JPH06124443A - Coating of magnetic paint - Google Patents

Abstract

PURPOSE:To obtain a uniform magnetic layer having excellent surface characteristic by conducting the coating while a variable magnetic field is applied to a magnetic paint without circulating the magnetic paint stored in the liquid tank through an external channel. CONSTITUTION:After a magnetic paint is dispersed by a dispersing device, it is sent to a liquid tank 12 from the rear side of a coater 11 passing a transporting pipe 14. In this case, a copper coil 15 covered with stainless is soaked within the liquid tank 12. A variable current is applied to the copper coil 15 from an external power supply through a lead wire 16. Thereby, the magnetic paint is maintained in the dispersed condition and flows into a slit 13 of the coater 11 passing the clearance of coil 15. The magnetic paint therefore is supplied from the end point of the slit, coating a base film. That is, a resin hardening agent is not added at a time to the magnetic paint stored in the stock case, but is it added a little by little from the intermediate portion of the transporting pipe depending on a flow rate of the magnetic paint to be sent to the coater 11. Thereby, the paint is again dispersed immediately before it is used for coating the base film and thereby a uniform magnetic layer having excellent surface characteristic can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coating method for uniformly coating a base film with a magnetic coating used in the production of magnetic recording media such as floppy disks and magnetic tapes.

[0002]

2. Description of the Related Art In recent years, a large amount of magnetic recording media such as floppy disks and magnetic tapes have been used for recording information in computers and word processors. The density of these information recording media has been increased, and defects on the surface of the magnetic layer have become a serious problem. The magnetic layer is formed by applying a magnetic coating that has been subjected to a dispersion treatment to the base film and then removing the organic solvent, but defects on the surface of the magnetic layer are observed when the dispersed state of the magnetic coating is insufficient. Most of them are due to aggregates of magnetic powder in the magnetic paint.

Usually, in order to obtain a good dispersion state of the magnetic paint, two rolls, a sand grinder, a high speed impeller,
Mechanical means such as high-speed impact mill, homogenizer, attritor, pebble mill, etc., and chemical means using various dispersants are used. Applied to the base film because the magnetic powder re-aggregates during transportation of the product, during storage in the stock container, during transportation from the stock container to the die coater (hereinafter referred to as the coater), or during the stay inside the coater. It is often seen that a defect is generated on the surface of the magnetic layer thus formed, which becomes a defect of an electric signal in recording and reproduction.

Therefore, in order to prevent such re-aggregation, a coating method using a paint circulation system as shown in FIG. 3 is generally used. In other words, the magnetic paint is circulated while passing through a coater, and shear stress is applied to the magnetic paint in a pipe used for this circulation to prevent reaggregation of magnetic powder and a part of the paint is used for coating. Is.

According to this method, the magnetic coating material that has undergone the dispersing step is dispersed by the dispersing machine once again, then passes through the transport pipe 4 and enters the liquid reservoir 2 from one side surface of the coater 1, and a part thereof enters the slit 3. Most of the paint is drained from the other side of the coater and then returned to the disperser, although it is poured from the tip and applied onto the base film. In this coating method, the liquid reservoir installed in front of the slit is always present in an ordinary coater, and is provided in order to make the paint extruded from the gap uniform at each location and to prevent pulsation of the paint. , Therefore, it requires a large volume. Therefore, in order to circulate the magnetic paint at a certain speed or more in this liquid reservoir as well, the amount of the magnetic paint required for circulation is inevitably large, usually 10 times the amount of the applied liquid. It is a degree.

[0006]

However, there are some problems in this coating method by the circulation method. In this circulation method, the flow rate of the magnetic coating material in the die coater does not control not only the film thickness of the magnetic layer formed simply by drying after coating but also the shear stress to the magnetic coating material, that is, the dispersion state of the magnetic coating material. must not. Therefore, there is a problem that it is difficult to control the flow rate of the magnetic paint.
Further, in this method, there is a problem in that the passage of the paint becomes long and thus a pipe loss occurs, resulting in a poor yield of the paint itself.

Further, in this circulation system, since it is difficult to continuously add the resin curing agent, a method is used in which a predetermined amount of the curing agent is added all at once to a container stocking the magnetic coating material. There is. As a result, there is also a problem that the magnetic coating material partially aggregates with time due to the influence of the resin curing agent added to the coating material, and the quality of the coating film deteriorates with the time of the coating operation. This effect agglomeration of the magnetic coating by the resin curing agent to some extent be avoided by increasing the circulation rate, the liquid reservoir in the coater reduces the flow rate inevitably magnetic coating composition for having a large volume, in this partial magnetic coating It is difficult to avoid agglomeration of. If the circulation speed is too high, the paint solvent itself may be charged with static electricity, which may cause an explosion.

An object of the present invention is to solve the above problems and to provide an improved method for applying a magnetic paint so that a better dispersion state of the magnetic paint can be obtained.

