Cholesteric liquid crystal display that prevents image sticking

Details Cholesteric liquid crystal display that prevents image sticking Cholesteric liquid crystal display that prevents image sticking

1999-12-22

2002-04-02

Wu, Shean C. (Department: 1756)

Liquid crystal cells, elements and systems

Particular excitation of liquid crystal

Electrical excitation of liquid crystal

C349S036000, C349S176000, C349S182000, C252S299500, C252S299610, C428S001100

Reexamination Certificate

active

06366330

ABSTRACT:

FIELD OF INVENTION
The present invention is directed to a chiral nematic liquid crystal material for use in a display device. More specifically, the invention relates to chiral nematic liquid crystal materials that include organic additives in an amount effective to avoid image sticking in a liquid crystal display device.
BACKGROUND OF THE INVENTION
Current cholesteric liquid crystal display devices, upon application of an electric field, exhibit a planar texture which selectively reflects a certain wavelength of light depending on the pitch length of the material. A focal conic texture is also present, in which light is scattered, not reflected. Such displays are preferably designed so that both the focal conic and planar textures are stable in the absence of an electric field.
Cholesteric displays are often used in public information signs and static image applications because of the wide viewing angle and its low power bistable operation modes. During operation of the liquid crystal display device, there are times when a pixel within the display remains in the focal-conic texture for some period and is then switched to the planar texture and vice versa. If the intensity level of this planar state is less than that of a pixel that was previously in the planar state, or conversely, if the intensity levels between focal conic textures of the same pixel are greater than that of a pixel that was previously observed in the earlier focal conic texture, the display is said to have image sticking. As a result of image sticking, the previous image of the display affects the new image of the display: a ghost image from the previous image appears in addition to the new image. For example, suppose the capital letter “T” is displayed by a cholesteric liquid crystal display device. The display may comprise a dark background wherein the light is scattered by the focal conic texture. The letter “T” would be defined by a brighter area within the dark background in which the liquid crystal is in the planar texture. Now suppose the letter is changed to the capital letter “I” and superimposes the position where the letter “T” was observed. If image sticking occurred, the outwardly extending horizontal portions of the letter “T” would still be slightly visible. In this example, the horizontal portions would be slightly lighter than the surrounding dark background but darker than the light portion used to display the letter “I”. Thus, the display would undesirably show a ghost image of the non-overlapping portions of the letter “T”. Generally, between updates, a display is refreshed to minimize the effect of image sticking. However, image sticking can be strong enough so that a refresh pulse is unable to completely erase the previous image.
Efforts have been made to eliminate image sticking or reduce the degree of image sticking by modification of driving waveforms. Various types of erasing waveform combinations have been discussed previously. For example, U.S. Pat. No. 5,644,330 entitled “Driving Method for Polymer Stabilized and Polymer Free Liquid Crystal Displays”, to Catchpole et al. discloses erasing waveforms. By employing a good erasing waveform, image sticking can be significantly reduced or even eliminated in some cases. However, it is noticed that in certain display configurations, image sticking cannot be completely removed solely by optimization of the driving waveforms. The degree of image sticking becomes very profound when the time duration between updates is long, i.e., greater than 24 hours. End users can usually heat the display above the isotropic transition temperature to eliminate the image sticking result from long time storage or shipping. However, this practice is not cost effective and user friendly. The invention addresses the problem of image sticking from a liquid crystal formulation approach.
SUMMARY OF THE INVENTION
The present invention is directed in general to a liquid crystal mixture for a display device which comprises a first chiral nematic liquid crystal component, and a second component that, by itself, exhibits no liquid crystalline phase at any temperature. The second component is present in an amount effective to avoid image sticking and/or reduce the bulk viscosity of the material by at least about 26% and, more preferably, by at least about 46%. The liquid crystal mixture preferably has a positive dielectric anisotropy. Display devices that include the present material exhibit greatly improved properties including higher contrast ratios, shorter pulse widths, lower driving voltages and no image sticking during display use.
In particular, at least in the case of viscosity lowering additives, the second component is achiral and has a molecular weight not exceeding about 205 grams/mole. The second component, when used as an image sticking additive, is preferably in an amount ranging greater than 0% to not greater than 5% by weight based upon the total weight of the material and, more preferably, in an amount ranging up to 3% by weight. Amounts of all chemical components herein are in % by weight based upon the total weight of the liquid crystal material, unless otherwise indicated.
A preferred embodiment of the invention is directed to a display device comprising a liquid crystal material including the first chiral nematic liquid crystal component, and the second component which exhibits no liquid crystalline phase at any temperature. The second component is effective for avoiding image sticking during display use and/or reducing viscosity of the liquid crystal material. Cell wall structure cooperates with the material to form focal conic and twisted planar textures that are stable in the absence of a field. A device applies an electric field for transforming at least a portion of the material to at least one of the focal conic and twisted planar textures.
The first component is a chiral nematic liquid crystal material. The second component may be a blend of additives that give the desired properties. More preferably, the second component is one compound that avoids image sticking. The material cooperates with the cell wall structure to form focal conic and twisted planar textures that are stable in the absence of a field.
The second component that helps avoid image sticking and/or lowers the viscosity of the material may be comprised of a compound having the following general formula I:
wherein R
1
is selected from the group consisting of an alkyl group, a substituted alkyl group and an alkoxy group each having from 1 to 9 carbon atoms.
More preferably, the second component may have the following formula:
wherein A comprises any group that renders the second component polar and soluble in the first component. In this regard, group A preferably comprises a cyano group. In the case wherein the additive is used for viscosity lowering properties, when group A is a cyano group the second component preferably does not include seven carbon atoms.
Preferred substituted phenyl compounds that satisfy the general formula I comprise the following: 1-phenylpentane, 1-phenylhexane, 1-cyclohexylbenzene, p-cyanopentylbenzene, 4-phenylbutyronitrile and p-cyanophenylheptane.
Alternatively, the second component for avoiding image sticking and/or lowering the viscosity of the material may be comprised of a compound having the following formula II:
R
2
—B  II.
wherein R
2
is selected from the group consisting of an alkyl group and an alkoxy group each having from 1 to 11 carbon atoms and B comprises any group that renders the second component polar and soluble in the first chiral nematic liquid crystal component. A preferred second component having a formula that satisfies formula II is undecyl cyanide.
The second component in the case of both formulas I and II preferably has a molecular weight not exceeding about 205 grams/mole at least in the case of viscosity lowering additives and possibly for additives that avoid image sticking. Groups A and B are preferably cyano groups. R
1
and R
2
are preferably alkyl, substituted alkyl groups or alkoxy groups.
The im

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