JPH0656652B2 - Binder for magnetic recording material - Google Patents

Description

The present invention relates to a binder for magnetic recording materials.

[Conventional technology]

Conventionally, a thermosetting type carboxyl group-containing polyurethane resin has been known as a binder for magnetic recording materials (JP-A-59-148127).

[Problems to be solved by the invention]

However, this product has a slightly insufficient dispersibility for a certain kind of magnetic powder, and the heat resistance is at a practical level. However, in recent years, the usage conditions of magnetic recording materials such as video tapes have become increasingly strict. It is coming, and further improvement in heat resistance is desired.

[Means for solving problems]

The present inventors have arrived at the present invention as a result of examining a binder for a magnetic recording material for the purpose of improving the dispersibility and heat resistance of the magnetic powder.

That is, the present invention provides a polymer polyol (A ₁ ) 5 to 70 having an [I] hydroxyl group equivalent (hereinafter abbreviated as equivalent) of 300 or more.
Low molecular weight polyol with an equivalent% and a hydroxyl equivalent of 250 or less (A
₂ ) 20 to 90 equivalent% and 0.2 to 30 equivalent% of a carboxyl group-containing diol (A ₃ ) having a hydroxyl equivalent of 150 or less.
(A), an active hydrogen-containing (meth) acrylate (B) having reactivity with isocyanate, and a urethane having less than 2.5 (meth) acryloyl groups in one molecule from polyisocyanate (C). It comprises an oligomer and at least one resin selected from the group consisting of [II] vinyl chloride-vinyl acetate resin, vinyl chloride-vinylidene chloride resin and cellulose resin, and the content of [II] is [I] ] And [II] in a total amount of 20 to 80% by weight, which is a binder for a radiation curable magnetic recording material.

Examples of the polymer polyol (A ₁ ) in the present invention include polyether polyol and polyester polyol.

As polyether polyols, low-molecular-weight glycols (ethylene glycol, propylene glycol, 1.4-butadiol, etc.); low-molecular-weight triols (glycerin, trimethylolpropane, hexanetriol, etc.), tetrafunctional or higher-functionality low-molecular-weight polyols (sorbitol, sucrose, etc.) ), Or an alkylene oxide (having 2 carbon atoms) of an amine (alkanolamine, aliphatic polyamine, etc.)
~ 4 alkylene oxides such as ethylene oxide,
Examples thereof include propylene oxide, butylene oxide) adducts, and ring-opening polymers of alkylene oxides, and specifically include polyethylene glycol, polypropylene glycol, and polytetramethylene glycol.

As the polyester polyol, a polyester polyol containing a terminal hydroxyl group of a polycarboxylic acid (aliphatic polycarboxylic acid such as adipic acid, maleic acid, dimerized linoleic acid; aromatic polycarboxylic acid such as phthalic acid) and a low molecular weight polyol or polyether polyol ,
Polycaprolactone polyols such as initiators [glycols (eg ethylene glycol), triols] based on (substituted) caprolactone (ε-caprolactone, α-methyl-ε-caprolactone, ε-methyl-ε-caprolactone) (Organometallic compounds, metal chelate compounds, fatty acid metal acylates, etc.)
Examples include polyols that are addition-polymerized in the presence of Also, polymer polyols and polybutadiene polyols can be used.

Among these, preferable are polymer polyols containing 50% by weight or more of a compound selected from the group consisting of polycaprolactone polyols, polyester polyols composed of polycarboxylic acids and low molecular weight polyols, and polytetramethylene glycol.

The equivalent of the high molecular weight polyol (A ₁ ) is 300 or more, preferably 400
~ 2000. If the equivalent weight is less than 300, the resulting urethane oligomer resin after radiation curing becomes brittle, and when used as a magnetic recording material, wear resistance and durability are deteriorated.

The equivalent 250 or lower molecular weight polyol in the present invention (A _2), a low molecular weight glycol (ethylene glycol, propylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, etc.), low-molecular triols (glycerol, tri Methylolpropane, hexanetriol, etc.), tetrafunctional or higher functional low molecular weight polyols (sorbitol, sucrose, etc.), low molar adducts of alkylene oxides (diethylene glycol, etc.) and low molar adducts of alkylene oxides (bisphenol, etc.) ( Bishydroxyethylbenzene etc.). Of these, preferred is a low molecular weight triol and / or a low molecular weight diol in combination with a low molecular weight triol.

