WO1997024714A1

WO1997024714A1 - Coating device for magnetic recording media

Info

Publication number: WO1997024714A1
Application number: PCT/KR1996/000206
Authority: WO
Inventors: Young Gak Choi
Original assignee: Young Gak Choi
Priority date: 1995-12-30
Filing date: 1996-11-19
Publication date: 1997-07-10
Also published as: KR970050277A; KR0164669B1

Abstract

This invention relates to a coating device for magnetic recording media, designed to coat two layers simultaneously wherein the front end is comprised of back edge, middle edge and front edge constructed in the order of substrate moving direction; the middle and front edge may be shaped suitable for smoothing the coating solution and towards the surface of the moving substrate, magnetic coating solution continuously extruded, is poured into the middle and front edge, respectively, through two slits between each edge; then, the solution is simultaneously coated in homogeneous thickness on the surface of a substrate.

Description

TITLE OF INVENTION

COATING DEVICE FOR MAGNETIC RECORDING MEDIA

TECHNICAL FIELD

This invention relates to a device for applying magnetic coating solution on the substrate and more particularly, to simultaneous dual layer coating device wherein the front end is comprised of back edge, middle edge and front edge constructed in the order of substrate moving direction; the middle and front edges is shaped suitable for smoothing the coating solution; and toward the surface of the moving substrate, magnetic coating solution extruded continuously through two slits between each edge is inserted into the middle and front edge, respectively, and simultaneously coated to the surface of a substrate in homogenous thickness.

BACKGROUND ART

The aforementioned coating device may be applied to a coated magnetic recorder, moreover to photograph film, X-ray film, print paper, adhesive tape, and to the multi-layer coating in the manufacture of various functional films using coating process.

In case of the conventional coating device to apply the coating solution to a substrate, several devices (e.g. , gravure roll coater, reverse roll coater, bar coater, knife coater, extrusion coater, slide coater and curtain coater) are recommended for use. Among these coating devices, the extrusion coating device has been most widely used since it was more advantageous than the conventional roll-type coaters, which had been widely used. However, a majority of them are coating devices forming a single coating layer. In case of forming the mutil-coating layers, a variety of coating methods have been suggested:

- Method of coating the substrate several times with same device;

- Method of coating the substrate in due seguence with a coater having several coating heads;

- Method of coating the substrate through a single coating head to form multilayer simultaneously.

As the coating device, which forms the plural coating layers simultaneously, there are multilayer-extrusion coater and slide coater. The slide coater is mainly used for coating the photograph film, but in case of high viscous soultion severely changeable in viscosity like the magnetic coating solution for magnetic recording media, it is inadeguate coating device.

As for the coating device of multi-layer extrusion type, mainly used as a magnetic recording media, it was mentioned in the Japanese Patent Laid Open No. 1988-88080 (U.S. Pat. No. 4854262) but it has recognized some disadvantages in that owing to the shape of back edge, the coating scope becomes narrower and give scratch in the prolonged time of coating.

DISCLOSURE OF THE INVENTION

This invention relates to a multi-layer (here, double layer) extrusion coating device designed to coat two kinds of magnetic coating solutions having different magnetic properties simultaneously, in order to manufacture a magnetic recording media representing remarkable signal properties by forming two magnetic layers to suit the freguency properities of each signal.

The inventor et al. have conducted a variety of simulations and tests so as to devise the double-layer extrusion coating device where the stabilized double

-layer, having more broad scope of coating and less coating defects, may be formed.

As a result, it is ascertained that the coating properties are determined by the dimensions and the shortest distances between each edge. Several tests shows that the coating device only, which may meet the above conditions, may form continuously two homogenous coating layers without such coating defects as scratch in the length direction, interlayer mixing, step in the transverse direction and bubble influx.

The simulation technigue is to select an optional coating system, in which material balance, momentum balance and momentum balance of substrate are simultaneously calculated by using finite element method so that pressure and current distribution at each portion are measured.

The simulations are performed by changing each dimension of the coating device in order to analyze the trends based on each parameter and its successive experimental verification may result in obtaining a proper coating device.

The object of this invention, therefore, is to provide a coating device for magnetic recording media, designed to coat two layers simutaneously wherein each of two solutions, continuously extruded through two slits at constant intervals, as formed among each edge of the front end consisting of back edge, middle edge and front edge in the moving direction of a substrate, is adhered in such a manner to coat the surface of the moving substrate in a predetermined moving route, characterized in that; the shape of each edge has a combination of flat surface without wedge or depressed part, protruded round-shape or a combination of protruded round surface and flat surface, the shortest distance (L.,) between vertical line passing through top point at back edge and extended line of slit at back edge is 0.05 to 0.5 mm, the shortest distance (L^) between vertical line passing through top point at middle edge and extended line of slit wall surface at middle edge is 0.1 to 3 mm, the shortest distance (H.,) between horizontal lines passing through each top point of back edge and middle edge is 0.1 to 0.5 mm, the shortest distance (H₂) between horizontal lines passing through each top point of middle edge and front edge is 0.1 times less than H, .

This invention is described in more detail by referring to the attached drawings.

