JPH0416081B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to a polarizing plate that is used for applications such as liquid crystal displays, and particularly has improved water resistance and moisture resistance. PRIOR ART Recently, liquid crystal displays using polarizing plates have begun to be used in a wide variety of applications, such as automobile display panels and other applications that require durability under high temperature and high humidity conditions. As attempts have been made, the demand for polarizing plates with better heat resistance and moisture resistance is increasing. A polyvinyl alcohol (hereinafter referred to as PVA) polarizing film is widely used as a polarizing plate, but this film has poor moisture resistance, and its polarizing performance is significantly degraded especially in a high temperature and humid environment. This is thought to be due to orientation relaxation due to water absorption of the PVA film. For this reason, PVA
It is common practice to protect the polarizing film by laminating a transparent film such as cellulose triacetate, which does not have double refraction properties, on the surface of the polarizing film, but the PVA film itself has poor moisture resistance.
Cellulose triacetate has poor moisture permeability, so it does not provide sufficient protection. For example, in an environment of 80° C. and 95% RH, even if the above-mentioned protective layer is provided, the PVA absorbs water and relaxes its orientation, resulting in discoloration and a decrease in the degree of polarization, resulting in insufficient moisture resistance. In addition, in order to eliminate such drawbacks, for example, in Japanese Patent Application Laid-open No. 12050/1983, an oxide layer such as SiO ₂ or TiO ₂ or a nitride layer such as Si ₃ N ₄ is directly provided on the surface of the polarizing film. In JP-A No. 55-114563, a method of improving moisture resistance by applying an oxide layer such as SiO ₂ , TiO ₂ , ZrO ₂ etc. to the surface of cellulose triacetate as a protective film via an anchor layer. A method has been proposed to improve moisture resistance by adding a Furthermore, it is required to improve heat resistance and moisture resistance. Purpose of the Invention In view of the above-mentioned current situation, the present invention has been carried out with the aim of providing a polarizing plate with excellent heat resistance and moisture resistance. This was done based on the knowledge that it has particularly excellent moisture permeability compared to _two- layered, etc., and that by applying it to the protective film of a polarizing plate, a polarizing plate with excellent performance could be obtained. Summary and Structure of the Invention The gist of the present invention is to provide a polarizing plate in which a cellulose acetate film is adhered to at least one surface of a polarizing film in which a polarizing element is adsorbed and oriented on a hydrophilic polymer film. The present invention relates to a polarizing plate characterized in that a transparent thin film layer of magnesium oxide is closely formed on at least one surface. The polarizing film used in the present invention is made by adsorbing polarizing elements such as iodine or dichroic dyes such as Direct Black 17, 19, 20, Direct Blue 1, 6, Direct Red 28, etc. to a hydrophilic polymer film such as PVA. , polarizing properties are imparted by orienting a polarizing element by stretching an adsorption film or the like. Cellulose acetate films include cellulose diacetate and cellulose triacetate, and cellulose triacetate is preferably used. In order to form a transparent thin film layer of magnesium oxide on the surface of a cellulose acetate film, a vapor deposition method such as a vacuum vapor deposition method, a reactive vapor deposition method, an ion plating method, or a sputtering method can be used. A thin layer of magnesium oxide can be formed in close contact with the surface of a cellulose acetate film. To explain this evaporation method in more detail, for example, when a vacuum evaporation method is used, magnesium oxide (MgO) is used as the evaporation material and the magnesium oxide is heated and evaporated with an electron beam, or a marminar crucible or a carbon crucible is used to evaporate the magnesium oxide. By heating and evaporating using a local wave induction heating method or a resistance heating method, a deposited film with good adhesion that does not require post-treatment such as heat treatment can be obtained. Vacuum deposition is 2x
Preferably, the reaction is carried out under a pressure of 10 ⁻² torr or less. or,
A transparent magnesium oxide film can also be formed by a reactive vapor deposition method in which magnesium is heated and evaporated in a vacuum vessel into which oxygen gas is introduced. Furthermore, the transparent film can also be formed by an ion plating method or a sputtering method, in which case a transparent thin film layer with better adhesion can be formed due to the high energy of the evaporated particles compared to the vacuum evaporation method. I can do it. The thickness of the transparent thin film layer is 100 to 5000 Å in order to improve the moisture resistance of the polarizing plate.
