JPS62243119A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic layer having excellent dispersibility of magnetic powder and excellent adhesiveness and wear resistance of a coated film by irradiating radiations to a resin incorporated therein with a specific resin as a resin component having radiation sensitive double bonds to polymerize and cure the coated film thereof. CONSTITUTION:A magnetic layer essentially consisting of ferromagnetic powder, resin having the radiation sensitive double bonds and thermoplastic resin without having the radiation sensitive double bonds is formed on a nonmagnetic base. The resin into which a (meth)acryl-modified polyurethane resin and the modified resin expressed by the formula are incorporated as the resin component having the radiation sensitive double bonds is irradiated with radiations by which the coated film is polymerized and cured. In the formula, R is a methyrol group or (meth)acrylate of the methyrol group and the molar ratio between the methyrol group and (meth)acrylate is in a 3:1-1:3 range. The magnetic layer having the good dispersibility of the magnetic powder and having the excellent electromagnetic conversion characteristic is thereby formed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to magnetic recording media such as magnetic chips and magnetic sheets used in audio and video equipment, computers, and the like. More specifically, the present invention relates to a magnetic recording medium using a specific radiation-curable resin as a binder component in a magnetic coating.

BACKGROUND OF THE INVENTION In recent years, the above-mentioned magnetic recording media have been trending toward high-density recording, and for this purpose, the recording wavelength has become shorter, the recording trunk width has become narrower, and the recording medium thickness has become thinner.

As a result, electromagnetic conversion characteristics such as reproduction output and S/N ratio are generally disadvantageous. As a countermeasure to this problem, the surface of the magnetic layer must be made even more flat in order to reduce as much as possible the distance loss between the magnetic recording medium and the magnetic head, which leads to a reduction in reproduction output during short wavelength recording. As a countermeasure to this problem, attempts have been made to make the magnetic powder finer, more highly dispersed, and use a highly smooth non-magnetic support (base film). However, in conventional magnetic recording media, a so-called thermosetting i-type paint, in which a highly reactive curing agent is added immediately before coating, is used on a non-magnetic support, mainly for the purpose of improving the durability of the magnetic layer. Since the magnetic recording medium was obtained by applying heat treatment for a relatively long time after coating it on the body, drying it, and calendering it, various methods are required to produce a magnetic recording medium with homogeneous properties and a highly smooth surface. A problem had arisen. For example, variations in coating film properties due to the short pot life of magnetic paint, or variations in the surface properties of the magnetic layer that are not limited to the dispersibility of magnetic powder,
It is extremely susceptible to the effects of the surface properties of the non-magnetic support on which the magnetic layer is formed and the surface properties of the back coat layer formed on the surface opposite to the magnetic layer on the non-magnetic support, and the degree of influence also depends on the product. Problems include having an inclination in the longitudinal direction. In order to solve these problems with conventional thermosetting magnetic recording media, a so-called radiation-curable magnetic recording medium was proposed, in which a magnetic layer containing a radiation-curable binder is irradiated with radiation to harden the coating. Many have already been done. (For example, Japanese Patent Publication No. 47-124231; Japanese Patent Publication No. 50-774
33 pulls.

The above-mentioned radiation-curable magnetic recording medium is expected to have the following advantages: the magnetic coating has a long pot life, the coating film can be cured instantaneously, and the manufacturing process can be simplified. In addition, the binders used in the manufacture of magnetic recording media not only have excellent dispersibility of magnetic powder and other fillers, but also have durability, such as being able to withstand repeated use under various environments. Due to the demand for superior properties, combinations of multiple types of binders have become common, as in the case of thermosetting magnetic recording media. In this case, the combination of radiation-curable binders or thermoplastic resins may be used, but the paint properties (pigment dispersibility, paint coatability, etc.), the flexibility of the paint film, the adhesion The latter combination has become more common. (For example, JP-A-57-136
1301 67-127926 Nori.

In these conventional examples, urethane-based resins (meth) are used as radiation-curable resins having radiation-sensitive unsaturated double bonds in their molecules in order to impart appropriate flexibility and abrasion resistance to coating films. Displaced acrylic materials are relatively often used. In addition, the thermoplastic resin used in combination with this is a vinyl chloride copolymer resin that has an appropriate glass transition temperature (T(J)) and relatively good properties in terms of friction and wear properties. often.

