It is known that, under certain conditions, stable diffuse glow discharges can be produced near or at atmospheric pressures. Articles that discuss stable glow discharges are listed in U.S. Pat. No. 5,558,843 issued Sep. 24, 1996 to Glocker et al, which is incorporated herein by reference. Work in this area has been limited and directed primarily at etching of photoresist and deposition of materials. These articles indicate that a reliable method of producing diffuse glow discharges at atmospheric pressure includes the use of helium gas. The work reported in the literature has been reproduced and found to be reliable.
As disclosed in the above-referenced patent, it was found that, by using mixtures of gases including helium, stable discharges can be produced at atmospheric pressure, which discharges are able to improve dramatically the wettability and adhesion of photographic emulsions to otherwise difficult-to-coat webs comprising thermoplastic polymeric materials such as polyethylene and polyesters such as polyethylene terephthalate, and polyethylene naphthalate.
It is known to glow-discharge treat plastic surfaces in substantial vacuum to improve both the wettability of the surface to aqueous coatings and the adhesion of such coatings to the surface when dried. Such treatment is performed at pressures in the range of 1 to 700 milliTorr in the presence of, for example, nitrogen and/or one or more of the noble gases (see Japanese Kokai Hei 71995!-166355 and Hei 71995!-166356). Improved wettability from vacuum glow discharge treatment, as measured by a decrease in the internal contact angle between a water drop and the plastic surface, can persist with little decay over a period of at least several weeks between treatment and aqueous coating. Improved adhesion can also remain virtually unchanged.
Glow discharge treating of webs in vacuum is most conveniently performed in a separate manufacturing step in a vacuum treatment machine. While operationally convenient, this method of treatment is very capital intensive and also adds substantially to the overall manufacturing cost of, for example, a photographic product using a thermoplastic resin web as the photographic film or paper base.
U.S. Pat. No. 5,558,843 issued Sep. 24, 1996, discloses method and apparatus for treating polyester web with glow discharge at atmospheric pressure, using gas mixtures including helium. The discovery is important because it permits treatment of conveyed rolls of web in an off-line machine at atmospheric pressure. Treatment is typically carried out on an apparatus wherein the web substrate is unwound from a stock roll, conveyed through a glow discharge region for treatment, and then wound into a treated stock roll. The roll may be placed in storage until needed for coating or it may be sent directly to a coating machine for application of layers to the treated surface.
At the time of the above discovery, it was believed that the effect produced on the web surface by the novel glow discharge treatment at atmospheric pressure was the same as that produced by previously-known glow discharge treatments at substantial sub-atmospheric pressures. The expected decrease in wettability contact angle and improvement in coated-layer adhesion supported this conclusion.
We now have found a shortcoming of such off-line treatment of webs, namely, a general decay in the improved adhesion effect (although, surprisingly, not in the wettability angle) as a function of elapsed time between glow discharge treatment at atmospheric pressure and coating. The present invention provides method and apparatus for avoiding this shortcoming.
The time period between treatment and coating is defined as the "latency" of the treatment. Since some treated rolls may be sent to storage while others are sent directly for coating, different rolls may have differing post-treatment histories and therefore differing latencies, which may result in roll-to-roll product variability for adhesion. Where the treatment effect decays rapidly, even the head and tail portions of an individual stock roll may exhibit substantially different adhesions.
Thus, a need exists for a process whereby all portions of all rolls being coated are provided with equal latency. Further, a need exists for a process whereby the latency is minimized, and preferably eliminated.
In the known manufacture of stock rolls of webs formed by melt cast extrusion of some thermoplastic resins, for example polyethylene terephthalate, the as-cast polymer ribbon typically is coated with a chloride-containing latex subbing layer, or primer, before lengthwise and widthwise stretching to achieve end use sheet dimensions. Typically, an additional gelatin-containing subbing layer must be applied to the latex sub before photographic layers can be coated. Omission of either the latex sub or the gel sub can result in adhesion failure of subsequently-coated photographic layers.
Thus, a need exists for a process to increase the coatability and adhesion properties of bare polymeric webs, especially webs containing a polyester polymer, sufficiently to permit omission of either or both of the subbing layers, at a substantial savings in manufacturing cost of coated product.
In the known manufacture of photographic paper including a cellulose-based web, the web typically is made water-proof, to minimize the uptake of processing chemicals during photographic development and fixing, by being impregnated on one or both sides with a bonded layer of a polyolefin, typically polyethylene. So-called "resin-coated" or "RC" paper as cast has unsatisfactory coatability and adhesion of aqueous formulations. Current practice is to treat the polyolefin surface with a corona discharge prior to the application of aqueous formulations.
This electrically intensive procedure can lead to irregular or non-uniform coatings and can cause serious damage to the photographic product if control is not tightly maintained.
Thus a need exists for a safe electrostatic process to increase the coatability and adhesion properties of polyolefin layers on resin-coated photographic papers.
It is a principal object of the invention to provide an improved apparatus and method for coating an aqueous formulation to a bare polymeric web substrate surface wherein the surface to be coated may be treated by glow discharge at atmospheric pressure shortly before coating.
It is a further object of the invention to provide an improved apparatus and method for coating an aqueous formulation to a bare polymeric web substrate surface wherein glow discharge treatment of the web is performed at atmospheric pressure in the web path on a coating machine ahead of the web coating application point.
It is a still further object of the invention to provide an improved apparatus and method for coating an aqueous formulation to a bare polymeric web substrate surface wherein glow discharge treatment of the web is performed at substantially atmospheric pressure (from 600 to 800 Torr) within two minutes prior to coating, and preferably within one second.
It is a still further object of the invention to provide an improved apparatus and method for atmospheric glow discharge treatment of successive stock rolls of a polymeric web substrate prior to coating whereby latency is constant and identical for all areas of all rolls.
It is a still further object of the invention to provide an improved apparatus and method for atmospheric glow discharge treatment of polymeric web substrates whereby one or more conventional latex and/or gelatin subbing layers on the web may be omitted.