Curtain coating apparatus

Abstract: A curtain coating method of coating a continuously travelling web with at least two kinds of coating solutions superposed upon one another in layers includes the steps of supplying said coating solutions to flow as a free falling curtain and impinging the coating solutions on the web, in which differences in gelatin concentration and viscosity between a coating solution for a lowest layer and a coating solution for an adjacent layer are adjusted to 0-2 weight percent and 0-20 cp respectively. Furthermore, a curtain coating apparatus for coating a web with a coating solution includes a hopper with a slide plane on which the coating solution flows and with a hopper lip from where the coating solution begins to freely fall down as a free falling curtain, and at least a pair of outlets which are each disposed in a position along the edge part of the free falling curtain and situated in a fixed distance down from the hopper lip, in which the auxiliary solutions are introduced from the width directions so as to maintain the curtain in the width direction thereof.

Description: 1. Field of the invention This invention relates to a curtain coating apparatus for uniform application of coating solutions and, more particularly, to the apparatus for curtain coating solutions on to a continuously travelling strip like support (hereinafter referred to as "web") which is employed in the production of photographic films, photographic printing papers, magnetic recording tapes, pressure-sensitive recording papers, offset plate materials and the like. 2. Description of the related arts Conventionally, a curtain coating technique has been applied to the production of photographic films, photographic printing papers and the like. First, descriptions will be given of conventional curtain coating methods as follows. The basic arts of curtain coating are described, e.g., in U.S.-A-3,508,947 and U.S.-A-3,632,374 which correspond to JP-B-49-24133 and JP-B-49-35447 respectively. Further, S.F. Kistler discloses a theory of curtain coating in "AlChe Winter National meeting" (1982), and describes the following three phenomena which predominantly determine the coating rate in the curtain coating method: (1) the phenomenon that fine bubbles are entrained in a gap between a web and a coating solution (This phenomenon is called "the air entrainment phenomenon" hereinafter), (2) the phenomenon that a foot-like cross-sectional shape of the impingement zone can develop a pronounced heel that can give rise to coating nonuniformity. (This phenomenon is called "the heel phenomenon" hereinafter, and it occurs in a case where a coating solution is made to flow down at a high flow rate), and (3) the phenomenon that a coating solution bounds at the web surface without adhering thereto (This phenomenon is called "the sagging phenomenon" hereinafter, and it occurs in the same case as the phenomenon (2), namely a case where a coating solution is made to flow down at a high flow rate). As for attempts to elevate the upper limit of coating speed in this curtain coating method, there is disclosed the means of inhibiting "the air entrainment phenomenon", e.g., by applying an electrostatic field between a web and a coating solution (JP-A-62-197176). In recent years, however, the coating operation has been performed at a high speed of 250 m/min or above and the flowing-down rate of a curtain of coating solution has also been increased. As a result thereof, the retardation of coating speed due to "the sagging phenomenon" has come to a greater problem than the retardation caused by the aforementioned "air entrainment phenomenon". Generally speaking, in order to get rid of nonuniformity in the thickness of the edge part of a free falling curtain, a curtain coating apparatus is usually equipped with guide plates for truing up both the edges of the coating solution on the slide plane, or edge guides for supporting both edge parts of the free falling curtain. In many cases, however, the sagging phenomenon occurs in the edge part of the free falling curtain, in which a thickness of the curtain is apt to be nonuniform. Methods for suppressing the sagging phenomenon and techniques to improve the conventional coating techniques are disclosed in the next two references. U.S.-A-5,393,571 which corresponds to JP-A-3-146172 discloses a method of conducting a curtain coating in which a total viscosity of coating solutions is specified and a surface roughness of web is adjusted to at least 0.3 µm in order to achieve a high-speed coating in a high flow rate range beyond 4 cc/cm/sec. U.S.