Magnetic recording medium

Abstract: A magnetic recording medium with improved dura­ bility is provided and comprises a substrate and a magnetic layer formed thereon which contains hexagonal system ferrite magnetic powder in a volume percentage of not larger than 35 % per whole volume of the magnetic layer.

Description: The present invention relates to a magnetic recording medium. More particularly, it relates to a magnetic recording medium with improved durability comprising a magnetic layer containing hexagonal system ferrite powder as magnetic powder. Conventionally, magnetic properties of the magnetic recording medium have been improved, for example, by orientating longer axis of acicular particles of magnetic powder in a direction along a longitudinal direction of the magnetic layer. However, in the magnetic recording medium which utilizes magnetic components in the longitudinal direction of the magnetic layer, strength of a diamagnetic field in the magnetic layer increases as a magnetic recording density increases. Therefore, the recording density has its own limit. In a perpendicular magnetic recording system which utilizes a magnetic component perpendicular to the plane of the magnetic layer, the strength of the demagnetizing field decreases as the recording density increases, so that such recording system is recognized to be suitable for high density recording. As the most suitable magnetic powder for the perpendicular magnetic recording system, used is hexagonal system ferrite powder each particle of which has a platelet shape and magnetic easy axis in a direction perpendicular to said platelet plane (cf. Japanese Patent Kokai Publication (unexamined) No. 209928/1985) However, a magnetic layer of the conventional magnetic recording medium contains hexagonal system ferrite powder in a volume percentage of 42 % or more based on the volume of the magnetic layer. Therefore, amounts of a binder component which bonds the magnetic power to the substrate, of an abrasive which reinforces the magnetic layer and of a lubricant which improves lubricity of the surface of the magnetic powder are decreased, so that durability of the magnetic layer is not satisfactory. DE-A-31 13 859 discloses magnetic recording media the magnetic layers of which contain magnetic particles in volume percentages which can be assumed to be not greater than 35 % based on the volume of the magnetic layer. The document discloses only acicular magnetic powder being present in the magnetic paint. DE-A-34 43 566 discloses the use of hexagonal system ferrite magnetic powders being useful magnetic powders for magnetic recording media. One object of the present invention is to provide a magnetic recording medium with improved durability which comprises a magnetic layer containing hexagonal system ferrite magnetic powder in a reduced volume percentage. Another object of the present invention is to provide a magnetic recording medium comprising a magnetic layer which contains hexagonal system ferrite magnetic powder in a reduced volume percentage and can contain increased amounts of a binder resin and other additives such as an abrasive and a lubricant. These and other objects are accomplished by a magnetic recording medium comprising a substrate and a magnetic layer having a porosity of not smaller than 20 % by Volume formed thereon which contains hexagonal system ferrite magnetic powder having a platelet Size of 0.05 to 0.2 µm in a volume percentage of not larger than 35 % per whole volume of the magnetic layer and a lubricant present in the pores in an amount of 0.1 to 25 % by weight of the magnetic powder. When the magnetic layer has porosity of not smaller than 20 % by volume, it is imparted with a cushioning effect so as to absorb shock caused by contact of the magnetic recording medium to a magnetic head. In addition, a sufficient amount of the lubricant is contained in the pores, whereby the durability of the magnetic layer is further improved. According to the present invention, the volume percentage of the hexagonal system ferrite magnetic powder per whole volume of the magnetic layer is not larger than 35 %, preferably not larger than 30 %. When the volume percentage of the hexagonal system ferrite magnetic powder is not larger than 35 %, the binder resin which bonds the magnetic powder to the substrate, the abrasive which reinforces the magnetic layer, the lubricant which increases lubricity of the surface of the magnetic layer or other additives can be contained in the magnetic layer in such amounts as to sufficiently improve durability of the magnetic recording medium. If the volume percentage of the hexagonal system ferrite magnetic powder is larger than 35 %, those additives cannot be contained