JPH0817036A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium having satisfactory electromagnetic transducing characteristics and excellent in still durability and running durability. CONSTITUTION:Two coating films are formed on the upper side of a nonmagnetic substrate. At least the upper one of the coating films is a magnetic layer, the thickness of the upper magnetic layer is 0.1-0.8mum and each of the coating films contains polyurethane resin having a number average mol.wt. of 5,000-25,000 and polyurethane resin having a number average mol.wt. of >25,000 to 100,000 as the essential components of a binder. Thus the objective magnetic recording medium having satisfactory electromagnetic transducing characteristics and excellent in still durability and running durability is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having excellent electromagnetic characteristics and excellent durability.

[0002]

2. Description of the Related Art A magnetic recording medium having a magnetic layer composed of at least two coating films has excellent characteristics such as excellent electromagnetic conversion characteristics and good durability. Traditionally,
A magnetic recording medium composed of a plurality of magnetic layers using polyurethane as a binder has been proposed. For example, in a magnetic recording medium in which a first magnetic layer and a second magnetic layer are provided in this order on the surface of a non-magnetic support, the first magnetic layer and the second magnetic layer both contain a polyurethane resin as a binder. ,
The number average molecular weight of the polyurethane resin contained in the first magnetic layer is 4/5 times or less the number average molecular weight of the polyurethane resin contained in the second magnetic layer. JP-A-64-79931), a magnetic recording medium in which a magnetic layer in which a ferromagnetic material is dispersed in a binder is multi-layered on a non-magnetic support, and the lower magnetic layer as the binder has a glass transition temperature. (Tg) contains at least one polyurethane resin having a temperature of −50 ° C. to −10 ° C.,
The upper magnetic layer contains at least one polyurethane resin having a glass transition temperature (Tg) of -20 ° C to 40 ° C, and the difference between the glass transition temperatures of the polyurethane resins used in the upper and lower magnetic layers is at least 10 ° C or more. In a magnetic recording medium (Japanese Patent Application Laid-Open No. 2-105326) having a plurality of magnetic layers on a non-magnetic support, the outermost magnetic layer comprises two or more types of polyurethane. A magnetic recording medium (Japanese Patent Laid-Open No. 3-84727) and a non-magnetic support which are characterized by containing a first magnetic layer and a second magnetic layer each containing a ferromagnetic powder and a binder. A magnetic recording medium having the same in sequence, the first magnetic layer containing an aliphatic polyurethane, and the second magnetic layer containing an aromatic polyurethane. 3-88119 JP), and the like.

[0003]

In the above-mentioned conventional magnetic recording medium, for example, the magnetic recording medium described in Japanese Patent Laid-Open No. 64-79931, the lower layer has excellent dispersibility of the ferromagnetic powder, and the surface of the magnetic layer is extremely smooth. By providing the upper layer, which is excellent in terms of durability, on the smooth surface of the lower layer, it is possible to obtain a magnetic recording medium having good electromagnetic conversion characteristics such as low-frequency characteristics and improved calender moldability. There is a problem in terms of durability.

The magnetic recording medium described in Japanese Patent Application Laid-Open No. 2-105326 has a lower layer without changing the use ratio of a binder having a high hardness (for example, vinyl chloride or vinyl acetate) and a soft binder (for example, polyurethane) as in the conventional case. The dispersibility and adhesion of the lower magnetic layer can be improved without lowering the Young's modulus (durability) of the magnetic layer.
There is a problem in running durability in low temperature and high temperature and high humidity environments. Among the two types of magnetic recording media described in JP-A-3-84727, one polyurethane improves the dispersibility of ferromagnetic powder, and the other polyurethane improves the dispersibility of abrasives. Dynamic noise and head turbidity can be improved, but there is a problem in running durability.

Further, the magnetic recording medium described in Japanese Patent Laid-Open No. 3-88119 can improve sliding noise and head turbidity without deteriorating electromagnetic conversion characteristics and still durability, but there is a problem in running durability. It was something. Therefore, there is a demand for a magnetic recording medium having excellent electromagnetic conversion characteristics and excellent durability and running properties at low temperature and high temperature and high humidity.

[0006]

In order to solve the above-mentioned problems, the inventors of the present invention have conducted intensive studies and found that in a magnetic recording medium comprising a magnetic layer having two coating films on the upper surface side of a non-magnetic support. The present inventors have found that the object can be achieved by using a polyurethane resin having two specific number average molecular weights as the binder for the two-layer coating film, and have arrived at the present invention. That is, the present invention provides (1) a magnetic recording medium having two layers of coatings on the upper surface side of a non-magnetic support, at least the upper layer of which is a magnetic layer, and the thickness of the coating of the upper magnetic layer Is 0.1 to 0.8 μm, and each of the coating films serves as a binder and has a number average molecular weight Mn of 5,000 ≦ Mn ≦ 25,
000 (Mn1) and 25,000 <Mn ≦ 100,0
A magnetic recording medium characterized by containing both of 00 (Mn2) polyurethane resin as an essential component, and (2) Mn1 and Mn2 of the polyurethane resin satisfy Mn2-Mn1 ≧ 3,000. (1) The magnetic recording medium according to (1), (3) the center line average roughness (Ra) of the magnetic layer surface of the magnetic recording medium is 1.0 nm ≦ Ra ≦ 10.0 nm. Magnetic recording medium of (4), of the two-layer coating film on the non-magnetic support, the lower coating film is a non-magnetic layer containing a non-magnetic inorganic powder (1), (2) or (3). (5) 2 on a non-magnetic support
The magnetic recording medium according to (1), (2) or (3), wherein all the coating films of the layers are magnetic layers, and (6) a backcoat layer on the lower surface side of the non-magnetic support.
The present invention relates to the magnetic recording medium according to (2), (3), (4) or (5).

