JPH05288930A - Phase difference plate having improved quality - Google Patents

Abstract

PURPOSE:To improve transmittance, retardation value and adhering workability without the dependency on the visual field angle by laminating two sheets of phase difference films consisting of the same base material and having the same retardation value in such a manner that the angle of deviation in a stretching axis direction attains a specific value. CONSTITUTION:Two sheets of the phase difference films consisting of the same base material and having the same retardation value are laminated in such a manner that the angle of deviation in the stretching axis direction attains 30 to 60 deg., more preferably 40 to 50 deg.. The base material of the phase difference films to be used include polyvinyl alcohol(PVA), hydrolyzed matter of ethylene- vinyl acetate copolymer, polyvinyl chloride, polymethyl methacrylate, polycarbonate, polypropylene, polystyrene, polyether, etc., and the use of the PVA films is more advantageous. The rate of hydrolysis of the PVA is preferably >=95%, more preferably >=99% and more particularly preferably >=99.5% and the degrees of polymn. thereof are preferably 1000, more preferably 1000 to 3000.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retardation plate having very little dependence on viewing angle, excellent optical characteristics such as transmittance and retardation value, and good workability.

[0002]

2. Description of the Related Art A retardation film is a film or sheet having birefringence. Since the light passing through the phase difference film has different refractive indexes in two directions orthogonal to each other, a phase difference occurs between the light rays orthogonal to each other after passing. A so-called quarter-wave plate having a function of producing a quarter-wave retardation with respect to the wavelength λ of an incident light beam is commercially available as a retardation film and is put to practical use. This is a mutual exchange element for linearly polarized light and circularly polarized light,
It is used as a beam splitter for laser beams, or in combination with a linear polarizer to form a circularly polarizing plate.

In addition, STN type liquid crystal display (STN
-LCD) elliptically polarizing plates and retardation films are used to prevent color shift. As the retardation film, various ones such as polyvinyl alcohol resin (PVA) or its derivative, polycarbonate, polystyrene, polypropylene and the like are known, and PVA and polycarbonate having excellent optical properties are known. The film or sheet is now in practical use.

[0004]

However, the LCD is
When viewing from the front only, it suffices to design the retardation film as a two-dimensional in-plane anisotropic body, but in reality, for example, in the technological development to enlarge the screen of LCD, by observing the tilt, There is a problem that the birefringence changes. Therefore, the viewing angle characteristic of the retardation film has become an important issue.

To solve this problem, the retardation film is regarded as a three-dimensional refractive index anisotropic body, and the in-plane refractive index is Nx.
(Stretching direction), Ny (orthogonal to Nx), and letting the refractive index in the thickness direction be Nz, the relationship between the refractive index Nz in the thickness direction and the refractive index Ny in the direction perpendicular to the optical axis is considered to improve the viewing angle characteristics. It is known that Nx>Nz> Ny. As a result, a retardation value within a certain range can be obtained when observed from any direction (for example, in practical use, the ratio of the retardation value at the incident angle of 40 ° to the retardation value at the incident angle of 0 °, that is, It is said that it is preferable to achieve R ₄₀ ° / R ₀ ° = 1.10 or less), and as a means therefor, 1) lamination of stretched films made of materials having positive and negative birefringence 2) thickness of stretched film It is considered effective to give the orientation in the depth direction.

As a method 1), for example, a direction of laminating retardation films of polycarbonate having a positive refractive index anisotropy and polystyrene having a negative refractive index anisotropy has been proposed. -235902 is available.
However, the effects of both methods are insufficient, and the adhesive used for lamination is not likely to be compatible with any of different types of substrates, and wrinkles and warps are likely to occur during the adhesive processing, which results in production. The problem remains in terms of efficiency.

[0007]

However, as a result of earnest studies to solve the above problems, the inventors of the present invention unexpectedly found that "two sheets made of the same base material and having the same retardation value" It has been found that the "retardation film obtained by laminating retardation films so that the deviation angle in the stretching axis direction is 30 ° to 60 °, preferably 40 ° to 50 °" has very little dependence on the viewing angle. The invention was completed.

In the present invention, since the same base material is used as described above, there are no problems with the adhesive during lamination, and the optical characteristics of the retardation plate, especially the viewing angle dependency are improved, and the incident light is improved. It has been found that there is almost no variation in retardation value even when the angle is considerably high. Hereinafter, the present invention will be described in detail.

