JP2005092977A - Magnetic recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a magnetic recording medium for an MR head, the noise of which is low, and which has high capacity and excellent running durability. <P>SOLUTION: The magnetic recording medium is provided with a magnetic layer made mainly of a ferromagnetic metal powder and a binder on a support, and it has such features that a carbon black is deposited to at least a part of the particle surface of the ferromagnetic metal powder, the average major axis length is 20-50nm, a volume rate of a nonmagnetic material existing in the magnetic layer is 50.0-64.5%, an average size of magnetic clusters at the DC elimination is ≥15,000 nm<SP>2</SP>and <45,000nm<SP>2</SP>, and also a square shape ratio in the longitudinal direction in the surface of the magnetic layer is 0.80-0.95. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a high-density magnetic recording medium excellent in electromagnetic conversion characteristics and running durability.

Conventionally, magnetic heads using electromagnetic induction as the operating principle (inductive magnetic heads) have been used and spread, but there is a limit to using them in a higher density recording / reproducing area, and MR (magnetoresistance) is used as the operating principle. A reproducing head is proposed and used in a hard disk or the like. MR heads can produce a reproduction output several times that of induction type magnetic heads and do not use induction coils, so that equipment noise such as impedance noise is greatly reduced. It has become possible to obtain an S / N ratio. A magnetic recording layer suitable for an MR head has been studied even in a coating type magnetic recording medium that is excellent in productivity and can be provided at a low price. A magnetic recording medium in which a magnetic layer formed by dispersing a substantially non-magnetic lower layer and ferromagnetic fine powder in a binder on a non-magnetic support in this order is provided on the magnetic layer while maintaining the smoothness of the surface. Many methods have been applied to today's high-density coated magnetic recording media as a method for realizing thinning (Patent Document 1).
In order to realize high-density recording, it is important to reduce the particle size of the magnetic powder. However, for example, during the development of a coating type magnetic recording medium exceeding the surface recording density of 0.3 Gbit / inch ² , there has been a problem that noise is increased particularly when the particle size of the magnetic particles is reduced. If the particle size of the magnetic powder is reduced, it becomes difficult to disperse in the binder during the preparation of the magnetic layer coating, and the smoothness of the magnetic layer surface cannot be maintained, resulting in the intended low noise medium. It becomes difficult. In order to suppress this noise, it is necessary to (1) eliminate aggregation of magnetic materials and (2) smooth the surface of the magnetic layer.
In addition to excellent electromagnetic conversion characteristics, magnetic recording tape is also required to have good running durability. When the magnetic powder is finely divided, the running durability tends to deteriorate even if the surface smoothness is comparable. Therefore, adding non-magnetic carbon black fine particle powder or the like to the magnetic recording layer is effective in ensuring running stability. However, excessive addition not only deteriorates electromagnetic conversion characteristics and inhibits high density recording, but also inhibits thinning of the magnetic recording layer, and the carbon black fine particle powder has a BET specific surface area value. Since it is difficult to disperse due to the large size and poor wettability with a solvent, it is difficult to obtain a magnetic recording medium having a smooth surface.
For example, Patent Document 2 proposes that carbon black is deposited on magnetic powder to improve running performance. However, the actually produced medium has a rough surface and a magnetic recording medium on which an MR head is mounted. It was not enough. Further, there is no specific description of improvement of electromagnetic conversion characteristics.
As described above, the conventional technology has not yet provided a magnetic recording medium having sufficiently good electromagnetic characteristics and running durability.

Japanese Examined Patent Publication No. 4-71244 JP 2001-143243 A

  An object of the present invention is to solve the problems of the prior art and to provide a magnetic recording medium for MR head that has low noise, high capacity, and good running durability.

As a result of intensive studies to achieve the above object, the present inventors have found that the magnetic metal powder in a magnetic recording medium provided with a magnetic layer mainly composed of a ferromagnetic metal powder and a binder on a support. Is coated with carbon black on at least a part of the particle surface, has an average major axis length of 20 to 50 nm, and has a volume ratio of nonmagnetic substance in the magnetic layer of 50.0 to 5.0. 64.5%, the average size of magnetic clusters at the time of DC erasure is 15000 nm ² or more and less than 45000 nm ² , and the squareness ratio in the longitudinal direction in the magnetic layer surface is 0.80 to 0.95. The goal was achieved by obtaining a magnetic recording medium characterized by this. The magnetic recording medium of the present invention is preferably reproduced by an MR head system.

  The present invention is excellent in CN ratio and running durability by defining the volume ratio, magnetic cluster size, and squareness ratio of the non-magnetic substance in the magnetic layer using the fine particle C-coated ferromagnetic metal powder of a specific size. A magnetic recording medium can be provided.

In the present invention, the carbon black powder is added in the same manner as in the prior art, and separately, the carbon black is deposited on the surface of the magnetic powder, so that the volume ratio of the non-magnetic substance in the magnetic layer is substantially increased, and noise is reduced. It was made by knowing that it would increase.
In the present invention, the magnetic cluster size that can cause noise by improving the runnability by depositing carbon black on the surface of the magnetic powder and further suppressing the volume ratio of the non-magnetic substance in the magnetic layer as much as possible. In addition, the electromagnetic conversion characteristics are improved by reducing the above and improving the degree of orientation of the magnetic powder.
As a method for controlling the volume ratio of the nonmagnetic substance, (1) the amount of carbon black to be deposited on the ferromagnetic metal powder, (2) carbon black other than the carbon black used in (1) above, which is used independently Examples include adjusting the addition amount of the powder and (3) the addition amount of the polyurethane resin for the binder.
The volume ratio of the non-magnetic substance is expressed as a percentage of the volume sum (Vn) of each material to the volume sum (Vt) of each material by estimating the volume of each material from the mass and specific gravity of each material contained in the magnetic layer. did. When the volume ratio of the non-magnetic substance is (V _{n / t} ), V _{n / t} (%) = 100 × Vn / Vt.
Here, the material constituting Vn or Vt means a material that is solid in a normal environment, and Vn means that they are all non-magnetic. The amount of carbon black deposited on the ferromagnetic metal powder is also included in Vn.

The ferromagnetic metal powder used for the magnetic layer in the present invention has carbon black deposited on at least a part of its particle surface, and has an average major axis length of 20 to 50 nm, preferably 20 to 45 nm.
The present invention can improve the C / N, weather resistance, and runnability of the magnetic recording medium by using such a ferromagnetic metal powder.
This ferromagnetic metal powder is obtained by adhering carbon black to part or all of the particle surface. In the present invention, the term “adhesion” means that carbon black is physically adsorbed on the particle surface. Alternatively, carbon black may be bonded to the particle surface by a covalent bond. However, it is preferable that both of them be held in the magnetic layer of the magnetic recording medium. By using the ferromagnetic metal powder obtained by depositing carbon black on the particle surface for the magnetic layer, the dispersibility is improved, so that the surface property of the magnetic layer is secured and the C / N is improved. The friction coefficient of the magnetic surface can be kept low, which can contribute to improvement of running durability.

