JPH0481266B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a magnetic recording medium, and particularly to a magnetic recording medium with improved C/N characteristics and suitable for high-density perpendicular magnetization recording. (Prior art) Conventionally, magnetic recording and reproduction involved coating a non-magnetic support with a coating liquid in which a ferromagnetic material consisting of needle-shaped crystals such as γ-Fe ₂ O ₃ or CrO ₂ was dispersed in a binder and aligning the material. Magnetic recording media have been widely used. Recently, there has been a strong desire to improve recording density in order to achieve larger recording capacity and smaller size. However, in order to obtain a recording medium suitable for high-density recording using conventional needle-shaped magnetic powder, It is necessary to make the maximum dimension of the shaped magnetic powder sufficiently smaller than the recording wavelength or recording bit length. Currently, acicular magnetic powder with a size of about 0.3 μm is already in practical use, and the shortest recording wavelength is approximately
1 μm was obtained. In order to obtain a medium capable of higher density recording in the future, it will be necessary to further reduce the dimensions of the acicular magnetic powder. However, such small needle-shaped magnetic powders are extremely thin, with a thickness of less than 100 Å.
Because the particle volume is extremely small, less than 10 ^-17 cm ³ , magnetic properties deteriorate due to thermal disturbances and surface effects, and sufficient orientation cannot be obtained even when a magnetic field is applied to the magnetic coating. There's a problem. Therefore, a magnetic recording medium was discovered in which the ferromagnetic material is a hexagonal ferrite having a flat plate shape and an axis of easy magnetization in the direction perpendicular to the plate surface, and the magnetic orientation is in the longitudinal direction in the plane of the magnetic recording medium (for example, Japanese Patent Publication No. 58-6525
No. 58-6526, etc.). Although magnetic recording media using such hexagonal ferrite enable high-density recording, they have the drawback of poor C/N characteristics. The inventors of the present invention have conducted intensive studies on hexagonal ferrite in order to solve this drawback, and have found that by using hexagonal ferrite fine powder with a specific plate diameter, and in which the volume fraction of the ferromagnetic material in the magnetic layer is above the specified value. We have discovered that the C/N ratio can sometimes be significantly improved, and have arrived at the present invention. (Objective of the Invention) That is, an object of the present invention is to provide a magnetic recording medium which has improved C/N characteristics and is suitable for high-density perpendicular magnetization recording. (Structure of the Invention) The object of the present invention is to provide a magnetic recording medium in which a magnetic layer containing a ferromagnetic material and a binder is provided on a non-magnetic support, wherein the ferromagnetic material has an average plate diameter of 0.015 to 0.07 μm.
This can be achieved by a magnetic recording medium which is made of hexagonal ferrite fine powder and has a volume fraction of ferromagnetic material in the magnetic layer of 0.2 or more. Examples of the ferromagnetic material used in the present invention include hexagonal ferrite powder, that is, barium ferrite, strontium ferrite, lead ferrite, calcium ferrite substitutes, manganese bismuth, hexagonal cobalt alloys, etc.
Particularly preferred are barium ferrite and strontium ferrite substituted with Co. The ferromagnetic material of the present invention has a plate diameter of 0.015 to 0.07 μm.
Especially preferred is 0.02 to 0.06 μm.
If the plate diameter is larger than 0.07μm, the output will be higher, but
The noise becomes larger than that, and the C/N becomes relatively low. Further, if the plate diameter is less than 0.015 μm, the noise will be reduced, but the output will be reduced and the ferromagnetism required for magnetic recording will not be exhibited. With a plate diameter of 0.015 μm, the reduction in noise is greater than the reduction in output. Noise becomes especially small when the plate diameter is 0.07μm or less. Therefore, the C/N, which is the difference between output and noise, depends on the plate diameter.
It is significantly improved when it is 0.015 to 0.07 μm. The plate thickness is
0.035 μm or less can be used, and 0.003 is particularly preferable.