-A-5,391,401 which corresponds to PCT publication translated in Japanese No. Hei6-503752 discloses a method of conducting a curtain coating stably in which a viscosity of coating solution to form the lowest layer is specified with using a pseudoplasticity solution as the coating solutions. In these methods, the viscosity is specified in order to conduct a high-speed coating stably. However, specifying only the total viscosity or the lowest layer viscosity is insufficient for realizing the high-speed coating and the uniform coated surface condition in multilayer coating. For instance, the coating solution for the lowest layer is likely to be designed so as to have low viscosity and low gelatin concentration from the viewpoint of high-speed coating suitability, but the coating solution for an upper layer is generally made as dense as possible from the viewpoints of ease of preparation, feeding, reduction of drying load and so on, thereby resulting in a rise of gelatin concentration. When the curtain coating method is adopted in the multilayer coating, it turned out that unevenness was caused in the coated film when there are large differences in gelatin concentration and viscosity between the coating solutions for the lowest layer and the directly upper layer. This is because, although the curtain is drawn out to be rendered thin at the times when it is formed from the slide plane and impinges on the web during the curtain coating operation, this drawn-out condition is not uniform in the direction of coating width in the case where there are large differences in gelatin concentration and viscosity between coating solutions for the lowest layer and the directly upper layer, resulting in generation of unevenness. Next, a description will be given of conventional curtain coating apparatuses as follows. In the steps of forming a free falling curtain, edge guides support both edge parts of the curtain formed, and various techniques thereto are disclosed. For example, Japanese examined patent publication No. Sho 58-37866 discloses a method of introducing an auxiliary liquid between the free falling curtain and the edge guides which are prepared at both the edge parts of the curtain, and Japanese examined patent publication No. Hei 6-61517 (hereinafter referred to as JP-B-6-61517) discloses a method of introducing an auxiliary liquid close to the hopper lip. Fig. 1 is a perspective view of a basic structure of a curtain coating apparatus as an embodiment of the present invention. A coating head 1 has a plurality of slits 12 respectively connected to manifolds 10, and the coating solutions extruded from each slit 12 flows down on a slide plane of a slide hopper 7 and falls down to form a free-falling curtain 5 of the coating solution. Both sides of the curtain 5 are supported by edge guides 4, and the curtain 5 falls and impinges at a line on a surface of a web 3 which travels in the arrow direction while supported by a backup roller 2, so that a coated film 8 is formed. Fig. 4 is a front view of a curtain coating apparatus to which an method disclosed in JP-B-58-37866 cited above is applied for introducing the auxiliary solution for the liquid curtain. Such an apparatus is shown as one example of conventional curtain coating apparatuses. Therein, each edge guide 4 is equipped with conduit 14 in a place situated downward at a fixed distance from the departing position of the liquid curtain from the hopper lip, and the conduit 14 is bent down so that its outlet turns to the lower side and each direction of the outlet are parallel to the falling direction of the free falling curtain 5. Fig. 5 is a side view of a curtain coating apparatus to which a method disclosed in JP-B-6-61517 cited above is applied for introducing the auxiliary solution into the free falling curtain. Such an apparatus is shown as another example of the conventional curtain coating apparatuses. Therein, an auxiliary solution supply pipe 15 is kept upright and arranged upward at a fixed distance apart from the departing position of the liquid curtain from the hopper lip. As shown in the above examples, the auxiliary liquid flows along each of the edge guides, so that the film formation can be stabilized and the coated film edge corresponding to the edge part of the liquid curtain is prevented from thickening. With respect to the conventional technique as shown in Fig. 4, the conduit 14 is disposed so as to go across the edge guide 4 and project