COATING APPARATUS

Abstract: Disclosed is a coating head for coating a solution on a flexible support which is moved so as to run along a coating surface of the coating head. The coating surface is slitted so as to form a front edge surface (2), a back edge surface (2) in order in the moving direction of the flexible support, and a slit (3) therebetween so that the coating head extrudes the solution through the slit (3) to the flexible support. The front edge surface has an arc surface with a curvature radius of r₁ and the back edge surface (2) wholly forms an arc surface with a curvature radius of r₂ in the following equations 3mm < r₂ < 20mm and 1mm < r₁ < r₂ / 2. Further, at least a part of the back edge surface (2) projects beyond the tangent line at the downstream end of the front edge surface.

Description: This invention relates to an extrusion-type coating apparatus, especially to a coating apparatus that allows trouble-free coating in manufacturing magnetic recording media. A variety of coating processes are available; these include roll coating, gravure coating, extrusion coating, slide-bead coating, and curtain coating. Magnetic recording media are available by coating magnetic coat solutions commonly by means of roll coating, gravure coating, and extrusion coating. Among these, extrusion coating is preferable since it provides uniform coat-film thicknesses. The magnetic recording medium itself, however, has improved rapidly in recent years; its material consists of an oxidized magnetic powder which has a high BET value, and barium ferrite. High viscosity coating solutions are also being increasingly used. For the purpose of higher productivity, high-speed coating is in growing demand. Conventional techniques in the extrusion coating process intended for manufacture of the magnetic recording medium is described in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 84771/1982, No. 104666/1983, and No. 238179/1985. In fact, said extrusion coating process provides uniformcoat film thicknesses, but only under good coating conditions limited to a narrow range; said extrusion coating process, however, often fails to attain desired coating under the high viscosity, high-speed coating conditions described above. Although a coating head with a specially designed shape enables high-speed coating at a rate of 300 m/min or more with a thick film thickness, said extrusion coating process cannot provide a marketable magnetic recording medium because of uneven thickness greatly out of tolerance, which is inevitable under said high-speed conditions. Under these types of coating conditions, thin film (30 µm or less before drying) coating in particular, said extrusion coating process causes failures which lead to major problems, for example, a coating film is partially peeled off by foreign matter, dust, coagulating particles, etc. which are deposited on the base material and transferred to or caught by the backedge face; the film thickness is partially thickened; and the base material is shaved by the front edge, especially the corner at the downstream end of the front edge, and the resulting base waste is deposited. In particular, a highviscosity coating is likely to cause a crosswise streak failure as a result of the stick-skip motion of the base material; it also causes noise and fluctuations of output. Various countermeasures are taken for said failures. A typical example is the technique disclosed in Japanese Patent O.P.I. Publication No. 238179/1985 (hereinafter referred to as prior technique). Said prior technique is derived from the technique described in Japanese Patent O.P.I Publication No. 104666/1983. In this technique the back-edge face 2' has a triangular cross section, as shown in a hypothetical line in Fig. 9. This is to prevent foreign matter from overriding the triangular cross section, and the foreign matter thus blocked is likely to deposit in the solution sump P. thereby causing a streak failure. In the Prior technique, not only the back-edge face 2' is smoothed as shown in a solid line in Fig. 9, but ϑ₁ and ϑ₂ are so designed as to satisfy the conditions of formula (1). ϑ₁ < ϑ₂ < 180° (1) In fact, the prior technique effectively reduces the incidence of failures in that no foreign matter is left in the solution sump P, but it is still subject to the failure caused by waste from the base material whose surface is shaved by the downstream end B of the front edge face 1′; this is where contact pressure between the base material and said downstream end B is concentrated because the running angle of the base material changes abruptly. On the other hand, in thick-film coating, the base waste failure is reduced to a certain extent because a contact pressure of the base material to the downstream end B is partly distributed to the back-edge face, as compared with thin-film coating. The coating film thickness is solely dependent on the balance between the pressure of the coating solution on the back-edge face, and the pressing force of the base material on the back edge face during running. This balance is so fine that the slightest unbalance results in stepped unevenness which means thicknesses differ along the running direction of the base material, or streaks along said running direction and with different thicknesses in the crosswise direction. The higher the coating speed, and the thicker the film thickness and the greater