JP4352270B2 - Hematite particle powder for nonmagnetic underlayer of magnetic recording medium and magnetic recording medium - Google Patents

Description

  The present invention provides a hematite particle powder suitable as a nonmagnetic powder for a nonmagnetic underlayer of a magnetic recording medium, which is excellent in filling properties and can be expected to improve surface smoothness by calendering.

  In recent years, as video recording and audio magnetic recording / reproducing devices have been recorded for a long time and reduced in size and weight, the performance of magnetic recording media such as magnetic tapes and magnetic disks has been improved, that is, higher recording density and higher output characteristics. In particular, there is an increasing demand for improved frequency characteristics and low noise.

  In particular, the recent demand for higher image quality on video tapes has been increasing, and compared to conventional video tapes, the frequency of the carrier signal to be recorded has shifted to the short wavelength region. The magnetization depth from the surface of the tape is remarkably shallow.

  In order to improve the high output characteristics of a magnetic recording medium, particularly the S / N ratio, with respect to a short wavelength signal, it is strongly required to reduce the thickness of the magnetic recording layer. In order to reduce the thickness of the magnetic recording layer, it is necessary to smooth the magnetic recording layer and reduce the thickness unevenness. For this purpose, the surface of the base film must also be smooth.

  In general, a magnetic recording medium is formed by forming a magnetic recording layer containing magnetic particle powder and a binder resin on a nonmagnetic support such as a base film, and smoothing the magnetic recording layer by applying a calendar to smooth the surface. It is carried out.

  In recent years, as the magnetic recording layer is further thinned, an underlayer (non-magnetic particle powder such as matite particle powder dispersed in a binder resin on a nonmagnetic support such as a base film is dispersed (in a binder resin). Hereinafter, it has been proposed and put into practical use to solve problems such as deterioration of the surface properties of the magnetic recording layer and electromagnetic conversion characteristics by providing a single layer of “non-magnetic underlayer”. Japanese Patent Publication No. 6-93297, Japanese Patent Application Laid-Open No. 62-159338, Japanese Patent Application Laid-Open No. 63-187418, Japanese Patent Application Laid-Open No. 4-167225, Japanese Patent Application Laid-Open No. 4-325915, Japanese Patent Application Laid-Open No. No. 5 -182177).

  In the case of the magnetic recording medium having the nonmagnetic underlayer, the nonmagnetic underlayer containing the nonmagnetic powder and the binder resin on the nonmagnetic support and the magnetic recording containing the magnetic particle powder and the binder resin. A layer is formed, and then a calendering process is performed so that the nonmagnetic underlayer absorbs the irregularities of the nonmagnetic support, thereby smoothing the surface of the magnetic recording layer. For example, in Japanese Patent Laid-Open No. 5-12650, “... a nonmagnetic buffer layer is provided with a nonmagnetic buffer layer. As a result, the lower layer (which is a nonmagnetic layer?) Acts as an absorbing layer, and the magnetic recording layer containing the upper hexagonal ferrite plate-like magnetic powder is smoothed. ing.

  In order to improve the surface smoothness of the nonmagnetic underlayer, it is required to use hematite particle powder that is excellent in dispersibility and filling properties and can be expected to improve the surface smoothness by calendering.

  Conventionally, various attempts have been made on nonmagnetic particle powders for nonmagnetic underlayers in order to improve various characteristics of magnetic recording media. For example, Patent Document 1 (Japanese Patent Laid-Open No. 6-60362) discloses Al. Non-magnetic underlayers for magnetic recording media containing non-magnetic particle powders consisting of acicular hematite particle powders coated with a compound are described. Patent Document 2 (Japanese Patent Laid-Open No. 7-129947) and Patent Document 3 (JP-A-8-279138) describes a magnetic recording medium using a hematite particle powder having a spindle shape as a nonmagnetic particle powder for a nonmagnetic underlayer. No. -334450) describes magnetic recording using acicular goethite fine particles in which no more than three identical crystal planes are bonded to each other in the crystallographic a-axis direction on a nonmagnetic underlayer of a magnetic recording medium. Body have been described.

Japanese Patent Laid-Open No. 6-60362 JP-A-7-129947 JP-A-8-279138 Japanese Patent Laid-Open No. 10-334450

  A hematite particle powder suitable as a nonmagnetic particle powder for a nonmagnetic underlayer capable of obtaining a magnetic recording medium with better surface smoothness is currently most demanded, but has not yet been obtained.

  That is, the hematite particle powder described in the above-mentioned Patent Document 1 (Japanese Patent Laid-Open No. 6-60362) does not have a structure arranged with directionality in the major axis direction, and it is difficult to obtain a surface smoothing effect by calendering. .

The spindle-shaped hematite particles described in Patent Document 2 (Japanese Patent Laid-Open No. 7-129947) and Patent Document 3 (Japanese Patent Laid-Open No. 8-279138) have a BET specific surface area value as shown in Comparative Examples described later. Since it is less than 100 m ² / g and the liquid absorption of cyclohexanone is less than 0.6 ml / g, it is presumed that the end of the spindle-shaped hematite particles is closed, and it is difficult to say that the filling property is excellent. is there.

  In addition, when the goethite particle powder described in Patent Document 4 (Japanese Patent Laid-Open No. 10-334450) is used as the nonmagnetic powder for the nonmagnetic underlayer, the surface of the goethite particle powder contains a large amount of crystal water. For this reason, the compatibility with the binder resin and the solvent is poor, and it is difficult to obtain a desired degree of dispersion.

  Therefore, the present invention is excellent in dispersibility and filling properties suitable as a nonmagnetic particle powder for a nonmagnetic underlayer of a magnetic recording medium excellent in surface smoothness, and can be expected to improve surface smoothness by calendering. Obtaining hematite particle powder is a technical problem.

  The technical problem can be achieved by the present invention as follows.

  That is, the present invention provides a spindle-shaped hematite particle powder having an average major axis diameter of 0.005 to 0.6 μm and an average minor axis diameter of 0.001 to 0.40 μm. This is a hematite particle powder for a non-magnetic underlayer of a magnetic recording medium, wherein the end of the spindle-shaped hematite particles to be opened is open (Invention 1).

Further, in the present invention, the spindle-shaped hematite particle powder of the present invention 1 has a BET specific surface area value of 100 to 250 m ² / g. It is a powder (Invention 2).

  The present invention also provides a hematite particle powder for a nonmagnetic underlayer of a magnetic recording medium, wherein the spindle-shaped hematite particle powder of the present invention 1 has a cyclohexanone absorption of 0.6 ml / g or more ( Invention 3).

  Further, the present invention is characterized in that the surface of the particle is coated with a surface coating composed of at least one selected from aluminum hydroxide, aluminum oxide, silicon hydroxide and silicon oxide. This is a hematite particle powder for a nonmagnetic underlayer of the magnetic recording medium of any one of Inventions 1 to 3 (Invention 4).

  The present invention also relates to a nonmagnetic support, a nonmagnetic underlayer comprising a nonmagnetic powder and a binder resin formed on the nonmagnetic support, and a magnetic particle powder and a binder formed on the nonmagnetic underlayer. A magnetic recording medium comprising a magnetic recording layer containing a resin, wherein the nonmagnetic powder is the hematite particle powder for a nonmagnetic underlayer according to any one of the first to fourth aspects of the present invention. There is (Invention 5).