into the edge part of the free falling curtain 5. Thus, the outlet of the conduit must be bent to the lower side. As a result, it is required for the conduit to have an adequate length for changing the direction of the outlet by bending. This requirement occupies the minimum distance between the hopper lip and the outlet. Accordingly, there will be no introduction of the auxiliary solution into the section formed between the hopper lip and the outlet. If the edge thickening occurs in this section, the coating is carried out under the condition of the thick edge. Moreover, it is necessary for a bent part of the conduit to be projected into the free falling curtain 5 to cause a turbulent flow therein. As a result, there comes a problem of defection of longitudinal streaks in the coated layer. With respect to the conventional technique as shown in Fig. 5, in taking into consideration a position where the auxiliary solution supply pipe 15 is disposed, the auxiliary solution is supplied from above to the edge of the liquid curtain in the thickness direction of the free falling curtain. In short, the auxiliary solution is supplied from outside of the free falling curtain 5. Therefore, it is difficult for the auxiliary solution to be supplied so as to go across the curtain edge in the thickness direction and uniformly reach the backside of the curtain 5. Specifically, in multilayer formation, such an uneven supply of the auxiliary solution makes a difference in edge part thickness between the upper layer and the lower layer. Moreover, it is very difficult to dispose the pipe 15 so that the auxiliary solution is supplied to the same position every time the coated layer is produced. Furthermore, since the auxiliary solution is brought into contact with a curtain surface, goes across the curtain and then reaches the edge part, the auxiliary solution is supplied from the direction in which the curtain flow maintaining pressure is weak. As a result, change in an auxiliary solution supplying pressure causes an external disturbance in the flow at the curtain surface, thereby causing a defection of longitudinal streaks. US-A-4,830,887 discloses an apparatus for a curtain coating method, wherein edge guides are provided which have a slit like opening extending over substantially the entire length of the edge guides. Auxiliary solutions are supplied through said slit like openings to the free falling curtain. EP-A-567 071 discloses an apparatus for curtain coating wherein an auxiliary solution is supplied onto a slide surface. Furthermore, edge guides are provided which extend also onto the slide surface. EP-A-636 423 discloses an edge guide lubricating fluid delivery apparatus. The edge guides are equipped with conduits which are bent down in the falling direction of the curtain. EP-A-566 503 discloses a method of controlling ripple by selecting an appropriate gelatin concentration during coating processes for photographic materials. SUMMARY OF THE INVENTION In view of the foregoing, it is the object of the present invention to provide a curtain coating apparatus which can stabilize a formation of a free falling curtain, inhibit the free falling curtain from causing a defection of longitudinal streaks, and prevent a coated film edges corresponding to edge parts of a free falling curtain from thickening. According to the present invention, there is provided a curtain coating apparatus for coating the web with a coating solution including a hopper with a slide plane on which the coating solution flows and the coating solution is freely fallen down from a hopper lip as a free falling curtain, and at least a pair of conduits each disposed in a position along the edge part of the free falling curtain and situated at a fixed distance down from the hopper lip, in which the auxiliary solution is introduced so as to support the free falling curtain in the width direction thereof, as defined in claim 1. Prefered embodiments are disclosed in the subclaims. In the above curtain coating apparatus, it is preferable that the fixed distance is adjusted to 0.1-1.5 mm. Also, a centerline of outlets for discharging the auxiliary solution may be sloped to the direction in which the coating solution flows down. It is preferable for each outlets to slope downward within 30 degrees from the horizontal line. It is preferable for each outlet to have a circular diametrical section of 0.4-1.5 mm in diameter. It is preferable that the amount of auxiliary solution discharged from each outlet is 3-8cc/min. Due to the