JPH05241164A - Liquid crystal oriented film and liquid crystal display element using that - Google Patents

Abstract

PURPOSE:To obtain a liquid crystal oriented film which hardly suffers from scratches during rubbing although the film contains solvent-soluble polyimide by using a polyimide resin having urethane coupling in the principal chain for the oriented film. CONSTITUTION:A transparent conductive film 2 is formed on the surface of a transparent substrate 1, on which an oriented film 3 is formed. These laminated structure of transparent substrate 1, transparent conductive film 2 and oriented film are provided in upper and lower sides, and a liquid crystal layer 4 is sealed between these to form a liquid crystal display element. The transparent substrate 1 consists of a transparent material such as glass, acryl resin, and polycarbonate resin, and the transparent conductive film 2 comprising tin oxide or ITO is formed on the substrate 1, and further an oriented film 3 containing a polyimide resin having urethane coupling in the principal chain is formed. Any kind of polyimide resin having urethane coupling in the principal chain may be used, and it is preferable that the ratio of imide coupling to urethane coupling is larger than 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal alignment film having the ability to align liquid crystals used in liquid crystal displays and the like, and a liquid crystal display device using the same.

[0002]

2. Description of the Related Art Conventionally, various polyimide materials have been mainly used industrially as an alignment film for aligning liquid crystals in a liquid crystal display device. Generally, polyimide materials are insoluble in solvents. Therefore, the polyimide alignment film is manufactured as follows.

That is, a polyimide precursor (polyamic acid) thin film is formed on a substrate and then heated at a temperature of about 250 ° C. for imidization to form a polyimide film. However, when considering colorization of the liquid crystal display, the heat resistance of the dye used for the color filter is not so high. Therefore, the heating temperature for film formation must be suppressed to 180 ° C. or lower.

However, when a conventional polyimide alignment film which heat-imidizes a polyamic acid is heated at 200 ° C. or lower, the alignment film remains with a chemically unstable polyamic acid structure. To overcome this problem, Hosaka et al. Have developed a solvent-soluble polyimide (see Journal of the Printing Society of Japan, Volume 27, No. 6, P. 16 to P. 21). This alignment film distorts the three-dimensional structure of the imide ring itself, weakens the interaction of molecular chains, and dissolves in a polar solvent. The production method is to prepare a soluble polyimide from polyamic acid from 2,3,5-tricarboxycyclopentyl acetic acid dianhydride and diamine. In this case, since the imidization reaction is not caused by heating, there is an advantage that chemically unstable polyamic acid does not remain in the alignment film.

[0005]

However, such a conventional solvent-soluble polyimide alignment film has a problem that the alignment film is easily scratched during rubbing for imparting alignment properties.

In view of the above problems of the conventional liquid crystal alignment film, the present invention provides a liquid crystal alignment film which is soluble and is not easily scratched during rubbing, and a liquid crystal device using the same. It is intended.

[0007]

The present invention is a liquid crystal alignment film containing a polyimide resin having a urethane bond in the main chain.

Further, the present invention is a liquid crystal display device using such a liquid crystal alignment film.

[0009]

The liquid crystal alignment film of the present invention contains a polyimide resin having a urethane bond in the main chain. Since the urethane bond has a high polarity and is flexible, it is easily dissolved in a polar solvent. Therefore, the liquid crystal alignment film of the present invention is soluble.

Further, it is considered that the liquid crystal alignment film is easily scratched when the adhesion between the liquid crystal alignment film and the substrate is poor. The liquid crystal alignment film of the present invention has a urethane bond as described above. Therefore, since the liquid crystal alignment film of the present invention forms a hydrogen bond with the surface of the substrate at the urethane bonding portion, the adhesiveness with the substrate becomes good. Therefore, scratches are less likely to occur during rubbing.

[0011]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the liquid crystal display device of the present invention.

A transparent conductive film 2 is formed on the surface of a transparent substrate 1, and an alignment film 3 is further provided on the surface thereof. Two sets of the transparent substrate 1, the transparent conductive film 2 and the alignment film 3 are vertically arranged, and the liquid crystal layer 4 is enclosed between them to form a liquid crystal display element. As the transparent substrate 1, known transparent materials such as glass, acrylic resin, and polycarbonate resin can be used.

Deposition / sputtering / CV on the transparent substrate 1
The transparent conductive film 2 made of tin oxide or ITO is formed by means such as D. Further, an alignment film 3 containing a polyimide resin having a urethane bond in the main chain is formed on the surface thereof.

The polyimide resin having a urethane bond in the main chain of the present invention may have any structure.
However, if the ratio of the urethane bond to the polyimide resin is high, the heat resistance is lowered, which is not suitable. The abundance ratio of the imide bond to the urethane bond is more preferably larger than 1.