  According to the present invention, in the magnetic recording medium of the present invention 5, a spindle-shaped metal magnetic particle powder whose main component is iron having an axial ratio of 7.0 or less is used as the magnetic particle powder. (Invention 6).

  When the hematite particle powder for nonmagnetic underlayer according to the present invention is used, a nonmagnetic underlayer excellent in surface smoothness can be obtained. When the nonmagnetic underlayer is used as a magnetic recording medium, surface smoothness is obtained. Therefore, it is suitable as a hematite particle powder for a nonmagnetic underlayer of a magnetic recording medium having a nonmagnetic underlayer.

  The magnetic recording medium according to the present invention is suitable as a high-density magnetic recording medium because it has excellent surface smoothness as described above.

  The configuration of the present invention will be described in more detail as follows.

  First, the hematite particle powder for nonmagnetic underlayer according to the present invention will be described.

  The average major axis diameter of the nonmagnetic underlayer hematite particle powder according to the present invention is 0.005 to 0.6 μm. When the average major axis diameter exceeds 0.6 μm, the particle size is too large. Therefore, when the nonmagnetic underlayer is formed using this, the surface smoothness of the coating film tends to be impaired. When the average major axis diameter is less than 0.005 μm, aggregation is likely to occur due to an increase in intermolecular force due to the refinement of the particles, so that dispersibility in the vehicle during the production of the nonmagnetic paint is reduced. Preferably it is 0.0075-0.45 micrometer, More preferably, it is 0.01-0.3 micrometer.

  The average minor axis diameter of the nonmagnetic underlayer hematite particle powder according to the present invention is 0.00025 to 0.3 μm. When the average minor axis diameter is less than 0.00025 μm, aggregation is likely to occur due to an increase in intermolecular force due to the refinement of the particles, so that dispersibility in the vehicle during the production of the nonmagnetic coating material is lowered. When the average minor axis diameter is 0.3 μm or more, it is difficult to obtain industrially. Preferably it is 0.0003-0.20 micrometer, More preferably, it is 0.0005-0.15 micrometer.

  As for the axial ratio (ratio of an average major axis diameter and an average minor axis diameter) (henceforth "axial ratio") of the hematite particle | grains for nonmagnetic underlayers concerning this invention, 2-20 are preferable. When the axial ratio is less than 2 or exceeds 20, the coating strength of the obtained nonmagnetic underlayer becomes small. More preferably 3-18, even more preferably 4-15, most preferably 5-10.

  The particle shape of the hematite particles constituting the non-magnetic underlayer hematite particle powder according to the present invention is such that the particle ends are open as shown in FIG. The conventional spindle-shaped hematite particles shown in FIG. 2 have closed particle ends, and the filling properties in the nonmagnetic underlayer are reduced.

  In the hematite particles constituting the hematite particle powder for a nonmagnetic underlayer according to the present invention, the particle width at the short axis diameter (W) of the hematite particles and the point (b) that is 1/10 of the long axis diameter from the particle end of the particles. The average value of the ratio (W / Wb) to (Wb) is preferably 1.20 or more. When the W / Wb ratio is less than 1.20, when the nonmagnetic underlayer is formed using the hematite particle powder, the dispersibility of the nonmagnetic coating material in the vehicle decreases. In consideration of dispersibility, it is preferably 1.21 to 3.0, more preferably 1.22 to 2.5.

The BET specific surface area value of the nonmagnetic underlayer hematite particle powder according to the present invention is preferably 90 to 300 m ² / g. When the BET specific surface area value exceeds 300 m ² / g, aggregation is likely to occur due to an increase in intermolecular force due to finer particles, so that the dispersibility in the vehicle during the production of the nonmagnetic coating material is lowered. When it is less than 90 m ² / g, the particle size is too large, which is not preferable from the viewpoint of improving the surface smoothness of the coating film. Considering the surface smoothness of the magnetic recording medium obtained, BET specific surface area value is more preferably 95~275m ² / g, and even more preferably 100 to 250 m ² / g.

  The liquid absorption amount of cyclohexanone in the nonmagnetic underlayer hematite particles according to the present invention is preferably 0.6 ml / g or more, more preferably 0.65 to 1.5 ml / g. If it is less than 0.6 ml / g, it is presumed that the end of the spindle-shaped hematite particles is closed, and the filling property in the nonmagnetic underlayer is lowered.

  The non-magnetic underlayer hematite particle powder according to the present invention preferably has a coating film shrinkage of 8.0 to 20% as measured by the measurement method described below. When the contraction rate of the coating film is less than 8.0%, a sufficient surface smoothing effect by the calendar process cannot be obtained. On the other hand, if it exceeds 20%, the film thickness of the coating film varies greatly, making it difficult to design a magnetic recording medium. More preferably, it is 8.5-19%, More preferably, it is 9.0-18%.

  The hematite particle powder for a nonmagnetic underlayer according to the present invention is at least one selected from the group consisting of an aluminum hydroxide, an aluminum oxide, a silicon hydroxide and a silicon oxide, if necessary. You may coat | cover with the surface coating which consists of. The hematite particle powder, the surface of which is coated with a surface coating, is well-familiar with the binder resin when dispersed in a vehicle, and a desired degree of dispersion is easily obtained.

The amount of the surface coating is 0.01 to each of aluminum hydroxide and aluminum oxide in terms of Al, and silicon hydroxide and silicon oxide in terms of SiO _{2 with} respect to the hematite particle powder. 50% by weight is preferred. When the amount is less than 0.01% by weight, there is almost no effect of improving dispersibility by coating. Considering the effect of improving dispersibility in the vehicle and industrial productivity, 0.05 to 20% by weight is more preferable.

When used in conjunction with an aluminum compound and a silicon compound, 0.01 to 50 wt% in total of Al equivalent amount and SiO ₂ equivalent amount with respect to hematite particles are preferred.

  The hematite particle powder coated with the surface coating according to the present invention has a particle size and a BET specific surface area value substantially the same as those of the hematite particle powder according to the present invention which is not coated with the surface coating.

  The hematite particle powder for nonmagnetic underlayer according to the present invention may contain aluminum inside the particle. By using hematite particles containing aluminum inside the particles, the durability of the obtained magnetic recording medium is improved. The amount of aluminum contained inside the particles is preferably 0.05 to 50% by weight, more preferably 0.05 to 40% by weight in terms of Al.

  In particular, in consideration of the corrosion resistance of the magnetic recording medium, highly purified hematite particle powder in which the content of soluble sodium salt, soluble sulfate, etc. in the hematite particle powder is reduced is preferable.

The highly purified hematite particle powder preferably has a soluble sodium salt content of 300 ppm or less, more preferably 200 ppm in terms of Na. Further, the content of soluble sulfate is preferably 150 ppm or less, more preferably 100 ppm or less in terms of SO ₄ . The powder pH value is preferably 8.0 or more.

  Next, a method for producing a hematite particle powder for a nonmagnetic underlayer according to the present invention will be described.

  The hematite particle powder for nonmagnetic underlayer according to the present invention can be obtained by subjecting spindle-shaped goethite particles to grinding treatment and then heat dehydration treatment in a temperature range of 200 to 540 ° C.

  The spindle-shaped goethite particle powder in the present invention can be obtained by a usual method, for example, obtained by reacting with a ferrous salt and an aqueous solution of an alkali carbonate aqueous solution or an alkali hydroxide / alkali carbonate aqueous solution. An oxygen-containing gas such as air can be aerated through the suspension containing the iron-containing precipitate.

