JPH1138418A - Liquid crystal alignment layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a liquid crystal alignment layer which imparts an excellent after-image characteristic to a liquid crystal display element by respectively specifying the time constant and electrode relaxation quantity respective expressed by specific equations to specific values or below. SOLUTION: The time constant Tc expressed by Tc =QR/<i>is <=250 sec. In the equation, Tc is a time constant (second), QR is a discharge charge quantity (C) obtainable from charge and discharge measurement in vacuum and <i> is an average charge current (A) obtainable from the charge and discharge measurement in vacuum. The electrode relaxation quantity Δε expressed by formula Δε=(QR/V)×(d/s)ε0 is <=10. In the equation, Δε is a polarization relaxation quantity, QR is discharge charge quantity (C) obtainable from the charge and discharge measurement in vacuum, V is an impressed voltage (V), (d) is a film thickness (m), (s) is an upper electrode area (m<2> ) and ε0 is a vacuum dielectric constant (F/m).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a liquid crystal alignment film. More specifically, the present invention relates to a liquid crystal alignment film having good liquid crystal alignment properties and providing excellent afterimage characteristics to a liquid crystal display device.

[0002]

2. Description of the Related Art Conventionally, the alignment of liquid crystal in a liquid crystal display device such as a TN type liquid crystal display device is usually realized by a liquid crystal alignment film provided with an alignment regulating ability of liquid crystal molecules by a rubbing treatment or the like. As a material of a liquid crystal alignment film constituting a liquid crystal display element, a resin having high electrical insulation such as polyimide has been used conventionally. However, since the residual voltage of the liquid crystal display element is large, there is a problem that an afterimage occurs or white spots occur during a reliability test of the liquid crystal display element. Not clear. It is generally interpreted that afterimages occur because impurity ions in the liquid crystal are adsorbed on the surface of the liquid crystal alignment film, but the adsorption mechanism related to the structure and physical properties of the liquid crystal alignment film is hardly discussed. In addition, the influence of the electrical properties of the liquid crystal alignment film, such as the specific resistance, the dielectric constant, and the dielectric loss, has been pointed out and studied. However, there is no example in which a clear quantitative correlation was found. At present, the design guidelines for the structure and physical properties required for the liquid crystal alignment film to realize low image lag are unknown.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to provide a liquid crystal alignment film which provides excellent afterimage characteristics to a liquid crystal display device. It is another object of the present invention to provide a liquid crystal alignment film that provides excellent afterimage characteristics specified by using electric characteristics as parameters. Still other objects and advantages of the present invention will become apparent from the following description.

[0004]

According to the present invention, the above objects and advantages of the present invention are as follows: T _c = Q _R / <i> (1) where T _c is a time constant (second) Where _QR is the discharge charge (C) obtained from the charge and discharge measurement in vacuum, and <i> is the average charge current (A) obtained from the charge and discharge measurement in vacuum. , The time constant T _c is 25
Sec or less, and the following formula (2) △ ε = (Q R / V) × (d / s) / ε O ··· (2) where, △ epsilon is polarization relaxation amount, of Q _R The definition is the same as the above equation (1), V is the applied voltage (V), d is the film thickness (m), s is the upper electrode area (m ² ),
Ε _O is a vacuum dielectric constant (F / m), and is achieved by a liquid crystal alignment film characterized in that the electrode relaxation amount で ε is 10 or less.

[0005] The sample film used for measuring the electrical characteristics of the liquid crystal alignment film of the present invention is prepared, for example, by coating using a roll coater method, a spinner method, a printing method, or the like, or by heating using a solvent casting method. Can be formed.

It is desirable that the thickness of a liquid crystal alignment film sample for measuring electrical characteristics is originally close to the thickness of a liquid crystal alignment film when actually used for a liquid crystal display device. However, when the film thickness is less than 2,000 angstroms, the generation of microvoids is unavoidable at the time of film formation, and the probability of conduction during measurement becomes extremely high due to the influence of the unevenness of the substrate. The thickness of the sample used for the charge / discharge measurement in the present invention must be 2,200 to 3,000 angstroms. More preferably, it is 2,500 to 2,700 angstroms. If it exceeds 3,000 angstroms, it becomes close to the bulk electrical properties, and it is not possible to obtain parameters characterizing the electrical properties of the thin film used in the present invention.

