JPH037921B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a polarizing plate, and more specifically to a polarizing plate protected with a transparent protective layer that does not generate colored interference fringes even with reflected or transmitted light from any viewing angle. When using a polarizing film in an environment where it is exposed to the outside air or when using it in a liquid crystal display, the surface of the polarizing film must be coated to protect it from external damage, moisture, or corrosion due to chemicals. It is necessary to provide a protective layer. Glass or plastic film is used as the material for forming the protective layer, but glass has the disadvantage that it cannot be made very thin due to its strength and is also heavy. Cellulose or polyacrylic resins have been put into practical use as plastic films;
It does not have excellent shape or dimensional stability, moisture resistance, or heat resistance, making it particularly unsuitable for outdoor use. In response, attempts have been made to use a film made by stretching polyethylene tereftate (abbreviated as PET in the following description), which is a type of polyester. Due to the stretching process, this film has high corrosion resistance against chemicals, as well as excellent heat and moisture resistance. cause refraction. Therefore, when viewing an object through a polarizing plate protected by this film, color unevenness may occur on the protective film due to light interference depending on the direction. Not suitable for membranes, etc. An object of the present invention is to eliminate the above-mentioned drawbacks of PET and other polyester films and to provide a polarizing plate protected by a polyester film that does not cause color unevenness. The purpose of the present invention is to obtain a polyester film that has been particularly strongly stretched in one direction parallel to the film surface, where Ny is the refractive index in the particularly strongly stretched direction, Nx is the refractive index in the direction perpendicular to this, and Nz is the film thickness. When the refractive index in the direction is [(1/Ny ² -1/Nx ² )/(1/Ny ² -1/Nz ² )] ^1/2 >0.8, and the film thickness is d. A polyester film that satisfies 2.3 [(1.642/1/N ² y + 0.642/N ² z) ^1/2 −Nx] d>4 is attached to at least one surface of the polarizing film via an adhesive layer. This is achieved by matching. Next, the theory underlying the present invention will be explained. As shown in Figure 1, birefringence exists between the two polarizers P ₁ and P ₂ of a crossed Nicol optical system consisting of two polarizers P ₁ and P ₂ arranged so that their optical axes are perpendicular to each other. When we have ^{a sample} with It is expressed as (1). Here, A is a constant and δ is a phase factor given by δ=2πΔn·d/λ. However, Δn is the difference in principal refractive index in a plane parallel to the surface of the sample, λ is the wavelength of light, and d is the thickness of the sample. Therefore, the expression
(1) can be expressed as I=Asin ² 2φ×sin ² (π・Δn・d/λ) (2). Note that Δn·d is called a letter dation. As can be seen from equation (2), φ=
Light does not pass through when 0, π/2, π, 3π/2, ..., but for example, even if 0<φ<π/2, the condition that Δn・d/λ is equal to N, where N is an integer and (N+
Between the conditions equal to 1)/2, the luminous intensity varies according to sin ² (π·Δn·d/λ) from 0 to the peak value. However, in reality, as N increases, the contrast of this change rapidly weakens, and light with a wavelength that satisfies such conditions does not actually contribute to color unevenness. Therefore, if N=1, d is a typical film thickness of 100 μm, and λ is the wavelength of 550 nm at the center of visible light, Δn is calculated to be approximately 0.005. This value is the value of Δn of PET film 0.08 to 0.14
Therefore, if λ and d are the same, PET is Δn・d/λ=N, or (N+1)/
The value of N that satisfies 2 is 10 or more, and there is no problem with color unevenness as far as the direction perpendicular to the film surface is concerned. That is, the letter d is Δn・d
If it is close to 10 μm, color unevenness will not be a problem as far as visible light is concerned in the vertical direction, and the above
In the case of PET film, the lettering is 8 to 14μ.