foregoing, the distance between the outlet of the conduit and the slide hopper lip can be made very short, and an edge thickening can be removed. Further, there is no factor disturbing a stream of the free falling curtain since only the leading end of each outlet is in direct contact with the edge of the free falling curtain, the remaining part of the conduit is not in contact with the free falling curtain at all, and the conduit is not projected into the curtain. Furthermore, the auxiliary solution is discharged so as to support the edge part of the free falling curtain, and so that the auxiliary solution comes almost simultaneously into contact with each face of the edge parts allowing the auxiliary solution be diffused uniformly. A pressure of the auxiliary solution supply is applied in the width direction of the free falling curtain. Since the free falling curtain is hardly effected in the width direction, the present apparatus enables the free falling curtain to be inhibited from causing a longitudinal streaks even when there are external disturbances including the discharge of the auxiliary solutions and fluctuation therein. BRIEF DESCRIPTION OF DRWINGS Fig. 1 is a perspective view of a curtain coating apparatus as an embodiment of the present invention. Fig. 2 is a front view of the curtain coating apparatus shown in Fig. 1. Fig. 3 is a front view of a curtain coating apparatus as another embodiment of the present invention. Fig. 4 is a front view of the curtain coating apparatus disclosed in a conventional art. Fig. 5 is a side view of another conventional curtain coating apparatus. DETAILED DESCRIPTION OF THE PRESENT INVENTION Embodiments will be given of the present invention as follows. [Curtain coating apparatus] Embodiments of the present curtain coating apparatus are described below in detail. As the basic constituent members of the present curtain coating apparatus shown in Fig. 1, edge guides and auxiliary solution supply conduits are attached. Additionally, the distance between a line 6 where the free falling curtain impinges and a lip part (end part) of the slide hopper 7 can be adjusted to, e.g. the order of 100mm. Fig. 2 is a front view of the curtain coating apparatus shown in Fig. 1. Therein each outlet of an auxiliary solution supply conduit 24 is fitted in each edge guide 4 guiding either edge situated along the curtain edge and 0.1-1.5 mm apart from the starting-point where the free falling curtain begins to fall at the end of the slide hopper 7. Each outlet for discharging an auxiliary solution opens to the width direction of the free falling curtain so as to supply the auxiliary solution so that each auxiliary solution maintains the free falling curtain, and does not project into the free falling curtain. Additionally, the distance between the outlet and the starting-point where the free falling curtain begins to fall is determined in taking account whether the free falling curtain can be held stably by the edge guides, the auxiliary solution can be supplied exactly to the edge part of the free falling curtain from the place very close to the hopper lip, and so on. The coating solution fed from a manifold 10 via a slit 12 which are respectively installed in a coating head 1 flows down on the slide plane of the slide hopper 7. Then, the coating solution reaches the hopper lip and falls down freely. While falling, the coating solution width is controlled by each edge guide 4 disposed on the both sides of the coating solution and the auxiliary solution is discharged from each outlet fitted in each of the edge guide. Although the auxiliary solution is discharged in a slight amount of from 3 to 8 cc/min, the presence of the auxiliary solution between the edge guide 4 and the free falling curtain 5 can have sufficient effect in preventing the emergence of thickness distribution of the free falling curtain between the edge part and the central part thereof. In addition, the outlets themselves have no interference in the free falling curtain, and the auxiliary solution discharged therefrom has no other appreciable influence on the free falling curtain 5 because the amount thereof is so small that the auxiliary solution flow pushing aside the free falling curtain is very thin. Fig. 3 is a front view of a curtain coating apparatus as another embodiment of the present invention. This apparatus resembles the embodiment shown in Fig. 2 in a structural feature that an outlet of an auxiliary solution supply conduit 34 is fitted in each edge guide 4 guiding either edge