Problems to be Solved by the Invention However, the reality is that the use of conventional radiation-curable resins has not yet been able to fully satisfy the various characteristics described above required for magnetic paints and magnetic coatings. there were.

In view of the above-mentioned problems, the present invention provides a radiation-curable magnetic recording medium having a magnetic layer having excellent dispersibility of magnetic powder and improved adhesive strength and abrasion resistance of the coating film.

Means for Solving the Problems In order to solve the above problems, the magnetic recording medium of the present invention comprises:
A magnetic layer mainly composed of ferromagnetic powder, a resin having radiation-sensitive unsaturated double bonds, and a thermoplastic resin having no radiation-sensitive unsaturated double bonds is formed on a non-magnetic support. A magnetic recording medium containing a (meth)acrylic modified polyurethane resin and a modified resin represented by the following structural formula as the resin component having the radiation-sensitive unsaturated double bond is irradiated with radiation, The weight of the coating film (where R is a methylol group or a (meth)acrylic ester of a methylol group, and the molar ratio of the methylol group to the (meth)acrylic ester is in the range of 3:1 to 1:3. Effects of the present invention With the above structure, it is possible to form a magnetic layer that can be cured by radiation, has excellent dispersibility of magnetic powder, and has excellent coating film adhesion and abrasion resistance. .

Examples The modified resin represented by the above structural formula used in the present invention is:
It is obtained by reacting tetramethylolbisphenol A, which is obtained by reacting 1 mole of bisphenol A with 4 moles of formaldehyde in an alkaline solution, and (meth)acrylic acid or a lower alcohol ester thereof. Bisphenol A/formaldehyde condensates have been known for a long time, but the present inventors added (meth)acrylic modified polyurethane resin and By using a combination of thermoplastic resins as a binder for magnetic powder, it is possible to simultaneously satisfy the conventional (meth)acrylic modified polyurethane resin alone and/or in combination with other radiation-curable resins and thermoplastic resins. The inventors have discovered that it is now possible to form a magnetic layer with improved radiation curability, adhesion and abrasion resistance of a coating film, which has been difficult, and has led to the present invention.

The molar ratio of the methylol group to the (meth)acrylic modified ester group in the modified tree shown in the above structural formula is 3:1.
If the ratio of methylol groups is too large, radiation curability will be poor. On the other hand, if the molar ratio is less than 1:3, that is, if the ratio of methylol groups is too small, the dispersibility of the magnetic powder will decrease and the brittleness of the coating film after radiation curing will increase too much, which is not preferable.

The (meth)acrylic modified polyurethane resin used in the present invention refers to polybasic acids (e.g. phthalic acid, isophthalic acid,
terephthalic acid, maleic acid, fumaric acid, succinic acid, adipic acid, sebacic acid, etc.) and polyhydric alcohols (such as ethylene glycol, diethylene glycol, glycerin, trimethylolpropane, propylene glycol, phthanesiol, hexanediol, pentaerythritol, sorbitol, A polyester polyol (resin) obtained by an esterification reaction with hydroquinone, bisphenol A, etc.), or a polyether polyol (resin) obtained by a polycondensation reaction of the above-mentioned polyhydric alcohols.
resin) and various diisocyanate compounds (e.g., tolylene diisocyanate, diphenylmethane 4,4'-diisocyanate, inphorone diincyanate, hexyl diisocyanate,
hydroxyalkyl (meth)acrylate (e.g., 2-hydroxyethyl (meth) Acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (
meth)acrylate, 2-hydroxyoctyl(meth)
acrylate, etc.) is used.

The average molecular weight of this (meth)acrylic modified polyurethane resin is not particularly limited, but is 1000 or more, 1
Less than 00000, preferably 5000 or more, 50000
Less than is better. If the molecular weight becomes too small, the flexibility of the coating film will be impaired, and if the molecular weight becomes too large, on the other hand, problems will arise such as a decrease in coatability and radiation sensitivity due to an increase in the viscosity of the coating film.

The thermoplastic resins used in the present invention together with the resins having radiation-sensitive unsaturated double bonds include polyester resins, polyurethane resins, cellulose resins, and vinyl chloride resins used in conventional magnetic recording media. In addition to polymer resins, examples include epoxy resins, phenoxy resins, butyral resins, etc., but a particularly preferred thermoplastic resin is a vinyl chloride copolymer resin. The reason for this is that the structure is fundamentally different from that of the resin having radiation-sensitive unsaturated double bonds used in the present invention, and the drawbacks of each can be compensated for, and as mentioned earlier, this resin has an appropriate Tq, It can be expected to have relatively good properties in terms of friction and wear characteristics, and in general, vinyl chloride copolymer resins are easily crosslinked by radiation irradiation even if they do not have radiation-sensitive double bonds. It will be done.

