JPS61220126A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To stabilize the dispersion of magnetic particles and to improve the packing density of the particles by allowing hexagonal ferromagnetic particles to be present in the magnetic layer as a unit aggregate obtained by laminating the particles in the direction of an axis easy for magnetization. CONSTITUTION:Hexagonal ferromagnetic particles 11 are washed with water and a surfactant and a dispersant are added to the particles. Then the hexagonal ferromagnetic particles 11 are laminated in the direction of the axis 12 easy for magnetization by mixing and gioing impact to form a unit aggregate 13. The aggregate 13 is then specifically treated and blended into a paint, which is coated on a film in 1-5mum thickness to form a magnetic recording medium. The dispersibility of the ferromagnetic particles is stabilized by the presence of the unit aggregate 13 obtained by laminating the particles in the direction of the axis easy for magnetization the packing density is increased, hence the coercive force is increased by 20-40% and a recording medium having excellent recording and reproducing outputs can be obtained.

Description

[Detailed description of the invention] [Technical field of invention] The present invention relates to improvements in magnetic recording media.

[Technical background of the invention]

A magnetic recording medium is produced by coating a support such as a polyester film with a magnetic coating consisting of a mixture of ferromagnetic particles, a binder resin, and optionally various additives, orienting it under a magnetic field, and drying and curing it. It is manufactured by forming a magnetic layer.

By the way, as ferromagnetic particles, needle-shaped γ-ferrite particles, CO metamorphosed ferrite particles, iron powder or iron nitride ferrite particles, and hexagonal ferrite particles typified by barium ferrite particles have conventionally been used. Regardless of the types of particles used, small-sized particles are increasingly being used for the purpose of improving magnetic recording density. Such small-diameter particles generally reduce noise during recording and reproduction, and particularly reduce noise in the high range where the recording wavelength is short, and are therefore suitable as a high-density recording medium.

[Problems with background technology]

However, as the particle size of the ferromagnetic particles is reduced,
Generally, the following problems occur.

The first problem is that as the particle size of ferromagnetic particles decreases, it becomes extremely difficult to stably disperse the particles in a single state, making it difficult to achieve the original goal of reducing noise. It is. For example, in the case of hexagonal ferrite particles,
As shown in FIG. 2, the magnetic easy axes of hexagonal ferrite particles 1 exist randomly in the magnetic layer with binder resin 2 interposed therebetween.

The second problem is that as the particle size of the ferromagnetic particles is reduced,
Since the specific surface area increases, a large amount of binder resin is required to bind each particle. As a result, the magnetization (Ms) per unit volume of the coating film becomes smaller, and the output that can be obtained becomes smaller. At this time, if an attempt is made to maintain the Ms of the magnetic layer, the strength inevitably decreases, leading to head clogging during recording and reproduction, and affecting the still durability or weather resistance of the magnetic recording medium.

The third problem is a decrease in orientation due to the use of small-sized ferromagnetic particles. For example, in a magnetic recording medium using hexagonal barium ferrite particles, the coating film is placed under a magnetic field in an undried state so that the axis of easy magnetization of the particles is perpendicular to the support surface, and then dried. Barium ferrite forms a vertically oriented magnetic layer, but when the barium ferrite particles have an ultrafine particle size of 0.1 μm or less, it becomes very difficult to obtain a high orientation rate. For this reason, at present, it has not been possible to improve the recording density of a perpendicular magnetic recording medium based on ultrafine hexagonal ferromagnetic particles.

[Purpose of the invention]

The present invention aims to provide a magnetic recording medium in which the dispersibility of hexagonal ferromagnetic particles in a binder resin is stabilized and the filling rate is improved.

[Summary of the invention]

The present invention provides a magnetic recording medium in which a magnetic layer is formed by coating hexagonal ferromagnetic particles together with a binder resin on a support, in which the ferromagnetic particles are formed as a unit assembly in which the ferromagnetic particles are laminated in the direction of their easy axis of magnetization. It is characterized by being present in the magnetic layer.

Hereinafter, the magnetic recording medium of the present invention will be explained in detail with reference to the manufacturing method.

