Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – With viewing layer of specified composition
Reexamination Certificate
2001-08-10
2003-11-18
Huff, Mark F. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
With viewing layer of specified composition
C428S001100
Reexamination Certificate
active
06649231
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an optical compensatory sheet comprising a transparent support, an orientation layer and an optically anisotropic layer formed from a liquid crystal compound. The invention also relates to an orientation film, an elliptically polarizing plate and a liquid crystal display.
BACKGROUND OF THE INVENTION
A liquid crystal display comprises a polarizing plate and a liquid crystal cell.
An optical compensatory sheet (phase retarder) is usually inserted between the polarizing plate and the liquid crystal cell to improve qualities of an image displayed by a TFT liquid crystal display of TN mode, as is described in Japanese Patent Provisional Publication No. 8(1996)-50206. The liquid crystal display having the optical compensatory sheet, however, is relatively thick.
Japanese Patent Provisional Publication No. 1(1989)-68940 discloses a liquid crystal display in which an optical compensatory sheet is provided on one face of a polarizing membrane and on the other face an elliptically polarizing plate having a protective film is provided. This liquid crystal display gives high front contrast, and its thickness is not too thick. However, the optical compensatory sheet in this display is liable to be warped (for example, by heat) to cause phase difference, and hence the liquid crystal display has poor durability. To improve the durability of the liquid crystal display, Japanese Patent Provisional Publication No. 7(1995)-191217 and European Patent No. 0,911,656 A2 disclose a liquid crystal display having an improved elliptically polarizing plate. In the display, an optical compensatory sheet comprising a transparent support and a thereon-provided optically anisotropic layer formed from a discotic compound is directly used as the protective film of the polarizing plate. This liquid crystal display has satisfying durability, and its thickness is not too thick.
The optical compensatory sheet is prepared by forming an orientation layer and an optically anisotropic layer in this order on a transparent support. The surface of the orientation layer is subjected to rubbing treatment (the surface is rubbed with, for example, cloth). The optically anisotropic layer is formed by the steps of: applying a liquid crystal compound on the orientation layer, aligning the molecules of the compound, and fixing the alignment. In the process of the rubbing treatment, the surface of the orientation layer is scraped, and the produced scrapings often aggregate to cause defects in the alignment of the liquid crystal compound. These defects lower the production yield of the optical compensatory sheet. For keeping a good production yield, Japanese Patent Provisional Publication No. 7(1995)-333613 discloses an attempt to remove the scrapings by means of an ultrasonic dust remover. This attempt solves the problem to a certain extent.
On a glass substrate of the liquid crystal cell, the optical compensatory sheet is laminated with an adhesive as a part (protective film) of the polarizing plate. In a practical production process, if a fault is found in the liquid crystal display in the step of product inspection, the polarizing plate is peeled from the glass substrate of the liquid crystal cell, and then the liquid crystal cell is reused because the cell is expensive. [Hereinafter, this step of peeling the polarizing plate from the glass substrate is referred to as the step of reworking.] In the step of reworking, the optically anisotropic layer of the optical compensatory sheet sometimes partially remains on the glass substrate. Actually, in the step of reworking, the interface between the orientation layer and the optically anisotropic layer is often destroyed, and also the orientation layer itself is often destroyed in its layer. The destruction of the interface can be prevented by the denatured polyvinyl alcohol described in Japanese Patent Provisional Publication No. 8(1996)-338913, and the destruction of the orientation layer can be also prevented, to a certain extent, by addition of the cross-linking agent described in Japanese Patent Provisional Publication No. 8(1996)-338913.
SUMMERY OF THE INVENTION
The scrapings produced in the rubbing treatment of the orientation layer impair the production yield of the optical compensatory sheet or the liquid crystal cell. Although the scrapings can be removed to a certain extent by means of an ultrasonic dust remover, the dust remover by no means directly inhibits the scrapings from being produced in the rubbing treatment. Since the liquid crystal display has been recently required to display an image more and more precisely, even a very small defect can not be allowed. Accordingly, a satisfying production yield of the liquid crystal display is hardly realized by the ultrasonic dust remover alone. On the other hand, if the liquid crystal cell is optically compensated by a thick optically anisotropic layer, a large distortion (stress) is applied on the orientation layer so that the film layer is often destroyed in the step of reworking.
An object of the present invention is to provide an orientation layer from which few scrapings are produced in the rubbing treatment, and thereby to reduce defects (caused by the scrapings) in the optical compensatory sheet or the liquid crystal display.
Another object of the invention is to provide a liquid crystal display having a large viewing angle, and an optical compensatory sheet or a polarizing plate having few defects.
A further object of the invention is to provide a liquid crystal display which has excellent displaying characters such as a large viewing angle and which can display an image with few defects.
The applicant has studied and finally found that the scrapings in the rubbing treatment can be reduced, while the aligning function of a conventional orientation layer is maintained, by controlling the surface roughness of the orientation layer. Further, the applicant assumes that the polymer of the orientation layer has such a low polymerization degree that interwindings among the polymer molecules are easily untied to break the film layer when a large stress is applied in the step of reworking. Therefore, it is thought that the destruction of the orientation layer can be prevented if a high polymer is used. However, the orientation layer must be subjected to the rubbing treatment to align molecules of a liquid crystal compound provided on the film. If the polymer of the orientation layer has a too high polymerization degree, large energy is needed for the rubbing treatment. According to the applicant's study, if two polymers having different polymerization degrees are used for the orientation layer, the optically anisotropic layer scarcely remains (i.e., the orientation layer is scarcely destroyed in its layer) in the step of reworking. Further, few scrapings are produced in the rubbing treatment from the orientation layer made of these two polymers.
The present invention provides an optical compensatory sheet which comprises a transparent support, an orientation layer and an optically anisotropic layer formed from a liquid crystal compound in the order, wherein the orientation layer has a surface roughness of 0.1 to 2.0 nm.
The invention also provides an orientation film for liquid crystal compound having a surface roughness of 0.1 to 2.0 nm.
The invention further provides an ellipsoidal polarizing plate comprising a polarizing membrane and an optical compensatory sheet laminated on at least one surface of the polarizing membrane, wherein the optical compensatory sheet comprises a transparent support, an orientation layer and an optically anisotropic layer formed from a discotic liquid crystal compound in the order, wherein the transparent support is a polymer film having a retardation in plane in the range of 20 to 200 nm, and a retardation along thickness direction in the range of 70 to 400 nm, and wherein the orientation layer has a surface roughness of 0.1 to 2.0 nm.
The invention furthermore provides a liquid crystal display comprising a liquid crystal
Burns Doane , Swecker, Mathis LLP
Huff Mark F.
Sadula Jennifer R.
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