The lower electrode is formed on a flat glass or silicon substrate in order to minimize irregularities on the electrode surface.
It can be formed by vapor deposition of a conductive metal such as ITO and gold. A film is formed on the lower electrode by a method such as spin coating, and the upper electrode is formed by vapor deposition of a conductive paste or a conductive metal such as gold without using a contact electrode. When a contact-type electrode is used, the influence of the contact pressure and the electrode surface unevenness is large, and the parameters used in the present invention cannot be accurately evaluated. The shape of the lower and upper electrodes may be any shape, but is preferably a circle or a strip so that the electrode area can be calculated accurately. Also, the electrode area is desirably several mm ² , and several electrodes must be formed on the same sample film to confirm that there is no continuity and that there is no variation in electrical characteristics. FIG. 1 shows a configuration diagram of the measuring device and the electrodes.

Before the measurement, the upper and lower electrodes are short-circuited for several hours to remove the influence of charging and polarization. When charging,
After confirming that the voltage-current is proportional and that dielectric breakdown does not occur, a voltage of 0.5 V is applied. During discharge, both electrodes are completely short-circuited. The charging time and the discharging time are both 10 minutes, and the degree of vacuum before the measurement is 10 ⁻² torr.
Below to avoid the effects of adhering substances such as adsorbed moisture.
Wait at least a minute before starting the measurement. The sampling time for the current measurement is 0.5 seconds. Since the charging current greatly attenuates during the charging time within 100 seconds, an average value from 100 seconds to 600 seconds is taken to define the average charging current <i>. The time integral of the discharge current is obtained by numerical integration, and 6 hours after the start of the discharge.
The discharge charge amount QR for 00 seconds is defined.

[0009] Thus the defined discharge charge quantity Q _R, because the sampling time is 0.5 seconds, does not include the influence of discharge in a short time. Changing the type of alignment layer, charge, was discharged measurement, <i> according to the above definition, when obtaining the polarization relaxation of constants and the equation (2) When the above equation (1) using a Q _R, the liquid crystal It was found that there was a close correlation with the afterimage of the display element. That is, a liquid crystal alignment film having a time constant of 250 seconds or less and a polarization relaxation amount of 10 or less can be a liquid crystal alignment film that provides excellent afterimage characteristics.

As the material polymer of the liquid crystal alignment film of the present invention, for example, resins such as polyimide, polyamide, polyamide imide and polyester can be preferably mentioned from the viewpoint of heat resistance, washing resistance and insulating properties. These liquid crystal alignment films are usually obtained by applying a liquid crystal alignment agent obtained by dissolving the material polymer or a precursor of the polymer in a solvent onto a substrate and drying the liquid crystal alignment agent.

The content of the polymer in the liquid crystal aligning agent used in producing the liquid crystal aligning film of the present invention is selected in consideration of viscosity, volatility, and the like. The content is in the range of 0.1 to 20% by weight, more preferably 1 to 10% by weight. When the content of the polymer is less than 0.1% by weight, the thickness of the alignment film may be too small to obtain a good liquid crystal alignment film, and when it exceeds 20% by weight. In addition, it is difficult to obtain a good liquid crystal alignment film due to an excessively large thickness of the alignment film, and the viscosity of the liquid crystal alignment agent may be increased, resulting in poor coating performance.

As a liquid crystal aligning agent for producing the liquid crystal alignment film of the present invention, a polyamic acid is synthesized by reacting a tetracarboxylic dianhydride with a diamine compound in an organic solvent, and if necessary, the polyamic acid is synthesized. A polymer containing an imide structure obtained by dehydrating and ring-closing a polyamic acid and an epoxy compound containing a nitrogen atom are particularly preferably used.

The ratio of the tetracarboxylic dianhydride and the diamine compound used in the synthesis reaction of the polyamic acid is as follows:
The ratio of the acid anhydride group of the tetracarboxylic dianhydride to 0.2 to 2 equivalents relative to 1 equivalent of the amino group contained in the diamine compound is desirable, and more preferably 0.3 to 1.2 equivalents. Percentage.

