JP4204486B2 - Magnetic recording medium - Google Patents

Description

  The present invention relates to a coating type magnetic recording medium, and more particularly to a magnetic recording medium that can achieve both a stable error rate and running durability in high-density recording.

  A magnetic recording medium in which a magnetic layer in which ferromagnetic powder is dispersed in a binder (binder) is provided on a nonmagnetic support is widely used as a recording tape, a video tape, a floppy (registered trademark) disk, or the like. . And various characteristics, such as the electromagnetic conversion characteristic, running durability, and running performance, are required to be at a high level. For example, an audio tape for recording and reproducing music is required to have a higher original sound reproduction capability. In addition, video tapes are required to have excellent electromagnetic conversion characteristics such as excellent reproduction of original images.

  In order to have excellent electromagnetic conversion characteristics to meet such demands, the ferromagnetic powder in the magnetic layer has been improved from γ iron oxide to a metal magnetic material, and has been increased in Hc and σs. In particular, metalization of 8 mm video tapes and video recording video tapes in the broadcasting field has been promoted. Recently, digital recording and reproduction of video and music has progressed, and it has become possible to faithfully reproduce the original picture and original sound by eliminating the reproduction deterioration of the original picture and original sound during the reproduction and editing process. An error rate (code error rate) at the time of reproduction is used for tape performance evaluation in this digitization, and Patent Document 1 proposes a method for separately evaluating an apparatus error rate and a tape error rate. Yes.

  In conventional coating-type magnetic recording media, in order to ensure the coating strength of the magnetic layer and the like, a curing agent mainly composed of an isocyanate compound is added to the coating solution for the magnetic layer and the like, followed by thermal curing treatment. Has been done. However, the isocyanate compound starts to cry on the surface of the magnetic layer, the irregularities of the back layer are transferred to the surface of the magnetic layer during the thermosetting treatment, roughen the surface of the magnetic layer, lower the SNR, and the protrusions on the back layer drop. A dent that causes out-out is generated in the magnetic layer, which deteriorates the error rate.

  In order to maintain a stable error rate on a high recording density medium, it is necessary to lower the high SNR and PW50 (half width of the pulse). In order to obtain a high SNR, it is effective to use a fine magnetic material, and in order to lower the PW50, it is effective to control the thickness of the magnetic layer (thinning).

Japanese Patent No. 2829972

However, there has been a problem that the coating strength of the magnetic layer deteriorates due to the finer magnetic material and the thinner magnetic layer, and the running durability of the magnetic recording medium deteriorates.
The present invention has been made to overcome the above-described problems of the prior art, and provides a magnetic recording medium that can achieve both a stable error rate and running durability in high-density recording.

  The present inventors diligently studied to achieve the above object. As a result, a binder containing a polyurethane resin having a glass transition temperature in the range of 100 to 200 ° C. is contained in the magnetic layer, a ferromagnetic powder having a minute average size is used in the thin magnetic layer, and the magnetic layer By making the layer of a high coercive force, we succeeded for the first time in obtaining a magnetic recording medium realizing a stable error rate and running durability, and completed the present invention.

That is, the object of the present invention is achieved by the following magnetic recording medium.
A magnetic recording medium in which a magnetic layer containing a ferromagnetic powder and a binder is provided on a nonmagnetic support,
The binder contained in the magnetic layer includes a polyurethane resin having a glass transition temperature of 100 to 200 ° C., does not contain an isocyanate compound, has a thickness of the magnetic layer of 20 to 150 nm, and a coercive force in the longitudinal direction of the magnetic layer. Is 160 to 240 kA / m, and the average size of the ferromagnetic powder is 20 to 60 nm.

  As described above, in the magnetic recording medium of the present invention, the magnetic layer is provided on the nonmagnetic support, the glass transition temperature of the polyurethane resin of the magnetic layer is 100 to 200 ° C., and the isocyanate compound is not contained in the magnetic layer. The thickness of the magnetic layer is 20 to 150 nm, the average size of the ferromagnetic powder in the magnetic layer is 20 to 60 nm, and the coercive force in the longitudinal direction of the magnetic layer is 160 to 240 kA / m. Thereby, it is possible to provide a magnetic recording medium capable of achieving both a stable error rate and running durability in high-density recording.

  The magnetic recording medium of the present invention will be described in detail below.

[Magnetic layer]
<Polyurethane resin having a Tg of 100 to 200 ° C.>
In the magnetic recording medium of the present invention, at least the binder used in the magnetic layer contains a polyurethane resin having a Tg of 100 to 200 ° C.

  In the present invention, when a binder containing a polyurethane resin having a high Tg at least in the magnetic layer is used, plastic flow of the magnetic layer due to frictional heat generated by sliding between the recording / reproducing head and the surface of the magnetic layer is suppressed. Coating strength can be obtained, and excellent running durability can be achieved. In particular, the effect is remarkable when the magnetic layer is a thin layer. Furthermore, if a binder containing the above polyurethane resin is used in the nonmagnetic layer described later, it is not necessary to add an isocyanate compound and perform a heat curing treatment in order to ensure the coating film strength. This is preferable because the out can be reduced.

  In the present invention, the Tg of the polyurethane resin contained in the binder is in the range of 100 to 200 ° C, preferably in the range of 120 to 170 ° C. When Tg is 100 ° C. or higher, good running durability can be obtained without lowering the coating film strength. Moreover, if Tg is 200 degrees C or less, the smoothing effect at the time of a calendar process will be acquired and a favorable electromagnetic conversion characteristic and driving | running | working durability will be acquired.

  The polyurethane resin preferably has a urethane group concentration in the range of 2.5 to 6.0 mmol / g, and more preferably 3.0 to 4.5 mmol / g. When the urethane group concentration is 2.5 mmol / g or more, the coating film has a high Tg and good durability can be obtained. When the urethane group concentration is 6.0 mmol / g or less, the solvent solubility is high and the dispersibility is good. It is. If the urethane group concentration is excessively high, it is inevitably impossible to contain a polyol, which makes it difficult to control the molecular weight.

  The polyurethane resin preferably has a weight average molecular weight (Mw) in the range of 30,000 to 200,000, and more preferably in the range of 50,000 to 100,000. When the molecular weight is 30,000 or more, the coating film strength is high and good durability can be obtained, and when it is 200,000 or less, the solvent solubility is high and the dispersibility is good.

The polar group of the polyurethane resin is preferably —SO ₃ M, —OSO ₃ M, —PO ₃ M ₂ , or —COOM, and —SO ₃ M, —OSO ₃ M (where M is a hydrogen atom, an alkali metal, More preferably, it is selected from ammonium. The polar group content is preferably 1 × 10 ⁻⁵ to 2 × 10 ⁻⁴ eq / g. When it is 1 × 10 ⁻⁵ eq / g or more, the adsorptivity to the ferromagnetic powder and the nonmagnetic powder is high and the dispersibility is good. On the other hand, when it is 2 × 10 ⁻⁴ eq / g or less, the solvent solubility is high and the dispersibility is good.

  The OH group content in the polyurethane resin is preferably 2 to 20 per molecule, and more preferably 3 to 15 per molecule. By containing two or more OH groups per molecule, it reacts well with the isocyanate curing agent, so that the coating strength is high and good durability can be obtained. On the other hand, when it contains 15 or less OH groups per molecule, the solvent solubility is high and the dispersibility is good. As a compound used for imparting an OH group, a compound having an OH group having a trifunctional or higher functional group such as trimethylolethane, trimethylolpropane, trimellitic anhydride, glycerin, pentaerythritol, hexanetriol, or a trifunctional or higher functional OH group Branched polyesters or polyether esters can be used. Among these, trifunctional ones are preferable. If it is tetrafunctional or more, the reaction with the curing agent becomes too fast and the pot life is shortened.

Examples of the polyol component of the polyurethane resin contained in the binder in the present invention include polyester polyols, polyether polyols, polycarbonate polyols, polyether ester polyols, diol compounds having ring structures such as polyolefin polyols and dimer diols, and long-chain alkyl chains. These known polyols can be used.
The molecular weight of the polyol is preferably about 500 to 2,000. When the molecular weight is within the above range, the weight ratio of the diisocyanate can be substantially increased, so that the urethane bond is increased, the interaction between molecules is increased, the glass transition temperature is high, and a coating film having high mechanical strength is obtained. be able to.

The diol component is preferably a diol compound having a cyclic structure and a long alkyl chain. Here, the long alkyl chain refers to an alkyl group having 2 to 18 carbon atoms. Since it has a bent structure when it has a cyclic structure and a long alkyl chain, it is excellent in solubility in a solvent. As a result, the spread of urethane molecular chains adsorbed on the surface of the magnetic material or non-magnetic material in the coating solution can be increased, so that there is an effect of improving dispersion stability and excellent electromagnetic conversion characteristics (SNR) can be obtained. . Moreover, polyurethane having a high glass transition temperature can be obtained by having a cyclic structure.
The diol compound having a cyclic structure and a long alkyl chain is particularly preferably a diol compound represented by the following formula.

In the formula, Z is a cyclic structure selected from a cyclohexane ring, a benzene ring and a naphthalene ring, R ₁ and R ₂ are alkylene groups having 1 to 18 carbon atoms, and R ₃ and R ₄ are 2 to 18 carbon atoms. It is an alkyl group.