The vinyl chloride copolymer resin used in the present invention preferably contains a hydrophilic group such as a hydroxyl group, a carboxyl group, or a sulfonic acid metal base for the purpose of improving the dispersibility of the magnetic powder, but is not particularly limited. isn't it.

The mixing ratio of the resin having a radiation-sensitive unsaturated double bond and the thermoplastic resin not having a radiation-sensitive unsaturated double bond as a binder for the magnetic layer used in the present invention is determined by weight ratio. It is preferable that each ratio is in the range of 7:3 to 3:7. If the ratio of the resin having radiation-sensitive unsaturated double bonds in the above ratio exceeds 7, it will be difficult to obtain the effect of combined use with the thermoplastic resin, and it will be difficult to simultaneously satisfy the coefficient of friction, flexibility, and adhesion of the coating film. It becomes difficult. On the other hand, when the mixing ratio of the resin is less than 3, the heat resistance and abrasion resistance of the coating film decrease due to the decrease in radiation curability of the coating film.

Next, a method for manufacturing the magnetic recording medium of the present invention will be explained using a magnetic tape as an example.

The magnetic layer is formed by using ferromagnetic powder, the above-mentioned specific binder component of the present invention, an organic solvent, and optional additive components (abrasive, antistatic agent, dispersant, lubricant, etc.). A magnetic paint consisting of is applied onto a non-magnetic support such as a polyester film, dried and subjected to a magnetic field orientation treatment, followed by a surface smoothing treatment using a calender roll, and then a hardening treatment of the coating film by irradiation with radiation. can get.

Furthermore, the magnetic tape of the present invention aims to improve electromagnetic conversion characteristics by highly smoothing the surface of the magnetic layer, and in order to provide tape running properties, a back coat with non-magnetic powder dispersed in a binder is used. A layer (hereinafter abbreviated as back layer) may be formed on the surface of the nonmagnetic support opposite to the magnetic stone surface.

The magnetic powder used in forming the magnetic layer is γ-Fe.
2o3. (FeO)-(Fe2o3)1-x, CrO
3゜Co coated r-Fe203. Examples include flat Ba7z light, Fe-Co-Ni alloy powder, Fe-Zn alloy powder, and the like.

As the polishing agent, Cr2O3, tl-Al 203. c.
t-Fe203. SiC, Ti0x (x=1 or 2)
There are high hardness powders such as

Examples of the antistatic agent include inorganic particles such as carbon black and graphite, as well as organic antistatic agents.

Examples of the dispersant include higher fatty acids, phosphoric acid esters, and sodium alkylbenzenesulfonate.

Examples of lubricants include silicone oil, furan oil, and higher fatty acid esters.

The back layer, which is formed as necessary, consists of non-magnetic fillers, binders, and organic solvents that are added for the purpose of reinforcing the coating film, imparting appropriate surface properties to the coating film, or imparting conductivity. Applying and drying a back layer paint consisting of additive components (e.g., dispersants, lubricants) used in magnetic materials on the opposite side of the nonmagnetic support from the side on which the magnetic layer is formed. obtained by. The binder used in this back layer may be either a thermosetting type or a radiation curing type, but in the present invention, the magnetic layer is a radiation curing type, and there is a possibility of simultaneous curing on the front and back sides, and The latter is suitable from the viewpoint of pot life, etc.

As the filler for the back layer, carbon black, Gura 7
Phyto, Ca CO3s Z n O, B a S O
ay talc.

There are inorganic powders such as Al2O3 and TiO2, which can be used singly or in combination of two or more. Additives added as necessary (dispersants, lubricants, etc.)
The same materials as those used in forming the magnetic layer can be used.

As the organic solvent for the magnetic paint (and back layer paint), methyl ethyl ketone, methyl isobutyl ketone, toluene, cyclohexanone, ethyl acetate, butyl acetate, etc. are used.

