Polymer for optical compensatory films for display panels

Details Polymer for optical compensatory films for display panels Polymer for optical compensatory films for display panels

2000-11-20

2003-05-13

Huff, Mark F. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

Alignment layer of specified composition

C568S818000, C585S352000

Reexamination Certificate

active

06562422

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a polymer for optical compensatory films, particularly to the material for optical compensatory films for display panels.
2. Description of the Prior Art
In recent times, with the rapid development of portable computers, the improvement of display panels has focused on the need for smaller sizes and lighter weights. Liquid Crystal Display, LCD, having characteristics of low voltage use, low power consumption, portability, compactness, high quality and mass productability, has become the most promising choice and is expansively used in digital watches, calculators, notebooks and other types of computer products.
Presently, LCD devices provide high quality images, yet the image becomes worse when looking at the panel at an angle. This problem results from the birefringence effect induced by the anisotropic liquid crystal. Owing to the positive birefringence of liquid crystal, i.e. the extraordinary refractive index (n⊥) is greater than the ordinary refractive index (n∥), the light passing by is phase retarded, which in turn induces the above problem when looking at the display panel at an angle. With the increasing size of LCD devices, the above problems could get even worse. Hence, a method to obtain a wide viewing angle effect has become the most important breakthrough in this field.
The improvement of viewing angle problems can be attained in a few different ways. For example, U.S. Pat. No. 5,774,179 proposed a method using electrodes to resolve the viewing angle problem, but the method has not effectively been used due to its high production cost and complex manufacturing processes.
Moreover, U.S. Pat. No. 5,416,621 uses ferroelectric liquid crystal to improve the viewing angle of the LCD. Although ferroelectric liquid crystal itself has the advantage of a wide viewing angle, there are still difficulties in mass production. The same is true of U.S. Pat. No. 5,781,267.
In addition, U.S. Pat. No. 5,689,322 discloses a method of partioning a liquid crystal display device into dozens of different areas, and by adjustment during the process, the alignment layer and liquid crystal are arranged to display different directions and pretilt angles. The method does not require any new material, but a few more steps are needed in the manufacturing process, consequently the production cost is increased.
Another important wide viewing angle technology is adhering a wide viewing angle film onto the display panels. In contrast to LCDs, the adhered film is birefringent and is able to achieve optical symmetry and obtain the results of compensation and colouring. No alterations for the manufacture of LCD devices is needed to apply this method, simply by adding a thin film, the viewing angle problem can be solved. Materials used in this method are mainly liquid crystal polymers (LCPs) disclosed in U.S. Pat. No. 5,883,685, photo-sensitive liquid crystal in U.S. Pat. No. 5,800,733, or polyimide in U.S. Pat. No. 5,344,916. Problems such as difficult manufacturing process and coating technique of LCPs and photo-sensitive liquid crystal have made this method unacceptable. Hence polyimide becomes the most important material that is commercially viable. Dissolubility, rigidity and transmittance of polyimide need to be taken into account to be commercially viable. At the moment, suitable material has not been discovered, therefore the invention tries to solve this problem and provide an applicable material for wide viewing angle technology.
SUMMARY OF THE INVENTION
The object of the invention is, to solve the above-mentioned problems and to provide a polymer for optical compensatory films, which has a wide viewing angle and can be used in display panels.
Another object of the invention is to provide a polymer of negative birefringence films having good coating properties, stability and is applicable to any display panels.
Another object of the invention is to provide a polymer of optical compensatory films that is able to counteract the birefringence effect of LCDs and obtain optically balanced characteristics. The high demands of the high transmittance are also met.
Moreover, an object of the invention is to provide a polymer which is able to give LCDs a wide viewing angle effect, and the polymer can be manufactured by common addition polymerization or reduction methods. This is in accordance with the requirement to be commercially viable.
Another object of the invention is to provide a polymer which is able to give LCDs a wide viewing angle effect, and the polymer can be coated on the LCDs by conventional coating machines, such as scrapper coating, spinning coating, etc.
DETAILED DESCRIPTION OF THE INVENTION
To achieve the above-mentioned object, the polymer of the optical compensatory films of the invention comprises derivatives of an aliphatic cyclic monomer 2,2-Bis-(4-aminophenyl)adamantane) with high transmittance, shown as formula I:
(wherein R represents alkyl or alkyl fluoride of 1 to 5 carbon atoms, such as H, CH
3
or CF
3
etc.; and m is 0, 1 or 2). The monomer is not a polar structure, and it hardens the high molecular polymer, which consequently attains good transmittance. The high molecular polymer of the invention is obtained by inserting the derivatives of monomer 2,2-bis(4-aminophenyl)adamantane in high molecular chains.
The invention is based on the theory that the rigid structure of the obtained high molecular polylmer, when forming films by coating or alignment agents, is able to produce a regular arrangement. Hence with appropriate thickness and the refractive index being different, i.e., d and &Dgr;n, the required phase retardation is achieved. An optical compensation effect is accomplished by exactly counteracting the negative birefringence of the film and the positive birefringence of the LCD. This results in good display quality when viewing from various angles, and the display contrast would not be diminished.
Factors affecting optical compensatory films are, for example, phase retardation, coating properties and transmittance, which are closely related to the structure and manufacturing process of the films. In terms of optics, phase retardation is the most critical because a wide angle viewing effect is only achieved when there is compensation between the film and the LCD. With regards to coating property, it affects the uniformity of film thickness and occurance of pinholes. If the thickness is not constant or having pinholes, the display quality, such as viewing angle and colouring of the LCD would be affected. The minimal acceptable transmittance is 92%.
For the purpose of controlling factors, such as film thickness, organic solvents may be used to adjust the viscosity of the polymer of the invention. Organic solvents preferably are N-Methyl Pyrrolidone; (NMP)
m-Cresol
r-butylacetone
N,N-dimethylacetamide; (DMAc)
N-N-dimethyl formamide; (DMF)
Ethylene glycol monobutyl ether; (BC)
Diethylene glycol monoethyl ether; (EC)
o-Chlorobenzene and Chloroform. The amount of BC and EC solvents must be below 90wt % and they must coexist with other solvents. Solid content of the polymer, i.e. the weight percent of the polymers with respect to solvents is preferably between 10% and 30%. After the reaction, solid content should be altered to 5~15% for the purpose of adjusting or diluting the viscosity of the polymer to control factors such as film thickness, etc.
Reaction time for the polymerization is between 3 minutes to 72 hours, preferably 10 minutes to 12 hours. Temperature of reaction is −20 to 180° C., preferably 10 to 60° C.
Cataylst may be added to increase the degree of polymerization and decrease the reaction time. Preferable catalysts are, for example, triethyl amine (TEA), diethyl amine, n-butyl amine (BuA) and pyridine, etc. The reason being that the value of pH can be adjusted and it acts as a catalyst to increase the degree of polymerization and viscosity and decrease the reaction time.
With the test of phase retardation and the coating procedure of wide viewing angle films,

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