Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2000-12-21
2003-01-28
Bareford, Katherine A. (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S470000, C427S540000, C427S402000, C427S420000
Reexamination Certificate
active
06511711
ABSTRACT:
FIELD OF THE INVENTION
The invention relates generally to the field of coating a liquid composition onto the surface of a moving web, and, more particularly, to a method for coating a shear-thinning liquid composition onto the surface of a moving web while using an electrostatic field to assist the coating operation.
BACKGROUND OF THE INVENTION
In all liquid coating systems, there is an upper speed limit, or critical speed, for coating at which the boundary layer of air carried on the substrate surface to be coated is no longer squeezed out at the coating point but rather becomes entrained under the impinging liquid composition. This is typically referred to as air entrainment. Air entrainment can disrupt the uniform application of composition to the web substrate and can result in unacceptable uniformity of coating. Air entrainment is a gross failure and can occur in all methods for coating moving web. Air entrainment occurs predominantly at high coating speeds.
It is known to those skilled in the art that an electrostatic force of attraction between the coating liquid and the surface of the web to be coated can be used to increase coating speed. This is typically referred to as a coating operation with an electrostatic assist. With an electrostatic assist the web and/or the coating apparatus is electrostatically charged to generate an electrostatic force of attraction between the coating liquid and the surface of the web to be coated. For example, a dielectric web carrying a net voltage bias on the surface can exhibit increased apparent 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 Nos. EP 0 390 774 B1 and EP 0 530 752 B1 and U.S. Pat. Nos. 4,835,004; 5,122,386; and 5,295,039.
In bead coating operations there are at least three other coating gross failures that may be encountered in addition to Air Entrainment. “Breaklines” (also known as low flow limit) are a running phenomenon in which a uniform, stable bead cannot be maintained, and the bead degenerates into an array of individual cells with gaps in between. This results in a crossweb array of regions of heavy coating interspersed with regions of no coating. “Pull-through” is a running phenomenon in which a portion of the composition in the bead is stripped from the underside of the bead by the stabilizing suction or vacuum pressure and is pulled down the suction drain, resulting in varying areas of thin or blotchy coating. Broad, irregularly-spaced streaks may appear in the coating at vacuum pressure levels below the vacuum pressure at which a portion of the coating composition is pulled down the drain. These streaks, which mark the onset of pull-through (also known as weeping or bleeding), are often labeled as Pull-Through, but they are also known in the art as “High Suction Streaks” (referred to herein as HSS). “Rakelines” (also known as ribbing) are a running phenomenon in which the bead assumes a regular array of alternately thick and thin areas, typically between 200 to 400 cycles per meter, resulting in a crossweb array of areas of heavy coating interspersed with areas of light coating. As observed in the process of discovering this invention, the application of electrostatic assist, while decreasing coating tendency toward Air Entrainment and Breaklines, can actually make some coatings more susceptible to Rakelines.
If the coating parameters are such that Breaklines and/or Air Entrainment may occur, then they will occur at relatively low levels of suction (vacuum pressure) and will often occur simultaneously. If the coating parameters are such that Pull-Through and/or Rakelines may occur, then they will occur at relatively high levels of suction (vacuum pressure) and may occur simultaneously. We define the “Coating Window” in terms of the range of vacuum pressure level at which “acceptable coating” can be performed. The term “acceptable coating” as used herein is intended to mean coating free of any and all gross failures defined above. The lower boundary of a coating window is the coating vacuum pressure level below which Breaklines or Air Entrainment occur. The upper boundary of a coating window is the coating vacuum pressure level above which Rakelines, high suction streaks, or Pull-Through occur. The “coating window” represents a commonly accepted measure of bead coating performance.
“Suction Noise Sensitivity” (referred to herein as Sensitivity) is the degree to which a given amplitude of suction variation or noise will modulate the thickness of the coating in the direction of the moving web. Sensitivity is also a common measure of bead coating performance, but it is less important than the Coating Window because the degree of non-uniformity induced by suction noise is usually less significant than the non-uniformity caused whenever a gross failure occurs. Sensitivity is dependent on many coating parameters, particularly the thickness of the layer(s), especially the bottom layer thickness, where a thinner bottom layer is more sensitive. Therefore, the uniformity requirements of the bottom layer may limit the thickness of the bottom layer to be greater than some minimum thickness.
It is known in the art that increasing the viscosity of compositions for coating can improve coating uniformity by increasing resistance to layer deformation by air currents both on the hopper slide and after application to a substrate. Preferably, for gelatin-based compositions, an increase in viscosity is achieved by reducing the amount of water in the composition. However, if the coating thickness is at a minimum due to sensitivity constraints, then increasing the amount of gelatin in the composition will increase the viscosity, but it is desirable to keep the gelatin fraction low (typically less than about 4%) to avoid premature reaction with other ingredients, like crosslinking agents. It is also known, however, that the bottom layer in a bead coating process, whether a single layer or the bottom layer of a multiple-layer pack, must exhibit a relatively low apparent viscosity, i.e. less than 10 centipoises and preferably less than 5 centipoises, at the point of dynamic wetting where the liquid composition first contacts the substrate surface. These requirements, that is, high viscosity under low shear conditions and low viscosity under high shear conditions, as well keeping the gelatin fraction low, may be met by formulating the bottom composition to be pseudoplastic, or non-Newtonian, by including in it an amount of a shear-thinning thickening agent. See, for example, U.S. Pat. No. 4,113,903 issued Sep. 12, 1978 to Choinski, the relevant disclosure of which is hereby incorporated by reference. Such agents are well known in the art of coating compositions, and may include, but are not limited to, sodium cellulose sulfate and other salts of cellulose; copolymers of methyl vinyl ether and maleic anhydride; salts of polyvinyl hydrogen phthalate; polystyrene sulfuric acid; sodium poly(styrenesulfonate); and sulfonated vinyltoluene polymers. It is known in the art that removing gelatin and adding shear-thinning thickening agents can increase the maximum coating speed permissible without Air Entrainment. However, chemical incompatibilities with some layer ingredients may prevent the use of those shear-thinning thickening agents that meet the preferred requirement of 5 centipoises at 10,000 sec
−1
, thus limiting the speed permissible without Air Entrainment and increasing the potential for Breaklines.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved coating method whereby coatings having improved thickness uniformity may be made.
It is a further object of the invention to provide an improved coating method whereby the coating speed for uniform coatings may be increased.
It is a still further object of the invention to provide an improved coating method whereby the size of the Coating Window is incr
Finnicum Douglas S.
Gros Alain E.
Joos Felipe M.
Quiel Robert R.
Bareford Katherine A.
Bocchetti Mark G.
Eastman Kodak Company
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