As illustrated in Fig. 1 and 2, the coating device (10) of this invention is designed to coat two layers simultaneously wherein each of two solutions, continuously extruded through two slits(13, 13') at constant intervals, as formed among each edge of the front end(14) consisting of back edge(b), middle edge(m) and front(f) in the moving direction of a substrate(30) , is adhered in such a manner to coat the surface of the moving substrate.

In case of a coating device for magenetic recording media of this invention designed to coat two layers simultaneously, each edge of front end (14) has a combination shape of flat surfaces without wedge or depressed portions or protruded round-shape or a combination shape of protruded round surface and flat surface .

The structure of cavity (11,11') is designed in a manner that the discharge of coating solution at front end of coating device (10) has a homogeneity in the direction of width; its shape should be made in a manner that the current of coating solution is neither turbulent nor stagnated.

The shape and size of cavity depend on the fluidity of coating solution. It is preferable that the crescent-shaped cavity in this invention be used.

The supply position of coating solution should be determined as center, both ends or one end of cavity (11,11') in the direction of width. The method of supplying the coating solution is to coat the whole amount of supplied coating solution or to coat part of supplied paint, while circulating the remaining amount. The device (10) adopts a method of coating the whole solution supplied from cavity (11, 11') at center in the direction of width.

To make the intervals and length of slit portions (13,13') proper may give homogeneity to the discharge pressure of coating solution and its width direction. If the inverval of slit portions (13, 13') is more than 1 mm, it is difficult to form streamline so that the turbulent and partial eddy flow can be occurred. Hence, it is preferable that the intervals of slit portions (13,13') be less than 1 mm. The longer the slit portions (13,13') may be, the more the homogeneity the discharge pressure becomes to have, while the more incresing the pressure required for discharge becomes. Therefore, a proper level of the silt portions should be determined, preferably within about 100 mm.

Front end (14) consists of front edge (f) , middle edge (m) and back edge (b) . As illustrated in Fig. 3, the shape of front end is a combination of flat surfaces, or as illustrated in Fig. 4 in more detail, it is preferable that the front end has a protruded round shape or their combination. The material of front end is preferable to be strong such as cimented carbides which is more than Rockwellhardness 80; If the marterial, less than Rockwellhardness 80, is employed, coating defects may occur since the front end is easily damaged by scratch. Further, in the event that the surface processing of each edge at front end (14) adjacent to the coating solution is poor, coating defects may occur, particulary, in case the mean surface illumination intensity in center line of each edge surface is larger than 5μm.

In case of the detailed structure of front end (14) , the shape of each edge has a combination of flat surface without wedge or depressed part, protruded round-shape or a combination of protruded round surface and flat surface. The thickness of each edge should be from 1 to 10 mm. If the thickness of each edge deviates the above scope, there are some difficulties in processing or actual application. It is preferable that the shortest distance

(L.,) between vertical line passing through top point at back edge (b) and extended line of slit (13) at back edge (b) is 0.05 to 0.5mm and 0.5 times less than ₁. The shortest distance (I^) between vertical line passing through top point at middle edge (m) and extended line of slit wall surface at middle edge (m) should be 0.1 to 3 mm and it does not need to be considered whether W₂, the thinckness of middle edge(m) , and ₂ are same or not. Further, the most important point in this coating device (10) is that the shortest distance (H,,) between horizontal lines passing through each top point of back edge (b) and middle edge (m) should be 0.1 to 0.5 mm, while the shortest distance (H.,) between horizontal lines passing through each top point of middle edge (m) and front edge (f) is 0.1 times less than H₁; hence, the top point of middle age should be the highest.

Also, the top point of middle edge should be higher than those of back edge and front edge. When the top point of back edge is in the highest position, the moving substrate reached the top point of back edge, whereby coating is unavailable with scratch. In addition, if the top point of front edge is higher, the pressure distribution of coating solution at each edge, being formed in the moving direction, is inhomogeneous. Under such circumstances, there is a high possibility that bubbles or spirals may occur owing to cavitation (local vaporization) and the coating properties become unstable.

In case of meeting the aforementioned conditions, a lot of tests have confirmed that favorable coating results may be obtained without any coating defect.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a drawing of coating device for magnetic recording media according to this invention,

Fig. 2 is a drawing showing the structure of coating device of Fig. 1,

Fig. 3 is a drawing showing each edge shape of front end in a coating device according to one example of this invention,

Fig. 4 is a drawing showing each edge shape of front end in a coating device according to other example of this invention.

Fig. 5 is a drawing showing each edge shape of front end in a coating device as disclosed in the Japanese Patent Laid Open No. 1988-88080.

* Code description of main parts in the drawings

10: Coating device 20; Driving roll 30: Substrate 40: Guide roll

11, 11': Cavity 12, 12': Inlet of coating 13, 13': Slit portion 14: Front end b : back edge m : middle edge f : front edge

^W1 the thickness of back edge the thickness of middle edge w, the thickness of front edge slit interval between back edge and middle edge silt interval between middle edge and front edge

BEST MODE FOR CARRYING OUT THE INVENTION

The coating results are represented in the following Example using the coating device of this invention but the present invention is not confined to said scope.

