Stock material or miscellaneous articles – Liquid crystal optical display having layer of specified... – Alignment layer of specified composition
Reexamination Certificate
2000-12-05
2004-02-03
Huff, Mark F. (Department: 1756)
Stock material or miscellaneous articles
Liquid crystal optical display having layer of specified...
Alignment layer of specified composition
C428S001400, C528S353000
Reexamination Certificate
active
06685997
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a varnish composition, more specifically to a varnish composition which is suited as a material for preparing a liquid crystal alignment film for a liquid crystal display as well as an insulating film and a protective film related to other electronic material fields.
BACKGROUND ART
At present, liquid crystal displays using nematic liquid crystal are mainly used, and well known are, for example, a matrix TN display in which a liquid crystal molecule is twisted by 90°, an STN display in which a liquid crystal molecule is twisted by 180° or more and a liquid crystal display of a so-called TFT mode in which a thin film transistor is used. Further, in recent years, an IPS (in-plane switching) type liquid crystal display of a lateral electric field type which is improved in a visual angle characteristic has been put to practical use.
A progress in a liquid crystal display depends largely on such modes, and in addition thereto, it is important to develop materials related to a liquid crystal display. In line with such object, an improvement in the materials has actively been investigated.
One of them is the development of an aligning agent for a liquid crystal alignment film. This is because a liquid crystal alignment film exerts a great influence on a display quality of a liquid crystal display and therefore an improvement thereof is further more desired in recent years.
At present, a practical liquid crystal alignment film is obtained in the form of a polyimide film by turning polyamic acid which is mainly an aligning agent component into a film and then turning it into imide or turning a soluble polyimide base polymer into a film. According to prior literature, various polymer base alignment films other than the above have been investigated, but most of them have problems on a heat resistance, a chemical (liquid crystal) resistance, electrical properties, display characteristics and alignment stability and therefore have scarcely been put to practical use.
In order to overcome this, investigated are films other than polyimide, for example, films using polyamide or polyamide having a structure in which H in an N—H portion of polyamide is substituted with other substituent, but they still have problems on an aligning property of the liquid crystal alignment film and electrical characteristics of the liquid crystal display.
Further, an aligning agent containing polyamide acid and polyamide is investigated as well, but an alignment film obtained from this has problems particularly on image sticking, a residual charge or a retention.
Thus, conventional liquid crystal alignment films have not been able to satisfy required characteristics synthetically with good balance.
DISCLOSURE OF THE INVENTION
An object of the present invention is to solve the defects of the conventional techniques described above and provide a varnish composition which can achieve well-balanced electrical characteristics such as a residual charge, a voltage-holding ratio and image sticking and various characteristics such as a pretilt angle, a coating property and an aligning property, which are desired to a liquid crystal alignment film.
Some of the problems described above are further explained next. First, with respect to the electrical characteristics, particularly a image sticking phenomenon is a problem in a TFT liquid crystal display, and therefore solution thereof is first required.
That is, caused in a TFT liquid crystal display is a so-called “image sticking phenomenon” that an image remains on a screen even after voltage applied is cut off.
This phenomenon is considered to be brought about by a lag on an element, which originates in a charge accumulated due to a DC voltage component applied. When displaying a dynamic picture, the lag thus produced brings about a so-called “tailing phenomenon” that a preceding image remaining as a lag is carried over and displayed on a subsequent image or a phenomenon that a trace of a preceding image remains after a scene is switched over. Accordingly, these defects have to be removed.
Usually employed as a method for evaluating lag is, as described later, a method based on a difference in a voltage of hysteresis in a C-V curve (capacitance-voltage curve). However, evaluation results obtained by this method sometimes do not meet an actual lag phenomenon in a display element, and therefore it has been concluded in the present invention to carry out visual evaluation in addition to evaluation according to the C-V curve mentioned above.
Next, the electrical characteristics have problems as well in terms of a voltage-holding ratio, and solution thereof is required.
That is, an image is displayed by applying voltage to a liquid crystal display element, but as referred to
FIG. 3
shown later, the above voltage is applied by repeating a series of a cycle comprising application of voltage (ON), OFF after short time passes, application of reverse voltage after fixed time passes and OFF after short time passes, thus the voltage is not continuously applied.
In order to optimize a picture, the voltage is preferably maintained at the same level as that in applying even when turning the applied voltage to OFF. In practice, however, this is impossible, and a reduction in the voltage to some extent is unavoidable. If this reduction in the voltage is great, a problem grows large particularly in TN type TFT and IPS elements. For example, while the voltage is turned to OFF and then the reverse voltage is turned to ON, the liquid crystal molecules lying in a horizontal direction in a TFT element of a TN type tilt to a vertical direction in the case where the liquid crystal used is an n type, and the liquid crystal molecules standing in a vertical direction tilt in a horizontal direction in the case of the p type, so that the contrast of the picture decrees.
Accordingly, these defects have to be removed by improving a liquid crystal material, a structure of a display element and an alignment film material, but it has not yet sufficiently been achieved.
Further, a problem on production of domains of liquid crystal is present in relation to a pretilt angle. That is, rising directions of the liquid crystal molecules have to be the same in a display element, but if a part where the rising directions become reverse is present, stripes called a domain are produced on the screen and may damage the display characteristics.
Such phenomenon is liable to be caused by a too small pretilt angle of liquid crystal molecules. Accordingly, in order to avoid this, various conditions related to a liquid crystal display have to be controlled so that the pretilt angle of the liquid crystal molecules is controlled to a fixed angle or more, and the rise directions thereof become the same. The pretilt angle described above falls suitably in a range of 3 to 12°, preferably 3 to 8° in the case of a TN type TFT element, and because liquid crystal molecules move in a horizontal direction in the case of an IPS type element, the pretilt angle does not have to be specifically enlarged and may be usually about 1°.
In order to achieve the object of the present invention, the present invention comprises the following structures.
(1) A varnish composition comprising a polymer composition comprising a polyamic acid represented by Formula (1):
(wherein R
1
represents a tetravalent organic residue originating in tetracarboxylic acids which is given by removing carboxyl groups, and R
2
represents a divalent organic residue originating in diamines which is given by removing amino groups) and a polyamide represented by Formula (2):
(wherein R
3
represents a divalent organic residue originating in dicarboxylic acids which is given by removing carboxyl groups; R
4
represents a divalent organic residue originating in diamines which is given by removing amino groups; at least one of R
5
and R
6
represents a group selected from monovalent organic groups, and the other presents hydrogen or a group selected from monovalent organic groups, but when both are groups selected from the
Fukui Kumiko
Kato Haruo
Murata Shizuo
Ono Hiroshi
Shimizu Itsuo
Chisso Corporation
Huff Mark F.
McDermott & Will & Emery
Sadule Jennifer R.
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