The means for depositing carbon black on the ferromagnetic metal powder is not particularly limited, but the surface of the ferromagnetic metal powder is preferably pretreated before depositing the carbon black on the ferromagnetic metal powder. The pretreatment is not particularly limited, and a conventionally known method is used. Examples of the pretreatment include depositing a resin such as a silicone resin, an acrylic resin, and an epoxy resin and other compounds on the surface of the ferromagnetic metal powder. Among these, a treatment for depositing a silane compound is preferable. A substance deposited on the ferromagnetic metal powder by these pretreatments is called a pretreatment product.
The amount of these pretreated products to be deposited is preferably 0.1 to 10% by mass, and more preferably 0.3 to 8% by mass with respect to the ferromagnetic metal powder. If it is less than 0.1% by mass, it is difficult to sufficiently deposit carbon black. If it exceeds 10% by mass, carbon black can be sufficiently deposited, but the effect is saturated and there is no point in adding it. By applying this pretreated product to the ferromagnetic metal powder, it is possible to improve the weather resistance of the ferromagnetic metal powder and consequently the weather resistance of the magnetic layer.

Hereinafter, although the method of pre-processing a ferromagnetic metal powder using a silicon compound is demonstrated, it applies mutatis mutandis also about another pre-processing thing.
The deposition treatment of the ferromagnetic metal powder with the silicon compound may be performed by mechanically mixing and stirring the ferromagnetic metal powder and the silicon compound, or mechanically mixing and stirring while spraying the silicon compound on the ferromagnetic metal powder. Here, the silicon compound may be used as it is or as a solution. Moreover, you may add catalysts, such as water and hydrochloric acid, to a solution suitably.
The silicon compound may be oligomerized or polymerized by a siloxane bond reaction as it is.
As a silicon compound, an alkoxysilane compound and a polysiloxane compound are preferable.
Examples of the alkoxysilane compound include tetraalkoxysilane, monoalkyltrialkoxysilane, dialkyldialkoxysilane, and trialkylalkoxysilane, which are used alone or in combination.

Examples of alkoxysilane compounds include tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, tetra-i-propoxysilane, methyltriethoxysilane, ethyltriethoxysilane, n-propyltriethoxysilane, dimethyldiethoxysilane, diethyldiethoxysilane, etc. Can be given.
As the polysiloxane compound, an organopolysiloxane compound is preferable. Substituents substituted on the organopolysiloxane compound include lower alkyl groups such as methyl and ethyl groups, phenyl groups, and functional groups such as hydroxyl, carboxyl, epoxy, vinyl, acryloyl, and methacryloyl groups. Is mentioned. The functional group is preferably bonded to the silicon atom as it is or through a divalent linking group composed of an alkylene group, a carbonyl group, an oxy group or the like alone or in combination.
The number of siloxane bonds, the weight average molecular weight, the number of substituents, and the like of the polysiloxane compound are appropriately selected depending on the type of ferromagnetic metal powder and carbon black used.
After the silicon compound is deposited on the surface of the ferromagnetic metal powder particles, carbon black is added, and mixed and stirred to deposit the carbon black on the silicon compound, followed by drying or heat treatment.

Further, the total amount of carbon black deposited in the present invention is preferably 0.5 to 40 parts by mass, more preferably 0.5 to 20 parts by mass with respect to 100 parts by mass of the untreated ferromagnetic metal powder. When the carbon black deposition amount is in the above range, a ferromagnetic metal powder having a low volume resistivity can be obtained, and it is preferable for securing V _{n / t} . Commercially available furnace black, channel black, etc. can be used for carbon black fine particle powder. The average particle size of the carbon black used for the deposition treatment is preferably 2 to 10 nm, more preferably 2 to 5 nm. When the average particle diameter of the carbon black used for the deposition treatment is in the above range, it is easy to handle the powder, and a small mechanical shearing force is required for uniform deposition on the adherend, which is industrially advantageous. is there.

The particle shape and particle size of the carbon black-coated ferromagnetic metal powder (hereinafter also referred to as C-coated ferromagnetic metal powder) in the present invention depend greatly on the particle shape and particle size of the ferromagnetic metal powder. It has a particle form similar to that of powder and has a slightly larger particle size than ferromagnetic metal powder. That is, the C-coated ferromagnetic metal powder in the present invention has an average major axis length of 20 to 50 nm, preferably 20 to 45 nm. The average acicular ratio {average of (major axis length / minor axis length)} of the C-coated ferromagnetic metal powder is preferably 2.0 to 6.5, and more preferably 2.5 to 5.5.
Prior to the above pretreatment, the C-coated ferromagnetic metal powder in the present invention may be prepared by subjecting the particle surface of the ferromagnetic metal powder to aluminum hydroxide, aluminum oxide, silicon hydroxide, and silicon, if necessary. You may coat | cover with 1 type, or 2 or more types of compounds chosen from oxide of these.

The ferromagnetic metal powder used for the C-coated ferromagnetic metal powder usually has an S _BET (BET specific surface area) of 40 to 100 m ² / g, preferably 50 to 80 m ² / g. The crystallite size is usually 8 to 15 nm, preferably 10 to 15 nm. When the average major axis length is 50 nm or more, when a magnetic layer is formed using this, the noise during short wavelength recording becomes large, and the S / N ratio tends to be impaired. When the average major axis length is less than 20 μm, aggregation is likely to occur due to an increase in intermolecular force due to particle miniaturization, and uniform pretreatment and dispersion treatment become difficult.
Examples of the ferromagnetic metal powder include a simple substance or an alloy such as Fe, Ni, Fe—Co, Fe—Ni, Co—Ni, and Co—Ni—Fe, and within a range of 20% by mass or less of the metal component, aluminum, Silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, yttrium, molybdenum, rhodium, palladium, gold, tin, antimony, boron, barium, tantalum, tungsten, rhenium, silver, lead, phosphorus, lanthanum, Cerium, praseodymium, neodymium, tellurium, bismuth, and the like can be included. Further, the ferromagnetic metal powder may contain a small amount of water, hydroxide or oxide. Methods for producing these ferromagnetic metal powders are already known, and the ferromagnetic metal powders used in the present invention can also be produced according to known methods. The shape of the ferromagnetic metal powder is not particularly limited as long as the needle-like ratio of the C-coated ferromagnetic metal powder can be obtained, and examples thereof include a needle shape, a spindle shape, and a rice grain shape.

  In the present invention, a binder, a curing agent, and a C-coated ferromagnetic metal powder are kneaded and dispersed together with a solvent such as methyl ethyl ketone, dioxane, cyclohexanone, or ethyl acetate, which is usually used in the preparation of a magnetic paint. A layer-forming paint is used. The kneading and dispersing can be performed according to a usual method.

In addition to the above components, the magnetic layer is usually used as an abrasive such as α-Al ₂ O ₃ or Cr ₂ O _3, an antistatic agent such as carbon black, a lubricant such as fatty acid, fatty acid ester or silicone oil, or a dispersant. The additives or fillers may be included.
Incidentally, since the antistatic agent such as carbon black is coated with the C-coated ferromagnetic metal powder of the present invention, the amount of the antistatic agent can be reduced more than usual and the antistatic effect is improved.
As binders, additives and the like used for the magnetic layer, those used for the back layer described later can be appropriately selected and used.

In the present invention, V _{n / t} , magnetic cluster size, and longitudinal squareness ratio (SQ) in the magnetic layer plane are controlled within a specific range. Although powder contributes, as described above, it is a more effective means to appropriately select the addition amount of carbon black alone and the addition amount of polyurethane resin to the magnetic layer.
As the carbon black material added to the magnetic layer alone, the same materials as those used for the back layer described later can be used. The size of the carbon black is preferably 10 to 50 nm, and more preferably 10 to 40 nm. The amount of carbon black added is preferably 0.1 to 2 parts by mass, more preferably 0.1 to 1 part by mass with respect to 100 parts by mass of the untreated ferromagnetic metal powder.

