JPH0535841B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a polarizing film used in a liquid crystal display device. [Prior art] As a polarizing film for liquid crystal display, a uniaxially oriented polyvinyl alcohol film containing iodine or dichroic dye is used as a polarizing element, and a non-axially oriented polyvinyl alcohol film obtained by a casting method is used as a supporting substrate. It is known that cellulose triacetate films are bonded to both sides of a polarizing element. However, in recent years, the applications of liquid crystal display devices have expanded, and they are being used in automobile instruments and external equipment meters, etc. However, the non-axially oriented cellulose triacetate film of the support substrate has high moisture permeability, and If exposed to sunlight for a long period of time, this film will hydrolyze, creating minute bubbles within the support substrate, which may reduce the clarity of images used as a polarizing film for liquid crystal displays, or cause the polarizing element to deteriorate. There are practical problems. As a solution to this problem, as seen in Japanese Patent Application Laid-Open No. 143344/1983, ``A non-axial or axial moisture-proofing film that eliminates the phase difference with the polarizing plate is laminated on the surface of a polarizing plate made of polyvinyl alcohol. There are also known ``coated uniaxial polyester films'' which are useful for manufacturing polarizers, as seen in JP-A-58-132523. A uniaxially oriented film (mainly a uniaxially oriented polyester film) has better moisture resistance and dimensional stability than a non-axial, non-oriented, unstretched film, and has less polarized light rotation than a biaxially oriented film. Polarizing films using uniaxially oriented films as supporting substrates are being put into practical use as polarizing films for liquid crystals. However, polarizing films using uniaxially oriented polyester films do not have sufficient thermal dimensional stability at high temperatures, making it difficult to manufacture polarizing films and incorporating them into liquid crystal display devices, and they also have poor ultraviolet shielding properties. It has been pointed out that this is insufficient and that the photodeterioration of liquid crystal display devices cannot be prevented completely. [Object of the Invention] An object of the present invention is to eliminate the above-mentioned drawbacks and to provide a polarizing film for liquid crystal display which has excellent moisture resistance, good thermal dimensional stability, and good ultraviolet shielding performance. [Structure of the Invention] The present invention uses a uniaxially oriented polyvinyl alcohol film containing a dichroic dye or iodine, which is a substance exhibiting optical dichroism, as a polarizing element, and a uniaxially oriented polyethylene naphthalate film as a support substrate. In the polarizing film, the refractive index in three directions of the polyethylene naphthalene dicarboxylate film serving as the supporting substrate has the following relationship, −0.03≦n ₁ +n ₃ /2−n ₂ ≦+0.05, where, n ₁ : refractive index in the main orientation direction n ₂ : refractive index in the direction perpendicular to n ₁ and n ₃ n ₃ : refractive index in the thickness direction, and when the polyethylene naphthalene dicarboxylate film was left at 150°C for 30 minutes,
A polarizing film for liquid crystal displays characterized by a heat shrinkage rate of 0.3% or less. As the polarizing element in the present invention, a polyvinyl alcohol film containing a dichroic dye may be used, but it is preferable to use a polyvinyl alcohol film containing iodine or an iodine compound from the viewpoint of polarizing performance. The polarizing element may be one in which an unstretched polyvinyl alcohol film is uniaxially stretched after first containing an iodine compound, or one in which iodine is adsorbed to a polyvinyl alcohol stretched film that has been uniaxially oriented. . In the present invention, polyethylene naphthalene dicarboxylate refers to a homopolymer of polyethylene 2,6 naphthalene dicarboxylate or a polymer consisting of an isomer containing 70% by weight or more of polyethylene 2,6 naphthalene dicarboxylate. Furthermore, copolymers and mixtures containing this polymer as a main component and polyester compositions that do not essentially lose the properties of this polyester can also be used. The polyethylene naphthalene dicarboxylate (hereinafter abbreviated as PEN) film in the present invention may be oriented uniaxially in the machine direction using a known roll stretching machine, or may be oriented in the width direction using a known stenter. It may be uniaxially oriented. Furthermore, it may be biaxially stretched in the mechanical direction and the width direction, or it may be substantially uniaxially oriented. The value of the following equation determined from the refractive index of the film in three directions must be in the range of -0.03 to +0.05. N=n ₁ +n ₃ /02−n ₂ If N is smaller than −0.03, it is inappropriate because a strong rainbow pattern will be formed on the surface of the polarizing film. Furthermore, if N is larger than +0.05, the film becomes fibrillated and tends to tear, and the polarizing film becomes easy to break when bent, which is not suitable. The preferred range of N is 0.00 to +0.03. Furthermore, as a PEN film, 25μm to 100μm
A film with a thickness of m is used as a supporting substrate,
The heat shrinkage rate when left at 150°C for 30 minutes without tension must be 0.3% or less. If the heat shrinkage rate exceeds 0.3%, the heat resistance as a support substrate will decrease, so when used in combination with a liquid crystal display, the display screen exposed to high temperatures may become distorted, and the transparency of the polarizing film may decrease. , it becomes impossible to obtain a bright and uniform LCD display screen. [Effects of the Invention] The polarizing film of the present invention uses a uniaxially oriented PEN film as a supporting substrate, which has a low thermal shrinkage rate and a refractive index characteristic that does not disturb the polarization performance of the polarizing element, so it can be used as a polarizing film for liquid crystal displays. It has good heat resistance and moisture resistance, and as a characteristic of PEN film, it blocks ultraviolet rays of 370 nm or less, which prevents deterioration of the liquid crystal molecules in liquid crystal displays, making liquid crystals that can withstand any changes in the external environment. It can be used as a polarizing film for display purposes. [Examples] The present invention will be further described below based on Examples. Note that various physical property values and characteristic values in the present invention were measured as follows. (1) Refractive index Measured according to ASTM-D542-50. The contact liquid used was an arsenic tribromide/disulfide formulation with a refractive index of 1820. (2) Heat shrinkage rate Measured after being left at 150°C for 30 minutes in a constant temperature bath without tension. If the original length is 1 ₀ and the measured length is 1, it is expressed as [(1 ₀ −1)/1 ₀ ]×100 (%). (3) Ultraviolet shielding property Double beam spectrophotometer manufactured by Hitachi, Ltd.
