JP2001176051A - Magnetic recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic recording medium excellent in dispensability, coated film flatness, electromagnetic conversion characteristics and long term storage properties, having reduced corrosion of a MR head and hardly exerting influence on the environment at the time of incineration of the medium. SOLUTION: The magnetic recording medium having at least one magnetic layer formed by dispersing a ferromagnetic powder and a bonding agent on a lower layer formed by dispersing non-magnetic powder or ferromagnetic powder and a bonding agent on a substrate is characterized in that the bonding agent contains a polyurethane resin containing a short chain diol having a cyclic structure and a molecular weight of below 500 and a long chain polyether diol having a molecular weight of 500-5,000 as diol components and the chlorine content in the magnetic recording medium is <=35 mg per m2 of the medium.

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 having a support on which a magnetic layer in which a ferromagnetic powder and a binder are dispersed is provided.

[0002]

2. Description of the Related Art Magnetic recording media are widely used as recording tapes, video tapes, floppy disks and the like. In a magnetic recording medium, a magnetic layer in which ferromagnetic powder is dispersed in a binder is laminated on a support.

[0003] A magnetic recording medium is required to be at a high level in various characteristics such as electromagnetic conversion characteristics, running durability and running performance. That is, audio tapes for recording and reproducing music are required to have higher original sound reproduction capabilities. Also, video tapes are required to have excellent electromagnetic conversion characteristics such as excellent original image reproduction capability. At the same time as having such excellent electromagnetic conversion characteristics, the magnetic recording medium is required to have good running durability as described above. As one of approaches for obtaining good running durability, a method of improving the dispersibility of a binder has been adopted.

[0004] Japanese Patent Application Laid-Open No. 8-293115 (hereinafter, referred to as
"Document A") includes polyether polyol 10
-50 wt%, short-chain diol having a cyclic structure 17-4
Japanese Patent Application Laid-Open No. 9-6 / 2004 discloses a polyurethane comprising 0 wt%.
No. 9222 (hereinafter referred to as “Document B”) discloses polyurethane of 10 to 50 wt% of a polyether polyol, 15 to 40 wt% of a short-chain diol having a cyclic structure, and a long-chain diol containing a polar group. JP-A-313481 (hereinafter referred to as "Document C") discloses a binder composition containing the urethane and an amino group-containing vinyl chloride resin described in Document A. The magnetic recording media using the binders described in Documents A and B have improved dispersion stability of powder, improved output, prevention of head contamination, improved still durability, and dropout (DO) (60 ° C90). % RH for 1 week). The magnetic recording medium using the binder described in Document C improves powder dispersibility and adsorbability,
Improvement of surface properties by surface binder, improvement of durability (output decrease, clogging), high-humidity storage (60 ° C 90% R)
H1 week μ value). The above-mentioned documents A, B and C are by the present applicant and are characterized by using a high-strength, highly-dispersible polyurethane as a binder. In addition, a vinyl chloride resin containing a polar group must be used in combination, and there have been problems such as head corrosion due to the generation of hydrochloric acid and insufficient environmental protection when discarding the magnetic recording medium. On the other hand, as a binder suitable for such environmental protection,
For example, JP-A-5-307734 (hereinafter referred to as "Document D") discloses a binder composed of only two or more polyester urethanes having different Tg.
Japanese Patent No. 539 (hereinafter referred to as "Document E") discloses a binder in which only polyester urethane is used as a binder, low molecular components are removed, and Tg is 60 to 80C. However, although Documents D and E can provide magnetic recording media useful for environmental protection, the polyurethane resins disclosed by them do not have sufficient dispersibility, smoothness, electromagnetic conversion characteristics, and storage stability. I couldn't say it. By the way, when used for a data medium requiring a large capacity, high speed and high reliability in recent years, an extremely small amount of hydrochloric acid gas generated from the magnetic recording medium adversely affects not only the storage stability of the tape but also the corrosion of the head. It turns out there is a possibility. MR (Magnet Resistance) used especially in data recording systems for computers
The head uses a metal thin film, and there is a concern that the characteristics may be degraded due to corrosion. In addition, when the tape is stored for a long period of time particularly under high temperature and high humidity, deterioration of the material in the tape due to hydrochloric acid gas, for example, fatty acids generated by hydrolysis of an ester lubricant may migrate and precipitate on the surface of the magnetic layer and crystallize. In addition, since the recording density has been improved as compared with the related art, there has been a problem that the influence of even finer foreign substances has been increasing. That is, a magnetic recording medium having excellent electromagnetic conversion characteristics, storage properties and environmental preservation properties has been desired, but has not been found yet.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide the following magnetic recording medium. Provided is a magnetic recording medium having excellent dispersibility, coating film smoothness, and electromagnetic conversion characteristics. Provided is a magnetic recording medium having excellent long-term storage properties. Provided is a magnetic recording medium with less MR head corrosion. Provided is a magnetic recording medium that has little effect on the environment when incinerated.

[0006]

According to the present invention, there is provided a magnetic layer comprising at least one ferromagnetic powder and a binder dispersed above a lower layer comprising a support and an inorganic powder or a ferromagnetic powder dispersed therein. Wherein the binder comprises a polyurethane resin containing a short-chain diol having a cyclic structure having a molecular structure of less than 500 and a long-chain polyether diol having a molecular weight of 500 to 5,000 as a diol component, and chlorine in the magnetic recording medium. A magnetic recording medium having a content of 35 mg or less per 1 m ^{2 of} the medium.

The preferred embodiments of the present invention are as follows. (1) A magnetic recording medium, wherein the binder is a composition containing a resin having a mass ratio of the polyurethane resin to the vinyl chloride resin containing a polar group in the range of 85/15 to 100/0. (2) The magnetic recording medium, wherein the ferromagnetic powder is a ferromagnetic metal powder containing Y. (3) The magnetic recording medium, wherein the magnetic layer and / or the lower layer contains an ester-based lubricant.

The present invention can satisfy these qualities at an extremely high level with the current state of the art magnetic recording media for high recording density. The present invention is a magnetic recording medium that has a low risk of generating harmful substances such as dioxins and has a small effect on the environment because not only the quality but also the chlorine content is low or does not contain chlorine even when incinerated for disposal. Can be provided.

[0009]

DETAILED DESCRIPTION OF THE INVENTION The binder used in the present invention is a polyurethane resin having a specific structure (hereinafter, also referred to as "polyurethane resin of the present invention"), that is, a short-chain diol having a cyclic structure and a molecular weight of less than 500 as a diol component. And molecular weight 500 ~ 50
It contains at least a polyurethane resin containing a 00 long-chain polyether diol.

The short-chain diol having a cyclic structure and having a molecular weight of less than 500 (hereinafter also referred to simply as “short-chain diol”) includes aromatic and alicyclic diols, and ethylene oxide or propylene oxide adducts thereof. Those that can be selected from are preferred.