In terms of chemical structure, a liquid crystal having a straight-chain structure and a partial crystal structure is most suitable because of high liquid crystal alignment.

The method for synthesizing the urethane-modified polyimide is not particularly limited, but it can be easily synthesized as follows.
That is, the synthesis is performed in the following two steps. The first step is to polymerize a diisocyanate and a bifunctional polyol to synthesize a terminal isocyanate compound having a urethane bond. Here, the terminal isocyanate can be obtained by setting the molar ratio of the diisocyanate component to the bifunctional polyol to be between 1 and 2. Here, as the polyol, polyethylene glycol having various molecular weights or polyesters having a terminal hydroxyl group obtained by polycondensing a diol and a dicarboxylic acid can be used. It is preferable to use a polyol having a linear hydroxyl group having a terminal hydroxyl group because the orientation of the liquid crystal is good. Diisocyanates such as 4,4′-diphenylmethane diisocyanate 2,4-toluene diisocyanate and 1,6-hexamethylene diisocyanate can be used as the isocyanate component.

As the second step, the terminal isocyanate compound having a urethane bond is reacted with tetracarboxylic dianhydride to form an imide bond. Any tetracarboxylic dianhydride may be used. That is, pyromellitic dianhydride, 3, 3 ′,
4,4′-benzophenone tetracarboxylic acid dianhydride,
Naphthalene-1,4,5,8-tetracarboxylic dianhydride, cyclobutanetetracarboxylic dianhydride, etc. can be used.

Here, the abundance ratio of the imide bond to the urethane bond must be greater than 1. When the abundance ratio of the imide bond to the urethane bond is 1 or less, the melting point is 100.
Since it is as low as about 0 ° C., the stability as the alignment film 3 is lost, which is not suitable.

The alignment film 3 of the present invention is a transparent substrate 1 in a solution state.
It is manufactured by applying it on the transparent electrode 2 and heating to remove the solvent. The polyimide having a urethane bond of the present invention is soluble in an aprotic polar solvent. For example, dimethylformamide, dimethylsulfoxide, dimethylacetamide, N-methyl-2-pyrrolidinone, γ-butyl lactone and the like can be used. The alignment film 3 of the present invention can be formed on a substrate by spin coating or printing which has been used conventionally. After forming the film, it is necessary to heat to remove the solvent. The heating temperature varies depending on the solvent used, but when N-methyl-2-pyrrolidinone is used as the solvent, heating on a hot plate at 100 ° C. for 1 minute is sufficient.

The alignment film 3 of the present invention is formed on the transparent electrode 2 formed on the transparent substrate 1 and on the surface in contact with the liquid crystal layer 4. An insulating layer may be formed between the transparent electrode 2 and the alignment film 3 to prevent a short circuit between the upper and lower substrates. Although the insulating layer may be made of any material, for example, silicon oxide and titanium oxide are excellent in insulating property and transparency. The presence of the insulating layer is more preferable because the adhesiveness between the alignment film 3 and the transparent substrate 1 is improved.

Then, the alignment film 3 thus formed has a liquid crystal layer 4
A rubbing treatment is performed in order to provide the ability to orient. The rubbing treatment is realized by bringing a nonwoven fabric such as nylon or rayon into contact with the alignment film 3 and rubbing it in a certain direction.

The two transparent substrates 1 having the alignment film 3 and the transparent electrode 2 rubbed in this manner are held at appropriate intervals so that the alignment films 3 on the respective transparent substrates 1 face each other. Subsequently, liquid crystal is injected into the gap to form a liquid crystal display element. At this time, in order to keep the distance between the two transparent substrates 1 constant, it is common to arrange spherical particles of glass or synthetic resin between the transparent substrates 1 as spacers. Then, it is necessary to bond the two transparent substrates 1 with an appropriate adhesive so that the liquid crystal does not disperse between the two transparent substrates 1 facing each other.

Further specific examples will be shown below.

Example 1 (Synthesis of urethane-modified polyimide) First, 68 g (1.1 mol) of ethylene glycol and 146 g of adipic acid were polycondensed to synthesize polyethylene adipate glycol having a molecular weight of about 1000. Next, 12.5 of polyethylene adipate glycol dehydrated in a flask under a nitrogen stream.
g (0.013 mol) and 5 mg of triethylenediamine were heated to 80 ° C. with vigorous stirring. In this container 4,4'-diphenylmethane diisocyanate 9.3g
(0.038 mol) was added and an NCO-terminated prepolymer was synthesized in 5 hours. This prepolymer was dissolved in DMF, aluminum isopropoxide was added as a catalyst while heating at 80 ° C., and a DMF solution of 8.3 g (0.038 mol) of pyromellitic dianhydride was added dropwise. The mixture was stirred for 6 hours to synthesize a urethane-modified polyimide. With respect to the urethane-modified polyimide of Example 1, the isocyanate: difunctional polyol: acid dianhydride (molar ratio) was 3: 1: 3, and the abundance ratio of the imide bond to the urethane bond was 2.