  The diol component is preferably contained in the polyurethane resin in an amount of 10 to 50 wt%, more preferably 15 to 40 wt%. When it is 10 wt% or more, the solvent solubility is high and the dispersibility is good, and when it is 50 wt% or less, a coating film having high Tg and excellent durability can be obtained.

  The polyurethane resin may be used in combination with a diol component other than the diol component as a chain extender. When the molecular weight of the diol component is increased, the diisocyanate content is inevitably reduced, so that the urethane bond in the polyurethane is reduced and the coating strength is inferior. Therefore, in order to obtain a sufficient coating strength, the chain extender used in combination is preferably a low molecular weight diol having a molecular weight of less than 500, preferably 300 or less.

  Specifically, ethylene glycol, 1,3-propanediol, propylene glycol, neopentyl glycol (NPG), 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,8 -Octanediol, 1,9-nonanediol, 2,2-dimethyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,2-diethyl-1,3-propane Aliphatic glycols such as diols, cyclohexanedimethanol (CHDM), cyclohexanediol (CHD), alicyclic glycols such as hydrogenated bisphenol A (H-BPA) and their ethylene oxide adducts, propylene oxide adducts, bisphenol A (BPA), bisphenol S, bisphenol P, bisphenol Aromatic glycols and their ethylene oxide adducts such Lumpur F, it is possible to use propylene oxide adducts. Particularly preferred is hydrogenated bisphenol A.

  As the diisocyanate used for the polyurethane resin, known ones can be used. Specifically, TDI (tolylene diisocyanate), MDI (diphenylmethane diisocyanate), p-phenylene diisocyanate, o-phenylene diisocyanate, m-phenylene diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, isophorone diisocyanate and the like are preferable.

  In the present invention, the polyurethane resin may be used in combination with a vinyl chloride synthetic 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.

  The polyurethane resin can be used in combination with various synthetic resins in addition to the vinyl chloride resin. Examples of such synthetic resins include ethylene / vinyl acetate copolymers, cellulose derivatives such as nitrocellulose resins, acrylic resins, polyvinyl acetal resins, polyvinyl butyral resins, epoxy resins, and phenoxy resins. These can be used alone or in combination.

  When using the said polyurethane resin and the said synthetic resin together, it is preferable that 10-90 mass% of polyurethane resins contained in a magnetic layer are contained in binder, and it is further contained 20-80 mass%. The content is preferably 25 to 60% by mass, particularly preferably. The vinyl chloride resin is preferably contained in the binder in an amount of 10 to 80% by mass, more preferably 20 to 70% by mass, and particularly preferably 30 to 60% by mass. preferable.

  Moreover, hardeners, such as a polyisocyanate compound, can be used with the binder of this invention. 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 weight of the resin component and the curing agent (that is, the binder) is preferably in the range of usually 15 to 40 parts by weight, more preferably 20 to 30 parts by weight with respect to 100 parts by weight of the ferromagnetic powder. It is.

<Ferromagnetic powder>
In the present invention, the average size of the ferromagnetic powder used in the magnetic layer is 20 to 60 nm. In the present invention, in order to obtain a stable error rate, a fine ferromagnetic powder having an average size for obtaining a higher SNR of 20 to 60 nm, preferably 20 to 50 nm, and more preferably 25 to 45 nm is used.

  In the present invention, the type of the ferromagnetic powder used in the magnetic layer is not particularly limited as long as it has an average size of 20 to 60 nm and has a predetermined coercive force described later. Examples of the ferromagnetic powder that can be used in the present invention include a ferromagnetic metal powder having an average major axis length of 20 to 60 nm or a hexagonal ferrite ferromagnetic powder having an average plate diameter of 20 to 60 nm.

(Ferromagnetic metal powder)
The ferromagnetic metal powder used in the magnetic layer in the present invention is a cobalt-containing ferromagnetic iron oxide or ferromagnetic alloy powder having an S _BET specific surface area of 40 to 80 m ² / g, preferably 50 to 70 m ² / g. Is preferred. The crystallite size is 12 to 25 nm, preferably 13 to 22 nm, and particularly preferably 14 to 20 nm. The average major axis length is 20 to 60 nm, preferably 20 to 50 μm, and particularly preferably 25 to 45 μm. Examples of the ferromagnetic powder include Fe, Fe—Co, Fe—Ni, and Co—Ni—Fe containing yttrium, and the yttrium content in the ferromagnetic powder is the ratio of yttrium atoms to iron atoms, Y / Fe is preferably 0.5 atomic% to 20 atomic%, and more preferably 5 to 10 atomic%. When the content is 0.5 atomic% or more, high saturation magnetization (σ _S ) of the ferromagnetic powder can be achieved, magnetic characteristics can be improved, and good electromagnetic conversion characteristics can be obtained. When the content is 20 atomic% or less, the iron content is appropriate, the magnetic characteristics are good, and the electromagnetic conversion characteristics are improved. Furthermore, within a range of 20 atomic% or less with respect to 100 atomic% of iron, aluminum, silicon, sulfur, scandium, titanium, vanadium, chromium, manganese, copper, zinc, molybdenum, rhodium, palladium, tin, antimony, boron, Barium, tantalum, tungsten, rhenium, gold, 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.

An example of the manufacturing method of the ferromagnetic powder which introduce | transduced cobalt and yttrium which can be used in this invention is shown. An example in which iron oxyhydroxide obtained by blowing an oxidizing gas into an aqueous suspension in which a ferrous salt and an alkali are mixed is used as a starting material can be given. As the type of this iron oxyhydroxide, α-FeOOH is preferable, and as its production method, ferrous salt is neutralized with alkali hydroxide to form an aqueous suspension of Fe (OH) _2. There is a first production method in which an oxidizing gas is blown into a needle-like α-FeOOH. On the other hand, there is a second production method in which ferrous salt is neutralized with alkali carbonate to form an aqueous suspension of FeCO ₃ , and oxidizing gas is blown into this suspension to form spindle-shaped α-FeOOH. Such an iron oxyhydroxide is obtained by reacting a ferrous salt aqueous solution and an alkaline aqueous solution to obtain an aqueous solution containing ferrous hydroxide and oxidizing it by air oxidation or the like. Is preferred. At this time, Ni salt, alkaline earth element salt such as Ca salt, Ba salt and Sr salt, Cr salt and Zn salt may coexist in the ferrous salt aqueous solution. The particle shape (axial ratio) etc. can be prepared by selecting and using.

  As the ferrous salt, ferrous chloride, ferrous sulfate and the like are preferable. As the alkali, sodium hydroxide, aqueous ammonia, ammonium carbonate, sodium carbonate and the like are preferable. Moreover, as a salt which can coexist, chlorides, such as nickel chloride, calcium chloride, barium chloride, strontium chloride, chromium chloride, zinc chloride, are preferable. Next, when introducing cobalt into iron, before introducing yttrium, an aqueous solution of a cobalt compound such as cobalt sulfate or cobalt chloride is stirred and mixed into the iron oxyhydroxide slurry. After preparing a slurry of iron oxyhydroxide containing cobalt, an aqueous solution containing a compound of yttrium can be added to the slurry and mixed by stirring.

  In addition to yttrium, neodymium, samarium, praseodymium, lanthanum, and the like can be introduced into the ferromagnetic powder of the present invention. These can be introduced using chlorides such as yttrium chloride, neodymium chloride, samarium chloride, praseodymium chloride, lanthanum chloride, and nitrates such as neodymium nitrate and gadolinium nitrate. Also good. There are no particular restrictions on the shape of the ferromagnetic powder, but needle-shaped, granular, dice-shaped, rice-grained and plate-shaped ones are usually used. It is particularly preferable to use acicular ferromagnetic powder.

The above resin component, curing agent and ferromagnetic 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 coating material to obtain a magnetic coating material. The kneading and dispersing can be performed according to a usual method. In addition to the above components, the magnetic paint contains an abrasive such as α-Al ₂ O ₃ and Cr ₂ O _3, an antistatic agent such as carbon black, a lubricant such as a fatty acid, a fatty acid ester, and silicone oil, a dispersion It may contain a commonly used additive or filler such as an agent.

(Hexagonal ferrite ferromagnetic powder)
Examples of the hexagonal ferrite contained in the magnetic layer in the present invention include barium ferrite, strontium ferrite, lead ferrite, calcium ferrite substitutes, and Co substitutes. Specific examples include magnetoplumbite-type barium ferrite and strontium ferrite, magnetoplumbite-type ferrite whose particle surface is coated with spinel, and magnetoplumbite-type barium ferrite and strontium ferrite partially containing a spinel phase. . In addition to predetermined atoms, Al, Si, S, Sc, Ti, V, Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg , Pb, Bi, La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, Ge, and Nb may be included. In general, elements added with Co-Zn, Co-Ti, Co-Ti-Zr, Co-Ti-Zn, Ni-Ti-Zn, Nb-Zn-Co, Sb-Zn-Co, Nb-Zn, etc. Can be used. Some raw materials and manufacturing methods contain specific impurities.