Examples of the carboxyl group-containing diol (A ₃ ) having an equivalent weight of 150 or less in the present invention include 4.4-bis (hydroxyphenyl)
Examples include butyric acid, 4.4-bis (hydroxyphenyl) valeric acid and dimethylolpropionic acid (DMPA).
Of these, DAPA is preferred.

The amount of (A ₁ ) in (A) is usually 5 to 70 equivalent%, preferably 10 to 50 equivalent%. When (A ₁ ) is less than 5 equivalent%, the resin after radiation curing of the urethane oligomer produced becomes brittle, and the wear resistance and durability of the resulting magnetic recording material deteriorate. If (A ₁ ) exceeds 70 equivalent%, the wear resistance of the magnetic recording material is lowered.

The amount of (A ₂ ) in (A) is usually 20 to 90 equivalent%, preferably 40 to 85 equivalent%. When (A ₂ ) is less than 20 equivalent%, the resin strength of the produced urethane oligomer after radiation curing is lowered, and the abrasion resistance and durability are lowered when it is used as a magnetic recording material. On the other hand, when (A ₂ ) is more than 90 equivalent%, the resin after radiation curing of the urethane oligomer produced becomes hard, and the wear resistance and durability of the magnetic recording material deteriorate.

The amount of (A) in the (A ₃₎ is usually 0.2 to 30 equivalent percent, preferably 0.5 to 20 equivalent%. When (A ₃ ) is less than 0.5 equivalent%, the dispersibility of the magnetic powder is inferior, and when it is more than 20 equivalent%, the viscosity of the magnetic coating becomes too high and the application becomes difficult.

Active hydrogen-containing (meth) acrylate (B) in the present invention
Examples include hydroxyalkyl (the carbon number of the alkyl group is usually 2 to 4) (meth) acrylate [hydroxyethyl (meth) acrylate, hydroxypropyl (meth) acrylate, etc.], alkyl oxide of hydroxyalkyl (meth) acrylate [carbon number 2-4 alkylene such as ethylene oxide (hereinafter abbreviated as EO), propylene oxide (hereinafter abbreviated as PO) and butylene oxide] and / or lactone (the carbon number is usually 3).
To 7) (addition mole number is usually 1 to 20, preferably 1 to 10), (meth) acrylamide, (meth) acrylic acid and the like. Hydroxyalkyl (meth) acrylate is preferable, and 2-hydroxyethyl (meth) acrylate is particularly preferable.

The polyisocyanate (C) in the present invention includes aromatic polyisocyanates such as aromatic polyisocyanates (tolylene diisocyanate (TDI), 4.4'-diphenylmethane diisocyanate (MDI), modified MD).
I, 1.5-naphthylene diisocyanate, m- and / or
Or p-xylylene diisocyanate and 2.2'-
Dimethyldiphenylmethane-4.4'-diisocyanate, etc.), aromatic polyisocyanate multimers (TDI,
NCO-terminated urethane prepolymers of dimers and trimers such as MDI) and polyols with aromatic polyisocyanates [reactants of low molecular weight polyols and excess aromatic polyisocyanates such as trimethylolpropane (1 mol) and TDI] (3 mol of reaction product),
Among the aromatic polyisocyanates, preferred is T
DI and MDI.

Also, non-aromatic polyisocyanates such as aliphatic polyisocyanates such as aliphatic diisocyanates (tetramethylene diisocyanate, hexamethylene diisocyanate, lysine diisocyanate, etc.) and alicyclic diisocyanates [hydrogenated TDI (hydrogenated tolylene diisocyanate), hydrogenated MDI (Hydrogenated 4.4'-diphenylmethane diisocyanate), isopropylidene bis (4-cyclohexyl isocyanate), isophorone diisocyanate (IPDI), etc.], trimer of non-aromatic polyisocyanate and polyol and non-aromatic polyisocyanate NCO-terminated urethane prepolymers (reactants of polyols such as low molecular weight polyols and excess non-aromatic polyisocyanate) can also be used. Among the non-aromatic polyisocyanates, preferred are IPDI and hydrogenated MDI.

Urethane oligomers from (A), (B) and (C) are (A),
Obtained by reacting (B) and (C). (A),
When reacting (B) and (C), (A) is usually 30-70.
Equivalent%, preferably 40-60 equivalent%, (B) is usually 0.5
-20 equivalent%, preferably 0.8-15 equivalent%. Also,
(C) is usually 20 to 70 equivalent%, preferably 30 to 60 equivalent%.

