Abstract:
A method for electrostatically assisting in coating a liquid composition from a hopper to a web supported by a backing roller, characterized by the steps of a) neutralizing all charges on both surfaces of the web by passing the web through an appropriate electrostatic environment ahead of the coating point, and b) providing a uniform electrostatic field around the backing roller, which field extends through the web to engender an electrostatic “pressure” for urging the liquid composition toward the coatable surface of the substrate at the coating point resulting in an increase in the maximum coating speed achievable without onset of air entrainment at the coating point, improved thickness uniformity of coating, and a reduction in residual charge on the web after coating.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a continuation-in-part of application Ser. No. 09/130,507, filed Aug. 6, 1998, entitled “IMPROVED COATING METHOD USING ELECTROSTATIC ASSIST” by Mark C. Zaretsky, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to methods for coating a liquid composition onto a moving support web, more particularly to methods for coating photographic emulsions onto a moving web of photographic support such as a plastic or resin-coated cellulose web, and most particularly to a method for increasing the speed of emulsion application and for improving the thickness uniformity of applied emulsions by controlling electrostatic charges on the web and coating apparatus. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of many commercial products, a liquid composition is applied as a coating to a receptor substrate. In many applications, and especially in photographic films and papers, the requirements for areal uniformity of coated thickness are highly demanding. Limitations on thickness variation of 1% or even less can be required. Density variations will form in the coating and when a uniform exposure is given to different density levels, streaks and non-uniform images are obtained. 
     Known coating apparatus typically includes a backing roller around which a web to be coated is wrapped and conveyed at a predetermined conveyance speed. A liquid composition is continuously delivered to and reshaped by an applicator, generally known as a hopper, from a jet flow at the applicator inlet into a broad ribbon of substantially uniform thickness at the applicator outlet from which it is dispensed onto the moving web. Typically, such an applicator is positioned either immediately adjacent to the moving web at a distance of typically less than 1 mm, a transverse, dynamic bead of composition being formed therebetween (bead coating), or above the web at a distance of typically several cm, the composition being allowed to fall as a curtain under gravity into continuous contact with the moving web (curtain coating). A liquid composition may be a single layer or a composite layer consisting of a plurality of coating compositions. 
     In all coating systems, there is an upper speed limit for coating at which the boundary layer of air carried on the surface of the web is no longer squeezed out by the advancing composition at the coating point but rather becomes entrained under the composition, disrupting the uniform application thereof to the web and resulting in unacceptable coating uniformity. 
     It is well known that electrostatic charging of a coating apparatus by electrifying the surface of the coating roller can be useful in increasing the upper limit of coating speed. See, for example, U.S. Pat. Nos. 3,335,026 issued Aug. 8, 1967; 4,837,045 issued Jun. 6, 1989; and U.S. Pat. No. 4,864,460 issued Sep. 5, 1989. However, the web can have random charge patterns created prior to the location of the ionizers, due to the unwinding and conveyance process as well as corona discharge treatment of the web. As charge nonuniformities on the web when entering upon the charged coating roller are not neutralized, or smoothed, by the charged roller but simply added to the electrostatic field imparted by the roller, the areal charge nonuniformity can result in a corresponding coating nonuniformity. 
     Methods and apparatus have been proposed to enhance coatability by removal of charge nonuniformities from both surfaces of a web by neutralizing charges on the web ahead of the coating roller. See U.S. Pat. Nos. 3,470,417 issued Sep. 30, 1969; U.S. Pat. No. 3,531,314 issued Sep. 29, 1970; U.S. Pat. No. 3,730,753 issued May 1, 1973; and 5,432,454 issued July 11, 1995. Such proposals avoid the problem of web charge nonuniformities created by processes such as corona discharge treatment but do not deal with the problem of providing an electrostatic assist to enhance web coatability and increase coating speed. 
     It is also well known that electrostatic charging of a web can be useful in increasing the upper limit of coating speed. For example, a dielectric web carrying a bound polar charge between opposite surfaces thereof can exhibit increased “wettability” and a consequent increase in acceptable coating speed when conveyed around a grounded coating roller. Means for applying such a charge to a web ahead of the coating point are disclosed, for example, in European Patent No EP 390774 issued Jul. 15, 1992; U.S. Pat. No. 4,835,004 issued May 30, 1989, U.S. Pat. No. 5,122,386 issued Jun. 16, 1992, U.S. Pat. No. 5,295,039 issued Mar. 15, 1994; and European Patent Application No. 0 530 752 A1 published Mar. 3, 1993. 
     Serious problems can arise in using electrostatic assist for coating in processes wherein the web is charged ahead of the coating point. For example, it can be difficult to apply the charge uniformly over the web. Ionizers must be rigorously maintained, and charging webs at high speeds can require prohibitively large and expensive installations. Apparatus and methods have been proposed for correcting the charge nonuniformity that can occur during the charge application process. See, for example, U.S. Pat. No. 4,835,004 and European Patent No. 0 530 752 A1 which propose to control charge uniformity by imposing strict environmental controls around the web. Such controls can be expensive to install and operate and also may be only marginally effective as heat and humidity are used to aid in the electrostatic assist by smoothing the charges and not removing them. This environmental control should not be required. Environmental control, such as heating the web, relies on changes in physical and electrical properties as the web. These changes can limit the choice of webs and/or sorting on these webs. 
