Coating method
BACKGROUND OF THE INVENTION 
     This invention relates in general to the art of coating and in particular to an improved method for carrying out a process of coating in which one or more layers of coating composition are applied to the surface of an object by advancing the object through a coating zone in which a flow of coating composition is applied thereto, for example, a process of bead coating or a process of curtain coating. More specifically, this invention relates to an improved coating method in the manufacturing of a photographic film, photographic printing paper, a magnetic recording tape, an adhesive tape, pressure-sensitive recording paper, an offset printing plate material or the like. 
     A method of applying an electrostatic field to act has been disclosed along with a conventional method of coating to a continuously moving web by applying the electrostatic field to act on the web. For example, there has been a device disclosed in the Japanese Patent No. 27423/71 in which a backup roller is kept at a high potential to generate an electrostatic field between a coating hopper and the backup roller to exercise a method of applying the electrostatic field to act between the coating hopper and the backup roller or between the coating hopper and a web. A device for subjecting a web to a corona discharge treatment to cause an electrostatic field to act between a coating hopper and the web was disclosed in the published Japanese Patent Application (OPI) No. 167750/82 (the term &#34;OPI&#34; as used herein means an &#34;unexamined published application&#34;). 
     Such a method of applying an electrostatic field to act produces an effect which prevents an air film often made on a rapidly moved web in coating to the web from hindering the liquid from wetting the web. However, there has not been a patent application in which the quality and structure of the surface of a web are descibed with regard to a method of coating to the web by applying an electrostatic field to act thereto. 
     In order to increase the speed of coating of a liquid to a web without increasing the load of drying the coated layers, it is necessary to increase the concentration of a solute in the coated layer, for example, the concentration of gelatin which is often used as the protective binder of a photographic photosensitive material. However, there is a problem that if the web is provided with an undercoating layer of gelatin to augment the adhesive power between the web and the coated layer, the speed of coating the web can hardly be increased by a method in which an electrostatic field is caused to act to the web to coat the liquid thereto. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a method in which the speed of coating to a web is increased without increasing the load of drying of the coated layers. 
     In the method, an electrostatic field is applied to act on a continuously moving web having an undercoating layer of gelatin, to thus coat the web. The method is characterized in that a surface active agent is previously added to the undercoating layer of gelatin. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a side view indicating the concept of a coating method which is an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, as shown in FIG. 1, a web 6 having an undercoating layer of gelatin to which a surface active agent is previously added is wound on a backup roller 3 so that the web is continuously advanced. When a liquid 2 flowing out of a coating hopper 1 is applied to the web 6 on the side of the undercoating, a pressure reduction chhamber 5 performs application stabilization based on pressure reduction and a high DC voltage source 4 performs coating stabilization based on the action of the electrostatic field. Since the web 6 is provided with the undercoating layer of gelatin to which the surface active agent is previously added, the degree of affinity between the web 6 and the coated layer is enhanced to promote the stabilization based on the action of the electrostatic field. This makes it possible to increase the speed of the coating. 
     The hydrophobic group of the surface active agent is a straight-chain alkyl group or an alkyl group having a side chain or a hydrocarbonaceous functional group having a benzene ring or the like, but the hydrophobic group is not confined to them. The hydrophilic group of the surface active agent is an anionic function group of sodium carboxylate, sodium sulfonate or the like, a cationic or ampholytic functional group of an ammonium salt, a sulfonium salt or the like, or a nonionic functional group of polyethylene oxide or the like, but the hydrophilic group is not limited thereto. 
     It is preferable that the quantity of the surface active agent per unit area of the web is 1×10 -3  mol/m 2  to 1×10 -6  mol/m 2 . 
     The web is paper, a plastic film, a resin-coated paper, synthetic paper or the like. The plastic film is made of a polyolefin such as polyethylene and polypropylene, a vinyl polymer such as polyvinyl acetate, polyvinyl chloride and polystyrene, a polyamide such as 6,6-nylon and 6-nylon, a polyester such as polyethylene terephthalate and polyethylene-2,6-naphthalate, polycarbonate, a cellulose acetate such as cellulose triacetate and cellulose diacetate, or the like. The resin for the resin-coated paper is a polyolefin such as polyethylene but is not confined thereto. 
     The composition of the liquid varies according to the use thereof. For example, the liquid may be used to form a photosensitive emulsion layer, undercoating layer, protective layer, back layer or the like of a photographic photosensitive material, a magnetic layer, undercoating layer, lubricant layer, protective layer, back layer or the like of a magnetic recording medium, an adhesive layer, a coloring layer, an anti-rusting layer or the like. The coating composition contains a water-soluble binder or an organic binder. 
     The coating method is a slide coating method, a roller bead coating method, a spray coating method, an extrusive coating method, a curtain coating method or the like. 
     A coating method, which is an embodiment of the present invention, is hereafter described in detail with regard to an actual use. 
     A coating composition for photographic purposes was prepared by adding 1.5 g/l of sodium dodecylbenzenesulfonate as a surface active agent and a red dye to an aqueous solution of alkali-treated gelatin (whose weight concentration was 10%). The liquid was caused to flow out of a coating hopper so that the quantity of the coating composition per unit area of a web was 40 ml/m 2 . 
     The web was a polyethylene terephthalate film and had a thickness of 100 μm. To the web was undercoated a layer of alkali-treated gelatin to which sodium dodecylbenzenesulfonate was added as a surface active agent. The undercoated layer of alkali-treated gelatin had a thickness of 0.3 μm. The quantity of the sodium dodecylbenzenesulfonate per unit area of the web was 2.0×10 -5  mol/m 2 . 
     For comparisoN, three undercoated layers were provided on polyethylene terephthalate films as webs: (1) an undercoating layer of only alkali-treated gelatin; (2) an undercoating layer of alkali-treated gelatin to which a polyoxyethylene alcohol ether having ten ethylene oxide groups was added; and (3) an undercoated layer of alkali-treated gelatin to which a potassium carbonate salt relating to carbon fluoride was added. Each of the films had a thickness of 100 μm. Each of the undercoated layers had a thickness of 0.3 μm. The quantity of each of the added substances per unit area of the web was 2.0×10 -5  mol/m 2 , which was the same as the sodium dodecylbenzenesulfonate. 
     A high DC voltage of 1,000 V was impressed on the backup roller 3. 
     The pressure of a pressure reduction chamber 5 was set to be 30 mm H 2  O lower than the atmospheric pressure. 
     The device shown in FIG. 1 was used to coat to each of the webs under the above-described conditions. The application speed at which a phenomenon that an air film accompanying the moving web hinders the liquid from wetting the web took place was measured with regard to each of the undercoating layers. The results of the measurements are shown in Table 1. 
     
                                           TABLE 1                                 
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Voltage impressed                                                         
         Additive to undercoating layer of gelatin on web                 
on application Sodium dodecylbenzene-                                     
                           Polyoxyethylene                                
                                    Potassium carbonate salt              
roller   None  sulfonate   alcohol ether                                  
                                    relating to carbon fluoride           
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   0 V   130 m/min.                                                       
               117 m/min.  128 m/min.                                     
                                    95 m/min.                             
1,000 V  135 m/min.                                                       
               350 m/min.  245 m/min.                                     
                                    96 m/min.                             
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     As shown in Table 1, it was proven that the coating speed with the action of an electrostatic field thereto could be much increased by adding the hydrocarbonaceous surface active agent to the undercoating layer of gelatin on the web in accordance with the present invention. 
     The present invention is not confined to the above-described embodiment but may be embodied in other various ways without departing from the spirit or character of the invention. For instance, an electric field may be applied to the web by corona discharge.