First, single-domain hexagonal ferromagnetic particles with uniform particle shape and particle size distribution washed out from the glass into the aqueous phase by the glass crystallization method are thoroughly washed with water. Next, after adding a dispersant such as a phosphate ester surfactant, a titanium coupling agent, or a silane coupling agent to the orthogonal ferromagnetic particles, a stirring mixer such as a sand grinder, blade mixer, or ribbon mixer is used. The hexagonal ferromagnetic particles with a single magnetic domain that had been randomly arranged were rearranged by mixing and impact using the ferromagnet, and by further drying, as shown in Figure 1, multiple single magnetic domain macrogonal ferromagnetic particles were formed. A unit aggregate 13 is created in which particles 11 are stacked in the direction of the easy magnetization axis 12. Subsequently, the unit aggregate is added to the solvent together with the binder resin, and a dispersant is added as necessary, using a dispersing machine such as a ball mill, sand mill, kneader, three-roll mill, colloid mill, paint shaker, homogenizer, dissolver, etc. Prepare the magnetic paint by mixing vigorously. Even in such a mixing process, the single-domain hexagonal ferromagnetic particles in the unstacked state are rearranged by the dispersant added in advance, and unit aggregates are formed in the resin in which they are stacked in the direction of the easy axis of magnetization. Furthermore, since the single-domain hexagonal ferromagnetic particles constituting the unit aggregate are strongly stacked on each other, the ferromagnetic particles do not fall apart during the mixing process with the binder resin.

Next, this magnetic paint is applied to a thickness of about 1 to 54 m on a support such as a polyester film, and dried to form a magnetic layer in which unit aggregates of hexagonal ferromagnetic particles exist, thereby producing a magnetic recording medium. . In this manufacturing process, before drying the magnetic paint film, the support is passed through a magnetic field perpendicular to its surface and the solvent is evaporated, so that the axis of easy magnetization of the unit aggregate is aligned with the surface of the support. A perpendicular magnetic recording medium having a magnetic layer oriented perpendicular to the magnetic field can be manufactured.

Examples of the hexagonal ferromagnetic particles include barium ferrite, strontium ferrite, calcium ferrite, or a part of the iron atoms constituting these ferrites.
Examples include those substituted with at least one element selected from the group of Ol'rt, Zr, N+, In5Cu, and Ge1Nb. Such macrogonal ferromagnetic particles are
As mentioned above, it is desirable that the particle shape and particle size distribution be uniform. Specifically, if the length of the diagonal of a hexagonal plate-shaped crystal is d and the thickness of the plate is t, then the length d) is O, O'3
~0.2 μm, the thickness of the plate relative to the diagonal length (d/
It is desirable to use one having l) of 3 to 10. In particular, hexagonal ferromagnetic particles having a large d/l are tabular and can be easily stacked in the direction of the axis of easy magnetization when applying the impact due to mixing described above. In addition, if the particle shape and particle size distribution of hexagonal ferromagnetic particles are irregular, small particles are sandwiched between large particles, resulting in a unit in which hexagonal ferromagnetic particles are stacked in the direction of the axis of easy magnetization. It becomes difficult to form an aggregate. Furthermore, it is desirable to use hexagonal ferromagnetic particles having a magnetization of 4Qemu/9 or more and a coercive force of 30008 or more.

Examples of the binder resin include vinyl chloride-vinyl acetate-vinyl alcohol copolymer, vinyl chloride-vinyl acetate-maleic anhydride copolymer, vinyl chloride-acrylic acid copolymer, and vinyl chloride-acrylic acid-maleic anhydride copolymer. Vinyl chloride copolymers typified by copolymers, or vinylidene chloride copolymers, polyvinyl alcohol, polyvinyl formal, polyvinyl butyral, polyvinyl acetal, polyester, polyurethane, polycarbonate, polyether, polyacrylic acid, polyvinylpyrrolidone, Thermoplastic resins such as poly-p-vinylphenol, polyamide, cellulose resin, phenoxy resin and their copolymers, or epoxy resin, unsaturated polyester, phenol resin, melamine resin, urea resin, furan resin, xylene resin, Examples include thermosetting resins typified by ketone resins and the like. These resins can usually be used in mixtures of one or more.