~0.02 μm. The plate ratio (plate diameter/plate thickness) is 2 or more, particularly preferably 3 to 15. The volume fraction of the ferromagnetic material of the present invention is preferably 0.2 or more, particularly preferably 0.25 or more. Here, the volume fraction of the ferromagnetic material can be calculated using the following formula. Volume fraction of magnetic material = Volume of ferromagnetic material / Volume of ferromagnetic material + Volume of non-magnetic particles + Volume of voids in the magnetic layer When the volume fraction of ferromagnetic material is less than 0.2, the output is low;
Therefore, the C/N is also low. The nonmagnetic material ratio is preferably 0.20 or more, particularly preferably 0.25 or more. The non-magnetic rate of is determined by the following formula. Non-magnetic material ratio = non-magnetic material volume / ferromagnetic material volume + non-magnetic material volume Here, the ratio of the ferromagnetic material volume to the non-magnetic material volume is calculated by converting the solid content weight in the magnetic paint to specific gravity. It can be found by The volume fraction of voids in the magnetic layer can be determined using a mercury intrusion porosimeter. If the nonmagnetic material ratio is less than 0.20, the adhesion to the nonmagnetic support will be poor, and the surface properties will be poor, making it unsuitable for use as a magnetic recording medium. The above relationship will also be explained based on the drawings. Figure 1 shows the relationship between the ferromagnetic volume fraction, the non-magnetic volume fraction, and the void volume fraction. A region where the population ratio is 0.2 or more (line A-C in FIG. 1) is preferable. More preferably, the ferromagnetic material volume fraction is 0.25 or more (straight line B'-C' in Figure 1) and the non-magnetic material fraction is 0.25.
This is the area described above (straight line A'-C' in Figure 1). The ferromagnetic material, ie, hexagonal ferrite fine powder, used in the present invention can be produced by a known method. Specifically, when using the glass method (Japanese Patent Application Laid-Open No. 1983-
No. 67904), by hydrothermal synthesis method (Unexamined Japanese Patent Publication No. 1983-
No. 149328), by coprecipitation method (JP-A-56-60002)
) can be mentioned. As the binder used in the present invention, conventionally known thermoplastic resins, thermosetting resins, reactive resins, and mixtures thereof are used. As a thermoplastic resin, the softening temperature is 150℃ or less, the average molecular weight is 10,000 to 200,000, and the degree of polymerization is approximately 200 to 2,000.
For example, vinyl chloride vinyl acetate copolymer, vinyl chloride vinylidene chloride copolymer, vinyl chloride acrylonitrile copolymer, acrylic ester acrylonitrile copolymer, acrylic ester vinylidene chloride copolymer, acrylic ester styrene. copolymer, methacrylic acid ester acrylonitrile copolymer, methacrylic acid ester vinylidene chloride copolymer, methacrylic acid ester styrene copolymer, urethane elastomer, polyvinyl fluoride, vinylidene chloride acrylonitrile copolymer, butadiene acrylonitrile copolymer,
Polyamide resin, polyvinyl butyral, cellulose derivatives (cellulose acetate butyrate,
cellulose diacetate, cellulose triacetate, cellulose propionate, nitrocellulose, etc.), styrene-butadiene copolymers, polyester resins, various synthetic rubber thermoplastic resins (polybutadiene, polychloroprene, polyisoprene, styrene-butadiene copolymers, etc.) and mixtures thereof are used. The thermosetting resin or reactive resin has a molecular weight of 200,000 or less in the coating liquid state, but when added after coating and drying, the molecular weight becomes infinite due to reactions such as condensation and addition. Also, among these resins, those that do not soften or melt before the resin is thermally decomposed are preferred. Specifically, for example, phenol-formalin-novolak resin, phenol-formalin-resol resin, phenol-furfural resin, xylene-formaldehyde resin, urea resin, melamine resin, drying oil-modified alkyd resin, carbonic acid resin-modified alkyd resin, maleic acid resin. Modified alkyd resins, unsaturated polyester resins, epoxy resins and curing agents (polyamines, acid anhydrides, polyamide resins, etc.),
Terminal isocyanate polyester moisture curable resin, terminal isocyanate polyether moisture curable resin, polyisocyanate prepolymer (compound having 3 or more isocyanate groups in one molecule obtained by reacting diisocyanate and low molecular weight triol, diisocyanate) trimers and tetramers), polyisocyanate prepolymers and resins with active hydrogen (polyether polyols,
polyether polyol, acrylic acid copolymer,
maleic acid copolymer, 2-hydroxyethyl methacrylate copolymer, parahydroxystyrene copolymer, etc.), and mixtures thereof. These binders may be used alone or in combination, and other additives may be added. The mixing ratio of ferromagnetic material and binder is ferromagnetic material by weight ratio.