  The average plate diameter of the hexagonal ferrite powder that can be used in the present invention varies depending on the recording density, but is usually 15 to 60 nm, preferably 20 to 50 nm, and particularly preferably 20 to 35 nm. Here, the plate diameter means the maximum hexagonal diameter of the hexagonal ferrite bottom surface of the hexagonal ferrite magnetic powder, and the average plate diameter is an arithmetic average thereof. In particular, in order to increase the track density, when reproducing with a magnetoresistive head, it is necessary to reduce the noise, and the plate diameter is preferably 35 nm or less. However, if it is in the range of 20 to 60 nm, the influence of thermal fluctuations is exerted. Since stable magnetization not received can be expected and noise can be suppressed, it is suitable for high-density magnetic recording. The plate ratio (plate diameter / plate thickness) is desirably 1 to 15, and preferably 1 to 7. If the plate ratio is 1 or more, sufficient orientation can be obtained while maintaining high filling in the magnetic layer. If the plate ratio is 15 or less, it is difficult to be affected by stacking between particles, and noise does not increase.

The specific surface area according to the BET method in the above particle size range is suitably 30 to ²⁰⁰ m ² / g. The specific surface area is generally referred to as an arithmetic calculation value from the particle plate diameter and plate thickness. The distribution of particle plate diameter and plate thickness is generally preferably as narrow as possible. Although numerical conversion is difficult, it can be compared by randomly measuring 500 particles from a particle TEM photograph. In many cases, the distribution is not a normal distribution, but when calculated and expressed as a standard deviation with respect to the average size, σ / average size = 0.1 to 2.0. In order to sharpen the particle size distribution, the particle generation reaction system is made as uniform as possible, and the generated particles are subjected to a distribution improvement process. For example, a method of selectively dissolving ultrafine particles in an acid solution is also known.

The coercive force (Hc) measured with the hexagonal ferrite ferromagnetic powder is in the range of about 119.4 to 318.4 kA / m (1500 to 4000 Oe). A higher coercive force (Hc) is advantageous for high-density recording, but is limited by the capacity of the recording head. The coercive force (Hc) can be controlled by the particle size (plate diameter / plate thickness), the type and amount of the contained element, the substitution site of the element, the particle generation reaction conditions and the like.
Saturation magnetization of the hexagonal ferrite ferromagnetic powder ([sigma] s) is suitably be 30~80A · m ² / kg, preferably a 40~60A · m ² / kg. It tends to be smaller as the particles become smaller. Production methods include a method of reducing the crystallization temperature or heat treatment temperature time, a method of increasing the amount of the compound to be added, and a method of increasing the surface treatment amount. It is also possible to use W-type hexagonal ferrite. When the magnetic material is dispersed, the surface of the magnetic material particles is also treated with a material suitable for the dispersion medium and the polymer.

  As the surface treatment agent, an inorganic compound or an organic compound is used. Typical examples of the main compound include oxides or hydroxides such as Si, Al, and P, various silane coupling agents, and various titanium coupling agents. The addition amount is 0.1 to 10% with respect to the magnetic substance. The pH of the magnetic material is also important for dispersion. Usually, about 4 to 12 has optimum values depending on the dispersion medium and polymer, but about 6 to 11 is selected from the chemical stability and storage stability of the medium. Water contained in the magnetic material also affects the dispersion. Although there is an optimum value depending on the dispersion medium and polymer, a range of 0.01 to 2.0% is usually selected.

  Hexagonal ferrite production methods include: (1) Barium oxide, iron oxide, metal oxide that replaces iron and boron oxide as a glass-forming substance are mixed so as to have a desired ferrite composition, and then melted and quenched. A glass crystallization method for obtaining barium ferrite crystal powder by washing and crushing after making it amorphous and then reheating, (2) neutralizing barium ferrite composition metal salt solution with alkali, and by-products After removal, liquid-phase heating at 100 ° C or higher, washing, drying and pulverization to obtain barium ferrite crystal powder, (3) neutralizing barium ferrite composition metal salt solution with alkali, by-product There is a coprecipitation method in which the product is removed, dried, treated at 1100 ° C. or lower, and pulverized to obtain a barium ferrite crystal powder, but the present invention does not select a production method.

<Thickness of magnetic layer and coercive force Hc>
In the present invention, the magnetic layer has a thickness of 20 to 150 nm. The thickness of the magnetic layer is preferably 20 to 100 nm, and more preferably 30 to 80 μm.
If the thickness of the magnetic layer is 150 nm or less, the value of PW50 (half width of pulse) can be lowered, and a stable error rate can be obtained during high density recording, which is preferable.

  In the present invention, the value of the coercive force (Hc) in the longitudinal direction or the in-plane direction of the magnetic layer reduces the self-demagnetization loss and achieves high density recording in the range of 160 to 240 kA / m (2000 to 3000 Oe). ) Is appropriate. If the coercive force (Hc) is 160 kA / m or more, good high density recording can be achieved. On the other hand, the higher the value of the coercive force (Hc), the higher the SNR is obtained at the time of high density recording. However, if the value is too high, the erasure rate decreases, so in the present invention the longitudinal direction or in-plane direction of the magnetic layer. It is appropriate that the upper limit value of Hc is 240 kA / m. The value of the coercive force (Hc) is preferably in the range of 160 to 207 kA / m (2200 to 2800 Oe), more preferably in the range of 183.1 to 214.9 kA / m (2300 to 2700 Oe).

  In the present specification, the longitudinal direction of the magnetic layer is a direction that coincides with the running direction of the tape-like magnetic recording medium, and is a direction that is perpendicular to the width direction. In the present specification, the in-plane direction of the magnetic layer is a direction along the disk surface in the disk-shaped magnetic recording medium.

In the present invention, in order to realize the coercive force (Hc) in the longitudinal direction or in-plane direction of the magnetic layer, in the present invention, for example, a fine ferromagnetic powder having an average size of 20 to 60 nm, It is appropriate to use a ferromagnetic powder in which (σs) is 110 to 155 Am ² / kg and the coercive force (Hc) of the ferromagnetic powder is 159 kA / m or more. In addition, it can be adjusted by changing the degree of filling of the magnetic layer by controlling the strength of the calendar process.

Next, additives that can be contained in the magnetic layer together with the binder and the ferromagnetic powder in the present invention will be described.
<Carbon black>
In the present invention, examples of the carbon black used in the magnetic layer include a furnace for rubber, a thermal for rubber, a black for color, and acetylene black. Specific surface area is 5 to 500 m ² / g, DBP oil absorption is 10 to 400 ml / 100 g, average particle size is 5 to 300 nm, pH is 2 to 10, water content is 0.1 to 10%, tap density is 0.1 ˜1 g / ml is preferred. Specific examples include those described in WO98 / 35345.

  Carbon black functions to prevent the magnetic layer from being charged, reduce the coefficient of friction, impart light-shielding properties, and improve the film strength. These differ depending on the carbon black used. Therefore, when the present invention has a multi-layer structure, the type, amount, and combination of each layer are changed, and the layer is properly used according to the purpose based on the above-described various characteristics such as particle diameter, oil absorption, conductivity, and pH. Of course, it is possible, rather it should be optimized at each layer.

<Abrasive>
In the present invention, the magnetic layer can contain an abrasive. As an abrasive, α-alumina, β-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, corundum, artificial diamond, silicon nitride, silicon carbide, titanium carbide, titanium oxide having an α conversion rate of 90% or more Known materials having a Mohs hardness of 6 or more, such as silicon dioxide and boron nitride, are used alone or in combination. Further, a composite of these abrasives (a product obtained by surface-treating an abrasive with another abrasive) may be used.

  In the present invention, the abrasive may contain a compound or element other than the main component, but the effect is not changed if the main component is 90% or more. The average particle size of these abrasives is preferably 0.01 to 2 μm, and in particular, in order to improve electromagnetic conversion characteristics (SNR), it is preferable that the particle size distribution is narrow. Further, in order to improve the durability, it is possible to combine abrasives having different particle diameters as necessary, or to obtain the same effect by widening the particle size distribution even with a single abrasive.

The tap density of the abrasive is preferably 0.3 to ² g / ml, the water content is 0.1 to 5%, the pH is 2 to 11, and the specific surface area is preferably ¹ to 30 m ² / g. The shape of the abrasive used in the present invention may be any of a needle shape, a spherical shape, and a dice shape, but those having a corner in a part of the shape are preferable because of high polishing properties. Specific examples include those described in WO 98/35345. Above all, the use of diamond as described therein is effective in improving running durability and electromagnetic conversion characteristics. The particle size and amount of the abrasive added to the magnetic layer should of course be set to optimum values.

<Other additives>
Examples of other additives that can be added to the magnetic layer in the present invention include those having a lubricating effect, an antistatic effect, a dispersing effect, a plasticizing effect, and the like, and a combination of these can improve the overall performance. . As a material that exhibits a lubricating effect, a lubricant that has a remarkable effect on adhesion caused by friction between the material surfaces is used. There are two types of lubricants. Lubricants used in magnetic recording media cannot be determined completely as fluid lubrication or boundary lubrication, but can be classified by general concepts such as higher fatty acid esters, liquid paraffin, silicone derivatives, etc. They are classified into long-chain fatty acids that exhibit boundary lubrication, fluorine-based surfactants, fluorine-containing polymers, and the like. In coated media, the lubricant exists in the state dissolved in the binder and partly adsorbed on the surface of the ferromagnetic powder, and the lubricant moves to the surface of the magnetic layer. Is determined by the compatibility of the binder and the lubricant. When the compatibility between the binder and the lubricant is high, the transition speed is low, and when the compatibility is low, the transition speed is low. One way of thinking about compatibility is to compare the solubility parameters of the two. Nonpolar lubricants are effective for fluid lubrication, and polar lubricants are effective for boundary lubrication.