The binder resin is preferably blended in an amount of 5 to 30 parts by weight per 1 part by weight of 100 hexagonal ferromagnetic particles. The reason for this is that if the amount of the resin is less than 5 parts by weight, the binding force to the ferromagnetic particles will decrease, while if the amount of the resin exceeds 30 parts by weight, the magnetic properties required for a magnetic recording medium may not be obtained. There is.

When adding the unit aggregate to the solvent together with the binder resin, it is also possible to add various additives in addition to the dispersant as necessary. Examples of additives that can be used include antistatic agents such as carbon and graphite, and Cr2O3.
, various abrasives represented by Aβ203, CaCO3, M
Inorganic powders such as GO1S102, surface lubricants such as various fatty acids, fatty acid esters, fatty acid amides, silicone oils, fluorocarbons, surfactants, stabilizers, mold release agents,
Examples include pigments, dyes, anti-aging agents, surface treatment agents, and plasticizers. These are often added in an amount of 50 parts by weight or less per 100 parts by weight of the binder resin, and 0.
Additives other than those mentioned above can be used as long as the amount is about 0.01 to 10 parts by weight.

The ratio of unit aggregates to the total hexagonal ferromagnetic particles present in the magnetic layer is determined to improve the dispersibility of the hexagonal ferromagnetic particles in the binder resin, improve the filling rate, and improve the orientation in the magnetic field. 30% or more, more preferably 5%
It is desirable to set it to 0% or more.

As described above, in the magnetic recording medium of the present invention, since a unit aggregate in which hexagonal ferromagnetic particles are stacked in the direction of the axis of easy magnetization is present in the magnetic layer, the ferromagnetic particles are randomly present in the magnetic layer. The coercive force can be reduced by 20-40% by stabilizing the dispersibility of ferromagnetic particles and improving the filling rate compared to the case where the ferromagnetic particles are
Can be increased.

Therefore, recording/reproducing output and S/N ratio can be improved, and the durability of the magnetic layer can be improved. Also,
As mentioned above, by orienting an undried coating film in a magnetic field and drying it to form a magnetic layer having ferromagnetic particles (unit aggregates) oriented perpendicularly to the support surface, non-orientation can be achieved. A perpendicular magnetic recording medium with a high recording density and superior recording/reproducing output in a high frequency range compared to a magnetic layer can be obtained.

[Embodiments of the invention]

The present invention will be explained in detail below.

Example 1 First, single-domain hexagonal ferrite particles washed out from the glass into the aqueous phase by the glass crystallization method are bound to 8a0.6.
It has a composition formula of ((Fe2-2xCoxTix)203) (where X is 0.87) and has an average particle size of 0008μ
Single-domain substituted barium ferrite particles with a coercive force (iHc) of 7000e were prepared. The barium ferrite particles have a particle size distribution of 90% by weight or more in the range of 0.04 μm to 0.10 μm, and have a thickness (d/l') of 5 with respect to the diagonal of the hexagonal plate-shaped crystal. Ta.

Next, after thoroughly washing the substituted barium ferrite particles with water, 1.5 parts by weight of a dialkyl phosphate having 18 carbon atoms and a mixed solvent of methyl ethyl ketone and toluene (mixing ratio 1) were added to 100 parts by weight of the barium ferrite particles. :1) 100 parts by weight and particle size 0.8 to 1.0 mg+
400 parts by weight of glass beads having a diameter of φ were added and mixed for about 5 hours in a sand grinder to give an impact, thereby producing a slurry having unit aggregates in which single-domain barium ferrite particles were stacked in the direction of their easy magnetization axis. Subsequently, this slurry was mixed with a mixed solvent of methyl ethyl ketone and toluene (mixing ratio 1:1) containing 10 parts by weight of vinyl chloride-acrylic acid copolymer having a molecular weight of about 20,000 and 10 parts by weight of urethane elastomer.
) was added thereto, and the mixture was further stirred and mixed using a sand grinder for 1 hour to prepare a magnetic paint. Subsequently, this magnetic coating material was filtered, coated on a polyester film to a thickness of about 3 μm using a reverse roll coater, and dried. Thereafter, the surface of the dried coating film (magnetic layer) was smoothed by calendering and cut into 1/2 inch width to produce a magnetic tape.