The specific structure of the present invention will be described in detail below. Magnetic Layer (Upper Layer) In the upper magnetic layer, as a binder, polyurethane resins 1 and 25 having a number average molecular weight Mn of 5,000 ≦ Mn ≦ 25,000 (Mn1), preferably 8,000 ≦ Mn ≦ 25,000. , 000 <Mn ≦ 100,000 (Mn
2) It preferably contains 25,000 ≦ Mn ≦ 80,000 of polyurethane resin 2 as an essential component. In the present invention, the dispersibility of the magnetic coating material is improved by containing the polyurethane resin 1 having Mn1 described above, and the strength of the coating film is increased by containing the polyurethane resin 2 having Mn2. Adhesion is reduced, driving safety is improved, and excellent electromagnetic conversion characteristics and running durability are obtained. If the Mn of the polyurethane resin 1 is too small, the strength of the coating film decreases, running stability deteriorates, and troubles during running easily occur. On the other hand, if Mn1 is too large, the dispersibility of the magnetic coating material is lowered and the surface roughness of the magnetic layer surface is deteriorated.

If the Mn of the polyurethane resin 2 is too small, running stability is deteriorated and troubles during running are likely to occur. On the other hand, if the Mn is too large, the dispersibility of the magnetic paint is lowered and the surface of the magnetic layer becomes rough. , The electromagnetic conversion characteristics such as reproduction output will deteriorate. The content of the polyurethane resin 1 is 10 to 50% by weight of the binder, more preferably 15 to 5%.
The content of the polyurethane resin 2 is 10 to 50% by weight, more preferably 15 to 50% by weight of the binder.
Is. If the content of the polyurethane resin 1 in the binder is too small, the dispersibility of the magnetic coating material deteriorates and the electromagnetic conversion characteristics deteriorate, while if it is too large, the coating strength decreases and running stability in the environment is increased. Becomes worse.

When the content of the polyurethane resin 2 in the binder is too small, the coating strength is lowered and the running stability is apt to be deteriorated. On the other hand, when the content is too large, the dispersibility of the magnetic coating is lowered and the surface of the magnetic layer is deteriorated. Becomes rough and the electromagnetic conversion characteristics such as reproduction output deteriorate.

The glass transition temperature Tg of the polyurethane resin 1 used is preferably −50 ° C. ≦ Tg ≦ + 80 ° C.
More preferably, it is −40 ° C. ≦ Tg ≦ + 80 ° C. If the glass transition temperature is too low, running stability in a high temperature environment is deteriorated, and troubles during running such as head clogging and guide adhesion are likely to occur. On the other hand, if it is too high, the calendering workability is deteriorated and the electromagnetic conversion characteristics are likely to be deteriorated. In addition, running stability in a low temperature environment deteriorates.

The glass transition temperature Tg of these resins may be measured by using a dynamic viscoelasticity measuring device.
Polyurethane resin 1 and polyurethane resin 2 are added in an amount of 20 to 100% by weight of the binder used, particularly 30 to 1
It is preferably 00% by weight. Further, the compounding ratio (weight ratio) of these polyurethane resins 1 and 2 is 1/10 to 10
It is preferably 10/1. If the amount of polyurethane resin 1 and polyurethane resin 2 added in the binder is too small, running stability tends to be deteriorated, or calendering processability is deteriorated to roughen the magnetic layer surface, resulting in reproduction output and the like. The electromagnetic conversion characteristics are likely to deteriorate. As the polyurethane resin used for the polyurethane resin 1 and the polyurethane resin 2, any polyurethane resin usually used for such magnetic recording media can be used as long as the number average molecular weight is satisfied. For example, a polyol such as a polyester polyol, a polyether polyol, a polycarbonate polyol, a polyester polycarbonate, a polyester polyether, or a polycaprolactone is reacted with an isocyanate and, if desired, a lengthening agent and the like. However, in order to improve the dispersibility of powder components contained in the magnetic layer such as ferromagnetic powder and the running durability of the magnetic layer, a polar group is contained in the molecule of polyurethane resin 1 and / or polyurethane resin 2. Is preferred. As the polar group, sulfonic acid group containing sulfur, sulfuric acid group or ester or salt thereof, phosphonic acid group containing phosphorus, phosphinic acid group, phosphoric acid group or ester or salt thereof, carboxylic acid or salt thereof, etc. Those containing one or more of are preferred. Particularly preferred is a sulfonic acid group (-
SO ₃ Y), a carboxylic acid group (-COOY), a phosphonic acid group (PO ₃ Y) or the like. However, Y may be either H or an alkali metal. It is preferable that 0.1 to 5 molecules of these polar groups are contained per 1 molecule of the polyurethane polymer.

The polyurethane resin used in the present invention includes an organic diisocyanate (A) and a molecular weight of 50.
A long-chain diol (B) having a molecular weight of 0 to 5,000 and a chain extender (C) having a molecular weight of less than 500 are reacted. As the organic diisocyanate (A) used in the production of the polyurethane resin, 2,4-tolylene diisocyanate,
2,6-tolylene 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-
Examples thereof include diisocyanate methylcyclohexane, 1,4-diisocyanate methylcyclohexane, 4,4′-diisocyanate cyclohexane, 4,4′-diisocyanate cyclohexylmethane and isophorone diisocyanate. The long-chain diol (B) used in the production of the polyurethane resin has a molecular weight of 500 to 500.
It is in the range of 0 and includes polyester diol, polyether diol, and polycarbonate diol. Examples of the carboxylic acid component of the polyester diol include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid and 1,5-naphthalic acid, aromatic compounds such as p-oxybenzoic acid and p- (hydroxyetokin) benzoic acid. Examples thereof include aliphatic dicarboxylic acids such as oxycarboxylic acid, succinic acid, adipic acid, azelaic acid, sebacic acid and dodecanedicarboxylic acid. Particularly, terephthalic acid, isophthalic acid, orthophthalic acid, adipic acid and sebacic acid are preferable. The glycol component of the polyester diol includes ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol, diethylene glycol, diethylene glycol, Propylene glycol, 2,2,4-trimethyl-1,3-pentanediol, cyclohexanedimethanol, bisphenol A ethylene oxide adduct and propylene oxide adduct, hydrogenated bisphenol A ethylene oxide adduct and propylene oxide adduct, etc. There is. Other polyester diols include lactone polyester diols obtained by ring-opening polymerization of lactones such as ε-caprolactone. Examples of the polyether diol include polyalkylene glycols such as polyethylene glycol, polypropylene glycol and polytetramethylene glycol. The polycarbonate diol is a long-chain diol represented by the general formula H-(-OR-OCO-) _n- ROH, and R is diethylene glycol, 1,4-butanediol, 1,6-hexanediol, bisphenol. A residue such as A. The long chain diol (B) is preferably polyester diol in order to enhance the mechanical properties of the polyurethane resin. The long-chain diol has a molecular weight of 500 to 5,000. Molecular weight is 50
If it is less than 0, the urethane group concentration will increase, and the flexibility and solvent solubility of the resin will decrease. If the molecular weight exceeds 5,000, the urethane group concentration will decrease, and the toughness, abrasion resistance, etc. peculiar to the polyurethane resin will deteriorate. Chain extender (C) having a molecular weight of less than 500 used in the production of polyurethane resin
Contains two or more active hydrogens in one molecule, and has the effect of adjusting the urethane group or urea group concentration in the resin and imparting toughness peculiar to the polyurethane resin. Specific compounds include ethylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,6-hexanediol, cyclohexanedimethanol, xylylene glycol, diethylene glycol, triethylene glycol, and bisphenol A ethylene oxide addition. Propylene oxide adduct of propylene glycol, propylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,2,4-trimethyl-1,3-pentanediol, bisphenol A Such as branched glycol, monoethanolamine, N-
Examples thereof include amino alcohols such as methylethanolamine, diamines such as ethylenediamine, hexamethylenediamine and isophoronediamine, and water.