Various kneaders are used to disperse magnetic paint (and back layer paint). For example, roll mill, attritor, high speed stone mill, ball mill, agitator mill, pebble mill, sand grinder, high speed mixer,
A homogenizer, an ultrasonic disperser, a high-speed impeller, a pressure kneader, etc. are used alone or in combination.

In the present invention, the radiation used to harden the magnetic layer (and back layer) includes electron beams using an electron beam accelerator as a radiation source, gamma rays using a Co60 radiation source, and X-ray generators as a radiation source. X-rays etc. are used. In particular, as a radiation source,
An electron beam irradiation method using an electron beam accelerator is advantageous from the viewpoint of controlling the absorbed dose, introducing it into a manufacturing process line, shielding ionizing radiation, etc.

A specific example of an electron beam accelerator is a low-dose type electron beam accelerator (manufactured by Energy Sciences, Inc. in the United States).
Electro Curtain System) and scanning low-dose type electron beam accelerator manufactured by Nissin High Voltage.

In addition, for radiation crosslinking, N2 gas, He gas, C
It is effective to irradiate the magnetic layer (and/or back layer) with radiation in an inert gas flow such as Q2 gas. Radiation irradiation in air is not preferable because polymerization and curing reactions of the resin are inhibited by the influence of oxygen and harmful ozone gas is generated.

Examples of the present invention will be specifically described below. Note that all parts of components mentioned in the examples indicate parts by weight.

Example 1 Ferromagnetic Co-containing γ-Fe2o3 acrylic modified polyurethane resin [number of acrylic groups in resin = 2.2; average molecular weight = 1,20000]...8
Acrylic modified resin [A]H3 having the following structural formula (wherein, R is a methylol group or an acrylic ester of a methylol group, and the molar ratio of the methylol group to the acrylic ester is 7:3)... ...4 parts vinyl chloride/vinyl acetate/vinyl alcohol copolymer resin
・・・・・・8 parts lecithin
・・・・・・1 part α-A12o
3 [Average particle size = 0.3 μm] ...4 parts Carbon black [Average particle size = 50 mμ] ...
・Tripartite myristic acid ・・・
...1 part butyl stearate...
...1 part organic solvent (MEK:) toluene:butyl acetate = 2:2:1] ...
220 parts of the above composition was mixed and dispersed using a pressure kneader and a sand grinder to prepare a magnetic paint.

The obtained magnetic paint was filtered through a filter with an average pore size of 1 μm to prepare a magnetic paint.

Next, the above-mentioned magnetic paint was applied onto a 15 μm thick polyester film, subjected to magnetic field orientation and drying treatment, and then applied to the original magnetic tape in an N2 atmosphere (02 concentration 4 , about 300p
After electron beam irradiation (low-dose electron beam irradiation equipment manufactured by Energy Science Co., Ltd.; used at an accelerating voltage of 170 kV) under conditions such that the absorbed dose was 6 Mrad), the original magnetic tape was wound up. One copy of this was cut into 2-inch width pieces to give a videotape sample (250 m long, magnetic layer thickness 4.8 μm).
t created.

Comparative Example 1 The acrylic modified polyurethane resin used in Example 1 and the vinyl chloride/vinyl acetate/vinyl alcohol copolymer resin used in Example 1 were both used in amounts of 10 parts without using the acrylic modified resin (A) used in Example 1. A videotape sample was prepared in exactly the same manner as in Example 1, except for the above.

Example 2 Preparation of 0 magnetic paint Methacrylic modified polyurethane resin [Number of methacrylic groups in resin - 2.6; Average molecular weight = 33,000]
......10 parts Acrylic modified resin [B] having the following structural formula (where R is a methylol group or an acrylic ester of a methylol group, and the molar ratio of the methylol group to the acrylic ester is 3 vs. 7)
...3 parts vinyl chloride, vinyl acetate, -1 part vinyl alcohol copolymer resin...
・・7 parts α-A1203 [average particle size = 0.3 μm] ・
...6 parts carbon black [average particle size = 50 mμ
] ・・・・・・Two parts palmitic acid
...1.6 parts butyl stearate
...1 part organic solvent [MEK: toluene:cyclohexanone = 3:3:2)
230 parts of the above composition was cross-dispersed using a pressure kneader and a sand grinder to prepare a magnetic paint. The obtained magnetic paint was filtered through a filter with an average pore size of 1 μm to prepare a magnetic paint.

