Liquid-crystalline medium and liquid-crystal display...

Details Liquid-crystalline medium and liquid-crystal display... Liquid-crystalline medium and liquid-crystal display...

1999-09-13

2002-07-16

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06419999

ABSTRACT:

The invention relates to liquid-crystalline media, in particular for use in liquid-crystal displays. It furthermore relates to liquid-crystal displays containing these liquid-crystalline media, in particular liquid-crystal displays having low addressing voltages. These liquid-crystal displays are operated in TN (twisted nematic) or STN mode. The addressing takes place directly or by time multiplexing at low multiplex ratios. The TN displays are preferably operated at the first Gooch and Tarry transmission minimum. The TN and STN displays are suitable, inter alia, for outdoor applications and, owing to their suitability for common applications, are also known as “common-use displays”.
Liquid crystals are principally used as dielectrics in display devices since the optical properties of these materials can be modified in a targeted manner by the action of an electric field.
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. Such devices are, for example, cells having dynamic scattering, DAP or ECB cells (DAP=deformation of lined phases; ECB=electrically controlled birefringence), guest/host cells, TN cells having a twisted nematic structure, STN (supertwisted nematic) cells, SPE (superbirefringence effect) cells, OMI (optical mode interference) cells, IPS (in-plane switching) cells or cells having a cholesteric-nematic phase conversion.
The most common display devices are TN cells. These are based on the Schadt-Helfrich effect and have a twisted nematic structure. Industrial use of such TN liquid-crystal displays requires liquid-crystalline materials which satisfy a multiplicity of conditions.
In order to achieve a long service life, the liquid-crystal materials must have, for example, good chemical and thermal stability and good stability to the action of electric fields and electromagnetic radiation.
In addition, the liquid-crystal materials must have a suitable mesophase at normal operating temperatures, i.e. in the broadest possible range above and below room temperature (20° C.). If they are intended to be suitable for outdoor applications, for example in automobiles or in avionics, they should have, in particular, good low-temperature behavior, which means, for example, that crystallization should not occur at low temperatures of −20° C. In addition, the mixtures should have clearing points T
N,I
above 55° C.
Operation of TN liquid-crystal displays at the first Gooch and Tarry transmission minimum favorably affects the size of the viewing angle at which the display can be viewed (U.S. Pat. No. 4,398,803). In this case, the optical anisotropy &Dgr;n of the liquid-crystal mixtures is selected so that the optical path difference (=product of the cell thickness d of the TN liquid-crystal display and the optical anisotropy &Dgr;n of the liquid-crystal mixture) is about 0.5 &mgr;m. For such applications, optical anisotropies &Dgr;n of from about 0.06 to 0.12 are preferred.
However, TN liquid-crystal displays can also be operated at a higher Gooch and Tarry transmission minimum. If, for example, they are operated at the second transmission minimum, the optical path difference is from about 1.0 &mgr;m to 1.1 &mgr;m. For such applications, liquid-crystal mixtures having optical anisotropies &Dgr;n of from about 0.10 to 0.21 are preferred. In general, TN liquid-crystal displays operated at the second or higher transmission minimum have greater viewing-angle dependencies than TN liquid-crystal displays operated at the first transmission minimum. On the other hand, they are usually characterized by being easier to produce and thus by better production yields.
In addition, the liquid-crystal materials should have the lowest possible rotational and flow viscosities. Low rotational viscosities &ggr;
1
favor, in particular, short response times. By contrast, low flow viscosities &mgr;
20
simplify the filling of TN liquid-crystal displays.
The liquid-crystal materials should give the lowest possible threshold voltages V
(10,0,20)
(also known as V
10
(0°, 20° C.), since the TN liquid-crystal display can then be operated at low operating voltages V
op
, which can be obtained, for example, by batteries. The three indices in the threshold voltages V
(10,0,20)
relate to the relative contrast (here 10%), the viewing angle (here &thgr;=0 degrees at &PHgr;=0 degrees) and the temperature (here 20° C.). The threshold voltage V
(10,0,20)
of a liquid-crystal mixture is influenced principally by the magnitude of the dielectric anisotropy &Dgr;∈, where the threshold voltage is lower the higher the dielectric anisotropy of the mixture. For example, mixtures having positive dielectric anisotropy &Dgr;∈ are used in common-use applications. These mixtures preferably have dielectric anisotropies &Dgr;∈ of from 1.5 to 39.
The liquid-crystal materials should give high contrast. In order that the TN liquid-crystal display can easily be read even at various viewing angles, the viewing-angle dependence of the contrast should in addition be as low as possible.
The liquid-crystal materials themselves, i.e. with the exception of any added dichroic dyes, must not absorb in the visible spectral region, i.e. they must be colorless, in order that the TN liquid-crystal displays have good contrast for black/white images and achieve the longest possible service life.
The liquid-crystal materials should have low electroconductivity, since otherwise a considerable part of the addressing voltage is lost due to conduction processes.
High-information TN liquid-crystal displays require liquid-crystalline substances having a steep electro-optical characteristic line which are characterized by a large ratio K
33
/K
11
of the elastic constants K
33
(bend) and K
11
(splay). STN displays, in particular, are used here. By contrast, such requirements are not made of liquid-crystal materials intended for use in TN liquid-crystal displays addressed at low multiplex ratios. By contrast, the use of substances having flatter electro-optical characteristic lines favors the display of grey shades.
Since no liquid-crystalline substance which satisfies all the demands simultaneously is known hitherto, liquid-crystal mixtures of from 5 to 30 compounds are generally prepared in order to obtain materials having suitable properties. The liquid-crystal materials must also be readily miscible with one another.
A major problem in the liquid-crystal displays of the prior art is the temperature dependence of the characteristic voltages, such as the threshold voltage, and thus the operating voltage. The liquid-crystal materials should have the lowest possible temperature dependence of the threshold voltage d[V
10,0,T)
/V
(10,0,20)
]/dT (or of the dielectric anisotropy d &Dgr;∈ (T)/dT). This has the advantage that temperature compensation of the addressing voltage can be omitted entirely or at least partly.
A known liquid-crystal mixture from the prior art which can be employed in TN liquid-crystal displays for common-use applications is, for example, E7 (Merck Ltd, Poole, England). This mixture comprises cyanobiphenyls and -terphenyls and has the following properties:
Clearing point T
N,I
: 60.5° C.
Optical anisotropy &Dgr;n: 0.225
Viscosity: 39 cSt
Threshold voltage V
(10,0,20)
: 1.5 V
In spite of an acceptable clearing point T
N,I
and relatively good viscosity, mixtures of this type are unsuitable for many applications, for example owing to their excessively high optical anisotropies &Dgr;n and/or excessively high threshold voltages V
(10,0,20)
.
In order, for example, to reduce the threshold voltage V
(10,0,20)
, highly polar nematic compounds having a terminal cyano group are added to the liquid-crystal mixtures known hitherto. However, the effective dipole moment of these compounds is reduced to a significant extent by a varying degree of antiparallel association of these molecules, meaning that a relatively large amount of polar compounds must be added. This in turn causes va

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