The method of introducing a polar group into the polyurethane resin of the present invention includes the following methods. A method in which a polyester diol is used as the long-chain diol and a compound having a polar group as a part of the dicarboxylic acid or glycol component of the polyester diol is used, and at least one component of a low molecular weight compound having two or more active hydrogens in the molecule is polar. Examples thereof include a method of using a compound containing a group. Among these methods, the method in which the polyester diol containing a metal sulfonate group is used as at least one component of the long-chain diol component is preferable in view of solubility in a general-purpose organic solvent, stability of production, and the like. Examples of the sulfonic acid metal base-containing compound include 5-sodium sulfoisophthalic acid, 5-potassium sulfoisophthalic acid, sodium sulfoterephthalic acid, 2-sodiumsulfo-1,4-butanediol,
2,5-dimethyl-3-sodium sulfo-2,5-hexanediol and the like can be mentioned. The polyurethane resin is, for example, polyester polyurethane resin, polyether polyurethane resin, polycarbonate polyurethane resin or the like, and the polyester polyurethane resin is polyester diol (1) / polyester diol (2) / organic diisocyanate (A) / chain extender. The reaction product of (C), the polyester diol (1) is a hard component which is a reaction product mainly composed of aromatic dicarboxylic acid / glycol, and the polyester diol (2) is an aliphatic dicarboxylic acid. / A soft component that is a reaction product mainly containing glycol. Further, Tg can be controlled by the blending ratio of this polyester diol (1) / polyester diol (2). In order to increase Tg, the blending ratio of the polyester diol (1) which is a hard component may be increased. Further, the blending ratio can be arbitrarily set within the range of 0/10 to 10/0.

Further, the binder used in the magnetic recording medium of the present invention may contain a resin other than the polyurethane resin 1 and the polyurethane resin 2 in an amount of less than 80% by weight of the binder. Such a resin is not particularly limited as long as it is a commonly used resin, and for example, Tg
Is a polyurethane resin in a range other than the above, vinyl chloride-acrylic acid ester-based copolymer, vinyl chloride-vinyl acetate-based copolymer, vinyl chloride-vinylidene chloride-based copolymer,
Vinyl chloride-acrylonitrile-based copolymer, acrylic ester-acrylonitrile-based copolymer, acrylic ester-vinylidene chloride-based copolymer, methacrylic acid ester-vinylidene chloride-based copolymer, methacrylic acid ester-ethylene-based copolymer , Polyvinyl fluoride-vinylidene chloride-acrylonitrile copolymer, acrylonitrile-butadiene copolymer, polyamide resin, polyvinyl butyral, cellulose derivative (cellulose acetate butyrate, cellulose diacetate, cellulose triacetate, cellulose propionate, nitro (Eg, cellulose), styrene-butadiene-based copolymer, polyester resin-chlorovinyl ether acrylate-based copolymer, amino resin and synthetic rubber-based thermoplastic resin. It can be. Of these binders, vinyl chloride resin is particularly preferable. These may be used alone or in combination of two or more. The content of these binders used in the magnetic layer is 1
5 to 40 parts by weight, particularly 10 to 30 parts by weight, is preferable to 00 parts by weight. If the content of the binder is too small, the strength of the magnetic layer is reduced, and the running durability is likely to deteriorate. On the other hand, if the amount is too large, the content of the ferromagnetic powder decreases, so that the electromagnetic conversion characteristics deteriorate. As the cross-linking agent for curing these binders, various polyisocyanates, particularly diisocyanates can be used, and one or more of tolylene diisocyanate, hexamethylene diisocyanate and methylene diisocyanate are particularly preferable. . These cross-linking agents are particularly preferably used as a cross-linking agent modified to have a plurality of hydroxyl groups such as trimethylolpropane or an isocyanurate-type cross-linking agent in which 3 molecules of a diisocyanate compound are bound, and a functional group contained in the binder resin is used. It bonds with groups and crosslinks the resin. The content of the crosslinking agent is preferably 10 to 30 parts by weight with respect to 100 parts by weight of the binder. To cure such a thermosetting resin, generally 12 to 4 at 50 to 70 ° C. in a heating oven.
It may be heated for 8 hours.

A radiation-curable resin can also be used as the binder used in the magnetic layer. The details are described in JP-B-63-66846, JP-B-6-12564, JP-A-61-59621 and JP-A-4-673.
The materials described in JP-A No. 14 and JP-A-6-131651 can be used.