In the present invention, it is preferable to combine a higher fatty acid ester exhibiting fluid lubrication having different properties and a long chain fatty acid exhibiting boundary lubrication, and it is more preferable to combine at least three kinds. A solid lubricant can also be used in combination with these.
As the solid lubricant, for example, molybdenum disulfide, tungsten disulfide graphite, boron nitride, fluorinated graphite and the like are used. Examples of long-chain fatty acids exhibiting boundary lubrication include monobasic fatty acids having 10 to 24 carbon atoms (which may include unsaturated bonds or branched), and metal salts thereof (Li, Na, K, Cu etc.). Examples of the fluorine-containing surfactant and fluorine-containing polymer include fluorine-containing silicones, fluorine-containing alcohols, fluorine-containing esters, fluorine-containing alkyl sulfates, and alkali metal salts thereof. Higher fatty acid esters exhibiting fluid lubrication include monobasic fatty acids having 10 to 24 carbon atoms (which may include unsaturated bonds or branched) and monovalent or divalent carbon atoms having 2 to 12 carbon atoms. Mono fatty acid ester, di fatty acid ester or tri fatty acid ester, alkylene, consisting of any one of trivalent, tetravalent, pentavalent and hexavalent alcohols (which may contain unsaturated bonds or may be branched) Examples thereof include fatty acid esters of monoalkyl ethers of oxide polymers. Liquid paraffin and dialkylpolysiloxane (alkyl is 1 to 5 carbon atoms), dialkoxypolysiloxane (alkoxy is 1 to 4 carbon atoms), monoalkyl monoalkoxypolysiloxane (alkyl is 1 to carbon atoms) 5; alkoxy has 1 to 4 carbon atoms), silicone oil such as phenyl polysiloxane and fluoroalkylpolysiloxane (alkyl is 1 to 5 carbon atoms), silicone with polar group, fatty acid-modified silicone, fluorine-containing silicone, etc. Is mentioned.

  Other lubricants include monovalent, divalent, trivalent, tetravalent, pentavalent, and hexavalent alcohols having 12 to 22 carbon atoms (which may contain an unsaturated bond or may be branched), and 12 carbon atoms. Alkoxy alcohol of -22 (which may contain an unsaturated bond or may be branched), alcohol such as fluorine-containing alcohol, polyolefin such as polyethylene wax and polypropylene, polyglycol such as ethylene glycol and polyethylene oxide wax, alkyl Examples thereof include phosphate esters and alkali metal salts thereof, alkyl sulfate esters and alkali metal salts thereof, polyphenyl ethers, fatty acid amides having 8 to 22 carbon atoms, and aliphatic amines having 8 to 22 carbon atoms.

  As an antistatic effect, a dispersion effect, a plasticizing effect, etc., phenylphosphonic acid, specifically, “PPA” of Nissan Chemical Co., Ltd., α-naphthylphosphoric acid, phenylphosphoric acid, diphenylphosphoric acid, p-ethylbenzenephosphone Acid, phenylphosphinic acid, aminoquinones, various silane coupling agents, titanium coupling agents, fluorine-containing alkyl sulfates and alkali metal salts thereof can be used.

  The lubricant used in the present invention is particularly preferably a fatty acid and a fatty acid ester, and specific examples include those described in WO 98/35345. In addition to these, different lubricants and additives can be used in combination.

In addition, nonionic surfactants such as alkylene oxide, glycerin, glycidol, alkylphenol ethylene oxide adducts, cyclic amines, ester amides, quaternary ammonium salts, hydantoin derivatives, heterocyclics, phosphonium or sulfoniums, etc. Cationic surfactants, anionic surfactants containing acidic groups such as carboxylic acid, sulfonic acid, phosphoric acid, sulfate ester group, phosphate ester group, amino acids, aminosulfonic acids, sulfuric acid or phosphate esters of amino alcohol, alkyl bedda An amphoteric surfactant such as in-type can also be used.
These surfactants are described in detail in “Surfactant Handbook” (published by Sangyo Tosho Co., Ltd.). These lubricants, antistatic agents, and the like are not necessarily 100% pure, and may contain impurities such as isomers, unreacted materials, side reaction products, decomposed products, and oxides in addition to the main components. These impurities are preferably 30% or less, more preferably 10% or less.
In the present invention, it is also preferable to use a combination of a monoester and a diester as described in WO 98/35345 as a fatty acid ester.

  In the present invention, the content of the lubricant in the magnetic layer is preferably 5 to 30 parts by mass with respect to 100 parts by mass of the ferromagnetic powder.

  These lubricants and surfactants used in the present invention have different physical actions, and the type, amount, and combination ratio of lubricants that produce a synergistic effect are optimally determined according to the purpose. It should be done. Control the leaching to the surface using fatty acids with different melting points in the nonmagnetic layer and magnetic layer, and control the amount of surfactant to control the leaching to the surface using esters with different boiling points, melting points and polarities. In this case, it is conceivable to improve the stability of coating and increase the lubricating effect by increasing the additive amount of the lubricant in the intermediate layer. Of course, it is not limited to the examples shown here. In general, the total amount of the lubricant is selected in the range of 0.1 to 50 parts by mass, preferably 2 to 25 parts by mass with respect to 100 parts by mass of the magnetic powder or nonmagnetic powder.

  Further, all or a part of the additives used in the present invention may be added in any step of magnetic coating production. For example, when mixed with a magnetic material before the kneading step, the magnetic material, the binder, and the solvent In the case of adding in the kneading step, the case of adding in the dispersing step, the case of adding after the dispersing, the case of adding just before coating, and the like. Moreover, after applying a magnetic layer according to the purpose, the object may be achieved by applying a part or all of the additive by simultaneous or sequential application. Depending on the purpose, a lubricant can be applied to the surface of the magnetic layer after calendering or after completion of the slit.

[Nonmagnetic layer]
The magnetic recording medium of the present invention can have a nonmagnetic layer as a lower layer of the magnetic layer. The nonmagnetic layer will be described in detail below.
The nonmagnetic layer in the present invention exhibits its effect if it is substantially nonmagnetic. For example, even if it contains a small amount of magnetic powder as an impurity, the effect of the present invention is exhibited. Needless to say, the configuration can be regarded as substantially the same as that of the present invention.
Here, the substantially nonmagnetic layer means that the residual magnetic flux density of the nonmagnetic layer is 10 T · m or less or the coercive force (Hc) is 8 kA / m (100 Oe) or less, preferably the residual magnetic flux density. And shows no coercive force. Further, when the nonmagnetic layer contains magnetic powder, it is preferable to contain less than half of the total inorganic powder of the nonmagnetic layer. Further, as a lower layer, a soft magnetic layer containing soft magnetic powder and a binder may be formed instead of the nonmagnetic layer. The thickness of the soft magnetic layer is the same as that of the nonmagnetic layer.

  In the present invention, the nonmagnetic layer is preferably composed mainly of a nonmagnetic inorganic powder and a binder. The nonmagnetic inorganic powder used in the nonmagnetic layer can be selected from inorganic compounds such as metal oxides, metal carbonates, metal sulfates, metal nitrides, metal carbides, metal sulfides, and the like. Examples of inorganic compounds include α-alumina, β-alumina, γ-alumina, θ-alumina, silicon carbide, chromium oxide, cerium oxide, α-iron oxide, hematite, goethite, corundum, silicon nitride with an α conversion rate of 90% or more. , Titanium carbide, titanium oxide, silicon dioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide, boron nitride, zinc oxide, calcium carbonate, calcium sulfate, barium sulfate, molybdenum disulfide, etc. are used alone or in combination . Particularly preferred are titanium dioxide, zinc oxide, iron oxide and barium sulfate because of their small particle size distribution and many means for imparting functions, and more preferred are titanium dioxide and α-iron oxide.

  The average particle diameter of the nonmagnetic inorganic powder is preferably 5 to 200 nm. Further, if necessary, nonmagnetic inorganic powders having different average particle diameters can be combined, and even a single nonmagnetic inorganic powder can have the same effect by widening the particle size distribution. The average particle size of the nonmagnetic inorganic powder is particularly preferably 10 to 200 nm. In particular, when the nonmagnetic inorganic powder is a granular metal oxide, the average particle diameter is preferably 80 nm or less, and when the nonmagnetic inorganic powder is an acicular metal oxide, the average major axis length is preferably 300 nm or less. More preferably, it is 200 nm or less.

The tap density is usually 0.05 to 2 g / ml, preferably 0.2 to 1.5 g / ml. The water content of the nonmagnetic inorganic powder is usually 0.1 to 5% by mass, preferably 0.2 to 3% by mass, and more preferably 0.3 to 1.5% by mass. The pH of the nonmagnetic inorganic powder is usually 2 to 11, but the pH is particularly preferably between 5.5 and 10. The specific surface area of the nonmagnetic inorganic powder is generally from 1 to 100 m ² / g, is preferably 5~80m ² / g, more preferably from 10 to 70 m ² / g. The crystallite size of the nonmagnetic inorganic powder is preferably 0.004 to 1 μm, and more preferably 0.04 to 0.1 μm. The oil absorption using DBP (dibutyl phthalate) is usually 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 usually from 1 to 12, and more preferably from 3 to 6. The shape may be any of a needle shape, a spherical shape, a polyhedron shape, and a plate shape. The Mohs hardness is preferably 4-10. SA (stearic acid) adsorption amount of nonmagnetic inorganic powder is 1 to 20 [mu] mol / m ^2, more preferably from 2 to 15 [mu] mol / m ^2,, further preferably 3 to 8 [mu] mol / m ^2. The pH is preferably between 3 and 6.