is the incidence of coating failure. The first aim of the invention is to provide a coating apparatus that allows excellent coating under conditions of a thick film thickness, and high-speed coating without causing said stepped unevenness and streaks. Fig. 9 shows said prior technique. Air is included between the front-edge face 1′ and the surface of the base material while the base material is running, and is blocked at the downstream end B of front-edge face 1′ whereby air is not allowed to enter the coating solution. However, this design causes base waste to deposit. The present inventors have found that a contact force of the base material is effectively distributed to the back-edge face by projecting at least a part of the back-edge face from the tangent line ℓ₁ at the downstream end B of the front-edge face. As the back edge face is projected, however, a contact force between the downstream end B and the base material is weakened accordingly. In particular, when the base material runs at a rather high rate, more boundary air on the base material surface flows over the downstream end B so that said air is not securely blocked, with the result that the pinhole failures on the sheet are likely to occur on the sheet after coating. The base material applies pressure to the front-edge face and back-edge face; the components of said force are then balanced with each other, as stated above. Said force and their balance or distribution of said components have so delicate an influence on coating properties that a coating head with an optimum shape is involved. However, there is a limitation on shape design for improving coating properties. For example, if coater heads are changed each time a base material with a different specification flows, productivity is reduced. In addition, it is extremely difficult to eliminate uneven thicknesses of the base material in the cross-wise direction, because a different tension acts the base material in that direction. The second aim of the invention is to provide a coating apparatus that successfully prevents coating failures such as uneven film thicknesses and allows coating conditions to be changed easily. The coater head is generally made of stainless steel. However, when a new roll of the base material is charged or any trouble occurs during coating, and the base material 1 is separated from the coater head CH as shown in Fig. 6, the coating solution A drips down along the surface of front edge 3 from slit 2. For this reason it is common practice to clean front edge 3 with a solvent before the coating apparatus is restarted. However, insufficient cleaning or an especially extended shutdown time causes the coating solution to solidify or for dry waste to remain on front edge 3, and said waste is coated on the base material after coating is restarted, causing a coating failure. The coating failure is diminished within several minutes, during which the highspeed coating produces a failure length of several hundreds meters from the restart position, causing reduction in productivity (yield). The third aim of the invention is to provide a coating apparatus that successfully prevents coating failures caused by waste of the coating solution which remains on the edge face of the coater head, and that ensures an improved yield. In addition to the extrusion-type coating apparatus as described above, the present invention discloses another method of manufacturing magnetic recording media while adding a curing agent in-line. Japanese Patent O.P.I Publication No. 10773/1983 discloses a method for in-line addition of a curing agent, when performing roll coating with a magnetic coating solution, for creating higher stability of the magnetic coating solution and improving characteristic such as squareness ratio. In fact, said method of in-line addition of a curing agent is a excellent method of improving stability of a magnetic coating solution. More particularly, if a curing agent is added before the magnetic coating solution is conditioned, the curing agent reacts with the binding agent in the solution tank, solution viscosity rises with time, and coating properties depart from the tolerance range, with the result that no magnetic recording medium is manufactured to the required quality level unless the magnetic coating solution is finely re-conditioned each time when required. In fact, the in-line addition method is thoroughly free from such trouble, thereby improving the stability of the coating solution. However, it should be noted that the in-line addition method has been employed mainly by the coating apparatus that applies a coating solution kept in a vat such as for gravure coating, as described in Japanese Patent O.P.I. Publication No. 10773/1983. More particularly, the coating solution in the coating solution supply line is not totally applied on the base material but only a part of said solution is applied, with the residue left in the vat. A residual solution in the vat causes the reaction to progress in the vat, the viscosity of the coating solution to rise with time, and the coating solution partially become highly viscous, with the result that the stability of the coating solution is insufficiently maintained. In particular, when a binding agent has a polar group, the performance of the tape is greatly deteriorated because of the high thixotropy of the coating solution. The fourth aim of the invention