STN liquid-crystal display

Details STN liquid-crystal display STN liquid-crystal display

1998-11-27

2001-02-06

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299010, C252S299610, C252S299630, C252S299660, C252S299670, C349S186000

Reexamination Certificate

active

06183821

ABSTRACT:

The invention relates to supertwist liquid-crystal displays (SLCDs) or supertwisted nematic (STN) displays having very short response times and good steepnesses and angle dependencies, and to the new nematic liquid-crystal mixtures used therein.
BACKGROUND OF THE INVENTION
SLCDs as defined in the preamble are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No. 11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21), 1468 (1987); H. A. van Sprang and H. G. Koopman, J. Arpl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term SLCD here covers any more highly twisted display element with a value for the twist angle of between 160° and 360°, such as, for example, the display elements of Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
SLCDs of this type are distinguished, in comparison to standard TN displays, by significantly better steepnesses of the electrooptical characteristic line and consequently better contrast values, and by significantly less angle dependence of the contrast. Of particular interest are SLCDs having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures were hitherto optimized using usually monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for all applications.
In order to achieve a steep electrooptical characteristic line in SLCDs, the liquid-crystal mixtures should have relatively large values for K
33
/K
11
and relatively small values for &Dgr;∈/∈
195
.
In addition to optimization of the contrast and the response times, further important requirements are made of mixtures of this type:
1. A broad d/p window
2. High long-term chemical stability
3. High electrical resistance
4. Low frequency and temperature dependence of the threshold voltage.
The parameter combinations achieved are still far from adequate, in particular for high-multiplex, but also for low- and medium-multiplex STNs ({fraction (1/400)}). This is in some cases attributable to the fact that the various requirements are affected in opposite manners by material parameters.
There thus continues to be a great demand for SLCDs, in particular for high-resolution displays (XGA), having very short response times and at the same time a large operating temperature range, high characteristic line steepness, good angle dependence of the contrast and low threshold voltage which meet the abovementioned requirements.
SUMMARY OF THE INVENTION
The invention has an object of providing SLCDs which do not have the above-mentioned disadvantages, or only do so to a lesser extent.
Upon further study of the specification and appended claims, further objects and advantages of this invention will become apparent to those skilled in the art.
It has now been found that the objects can be achieved if nematic liquid-crystal mixtures are used which comprise one or more compounds of the formula IA
in which
R
a
is alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 12 carbon atoms,
R
b
is alkyl or alkoxy having 1 to 5 carbon atoms,
Z is —COO—, —CH
2
CH
2
— or a single bond, and
 are each, independently of one another,
and
one of the rings
 may alternatively be
and one or more compounds of the formula IB
in which
R
3
is an alkenyl group having 2 to 7 carbon atoms,
R
4
is as defined for R
a
in the formula IA, and
c is 0 or 1.
The use of compounds of the formulae IA and IB in the mixtures for SLCDs according to the invention produces, for example, long shelf lives in the display at low temperatures. These shelf lives are, for example, longer than in the case of the use of analogous liquid-crystal mixtures which comprise corresponding non-fluorinated tolane compounds instead of the compounds of the formula IA.
Furthermore, the mixtures according to the invention are distinguished by the following advantages:
they have low viscosity,
they have low temperature dependence of the threshold voltage and the operating voltage.
The invention also relates to a liquid-crystal display containing
two outer plates which, together with a frame, form a cell,
electrode layers with alignment layers on the insides of the outer plates,
a pretilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from about 1 degree to 30 degrees, and
a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 100° and 600°, and
a nematic liquid-crystal mixture of positive dielectric anisotropy which is present in the cell, consisting of
a) 5-90% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 10-80% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between −1.5 and +1.5;
c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below −1.5; and
d) an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,
 characterized in that the liquid-crystal mixture comprises one or more compounds of the formula IA
in which
R
a
is alkyl, alkoxy, alkenyl or alkenyloxy having 1 to 12 carbon atoms,
R
b
is alkyl or alkoxy having 1 to 5 carbon atoms,
Z is —COO—, —CH
2
CH
2
— or a single bond, and
 are each, independently of one another,
 and one of the rings
 may alternatively be
 and component B comprises one or more compounds of the formula IB
in which
R
3
is an alkenyl group having 2 to 7 carbon atoms,
R
4
is as defined for Ra in the formula IA, and
c is 0 or 1.
The invention also relates to corresponding liquid-crystal mixtures for use in SLCDs.
Preferred compounds of the formula IA are those compounds in which Z is a single bond.
Particularly preferred compounds of the formula IA are the compounds of the subformulae IA1 to IA10
in which R
a
and R
b
are each, independently of one another, as defined under the formula IA and are preferably each, independently of one another, straight-chain alkyl or alkoxy having 1 to 5 carbon atoms, vinyl or straight-chain 1E- or 3E-alkenyl having 3 to 7 carbon atoms, in particular R
a
is straight-chain alkyl having 1 to 5 carbon atoms, vinyl or straight-chain 1E- or 3E-alkenyl having 3 to 7 carbon atoms, and R
b
is straight-chain alkyl having 1 to 5 carbon atoms.
Of the particularly preferred compounds of the formulae IA1 to IA10, particular preference is given to those compounds in which
Particular preference is furthermore given to those compounds in which
From the group of compounds of the formulae IA1 to IA10, the compounds of the formulae IA2, IA4, IA8 and IA10 are very especially preferred.
The formula IB embraces the following compounds
in which R
3
is an alkenyl group having 2 to 7 carbon atoms, and R
4
is as defined for R
a
in the formula IA. Preference is given to compounds of the formulae IB1 and IB2 in which R
3
is a vinyl or 1E- or 3E-alkeny

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