It is preferable that Al ₂ O ₃ , SiO ₂ , TiO ₂ , ZrO ₂ , SnO ₂ , Sb ₂ O ₃ , ZnO, and Y ₂ O ₃ exist on the surface of these nonmagnetic inorganic powders by surface treatment. Particularly preferred for dispersibility are Al ₂ O ₃ , SiO ₂ , TiO ₂ , and ZrO ₂ , but more preferred are Al ₂ O ₃ , SiO ₂ , and ZrO ₂ . These may be used in combination or may be used alone. Further, a co-precipitated surface treatment layer may be used depending on the purpose, and a method in which alumina is first present and then silica is present in the surface layer, or vice versa can be employed. 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.
As a specific example and manufacturing method of the nonmagnetic inorganic powder used for the lower layer of the present invention,
The thing of WO98 / 35345 is illustrated.

  An organic powder can also be added to the nonmagnetic layer according to the purpose. Examples thereof include acrylic styrene resin powder, benzoguanamine resin powder, melamine resin powder, and phthalocyanine pigment. Polyolefin resin powder, polyester resin powder, polyamide resin powder, polyimide resin powder, and polyfluorinated ethylene resin can also be used. As the production method, those described in JP-A Nos. 62-18564 and 60-255827 can be used.

  The binder, lubricant, dispersant, additive, solvent, dispersion method, content, etc. of the nonmagnetic layer can be those of the above magnetic layer. In particular, known techniques relating to the magnetic layer can be applied to the amount, type, additive, and dispersant added amount, type.

[Non-magnetic support]
The support used for the magnetic recording medium of the present invention is preferably a nonmagnetic flexible support.
These supports include known films such as polyesters such as polyethylene terephthalate and polyethylene naphthalate, polyolefins, cellulose triacetate, polycarbonate, aromatic or aliphatic polyamide, polyimide, polyamideimide, polysulfone, polyaramid, and polybenzoxazole. Can be used. It is preferable to use a high strength support such as polyethylene naphthalate or polyamide. If necessary, the surface roughness of the magnetic layer surface and the base surface of the support can be changed. For example, a laminated type support as disclosed in JP-A-3-224127 can be used. These supports may be subjected in advance to corona discharge treatment, plasma treatment, easy adhesion treatment, heat treatment, dust removal treatment and the like. Moreover, it is also possible to apply aluminum or a glass substrate as a support body of this invention.

Preferably, the support has a center surface average surface roughness Ra of 4.0 nm or less, preferably 2.0 nm or less, measured using WYKO HD-2000 type as a support. These supports preferably have not only a small center plane average surface roughness but also no coarse protrusions of 0.5 μm or more. The surface roughness shape is freely controlled by the size and amount of filler added to the support as required. Examples of these fillers include oxides and carbonates such as Ca, Si and Ti, and acrylic organic powders. The maximum height Rmax of the support is 1 μm or less, the ten-point average roughness Rz is 0.5 μm or less, the center surface mountain height Rp is 0.5 μm or less, the center surface valley depth Rv is 0.5 μm or less, and the center surface area ratio Sr is preferably 10 to 90%, and the average wavelength λa is preferably 5 to 300 μm. In order to obtain desired electromagnetic conversion characteristics and durability, the surface protrusion distribution of these supports can be arbitrarily controlled by a filler, and each having a size of 0.01 to 1 μm is 0 to 2000 per 0.1 mm ² . It can be controlled within a range.

The F-5 value of the support used in the present invention is preferably 49 to 490 MPa (5 to 50 kg / mm ² ). Further, the thermal contraction rate of the support at 100 ° C. for 30 minutes is preferably 3% or less, and more preferably 1.5% or less. The heat shrinkage rate at 80 ° C. for 30 minutes is preferably 1% or less, and more preferably 0.5% or less. Furthermore, it is preferable that the heat shrinkage rate at 100 ° C. for 30 minutes and the heat shrinkage rate at 80 ° C. for 30 minutes of the support are equal to each other within 10% of the in-plane direction of the support. The breaking strength is preferably 49 to 980 MPa (5 to 100 kg / mm ² ). The elastic modulus is preferably 980 to 19600 MPa (100 to 2000 kg / mm ² ). The temperature expansion coefficient is 10 ⁻⁴ to 10 ⁻⁸ / ° C., and preferably 10 ⁻⁵ to 10 ⁻⁶ / ° C. The humidity expansion coefficient is 10 ⁻⁴ / RH% or less, preferably 10 ⁻⁵ / RH% or less. These thermal characteristics, dimensional characteristics, and mechanical strength characteristics are preferably substantially equal with a difference of 10% or less in each in-plane direction of the support.

[Back coat layer]
In the magnetic recording medium of the present invention, a backcoat layer can be provided on the surface on the nonmagnetic support opposite to the surface having the magnetic layer, if necessary. Although a back coat layer can be provided even on a magnetic disk, in general, a magnetic tape for computer data recording is strongly required to have repeated running characteristics as compared with a video tape and an audio tape. In order to maintain such high running durability, the back coat layer preferably contains carbon black and inorganic powder.

  The carbon black used in the back coat layer is preferably used in combination of two types having different average particle diameters. In this case, it is preferable to use a combination of fine particle carbon black having an average particle size of 10 to 20 nm and coarse particle carbon black having an average particle size of 230 to 300 nm. In general, by adding fine carbon black as described above, the surface electrical resistance of the backcoat layer can be set low, and the light transmittance can also be set low. Some magnetic recording devices utilize the light transmittance of the tape and are used for the operation signal. In such a case, the addition of particulate carbon black is particularly effective. In addition, particulate carbon black is generally excellent in retention of liquid lubricant, and contributes to reduction of the friction coefficient when used in combination with lubricant. On the other hand, coarse carbon black having an average particle size of 230 to 300 nm has a function as a solid lubricant, and also forms fine protrusions on the surface of the backcoat layer to reduce the contact area, thereby reducing friction. Contributes to reducing the coefficient.

In the present invention, when a commercially available product is used as the fine particulate carbon black and the coarse particulate carbon black used for the backcoat layer, specific products may include those described in WO 98/35345.
In the case of using two types of backcoat layers having different average particle diameters, the content ratio (mass ratio) of fine particle carbon black of 10 to 20 nm and coarse particle carbon black of 230 to 300 nm is the former: the latter = It is preferably in the range of 98: 2 to 75:25, and more preferably in the range of 95: 5 to 85:15.
The content of carbon black in the backcoat layer (the total amount when two types are used) is usually in the range of 30 to 80 parts by mass with respect to 100 parts by mass of the binder, It is the range of 45-65 mass parts.

The inorganic powder used in the backcoat layer is preferably used in combination of two types having different hardness. Specifically, it is preferable to use a soft inorganic powder having a Mohs hardness of 3 to 4.5 and a hard inorganic powder having a Mohs hardness of 5 to 9. By adding a soft inorganic powder having a Mohs hardness of 3 to 4.5, the friction coefficient can be stabilized by repeated running. Moreover, when the hardness is within this range, the sliding guide pole is not scraped. Moreover, it is preferable that the average particle diameter of this inorganic powder exists in the range of 30-50 nm.
Examples of the soft inorganic powder having a Mohs hardness of 3 to 4.5 include calcium sulfate, calcium carbonate, calcium silicate, barium sulfate, magnesium carbonate, zinc carbonate, and zinc oxide. These can be used alone or in combination of two or more.
The content of the soft inorganic powder in the back coat layer is preferably in the range of 10 to 140 parts by mass, more preferably 35 to 100 parts by mass with respect to 100 parts by mass of the carbon black.

By adding a hard inorganic powder having a Mohs hardness of 5 to 9, the strength of the back layer is enhanced and the running durability is improved. When these inorganic powders are used together with carbon black or the soft inorganic powder, there is little deterioration even with repeated sliding, and a strong backcoat layer is obtained. Moreover, by adding this inorganic powder, an appropriate polishing force is imparted, and adhesion of shavings to a tape guide pole or the like is reduced. In particular, when used in combination with soft inorganic powder, the sliding characteristics with respect to the guide pole having a rough surface are improved, and the friction coefficient of the backcoat layer can be stabilized.
The average particle size of the hard inorganic powder is preferably in the range of 80 to 250 nm, and more preferably in the range of 100 to 210 nm.
Examples of the hard inorganic powder having a Mohs hardness of 5 to 9 include α-iron oxide, α-alumina, and chromium oxide (Cr ₂ O ₃ ). These powders may be used alone or in combination. Among these, α-iron oxide or α-alumina is preferable. The content of the hard inorganic powder is usually 3 to 30 parts by mass and preferably 3 to 20 parts by mass with respect to 100 parts by mass of the carbon black.

When the soft inorganic powder and the hard inorganic powder are used in combination in the back coat layer, the difference in hardness between the soft inorganic powder and the hard inorganic powder is 2 or more (more preferably 2.5 or more, particularly 3 or more). It is preferable to select and use a soft inorganic powder and a hard inorganic powder.
The backcoat layer preferably contains the two types of inorganic powders having different specific Mohs hardnesses having the specific average particle size and the two types of carbon blacks having different average particle sizes.