The above reaction can be carried out in the presence or absence of a solvent inert to the isocyanate group. Examples of this solvent include aromatic hydrocarbon solvents (toluene, xylene, etc.), ketone solvents (cyclohexanone, methyl ethyl ketone,
Examples include methyl isobutyl ketone, methyl isobutyl ketone, etc.), ester solvents (ethyl acetate, butyl acetate, etc.), ether solvents (dioxane, tetrahydrofuran, etc.), and mixed solvents of two or more thereof.

As the reaction method, (A), (B) and (C) are collectively charged into a reaction vessel to react, (A), (B) and (C) are divided to carry out a multistage reaction and ( A ₁ ), (A ₃ ), and (A
_Examples include a method in which a part of ₂ ) is reacted with (C), then the rest of (A ₂ ) is reacted, and then (B) is reacted.

The reaction temperature is generally 40 to 130 ° C, preferably 60 to 90 ° C. In the reaction, a catalyst used in a usual urethane reaction for promoting the reaction, for example, a tin-based catalyst (trimethyltin laurate, trimethyltin hydroxide, dimethyltin dilaurate, dibutyltin dilaurate, stannas octoate, etc.), a lead-based catalyst (Led oleate, red 2-ethylhexoate, etc.) can also be used.

The obtained urethane oligomer has one or more and less than 2.5 (meth) acryloyl groups in one molecule. When the number of (meth) acryloyl groups is less than 1 in 1 molecule, the wear resistance and durability of the magnetic recording material are deteriorated. The urethane oligomer has a number average molecular weight of usually 1500 to 50,000, preferably 2000 to 30,000, and an acid value (KOHmg / g) of usually 0.5 to
50, preferably 1-30.

In the present invention, the above urethane oligomer and at least one resin selected from the group consisting of vinyl chloride-vinyl acetate resins, vinyl chloride-vinylidene chloride resins and cellulose resins are essential components.

As vinyl chloride-vinyl acetate resin, vinyl chloride-
Vinyl acetate copolymer (VYHH, UCC; Eslek C, Sekisui Chemical, etc.), vinyl chloride-vinyl acetate-vinyl alcohol copolymer (VAGH, Eslek A, etc.), vinyl chloride-vinyl acetate-maleic acid copolymer And so on.
Examples of vinyl chloride-vinylidene chloride resins include vinyl chloride-vinylidene chloride copolymers and vinyl chloride-vinylidene chloride-acrylonitrile copolymers (made by Saran Asahi Dow) and the like. Examples of the cellulose-based resin include esterified cellulose, such as nitrocellulose (having a nitrification degree of 8 to 12%, specifically RS1 / 2H, manufactured by Daicel), acetylated cellulose and the like.

The content of the above resin is usually based on the weight of the binder.
It is 20 to 80%, preferably 30 to 70%, and the content of urethane oligomer is usually 20 to 80%, preferably 30 to 70%. When the content of the urethane oligomer is less than 20%, the wear resistance of the magnetic recording material is lowered.

The binder of the present invention can be made into a magnetic coating material by incorporating a magnetic material into it.

Examples of this magnetic material include iron oxides such as γ-Fe ₂ O ₃ (γ-
Hematite), CrO ₃ (chromium trioxide) and alloy-based magnetic materials such as Co-γ-Fe ₂ O ₃ (cobalt ferrite or cobalt dub γ-iron oxide), Fe-Co-Cr, pure iron Fe, and barium. Ferite can be given. Particularly in the present invention, the recent more finely pulverized Co-γ-Fe ₂ O ₃
It is useful for (for example, a particle major axis of about 0.3 μm) and pure iron (for example, a particle major axis of about 0.2 μm).

The magnetic substance is magnetic powder and its shape is usually granular or acicular. The diameter is 0.02 if an example of the size (μm) of the magnetic material is shown.
.About.0.70, for example, long diameter 0.2 to 0.7, short form 0.02 to 0.1.

If necessary, a dispersant (lecithin, anion, nonion, cationic surfactant such as sodium dodecylbenzenesulfonate) and a lubricant (higher fatty acid ester such as butyl stearate) can be added.

A solvent is used for the magnetic paint to adjust the viscosity. As this solvent, the same solvents as those described in the urethane oligomer formation reaction, that is, ester solvents (ethyl acetate, butyl acetate, etc.), ether solvents (dioxane,
Tetrahydrofuran etc.), Ketone solvents (cyclohexane, methyl ethyl ketone, methyl isobutyl ketone etc.), Aromatic hydrocarbon solvents (toluene, xylene etc.)
And a mixed solvent of two or more of these can be used. Among these, preferred are mixed solvents of ketone type and aromatic hydrocarbon type.