     Further, even when charge has been applied uniformly, the uniformity can be compromised by any of various well known contacts or exposures between the charge application point and the coating point. It has been observed that a significant loss in charge from a charged web surface can occur upon contact with conveyance rollers that typically are conductive and electrically grounded. These rollers may have a surface pattern such as a series of circumferential grooves to provide traction. The charge loss experienced by a charged web surface when conveyed over these rollers occurs in a manner corresponding to the surface pattern. Areal variations in charge on the web when it reaches the coating point typically result in variations in layer thickness and consequent density nonuniformity also corresponding to the surface pattern. None of the prior art discusses this charge loss issue between the charge application point and the coating point. 
     Further, charge remaining on the web after coating can be a shock hazard to operators and can be a marking or fogging hazard to light-sensitized product later in coating and in finishing. 
     Thus there is a need for a method for coating a liquid composition to a moving web at high speed whereby the web is rendered substantially discharged ahead of the coating point, and whereby the coating bead or curtain is subjected to a highly uniform electrostatic field widthwise of the web at the point of coating. 
     SUMMARY OF THE INVENTION 
     It is a principal object of the invention to provide an improved web coating method whereby a predetermined, uniform electrostatic charge on a coating roller assists in providing a coating having excellent thickness uniformity. 
     It is a further object of the invention to provide an improved web coating method whereby webs may be coated to an excellent level of uniformity at increased coating speeds. 
     It is a still further object of the invention to provide an improved, more operationally robust web coating method which is more tolerant of other operational variability. 
     It is a still further object of the invention to provide an improved web coating method whereby minimal charge remains on a web after coating. 
     The apparatus and method of the present invention are useful in providing coated substrates having a high level of coated layer uniformity, manufactured at higher substrate coating speeds than would be possible without the invention. 
     Briefly described, the present invention includes a method for electrostatically assisting in coating, characterized by the steps of a) preparing the web by neutralizing all charges on both surfaces of the web ahead of the coating point by passing the web through an appropriate electrostatic environment, and b) providing a uniform electric charge over the surface of the backing roller at the coating station to create a uniform electrostatic field around the backing roller, which field extends through the web to engender an electrostatic “pressure” urging the liquid composition toward the coatable surface of the substrate at the coating point. 
     In a preferred method and apparatus in accordance with the invention, a substantially dielectric web to be coated with a liquid coating such as a gelatin-based aqueous emulsion, for example a web formed from polyethylene terephthalate, is first passed through means for dissipating all surface charges on the web. Preferably such means is disposed in the web conveyance path of a coating machine a short distance ahead of the point of entrance of the web onto the coating backing roller. An example of a suitable means for dissipating charges is an ionizer similar to that disclosed in U.S. Pat. No. 3,730,753 issued May 1, 1973 to Kerr, hereby incorporated by reference, wherein the web is exposed sequentially to one or more high positive charges and high negative charges to “flood” pre-existing charge variations on the web and is then discharged. Preferably, the web is also conditioned for coating by removal of residual free charge by treatment, for example, in accordance with the disclosure of U.S. Pat. No. 5,432,454, hereby incorporated by reference. 
     After being electrically neutralized, the web is entered onto an electrically-isolated backing roller at a coating station wherein a coating hopper provides a ribbon of liquid composition for coating. The hopper is maintained at ground potential, and the roller is maintained at a predetermined DC potential, either positive or negative, with respect to ground, creating an electrostatic field around the roller. The electrostatic field exerts an electrostatic force which acts through the web to draw the emulsion against the web. The practical result is enhanced apparent “wettability” of the web surface and an increase in the maximum coating speed achievable without onset of air entrainment at the coating point. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objectives, features, and advantages of the invention will be apparent from the following more particular description, including the presently preferred embodiment of the invention, as illustrated in the accompanying drawings in which: 
     FIG. 1 is a schematic cross-sectional view of an apparatus for discharging a web and electrifying the coating backing roller prior to bead coating of the web in accordance with the invention; 
     FIG. 2 is a schematic view like that in FIG. 1, shown for curtain coating of the web; and 
     FIG. 3 is a schematic view like that in FIG. 2, showing the coating hopper as being electrified and the coating backing roller as being grounded. 
    
