JPH06194520A - Phase-difference film - Google Patents

Abstract

PURPOSE:To improve the visual field characteristic and display characteristic of an STN liquid crystal display element by minimizing the change in the hue of the black-and-white display by coloring against a large change in the viewing angle. CONSTITUTION:When the phase-difference film is made of a polycarbonate, the film is formed with a polycarbonate copolymer of a dihydroxy compd. (a) capable of forming a polycarbonate A and a dihydroxy compd. (b) capable of forming a polycarbonate B so that the solid photoelastic constant of the polycarbonate A is larger than that of the polycarbonate B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a retardation film used for black and white display of STN-LCD.

[0002]

2. Description of the Related Art A phase difference compensating plate is necessary for compensating a phase difference due to a birefringence effect of an STN liquid crystal display element and making a display black and white. Initially, the same type of liquid crystal cell was used for compensation. However, recently, a retardation film made of a plastic film has been used because of its light weight and low cost. As the resin used, there are polycarbonate, polymethylmethacrylate, polyvinyl alcohol, polystyrene, etc., which are produced by uniformly uniaxially stretching a film made of these resins. This utilizes the birefringence effect caused by the uniaxial orientation. However, the stretched plastic film has a large change in retardation when viewed from an oblique direction. Therefore, even if it is displayed in black and white when viewed from a vertical direction, when it is viewed from an oblique direction, remarkable coloring occurs or the display is reversed. As a result, the field of view is limited.

Various studies have been conducted to solve this problem. As an example of attempting to solve the problem by improving and compounding the resin, a retardation film made of a resin which produces positive birefringence and a negative birefringence film are given. A method of combining retardation films made of a resin that causes refraction (Japanese Patent Laid-Open No. 3-206422),
A method of producing a retardation film by copolymerizing or mixing a resin that produces positive birefringence and a resin that produces negative birefringence (JP-A-4-195103, 4-194902) has been proposed. . Among the proposals so far, in the method of stacking two retardation films having different signs of birefringence,
It has drawbacks that it is difficult to align the axes of two sheets, the transmittance is reduced due to the use of two sheets, the display is dark, and the material cost is high because of the two sheets. Further, the method of copolymerizing or mixing resins having different signs of birefringence has the drawbacks that the photoelastic constant is large, the heat resistance is high, and there are very few combinations showing optical perfect compatibility.

[0004]

DISCLOSURE OF THE INVENTION In order to improve these drawbacks, the present invention has optical transparency, heat resistance and a large photoelastic constant, so that a thin retardation film is possible. It also provides a retardation film which is advantageous in terms of cost. The present invention uses a single resin,
It is intended to provide a retardation film having a wide viewing angle and a wide viewing angle with a single film.

[0005]

DISCLOSURE OF THE INVENTION The present invention provides a dihydroxy compound a capable of producing a polycarbonate A having a solid photoelastic constant of polycarbonate A> a solid photoelastic constant of polycarbonate B when made into a polycarbonate. And a dihydroxy compound b capable of producing a polycarbonate B, is a retardation film formed of a polycarbonate copolymer.

[0006]

In the present invention, by using the resin having the above-mentioned constitution, it is possible to suppress the decrease in the refractive index in the thickness direction of the retardation film after stretching, and thus the relative refractive index in the thickness direction As a result, the angle dependence of retardation is reduced and the viewing angle can be widened. In the present invention, polycarbonate A
Photoelastic constant of Polycarbonate B> more effectively, the photoelastic constant of Polycarbonate A-the photoelastic constant of Polycarbonate B ≧ 10 Brewster, preferably 20 Brewster desirable.

As the dihydroxy compound a used in the present invention, various compounds can be used.
Dihydroxy aromatic compounds such as bisphenol A and its derivatives are suitable. The dihydroxy compound b is preferably a dihydroxy aromatic compound from the viewpoint of chemical properties and physical properties, but particularly, the methyl group at the central carbon of bisphenol A is substituted with a benzene ring, a cyclohexane ring or an aliphatic hydrocarbon group. A derivative or a dihydroxy aromatic compound in which an aliphatic group, a benzene ring, a cyclohexane ring or the like is introduced into each benzene ring of bisphenol A asymmetrically with respect to the central carbon is preferable. That is, it is desirable to use, as the dihydroxy compound b, a compound that gives a structure in which the anisotropy in the unit molecule of the resulting polycarbonate is as small as possible.