Examples of the ferromagnetic powder used in the magnetic recording medium of the present invention include γ-Fe ₂ O ₃ and Co-containing γ-Fe.
₂ O ₃ , Fe ₃ O ₄ , Co-containing Fe ₃ O ₄ , CrO ₂ , barium ferrite, strontium ferrite, or other fine oxide powder, or Fe, Co, Ni, or an alloy fine powder of these known magnetic powders, There is no particular limitation as long as an appropriate one is selected according to the purpose. The shape of the ferromagnetic powder is not limited as long as it is a commonly used shape such as a needle shape, a spindle shape, a granular shape, or a plate shape. It is preferably needle-shaped or spindle-shaped because the effect of orientation treatment can be expected to be higher and the strength of the magnetic layer itself in the longitudinal direction is increased. The average major axis of the powder having such a shape, the average axial ratio, and the like are not particularly limited as long as they are in a commonly used range,
It may be selected according to the intended structure of the magnetic layer, but usually, the average major axis is about 0.05 to 0.5 μm, and the average axial ratio is 3 to.
About 15 is preferable. Furthermore, coercive force Hc, saturation magnetization σ
s and the like are not particularly limited and may be appropriately selected depending on the purpose, but normally, Hc is preferably about 350 to 25,000 e and σs is preferably about 50 to 180 emu / g. Such ferromagnetic powder may be contained in the magnetic layer composition in an amount of about 70 to 90 parts by weight. If the content of the ferromagnetic powder is too large, the content of the binder is reduced, so that the surface smoothness due to calendering tends to deteriorate, while if it is too small, a high reproduction output cannot be obtained. Further, in the magnetic layer, α-Al ₂ O ₃ , Cr ₂ O ₃ , TiO ₂ , SiC, and α-Fe ₂ are used to increase the mechanical strength of the magnetic layer and to prevent clogging of the magnetic head.
It is preferable to contain at least one kind of inorganic fine particles such as O ₃ . These usually have an irregular shape, prevent clogging of the magnetic head, and improve the strength of the coating film. The average particle size of the non-magnetic inorganic powder is preferably 0.05 to 0.8 μm. If the average particle diameter is too large, the amount of protrusion from the surface of the magnetic layer becomes large, resulting in deterioration of electromagnetic conversion characteristics, increase in dropout, increase in head wear, and the like. If the average particle size is too small, the amount of protrusion from the surface of the magnetic layer becomes small, and the effect of preventing head clogging becomes insufficient. The average particle size is usually measured by a transmission electron microscope.

If desired, a dispersant such as a surfactant, a lubricant such as a higher fatty acid, a fatty acid ester, or a silicone oil, and various other additives may be added to the magnetic layer. The coating material for forming the magnetic layer is prepared by adding an organic solvent to the above components. The organic solvent used is not particularly limited, and one or more of various solvents such as a ketone solvent such as methyl ethyl ketone, methyl isobutyl ketone, and cyclohexanone and an aromatic solvent such as toluene may be appropriately selected and used. The amount of the organic solvent added may be about 100 to 900 parts by weight based on 100 parts by weight of the total amount of the solid content (magnetic powder, various inorganic particles, etc.) and the binder. The thickness of the magnetic layer provided as the upper layer of the medium is 0.1 to 0.8 μm,
The thickness is preferably 0.15 to 0.6 μm. If the magnetic layer is too thin, the coatability of the coating film deteriorates and the coating film strength decreases. On the other hand, if the upper magnetic layer is too thick, when the lower coating film is a magnetic layer, the electromagnetic conversion characteristics tend to deteriorate during recording / reproduction of a reduced signal depending on the lower magnetic layer. Further, when the lower coating film is a non-magnetic layer, self-demagnetization loss and thickness loss increase. Line average roughness ± (R
It is preferable that a) is 1.0 nm ≦ Ra ≦ 10.0 nm. If (Ra) is small, the surface is too smooth and running stability is deteriorated, and troubles during running are likely to occur. If it is large, the surface of the magnetic layer becomes rough and electromagnetic conversion characteristics such as reproduction output deteriorate.

Magnetic Layer (Lower Layer) When the lower layer is a magnetic layer, the structure of the lower magnetic layer may be substantially the same as that of the upper magnetic layer. However, since the non-magnetic inorganic powder in the upper magnetic layer mainly plays a role in preventing head clogging, it is preferable that the amount of the non-magnetic inorganic powder in the lower layer is smaller than that in the upper magnetic layer in order to suppress deterioration of electromagnetic conversion characteristics. The thickness of the lower magnetic layer is 0.1 to 3.0 μm, preferably 0.5 to 2.5 μm. If the lower layer is too thin, it is likely to be affected by the surface property of the non-magnetic support, and as a result, the surface property of the lower layer deteriorates and the surface property of the upper layer tends to deteriorate, and the electromagnetic conversion characteristics tend to deteriorate. . Further, since the light transmittance becomes high, it becomes a problem when the tape end is detected by the change of the light transmittance. Further, even if the lower layer is thickened to some extent, the performance is not particularly improved. Non-Magnetic Layer (Lower Layer) The non-magnetic layer contains at least non-magnetic powder and a binder. Various inorganic powders can be used for the non-magnetic powder,
For example, acicular non-magnetic iron oxide (α-Fe ₂ O ₃ ) or the like can be used. However, by using spherical ultrafine iron oxide particles, high dispersibility can be obtained, and the filling rate of particles in the nonmagnetic layer can be increased. Therefore, the surface property of the non-magnetic layer itself is improved, and the surface property of the magnetic layer is improved, and the electromagnetic conversion characteristics are improved. In addition, JP-A-63-191315 and JP-A-63-19131
Carbon black, CaC as disclosed in Japanese Patent No. 8
Various non-magnetic powders such as O ₃ , titanium oxide, barium sulfate and α-Al ₂ O ₃ may be used. The binder, solvent, abrasive, lubricant, etc. used for the non-magnetic layer may be the same as those used for the magnetic layer described above. The thickness of the non-magnetic layer is 0.1-3.
It is 0 μm, preferably 0.5 to 2.5 μm. If the non-magnetic layer is too thin, it is likely to be affected by the surface properties of the non-magnetic support, and as a result, the surface properties of the non-magnetic layer will deteriorate and the surface properties of the magnetic layer will also easily deteriorate, resulting in poor electromagnetic conversion characteristics. It tends to decrease. Further, since the light transmittance becomes high, it becomes a problem when the tape end is detected by the change of the light transmittance.
Further, even if the non-magnetic layer is thickened to some extent or more, the performance is not improved. In the magnetic recording medium of the present invention, a back coat layer may be provided on the lower surface side of the non-magnetic support, if necessary.