  The spindle-shaped goethite particles preferably have an average major axis diameter of 0.007 to 0.80 μm, an average minor axis diameter of 0.001 to 0.35 μm, and an axial ratio of 2 to 20.

  If necessary, the surface of the spindle-shaped goethite particles may be coated with a sintering inhibitor such as P, Si, B, Zr, or Sb.

  Spindle-shaped goethite particles are ground by spinning the suspension containing the spindle-shaped goethite particles obtained by the reaction of forming the spindle-shaped goethite particles, and the wet cake washed with water and dispersed again in water. Or a suspension containing the spindle-shaped goethite particles obtained by the reaction for producing the spindle-shaped goethite particles is filtered off, washed with water, dried, and taken out as spindle-shaped goethite particle powder. An aqueous suspension containing spindle-shaped goethite particles dispersed in may be used. It is preferable to use a suspension containing spindle-shaped goethite particles in which a wet cake of spindle-like goethite particles is dispersed again in water.

  The grinding treatment of the spindle-shaped goethite particles is carried out by adjusting the concentration of the slurry containing the spindle-shaped goethite particles to 30 to 500 g / l, preferably 40 to 250 g / l, more preferably 50 to 200 g / l, and then rotating the slurry. It is carried out by grinding by applying a shearing force of several thousand to 9000 rpm, preferably 1200 to 5000 rpm.

  As an apparatus for grinding a slurry containing spindle-shaped goethite particles, an apparatus capable of applying a shearing force to the slurry is preferable. For example, a wet grinding machine such as a grinding atomizer or an ultrafine grinding machine is used. Can do.

  Specific examples of the wet mill include supermass colloiders, serendipeaters (manufactured by Masuko Sangyo Co., Ltd.), and T.C. K. There are Myco Loader M type (Special Machine Industry Co., Ltd.).

  Next, the spindle-shaped goethite particle powder after the grinding treatment is heat-treated in a temperature range of 200 to 540 ° C. to obtain a hematite particle powder. When the heating temperature is less than 200 ° C., it takes a long time for the dehydration reaction. When the temperature exceeds 540 ° C., the dehydration reaction occurs abruptly, and the shape of the particles tends to be collapsed or sintering between the particles is not preferable. The temperature of the heat treatment is preferably 250 to 500 ° C, more preferably 280 to 450 ° C. The heat treatment time is preferably 30 minutes to 3 hours.

  The highly purified hematite particle powder can be obtained by subjecting the heat-treated hematite particle powder to heat treatment in an alkaline aqueous solution after filtration, filtering and washing with water.

  The pH value of the alkaline aqueous solution is preferably 13.0 or more. The temperature of the heat treatment is preferably 80 ° C. or higher, more preferably 90 ° C. or higher.

  The hematite particle powder coated with the surface coating in the present invention is obtained by dispersing the hematite particle powder obtained by the heat treatment after the grinding treatment, in an aqueous suspension obtained by dispersing the aluminum compound, the silicon compound, or both of them. By adding a compound and mixing and stirring, or if necessary, adjusting the pH value after mixing and stirring, the particle surface of the hematite particle powder is mixed with aluminum hydroxide, aluminum oxide, silicon water. It coat | covers with the 1 type, or 2 or more types of compound chosen from the oxide and the oxide of a silicon | silicone, Then, it filters, wash | cleans, dries, and grind | pulverizes. If necessary, a deaeration / consolidation process may be further performed.

  As the aluminum compound, aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride, and aluminum nitrate, and alkali aluminates such as sodium aluminate can be used. As the silicon compound, No. 3 water glass, sodium orthosilicate, sodium metasilicate and the like can be used.

  Next, the magnetic recording medium according to the present invention will be described.

  The magnetic recording medium according to the present invention comprises a nonmagnetic support, a nonmagnetic underlayer formed on the nonmagnetic support, and a magnetic recording layer formed on the nonmagnetic underlayer.

  Examples of the nonmagnetic support include polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, synthetic resin films such as polyamide, polyamideimide, and polyimide, which are widely used for magnetic recording media, and metal foils such as aluminum and stainless steel. A board and various types of paper can be used. The thickness varies depending on the material, but is usually preferably 1.0 to 300 μm, more preferably 2.0 to 200 μm.

  In the case of a magnetic disk, polyethylene terephthalate is generally used, and its thickness is usually 50 to 300 μm, preferably 60 to 200 μm. In the case of a magnetic tape, polyethylene terephthalate, polyethylene naphthalate, polyamide or the like is used. The thickness of polyethylene terephthalate is usually 3 to 100 μm, preferably 4 to 20 μm, and the thickness of polyethylene naphthalate is usually 3 to 50 μm, preferably Is 4 to 20 μm, and the thickness of polyamide is usually 2 to 10 μm, preferably 3 to 7 μm.

  The nonmagnetic underlayer in the present invention comprises the nonmagnetic underlayer hematite particle powder according to the present invention or the nonmagnetic underlayer hematite particle powder coated with the surface coating according to the present invention and a binder resin.

As binder resins, vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, which are currently widely used in the production of magnetic recording media, are used. Polymers, cellulose derivatives such as polyvinyl butyral, nitrocellulose, synthetic rubber resins such as polyester resins and polybutadiene, epoxy resins, polyamide resins, polyisocyanates, electron beam curable acrylic urethane resins, and mixtures thereof can be used. Each binder resin contains polar groups such as —OH, —COOH, —SO ₃ M, —OPO ₂ M ₂ , —NH ₂ (where M is H, Na, K). May be. Considering the dispersibility of the hematite particle powder and magnetic particle powder in the vehicle, a binder resin containing —COOH and —SO ₃ M as polar groups is preferable.

  The blending ratio of the hematite particle powder and the binder resin in the present invention is 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight of the hematite particle powder with respect to 100 parts by weight of the binder resin.

  When the hematite particle powder is less than 5 parts by weight, the amount of hematite particle powder in the non-magnetic coating is too small, and when the coating film is formed, a continuously dispersed layer of hematite particle powder cannot be obtained. The smoothness and the strength of the coating film are insufficient. When the amount exceeds 2000 parts by weight, the amount of hematite particle powder is too much with respect to the amount of the binder resin, so that the hematite particle powder is not sufficiently dispersed in the nonmagnetic paint, and as a result, when a coating film is formed, It is difficult to obtain a coating film having sufficient surface smoothness. Further, since the hematite particle powder is not sufficiently bound by the binder resin, the obtained coating film tends to be brittle.

  The coating thickness of the nonmagnetic underlayer formed on the nonmagnetic support is preferably 0.2 to 10 μm. When it is less than 0.2 μm, it is difficult to improve the surface roughness of the nonmagnetic support, and the strength tends to be insufficient. Considering the thinning of the magnetic recording medium and the strength of the coating film, the coating film thickness is more preferably 0.5 to 5 μm.

  Note that a lubricant, an abrasive, an antistatic agent, and the like that are usually used in the manufacture of magnetic recording media may be added to the nonmagnetic underlayer.

  The nonmagnetic underlayer using the hematite particle powder of the present invention 1 to 3 has a coating film glossiness of 182-300%, preferably 187-300%, and a coating film surface roughness Ra of 0.5- The strength of the coating film is 7.5 nm, preferably 0.5 to 7.3 nm. The Young's modulus (relative value) of the coating film is 126 to 160, preferably 128 to 160, and the contraction ratio of the coating film is 8. It is 0 to 20%, preferably 8.5 to 19%, more preferably 9.0 to 18%.

