This invention relates to a coating apparatus according to the precharacterising part of claim 1 and to a method for applying a coating solution onto a belt-shaped flexible support.
An apparatus and a method of this type are known from e.g. JP-A-2 223 945. Such an apparatus and such a coating method are used for applying a variety of liquid-phase compound materials to a belt-shaped flexible support (hereinafter referred to as "a web", when applicable) which is run continuously, to manufacture photographing photo-sensitive materials such as photographing film and photographic paper, photographic printing plate, pressure-sensitive sheets, heat-sensitive sheets, and so forth.
Examples of a conventional coating method in which a web is electrostatically charged to coat it with a coating solution, are for instance as follows:
The first and second coating methods suffer from the following problem: In the case where the surface of the web is relatively large in the number of polar groups; for instance in the case where, as in the case of the second coating method, the web contains surface active agent, the coating condition of the surface of the web is improved which is probably due to the orientation of the polar groups in the surface of the web as well as an electrostatic attraction force provided by the electric field applied thereto, as a result of which the coating speed can be increased. On the other hand, in the case where the surface of the web is relatively small in the number of polar groups (the number of polar groups per unitary area of the part of the web which is 1 µm in thickness from the surface being 4 x 10-3 equivalent/m2 or less), the coating condition of the surface of the web is not bettered, and the coating speed cannot be increased so much as expected.
In the third coating method, the surface of the web is made uniform in potential distribution by heating the web which has been electrostatically charged, whereby the difficulty is eliminated that streaks are formed on the coating solution layer on the support. However, in the part of the web which is heated, the surface potential is lowered, as a result of which it is difficult to maintain the surface potential so high, and the coating speed cannot be increased so much as expected. Another method and apparatus for applying a coating liquid to a moving web is known from EP-A-0 299 492.
Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional coating method.
More specifically, an object of the invention is to provide a coating method in which, even in the case where the surface of a web is relatively low in the number of polar groups, a relatively high potential can be uniformly applied to the surface of the web, and the critical coating speed can be greatly increased.
The foregoing object of the invention has been achieved by the coating apparatus as defined in claim 1 and by the coating method as defined in claim 2.
The nature, principle, and utility of the invention will be more clearly understood from the following detailed description of the invention when read in conjunction with the accompanying drawing.
In the accompanying drawing:
With a coating apparatus shown in FIG. 1, the surface of a web 1 is made single polar by electrostatically charging it by corona discharge. The web 1 is heated to 35°C to 45°C while passing through a heating zone 2. The web 1 thus heated is electrostatically charged by a corona discharge electrode 7 which is connected to a DC high voltage source 3 and is confronted with a grounding electrode 8, so that the surface of the web is made single polar, having a potential of 0.5 to 2 kV. The web 1 thus processed is delivered to a coating backup roller 4 while being maintained sufficiently high in temperature, where a coating solution 6 from a coating nozzle 5 is applied to the web.
In the case of FIG. 2, a DC high voltage is applied to a web 1 at a backup roller. Similarly as in the case of FIG. 1, the web 1 is heated to 35°C to 45°C at a heating zone 2. The web 1 thus heated is delivered to a coating backup roller 4 to which a DC high voltage is applied by a DC high voltage source 3, where a coating solution 6 from a coating die 5 is applied to the web 1.
As was described above, in the coating method of the invention, the web is heated to 35°C to 45°C. This will be described in more detail.
Heretofore, a coating operation is carried out at a room temperature of 20°C to 25°C. In the invention, the web is heated to 35°C to 45°C by convection by using hot air, or by conduction by bringing it into contact with a high temperature pass roll, or by radiation by using infrared rays. If the temperature of the web thus heated is less than 35°C, then the macromolecules in the surface of the web are greatly limited in mobility, so that the coating speed cannot be increased so much as expected. In addition, when the temperature of the web is higher than 45°C, then the coating solution setting conditions are not satisfied.
Furthermore, in the coating method of the invention, the surface potential of the web on the backup roller is set to 0.5 to 2.0 kV. For this purpose, two methods are employed; in the first method, the web is charged by corona discharge; and in the second method, a DC voltage is applied to the backup roller.
In the first method, preferably a wire-like conductor 100 µm to 200 µm in diameter is laid tight with a tension of 1 kg or higher, and intermediate frames support the wired at points spaced at an interval of not more than 300 mm. Each intermediate holding frame is shifted in the longitudinal direction on each wire form a superposed (aligned) position on the wires in the transfer direction with a shift ≥ 20 mm from the immediately previous adjacent wire so that the distance between the wire-like, electrode and the web is smaller at the end of the wire than at the middle. In this case, the variations in potential in the direction of conveyance of the web and in the direction of width of the latter can be minimized, and therefore, the difficulty can be substantially eliminated that the coating solution layer becomes non-uniform in thickness because of the variations in potential (US Patent No. 5,138,971).