in the magnetic layer in sufficient amounts so that durability of the magnetic recording medium is not satisfactorily improved. The magnetic layer has porosity of not smaller than 20 %. When the porosity is not smaller than 20 %, the magnetic layer is imparted with the cushioning effect so that it can absorb shock caused by the contact of the magnetic recording medium to the magnetic head. Further, when the lubricant is contained in the pores, lubricity and in turn durability of the magnetic recording medium are further improved. Specific examples of the hexagonal system ferrite magnetic powder are barium ferrite magnetic powder, strontium ferrite magnetic powder, lead ferrite magnetic powder, calcium ferrite magnetic powder and the like. In such ferrite magnetic powder, a part of iron atoms may be replaced with other metal atom such as cobalt, titamium, zinc, zirconium, nickel, manganese, niobium and indium. A platelet of the hexagonal system ferrite magnetic powder has a size of 0.05 to 0.2 µm. When the particle size of the hexagonal system ferrite magnetic powder is smaller than 0.05 µm, a volume of the platelet becomes too small so that strength of the magnetic layer is not improved. The hexagonal system ferrite magnetic powder having a platelet size of not smaller than 0.05 µm can effectively reinforce the magnetic layer and improve durability of the magnetic recording medium. When the platelet size of the hexagonal system ferrite magnetic powder is larger than 0.2 µm, a noise level of the magnetic recording medium is unfavorably increased. To efficiently carry out high density recording, the hexagonal system ferrite magnetic powder preferably has saturation magnetization of 377 to 880·10⁻¹⁰ T/g (30 to 70 emu/g) and coercive force of 159 to 1592 A/cm (200 to 2,000 Oe). The lubricant contained in the magnetic layer can be any one of conventionally used lubricants. Preferred examples of the lubricants are aliphatic lubricants such as aliphatic acid esters, aliphatic acids, metal salts of aliphatic acids, aliphatic acid amide and aliphatic alcohols, fluorine-containing lubricants, silicone type lubricants, hydrocarbon type lubricants and the like. Preferred examples of the aliphatic acid ester are oleyl oleate, 2-ethylhexyl oleate, butyl stearate, octyl myristate, monoglyceride stearate, monoglyceride palmitate, monoglyceride oleate, pentaerythritol tetrastearate and the like. Preferred examples of the aliphatic acid are lauric acid, myristic acid, palmitic acid, oleic acid, stearic acid, behenic acid and the like. Preferred examples of the metal salts of the aliphatic acid are lithium salt, sodium salt, calcium salt, magnesium salt, aluminum salt, iron salt, cobalt salt, zinc salt, barium salt and lead salt of the above exemplified aliphatic acids. Preferred examples of the aliphatic acid amide are caproic amide, caprylic amide, lauric amide, palmitic amide, behenic amide, oleic amide, linolic amide, methylenebisstearic amide and the like. Preferred examples of the aliphatic alcohol are stearyl alcohol, myristyl alcohol and the like. As the fluorine-containing lubricant, there are exemplified trichlorofluoroethylene, perfluoropolyether, perfluoro(alkyl polyether), perfluoroalkyl carboxylic acid. Examples of commercially available fluorine-containing lubricants are Daiflon® #20, Crytox® M, Crytox® H and Bidax®, Fomblin® Z and the like. Preferred examples of the silicone type lubricant are silicone oil and modified silicone oil. Preferred examples of the hydrocarbon type lubricant are liquid paraffin, squalane and synthetic squalane. The lubricant is added to the magnetic layer in an amount of 0.1 to 25 % by weight based on the weight of the hexagonal system ferrite magnetic powder. When the amount of the lubricant is less than 0.1 % by weight, lubricity of the magnetic layer is not effectively improved. When it is larger than 25 % by weight, adhesivity of the magnetic layer with the substrate made of, for example, a polyester film is deteriorated. The magnetic recording medium of the present invention may be produced by a per se conventional method. For example, the hexagonal system ferrite powder is mixed with and dispersed in a binder resin, an organic solvent and other conventional additives so that the volume percentage of the hexagonal system ferrite magnetic powder and the porosity are adjusted to prepare a magnetic paint composition in the ranges as defined according to the present invention. Then, the composition is applied on a substrate made of, for example, a polyester film by a conventional coating method, for example, by means of a roll coater and dried to form a magnetic layer on the substrate. If necessary, the coated magnetic layer is calendered to adjust the