As the polyurethane resin used for the magnetic layer, the following are preferable.
The structure of the polyurethane resin includes: a polyester polyol composed of an aliphatic dibasic acid and a diol component; an aliphatic diol having an alkyl branched side chain with a total of 3 or more carbon atoms in the branched side chain as a chain extender; and an organic diisocyanate. And a polyurethane resin in which 70 mol% or more of the diol component in the polyester polyol is an aliphatic diol having no cyclic structure having an alkyl branched side chain.
In the magnetic recording medium of the present invention, the polyurethane resin contains no or reduced cyclic structure that adversely affects solubility in a solvent such as an aromatic ring or a cyclohexane ring, and has an alkyl branched side chain. By preventing steric hindrance of the association between urethane bonds and ester bonds, the solubility in solvents can be improved by reducing the intermolecular interaction, and especially the dispersibility of magnetic materials that tend to aggregate with magnetic energy. It seems to contribute to In addition, it is generally less susceptible to thermal decomposition than polyether-based materials. Especially when used for the uppermost magnetic layer that contacts the head of a video tape recorder, the strength of the coating film decreases due to the sliding heat between the head and the tape surface. It has the feature which can prevent.

  Examples of the aliphatic diol having no cyclic structure having an alkyl branched side chain that can be used for the polyester polyol of the polyurethane resin include 2,2-dimethyl-1,3-propanediol, 3,3-dimethyl-1, 5-pentanediol, 2-methyl-2-ethyl-1,3-propanediol, 3-methyl-3-ethyl-1,5-pentanediol, 2-methyl-2-propyl-1,3-propanediol, 3-methyl-3-propyl-1,5-pentanediol, 2-methyl-2-butyl-1,3-propanediol, 3-methyl-3-butyl-1,5-pentanediol, 2,2-diethyl -1,3-propanediol, 3,3-diethyl-1,5-pentanediol, 2-ethyl-2butyl-1,3-propanediol, 3-ethyl -3-butyl-1,5-pentanediol, 2-ethyl-2-propyl-1,3-propanediol, 3-ethyl-3-propyl-1,5-pentanediol, 2,2-dibutyl-1, 3-propanediol, 3,3-dibutyl-1,5-pentanediol, 2,2-dipropyl-1,3-propanediol, 3,3-dipropyl-1,5-pentanediol, 2-butyl-2- Propyl-1,3-propanediol, 3-butyl-3-propyl-1,5-pentanediol, 2-ethyl-1,3-propanediol, 2-propyl-1,3-propanediol, 2-butyl- 1,3-propanediol, 3-ethyl-1,5-pentanediol, 3-propyl-1,5-pentanediol, 3-butyl-1,5-pentanediol, 3- Cutyl-1,5-pentanediol, 3-myristyl-1,5-pentanediol, 3-stearyl-1,5-pentanediol, 2-ethyl-1,6-hexanediol, 2-propyl-1,6- Hexanediol, 2-butyl-1,6-hexanediol, 5-ethyl-1,9-nonanediol, 5-propyl-1,9-nonanediol, 5-butyl-1,9-nonanediol, etc. Can do. Among these, preferred are 2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, and 2,2-diethyl-1,3-propanediol. be able to. As for content of the aliphatic diol which does not have the cyclic structure which has the said alkyl branched side chain in the diol component used for polyester polyol, 80-100 mol% is preferable. More preferably, it is 90-100 mol%.

  The aliphatic diol having an alkyl branched side chain having a total of 3 or more carbon atoms in the branched side chain does not necessarily exclude groups other than the alkyl branched side chain at all. If desired, for example, an alicyclic group, It may have a halogen atom, an alkoxy group or the like. The alkyl branched side chain preferably includes an ethyl group, a propyl group, and a butyl group, and preferably has 2 to 3 of these groups. The main chain of this diol preferably has 3 to 6 carbon atoms. Examples of the diol include 2-methyl-2-ethyl-1,3-propanediol, 3-methyl-3-ethyl-1,5-pentanediol, 2-methyl-2-propyl-1,3-propane Diol, 3-methyl-3-propyl-1,5-pentanediol, 2-methyl-2-butyl-1,3-propanediol, 3-methyl-3-butyl-1,5-pentanediol, 2,2 -Diethyl-1,3-propanediol, 3,3-diethyl-1,5-pentanediol, 2-ethyl-2-butyl-1,3-propanediol, 3-ethyl-3-butyl-1,5- Pentanediol, 2-ethyl-2-propyl-1,3-propanediol, 3-ethyl-3-propyl-1,5-pentanediol, 2,2-dibutyl-1,3-propanediol, , 3-Dibutyl-1,5-pentanediol, 2,2-dipropyl-1,3-propanediol, 3,3-dipropyl-1,5-pentanediol, 2-butyl-2-propyl-1,3- Propanediol, 3-butyl-3-propyl-1,5-pentanediol, 2-ethyl-1,3-propanediol, 2-propyl-1,3-propanediol, 3-propyl-1,5-pentanediol 3-butyl-1,5-pentanediol, 3-octyl-1,5-pentanediol, 3-myristyl-1,5-pentanediol, 3-stearyl-1,5-pentanediol, 2-propyl-1 , 6-hexanediol, 2-butyl-1,6-hexanediol, 5-propyl-1,9-nonanediol, 5-butyl-1,9-nonanediol Mention may be made of the Le and the like. Of these, preferred are 2-ethyl-2-butyl-1,3-propanediol and 2,2-diethyl-1,3-propanediol.

  Examples of the aliphatic dibasic acid that can be used in the polyester polyol include succinic acid, adipic acid, azelaic acid, sebacic acid, malonic acid, glutaric acid, pimelic acid, and suberic acid. Among these, succinic acid, adipic acid, and sebacic acid are preferable. And the content of an aliphatic dibasic acid is 70 mol% or more among all the dibasic acid components of polyester polyol. If it is less than 70 mol%, the dibasic acid component having a cyclic structure such as an aromatic dibasic acid substantially increases, so that the solubility in a solvent is lowered and the dispersibility is lowered.

  The organic diisocyanate that can be used in the present invention includes 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'-diphenylpropane diisocyanate, m- Aromatic diisocyanates such as phenylene diisocyanate, p-phenylene diisocyanate, naphthylene-1,4-diisocyanate, naphthylene-1,5-diisocyanate, 3,3′-dimethoxydiphenyl-4,4′-diisocyanate, Tiger diisocyanate, hexamethylene diisocyanate, aliphatic diisocyanates such as lysine diisocyanate, isophorone diisocyanate, hydrogenated tolylene diisocyanate, can be mentioned alicyclic diisocyanates such as hydrogenated diphenylmethane diisocyanate. Aromatic diisocyanates are preferable, and 4,4-diphenylmethane diisocyanate, 2,2-tolylene diisocyanate, p-phenylene diisocyanate, and isophorone diisocyanate are more preferable.

In addition, the polyurethane resin may have a polar group, -SO ₃ M, -OSO ₃ M, -COOM, -PO ₃ MM ', -OPO ₃ MM', -NRR ', -N ⁺ RR. 'R "COO ^- (wherein, M and M' are each independently a hydrogen atom, an alkali metal, alkaline earth metal or ammonium, R and R are each independently an alkyl group having 1 to 12 carbon atoms, R ″ Represents an N-linked alkylene group having 1 to 12 carbon atoms), a sulfobetaine group, a phosphobetaine group, a sulfamic acid, and a sulfamic acid group are preferably used.
The polar group may be polymerized using a monomer having the polar group to form a polyurethane resin, or a polar group may be introduced into the produced polyurethane resin.
The amount of these polar groups is preferably 1 × 10 ^{−5 to} 5 × 10 ⁻⁴ eq / g, particularly preferably 5 × 10 ^{−5 to} 3 × 10 ⁻⁴ eq / g. When the amount is less than 1 × 10 ⁻⁵ eq / g, the powder tends to be insufficiently adsorbed, so that the dispersibility tends to decrease. When the amount exceeds 5 × 10 ⁻⁴ eq / g, the solubility in the solvent decreases. Therefore, dispersibility tends to decrease.