The epoxy compound includes an epoxy compound having a nitrogen atom having an aromatic monoamine as a mother nucleus, an epoxy compound having a nitrogen atom having an aromatic diamine as a mother nucleus, and an alicyclic diamine having a mother nucleus. And an epoxy compound having a nitrogen atom. Of these, N, N-diglycidylaniline, N,
N, N ', N'-tetraglycidyl-4,4'-diaminodiphenylmethane and 1,3-bis (N, N'-diglycidylaminomethyl) cyclohexane are particularly preferred.

The content of the nitrogen-containing epoxy compound is 0.01 to 50 parts by weight, preferably 0.1 to 40 parts by weight, per 100 parts by weight of the polymer in the liquid crystal aligning agent.
Parts by weight, particularly preferably 0.1 to 30 parts by weight. If the content of the epoxy compound is less than 0.01 parts by weight,
In some cases, the effect of improving the afterimage characteristics is hardly exhibited. When the amount exceeds 50% by weight, the storage stability of the liquid crystal alignment agent deteriorates. The liquid crystal aligning agent used in the present invention may contain a functional silane-containing compound from the viewpoint of improving the adhesiveness between the polymer and the surface of the substrate to be coated.

[0016]

The afterimage of a liquid crystal display device can be improved by controlling the electrical characteristics of a liquid crystal alignment film obtained by forming a liquid crystal alignment agent. By using the liquid crystal alignment film as described above, a liquid crystal display device having excellent afterimage characteristics can be manufactured.

[0017]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples. The evaluation method for each liquid crystal display element prepared by the following Examples and Comparative Examples is shown below. [Polymer imidation rate] The polymer was dried under reduced pressure at room temperature, dissolved in deuterium or dimethyl sulfoxide, and ¹ H-NMR was measured at room temperature using tetramethylsilane as a reference substance. I asked. The imidation ratio of the polyamic acid which was not subjected to the dehydration ring closure reaction was set to 0%.

Imidation ratio (%) = (1−A ₁ / A ₂ ) × 100 (3) A ₁ : peak area derived from proton of NH group (10 pp)
m) A ₂ : peak area derived from other protons α: N in the precursor (polyamic acid) of the polymerization _couple
Number ratio of other protons to one proton of H group

[Afterimage erasing time] After applying an 8 V DC voltage to the liquid crystal display device for 3 hours, the voltage was turned off, and the time until the afterimage was erased was visually measured.

The polymers used in Examples and Comparative Examples are shown below. 2,3,5-tricarboxycyclopentylacetic acid dianhydride, p-phenylenediamine and the following formula

[0021]

Embedded image

The diamine compound represented by the formula was polymerized in this order at a molar ratio of 1: 0.9: 0.1 to obtain a polymer (A-1) having an imidation ratio of 0%. This was dehydrated and ring-closed to give a polymer (A-2) having an imidation ratio of 90%, a polymer (A-3) having an imidation ratio of 20%, and a polymer (A-) having an imidation ratio of 35%.
4) was obtained. As tetracarboxylic dianhydride, 1,3,
3a, 4,5,9b-Hexahydro-8-methyl-5-
(Tetrahydro-2,5-dioxo-3-furanyl)-
Except for using naphtho [1,2-c] furan-1,3-dione, a polymer (A-
5) was obtained. This was dehydrated and ring-opened to obtain a polymer (A-6). In the synthesis of the polymer (A-1), except that p-phenylenediamine was used as a diamine compound in a molar ratio of 1: 1 in the same manner as in the synthesis of the polymer (A-1), a polymer having an imidation ratio of 0% was used. A compound (A-7) was obtained. Pyromellitic dianhydride and p-phenylenediamine were polymerized at a molar ratio of 1: 1 to obtain a polymer (A-8) having an imidation ratio of 0%.

Example 1 2.0 g of the polymer (A-3) and N, N as an epoxy compound
0.5 g of N, N ′, N′-tetraglycidyl-4,4′-diaminodiphenylmethane is dissolved in γ-butyrolactone, and this solution is filtered through a 1 μm-diameter filter,
The liquid crystal aligning agent was adjusted. The average imidation ratio of the liquid crystal aligning agent was 20%. The above-mentioned liquid crystal aligning agent was applied on a glass substrate with an ITO transparent electrode, and dried on a hot plate at 180 ° C. for 1 hour to prepare a sample film having a film thickness of 2600 angstroms for evaluating electric characteristics. An upper electrode having a diameter of 5 mm was prepared using a silver paste, charge and discharge were measured at an applied voltage of 0.5 V, and the time constant _Tc and the amount of polarization relaxation Δ
ε was determined.