is to provide a method of manufacturing magnetic recording media with excellent characteristics by improving the stability of the coating solution. The magnetic tape, which is a magnetic recording medium, has a back-coating layer formed on the surface of a base support material which is opposite to the magnetic layer surface, whether through an uncoated layer or not. This is done to improve the running stability and to reduce the friction resistance between tapes. After the magnetic layer is coated, said back coating layer is commonly coated by means of gravure coating, reverse coating, or kiss-roll coating. However, when the back-coating layer is formed by conventional means after the magnetic layer is coated, the magnetic tape drops out because the magnetic layer is nipped by the rubber roll while the back coating layer is coated, dirt deposited on said roll or contained in the coating solution is pressed to the magnetic layer to produce blemishes or scratches. In recent years, magnetic recording media with lower dropouts are in growing demand, and require an improved coating method. The fifth aim of the invention is to provide a method of manufacturing magnetic recording media by forming a backcoating layer in a series of coating processes while reducing dropouts. The primary aim of the invention is to provide a coating apparatus that exhibits excellent coating properties under conditions of thicker film and high-speed coating, without causing failures such as stepped unevenness and streaks; this is successfully attained with the configuration described below. The invention provides an apparatus according to Claim 1. The reason said configuration of the present invention has excellent coating properties even in thick-film and highspeed coating is not clear, but it may be attributed to the following points: (1) The back-edge projected from the tangent line at the end of the front edge prevents the base material from coming into contact with the end of said front edge and protects said base material from being shaved, thus preventing occurrence of base waste. Therefore, even if the base material is forcibly pressed to the coating head, a waste failure may not result. It follows that the pressure of extruding the coating solution is well balanced in high level with the pressing force of the base material onto said back edge whereby sensitivity to factors of changing film thickness may be sluggish and stepped unevenness or streaks are successfully prevented even under conditions of thick film and high-speed coating. On the other hand, even if stick-slip (wavy) motion is added, the base material keeps in contact with any point at the end of said front edge. This means that no waving occurs, eliminating stepped unevenness. (2) In addition, the curvature radius r₁ exceeding 1 mm prevents the base material from having contact with said front edge at an acute angle and prevents base waste from occurring, while said radius less than r₂/2 prevents boundary air from intruding through a clearance between said front edge and base material. On the other hand, a curvature radius r₂ exceeding 3 mm inhibits the coating solution from an abrupt change in flow rate between the slit outlet and the end of said back edge, while the same radius less than 20 mm allows a shearing force to act on the coating solution until the solution reaches the end of said back edge when performing high speed coating, thus assuring coating to a uniform film thickness. The second aim of the invention is to provide the coating apparatus that allows easily changing coating conditions while preventing coating failures such as uneven thicknesses in the crosswise direction. This is successfully attained with the configuration described below. In a coating apparatus comprising the coater head which coats the coating solution on the surface of the flexible base material, which continuously runs along the front edge surface and back edge surface of the coater head, by continuously extruding said coating solution from a slit between both edges surface, and a pair of support rolls arranged on the upstream and downstream sides of said coater head in order to force said base material onto said coater head, a fluid pressing means is provided between said support rolls. This means applies fluid pressure to the outer surface of said base material from the side opposite to the coater head side of the base material. As the fluid pressure means related to the invention (pressurizing said base material with fluid), on the outer surface of said base material at the position of said back edge surface the discharge pressures of air, etc. may be changed in the crosswise direction of said base material. In this way film thickness in the crosswise direction may be controlled for uniformity. The fluid pressure means may be provided at said front edge surface so that a fluid pressure is added to a tension of said base material. This design prevents inclusion of air and resulting pinhole failures. Or, the fluid pressure means may be provided at said back edge face so that a high viscosity coating solution or thin film can be coated easily by regulating the pressure due to a tension on said front edge surface. A lower tension is sometimes necessary, for example, when a base material with an underlayer is overcoated or when dual layers are coated. In these cases, the fluid pressure means may be provided at