The molecular weight of the polyurethane resin is preferably a weight average molecular weight Mw of 30,000 to 70,000. More preferably, it is 40000-60000. When it is less than 30000, the coating film strength is lowered and the durability is lowered. Moreover, if it is 70000 or more, the solubility to a solvent will fall and a dispersibility will fall.
The glass transition temperature (Tg) of the polyurethane resin is preferably 40 ° C to 150 ° C. More preferably, it is 70 degreeC-120 degreeC, More preferably, it is 80 degreeC-100 degreeC. When Tg is in the above range, the coating strength at high temperatures is maintained, durability and storage stability are ensured, calendar moldability is good, and electromagnetic conversion characteristics are ensured.
The polyurethane resin may have a polar group, and the polar group is preferably —SO ₃ M, —OSO ₃ M, —PO ₃ M ₂ , —COOM, more preferably —SO ₃ M, — OSO ₃ M. Further, the content of the polar group is preferably 1 × 10 ⁻⁵ eq / g to 2 × 10 ⁻⁴ eq / g, and within this range, the adsorption to the magnetic material is sufficient and the dispersibility is ensured, And since the solubility to a solvent is ensured, dispersibility is ensured. The OH content of the polyurethane resin is preferably 2 / molecule to 20 / molecule, more preferably 3 / molecule to 15 / molecule. When the OH content is within this range, the coating film strength is ensured because the reactivity with the isocyanate curing agent is ensured, and as a result, the durability is ensured and the solubility in the solvent is ensured. Secured.

  A vinyl chloride synthetic resin may be used in combination with the polyurethane resin. The degree of polymerization of the vinyl chloride resin that can be used in combination is preferably from 200 to 600, particularly preferably from 250 to 450. The vinyl chloride resin may be a copolymer of vinyl monomers such as vinyl acetate, vinyl alcohol, vinylidene chloride, acrylonitrile and the like.

  As the polyurethane resin, various synthetic resins can be used in addition to the vinyl chloride resin. For example, ethylene / vinyl acetate copolymer, cellulose derivatives such as nitrocellulose resin, acrylic resin, polyvinyl acetal resin, polyvinyl butyral resin, epoxy resin, and phenoxy resin. These can be used alone or in combination.

  The magnetic layer can use a curing agent such as a polyisocyanate compound together with a resin as a binder. Examples of the polyisocyanate compound include a reactive product of 3 mol of tolylene diisocyanate and 1 mol of trimethylolpropane (eg, Desmodur L-75 (manufactured by Bayer)), diisocyanate 3 such as xylylene diisocyanate or hexamethylene diisocyanate. Reaction product of 1 mol of trimethylolpropane, 3 moles of burette addition compound with 3 moles of hexamethylene diisocyanate, 5 moles of isocyanurate compound of tolylene diisocyanate, isocyanurate addition of 3 moles of tolylene diisocyanate and 2 moles of hexamethylene diisocyanate Mention may be made of compounds, polymers of isophorone diisocyanate and diphenylmethane diisocyanate.

  The polyisocyanate compound contained in the magnetic layer 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. Moreover, when performing the hardening process by electron beam irradiation, the compound which has reactive double bonds like urethane acrylate etc. can be used. The total mass of the resin component and the curing agent (that is, the binder) is preferably in the range of usually 15 to 40 parts by mass, more preferably 20 to 30 parts by mass with respect to 100 parts by mass of the magnetic powder. is there.

  The magnetic recording medium of the present invention is not particularly limited as long as it has a structure having a magnetic layer on at least one surface of the support, and widely includes, for example, those having a back layer on the opposite surface of the support. Accordingly, the magnetic recording medium of the present invention includes those having layers other than the magnetic layer and the back layer. For example, a nonmagnetic layer containing nonmagnetic powder, a soft magnetic layer containing soft magnetic powder, a second magnetic layer, a cushion layer, an overcoat layer, an adhesive layer, and a protective layer may be included. These layers can be provided at appropriate positions so that their functions can be effectively exhibited. The magnetic recording medium of the present invention is preferably a magnetic recording medium having a lower nonmagnetic layer containing a nonmagnetic inorganic powder and a binder between the support and the magnetic layer. The thickness of the layer can be, for example, 0.03 to 1 μm for the magnetic layer and 0.5 to 3 μm for the lower nonmagnetic layer. The lower nonmagnetic layer is preferably thicker than the magnetic layer. When a soft magnetic layer is provided between the support and the magnetic layer, for example, the magnetic layer is 0.03 to 1 μm, preferably 0.05 to 0.5 μm, and the soft magnetic layer is 0.8 to 3 μm. be able to.

Next, the lower nonmagnetic layer or the lower magnetic layer (hereinafter, the lower nonmagnetic layer or the lower magnetic layer is also referred to as the lower layer) will be described. The inorganic powder used for the lower layer of this invention does not ask | require magnetic powder and nonmagnetic powder. For example, in the case of nonmagnetic powder, it can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and nonmagnetic metals. As an inorganic compound, for example, titanium oxide (TiO ₂ , TiO), α-alumina, β-alumina, γ-alumina, α-iron oxide, chromium oxide, zinc oxide, tin oxide, oxidized with an α conversion rate of 90-100% Tungsten, vanadium oxide, silicon carbide, cerium oxide, corundum, silicon nitride, titanium carbide, silicon dioxide, magnesium oxide, zirconium oxide, boron nitride, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, goethite, hydroxide Aluminum or the like can be used alone or in combination. Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate, and more preferred are titanium dioxide described in JP-A-5-182177, and JP-A-6-60362 and JP-A-9-170003. Α-iron oxide described in the publication. Nonmagnetic metals include Cu, Ti, Zn, Al and the like. These nonmagnetic powders preferably have an average particle size of 0.005 to 2 μm. However, if necessary, nonmagnetic powders having different average particle sizes may be combined or a single nonmagnetic powder may have a wide particle size distribution. The same effect can be given. Particularly preferred is a nonmagnetic powder having an average particle size of 0.01 μm to 0.2 μm. The pH of the nonmagnetic powder is particularly preferably 6-9. The specific surface area of the nonmagnetic powder is 1 to 100 m ² / g, preferably 5 to 50 m ² / g, more preferably 7 to 40 m ² / g. The crystallite size of the nonmagnetic powder is preferably 0.01 μm to 2 μm. The oil absorption using DBP is 5 to 100 ml / 100 g, preferably 10 to 80 ml / 100 g, and more preferably 20 to 60 ml / 100 g. The specific gravity is 1 to 12, preferably 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape.

As soft magnetic powders, granular Fe, Ni, granular magnetite, Fe-Si, Fe-Al, Fe-Ni, Fe-Co, Fe-Co-Ni, Fe-Al-Co (Sendust) alloy, Mn-Zn ferrite Ni-Zn Ferrite, Mg-Zn Ferrite, Mg-Mn Ferrite, and others, N. Kakunobu, “Physics of Ferromagnetic Materials (below) Magnetic Properties and Applications” (Yuhuabo, 1984), 368-376) And the like described in. The surface of these nonmagnetic powders and soft magnetic powders is surface-treated so that at least a part thereof is coated with Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO. It is preferable to leave. Of these, particularly providing good dispersibility is _{Al 2 O 3, SiO 2,} TiO 2, ZrO 2, further preferred is _{Al 2 O 3, SiO 2,} ZrO 2. These may be used in combination or may be used alone. Further, a surface-treated layer co-precipitated depending on the purpose may be used, or a method of first treating the surface layer with alumina and then treating the surface layer with silica, or vice versa. May be. The surface treatment layer may be a porous layer depending on the purpose, but it is generally preferable that the surface treatment layer is homogeneous and dense.