The content of the binder in the magnetic paint is usually 3% by weight or more, preferably 5 to 30% by weight based on the weight of the paint.
By including the binder in an amount of 3% by weight or more, the object of the present invention can be sufficiently achieved.

The amount of magnetic substance in the magnetic paint is usually 15 to 80% by weight.

As a method for producing a magnetic coating material, there is a method in which a binder, a medium and a magnetic material are preliminarily mixed if necessary with a premixer or the like and then dispersed with a mixing / dispersing machine (such as a ball mill) and then passed.

This magnetic paint can be applied to a support film for a magnetic recording medium to prepare a magnetic recording medium (such as a magnetic tape). This support film includes paper, cellophane,
Examples of the film include acetate, polyester (PET), reinforced polyester, and polyimide. Of these films, polyester film is often used.

When the magnetic coating material is applied to the support film, the coating thickness is usually several μ to several tens μ. Examples of applicable methods include doctor blade methods and transfer printing methods (gravure method, reverse roll method, etc.) and the like.

The film coated with the magnetic paint is then subjected to processes such as orientation, drying, surface processing, cutting and winding to obtain a magnetic recording medium.

A magnetic recording medium is generally composed of a support film and a magnetic layer (magnetic binder), but one having an intermediate layer (undercoat layer) provided between the support film and the magnetic layer, or a support film. There are also those having magnetic layers on both sides, those having magnetic layers having different magnetic characteristics stacked, and those having a protective layer provided on the magnetic layer.

The urethane oligomer in the present invention is cured by radiation. Examples of this radiation include electron rays, neutron rays, and γ rays, and electron rays are preferable. Examples of electron beam accelerators include a scanning method, a curtain beam method, and a broad beam curtain method. The accelerating voltage of the electron beam is usually 100 ~ 1000KV, preferably 100 ~ 1000KV
It is 300KV. The absorbed dose is usually 0.5 to 20 megarats, preferably 3 to 15 megarats. If the accelerating voltage is less than 100 KV, the electron beam transmission film thickness will be insufficient and 100
If it exceeds 0 KV, the energy efficiency used for curing is reduced and it is not economical. If the absorbed dose exceeds 20 megarats, the energy efficiency used for curing decreases.

〔Example〕

The present invention will be further described below with reference to examples, but the present invention is not limited thereto. Parts in the examples indicate parts by weight.

Example 1-4, Comparative Example 1-3 A binder having the composition shown in Table 1 was obtained using the following oligomers.

Oligomer I In a four-neck flask equipped with a stirrer and a cooling tube, 57.6 parts of polytetramethylene glycol (hereinafter abbreviated as PTMG) having an average molecular weight of 1000, 13.2 parts of ethylene glycol (hereinafter abbreviated as EG), 2.4 parts of DMPA, Take 90.6 parts of IPDI, 195.5 parts of cyclohexanone and 0.005 parts of dibutyltin dilaurate, react at 80-85 ° C for 5 hours, then add 11.0 parts of 1.4-butanediol (hereinafter abbreviated as 1.4-BG) and react for another 5 hours. Let Thereafter, 10.2 parts of 2-hydroxyethyl acrylate (hereinafter abbreviated as HEA) and 0.15 parts of monomethylhydroquinone (hereinafter abbreviated as MQ) are added, and the reaction is further continued for 20 hours to give a urethane oligomer I having two acryloyl groups in one molecule. Got This product had a number average molecular weight of 3770 and an acid value of 5.4 (converted to solid content, hereinafter omitted).

Oligomer II In the same manner as in Example 1, 48.5 parts of butylene adipate having an average molecular weight of 1000, 19.0 parts of neopentyl glycol (hereinafter abbreviated as NPG), 1.6 parts of DMPA, 63.2 parts of MDI and 134.0 parts of cyclohexanone at 60 to 70 ° C. The reaction was carried out for 2 hours. After that, 0.36 parts of trimethylolpropane (hereinafter abbreviated as TMP),
Add 1.4-BG 0.06 parts, HEA 1.04 parts and MQ 0.13 parts,
After reacting for 6 hours, the acryloyl group in one molecule 2.
A urethane oligomer II having 4 units was obtained. This one is
The number average molecular weight was 26,000 and the acid value was 5.0.

