Liquid crystal cells – elements and systems – Nominal manufacturing methods or post manufacturing...
Reexamination Certificate
2002-11-05
2004-10-12
Kim, Robert H. (Department: 2871)
Liquid crystal cells, elements and systems
Nominal manufacturing methods or post manufacturing...
C428S001300
Reexamination Certificate
active
06803985
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure. The invention further relates to an optical element obtained by the process, optical films using the optical element, and an illuminator and a liquid crystal display each using the optical element or optical films.
DESCRIPTION OF THE RELATED ART
Conventional processes for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure include, for example, a process which comprises sandwiching a liquid crystal mixture comprising a polymerizable liquid crystal monomer and a chiral monomer between two parallel oriented substrates, and polymerizing and curing the mixture with ultraviolet rays or the like to produce a cholesteric polarizer having a helically twisted molecular structure (see Japanese Paten Laid-Open No. 6-281814). However, the process disclosed in this publication, in which a liquid crystal mixture sandwiched between two oriented substrates is polymerized, involves poor thickness precision and in-plane unevenness in optical properties due to the poor thickness precision. As a result, this process has disadvantages that brightness improvement is insufficient and front-view hue/slant-view hue characteristics decrease. In addition, there is a problem that a large amount of industrial wastes is unavoidable because of the necessity of oriented substrates on both sides of the liquid crystal mixture.
The cholesteric polarizers actually obtained and reported in the publication cited above have a thickness as large as from 18 to 20 &mgr;m. Such a large product thickness results in poor thickness precision, which causes in-plane unevenness in optical properties. The related art technique hence has disadvantages that brightness improvement is insufficient and front view hue/slant view hue characteristics decrease. Still another problem is that the large product thickness increases the material cost.
Furthermore, the related art process described above requires exposure with an ultraviolet source (365 nm) at an irradiation intensity as extremely low as from about 0.06 to 5 mW/cm
2
for a period of time as extremely long as from 5 to 60 minutes. This process hence has a considerably poor production efficiency. This related art process adds a dye to the liquid crystal mixture or disposes a spacer along the edges of the alignment substrates, and realizes the spread of a wavelength range of a cholesteric polarizer. In this case, however, orientation properties of the liquid crystal become insufficient and this causes in-plane unevenness in optical properties. This technique hence has the disadvantages that brightness improvement is insufficient and front view hue/slant view hue characteristics decrease. Moreover, in this related art process, the shearing operation with the two substrates for inhibiting the formation of discontinuity is conducted in such a manner that the two substrates are sheared over a short distance until planeness is obtained. This makes it difficult to conduct roll-to-roll production and the process hence has a considerably low production efficiency.
SUMMARY OF THE INVENTION
Accordingly one object of the present invention is to provide a process for producing an optical element which comprises a polymeric liquid crystal layer having a helically twisted molecular structure and has satisfactory thickness precision and satisfactory optical properties.
Another object of the present invention is to provide an optical element obtained by the process.
Still another object of the present invention is to provide optical films using the optical element.
Further object of the present invention is to provide an illuminator and a liquid crystal display each using the optical element or optical film.
As a result of intensive investigations to overcome the problems described above, it has been found that above-described objects can be accomplished with the production process described below. The invention has thus been completed based on this finding.
The present invention provides a process for producing an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure, which comprises:
applying a mixed solution containing a polymerizable nematic liquid crystal compound and a polymerizable chiral reagent to an oriented substrate, followed by drying, to orient the mixed solution; and
irradiating the mixed solution applied with radiation from the oriented substrate side while maintaining the mixed solution in contact with a gas comprising oxygen to thereby polymerize and cure the mixed solution.
According to the process of the present invention, the mixed solution containing a liquid crystal material is oriented from one side thereof by one oriented substrate and then irradiated with radiation from the side opposite the mixed solution, i.e., from the oriented substrate side, while maintaining the mixed solution in contact with a gas comprising oxygen. This process improves orientation properties of the resulting liquid crystal layer in film thickness direction of the liquid crystal layer, and controls the rate of polymerization and curing by the difference in oxygen concentration. As a result, this process can attain an improvement in thickness precision and, hence, a decease in product thickness. This makes it possible to produce an optical element comprising a polymeric liquid crystal layer having a helically twisted molecular structure in the form of a thin film having a thickness of from 2 to 15 &mgr;m. Further, the improved thickness precision brings about in-plane evenness in optical properties. As a result, brightness-improving characteristics and front view hue/slant view hue characteristics are improved, and this can realize the spread of a wavelength range of an optical element.
Furthermore, the process of the invention, in which one side orientation is conducted, is advantageous in that the amount of oriented substrates required is a half of that necessary in the related art process in which two oriented substrates are used to conduct both sides orientation, and thus attains decrease in the amount of industrial wastes. In addition, since high thickness precision and decreased product thickness can be attained, the cost of liquid crystal materials can be decreased. Still a further advantage of the invention is as follows. For the spread of a wavelength range of a dye-containing system in both-sides orientation embodiment, a certain extent of layer thickness is required for the reason that the rate of crosslinking reaction should be controlled based on light absorption by the dye. In contrast, according to the process of the present invention, orientation properties and thickness precision are improved, and hence, brightness-improving characteristics and front view hue/slant view hue characteristics are improved. Consequently, the optical element can be used in a wider wavelength range with the necessary and minimum thickness, whereby decrease in film cost can be attained. Moreover, roll-to-roll production is possible and improvement in production efficiency can hence be attained.
In the process for producing an optical element, the temperature during the irradiation with radiation is preferably 40° C. or higher. By regulating the temperature during the irradiation with radiation to 40° C. or higher, the liquid crystal material can be polymerized and cured in a satisfactorily oriented state. The temperature during the irradiation with radiation is more preferably from about 50 to 90° C.
In the process for producing an optical element, the gas comprising oxygen preferably has an oxygen concentration of 0.5 vol % or higher. Air is preferably used as the gas.
The irradiation with radiation in the process for optical-element production is preferably conducted at an intensity of from 10 to 1,000 mW/cm
2
. According to the present invention, when an ultraviolet source (365 nm) is used
Kawamoto Ikuo
Motomura Hironori
Shiraogawa Miki
Chung David Y.
Nitto Denko Corporation
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