Coating apparatus – Solid applicator contacting work – With cleaner or conditioner for applicator
Reexamination Certificate
1998-07-13
2001-04-10
Lamb, Brenda A. (Department: 1734)
Coating apparatus
Solid applicator contacting work
With cleaner or conditioner for applicator
C118S410000, C118S325000, C118SDIG002, C118SDIG003
Reexamination Certificate
active
06214111
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for coating liquids containing volatile solvents, and in particular, to a spraying system and method for maintaining a uniform wetting line of the coating bead.
BACKGROUND OF THE INVENTION
The production of high quality articles, particularly photographic, photothermographic, and thermographic articles, consists of applying a thin film of a coating solution on to a continuously moving substrate or web. Thin films can be applied using a variety of techniques including: dip coating, forward and reverse roll coating, wire wound rod coating, blade coating, slot coating, slide coating, and curtain coating. Coatings can be applied as a single layer or as two or more superimposed layers. Although it is usually most convenient for the substrate to be in the form of a continuous web, it may also be formed of a succession of discreet sheets.
Slide coaters have been used extensively since the 1950's in the photographic and related industries for coating aqueous photographic emulsions with relatively low viscosity (less than 100 cP). In slide coating, it is well known to start and stop coating of a moving web by means known as “pick-up.” In the pick-up phase, the flow of the coating liquid is established with the coater die retracted from the web. The coating liquid drains over the die edge into a vacuum box and drain. Once the flows of all the coating liquids are stabilized from all the feed slots of the slide coating die, the die and vacuum box are moved into the coating position in a rapid manner with the web moving at the desired coating speed. When the die is in close proximity to the moving web, the coating liquid forms a coating bead that coats the web rather than draining over the die edge. If the coating process needs to be interrupted (for example, as a web splice is passing in the slide coating die), the die and vacuum box assembly are simply retracted from the web until resumption of the coating is desired.
Streak-type defects can be formed by disturbances of the coating bead. Mechanical disturbances include nicks in the die edge. Contamination disturbances that may cause streaking include dirt particles lodged near the coating bead, dried or semi-dried particles of coating compound, and non-uniform wetting of the contact line of the coating liquid on the coating die edge. Non-uniform wetting on the die edge, especially after pick-up, appears to be an important factor when coating fluids containing volatile solvents. For example, contamination may adhere to the front face and/or die edge of the slide coating die. That contamination may lead to a non-uniform wetting line and possible streaking of the coating compound.
FIG. 1
illustrates an exemplary slide coating die
20
in which a coating fluid
22
is flowing along a slide surface
24
to a die edge
26
. A static wetting line
28
is formed along a front face
30
of the slide coating die
20
. The irregular shape of the static wetting line
28
is likely to cause unevenness and streaking of the coating fluid as it is applied to the moving web (not shown).
Another problem related to slide coating is contamination of vacuum ports and drains in the vacuum box when the die is retracted from the moving web and the coating liquid is flowing freely. Contamination of the vacuum ports and drains can lead to unstable vacuum operation causing defects and eventually requiring cessation of the coating operation to clean the vacuum box and ports. This problem is exacerbated with high viscosity fluids 100-10,000 cP) that contain volatile solvents that dry much faster than water (such as methyl ethyl ketone, tetrahydrofuran, or methanol).
SUMMARY OF THE INVENTION
The present invention relates to a method and apparatus for coating a moving web with a coating fluid. The present invention is also directed to a spraying system for spraying the die edge region of a coating die with a cleaning fluid for a short duration subsequent to pick-up of the coating fluid onto the moving web.
The present spraying system cleans the front face of the coating die below the die edge so that a uniform wetting line of the coating bead is established. Additionally, a continuous low flow of cleaning fluid from the spray system may be maintained to keep the vacuum box, vacuum ports and drain tubes clean during the coating process. The spray system may be an arrangement of holes, slots, atomizers, spray nozzles, and a variety of other configurations.
The coating apparatus includes a coating die having at least one feed slot for extruding the coating fluid onto the moving web. The feed slot is demarked from a front surface of the die by a die edge. A guide mechanism guides the moving web in a first direction past the coating die such that a coating bead is formed in a coating gap between the moving web and the die edge.
A spraying system sprays a cleaning fluid at a first flow rate on at least a portion of the front face of the coating die. The first flow rate may generate an atomizing spray or a continuous stream of cleaning fluid. The spraying system includes a plurality of cleaning fluid ejection means arranged parallel to the width of the moving web and below the die edge. The spraying system preferably directs the cleaning fluid to a portion of the front surface about 1 to 20 mm below the die edge, but does not contact the moving web. The spraying system may also spray cleaning fluid at a second flow rate less then the first flow rate. In an alternate embodiment, a first cleaning fluid is sprayed at the first flow rate and a second cleaning fluid is sprayed at the second flow rate.
In one embodiment of the present invention, at least a portion of the coating bead is formed on the front face of the slide coating die. The coating bead has a substantially linear static wetting line on the coating die generally perpendicular to the first direction of the moving web. The wetting line is generally located on the front face.
The cleaning fluid is preferably a solvent of the coating fluid, such as methyl ethyl ketone, tetrahydrofuran, and methanol. It is understood that for aqueous coating fluids, the cleaning fluid may simply be water. The cleaning fluid serves a variety of purposes, such as for example pre-wetting critical surfaces of the coating system, preventing premature drying of the coating fluid, providing a vapor pressure to retard drying of the coating fluid, washing-off surfaces of the coating die to remove debris, and cleaning the vacuum box and vacuum ports.
The present spraying system may be used with a variety of die configurations, including a slide coating die, extrusion or slot coating die, or curtain coating die. For slide coating dies, the coating gap is between 0.0254 mm and 3.81 mm.
The present coating apparatus may also include a vacuum system for generating a reduced pressure condition below the coating bead and between the front face and the moving web. The vacuum system includes a drain chamber separated from a vacuum source by a partition. In another aspect of the present invention, the vacuum source and sensor ports are physically separated from possible contact with the coating fluid so as to prevent contamination.
The method of the present invention includes extruding the coating fluid through the feed slot(s) on a coating die. The coating die has a front face demarked from the feed slot by a die edge. The moving web and the coating die are positioned such that a coating bead is formed in a coating gap between the moving web and the die edge. Cleaning fluid is sprayed at a first flow rate on at least a portion of the front face of the coating die. Cleaning fluid is sprayed at a second flow rate such that the coating bead is not disturbed and a generally linear static wetting line is formed on the front face. The second flow rate is generally less than the first flow rate. The method of the present invention may also include generating a reduced pressure condition below the coating bead and between the front face and the moving web.
In one embodiment, th
Bhave Aparna V.
Iverson Hans E.
Milbourn Thomas M.
Norton Daniel V.
Wallace Lawrence B.
3M Innovative Properties Company
Lamb Brenda A.
Sprague Robert W.
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