Backcoat Layer The backcoat layer is provided to improve running stability and prevent electrification of the magnetic layer.

The back coat layer comprises various synthetic resins as binders and various additives. The synthetic resin that is a binder may be the same as that used in the magnetic layer coating material described above. The polyurethane resin preferably has a glass transition temperature Tg of −50 ° C. ≦ Tg ≦ 80 ° C. and a number average molecular weight of 5,000 to 100,000, and may be used alone or in combination of two or more.

The back coat layer preferably contains 30 to 80% by weight of carbon black. If the content of carbon black is too small, the antistatic effect tends to decrease, and running stability tends to decrease. In addition, since the light transmittance tends to be high, it becomes a problem in the method of detecting the tape end by the change of the light transmittance. on the other hand,
When the content of carbon black is too large, the strength of the back coat layer is lowered and the running durability is apt to be deteriorated. The carbon black may be any carbon black as long as it is usually used, and the average particle size thereof is 5 to 500 n.
About m is preferable. The average particle size is usually measured by a transmission electron microscope. In addition to the carbon black, the back coat layer may contain non-magnetic inorganic powder such as various abrasives mentioned in the description of the magnetic layer in order to enhance mechanical strength. The content of the non-magnetic inorganic powder is preferably 0.1 to 5 parts by weight, more preferably 0.5 to 2 parts by weight, based on 100 parts by weight of carbon black. The average particle size of the non-magnetic inorganic powder is preferably 0.1 to 0.5 μm. If the content of such a non-magnetic inorganic powder is too small, the mechanical strength of the back coat layer tends to be insufficient, and if it is too large, the amount of wear of the guides in the tape sliding contact path tends to increase. In addition, a dispersant such as a surfactant, a higher fatty acid, a fatty acid ester, a lubricant such as silicone oil, and other various additives may be added, if necessary. The binder, cross-linking agent, solvent and the like used for the back coat layer may be the same as those used for the magnetic layer coating material described above.
The content of the binder is preferably 15 to 200 parts by weight, more preferably 5 parts by weight, based on 100 parts by weight of the total solid content.
0 to 180 parts by weight. If the content of the binder is too large, the friction with the medium sliding contact path becomes too large, the traveling stability is deteriorated, and a traveling accident is likely to occur. In addition, problems such as blocking with the magnetic layer occur. When the content of the binder is too small, the strength of the back coat layer is lowered and the running durability is likely to be lowered. The thickness of the back coat layer (after calendering) is 1.0 μm or less, preferably 0.1 to 1.0 μm, more preferably 0.2 to 0.
It is 8 μm. If the back coat layer is too thick, friction with the medium sliding contact path becomes too large, and running stability tends to decrease. On the other hand, if it is too thin, the surface property of the back coat layer is lowered due to the effect of the surface property of the non-magnetic support. Therefore, when the back coat is thermally cured, the roughness of the surface of the back coat layer is transferred to the surface of the magnetic layer, and high range output, S /
N and C / N are lowered. If the back coat layer is too thin, the back coat layer will be scraped when the medium runs.

Non-magnetic support In the present invention, the material used as the non-magnetic support is not particularly limited, and selected from various flexible materials and various rigid materials according to the purpose, and tape-shaped according to various standards. The predetermined shape and size may be set. Examples of flexible materials include polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins such as polypropylene, and various resins such as polyamide, polyimide and polycarbonate. The thickness of these non-magnetic supports is preferably 4.0 to 75.0 μm.
The form of the non-magnetic support is not particularly limited, and may be tape-shaped, sheet-shaped, card-shaped, disk-shaped, or the like, and various materials are selected and used depending on the shape and as needed. can do. The surface roughness of the non-magnetic support used in the present invention is 2 at the center line average surface roughness Ra.
It is 5 nm or less, preferably 20 nm or less. The surface roughness of the non-magnetic support is freely controlled by the size and amount of the filler added to the non-magnetic support as needed. Examples of these fillers include Ca, Si, Ti, Al
In addition to oxides and carbonates such as the above, organic resin fine powders such as acrylics are listed, and a combination of Al ₂ O ₃ and organic resin fine powders is preferable.