The aromatic ring constituting the aromatic includes a heterocyclic ring having 6 to 12 carbon atoms, and the hetero atom includes nitrogen, oxygen, sulfur and the like. The alicyclic ring constituting the alicyclic group includes one having 6 to 12 carbon atoms, and may have nitrogen, oxygen, sulfur, or the like as a hetero atom. As the short-chain diol, bisphenol A, hydrogenated bisphenol A, bisphenol S,
Examples thereof include hydrogenated bisphenol S, bisphenol P, hydrogenated bisphenol P, cyclohexanedimethanol, cyclohexanediol, and hydroquinone. Among these, preferred are bisphenol A,
Examples include hydrogenated bisphenol A and their ethylene oxide adducts and propylene oxide adducts. More preferred is hydrogenated bisphenol A. The content of the short-chain diol in the polyurethane resin is preferably 15 to 40% by mass, and more preferably 20 to 35% by mass.
% By mass. If the amount is less than 15% by mass, the resulting coating film will be soft, and sufficient strength will not be obtained, and still durability tends to decrease. On the other hand, if it exceeds 40% by mass, the solubility in a solvent decreases, the dispersibility of the ferromagnetic powder decreases, and it tends to be difficult to obtain a magnetic recording medium having excellent electromagnetic conversion characteristics.

As the long-chain polyether diol having a molecular weight of 500 to 5,000 (hereinafter also simply referred to as "long-chain diol"), polyalkylene glycol, cyclic diol, or an oxide adduct thereof is preferable. As the polyalkylene glycol, an alkylene group having 1 to 4 carbon atoms
And specifically, polypropylene glycol, polyethylene glycol, polytetramethylene glycol and the like. Examples of the basic structure of the cyclic diol include the same aromatic and / or alicyclic cyclic diols as those used in the above short-chain diol.

As the long-chain diol, preferred are propylene oxide adduct of bisphenol A, ethylene oxide adduct of bisphenol A, ethylene oxide adduct of hydrogenated bisphenol A, and propylene oxide adduct of hydrogenated bisphenol A. The molecular weight of the long-chain diol is preferably 600-3000.
When the molecular weight exceeds 5,000, the Tg of the coating film decreases, the strength also decreases, and the durability decreases.

The ether content of the polyurethane resin of the present invention is preferably 1 to 5 mmol / g. Particularly preferably, it is 2 to 4 mmol / g. When the content of the ether group is less than 1 mmol / g, the adsorptivity to the magnetic substance is reduced, and the dispersibility tends to be reduced.
If it exceeds 5 mmol / g, the solubility in the solvent tends to decrease, and the dispersibility tends to decrease. Since the polyurethane resin of the present invention has a cyclic structure, the strength of the coating film is high, the durability is excellent, the solubility in a solvent is high, and the dispersibility is excellent.

The polyurethane resin of the present invention preferably has a polar group. As the polar group, -SO ₃ M, -
_{OSO 3 M, -COOM, -P =} O (OM) 2, -O-P
ＯO (OM) ₂ , —NR ₂ , —N ⁺ R ₂ R′COO ⁻ (where M is a hydrogen atom, an alkali metal, ammonium, R,
R ′ represents an alkyl group having 1 to 12 carbon atoms). The polar group may be polymerized using a monomer introduced into a short-chain diol and / or a long-chain diol, or another polyol component to form the polyurethane resin of the present invention. Alternatively, the polar group may be introduced into the resulting resin. Is also good.

As the polyol component which can be used in combination, polyester polyol, polyether polyol, polyether ester polyol, polycarbonate polyol and the like are used. As the monomer having a polar group, specifically, 5-sodium sulfoisophthalic acid,
5-potassium isophthalic acid, sodium sulfoterephthalic acid, potassium terephthalic acid, 2-sodium sulfo-
1,4-butanediol, 2-potassium sulfo-1,4
A polar group-containing polyester polyol obtained by dehydration condensation with butanediol, sodium bis (2-hydroxyethyl) phosphinate, dimethylolpropionic acid, sodium dimethylolpropionate, sodium sulfosuccinic acid, or other glycols and dicarboxylic acids A polar group-containing polyester polyol obtained by ring-opening polymerization of a lactone such as ε-caprolactone using the polar group-containing diol as an initiator, or a polar group-containing poly obtained by adding an alkylene oxide such as ethylene oxide or propylene oxide to a polar group-containing diol. And ether diols.

Glycols used in the polyester polyol include ethylene glycol, 1,3-propanediol, propylene glycol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, There are aliphatic glycols such as 1,8-octanediol, 1,9-nonanediol, diethylene glycol and dipropylene glycol, and alicyclic glycols such as cyclohexanediol and cyclohexanedimethanol. As other dicarboxylic acids, aliphatic or aromatic dicarboxylic acids such as succinic acid, adipic acid, sebacic acid, azelaic acid, phthalic acid, isophthalic acid, terephthalic acid and naphthalenedicarboxylic acid are used.

Preferred are, for example, polyester polyols obtained by dehydrating and condensing 5-sodium sulfoisophthalic acid / isophthalic acid / neopentyl glycol.
Polyester polyol obtained by ring-opening polymerization of ε-caprolactone using ethylene oxide adduct of 5-potassium sulfoisophthalic acid as an initiator, 2-sodium sulfo-1,
Examples thereof include polyether polyol obtained by adding propylene oxide to 4-butanediol. The molecular weight is preferably 500-5000, more preferably 600-2.
500. In the polyurethane resin of the present invention into which a polar group has been introduced using this polyol, the polar group is not concentrated in the polyurethane molecular chain but exists uniformly, so that the dispersibility and the dispersion stability are improved. If the molecular weight is too large, the amount of polar groups that can be introduced into the polyurethane resin of the present invention tends to decrease, and the dispersibility tends to decrease. The amount of the polar group in the polyurethane resin of the present invention is from 1 × 10 ⁻⁵ eq / g to 2 × 10 ⁻⁴ e.
q / g is preferred. When the number increases, the viscosity of the solution tends to increase due to association between polar groups, and the dispersibility tends to decrease.

The glass transition temperature Tg of the polyurethane resin of the present invention is preferably 45 to 120 ° C., and more preferably 50 to 90 ° C. Those having a Tg of less than 45 ° C have a low coating strength and a reduced running durability.
If the ratio exceeds the above, calender moldability decreases, and electromagnetic conversion characteristics decrease. Further, the mass average molecular weight of the polyurethane resin of the present invention is usually 10,000 to 200,000, preferably 10,000 to 100,000,
000 to 800,000, more preferably 30,000
0 to 70,000 is particularly preferred. If the weight average molecular weight is less than 10,000, the strength of the coating film tends to decrease, and the running durability tends to decrease. When the weight average molecular weight exceeds 200,000, the solubility in a solvent tends to decrease, and the dispersibility tends to decrease.