  The nonmagnetic underlayer using the hematite particle powder of the present invention 4 has a coating film glossiness of 187 to 300%, preferably 192 to 300%, and a coating film surface roughness Ra of 0.5 to 7. The strength of the coating film is 3 nm, preferably 0.5 to 7.0 nm, and the Young's modulus (relative value) is 128 to 160, preferably 130 to 160, and the contraction rate of the coating film is 8.5 to 20%, preferably 9.0 to 19%, more preferably 10.5 to 18%.

  The magnetic recording layer in the present invention comprises magnetic particle powder and binder resin.

As magnetic particle powder, Co or Co and Fe are added to magnetic iron oxide particle powder such as maghemite particle powder (γ-Fe ₂ O ₃ ) and magnetite particle powder ( FeO _x · Fe ₂ O ₃ , 0 <x ≦ 1). Co-coated magnetic iron oxide particle powder deposited, and the Co-coated magnetic iron oxide particle powder contains different elements such as Co, Al, Ni, P, Zn, Si, B, rare earth metals other than Fe Co-coated magnetic iron oxide particle powder, acicular metal magnetic particle powder containing iron as a main component, acicular shape containing Co, Al, Ni, P, Zn, Si, B, rare earth metal, etc. other than iron Iron alloy magnetic particle powder, magnetoplumbite type plate-like ferrite particle powder containing Ba, Sr or Ba-Sr, and Co, Ni, Zn, Mn, Mg, Ti, Sn, Zr, Nb, Cu, Mo Divalent and tetravalent metals such as It can be used any of such plate-type ferrite particles which contains one or more al selected coercivity reducing agent.

  In consideration of recent short-wavelength recording and high-density recording, acicular metal magnetic particle powder mainly composed of iron, Co, Al, Ni, P, Zn, Si, B, rare earth metals, etc. other than iron The acicular iron alloy magnetic particle powder contained is preferable.

  The magnetic particle powder has an average major axis diameter (average plate surface diameter in the case of plate-like particles) of 0.01 to 0.5 μm, preferably 0.03 to 0.3 μm. The particle shape of the magnetic particle powder is preferably a needle shape or a plate shape. Here, the “needle shape” means not only a literal needle shape but also a spindle shape or a rice grain shape.

  Further, when the particle shape of the magnetic particle powder is needle-shaped, the axial ratio is 3 or more, preferably 5 or more, and the upper limit is 15 and preferably 10 considering the dispersibility in the vehicle. . Further, the axial ratio is preferably 7.0 or less in consideration of the filling rate of the magnetic particle powder in the magnetic recording layer.

  When the particle shape of the magnetic particle powder is plate-like, the plate-like ratio (ratio of average plate surface diameter to average thickness) (hereinafter referred to as “plate-like ratio”) is 2 or more, preferably 3 or more. The upper limit is 50, preferably 45, taking into account the dispersibility at.

The magnetic properties of the magnetic particle powder include a coercive force value of 39.8 to 318.3 kA / m (500 to 4000 Oe), preferably 43.8 to 318.3 kA / m (550 to 4000 Oe), and a saturation magnetization value. Is 50 to 170 Am ² / kg (50 to 170 emu / g), preferably 60 to 170 Am ² / kg (60 to 170 emu / g).

In consideration of high-density recording and the like, the magnetic characteristics when the acicular metal magnetic particle powder or acicular iron alloy magnetic particle powder mainly containing iron is used as the magnetic particle powder have a coercive force value of 63.7. 278.5 kA / m (800-3500 Oe), preferably 71.6-278.5 kA / m (900-3500 Oe), saturation magnetization value 90-170 Am ² / kg (90-170 emu / g), preferably 100 -170 Am < ² > / kg (100-170 emu / g).

  As the binder resin, the binder resin used for forming the nonmagnetic underlayer can be used.

  The coating thickness of the magnetic recording layer provided on the nonmagnetic underlayer is in the range of 0.01 to 5 μm. If it is less than 0.01 μm, uniform coating is difficult, and phenomena such as uneven coating tend to occur, which is not preferable. When it exceeds 5 μm, it is difficult to obtain desired electromagnetic characteristics due to the influence of the demagnetizing field. Preferably it is the range of 0.05-1 micrometer.

  The blending ratio of the magnetic particle powder and the binder resin is 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight, based on 100 parts by weight of the binder resin.

  In the magnetic recording layer, commonly used lubricants, abrasives, antistatic agents and the like may be added.

  The magnetic recording medium according to the present invention uses the magnetic particle powder as a magnetic particle powder, and the hematite particle powder for a nonmagnetic underlayer according to any one of the present invention 1 to 3 as a nonmagnetic powder for a nonmagnetic underlayer. Includes a coercive force value of 39.8 to 318.3 kA / m (500 to 4000 Oe), preferably 43.8 to 318.3 kA / m (550 to 4000 Oe), and a squareness ratio (residual magnetic flux density Br / saturated magnetic flux density). Bm) is 0.85 to 0.95, preferably 0.86 to 0.95, the glossiness of the coating film is 172 to 300%, preferably 177 to 300%, and the coating film surface roughness Ra is 7.8 nm or less. The Young's modulus (relative value) is preferably 2.0 to 7.3 nm and 128 to 160, preferably 130 to 160. Further, the shrinkage ratio of the coating film composed of the nonmagnetic underlayer and the magnetic recording layer according to the evaluation method described later is 7.5 to 18%, preferably 8.0 to 17%, more preferably 8.5 to 16%. is there.

  The magnetic recording medium according to the present invention uses the magnetic particle powder as the magnetic particle powder, and the coercive force when the nonmagnetic underlayer hematite particle powder of the present invention 4 is used as the nonmagnetic underlayer nonmagnetic powder. The value is 39.8 to 318.3 kA / m (500 to 4000 Oe), preferably 39.8 to 318.3 kA / m (550 to 4000 Oe), and the squareness ratio (residual magnetic flux density Br / saturated magnetic flux density Bm) is 0. 85 to 0.95, preferably 0.86 to 0.95, the glossiness of the coating film is 177 to 300%, preferably 182 to 300%, and the coating film surface roughness Ra is 7.3 nm or less, preferably 2. The Young's modulus (relative value) is 0 to 7.0 nm and 130 to 160, preferably 132 to 160. Further, the shrinkage ratio of the coating film composed of the nonmagnetic underlayer and the magnetic recording layer according to the evaluation method described later is 7.5 to 18%, preferably 8.0 to 17%, more preferably 8.5 to 16%. is there.

  In consideration of high density recording or the like, the present invention 1 to 3 are used as nonmagnetic powders for nonmagnetic underlayers by using acicular metal magnetic particle powder or acicular iron alloy magnetic particle powder mainly composed of iron as magnetic particle powder. When the nonmagnetic underlayer hematite particle powder is used, the coercive force value is 63.7 to 278.5 kA / m (800 to 3500 Oe), preferably 71.6 to 278.5 kA / m ( 900 to 3500 Oe), the squareness ratio (residual magnetic flux density Br / saturated magnetic flux density Bm) is 0.86 to 0.95, preferably 0.87 to 0.95, and the glossiness of the coating film is 202 to 300%, preferably The coating film surface roughness Ra is 6.8 nm or less, preferably 2.0 to 6.3 nm, and the Young's modulus (relative value) is 128 to 160, preferably 130 to 160. Further, the shrinkage ratio of the coating film composed of the nonmagnetic underlayer and the magnetic recording layer according to the evaluation method described later is 7.5 to 18%, preferably 8.0 to 17%, more preferably 8.5 to 16%. is there.