By mixing carbon black in the lower layer, the surface electrical resistance Rs can be lowered and the desired micro Vickers hardness can be obtained. The average particle diameter (arithmetic average of particle diameter) of carbon black is usually 5 nm to 80 nm, preferably 10 to 50 nm, and more preferably 10 to 40 nm. Specifically, the same carbon black that can be used for the back layer described later can be used. In the lower layer of the present invention, a magnetic powder can also be used as the inorganic powder. As the magnetic powder, γ-Fe ₂ O ₃ , Co-modified γ-Fe ₂ O ₃ , an alloy containing α-Fe as a main component, CrO ₂ or the like is used. The lower layer magnetic body can be selected according to the purpose, and the effect of the present invention does not depend on the type of the magnetic body. However, it is known that the performance is changed in the upper and lower layers according to the purpose. For example, in order to improve the long wavelength recording characteristics, it is desirable to set the Hc of the lower magnetic layer lower than the Hc of the upper magnetic layer, and to set Br of the lower magnetic layer higher than Br of the upper magnetic layer. It is valid. In addition, the advantage by taking a well-known multilayer structure can be provided. As the binder, lubricant, dispersant, additive, solvent, dispersion method and the like of the lower magnetic layer or lower nonmagnetic layer, those of the magnetic layer can be applied. In particular, known techniques relating to the magnetic layer can be applied to the amount, type, additive, and amount of dispersant added, and type.

  Examples of the support that can be used in the present invention include biaxially stretched polyethylene naphthalate (PEN), polyethylene terephthalate (PET), polyamide, polyimide, polyamideimide, aromatic polyamide, and polybenzoxidazole. it can. These nonmagnetic supports may have been subjected to corona discharge, plasma treatment, easy adhesion treatment, heat treatment, and the like in advance. Further, the nonmagnetic support that can be used in the present invention has a center surface average surface roughness of usually 0.1 to 20 nm, preferably 1 to 10 nm at a cutoff value of 0.25 mm, and has an excellent surface. It is preferable to have smoothness. These non-magnetic supports preferably have not only a small center line average surface roughness but also no coarse protrusions of 1 μm or more. The thickness of the nonmagnetic support is usually 4 to 15 μm, preferably 4 to 9 μm. In the case of being thin, the unevenness of the back layer is easily reflected by the handling tension, and this can be effectively suppressed by using a polyurethane resin used for the back layer described later as the uppermost layer of the magnetic layer. When the thickness is 7 μm or less, it is preferable to use an aromatic polyamide such as PEN or aramid. Most preferred is aramid.

In the magnetic recording medium of the present invention, a back layer is preferably provided on the support surface opposite to the surface on which the magnetic layer is provided. Further, by using a back layer having a specific composition as described below, the reflection of the back surface on the magnetic layer is reduced, which can contribute to an improvement in C / N.
It is preferable to use inorganic oxide powder for the back layer. As the inorganic oxide powder, titanium oxide, α-iron oxide, or a mixture thereof is preferably used. As titanium oxide and α-iron oxide, those usually used can be used. Further, the shape of the particles is not particularly limited. When spherical titanium oxide or α-iron oxide is used, one having a particle size of 0.01 to 0.1 μm is preferable for securing the film strength of the back layer itself. Further, when acicular titanium oxide or α-iron oxide is used, one having an acicular ratio of 2 to 20 is appropriate, and one having 3 to 10 is more preferable. Further, it is preferable that the major axis length is 0.05 to 0.3 μm and the minor axis length is 0.01 to 0.05 μm. In particular, at least a part of the surface of the inorganic oxide powder is at least one compound selected from Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ and ZnO ₂ , especially Al _2. Those coated with at least one compound selected from O ₃ , SiO _2, and ZrO ₂ are preferred in terms of excellent dispersibility in the binder. Such an inorganic oxide powder is prepared by synthesizing titanium oxide or α-iron oxide particles and then treating the surface so that another compound as described above is deposited or coated on the surface. - can be obtained by a method of coprecipitating at least one compound selected from iron oxide and _{Al 2 O 3, SiO 2,} TiO 2, ZrO 2, SnO 2, Sb 2 O 3 and ZnO _2.

  Moreover, such inorganic oxide powder can also be obtained from a commercial item. For example, DPN-245, DPN-250, DPN-250BX, DPN-270BX, DPN-550BX, DPN-550RX, TF-100, TF-120 manufactured by Toda Kogyo Co., Ltd., TTO- manufactured by Ishihara Sangyo Co., Ltd. 51A, TTO-51B, TTO-51C, TTO-53B, TTO-55A, TTO-55B, TTO-55C, TTO-55D, TTO-55N, TTO-55S, TTO-S-1, TTO-S-2, TTO-M-1, TTO-M-2, TTO-D-1, TTO-D-2, SN-100, E270, E271, STT-4D, STT-30D, STT-30 manufactured by Titanium Industry Co., Ltd. , STT-65C, MT-100F, MT-100S, MT-100T, MT-150W, MT-500B, MT-500HD manufactured by Teika Co., Ltd. MT-600B, manufactured by Nippon Aerosil Co., Ltd. of TiO2P25, mention may be made of a super Thailand Tania, such as manufactured by Showa Denko (Ltd.).

It is preferable to use carbon black for the back layer to prevent electrification. As the carbon black used for the back layer, those commonly used for magnetic recording media can be widely used. For example, furnace black for rubber, thermal black for rubber, carbon black for color, acetylene black, and the like can be used. In order to prevent the irregularities of the back layer from appearing on the magnetic layer, the particle size of the carbon black is preferably 0.3 μm or less. Here, the particle size is the particle size of a single particle without aggregation. A particularly preferable particle diameter is 0.01 to 0.1 μm. Carbon black preferably has a pH of 2 to 10, a moisture content of 0.1 to 10%, and a tap density of 0.1 to 1 g / ml. The specific surface area is usually, 100~500m ² / g, preferably from 150~400m ² / g, DBP oil absorption 20 to 400 ml / 100 g, preferably 30 to 200 ml / 100 g. As specific examples, BLACK PEARL S2000, 1300, 1000, 900, 800, 880, 700, VULCAN XC-72 manufactured by Cabot Corporation; # 3050B, # 3150B, # 3250B, # 3250B, #, manufactured by Mitsubishi Chemical Corporation 3750B, # 3950B, # 950, # 650B, # 970B, # 850B, MA-600; Columbian Carbon's CONDUCTEX SC, RAVEN8800, 8000, 7000, 5750, 5250, 3500, 2100, same 2000, 1800, 1500, 1255, 1250; Akzo Ketchen Black EC, Asahi Carbon Corporation # 55, # 50, # 30; Colombian Carbon Corporation RAVEN450, 430; Kerncarb Corporation Thermax MT, etc. I can make it.

  For the binder for the back layer of the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins and the like can be used. Preferred binders are vinyl chloride resins, vinyl chloride-vinyl acetate resins, fibrous resins such as nitrocellulose, phenoxy resins, and polyurethane resins. Among them, it is more preferable to use a vinyl chloride resin, a vinyl chloride-vinyl acetate resin, or a polyurethane resin because the hardness of the back layer becomes close to the hardness of the magnetic layer and the back shot can be reduced.

