Liquid crystalline medium

Details Liquid crystalline medium Liquid crystalline medium

1998-01-30

2003-01-14

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06506462

ABSTRACT:

The present invention relates to a liquid-crystalline medium, to the use thereof for electro-optical purposes, and to displays containing this medium.
BACKGROUND OF THE INVENTION
Liquid crystals are used, in particular, as dielectrics in display devices, since the optical properties of such substances can be modified by an applied voltage. Electro-optical devices based on liquid crystals are extremely well known to the person skilled in the art and can be based on various effects. Examples of such devices are cells having dynamic scattering, DAP (deformation of aligned phases) cells, guest/host cells, TN (twisted nematic) cells, STN (supertwisted nematic) cells, SBE (superbirefringence effect) cells and OMI (optical mode interference) cells. The most common display devices are based on the Schadt-Helfrich effect and have a twisted nematic structure.
The liquid-crystal materials must have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have relatively low viscosity and give short addressing times, low threshold voltages and high contrast in the cells.
Furthermore, they should have a suitable mesophase, for example a nematic or cholesteric mesophase for the abovementioned cells, at conventional operating temperatures, i.e. in the broadest possible range above and below room temperature. Since liquid crystals are generally used in the form of mixtures of a plurality of components, it is important that the components are readily miscible with one another. Further properties, such as the electrical conductivity, the dielectric anisotropy and the optical anisotropy, must satisfy different requirements depending on the cell type and area of application. For example, materials for cells having a twisted nematic structure should have positive dielectric anisotropy and low electrical conductivity.
For example, media of large positive dielectric anisotropy, broad nematic phases, relatively low birefringence, very high resistivity, good UV and temperature stability and low vapour pressure are desired for matrix liquid-crystal displays having integrated nonlinear elements for switching individual pixels (MLC displays).
Matrix liquid-crystal displays of this type are known. Examples of nonlinear elements which can be used for individual switching of individual pixels are active elements (i.e. transistors). This is then referred to as an “active matrix”, and a differentiation can be made between two types:
1. MOS (metal oxide semiconductor) or other diodes on silicon wafers as substrate.
2. Thin-film transistors (TFTs) on a glass plate as substrate.
Use of monocrystalline silicon as the substrate material limits the display size, since even modular assembly of the various part-displays results in problems at the joints.
In the case of the more promising type 2, which is preferred, the electro-optical effect used is usually the TN effect. A differentiation is made between two technologies: TFTs comprising compound semiconductors, such as, for example, CdSe, or TFTs based on polycrystalline or amorphous silicon. Intensive work is being carried out worldwide on the latter technology.
The TFT matrix is applied to the inside of one glass plate of the display, whilst the other glass plate carries the transparent counter electrode on the inside. Compared with the size of the pixel electrode, the TFT is very small and has virtually no adverse effect on the image. This technology can also be extended to fully colour-compatible image displays, where a mosaic of red, green and blue filters is arranged in such a way that each filter element is located opposite a switchable pixel.
The TFT displays usually operate as TN cells with crossed polarizers in transmission and are illuminated from the back.
The term MLC displays here covers any matrix display containing integrated nonlinear elements, i.e., in addition to the active matrix, also displays containing passive elements, such as varistors or diodes (MIM=metal-insulator-metal).
MLC displays of this type are particularly suitable for TV applications (for example pocket TV sets) or for high-information displays for computer applications (laptops) and in automobile or aircraft construction. In addition to problems with respect to the angle dependence of the contrast and the response times, problems arise in MLC displays owing to inadequate resistivity of the liquid-crystal mixtures [TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K., TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September 1984: A 210-288 Matrix LCD Controlled by Double Stage Diode Rings, p. 141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Design of Thin Film Transistors for Matrix Addressing of Television Liquid Crystal Displays, p. 145 ff, Paris]. With decreasing resistance, the contrast of an MLC display drops, and the problem of after-image, elimination can occur. Since the resistivity of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the internal surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable service lives. In particular in the case of low-voltage mixtures, it was hitherto not possible to achieve very high resistivities. It is furthermore important that the resistivity increases as little as possible with increasing temperature and after heating and/or exposure to UV radiation. Also particularly disadvantageous are the low-temperature properties of the mixtures from the prior art. It is required that crystallization and/or smectic phases do not occur, even at low temperatures, and that the temperature dependence of the viscosity is as low as possible. MLC displays of the prior art thus do not satisfy current requirements.
There thus continues to be a great demand for MLC displays having very high resistivity at the same time as a broad operating temperature range, short response times, even at low temperatures, and low threshold voltage which do not have these disadvantages or only do so to a reduced extent.
In the case of TN (Schadt-Helfrich) cells, media are desired which facilitate the following advantages in the cells:
broadened nematic phase range (in particular down to low temperatures),
switchability at extremely low temperatures (outdoor use, automobiles, avionics),
increased stability on exposure to UV radiation (longer life).
The media available from the prior art do not enable these advantages to be achieved while simultaneously retaining the other parameters.
In the case of supertwisted cells (STN), media are desired which enable greater multiplexibility and/or lower threshold voltages and/or broader nematic phase ranges (in particular at low temperatures). To this end, a further extension of the parameter latitude available (clearing point, smectic-nematic transition or melting point, viscosity, dielectric quantities, elastic quantities) is urgently desired.
SUMMARY OF THE INVENTION
The invention has the object of providing media, in particular for MLC, TN or STN displays of this type, which do not have the abovementioned disadvantages, or only do so to a reduced extent, and preferably at the same time have very high resistivities and low threshold voltages.
It has now been found that this object can be achieved when novel media are used in displays.
The invention thus relates to a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I,
in which
R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms which is unsubstituted, monosubstituted by CN or CF
3
or at least monosubstituted by halogen, it also being possible for one or more CH
2
groups in these radicals to be replaced, in each case independently of one another, by —O—, —S—,
 —CO—, —CO—O, —O—CO—, or —O—CO—O— in such a way that O atoms are not linked directly to one another,
Y is F, Cl, halogenated

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