Nematic liquid crystal composition for active matrix...

Details Nematic liquid crystal composition for active matrix... Nematic liquid crystal composition for active matrix...

2000-02-17

2003-02-25

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

With viewing layer of specified composition

C252S299630, C252S299670, C428S001100, C349S177000, C349S180000, C349S181000, C349S186000

Reexamination Certificate

active

06524666

ABSTRACT:

SUMMARY OF THE INVENTION
The invention relates to nematic liquid crystal compositions with positive dielectric anisotropy, high voltage holding ratio and very low birefringence. They are suited for active matrix application and especially suited for reflective displays. The invention also relates to liquid crystal displays comprising the liquid crystal mixtures and to reflective liquid crystal displays with low optical retardation and with improved properties.
Active matrix displays (AMDs) are highly favored for displays with high information content. They are widely used for screens for small portable TV sets and for monitors for laptop computers. Recently they are even entering the desktop monitor domain, which is more demanding, at least with respect to optical performance. However, through AMDs do require only very little power to display information, which is a feature characteristic for almost all LCDs, their use in mobile displays so far is rather limited. The main reason for this is the fact that the intrinsically passive (not-light emitting) LCDs are equipped with a rather power consuming back light. This does even severely limit the active operation times of portable TVs and of laptop computers. And it has proven prohibitive so far to the introduction of high information content LCDs, in particular of AMDs to most battery driven applications.
Thus there is a substantial need for reflective mode AMDs with good optical performance. Liquid crystal mixtures for application in AMDs are already known e.g. from EP 365,962, EP 406,468, EP 473,821, EP 502,406, DE 4,426,904, DE 4,426,905 and EP 770,117.
Though the mixtures disclosed in the prior art, especially the documents cited above, are already characterized by a reasonable or even good voltage holding ratio, they generally have birefringence values (&Dgr;n) which are too high for displays in reflective mode. This does by no means only hold for those patent applications directed to projection type LCDs like EP 770,147, but applies quite generally to the known liquid crystal mixtures.
Even if in the mixture concepts of the prior art there are various possibilities to reduce the birefringence of the liquid crystal mixtures, these are accompanied by undesired and even not tolerable side effects like increases in the characteristic voltages and/or decreases of the stability of the nematic phase especially at low temperatures.
Furthermore, liquid crystal mixtures for reflective AMDs preferably have suitable dielectric properties such as &Dgr;&egr; at 20° C. of at least 5 and prefeably of less than 13, and &Dgr;&egr; at 20° C. of at most 7 (all at 1 kHz).
The inventive liquid crystal mixtures are especially suited for active matrix addressed LCDs, e.g. operating in the “first minimum” TN-mode, as disclosed in DE 3,022,818.
It has now been found that, surprisingly, this task could be achieved by using a liquid crystal mixture which comprises
5 to 15 % of one or more compounds from group 1:
group 1:
wherein R
1
denotes a straight-chain alkyl group of 5 to 7 carbon atoms;
30 to 40% of two or more compounds from group 2:
group 2:
wherein R
21
denotes a straight-chain alkyl group of 3 to 7 carbon atoms and R
22
denotes a straight-chain alkoxy group of 1 to 3 carbon atoms or an 1 E-alkenyl group with 2 or 3 carbon atoms;
25 to 35 % of three or more compounds from group 3:
group 3:
wherein R
3
denotes a straight-chain alkyl group of 2 to 5 carbon atoms;
10 to 20% of two or more compounds from group 4:
group 4:
wherein R
4
denotes a straight-chain alkyl group or a 1 E-alkenyl group, both with 2 or 3 carbon atoms and Z
4
denotes a —CH
2
CH
2
— group or a single bond;
8 to 12% of three compounds of group 5:
group 5:
wherein R
51
denotes n-alkyl with 3 or 4 carbon atoms and R
52
denotes n-alkyl with 3 or 5 carbon atoms; and