○ Adjustment of back layer paint Acrylic modified polyester [Average molecular weight - 1,8000]
...30 parts Nitrocellulose resin ...20 parts Carbon black [average particle size 290 mμ] ...60 parts γ-A1203 [average particle size = 0.2 μm] ...・・・
・・・ 2 parts organic solvent [MEK: toluene: cyclohexanone = 2:2:1) ・・・・・・
After mixing and dispersing 320 parts of the above composition in a ball mill for 48 hours to remove contaminants, the mixture was filtered through a filter with an average pore size of 1 μm to prepare a paint for the back layer.

Next, the magnetic paint was applied onto a 14.6 μm thick polyester film, oriented in a magnetic field, and dried in an N2 atmosphere (o
22 parts, approximately 300 ppm), absorbed dose is 6 Mra
Electron beam irradiation (equipment) under the conditions of d.

The acceleration voltage was the same as in Example 1), and then the original magnetic tape was wound up. (Magnetic layer thickness, 4.6 μm) Next, the above-mentioned back layer paint was applied to the opposite side of the magnetic tape roll from the magnetic layer and dried, and then the surface of the back layer was coated with a calender roll. While performing smoothing treatment, the original magnetic tape was exposed to N2 atmosphere (o22 part,
After electron beam irradiation (apparatus and acceleration speed voltage were the same as in Example 1) under conditions such that the absorbed dose was 5 Mrad (approximately 500 ppm), the original magnetic tape was wound up.

(Back layer thickness: 0.7 μm) One copy of this was cut into a 2-inch width to prepare a videotape sample.

(250 m length) Comparative Example 2 A videotape sample was prepared in exactly the same manner as in Example 2, using pentaerythritol instead of the acrylic modified resin CB) in the binder for the magnetic layer.

Example 3 Preparation of 0 magnetic paint Ferromagnetic (Fe, Go, Ni) alloy acrylic modified polyurethane resin [Number of acrylic groups in resin = 1.8; average molecular weight = 21,000]...4 parts A methacrylic modified tree (C3 (where R is a methylol group or a methacrylic ester of a methylol group, and the molar ratio of the methylol group to the methacrylic ester is 1:1) having the following structural formula:
... 4-part sodium sulfonate group-containing vinyl chloride/vinyl alcohol copolymer resin ...
・12 parts α-A1203 [average particle size = 0.2 μm]
-5 parts carbon black [average particle size = 20 m /
J〕・・・・・・ 22 parts oleic acid
・・・・・・ 1 part myristic acid
・・・・・・ 1 part ethyl stearate
...... 1 part organic solvent (MEK:) toluene: cyclohexanone = 3:3:2)
・・・・・・220 parts The above composition was mixed and dispersed using a pressure kneader and a sand grinder to prepare a magnetic paint. The obtained magnetic paint was filtered through a filter with an average pore size of 1 μm to obtain a magnetic paint. preparations were made.

Preparation of paint for O back layer Acrylic modified polyurethane [Average molecular weight = 18,000]
...30 parts Nitrocellulose resin ...20 parts carbon black [average particle size = 40 mμ] ...60 parts γ-
AI O [average particle size = 0.2 μm] ...
・2 parts organic solvent (MEK: toluene: cyclohexanone = 2:2:1) ・・・・・・32
0 parts The above composition was mixed and dispersed in a ball mill for 60 hours to remove contaminants, and then filtered through a filter with an average pore size of 1 μm to prepare a paint for the back layer.

Next, the above-mentioned magnetic paint was applied onto a 10.0 μm thick polyester film, subjected to magnetic field orientation and drying treatment, and then mirror-finished the magnetic layer using a calendar roll while the original magnetic tape was placed in an N atmosphere (O22 part, about soo
After electron beam irradiation (apparatus and acceleration voltage were the same as in Example 1) under conditions such that the absorbed dose was 5 Mrad (ppm) and the absorbed dose was 5 Mrad, the original magnetic tape was wound up. (Magnetic layer thickness, 3.6
Next, the above-mentioned back layer paint is applied to the surface opposite to the magnetic layer on the raw magnetic tape roll, and while drying, the raw magnetic tape is heated in an N atmosphere (02111 degrees, approximately 200 ppm). ), electron beam irradiation under conditions where the absorbed dose is 5Mxad (equipment and acceleration voltage are the same as in Example 1)
After that, the original magnetic tape was wound up. (Back layer thickness: 0.7 μm) One copy of this was cut into a 2-inch width to prepare a videotape sample. (250 m length) Comparative Example 3 The same procedure as in Example 3 was used except that the same amount of trimethylolpropane triacrylate was used instead of the methacrylic modified resin [C] in the magnetic layer binder used in Example 3. A videotape sample was made.

