JPH0324502A - Phase difference plate - Google Patents

Abstract

PURPOSE:To reduce the angle dependency of an optical path difference by setting the main refractive index n3 of the phase difference plate in the thickness direction between the main refracting indexes n1 and n2 in direction parallel to the surfaces of the phase difference plate. CONSTITUTION:The phase difference plate is formed by placing an anisotropic sheet or film made of resin whose characteristic birefringent value is positive and an anisotropic film or sheet made of resin whose characteristic birefringent value is negative one over the other so that the directions wherein the degrees of orientation are large are substantially at right angles to each other. Then n1 < n3 < n2, where n1 and n2 are the main refractive indexes in the direction parallel to the surfaces of the phase difference plate and n3 is the main refrac tive index in the thickness direction. The angle dependency of the phase differ ence plate satisfying the relation n1<n3<n2 is improved remarkably and, spe cially, when n1-n3 = n3-n2, the incidence angle dependency of the optical path difference is specially small.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a retardation plate with excellent transparency, and particularly to a retardation plate with low incidence angle dependence. [Prior Art] Retardation plates have become increasingly important with the recent development of optical technology. For example, it is expected to be used as a compensating retardation plate in black and white liquid crystal displays that have been made colorless by compensating for the birefringence inherent in liquid crystals. Conventionally, anisotropic polycarbonate films or sheets have been used as such retardation plates. However, polycarbonate anisotropic films or sheets have the disadvantage that the optical path difference changes greatly depending on the incident angle of light, and when viewed from an angle, compensation is not performed properly and the viewing angle range becomes narrow. .

[Problem to be solved by the invention]

