JPH0459691B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a two-layer magnetic recording medium for high-density recording, which has excellent overall characteristics including high S/N characteristics. Ferromagnetic powders conventionally used in magnetic recording media include γ-Fe ₂ O ₃ and cobalt-containing γ.
-Oxide-based magnetic powders such as Fe ₂ O ₃ , Co-containing Fe ₃ O ₄ , and CrO ₂ were present. However, the coercive force (Hc), saturation magnetization (σs), and other magnetic properties of these ferromagnetic oxide powders are inadequate for use in magnetic recording media for high-sensitivity and high-density recording applications that will be required in the future market. However, it is not very suitable for magnetic recording of signals with short recording wavelengths of approximately 1 μm or less or narrow track widths (approximately 20 μm or less). As the requirements for such magnetic recording media become stricter, ferromagnetic powders with characteristics suitable for high-density recording are being actively developed, and ferromagnetic metal powder is one of the target materials. Ferromagnetic metals include Fe, Co, Ni, and
These alloys such as Fe-Co, Fe-Ni, Fe-Ni-Co, Co-Ni, and these alone or in alloys
It includes those to which metals such as Cr, Mn, Zn, Cu, Zr, Al, Tl, and Pt are added, as well as those to which small amounts of B, C, Si, P, etc. are added. The term "metal" is used herein to include single metals and alloys. At present, the following six methods can be considered as basic methods for producing these ferromagnetic metal powders: (1) Reduction of metal oxalates, which can form ferromagnetic materials, at high temperatures in a hydrogen stream. Method. (2) A method of reducing goethite or acicular γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃ , Fe ₃ O ₄ , Co-containing Fe ₃ O ₄ , etc. in a high-temperature air flow (dry reduction method) . (3) A method of evaporating a ferromagnetic material in an inert gas (evaporation method). (4) A method of decomposing metal carbonyl compounds that can create ferromagnets. (5) A method in which a ferromagnetic metal is electrodeposited in mercury using a mercury cathode, and then heated to separate it from the mercury. (6) A method in which a ferromagnetic powder is obtained by reducing a metal salt capable of forming a ferromagnetic substance using a reducing substance (such as a boron hydride compound) in an aqueous solution. The ferromagnetic metal powder produced by these production methods has unique properties due to the production method, but among them, the ferromagnetic metal powder obtained by the dry reduction method (2) has an acicular shape of the matrix. It inherits the needle-like shape. The particle size is 200 to 1000 Å along the short axis, and those with an axial ratio of 3 to 20 are suitable for use in magnetic recording media. As for magnetic properties, coercive force (Hc)
is more than 800 Oe, generally in the range of 800 to 2000 Oe, and the saturation magnetization (σs) is more than 100 emu/g, generally in the range of 100 to 180 emu/g. In addition, the dry-reduced ferromagnetic metal powder has good compatibility with the binder, produces a magnetic recording coating with good dispersibility, and has good orientation when made into a tape. However, dry reduction method ferromagnetic metal powder is difficult to make fine particles, so when it is made into a tape, the packing density per unit volume is low, the residual magnetic flux density (Br) is around 2000G, and the long wavelength (10 to 20 μm)
It is difficult to obtain an output, and therefore there are many noise components.
It has the disadvantage of poor signal-to-noise ratio. The ferromagnetic metal powder obtained by the evaporation method (3) has a chain shape in which several or more fine particles with a particle diameter of 100 to 500 Å and an average length of about 200 Å and a length of 500 to 10,000 Å are connected. As for magnetic properties, Hc is
It is easy to obtain ferromagnetic metal powder with a value of 1000 Oe or more and a σs of 100 emu/g or more. In addition, since the evaporation method ferromagnetic metal powder is fine particles as mentioned above, it has a high packing density per unit volume when made into a tape, and a residual magnetic flux density (Br) of about 3500G can be easily obtained.
Therefore, a high output can be obtained with less noise components, and a high S/N ratio can be achieved. However, evaporation method ferromagnetic metal powder has disadvantages such as poor running durability when made into a tape, easy head clogging, and frequent dropouts, so despite its excellent magnetic properties as mentioned above, However, no plans have yet been made to put this powder into practical use. As the demand for ferromagnetic metal tapes for high-density recording increases, the demand for improved overall properties including high output, low noise, running durability, reduced head clogging, and other properties is also increasing. At present, there is a limit to the improvement of overall properties when using only the two types of powder produced by the above-mentioned typical manufacturing method. In order to meet these demands, the inventors of the present invention believe that it is best to take full advantage of the advantages of the two types of ferromagnetic metal powders and find an appropriate combination of them.As a result of intensive research, the inventors found that We succeeded in obtaining an excellent high-density magnetic recording medium. That is, the present inventors have developed a lower layer consisting of a ferromagnetic metal powder obtained by a dry dry source method, which has a strong coating structure and good physical properties, on a non-magnetic support, and a ferromagnetic metal powder obtained by an evaporation method. 2 with an upper layer consisting of
A layered structure is formed, and the coating thickness is 1.0μ or less with respect to the upper layer made of the evaporation method ferromagnetic metal powder, and the weight ratio (P/B) of the magnetic powder to resin in the coating is 8/1. It was discovered that by adding regulations to the range of ~12/1, it was possible to improve poor physical properties such as running stability and head clogging, which had been a serious drawback until now. It is. Magnetic recording media with a two-layer structure have been developed in the past, but it is difficult to improve characteristics such as video S/N, running durability, and head clogging in magnetic tapes with a two-layer coating structure made of ferromagnetic metal powder. There are few targets in particular, and therefore, conventional two-layer tapes have not had good overall characteristics. On the other hand, the present invention improves running durability, physical properties such as still image characteristics, and video S/N by appropriately combining dry reduction method ferromagnetic metal powder and evaporation method ferromagnetic metal powder. It also creates a much better high-density recording medium than if they were used alone. The advantages of the present invention are as follows. (1) Since the upper layer is made of evaporation-produced metal powder, which is currently the finest particle of ferromagnetic metal powder, good surface smoothness can be obtained when it is made into a tape. In addition, since the noise component is small and high filling can be achieved, the residual magnetic flux density (Br) is high, and the spacing loss with the head is small, so a high-output, low-noise magnetic recording medium can be obtained. (2) Since the lower layer is made of ferromagnetic metal powder obtained by a dry reduction method, the physical strength of the lower layer coating can be reinforced when it is made into a tape. (3) Since it is not necessary to add non-magnetic powder such as an abrasive to the lower layer, a magnetic recording medium with a further improved S/N ratio can be obtained. (4) By adjusting the coercive force between the upper layer and the lower layer and other magnetic properties, a magnetic recording medium having desired magnetic properties can be suitably manufactured. In addition, since the upper layer coating thickness is as thin as 1μ or less, even if the upper layer is made to have a high Hc, the Hc of the coating as a whole will not be affected too much, and high output can be achieved without compromising compatibility with other tapes. Preferably, the upper layer retention force is equal to or higher than the lower layer retention force. The metal powder in each layer of the two-layer structure is kneaded with a binder, and then applied and surface-treated according to conventional methods. The thickness of the upper layer is 1.0μ or less, preferably 0.5μ or less, while the thickness of the lower layer may be about 1.5 to 3.0μ, which is the thickness of a normal magnetic recording medium, and the lower layer is selected depending on the intended use. sell. Lower layer is 2.0~3.0μ
One preferred example is one in which the upper layer has a thickness of 0.3 to 0.7 μm. As the binder, any combination of conventionally used vinyl chloride-vinyl acetate, acrylonitrile, urethane, polyester, epoxy, phenoxy, nitrocellulose, etc. can be used, especially in the lower magnetic layer. The binder species used for this purpose eliminates direct contact with the head and guide system.
Since the dispersibility of the magnetic powder and the adhesion to the non-magnetic support can be selected as main factors, it is possible to use a wide range of binders, from a single binder to a mixture of multiple binders. In addition, it may contain a dispersant, an abrasive, a lubricant, an antistatic agent, etc., but it is advantageous to use these additives separately for the upper layer and the lower layer. For example, it is not necessary to add an abrasive, which is a non-magnetic powder, to the lower layer, and thereby the S/N ratio of the lower layer can be improved more than before. A lubricant may also be added with particular emphasis on the upper layer. On the other hand, an antistatic agent such as carbon black can be added to the lower layer in more than the usual amount, thereby making it possible to further reduce the electrical resistance. The present invention will be explained below based on comparative examples and examples. Comparative example 1 (Magnetic powder alone using evaporation method magnetic powder) Evaporation method alloy powder 400 parts by weight Polyurethane resin 50 〃 Epoxy resin 30 〃 Nitrified cotton 20 〃 Dispersant 3 〃 Abrasive 20 〃 Lubricant 6 〃 Methyl ethyl ketone 600 〃 Methyl isobutyl ketone 300 〃 Cyclohexanone 200 〃 Put the above formulation into a paint dispersion machine, thoroughly knead and disperse, and add crosslinking agent polyisocyanate (Coronate L manufactured by Nippon Polyurethane Co., Ltd.) to the resin component.
A paint was prepared by adding 15 parts by weight and mixing and stirring until uniform. The appropriate amount of the curing agent is preferably 10 to 30 parts by weight (preferably 15 to 20 parts by weight) in order to expect a sufficient effect on the resin component used.
