Method for coating a plurality of fluid layers onto a substrate

Details Method for coating a plurality of fluid layers onto a substrate Method for coating a plurality of fluid layers onto a substrate

1999-11-15

2001-03-13

Bareford, Katherine A. (Department: 1762)

Coating processes

Applying superposed diverse coating or coating a coated base

C427S420000, C118S411000, C118SDIG004

Reexamination Certificate

active

06200641

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a method for coating a plurality of fluid layers onto a substrate and more particularly to a method for coating a plurality of fluid layers onto a substrate to create, for example, a photothermographic, thermographic, or photographic element, or a data storage element (e.g., a magnetic computer tape and floppy or rigid disks or diskettes, and the like).
BACKGROUND OF THE ART
A construction of a known photothermographic dry silver film or paper product
10
is shown in FIG.
1
. This construction can be created by coating a plurality of layers onto a substrate. One of the layers is a photothermographic emulsion layer
14
made up of a photosensitized silver soap in a binder resin which can include toners, developers, sensitizers and stabilizers. To improve adhesion of the photothermographic emulsion layer
14
to the substrate, a primer layer
16
can be positioned between them. A topcoat layer
12
can be positioned above the photothermographic emulsion layer
14
and can be made up of a mar-resistant hard resin with toners and slip agents. The substrate
18
can be a paper-based substrate or a polymeric film-based substrate. An antihalation layer
20
can be applied to the surface of the substrate
18
opposite the surface on which the primer, photothermographic emulsion, and topcoat layers
16
,
14
,
12
can be positioned. The compositions of layers
16
,
14
and
12
are chosen for product performance reasons, and components comprising adjacent coating layers could be incompatible.
It is desirable to determine how to coat the fluids that form (i.e., the precursors) for the primer, photothermographic, and topcoat layers
16
,
14
,
12
, respectively, using a simultaneous multilayer coating method. Slide coating, as described in U.S. Pat. No. 2,761,419 (Mercier et al., 1956) and elsewhere (see E. D. Cohen and E. B. Gutoff,
Modern Coating and Drying Technology,
VCH Publishers, 1992), is a method for multilayer coating, i.e., it involves coating a plurality of fluid layers onto a substrate. The different fluids comprising the multiple layer precursors flow out of multiple slots that open out onto an inclined plane. The fluids flow down the plane, across the coating gap and onto an upward moving substrate. It is claimed that the fluids do not mix on the plane, across the coating gap, or on the web, so that the final coating is composed of distinct superposed layers. A number of developments have been reported in this area regarding the use of slot steps, chamfers, and have been described in literature (see E. D. Cohen and E. B. Gutoff,
op. cit.
).
The application of multilayer slide coating as described in the above references to the coating of a product such as is described in
FIG. 1
, that involves coating layers comprising incompatible solutes in miscible solvents, can lead to a problem of “strikethrough” that is described herewith. Incompatible solutes are solutes that do not mix in some or all concentration ranges, whereas miscible solvents are solvents that mix in any proportion.
Occasionally during coating, a disturbance causes one of the coating layers above the bottom-most coating layer to penetrate through the bottom-most coating layer to the slide surface. When the solute of the coating layer(s) above the bottom-most coating layer is sufficiently incompatible with the solute of the bottom-most layer, the penetrating coating layer attaches to slide surface
53
and is not quickly self-cleaned by the bottom-most coating layer. This phenomenon is referred to as strikethrough. (The term “self-clean” means the process which occurs when the flow of the bottom-most coating layer (or the bottom-most coating layer and one or more adjacent coating fluid layers) cleans off the penetrant coating fluid layer that sticks to the slide surface.)
When strikethrough occurs, the flow of the coating fluid down the slide surface
53
is disturbed which can lead to streaking defects in the coated product. Streaking defects can, in turn, reduce product quality to the point where the final product is outside specifications and cannot be used.
Another problem encountered during multilayer slide coating of product constructions involving different solvents in different layers is that the interdiffusion of solvents between these layers can cause phase separation of one or more solutes within one or more layers. This phase separation can result in the inability to coat such a construction using a multi-layer coating technique due to formation of defects such as streaks or fish-eyes, or due to a disruption of flow and the intermixing of separate fluid layers.
Traditional slide coating, as described in U.S. Pat. No. 2,761,419 (Mercier et al., 1956), is restricted to coating solutions that are relatively low in viscosity. The use of a “carrier layer” in slide coating was first described by U.S. Pat. No. 4,001,024 (Dittman and Rozzi, 1977), where the authors claimed an improvement over a previously-described method of slide coating “by coating the lowermost layer as a thin layer formed from a low viscosity composition and coating the layer above the lowermost layer as a thicker layer of higher viscosity.” Furthermore, the authors state that due to the vortical action of the coating bead that is confined within the two bottom layers, intermixing occurs between the two bottom layers, and, therefore, the coating compositions of these two layers must be chosen such that the interlayer mixing is not harmful to the product. However, this patent does not address strikethrough or phase separation.
U.S. Pat. No. 4,113,903 (Choinski, 1978) teaches that a low viscosity carrier layer tends to be unstable “in the bridge between the coater lip and the web in the bead formed with a bead coater” and can limit the web speed at which the method can be applied. To overcome this problem, Choinski suggests use of a non-Newtonian pseudoplastic liquid as the carrier, such that it has a high viscosity on the slide and in the bead where the shear rate is low, and a low viscosity near the dynamic contact line where the shear rate is high. In U.S. Pat. No. 4,525,392 (Ishizaki and Fuchigami, 1985), it is further specified that the non-Newtonian (or shear thinning) carrier layer viscosity should be within 10 cp of the next layer at low shear rates, but lower at high shear rates. However, these patents do not address strikethrough or phase separation.
Interlayer mixing between the bottom two layers “caused by a whirl formation in the meniscus” is cited as a limitation of the above patents, and a method of overcoming this interlayer mixing by adjustment of coating gap is described in U.S. Pat. No. 4,572,849 (Koepke et al., 1986). This method also employs a low viscosity accelerating layer as the lowermost layer over which other higher viscosity layers can be arranged. A slightly different layer arrangement is also described where a low viscosity spreading layer is used as the uppermost layer in addition to the lowermost low viscosity accelerating layer. The same arrangement is used for curtain coating in related patent U.S. Pat. No. 4,569,863 (Koepke et al., 1986). However, neither patent addresses the problem of strikethrough or phase separation that occurs on the slide surface.
U.S. Pat. No. 4,863,765 (Ishizuka, 1988) teaches that using a thin layer of distilled water as carrier allows high coating speeds and also eliminates mixing between the two lowermost layers. In related patents U.S. Pat. No. 4,976,999 and U.S. Pat. No. 4,977,852 (Ishizuka, 1990a and 1990b), the carrier slide construction with water as carrier (as described in U.S. Pat. No. 4,863,765) is used, and it is noted that streaking is reduced by using smaller slot heights for the carrier layer and that bead edges are stabilized by extending the width of the carrier layer beyond the width of the other layers coated above the carrier. This patent also does not address strikethrough or phase separation.
In summary, U.S. Pat. Nos. 4,001,024, 4,113,903, and 4,525,392 require that the composition of the tw

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