MAGNETIC RECORDING MEDIUM AND PRODUCTION THEREOF

Abstract: A magnetic recording medium comprising a non­ magnetic substrate and a magnetic layer formed thereon which contains chromium oxide ferromagnetic powder comprising chromium oxide of the formula: CrO x wherein x is not smaller than 1.74 and not larger than 1.91, which has improved long term storage stability.

Description: The present invention relates to a magnetic recording medium and production thereof. More particularly, it relates to a magnetic recording medium comprising a magnetic layer containing chromium oxide, which has improved long term storage stability. It is well known that chromium dioxide (CrO₂) powder has a ferromagnetic property and is used as a magnetic powder of a magnetic recording medium. Chromium dioxide powder is commercially produced by oxidation of Cr₂O₃ and CrO₃ according to following reactions: Coercive force and/or a particle size of the chromium dioxide powder can be adjusted by the addition of Sb₂O₃, Fe₂O₃, TeO₂, RhO₂, IrO₂ or the like to produce magnetic powder for magnetic recording. Recently, the amount of the chromium dioxide powder consumed as the magnetic powder is slightly decreasing since a magnetic powder comprising iron oxide or iron oxide covered with cobalt has been developed. This is because chromium dioxide contains Cr(IV) which is chemically unstable so that it is inevitable that the saturation magnetization of chromium dioxide is inherently deteriorated in the air with the passage of time. However, the chromium dioxide powder has an excellent property such that when recorded information is overwritten, namely when the recorded information of the magnetic recording medium is erased for rewriting, the information is satisfactorily erased. Since such good overwriting property of the chromium dioxide powder is suitable for a magnetic recording medium for use as an external memory of a computer which should have good repeated recording properties, the chromium dioxide powder is being reevaluated. Under such circumstances, some attempts have been made to suppress the deterioration of saturation magnetization with the passage of time due to chemical unstability of chromium dioxide with maintaining such inherent advantage of chromium dioxide. The first attempt is to convert the surface layer of the chromium dioxide powder particle from dioxide to hydroxide of Cr(III) which is more stable than chromium dioxide and comprises treating the chromium dioxide powder with a reducing agent such as sodium sulfite (cf. Italian Patent No. 27333A/79 and U.S. Patent Nos. 3,512,930 and 3,529,930). The second one is to remove reactive materials such as adsorbed water present on the surface of the chromium dioxide powder particles and comprises heating the chromium dioxide powder at a temperature of 330 to 370°C in a nitrogen atmosphere (cf. G. Bassile, Material Review Bulletin, 17 , (1982) 1197). However, the above attempts cannot prevent deterioration of saturation magnetization of the chromium dioxide powder with the passage of time, and a saturation magnetic flux density of a magnetic recording medium comprising such modified or thermally treated chromium dioxide powder decreases by 10 to 30 % from the original value after the recording medium is kept standing in the air at 60°C for 4 weeks. This may be partly because the particle size of the chromium dioxide powder is made smaller to satisfy the requirements for high density recording and in turn its specific surface area increases so that reactivity of the particle surfaces increases and thereby the saturation magnetization is more easily deteriorated than the conventional chromium dioxide powder having a larger particle size, and partly because mutual reactivity between a binder and the chromium dioxide powder is increased by functional groups of an additive which is added to increase dispersibility of the chromium dioxide powder in the binder. According to the study of the present inventors to find a good technique to prevent the deterioration of the saturation magnetization of the chromium dioxide powder with the passage of time, we came to a conclusion that, by the above first attempt, although CrOOH is formed on the surface of the chromium dioxide powder particle, it may deteriorate the chemical stability of the chromium dioxide powder. Namely, according to the first attempt, even when CrOOH is formed on the surface of the chromium dioxide powder particle, it easily attracts water because of its hydrophilicity. This results in insufficient stability of the chromium dioxide powder. By the above second attempt, the chromium dioxide powder is heated at a temperature of 330 to 370°C in an atmosphere of inert gas such as nitrogen to remove crystallization water of chromium dioxide or adsorbed water on the particle surface in order to increase the stability of chromium dioxide. However, the result is not satisfactory. This is because not only CrOOH but also chromium dioxide has strong bonding force with a reactive material such as water. Japanese