Further, as the short-chain diol used in the polyurethane resin of the present invention, other diols other than those described above can be used in combination. Specifically, ethylene glycol, 1,3
-Propylenediol, 1,2-propyleneglycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-dimethylpropanediol, 1,8-octanediol, 1,9 -Aliphatic diols such as nonanediol and diethylene glycol.

Examples of the organic diisocyanate compound contained as a raw material of the polyurethane resin of the present invention include 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, xylene-1,4-diisocyanate, xylene-1 , 3-diisocyanate, 4,
4'-diphenylmethane diisocyanate, 4,4-diphenyl ether diisocyanate, 2-nitrodiphenyl-4,4'-diisocyanate, 2,2'-diphenylpropane-4,4'-diisocyanate, 4,4 '
Aromatic diisocyanates such as -diphenylpropane diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, and 3,3'-dimethoxydiphenyl-4,4'-diisocyanate And aliphatic diisocyanates such as tetramethylene diisocyanate, hexamethylene diisocyanate, and lysine diisocyanate, and alicyclic diisocyanates such as isophorone diisocyanate, hydrogenated tolylene diisocyanate, and hydrogenated diphenylmethane diisocyanate. The diisocyanate compound contained in the polyurethane is preferably contained in the binder in the range of 10 to 50% by mass, and more preferably in the range of 20 to 40% by mass.

The polyurethane resin of the present invention contains at least a binder component in the lower layer and the magnetic layer. The binder used for the lower layer and the magnetic layer may be the polyurethane resin of the present invention alone, or may be used in combination with another resin, and is usually a composition containing a curing agent such as polyisocyanate. .

Examples of the resin used in combination include vinyl chloride resins. The polymerization degree of vinyl chloride resin is 100
To 500 are preferable, and 150 to 400 are more preferable.
200 to 300 are particularly preferred. The vinyl chloride resin may be a copolymer of a vinyl monomer, for example, vinyl acetate, vinyl alcohol, vinylidene chloride, acrylonitrile and the like.

Among them, a copolymer containing vinyl chloride and vinyl acetate is preferable as the vinyl chloride resin. When vinyl acetate is contained in the copolymer preferably in an amount of 1 to 15% by mass, the compatibility with the polyurethane resin of the present invention is high,
In addition, the viscosity of the coating solution at a high shear rate is reduced, and an extremely smooth magnetic layer can be obtained.

The vinyl chloride resin preferably has the same polar group as the polyurethane resin of the present invention, and the content of the polar group is 1 × 10 ⁻⁵ to 1 × 10 ⁻³ eq / g. preferable. If it exceeds this range, the viscosity tends to be high and the dispersibility tends to decrease. At least the dispersibility tends to be lower than this range. Further, the vinyl chloride resin preferably has an epoxy group, and the amount of the epoxy group contained is preferably 1 × 10 ⁻⁴ to 1 × 10 ⁻² eq / g, more preferably 5 × 10 ⁻² eq / g. ^{−4 to} 2 × 10 ⁻³ eq / g.

Further, the vinyl chloride resin preferably has an OH group. It is preferable to introduce an OH group into the vinyl chloride resin because it reacts with the isocyanate curing agent to form a crosslinked structure and improves the mechanical strength. As a method of introducing an OH group, a method in which the OH group is bonded via a hydrocarbon chain or a polyalkylene glycol chain from the main chain is more preferable than a method in which the OH group is directly bonded to the polymer main chain, as in vinyl alcohol. The OH group is preferably secondary or primary. Introduction of an OH group into a vinyl chloride-based resin is performed by adding a vinyl monomer such as 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, or 4-hydroxybutyl allyl ether. It can be carried out by copolymerization.

The OH group content of the vinyl chloride resin is preferably 1 × 10 ^{−4 to} 5 × 10 ⁻³ eq / g, more preferably 2 × 10 ⁻⁴ to 2 × 10 ⁻³ eq / g. . The vinyl chloride resin can usually contain another copolymerizable monomer in the range of 0 to 15% by mass. Examples of such copolymerizable monomers include alkyl (meth) acrylate, vinyl carboxylate, allyl ether, styrene, glycidyl (meth) acrylate, 3-hydroxypropyl methacrylate, 2-hydroxypropyl methacrylate, Hydroxypropyl allyl ether and other vinyl monomers.

In the present invention, other binder components used in combination include cellulose derivatives such as nitrocellulose resins, acrylic resins, polyvinyl acetal resins, and the like.
Examples thereof include a polyvinyl butyral resin, an epoxy resin, and a phenoxy resin, and these can be used alone or in combination. When another synthetic resin is used in combination, its amount must be adjusted so that the chlorine content falls within the range of the present invention. Usually, the polyurethane resin of the present invention contains 50 to 100 mass% in the binder. %, More preferably 70 to 100% by mass. Particularly preferred is an amount of 80 to 100% by mass. If it is less than 50% by mass, the dispersibility will be reduced.

The resin used in combination preferably has a polar group, and the polar group and the amount used are the same as those of the polyurethane resin of the present invention unless otherwise mentioned. When used in combination with a vinyl chloride resin, the mass ratio of the polyurethane resin of the present invention to the polar group-containing vinyl chloride resin as the binder is preferably 85 / 15-10.
0/0, more preferably 90/10 to 100/0, particularly preferably a composition containing a resin in the range of 95/5 to 100/0.

Examples of the curing agent serving as a binder component include a polyisocyanate curing agent and an epoxy curing agent, and a polyisocyanate curing agent is preferred. Examples of the polyisocyanate curing agent include the organic diisocyanate compound as a constituent of the polyurethane resin of the present invention, a reaction product of the diisocyanate with a polyhydric alcohol such as trimethylolpropane or glycerin, for example, 3 mol of tolylene diisocyanate and trimethylol Examples include a reaction product of 1 mol of propane, xylylene diisocyanate, or a reaction product of 3 mol of hexamethylene diisocyanate and 1 mol of trimethylolpropane. Further, as the isocyanurate type polyisocyanate obtained by polymerizing a diisocyanate compound, there are trimer, isomer, isomer such as tolylene diisocyanate and hexamethylene diisocyanate. Further, there is a polymeric MDI which is a multimer of MDI (4,4-diphenylmethane didiisocyanate). When performing a curing treatment by electron beam irradiation,
A compound having a reactive double bond, such as urethane acrylate, can be used. The mass of the sum of the resin component and the curing agent (that is, the binder) is 100 masses of the ferromagnetic powder.
Usually, it is preferably in the range of 15 to 40 parts by mass, more preferably 20 to 30 parts by mass with respect to parts by mass.

