JPS61182631A - Production of magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve the dispersibility of magnetic powder by freezing quickly a suspension consisting of three components; ferromagnetic powder, dispersant and liquid medium, evaporating away the medium component in the frozen state and using the ferromagnetic powder obtd. in such a manner as a magnetic coating paint. CONSTITUTION:The suspension consisting of the three components; the ferromagnetic powder, the dispersant which assists the dispersibility of the ferromagnetic powder by adsorbing chemically on the surface of said powder and the liquid medium, for example, water or the medium, for example, dioxane which can be easily frozen and has the high vapor pressure in the frozen state is prepd. After such suspension is quickly frozen, the medium component is removed by a freeze drying. The magnetic paint is prepd. by using the resulted magnetic powder. Such paint is coated on a non-magnetic base to form a magnetic layer. The dispersibility of the magnetic powder is thus improved and the improvement in the surface characteristic of the magnetic recording medium is made possible.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method of manufacturing a magnetic recording medium.
In particular, the present invention is characterized by a means for producing a magnetic coating material with high dispersibility of ferromagnetic particles in order to obtain a so-called coated type high-density magnetic recording medium.

Conventional technology As is well known, coating-type magnetic recording media involves preparing a magnetic coating consisting of ferromagnetic powder, an organic binder, necessary additives, and a solvent, and applying this coating onto a non-magnetic base material. Manufactured. The additives needed here include surfactants, which are important when making paints, and materials added to improve the properties of recording media, such as alumina powder to strengthen the magnetic layer, and materials to impart conductivity. carbon powder,
Refers to organic and inorganic substances such as lubricants that provide lubricity. The method for manufacturing this magnetic recording medium is of course comprised of many steps, but the most important of these is the step of obtaining a highly dispersible magnetic paint. The magnetic paint is not simply a mixture of the above-mentioned paint components, but it is necessary that the ferromagnetic particles are not agglomerated and preferably each particle is dispersed separately. This is because it is known that important characteristics of a magnetic recording medium, such as amplitude versus frequency characteristics, modulation noise, and discontinuous noise, strongly depend on the degree of dispersion of the magnetic gold powder used. For these reasons, many methods have been proposed and implemented to obtain highly dispersible magnetic paints. A commonly used method is to add a component called a dispersant as one of the constituent components of the paint to carry out the dispersion operation. Dispersants used include substances such as phosphoric acid ester substances, higher fatty acids and their derivatives, which chemically adsorb to the surface of the ferromagnetic powder and change the properties of the surface. It is also possible to surface-treat the magnetic powder in advance and then use it as a paint; for example, the method disclosed in Japanese Patent Application Laid-Open No. 130838/1983, in which colloidal silica particles are attached to the surface of the magnetic powder to prevent agglomeration of the magnetic particles. is one example. Further, JP-A-48-104505 discloses a method for processing magnetic powder that improves dispersibility by reducing oil absorption by crushing the magnetic powder after heating and drying it. Naturally, both the surface modification treatment of the magnetic powder and the addition of a dispersant when forming the powder into a paint are used in combination.

However, in recent years, in order to meet the demand for even higher density magnetic recording, the particle size of magnetic powder has become increasingly finer, which makes dispersion difficult, so conventional methods such as those mentioned above have become more and more difficult. However, there was a problem in that the dispersion could not be sufficiently achieved using this method.

Problems to be Solved by the Invention As described above, the preparation of magnetic paint using magnetic fine particles with a particle size of a fraction of a micrometer requires high shear stress to be applied to the paint, while employing dispersants and measures to prevent agglomeration. is added and kneaded to break up the fine particle aggregates and create a highly dispersed state.
However, it is still insufficient. Excessive cross-wire processing leads to destruction of magnetic particles. The present invention provides a method to solve this problem.

Means for Solving the Problems The present invention provides a highly efficient solution by pre-treating ferromagnetic powder by freeze-drying during the powder surface treatment, and then converting it into a paint using conventional dispersion means. This is based on the knowledge that dispersed paints can be easily obtained. That is, in this method of manufacturing a magnetic recording medium, first, a ferromagnetic powder, a dispersant that is chemically adsorbed onto the surface of the powder and has the effect of aiding its dispersibility, and a suitable liquid medium such as water or dioxane that freezes easily. A suspension consisting of 3 components is made, which has a high vapor pressure in the frozen state. This suspension is then flash frozen. Thereafter, the medium components are removed by a conventional freeze-drying method.

The magnetic powder thus obtained is extremely bulky. This powder is used as a magnetic paint, and this paint is applied onto a non-magnetic support to form a magnetic layer. The magnetic powder herein may be an oxide, metal, or other type of powder used in magnetic recording media, and its shape and particle size are not particularly limited. Furthermore, the above-mentioned magnetic paint is composed of surface-treated and freeze-dried ferromagnetic powder, an organic binder, and usually necessary inorganic and organic additives and solvents, which are additive components unique to the present invention. does not contain anything. Further, the term "dispersant" refers to a compound known in this field that adsorbs to the surface of magnetic powder and aids in its dispersion, and does not refer to a new compound uniquely required for the present invention.

Furthermore, the dispersion equipment used for making the paint may be any commonly used dispersion equipment, such as a ball mill,
Attritor, roll mill, pebble mill/sand grinder, stone mill, mixer.

Negoo, Disper, Impact Mill, Colloid Mill.

A homogenizer, an ultrasonic disperser, etc. can be used alone or in combination as appropriate.

The first point of action appears to be that when the suspension freezes, the volume of the ferromagnetic powder increases, and at this time the ferromagnetic powder aggregates collapse. Furthermore, the second point of action appears to be that the powder surface-treated in the liquid phase is dried directly from the frozen state, so that the powder is not given a chance to reagglomerate.

Example Next, the present invention will be explained with reference to examples.

Example 1 BICT surface area 40(v) Co-coated γ-Fe2O3 powder 5oot tSodium stearate 16f.

Mix 1 cup of pure water in a ball mill to make a suspension.

This suspension is injected into liquid nitrogen through a nozzle using compressed air and frozen. The frozen product is treated with a freeze dryer to evaporate the frozen water and obtain dry magnetic powder. A magnetic paint with the following composition is made using this magnetic powder.

Magnetic powder 100 parts by weight Polyurethane 1j 1 Nitrocellulose
11 1 carbon black
4I alumina fine powder 5I stearic acid 2I oleyl oleate
2 Solvent 320
N A composition having the above-mentioned mixing ratio was dispersed using a colloid mill and a sand mill sequentially, and 6 parts of polyisocyanate as a curing agent was added and mixed to obtain a magnetic paint. The paint is pressure filtered through a nominal 1 micron filter and immediately applied onto polyethylene terephthalate film.

This is dried and hardened to obtain a magnetic recording medium sample. A magnetic paint having the above-mentioned compounding ratio is one of the popular compositions in this field.

Example 2 A magnetic recording medium was prepared in the same manner as in Example 1 except that the same amount of lecithin was used instead of sodium stearate and the same amount of dioxane was used instead of pure water.

Comparative Example 1 A comparative sample was prepared in the same manner as in Example 2 except for the following two changes.

(a) A suspension consisting of magnetic powder, lecithin as a dispersant and a medium is not created, and therefore no freeze-drying treatment is performed.

(b) When making a paint, 3 parts by weight of lecithin is added.

The measured values obtained for the above three samples are listed below. ■ Filtration rate of magnetic paint: Pressure filtration rate at 2.5 Kf/cII using a nominally 1 micron filter plate.

■ Magnetic layer glossiness: 45 degree glossiness of the magnetic layer before calendering 0 ■ Magnetic layer surface roughness: R of the magnetic layer surface after calendering
Surface roughness expressed as a (center line average roughness).

■ Video characteristics: Playback output value at 5MHz using a VTR. Let the comparative sample be odB.

Effects of the Invention As can be seen from the above Examples and Comparative Examples, the magnetic recording medium produced by the method of the present invention has excellent surface properties in terms of both the glossiness of the magnetic layer before calendering and the surface roughness of the magnetic layer after calendering. The effect of this improvement is clearly visible in the video characteristics. When considered in conjunction with the value of paint filtration rate, this can be said to mean that the dispersibility of the magnetic powder is improved, and this method has an excellent feature.

Claims (1)

[Claims] A suspension consisting of three components: a ferromagnetic powder, a dispersant that chemically adsorbs to the surface of the ferromagnetic powder to help its dispersibility, and a liquid medium is rapidly frozen, and the medium components are removed by evaporation while in the frozen state. 1. A method for manufacturing a magnetic recording medium, which comprises using the obtained ferromagnetic powder as a magnetic coating material, and coating the coating material on a non-magnetic support to form a magnetic layer.