    
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following detailed description and appended claims in connection with the preceding drawings and description of some aspects of the invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a first embodiment  10  of an electrostatic coating assist apparatus for coating a liquid composition to a web which can be coated with a non-light sensitive coating or uncoated in accordance with the invention includes a web charge-elimination section  12  and an electrified coating section  14  for bead coating of the web. 
     A continuous web  16  having first and second surfaces  18 , 20 , is supplied to section  12  from a conventional unwinding and conveyance apparatus (not shown) and may be conveyed conventionally through the apparatus on generic rollers  17 . Web  16  may be formed of any substantially non-conductive material including, but not limited to, 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, and polyethylene-2 and -6 naphthalate; polycarbonate; and cellulose acetates such as cellulose diacetate and cellulose triacetate. The web may carry one or more coats of subbing material on one or both surfaces. The resin employed for resin-coated paper is typically a polyolefin such as polyethylene. 
     Web  16  may have patches of electrostatic charges disposed randomly over one or both surfaces  18 , 20 . In Section  12 , charges on the web are neutralized. Various methods and apparatus known in the art, including but not limited to those disclosed in the patents recited hereinabove, may be suitable for charge removal in accordance with the invention such as the use of DC ionizers, AC ionizers or conductive brushes. 
     In a presently preferred embodiment, web  16  is wrapped and conveyed around a grounded, conductive backing roller  22  with web surface  20  in intimate contact with the conductive surface  23  of roller  22 . Web surface  18  is exposed to negatively charged electrodes  24 , 26  which “flood” a large amount of negative charges onto surface  18 . Electrodes  24 , 26  may be electrically connected to the negative terminal of an adjustable 0 to −20 kV, 0 to −15 mA source  28  of DC potential. Grounded roller  22  acts as a counter electrode for electrodes  24 , 26 . 
     One or a set of electrodes can be electrically connected to a negative terminal and the other electrode or set of electrodes can be electrically connected to a positive terminal. 
     As web  16  is advanced along roller  22 , it moves beneath electrodes  30 , 32  which may be electrically connected to the positive terminal of a DC potential source  33  similar to source  28 . Electrodes  30 , 32  deposit a large amount of positive charges onto web surface  18  which can be used to neutralize the negative charges previously imparted to this surface by electrodes  24 , 26 . Grounded roller  22  at this point finctions as a counter electrode for electrodes  30 , 32 . 
     In the preferred embodiment, web  16  is conveyed about grounded roller  52  so that web surface  20  is in intimate contact with roller  52 , the opposing web surface  18  being exposed to an induction probe  53  (such as a non-contacting electrostatic voltmeter) of a feedback control system comprising probe  53  and controller  56 , which controller is responsive to the level of charge sensed by probe  53  and automatically adjusts the level of charge applied by DC source  33  to electrodes  30 , 32  to control to a desired set-point. This setpoint may be set to zero so as to minimize the steady-state charge on surface  18 , or to a non-zero level so as to provide some electrostatic assist for the coating process. 
     The just-described electrostatic web treatment typically is sufficient to completely control the charge distribution on surface  18  of the web and some of the charge distribution on surface  20 . However, some webs may retain an unacceptable level of residual charge on surface  20  which also must be removed. 
