Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2003-02-05
2004-10-26
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299630, C252S299660, C252S299670
Reexamination Certificate
active
06808764
ABSTRACT:
The present invention relates to a liquid-crystalline medium, and to electro-optical displays containing this medium.
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 cells having a twisted nematic structure, 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 generally have good chemical and thermal stability and good stability to electric fields and electromagnetic radiation. Furthermore, the liquid-crystal materials should have low viscosity and give short response times, low threshold voltages and high contrast in the cells.
They should furthermore have a suitable mesophase, for example a nematic 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 as 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 various 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, liquid-crystalline media having large positive dielectric anisotropy, broad nematic phase range, high resistivity, good UV and temperature stability and low vapour pressure are desired for matrix liquid-crystal displays containing integrated non-linear elements for switching individual pixels (MLC displays).
Matrix liquid-crystal displays of this type are known. Non-linear elements which can be used for individual switching of the individual pixels are, besides passive elements such as varistors or diodes, active elements such as transistors. This case is then referred to as an “active matrix”.
In the promising TFT (thin film transistor) displays, the electro-optical effect utilized is usually the TN effect. A distinction is made between TFTs comprising compound semiconductors, for example CdSe, or TFTs based on polycrystalline or amorphous silicon.
The TFT matrix is applied to the inside of one glass plate of the display, while the other glass plate carries the transparent counterelectrode on its 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 expanded to fully colour-compatible displays, in which 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 backlit.
MLC displays of this type are used as displays in notebook computers, for televisions (pocket TVs) or in automobile or aircraft construction. Besides problems regarding the angle dependence of the contrast and the response times, difficulties also arise in these MLC displays due to inadequate resistivity of the liquid-crystal mixtures. With decreasing resistance, the contrast of an MLC display drops, and the problem of “image sticking” can occur. Since the resistivity of the liquid-crystal mixture generally drops over the life of an MLC display owing to interaction with the interior surfaces of the display, a high (initial) resistance is very important in order to obtain acceptable service lives. In particular in the case of mixtures having a low threshold voltage, it was hitherto impossible to achieve very high resistivities, since liquid-crystalline materials having a high positive dielectric anisotropy &Dgr;∈ in general also have a relatively high electrical conductivity. It is furthermore important that the resistivity increases as little as possible with increasing temperature and after heating and/or UV exposure. To achieve short display response times, the mixtures must furthermore have a small rotational viscosity. To be able to use the displays even at low temperatures, for example for outdoor, automobile or avionics applications, 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.
To this end, liquid-crystalline media are desired which have the following properties:
expanded nematic phase range, in particular down to low temperatures, and low temperature dependence of the viscosity for use of the displays even at low temperatures;
high resistance to UV radiation for a long life of the displays;
high positive dielectric anisotropy &Dgr;∈ for a low threshold voltage V
th
;
low rotational viscosity &ggr;
1
for short response times.
It is an object of the invention to provide liquid-crystalline media for IPS, MLC, TN or STN displays, especially for IPS displays, which have very high resistivities, low threshold voltages and short response times while maintaining the other boundary conditions.
To be able to reduce layer thicknesses in IPS displays to e.g. 2 to 3 &mgr;m, liquid-crystal mixtures having a high optical birefringence &Dgr;n are desirable. The aim of the reduction of the layer thicknesses is to reduce the response time and render the IPS displays video-capable. At the same time, new display filling methods lead to higher UV exposure of the liquid-crystal mixtures.
It is therefore also a particular object of the invention to provide liquid-crystal mixtures having a high birefringence An and improved UV stability for IPS displays.
The object is achieved by a liquid-crystalline medium comprising the compound of the formula (I):
The liquid-crystalline medium of the invention generally contains from 1 to 30% by weight, preferably from 2 to 20% by weight, particularly preferably from 3 to 30% by weight, of the compound of the formula (I).
Preferred liquid-crystalline media comprise
a) from 3 to 10% by weight of the compound of the formula (I),
b) from 10 to 50% by weight of one or more compounds of the general formula (II)
are each, independently of one another:
Z
1
, Z
2
are each, independently of one another, a single bond, —CH
2
—CH
2
—, CF
2
CF
2
, —COO— or —CF
2
O—,
X is F, Cl, —OCF
3
or —OCF
2
CF
3
, preferably F or —OCF
3
, and
R is H or an alkyl or alkenyl radical having 1 to 15 carbon atoms and 2 to 15 carbon atoms, respectively, which is unsubstituted, monosubstituted by —CN or —CF
3
or at least monosubstituted by halogen, where one or more —CH
2
— groups in these radicals may, in each case independently of one another, be replaced by —O—, —S—,
—CO—, —COO—, —OCO— or —OCOO— in such a way that O atoms are not linked directly to one another,
c) from 10 to 50% by weight of one or more compounds of the general formula (III), and/or (IV):
in which
R and X are as defined above, R
1
and R
2
are each independently as defined for R, and Z is as defined for Z
1
,
d) from 10 to 40% by weight of one or more compounds of the general formulae (V) and/or (VI):
in which R
1
and R
2
are as defined above, and
e) from 0 to 15% by weight of one or more further liquid-crystalline compounds.
In those compounds of the general formula (II) which are preferably used as component b):
are each, independently of one another:
In further compounds of the general formula (II) which are preferably used as component b), at least one of the groups Z
1
or Z
2
is a single bond, and preferably both groups Z
1
and Z
2
are a single bond.
In the general formulae (II), (III), (IV), (V) and (VI), R
Heckmeier Michael
Reiffenrath Volker
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
Wu Shean C.
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