When the surface and cross-section of the magnetic layer in the obtained magnetic tape was observed with a scanning electron microscope, it was found that single-domain barium ferrite particles were stacked in the direction of their easy magnetization axis, and that a unit aggregate of 40 barium ferrite particles was formed in the entire barium ferrite. It was clearly recognized that the presence of

Example 2 Before drying the coating film produced by the reverse roll coater in Example 1, while passing it through a vertically oriented magnetic field of 5 kOe,
It was dried and solidified to form a dry coating film with grayish white gloss. Thereafter, it was calendered in the same manner as in Example 1 and cut into 1/2 inch width to produce a magnetic tape having a magnetic layer with a smooth surface. When the surface and cross-section of the magnetic layer in the obtained magnetic tape was observed with a scanning electron microscope, it was found that single-domain barium ferrite particles were stacked in the direction of their easy magnetization axis, and that the unit aggregate was 60% in the whole barium ferrite. %, and it was clearly observed that the hexagonal plate surfaces of the unit aggregates were vertically oriented so as to be parallel to the polyester film surface.

Comparative Example 1 The number of carbon atoms is 1 per 100 parts by weight of single-domain macrogonal barium ferrite particles made by the glass crystallization method in the same manner as in Example 1.
1.5 parts by weight of dialkyl phosphate ester of No. 8, mixed solvent of methyl ethyl ketone and toluene (mixing ratio 1:1) 16
0 parts by weight, 10 parts by weight of a vinyl chloride-acrylic acid copolymer with a molecular weight of about 20,000, and 710 parts by weight of urethane elastomer were added to form a mixed solution, and to this was added a solution with a particle size of 0.8 to 1, O
After adding 400 parts by weight of sφ glass beads, the mixture was mixed in a sand grinder for about 5 hours. Subsequently, this magnetic coating material was filtered, and then coated onto a polyester film to a thickness of about 3 μm using a reverse roll coater and dried. After this,
The surface of the dried coating film (magnetic layer) was smoothed by calendering and cut into 1/2 inch width to produce a magnetic tape.

The surface and cross section of the magnetic layer in the obtained magnetic tape were examined using a scanning electron microscope. As a result, it was found that single-domain barium ferrite particles were in the form of a lump, and the existence of unit aggregates like those in Example 1.2 was hardly observed.

Comparative Example 2 A magnetic paint similar to that in Comparative Example 1 was prepared using single-domain hexagonal barium ferrite particles made by the glass crystallization method in the same manner as in Example 1, and coated on a polyester film thickly using a reverse roll coater. A thickness of about 3 μm was applied. Subsequently, the coating film before drying was passed through a vertical alignment magnetic field of 5 koe for drying and assimilation. Thereafter, it was calendered and cut into 1/2 inch width to produce a magnetic tape having a magnetic layer with a smooth surface. When the surface and cross section of the magnetic layer in the obtained magnetic tape was observed with a scanning electron microscope, it was found that single-domain barium ferrite particles were present in clusters, and unit aggregates like those in Examples 1 and 2 were not present. It was hardly recognized.

The magnetostatic characteristics and recording/reproducing output characteristics of the magnetic tapes of Example 1.2 and Comparative Example 1.2 were measured. The results are shown in the table below.

In addition, the tape coercive force in the table is the value measured by the perpendicular magnetization curve, the perpendicular orientation ratio is the value after demagnetizing field (4πMs) correction, and the recording/reproducing output and S/N ratio are the values measured by the relative speed of the tape and the head by 3. 75 m/sec and a carrier frequency of 5 MHz, respectively.

