Liquid-crystalline mixtures

Details Liquid-crystalline mixtures Liquid-crystalline mixtures

2002-07-26

2004-11-23

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299610, C252S299630, C252S299660, C252S299670

Reexamination Certificate

active

06821582

ABSTRACT:

One aspect of the invention relates to liquid-crystalline mixtures for twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies. The mixtures according to the invention are furthermore also suitable for IPS (in plane switching) displays.
TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays 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. Appl. 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 STN here covers any relatively highly twisted display element having a twist angle with a value of between 160° and 360°, such as, for example, the display elements according to 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)).
STN displays are distinguished from standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, at moderate and relatively high multiplex rates, for example from 32 to 64, by better contrast values. On the other hand, the contrast in TN displays is generally higher, owing to the better dark value, and the angle dependence of the contrast is lower than in STN displays having low multiplex rates of, for example, less than 32.
Of particular interest are TN and STN displays having very short response times, in particular at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimised using mostly monotropic additives having relatively high vapor pressure. However, the response times achieved were not adequate for every application.
In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K
33
/K
11
and relatively small values for &Dgr;&egr;/&egr;
⊥
, where &Dgr;&egr; is the dielectric anisotropy and &egr;
⊥
is the dielectric constant perpendicular to the longitudinal molecular axis.
In addition to optimisation of the contrast and response times, further important requirements are made of mixtures of this type:
1. 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 in the art are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about {fraction (1/400)}), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about {fraction (1/64)} and {fraction (1/16)} respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.
Thus, there continues to be a great demand for TN and STN displays, in particular for moderate- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high characteristic-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.
One of the objects of the invention is to provide TN and STN displays which do not have the above-mentioned disadvantages or do so only to a lesser extent, and at the same time have short response times, in particular at low temperatures, and very good steepnesses.
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.
These objects can be achieved by using nematic liquid-crystal mixtures which comprise one or more compounds of formula IA,
one or more compounds of formula IB,
 and
one or more compounds of formula IC,
wherein
R
a
, R
b
and R
d
are each, independently of one another, an alkyl group having up to 12 carbon atoms which is unsubstituted or substituted by at least one halogen atom and in which one or two non-adjacent CH
2
groups may be replaced by —O—, —CH═CH—, —CO—, —OCO— or —CO—O— in such a way that O atoms are not linked directly to one another, where, in the case where a=1, R
d
may alternatively be F, Cl, OCHFCF
3
, CF
3
or OCF
3
,
R
c
is alkenyl or alkenyloxy having from 2 to 6 carbon atoms,
Z is —CO—O—, —CF
2
O—, —OCF
2
— or a single bond,
L
1
to L
4
are each, independently of one another, H or F, and
a is 0 or 1,
where the proportion of compounds of formula IC in the mixture is at least 25% by weight.
The use of the compounds of the formulae IA, IB and IC in the mixtures for TN and STN displays according to the invention results in at least
high steepness of the electro-optical characteristic line,
low temperature dependence of the threshold voltage, and
very fast response times, in particular at low temperatures.
The compounds of the formulae IA, IB and IC significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and low temperature dependence of the threshold voltage.
The mixtures according to the invention are furthermore distinguished by at least the following advantages:
they have low viscosity,
they have low threshold voltage and operating voltage, and
they effect long shelf lives in the LC display at low temperatures.
The mixtures according to the invention are also suitable for IPS displays.
The mixtures according to the invention are, in particular, suitable for liquid-crystal displays having
two outer plates, which, together with a frame, form a cell,
a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,
electrode layers with alignment layers on the insides of the outer plates,
a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 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 22.5° and 600°.
Liquid-crystal displays of this type preferably contain a nematic liquid-crystal mixture comprising
a) 15-80% by weight of a liquid-crystalline component A containing one or more compounds having a dielectric anisotropy of greater than +1.5;
b) 25-85% by weight of a liquid-crystalline component B containing 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 containing one or more compounds having a dielectric anisotropy of below −1.5, and
d) if desired, 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 greater than about 0.2, preferably 0.2 to 1.3,
characterised in that they comprise at least one compound of formula IA,
one or more compounds of formula IB,
 and
one or more compounds of formula IC,
wherein
R
a
, R
b
and R
d
are each, independently of one another, an alkyl group having up to 12 carbon atoms which is unsubs

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