said back edge face for uniform and stable coating. The third aim of the invention is to provide the coating apparatus that allows yield to be increased by preventing the coating failure caused by coating solution waste from depositing on the edge surface of said coater head. This is successfully attained by making at least one edge surface of the coater head with a material whose contact angle with water ϑ is 63° or more. The invention uses at least one edge surface made up of the material whose contact angle with water is 63° or more, thereby the coater head is provided with excellent liquid repellency. As a result, even if coating is suspended, the coating solution immediately drips from said edge surface, leaving hardly any residue on said edge surface. Therefore, an extended time before a restart never cause or is least likely to cause waste to deposit, thus reducing coating failures to a minimum. The fourth aim of the invention is to provide a method of manufacturing magnetic recording media with excellent characteristics by improving the stability of the coating solution; this is successfully attained by the following means. A curing agent is added to the magnetic coating solution containing fine ferromagnetic powder dispersed in a binder, while the mixture thus prepared is left under a shearing stress of 1 x 10² - 1 x 10⁵ dyne/cm² for at least five seconds before being coated on the base material. Said magnetic coating solution (with a curing agent) is then led to the coating apparatus and is continuously extruded from the slit between said front edge surface and back edge surface. The invention adds a curing agent to a magnetic coating solution that has been prepared as specified whereby the final coating solution has been improved it's stability. In addition, the invention uses the extrusion-type coating apparatus, which discharges the specified total quantity of said final solution through the supply line from the slit of the coater head, with no coating solution left in a vat, etc. on the way. This design prevents said coating solution from time-dependent deterioration and assures excellent performance of the magnetic recording medium. In addition, said coating solution is free from a change with time elapsed, eliminating the need of adjusting the coater head for each time that some deterioration of said coating solution is detected, thereby improving productivity. The fifth aim of this invention is to provide a method of manufacturing the magnetic recording medium with the back coating layer formed in a series of coating processes while reducing dropouts, and is successfully attained in the following process. The base material is coated with a magnetic layer on one surface, dried and calendered. Then, the other surface of said base material is forced to the front edge surface and back edge surface, while being coated with the coating solution extruded from the slit between said front edge surface and back edge surface in order to continuously form a back coating layer. This invention completes a series of magnetic layer coating operations, including coating, drying, and calendering of the magnetic layer, before forming the back coating layer, thus preventing dropouts. More particularly, the extrusion coating head does not extrude the coating solution to the magnetic layer surface of said base material but only to the reverse surface. It follows that dirt, if there is any in the coating solution, is sequentially extruded onto said reverse surface and never influences the magnetic layer surface. In addition, said back coating layer is coated after a series of magnetic layer coating operations has been completed and before the base material is rewound. Therefore, said back coating layer may not be transferred to a group of feed rolls and calender rolls to which said base material is subjected before the completion of magnetic layer coating process. On the other hand, to prevent dirt causing dropouts in the magnetic layer, a knife coater seems to be an alternative instead of the roll-type coating system in forming said back coating layer. However, considering that said back coating layer is generally an extremely thin film of 3 µm or less and that uniformity in the crosswise direction is required; the knife coater system is not feasible because of failure to assure uniformity of film thickness. The present inventors have found that only the extrusion system meets the requirements of feasibility. Brief Description of the Drawings Fig. 1 shows the cross section of the main part of the coater head in the coating apparatus of the invention. Fig. 2 is the whole view of the coating apparatus. Fig. 3 is the cross sectional view of the enlarged main part of the coater head. Fig. 4 is the oblique view of the blow-out nozzle viewed from the diffuser (slit) of said nozzle Fig. 5 is the comparative view showing the distribution of film thickness in the crosswise direction. Fig. 6 shows the outline of the coating apparatus of the invention and the typical layout thereof. Fig. 7 shows the outline of the equipment with which the method of manufacturing magnetic recording media according to the invention is put into practice. Fig. 8 shows how shearing rate is calcualated. Fig. 9 is the cross sectional view showing the whole of conventional coating apparatus. Detailed Description of the Invention Detailed description of the invention follows. Fig. 