Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-10-26
2003-02-11
Bareford, Katherine A. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S470000, C427S540000, C427S420000
Reexamination Certificate
active
06517909
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to methods and apparatus for coating a liquid composition onto a moving support web, and more particularly, to methods and apparatus for increasing the speed of coating application and for improving coating thickness uniformity of applied compositions in instances in which a high degree of uniformity is required.
BACKGROUND OF THE INVENTION
In the manufacture of many commercial products, such as photographic materials, a liquid composition is applied as a coating to a substrate. In many applications, and especially in imaging films and papers, the requirements for areal uniformity of coated thickness are highly demanding; thickness variations of less than 1% can be unacceptable for some products.
Known curtain 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. Such an applicator is generally positioned above the web at a distance of typically several centimeters, 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 individual layers of coating compositions.
It is well known that electrostatic field variations created by non-uniform surface charge on the moving web can create objectionable coating non-uniformities. For example, corona discharge treatment of plastic-coated paper to improve adhesion of emulsion to the paper also creates non-uniform charge patterns that cause coating disturbances such as crosslines or mottle. Means for eliminating such charge patterns are disclosed in U.S. Pat. No. 3,531,314 using charged rollers and in U.S. Pat. No. 3,729,648 using ionizers.
The moving web carries with it a boundary layer of air on the front side (the side to be coated) and the back side (the side facing the backing roller). To prevent upsets in the coating and resulting coated thickness nonuniformities, each boundary layer of air must be eliminated before or at the coating point. The elimination of boundary layers becomes more difficult as coating speed is increased.
In all coating systems, there is an upper speed limit for coating at which the boundary layer of air carried on the web surface to be coated 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 non-uniformity.
It is well known that electrostatic charging of a web and/or coating apparatus can be useful in increasing this limit on coating speed, such process being referred to herein as electrostatic assist. 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; U.S. Pat. Nos. 4,835,004; 5,122,386; 5,295,039; and European Patent Application No. 0 530 752 A1.
Apparatus and methods also have been proposed for maintaining a uniform charge on a web between the charging apparatus and the coating roller. See, for example, U.S. Pat. No. 4,835,004 and European Patent Application No. 0 530 752 A1 which propose to prevent degradation of charge uniformity by imposing strict environmental controls around the web.
It is also known to apply electrostatic charge at the coating point by electrifying the surface of the coating roller itself. See, for example, U.S. Pat. Nos. 3,335,026; 4,837,045; and 4,864,460.
All of these techniques can be useful in electrostatically assisting the coating of a composition to a web by providing an electrostatic field between the composition and the backing roller at the point of coating. Such an assist acts to cause the composition to be drawn more aggressively toward the backing roller and thus to more forcefully squeeze out the front side boundary layer of air, permitting thereby an increase in coating speed which can be economically beneficial.
As noted above, a moving web also carries a boundary layer of air on its back side or surface as does the backing roller surface prior to engagement with the web. For every conveyance system there exists a speed at which conveyance is limited by back surface air entrainment between the web and the conveying roller. It is known to provide means to remove or exhaust the boundary layers of air being carried on the back surface of a web and the surface of a roller when the two come into contact, increasing thereby the tractional contact of the web with the roller. Such means may include, for example, a pressure-loaded nip roller urged toward the conveying roller, the web passing therebetween. However, use of a nip roller may not be particularly desirable, as it adds mechanical complexity to the apparatus, and a face-side nip roller can mar the surface of the web to be coated and can cause electrostatic disturbance of either or both of the web surfaces, resulting in coating non-uniformities.
Such means may also include a relief pattern formed in the surface of the conveying roller into which the back-side boundary layer air may be exhausted from the web and escape. See U.S. Pat. No. 3,405,855 issued Oct. 15, 1968 to Daly et al., for example. In this patent, Daly et al. teach the use of a roller having peripheral venting grooves and supporting land areas to vent air carried by the underside of the traveling web. Typically, for example, approximately 10% to 40% of the roller surface consists of grooves 0.5 mm to 2.4 mm in depth, 0.5 mm to 2.3 mm in width, and arranged from 5 mm to 15 mm apart. Another example is provided by U.S. Pat. No. 4,426,757 issued Jan. 24, 1984 to Hourticolon, et al. In this patent, Hourticolon, et al. teach the manufacture and use of a roller having a surface relief consisting of a “finely branched network of compression chambers,” allowing the entrained air to be compressed into pockets rather than reducing the web traction. Both of these patents deal with purely conveyance roller issues and neither patent addresses the issue of electrostatic assist with such a roller surface pattern. A pattern on a backing roller creates a variable gap between the surface of the roller and the support being backed; that is, the gap to non-relieved areas of the roller surface is substantially zero, whereas the distance to the bottom of the relief may be up to 2.4 millimeter deep. This variable gap changes the capacitive relationship between the coating fluid and the backing roller, causing non-uniform electrostatic fields at the locus of the coating line. These electrostatic field variations can be comparable in magnitude to the previously described variations arising from non-uniform charge patterns created by corona discharge treatment and resulting in coating disturbances and coated thickness non-uniformities.
U.S. Ser. No. 09/185,045 teaches that the loss in electrostatic force over the relieved portions of the roller is less detrimental than the loss in electrostatic force caused by the intermittent lifting of the web from the backing roller while conveying at high coating speed (≧75 meters per minute or 125 centimeters per second) over a backing roll of large diameter (≧10 centimeters). Therefore, to prevent air entrainment between the web and the coating fluid, it is advantageous to use a relieved backing roller when using electrostatic assist at high coating speeds with a large backing roller.
For many coatings requiring only a modest level of uniformity, coating thickness variations on the order of
Billow Steven A.
Zaretsky Mark C.
Bareford Katherine A.
Bocchetti Mark G.
Eastman Kodak Company
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