Similarly, the above liquid crystal aligning agent was applied on a glass substrate provided with an ITO transparent electrode and dried to form a coating film having a thickness of 0.05 μm. Using a rubbing machine, this coating film
Roll rotation speed 500rpm, stage moving speed 1cm
A rubbing treatment was performed at a hair foot pushing length of 0.4 mm / sec. Next, an epoxy adhesive containing aluminum oxide spheres having a diameter of 17 μm is applied to the outer edges of the pair of rubbed liquid crystal holding substrates, and then superposed such that the liquid crystal alignment films face each other and the rubbing directions are antiparallel. Crimping was performed to cure the adhesive. After filling a nematic liquid crystal (MLC-5081 manufactured by Merck) between a pair of substrates from a liquid crystal injection port, the liquid crystal injection port is sealed with an epoxy-based adhesive, and deflection plates are provided on both outer surfaces of the substrate. The polarizing plates were stuck together so that the polarization direction of the polarizing plates coincided with the rubbing direction of the liquid crystal alignment film of each substrate, thereby producing a liquid crystal display device. The time constant is 150 seconds,
The polarization relaxation amount was as small as 10, and the afterimage erasing time was as small as 0.9 seconds.

Example 2-9 A liquid crystal aligning agent was prepared in the same manner as in Example 1 except that a polymer (component (A)) and an epoxy compound having a nitrogen atom (component (B)) were used according to the formulation shown in Table 1. Was prepared, and a sample film for evaluating electrical characteristics and a liquid crystal display device were prepared. In all the examples, the time constant was 250 seconds or less, the amount of polarization relaxation was as small as 10 or less, and the afterimage erasing time was as small as 2 seconds or less.

[0026]

[Table 1]

Comparative Example 1-3 A liquid crystal aligning agent was prepared in the same manner as in Example 1 except that a polymer ((A) component) and an epoxy compound having a nitrogen atom ((B) component) were used according to the formulation shown in Table 2. Was prepared, and a sample film for evaluating electrical characteristics and a liquid crystal display device were prepared. In each of the comparative examples, the values of the time constant and the amount of polarization relaxation were large, and the afterimage erasing time was also long.

[0028]

[Table 2]

The compound names of the epoxy compounds in Tables 1 and 2 are as follows. B-1: N, N, N ', N'-tetraacrylic-4,4'-aminoaminophenylphenylmethane B-2: 1,3-Phase (N, N'-cyclic aminomethyl) cyclohexane b-3: Trimethylphenyl Triglyceryl ether

[0030]

As described above, a quantitative correlation can be found for the first time by introducing parameters such as the time constant and the amount of polarization relaxation, for the afterimage phenomenon, in which the relationship with the electrical characteristics of the alignment film itself has not been clarified conventionally. Was. Furthermore, according to the present invention, when a liquid crystal display device is used, a highly reliable liquid crystal alignment film with little afterimage is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a measuring device and electrodes for measuring electric characteristics of a liquid crystal alignment film of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yasuo Matsuki 2--11-24 Tsukiji, Chuo-ku, Tokyo Inside Nippon Gosei Rubber Co., Ltd. (72) Masayuki Kimura 2-11-24 Tsukiji, Chuo-ku, Tokyo Nippon Synthetic Rubber Co., Ltd.

Claims (1)

[Claims] 1. A formula _{(1) T c = Q R} / <i> ··· (1) where, T _c is the time constant (sec), Q _R is charged in a vacuum, a discharge measurement a discharge charge quantity obtained from (C), and <i> constant T _c when the charge is the average charging current obtained from the discharge measurement (a), in represented in vacuum 25
0 seconds or less, and the following equation (2) Δε = (Q _R / V) × (d / s) / ε _O (2) where Δε is the polarization relaxation amount, and Q _R Is the same as in the above equation (1), V is the applied voltage (V), d is the film thickness (m), s is the upper electrode area (m ² ),
The liquid crystal alignment film is characterized in that the electrode relaxation amount Δε expressed by ε _O is a vacuum dielectric constant (F / m) is 10 or less.