Compositions – Liquid crystal compositions
Patent
1996-02-12
1998-02-03
Kelly, C. H.
Compositions
Liquid crystal compositions
25229963, 25229966, 25229967, 2522996, C09K 1952, C09K 1930, C09K 1912, C09K 1920
Patent
active
057140871
DESCRIPTION:
BRIEF SUMMARY
The invention relates to supertwist liquid-crystal displays (STN displays) having extremely short switching times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.
STN displays as in the heading are known, for example from EP 0,131,216 B1; DE 3,423,993 A1; EP 0,098,070 A2; M. Schadt and F. Leenhouts, 17th Freiburg conference 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-L1786 (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 SFA here includes any relatively highly twisted display element having a twist angle with a value between 160.degree. and 720.degree., 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), the STN-LCDs (DE OS 3,503,259), 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 OS 0,246,842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).
Compared with standard TN displays, STN displays of this type are distinguished by significantly better steepnesses of the electrooptical characteristic and, associated therewith, better contrast values, and by a significantly lower angle dependence of the contrast. Of particular interest are STN displays with extremely short switching times, in particular also at relatively low temperatures. In order to achieve short switching times, the viscosities, in particular of the liquid crystal mixtures have hitherto been optimized using usually optimized combinations of liquid crystal components and optionally also monotropic additives of relatively high vapour pressure. However, the switching times achieved were not adequate for all applications.
Shorter switching times can also be achieved by reducing the thickness of LC layer of the STN display and using liquid-crystal mixtures with a higher birefringence .DELTA.n.
All these approaches to shorter switching times, however, still end up with mixtures which were not adequate for every use.
The European patent EP 0 366 985 describes STN displays with comparable short switching times with 180.degree. twist based on 4-alkyl-4'-alkoxybicyclohexyles. There is still a demand to improve these switching times.
Further demands for STN display are a higher multiplexability, lower threshold voltages and a steep characteristic curve.
Optimal parameters, however, cannot be achieved simultaneously for all the properties mentioned above because of opposite influence of different material parameters such as dielectric and elastic properties.
Therefore, there continues to be a great demand for improved STN displays having short switching times and, at the same time, a broad service temperature range, high characteristic line steepness, good angle dependency of the contrast and low threshold voltage.
The invention has the object of providing STN displays which only have the abovementioned disadvantages to a small extent, or not at all, and at the same time have very useful overall properties.
It has now been found that this object can be achieved if nematic liquid-crystal mixtures are used which contain one or more compounds selected from the formulae I1, I2, II1 and II2 ##STR4## wherein R.sup.3 and R.sup.4 are each independently alkyl with 1 to 8 C atoms, ##STR5## wherein R.sup.1 is alkyl or alkoxy with 1 to 8 C atoms, ##STR6## wherein R.sup.2 is oxaalkyl, alkenyl with up to 8 C atoms.
The nematic liquid-crystal mixture preferably has a nematic phase range of at least
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Bremer Matthias
Ichinose Hideo
Junge Michael
Kojima Akihiro
Numata Hiroshi
Kelly C. H.
Merck Patent Gesellschaft mit beschrankter Haftung
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