Coating apparatus – Immersion or work-confined pool type – Work-confined pool
Reexamination Certificate
1999-07-22
2001-09-11
Beck, Shrive P. (Department: 1762)
Coating apparatus
Immersion or work-confined pool type
Work-confined pool
C118SDIG004, C427S420000
Reexamination Certificate
active
06287384
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a curtain coating method and a curtain coating apparatus for uniform application of coating solutions and, more particularly, to the method and the apparatus for curtain coating solutions on to a continuously travelling strip like support (hereinafter referred to as “web”) which is employed in the production of photographic films, photographic printing papers, magnetic recording tapes, pressure-sensitive recording papers, offset plate materials and the like.
2. Description of the Related arts
Conventionally, a curtain coating technique has been applied to the production of photographic films, photographic printing papers and the like.
First, descriptions will be given of conventional curtain coating methods as follows.
The basic arts of curtain coating are described, e.g., in U.S. Pat. No. 3,508,947 and U.S. Pat. No. 3,632,374 which correspond to JP-B-49-24133 and JP-B-49-35447 respectively (The term “JP-B” as used herein means an “examined Japanese patent Publication”).
Further, S. F. Kistler discloses a theory of curtain coating in “AlChe Winter National meeting” (1982), and describes the following three phenomena which predominantly determine the coating rate in the curtain coating method:
(1) the phenomenon that fine bubbles are entrained in a gap between a web and a coating solution (This phenomenon is called “the air entrainment phenomenon” hereinafter),
(2) the phenomenon that a foot-like cross-sectional shape of the impingement zone can develop a pronounced heel that can give rise to coating nonuniformity. (This phenomenon is called “the heel phenomenon” hereinafter, and it occurs in a case where a coating solution is made to flow down at a high flow rate), and
(3) the phenomenon that a coating solution bounds at the web surface without adhering thereto (This phenomenon is called “the sagging phenomenon” hereinafter, and it occurs in the same case as the phenomenon (2), namely a case where a coating solution is made to flow down at a high flow rate).
As for attempts to elevate the upper limit of coating speed in this curtain coating method, there is disclosed the means of inhibiting “the air entrainment phenomenon”, e.g., by applying an electrostatic field between a web and a coating solution (JP-A-62-197176). (The term “JP-A” as used herein means an “unexamined published Japanese patent application”).
In recent years, however, the coating operation has been performed at a high speed of 250 m/min or above and the flowing-down rate of a curtain of coating solution has also been increased. As a result thereof, the retardation of coating speed due to “the sagging phenomenon” has come to a greater problem than the retardation caused by the aforementioned “air entrainment phenomenon”.
Generally speaking, in order to get rid of nonuniformity in the thickness of the edge part of a free falling curtain, a curtain coating apparatus is usually equipped with guide plates for truing up both the edges of the coating solution on the slide plane, or edge guides for supporting both edge parts of the free falling curtain. In many cases, however, the sagging phenomenon occurs in the edge part of the free falling curtain, in which a thickness of the curtain is apt to be nonuniform.
Methods for suppressing the sagging phenomenon and techniques to improve the conventional coating techniques are disclosed in the next two references.
U.S. Pat. No. 5,393,571 which corresponds to JP-A-3-146172 discloses a method of conducting a curtain coating in which a total viscosity of coating solutions is specified and a surface roughness of web is adjusted to at least 0.3 &mgr;m in order to achieve a high-speed coating in a high flow rate range beyond 4 cc/cm/sec.
U.S. Pat. No. 5,391,401 which corresponds to PCT publication translated in Japanese No. Hei6-503752 discloses a method of conducting a curtain coating stably in which a viscosity of coating solution to form the lowest layer is specified with using a pseudoplasticity solution as the coating solutions.
In these methods, the viscosity is specified in order to conduct a high-speed coating stably. However, specifying only the total viscosity or the lowest layer viscosity is insufficient for realizing the high-speed coating and the uniform coated surface condition in multilayer coating. For instance, the coating solution for the lowest layer is likely to be designed so as to have low viscosity and low gelatin concentration from the viewpoint of high-speed coating suitability, but the coating solution for an upper layer is generally made as dense as possible from the viewpoints of ease of preparation, feeding, reduction of drying load and so on, thereby resulting in a rise of gelatin concentration.
When the curtain coating method is adopted in the multilayer coating, it turned out that unevenness was caused in the coated film when there are large differences in gelatin concentration and viscosity between the coating solutions for the lowest layer and the directly upper layer. This is because, although the curtain is drawn out to be rendered thin at the times when it is formed from the slide plane and impinges on the web during the curtain coating operation, this drawn-out condition is not uniform in the direction of coating width in the case where there are large differences in gelatin concentration and viscosity between coating solutions for the lowest layer and the directly upper layer, resulting in generation of unevenness.
Next, a description will be given of conventional curtain coating apparatuses as follows.
In the steps of forming a free falling curtain, edge guides support both edge parts of the curtain formed, and various techniques thereto are disclosed. For example, Japanese examined patent publication No. Sho 58-37866 discloses a method of introducing an auxiliary liquid between the free falling curtain and the edge guides which are prepared at both the edge parts of the curtain, and Japanese examined patent publication No. Hei 6-61517 (hereinafter referred to as JP-B-6-61517) discloses a method of introducing an auxiliary liquid close to the hopper lip.
FIG. 1
is a perspective view of a basic structure of a curtain coating apparatus as an embodiment of the present invention.
A coating head
1
has a plurality of slits
12
respectively connected to manifolds
10
, and the coating solutions extruded from each slit
12
flows down on a slide plane of a slide hopper
7
and falls down to form a free-falling curtain
5
of the coating solution.
Both sides of the curtain
5
are supported by edge guides
4
, and the curtain
5
falls and impinges at a line on a surface of a web
3
which travels in the arrow direction while supported by a backup roller
2
, so that a coated film
8
is formed.
FIG. 4
is a front view of a curtain coating apparatus to which an method disclosed in JP-B-58-37866 cited above is applied for introducing the auxiliary solution for the liquid curtain. Such an apparatus is shown as one example of conventional curtain coating apparatuses. In
FIG. 4
, each edge guide
4
is equipped with conduit
14
in a place situated downward at a fixed distance from the departing position of the liquid curtain from the hopper lip, and the conduit
14
is bent down so that its outlet turns to the lower side and each direction of the outlet are parallel to the falling direction of the free falling curtain
5
.
FIG. 5
is a side view of a curtain coating apparatus to which a method disclosed in JP-B-6-61517 cited above is applied for introducing the auxiliary solution into the free falling curtain. Such an apparatus is shown as another example of the conventional curtain coating apparatuses. Therein, an auxiliary solution supply pipe
15
is kept upright and arranged upward at a fixed distance apart from the departing position of the liquid curtain from the hopper lip.
As shown in the above examples, the auxiliary liquid flows along each of the edge guides, so that the film formation can be stabilized and the coated film edge corresponding to the edge pa
Katagiri Yoshinobu
Oki Kazuhiro
Beck Shrive P.
Cleveland Michael
Fuji Photo Film Co. , Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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