1 shows the critical part of the extruder related to the invention. Said extruder is equipped with front-edge face 1 with curvature radius r₁ on the surface on the upstream side, back-edge face 2 with curvature radius r₂ on the surface on the downstream side, and slit 3 which is located between said front edge face and back edge face and which is connected to coating solution pocket 4. (See Fig. 9.) There is the end part 1a of curvature radius r₁ on front edge 1. Said end part 1a is of such length that a line connecting curvature radius center O₁ to end B is at an angle of 30° to 120°, preferably of 75° to 100°, with a line connecting said center O₁ to turning point 1b. An introducing face 1c to the turning point 1b may be a straight line or a curve. On back-edge face 2, on the other hand, curvature radius r₂ is designed to meet requirement 3 mm < r₂ < 20 mm over from exit line C of slit 3 to it's end. In relation to curvature radius r₂ on back edge face 2, curvature radius r₁ at end part 1a of front edge face 1 is designed to meet requirement 1 mm < r₁ < r₂/2 . In addition, a part of back-edge face 2 is projected (almost upward in Fig. 2) from tangent line ℓ₁ at the downstream end B of front-edge face 1. This condition may be expressed in the following formula (2) where said tangent line ℓ₁ is at angle α₂ with line ℓ₃ connecting back-edge face 2 to downstream end B, and said tangent line ℓ₁ is at angle α₁ with tangent line ℓ₂ at downstream end A of back edge face 2. α₂ < α₁ < 180° (2) For the base material related to the invention, a variety of materials may be used including plastic film, such as polyester film, paper, the sheet laminating plastic film with paper, and metallic sheet. Any flexible material may be used irrespective of quality and formation. Comparative Test (1) The effect of the invention is clarified using the following examples. Example 1-1 A magnetic recording sheet is made up of polyethylene terephthalate film of 15 µm in thickness as basic support material on which a high viscosity magnetic coating solution of 4000 cps is coated to a wet film thickness of 40 µm; said coating solution contains metal powder (with a BET value of 60 m³/g). For the purpose of the present test, two coating apparatus were prepared: One is related to the present invention, and the other to the prior technique. Tests were performed for coating properties, with coating rate being changed; Table 1 lists the test results . Coating rate Prior technique Present invention 100 m/min o o 200 m/min o o 300 m/min o - Δ o 400 m/min Δ o 500 m/min x o Symbols: x : Excessive uneven thicknesses are obviously found. Δ : Many uneven thicknesses are found. Δ - o : Uneven thicknesses are found in some sections. o : Coating properties are satisfactory. The base material was coated over a length of 500 m or more, except for those indicated by symbols x and Δ , where coating was stopped in a length of about 100 m because unevenness is clearly seen in visual inspection. For the purpose of the present test, curvature radii r₁ and r₂ of the coating head of the invention were 3 mm and 8 mm, respectively. Example 1-2 The test was performed under the same coating conditions as the test in Example 1-1, except that a coating rate of 400 m/min was used. For the purpose of the present test, curvature radii r₁ and r₂ were changed to check for coating properties. Table 2 lists the test results. r₁ r₂ Coating properties Remarks 0.5 mm 8 mm x Failure results from air inclusion. 1.5 mm 8 mm o 3 mm 2.5 mm x Many streaks 3 mm 25 mm x Apparent crosswise unevenness 4 mm 6 mm x Apparent unevenness. The above tests have proven that the present invention ensures good coating properties even in thick-film and highspeed coating. The configuration intended to attain the second aim of the invention is now described in detail; see Figs. 2 to 5. Fig. 3 shows the positions between the critical part of extruder coater head CH (Refer to Fig. 1.) and the fluid pressure means. On the other hand, support rolls 4 and 5 are located on upstream and downstream sides of coater head CH, as shown in Fig. 2. The base material is led to the downstream side through upstream support roll 4, front edge face 1, back edge face 2, and downstream support roll 5 in that order. While the base material 6 runs toward the downstream side, it is forced to front edge face 1 and back edge face 2 by coater head CH, which projects from the base material running along line L. In the present invention, a fluid pressure means 7 is provided between upstream support roll 4 and downstream support roll 5 in order that said means 7 blows fluid, such as air to front edge face 1 and back edge face 2 on the side opposite to coater head CH of base material 6. Figs 2 and 3 show a typical example of fluid pressure means 7. The blow-out nozzle is formed by slit 7B on tubular header 7A which is connected to compressor 7C. The blow-out nozzle has a length almost equal to the width of base material 6. To change pressures blown in the crosswise direction of base material 6, a preferable means is as follows: slit 7B is separated by partition wall 7D, the plural exit tubes of compressor 7C are branched on the way, and each branch is provided with pressure control value 7E. Air blown out from the blow-out nozzle forces base material 6 toward coater head CH. Gas such as air should preferably be used as a working fluid in view of easy handling and maintenance, but a liquid such as water may be substituted for it. The blow-out nozzle may be located in an appropriate position, subject to conditions described in above pressure regulaton in crosswise direction. A blow-out nozzle may be provided on each of front-edge face 1 and back-edge face 2; each blow-out nozzle may be designed to provide a pressure different from that of the other nozzle. Blow-out pressure may be set to any appropriate value depending on the distance to base material 6, tension of base material 6, coating rate, viscosity of the coating solution, intended film thickness, etc. In a preferable application, running line L of said base material 6 should be at angle ϑ satisfying the following formula (2) with tangent line ℓ₂ at downstream end A of back edge face 2; see Fig. 2. 