Polyurethane resin, _-SO 3 _M in the _{molecule, -OSO 3 M, -COOM, -PO} 3 MM ', - OPO 3 MM', - NRR ', - N + RR'R "COO - ( wherein, M And M ′ are each independently a hydrogen atom, an alkali metal, an alkaline earth metal, or ammonium, R and R are each independently an alkyl group having 1 to 12 carbon atoms, and R ″ is an N bond having 1 to 12 carbon atoms. It preferably contains at least one polar group selected from (which represents an alkylene group), particularly preferably —SO ₃ M or —OSO ₃ M. The amount of these polar groups is preferably 1 × 10 ⁻⁵ to 2 × 10 ⁻⁴ eq / g, particularly preferably 5 × 10 ⁻⁵ to 1 × 10 ⁻⁴ eq / g. If the amount is less than 1 × 10 ⁻⁵ eq / g, the powder tends to be insufficiently adsorbed, so that the dispersibility tends to decrease. If the amount exceeds 2 × 10 ⁻⁴ eq / g, the solubility in the solvent decreases. Therefore, dispersibility tends to decrease.

  The number average molecular weight (Mn) of the polyurethane resin is preferably 5,000 to 100,000, more preferably 10,000 to 50,000, and particularly preferably 20,000 to 40,000. If it is less than 5000, the strength and durability of the coating film are low. Moreover, when more than 100,000, the solubility to a solvent and a dispersibility are low. As the polyurethane resin, those having a cyclic structure and an ether group are preferable. The cyclic structure of the polyurethane resin contributes to rigidity, and the ether group contributes to flexibility. The polyurethane resin preferably has a Tg of 80 ° C to 140 ° C.

  In the back layer of the magnetic recording medium of the present invention, components other than inorganic oxide powder, carbon black, and binder can be appropriately contained. The back layer preferably contains a lubricant. Examples of the lubricant include fatty acids, fatty acid esters, and fatty acid amides. In particular, the inclusion of a fatty acid is essential for suppressing an increase in the coefficient of friction during repeated running while maintaining the strength. Further, by containing a fatty acid ester or an abrasive having a Mohs hardness of 8 or more, an increase in the coefficient of friction during repeated running can be suppressed and sliding durability can be improved. Furthermore, an increase in friction coefficient can be suppressed by adding an aromatic organic acid compound or a titanium coupling agent to improve dispersibility and increase strength. Moreover, an organic powder can be contained to suppress an increase in the coefficient of friction and to reduce the reflection. Examples of fatty acids that can be added include monobasic fatty acids having 8 to 24 carbon atoms. Among these, monobasic fatty acids having 8 to 18 carbon atoms are preferable. Specific examples thereof include lauric acid, caprylic acid, myristic acid, palmitic acid, stearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, and elaidic acid. In addition, amides of these fatty acids are also used.

Examples of fatty acid esters include monobasic fatty acids having 10 to 24 carbon atoms (which may include unsaturated bonds or branched) and monovalent, divalent, trivalent, and tetravalent carbon atoms having 2 to 12 carbon atoms. And mono-fatty acid ester, di-fatty acid ester or tri-fatty acid ester formed of any one of monovalent, pentavalent, and hexahydric alcohols (which may include an unsaturated bond or may be branched). . Specific examples thereof include butyl stearate, octyl stearate, amyl stearate, isooctyl stearate, octyl myristate, butoxyethyl stearate, anhydrosorbitan monostearate, anhydrosorbitan distearate, anhydrosorbitan tristearate. Rate.
The addition amount of the lubricant is preferably 0.1 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, when the total amount of the inorganic oxide powder and carbon black is 100 parts by mass.

  Examples of the abrasive having a Mohs hardness of 8 or more include α-alumina, chromium oxide, artificial diamond, and carbon-modified boron nitride (CBN). Among them, it is preferable to use those having an average particle size of 0.2 μm or less and a particle size of the back layer or less. Further, the height of the protrusion from the back layer surface is preferably 30 nm or less in order to reduce back shot. In the present invention, since the back layer can be made thin, sufficient sliding durability can be ensured only by adding a small amount of abrasive. As the aromatic organic acid compound, phenylphosphonic acid is preferable. The amount used is 0.03 to 10 parts by mass, preferably 0.5 to 5 parts by mass, when the total amount of the inorganic oxide powder and carbon black is 100 parts by mass.

  Examples of the organic powder include acrylic-styrene copolymer resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment.

  The glass transition temperature of the back layer is preferably 60 to 120 ° C., and the dry thickness is usually about 0.05 to 1.0 μm.

  The magnetic recording medium of the present invention can reduce the thickness of the tape to 4 to 9 μm because the back layer is difficult to be reflected on the magnetic layer even when it is wound and stored with high tension.

The magnetic recording medium of the present invention can be produced, for example, by depositing or applying a paint on the surface of the non-magnetic support under running so that the layer thickness after drying is within the predetermined range described above. It can be carried out. A plurality of magnetic paints or non-magnetic paints may be applied successively or simultaneously. As an application machine for applying 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 coating, spin coating, etc. can be used. As for these, for example, “Latest Coating Technology” (May 31, 1983) issued by General Technology Center Co., Ltd. can be referred to. When manufacturing a magnetic recording medium having two or more layers on one side, for example, the following method can be used.
(1) First, the lower layer is first applied by a coating apparatus such as a gravure, roll, blade, or extrusion, which is generally applied in the application of magnetic paint, and before the lower layer is dried, Japanese Patent Publication No. 1-44686, A method of coating the upper layer using a support pressure type extrusion coating apparatus or the like disclosed in JP-A-60-238179, JP-A-2-265672, and the like.
(2) Using a single coating head having two coating liquid passage slits disclosed in JP-A-63-88080, JP-A-2-17971, and JP-A-2-265672, A method of applying the lower layer almost simultaneously.
(3) A method of applying the upper and lower layers almost simultaneously using an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

The back layer can be prepared by applying a coating material for forming a back layer in which particulate components such as an abrasive and an antistatic agent and a binder are dispersed in an organic solvent to the surface opposite to the magnetic layer. Sufficient dispersibility can be secured if the amount of inorganic oxide powder used is larger than that of carbon black, so that it is possible to prepare a coating material for forming a back layer without carrying out roll kneading, which has been conventionally required. it can. Further, if the carbon black content ratio is low, the amount of residual cyclohexane after drying can be reduced even if cyclohexanone is used as a solvent.
The applied magnetic layer is dried after the magnetic powder orientation treatment of the ferromagnetic powder contained in the magnetic layer. The magnetic field orientation treatment can be appropriately performed by a method well known to those skilled in the art. The magnetic layer is subjected to a surface smoothing treatment using a super calender roll or the like after drying. By performing the surface smoothing treatment, voids generated by the removal of the solvent at the time of drying disappear and the filling rate of the ferromagnetic powder in the magnetic layer is improved. For this reason, a magnetic recording medium having high electromagnetic conversion characteristics can be obtained. As the calendering roll, a heat-resistant plastic roll such as epoxy, polyimide, polyamide, polyamideimide or the like is used. Moreover, it can also process with a metal roll.