It's becoming m. However, in the case of diagonal direction,
It may be possible for Δn to be close to 0.005, and if the film is viewed from such a direction, colored interference fringes will appear. We will discuss this further below. Figure 2 shows that when the principal refractive index is nx, ny, nz,
It is a diagram of a refractive index ellipsoid expressed by the formula X ² /nx ² +y ² /ny ² +z ² /nz ² =1 (3). When the cross section of this ellipsoid passing through the origin is a circle, if the film is viewed from a direction close to the normal direction of the cross section, Δn will be extremely small (if viewed from the normal direction, Δn =
0), in this case, specifically, when viewing the film from a direction where Δn=0.005, colored interference fringes will be observed. There are two cross sections that are circular in shape and are symmetrical about the z-axis. The conditions for a circular cross section can be found as follows. That is, origin 0
, the normal line makes an angle θ' with the z-axis, and the ellipsoid of equation (3) is cut by a plane perpendicular to the y-z plane. The created figure) is expressed as x ² /nx ² +z′ ² (cos ² θ′/ny ² +sin ² θ′/nz ² )…(4). Here, z′ is the normal to the cross section (y′ axis)
is the coordinate axis chosen perpendicular to the x-axis. This formula (4)
The condition for this to represent a circle is that the coefficients of x ² and z′ ² are equal, that is, 1/nx ² = cos ² θ′/ny ² + sin ² θ′/nz ² …(5) holds true. It is. Here, the method for deriving equation (4) will be explained using FIG. 8. This figure is a diagram explaining the relationship between the coordinate axes y, y', z, and z' in Figure 2. Through such coordinate transformation, y, y',
Between z and z′, y=y′cos(90°−θ′)−z′sin(9°−θ)=y′sinθ′−z′cosθ′…(4a) z=y′sin (90°−θ′) + z′cos (90°−θ′) y′cosθ′+z′sinθ′ …(4b) holds. Expressions (4a) and (4b) are expressed as
Substituting into (3), x ² /nx ² + (y′sinθ′−z′cosθ′) ² /ny ² + (y′cosθ′+z′sinθ′) ² /nz ² = 1 … (4c) is obtained. That is, equation (4c) becomes equation (3), and equation (4a)
is obtained by applying the coordinate transformation shown in equation (4b). Therefore, the normal line makes an angle θ′ with the z-axis, and y
By setting y′=0 in equation (4c), the equation representing the cross section of the ellipsoid in equation (3) taken by a plane orthogonal to the -z plane becomes x ² /nz ² +z′ ² (cos ² θ ′/ny ² +sin ² θ′/nz ² )=1. That is, equation (4) is derived in this way. From equation (5), sinθ' = [(1/ny ² - 1/nx ² )/ (1/ny ² - 1/nz ² )] ^1/2 ...(6) is derived, and equation (6) is Colored interference fringes appear when viewed from a direction close to the direction of the satisfied angle θ'. However, the angle θ' found here is the angle within the medium film, and in reality, taking into account the refraction of light at the film surface, the angle θ when observed from air (see Figure 3) is need to be converted. If the refractive index of air (which can be considered as 1 in fact) is n ₁ and the refractive index of film is n ₂ , then
There is a relationship between θ and θ′ as n ₁・sinθ=n ₂・sinθ′ (7), and if θ is close to 90°, it is actually difficult to observe the film surface from an oblique direction. No colored interference fringes appear. Here, from equations (6) and (7), n ₁ = 1, n ₂
θ as = 1.6 (average refractive index of PET film)
If we find the condition for >90°, we get sinθ′=[(1/Ny ² −1/Nx ² ) /(1/Ny ² −1/Nz ² )] ^1/2 >0.8 …(8) . Therefore, the principal refractive indices Nx, Ny, Nz are expressed as
If the film satisfies (8), colored interference fringes with strong contrast will not appear even when the film is viewed from an oblique direction. Furthermore, in order to eliminate the interference fringe orders of several orders where the contrast is weak and to make it perfect, calculate the fringe order when tilted most diagonally, that is, under the worst conditions, and this is a constant value. All you have to do is go in the direction of eliminating the contrast. Specifically, Nz=1.60,
It is sufficient to calculate the fringe order when θ=90° (in this case, θ′=38.7°). FIG. 4 shows a diagram of the index ellipsoid in such a case. In this figure, ΔN in the direction of light incidence
If you calculate Using this to calculate the fringe order, N=ΔNd/cosθ'/λ...(10) Since no interference fringes are actually observed when the N value is 4 or more, N>4...(11) Equation (11) By substituting (9), (10) and θ′=38.7°, the condition of 2.3 [(1.642/1/Ny ² +0.642/N ² z) ^1/2 −Nx] d>4 is obtained. That is, by using a film that satisfies equations (8) and (12) as a protective film for a polarizing film, it is possible to obtain a polarizing plate in which no interference fringes are observed when viewed from any direction. The following table shows the values on the right side of equations (8) and (12) that were verified for PET film samples prototyped in the present invention.

【table】

[Table] Since sin θ' = 0.50 in the sample, interference fringes with strong contrast were observed. In addition, in the sample, although sin θ'>0.8, since N=2.2, faint interference fringes with weak contrast were observed. In the sample, both sin θ'>0.8 and N=4 were satisfied, and no interference fringes were observed at all when viewed from any direction. Next, we will discuss the material of the polarizing film used in the polarizing plate of the present invention and its processing method, the type and processing method of the polyester film that does not produce colored interference fringes and is attached as a protective film to the surface of the polarizing film, and the polarizing film and protective film. We will explain the types of adhesives used for bonding. The polarizing film used in carrying out the present invention is a polyvinyl alcohol film, a partially formalized polyvinyl alcohol film, or a saponified ethylene-vinyl acetate copolymer (EVOH).
