Compositions – Liquid crystal compositions – Containing nonsteryl liquid crystalline compound of...
Patent
1997-01-27
1998-05-19
Kelly, C. H.
Compositions
Liquid crystal compositions
Containing nonsteryl liquid crystalline compound of...
25229966, C09K 1930, C09K 1912
Patent
active
057531420
DESCRIPTION:
BRIEF SUMMARY
This application is a 371 of PCT EP96/04787.
The invention relates to a nematic liquid-crystal composition based on terminally and laterally fluorinated compounds. This composition is especially useful for active matrix application.
Active matrix displays (AMD) are highly favored for commercially interesting displays with a high information content. Such AMDs are used for TV application and also for displays with high information content for computer terminals, automobiles and aeroplanes.
AMDs have non-linear electrical switching elements which are integrated at each picture element. As non-linear driving elements thin film transistors metal insulator metal: MIM) (Niwa, K., et al., 1984, SID 84, Digest, pp. 304-307! can be applied. These non-linear driving elements allow to use an electro-optical effect with a rather flat electro-optical characteristic if a good viewing angle characteristic can be obtained. So a TN-type cell twist angle of 90.degree. can be used. For these applications some physical properties of the liquid crystal become more important than for passive TN displays. Some of the decisive properties for the performance S., Sekiguchi, K., Tanabe, H., Yamamoto, E., Sorimachi, K., Kajima, E., Watanabe, H., Shimuzu, H., Proc. Eurodisplay 84, Sept. 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 144 ff, Paris; Stromer, M., Proc. Eurodisplay 84, Sept. 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris!.
In an AMD the non-linear switching elements are addressed in a multiplex scheme. So they charge the electrodes of a pixel in the limited time they are active. Then they become inactive until they are addressed again in the next cycle. Consequently the change of the voltage on an activated (charged) pixel is a nondesired but a very decisive feature of such a display. The discharge of a pixel is determined by two factors. They are the capacity of the electrodes of the pixel and resistivity of the dielectric material between the electrodes, namely the liquid crystal. The characteristic time constant of the decay of the voltage at a pixel (RC-time) has to be significantly bigger than the time between two addressing cycles (tadr.). A parameter frequently used to describe the performance of an AMD is the voltage holding ratio HR of a picture element: ##EQU1##
As the voltage at a pixel decays exponentially an increase of the holding ratio necessitates liquid crystal materials with exceptionally high resistivities.
There are several points of importance for the resistivity of the liquid crystal inside a display, e.g. orientation layers, curing condition of the orientation material. But by no means less important are the electrical properties of the liquid crystal used. Especially the resistivity of the liquid crystal in the display determines the magnitude of the voltage drop at the pixel.
Similar compositions are disclosed in the following references WO-A-9202597, WO-A-9116399, WO-A-9116400, WO-A-9116397, WO-A-9115555, WO-A-9116398, WO-A-9103450 and WO-A-9302153. Most of the known compositions are, however, used in the so-called first minimum of transmission and consequently have relatively low values for the optical birefringence.
For projection panels higher values for optical birefringence are requested and there is thus still a great need for liquid-crystal composition having an optical birefringence .DELTA.n in the range from 0.12 to 0.13, a high resistivity and other suitable material properties for use in AMDs such as a low threshold voltage, a broad nematic mesophase range with an extremely low transition temperature smectic-nematic and no crystallization at low temperatures.
The invention has for its object to provide a liquid-crystal composition with a very high resistivity which meets also the other demands.
It has been found that a nematic liquid-crystal composition having an optical birefringence .DELTA.n in the range from 0.12 to 0.13 and essentially consisting of compounds from the following groups 1 to 5
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Bartmann Ekkehard
Fujioka Kazuyoshi
Ichinose Hideo
Kozaki Syuichi
Nakamura Masako
Kelly C. H.
Merck Patent Gesellschaft mit beschrankter Haftung
Sharp Corporation
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