Table Example 3 First, the sro e of single-domain hexagonal ferrite particles washed out from the glass into the aqueous phase by the glass crystallization method.
6 ((Fe2-2xCoXT ix) 203 )(
However, X indicates 0.87) and has an average particle size of 0.
．． Single-domain substituted strontium ferrite particles with a diameter of 08 μm and a coercive force (iHc) of 700 Qe were prepared. The strontium ferrite particles have a diameter of 0.04 um to 0.04 um.
It has a particle size distribution of 90% by weight or more in the range of 10 μm f7), and the thickness (d/l
) was 5.

Next, after thoroughly washing the substituted strontium ferrite particles with water, the strontium ferrite particles 10
0! 1.5 parts by weight of dialkyl phosphate ester having 18 carbon atoms, 100 parts by weight of a mixed solvent of methyl ethyl ketone and toluene (mixing ratio 1:1), and particle size 0.8 to 1.
．． 400 parts by weight of glass beads with a diameter of 0 mm were added and mixed in a sand grinder for about 5 hours to give an impact, thereby producing a slurry having unit aggregates in which single-domain strontium ferrite particles were stacked in the direction of their easy magnetization axis.

Next, using this slurry, in the same manner as in Example 1, magnetic paint was prepared, applied onto a polyester film, and 5kO
A vertically oriented magnetic tape was manufactured by drying in a vertically oriented magnetic field, calendering, and cutting into 1/2 inch width.

The magnetostatic properties and recording/reproduction output properties of the obtained magnetic tape were measured. As a result, the tape coercive force measured by the perpendicular magnetization curve was 720Qe, and the demagnetizing field (4πM
e') The vertical orientation ratio after correction was 0.85. Also,
When the relative speed between the tape and the head was 3.75 m/sec and the carrier frequency was 5 MH2, the recording/reproducing output was +1. compared to the magnetic tape obtained by coating the strontium ferrite as a paint according to Comparative Example 1. 5dB.

The S/N ratio was +2.0 dB, showing good characteristics.

On the other hand, when the surface and cross-section of the magnetic layer in the obtained magnetic tape was observed using a scanning electron microscope, it was found that single-domain strontium ferrite particles were stacked in the direction of their easy magnetization axis, and that unit aggregates were present in the entire strontium ferrite. It was clearly observed that the hexagonal plate surface of the unit aggregate was vertically oriented so that it was parallel to the polyester film surface.

〔Effect of the invention〕

As detailed above, according to the present invention, the hexagonal ferromagnetic particles are stabilized in the magnetic layer by being present in the magnetic layer as a unit aggregate stacked in the direction of the easy axis of magnetization. A magnetic material that can be widely dispersed, improve the filling rate, and increase the saturation magnetization per unit volume of the magnetic layer, thereby improving recording/reproducing output, S/N ratio, and durability of the magnetic layer. Can provide recording media. Furthermore, by vertically orienting a magnetic layer having a unit aggregate in which the hexagonal ferromagnetic particles are laminated in the direction of their easy axis of magnetization, a high perpendicular orientation rate can be achieved without reducing the saturation magnetization of the magnetic layer. Therefore, it is possible to provide a perpendicularly oriented magnetic recording medium that is capable of high-density recording and has superior output characteristics in a high frequency range as compared to a non-oriented magnetic layer.

[Brief explanation of drawings]

Figure 1 is a schematic diagram showing a unit assembly in which hexagonal ferromagnetic particles present in the magnetic layer of the magnetic recording medium of the present invention are laminated in the direction of their easy axis of magnetization, and Figure 2 is a schematic diagram showing the magnetic properties of a conventional magnetic recording medium. FIG. 2 is a schematic diagram showing the state of hexagonal ferromagnetic particles present in a layer. 11... Hexagonal ferromagnetic particle, 12... Easy axis of magnetization,
13...Unit aggregate. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2

Claims (2)

[Claims] (1) In a magnetic recording medium in which a magnetic layer is formed by coating hexagonal ferromagnetic particles together with a binder resin on a support, the magnetic layer is formed as a unit aggregate in which the ferromagnetic particles are laminated in the direction of their easy magnetization axis. A magnetic recording medium characterized by being present in a layer. (2) The unit aggregate of hexagonal ferromagnetic particles is present in the magnetic layer with its axis of easy magnetization oriented perpendicularly to the surface of the support. Magnetic recording medium described in Section 1.