[0009]

As a result of studying such a coating method of a magnetic paint, the present inventors have found that among the coating methods using a coater, the magnetic paint in the liquid reservoir is circulated through an external flow path. The present inventors have found that a uniform magnetic layer having excellent surface properties can be formed by using a method of applying the magnetic paint without changing the magnetic paint while applying a varying magnetic field to the magnetic paint.

That is, the gist of the present invention is to provide a method for applying a magnetic coating material using a die coater in which a liquid reservoir is formed inside and a coating slit is formed at a tip end extending from the liquid reservoir. A magnetic paint coating method is characterized in that coating is performed without circulating the paint through an external channel and while applying a fluctuating magnetic field to the magnetic paint.

The present invention will be described in detail below. The magnetic coating material used for coating in the present invention contains magnetic powder, a binder resin, an organic solvent, and may further contain an abrasive, a dispersant, a lubricant, a conductivity-imparting agent, and the like. The components constituting these magnetic paints are not particularly limited as long as they are used in the production of ordinary magnetic recording media.

The fluctuating magnetic field applied in the present invention is
It means a magnetic field whose direction and / or strength changes with time, and examples thereof include a stationary alternating magnetic field and an alternating magnetic field whose strength periodically decays to substantially zero. Due to these fluctuating magnetic fields, a good dispersion state of the magnetic paint can be achieved. For example, in the case of a needle-shaped magnetic powder that is preferably used in the present invention, the magnetic field of each magnetic powder is momentarily aligned in a constant direction by a fluctuating magnetic field, and then the magnetic powders are repulsed by the magnetic powders. By being separated,
A high degree of dispersion is achieved. The conditions such as the frequency and strength of these fluctuating magnetic fields are appropriately selected and optimized according to the properties of the magnetic paint and the shape of the magnetic powder, but generally the maximum magnetic field strength is the average of the magnetic powder in the magnetic paint. 1-3 of coercive force
The standard is about double, and there is a tendency that finer magnetic powders require higher frequencies.

As an example of a method of applying a fluctuating magnetic field applied in the present invention, power is supplied to these coils by using a method of installing coils outside the coater or directly installing coils inside the liquid reservoir. It is possible to apply a variable current from In this case, as the shape of these coils, in addition to a simple solenoid, a short solenoid coil wound around a rod-shaped winding core in an opposite direction may be used. The latter has the advantage that a large magnetic field is generated outside the coil, although the direction of the magnetic field is not constant. In addition, in order to prevent heating of the coils, the conductors forming these coils may be hollow inside to allow a coolant such as water to flow therein.

1 and 2 are schematic views showing an example of a coater for carrying out the method of the present invention. After the magnetic paint is dispersed by the disperser, the magnetic paint passes through the transportation pipe 14 and enters the liquid reservoir 12 from the back surface of the coater 11. Usually this transportation process,
In addition, although magnetic powder reaggregates in the liquid reservoir, a copper coil 15 coated with stainless steel is immersed in the liquid reservoir, and a variable current is supplied from an external power source through a conductor 16, The magnetic coating material receives a fluctuating magnetic field and becomes in a good dispersion state, then flows into the slit 13 of the coater through the gap of the coil, and is applied to the base film from the tip thereof.

In the present invention, since the magnetic coating material existing in the liquid reservoir is not circulated through the external flow path, it is possible to continuously add the resin curing agent to the magnetic coating material, which was difficult in the conventional circulation system. Become. That is, in the method of the present invention,
Instead of adding the resin curing agent to the magnetic paint in the stock container all at once, depending on the flow rate of the magnetic paint sent to the coater,
A method of adding a resin curing agent little by little from the middle of the transportation pipe can be adopted. As the resin curing agent used at this time, any resin curing agent usually used in the production of magnetic recording media can be used.

[0016]

In the coating method according to the present invention, since a varying magnetic field is applied to the magnetic paint in the liquid reservoir existing immediately before the slit of the coater, reaggregation is caused while the magnetic paint is transported from the disperser to the coater. Even if it occurs, since it is dispersed again immediately before being applied to the base film, the magnetic layer formed by drying after application is uniform and the surface property becomes extremely good. Further, since the resin curing agent is continuously added to the magnetic coating material without circulating the magnetic coating material in the liquid reservoir, deterioration of the quality of the magnetic coating material due to the influence of the resin curing agent over time is reduced.

[0017]

EXAMPLES Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist. Examples 1 and 2 Magnetic paints were prepared according to the compositions shown below.