     As shown in FIGS. 1 and 2, after leaving roller  22 , web  16  may be passed by two fixed voltage or fixed DC current ionizers  34 , 36  which are mounted near and facing surface  20  of web  16  on a free span of travel. The ionizers  34 , 36  are mounted so that the central axis of each ionizer lies parallel to the web in the transverse direction of the web. Each ionizer is electrically connected to a separate DC high voltage power supply  38 , 40 . A conductive plate  42  which is electrically isolated from ground is positioned opposite ionizers  34 , 36  and facing surface  18  of web  16 . Plate  42  can be of various shapes, designs, constructions, or materials, including both solid materials and screens, but plate  42  should incorporate at least a layer of conductive material that can act as an equipotential surface to attract charge from ionizers  34 , 36 . A controllable bipolar high voltage source  44  is electrically coupled to plate  42  to deliver voltage to the plate over a wide range of positive and negative voltages. A feedback control system  46  may have a sensor  48  (such as a non-contacting electrostatic fieldmeter) or sensor array responsive to the mean charge density residual on the web after treatment by the ionizers. Source  44  may be adjusted manually to adjust the voltage level on plate  42  so that the plate voltage increases in the same polarity as a direct function of the residual charge density on the web. Preferably, such adjustment is controlled automatically by an electronic controller  50  to minimize the steady-state residual charge on surface  20 , preferably near or at zero. 
     As shown in FIGS. 1 and 2, in section  14  an electrically-isolated coating backing roller  54  is electrically connected to a high voltage DC source  55  to place a high potential on the surface of backing roller  54 , for example, from 0.1 kV to 5 kV and preferably 1 kV, creating a standing electric field around roller  54 . Coating hopper  58  (which may be a bead coating hopper as  58   a  in FIG. 1 or a curtain coating hopper as  58   b  in FIG. 2 or an extrusion coating hopper, as is well known in the art) is electrically grounded. Air pressure within the bead coating hopper can be varied with the use of a vacuum trough  59 . Web  16  is entered upon and wrapped partially around roller  54 . The angle of wrap which may be from 45° to 200° includes coating point  60 . 
     An identical electrostatic attractive force at the coating point may be generated by exchanging the roles of the coating roller and hopper, as shown in FIG. 3, such that the roller is grounded and the hopper is electrified. Because photographic compositions typically are electrically conductive, in such a configuration the entire delivery system must be electrically isolated to maintain the desired potential at the coating point. Further, there is increased risk of electric shock to operating personnel, and of fogging of product from inadvertent discharges. Therefore, in the preferred embodiment the hopper is grounded and the coating roller is electrified. 
     A further benefit of coatings in accordance with the invention is that little or no net charge is applied to the web after the charge-elimination section, and thus the coated web carries substantially little or no charge on either side of the web upon leaving the coating station. 
     The following example shows unexpected beneficial results obtained when a web to be coated is first completely neutralized and then a coating applied using electrostatic assist as supplied by a voltage on the coating roller as opposed to part or all of the electrostatic assist supplied by depositing charge on the web to be coated. 
     EXAMPLE 
     An aqueous composition was bead coated to a web of polyethylene coated paper 0.25 mm thick being conveyed at a speed of 1.5 m/s on a backing roller with a diameter of 10 cm. Web conveyance rollers  17  had relieved surfaces consisting of circumferential grooves with a nominal groove depth (in the radial dimension) of 0.05 mm, a nominal width of 0.43 mm and a nominal pitch of 1 groove/mm. The aqueous composition consisted of: 1) a bottom layer having a viscosity of about 23 cP and a wet laydown of about 30 g/m 2 , containing about 5% gelatin and surfactants, as well as carbon black, and 2) a top layer having a viscosity of about 60 cP and a wet laydown of about 26 g/m 2 , containing about 13.5% gelatin and surfactants. The application hopper coating hopper was spaced about 125 μm from the web and the level of air pressure in the vacuum trough was nominally 200 Pa. 
     The charge on the web, as controlled by charge-elimination section  12 , was varied between levels of 0, 280 and 670 volts. Similarly, the level of voltage imposed by source  55  on coating roller  54  was varied between 0, 280 and 670 volts. The coating uniformity was examined visually for the appearance of groove lines in the coating corresponding to the 1 groove/mm pattern on the web conveyance rollers. The coating uniformity was also captured digitally and a power spectrum analysis was performed. The power spectrum rms density value at a spatial frequency of 1 cycle/mm, indicative of the coating non-uniformity, was recorded. The results are tabulated below. 
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                   
                   