Method for Producing Magnetic Recording Medium The magnetic recording medium of the present invention is produced by applying the coating material for the lower coating film and the coating material for the upper magnetic layer prepared by the above materials in this order on the non-magnetic support. be able to. The step of producing the coating material for the upper and lower coating films of the magnetic recording medium of the present invention comprises at least a kneading step, a dispersing step, and before and after these steps, if necessary, a mixing step, a viscosity adjusting step and a filtering step. Each process may be divided into two or more stages. The ferromagnetic powder, the non-magnetic inorganic powder, the binder, the abrasive, the carbon black, the lubricant, the solvent and the like used in the present invention may all be added at the beginning or in the middle of any step. In addition, individual materials may be divided and added in two or more steps. For the kneading / dispersion of the paint, it goes without saying that conventionally known manufacturing techniques can be used for some or all of the steps, but in the kneading step, those having a strong kneading force such as a continuous kneader or a pressure kneader are used. It is preferable. When a continuous kneader or a pressure kneader is used, the ferromagnetic powder or the non-magnetic inorganic powder and all or part of the binder (however, 10% by weight or more of the total binder is preferable) and the ferromagnetic powder or the non-magnetic inorganic powder 100 5 to parts by weight
Kneading is performed in the range of 30 parts by weight. The slurry temperature during kneading is preferably 50 ° C to 110 ° C. In addition, it is desirable to use a dispersion medium having a high specific gravity for dispersing the paint that can be used in each step, and a ceramic medium such as zirconia is suitable, but conventionally used glass beads, metal beads, and alumina. Beads and the like can be selectively used depending on the composition and composition. The paint filtering step is preferably provided after each manufacturing step. The presence of undispersed material such as magnetic powder or agglomerates, or resin insoluble material in the magnetic paint causes defects when the magnetic coating film is formed, resulting in an increase in dropout and an increase in error rate. The main purpose of the filtration step is to remove foreign substances in the magnetic paint. The details thereof have been previously proposed by the present inventors (Japanese Patent Application No. 5-346251, Japanese Patent Application No. 5-341819,
Japanese Patent Application No. 5-340794, Japanese Patent Application No. 5-329944
And Japanese Patent Application No. 5-329943).
There is no particular limitation on the method of applying the coating material prepared by the above-mentioned manufacturing method to the non-magnetic support, and any method that is commonly used may be used. For example, in the case of forming a coating film having a multi-layered structure including a magnetic layer, a so-called wet-on-wet coating method in which a lower layer and an upper layer are applied in a wet state on a non-magnetic support, or a lower layer is applied and dried. Although any of the so-called wet-on-dry coating methods for coating the upper layer can be used, the wet-on-wet coating method is preferably used. At the time of coating the upper layer in the wet-on-wet coating method, it is preferable that 10% or more of the organic solvent remains in the lower layer coating film. The back coat layer may be applied before or after applying the lower layer and the upper layer, or may be applied simultaneously. Further, in the present invention, in the case of a magnetic tape, it is preferable to apply a magnetic field after orienting a magnetic layer to orient the magnetic particles in the layer, and the orientation direction depends on the purpose and the running direction of the medium. On the other hand, in the longitudinal direction, in the vertical direction, or in the oblique direction, the permanent magnet such as a ferrite magnet or a rare earth magnet, an electromagnet, or the like in order to orient in a predetermined direction.
It is preferable to apply a magnetic field of 1000 G or more with a solenoid or the like, or to use a plurality of these magnetic field generating means in combination, and further to provide an appropriate drying step before orientation so that the orientation after drying becomes the highest. Orientation may be performed by, for example, drying at the same time as orientation, or in the case of a floppy disk, magnetic powder naturally oriented by coating is made as non-oriented as possible with a permanent magnet, electromagnet, solenoid, etc. May be. After being applied in this way, the paint subjected to the orientation treatment is usually dried and fixed by known drying and evaporation means such as hot air, far infrared rays, an electric heater, and a vacuum device provided inside the drying furnace. It The drying temperature may range from room temperature to about 300 ° C., and may be appropriately selected depending on the heat resistance of the non-magnetic support, the solvent species, the concentration, and the like, and a temperature gradient may be provided in the drying furnace. Further, as the gas atmosphere in the drying furnace, general air or inert gas may be used. After drying the magnetic layer in this way, if necessary, calendering is carried out as a surface smoothing treatment. As a calendering roll, heat-resistant plastic such as epoxy, polyester, nylon, polyimide, polyamide, or polyimideamide can be used. A combination of rolls (which may be kneaded with carbon, metal or other inorganic compound) and metal rolls (combination of 3 to 7 stages) is used. Further, it is also possible to perform processing by using metal rolls. The treatment temperature is preferably 90 ° C. or higher, more preferably 100 ° C. or higher. Linear pressure is preferably 200 kg / cm or more, more preferably 250 kg
/ Cm or more, and the processing speed is in the range of 20 m / min to 900 m / min. The effect of the present invention is 250 at a temperature of 100 ° C. or higher.
The effect can be further enhanced with a linear pressure of kg / cm or more.

[0024]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited to these examples. The polyester polyurethane resin used in this example has the following composition. Polyester diol (1) / Polyester diol (2) / Neopentyl glycol /
It is a reaction product of 4,4′-diphenylmethane diisocyanate, and as the polyester diol (1), terephthalic acid / isophthalic acid / 5-sodium sulfoisophthalic acid / ethylene glycol / neopentyl glycol = 49/49/2/50 / 50 molar ratio,
As the polyester diol (2), adipic acid / 5
-Sodium sulfoisophthalic acid / 1,4-butanediol / neopentyl glycol = 98/2/75/25
It is a molar ratio. Examples 1 to 17 and Comparative Examples 1 to 13 1) Preparation of coating the upper magnetic layer (1) binder solution prepared vinyl chloride resin (Nippon Zeon Co., Ltd.: MR-110) 10 parts by weight polyester polyurethane resin (-SO ₃ Na group-containing, Mn = 23000, T g = 20 ° C.) 4 parts by weight Polyester polyurethane resin (—SO ₃ Na group-containing, Mn = 80000, T g = 20 ° C.) 3 parts by weight MEK 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight The above composition was put into a hyper mixer and mixed for 6 hours.
Stir to give a binder solution. The binder solution was subjected to circulation filtration for 8 hours using a depth filter with 95% cut filtration accuracy = 5.0 μm. (2) Kneading The following composition was put into a pressure kneader and kneaded for 2 hours. α-Fe magnetic powder (Hc = 16500e, σs = 126emu / g, BET = 57 m ² / g, major axis length = 0.10 μm) 100 parts by weight α-Al ₂ O ₃ (Sumitomo Chemical Co., Ltd .: HIT-60A 5 parts by weight Cr ₂ O ₃ (manufactured by Nippon Kagaku Kogyo Co., Ltd .: U-1) 5 parts by weight Binder solution 40 parts by weight The following composition was added to the slurry after kneading to adjust the viscosity to the optimum level for dispersion treatment. Binder solution 40 parts by weight MEK 15 parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts by weight Slurry after viscosity adjustment is 95% cut Filtration accuracy = 75
Circulation filtration was performed for 4 hours using a μm depth filter. (3) Dispersion The slurry was subjected to a dispersion treatment with a sand mill. The dispersed slurry was subjected to circulation filtration for 4 hours using a depth filter with 95% cut filtration accuracy = 75 μm. (4) Preparation of viscosity adjusting liquid The following composition was put into a hyper mixer and mixed for 1 hour.
The mixture was stirred to obtain a viscosity adjusting liquid. The viscosity adjusting solution was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm. Stearic acid 0.5 parts Myristic acid 0.5 parts Butyl stearate 0.5 parts MEK 65 parts Toluene 65 parts Cyclohexanone 65 parts (5) Viscosity adjustment After mixing and stirring the above solution in the slurry Then, it was dispersed again by a sand mill to obtain a paint. The coating material was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm to adjust the viscosity to 50 cp. (The clay at a liquid temperature of 20 ° C. and a shear rate of 3000 sec to ¹ was determined by using an MR-300 solid liquid meter manufactured by Rheology Co., Ltd.) (6) Final paint 100 parts by weight of the paint after filtration was mixed with an isocyanate compound (Japan. 0.8 parts by weight of polyurethane, Coronate L) was added and stirred and mixed to obtain a final coating material for the magnetic layer.