  The non-magnetic according to the present invention is coated with a surface coating as a non-magnetic powder for a non-magnetic underlayer using an acicular metal magnetic particle powder or acicular iron alloy magnetic particle powder containing iron as a main component as the magnetic particle powder. When the hematite particle powder for the underlayer is used, the coercive force value is 63.7 to 278.5 kA / m (800 to 3500 Oe), preferably 71.6 to 278.5 kA / m (900 to 3500 Oe), square The ratio (residual magnetic flux density Br / saturated magnetic flux density Bm) is 0.87 to 0.95, preferably 0.88 to 0.95, and the glossiness of the coating film is 207 to 300%, preferably 212 to 300%. The film surface roughness Ra is 6.3 nm or less, preferably 2.0 to 5.8 nm, and the Young's modulus (relative value) is 130 to 160, preferably 132 to 160. Further, the shrinkage ratio of the coating film composed of the nonmagnetic underlayer and the magnetic recording layer according to the evaluation method described later is 7.5 to 18%, preferably 8.0 to 17%, more preferably 8.5 to 16%. is there.

  Further, when the nonmagnetic underlayer hematite particle powder according to the present invention containing aluminum inside the particle is used as the nonmagnetic underlayer nonmagnetic powder, the durability of the magnetic recording medium is improved and the durability is improved. Among them, the running durability is 20 minutes or more, preferably 22 minutes or more, and the scratch property is A or B, preferably A.

  In particular, a non-magnetic under magnetic powder according to the present invention is used as a non-magnetic powder for a non-magnetic underlayer using acicular metal magnetic particle powder or acicular iron alloy magnetic particle powder mainly composed of iron as the magnetic particle powder. When the hematite particle powder for the formation is used, the corrosivity shown by the change rate (%) of the coercive force value of the coating film is 10.0% or less, preferably 9.5% or less, and the change rate of the saturation magnetization value ( %) Is 10.0% or less, preferably 9.5% or less.

  Next, a method for manufacturing a magnetic recording medium according to the present invention will be described.

  In the magnetic recording medium according to the present invention, a nonmagnetic undercoat layer is formed by applying a nonmagnetic coating material containing a hematite particle powder, a binder resin and a solvent onto a nonmagnetic support by a conventional method, and drying the nonmagnetic underlayer. It can be obtained by applying a magnetic coating material containing magnetic particle powder, binder resin and solvent on the base layer to form a coating film, orienting in a magnetic field, then calendering and curing.

  As the solvent used for forming the non-magnetic underlayer and the magnetic recording layer, methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran and a mixture thereof, which are widely used for magnetic recording media, can be used.

  The usage-amount of a solvent is 50-1000 weight part in the total amount with respect to 100 weight part of hematite particle powder or magnetic particle powder. If it is less than 50 parts by weight, the viscosity becomes too high when applied as a paint, making application difficult. When it exceeds 1000 parts by weight, the volatilization amount of the solvent when forming the coating film becomes too large, which is industrially disadvantageous.

<Action>
The most important point in the present invention is that the hematite particle powder according to the present invention is excellent in dispersibility and filling properties, and the magnetic recording medium having a nonmagnetic underlayer using the hematite particle powder is excellent in surface smoothness. Is a point.

  The present inventor considers the reason why the dispersibility and filling property of the hematite particle powder according to the present invention are excellent as follows.

  In general, when the particle shape is needle-like, the particles are easily entangled in the vehicle, and it is difficult to obtain a desired dispersibility. The spindle-shaped goethite particles are milled to make the spindle-shaped goethite particles open, and heat-treated in a temperature range that does not destroy the particle morphology to form hematite particles. In addition, since the particles are hematite particles having an open end, it is easier to disperse in the vehicle than the acicular particles, and as a result, it is considered that the filling property is improved.

  Further, for the reason why the surface smoothness of the magnetic recording medium having a nonmagnetic underlayer using the nonmagnetic underlayer hematite particle powder according to the present invention is excellent, the present inventor As described above, it is presumed that since the particle shape is such that the particle ends are open, the compression by the calendar process is easily applied, and the effect of improving the surface smoothness is easily obtained.

  A typical embodiment of the present invention is as follows.

  The average major axis diameter, average minor axis diameter or average plate surface diameter, and average thickness of hematite particles, spindle-shaped goethite particles, and magnetic particles are shown in photographs obtained by enlarging the electron micrographs four times in the vertical and horizontal directions, respectively. The major axis diameter and minor axis diameter of each of about 350 particles were measured and indicated by the average value.

  Further, the ratio between the minor axis diameter (W) of the hematite particles and the particle width (Wb) of the point (b) that is 1/10 of the major axis diameter from the particle end of the hematite particles is similarly determined from the above electron micrograph. Then, the particle width (Wb) at the point (b) of 1/10 of the major axis diameter from the particle end of the hematite particle is measured, and the minor axis diameter (W) and 1/10 of the major axis diameter from the particle end of the hematite particle are measured. The average value of the ratio (W / Wb) to the particle width (Wb) at point (b).

  The axial ratio is the ratio between the average major axis diameter and the average minor axis diameter, and the plate ratio is the ratio between the average plate surface diameter and the average thickness.

  The specific surface area value was indicated by a value measured by the BET method.

  The amount of cyclohexanone absorbed in the hematite particle powder is as follows: 1.0 g of a sample is placed in a round bottom flask with a stopper, cyclohexanone is added dropwise from a burette, and the round bottom flask is shaken to absorb cyclohexanone into the sample. The amount of cyclohexanone dropped when the sample becomes agglomerated and no longer absorbs is taken as the amount of absorption.

  The amounts of Al, Si, and Si in the anti-sintering agent and P in the hematite particle powder and magnetic particle powder are “X-ray fluorescence analyzer 3063M type” (Rigaku Denki Kogyo Co., Ltd.) And manufactured according to JIS K0119 “General Rules for Fluorescence X-ray Analysis”.

  The pH value of the powder was measured by weighing 5 g of a sample into a 300 ml Erlenmeyer flask, adding 100 ml of boiled pure water, heating and holding the boiled state for about 5 minutes, then plugging it and letting it cool to room temperature. After adding water corresponding to the above, stoppered again, shaken for 1 minute, allowed to stand for 5 minutes, and then measured the pH of the obtained supernatant according to JIS Z 8802-7. It was.

The content of the soluble sodium salt and the content of the soluble sulfate were determined by changing the supernatant prepared for measurement of the powder pH value as No. Filtration was performed using 5C filter paper, and Na ⁺ and SO ₄ ²⁻ in the filtrate were measured using an inductively coupled plasma emission spectrometer (manufactured by Seiko Denshi Kogyo Co., Ltd.).

The coating viscosity at 25 ° C. of the obtained coating was measured using an E-type viscometer EMD-R (manufactured by Tokyo Keiki Co., Ltd.) and indicated by a value at a shear rate D = 1.92 sec ⁻¹ . .

  The glossiness of the coating surface of the nonmagnetic underlayer and magnetic recording layer was determined by measuring the 45 ° glossiness of the coating using “Gloss Meter UGV-5D” (manufactured by Suga Test Instruments Co., Ltd.).

