Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified...
Reexamination Certificate
2001-12-28
2004-06-08
Huff, Mark F. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
C428S473500
Reexamination Certificate
active
06746730
ABSTRACT:
TECHNICAL FIELD
This invention relates to varnish compositions which provide excellent characteristics in respect of electrical properties (voltage holding ratio, residual charge or image sticking resistance), acceptable pretilt angle, coating property, or alignment property of a liquid crystal molecule, etc., when used as a liquid crystal aligning film. Since the varnish composition of the invention has excellent electrical properties, it can be also used for an insulating coat, a protective coat, etc. in the field of electronic materials in addition to the liquid crystal aligning film.
BACKGROUND ART
In a liquid crystal display element, a display element using a nematic liquid crystal is now prevailing. Display elements by various systems are put to practical use, such as a matrix TN element twisted 90 degrees, a STN element usually twisted 180 degrees or more, a TFT liquid crystal display element using a thin film transistor, and an IPS (In Plane Switching) liquid crystal display element of horizontal electric field (electric field directed in parallel to a circuit board plane) type which is improved in visual angle characteristics. However, progress of a liquid crystal display element is not necessarily seen only in the development of display system. For the purpose of improving the properties of a liquid crystal display element, its associated material has also been improved actively. A liquid crystal aligning film is one of the important elements concerning the display quality of a liquid crystal display element, and its role becomes larger as a display element of high quality is required.
The liquid crystal aligning agents that are mainly used now include polyamic acid and soluble polyimide resins. Of these conventional materials for the liquid crystal aligning film, however, there have not yet been found those which have comprehensively sufficient characteristics in respect of electrical properties as a liquid crystal display element, coating property of the aligning agent, pretilt angle (large angle is desired), alignment of a liquid crystal molecule, etc.
Methods of mixing a soluble polyimide with a polyamic acid have been studied for improving electrical properties (particularly, small residual charge, large voltage holding ratio) of a liquid crystal display element. In this case, a soluble polyimide with a side chain group does not give so large pretilt angle (refer to Comparative Examples as given later). For this reason, when a certain degree of pretilt angle is required in a TFT element, a STN element, etc., there are the problems of easy occurrence of domain, etc. Since a soluble polyimide has poor solubility as compared with a polyamic acid, the selection range of the solvent is restricted. Even if a polyimide having the solubility as good as the polyamic acid is prepared, however, a polyimide aligning film made therefrom may easily be dissolved by a liquid crystal because of its good solubility, and thus the alignment of a liquid crystal molecule may be disturbed.
Moreover, most of good solvents to a polyimide are aprotic polar solvents generally having large surface tension and have the problems of cissing apt to occur upon coating, etc. When a polyamic acid is used, a solvent having small surface tension such as cellosolve and carbitol solvents is usually added in these solvents for the purpose of more improving the coating property, etc. However, if these solvents are used as a solvent for a polyimide varnish in such a large amount as to improve the coating property, there may arise the problem that a polyimide is easy to precipitate. Moreover, the reason that the coating property of a polyimide varnish is inferior to a polyamic acid varnish may be the fact that a polyimide does not have a carboxyl group, in addition to the above-mentioned problem that the selection range of the solvent is restricted.
Moreover, there is a problem of the voltage holding ratio concerning electrical properties of a liquid crystal display element. That is, voltage is not applied continuously to the liquid display element during the display of the image. As shown in
FIG. 2
, voltage is turned OFF in a short time after the application of voltage, and subsequently a reverse voltage is applied for a short time after a fixed time, and voltage is again turned OFF, thereby performing the display of image. In this manner, it is desirable to hold an applied voltage as it is when voltage is turned OFF, but voltage actually drops without holding the applied voltage as it is. If this voltage drop is large, the contrast of a display image will be lowered. Thus, it is preferred that the voltage holding ratio is larger.
On the other hand, mixing a polyamic acid free from a side chain with a polyamic acid having a long side chain may prepare a liquid crystal display element having small residual charge, depending on the combination of polyamic acids, but the improvement in image sticking is insufficient even in that case.
If a liquid crystal display element is left for some time with voltage applied, a charge is accumulated. The accumulated charge remains as it is when voltage is turned OFF, and the display element may be in the state as if voltage were applied. When voltage is turned OFF or the image is switched to another one, in such a state, a phenomenon will occur wherein a previous image remains displayed or a vague image outline remains. This phenomenon is usually called “image sticking”.
It is usually said that small residual charge can overcome this image sticking. Although this may generally be right, the mechanism for the generation of an image sticking is complicated and the image sticking may not be fully estimated by measurement of only a residual charge (In fact, the mechanism of an image sticking is not fully solved).
In the present invention, the estimation of an image sticking is not based on the measurement of a residual charge, but is actually performed by visual observation of the image sticking of a cell.
Such a phenomenon poses a great problem especially in a TN type TFT or an IPS element.
An object of the present invention is to provide an improved aligning agent which can overcome the above-mentioned disadvantages in the case of mixing a polyamic acid with a soluble polyimide in the varnish for aligning agents, which has a great influence on the display characteristics of a liquid crystal display element.
More particularly, a subject of the invention is to provide a varnish composition for aligning agents which can adjust a pretilt angle arbitrarily (it hardly generates a domain accordingly), gives no or little image sticking, and has excellent coating property (no cissing occurs and the coating of uniform thickness is obtained).
DISCLOSURE OF THE INVENTION
As a result of our intensive studies for the purpose of overcoming the above-mentioned problems in the prior art, the inventors have found that a varnish composition in a specific combination is useful as an aligning film material used for a liquid crystal display element, thus leading to the completion of the present invention.
The present invention includes the following items (1) to (20).
(1) A varnish composition which comprises a polymer component containing a polyamic acid B represented by formula (1), a polyamic acid A represented by formula (2) and a soluble polyimide represented by formula (3), and a solvent for dissolving the polymer component, in which the weight ratio of the polyamic acid A to the soluble polyimide in the polymer component is 1/99 to 99/1, the sum of the polyamic acid A and the soluble polyimide accounts for 1 to 80% by weight of the polymer component, the polyamic acid B accounts for 99 to 20% by weight of the polymer component, and the proportion of the polymer component in the whole varnish composition is 0.1 to 40% by weight;
wherein R
1
is a tetravalent organic radical derived from tetracarboxylic acids, R
2
is a divalent organic radical derived from a diamine, each of which is a radical having no side chain, a radical having an alkyl side chain of less than 3 carbon ato
Fukui Kumiko
Murata Shizuo
Ono Hiroshi
Shimizu Itsuo
Tanioka Satoshi
Chisso Corporation
Huff Mark F.
Sadule Jennifer R
Wenderoth , Lind & Ponack, L.L.P.
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