The binder is used in an amount of 8 to 400 parts by weight, preferably 10 to 200 parts by weight per 100 parts by weight. Dispersants (pigment wetting agents) include caprylic acid, capric acid, lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, elaidic acid, linoleic acid, linolenic acid, stearolic acid, etc. with 12 to 18 carbon atoms. Fatty acids (R ₁ COOH, R ₁ is an alkyl or alkenyl group with 11 to 17 carbon atoms):
Alkali metals (Li, Na, K, etc.) of the above fatty acids
or metal soaps made of alkaline earth metals (Mg, Ca, Ba): fluorine-containing compounds of the above fatty acid esters: amides of the above fatty acids: polyalkylene oxide alkyl phosphate esters: lecithin: trialkyl polyolefin oxy Tetraammonium salts (alkyl having 1 to 5 carbon atoms, olefin being ethylene, propylene, etc.) are used. In addition to these, higher alcohols having 12 or more carbon atoms and sulfuric esters can also be used. These dispersants are added in an amount of 0.5 to 2.0 parts by weight based on 100 parts by weight of the binder. As a lubricant, dialkyl polysiloxane (alkyl has 1 to 5 carbon atoms), dialkoxypolysiloxane (alkoxy has 1 to 4 carbon atoms), monoalkyl monoalkoxy polysiloxane (alkyl has 1 to 5 carbon atoms, alkoxy has 1 to 4 carbon atoms
), phenylpolysiloxane, fluoroalkylpolysiloxane (alkyl has 1 to 5 carbon atoms), and other silicone oils; conductive fine powders such as graphite; inorganic fine powders such as molybdenum disulfide and tungsten disulfide; polyethylene , plastic fine powder such as polypropylene, polyethylene vinyl chloride copolymer, polytetrafluoroethylene;
α-olefin polymer: Unsaturated aliphatic hydrocarbon that is liquid at room temperature (a compound in which an n-olefin double bond is bonded to the terminal carbon, approximately 20 carbon atoms); carbon number 12 to
Fatty acid esters consisting of 20 monobasic fatty acids and monohydric alcohols having 3 to 12 carbon atoms, fluorocarbons, etc. can be used. These lubricants are added in an amount of 0.2 to 20 parts by weight based on 100 parts by weight of the binder. Commonly used abrasive materials include fused alumina, silicon carbide, chromium oxide ((Cr ₂ O ₃ ), corundum, artificial corundum, diamond, artificial diamond, garnet, and emery (main components: corundum and magnetite). These abrasives have a Mohs hardness of 5 or more and an average particle size of
A size of 0.05 to 5μ is used, particularly preferably 0.1 to 2μ. These abrasives are added in an amount of 0.5 to 20 parts by weight based on 100 parts by weight of the binder. In addition, the magnetic recording medium of the present invention is
It can be prepared using the material manufacturing method described in No. 26890. (Example) The present invention will now be described in more detail with reference to Examples. In addition, "parts" in the examples indicate "parts by weight." Example 1 Ba ferrite substituted with Co, average particle size of 0.015, 0.02, 0.04, 0.06, 0.07, 0.10, 0.15μm, plate ratio: 4, coercive force: 800Oe, 550 parts, graphite powder, 15 parts, vinyl chloride - Vinylidene chloride copolymer (copolymerization ratio
80:20, molecular weight 45000) 45 parts amyl stearate 10 parts lecithin 3 parts Cr ₂ O ₃ 5 parts methyl ethyl ketone 350 parts toluene 350 parts For each of the above ferromagnetic substances, add each component composition and use a sand grinder while circulating. The mixture was mixed and dispersed for 10 hours. Next, 50 parts of polyester polyol was added and mixed uniformly, and then 30 parts of polyisocyanate was added and mixed and dispersed again to obtain a curable magnetic paint. On the other hand, the above coating material was applied to a 25 μm polyethylene terephthalate film which had been subjected to corona discharge treatment using a gravure roll so as to have a dry thickness of 4 μm, and then dried to obtain a magnetic tape. The C/N and squareness ratio of the obtained magnetic tape were measured, and the relationship with the average plate diameter is shown in FIGS. 2 and 4. Example 2 500 Oe in the vertical direction of the web coated in Example 1