  The surface roughness Ra of the coating film of the nonmagnetic underlayer and the magnetic recording layer was determined by measuring the centerline average roughness of the coating film using “Surfcom-575A” (manufactured by Tokyo Seimitsu Co., Ltd.).

  The coating strength of the nonmagnetic underlayer and magnetic recording layer was measured by measuring the Young's modulus of the coating using “Autograph” (manufactured by Shimadzu Corporation), and the commercially available video tape “AV T-120 (Nippon Victor Co., Ltd.). It was expressed as a relative value with the Young's modulus of “made by the company”. It shows that the intensity | strength of a coating film is so favorable that a relative value is high.

  The thickness of each layer of the nonmagnetic support, the nonmagnetic underlayer and the magnetic recording layer constituting the magnetic recording medium was measured by the following measurement method.

  First, the film thickness (A) of the nonmagnetic support is measured using a digital electronic micrometer K351C (manufactured by Anritsu Electric Co., Ltd.). Next, the thickness (B) of the nonmagnetic support and the nonmagnetic underlayer formed on the nonmagnetic support (the sum of the thickness of the nonmagnetic support and the nonmagnetic underlayer) was measured in the same manner. To do. Further, the thickness (C) of the magnetic recording medium obtained by forming the magnetic recording layer on the nonmagnetic underlayer (the sum of the thickness of the nonmagnetic support, the thickness of the nonmagnetic underlayer, and the thickness of the magnetic recording layer). ) Is measured in the same manner. The thickness of the nonmagnetic underlayer is indicated by (B)-(A), and the thickness of the magnetic recording layer is indicated by (C)-(B).

  The shrinkage ratio of the coating film of the nonmagnetic underlayer was measured by the following measuring method.

(1) 12 g of hematite particle powder, binder resin solution (vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group 30% by weight and cyclohexanone 70% by weight) and cyclohexanone are mixed to obtain a mixture (solid content ratio 72% This mixture was further kneaded with a plastmill for 30 minutes to obtain a kneaded product.
(2) The kneaded product is 95 g of 1.5 mmφ glass beads, an additional binder resin solution (polyurethane resin having a sodium sulfonate group 30 wt%, solvent (methyl ethyl ketone: toluene = 1: 1) 70 wt%), cyclohexanone, It was added to a 140 ml glass bottle together with methyl ethyl ketone and toluene, and mixed and dispersed for 6 hours with a paint shaker to obtain a nonmagnetic paint.
100.0 parts by weight of hematite particle powder,
Has sodium sulfonate group
10.0 parts by weight of vinyl chloride-vinyl acetate copolymer resin,
10.0 parts by weight of a polyurethane resin having a sodium sulfonate group,
44.6 parts by weight of cyclohexanone,
111.4 parts by weight of methyl ethyl ketone,
66.9 parts by weight of toluene.
(3) Next, the obtained nonmagnetic coating material was applied on a nonmagnetic support to a thickness of 55 μm using an applicator and dried to form a nonmagnetic underlayer.
(4) The dried coating film after coating is calendered four times at 85 ° C. and 200 kg / cm, and the film thickness t ₀ (μm) of the coating film before calendar processing and the film thickness t ₁ after calendar processing. From (μm), the shrinkage rate (%) of the coating film of the nonmagnetic underlayer was determined according to the following formula.
Shrinkage rate of coating film (%) = {(t ₀ −t ₁ ) / t ₀ } × 100
t ₀ : thickness of nonmagnetic underlayer before calendar (μm)
t ₁ : thickness of nonmagnetic underlayer after calendar (μm)

  The shrinkage ratio of the coating film composed of the nonmagnetic underlayer and the magnetic recording layer was measured by the following measuring method.

(1) 12 g of hematite particle powder, binder resin solution (vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group 30% by weight and cyclohexanone 70% by weight) and cyclohexanone are mixed to obtain a mixture (solid content ratio 72% This mixture was further kneaded with a plastmill for 30 minutes to obtain a kneaded product.
(2) The kneaded product is 95 g of 1.5 mmφ glass beads, an additional binder resin solution (polyurethane resin having a sodium sulfonate group 30 wt%, solvent (methyl ethyl ketone: toluene = 1: 1) 70 wt%), cyclohexanone, It was added to a 140 ml glass bottle together with methyl ethyl ketone and toluene, and mixed and dispersed for 6 hours with a paint shaker to obtain a nonmagnetic paint.
100.0 parts by weight of hematite particle powder,
Has sodium sulfonate group
10.0 parts by weight of vinyl chloride-vinyl acetate copolymer resin,
10.0 parts by weight of a polyurethane resin having a sodium sulfonate group,
44.6 parts by weight of cyclohexanone,
111.4 parts by weight of methyl ethyl ketone,
66.9 parts by weight of toluene.
(3) Next, the obtained nonmagnetic coating material was applied on a nonmagnetic support to a thickness of 55 μm using an applicator and dried to form a nonmagnetic underlayer.
(4) Magnetic particle powder 12 g, abrasive (alumina) 1.2 g, carbon black fine particle powder 0.12 g, binder resin solution (vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group 30% by weight and cyclohexanone 70 % By weight) and cyclohexanone were mixed to obtain a mixture (solid content ratio 78%), and this mixture was further kneaded with a plast mill for 30 minutes to obtain a kneaded product.
(5) 95 g of 1.5 mmφ glass beads, additional binder resin solution (30% by weight of polyurethane resin having sodium sulfonate group, 70% by weight of solvent (methyl ethyl ketone: toluene = 1: 1)), cyclohexanone, methyl ethyl ketone And it added to a 140 ml glass bottle with toluene, and mixed and disperse | distributed for 6 hours with the paint shaker, and obtained the magnetic coating material. Then, a lubricant and a curing agent were added, and further mixed and dispersed for 15 minutes with a paint shaker.
Magnetic particle powder 100.0 parts by weight Vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group 10.0 parts by weight Polyurethane resin having sodium sulfonate group 10.0 parts by weight Polishing agent (alumina) 10.0 parts by weight Carbon black fine particle powder 1.0 part by weight Lubricant (myristic acid: butyl stearate = 1: 2) 3.0 part by weight Curing agent (polyisocyanate) 5.0 part by weight Cyclohexanone 65.8 part by weight Methyl ethyl ketone 164.5 part by weight Part 98.7 parts by weight of toluene (6) The obtained magnetic coating material was applied on a substrate having the above-mentioned nonmagnetic underlayer to a thickness of 15 μm using an applicator, and then oriented and dried in a magnetic field.
(7) The dried coating film after application is calendered four times at 85 ° C. and 200 kg / cm, and the film thickness t ₂ (μm) before the calendering process and the film thickness t ₃ after the calendering process. From (μm), the shrinkage ratio (%) of the coating film is determined according to the following formula.
Shrinkage rate of coating film (%) = {(t ₂ −t ₃ ) / t ₂ } × 100
t ₂ : Sum of the thickness of the nonmagnetic underlayer before the calendar and the thickness of the magnetic recording layer (μm)
t ₃ : Sum of the thickness of the nonmagnetic underlayer after the calendar and the thickness of the magnetic recording layer (μm)

  The magnetic properties of the magnetic particle powder and the magnetic recording medium were measured using an “vibrating sample magnetometer VSM-3S-15” (manufactured by Toei Kogyo Co., Ltd.) up to an external magnetic field of 795.8 kA / m (10 kOe). did.