2) Preparation of coating for lower magnetic layer (1) Preparation of binder solution Vinyl chloride resin (manufactured by Zeon Corporation: MR-110) 10 parts by weight Polyester polyurethane resin (containing —SO ₃ Na group, Mn = 23000) Tg = 20 ° C.) 4 parts by weight Polyester polyurethane resin (containing —SO ₃ Na group, Mn = 50000, T g = 20 ° C.) 3 parts by weight MEK 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight The above composition is hyper. Put in a mixer and mix for 6 hours.
Stir to give a binder solution. The binder solution was subjected to circulation filtration for 8 hours using a depth filter with 95% cut filtration accuracy = 5.0 μm. (2) Kneading The following composition was put into a pressure kneader and kneading was performed for 7 hours. Co-containing γ-Fe ₂ O ₃ (Hc = 7000e, σs = 75 emu / g, BET = 44 m ² / g, major axis length = 0.20 μm) 100 parts by weight carbon black (Colombian Carbon Co., central particle size = 21 nm, BET = 220 m ² / g) 9 parts by weight Cr ₂ O ₃ (Nippon Kagaku Kogyo Co., Ltd .: U-1) 5 parts by weight Binder solution 40 parts by weight The following composition was added to the slurry after kneading for dispersion treatment. Adjust to the optimum viscosity. Binder solution 40 parts by weight MEK 15 parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts by weight Slurry after viscosity adjustment is 95% cut Filtration accuracy = 75
Circulation filtration was performed for 4 hours using a μm depth filter. (3) Dispersion The slurry was subjected to a dispersion treatment with a sand mill. The dispersed slurry was subjected to circulation filtration for 4 hours using a depth filter with 95% cut filtration accuracy = 75 μm. (4) Preparation of viscosity adjusting liquid The following composition was put into a hyper mixer and mixed for 1 hour.
The mixture was stirred to obtain a viscosity adjusting liquid. The viscosity adjusting solution was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm. Stearic acid 0.5 parts Myristic acid 0.5 parts Butyl stearate 0.5 parts MEK 65 parts Toluene 65 parts Cyclohexanone 65 parts (5) Viscosity adjustment After mixing and stirring the above solution in the slurry Then, it was dispersed again by a sand mill to obtain a paint. The coating material was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm to adjust the viscosity to 50 cp. (6) Final paint 0.8 parts by weight of an isocyanate compound (Nippon Polyurethane, Coronate L) was added to 100 parts by weight of the paint after filtration and stirred and mixed to obtain a final paint for the magnetic layer.

3) Preparation of Coating Material for Lower Non-Magnetic Layer (1) Preparation of Binder Solution Vinyl Chloride Resin (MR-110, manufactured by Nippon Zeon Co., Ltd.) 10 parts by weight Polyester polyurethane resin (containing —SO ₃ Na group, Mn = 23000) , T g = 20 ° C.) 4 parts by weight Polyester polyurethane resin (containing —SO ₃ Na group, Mn = 50000, T g = 20 ° C.) 3 parts by weight MEK 21 parts by weight Toluene 21 parts by weight Cyclohexanone 21 parts by weight Put in a hyper mixer and mix for 6 hours.
Stir to give a binder solution. The binder solution was subjected to circulation filtration for 8 hours using a depth filter with 95% cut filtration accuracy = 5.0 μm. (2) Kneading The following composition was put into a pressure kneader and kneading was performed for 7 hours. Needle-shaped α-Fe ₂ O ₃ (major axis length = 0.15 μm, axial ratio = 6) 100 parts by weight Carbon black (Colombian Carbon Co., central particle size = 21 nm, BET = 220 m ² / g) 9 parts by weight Binder solution 40 parts by weight Add the following composition to the slurry after kneading to adjust the viscosity to the optimum level for dispersion treatment. Binder solution 40 parts by weight MEK 15 parts by weight Toluene 15 parts by weight Cyclohexanone 15 parts by weight Slurry after viscosity adjustment is 95% cut Filtration accuracy = 75
Circulation filtration was performed for 4 hours using a μm depth filter. (3) Dispersion The slurry was subjected to a dispersion treatment with a sand mill. The dispersed slurry was subjected to circulation filtration for 4 hours using a depth filter with 95% cut filtration accuracy = 75 μm. (4) Preparation of viscosity adjusting liquid The following composition was put into a hyper mixer and mixed for 1 hour.
The mixture was stirred to obtain a viscosity adjusting liquid. The viscosity adjusting solution was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm. Stearic acid 0.5 parts Myristic acid 0.5 parts Butyl stearate 0.5 parts MEK 65 parts Toluene 65 parts Cyclohexanone 65 parts (5) Viscosity adjustment After mixing and stirring the above solution in the slurry Then, it was dispersed again by a sand mill to obtain a paint. The coating material was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm to adjust the viscosity to 50 cp. (6) Final paint 0.8 parts by weight of an isocyanate compound (Coronate L, manufactured by Nippon Polyurethane Co., Ltd.) was added to 100 parts by weight of the filtered paint, and the mixture was stirred and mixed to obtain a final paint for the non-magnetic layer.