This paint was applied onto a polyester film base with a thickness of 15μ, subjected to surface treatment using a super calendar, and cross-linked at 60°C for 24 hours to produce a magnetic recording medium with a coating thickness of 2.0μ. This is referred to as Comparative Example 1. The above alloy has a weight ratio of Fe:Ni:Co-70:
A 20:10 alloy was placed in a sealed container equipped with an air-core coil of a plasma generator, and the container was brought to a low pressure state using a pump, and argon was introduced to bring the pressure to 2 mmHg.Then, it was heated with an argon plasma jet, evaporated, and then evaporated. The metal vapor was collected after being applied with a magnetic field of 900 Oe using an air-core coil as it condensed. Comparative Example 2 (Magnetic layer alone using dry reduction method magnetic powder) A coating material was prepared using the ferromagnetic alloy powder obtained by the dry reduction method in the same manner as in Comparative Example 1, and a magnetic layer was prepared through the same process. A recording medium was used, and the obtained sample was used as Comparative Example 2. The above alloy powder is produced by heat-treating cobalt-coated goethite to form iron-based acicular iron oxide containing cobalt, reducing it in a high-temperature hydrogen stream, cooling, and immersing it in sodium oleate to form acicular and strong iron oxides. It was prepared by obtaining magnetic alloy powder. Example 1 and Comparative Example 3 In the same manner as in Comparative Example 1, the weight ratio of evaporation alloy powder and binder was changed to 7/1, 8/1,
10/1, 12/1, and 13/1, the paint was adjusted and applied as an upper layer on the coating film obtained in Comparative Example 2, and the surface treatment was performed using a super calender at 60°C for 24 hours. Crosslinked, upper layer thickness 0.5μ, 1.0μ
and 2.0μ samples were obtained. This was cut into 1/2 inch width to obtain a video magnetic recording medium. The samples obtained in various combinations in this manner are listed in the table below along with their various properties. In this method, the upper layer and the lower layer are prepared separately, but the same effect can be obtained even if the two layers are simultaneously coated using a coating machine having a multilayer coating function. Those within the bold frame meet the conditions of the present invention, and the rest represent Comparative Example 3 outside the conditions of the present invention.

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[Table] Regarding each characteristic shown in the table, "Video S/
"N" is a measured value with Comparative Example 2 set as 0 (dB). "Still still image" is a still image state at 20° C. and 60%, and is shown as the time required for the 3MHz output to attenuate by 2 (dB) from the initial value.
"Running durability" is the number of runs until the video S/N decreases by 2.0 (dB) when continuously running in an environment of 20°C and 60%. "Head adhesion stain" was determined by visually observing the degree of stain on the head surface using a microscope after 120 minutes of initial running. × indicates head stains, △ indicates head stains are slightly present, and ○ indicates head stains are not present. As is clear from the above tables, the two-layer magnetic recording medium of the present invention is significantly improved in video S/N compared to those used alone in the past, and is also superior in various physical properties. Overall, it is an excellent magnetic recording medium that has never existed before.

Claims (1)

[Scope of Claims] 1. A lower coating film made of acicular ferromagnetic metal powder obtained by a dry reduction method on a non-magnetic support, and an upper coating film made of ferromagnetic metal powder obtained by an evaporation method. and in the upper coating film, the weight ratio of the magnetic powder to the binder in the coating film is 8/1 to 12/
1, and the coating film thickness is 1μ or less. 2. A patent claim in which the acicular ferromagnetic metal powder obtained by the dry reduction method is obtained by reducing acicular oxyhydroxides or acicular oxides obtained from these oxyhydroxides in a reducing air flow. The magnetic recording medium according to scope 1. 3. The magnetic recording medium according to claim 2, wherein the acicular oxide obtained by a dry reduction method is reduced in a high-temperature hydrogen stream. 4. The magnetic recording medium according to claim 1, wherein the ferromagnetic metal powder obtained by the evaporation method is a chain of ferromagnetic metals evaporated in an inert gas. 5 Acicular ferromagnetic metal powder produced by dry reduction method,
It has a short axis dimension of 200 to 1000 Å and an axial ratio of 3 to 20,
The particles have a coercive force (Hc) of 800 Oe or more and a saturation magnetization (σs) of 100 emu/g or more, and the chain-like ferromagnetic metal powder produced by the evaporation method has a particle size of 100 to 500 Å and a length of 500 to 10000 Å. The magnetic recording medium according to claim 1, which is a particle having a coercive force (Hc) of 1000 Oe or more and a saturation magnetization (σs) of 100 emu/g or more.