The following evaluation tests were conducted on the videotape samples obtained in each of the above Examples and Comparative Examples.

(1) Surface roughness μm) - The surface roughness of the magnetic layer was measured using a Talystep type surface roughness meter manufactured by Taylor Hopson.

The value is the square of the peak height on the roughness chart/&3T
une 1982 p, 52o)) (2) C/N(
dB) - Each sample tape was wound into a cassette huff and the C/N at 5 MHz was measured. A VH8 type VTR (manufactured by Matsushita Electric Industrial Co., Ltd., Nv-aaoo) was used as a video tape recorder (VTR) for C/N measurement. However, sample tapes using ferromagnetic alloy powder (Example 3 and Comparative Example 3)
), measurements were made by replacing the recording/reproducing head of the VTR with a Sendust alloy. Standard tapes include Super HG videotape (manufactured by Matsushita Electric Industrial Co., Ltd.).
(7) C/N in odB
And so.

(3) Gelation rate of magnetic coating film - The magnetic layer of each sample tape was forcibly peeled off and extracted in methyl ethyl ketone (sample concentration: 2%) at 60°C for 11 hours. The gelation rate (insolubilization rate) of the organic component in the magnetic coating film was calculated by determining the amount of the solvent component using a gravimetric method.

(4) Adhesiveness of the magnetic coating film The adhesiveness of the magnetic coating film is determined from the degree of peeling of the magnetic coating film when the adhesive tape attached to the magnetic side of each sample tape is rolled at a constant speed. A six-level evaluation was performed: no peeling, almost no peeling, slight peeling, almost no peeling, and ``complete peeling.''

(5) Running durability/Running durability evaluation of the sample tape was performed using the same VT as in (2).
The number of dropouts (D, O) after each sample tape was subjected to a 100x running test (3.30 g/sec) at 40"G and 80% RH using R.

This was done by observing changes in

The evaluation test results for each sample tape are shown in the table below.

(><”T-Compound) Effects of the Invention A magnetic layer mainly composed of ferromagnetic powder, a resin having radiation-sensitive unsaturated double bonds, and a thermoplastic resin having no radiation-sensitive unsaturated double bonds. As is clear from the above table, in a magnetic recording medium formed on a non-magnetic support,
The magnetic recording medium obtained by combining the resins of the present invention has good dispersion of magnetic powder, makes it possible to form a magnetic layer with excellent electromagnetic conversion characteristics, and has good crosslinking properties of the coating film by radiation irradiation. , good adhesion and extremely excellent results were obtained in terms of durability.

As described above, according to the present invention, it has high electromagnetic conversion characteristics,
In addition, it is possible to obtain a radiation-curable magnetic recording medium with excellent durability, so it has great practical value.

In the above embodiments, a video tape was explained, but the present invention can also be applied to audio chips, cop/computer tapes, and magnetic recording media such as magnetic disks and magnetic cards. Needless to say.

Claims (3)

[Claims] (1) A magnetic layer mainly composed of ferromagnetic powder, a resin having radiation-sensitive unsaturated double bonds, and a thermoplastic resin having no radiation-sensitive unsaturated double bonds is formed on a non-magnetic support. A magnetic recording medium comprising a (meth)acrylic modified polyurethane resin and a modified resin represented by the following structural formula as the resin component having radiation-sensitive unsaturated double bonds is exposed to radiation. A magnetic recording medium characterized in that the coating film is polymerized and cured by irradiation. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (However, R is a methylol group or a (meth)acrylic acid ester of a methylol group, and the molar ratio of the methylol group to the (meth)acrylic acid ester is 3:1 to 1. (within the range of 3) (2) The mixing ratio of the resin with radiation-sensitive unsaturated double bonds and the thermoplastic resin without radiation-sensitive unsaturated double bonds is in the range of 7:3 to 3:7, respectively, in terms of weight ratio. A magnetic recording medium according to claim 1, characterized in that: (3) The magnetic recording medium according to claim 1, wherein the thermoplastic resin having no radiation-sensitive unsaturated double bonds is a vinyl chloride copolymer resin.