4) Preparation of coating for back coat layer (1) Preparation of binder solution Vinyl chloride-vinyl acetate-vinyl alcohol copolymer (monomer weight ratio = 92: 3: 5, average degree of polymerization = 420) 40 parts by weight Chloride Vinyl-vinyl acetate-vinyl alcohol copolymer (monomer weight ratio = 91: 2: 7, nitrogen atom = 390 ppm, average degree of polymerization = 340) 25 parts by weight Polyurethane resin (containing —SO ₃ Na group, Mn = 40000, Tg) = 20 ° C.) 35 parts by weight MEK 260 parts by weight Toluene 260 parts by weight Cyclohexanone 260 parts by weight The above composition is charged into a hyper mixer and mixed for 6 hours.
Stir to give a binder solution. The binder solution was subjected to circulation filtration for 8 hours using a depth filter with 95% cut filtration accuracy = 5.0 μm. (2) Dispersion The following composition was placed in a ball mill and dispersed for 24 hours. Carbon black (manufactured by Colombian Carbon Co., central particle size = 21 nm, BET = 220 m ² / g) 80 parts by weight Carbon black (manufactured by Colombian Carbon Co., central particle size = 350 nm, BET = 8 m ² / g) 1 part by weight Part α-Fe ₂ O ₃ (average particle size = 0.1 μm) 1 part by weight Binder solution 880 parts by weight Slurry after dispersion is 95% cut Filtration accuracy = 75 μm
Circulation filtration was performed for 4 hours using the depth filter of. (3) Preparation of viscosity adjusting liquid The following composition was put into a hyper mixer and mixed for 1 hour.
The mixture was stirred to obtain a viscosity adjusting liquid. The viscosity adjusting solution was circulated and filtered for 8 hours using a depth filter having a 95% cut filtration accuracy of 1.2 μm. Stearic acid 1 part by weight Myristic acid 1 part by weight Butyl stearate 2 parts by weight MEK 210 parts by weight Toluene 210 parts by weight Cyclohexanone 210 parts by weight (4) Viscosity adjustment After mixing and stirring the above solution in the dispersion slurry, it is dispersed again in a ball mill. The treatment was carried out for 3 hours. 9 of the above paint
Circulation filtration was performed for 8 hours using a depth filter with 5% cut filtration accuracy = 1.2 μm, and the viscosity was adjusted to 50 cp. (5) Final paint 1 part by weight of an isocyanate compound (Coronate L, made by Nippon Polyurethane) was added to 100 parts by weight of the paint after filtration, and the mixture was stirred and mixed to obtain a back coat paint.

5) Preparation of magnetic tape When a lower layer coating material is applied to the surface of a non-magnetic support (a polyethylene terephthalate film having a thickness of 8.3 μm; 8WQ05P manufactured by Toray Co., Ltd.) and the coating film of the lower layer coating material is in a wet state. Apply the upper layer paint to, while both layers are in a wet state,
After orientation treatment and drying of both coating films, treatment was performed at a temperature of 100 ° C. and a linear pressure of 250 kg / cm with a seven-stage calender. The thicknesses of the upper and lower layers after calendering are shown in each table. Further, the back surface of the non-magnetic support was coated with the back coat layer coating material. After drying, use a 7-step calender at 1
The treatment was performed at 00 ° C. and a linear pressure of 250 kg / cm. The thickness of the back coat layer after calendering was 0.
5 μm. 2 each coating in a heating oven at 60 ℃
After curing for 4 hours, it was cut into a width of 8 mm and incorporated in a cassette to prepare a magnetic tape sample. The measuring method of each characteristic of the magnetic tape is as follows. <Electromagnetic conversion characteristics> ・ 7 MHz output 7 MHz when a 7 MHz signal was recorded using a Sony EV-S900 Hi8 video deck and this signal was reproduced.
The MHz signal reproduction output was measured with an oscilloscope. It was measured as a relative value when the 7 MHz output of Comparative Example 10 was set to 0 dB.

・ Y-C / N 6 MHz when a 7 MHz signal is recorded using a Sony EV-S900 Hi8 VCR and this signal is reproduced.
The noise generated at MHz was measured with a spectrum analyzer, and the ratio of the reproduced signal to this noise was measured.
It was measured as a relative value when Y-C / N of Comparative Example 10 was set to 0 dB.

<Still endurance> Each tape sample was reproduced in still mode using a Sony EV-S900 Hi8 video deck, and the reproduction output at 7 MHz was 6 dB.
The time to decrease was measured.

<Durability Running> VTR 100 times reciprocating running at 20 ° C. and 60% RH environment is performed on 50 tape samples, and running troubles such as running stop and head clogging are judged based on the following criteria. did. Deck used: Sony EV-S900 (Hi8 format V
TR) ◎: Trouble occurrence 0 rolls ○: Trouble occurrence 1 roll ×: Trouble occurrence 2 rolls or more <Surface roughness> Taylor Hobson's stylus type surface profile measuring instrument, TALYSTEP system is used, JIS B
The measured value is obtained by the method described in -0601.
The measurement conditions are: filter condition: 0.18-9 Hz, needle pressure: 2 mg, needle used: 0.1 × 2.5 μm special stylus, scan speed: 0.03 mm / sec. Scan length: 500 μm. Ra was determined from the obtained results.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

INDUSTRIAL APPLICABILITY The present invention has a two-layer coating film on the upper surface side of a non-magnetic support, and in a magnetic recording medium in which at least the upper layer is a magnetic layer, the thickness of the coating film of the upper magnetic layer is 0.1
.About.0.8 μm, a polyurethane resin having a number average molecular weight Mn of 5000 ≦ Mn ≦ 25000 (Mn1) and a number average molecular weight Mn of 25000 <Mn ≦
By using both the 100,000 (Mn2) polyurethane resin as an essential component, it is possible to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics and excellent still durability and running durability.

Claims (6)

[Claims] 1. A magnetic recording medium having a two-layer coating film on the upper surface side of a non-magnetic support, at least the upper layer of which is a magnetic layer, and the thickness of the coating film of the upper magnetic layer is 0. 1 to 0.
It is 8 μm, and each of the coating films must have both a number average molecular weight Mn of 5,000 ≦ Mn ≦ 25,000 (Mn1) and a polyurethane resin of 25,000 <Mn ≦ 100,000 (Mn2) as a binder. A magnetic recording medium characterized by containing as a component. 2. The Mn1 and M of the polyurethane resin
The magnetic recording medium according to claim 1, wherein n2 satisfies Mn2-Mn1≥3,000. 3. The center line average roughness (Ra) of the magnetic layer surface of the magnetic recording medium is 1.0 nm ≦ Ra ≦ 10.0 nm.
The magnetic recording medium according to claim 1, wherein 4. The magnetic recording medium according to claim 1, wherein the lower coating film among the two coating films on the non-magnetic support is a non-magnetic layer containing a non-magnetic inorganic powder. 5. The magnetic recording medium according to claim 1, wherein all of the two coating films on the non-magnetic support are magnetic layers. 6. The magnetic recording medium according to claim 1, further comprising a back coat layer on the lower surface side of the non-magnetic support.