According to the present invention, the chlorine content in a magnetic recording medium is 35 mg or less, preferably 20 mg / m ^{2 of} the medium.
Below, more preferably 10 mg or less. Here, the chlorine content refers to the F content obtained by subjecting a magnetic recording medium to X-ray fluorescence analysis.
e means a value obtained by quantifying the amount of Cl from the peak area of the Cl element and converting the amount to the Cl mass per 1 m ^{2 of} the magnetic recording medium.
Therefore, the form, location, and the like of chlorine in the magnetic recording medium are arbitrary, but are generally considered to be mostly derived from the binder. Also, per 1 m ^{2 of} magnetic recording medium means
Naturally, it includes the entire thickness direction, that is, the upper magnetic layer, the lower layer, the back layer, the support, and the like.

The ferromagnetic powder used in the magnetic recording medium of the present invention is a ferromagnetic iron oxide, cobalt-containing ferromagnetic iron oxide or ferromagnetic alloy powder having a specific surface area (S _BET ) of usually 40 to 80 m by the BET method. ² / g, preferably 50-70 m ² /
g. The crystallite size is usually 12 to 25 nm, preferably 13 to 22 nm, particularly preferably 14 to 2 nm.
0 nm.

As the ferromagnetic metal powder, Fe, Ni, Fe
-Co, Fe-Ni, Co-Ni, Co-Ni-Fe and the like. In the present invention, those mainly composed of Fe used for a high recording density medium are preferable. 50 atom% or more of the powder, preferably 55 to
90 at%, and elements that can be used in combination are Y, C
o and the like are preferred. In the present invention, it is particularly preferable because the saturation magnetization σs can be increased and a dense and thin oxide film can be formed. The content of Co is 5 to 50 with respect to Fe atoms.
Atomic% is preferable, and more preferably 10 to 45 atomic%.
The most preferable is 15 to 40%. Also, Ni
Is usually used in an amount of 0 to 10 atomic% and has a reduction promoting effect. It is known that Co and Ni are partially doped in the raw material, then the required amount is deposited on the surface, added to the raw material, and alloyed by reduction. The ferromagnetic metal powder usable in the present invention generally contains Al and S in a total amount of 20 atomic% or less, preferably 7 to 20 atomic% with respect to Fe atoms.
i, S, Ti, V, Cr, Cu, Y, Sn, Sb, T
e, Ba, Sr, W, La, Ce, Pr, Nd, P, M
It may contain atoms such as n, Zn, B, Ca, and Mg.

The rare earth element (including Y) is usually 2 to 10 at%, preferably 4 to 9 at% based on Fe. The content of Y is preferably in the range of 40 to 100 atomic%, more preferably 55 to 100 atomic%, based on the total of the rare earth elements. Al is usually 5 to 20 with respect to Fe.
Atomic%, preferably 6 to 15 atomic%, and it is also useful to use Al in combination with a rare earth element (including Y). These elements are effective not only for controlling the shape of the starting material but also for preventing sintering between particles and promoting reduction, and controlling the shape of the reduced ferromagnetic metal powder and the unevenness of the particle surface. In particular, Al is preferably contained at least as a sintering inhibitor.

The method for producing these ferromagnetic powders is already known, and the ferromagnetic powder used in the present invention can be produced according to a known method. In the shape of ferromagnetic powder,
Needle-like, granular, dice-like, rice-granular, and plate-like ones are used. It is particularly preferable to use acicular ferromagnetic powder.

As is well known, an oxide film is formed on the surface of the ferromagnetic metal powder by gradual oxidation to stabilize it chemically. The ferromagnetic metal powder may include a small amount of hydroxide or oxide. If carbon dioxide gas is contained in the gas used at the time of slow oxidation, it will be adsorbed at basic points on the surface of the ferromagnetic metal powder, and thus such carbon dioxide gas may be contained.

In order to reduce the surface roughness of the magnetic recording medium, the average major axis length of the ferromagnetic metal powder is usually 0.04 to 0.04.
0.15 μm, more preferably 0.05 to 0.12 μm
m, the average acicular ratio is usually 4 to 10, and preferably 4 to 8.

When observing the crystals in the magnetic metal powder particles,
When the ratio of the particles formed of a single crystal to the total particles is defined as the crystallinity, the crystallinity is preferably 30 to 100%, more preferably 35 to 100%. Saturation magnetization σs of the ferromagnetic metal powder of the present invention is preferably not less than 100A · m ² / kg, more preferably from 110~160A · m ² / kg. The coercive force Hc of the ferromagnetic metal powder is 1800 to 300
0 Oersted (1.43 to 2.39) × 10 ⁵ A /
m} is preferred, and more preferably 1900 to 2800 Oersted {(1.51 to 2.23) × 10 ⁵ A / m}.
It is. Further, the ferromagnetic metal powder, a dispersant described below,
Before the dispersion, a treatment with a lubricant, a surfactant, an antistatic agent or the like can be performed. Specifically,
No. -14090, Japanese Patent Publication No. 45-18372,
JP-B-47-22062, JP-B-47-2251
No. 3, JP-B-46-28466, JP-B-46-28466
JP-B-38-755, JP-B-47-4286, JP-B-47-12422, and JP-B-47-17284.
JP, JP-B-47-18509, JP-B-47-18509
No. 18573, JP-B-39-10307, JP-B-48-39639, U.S. Pat.
No. 3031341, No. 3100194, No. 32
No. 42005, No. 3389014 and the like.

It is desirable that the water content of the ferromagnetic metal powder be 0.01 to 2% by mass. It is desirable to optimize the water content of the ferromagnetic metal powder depending on the type of the binder described below.
The ferromagnetic metal powder has a tap density of 0.2 to 0.8 g / c.
c is desirable. If it exceeds 0.8 g / cc, it is difficult to handle the ferromagnetic metal powder safely because the ferromagnetic metal powder is not gradually oxidized when gradually oxidized. Tend to decrease. If it is less than 0.2 cc / g, the dispersion tends to be insufficient.

The above-mentioned resin component, curing agent and ferromagnetic powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone and ethyl acetate which are usually used in the preparation of a magnetic paint to obtain a magnetic paint. The kneading and dispersion can be performed according to a usual method. In addition, in the magnetic paint, α-Al ₂ O ₃ , Cr ₂ O ₃
And other commonly used additives or fillers such as abrasives such as carbon black, antistatic agents such as carbon black, fatty acids, fatty acid esters, silicone oils and other lubricants, and dispersants.

Since the magnetic recording medium of the present invention has an extremely low chlorine content, the amount of generated hydrochloric acid is extremely low, and a lubricant decomposed by hydrochloric acid, especially an ester-based lubricant, can be effectively used for the upper magnetic layer and / or the lubricant. It can be contained in the lower layer and is effective in improving running durability such as reduction of dropout. Lubricants which can be preferably used in the present invention include dialkoxypolysiloxane (alkoxy has 1 to 4 carbon atoms), monoalkylmonoalkoxypolysiloxane (alkyl has 1 to 5 carbon atoms, and alkoxy has 1 carbon atom. Silicon oils such as ４4);
Fatty acid esters, alkyl phosphates, saturated fatty acids having 10 to 22 carbon atoms, unsaturated fatty acids and fatty acid amides comprising zero monobasic fatty acids and monohydric or polyhydric alcohols having 3 to 12 carbon atoms And the like.