  The magnetic recording medium of the present invention preferably has a surface with good smoothness. In order to improve the smoothness, for example, it is effective to subject the magnetic layer formed by selecting a specific binder as described above to the calendar treatment. In the calendering, the temperature of the calender roll is set to 60 to 100 ° C., preferably 70 to 100 ° C., particularly preferably 80 to 100 ° C., and the pressure is usually 100 to 500 kg / cm (98 to 490 kN / m), preferably 200 450 kg / cm (196 to 441 kN / m), particularly preferably 300 to 400 kg / cm (294 to 392 kN / m). The obtained magnetic recording medium can be cut into a desired size using a cutting machine or the like. The magnetic recording medium that has undergone the calendar process is generally heat-treated. Recently, it is important to reduce the thermal contraction rate for the linearity of the high-density magnetic recording medium (to ensure off-track margin). In particular, as the track becomes narrower, the shrinkage rate in the MD direction (longitudinal direction) under the use environment is required to be suppressed to 0.07% or less. As a means for reducing the heat shrinkage rate, there are a method in which heat treatment is performed in a web shape while handling at a low tension, and a method in which heat treatment is performed in a form in which tapes are laminated as in a bulk or cassette (thermo treatment).

EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples. In addition, unless otherwise indicated, the display of "part" in an Example shows a "mass part".
[Ferromagnetic powder type]
The ferromagnetic acicular metal powders used were classified as shown in the following table according to the average major axis length and the presence or absence of carbon black deposition on the surface. The average major axis length is a value after deposition of carbon black.

[Synthesis Example 1]
A synthesis example of polyurethane resin A used in the following magnetic layer coating is shown below. Polyester polyol (6 parts of polyester polyol comprising adipic acid / 2,2-dimethyl-1,3-propanediol / 1,3-propanediol = 50/35/15 and adipic acid / sulfoisophthalic acid / 2,2- 3 parts of a polyester polyol comprising dimethyl-1,3-propanediol / 1,6-hexanediol = 45/5/35/15) and 100 parts of 2-ethyl-2-butyl-1,3-propanediol are refluxed. It was equipped with a cooler and a stirrer, and dissolved in a 30% cyclohexanone solution at 60 ° C. under a nitrogen stream in a container previously purged with nitrogen. Next, 60 ppm of di-n-dibutyltin dilaurate was added as a catalyst and further dissolved for 15 minutes. Furthermore, 107 parts of 4,4-diphenylmethane diisocyanate (MDI) was added and heated at 90 ° C. for 6 hours to obtain polyurethane resin A having a mass average molecular weight of 51,000.

[Example 1]
Magnetic layer paint Ferromagnetic metal powder A 100 parts Polyurethane resin A 18 parts α-Al ₂ O ₃ (average particle diameter: 0.2 μm) 10 parts Carbon black (average particle diameter: 20 nm) 0.1 part Cyclohexanone 110 parts Methyl ethyl ketone 100 Parts Toluene 100 parts Butyl stearate 0.7 parts Stearic acid 1.5 parts

Lower layer paint α-iron oxide 85 parts Average major axis length: 0.15 μm, Average needle ratio: 7, BET specific surface area: 52 m ² / g
Carbon black 15 parts Average particle size: 20 nm
13 parts of vinyl chloride copolymer (MR110: manufactured by Nippon Zeon Co., Ltd.)
6 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
Phenylphosphonic acid 3 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Butyl stearate 2 parts Stearic acid 1 part

Back layer coating material α-iron oxide 80 parts Average major axis length: 0.15 μm, average needle ratio: 7, BET specific surface area: 52 m ² / g
Carbon black 20 parts Average particle size: 20 nm
Carbon black 3 parts Average particle size: 100 nm
13 parts of vinyl chloride copolymer (MR110 manufactured by Nippon Zeon Co., Ltd.)
6 parts of polyurethane resin (UR8200: Toyobo, sulfonic acid group-containing polyurethane resin)
Phenylphosphonic acid 3 parts α-Al ₂ O ₃ (average particle size 0.2 μm) 3 parts Cyclohexanone 140 parts Methyl ethyl ketone 170 parts Stearic acid 3 parts

About each of the said magnetic layer coating material, the coating material for lower layers, and the coating material for back layers, after kneading each component for 60 minutes with an open kneader, dispersion | distribution with a sand mill is performed for 540 minutes for a magnetic coating material, For 180 minutes. Trifunctional low molecular weight polyisocyanate compound was added to the obtained dispersion liquid in 3 parts for the magnetic layer paint, 5 parts for each of the lower layer paint and the back paint, and 40 parts of cyclohexanone was added to each, and an average of 1 μm was added. It filtered using the filter which has a hole diameter, and prepared the coating liquid of a magnetic layer, a lower layer, and a back layer.
Thickness of the obtained nonmagnetic layer coating solution so that the thickness of the lower layer after drying is 1.2 μm, and immediately after that, the thickness of the magnetic layer is 0.07 μm. Simultaneous coating was performed on an aramid support having a center surface average surface roughness of 2 nm at 6 μm, and the layers were oriented with a magnet having a magnetic force of 0.3 T while both layers were still wet. After drying, the surface was smoothed at a speed of 100 m / min, a linear pressure of 300 kg / cm (294 kN / m), and a temperature of 90 ° C. with a seven-stage calendar composed only of a metal roll. Thereafter, a back layer having a thickness of 0.5 μm was applied. Then, slit the tape to a 1/4 inch width, feed the slit product, and remove the magnetic layer with a tape cleaning device attached so that the nonwoven fabric and the razor blade press against the magnetic surface. The surface was cleaned to obtain a tape sample.

[Example 2]
In Example 1, the ferromagnetic metal powder of the magnetic layer paint was changed to B, and the addition amounts of carbon black and polyurethane resin A in the magnetic layer paint were changed to 0 part and 24 parts, respectively, as in Example 1. The tape of Example 2 was prepared by the method described above.

[Example 3]
In Example 1, the ferromagnetic metal powder of the magnetic layer coating was changed to C, and the addition amounts of carbon black and polyurethane resin A of the magnetic layer coating were changed to 0 part and 12 parts, respectively, as in Example 1. The tape of Example 3 was prepared by the method described above.

[Comparative Example 1]
In Example 1, the tape of Comparative Example 1 was prepared in the same manner as in Example 1 except that the addition amounts of carbon black for the magnetic layer coating and polyurethane resin A for the magnetic layer coating were changed to 0 part and 6 parts, respectively. did.

[Comparative Example 2]
In Example 1, the tape of Comparative Example 1 was prepared in the same manner as in Example 2 except that the ferromagnetic metal powder of the magnetic layer paint was changed to B and the polyurethane resin A of the magnetic layer paint was changed to 36 parts. did.

[Comparative Example 3]
The tape of Comparative Example 3 was prepared in the same manner as in Example 1, except that the ferromagnetic metal powder of the magnetic layer coating material was changed to D and the amount of carbon black added to the magnetic layer coating material was changed to 0 part. It was created.

[Comparative Example 4]
In Example 1, the ferromagnetic metal powder of the magnetic layer coating was changed to E, the amount of carbon black added to the magnetic layer coating was changed to 0 part, and the polyurethane resin A of the magnetic layer coating was changed to polyurethane resin (UR8200; Toyobo). A tape of Comparative Example 4 was prepared in the same manner as in Example 1 except that the product was changed to “manufactured”.

[Comparative Example 5]
In Example 1, the ferromagnetic metal powder of the magnetic layer paint was changed to F, the polyurethane resin A of the magnetic layer paint was changed to a polyurethane resin (UR8200; manufactured by Toyobo), and the addition amount of the polyurethane resin was changed to 36 parts. A tape of Comparative Example 5 was produced in the same manner as in Example 1 except that.