A magnetic field was applied to vertically align the material in the magnetic field, and the material was further dried to obtain a magnetic tape. The C/N and squareness ratio of the obtained magnetic tape were measured, and the results are shown in FIGS. 2 and 4. Example 3 Co-substituted Ba ferrite Each average plate diameter is 0.015, 0.02, 0.04, 0.06, 0.07, 0.10, 0.15 μm, plate ratio 12, coercive force: 800 Oe Each of the above Co-substituted Ba ferrites as a ferromagnetic material A magnetic tape was obtained by processing in exactly the same manner as in Example 1, except that a magnetic tape was used. The C/N and squareness ratio of the obtained magnetic tape were measured, and the results are shown in FIGS. 3 and 5. Example 4 500 Oe in the vertical direction of the web coated in Example 3
A magnetic tape was obtained by applying a magnetic field of 100 mL and drying in a vertical orientation in a magnetic field. The C/N and squareness ratio of the obtained magnetic tape were measured, and the results are shown in FIGS. 3 and 5. The volume fraction of the ferromagnetic material in each sample of Examples 1 to 4 was between 0.38 and 0.40. Example 5 Using Co-substituted Ba ferrite with an average plate diameter of 0.04 μm, plate ratio: 4, and coercive force: 800 Oe, a magnetic tape having the composition shown in Fig. 1 was produced.

[Table] Next, the measurement method is shown below. The evaluation was carried out using a β-type VTR machine with a ferrite head at a relative speed of 3.5 m/sec. (1) C/N The C/N value is the carrier (4.5MHz) measured with a spectrum analyzer at a bandwidth of 10KHz.
The ratio of the peak level of 3.5MHz to the noise level of 3.5MHz was taken. (2) Void volume ratio The void volume and void volume ratio in the magnetic layer were determined using a mercury intrusion porosimeter manufactured by Amco. (3) Squareness ratio This is the ratio of saturation magnetization to residual magnetization when an external magnetic field is 5KOe. (Effect of the invention) As is clear from FIGS. 2 and 3, Example 1,
In all cases of 2, 3, and 4, the average plate diameter is 0.015~
It can be seen that the C/N ratio is significantly improved in the range of 0.07 μm. The C/N becomes higher when the magnet is oriented, and the C/N becomes higher when a ferromagnetic material with a large plate ratio is used. This is thought to be because the oriented ferromagnetic material and the ferromagnetic material with a larger plate-like ratio have a larger squareness ratio in the direction perpendicular to the magnetic layer, improving the reduction output.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the average plate diameter and C/N for non-oriented and oriented particles with a plate ratio of 4. Second
The figure shows the relationship between the average plate diameter and C/N for non-oriented and oriented particles with a plate ratio of 12. FIG. 3 is a diagram showing the relationship between the average plate diameter of non-oriented and oriented particles and the squareness ratio in the vertical direction for particles having a plate-like ratio of 4. FIG. 4 is a diagram showing the relationship between the average plate diameter of non-oriented and oriented particles and the squareness ratio in the vertical direction for particles with a platelet ratio of 12. FIG. 5 is a volume fraction correlation diagram of magnetic material, non-magnetic material, and voids. FIG. 6 is a diagram showing the relationship between the volume fraction of magnetic particles and C/N with an average plate diameter of 0.04 μm, a plate ratio of 4, and no orientation.

Claims (1)

[Claims] 1. In a magnetic recording medium in which a magnetic layer containing a ferromagnetic material and a binder is provided on a non-magnetic support, the ferromagnetic material has an average plate diameter of 0.015 to 0.07 μm and a plate ratio of 3 to 15.
1. A magnetic recording medium comprising a hexagonal ferrite fine powder, characterized in that the volume fraction of ferromagnetic material in the magnetic layer is 0.2 or more, and the volume fraction of nonmagnetic material is 0.20 or more.