Of the above, fatty acid esters are preferred.
Ethanol, butanol, phenol, benzyl alcohol,
2-methylbutyl alcohol, 2-hexyldecyl alcohol, propylene glycol monobutyl ether, ethylene glycol monobutyl ether, dipropylene glycol monobutyl ether, diethylene glycol monobutyl ether, mono-alcohols such as sec-butyl alcohol, ethylene glycol, diethylene glycol, neopentyl glycol And polyhydric alcohols such as glycerin and sorbitan derivatives. Acetic acid, propionic acid,
Aliphatic carboxylic acids such as octanoic acid, 2-ethylhexanoic acid, lauric acid, myristic acid, stearic acid, palmitic acid, behenic acid, arachinic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, palmitoleic acid and the like; Mixtures are mentioned. Specific examples of the fatty acid ester include butyl stearate, sec-butyl stearate, isopropyl stearate, butyl oleate, amyl stearate, 3-methylbutyl stearate, 2-ethylhexyl stearate, 2-hexyl decyl stearate, Butyl palmitate, 2-ethylhexyl myristate, a mixture of butyl stearate and butyl palmitate, butoxyethyl stearate,
Various esters such as butoxy-1-propyl stearate, dipropylene glycol monobutyl ether esterified with stearic acid, diethylene glycol dipalmitate, hexamethylenediol acylated with myristic acid to form a diol, glycerin oleate, etc. Compounds can be mentioned.

The lubricant composition used in the present invention is more preferably a combination of a fatty acid ester and a fatty acid.
As the fatty acid, for example, a saturated fatty acid (having 10 to 22 carbon atoms) which is solid at normal temperature can be mentioned.

Further, in order to further reduce the hydrolysis of fatty acid esters that often occur when the magnetic recording medium is used under high humidity, the raw material fatty acid and alcohol are branched / removed.
It is possible to select an isomer structure such as linear or cis / trans, and a branch position. These lubricants are added in the range of 0.2 to 20 parts by mass based on 100 parts by mass of the binder. In particular, the fatty acid is usually used in an amount of 0.1 to 2.0 parts by mass based on 100 parts by mass of the ferromagnetic powder (the upper magnetic layer or the lower magnetic layer) or the nonmagnetic powder (for the main powder of the lower nonmagnetic layer). Parts, preferably 0.3 to 1.5 parts by mass, and the fatty acid ester is a ferromagnetic powder (upper magnetic layer or lower magnetic layer) or a non-magnetic powder (for a powder that is a main component of the lower non-magnetic layer) The amount is usually 0.5 to 3.0 parts by mass, preferably 0.7 to 2.5 parts by mass, per 100 parts by mass.

Next, the layer structure of the magnetic recording medium of the present invention will be described.
Will be explained. In the present invention, a structure in which a magnetic layer is provided on a lower layer is provided.
There is no particular limitation as long as it is successful. The lower layer is made of non-magnetic powder or
Is obtained by dispersing a ferromagnetic powder and a binder,
Non-magnetic layer when mainly non-magnetic powder
However, when ferromagnetic powder is mainly selected, the magnetic layer is
Can be configured as When the lower layer is a non-magnetic layer,
When the magnetic layer and the lower layer are magnetic layers, they are also called lower magnetic layers.
When they are collectively referred to, they are simply referred to as lower layers. In addition,
Magnetic layer (also called “upper magnetic layer”)
) May be a single layer or multiple layers. For lower non-magnetic layer
Non-magnetic powders include metal oxides, metal carbonates, metal sulfates
Mineralization of salts, metal nitrides, metal carbides, metal sulfides, etc.
Compounds can be selected. Specifically, the α conversion rate is 9
0 to 100% α-alumina, β-alumina, γ-alumina
Mina, silicon carbide, chromium oxide, cerium oxide, α-acid
Iron fossil, corundum, silicon nitride, titanium carbide, oxide
Titanium, silicon dioxide, tin oxide, magnesium oxide,
Tungsten oxide, zirconium oxide, boron nitride, acid
Zinc fossil, calcium carbonate, calcium sulfate, barium sulfate
Or molybdenum disulfide used alone or in combination
It is. Particularly preferred are titanium dioxide, zinc oxide, and oxide.
Iron and barium sulfate, more preferably titanium dioxide
It is. The average particle size of these nonmagnetic powders is 0.005.
To 2 μm is preferable, but if necessary, the average particle diameter may vary.
Combination of non-magnetic powders
The same effect can be obtained by broadening the particle size distribution.
You. Particularly preferably, the average particle size of the nonmagnetic powder is
It is 0.01 μm to 0.2 μm. The pH of non-magnetic powder is
Particularly preferred is between 6 and 9. The specific surface area of non-magnetic powder
Always 1-100m^Two/ G, preferably 5 to 50 m^Two/ G,
More preferably, 7 to 40 m^Two/ G. Non-magnetic powder
The crystallite size is preferably from 0.01 μm to 2 μm. DB
The oil absorption by P is usually 5 to 100 ml / 100 g, preferably
Preferably 10 to 80 ml / 100 g, more preferably 2
It is 0 to 60 ml / 100 g. Specific gravity is usually 1-1.
2, preferably 3 to 6. Needle-shaped, spherical, multi-faceted
Any of a body shape and a plate shape may be used. Table of these non-magnetic powders
The surface is treated with Al_TwoO_Three, SiO_Two, Ti
O_Two, ZrO_Two, SnO_Two, Sb_TwoO_Three, With the presence of ZnO
Preferably. Particularly preferred for improving dispersibility are
Al _TwoO_Three, SiO_Two, TiO_Two, ZrO_TwoBut also
Preferred is Al_TwoO_Three, SiO_Two, ZrO_TwoIt is. This
These may be used in combination or used alone.
Can also be. It is also possible to use a co-precipitated surface treatment layer.
First, after the presence of alumina, silica
Use a method that causes
You can also. Also, the surface treatment layer may be a porous layer.
Good but homogenous and dense are generally preferred.

By mixing carbon black in the lower layer, Rs, which is a known effect, can be reduced, and a desired micro Vickers hardness can be obtained. For this purpose, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black and the like can be used. The specific surface area of the carbon black is 100 to 500 m ² / g, preferably 150 to 400 m ²
/ G, DBP oil absorption is 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g. The average particle size of the carbon black is 5 to 80 nm, preferably 10 to 80 nm.
It is 50 nm, more preferably 10 to 40 nm. The carbon black has a pH of 2 to 10 and a water content of 0.1 to 1
0% and the tap density are preferably 0.1 to 1 g / ml. Specific examples of the carbon black used in the present invention include BLACKPEARLS 200 manufactured by Cabot Corporation.
0, 1300, 1000, 900, 800, 880, 7
00, VULCAN XC-72, manufactured by Mitsubishi Chemical Corporation, # 3
050B, 3150B, 3250B, # 3750B, #
3950B, # 950, # 650B, # 970B, # 8
50B, MA-600, manufactured by Columbia Carbon, CO
NDUCTEX SC, RAVEN 8800, 800
0, 7000, 5750, 5250, 3500, 210
0, 2000, 1800, 1500, 1255, 125
And Ketjen Black EC manufactured by Akzo.