[Comparative Example 6]
In Example 1, the ferromagnetic metal powder of the magnetic layer coating was changed to G, and the addition amounts of carbon black and polyurethane resin A of the magnetic layer coating were changed to 0 part and 9 parts, respectively, as in Example 1. The tape of Comparative Example 6 was prepared by the method described above.

[Comparative Example 7]
The tape of Comparative Example 7 was prepared in the same manner as in Example 1 except that the ferromagnetic metal powder of the magnetic layer coating material was changed to H and the amount of polyurethane resin A added to the magnetic layer coating material was changed to 36 parts. It was created.

[Comparative Example 8]
The tape of Comparative Example 8 was prepared in the same manner as in Example 1, except that the ferromagnetic metal powder of the magnetic layer coating material was changed to H and the amount of carbon black added to the magnetic layer coating material was changed to 5 parts. It was created.

The obtained samples were evaluated by the following, and the results are shown in Table 2.
(Measuring method)
1. Electromagnetic conversion characteristics The electromagnetic conversion characteristics were measured using a drum tester. Write a signal with a recording wavelength of 0.2 μm using a 1.5T MIG head, reproduce it with an MR head, measure the voltage at a position ± 0.5 MHz away from the output obtained by a spectrum analyzer and noise as C / N value was determined. The relative speed of the tape and the head is 5 m / s, the C / N of Comparative Example 8 is 0 dB, and 0 dB or more is considered good.

2. The friction coefficient of the 100th pass of the tape The friction coefficient was measured by wrapping the tape 180 ° on a 6 mmφ (roughness 0.6 s) SUS420J bar and running 100 passes in a 23 ° C. 50% RH environment with a load of 20 g. The tension when the 100th pass tape was pulled was measured, and the coefficient of friction was calculated using Euler's equation. Less than 0.35 is considered good.

3. Calculation of the average major axis length The average major axis length of the C-coated ferromagnetic metal powder is shown in a photograph obtained by enlarging the electron micrograph (accelerating power: 20 kV, magnification: 50000 times) 4 times in the vertical and horizontal directions, respectively. The major axis length was measured for about 500 particles, and the average value was shown.

4). Calculation of volume ratio of non-magnetic substance The volume ratio of the non-magnetic substance was expressed by the volume estimated from the mass and specific gravity of each material.

5). Magnetic Cluster Size Measurement The obtained sample was subjected to DC erase by applying a magnetic field of 796 kA / m (10 kOe) using a vibrating sample magnetometer (manufactured by Toei Kogyo Co., Ltd.) and then taking it out. Using the magnetic force microscope mode of Nanoscope III manufactured by Digital Instruments, the erased sample was measured in a range of 5 × 5 μm at a lift height of 40 nm to obtain a magnetic force image. 70% of the standard deviation (rms) value of the magnetic force distribution was set as a threshold value, and the image was binarized to display only a portion having a magnetic force of 70% or more. This image was introduced into an image analyzer (KS400), and after noise removal and hole filling processing, an average area was calculated.

(Description of Examples and Comparative Examples)
In Examples 1 to 3, the average major axis length of the magnetic powder, the presence / absence of carbon black deposition on the surface of the magnetic powder, the volume ratio of the nonmagnetic substance, the squareness ratio in the longitudinal direction, and the magnetic properties during DC erasure It is an example of the tape whose cluster size is the range of a claim. In Examples 1 to 3, since carbon black is deposited on the surface of the magnetic powder, the friction coefficient is as low as less than 0.35, the volume ratio of the nonmagnetic substance is within the specified range, and the squareness ratio is 0. The CN ratio is good because it is as large as 80 or more and the magnetic cluster size is small.

Comparative Example 1 is an example of a tape that satisfies the scope defined by the claims except that the volume ratio of the nonmagnetic material is as small as 45.3% and the magnetic cluster size is less than 15000 nm ² . Although the squareness ratio is 0.88 and the magnetic cluster size is as small as 14800 nm ² , the amount of the resin for the present invention is reduced in order to reduce the volume ratio of the nonmagnetic substance. Thus, the coefficient of friction and electromagnetic conversion characteristics could not be measured.

Comparative Example 2 is an example of a tape that satisfies the scope defined by the claims except that the volume ratio of the nonmagnetic material is larger than 64.5% and the magnetic cluster size is 45000 nm ² or more. Since carbon black is deposited on the surface of the magnetic powder, the friction coefficient is as low as 0.25. The squareness ratio was good at 0.80, but the volume ratio of the nonmagnetic material was as large as 71.6%, and the noise increased because the magnetic cluster size increased, resulting in a decrease in the CN ratio.

Comparative Examples 3 and 6 are examples of tapes that satisfy the scope defined by the claims except that the average major axis length of the magnetic powder is larger than 50 nm and the magnetic cluster size is 45000 nm ² or more. In either case, the coefficient of friction is less than 0.35 because carbon black is deposited on the surface of the magnetic powder. The volume ratio of the non-magnetic substance is within the specified range and the squareness ratio is 0.80 or more, but the magnetic cluster size increases and the CN ratio deteriorates because the average major axis length of the magnetic powder is larger than 50 nm. .

Comparative Example 4 is an example of a tape that satisfies the scope defined by the claims except that the squareness ratio is less than 0.80. Since carbon black is deposited on the surface of the magnetic powder, the friction coefficient is less than 0.35. In addition, the volume ratio of the non-magnetic substance is 64.1% and the magnetic cluster size is 43200 nm ² within the specified range, so the noise is small, but the squareness ratio is as small as 0.62, so the output deteriorates and the CN ratio is Declined.

Comparative Example 5 is a tape that satisfies the range defined in the claims except that the volume ratio of the nonmagnetic material is larger than 64.5%, the magnetic cluster size is 45000 nm ² or more, and the squareness ratio is less than 0.80. It is an example. The coefficient of friction is low because carbon black is deposited on the surface of the magnetic powder, but the volume ratio of non-magnetic material is large and the magnetic cluster size is increased, and the CN ratio is degraded due to poor orientation. .

In Comparative Example 7, carbon black is not deposited on the surface of the magnetic powder, the ratio of the nonmagnetic substance is as large as 68.8%, and the size of the magnetic cluster size is as large as 45800 nm ^2. It is an example of a tape that satisfies the range. Since the surface of the magnetic powder is not coated with carbon black, the coefficient of friction is as large as 0.41 and the squareness ratio is good at 0.81, but the volume ratio of nonmagnetic material is large and the magnetic cluster size is increased. As a result, noise increased and the CN ratio deteriorated.

  Comparative Example 8 is an example of a tape that satisfies the scope defined by the claims except that the surface of the magnetic powder is not coated with carbon black. The CN ratio is 0 dB because the volume ratio of the non-magnetic substance is small, the magnetic cluster size is within the specified range, and the orientation is good, but the coefficient of friction is not applied to the surface of the magnetic powder. Because of the large sticking, running up to 100 passes was impossible.

Claims (2)

In a magnetic recording medium provided with a magnetic layer mainly composed of a ferromagnetic metal powder and a binder on a support, the ferromagnetic metal powder has carbon black deposited on at least a part of its particle surface, and The average major axis length is 20 to 50 nm, the volume ratio of the non-magnetic material present in the magnetic layer is 50.0 to 64.5%, and the average size of the magnetic cluster at the time of DC erase is 15000 nm. A magnetic recording medium characterized by being ² or more and less than 45000 nm ² and having a squareness ratio in the longitudinal direction in the plane of the magnetic layer of 0.80 to 0.95.   2. The magnetic recording medium according to claim 1, wherein the magnetic recording medium is reproduced by an MR head system.