Use of film obtained by a casting method as a retardation compensating film.

Abstract: A retardation compensating film forming method for giving birefringence by expanding a macromolecular film formed by casting method. A solvent content value of said film is adjusted within a range of 2 - 10 % based on a solid content before said expanding.

Description: The present invention relates to the use of a film obtained by a casting method as a retardation compensating film Conventionally, a retardation compensating film of a liquid crystal display is produced by expanding a film formed by a film casting method in the length or width direction in order to provide the film with birefringence characteristics. It is important for the quality that the cast film utilizing the birefringence, in addition to transparency, is extended and expanded without unevenness; however, unevenness often occurs. JP-A-63-261 201 discloses a method for producing an oriented film made of polyvinyl alcohol or its derivatives for use in a polarizing film. It is the object of the present invention to provide a retardation compensating film in a liquid crystal display having better quality without being uneven. It was found that the optical unevenness of the retardation compensating film is determined in a casting process, and that the unevenness is reduced to some degree by restricting the solvent quantity included in the cast film at the time of the expanding. The object of the present invention is, thus, accomplished by the use of a film obtained by expanding a macromolecular film formed by a casting method, said macromolecular film having a solvent content of from 2 to 10% based on the solid content, as a retardation compensating film in a liquid crystal display. As regards restricting the solvent content of the film formed by a film casting method to 2 - 10 % based on the solid content, it means that an orientation of the molecules is relaxed for a time during the expanding step. It means that the solvent quantity of reeled products is restricted during the casting, the quantity based on the solid content is restricted within 2 - 10 %, more preferably 2.5 - 7%. When the value is less than 2 %, the unevenness remains. When the value is more than 10%, the orientation of the molecules hardly occurs. Further, in the present invention, the solvent content in the film formed by a film casting method indicates the content in the film just before the expanding step. A method of adjusting the solvent content within the aforesaid range during reeling after the film casting can be used, or a method of exposing the reeled film to the atmosphere of the solvent for adjusting the solvent content to the target value can be used. As the expanding method in the present invention, a method providing an expanding device directly linked with the casting device for performing in on-line may be used. Otherwise, a method of using an expanding device after reeling the cast film may be used. The direction to expand the film is not restricted to the length direction or the width direction. On the other hand, an expanding method in length direction by means of two rolls having different revolving speed as disclosed in Japanese Patent Unexamined Publication No. Sho. 63-189804, or an expanding method in width direction by Tenter method is used as the expanding method. With respect to the film casting method, either a band type film casting method or a drum type film casting method may be used. A macromolecular solution is made to spread onto a running endless support of either band or drum, dried to some extent, stripped from the support, and after-dried to be formed as a cast film. The method is, for example, disclosed in Japanese Patent Unexamined Publication No. Hei. 2-111511. As the macromolecular material used in the present invention, polystyrene, polymethyl methacrylate, polypropylene, polyvinyl chloride, styrene-acrylonitrile copolymer, polyethylene, polyacrylate, polyether sulfone, polyphenylene oxide, diacetyl cellulose and/or triacetyl cellulose may be used in addition to polycarbonate. The organic solvent to be used according to the present invention may be selected from a compound having a chlorinated hydrocarbon group such as methylene chloride, a compound having a ketone group such as acetone or methyl ethyl ketone, and a compound having an aromatic group such as toluene. Additionally, the solvent content based on the solid content indicates the solvent quantity included in the dried solute. Namely, when the solute is W (g), and the solvent quantity is w (g), the solvent content is indicated by the formula: w/W x 100 (%). The present invention will be described in more detail with reference to the Examples. Examples The film forming method comprised the steps of making a methylene chloride solution of polycarbonate (18 weight percent) and spreading it onto a running support by using a die and drying the solution. Cast films having solvent content rates as shown in Table 1, respectively, in the reeling step were produced, and each film was expanded in length direction by an expanding device. sample solvent content rate in the reeling step A 1.5% B 4.0% C 7.0% D 11.0% *(Based on the solid content) Upon inspection of the surface during the reeling step by the film casting machine, Samples A - D comprised almost the same degree of unevenness. However, after the expanding step by the expanding device the results of Table 2 were obtained. sample expanding rate retardation (nm) surface condition Comparative Example 1 A 1.20 570 large unevenness Example 1 according to the invention B 1.25 570 even or weak unevenness Example 2 according to the invention C 1.35 570 even or weak unevenness Comparative Example 2 D 1.50 450 large expansion and difficult orientation As can be seen in Sample A a retardation [(Re=Δn.d) = (birefringence) x (film thickness)] by the expanding step was observed but high unevenness remained. Samples B and C had a better surface quality, and the same retardation as for Sample A by means of the same expanding rate was obtained. With respect to Sample D, the film was expanded by the expanding step; however, because of the high amount of remaining solvent, an orientation hardly occurred. Furthermore, Samples A', B', C', and D' which had the same expanding rate of 1.20 as Samples A, B, C and D and having solvent content rates in the reeling step as indicated in Table 1, were expanded resulting in retardation and surface conditions as shown in Table 3. sample expanding rate retardation (nm) surface condition Comparative Example 3 A' 1.20 570 high retardation, but large unevenness Example 3 according to the invention B' 1.20 420 even or weak unevenness Example 4 according to the invention C' 1.20 240 even Comparative Example 4 D' 1.20 100 even, but low retardation Sample A', having a high retardation, but being largely uneven, cannot be used as a retardation compensating film. Sample D' is uniform (even) but has a low retardation. Sample C' and D' have a retardation for practical use and uniform surface condition. The retardation range for practical use as a retardation compensating film is generally 140 nm to 1000 nm. By the use of a film according to the present invention, as shown in the examples, a retardation compensating film comprising high quality without the occurrence of unevenness can be obtained.

Claims: Use of a film obtained by expanding a macromolecular film formed by a casting method, said macromolecular film having a solvent content of from 2 to 10% based on the solid content, as a retardation compensating film in a liquid crystal display. The use of claim 1, wherein the macromolecular film comprises a polycarbonate. The use of claim 2, wherein the macromolecular film comprises polystyrene, polymethyl methacrylate, polypropylene, polyvinyl chloride, styrene-acrylonitrile copolymer, polyethylene, polyacrylate polyether sulfone, polyphenylene oxide, diacetyl cellulose and/or triacetyl cellulose in addition to the polycarbonate. The use of any of claims 1 to 3, wherein the solvent is an organic solvent. The use of claim 4, wherein the solvent is selected from chlorinated hydrocarbons, ketones and aromatic compounds.

Inventors: YOSHIDA TORU, YOSHIDA, TORU, Yoshida, Toru, c/o FUJI PHOTO FILM CO., LTD.
Applicants: FUJI PHOTO FILM CO LTD, FUJI PHOTO FILM CO., LTD.
Application year: 1991
Main classification: B29C41
IPC codes: B29C55, G02B5, B29C41, B29D7, B29L7