  The running durability of the magnetic recording medium was measured by using “Media Duability Tester MDT-3000” (manufactured by Steinberg Associates) with a load of 1.96 N (200 gw) and an actual moving time at a relative speed of 16 m / s between the head and the tape. evaluated. The longer the actual movable time, the better the running durability.

  For the scratch characteristics, the surface of the tape after running was observed with a microscope, the presence or absence of the scratch was visually evaluated, and the following four grades were evaluated.

A: No scratch B: Slight scratch C: Scratch D: Severe scratch

  The change over time in the magnetic properties of the magnetic recording medium accompanying the corrosion of the metal magnetic particle powder containing iron as a main component in the magnetic recording layer is that the magnetic recording medium is left in an environment of a temperature of 60 ° C. and a relative humidity of 90% for 14 days. The coercive force value and saturation magnetic flux density value before and after standing were measured, and the value obtained by dividing the amount of change by the value before standing was expressed as a percentage.

<Embodiment 1>
<Manufacture of hematite particle powder>
Spindle-shaped goethite particle powder obtained by dispersing a wet cake of spindle-shaped goethite particles in water (average major axis diameter 0.196 μm, average minor axis diameter 0.0261 μm, axial ratio 7.5, W / wb 1.31, 1384 g of No. 3 water glass (corresponding to 2.0 wt% in terms of SiO _{2 with} respect to the spindle-shaped goethite particle powder) is added to an aqueous suspension containing 20 kg of an Al content of 1.35 wt% inside, After mixing and stirring, the mixture was filtered, washed with water, and dried to prevent sintering.

  Next, the concentration of the slurry containing the spindle-shaped goethite particle powder subjected to the sintering prevention treatment is adjusted to 100 g / l, and an ultrafine grinding machine “Super Mass Colloid” (product name, manufactured by Masuko Sangyo Co., Ltd.) is used. After passing through 5 times at a shaft rotational speed of 2000 rpm, grinding was performed, followed by filtration, washing with water and drying.

  The obtained spindle-shaped goethite particle powder was heat-treated at 350 ° C. for 60 minutes to obtain spindle-shaped hematite particle powder.

The spindle-shaped hematite particles obtained here have an average major axis diameter of 0.168 μm, an average minor axis diameter of 0.0280 μm, an axial ratio of 6.0, W / Wb of 1.32, and a BET specific surface area value of 101.6 m ^2. / G, the amount of cyclohexanone absorbed was 0.85 ml / g, the Al content was 1.48 wt%, and the sintering inhibitor was 1.95 wt% in terms of SiO ₂ .

  As a result of observation with a transmission electron microscope (TEM) photograph (× 30,000), it was confirmed that the obtained spindle-shaped hematite particles were in a state where both ends of the particles were open.

<Manufacture of nonmagnetic underlayer>
12 g of the spindle-shaped hematite particle powder obtained here, a binder resin solution (30% by weight of vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group and 70% by weight of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content The mixture was further kneaded with a plast mill for 30 minutes to obtain a kneaded product.

  This kneaded product was mixed with 95 g of 1.5 mmφ glass beads, an additional binder resin solution (polyurethane resin having a sodium sulfonate group 30 wt%, solvent (methyl ethyl ketone: toluene = 1: 1) 70 wt%), cyclohexanone, methyl ethyl ketone and toluene. The mixture was added to a 140 ml glass bottle and mixed and dispersed for 6 hours with a paint shaker to obtain a coating composition.

  The composition of the obtained nonmagnetic coating material was as follows.

100 parts by weight of spindle-shaped hematite particle powder,
Has sodium sulfonate group
10 parts by weight of vinyl chloride-vinyl acetate copolymer resin,
10 parts by weight of a polyurethane resin having a sodium sulfonate group,
44.6 parts by weight of cyclohexanone,
111.4 parts by weight of methyl ethyl ketone,
66.9 parts by weight of toluene.

  The obtained nonmagnetic paint had a paint viscosity of 2,637 cP.

  Next, the obtained nonmagnetic coating material was applied on a polyethylene terephthalate film having a thickness of 12 μm to a thickness of 55 μm using an applicator and dried to form a nonmagnetic underlayer.

  The coating thickness of the obtained nonmagnetic underlayer was 3.50 μm, the gloss was 216%, the surface roughness Ra was 5.5 nm, and the Young's modulus (relative value) was 135. The thickness of the nonmagnetic underlayer after the calendar treatment was 3.05 μm, and the compression ratio of the coating film of the nonmagnetic underlayer was 12.8%.

<Manufacture of magnetic recording media>
Acicular metal magnetic powder mainly composed of iron (average major axis diameter 0.103 μm, average minor axis diameter 0.0155 μm, axial ratio 6.6, coercive force 181.9 kA / m (2,286 Oe), saturation magnetization Value 138 Am ² / kg (138 emu / g)) 12 g, carbon black fine particle powder 0.36 g and alumina particle powder 1.2 g as an abrasive, binder resin solution (vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group) 30% by weight and 70% by weight cyclohexanone) and cyclohexanone were mixed and kneaded at a solid content of 78% using a plastmill for 30 minutes to obtain a kneaded product.

  This kneaded product is placed in a 140 ml glass bottle with 95 g of 1.5 mmφ glass beads and an additional resin binder solution (consisting of 30% by weight of polyurethane resin having a sodium sulfonate group, 35% by weight of toluene and 35% by weight of methyl ethyl ketone), and cyclohexanone, toluene, methyl ethyl ketone. Was added and mixed and dispersed in a paint shaker for 6 hours. Thereafter, a lubricant and a curing agent were added to the obtained coating composition, and further mixed and dispersed for 15 minutes with a paint shaker.

The composition of the obtained magnetic paint is as follows.
100.0 parts by weight of acicular metal magnetic particle powder mainly composed of iron,
Abrasive (alumina particle powder) 10.0 parts by weight,
3.0 parts by weight of carbon black fine particle powder,
Vinyl chloride-vinyl acetate copolymer resin having sodium sulfonate group
10.0 parts by weight,
10.0 parts by weight of a polyurethane resin having a sodium sulfonate group,
Lubricant (myristic acid: butyl stearate = 1: 2) 3.0 parts by weight,
Curing agent (polyisocyanate) 5.0 parts by weight,
65.8 parts by weight of cyclohexanone,
164.5 parts by weight of methyl ethyl ketone,
98.7 parts by weight of toluene.

  The resulting magnetic paint had a paint viscosity of 8,499 cP.

  The obtained magnetic paint was applied on a substrate having the nonmagnetic underlayer to a thickness of 15 μm using an applicator, and then oriented and dried in a magnetic field. At this time, the thickness of the magnetic layer was 1.01 μm, and the total thickness of the coating layer was 4.86 μm.

  Next, after calendering four times at 85 ° C. and 200 kg / cm, a curing reaction was performed at 60 ° C. for 24 hours, and slitting to a width of 1.27 cm (0.5 inch) gave a magnetic tape. The compressibility of the coating film before and after the calendar treatment was 11.5%. The total thickness of the coated layer of the obtained magnetic tape was 4.30 μm, the coercive force value was 185.6 kA / m (2,332 Oe), the squareness ratio was 0.90, the gloss was 236%, and the surface roughness Ra was 5 1.6 nm, Young's modulus (relative value) was 136, running durability was 30 minutes or more, and scratch characteristics were A.

  Next, examples and comparative examples manufactured according to the first embodiment will be given.