In the present invention, the composition ratio of the polycarbonate copolymer is such that the weight ratio of the dihydroxy compound a and the dihydroxy compound b in the copolymer is 95: 5 to 5:95, more effectively 90:10 to. 10:90 is preferable. When the dihydroxy compound b is less than 5, the angle dependence is not improved.
If the dihydroxy compound b is more than 95, the photoelastic constant becomes too small, a large retardation cannot be obtained even when stretched, stable processing cannot be performed, and it becomes difficult to obtain a uniform film.

In terms of cost, it is desired to find a composition having the best optical characteristics when the ratio of the special dihydroxy compound b is as small as possible. The viscosity average molecular weight of the copolymer thus obtained is preferably 20000 to 100000.

[0010]

【Example】

Example 1 A polycarbonate copolymer having a weight ratio of bisphenol A and 4,4′-hydroxytetraphenylmethane of 90:10 was produced. In this case, the solid photoelastic constant of the polycarbonate using only bisphenol A is 71 Brewster, and the solid photoelastic constant of the polycarbonate using only 4,4'-hydroxytetraphenylmethane is 22 Brewster. It was The viscosity average molecular weight of the copolymer was 35,000. next,
This copolymer was dissolved in methylene chloride to prepare a varnish having a solid content of 20%, and a film having a thickness of 70 μm was produced by a casting method. Furthermore, this film was 165
It was heated to ℃, and uniaxially stretched 1.2 times.

When the optical properties of the obtained film were evaluated, the retardation was 370 nm, and the angle dependence of the retardation was 9% in average in the increasing and decreasing directions at an incident angle of 45 degrees. there were. This value has been improved several times compared to the current product of Comparative Example 1, and when it was attached to a liquid crystal cell and used as a monochrome display panel, the display characteristics were improved in a wide viewing angle, and it was easy to use. There was little feeling of fatigue.

(Example 2) Bisphenol A and 2,2'-
A polycarbonate copolymer having a weight ratio of 70:30 with bis (3-cyclohexyl-4-hydroxyphenyl) propane was produced. In this case, the solid-state photoelastic constant of the polycarbonate using only 2,2′-bis (3-cyclohexyl-4-hydroxyphenyl) propane was 29 Brewster. Next, in the same manner as in Example 1, 80μ
A film with a thickness of m is produced, and the
Uniaxial stretching was carried out at 1.2 ° C and 1.2 times. When the optical properties of the retardation film thus obtained were evaluated, the retardation was 410 nm, and the angle dependence of the retardation was a value obtained by averaging the phase increasing direction and the phase decreasing direction at an inclination of an incident angle of 45 degrees. 7%, showing a wide viewing angle.

(Comparative example) A 90 μm film was prepared in the same manner as in Example 1 using a polycarbonate containing only bisphenol A having a viscosity average molecular weight of 70,000. Next, uniaxial stretching was performed with a tenter type stretching machine at 170 ° C. and 1.2 times. When the optical properties of the obtained retardation film were evaluated, the retardation was 450 nm,
The angle dependence of retardation was 25% on average in the increasing and decreasing directions at an incident angle of 45 degrees. When this phase difference film was attached to the liquid crystal panel, the field of view was quite narrow,
The field of view was easily inverted, the displayability was poor, the viewing angle of the user was limited, and the user felt tired.

[0014]

The use of the retardation film of the present invention minimizes the change in color tone of black and white display due to coloring even with a large change in viewing angle, and improves the visual field characteristics of the STN liquid crystal display device. The display characteristics can be improved.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shuji Sakamoto 1280 Kamizumi, Sodegaura, Chiba Prefecture Central Research Institute of Idemitsu Kosan Co., Ltd. (72) Hideyuki Miyamoto 1280, Uezumi, Sodegaura, Chiba Idemitsu Kosan Co., Ltd. Central Research Center

Claims (1)

[Claims] 1. A dihydroxy compound a capable of producing a polycarbonate A and a polycarbonate B capable of producing a polycarbonate having a photoelastic constant of a solid of polycarbonate A> a solid photoelastic constant of polycarbonate B when formed into a polycarbonate. A retardation film formed of a polycarbonate copolymer with a dihydroxy compound b.