Coating processes – Falling curtain of coating material utilized
Patent
1993-06-17
1995-02-21
Acquah, Samuel A.
Coating processes
Falling curtain of coating material utilized
4274071, 4274342, 427439, 118DIG4, B65D 130
Patent
active
053914015
DESCRIPTION:
BRIEF SUMMARY
This invention relates to coating processes and is more particularly concerned with curtain coating processes.
Curtain coating processes are well-known and widely used for the application of one or more liquid layers on to the surface of a moving support. In particular, curtain coating may be used for coating photographic products.
Bead coating is the original method for the simultaneous coating of multiple layers, and its implementation led to highly efficient manufacture of photographic films and papers.
U.S. Pat. No. 2,761,791 discloses a bead coating method in which a low viscosity bottom layer is required to wet the support at high coating speeds. It is preferable that the bottom layer be relatively thick, to achieve the high degree of coating uniformity usually required in photographic products. Bottom layers having viscosity from about 3 to 10 mPas and wet thickness from about 40 to 100 .mu.m are disclosed in U.S. Pat. No. 4,001,024.
However, the combination of a relatively thick and low viscosity bottom layer can be inconsistent with the requirements of the product, and can also result in a high load on the drier thereby restricting manufacturing speeds. Even with a relatively thick and low viscosity bottom layer, it is still generally necessary, in order to stabilize the bead, to apply at least a small pressure difference or suction across the bead, of the order of 1 cm of water, and to maintain a small gap, of the order of 300 .mu.m, between the lip of the hopper and the support.
As a result, complex apparatus is required to produce and control a smooth suction in contact with the bead.
Moreover, producing and maintaining an accurate small gap between the hopper lip and the support is difficult and expensive. With so small a gap between hopper lip and support, bubbles and debris can become trapped under or on the lip, producing line and streak non-uniformities in the coating. Bubbles can be generated at coating starts and at splices, for example, and debris can be brought in on the surface of the support, or even as an imperfection of the support surface itself.
U.S. Pat. No. 4,001,024 discloses a bead coating method in which the limitations and disadvantages of bead coating methods are mitigated. The bead coating method described in U.S. Pat. No. 4,001,024 employs a thin, low viscosity bottom layer, with a sufficiently thick layer above the bottom layer which has a higher viscosity. For best results, the compositions of the bottom two layers should be such that some mutual displacement or intermixing of the bottom two layers can be tolerated. Bottom layers with viscosity from about 1 to 8 mPas and with wet thickness from about 2 to 12 .mu.m, and a layer above the bottom layer with viscosity from about 10 to 100 mPas and wet coverage from 15 to 50 .mu.m, are contemplated. Though drying loads can often be reduced in this manner, restrictions due to the need for uniformity of the bottom layer may still be encountered.
U.S. Pat. No. 4,113,903 discloses the use of a thin, pseudoplastic bottom layer in bead coating. The viscosity of the bottom layer is low, less than about 10 mPas, at the wetting line where shearing rates are high. This is to assist the dynamic wetting of the support. The bottom layer is also such that its viscosity is higher at the lower rates of shearing in the coating bead, away from the wetting line. The higher viscosity in the bead strengthens it, so that a larger gap between the lip and support can be used. Higher suctions, up to 25 cm of water, may be required to stabilize such a bead. The disadvantages of this method include the need to produce a high suction in contact with the bead, and the need for a small gap between the hopper lip and support. As described in U.S. Pat. No. 4,001,024, restrictions around the uniformity of the thin bottom layer can still be encountered.
The major disadvantages of bead coating methods were largely alleviated or solved by curtain coating methods. One method is described in U.S. Pat. No. 3,632,374.
In U.S. Pat. No. 3,632,374, a support is coat
REFERENCES:
patent: 2761791 (1956-09-01), Russell
patent: 3508947 (1970-04-01), Hughes
patent: 3632374 (1972-01-01), Greiller
patent: 3811897 (1974-05-01), Babbit et al.
patent: 3867901 (1975-02-01), Greiller
patent: 4001024 (1977-01-01), Dittman et al.
patent: 4113903 (1978-09-01), Choinski
patent: 4569863 (1986-02-01), Koepke et al.
Batts Gregory N.
Blake Terence D.
Dobson Rosemary
Harrison William J.
Acquah Samuel A.
Eastman Kodak Company
Ruoff Carl F.
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