Spindle-shaped goethite particles A and B and acicular goethite particles C:
As the spindle-shaped goethite particle powder, spindle-shaped goethite particle powder and needle-like goethite particle powder having the characteristics shown in Table 1 were prepared.

<Milling process>
Examples 1 and 2, Comparative Examples 1 to 4, Reference Example 1:
Except for changing the type of goethite particles, the type and amount of the sintering inhibitor in the anti-sintering treatment, the presence or absence of the grinding treatment, the slurry concentration and rotation speed in the grinding treatment, and the heating temperature and time in the heat treatment. A nonmagnetic particle powder was obtained in the same manner as in the embodiment of the invention.

  The production conditions at this time are shown in Table 2, and various properties of the obtained spindle-shaped hematite particle powder are shown in Table 3.

  Comparative Example 5 is a hematite particle powder that is heat-dehydrated at 340 ° C. and then heat-treated at 650 ° C. without grinding the goethite particles A. Reference Example 1 is a hematite particle powder that is heat-treated at 340 ° C. after the goethite particles C are ground.

  As a result of observation with a transmission electron microscope (TEM), the hematite particle powders obtained in Examples 1 and 2 were found to have a shape with open ends.

  On the other hand, it was confirmed that the hematite particle powder obtained in Comparative Example 1 had a shape in which the particle ends were closed.

<Heat treatment in alkaline aqueous solution>
Example 3
600 g of the hematite particle powder obtained in Example 1 was put into 3.5 l of pure water, and peptized for 60 minutes using a homomixer (manufactured by Tokushu Kika Kogyo Co., Ltd.).

  Next, the slurry of the obtained hematite particle powder was mixed and dispersed for 3 hours at a shaft rotational speed of 2000 rpm while circulating through a horizontal SGM (Dispamat SL: manufactured by ESC Adchem). The sieve residue in the 325 mesh (aperture 44 μm) of the hematite particle powder in the obtained slurry was 0%.

  After the concentration of the resulting hematite particle powder slurry was adjusted to 100 g / l, 5 l of the slurry was taken. While stirring this slurry, 6N NaOH aqueous solution was added to adjust the pH value of the slurry to 13.5. Next, this slurry was heated with stirring to a temperature of 95 ° C. and held at that temperature for 180 minutes.

  Next, this slurry was washed with water by a decantation method to obtain a slurry having a pH value of 10.5. To ensure accuracy, the slurry concentration at this point was confirmed to be 96 g / l.

  Next, the solution is filtered using a Buchner funnel, and pure water is passed through, and the filtrate is washed with water until the electric conductivity of the filtrate is 30 μs or less. Particle powder was obtained.

  The production conditions at this time are shown in Table 4, and various characteristics of the obtained hematite particle powder are shown in Table 5.

Example 4:
A highly purified hematite particle powder was obtained in the same manner as in Example 3 except that the type of hematite particle powder, the pH value of the alkaline aqueous solution, the heating temperature and the heating time were variously changed.

  The production conditions at this time are shown in Table 4, and various characteristics of the obtained hematite particle powder are shown in Table 5.

  As a result of observation of a transmission electron microscope (TEM) photograph of the obtained hematite particle powder, the hematite particle powder obtained in Examples 3 and 4 has a shape in which the particle ends are open even after heat treatment in an alkaline aqueous solution. It was recognized that

<Surface coating treatment>
Example 5:
A slurry containing hematite particle powder was obtained using 10 kg of the hematite particle powder of Example 1 and 75 l of water. The pH value of the redispersed slurry containing the obtained hematite particle powder was adjusted to 10.5 using an aqueous sodium hydroxide solution, and water was added to the slurry to adjust the slurry concentration to 98 g / l. 75 l of this slurry was heated to 60 ° C., and 4083 ml of 1.0 mol / l sodium aluminate solution (corresponding to 1.5% by weight in terms of Al with respect to the hematite particle powder) was added to this slurry, and 30 minutes After holding, the pH value was adjusted to 7.5 using acetic acid. After maintaining for 30 minutes in this state, filtration, washing, drying and pulverization were performed to obtain a hematite particle powder whose particle surface is coated with an aluminum hydroxide.

  The production conditions at this time are shown in Table 6, and the properties of the obtained surface-treated hematite particles are shown in Table 7.

Examples 6-8:
The nonmagnetic particle powder in which the particle surface is coated with a coating in the same manner as in Example 5 except that each hematite particle powder of Examples 2 to 4 is used and the type and amount of the surface coating are variously changed. Got.

  Table 6 shows the production conditions at this time, and Table 7 shows the characteristics of the obtained surface-treated hematite particle powder.

  The types of coatings shown in Table 6 indicate that A is a hydroxide of aluminum and S is an oxide of silicon.

  As a result of observation of a transmission electron microscope (TEM) photograph (× 30,000) of the obtained hematite particle powder, each hematite particle powder obtained in Examples 5 to 8 was opened at the particle end even after the surface coating treatment. It was found to have a shape.

<Manufacture of nonmagnetic underlayer>
Examples 9 to 16, Comparative Examples 5 to 9, Reference Example 2:
A nonmagnetic underlayer was obtained in the same manner as in the above embodiment except that the kind of nonmagnetic particle powder was variously changed.

  Table 8 shows the manufacturing conditions and the characteristics of the obtained nonmagnetic underlayer.

Magnetic particle powders (1) to (4):
Magnetic particle powders (1) to (4) having the characteristics shown in Table 9 were used as the magnetic particle powder.

<Manufacture of magnetic recording media>
Examples 17-24, Comparative Examples 10-14, Reference Example 3:
A magnetic recording medium was obtained in the same manner as in the above embodiment except that the type of the nonmagnetic underlayer and the type of the magnetic particles were variously changed.

  Tables 10 and 11 show the manufacturing conditions and the characteristics of the obtained magnetic recording medium.

Conceptual diagram of hematite particles according to the present invention Conceptual diagram of conventional spindle-shaped hematite particles

Claims (6)

A spindle-shaped hematite particle powder having an average major axis diameter of 0.005 to 0.6 μm and an average minor axis diameter of 0.001 to 0.40 μm, and the spindle-shaped hematite particles constituting the hematite particle powder A hematite particle powder for a nonmagnetic underlayer of a magnetic recording medium, wherein the particle ends are open. The hematite particle powder for a nonmagnetic underlayer of a magnetic recording medium according to claim 1, wherein the spindle-shaped hematite particle powder according to claim 1 has a BET specific surface area value of 100 to 250 m ² / g. A hematite particle powder for a nonmagnetic underlayer of a magnetic recording medium, wherein the spindle-shaped hematite particle powder according to claim 1 has a cyclohexanone absorption of 0.6 ml / g or more. The surface of the particle is coated with a surface coating made of at least one selected from aluminum hydroxide, aluminum oxide, silicon hydroxide, and silicon oxide. The hematite particle powder for nonmagnetic underlayers of the magnetic recording medium according to claim 3. Nonmagnetic support, nonmagnetic underlayer containing nonmagnetic powder and binder resin formed on nonmagnetic support, and magnetic recording layer containing magnetic particle powder and binder resin formed on nonmagnetic underlayer A magnetic recording medium comprising: a nonmagnetic underlayer hematite particle powder according to any one of claims 1 to 4. 6. The magnetic recording medium according to claim 5, wherein a spindle-shaped metal magnetic particle powder mainly composed of iron having an axial ratio of 7.0 or less is used as the magnetic particle powder.

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