[0018]

Table 1 Parts by weight Fe-Ni alloy (Hc = 1500 Oe, SSA = 45 m ² / g) (hereinafter abbreviated as PA) or Co-γ-Fe ₂ O ₃ (Hc = 700 Oe, SSA = 20 m ² / g) (Hereinafter, abbreviated as PB) 100 Polyurethane (manufactured by Nippon Polyurethane; N2304, MDI polyester polyurethane) 18 Vinyl chloride / vinyl acetate / vinyl alcohol copolymer (UCC; VAGH) 18 α-alumina (particle diameter: 0. 5 μm) 3 Carbon black (manufactured by Mitsubishi Kasei; # 3250B) 8 Lecithin 5 Butyl stearate 5 Methyl ethyl ketone 170 Cyclohexanone 170

After passing through the dispersing step, the above-mentioned magnetic paint was put into a stock container, sent to a coater shown in FIGS. 1 and 2 through a transport pipe, and applied on a polyethylene terephthalate base film. At this time, immediately after the magnetic paint was sent from the stock container to the transportation pipe, a resin curing agent [polyisocyanate (manufactured by Nippon Polyurethane; Coronate L, trimethylolpropane TDI adduct)]
(10 parts by weight of polyisocyanate, 25 parts by weight of methyl ethyl ketone, 25 parts by weight of cyclohexanone)
12 parts by weight of a resin curing agent solution was continuously added to 100 parts by weight of the magnetic paint. A constant alternating current of 50 Hz was applied to the coils of FIGS. 1 and 2 while flowing cooling water inside, and a varying magnetic field was applied to the magnetic paint in the liquid reservoir. The maximum strength of the applied fluctuating magnetic field is about 1800 Oe. The magnetic layer formed on the base film after drying the magnetic paint is
Gloss was measured using a gloss meter manufactured by Suga Test Instruments Co., Ltd. without calendering (incident angle 60
°). The results are shown in Table-1.

Example 3 The same procedure as in Example 1 was repeated except that the solution of the resin curing agent was added to the magnetic coating material used in Example 1 in a stock container just before the start of the coating operation. Then, the magnetic paint was applied and the gloss of the formed magnetic layer was measured. The results are shown in Table-1.

Comparative Examples 1 and 2 Gloss was measured in the same manner as in Examples 1 and 2 except that the varying magnetic field was not applied. The results are shown in Table-1.

Comparative Examples 3 to 4 To the magnetic coating materials used in Examples 1 and 2, a solution of a resin curing agent was added all at once in advance just before the start of the coating operation, and the conventional circulation system shown in FIG. 3 was used. Coating was performed on a polyethylene terephthalate base film using a coater to form a magnetic layer. The gloss is measured in Examples 1 and 2
Is the same as. The results are shown in Table-1.

[0023]

[Table 2]

According to Table 1, when the fluctuating magnetic field was applied to the magnetic paint in the liquid reservoir (Examples 1 to 3), the magnetic layer was compared with the case where the fluctuating magnetic field was not applied (Comparative Examples 1 to 4). It can be seen that the surface has excellent gloss (%) and a uniform magnetic layer is formed. Further, in these cases, even though the magnetic coating material was not circulated, even if it was left for a long time (5 hours) after the dispersion process was completed, the surface properties of the magnetic layer were not affected (Examples 1 and 2). I understand. Further, when the resin curing agent is added to the magnetic coating material all at once, the change in gloss with time occurs after the start of coating (Example 3, Comparative Example 3).
It can be seen that the gloss change with time is extremely small when continuously added (Example 1).

[0025]

According to the coating method of the present invention, since the magnetic coating material is in a good dispersed state immediately before being coated on the base film, the magnetic layer formed by drying after coating has a uniform surface property. Will be very good. Further, even if the magnetic paint re-aggregates while being transported from the disperser to the coater, there is no major problem in the manufacturing process, and it is necessary to control the transport speed of the magnetic paint from the viewpoint of maintaining a good dispersed state. Disappear. Therefore, the control of the flow rate of the coating material may be simply considered for the control of the film thickness of the magnetic layer formed after coating, and the process control becomes extremely simple. Furthermore, by not circulating the magnetic paint, the piping loss is reduced and the yield of the paint is increased. The change with time is also eliminated, and the variation in quality as a magnetic recording medium is reduced. Like this
A magnetic recording medium coated by a coating method capable of preventing reaggregation of magnetic powder has reduced noise and an extremely low error rate, and thus can be used as a medium for high recording density.

[Brief description of drawings]

FIG. 1 is a schematic view showing an example of a coater for carrying out the method of the present invention.

FIG. 2 is a sectional view taken along line II-II of the coater shown in FIG.

FIG. 3 is a schematic view of a coater using a conventional circulation system.

[Explanation of symbols]

 1,11 Coater 2,12 Liquid reservoir 3,13 Slit 4,14 Transport pipe 15 Coil 16 Conductor

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masahiro Yamamoto Inventor Masahiro Yamashita Kasei Co., Ltd.

Claims (2)

[Claims] 1. A method of applying a magnetic paint using a die coater having a liquid reservoir formed inside and a coating slit formed at a tip extending from the liquid reservoir, wherein the magnetic paint in the liquid reservoir is passed through an external flow path. A method for applying a magnetic paint, characterized in that the magnetic paint is applied without being circulated through the magnetic paint while applying a fluctuating magnetic field to the magnetic paint. 2. The resin curing agent is continuously added to the magnetic coating material in the supply path to the die coater.
The method for applying the magnetic paint according to.