by the method of Synthesis Example 2 using the raw materials shown in Table 1.

Comparative Synthesis Examples 1 to 6 Polyurethane resins (■) to (Xm) were obtained by the method of Synthesis Example 2 using the raw materials shown in Table 1. Comparative synthesis examples 1-2
In Comparative Synthesis Examples 3 and 4, a trifunctional component was used as a raw material for producing a polyurethane resin.

In the table, the abbreviations indicate the following.

(g); Polyester diol of Synthesis Example 1); Polyester polyol (terephthal rs/isophthalic acid 15
-Sulfonodium isophthalic acid/trimellitic anhydride/ethylene glycol/neopentyl glycol = 47
/4 s/37s750150, molar ratio) molecule fi23
00 (C); Polyester polyol (adipate 15-sulfonodium isophthalic acid/trimellitic anhydride l neopentyl glycol/1,6-hexanediol = 8
7/3/10/30/70, molar ratio) molecule [1800 ■); Polyester polyol (adipine [/)' dimethylolpropane/1.4-butanediol = 100
/ 5 / 95, molar ratio) molecule [2000 ■); Polyester polyol (isophthalic acid / phthalic anhydride l glycerin / ethylene glycol / neopentyl glycol = 50 / 50 / 3 / 47150, molar ratio) molecular weight 1500 MD I; 4.4'-diphenylmethane diisocyanate IPDI: Isophorone diisocyanate TDI: 2,
4-Tolylene diisocyanate NPG: 10 parts of neopentyl glycol; neopentyl hydroxybivalate CF'); polyester diol (adipic acid/neopentyl glycol/1,6-hexanediol = 100/25/75, molar ratio) Molecule: 120
00(G); Polycaprolactone diol molecular view 20
00 (Company); Polyester diol (terephthalic acid/inphthalic acid/ethylene glyfur/neopentyl glycol = 50/50/50/50, molar ratio) molecular weight 2500 CI): Polyester diol (adipate 15-sulfonodium inphthalate) / 1, 6-hexane di,t-ol/*opentyl glycol = 98 / 2
/70/30. Molar ratio) Molecular weight: 2000 Example 1 Using the polyurethane resin (I) obtained in Synthesis Example 2, a composition with the following blending ratio was placed in a ball mill and dispersed for 48 hours, and then mixed with incyanate compound Coronate 203.
0 (manufactured by Nippon Polyurethane Industry Co., Ltd.) as a curing agent.
The mixture was further mixed for 11 hours to obtain a magnetic paint. This was covered with polyethylene having a thickness of 12 μm. After leaving at 60℃ for 1 day 1
A magnetic tape was obtained by slitting the tape into a width of /2 inch.

Polyurethane resin (I) solution ioo part (
Solids content 30%, MEK/) Luene = l/1 solution) Cobalt deposited r-Fl! 203 120 years old leave night 1 part MEK
100 parts toluene
100 parts Examples 2 to 7, Comparative Examples 8 to 11 Polyurethane resins (II) obtained in Synthesis Examples 2 to 7 and Comparative Synthesis Examples 8 to 13 in place of the polyurethane resin <1) used in Example 1 Example 1 using (XI[[)
A magnetic tape was obtained in the same way. Examples 2 to 1O1 and Comparative Examples 1 to 6, respectively.

Table 2 shows the characteristics of the magnetic tapes of the above examples and comparative examples.

Table 2 (*1) Gloss, 60 degree reflectance measured using a meter (*2) Number of times the magnetic layer was rubbed with a cotton swab soaked in methyl ethyl ketone until the magnetic layer was completely removed.

Nitrified cotton - R8I/2 manufactured by Daicel Corporation PVC/vinyl acetate resin - manufactured by U, C, C, Inc. "VAGHJ blending ratio indicates weight ratio.

Example 8 Polyurethane resin (II) solution obtained in Synthesis Example 3 (
The following composition having a solid content of 30% and a solvent (MEK/) luene) as a binder was applied to the opposite side of the magnetic layer of the magnetic tape obtained in Example 2 so that the thickness after drying was 0°7μ. to form a back layer.

Polyurethane resin (II) solution 100 parts Polyisocyanate (Coronet manufactured by Nippon Polyurethane Co., Ltd.)
(L)
10 parts Calcium carbonate (Homocal D manufactured by Shiraishi Kogyo) 50 parts Methyl ethyl ketone
300 parts Comparative Example 5 Polyurethane resin (X) obtained in Comparative Synthesis Example 3 was used instead of polyurethane resin (I[) used in Example 8.
A back layer was formed in the same manner using a solution (LTTL type 3 treatment 30% M E K / toluene).

Table 3 shows the measurement results of the characteristics of the magnetic tubes obtained in Example 8 and Comparative Example 5.

(*1) Dynamic friction coefficient ratio of the back coat layer of the magnetic tape to the stainless steel ball before the running test and after running 100 times.

Example 9 A polyisocyanate compound, Coronate 2030 (Nippon Polyurethane Co., Ltd. M) was added to the polyurethane resin (■) obtained in Synthesis Example 3 at 10% of the resin content, and this solution was spread on a polyethylene terephthalate film with a thickness of 12 μm. The coating was applied so that the thickness after drying was 0.1 μm to provide an anchor coat layer. The magnetic paint used in Example 1 was applied on this coating layer in the same manner as in Example 1 to obtain a magnetic tape.

Comparative Example 7 In place of the polyurethane resin used for the anchor foot layer in Example 9, the polyurethane resin obtained in Comparative Synthesis Example 4 was used.

An anchor coat layer was provided in the same manner as in Example 9 using urethane resin (XI) to obtain a magnetic tape. A peel test using cellophane tape was performed on the surfaces of the magnetic tapes obtained in Example 9, Comparative Example 7, and Example 1 (in the case where the anchor coat layer was not provided in Example 9). The results are shown in Table 4. In the magnetic tape of Example 9, no peeling of the magnetic layer was observed.

According to the above Examples and Comparative Examples, the magnetic recording medium of the present invention has excellent dispersibility of magnetic particles, surface smoothness, solvent resistance, runnability,
It can be seen that the wear resistance is improved.

(Effect of the invention) Since the thermoplastic polyurethane resin used in the present invention has hydroxyl groups not only at the ends but also in the side chains, it is highly reactive with polyisocyanate curing agents, and therefore has excellent heat resistance, durability, and abrasion resistance. An excellent coating layer is obtained. In particular, when used as a binder for a magnetic layer, it has excellent abrasion resistance and durability, when used as a binder for a back coat layer, it has excellent runnability and durability, and when used as an anchor foot agent, A magnetic recording medium with improved adhesiveness can be obtained. Furthermore, compared to a polyurethane resin having a hydroxyl group in a side chain using a trifunctional component as part of a chain extender, the dispersibility of magnetic particles is excellent, and a magnetic recording medium with good magnetic properties and electromagnetic conversion properties can be obtained.

Claims (1)

[Claims] (1) In a magnetic recording medium in which a coating layer is provided on a nonmagnetic support, the coating layer reacts an organic diisocyanate (A), a long chain polyol (B) with a molecular weight of 500 to 8,000, and a chain extender with a molecular weight of less than 500. At least 50% by weight of the long chain polyol (B) is a polyester polyol in which 1 to 15 mol% of a trifunctional component is copolymerized with respect to the total carboxylic acid component and/or total glycol component. A magnetic recording medium characterized by: