JPS61180933A - Production of magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain easily a highly dispersed paint by freeze-drying ferromagnetic fine powder as the pretreatment, and then dispersing the powder in the paint by a conventional dispersing method. CONSTITUTION:Magnetic powder is suspended in a liq. such as water or dioxane which is easily freezed and has a high vapor pressure in a freezed state. Then the suspension is freezed. The liq. component is succeedingly removed by a conventional freeze-drying method. The obtained magnetic powder is extremely voluminous. A magnetic paint is prepared by using the powder, and the paint is coated on a nonmagnetic carrier to form a magnetic layer. Oxides, metals or other kinds to be used for a magnetic recording medium can be used as the magnetic powder, the shape of the particle is not specified, and acicular, cubic, platy, spherical particles, etc. can be used.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a method for producing a magnetic recording medium, and in particular, to produce a magnetic paint with high dispersibility of ferromagnetic particles in order to obtain a coated high-density magnetic recording medium. It is characterized by its means.

Conventional technology coated magnetic recording media use ferromagnetic powder, organic binder,
It is manufactured by preparing a magnetic paint consisting of necessary additives and solvents and coating this paint on a non-magnetic base material. Of course, this process includes many steps, but the most important among them is the step of obtaining a highly dispersed paint.

The magnetic paint is not simply a mixture of the aforementioned paint constituents, but must be in a state in which ferromagnetic particles are dispersed without agglomeration. This is because important characteristics of the magnetic recording medium, such as amplitude versus frequency characteristics, modulation noise, and discontinuous noise, are strongly influenced by the degree of dispersion of the magnetic paint used. For these reasons, a large number of methods have been proposed and implemented to obtain highly dispersed magnetic paints. A commonly used method is to perform a dispersion operation with a component called a dispersant added as one of the constituent components of the paint.

Specific examples of dispersants include phosphoric acid ester substances. It is also possible to apply a surface treatment to the magnetic particles in advance to improve their dispersion and then turn them into a paint, Publication No. 56-
No. 130,838 discloses that colloidal silica particles are attached to the surface of magnetic powder to prevent agglomeration of magnetic particles.

Further, 48-104605 discloses a method for processing magnetic powder, in which the dispersibility can be improved by crushing the magnetic powder after heating and drying it to reduce oil absorption. The combined use of such surface modification treatment of magnetic powder and the addition of dispersants to paints has become popular. However, in recent years, the particle size of magnetic powder has become finer in order to meet the demand for even higher density magnetic recording, which makes dispersion difficult. There was a problem that it could not be distributed.

Problems to be Solved by the Invention There is a limit to how high shear stress can be applied to a paint to break up aggregates of fine powder, and excessive processing will lead to destruction of the magnetic particles. The present invention provides a method to solve these problems.

Means to Solve the Problems The present invention provides a novel method in which a highly dispersed coating material can be easily obtained by freeze-drying the ferromagnetic fine powder as a pretreatment and then converting it into a coating material using ordinary dispersion means. This is based on extensive knowledge. That is, in this method of manufacturing a magnetic recording medium, magnetic powder is first suspended in a suitable liquid, such as water or dioxane, which can be easily frozen and has a high vapor pressure in a frozen state. This suspension is then frozen. The liquid component is then removed by conventional freeze-drying. The magnetic powder thus obtained is extremely bulky. This is used as a magnetic coating material, and this coating material is applied onto a non-magnetic support to form a magnetic layer. Here, the magnetic powder may be an oxide, metal, or other type of powder used in magnetic recording media, and the particle shape is not particularly limited to acicular, cubic, plate-shaped, bispherical, etc. Further, the above-mentioned magnetic paint is composed of freeze-dried ferromagnetic powder, an organic binder, and normally necessary additives and solvents, and does not contain any new additive components unique to the present invention. Further, the dispersion equipment used in making the paint may be one that is normally used. That is, for example, a ball mill.

Attritors, three-roll mills, pebble mills, sand grinders, stone mills, mixers, kneaders, dispersers, impact mills, colloid mills, homogenizers, ultrasonic dispersers, and the like are used singly or in combination.

Effect: This is thought to be because when the liquid freezes, the volume of the liquid increases, and at this time, the aggregates of the ferromagnetic powder are broken down. −
Also, since dry powder is obtained without going through a liquid phase, the powder does not have a chance to coagulate, which seems to be effective.

EXAMPLE Next, a method for manufacturing a magnetic recording medium according to the present invention will be explained using a specific example.

(Example 1) Co-coated γ-F e2 with BET specific surface area of 38 d/g
03 powder 600.9 and pure water 11 are mixed in a ball mill to make a suspension. This suspension is injected into liquid nitrogen using a nozzle and frozen. The frozen product is treated with a freeze dryer to evaporate 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 11 Nitrocellulose
11 parts by weight Phosphate ester dispersant 3 Carbon black 4 Alumina powder
5 stearic acid 2 oleyl oleate 2 solvent 320 A composition having the above blending ratio was dispersed using a colloid mill and a sand mill in sequence, and 6 parts of polyisocyanate as a curing agent was added and mixed to obtain a magnetic paint.

This paint is filtered under pressure using a nominally 1 micron filter,
Apply immediately 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 sample was prepared in the same manner as in Example 1 except that dioxane was used instead of pure water.

(Comparative Example 1) A magnetic recording medium sample was prepared in the same manner as in Example 1 except that the magnetic powder was not subjected to freeze-drying using pure water.

The measured values obtained for the above three examples are listed below.

■Filtration rate of magnetic paint: Pressure filtration rate of 2.5 ~ with a nominal 1 micron filter ■Glossiness of magnetic layer: 4 of the magnetic layer before calendering
5 degree gloss.

■Magnetic layer surface roughness: Surface roughness expressed as Ra (center line average roughness) of the magnetic layer after calendering.

■Video characteristics: Measure the playback output at 5M) (z) using a VTR.The comparative sample is set to odB.

As can be seen from the above description, the magnetic recording medium produced by the method according to the present invention has improved surface properties in terms of both the glossiness of the magnetic layer before calendering and the surface roughness of the magnetic layer after calendering. This is clearly reflected in the video characteristics. Considering this together with the value of filtration rate, it can be said that the dispersibility of the magnetic powder has been improved, and this method has an unprecedented feature.

Claims (1)

[Claims] A suspension consisting of a ferromagnetic powder and a liquid medium is frozen, and the medium components are evaporated off while in the frozen state.The obtained ferromagnetic powder is used to make a magnetic paint, and the paint is placed on a non-magnetic support. A method for manufacturing a magnetic recording medium, which comprises forming a magnetic layer by coating.