In the case of the lower magnetic layer, the ferromagnetic powder includes γ-Fe ₂ O ₃ , Co-modified γ-Fe ₂ O ₃ , α-F
An alloy containing e as a main component, CrO ₂ or the like is used. Particularly, Co-modified γ-Fe ₂ O ₃ is preferable. The ferromagnetic powder used in the lower magnetic layer of the present invention is selected to have a different composition and / or magnetic property from the ferromagnetic powder used in the upper magnetic layer. For example, in order to improve long-wavelength recording characteristics, the coercive force Hc of the lower magnetic layer is desirably set lower than that of the upper magnetic layer, and the lower magnetic layer has a ferromagnetic iron oxide powder and the upper magnetic layer has a ferromagnetic metal powder. Can be applied.
It is also effective to make the residual magnetic flux density Br of the lower magnetic layer higher than that of the upper magnetic layer. The same binder, lubricant, dispersant, additive, solvent, dispersing method and the like as those used for the upper magnetic layer can be applied to the production of the lower magnetic layer or the lower nonmagnetic layer.

As the support that can be used in the present invention, known supports such as biaxially stretched polyethylene naphthalate, polyethylene terephthalate, polyamide, polyimide, polyamide imide, aromatic polyamide, and polybenzoxdazole can be used. . Preferred are polyethylene naphthalate and aromatic polyamide. These supports may be subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment, or the like in advance. The support that can be used in the present invention has a surface having excellent smoothness such that the center plane average surface roughness is in the range of 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm. preferable. In addition, it is preferable that these supports not only have a small center plane average surface roughness but also have no coarse protrusions of 1 μm or more.

In the method for producing a magnetic recording medium of the present invention, for example, a lower layer coating solution and an upper layer magnetic layer coating solution are coated on the surface of a running support, and the dry thickness of the upper layer magnetic layer is preferably 0.1 mm.
In the range of 0.05 to 5 μm, more preferably 0.07 to 1 μm
m, so that the dry thickness of the lower layer is in the range of 0.05 to 5 μm, more preferably 0.07 to 3 μm. Here, the lower layer coating solution and the upper magnetic layer coating solution may be sequentially or simultaneously multi-layer coated.

As a coating machine for applying the magnetic paint,
Air doctor coat, blade coat, rod coat, extrusion coat, air knife coat, squeeze coat, impregnation coat, reverse roll coat, transfer roll coat, gravure coat, kiss coat, cast coat, spray coat, spin coat and the like can be used. These are described in, for example, “Latest Coating Technology” published by Sogo Gijutsu Center (May 3, 1983).
1 day).

The following can be proposed as examples of the coating apparatus and method applied to the manufacture of the magnetic recording medium of the present invention. (1) Gravure generally applied in the application of magnetic paint,
First, the lower layer is applied by a coating device such as a roll, a blade, an extrusion or the like.
No. 9, JP-A-2-265672, etc., the upper magnetic layer is applied by a support pressure type extrusion coating apparatus. (2) JP-A-63-88080, JP-A-2-17
No. 971 and JP-A-2-265672, the upper magnetic layer and the lower layer are applied almost simultaneously by one coating head having two coating liquid passage slits. (3) The upper magnetic layer and the lower layer are coated almost simultaneously by an extrusion coating device with a backup roll as disclosed in JP-A-2-174965.

A back coat layer (backing layer) may be provided on the surface of the support used in the present invention on which the magnetic paint is not applied. The back coat layer was provided by applying a back coat layer forming paint in which a particulate component such as an abrasive and an antistatic agent and a binder were dispersed in an organic solvent on the surface of the support on which the magnetic paint was not applied. Layer. Various inorganic pigments and carbon black can be used as the particulate component, and resins such as nitrocellulose, phenoxy resin, vinyl chloride resin, and polyurethane can be used alone or as a mixture of these as a binder. . Note that an adhesive layer may be provided on the surface of the support on which the magnetic paint and the backcoat layer forming paint are applied.

These coating layers are dried after subjecting the ferromagnetic powder contained in the coating layer to a magnetic field orientation treatment. After being dried in this manner, the coating layer is subjected to a surface smoothing treatment. For the surface smoothing treatment, for example, a super calender roll or the like is used. By performing the surface smoothing treatment, voids generated by the removal of the solvent during drying disappear, and the filling rate of the ferromagnetic powder in the magnetic layer is improved, so that a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. it can. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, or polyamideimide is used. Moreover, it can also process with a metal roll.

The magnetic recording medium of the present invention has a center line average roughness of 0.1 to 0.1 at a cutoff value of 0.25 mm.
It is preferable that the surface has an extremely excellent smoothness of 4 nm, preferably 1 to 3 nm. As a method for this, for example, a magnetic layer formed by selecting a specific ferromagnetic powder and a binder as described above is subjected to the above calendering treatment. As the calendering conditions, the temperature of the calender roll is in the range of 60 to 100 ° C, preferably in the range of 70 to 100 ° C, particularly preferably 80 to 10 ° C.
0 ° C. and the pressure is usually 100-500 Kg /
cm (980-4900 N / cm), preferably 200-450 Kg / cm (1960-4410).
N / cm), and particularly preferably 300 to 40.
It is preferable to perform the operation by operating under the condition of 0 kg / cm (2940 to 3920 N / cm). The laminate thus cured is then formed into a desired shape. The obtained magnetic recording medium can be used after being cut into a desired size using a cutting machine or the like.

Since the polyurethane resin of the present invention contains a large amount of short-chain diols having a cyclic structure such as an aromatic or alicyclic group, high strength and high Tg can be obtained as compared with conventional polyurethane resins. In particular, it is excellent for repeated running in a high temperature environment.
Further, since the short-chain diol content is large, the concentration of urethane bonds in the polyurethane resin substantially increases, so that the intermolecular hydrogen bonding by urethane bonds further increases the strength and Tg.
was gotten. In addition, the conventional polyurethane resin becomes brittle when the cyclic structure and urethane bond are increased, and the cut surface formed when cutting the magnetic layer is not uniform, and the magnetic layer at the edge cracks and the magnetic layer breaks during running. There was a problem that the powder dropped out and the dropout increased. However, in the polyurethane resin of the present invention, the balance between the long-chain polyether polyol and the short-chain diol having a cyclic structure was well-balanced, and high strength and high Tg were obtained, while relatively large elongation at break and tough film properties were obtained. As a result, it is possible to provide a magnetic recording medium which is excellent in durability such as a problem, an increase in dropout, a head contamination, and the like, when the magnetic layer is easily broken and becomes powdery by repeated running. Further, in the conventional polyurethane resin, when the cyclic structure or the urethane bond concentration increases, the solubility in a solvent decreases and the dispersibility decreases, but the polyurethane resin of the present invention has an advantage that the solubility in a solvent is excellent. . This is presumably because the polyurethane of the present invention contains an ether group in an appropriate amount, so that the solvent solubility is improved, and the polyurethane is easily adsorbed on a magnetic substance, so that the dispersibility is improved. Furthermore, since the short-chain diol has a cyclic structure, steric hindrance is imparted in the vicinity of the urethane bond, and it is difficult for the urethane bond to associate between molecules in the coating solution. Since the solubility is not reduced, the dispersibility is improved. Of the short-chain diols, alicyclic short-chain diols are preferred because of their higher solvent solubility.

The urethane resin of the present invention has a polar group in order to improve dispersibility. Polar group -SO ₃ M, -
_{OSO 3 M, -COOM, -P =} O (OM) 2, -O-P
ＯO (OM) ₂ , —NR ₂ , —N ⁺ R ₂ R′COO ⁻ (where M is a hydrogen atom, an alkali metal, ammonium, R,
R ′ represents an alkyl group having 1 to 12 carbon atoms), and several strong polar groups are present in the polyurethane molecule and are adsorbed on the surface of the magnetic material. It is advantageous to the dispersion stability to take the form of a molecular chain that has spread to a large extent. Therefore, when the amount of the polar group in the molecule is too large, the dispersibility tends to decrease because the entire polyurethane molecular chain is adsorbed on the surface of the magnetic material. On the other hand, if the amount is too small, the proportion of polyurethane molecules having no polar group increases, so that the dispersibility tends to decrease. In particular, even with the same amount of polar groups, in a mixture of molecules having many polar groups and molecules having no polar groups, the amount of polyurethane resin adsorbed on the surface of the magnetic material is reduced and dispersibility is reduced.
In particular, the dispersion stability tends to decrease. It is important to have a uniform polar group in any polyurethane molecular chain in order to improve dispersion stability. The polyurethane resin of the present invention preferably uses a polar group-containing long-chain polyol having high solubility in a polyurethane polymerization solvent in order to uniformly introduce a polar group. Since the long-chain polar group-containing polyol has a high solubility in a solvent, it is uniformly mixed with other components when the polyurethane is polymerized, so that the polyol is uniformly incorporated into the polyurethane molecule. As a result, a molecule having no polar group is hardly generated, and the dispersibility can be improved. In particular, the dispersion stability was also improved.
Further, the polyurethane of the present invention is particularly effective in a multilayer magnetic recording medium having a thin magnetic layer. Such a magnetic recording medium has a magnetic layer surface which is smooth and is thin, so that it is liable to be scraped by repeated running in a still state or the like, resulting in reduced still durability. It is considered that the resin has high strength and high Tg, so that the magnetic layer is less likely to flow due to sliding heat with the head, and the still durability is improved. Further, since the polyurethane resin of the present invention has a low chlorine content or does not contain chlorine, it is possible to effectively prevent corrosion of the head due to generation of hydrochloric acid and effectively suppress generation of dioxins and the like accompanying incineration. be able to.

[0057]

The present invention will be described below in more detail with reference to Examples of the present invention. “Parts” in the examples indicates “parts by mass”. Synthesis Example 1 (Synthesis of Polyurethane Resins A, B, and C of the Present Invention) In a vessel equipped with a reflux condenser and a stirrer and previously purged with nitrogen, the diol shown in Table 1 was placed in cyclohexanone at 60 ° C. under a nitrogen stream. And dissolved. Then, as a catalyst, di-n
-Dibutyltin dilaurate was added in an amount of 60 ppm based on the total amount of the raw materials, and was further dissolved for 15 minutes. Next, the MDI shown in Table 1 was added, and the mixture was heated and reacted at 90 ° C. for 2 hours.
A terminal NCO polyurethane prepolymer was synthesized. The NCO content of the obtained prepolymer was quantified, trimethylolpropane having the same mole as the NCO content was added, and the mixture was further heated and reacted for 4 hours to obtain polyurethane resins A, B, and C having terminal branched OH. The OH content was adjusted by adjusting the MDI content shown in Table 1. Table 1 shows the OH content and molecular weight of the obtained polyurethane. The content of the polar group is shown by 10 ^-5 eq / g. The OH content of the polyurethane resin was represented by the number of OH groups per molecule from the OH value obtained by the test method of JIS K0070 and the number average molecular weight in terms of polystyrene obtained by using GPC.

Synthesis Example 2 (Synthesis of Polyurethane Resin P (Comparative)) Bisphenol A was obtained by adding ethylene oxide to bisphenol A in the same manner as described in Example 1 of JP-A-1-267829. Polyol and 4,
Polyurethane resin P was synthesized using 4-diphenylmethane didiisocyanate as the polyisocyanate component and is shown in Table 1.

[0059]

[Table 1]

In Table 1, HBpA: hydrogenated bisphenol A compound A: bisphenol A PO (propylene oxide) adduct (molecular weight 600) n = 3-4 MDI: 4,4-diphenylmethane didiisocyanate TMP: trimethylolpropane polar group Long-chain diol containing SIS / IP / NPG polyester Molecular weight 10
00 a DEIS ε-caprolactone adduct molecular weight 200
0 SIS: 5-sodium sulfoisophthalic acid DEIS: 5-sodium sulfoisophthalic acid ethylene oxide adduct NPG: neopentyl glycol IP: isophthalic acid

Example 1 "Parts" in the examples indicate "parts by mass" (paint for upper magnetic layer) Ferromagnetic alloy powder (composition: 89 at% Fe, 5 at% Co, Y 6 Atomic%, Hc: 2000 Oe (1.59 × 10 ⁵ A / m), crystallite size: 15 nm, S _BET : 59 m ² / g, average major axis length: 0.12 μm, average needle ratio: 7, σs : 150 A · m ² / kg) 100 parts were pulverized with an open kneader for 10 minutes, then 10 parts of polyurethane A (solid content) 1.7 parts of vinyl chloride resin (vinyl chloride / vinyl acetate / glycidyl methacrylate = 86/9 / Compound obtained by adding hydroxyethylsulfonate sodium salt to the copolymer of No. ⁵ , SO ₃ Na = 6 × 10 ⁻⁵ eq / g, epoxy = 10 ⁻³ eq / g, Mw: 30,000) cyclohexanone 60 And kneading for 60 minutes, then 2 parts of abrasive (Al ₂ O ₃ ) (average particle diameter: 0.3 μm) 2 parts Carbon black (average particle diameter: 40 nm) 2 parts Methyl ethyl ketone / toluene = 1/1 200 parts And dispersed in a sand mill for 120 minutes. 5 parts of polyisocyanate (solid content) (Coronate 3041 manufactured by Nippon Polyurethane) 2 parts of butyl stearate 1 part of stearic acid 1 part of methyl ethyl ketone 50 parts were further added, and the mixture was further stirred and mixed for 20 minutes, and then a filter having an average pore diameter of 1 μm was used. And filtered to prepare an upper layer magnetic paint.

(Lower layer non-magnetic paint) α-Fe ₂ O ₃ 85 parts (average particle diameter: 0.15 μm, S _BET : 52 m ² / g, surface-treated Al ₂ O ₃ , SiO ₂ , pH: 6.5) ~ 8.0) 15 parts of carbon black (average particle diameter: 40 nm) was pulverized by an open kneader for 10 minutes, and then 10 parts of polyurethane resin A (solid content) 1.7 parts of vinyl chloride resin (vinyl chloride / vinyl acetate / Compound obtained by adding sodium salt of hydroxyethylsulfonate to a copolymer of glycidyl methacrylate = 86/9/5, SO ₃ Na = 6 × 10 ⁻⁵ eq / g, epoxy = 10 ⁻³ eq / g, Mw: 30 , 00) Cyclohexanone 60 parts and kneaded for 60 minutes, then methyl ethyl ketone / cyclohexanone = 6/4 200 parts was added and dispersed by sand mill for 120 minutes. . To this, 2 parts of butyl stearate, 1 part of stearic acid, 1 part of methyl ethyl ketone, and 50 parts of methyl ethyl ketone were added, and the mixture was further stirred and mixed for 20 minutes. Then, the mixture was filtered using a filter having an average pore diameter of 1 μm to prepare a lower layer coating material.

A sulfonic acid-containing polyester resin was applied as an adhesive layer to the surface of an aramid support having a thickness of 4 μm using a coil bar so that the thickness after drying was 0.1 μm. Next, the obtained lower layer coating material was simultaneously coated with a reverse roll so as to have a thickness of 1.0 μm, and immediately thereafter, the upper layer magnetic coating material had a thickness of 0.1 μm after drying. A nonmagnetic support coated with a magnetic paint is coated with a 0.5T (Tesla) Co magnet and a 0.5T
The magnetic field is oriented by a solenoid magnet of the above, and the applied material is subjected to a calendering treatment by a combination of a metal roll-metal roll-metal roll-metal roll-metal roll-metal roll-metal roll (speed 100 m / min, linear pressure 300 kg / c).
m (linear pressure 2840 N / cm), temperature 90 ° C., and then slit into 3.8 mm width.

(Examples 2 to 8 and Comparative Examples 1 to 4) Example 1 was changed by changing the kind of the polyurethane resin, the amount of the vinyl chloride resin added, and the coating thickness (after drying) of the lower nonmagnetic layer as shown in Table 2.
It was created in the same way as. The obtained sample was measured and evaluated as follows, and the results are shown in Table 2.

Measurement method: Chlorine content: The amount of Cl was quantified from the peak areas of the Fe and Cl elements by X-ray fluorescence analysis, and the Cl content per m ^{2 of} the tape was determined.
It was converted to mass. Head corrosion: Permalloy, which is a metal member of the MR head, is left in contact with a magnetic layer for 2 weeks in an environment of 60 ° C. and 90% RH. Then, the surface of the permalloy member is observed with an optical microscope to examine changes in the surface. Was. When discoloration such as corrosion was observed, x was given, and when discoloration was not seen, x was given. Long-term storage: 8 hours at 60 ° C and 90% RH
After storing for a week, the surface of the magnetic layer was observed with a differential interference microscope (× 200). The case where fine crystals were generated was evaluated as x, and the case where no change was observed was evaluated as ○. Dropout after storage (DO): The tape is wound up on a reel, stored at 60 ° C. and 90% RH for 8 weeks, and a digital video tape recorder made by Matsushita Electric (NV-
After repeating running 100 times for 5 minutes under the environment of 40 ° C. and 10% RH using BJ1), the number of dropouts whose output decreased by -10 dB or more for 15 μs or more per minute was examined, and was indicated as DO increase. Reproduction output: Using a drum tester (manufactured by Koyo Seisakusho) on a sample tape, recording wavelength 0.5 μm, head speed 10 m /
Recording and playback were performed under the condition of seconds. When the playback output of the tape of Comparative Example 3 was set to 0 dB, the relative playback output of each sample tape was evaluated.

[0066]

[Table 2]

As can be seen from the above table, the polyurethane resin of the present invention was used, but Comparative Examples 1, 2 and 4 in which the chlorine content was out of the range of the present invention due to the combined use with a vinyl chloride resin, the reproduction output was However, it can be seen that head corrosion, long-term storage stability, and DO increase after storage are inferior. Comparative Example 3 using a polyurethane resin other than the polyurethane resin of the present invention and having a chlorine content within the scope of the present invention.
The head corrosion, long-term storage and DO increase after storage are
It can be seen that the reproduction output is inferior to that of the embodiment, although it is almost the same as that of the embodiment.

[0068]

The magnetic recording medium containing the polyurethane resin of the present invention has the following effects. Electromagnetic conversion characteristics have been improved. Improved head corrosion. Improved long-term storage. It is possible to provide a magnetic recording medium having a small effect on the environment.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Katsuhiko Meguro 2-12-1, Ogimachi, Odawara-shi, Kanagawa F-film F-term (reference) 4H104 BB32A BB34A LA02 LA06 LA20 PA16 4J038 CD042 DG051 DG131 DL032 GA03 GA06 GA07 GA09 GA13 GA14 HA026 HA066 HA216 HA286 HA316 HA356 HA376 HA436 HA446 JA38 JA56 JA57 JB12 JC24 KA03 KA07 KA20 MA14 NA22 PB11 PC08 5D006 BA09 BA15 BA16 CA01 DA00 FA01 FA09

Claims (4)

[Claims] 1. A magnetic recording medium having a magnetic layer in which at least one ferromagnetic powder and a binder are dispersed on a lower layer in which a nonmagnetic powder or a ferromagnetic powder and a binder are dispersed on a support, The binder has a cyclic structure as a diol component and a short-chain diol having a molecular weight of less than 500 and a molecular weight of 500 to
It contains a polyurethane resin containing 5000 long-chain polyether diols and has a chlorine content of 1 in the magnetic recording medium.
A magnetic recording medium characterized by being 35 mg or less per m ² . 2. The binder according to claim 1, wherein the mass ratio of the polyurethane resin to the polar group-containing vinyl chloride resin is 85 / 15-10.
2. The magnetic recording medium according to claim 1, which is a composition containing a resin in a range of 0/0. 3. The magnetic recording medium according to claim 1, wherein the ferromagnetic powder is a ferromagnetic metal powder containing Y. 4. The magnetic recording medium according to claim 1, wherein the magnetic layer and / or the lower layer contains an ester-based lubricant.