228A was inserted into the polarizing plate with its absorption axis tilted at 45 degrees, and the light transmittance at 365 nm was measured. A light transmittance of 5% or less was considered good, a value of 5 to 20% was considered acceptable, and a light transmittance of 20% or more was judged as poor. (4) Transmittance variation Double beam spectrophotometer manufactured by Hitachi, Ltd.
228A, the absorption axes of two polarizing plates are orthogonal,
Insert this orthogonal axis at an angle of 45° and insert from 385nm to 785n.
The light transmittance was measured at a wavelength of m. When a strong rainbow pattern appears, it is known that the light transmittance varies depending on the wavelength, so cases where the light transmittance exceeds 1% due to this variation are considered unacceptable, and cases where the light transmittance is 1% or less are considered acceptable. (5) Heat resistance Immediately after the polarizing plate was left at 130° C. for 1 hour without tension, the light transmittance at 550 nm was measured in the same manner as in section (3). When the initial transmittance is T ₀ and the transmittance after leaving is T [T/T ₀ ] x 100 (%)
5% or less was considered good, 5 to 10% was acceptable, and more than 10% was unacceptable. (6) Moisture resistance In section (5), the polarizing plate was heated to 90% RH at 60℃.
It was left to stand for 300 hours and judged in the same way. (7) Flexibility Cracks were evaluated when the polarizing plate was bent 10 times. The case where there were no cracks or cracks was judged as good, and the case where there were cracks or cracks was judged as poor. Example 1 Pellets of PEN homopolymer with an extreme viscosity of 0.65 were
It was dried at 170°C for 5 hours. The pellets were melt-extruded at 300°C using a T-shaped die in accordance with a conventional method to create an unstretched film with a thickness of 340 μm, which was rolled in the machine direction at 120°C to a factor of 4.0, and then using a stenter at 245°C for 300°C. Heat set for seconds to a thickness of 85μ
A uniaxially oriented PEN film for use as a supporting substrate of m was prepared. Table 1 shows the refractive index and thermal shrinkage rate of this support substrate film. On the other hand, a polyvinyl alcohol film containing 2.0% by weight of iodine was stretched 4.0 times in the machine direction using rolls to obtain a film for a polarizing element having a thickness of 60 μm. After the supporting substrate film was subjected to corona treatment, an adhesive was uniformly applied to both sides of the polarizing element film, and the adhesive was pressed onto both sides of the polarizing element film to obtain a polarizing plate film. The properties of the polarizing film thus obtained are shown in Table 1, and it has good ultraviolet shielding properties, transmittance fluctuations, heat resistance, moisture resistance, and flexibility, and is satisfactory as a polarizing film for liquid crystal displays. Ta. Comparative Example 1 In Example 1, the unstretched film thickness was 680 μm.
Stretched 4.0 times in the machine direction, then stretched in the width direction
A polarizing plate was prepared in the same manner as in Example 1, except that the film was stretched 2.0 times at 120°C. The physical properties of the polarizing film in this case are shown. Comparative Example 2 In Example 1, the unstretched film thickness was 510 μm.
The film was stretched 6.0 times in the machine direction. Table 1 shows the physical properties of the polarizing film at this time. Comparative Example 3 In place of PEN in Example 1, polyethylene terephthalate (hereinafter abbreviated as PET) pellets having an intrinsic viscosity of 0.65 were used. The drying temperature, melting temperature and stretching temperature were adjusted as appropriate. Comparative Example 4 Instead of the PEN film of Example 1, a 100 μm thick cellulose triacetate (abbreviated as TAC) film manufactured by Fuji Photo Industries Co., Ltd. and containing an ultraviolet absorber was used as a supporting substrate. Other conditions are the same as in Example 1. The physical property values and polarizing plate characteristics of Comparative Examples 1 to 4 are shown in the table.
1, but none satisfied all of the characteristic values. 【table】

Claims (1)

[Scope of Claims] 1. A polarizing film in which a polarizing element is uniaxially oriented polyvinyl alcohol containing a substance having photodichroism, and a uniaxially oriented thermoplastic polymer film is used as a support substrate, wherein polyethylene naphthalene is used as the support substrate. A polarizing film for liquid crystal display characterized by using a uniaxially oriented dicarboxylate film. 2. Polyethylene naphthalene dicarboxylate film satisfies the relationship of the following formula, and the heat shrinkage rate when left under no tension at 150℃ for 30 minutes is
A polarizing film for liquid crystal display according to claim 1, wherein the content is 0.3% or less. −0.03≦n ₁ +n ₃ /2−n ₂ ≦0.05 [Here, n ₁ is the refractive index in the main orientation direction, n ₂ is the refractive index perpendicular to n ₁ , and n ₃ is the refractive index in the thickness direction. . ]