Examples of the base material of the retardation film used in the present invention include well-known materials such as PVA, saponified ethylene-vinyl acetate copolymer, polyvinyl chloride, polymethylmethacrylate, polycarboneto, polypropylene, polystyrene and polyester. However, the use of PVA-based films is advantageous. The retardation film to be laminated in the present invention has a uniaxial stretching ratio of 1.05 to 2.0.
0, and a retardation value of about 30 to 800 nm is practical. The PVA type retardation film will be specifically described.

As PVA-based materials, PVA and PVA
Examples thereof include, but are not limited to, so-called polyvinyl acetal resins such as polyvinyl butyral resins and polyvinyl formal resins, and other PVA derivatives obtained by copolymerization, which are obtained by reacting aldehydes with aldehydes in the presence of an acid. Of these, PVA having a high degree of saponification and a high degree of polymerization is preferable because it has good heat resistance. That is, the saponification degree is preferably 95% or more, more preferably 99% or more, and particularly preferably 99.5% or more, and the polymerization degree is 1
000 or more, preferably 1000 to 3000 are preferable.

The PVA type retardation film means PVA.
The original film or sheet in which the base resin is formed into a film by a usual method such as a casting method, an extrusion method or a calendering method is used in 1.1
~ 4 times uniaxially stretched, 10 ~ 30 at 150 ~ 250 ° C
The thickness is about 30 to 60 μm, preferably about 40 to 50 μm, which is obtained by heat treatment for about 10 seconds at about 200 seconds to preferably about 200 to 230 ° C., and the retardation value is 30 to 800 nm in terms of physical properties. It is a base material.

In order to obtain a retardation plate having a constant optical principal axis and less optical color spots, the original film or sheet should have good thickness accuracy and be as optically uniform as possible. preferable. It is not preferable that a die line or the like is generated during film formation on the film or sheet. In order to obtain a retardation film having an optically small color spot in the present invention, a neck-in rate (100 × (A− is defined as a film width A before stretching and a film width B after stretching.
It is preferable to control B) / A) to 20% or less.

The uniaxial stretching method includes, for example, preheating the film by passing the film between a large number of rolls and then stretching the film at a predetermined ratio with two pairs of stretching rolls. A method in which preheating and stepwise stretching are performed concurrently while passing through the film, and a method in which a predetermined magnification is obtained, a method such as a lateral uniaxial stretching method in which the film is stretched in the width direction by a tenter method may be employed.

Stretching is appropriately performed depending on the intended use of the retardation film. That is, the retardation value R is defined by the product Δnd of the refractive index difference Δn and the thickness d of the retardation film or sheet in the main stretching direction (MD direction) and the direction (TD direction) perpendicular thereto. .. Various additives such as an ultraviolet absorber and a stabilizer may be added to the PVA type retardation film according to the present invention. Also,
The heat treatment performed after the stretching makes the molecular orientation permanent.

A feature of the present invention is that two retardation films having the same base material and the same retardation value are superposed on each other, and the deviation angle in the stretching axis direction at the time of superposition is 30 ° to 60 °. A retardation film having no viewing angle dependence can be obtained by specifying preferably at 40 ° to 50 °. The two retardation films are adhered using an ordinary adhesive or pressure-sensitive adhesive, but they may be simply superposed if the laminated state is maintained. Any adhesive or pressure-sensitive adhesive may be used, such as vinyl acetate resin, acrylic resin, urethane resin, epoxy resin, PV.
Examples thereof include solvent solutions, aqueous solutions and dispersion solutions of A-based resins, cellulose-based resins, polyester-based resins and the like.

In the present invention, since the base material of the retardation film is the same, the adhesive and the pressure sensitive adhesive are compatible with any of the films, so that wrinkles and warps are generated at the time of the bonding process, resulting in problems such as poor adhesion. There is no worry about it, and the phase difference film can be manufactured extremely efficiently. The retardation plate manufactured as described above can be sufficiently used in practical use, and the effects of the present invention can be sufficiently exhibited. However, if necessary, a cellulose-based film, a polystyrene-based film, a polyester-based film, a polysulfone-based film, etc. There is no problem even if the water-resistant polymer film is laminated.

