JPS593729B2 - Substrate for liquid crystal display device and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a substrate for a liquid crystal display device having a coating for liquid crystal alignment.

A liquid crystal display device generally has two substrates sandwiching a liquid crystal, and a liquid crystal alignment material is applied to the surface of at least one of the substrates that comes into contact with the liquid crystal.

Here, the term "substrate" includes the electrode film if an electrode film is applied thereto. Therefore, when an electrode film is present on a part of the substrate, a part of the surface of the substrate that comes into contact with the liquid crystal is the electrode film surface. Conventionally, as a method for obtaining uniform parallel alignment with a small tilt angle, a method is known in which the surface of a substrate is coated with a substance having good parallel alignment properties and the surface is rubbed. As such organic substances, those having a structure close to a monomolecular film such as silane are known, but they lack long-term stability and require a separate protective film such as SiO2. On the other hand, if a good polymer membrane is available, it would be advantageous in terms of stability. Polyvinyl alcohol (PVA) has already been reported as a polymer membrane, but
Its stability and liquid crystal orientation have not yet been fully clarified. Accordingly, one object of the present invention is to provide a liquid crystal aligning substance that has long-term stability and provides good alignment to liquid crystals in place of silane. Another object of the present invention is to provide a method for increasing the stability of liquid crystal orientation in PVA that has been used in the past. PVA is applied to the substrate as an aqueous solution and then dried to form a PVA coating on the substrate. The alignment performance of the PVA film formed in this manner deteriorates over time. However, it has now been discovered that when a PVA film is heat treated at a temperature above its melting point, the alignment performance does not deteriorate and the stability increases. However, if the temperature is too high, there is a risk that the PVA will discolor, so it is safer if the temperature is not too high. This upper limit temperature cannot be particularly limited because it depends on the impurities contained in the PVA, but a temperature within a range that does not cause discoloration can be used. In this application, such a range will be referred to as the melting point or a temperature slightly higher than the melting point. The present invention will be explained in more detail below with reference to Examples.

Example Table 1 shows the various polymer materials used in the experiment.

The substrate used was soda glass partially covered with an In2O3 transparent electrode, which was immersed in a solution of a polymeric material and dried to form a film with a thickness of about 1000λ. A liquid crystal cell was prepared by wrapping two substrates with gauze, sandwiching a polyamide film around the periphery as a cover, bonding them under heat, and vacuum sealing the liquid crystal in the space between the substrates. The cell thickness is 12 μm and the area of one electrode is approximately 2.24 cd. The resistance of the cell was measured using a universal bridge manufactured by Yokogawa Electric Corporation. The measurement voltage was 1KHz 150mV, and the biphenyl mixed liquid crystal GR for each polymer material was measured.
Table 1 shows the orientation of -41 (manufactured by Chitsuso Corporation). P
It can be seen that good parallel alignment can be obtained with PAN and PAAm in addition to VA. Next, we investigated the alignment of the liquid crystals shown in Table 2 with respect to these three types of polymer films, and as a result, it was confirmed that all liquid crystals were aligned parallel to these polymer films with good uniformity. Next, PVA, PAN
, 50 pressure, 20
We investigated the change in resistance over time when an AC voltage of V was applied.

The results are shown in FIG. The liquid crystal used here is a biphenyl mixed liquid crystal GR-4M (manufactured by Chitsuso Corporation) for TN cells. The figure also shows the characteristics of a cell without a polymer membrane for comparison. The figure shows that the resistance of the cell without a polymer film and the cell using PAN hardly changes, whereas the resistance of the cell using PVA decreases over time. Also P
With AAm, there was no change in resistance for up to 600 hours, but after that, the orientation deteriorated and the resistance suddenly decreased. For comparison, we also investigated the change in resistance over time when using an alkoxysilane, γ-morpholinopropyl-trimethoxysilane (MOMS), but no significant change was observed over the 2000 hours tested. . It is thought that the decrease in resistance in PVA is due to ionic impurities being eluted from the PVA film into the liquid crystal.

Therefore, we created a cell by subjecting it to a heat treatment known as a hardening treatment for PVA fibers for 10 minutes. Figure 2 shows the change in resistance over time in this case. It can be seen from the figure that the resistance of the cell does not change when the treatment is performed at a temperature of 220° C. or higher. 220° C. corresponds to the melting point of the PVA used, and it is known that heat treatment at a temperature higher than this promotes crystallization of the PVA film and makes the structure denser.

As a result of X-ray diffraction analysis of the PVA film in this study, it was confirmed that crystallization is promoted at temperatures of 220° C. or higher. It is thought that the reason why the film is stabilized by heat treatment is that such a change in the film prevents the elution of ionic impurities. However, if the heat treatment is carried out at a temperature higher than 220°C, the coloring of the film will become noticeable, so it is better not to heat it too high, and 220°C is the optimum temperature. PVA
is the structural formula shown in Table 1, and the melting point changes depending on the number of n, so it can be said that the heat treatment required in this application is generally within the range of the melting point of PVA or a temperature slightly higher than that. Dew.

From the results of the above examples, PV heat treated under the above temperature conditions
It will be appreciated that the coating of A is suitable as a liquid crystal aligning material for a substrate for a liquid crystal display device.

[Brief explanation of drawings]

FIG. 1 is a diagram showing changes in resistance values of liquid crystal cells coated with various polymer films as liquid crystal aligning substances with respect to voltage application time.

Claims (1)

[Scope of Claims] 1. A liquid crystal display device substrate having a polymer film made of polyvinyl alcohol heat-treated at or slightly higher than its melting point is applied as a liquid crystal alignment film to the surface of the substrate in contact with the liquid crystal. Substrates for liquid crystal display devices. 2. A solvent solution of a polymer made of polyvinyl alcohol is applied to the surface of the substrate for a liquid crystal display device in contact with the liquid crystal, and a film of the polymer is formed by evaporating and removing the solvent. A method for manufacturing a substrate for a liquid crystal display device having a liquid crystal aligning film, which comprises heat treatment at a temperature at or slightly higher than the melting point. 3. The manufacturing method according to claim 2, wherein the solvent solution of the polymer is an aqueous solution.