                 Power Spectrum 
               
               
                   
                 Charge on 
                 Voltage 
                   
                 @ 1 cycle/mm 
               
               
                 Trial 
                 Web 
                 on Roller 
                 Visual Observation 
                 (rms density) 
               
               
                   
               
             
             
               
                 1 
                  0 V 
                  0 V 
                 No coating 
                 — 
               
               
                 2 
                 680 V 
                  0 V 
                 Strong groove lines 
                 1.2E-3 
               
               
                 3 
                 280 V 
                 280 V 
                 Moderate groove lines 
                 5.5E-4 
               
               
                 4 
                  0 V 
                 680 V 
                 No groove lines 
                 1.5E-4 
               
               
                   
               
             
          
         
       
     
     In trial 1, with no electrostatic assist provided, no stable coating was obtained. In trials 2, 3 and 4, with electrostatic assist provided, a stable coating was formed, demonstrating the benefit of this process with respect to a more operationally robust web coating method In trial 2, with all the electrostatic assist provided by depositing charge on the web upstream of the coating roller, a significant loss of charge in a pattern-wise fashion corresponding to the grooves on the web conveyance rollers and due to face side contact of the charged web with these rollers, has resulted in a significant coating non-uniformity. This is observed visually and confirmed by the large rms density in the power spectrum. In trial 3 this charge loss and concommitant coating non-uniformity has again occurred, though to a lesser extent due to less charge being initially deposited upon the web and more of the electrostatic assist being provided by the voltage on the coating roller. Note that the power spectrum rms density is roughly one-half the level of trial 2. In trial 4, with all the electrostatic assist provided by voltage on the coating roller and the web charge only being neutralized by charge-elimination section  12 , no coating non-uniformity is observed. Note that the power spectrum rms density is an order of magnitude lower than in trial 1 and is comparable to the background noise level for rms density at other spatial frequencies. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 10 
                 first embodiment apparatus 
               
               
                 12 
                 charge-elimination section 
               
               
                 14 
                 electrified coating section 
               
               
                 16 
                 continuous web 
               
               
                 17 
                 web conveyance rollers 
               
               
                 18 
                 first web surface 
               
               
                 20 
                 second web surface 
               
               
                 22 
                 conductive backing roller in 12 
               
               
                 23 
                 conductive surface of 22 
               
               
                 24 
                 first negative electrode 
               
               
                 26 
                 second negative electrode 
               
               
                 28 
                 DC source to drive 24,26 
               
               
                 30 
                 first positive electrode 
               
               
                 32 
                 second positive electrode 
               
               
                 33 
                 DC source to drive 30,32 
               
               
                 34 
                 first DC ionizer 
               
               
                 36 
                 second DC ionizer 
               
               
                 38 
                 power supply for 34 
               
               
                 40 
                 power supply for 36 
               
               
                 42 
                 conductive plate 
               
               
                 44 
                 bipolar high voltage source 
               
               
                 46 
                 feedback control system 
               
               
                 48 
                 sensor 
               
               
                 50 
                 electronic controller 
               
               
                 52 
                 grounded roller 
               
               
                 53 
                 induction probe 
               
               
                 54 
                 coating backing roller 
               
               
                 55 
                 high voltage DC source 
               
               
                 56 
                 controller 
               
               
                 58 
                 coating hopper 
               
               
                 59 
                 vacuum trough 
               
               
                 60 
                 coating point 
               
               
                 62 
                 curtain of coating composition