Journal of Applied Physics, vol. 6, No. 12, December 1967, pages 1397-1399, discloses the reduction of CrO₃ to CrO₂ and Cr₂O₃ under very high pressure and high temperature. It is stated that high pressure and high temperature region under which chromium dioxide can be synthesized was examined by using split sphere type of high pressure equipment. The experiments were performed under pressures of up to 85 Kb and at a temperature up to 1600 °C. GB-A-1 006 610 discloses crystalline ferro-magnetic chromium dioxide and its preparation. It is claimed a ferromagnetic chromium dioxide of rutile structure obtained by the thermal decomposition of a mixture of chromium trioxide and tellurium compounds, the grain form of which is rectangular to needle, having a specific grain size and coercive force. One object of the present invention is to provide novel chromium oxide ferromagnetic powder. Another object of the present invention is to provide a magnetic recording medium having good overwriting properties and improved stability so that its magnetic properties are less deteriorated after long term storage. Further object of the present invention is to provide a method for preventing deterioration of magnetic powder comprising chromium oxide by simple thermal treatment. A yet another object of the present invention is to provide a method for producing a magnetic recording medium having good overwriting properties and magnetic stability. Fig. 1 shows changes of saturation magnetization (black circles) and coercive force (white circles) when the chromium dioxide powder having saturation magnetization of 86.3 emu/g and coercive force of 516 Oe is heated in the air for one hour at various temperatures, and Fig. 2 shows changes of saturation magnetization (black circles) and coercive force (white circles) when magnetic powder having saturation magnetization of 77.4 emu/g and coercive force of 505 Hc, which is produced by hydroxidising the surfaces of the chromium dioxide powder particles and supposed to have CrOOH on the surfaces, is heated in the air for one hour at various temperatures. In view of the above described circumstances, the present inventors have tried to replace a part of chromium dioxide with more stable chromium (III) oxide, since the chromium dioxide powder inherently easily bonds with water and the like and its properties are deteriorated. To achieve this, reaction conditions have been carefully selected with keeping it in mind that oxides of chromium other than chromium (IV) are easily formed since chromium dioxide is of course an oxide of chromium, and that the formed chromium oxides should be stably present and not disappear. As the result, it has been found that a desirable result can be obtained when the chromium dioxide powder is heated at a temperature not lower than 410°C till saturation magnetization decreases to 46 to 87 % of that of the untreated chromium dioxide (σ s : 86.3 emu/g). Therefore, according to one aspect of the present invention, there is provided chromium oxide ferromagnetic powder comprising chromium oxide of the formula: CrO x wherein x is not smaller than 1.74 and not larger than 1.91 (1.74 ≦ x ≦ 1.91). According to another aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and at least one magnetic layer formed thereon which contains the chromium oxide ferromagnetic powder of the present invention. Fig. 1 shows changes of saturation magnetization σ s (black circles) and coercive force Hc (white circles) when the chromium dioxide powder having saturation magnetization of 86.3 emu/g and coercive force of 516 Oe is heated in the air for one hour at various temperatures. From these results of Fig. 1, it is seen that saturation magnetization and coercive force due to magnetism of the chromium dioxide powder are substantially not deteriorated when the powder is heated at a temperature lower than 400°C. Such results indicate that any chemical change is not caused. Fig. 2 shows changes of saturation magnetization (black circles) and coercive force (white circles) when magnetic powder having saturation magnetization of 77.4 emu/g and coercive force of 505 Hc, which is produced by hydroxidising the surfaces of the chromium dioxide powder particles and supposed to have CrOOH on the surfaces, is heated in the air for one hour at various temperatures. From the fact that saturation magnetization of the magnetic powder increases in a temperature range lower than 400°C and decreases in a temperature range higher than 400°C, it can be said that when the magnetic powder is heated at a temperature higher than 410°C, CrOOH is reconverted to CrO₂ and substantially no CrOOH is present on the surfaces of the chromium dioxide powder particles. The reason why the chromium dioxide powder is heated at a temperature not lower than 410°C according to the present invention is to produce desired chromium oxide and to have CrOOH disappeared in order to stabilize the chromium dioxide powder. The gradual conversion from CrO₂ to a non-magnetic material comprising Cr₂O₃ (Cr(III) oxide) by heating at a temperature not lower than 410°C can be traced by measuring decrease of saturation magnetization or weight change. While the theoretical weight decrease is 9.5 % when all the chromium dioxide powder is converted to Cr₂O₃, the weight loss found in the completely nonmagnetized powder is about 9 % for the raw material chromium dioxide powder of the present invention. This means that the thermally treated magnetic powder according to the present invention comprises a major amount of chromium (III) oxide and a slight amount of chromium (IV) oxide. An average valency of chromium in the oxide is closer to trivalency than to tetravalency and x in CrO x falls in the range from 1.71 to 1.91. In this connection, U.S. Patent No. 3,512,930 discloses an intermediate chromium oxide of Cr(IV) and Cr(III) of the formula: Cr y O z wherein a ratio of z/y (=x) is not smaller than 2.0 and not larger than 3.0. However, such intermediate chromium oxide does not have sufficient effect on improvement of stability of chromium dioxide. On the contrary, the magnetic recording medium utilizing the chromium oxide ferromagnetic powder treated according to the present invention shows excellent stability. That is, when the chromium oxide ferromagnetic powder according to the present invention is kept standing in the air at 60°C for 4 weeks, a deterioration ratio of saturation magnetization does not exceed 6 %. It is easily recognized by the conventional gravimetric method that the atomic composition of the extremely stabilized chromium oxide ferromagnetic powder according to the present invention is such that the atomic ratio of chromium and oxygen, namely x in the formula: CrO x is not smaller than 1.74 and not larger than 1.91, and that preferably the weight ratio of chromium(IV) oxide to a total weight of chromium (IV) oxide and other chromium oxides is at least 48 % by weight. Further, by a sample vibration type magnetometer, the chromium oxide according to the present invention is analyzed to have saturation magnetization of 40 to 75 emu/g. For effectively producing the chromium oxide according to the present invention, the chromium dioxide powder containing substantially no CrOOH is heated at a temperature not lower than 410°C till saturation magnetization decreases to 46 to 87 % of original saturation magnetization of untreated chromium dioxide powder (σs = 86.3 emu/g) to produce CrO x wherein x is as defined above. When the chromium dioxide powder contains CrOOH, it may be heated at a temperature of 250 to 400°C in a nonreducing atmosphere to reconvert CrOOH to chromium dioxide and then heated at a higher temperature as described in the above, or the chromium dioxide powder containing CrOOH as such may be heated at a higher temperature as described in the above. The thermally treated ferromagnetic powder of the present invention is used as such or coated with a conventional organic material, and then dispersed in a synthetic resin base binder together with an organic solvent to prepare a magnetic paint, which is coated on a non-magnetic substrate made of, for example, a synthetic resin film or a non-magnetic metal plate and dried to produce a stabilized magnetic recording medium of the present invention. As described in the above, the chromium dioxide powder should be heated at a temperature not lower than 410°C. In view of uniformity and efficiency of the thermal treatment, preferably the chromium dioxide powder is heated at a temperature of 410 to 600°C, more preferably 420 to 500°C. The heating atmosphere should not be such atmosphere which prohibits the formation of CrO x wherein x is as defined in the above. Preferably, the heating is carried out in the air or a non-reducing atmosphere. The heating time is usually 0.2 to 6 hours. The ferromagnetic chromium oxide powder according to the present invention has improved affinity with a highly reactive resin having various polar groups and/or functional groups. The present invention will be explained further in detail by following examples in which parts are by weight unless otherwise indicated. Example 1 Each of following ferromagnetic chromium oxide powder (1,500 g) was heated in a muffle furnace of 50 cm in length, 30 cm in width and 30 cm in depth at a temperature of Table 1 and for a period of time of Table 1 in the air under 1 atm. Ferromagnetic chromium oxide powder (a) Ferromagnetic chromium dioxide powder Major axis: 0.4 µm Axis ratio: 10 Specific surface area: 25 m²/g Saturation magnetization: 86.3 emu/g Coercive force: 516 Oe Deterioration ratio: 29 % (after kept standing at 60°C in the air for 4 weeks) (b) Ferromagnetic chromium oxide powder particle surfaces of which were converted to the hydroxide of chromium (III) according to U.S. Patent No. 3,512,930 Major axis: 0.4 µm Axis ratio: 10 Specific surface area: 25 m²/g Saturation magnetization: 77.4 emu/g Coercive force: 505 Oe Deterioration ratio: 24 % (after kept standing at 60°C in the air for 4 weeks) Properties of each sample ferromagnetic powder are