part of the free falling curtain 5 at a place which is situated along the curtain edge and 0.1-1.5 mm apart from the hopper lip. However, the different point from the embodiment shown in Fig. 2 is the opening direction of the outlets. More specifically, each of the outlets is disposed so as to slope down to the falling direction of the free falling curtain from the width direction of the free falling curtain. On the other hand, it is similar to the embodiment shown in Fig. 2 that the auxiliary solution is introduced so as to maintain the edge part of the free falling curtain and there is no projection into the free falling curtain. By such a disposition of outlets also, the coating apparatus can have function and effect equal to or better than those of the coating apparatus shown in Fig. 2. Additionally, it is desirable that the angle of the foregoing slope is within 30 degrees. Now, the advantages of the present coating apparatus will be illustrated more clearly by reference to the following examples. In these examples, the coating operations for preparing photographic paper or lithographic film can be carried out. Auxiliary Solution 1 Auxiliary Solution 2 Auxiliary Solution 3 Water 100 65 62 Methanol -- 35 35 Gelatin -- - 2 Samples of lithographic film having the layer structure described below were each prepared as prescribed below. The coating operations therein were performed using the curtain coating apparatus as shown in Fig. 1. The first subbing layer according to the formula (1) and the second subbing layer according to the formula (2) were coated successively on both surfaces of a 100 µm-thick biaxially stretched polyethylene terephthalate film. Formula (1) for First Subbing Layer: Vinylidene chloride latex (Aqueous dispersion of core/shell latex constituted of 90 weight percent core part and 10 weight percent shell part) 15 parts by weight Core part; vinylidene chloride/methylacrylate/methylmethacrylate/acrylonitrile/acrylic acid (93/3/3/0.9/0.1) Shell part; vinylidene chloride/methylacrylate/methylmethacrylate/acrylonitrile/acrylic acid (90/3/3/2/2) 2,4-Dichloro-6-hydroxy-s-triazine 0.25 parts by weight Fine particles of polystyrene (Average diameter: 3 µm) 0.05 parts by weight Distilled water to make 100 parts by weight Thereto, a 10 weight percent KOH was further added to adjust the pH to 6 ± 0.3. In coating this coating solution, the solution temperature was kept at 10°C. The layer coated was dried at 180°C for 2 minutes. The coating condition of this coating solution was set so that the layer coated had a dry thickness of 1 µm. Formula (2) for Second Subbing Layer: Gelatin 1 parts by weight Methyl cellulose 0.05 parts by weight Compound (1) 0.02 parts by weight C 12 H 25 O(CH 2 CH 2 O) 10 H 0.03 parts by weight Compound (2) 3.5×10 -3 parts by weight Acetic acid 0.2 parts by weight Water to make 100 parts by weight This coating solution was coated so as to have a dry thickness of 0.1 µm. Therein, the drying was carried out at 170°C for 2 minutes. On one side of the thus obtained support, two layers, a conductive layer of the following formula (3) and a backing layer of the following formula (4), were coated simultaneously. Solutions as shown in Table B1 are obtained by adding a backing layer shown below to each of LiCl, NaCl and KCl. Formula (3) for Conductive Layer: SnO 2 /Sb (9/1 by weight, average grain size: 0.25 µ) 300 mg/m 2 Gelatin 170 parts by weight Compound (2) 7 parts by weight Sodium dodecylbenzenesulfonate 10 parts by weight Sodium dihexyl-α-sulfosuccinate 40 parts by weight Sodium polystyrenesulfonate 9 parts by weight Formula (4) for Backing Layer: Gelatin 2.9 g/m 2 Compound (2) 10 parts by weight Sodium dodecylbenzenesulfonate 70 parts by weight Sodium dibenzyl-α-sulfosuccinate 15 mg/m 2 1,1'-Bis(vinylsulfonyl) methane 150 parts by weight Ethylacrylate latex (Average particle size: 0.05 µ) 500 parts by weight Lithium perfluorooctanesulfonate 10 parts by weight Finely pulverized silicon dioxide (Average particle size: 4 µ, pore diameter: 170 A, surface area: 300 m 2 /g) 35 parts by weight On the other side of the support, the silver halide emulsion layer of the following formula (5) and the protective layer of the following formula (6) were coated successively. Formula (5) for Silver Halide Emulsion Layer Solution I; Water: 300 ml, gelatin: 9 g. Solution II; AgNO 3 : 100 g, water: 400 ml Solution III; NaCl: 37 g, (NH 4 ) 3 RhCl 6 : 1.1 ml, water: 400 ml Solutions II and III were added simultaneously at a constant rate to Solution I kept at 45°C to