porosity. The binder resin may be any of the conventional ones and includes vinyl chloride-vinyl acetate copolymers, polyvinyl butyral resins, cellulose type resins, polyurethane resins, polyester resins, isocyanate compounds and the like. The organic solvent is also any of the conventional ones and includes toluene, methyl isobutyl ketone, methyl ethyl ketone, cyclohexanone, tetrahydrofuran, ethyl acetate and mixtures thereof. Examples of other additives are a dispersant, a lubricant, an abrasive and an antistatic agent. The magnetic recording medium according to the present invention includes various forms such as a magnetic tape having a magnetic layer on one surface of the plastic tape substrate, a magnetic disc having magnetic layers on both surfaces of the disc substrate, and a magnetic drum. The present invention will be hereinafter explained further in detail by following examples, wherein parts are by weight. Examples 1-11 Hexagonal system ferrite magnetic powder in which a part of iron atoms are replaced with cobalt and titanium atoms having the properties as shown in Table 1 was used. Magnetic paint components as shown in Table 2 were compounded in a ball mill for 72 hours to prepare a magnetic paint. The magnetic paint was coated on both surface of a polyester film having a thickness of 75 µm and dried to form dry magnetic layers each having a thickness of 1.0 µm followed by calendering under conditions shown in Table 3. Then the film with the magnetic layers was blanked to produce a magnetic disc having a diameter of 20.02 cm. Comparative Examples 1-8 In the same manner as in Examples but using hexagonal system ferrite magnetic powder as shown in Table 1 and a magnetic paint composition as shown in Table 2 and calendering the film with the magnetic layers under conditions as shown in Table 3, a magnetic disc was produced. Example No. Hexagonal system ferrite magnetic powder Diameter (µm) Thickness (µm) Coercive force (Oe) A/cm Saturation magnetization (emu/g) 10⁻¹⁰ T/g Specific surface area by BET method (m²/g) 1 0.06 0.012 (500) 398 (54) 679 52 2 0.06 0.012 (500) 398 (54) 679 52 3 0.06 0.012 (500) 398 (54) 679 52 4 0.08 0.016 (490) 390 (55) 691 50 5 0.06 0.012 (500) 398 (54) 679 52 6 0.12 0.024 (480) 382 (56) 703 45 7 0.06 0.012 (500) 398 (54) 679 52 8 0.06 0.012 (500) 398 (54) 679 52 9 0.06 0.012 (500) 398 (54) 679 52 10 0.05 0.010 (510) 406 (53) 666 56 11 0.04 0.008 (520) 414 (52) 653 60 Comp. 1 0.06 0.012 (500) 398 (54) 679 52 2 0.06 0.012 (500) 398 (54) 679 52 3 0.06 0.012 (500) 398 (54) 679 52 4 0.06 0.012 (500) 398 (54) 679 52 5 0.06 0.016 (490) 390 (55) 691 50 6 0.12 0.024 (480) 382 (56) 703 45 7 0.05 0.010 (510) 406 (53) 666 56 8 0.04 0.008 (520) 414 (52) 653 60 Calendering Conditions Example No. Temp. (°C) Pressure (kg/cm²) Rate (m/min.) 1 50 175 40 2 50 150 40 3 50 150 40 4 50 150 40 5 40 150 40 6 50 150 40 7 50 200 40 8 50 150 40 9 50 100 40 10 50 150 40 11 50 150 40 Comp. 1 70 200 40 Comp. 2 40 200 40 Comp. 3 50 200 40 Comp. 4 50 150 40 Comp. 5 70 200 40 Comp. 6 50 205 40 Comp. 7 50 200 40 Comp. 8 50 200 40 The volume percentage of the hexagonal system ferrite magnetic powder and porosity in each of the magnetic discs produced in Examples and Comparative Examples were measured, and durability of the magnetic disc was tested by driving it in a floppy disc drive and counting the drive numbers when flaws appeared on the surface of the disc. The results are shown in Table 4. Example No. Volume percentage (vol%) Porosity (vol%) Durability (x 10⁶) 1 34.5 24.8 4.5 2 32.0 26.1 5.2 3 29.2 24.4 5.0 4 29.4 24.2 5.8 5 27.2 25.9 6.4 6 29.6 24.0 7.0 7 27.0 17.4 3.8 8 25.1 23.2 6.6 9 23.2 29.1 9.5 10 28.8 24.8 4.2 11 28.6 25.0 3.1 Comp. 1 36.9 19.6 1.5 Comp. 2 36.7 24.5 2.4 Comp. 3 39.0 24.1 1.7 Comp. 4 39.2 28.1 2.2 Comp. 5 42.7 21.7 0.25 Comp. 6 39.4 23.7 2.0 Comp. 7 38.7 24.4 1.4 Comp. 8 38.4 24.7 1.0 As is understood from the results of Table 4, the magnetic discs produced in Examples 1 to 11 had better durability than those produced in Comparative Examples 1 to 8.

Claims: A magnetic recording medium comprising a substrate and a magnetic layer having a porosity of not smaller than 20 % by volume formed thereon which contains hexagonal system ferrite magnetic powder having a platelet size of 0.05 to 0.2 µm in a volume percentage of not larger than 35 % per whole volume of the magnetic layer and a lubricant present in the pores in an amount of 0.1 to 25 % by weight of the magnetic powder. The magnetic recording medium according to claim 1, being a magnetic disc.

Inventors: KISHIMOTO MIKIO, MIYAKE AKIRA, KISHIMOTO, MIKIO, MIYAKE, AKIRA
Applicants: HITACHI MAXELL, HITACHI MAXELL LTD.
Application year: 1987
Main classification: G11B5
IPC codes: C09D5, G11B5, C09D7