shown in Table 2. Each ferromagnetic chromium oxide powder having the above properties (100 parts) was mixed with vinyl chloride/ vinyl acetate/vinyl alcohol copolymer (VAGH manufactured by U.C.C.) (9 parts), polyurethane resin (Estan 5701 manufactured by Goodrich) (8 parts), myristic acid (1 part), n-butyl stearate (0.5 part), carbon black (2 parts), cyclohexanone (144 parts) and toluene (144 parts) and dispersed in a ball mill for 75 hours. Thereafter, polyisocyanate compound (Colonate L manufactured by Nippon Urethane) (3 parts) was added and thoroughly mixed to prepare a magnetic paint. Then, the magnetic paint was coated on a substrate made of a polyester film having a thickness of 20 µm with a gravure coater to a thickness of 4 µm after drying and planished. The film with the magnetic coating was cut to produce a magnetic tape, which was assigned the same number as that of the sample of the ferromagnetic powder. Tape No. Residual magnetic flux density Br (G) (10⁻⁴ T) Squareness ratio Br/Bs Coercive force (Oe) Deterioration ratio (%) 1 1,760 0.80 500 12.0 2 1,720 0.81 485 11.7 3 1,510 0.82 465 5.5 4 1,480 0.79 450 4.6 5 1,605 0.80 460 4.0 6 1,650 0.81 440 3.0 7 1,710 0.80 470 10.9 8 1,700 0.79 450 10.1 9 1,635 0.80 440 4.9 10 1,610 0.81 435 4.2 11 1,770 0.81 505 16.3 12 1,780 0.81 500 14.0 Properties of the ferromagnetic powder and the magnetic tape shown in Tables 2 and 3 were measured as follows: Deterioration ratio Deterioration ratios of the ferromagnetic powder and the magnetic tape are calculated from the original saturation magnetization and residual magnetic flux density and those measured after the ferromagnetic powder and the magnetic tape are kept standing in the air at 60°C for 4 weeks. Atomic ratio x (CrO x ) The atomic ratio x is calculated from the weight of CrO x and saturation magnetization, and the weight of the sample after heating it till saturation magnetization becomes 0 (zero) with assuming that only CrO₂ component has magnetism. Cr(IV)/non-Cr(IV) ratio This ratio is calculated from the weight change of the sample caused by the thermal treatment carried out in measuring the atomic ratio x. Saturation magnetization, Coercive force and Saturated magnetic flux density These properties are measured with a sample vibration type magnetometer (VSM-3 manufactured by Toei Kogyo) by applying a magnetic field of 5kOe. As understood from the above results, the ferromagnetic powder samples and the magnetic tapes of the present invention (Nos. 3, 4, 5, 6, 9 and 10) suffer from less deterioration than the conventional ones.

Claims: A chromium oxide ferromagnetic powder comprising chromium oxide of the formula: CrO x wherein x is not smaller than 1.74 and not larger than 1.91. The chromium oxide ferromagnetic powder according to claim 1, which has saturation magnetization of 40 to 75 emu/g. The chromium oxide ferromagnetic powder according to claim 1, which contains at least 48 atomic % of Cr(IV) based on the total of Cr(IV) and Cr other than Cr(IV). The chromium oxide ferromagnetic powder according to claim 1, the deterioration ratio of saturation magnetization of which after kept standing in the air at 60°C for 4 weeks is not more than 6 %. A magnetic recording medium comprising a nonmagnetic substrate and a magnetic layer formed thereon which contains chromium oxide ferromagnetic powder comprising chromium oxide of the formula: CrO x wherein x is not smaller than 1.74 and not larger than 1.91. The magnetic recording medium according to claim 5, wherein the chromium oxide ferromagnetic powder has saturation magnetization of 40 to 75 emu/g. The magnetic recording medium according to claim 5, wherein the chromium oxide ferromagnetic powder contains at least 48 atomic % of Cr(IV) based on the total of Cr(IV) and Cr other than Cr(IV). The magnetic recording medium according to claim 5, wherein the deterioration ratio of saturation magnetization of the chromium oxide ferromagnetic powder after kept standing in the air at 60°C for 4 weeks is not more than 6 %. A process for producing a magnetic recording medium which comprises steps of heating chromium dioxide powder containing substantially no CrOOH at a temperature not lower than 410°C till saturation magnetization decreases to 46 to 87 % of original saturation magnetization of untreated chromium dioxide powder, preparing a magnetic paint containing the thermally treated chromium oxide ferromagnetic powder in the previous step, coating the magnetic paint on a non-magnetic substrate and drying it. The process according to claim 9, wherein the chromium dioxide ferromagnetic powder is heated at a temperature of 410 to 600°C. The process according to claim 9, wherein the chromium dioxide ferromagnetic powder is heated in a non-reducing atmosphere.

Inventors: HOSOO, SHIGEYUKI, MATSUURA, TAKESHI, SHIBATA, TAKESHI C/O HITACHI MAXELL KOYO RYO, TERAZAWA, MASATOSHI C/O TOYO UNPANKI HOKUSETSU RYO
Applicants: HITACHI MAXELL LTD.
Application year: 1987
Main classification: G11B5
IPC codes: C01G37, G11B5, C09D7, C09D5, H01F1