0.5° ≦ ϑ ≦ 10° (2) Of ϑ < 0.5°, a force sufficient to press base material 6 against coater head CH is not available, and failures such as streaks are likely to occur. If ϑ > 10°, smooth coat film is not available, and film thickness is subject to greater variations in the longitudinal direction of the sheet. There is space S between downstream end A of back edge face 2 and the center of downstream support roll 5. Space S should preferably be 5 - 50 mm. If S < 5 mm, base material 6 has to abruptly change its directions at downstream end A of back edge face 2, thereby jeopadizing the smoothness of the coated film If S > 50 mm, a force pressing base material 6 is reduced, the coating film thickness is likely to vary. Base material 6 may be introduced into upstream support roll 4 and taken out of downstream support roll 5 in any direction. Comparative Test (2) The effect of the invention is clarified using examples of coating. A magnetic recording sheet is made up of polyethylene terephthalate film of 15 µm to which a high viscosity magnetic coating solution is coated to an intended dry film thickness of 3 µm; said solution contains metallic powder (with a BET value of 60 m³/g) at 3000 cps. The present test was performed in two ways: One is to use said fluid pressure means, and the other is not. In both ways, the average distribution of film thickness in the crosswise direction was measured; the test results are shown in Fig. 5. In the former way, the blow-out nozzle was internally divided into six chambers evenly with partition walls, and was provided with pressure control valves to control blow-out pressure. Fig. 5 shows that variations of film thickness distribution are inhibited to an extremely low level. For the purpose of experiment, various coating heads were manufactured and used together with said blow-out nozzle, which was located on front edge face 1 and back edge face 2, to find the optimal blow-out pressure. The inventor has found that said optimal pressure results in excellent coating properties as compared with those where no blow-out nozzle is used. The present test has proven that this invention ensures excellent coating properties while preventing uneven thicknesses and coating failures. The third aim of the invention is to prevent coating failures caused by waste of the coating solution deposited on edge faces of the coater head, and is successfully attained using liquid repellent edges. More particularly, front-edge face 3 (and preferably alose back-edge face 4) is made up of a material with liquid repellent material. Such material should have contact angle ϑ with water of 63° or more, more preferably 68° or more. Typical examples include polyacetal resin, high-density polyethylene, fluorocarbon resin, and polyethylene terephthalate resin. The present invention defines and measures contact angle Owith water as follows. Ionized water of about 5 µℓ is dripped onto a test specimen, when a water drop becomes a sphere on the surface of the test specimen. At the contact point between the sphere and surface of the test specimen, an angle formed by the tangent line of the sphere and the surface of the test specimen is measured 30 sec after said ionized water is dripped, using a contact angle measuring device from Elma Optics Co. Comparative Test (3) The comparative test (3) was performed as follows: A polyethylene terephthalate film of 15 µm was used as the base material, to which a magnetic coating solution was coated to a wet film thickness of 40 µm at a coating rate of 100 m/min; said solution contains metallic powder (with a BET value of 55 m³/g) at 3000 cps. For the purpose of the test, coater heads made up of various materials were prepared and used to find the average of defect lengths caused by coating failures. Table 3 lists the test results. Material Contact angle ϑ Defect length due to coating failure With cleaning device With no cleaning device Selected example 1 Teflon 1 95° 0 m 0.5 m Selected example 2 Polyfron 2 90° 0 m 1.2 m Selected example 3 High-density polyethylene 70° 5 m 50 m Selected example 4 Polyacetal 64° 12 m 59 m Comaparative example 1 Iron 55° 199 m 248 m Comaparative example 2 Stainless steel (SUS 304) 60° 212 m 293 m The present test has proven that, the greater the contact angle, the fewer are the coating failures. For example, when a front edge face made up of polyacetal with a contact angle of 64° is used and cleaned as required, a coating failure may be limited within a range in which no trouble occurs in practical use. In addition, the test indicates that when a conventional coater head is used a defective