It is best to keep it within the range. In addition, even if the thickness of the transparent thin film layer exceeds 5000 Å, the transparency will not be impaired, but due to the stress inside the transparent thin film layer, the curling property of the cellulose acetate film may become a problem, and cracks and peeling may easily occur. Sometimes. Furthermore, if the thickness of the transparent thin film layer is less than 100 Å, the film may have an island-like structure, resulting in insufficient moisture permeation resistance. In the present invention, sufficient moisture permeability can be achieved by providing a magnesium oxide layer on only one side of the cellulose acetate film, but it may be formed on both sides if necessary. The polarizing plate of the present invention is formed by adhering to at least one surface of the polarizing film a cellulose acetate film on which the transparent thin film layer of magnesium oxide is closely formed, and the adhesive used for this adhesion is The material is urethane,
Acrylic and epoxy adhesives can be used. When a cellulose acetate film with a transparent thin film layer formed on only one side is adhered to a polarizing film, the thin film layer may be placed in contact with the polarizing film, or the thin film layer may be placed in contact with the polarizing film. You can place it opposite to the film side, but
Adhesion is carried out by placing the thin film layer in contact with the polarizing film in order to prevent the transparent thin film layer from being damaged by contact with other objects, etc., and because the thin film layer has excellent adhesion with adhesives. is preferable. The polarizing plate of the present invention has the structure as described above, and has excellent moisture resistance under high temperature and high humidity conditions due to the transparent thin film layer of magnesium oxide closely formed on at least one surface of the cellulose acetate film used therein. Because it has been given a gender,
It can be used in applications that require moisture resistance under harsh conditions, such as liquid crystal display panels for automobiles, and can exhibit excellent effects. EXAMPLES The present invention will be described below based on examples. Example 1 An unstretched PVA film with a degree of saponification of 99.8% was washed with water at room temperature and then dyed by passing it through an aqueous dyeing solution at about 40°C containing dichroic dye Direct Blue 15. Dry with hot air at ℃. This film was dry-heat stretched approximately 4 times in the uniaxial direction at 140°C, and then stretched at 170°C for 5 times while maintaining tension.
The film was heated for 1 minute, and then immersed in a 3% by weight boric acid aqueous solution at 55°C for 10 minutes while maintaining tension to prepare a PVA polarizing film. On the other hand, a magnesium oxide transparent film with a thickness of 1030 Å was formed on a cellulose triacetate film with a thickness of 80 μm using a vacuum evaporation method under the following conditions. Film formation method: Vacuum deposition method (electron gun heating method) Evaporation source: Magnesium oxide with 99% purity Vacuum degree: 2 × 10 ^-4 Torr Film thickness: 1030 Å (measured with a crystal oscillation monitor) Film formation surface: One side of the film Next Next, the vapor-deposited film was adhered to both surfaces of the PVA polarizing film with a urethane adhesive so that the magnesium oxide layer of the vapor-deposited film prepared above was in contact with the PVA polarizing film surface. The adhesion between the PVA polarizing film and the magnesium oxide vapor-deposited surface of the vapor-deposited film was extremely good. Regarding the polarizing plate prepared in this way, 80℃, 95%
A moisture resistance test was conducted under RH conditions for 500 hours.
Before and after the test, the single plate transmittance T (%) at a wavelength of 610 nm and the degree of polarization P (%) at a wavelength of nm were measured, and the results shown in Table 1 were obtained. Note that the degree of polarization P was determined based on the following formula. T ₁ : Parallel transmittance (λ = 610 nm) of two polarizing plates stacked with their polarization axes parallel to each other T ₂ : Orthogonal transmittance (λ = 610 nm) of two polarizing plates stacked with their polarization axes perpendicular to each other Comparative Examples 1 to 3 In the same manner as in Example 1 except that SiO ₂ , TiO ₂ , and ZrO ₂ were used as evaporation sources, they were each vapor-deposited on one side of a cellulose triacetate film (thickness: 80 μm), and then deposited on one side of the film. SiO ₂ thin film with a thickness of 980 Å (Comparative Example 1),
TiO ₂ thin film with a thickness of 1015 Å (comparative example 2) and thickness
Cellulose triacetate films each provided with a 1080 Å ZrO ₂ thin film (Comparative Example 3) were prepared. These vapor-deposited films were coated on both sides of the same PVA polarizing film prepared in Example 1.
Three types of polarizing plates were created by bonding in the same manner as above. The three types of polarizing plates thus prepared were subjected to the same moisture resistance test as in Example 1, and the results shown in Table 1 were obtained. 【table】

Claims (1)

[Scope of Claims] 1. A polarizing plate in which a cellulose acetate film is adhered to at least one surface of a polarizing film in which a polarizing element is adsorbed and oriented on a hydrophilic polymer film, and the cellulose acetate film is attached to at least one surface of the polarizing film. A polarizing plate characterized in that a transparent thin film layer of magnesium oxide is closely formed. 2. The polarizing plate according to claim 1, wherein the hydrophilic polymer film is a polyvinyl alcohol film.