  The back coat layer can contain a lubricant. The lubricant can be appropriately selected from the above-mentioned lubricants that can be used for the nonmagnetic layer or the magnetic layer. In the back layer, the lubricant is usually added in the range of 1 to 5 parts by mass with respect to 100 parts by mass of the binder.

  For the binder, lubricant, dispersant, additive, solvent, dispersion method, content, etc. of the backcoat layer, those of the magnetic layer and nonmagnetic layer described above can be applied. In particular, known techniques relating to the magnetic layer can be applied to the amount, type, additive, and dispersant added amount, type.

[Undercoat layer]
In the magnetic recording medium of the present invention, an undercoat layer may be provided between the nonmagnetic support and the magnetic layer or nonmagnetic layer as necessary. By providing the undercoat layer, the adhesive force between the nonmagnetic support and the magnetic layer or nonmagnetic layer can be improved. As the undercoat layer, a solvent-soluble polyester resin is used.

  The magnetic recording medium of the present invention can be used in a magnetic recording / reproducing system in which a signal magnetically recorded on a magnetic recording medium is reproduced by an MR head. The MR head used for reproduction is not particularly limited, and for example, a GMR head or a TMR head can be used. The saturation magnetization amount of the head used for magnetic recording is not particularly limited, but the saturation magnetization amount is 1.0 T or more, preferably 1.5 T or more.

[Layer structure]
In the thickness structure of the magnetic recording medium of the present invention, the nonmagnetic support is usually 2 to 100 μm, preferably 2 to 80 μm. The non-magnetic support of the computer tape has a thickness in the range of 3.0 to 8 μm (preferably 3.0 to 7.5 μm, more preferably 4.0 to 7 μm).

  The thickness of the undercoat layer is 0.1 to 1.0 μm, preferably 0.1 to 0.3 μm. Moreover, when providing a backcoat layer, the thickness of a backcoat layer is 0.2-1.0 micrometer, Preferably it is 0.3-0.7 micrometer.

  The thicknesses of the nonmagnetic layer and the magnetic layer of the magnetic recording medium of the present invention are optimized depending on the saturation magnetization of the head used, the head gap length, and the recording signal bandwidth. In the present invention, as described above, the thickness of the magnetic layer is preferably 20 to 150 nm, preferably 20 to 100 nm, and more preferably 30 to 90 μm. The thickness of the nonmagnetic layer is usually 0.5 to 3 μm, preferably 0.5 to 2 μm, and more preferably 0.5 to 1.5 μm.

  In the case of a magnetic recording medium having two magnetic layers, a nonmagnetic layer or a soft magnetic layer may or may not be provided. For example, the thickness of the magnetic layer far from the support is 0.01 to 0.1 μm, preferably Can be 0.01 to 0.05 μm, and the thickness of the magnetic layer closer to the support can be 0.05 to 0.15 μm. In addition, when it has a magnetic layer independently, as mentioned above, the thickness of a magnetic layer shall be 20-150 nm.

[Production method]
The process for producing the magnetic coating material of the magnetic recording medium of the present invention comprises at least a kneading process, a dispersing process, and a mixing process provided as necessary before and after these processes. Each process may be divided into two or more stages. All raw materials such as magnetic powder, non-magnetic powder, radiation curable resin, binder, carbon black, abrasive, antistatic agent, lubricant, solvent, etc. used in the magnetic recording medium of the present invention It may be added in the middle. In addition, individual raw materials may be added in two or more steps. For example, polyurethane may be divided and added in a kneading step, a dispersing step, and a mixing step for adjusting the viscosity after dispersion.

  In order to achieve the object of the present invention, a conventional known manufacturing technique can be used as a partial process. In the kneading step, it is preferable to use a kneading force such as an open kneader, a continuous kneader, a pressure kneader, or an extruder. When a kneader is used, the magnetic powder or non-magnetic powder and all or part of the binder (preferably 30% or more of the total binder) are in the range of 15 to 500 parts by mass with respect to 100 parts by mass of the magnetic powder. Kneaded. Details of these kneading treatments are described in JP-A-1-106338 and JP-A-1-79274. In order to disperse the coating material for the magnetic layer and the coating material for the nonmagnetic layer, glass beads can be used, but zirconia beads, titania beads, and steel beads, which are high specific gravity dispersion media, are suitable. The particle diameter and filling rate of these dispersion media are optimized. A well-known thing can be used for a disperser.

  The non-magnetic layer coating and the magnetic layer coating may be applied sequentially or simultaneously, and when there are two magnetic layers, the lower magnetic layer coating and the upper magnetic layer coating are sequentially performed. Or it may be applied in multiple layers simultaneously. Preferably, the nonmagnetic layer and the magnetic layer are coated on a nonmagnetic support with a nonmagnetic powder and a binder and then dried to form a nonmagnetic layer, and then the nonmagnetic layer is formed on the nonmagnetic layer. A magnetic layer is formed by applying a magnetic layer coating material containing a ferromagnetic powder and a binder and then drying to form a magnetic layer, that is, forming a magnetic layer by a coating method after drying (wet on dry coating method).

  Examples of the coating machine for applying the magnetic layer coating material or the nonmagnetic layer coating material include air doctor coating, blade coating, rod coating, extrusion coating, air knife coating, squeeze coating, impregnation coating, reverse roll coating, transfer roll coating, and gravure coating. A coat, a kiss coat, a cast coat, a spray coat, a spin coat, etc. can be used. For example, “Latest coating technology” (May 31, 1983) issued by the General Technology Center Co., Ltd. can be referred to.

In the present invention, when a magnetic recording medium having a multilayer structure is applied, the following method is preferably used.
(1) A lower layer is first applied by a gravure coating, a roll coating, a blade coating, an extrusion coating apparatus, etc. that are generally used in the application of a magnetic paint. A method of coating the upper layer with a support pressure type extrusion coating apparatus disclosed in JP-A-60-238179 and JP-A-2-265672.
(2) One coating head containing two coating liquid passage slits as 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 in which the upper and lower layers are applied almost simultaneously using an extrusion coating apparatus with a backup roll disclosed in JP-A-2-174965.

  In order to prevent the deterioration of the electromagnetic conversion characteristics of the magnetic recording medium due to the aggregation of the magnetic particles, the inside of the coating head is formed by a method as disclosed in Japanese Patent Application Laid-Open Nos. 62-95174 and 1-2236968. It is desirable to apply shear to the coating solution. Furthermore, the viscosity of the coating solution needs to satisfy the numerical range disclosed in Japanese Patent Laid-Open No. 3-8471.

  In the case of a disk-shaped magnetic recording medium, a sufficiently isotropic orientation may be obtained even without orientation without using an orientation device, but cobalt magnets are alternately arranged obliquely, and an alternating magnetic field is applied by a solenoid. It is preferable to use a known random orientation device. Hexagonal ferrite generally tends to be in-plane and vertical three-dimensional random, but it can also be in-plane two-dimensional random. Further, isotropic magnetic characteristics can be imparted in the circumferential direction by using a well-known method such as a counter-polarized magnet and making it vertically oriented. In particular, when performing high density recording, vertical alignment is preferable. Moreover, it is good also as circumferential orientation using a spin coat.

In the case of a tape-like magnetic recording medium, it is oriented in the longitudinal direction using a cobalt magnet or a solenoid. It is preferable that the drying position of the coating film can be controlled by controlling the temperature, air volume, and coating speed of the drying air, the coating speed is preferably 20 to 1000 m / min, and the temperature of the drying air is preferably 60 ° C. or higher. Moreover, moderate preliminary drying can also be performed before entering a magnet zone.
The calendering roll is treated with a heat-resistant plastic roll or metal roll such as epoxy, polyimide, polyamide, polyimide amide or the like. In particular, when a double-sided magnetic layer is used, it is preferably treated with metal rolls. The treatment temperature is preferably 50 ° C. or higher, more preferably 100 ° C. or higher. The linear pressure is preferably 196 kN / m (200 kg / cm) or more, more preferably 294 kN / m (300 kg / cm) or more.

[Physical properties]
In the case of a disk-shaped magnetic recording medium, the squareness ratio is usually 0.55 to 0.67 and preferably 0.58 to 0.64 in the case of two-dimensional randomness. In the case of three-dimensional randomness, the squareness ratio is preferably 0.45 to 0.55. In the case of vertical alignment, the squareness ratio is usually 0.6 or more in the vertical direction, and preferably 0.7 or more. Further, when demagnetizing field correction is performed, it is usually 0.7 or more, and preferably 0.8 or more. The orientation degree ratio is preferably 0.8 or more for both two-dimensional random and three-dimensional random. In the case of two-dimensional randomness, the squareness ratio in the vertical direction, Br in the vertical direction, and Hc in the vertical direction are preferably within 0.1 to 0.5 times the in-plane direction. In the case of a tape-like magnetic recording medium, the squareness ratio is 0.7 or more, preferably 0.8 or more.

