Drying method and drying apparatus for coating layer

Abstract

A coating solution is applied to a web to form a coating layer. Then the web is transported into a drying apparatus, in which a guide roller guides the web such that an angle of the web to a horizontal direction is smaller downstream from the guide roller. The angles at entrance and exit of the drying apparatus are named entrance and exit angles theta1, theta3, respectively, and satisfy a condition theta1>theta3. The coating layer has the temperature T1 at the entrance, the temperature T2 at the exit, and the temperature T3 in the drying apparatus. The differences |T2-T1| and |T3-T1| are at most 5° C. In the drying apparatus, as the organic solvent evaporates uniformly, the generation of the unevenness is reduced. Thereafter, the drying is made at the large drying speed in the blow-drying apparatus to obtain a film product.

Description

[0001] 1. Field of the Invention[0002] The present invention relates to a drying method and a drying apparatus for drying a coating layer, and particularly to a drying method and a drying apparatus for drying a coating layer on a continuous and wide coating surface of a web as a continuously running flexible substrate while at least one liquid composition is applied on the web so as to form the coating layer.[0003] 2. Description Related to the Prior Art[0004] E. B. Gutoff & E. D. Cohen teaches in the publication "Coating and Drying Defects" (Wiley-Interscience, John Wiley & sons, Inc) some methods and apparatuses for drying a wet coating layer on a continuous and wide coating surface of a web as a continuously running flexible substrate while at least one liquid composition is applied onto the web so as to form the coating layer. In the method and apparatus for drying, an wind is blown from an air nozzle to dry a wet coating layer on a coating surface of the web while another non-c...

AI Technical Summary

Problems solved by technology

Although the blow-drying method is excellent in the drying efficiency, the wind is blown to the coating layer directly or through a porous plate, straightening plate, or the like, which causes the bad conditions for the coating layer.

Accordingly, the thickness of the coating layer loses uniformity, and unevenness or mura is generated.

Further, the convection of the wind makes the evaporation speed of the solvent from the coating layer on coating surface nonuniform.

Especially when an organic solvent is contained in a coating solution, the unevenness is generated so much.

The generation of the unevenness is a considerable coating defect.

Further, when the liquid crystal compounds are contained in the coating layer, it is hard to obtain the predetermined and uniform orientation by blowing the wind to the substrate.

However, as described in Japanese Patent Laid-Open Publication No. 2001-170547 (Page 3-5, FIG. 1), the above two methods are not adequate for the high speed coating, and increase the drying time, which causes the extremely low productivity.

When a drying apparatus which has a casing surrounding the web is provided just after the coating process of the coating layer, the unevenness is easily generated especially in early dry period while the content of the solvent in the coating layer is large.

Great Britain Patent No.1401041 teaches that when the coating surface is directed downwards, the downward diffusion of the solvent vapor is promoted, and it is hard to keep the concentration of the solvent vapor high near the coating layer.

Therefore the evaporation is not made uniformly.

In this case, the solvent vapor easily escapes in a widthwise direction of the web, which causes the generation of the unevenness.

Actually, however, when the temperature of the surfaces becomes too high, the solvent evaporates at a high speed.

In this case, the coating layer is often not uniformly dried, or the high temperature decreases the viscosity of the coating layer on the substrate such that the streaming of the solution occurs in the coating layer.

The publications of the prior arts disclose the methods of recovering the solvent, but do not teach any concrete methods of reducing the generation of the unevenness just after the drying.

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