Oligomer III As in Example 1, 61.7 parts of polycaprolactone diol having an average molecular weight of 1250 (hereinafter abbreviated as PCL), 1.4-BG
3.9 parts, DMPA 0.8 parts, IPDI 28.9 parts, cyclohexanone
Take 100 parts and dibutyltin dilaurate 0.003 parts,
After reacting at 80 to 85 ° C. for 5 hours, 3.7 parts of HEA, 1.0 part of methanol and 0.1 part of MQ were added, and the reaction was continued for another 20 hours to obtain a urethane oligomer III having one acryloyl group in one molecule. It has a number average molecular weight of 3
It had an acid value of 160 and an acid value of 3.5.

Oligomer Y 47.5 parts of PTMG having an average molecular weight of 1000, EG as in Example 1
8.2 parts, DMPA 2.0 parts, IPDI 71.3 parts, cyclohexanone 150
Part and dibutyltin dilaurate 0.003 parts, 80
React at ~ 85 ° C for 5 hours, then add 5.8 parts of TMP,
The reaction was continued for 5 hours. Thereafter, 15.2 parts of HEA and 0.15 part of MQ were added, and the reaction was further continued for 20 hours to obtain a urethane oligomer Y having 3 acryloyl groups in one molecule. This product had a number average molecular weight of 3510 and an acid value of 5.5.

Oligomer Z In the same manner as in Example 1, 48.5 parts of butylene diadipate having an average molecular weight of 1000, 19.0 parts of NPG, 1.6 parts of DMPA, 63.2 parts of MDI and 134.0 parts of cyclohexanone were reacted at 65 to 75 ° C for 3 hours. Then, 0.36 part of TMP and 0.74 part of EG were added and reacted for 6 hours to obtain a urethane oligomer Z having an OH group at the terminal. This product has a number average molecular weight of 26200 and an acid value of
It was 5.1.

Use Example 1 The binders of Examples 1-4 and Comparative Examples 1 and 2 were blended as shown below, and shaken for 4 hours with a paint conditioner (manufactured by Lett Devil Co.) to prepare magnetic paints.

Combined binder (solid content) 18 parts Pure iron (major axis 0.21μ, surface area 50m ² / g 72 parts Toluene 105 parts Cyclohexane 105 parts) The obtained magnetic coating was applied to a 15μ polyester film using a doctor blade and solid content. Was applied to give a coating thickness of 5 to 6 μm, and this was immediately mixed with a magnetic field through a parallel magnetic field of 6000 oersteds, and then the solvent was dried (100 ° C.,
For 1 minute). After that, it was calendered and the surface was smoothed, and then an accelerating voltage of 150 KV, using an electron beam accelerator.
A magnetic film was obtained by irradiating an electron beam in a nitrogen gas atmosphere under the condition of absorbed dose of 5 megarads.

In addition, the binder of Comparative Example 3 was further treated with Coronate L (manufactured by Nippon Polyurethane Co., Ltd., solid content) of 0.9 when the magnetic paint was prepared.
Parts were added to prepare a magnetic paint. The obtained magnetic paint,
In the same manner as described above, the polyester film was applied, magnetic field oriented, and then the solvent was dried. Then, it was calendered to make the surface smooth, and then allowed to stand in a constant temperature bath at 70 ° C. for 24 hours to cure the magnetic layer to obtain a magnetic film. Table 2 shows the test results.

Example 5 A magnetic film was prepared in the same manner as in Example 1 using 50 parts of Oligomer III and 50 parts of vinyl chloride-vinylidene chloride copolymer. The test results of this magnetic film are shown in the table.
3 shows.

(Comparative Example 4) Polybutylene diadipate 10 having an average molecular weight of 2,000
0 part and 25 parts of 4,4'-diphenylmethane diisocyanate in 200 parts of methyl ethyl ketone to 70-8
The reaction was carried out at a temperature of 0 ° C. for 3 hours. Next, 48 parts of trimethylolpropane diacrylate and 0.1 part of hydroquinone monomethyl ether were added, and the mixture was further added at the same temperature for 10 minutes.
After reacting for a time, an oligomer U was obtained. This had a number average molecular weight of 2,500. This oligomer U60
Parts and 40 parts of vinyl chloride-vinyl acetate copolymer,
A magnetic film was prepared in the same manner as in Use Example 1. The test results of this magnetic film are shown in Table-3.

