Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-08-29
2003-05-13
Wu, Shean C. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C252S299630, C252S299660, C252S299670, C349S179000, C349S186000
Reexamination Certificate
active
06562421
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a supertwisted nematic liquid crystal device having reduced dependence of threshold voltage on temperature and frequency.
BACKGROUND OF THE INVENTION
Supertwisted nematic liquid crystal displays (hereinafter abbreviated as STN-LCDS) have found wide uses and have been required to satisfy various characteristics to cope with still wide applications. For example, STN-LCDs for use in personal digital assistances (PDAS) have been required to be capable of displaying a vast volume of information and to exhibit satisfactory display quality over a broad operating temperature. Addition of an electrical temperature-compensating circuit to the driving circuit has been adopted to reduce the influences of the surrounding temperature. However, the addition of an extra circuit involves process complication, which results in a reduction of yield. Therefore, it has been desired that the threshold voltage of LCDs be insusceptible to the temperature influences. Further, in a multiplexing drive system for displaying a large volume of information, it has also been demanded that the threshold voltage of STN-LCDs should show no fluctuation in a frequency range of the driving voltage which varies according to the duty ratio and the contents of display. That is, STN-LCDs, the threshold voltage of which has reduced temperature dependence as well as reduced frequency dependence within an operating temperature range, have been demanded. Fulfillment of these requirements will make it possible to obtain a satisfactory display that is not influenced by the surrounding temperature even when placed under severe temperature conditions, such as in outdoor use. Although various approaches have been proposed as described in JP-A-4-296387 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”), JP-A-4-300681, JP-A-7-209624, JP-A-9-157654, WO89/08102 and WO91/08184, the degrees of improvements in temperature dependence or frequency dependence of threshold voltage by these approaches are not satisfactory, and further improvements in liquid crystal compounds, liquid crystal compositions or STN-LCDs have still been demanded.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an STN-LCD, the threshold voltage of which has reduced temperature dependence and frequency dependence in its operating temperature range.
Other objects and effects of the present invention will be apparent from the following description.
The objects of the invention have been achieved by providing a supertwisted nematic liquid crystal display comprising:
a pair of substrates each having a liquid crystal orientation-controlling layer and a transparent electrode;
a liquid crystal material held between said substrates; and
at least one polarizing sheet provided on at least one of said substrates,
wherein said liquid crystal material comprises:
(a) a compound represented by formula (I):
wherein R
1
represents an alkyl group having 1 to 16 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, an alkenyloxy group having 3 to 16 carbon atoms; and X
1
represents a hydrogen atom or a fluorine atom; and
(b) at least one compound selected from the group consisting of: compounds represented by formula (II):
wherein R
2
represents a fluorine-substituted or unsubstituted alkyl group having 1 to 16 carbon atoms, a fluorine-substituted or unsubstituted alkoxy group having 1 to 16 carbon atoms, an alkenyl group having 2 to 16 carbon atoms, an alkoxyalkyl group having 2 to 16 carbon atoms, or an alkenyloxy group having 3 to 16 carbon atoms; and compounds represented by formula (III):
wherein R
3
has the same meaning as R
2
,
wherein said liquid crystal display satisfies:
condition formula (i):
|
V
th
(
-
20
°
C
.
)
-
V
th
(
50
°
C
.
)
70
°
C
.
|
≦
3
m
V
/
°
C
.
(
i
)
wherein V
th
(−20° C.) and V
th
(50° C.) represent the threshold voltage of the liquid crystal display measured at a temperature of −20° C. and 50° C., respectively, with an applied voltage having a square wave form and a frequency of 100 Hz; and
condition formula (ii):
|
V
th
(
5000
Hz
)
-
V
th
(
100
Hz
)
V
th
(
100
Hz
)
|
≦
0.3
(
ii
)
wherein V
th
(5000 Hz) and V
th
(100 Hz) represent the threshold voltage of the liquid crystal display measured at −20° C. with an applied voltage having a square wave form and a frequency of 5000 Hz and 100 Hz, respectively.
The temperature dependence of threshold voltage is represented by the left side of condition formula (i). If this value exceeds 3 mV, satisfactory display quality cannot be secured because the display has too faint image in a low temperature range and too deep image in a high temperature range. The frequency dependence of threshold voltage is defined by the left side of condition formula (ii). Since the frequency of applied voltage varies according to the duty ratio or the contents of a display in a multiplexing driving system, which is a driving system for STN-LCDs, a threshold voltage frequency dependence exceeding 0.3 results in display unevenness due to visible changes of threshold voltage with the display contents. Because the threshold voltage of the STN-LCD according to the present invention has a temperature dependence of not more than 3 mV and a frequency dependence of not more than 0.3, the STN-LCD of the invention exhibits satisfactory display quality with no unevenness in its operating temperature range.
DETAILED DESCRIPTION OF THE INVENTION
In formula (I) representing the component (a) for use in the present invention, R
1
is preferably an alkyl group having 1 to 8 carbon atoms or an alkenyl group having 2 to 8 carbon atoms. For increasing the contrast, R
1
is still preferably an alkenyl group represented by formula (p) or (q):
(the group of formula (p) or (q) is bonded at its right end to the ring). The alkenyl group of formula (q) is particularly preferred. X
1
is preferably a hydrogen atom.
The compound (I) is preferably a compound represented by formula (I-a) or (I-b):
wherein R
13
represents an alkyl group having 1 to 16 carbon atoms or an alkenyl group having 2 to 8 carbon atoms.
The content of the compound (I ) in the liquid crystal material is preferably 5 to 40% by weight. The liquid crystal material of the invention preferably comprises one to four kinds, particularly preferably one or two kinds, of each of the compound (I-a) and the compound (I-b). The total content of the compounds (I-a) and the compound (I-b) in the liquid crystal material is preferably 5 to 40% by weight, more preferably 5 to 30% by weight, particularly preferably 5 to 20% by weight.
The content of the component (b), selected from the group consisting of compounds represented by formula (II) and compounds represented by formula (III), in the liquid crystal material is preferably 5 to 40% by weight, more preferably 10 to 30% by weight. While the liquid crystal material contains either one of the compound (II) and the compound (III), it is preferred to contain the compound (III) for reducing the temperature dependence and the frequency dependence of threshold voltage. For ensuring reduction in temperature dependence and frequency dependence of threshold voltage, it is more preferred for the liquid crystal material to contain both the compound (II) and the compound (III).
In formulae (II) and (III), R
2
and R
3
each preferably represent an alkyl group having 1 to 16 carbon atoms or an alkenyl group having 2 to 16 carbon atoms, still preferably represent an alkyl group having 1 to 8 carbon atoms or an alkenyl group having 2 to 8 carbon atoms, particularly preferably represent an alkenyl group represented by any one of the following formulae (p) to (t):
(each structural formula is bonded at its right end to the ring).
In particular, the alkenyl group represented by formula (r) or (s)
Kawakami Shotaro
Kuriyama Takeshi
Ohnishi Hiroyuki
Sudo Go
Takatsu Haruyoshi
Dainippon Ink and Chemicals Inc.
Wu Shean C.
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