Solvent casting process, polarizing plate protective film, optically functional film and polarizing plate

Abstract

A process for solvent casting which comprises, casting a dope from a casting die onto a casting support, drying the cast dope on the casting support to form film, stripping off the film from the casting support, and further drying the film, wherein the dope has a solid matter content of from 17 mass % to 30 mass %, and a mean drying rate between the casting of the dope and the stripping off is more than 300 mass % / minute and not more than 1000 mass % / minute is proposed. The process for solvent casting for producing a film comprising the steps of casting a dope from a casting die to a casting support, forming the film by drying the dope to some extent, stripping off the film from the casting support and further drying the film while conveying the film with carrying it at both side edges of the film by a tenter under a specified condition is also proposed. The high-speed solution film forming methods enable to produce the film having good quality with stable conveying and with stable drying especially in the conveying of the film by the tenter machine in the drying steps after stripping off from the casting support.

Description

[0001] The present invention relates to a solvent casting process for producing cellulose triacetate film, etc. and polarizing plate protective film, optically functional film, or polarizing plate using the film produced by the solvent casting process.[0002] Generally, a part of films, such as cellulose acetate, polycarbonate, and cellophane are produced by solvent casting. In the solvent casting process, dope being a polymer solution is cast from a casting die onto a casting support of belt type or drum type, stripped off after being dehumidified to some extent on the casting support, dried while being carried by a tenter machine with being held at both side edges, and further dried while being conveyed over rollers (JP 62-46625A, JP 62-46626A, etc.).[0003] In order to improve productivity in such a solvent casting process, it is necessary to rapidly strip off the dope cast on the casting support. As a result, various technics for stripping off the dope quickly have been developed,...

AI Technical Summary

Benefits of technology

[0007] Solving the foregoing problems, an object of the present invention is to provide a process for solvent casting enabling to produce a film having good quality with stability and with high productivity.

[0008] Another object of the present invention is to provide a process for solvent casting enabling to produce a film having good quality at a high speed with stable conveyance and with stable drying especially while the conveyance of the film by the tenter machine in drying after stripping off from the casting support.

[0009] The inventors investigated eagerly in order to achieve the objects of the present invention, and they found that a quick stripping off of the film from the casting support is possible by adjusting drying conditions between the casting of the dope onto the casting support and the stripping off the film in a state where the dope solidified in some degree from the casting support. The film is highly clear, optically isotropic and excellent in mechanical strength, in dimensional stability and in durability. Especially, the present invention was completed by finding a desirable drying rate in terms of distance and time between the casting and the stripping off.

[0010] They also found that the occurrence of holes, break and foams in the film were caused depending on the thickness of the film, quantity of the volatile components remaining in the film, the temperature of a tenter, atmosphere, drying conditions, etc. and during investigating these optimum conditions, they found a relationship between the thickness of the film at both side edges being carried and the product portion for ensuring stable conveyance and drying. By keeping the relationship, rupture by stress caused by contraction of drying could be eliminated. In addition, foaming problem could be eliminated by adjusting the content of residual volatile components at the portion being carried by the tenter machine in a specific range.

Problems solved by technology

Therefore, particularly great drying rate is required between the casting and the point of being stripped off.

However, it was found that, according to drying conditions, problems occur, such as foaming and degradation of film properties, such as, mechanical strength, transparency, retardation, flatness, etc.

On the other hand, in the case that the cast film containing volatile components in a high concentration is stripped off from a casting support by a tenter machine of a clip type, the film breaks at the portion caught by the clip, and it is difficult to continue carring the film by the tenter.

Therefore, the high-speed casting and improvement in the manufacturing speed of the film are difficult.

In addition, since the film stripped off from the casting support contains volatile components in quality, the film is occasionally foamed at the part caught by a clip tenter heated to a high temperature by drying wind.

Then, the foams generated in the film edges are possible to extend to product portion, which become a problem in quality.

Moreover, break of the film also occurs occasionally caused by rupture of foamed portions.

When the thickness of this penetration area of the film is thin, the film edge tears by the tension caused by contraction on drying, and it is also possible that the film breaks while releasing the film from the tenter machine.

On the other hand, when the thickness of this penetration portion of the film is too thick, the pins are contaminated by the adhesion of the dope not sufficiently dried in the film, which causes a conveyance trouble.

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