(Comparative Example 5) 100 parts of polyether triol having an average molecular weight of 3,000 (Sannickus GP-3000; manufactured by Sanyo Kasei) and 25 parts of 4,4'-diphenylmethane diisocyanate were added in 200 parts of cyclohexane at a temperature of 70 to 80 ° C. The reaction was carried out for 3 hours. Then 19 parts of hydroxyethyl acrylate and 0.1 parts of hydroquinone monomethyl ether
Then, 70 parts of polybutylene adipate having an average molecular weight of 2,000 was added, and the mixture was further reacted for 5 hours to obtain an oligomer V. This product has a number average molecular weight of 5,500 and a hydroxyl value of 35.
Met. A magnetic film was prepared in the same manner as in Use Example 1 using 60 parts of this oligomer V and 40 parts of a vinyl chloride-vinyl acetate copolymer. The test results of this magnetic film are shown in Table-3.

(Comparative Example 6) Polybutylene adipate 48.5 having a hydroxyl equivalent of 112.
Part, neopentyl glycol (hydroxyl equivalent = 52) 19
And 60.2 parts of 4,4'-diphenylmethane diisocyanate in 130 parts of cyclohexane in an amount of 60-70.
The reaction was carried out at a temperature of ° C for 2 hours. Next, 0.36 parts of trimethylolpropane and 0.06 of 1,4-butylene glycol
Parts, hydroxyethyl acrylate 1.04 parts and hydroquinone monomethyl ether 0.1 parts were added and further reacted at the same temperature for 6 hours to obtain an oligomer W. This had a number average molecular weight of 25,000. A magnetic film was prepared in the same manner as in Use Example 1 using 40 parts of this oligomer W and 60 parts of a vinyl chloride-vinyl acetate copolymer. The test results of this magnetic film are shown in Table-3.

* 1) Using a BH curve tracer, measure the saturation magnetic flux flux density Bm and the residual magnetic flux density Br in the orientation direction of the magnetic film,
The ratio Br / Bm of the two was determined. The higher the value, the better the dispersibility of the magnetic powder.

* 2) Apply a load of 1 kg to one end of a magnetic film cut into 1/2 inch width and wrap it around a glass tube with a diameter of 40 mm, 60 ° C, 80%
After left for 1 week under the condition of RH, when the magnetic film was unwound, the presence or absence of resistance was evaluated. Those with less resistance were considered good.

* 3) The magnetic film is made into a disk shape and put on a commercially available taper type scratch tester (manufactured by Toyo Seiki Co., Ltd.), and the degree of wear of each magnetic film is evaluated relative to each other. And

* 4) Simulated head of test magnetic tape with a length of 50 m is 5 m / se.
After repeatedly sliding back and forth at a speed of c, the tape surface after that was visually and microscopically measured. A coating film free from scratches and dust was considered good.

〔The invention's effect〕

The binder for magnetic recording materials of the present invention has improved dispersibility of magnetic powder and heat resistance as compared with conventional binders. In addition to the above effects, the binder of the present invention has excellent wear resistance and durability.

The binder of the present invention is useful as a binder for magnetic recording materials such as audio tapes, video tapes, computer tapes, data recorder tapes, video sheets, magnetic disks, and magnetic cards.

Claims (2)

[Claims] 1. [I] 5-70 equivalent% of a high molecular weight polyol (A ₁ ) having a hydroxyl equivalent of 300 or more, 20-90 equivalent% of a low molecular polyol (A ₂ ) having a hydroxyl equivalent of 250 or less, and 150 or less hydroxyl equivalent. carboxyl group-containing diol (a ₃₎ 0.2 to 30 consisting equivalent% polyol (a), in one molecule of the active hydrogen-containing having isocyanate reactive (meth) acrylate (B), and the polyisocyanate (C) Less than 2.5 (meta)
A urethane oligomer having an acryloyl group, [II]
At least one resin selected from the group consisting of vinyl chloride-vinyl acetate-based resin, vinyl chloride-vinylidene chloride-based resin and cellulosic resin, wherein the content of [II] is [I] and [II] A binder for a radiation-curable magnetic recording material, which is 20 to 80% by weight based on the total amount. 2. A compound in which (B) is selected from the group consisting of hydroxyalkyl (meth) acrylate, adduct of hydroxyalkyl (meth) acrylate with alkylene oxide and / or lactone, (meth) acrylamide and (meth) acrylic acid. The binder according to claim 1, wherein