The coefficient of friction with respect to the head of the magnetic recording medium of the present invention is usually 0.5 or less and preferably 0.3 or less in the range of temperature -10 to 40 ° C. and humidity 0 to 95%. The surface specific resistance is preferably a magnetic surface of 10 ⁴ to 10 ¹² Ω / sq, and the charged position is preferably −500 V to +500 V. The elastic modulus at 0.5% elongation of the magnetic layer is preferably 980 to 19600 MPa (100 to 2000 kg / mm ² ) in each in-plane direction, and the breaking strength is 98 to 686 MPa (10 to 70 kg / mm ^2). ) Is preferable. The elastic modulus of the magnetic recording medium is preferably 980 to 14700 MPa (100 to 1500 kg / mm ² ) in each in-plane direction. The residual elongation is preferably 0.5% or less, the thermal shrinkage at any temperature of 100 ° C. or less is preferably 1% or less, more preferably 0.5% or less, and 0% More preferably, it is 1% or less.

The glass transition temperature of the magnetic layer (the maximum point of the loss modulus of dynamic viscoelasticity measured at 10 Hz) is 80 to 200 ° C. The nonmagnetic layer is made of the polyurethane resin used in the present invention for the nonmagnetic layer. When using the binder containing, it is 100-200 degreeC, and when not using, it is preferable that it is 80-120 degreeC. The loss elastic modulus is preferably in the range of 1 × 10 ⁵ to 8 × 10 ⁸ Pa, and the loss tangent is preferably 0.2 or less. If the loss tangent is too large, adhesion failure is likely to occur. These thermal characteristics and mechanical characteristics are preferably within 10% in each in-plane direction of the medium, and are preferably substantially equal. The residual solvent contained in the magnetic layer is preferably 100 mg / m ² or less, and more preferably 10 mg / m ² or less. The porosity of the coating layer is preferably 30% by volume or less, and more preferably 20% by volume or less for both the nonmagnetic layer and the magnetic layer. The porosity is preferably small in order to achieve high output, but it may be better to ensure a certain value depending on the purpose. For example, in the case of a disk medium in which repeated use is important, a larger void ratio is often preferable for running durability.

The center plane average surface roughness Ra and the 10-point average roughness Rz in the magnetic layer of the present invention are preferably 5 to 50 nm, respectively. Further, the maximum height R _max of the magnetic layer is 0.5 μm or less, the center surface peak height Rp is 0.3 μm or less, the center surface valley depth Rv is 0.3 μm or less, the center surface area ratio Sr is 20 to 80%, The average wavelength λa is preferably 5 to 300 μm. The number of protrusions on the surface of the magnetic layer is preferably in the range of 5 to 1000 per 100 μm ² having a size of 10 to 20 nm. These can be easily controlled by controlling the surface properties by the filler of the support, the particle size and amount of the powder added to the magnetic layer, the roll surface shape of the calendering process, and the like. The curl is preferably within ± 3 mm. In the magnetic recording medium of the present invention, it is easily estimated that these physical characteristics can be changed between the nonmagnetic layer and the magnetic layer according to the purpose. For example, the elastic modulus of the magnetic layer is increased to improve running durability, and at the same time, the elastic modulus of the nonmagnetic layer is made lower than that of the magnetic layer to improve the contact of the magnetic recording medium with the head.

The need for image recording has become stronger not only in the industrial world but also at home, and the magnetic recording medium of the present invention is not only data such as characters and numbers, but also functions as an image recording medium. It has the ability to fully meet the cost requirements.
The magnetic recording medium of the present invention can be suitably used in a magnetic recording / reproducing system using a magnetoresistive reproducing head (MR head). The type of MR head is not particularly limited, and a GMR head or a TMR head can also be used. The head used for recording is not particularly limited, but the saturation magnetization is preferably 1.2 T or more, and more preferably 2.0 T or more.
The magnetic recording medium of the present invention is suitable for computer data recording.

Specific examples of the present invention will be described below, but the present invention should not be limited thereto. In addition, the following “parts” means “parts by mass” unless otherwise specified.
Example 1
Preparation of coating solution for magnetic layer formation (component for magnetic layer formation)

Ferromagnetic metal powder 100 parts Coercive force (Hc): 189.6 kA / m (2400 Oe)
Specific surface area by BET method: 75 m ² / g
Crystallite size: 13 nm (130 cm)
Saturation magnetization (σs): 120 A · m ² / kg (120 emu / g)
Particle size (average major axis diameter): 45 nm
Needle ratio: 5.5
Composition Fe / Co = 100/30
Surface treatment layer: Al ₂ O ₃ , Y ₂ O ₃
Polar group (-SO ₃ Na group) -containing vinyl chloride copolymer 10 parts MR-110 made by Nippon Zeon
Polar group (—SO ₃ Na group) -containing dimer diol type 10 parts polyurethane resin [Tg 160 ° C. —SO ₃ Na group content: 6 × 10 ⁻⁵ eq / g]
α-alumina (particle size: 0.1 μm) 5 parts carbon black (particle size: 0.08 μm) 0.5 part butyl stearate 1 part stearic acid 2 parts methyl ethyl ketone 90 parts cyclohexanone 30 parts toluene 60 parts

(Nonmagnetic layer forming component)
Nonmagnetic powder αFe ₂ 0 ₃ hematite 80 parts BET specific surface area: 110 m ² / g
Particle size (average major axis length): 0.15 μm
pH: 9.3
Tap density: 0.98 g / ml
Surface treatment layer: Al ₂ O ₃ , SiO ₂
Carbon black 20 parts Average primary particle size: 16 nm (16 mμ)
DBP oil absorption: 80ml / 100g
pH: 8.0
Specific surface area by BET method: 250 m ² / g
Volatile content: 1.5%
Polar group (—SO ₃ Na group, epoxy group) 12 parts-containing vinyl chloride resin [MR-104, manufactured by Nippon Zeon Co., Ltd.]
Polar group (—SO ₃ Na group) -containing dimer diol type 10 parts polyurethane resin [Tg 180 ° C. —SO ₃ Na group content: 6 × 10 ⁻⁵ eq / g]
Butyl stearate 1 part Stearic acid 2 parts Methyl ethyl ketone 100 parts Cyclohexanone 50 parts Toluene 50 parts

  The components forming the magnetic layer and the nonmagnetic layer were kneaded with an open kneader and then dispersed using a sand mill. To each of the obtained dispersions, 40 parts of methyl ethyl ketone was added and filtered using a filter having an average pore size of 1 μm to prepare a coating solution for forming a nonmagnetic layer and a coating solution for forming a magnetic layer, respectively.

(Back coat layer forming component)
Particulate carbon black powder 100 parts [Furnace black, average particle size: 17 nm]
Coarse particulate carbon black powder 30 parts [Furnace black, average particle size: 100 nm]
α-alumina (hard inorganic powder) 5 parts [Average particle size: 100 nm, Mohs hardness: 9]
Nitrocellulose resin 140 parts Polyurethane resin 15 parts Polyester resin 5 parts Methyl ethyl ketone 1200 parts Butyl acetate 300 parts Toluene 600 parts

  Each component forming the back coat layer was kneaded with a chilled roll and then dispersed using a sand mill. After adding 40 parts of polyisocyanate (Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.) and 1000 parts of methyl ethyl ketone to the obtained dispersion, the mixture is filtered using a filter having an average pore diameter of 1 μm, and a coating solution for forming a back coat layer is formed. Was prepared.

[Manufacturability of magnetic tape]
The magnetic layer forming coating solution and the nonmagnetic layer forming coating solution obtained as described above were prepared by using a polyethylene naphthalate (PEN) support (thickness: 5 μm, Young's modulus in the length (MD) direction of 800 kg / mm. ² , on the width (TD) direction Young's modulus 750 kg / mm ² , on the magnetic layer coated surface centerline average surface roughness Ra (cutoff value: 0.25 mm), 2 nm) on the magnetic layer, nonmagnetic layer drying Simultaneous multilayer coating was applied so that the subsequent thicknesses were 0.1 μm and 1.5 μm, respectively. Next, while the magnetic layer was still wet, orientation treatment was performed using a Co magnet having a magnetic flux density of 0.3T (3000 gauss) and a solenoid magnet having a magnetic flux density of 0.15T (1500 gauss). Thereafter, the magnetic layer was formed by drying.

Thereafter, the back coating layer forming coating solution is applied to the other side of the support (the side opposite to the magnetic layer) so that the thickness after drying is 0.5 μm and dried to form the back coating layer. Formed. A magnetic recording medium roll having a magnetic layer on one side of the support and a backcoat layer on the other side was obtained.
The above roll is calendered by passing through a seven-stage calendering machine (temperature 90 ° C., linear pressure 300 kg / cm, speed 300 m / min) composed of a heated metal roll and an elastic roll coated with a thermosetting resin on a core metal. And wound up with a tension of 5 kg / m. The material of the heating metal roll is a chromium molybdenum steel plated with hard chrome, and the surface roughness Ra is 0.005 μm (cutoff value 0.25 mm). The thermosetting resin of the elastic roll is obtained by reacting a bis (2-oxozoline) processed product, an aromatic diamine and an epoxy compound.
Next, the roll was slit to 1/2 inch width, and then demagnetized by passing through a solenoid having a magnetic flux density of 0.3 T (3000 G).

(Example 2)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that a dimer diol polyurethane resin having a Tg of 105 ° C. was used for both the magnetic layer and the nonmagnetic lower layer.
(Example 3)
A magnetic tape according to the present invention was prepared in the same manner as in the production of the magnetic tape of Example 1, except that a dimer diol polyurethane resin having a Tg of 195 ° C. was used for both the magnetic layer and the nonmagnetic lower layer.