length of 200 m or more occurs. The present invention ensures that coating failures are greatly reduced, which are caused by waste of a coating solution left on edge faces of the coater head, and that an improved yield is attained. Fig. 7 outlines the equipment necessary for manufacturing magnetic recording media; the fourth aim of the invention. Base material 1 fed from roller 2 is coated, on one surface, with a magnetic coating solution Lm discharged from extrusion coater head No. 1 (4A) before reaching drying apparatus No. 1 (3). Magnetic coating solution Lm thus coated is dried by drying apparatus No. 1 (3) and is turned into a magnetic layer, which is then smoothed by calender roll 5. In the next, base material 1 is coated, on the other surface, with a back-coating solution Lb discharged from coater head No. 2 (4B). Said solution Lb thus coated is dried by drying apparatus No. 2 (6) and is turned into a back-coating layer before base material 6 is wound by rewinder 7. Said coater heads 4A and 4B have front edge face Sf on the upstream side in the running direction of base material 1, and back-edge face Sb on the downstream side. Slit G, provided between front-edge Sf and back-edge Sb, receives a coating solution continuously supplied by pocket P, and extrudes said solution onto the surface of base material 1. Said magnetic coating solution is prepared as follows. Make-up tank 10 holds a solution which contains magnetic powder which has been pulverized with a ball mill, sand mill, or the like, and is dispersed in said solution. To the dispersion solution, additives 11 and 12 are added as required. Then, curing agent 17 is added to the dispersion solution from curing agent tank 16 by pump 18 before the dispersion solution is fed to disperser 15 through filter 13 by pump 13. The magnetic coating solution (containing a curing agent) undergoes a shear stress (shearing force) of 1 x 10² - 1 x 10⁵ dyne/cm² in disperser 15 for at least 5 seconds, preferably for 10 - 60 seconds, and thereafter is led to pocket P of coater head No. 1 (4A) and is discharged onto base material 1 through slit G. The magnetic coating solution may preferably be applied within 20 seconds after said magnetic powder is dispersed in said magnetic solution. Coater head 4A may be the coater head as described earlier, that is, a known one. To prevent various coating failures, it is preferable to use the coater head according to the invention, that is, coater head 4A may preferably have a part of back-edge face Sb projecting from tangent line ℓ 1. As a disperser for applying a required shear stress, a sand mill, high-speed dissolver, etc. may be used, but a static mixer or low-speed dissolver is not acceptable because of incapability of providing the necessary shear stress. The shear stress is measured indirectly by measuring the viscosity of a coating solution using a precision viscometer such as a Harke viscometer. A simplified alternative is to find the shear stress using the following equations (1) and (2) assuming that the rate gradient is linear. Symbols in these equations are as described in Fig. 8. The present invention uses a variety of fine ferromagnetic powder; ferromagnetic oxide powder such as r-Fe₂O₃, r with Co-Fe₂O₃, Co coated r-Fe₂O₃, Fe₃O₄, Co-Fe₃O₄ and CrO₂, and metallic ferromagnetic powder with the content of Fe, Ni, Co, and Cr, such as Fe-Co-Ni alloy, Fe-Al alloy, MnBi alloy, Fe-Al-P alloy, Fe-Co-Ni-Cr alloy, Fe-Ni-Zn alloy, Fe-Co-Ni-P alloy, Fe-Ni alloy, Co-Ni-P alloy, Co-Ni alloy, Co-P alloy, Fe-Mn-Zn alloy, Fe-Ni-Mn alloy, Fe-Ni-Cr-P alloy, and Fe-Ni-Co-Zn alloy. The binder used by the invention includes a vinylchloride-vinyleacetate copolymer, nivylchloridevinylidene chloride copolymer, vinylidene chlorideacrylonitrite copolymer, buthy acrytate-acrylonitrile copolymer, cellulose-base resin, epoxy resin, polyvinyl butyral resin, polyurethane resin, polyester resin, and synthetic-rubber-base resin. The coating solution is dissolved in common organic solvents. To the coating solution added are, as required, the dispersion agent, lubricant, abrasive, destaticizer, curing agent, plasticizer, surface active agent, etc. The present invention is preferably applied when fine ferromagnetic powder has a specific area of 40 m³/g or more and the binder has any of the following polar groups. Here M stands for a hydrogen atom, lithium, potassium, or sodium, while M₁ and M₂ for a hydrogen atom, lithium, potassium, sodium, or arkyl base. Comparative Test (4) The effect of the invention is clarified using the following examples. Test examples 4-1, 4-2, and 4-3 A polyethylene terephthalate film 1000 mm in width and 15 µm in thickness was used as the base material, on which a magnetic layer was formed using the coating apparatus illustrated in Figs. 1 and 7. The magnetic coating solution for the test contained metal powder (with a BET value of 55 m³/g) and a vinylchrolide vinyl acetate copolymer, and was applied at a viscosity of 2500 cps to a wet film thickness of 30 µm. The coating rate was 100 m/min. Test examples 4-1, 4-2, and 4-3 were obtained by a high speed dissolver, sand mill A, and sand mill B, respectively. Comparative examples 4-1 to 4-6 Comparative examples 4-1 to 4-6 show the result of tests which were intentionally deviated from requirements of the invention in the presence or absence of dissolver, the difference of dissolver, and binder with a polar