Coating apparatus – Immersion or work-confined pool type – Work-confined pool
Reexamination Certificate
1998-12-17
2002-02-26
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Immersion or work-confined pool type
Work-confined pool
C118SDIG004
Reexamination Certificate
active
06350318
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to apparatus for coating a liquid composition onto a substrate surface to form a coating thereupon and a method for determining the physical parameters of and making said hopper and, more particularly to a hopper for coating one or more liquid compositions from a plurality of distribution slots onto a substrate surface to form a coating, and most particularly to a multiple-slide hopper having a plurality of internal distributional cavities, internal slots, distribution slots, slide surfaces, and offsets between slide surfaces wherein the shapes of and relationships among these elements are optimized to permit transversely-uniform coatings of compositions having a wide range of Newtonian and shear-thinning Theological properties.
BACKGROUND OF THE INVENTION
In forming a flowing sheet of a liquid composition for coating onto a substrate surface, the composition is reshaped typically from collimated flow in a pipe to sheet flow for application by an apparatus known variously in the art as a die, a distributor, an extruder, and a hopper. As used herein, all such types of apparatus are referred to collectively as hoppers. A hopper may comprise one or more parallel longitudinal members which are oriented transversely of the direction of liquid flow, which members may be bolted together or otherwise attached to form a hopper unit. A primary member may be referred to as a “hopper body,” and one or more secondary members as“hopper bars.” Within a hopper, a flow path for liquid composition typically includes an inlet, one or more transverse distributional voids known as cavities or channels, and a slotted exit from each cavity communicating with either a successive cavity or the exterior of the hopper. The last such slot in the flow path is commonly known as an exit slot.
In extrusion/slide hoppers, as are used typically in the manufacture of photographic films and papers, composition is extruded upwards from the exit slot onto an inclined slide surface terminating at a lower edge in a coating lip. The extruded sheet flows down the slide surface under gravity and is transferred to the passing substrate either through a dynamic longitudinal bead, as in bead coating, or a falling curtain, as in curtain coating.
It is well known that a hopper unit may combine a plurality of individual distribution systems to permit the simultaneous application of a plurality of compositions and/or the split flows of a single composition having an exceptionally high flow rate (see, for example, U.S. Pat. No. 5,143,758 issued Sep. 1, 1992 to Devine). Such a hopper is known in the art as a multiple-slot hopper. The full stack of compositions to be coated is assembled as the liquids flowing from the exit slots of the individual distribution systems progressively become stacked on the hopper slides, each additional liquid sheet becoming the bottom-layer conveyance for the already-assembled stack of sheets sliding onto it from higher on the inclined hopper surface.
It is known that each slide surface is preferably offset vertically from (generally slightly lower than) the surface of the next slide farther from the substrate, to accommodate the new flow being added to the bottom of the stack. If an offset is too small or too large, the layers may not join smoothly; in particular, eddies and bubble traps may form that can result in nonuniformly coated layers. In the extreme, the sliding layers can form stable longitudinal ridges at an offset that is too high. Means for determining the optimum offset to accommodate a wide range of Newtonian and non-Newtonian rheologies has not heretofore been disclosed.
The slide surface of an extrusion/slide hopper terminates immediately above the uppermost slot in a wall, or “backland,” for attachment of the free upper surface of the coating pack. It is known that if the backland is too high, attachment may occur irregularly along the vertical face of the wall and cause streaks; moreover, the upwardly curved meniscus traps bubbles. If the backland is too low, such attachment occurs irregularly along the hopper surface above the backland. A proper height backland permits the upper layer to attach uniformly at the well-defined upper edge of the backland while minimizing the risk of trapping bubbles. Typically, backlands may be about 2.5 mm high; however, means for determining the optimum backland height have not heretofore been disclosed.
It is standard practice in the coating art to fill all the slots in a hopper in use. Thus, for example, if a four-composition coating is to follow a five-composition coating using a five-slot hopper, either a four-slot hopper must be substituted between coatings or one of the compositions in the second coating must be split and delivered through two adjacent slots (See U.S. Pat. No. 5,143,758 supra). The former alternative requires the building of two entirely separate, expensive hopper units. In a large manufacturing practice, a large fleet of hoppers having different numbers of delivery slots may be required, which can be very expensive to fabricate and maintain. The latter alternative can require undesirably low flow rates through some slots or undesirable dilution of layers to artificially increase flow rates.
An alternative, not successfully practiced heretofore, is to use only the number of slots that is optimal for the compositions to be coated. In the above example, the fifth slot of the five-slot hopper could be left empty for the second coating if advantageous. A serious practical problem arises in so doing, however, as the fifth/fourth slide offset must now function as the backland, and typically such offset is substantially smaller than that conventionally used for a backland. Such a hopper, therefore, is not properly versatile.
Means for determining an acceptable height for an offset which can function either as a flow offset within a coating stack or as a backland at the top of a hopper slide, to increase the versatility of a hopper, has not heretofore been disclosed.
Distribution arrangements for each flow within a hopper typically include a flared central inlet (see, for example, U.S. Pat. No. 5,256,052 issued Oct. 26, 1993 to Cloeren) connecting a feed pipe to the center of a generally bilaterally-symmetrical first distributional cavity disposed transversely of the required sheet flow and web conveyance direction. A first slot connects the first cavity with a second cavity generally parallel with the first cavity. A second slot connects the second cavity with the slide surface of the hopper. Such a hopper may be referred to in the art as a “dual cavity” hopper having a primary cavity and slot and a secondary cavity and slot.
The functions of these hopper flow elements are as follows. The inlet is flared downstream where it joins the first cavity to begin the conversion of composition flow from flow in a conduit to cavity flow transversely of the direction of coating. The first or primary cavity provides the initial and principal widthwise distribution of composition. Because velocity is lost as the liquid flows along a cavity of constant cross-sectional area, the primary cavity preferably is tapered in continuously-declining cross-sectional area between the center and the two ends (See, for example, U.S. Pat. No. 4,285,655 issued Aug. 25, 1981 to Matsubara; U.S. Pat. No. 5,234,649 issued Aug. 10, 1993 to Cloeren; and U.S. Pat. No. 5,494,429 issued Feb. 27, 1996 to Wilson et al). The cross-sectional shape may be generally circular, generally rectangular, or a combination of the two (See, for example, U.S. Pat. No. 4,222,343 issued Sep. 16, 1980 to Zimmermann et al.; U.S. Pat. No. 5,256,052 issued Oct. 26, 1993 to Cloeren; U.S. Pat. No. 5,593,734 issued Jan. 14, 1997 to Yuan et al.; and U.S. Pat. No. 5,643,363 issued Jul. 1, 1997 to Hosogaya et al).
The slot adjoining the primary cavity, the primary slot, is small enough in height, typically about 0.025 cm or less, to create a substantial back pressure in the primary cavity. The length of the slot between the cavities may be cons
Ruschak Kenneth J.
Suter Daniel J.
Weinstein Steven J.
Bocchetti Mark G.
Eastman Kodak Company
Lamb Brenda A.
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