4 to 8% of two or more compounds of group 6:
group 6:
wherein R
61
denotes n-alkyl with 3 carbon atoms and R
62
denotes n-alkyl with 3 or 4 carbon atoms.
Preferably the liquid crystal mixture contains
one compound from group 1, preferably with R
1
n-heptyl;
one compound each of subgroups 2
a
and 2
b
Subgroup 2
a
:
wherein R
11′
denotes straight-chain alkyl with 3 or 5, preferably 5, carbon atoms and R
12′
denotes H or CH
3
, preferably H,
Subgroup 2
b
:
wherein R
11″
denotes straight-chain alkyl with 3 or 5, preferably 5, carbon atoms, and R
12″
denotes straight-chain alkyl with 1 or 2, preferably 1, carbon atoms;
three compounds of group 3, preferably with R
3
ethyl, n-propyl and n-pentyl, respectively;
one compound each of subgroups 4
a
and 4
b:
Subgroup 4
a
:
wherein R
41
denotes n-alkyl with 2 to 5 carbon atoms or preferably 1 E-alkenyl with 2 or 3, preferably 2, carbon atoms
Subgroup 4
b
:
wherein R
42
denotes n-alkyl with 2 or 3, preferably 3, carbon atoms;
three compounds of group 5, and
two compounds of group 6.
The preferred concentration ranges for the compounds of groups 1 to 6 are as given in the following table (Table 1).
TABLE 1
Preferred concentration ranges of groups respectively
subgroups
group
concentration/%
group 1
 6 to 12
group 2
31 to 35
group 3
28 to 33
group 4
12 to 16
group 5
 8 to 12
group 6
4 to 8
subgroup
concentration/%
group 2a
15 to 20
group 2b
14 to 18
group 4a
 8 to 10
group 4b
5 to 8
The preferred concentration ranges of the subgroups 2
a
, 2
b
, 4
a
and 4
b
are also given in Table 1.
The inventive mixtures have a nematic phase range from at least −20 ° C. to 80 ° C. preferably from at least −30 ° C. to 85 ° C.
The liquid crystal compositions according to the instant invention can contain suitable additives known to the expert such as e.g. chiral dopants and dyes. The preferred concentrations for these additives is 0 to 3%, preferably 0.1 to 1% per compound and 0 to 10%, preferably 0.1 to 3% total for all additives, relative to the other components.
When the concentrations of the other components of the liquid crystal mixture are specified the additives are not considered.
The compounds used in the inventive mixtures and their preparation is know to the expert or they are prepared analogously to known compounds.
The compounds of groups 1 to 5 especially are known from EP 0 003 215, EP 0 014 840, EP 0 087 032, EP 0 122 389, EP 0 125 563, EP 0 132 553 and EP 0 441 932.
All physical properties in this application relate to and are determined at a temperature of 20° C., all concentrations are given in mass %, both unless explicitly stated otherwise.
The physical properties of the liquid crystals and of the displays have been determined as described in “Merck Liquid Crystals, Physical Properties of Liquid Crystals, Description of the Measurement Methods” ed. W. Becker, states November 1997, unless explicitly stated otherwise.
The characteristic optical properties have been determined in TN test cells manufactured by Merck KGaA, however also commercially available cells may be used, with crossed polarizes, perpendicular to the rubbing direction of the adjacent substrates. The test cells had orientation layers of AL-1051 from Japan Synthetic Rubber Co., Japan. Characteristic voltage values for 10%, 50% and 90% relative contracts have been determined using a He-Ne layer as a light source.
Storage tests at deep temperature have been performed in sealed test cells of the same type used for the electro-optical investigation, as used in the electrooptical investigations. The test cells were provided with crossed adhesive polarizes. Five test cells were investigated in parallel at several fixed temperatures (−20° C., −30° C. and optionally −40° C.). The time for storage was given as the time at which, after storage at a given temperature, under visual inspection, still none of five test cells showed any defects.
Alternatively, the liquid crystal mixtures were stored in three separate bottles each. The quantity of liquid crystal material was 1 g each in a 3 cm
3
bottle. The evaluation was again by inspection by eye.
The preparation of the compositions according to the invention proceeds in conventio

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