A polarizing element such as iodine and/or dichroic dye is adsorbed and oriented on a hydrophilic polymer film such as a film, and an iodine and/or dichroic dye polarizing film or a polyvinyl alcohol film is dehydrated. Alternatively, there may be a polyene polarizing film made by subjecting a polyvinyl chloride film to dehydrochloric acid treatment to form polyene and orienting it. The protective polyester film bonded to one or both sides of the polarizing film via an adhesive layer is a film made of polyester such as polyethylene isophthalate or polybutylene terephthalate, which is subjected to an axial stretching process,
Provides corrosion resistance against chemicals. In addition to adhesive properties, the adhesive composition used for laminating the polarizing film and polyester film has optical transparency at a thickness of about 2 to 50 μm, and depending on the ingredients, the polarizing properties of the polarizing film can be improved. It is necessary that the adhesive does not disappear or deteriorate, and suitable adhesives include polyester adhesives, polyacrylic adhesives, epoxy adhesives, cyanoacrylate adhesives, polyurethane adhesives,
Examples include spirane adhesives. Furthermore, when bonding both films together,
In order to obtain sufficient adhesive strength, it is desirable to surface-treat the bonding interface between both films. As a surface treatment method for polyester film,
A sputtering method, an oxidation fire method, a primer treatment method, an alkali treatment method, etc. can be used. As a surface treatment method for the polarizing film, a primer treatment method using a silane coupling agent, a polyisocyanate compound, etc. can be used. In addition, the surface of the polyester film (exposed surface)
In addition, it is possible to extend the life of the polarizing plate and improve its polarizing properties by coating it with a silicone-based resin to make it scratch resistant, or by vapor-depositing magnesium fluoride to improve its transparency. It is preferable. Next, embodiments of the present invention will be described based on the drawings. FIG. 5 is a sectional view of this embodiment, and FIG. 6 is a partially cutaway perspective view of this embodiment. In both figures, the polarizing film 2 is a polarizing film made by stretching a polyvinyl alcohol film on which iodine has been adsorbed to about four times the length to give orientation to the iodine molecules and giving it polarizing properties. surface treatment with a polyisocyanate compound. Protective film 1
and 3 have a thickness of 100 μm, a difference in principal refractive index in the plane parallel to the surface of 0.14, and a principal refractive index in the thickness direction.
A polyethylene terephthalate film that has been stretched to have an angle of 1.51 and is adhered to both sides of the polarizing film 2 using a polyester adhesive so that the stretching direction is perpendicular to the polarization axis of the polarizing film. The adhesive layer is not shown in the figure. In the figure, solid arrows indicate the direction of the polarization axis of the polarizing film 2, and dotted arrows indicate the direction of extension of the protective films 1 and 3. Next, as another embodiment of the present invention, using exactly the same polarizing film, protective film and adhesive as above,
FIG. 7 shows a partially cutaway perspective view of a polarizing plate in which protective films are adhered to both sides of a polarizing film with the polarizing axis of the polarizing film aligned with the extending direction of the protective film. In FIG. 7, components corresponding to the embodiment shown in FIG. 6 are given the same numbers as in FIG. 6. Also in FIG. 7, the solid line arrows indicate the polarization of the polarization axis of the polarizing film 2, and the dotted line arrows indicate the direction of extension of the protective films 1 and 3. In both of the Examples shown above, no colored interference fringes were observed when these polarizing plates were observed from any direction under irradiation with white light. That is, as can be seen from the theoretical considerations described above, the stretched polyester film used as a protective film in the present invention alone satisfies the optical conditions for not producing colored interference fringes. When pasting this on a polarizing film to construct the polarizing plate of the present invention,
The relationship between the stretching direction of the polyester film and the polarization axis direction of the polarizing film does not need to be considered and may be arbitrary, but as an extreme example, both of the above examples in which the two were made perpendicular or parallel supported this. Become. In addition, as in the case of use in a liquid crystal display, the polarizing plates according to both of these embodiments may be arranged parallel to each other.
Moreover, even when observation was performed with the respective polarization axes being held orthogonal to each other, no colored interference fringes were observed. As is clear from the above description, the present invention provides a polarizing plate that does not produce colored interference fringes even when used alone or arranged so that the optical axes are orthogonal to each other.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of the orthogonal Nicol optical system, Figures 2 and 4 are diagrams showing the refractive index ellipsoid, and Figure 3 is the PET
FIG. 3 is a diagram showing the state of refraction of light at the contact surface between a film and air. 5 and 6 are a sectional view and a partially cutaway perspective view, respectively, showing the structure of an embodiment of the present invention. FIG. 7 is a partially cutaway view showing the configuration of another embodiment of the present invention. FIG. 8 is a diagram showing the state of coordinate transformation.

Claims (1)

[Claims] 1. In a polyester film that has been particularly strongly stretched in one direction parallel to the film surface, Ny is the refractive index in the particularly strongly stretched direction, Nx is the refractive index in the direction perpendicular to this, and Nz is the film thickness. When the refractive index in the direction is [(1/Ny ² -1/Nx ² )/(1/Ny ² -1/Nz ² )] ^1/2 >0.8, and the film thickness is d. A polyester film satisfying 2.3 [(1.642/1/Ny ² +0.642/Nz ² ) ^1/2 −Nx] d>4 is attached to at least one surface of the polarizing film via an adhesive layer. Combined polarizing plates. 2. A patent claim in which the adhesive is at least one selected from the group of polyester adhesives, polyacrylic adhesives, epoxy adhesives, cyanoacrylate adhesives, polyurethane adhesives, and spirane adhesives. The polarizing plate according to the range 1 above.