JPH0226290B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a magnetic recording medium. In particular, the object is to provide a magnetic recording medium with excellent electromagnetic conversion characteristics and running stability. In general, magnetic recording media include magnetic tape, magnetic sheet, magnetic card, magnetic disk, magnetic drum, etc., and each is widely used for audio, video, and computer purposes, and has recently undergone remarkable development. Be looked at. In particular, with the spread of home video, the demand for videotapes has increased significantly. Video decks have ranged from single-function types that simply run a rotating magnetic head and tape at a constant high speed, to still image, variable speed low-to-high speed types, long-time recording, audio multiplexing for stereo, and even portable ones that aim to be smaller and lighter. As more and more functions are being added, magnetic tapes are being forced to be used in harsher ways than before. For this reason, there is an increasing demand for magnetic tapes that have improved image quality through improved electromagnetic conversion characteristics, long-term running stability, and environmentally resistant durability. In order to obtain such high-performance, high-quality magnetic tapes, materials and tape manufacturing techniques have been intensively studied, but the current situation is that they have not yet reached a fully satisfactory level. Magnetic tape is manufactured through approximately the following steps. (1) A manufacturing process for a magnetic coating liquid, which is made by mixing magnetic powder, a binder, and various additives added as necessary with an organic solvent. (2) A coating step in which the magnetic coating solution is coated on a non-magnetic support such as a polyester film and dried. (3) A surface forming process that smooths the surface of the applied magnetic film. (4) A shredding process in which the magnetic film after surface molding is cut into a predetermined width. (5) Assembly process to finish the product. When manufacturing a magnetic tape for video using the above manufacturing method, it is necessary to improve the surface properties of the magnetic layer from the viewpoint of sensitivity, S/N, or dropout, and to lower the coefficient of friction of the magnetic tape. Raw materials and process technology are rigorously tested to ensure stable running at all times due to high-speed sliding contact with heads and cylinders. Conventionally, the following methods have been known as main methods for improving electromagnetic conversion characteristics, mainly sensitivity and S/N. (1) Uniformly disperse magnetic powder in the magnetic layer. (2) The particle size of the magnetic powder is made small enough to prevent it from becoming superparamagnetic. (3) Increase the residual magnetization of the magnetic layer. (4) Improve the surface finish of the magnetic layer and reduce gap loss. (5) Reduce the surface roughness of non-magnetic supports such as polyester films. Although sensitivity, S/N, etc. can be considerably improved by such a method, the coefficient of friction on the tape surface increases, making running unstable and reducing durability. However, there was a problem in that it was generally disadvantageous in terms of practical running, and the initial electromagnetic conversion characteristics could not be maintained. In particular, in order to further improve the dispersibility of magnetic powder in the magnetic layer, a dispersant is generally used when producing a magnetic paint. However, if a large amount of dispersant is added to improve the dispersibility of magnetic powder, a blooming phenomenon will occur on the surface of the magnetic layer that will be formed later, resulting in a decrease in sensitivity and S/N.
In addition, clogging of the magnetic head, powder adhesion, and increase in dropout occur, which is undesirable. On the other hand, if the amount of dispersant is small, the dispersion effect will be small and the electromagnetic characteristics of the magnetic tape will not be improved. For this reason, a major challenge was how to improve the dispersibility of magnetic powder by reducing the amount of dispersant. The present invention is based on the discovery that by studying the method of adding a dispersant, it is possible to obtain a magnetic recording medium consisting of a magnetic layer with excellent dispersibility of magnetic powder with a smaller amount of dispersant than before. It is something. Another object of the present invention is to provide a magnetic recording medium with significantly improved coefficient of friction and stable running. That is, in the present invention, a part of the dispensing agent is included in advance in a composition consisting of an inorganic powder consisting of magnetic powder, carbon black and an abrasive, and other necessary components, and the mixture is mixed and dispersed to form a kneaded product. After that, by adding the remaining dispersant and stirring and dispersing, a magnetic coating liquid with high dispersibility is obtained with a small total amount of dispersion, and the magnetic tape manufactured using this magnetic coating liquid can be used for electromagnetic conversion. Excellent characteristics,
It is based on the fact that the coefficient of friction is small. If the entire amount of the dispersant is added at once at the stage of preparing the above-mentioned composition and making a kneaded product as in the past, more dispersant than necessary will be adsorbed on the surface of the magnetic powder and inorganic fine powder such as carbon black. In order to incorporate the required dispersant into the organic material, which is mainly composed of a binder, a larger amount of the dispersant is required. This is thought to be the cause of the blooming phenomenon on the surface of the magnetic layer when it is later made into a magnetic tape.In contrast, when making a magnetic coating liquid as in the present invention, two dispersants are used. It is surmised that if the dispersant is added in stages, the dispersant can sufficiently produce its function with a small total amount of dispersant, and the above-mentioned drawbacks can be overcome. Dispersants used in the present invention include higher fatty acids, metal soaps prepared by saponifying them with alkali metals, higher alcohols, lecithin, and the like alone or in combination of two or more thereof. Among these, higher fatty acids having 12 to 18 carbon atoms and lecithin are particularly preferred. The total amount of these dispersants is
The amount is preferably 0.2 to 4 parts by weight per 100 parts by weight, and the ratio of pre-addition to post-addition is preferably within the range of 1/9 to 9/1. When the amount of the above-mentioned component is less than 0.2 parts by weight, the dispersion effect is small, and when it is more than 4 parts by weight, a blooming phenomenon occurs. Additionally, if the addition ratio is less than 1/9, pre-dispersion becomes difficult;
When it is larger than 9/1, it is no different from conventional bulk addition. The magnetic powder used in the method of the present invention is
Conventionally known magnetic iron oxide powder (Fe ₃ O ₄ ,
Co-containing-Fe ₃ O ₄ , γ-Fe ₂ O ₃ , Co-containing γ-Fe ₂ O ₃
etc.), chromium dioxide powder, or metal magnetic powder. The abrasive is mixed in an appropriate _amount in _the magnetic layer to prevent the surface of the magnetic head from becoming rough.
High hardness fine powders such as Al ₂ O ₃ , SiC, TiO ₂ , α-Fe ₂ O ₃ and the like can be used. However, care must be taken because if this amount is too large, the amount of wear on the magnetic head will increase, giving adverse results. Antistatic agents reduce the surface electrical resistance of the tape to 10 ⁸ to 10 ¹⁰ in order to prevent drop-outs and noise generation due to charging of the magnetic tape made of insulating material.
This is added because it is necessary to lower the resistance to Ω/□. For example, conductive materials such as carbon particles, graphite particles, or a surfactant are included in the magnetic layer. As for the support, polyester film, acetate film, polyimide film, polyamide film, paper, etc. are used. The binder serves as a skeleton material for binding magnetic powder to form a magnetic layer, and many resins are known. Typical examples include polyurethane resin, vinyl chloride-vinyl acetate resin, epoxy resin, phenolic resin, acrylic resin, cellulose resin, butyral resin, polyamide resin,
There are urea resins and the like, and in reality, these may be used alone or in combination of two or more. Furthermore, in the case of video magnetic tape, durability and heat resistance are required because it comes into sliding contact with the magnetic head at a high relative speed, so a hardening agent is added to the binder to form a thermosetting resin. Common. Examples of curing agents used at this time include isocyanade, polyamine, and polyamide compounds. Lubricants include higher fatty acid ester-based lubricants, silicone oil-based or fluorine oil-based lubricants, all of which have small additions that lower frictional resistance during running, stabilize running performance, and improve performance. It is used for the purpose of improving the abrasion resistance of the tape itself. However, if too much lubricant is used for this purpose, a blooming phenomenon will occur, so
Excessive addition is not preferred. A part of the dispersant mentioned above is added to the magnetic fine powder, binder, and trace additives such as antistatic agents, abrasives, and lubricants added as needed, and mixed with an organic solvent. Thoroughly mix and disperse with a machine to make a kneaded product. Next, after adjusting the viscosity of this kneaded product, the remainder of the above-mentioned dispersant is added and sufficiently stirred and mixed using a dispersing machine to form a magnetic coating liquid. The organic solvents used here are mainly:
These include methyl ethyl ketone, toluene, methyl isobutyl ketone, cyclohexanone, and ethyl acetate. As the mixer and dispersion machine, a ball mill, sand mill, dissolver, attritor, high-speed mixer, kneader, etc. may be used. The magnetic coating liquid prepared in this way is applied onto a support such as a polyester film and dried. The coating method may be a doctor blade method, a gravure method, or a reverse roll method. At this time, in order to further improve the smoothness of the coating film, a smoother is brought into contact immediately after coating. This smoother includes bar smoother, wire smoother,