The retardation plate thus obtained has very little dependence on the viewing angle, is extremely excellent in optical characteristics such as transmittance and retardation value, and can sufficiently meet the recently required high physical properties. The retardation plate is appropriately cut at the time of use and is useful for liquid crystal display applications by laminating it with a polarizing film, glass, or other substrate which is a mating substrate.

[0018]

[Working] The retardation plate of the present invention has no dependence on the viewing angle, has excellent transmittance and retardation value, and has excellent adhesive workability by bonding two retardation films of the same kind at a specific angle. Is also good.

[0019]

The present invention will be described in more detail below with reference to examples. Example 1 A uniaxially stretched PVA film having an average degree of polymerization of 2000 and an average degree of saponification of 99.7 mol% and a thickness of 50 μm (stretching ratio 1.2
After being doubled and stretched, it is heat-treated at 220 ° C. for 15 seconds, and the two sheets have a retardation value of 380 nm), and the ethylene-vinyl acetate copolymer emulsion is used as an adhesive, and the deviation angle in the stretching direction is 30 °, 40 °, 45 °, 50. Five kinds of retardation films were manufactured by laminating them so as to form an angle of 60 °. The dependence of the viewing angle of this retardation plate was measured by the following method. In addition, the ratio of the retardation value at the incident angle of 40 ° to the retardation value at the incident angle of 0 ° (R ₄₀ ° / R ₀ °), and the retardation value at the incident angle of 50 ° to the retardation value at the incident angle of 0 °. The ratio (R ₅₀ ° / R ₀ °) was shown. or,
As a reference example, the same measurement was performed for the retardation plate in which the two films were laminated in parallel (deviation angle of 0 °) and orthogonally laminated (deviation angle of 90 °), and the single-layer film. The results are shown in Tables 1 and 2.

(Method of measuring viewing angle dependency) Tilt type automatic birefringence meter (Model KOBRA-21A manufactured by KS Systems Co., Ltd.)
The relationship between the incident angle and the retardation value is obtained by inclining the retardation plate to the front, rear, left and right according to D). (Measurement of Retardation Value) The sample is irradiated with polarized light and the birefringence is determined from the angle dependence of the intensity of the transmitted light, and the product with the sample thickness is taken as the retardation value.

[0021]

[Table 1]

[0022]

[Table 2]

Example 2 In Example 1, a uniaxially stretched PVA film having an average degree of polymerization of 1700 and an average degree of saponification of 99.9 mol% and a thickness of 60 μm (stretching ratio of 1.25 times, heat treatment at 200 ° C. for 15 seconds after stretching). The same experiment was performed, except that two sheets were used, the retardation value being 680 nm). The results are shown in Table 3 and Table 4.
Shown in.

[0024]

[Table 3]

[0025]

[Table 4]

Example 3 The retardation value of 425 was used instead of the PVA of Example 1.
nm Polycarbonate film (Molecular weight 5 × 10 ⁴ )
Experiment was carried out using. The results are shown in Tables 5 and 6.

[0027]

[Table 5]

[0028]

[Table 6]

Example 4 Instead of the PVA of Example 1, a retardation value of 400 was used.
Experiments were carried out using nm nm polyester film. The results are shown in Tables 7 and 8.

[0030]

[Table 7]

[0031]

[Table 8]

Example 5 The PVA of Example 1 was replaced by a retardation value of 380.
Experiments were conducted using nm polyphenylene sulfide film. The results are shown in Tables 9 and 10.

[0033]

[Table 9]

[0034]

[Table 10]

[0035]

[Effect] In the present invention, two retardation films made of the same base material and having the same retardation value have a deviation angle in the stretching axis direction of 30 ° to 60 °, preferably 40 ° to 50 °. The retardation plate laminated so as to have almost no viewing angle dependency, has excellent transmittance and retardation value, and has good adhesive workability.

Claims (4)

[Claims] 1. A quality obtained by laminating two retardation films, which are made of the same base material and have the same retardation value, so that a shift angle in a stretching axis direction is 30 ° to 60 °. Improved retardation plate. 2. The retardation plate according to claim 1, wherein the retardation film is a polyvinyl alcohol-based uniaxially stretched film. 3. A quality obtained by laminating two retardation films, which are made of the same base material and have the same retardation value, so that the deviation angle in the stretching axis direction is 40 ° to 50 °. Improved retardation plate. 4. The retardation plate according to claim 3, wherein the retardation film is a polyvinyl alcohol-based uniaxially stretched film.