In the case of the second method, it is preferable that the journal of the backup roller is insulated with a ceramic cover and supported with a fluid bearing, and a connecting terminal other than a brush type slip ring is provided at a rotary part which is an extension of the journal, and connected to the DC high voltage source. In the second method, when compared with the conventional method in which a ball bearing is employed, the rotation is steady. Further-more, when compared with the case where the brush-type slip ring is employed, a high voltage of 0.5 to 3 kV can be positively applied (cf. Japanese Patent Application (OPI) No. 251266/1990.
Employment of the above-described two methods in combination, the surface of the web on the backup roller can be set uniformly to 0.5 to 2.0 kV, preferably 0.5 to 1.2 kV. As a result, the polar groups of macromolecules are orientated in the surface of the web, thus increasing the web's surface energy, which improves the coating conditions. In addition, the electrostatic attraction force by the electrostatic field facilitates the sticking of the coating solution to the surface of the web.
In the invention, the term "belt-shaped flexible support" or "web" as used herein is intended to include those of paper, plastic film, resin-coated paper, and synthetic paper. Examples of the material of the plastic film are polyolefins such as polyethylene and polypropylene, vinyl copolymers such as polyvinyl acetate, polyvinyl chloride and polystyrene, polyamide such as 6,6-nylon and 6-nylon, polyesters such as polyethylene terephthalate, polyethylene-2, and 6-napthalate, and polycarbonate, and cellulose acetates such as cellulose triacetate and cellulose diacetate. An undercoat layer of gelatin may be formed on the web as the case may be. The resin employed for manufacture of the resin-coated paper is typically polyolefin such as polyethylene. A web of resin-coated paper is not limited in surface roughness; that is, not only a web of resin-coated paper whose surface is smooth, but also one whose surface is rough can be employed.
Even in the case where the web is such that its surface is relatively low in the number of polar groups (the number of polar groups per unitary area of the part of the web which is 1 µm in thickness from the surface being 4 x 10-3 equivalent/m2 or less), the occurrence of an air entrainment phenomenon (obstructing the application of a coating solution to a web) which limits the critical coating speed is prevented accordingly to the invention. That is, the critical coating speed is increased according to the invention.
The term "coating solution" as used herein is intended to include a variety of coating solutions, such as those used to form a photo emulsion layer, undercoat layer, protective layer and a back layer thereby to form a photographing photo-sensitive material, or those for forming an adhesive layer, coloring layer, rust-proofing layer, etc. Those coating solutions may contain water-soluble binder, or organic binder.
In applying the coating solution to the web, for instance a slide coating method, roller bead coating method, spray coating method, extrusion coating method, and curtain coating method may be employed.
As conducive to a full understanding of the invention, its concrete example will be described. However, it should be noted that the invention is not limited thereto or thereby.
A web was prepared by forming a gelatin layer 0.3 µm in thickness on a coating surface of a polyethylene resin coated sheet 220 µm in thickness, and an alumina-sol layer on the rear surface. The coating surface had an average surface roughness (Ra) of 4 to 5 µm. The surface of the web was 3 x 10-3 equivalent/m2 in the number of polar groups.
Color paper coating solutions were employed. The layer structure was made up of three photo-sensitive layers containing silver halide, three color filter layers, and one protective layer. A total quantity of coating solutions supplied per unitary web area 1 m2 was 106 ml. As for the lowermost layer, namely, a yellow coloring layer, the density of gelatin was 6% by weight, the viscosity was 40 cp (with a shear rate of 50 sec-1), and the quantity of supply was 14.4 ml/m2.
A slide bead coating method was employed. The pressure of the pressure reducing chamber was set to a value lower by 40 mm H2O than the atmospheric pressure. The gap between the end of the coating nozzle and the web was about 400 µm.
A heating method using a hot air at 70°C to 50°C was employed. The length of the web in the heating zone was 18 m. The temperature of the web which was going to be coated being on the coating backup roller was measured as a web temperature.
The web was electrostatically charged as shown in FIG. 1. In this operation, the distance between the backup roller 4 and the corona discharge electrode 7 adapted to electrostatically change the web by corona discharge so as to make the web's surface single polar was short, 2 m. Thus, the web was substantially free from temperature difference.
The surface potential of the web which was going to be coated being on the coating backup roller was measured as a web temperature. The critical speeds were measured when, depending on the web potential and web temperature, an air layer accompanying the web obstructed the application of the coating solution to the web. The results of measurement are as indicated in the following Table 1:
In the concrete example, the web temperatures were 38°C and 45°C, and the web potentials were 0.5 kV, 1.0 kV, and 1.2 kV, as indicated in Table 1. In the prior art, with 22°C and 0.5 kV, the critical coating speed was 200 m/min, as shown in Table 1. However, this critical coating speed was increased to 240 m/min. by raising the web temperature to 38°C according to the invention. In addition, in the prior art, with 22°C and 1.2 kV, the critical coating speed was 203 m/min. This critical coating speed was increased to 270 m/min.
Furthermore, in the concrete example, the surface of the product was satisfactory being free from defects such as streaks and non-uniform thickness.
As is apparent from the above-description, according to the coating method of the invention, even in the case where the surface of the web is relatively low in the number of polar groups, relatively high potential can be provided uniform on the surface of the web, and the critical coating speed can be greatly increased.
While there has been described in connection with the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined in claim 1 and in claim 2.