(Example 4)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 189.6 kA / m (2400 Oe)
Specific surface area by BET method: 70 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 60 nm
Needle ratio: 6
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Example 5)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 189.6 kA / m (2400 Oe)
Specific surface area by BET method: 65 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 45 nm
Needle ratio: 6.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Example 6)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 189.6 kA / m (2400 Oe)
Specific surface area by BET method: 65 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 25 nm
Needle ratio: 6.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Example 7)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 189.6 kA / m (2400 Oe)
Specific surface area by BET method: 65 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 55 nm
Needle ratio: 6.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Example 8)
A magnetic tape according to the present invention was produced in the same manner as in the production of the magnetic tape of Example 1, except that the magnetic layer thickness was changed to 25 nm.
Example 9
A magnetic tape according to the present invention was prepared in the same manner as in the production of the magnetic tape of Example 1, except that the thickness of the magnetic layer was changed to 145 nm.

(Comparative Example 1)
In the production of the magnetic tape of Example 1, both the magnetic layer and the nonmagnetic lower layer use a polar polyester polyurethane resin having the following Tg of 80 ° C., and magnetically use a polyisocyanate curing agent [Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.]. A magnetic tape was prepared in the same manner except that 50 parts were added to the layer and nonmagnetic underlayer coating solution, and the calendered magnetic recording medium roll was stored in a 50 ° C. dry environment for 24 hours and subjected to thermosetting treatment.

Polar group (-SO ₃ Na group) -containing polyester polyurethane resin [neopentyl glycol / caprolactone polyol / MDI = 0.9 /
2.6 / 1 (weight ratio)
—SO ₃ Na group content: 1 × 10 ⁻⁴ mol / g]

(Comparative Example 2)
In the production of the magnetic tape of Comparative Example 1, a magnetic tape was produced in the same manner except that the thermosetting treatment was not performed.
(Comparative Example 3)
In the production of the magnetic tape of Comparative Example 2, a magnetic tape was produced in the same manner except that a polyisocyanate curing agent [Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.] was not added.
(Comparative Example 4)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that a polyurethane resin having a Tg of 95 ° C. was used for both the magnetic layer and the nonmagnetic lower layer.
(Comparative Example 5)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that a polyurethane resin having a Tg of 205 ° C. was used for both the magnetic layer and the nonmagnetic lower layer.

(Comparative Example 6)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 151.2 kA / m (1900 Oe)
Specific surface area by BET method: 75 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 45 nm
Needle ratio: 5.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Comparative Example 7)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic materials were used.

Ferromagnetic metal powder Coercive force (Hc): 151.2 kA / m (1900 Oe)
Specific surface area by BET method: 75 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 45 nm
Needle ratio: 5.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Comparative Example 8)
In the production of the magnetic tape of Example 1, a magnetic tape was produced in the same manner except that the following magnetic material was used and the thickness of the magnetic layer was changed to 101 nm.

Ferromagnetic metal powder Coercive force (Hc): 151.2 kA / m (1900 Oe)
Specific surface area by BET method: 75 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 15 nm
Needle ratio: 5.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Comparative Example 9)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that the following magnetic material was used and the thickness of the magnetic layer was changed to 102 nm.

Ferromagnetic metal powder Coercive force (Hc): 151.2 kA / m (1900 Oe)
Specific surface area by BET method: 75 m ² / g
Crystallite size: 130Å
Saturation magnetization (σs): 125 A · m ² / kg (125 emu / g)
Particle size (average major axis length): 65 nm
Needle ratio: 5.5
Fe / Co = 100/30
Surface treatment layer Al ₂ O ₃ , Y ₂ O ₃

(Comparative Example 10)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that the thickness of the magnetic layer was changed to 15 nm.
(Comparative Example 11)
A magnetic tape was produced in the same manner as in the production of the magnetic tape of Example 1, except that the thickness of the magnetic layer was changed to 155 nm.

[Evaluation of magnetic tape]
(1) Magnetic layer surface roughness Ra
Using an optical three-dimensional roughness meter TOPO3D manufactured by WYKO, an average central roughness Ra having an area of about 250 nm × 250 nm was measured on the medium surface by the MIRAU method. A 512 × 512 pixel array detector, 20 × magnification was used.
(2) Number of magnetic layer recesses Using a Zygo optical three-dimensional surface analyzer, the number of recesses of 20 nm or more in the area of 340 nm × 250 nm on the surface of the magnetic layer was measured.
(3) Dropout (D.O.)
Reproduction output decreased by 50% or more at a length of 2 μm or less per 1 m of tape length. O. Number was measured.
(4) Running durability After the surface of the magnetic layer was reciprocated 20,000 times with respect to the DLT4 drive recording / reproducing head in an environment of 40 ° C. and 80%, the amount of dirt adhered to the head was evaluated by a five-point method. The tape length is 20 m, the running tension is 100 gr, and the running speed is 3 m / sec.

5 points: No head contamination 4 points: There is head contamination but does not enter the sliding surface with the tape 3 points: Head contamination enters the tape sliding surface 2 points: Contamination adheres to the vicinity of the head gap 1 point: The head gap is covered with dirt.

(5) PW50 measurement method An original running system for running the tape between two reels was assembled, and a tape guide was provided between them to incorporate a device for running with high accuracy. Servo light is not running.
The recording head employs a SAL type merge head having an MR sensor with a track width of 25 μm and a gap width of 0.3 μm, and the reproducing head having a track width of 12 μm and a shield interval changed. An optimum recording current was set at 150 kfci, and a 100 kHz rectangular wave was recorded and reproduced, and the PW50 (half-value width of the isolated wave) of the isolated wave was measured.
The traveling speed was 2.55 m / sec.

(6) SNR measurement method [SNR of magnetic tape]
A recording head (MIG, gap 0.15 μm, 1.8 T) and a reproducing head MR head (optimum Br · t: 0.035 T · μm) were attached to a modified DLT7000 drive. These heads are fixed heads.
The SNR was obtained from reproduction output at a linear recording density of 100 kFCI and noise (a signal component at a frequency 1 MHz away from the carrier frequency). Reproduction output and SNR were evaluated relative to the magnetic tape of Comparative Example 2.

  From Table 1, the Tg of the polyurethane resin used in the magnetic layer is 100 to 200 ° C., the average major axis length of the ferromagnetic metal powder is 20 to 60 nm, and the thickness of the magnetic layer is 20 to 150 μm. The magnetic recording medium having a coercive force of 160 to 240 kA / m and containing no isocyanate compound in the magnetic layer had good SNR and PW50, little dropout, and good running durability.

On the other hand, when the Tg of the polyurethane resin is less than 100 ° C., it is necessary to add a curing agent and heat curing treatment in order to ensure running durability. The protrusion is transferred to the surface of the magnetic layer, and a dent is formed in the magnetic layer. For this reason, SNR falls and dropout (DO) also increases (comparison of comparative example 1 and comparative examples 2 and 3). When the Tg of the polyurethane resin was less than 100 ° C., the number of dents in the magnetic layer increased and the dropout increased. Moreover, since the strength of the magnetic layer decreased, the running durability deteriorated (Comparative Examples 3 and 4). When the Tg of the polyurethane resin exceeded 200 ° C., the surface of the magnetic layer became rough and the SNR deteriorated (Comparative Example 5).

When the magnetic layer longitudinal direction Hc was less than 160 kA / m, the SNR was lowered and the PW50 was increased (Comparative Example 6). When the longitudinal direction Hc of the magnetic layer exceeded 240 kA / m, the recording head was saturated and sufficient recording could not be performed, so the SNR was lowered (Comparative Example 7).
When the long axis length of the magnetic material was less than 20 nm, the dispersibility of the magnetic layer coating solution became poor, the surface of the magnetic layer became rough, and the SNR was lowered. Due to the poor dispersibility of the magnetic layer, the strength of the magnetic layer was lowered and the running durability was deteriorated (Comparative Example 8). When the long axis length of the magnetic material exceeded 60 nm, the surface of the magnetic layer became rough and the SNR was lowered. And PW50 was also increased (Comparative Example 9).
When the thickness of the magnetic layer was less than 20 nm, the magnetic layer could not be applied uniformly at the time of application, and a sample could not be produced (Comparative Example 10). When the thickness of the magnetic layer exceeded 150 nm, the SNR decreased and the PW50 increased (Comparative Example 11).

  From the above, the Tg of the binder (polyurethane resin) used in the magnetic layer is increased, the average major axis length (average plate diameter) of the ferromagnetic powder is decreased, the thickness of the magnetic layer is decreased, and the longitudinal resistance is increased. By making the magnetic force appropriate and not containing an isocyanate compound in the magnetic layer, stable durability, high SNR and low PW50 value can be obtained compared to conventional magnetic recording media. O. It can be seen that the error rate can be stabilized.

Claims (1)

A magnetic recording medium in which a magnetic layer containing a ferromagnetic powder and a binder is provided on a nonmagnetic support,
The binder contained in the magnetic layer includes a polyurethane resin having a glass transition temperature of 100 to 200 ° C., does not contain an isocyanate compound, has a thickness of the magnetic layer of 20 to 150 nm, and a coercive force in the longitudinal direction of the magnetic layer. Is 160 to 240 kA / m, and the average size of the ferromagnetic powder is 20 to 60 nm.