An object of the present invention is to solve the problems of the prior art described above, namely, to develop a retardation plate with less angular dependence of optical path difference. [Means for Solving the Problems] An object of the present invention is to change the principal refractive index parallel to the plane of the retardation plate to nl.
, n2, and when the principal refractive index in the thickness direction is expressed as n3, n3
is achieved by a retardation plate with a value between n1 and n2. Figure 1 shows, as an example, a retardation plate with a thickness of 200μ and an optical path difference of 600 nm (nl-n2=0.003) in the direction perpendicular to the retardation plate surface, and the size and optical path difference of n3, n1, and n2. This shows the angle dependence of . The average refractive index of the resin is 1.586. Here, the angular dependence is the absolute amount of change in the optical path difference when the incident light beam is tilted by 45@ in the n2 direction compared to the case where the light beam is incident on the retardation plate at right angles, expressed as a percentage. When the incident light beam is tilted in this direction and the 01 direction, the change in the optical path difference becomes the largest, and in the former case, the optical path difference changes by almost the same amount to the high optical path difference side, and in the case of the wanderer, it changes by almost the same amount to the low optical path difference side. In conventional resin retardation plates represented by anisotropically oriented polycarbonate films, for both uniaxially rolled and biaxially rolled materials, the intrinsic birefringence@value is positive, n1, n2≧03, and negative. nl, n2≦n3, and there was no case where n3 had a value between n1 and 02. Although FIG. 1 shows a case where the characteristic torsion FT value is positive, it is not possible to obtain excellent angle dependence with such a device. On the other hand, n3 of the present invention is n1 and n
It can be seen that for retardation plates with values between 2 and 2, the angle dependence is significantly improved. Especially nl-n3=n3-n
When the second condition is satisfied, the dependence of the optical path difference on the angle of incidence is particularly small, making the retardation plate suitable for liquid crystal displays. Specifically, in the retardation plate of the present invention, for example, an anisotropic film or sheet made of a resin with a positive intrinsic birefringence value and an anisotropic film or sheet made of a resin with a negative intrinsic birefringence value are arranged such that the direction of high orientation is substantially It is realized by superimposing objects at right angles to each other. In this case, materials with positive intrinsic birefringence values include polyester resins such as polycarbonate resins, cellulose diacetate resins, polyphenylene oxide resins, and polyethylene terephthalate, and materials with negative intrinsic birefringence values include poly(meth)acrylate resins and polystyrene. A retardation plate made of a transparent homopolymer or cobolimer such as a resin of an unsaturated aromatic compound, or a blend or polymer alloy containing these as the main component can be used. Particularly preferred are polycarbonate resins with excellent transparency, cellulose diacetate resins, acrylic resins containing methacrylic acid ester as a main component, and styrene resins containing styrene as a main component. Anisotropically oriented films or sheets are produced by, for example, forming the above-mentioned raw material resin into a film or sheet by extrusion molding, and then uniaxially stretching it at a temperature 10 to 40°C higher than the glass transition temperature of the resin, or by forming it into a film or sheet so as to have anisotropy. It is obtained by biaxial stretching under the following conditions. In addition, it is preferable that the deviation from the perpendicular direction be within 35° during overlapping. Although the thickness of the retardation plate of the present invention is not essentially limited, it is preferably 25μ to 5sn from the viewpoint of ease of handling. A protective layer may be laminated on one or both sides of the retardation plate of the present invention. The method for measuring physical property values and the method for evaluating the angular dependence of optical path difference used in the description of the invention are shown below.・Optical path difference measurement method: Polarizing microscope (manufactured by Nippon Kogaku Kogyo Co., Ltd., LABOPH)
OT-POL) according to a conventional method. The angle dependence was determined by fixing the sample on a sample stand at a predetermined angle.・Principal refractive index: R. S. Stein's method (Journa
l of Polymer Science 24.3
83-386 (1957)) and the average refractive index of the resin. In addition, the larger principal refractive index parallel to the phase difference plate surface is expressed as nl, and the smaller one is expressed as n2. In the multilayer retardation plate, the intrinsic birefringence value was matched to that of the positive anisotropic sheet.・Evaluation of angle dependence: Based on the case where the light beam is incident on the retardation plate at right angles, the absolute amount of change in the optical path difference when the incident light beam is tilted in the n1 direction and the n2 direction is calculated as a percentage, and both Evaluation was made using the average value. The angle of incidence indicates the angle of inclination. 1 Example] The present invention will be specifically explained with reference to Examples. Example 1 Polycarbonate resin (manufactured by Idemitsu Petrochemical Co., Ltd., A-2
500) and a polystyrene resin (manufactured by Mitsubishi Monsanto Co., Ltd., Dialex HF-77) and an anisotropic film with a thickness of 100 μ produced by uniaxial stretching at a constant width of 2.2 times at a stretching temperature of 180 ° C. An anisotropic film with a thickness of 101 μm, which was produced by uniaxially stretching a constant width at a stretching temperature of 120° C. and a stretching ratio of 2.2 times, was stacked so that the stretching direction was perpendicular to produce a retardation plate. The principal refractive index of this is that n1 is 1. 5 8 9 8, n2
is 1.5883, n3 is 1.5890 and n3 is n1 and n
The size was between 2 and approximately satisfied the condition n2-n3=n3-nl. Even when the incident angle was 45°, the optical path difference did not change by more than 5%, and the angle dependence was small. Examples 2 to 5 An anisotropic film of polycarbonate resin and an anisotropic film of polystyrene resin produced by uniaxial stretching or two-wheel stretching are superimposed so that the direction with the highest degree of orientation is at right angles, and n3 is between n1 and n2. A retardation plate with the size of was fabricated. As shown in Table 1, these had small angular dependence of the optical path difference, and even at an incident angle of 45°, the amount of change in the optical path difference was 10% or less ● Comparative Examples 1 to 3 Polycarbonate used in Example 1 Anisotropic films or sheets were produced from resin by uniaxial stretching or two-wheel stretching. As shown in Table 1, n3 is nl and n2
However, the angle dependence of the optical path difference is large (we could not obtain a change in the optical path difference of less than 10% at an incident angle of 45°). Comparative Examples 4-5 Uniaxial l! from the polystyrene resin used in Example 1! Anisotropic films were prepared by horizontal or biaxial stretching. As shown in Table 1, these had n3 not between 01 and 02, and the angle dependence of the optical path difference was large. Examples 6 to 8 Anisotropic films of polycarbonate resin and acrylic resin (Parapet SH, manufactured by Kyowa Gas Chemical Industry Co., Ltd.) prepared in the same manner as in Examples 1 to 5 were prepared so that the direction with the highest degree of orientation was at right angles. A retardation plate in which the overlap n3 has a size between nl and n2 was manufactured as shown in FIG. As shown in Table 1, the angle dependence of the optical path difference is small as in Examples 1 to 5, and in particular, approximately n2-n3=
In the case of satisfying the n3-nl condition, the optical path difference did not change by even 5% even when the incident angle was 45°, and the angle dependence was small. [Effect of the invention] When the principal refractive index is expressed as nl, n2, and the principal refractive index in the thickness direction is expressed as 03, the angle dependence of the optical path difference, which was previously impossible, is reduced due to the retardation plate where n3 is a bond between n1 and n2. A retardation plate became possible. this is,
For example, it is suitably used as a compensating retardation plate for a black and white liquid crystal display.

[Brief explanation of drawings]

FIG. 1 shows the angular dependence of the optical path difference between the retardation plate of the present invention and a conventional retardation plate.

Claims (1)

[Claims] (1) When the principal refractive index parallel to the plane of the retardation plate is expressed as n1, n2, and the principal refractive index in the thickness direction is expressed as n3, n3 is n1 and n
Retardation plate with a value between 2.