Examples include film smoothers and knife smoothers. After this, in order to align the magnetic powder in one direction,
A magnetic field of appropriate strength is applied through a magnetic field orientation device. The strength of the magnetic field at this time varies depending on the coercive force of the magnetic powder, but is preferably about 2 to 4 times as strong. The magnet may be either a permanent magnet or an electromagnet. After orientation, the film is dried to remove the solvent. After coating and drying, the surface of the magnetic film is calendered to make it even smoother. At this time, the surface roughness of the mirror roll and elastic roll of the calender device has a large effect on the electromagnetic conversion characteristics of the tape. Calendar conditions include temperature
50~100℃, pressure 50~400Kg/cm, speed 20~200
m/min is preferred. The surface-molded magnetic film is shredded to a predetermined width, but it is important that the width accuracy is high and width fluctuations are small without being stretched to one side. During the final assembly process, care must be taken to ensure that the tape is free from dust and other deposits that may cause dropouts. In the magnetic tape obtained as described above, the magnetic powder is dispersed in a better state in the magnetic layer than in the conventional case where the entire amount of dispersant is added at once at the initial stage of making magnetic paint. Moreover, since no blooming phenomenon was observed, it was found that the S/N ratio, electromagnetic conversion characteristics of sensitivity, and runnability were very excellent. It goes without saying that the gist of the present invention is applicable not only to magnetic tapes but also to the manufacture of magnetic recording media such as magnetic sheets, magnetic cards, magnetic disks, and magnetic drums. Examples of the method of the present invention will be specifically described below. It should be noted that all parts in the composition ratios described in the examples are parts by weight. Example Co-containing magnetic iron oxide (Hc=650 _Oe ) 100 parts Polyurethane resin 10 parts Vinyl chloride-vinyl acetate copolymer 4 parts Nitrocellulose resin 6 parts Carbon black 5 parts Toluene 120 parts Methyl ethyl ketone 120 parts To the above composition, Add the dispersant shown in the "Pre-addition" column in the table, mix and disperse in a ball mill for 48 hours, take out the kneaded product, and then add the amount of dispersant shown in the "Post-addition" column and 4 parts of the polyisocyanate compound as described above. The mixture was added to the kneaded material, stirred and dispersed for 30 minutes using a high-speed dissolver, and then filtered through a filter with an average pore size of 3 μm to prepare a magnetic coating liquid. After each of the magnetic coating solutions described above was applied onto a polyester film with a thickness of 15 μm and dried, the surface treatment of the magnetic layer was performed using a super calendar roll to obtain a wide magnetic film with a magnetic layer thickness of about 5 μm.
These were cut into pieces 1/2 inch (12.65 mm) wide to produce video magnetic tape. Sample each magnetic tape obtained in the above manner.
The squareness ratio, S/N, sensitivity, and friction coefficient of each No. 1 to No. 9 are summarized in the table below. Comparative Example A magnetic tape was produced in the same manner as in the Example, except that the pre-addition and post-addition of the dispersant performed in the Example were changed to only the pre-addition (see the table below for the amount of dispersant). The samples at this time were designated as Nos. 11 to 16, and the tape characteristics of each are shown in the table below.

【table】

[Table] Among the results in the table above, the results for myristic acid as a dispersant are shown in the drawing. As is clear from the above table and drawings, according to the method of the present invention, various characteristics such as S/N, blooming, and coefficient of friction are all improved.

[Brief explanation of drawings]

The figure shows the effects of the magnetic recording medium manufacturing method according to the present invention and the conventional method in comparison.

Claims (1)

[Claims] 1. A part of a dispersant is added in advance to a composition consisting of an inorganic powder consisting of at least magnetic powder, carbon black, and an abrasive, a binder, and other necessary components, and the composition is mixed and dispersed. process and
Thereafter, the method includes the steps of: adding the remainder of the dispersant and stirring and dispersing it to obtain a magnetic coating solution; and coating the magnetic coating solution on a non-magnetic support and drying it to form a magnetic layer. A method of manufacturing a magnetic recording medium characterized by: 2. The method for manufacturing a magnetic recording medium according to claim 1, wherein a higher fatty acid having 12 to 18 carbon atoms or lecithin is used as the dispersion.