[Example]

The magnetic paint of upper and lower layers are prepared by dispersion based on the following composition. < Composition of lower magnetic layer >

Mill Mix

partsperweight

Denatured cobalt magnetic substance 100

(Coercive force: 650 Oe, long axial field of magnetic substance 0.3 μm) Carbon black 10

Vinyl chloride-vinyl acetate-anhydrous maleate copolymer resin 9

Cyclohexanone 80

Methylethylketone 60 Toluene 50

Let Down

Polyester polyurethane resin 12 Normal butylstearate 1.5

Myristic acid 1.5

Cyclohexanone 40

Methylethylketone 60

Toluene 50 Polyisocyanate 3

< Composition of upper magnetic layer >

parts per weight Mill Mix

Denatured cobalt magnetic substance 100

(Coercive force: 700 Oe, long axial field of magnetic substance 0.2μm

DBP adsorptive amount 75g/magnetic substance lOOg) Carbon black 2 Alpha-alumina 4 Vinyl chloride-vinyl acetate-anhydrous maleate copolymer resin 11

Cyclohexanone 80 Methylethylketone 60

Toluene 60

Let Down

Polyester polyurethane resin 8 Normal butylstearate 0.5 Myristic acid 1.5

Cyclohexanone 40

Methylethylketone 60

Toluene 50 Polyisocyanate 5

Each magnetic paint of upper and lower layer in said two types was prepared in the following process: First, the mill mix paint consisting of magnetic substance, carbon black, or alumina etc. , in powder form and vinyl binders resin solution was dispersed by dissolver, agitator, mill dispersing machine and then, urethane resin solution, fatty acid and fatty acid ester lubricant and solvent were added to the 1st dispersed paint. The resulting mixture was secondly dispersed by agitator and mill dispersing machine, and polyisocyanate, a hardening agent, was added thereto and stirred prior to final filtration. The magnetic paint of each layer, so manufactured, was finally filtered and coated simultaneously to polyethyleneterephthalate film, substrate in 14.5 μm thickness, at coating head; the coating thickness of upper layer in nondrying state was 5 μm, while that of lower layer was 15 μm. When the paint's fluidity was measured at 25 °C by Haake visco eter RV-20 (Germany) , the following results were observed:

With coating solution of upper layer, when the shearing stress is 1000/sec, viscosity is 61.5 centipoise and in case of 5000/sec, viscosity is 28.1 centipoise. With coating solution of lower layer, when the shearing stress is 1000/sec, viscosity is 82.3 centipoise and in case of 5000/sec, viscosity is 38.5 centipoise.

The coating device was coated in a structure as shown in Fig.3 and coating conditions and the dimensions of each coating device are as follows:

Coating speed 200 m/min Coating width 150 mm

Coating thickness of upper layer 15μm in a non-drying state

Coating thickness of lower layer 5μm in a non-drying state Substrate polyethylene terephthalate film, thickness 14.5μm

Drying temperature 90°C (drying by heat gun)

Transit tension 1.5kg/150 mm

Slit intervals of upper layer d., 0.25 mm

Slit intervals of lower layer d₂ 0.25 mm

Edge's material Cemented carbide (Rockwellhardness 90) Surface illumination intensity of edge (Ra) 4μm Thickness of back edge W₁ 3 mm Thickness of middle edge W₂ 2 mm Thickness of front edge ₃ 1 mm Radius of cavity 8 mm (same value in the coating solution of upper and lower layers)

Table l.

Table 2 .

As shown in the above table 1 and 2, the application of double-layer extrusion coating device according to this invention may lead to obtainment of good coating surface without coating defects such as scratch, interlayer mixing, transverse step, stripe in the length direction, pin hole and overflow. In addition to that, the superior physical properties of final product may be obtained.

Claims

WHAT IS CLAIMED IS:

1. A coating device for magnetic recording media, designed to coat two layers simultaneously wherein each of two solutions, continuously extruded through two slits(13, 13') at constant intervals, as formed among each edge of the front end(14) consisting of back edge(b) , middle edge(m) and front(f) in the moving direction of a non¬ magnetic substrate(30) , is adhered in such a manner to coat the surface of the moving substrate in a predetermined moving route, characterized in that; the shape of each edge has a combination of flat surface without wedge or depressed part, protruded round- shape or a combination of protruded and flat surface, the shortest distance(L.,) between vertical line passing through top point of back edge and extended line of slit at back edge is 0.05 to 0.5 mm, the shortest distance (L₂) between vertical line passing through top point of middle edge and extended line of slit wall surface in back edge direction at middle edge is 0.1 to 3 mm, the shortest distance (H,,) between horizontal lines passing through each top point of back edge and middle edge is 0.1 to 0.5 mm, the shortest distance (H₂) between horizontal lines passing through each top point of middle edge and front edge is 0.1 times less than H₁.

2. A coating device for magnetic recording media according to claim 1 wherein the top point of middle edge is higher than that of the back edge and the front edge.

3. A coating device for magnetic recording media :ording to claim 1 wherein illumination intensity of the surface adjacent to coating solution within a coating device is less than 5μm as a mean surface illumination intensity value in the center line.