In this way, a DMF solution of urethane-modified polyimide is prepared.

Next, on a surface having an electrode 2 on a glass substrate 1 having a circular electrode with an area of 2 cm ² using ITO,
A DMF solution of the above urethane-modified polyimide was applied and dried to prepare an alignment film 3 of a urethane-modified polyimide film having a film thickness of 70 nm.

Further, the urethane-modified polyimide thin film formed on the substrate was rubbed in one direction with rayon nonwoven fabric for rubbing treatment. Further, the two rubbed substrates were combined so that the urethane-modified polyimide thin films face each other and the rubbing directions were orthogonal to each other. Substrate spacing is 6 μm
After sealing the periphery of the substrate with an epoxy resin except at one place, a nematic liquid crystal (trade name: 5CB, manufactured by Merck) was sealed between the substrates by a vacuum injection method to prepare a liquid crystal display element. This liquid crystal display element was sandwiched between two deflecting plates arranged so that their deflection axes were orthogonal to each other. Further, when a rectangular wave of 5 V and 60 Hz was applied to the liquid crystal display element, light was not transmitted through the electrode portion and light was transmitted through the other portions.

Examples 2 to 4 Liquid crystal display devices were produced in the same manner as in Example 1. However, the urethane modified polyimide was prepared by changing the compounding ratio, the type of isocyanate and the type of acid dianhydride as shown in (Table 1).

[0030]

[Table 1]

Comparative Example 1 A urethane-modified polyimide was synthesized in the same manner as in Example 1. However, the isocyanate was 4,4′-diphenylmethane diisocyanate as in Examples 1 and 2, and the ratio of the isocyanate to the bifunctional polyol and the acid dianhydride was set to 2: 1: 2 as shown in (Table 1). In this case, the ratio of imide bond to urethane bond is 1.

However, this urethane-modified polyimide melted when heated to 120 ° C., so that it could not withstand the sealing resin curing step in the liquid crystal display element manufacturing step.

Comparative Example 2 Solvent-soluble polyimide (trade name Optomer AL105
1, manufactured by Nippon Synthetic Rubber Co., Ltd. was coated on the substrate in the same manner as in the example. Further, a liquid crystal display element was manufactured in the same manner as in the example using the substrate coated with this polyimide. In this liquid crystal display element as well, it was possible to control the transmission / non-transmission of light by applying a rectangular wave of 5 V and 60 Hz as in the example. (Comparison of Scratchability by Rubbing) In order to examine how easily the alignment film is easily scratched, a sample is prepared by coating the polyimide thin film (thickness 70 nm) of Examples 1 to 4 and Comparative Example 2 on a glass substrate. did.

The same sample was rubbed a plurality of times using an RM-50 type rubbing machine manufactured by EHC Co., Ltd., and the number of times when scratches could be visually observed was measured as the rubbing resistance. It can be said that an alignment film having a higher rubbing resistance is more resistant to scratches. The results are shown in (Table 2).

[0035]

[Table 2]

As is clear from Table 2, the polyimide alignment film of Comparative Example 2 has a rubbing resistance lower than that of any of the alignment films of Examples 1 to 4. That is, the alignment films of Examples 1 to 4 are less likely to be scratched during rubbing than the alignment films of Comparative Example.

In the above description, a TN type liquid crystal display element has been described as an example, but a liquid crystal display element of a type using another liquid crystal alignment film, for example, STN type, SBE.
The present invention can be applied to liquid crystal display devices such as die, ferroelectric liquid crystal type, and SH type.

[0038]

As is clear from the above description, the present invention uses a polyimide resin having a urethane bond in the main chain as a liquid crystal alignment film, so that it is a solvent-soluble polyimide. It has the advantage of being less likely to be scratched during rubbing.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a liquid crystal display element according to an embodiment of the present invention.

[Explanation of symbols]

 1 transparent substrate 2 transparent conductive film 3 liquid crystal alignment film 4 liquid crystal layer

Claims (3)

[Claims] 1. A liquid crystal alignment film comprising a polyimide resin having a urethane bond in the main chain. 2. The liquid crystal alignment film according to claim 1, wherein the abundance ratio of the imide bond to the urethane bond is greater than 1. 3. A liquid crystal in which a pair of members, each having an electrode formed on a substrate and a liquid crystal alignment film formed thereon, are assembled so that the liquid crystal alignment films face each other, and the liquid crystal is sandwiched therebetween. In the display element, the liquid crystal alignment film is the liquid crystal alignment film according to claim 1 or 2.