group. <Result> Each of said nine examples was checked for a rise in viscosity as an index of stability of the coating solution and for deterioration of the magnetic recording medium obtained. Table 4 lists the test results. Coating apparatus Polar group in binder Disperser Shear stress Rise in viscosity RF loss (dB) Test example 4-1 Extrusion Present High speed dissolver (4000rpm) 1.1x10² None -0.1 Test example 4-2 Extrusion Present Sand mill 1.6x10⁴ None 0 Test example 4-3 Extrusion None Sand mill 1.6x10⁴ None 0 Comparative example 4-1 Extrusion Present Static mixer 0.5x10 None -0.5 Comparative example 4-2 Extrusion Present Dissolver (500 rpm) 1.3x10 None -0.5 Comparative example 4-3 Extrusion Present None - None -1.5 Comparative example 4-4 Extrusion None None - None -1.2 Comparative example 4-5 Gravure Present Sand mill 1.6x10⁴ 2 timmes -0.5 Comparative example 4-6 Gravure None Sand mill 1.6x10⁴ 1.5 times -0.4 In conclusion, the invention improves stability of the magnetic coating solution and provides magnetic recording media having excellent recording and playback characteristics. The fifth aim of the invention is to form a back-coating layer in a series of coating operations in the manufacturing process of magnetic recording media, and is successfully attained by use of the coating apparatus illustrated in Fig. 7. The back coating solution is prepared in the same way as the magnetic coating solution, except that fine magnetic powder is not used. An inorganic filler, carbon, CaSO₄, TiO₂, etc. may be used. Materials of the back-coating layer may be selected from those of the magnetic layer; the binder, coating solution, and additive are as described earlier. The back-coating layer may be formed to a thickness of 0.1 - 5 µm, but more preferably to a thickness of 0.2 - 0.3 µm. Comparative Test (5) The effect of the invention is clarified using the following examples. Test example 5-1 A polyethylene terephthalate film 1000 mm in width and 15 m in thickness was used as the base material, on which the magnetic layer and back-coating layer were formed using the coating apparatus illustrated in Figs. 1 and 7. The coating apparatus was run at a coating rate of 100 m/min. The magnetic coating solution contained metal powder (with a BET value of 60 m³/g) and a vinylchloride and vinylacetate copolymer, and was applied at a viscosity of 2500 cps to a wet film thickness of 30 µm. The back-coating solution had a viscosity of 1500 cps, consisting mainly of polyurethane resin having fluorine, and was applied to a dry film thickness of 2 µm. Comparative examples 5-1 to 5-3 Comparative examples 5-1, 5-2, and 5-3 were obtained using the gravure roll, reverse roll, and knife coater, respectively, which substituted for the extrusion-type coating apparatus to apply the back-coating solution. Any other coating conditions were left unchanged. <Result> The magnetic recording medium thus obtained was checked for dropouts at -12 dB and variations of film thickness in the longitudinal direction; Table 5 lists the test results. D/O (10 µS/3 µS) Rate of variation Test example 5-1 Extrusion 0/2 ± 5% Comparative example 5-1 Gravure roll 1/7 ± 8% Comparative example 5-2 Reverse roll 2/8 ± 20% Comparative example 5-3 Knife coater 2/7 ± 35% The present test has proven that the invention ensures uniform distribution of film thickness in the longitudinal direction and provides magnetic recording media with fewer dropouts. Thus, the invention can manufacture magnetic recording media which feature reduced dropouts and whose back-coating layer has fewer variations in film thickness.

Claims: An apparatus for coating a solution on a flexable support (6) which is moved so as to run along a coating surface of said apparatus; wherein said coating surface is slitted so as to form a front edge surface (1), a back edge surface (2) in order in the moving direction of the flexible support (6), and a slit (3) therebetween so that said apparatus extrudes the solution through the slit (3) to the flexible support (6), and said front edge surface (1) includes an introduction segment (1c), a turning portion (1b), and an end section which is defined by an arc surface (1a) with a curvature radius of r₁; and said back edge surface (2) wholly forms an arc surface with a curvature radius of r₂; characterized in that said radii r₁ and r₂ satisfy the following inequalities:- 3mm < r₂ < 20mm 1mm < r₁ < r₂/2 ; and, at least a part of said back edge surface (2) projects beyond the tangent line (ℓ₁) at the end (B) of said front edge surface (1) adjacent said slit (3), with respect to the radius centre (O₂) of said back edge surface radius (r₂). An apparatus according to Claim 1, wherein at least one of said front and back edge surfaces (1,2) is made of a material which has a contact angle of not less than 68 degrees with water. An apparatus according to Claim 1 or Claim 2, which has a pair of support rolls (4,5) arranged on the upstream and downstream sides of said coating surface (1,2) in order to press said flexible support (6) onto said coating surface, and a fluid pressure means (7) which provides a fluid pressure toward the outer surface of said flexible support (6) on said coating surface (1,2) between said support rolls (4,5).

Inventors: KAWABE, SHIGETOSHI, NAMIKI, TAKEMASA, TOBISAWA, SEIICHI
Applicants: KONICA CORPORATION
Application year: 1989
Main classification: B05C5
IPC codes: B05C5