make an emulsion. After removal of soluble salts from the emulsion in the usual way well known to one skilled in the art, the resulting emulsion was admixed with gelatin, and thereto 6-methyl-4-hydroxy-1,3,3a,7-tetraazaindene as a stabilizer wad further added. The thus made emulsion was a monodisperse emulsion having an average grain size of 0.20 µm and the gelatin content therein was 60 g per kg of emulsion. To the thus prepared emulsion, the following ingredients were further added: Compound (3) 6×10 -3 mole/mole Ag Compound (4) 60 mg/m 2 Compound (5) 9 mg/m 2 Compound (2) 10 mg/m 2 Sodium polystyrenesulfonate 40 mg/m 2 Sodium N-oleoyl-N-methyltaurine 50 mg/m 2 1,1'-Bis(vinylsufonyl)methane 70 mg/m 2 1-Phenyl-5-mercaptotetrazole 3 mg/m 2 Ethylacrylate latex (average particle size: 0.05 µm) 0.46 g/m 2 The thus prepared emulsion was coated so as to have a silver coverage of 1.3 g/m 2 . Formula (6) for Protective Layer: Gelatin 1.0 g/m 2 Lipoic acid 5 mg/m 2 Sodium dodecylbenzenesulfonate 5 mg/m 2 Compound (6) 20 mg/m 2 Sodium polystyrenesulfonate 10 mg/m 2 Compound (7) 20 mg/m 2 Ethylacrylate latex (average particle diameter: 0.05 µ) 200 mg/m 2 As can be seen from Table B3, neither longitudinal streaks nor edge thickening were caused in the production of lithographic films by means of the present curtain coating apparatus. In a coating apparatus according to the present invention, as mentioned above, the distance between each outlet and the hopper lip can be made very short to exclude the factor thickening the edges of the free falling curtain; as a result, the coated film edges are inhibited from thickening. Further, there is no factor disturbing the stream of the free falling curtain since only the leading end of each outlet is in direct contact with the edge of the free falling curtain, and so each pipe does not project into the curtain; as a result, no longitudinal streaks appears. Furthermore, the auxiliary solution is discharged so as to maintain the edge part of the free falling curtain, and thereby it comes almost simultaneously into contact with all faces of the edge part to effect the uniform dispersion into the edge part. Accordingly, no difference in thickness is made between the front and backsides of the curtain edge. In addition, the pressure of auxiliary solution supply is applied in the width direction of the free falling curtain. Since this direction is the most resistant to the influence of external disturbance, the present apparatus enables the curtain to be inhibited from causing longitudinal streaks in spite of the external disturbance due to discharge of an auxiliary solution. Thus, the present coating apparatus enables the stabilized formation of the free falling curtain, the prevention from longitudinal streaks, and the prevention of the thickening of the coated film edges corresponding to edge parts of the free falling curtain.

Claims: An apparatus for coating a web to be coated with a coating solution as a free falling curtain comprising: a hopper with a slide plane on which said coating solution flows and with a hopper lip from where said coating solution begins to fall down as said free falling curtain; a pair of edge guides guiding both edge parts of said curtain to the falling direction; and at least a pair of outlets for discharging an auxiliary solution which are disposed in said edge guides in the position along each edge parts of said curtain and at a fixed distance downward from the hopper lip; and wherein each of said auxiliary solutions is introduced between said free falling curtain and said edge guides from the width directions so as to maintain said curtain in the width direction, wherein each of said auxiliary solutions is introduced to said outlets through conduits penetrating said edge guides. The apparatus as claimed in claim 1, wherein said fixed distance is 0.1-1.5 mm. The apparatus as claimed in claim 1, wherein each of said outlets has a circular diametrical section of 0.4-1.5 mm in diameter. The apparatus as claimed in claim 1, wherein a discharging rate from each of said outlets is 3-8cc/min. The apparatus as claim in claim 1, wherein a centreline of said outlets is sloped to the flowing direction. The apparatus as claimed in claim 5, wherein an angle between the centerline of said outlets and a horizontal line is within 30 degrees.

Inventors: KATAGIRI YOSHINOBU, OKI KAZUHIRO, KATAGIRI, YOSHINOBU, OKI, KAZUHIRO
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1999
Main classification: G03C1
IPC codes: B05C5, G03C1, B05D1