Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2002-08-15
2004-10-05
Buttner, David J. (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S001310, C524S190000, C524S205000, C524S208000, C524S246000, C524S293000, C524S369000, C524S481000, C524S484000, C524S485000, C524S486000, C353S020000
Reexamination Certificate
active
06800366
ABSTRACT:
BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to acrylic pressure-sensitive adhesive compositions for a polarizing film, and the polarizing film using the same.
(b) Description of the Related Art
A liquid crystal display generally includes a liquid crystal cell and polarizers, and the liquid crystal cell and polarizers are assembled to produce a final device by using suitable pressure-sensitive adhesive (PSA) layers. In addition, the liquid crystal display may include phase retardation films, compensation films for wide viewing angle performance, and films for brightness enhancement in order to improve the function of the liquid crystal display.
Two main components of the liquid crystal display include 1) a liquid crystal layer aligned properly between two glass plates, in which a color filter and a transparent electrode layer are constructed inside, and 2) polarizers, if necessary including phase retardation films, and additional functional films laminated on the glass plates by suitable adhesive or PSA layers. The polarizers include polarizing elements such as an iodine-based compound or a dichloric polarizing material aligned in a constant direction by stretching the molecular chains of polyvinyl alcohol (PVA)-based film, or a structure of polyene prepared by hydrolysis of PVA film or by a dehydrochloric acid reaction of polyvinyl chloride (PVC) film, and at least one side of the polarizing element is protected by a protecting film such as triacetyl cellulose (TAC) film. Additionally, the polarizer may include retardations film that have an anisotropic molecular alignment, and/or functional films for wide-viewing-angle compensation and brightness enhancement such as an optically designed liquid crystal film.
The aforementioned films have different physical, chemical, and optical characteristics, since the films are prepared from materials that have different molecular structures and compositions. When a liquid crystal display is used for a long time, the composing materials having different molecular structures and composition from each other become aged, and the molecular structure of the materials are changed. For example, 1) the difference in linear expansion coefficients among materials according to temperature change results in a continuous stress generation in the composing layers, and 2) materials having a preexisting molecular alignment are deformed by stress relaxation, so optical properties of polarizers change resulting in a severe light leakage from the liquid crystal display device.
In order to prevent light leakage from the polarizer, the stress resulting from the polarizer shrinkage should be first eliminated, but it is difficult to remove the stress build-up among the materials, since the polarizer and the PSA on the glass plate have different characteristics of dimensional stability in terms of shrinking and expansion behavior in high temperature and humidity conditions.
As an attempt for stress relaxation between a polarizer and a PSA layer on a glass plate, U.S. Pat. No. 5,795,650 discloses that an adhesive layer comprising plasticizer components is effective in relaxing a stress due to a polarizer shrinkage. However, the light leakage is not completely eliminated and since the plasticizer components may deteriorate the adhesion property of the PSA, one may have durability problems, such as bubbles and edge lifting problems. In addition, when a polarizer product is cut by a knife, the polarizer may be contaminated by the plasticizer component due to the increase in softness of the adhesive composition.
As another attempt, Japanese Patent Hei 10-279907 discloses that a mixture of an acryl-based polymer having a high molecular weight and an acryl-based polymer having a molecular weight of less than 30,000 relieves the stress in order to prevent the light leakage from the device. However, the light leakage is not completely eliminated, and the adhesion reliability is suspect due to the possibility of generation of large bubbles and edge lifting. Also, the polarizer may be contaminated by a cutting operation of the polarizer, the same as in the case of U.S. Pat. No. 5,795,650.
In general, PSAs include rubber-based, acryl-based, and silicon-based materials, and the acryl-based PSA among them has been extensively used due to their high optical performance and adhesion quality. The acryl-based adhesives exhibit good adhesion characteristics when they are lightly pressurized at room temperature, since the acryl-based adhesives can be designed to have proper chain mobility of molecules by the light pressure. The adhesion force of acrylic PSAs ranges generally from 100 to 3000 gm/in. Molecular characteristics of acryl-based PSA materials, such as molecular weight and distribution, cross-linking density, and composition, mainly influence the durability of PSAs, which may be controlled by the high adhesive strength and cohesive strength.
However, generally, using a PSA with the capacity for stress relaxation to relieve the stress build-up in a polarizer unit shows insufficient durability in resisting the formation and growth of bubbles and edge lifting in high temperature and high humidity conditions which are critical to the performance of a display panel. In addition, when a polarizer product comprising an adhesive sheet having high mobility is cut precisely, the adhesive is easily elongated out of the product, resulting in contamination of the product.
Therefore, the improvement in light leakage of PSAs must be achieved with little change in the main requirements of polarizer products such as durability and cuttability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an acrylic PSA composition that greatly improves light leakage from the polarizer-unit with little change in major requirements of polarizer products such as durability and cuttability.
It is another object to provide a polarizer film using the same.
In order to achieve these objects, the present invention provides an acryl-based PSA composition comprising a component having a positive stress optical coefficient, and a polarizer film using the same.
DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventors of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive.
The present invention provides an acryl-based PSA composition comprising a component having a positive stress optical coefficient, and a polarizer film using the same.
When a liquid crystal display is used for a long time, a complicated optical distortion occurs by the change in optical properties of composing films mainly due to the shrinking stress of a polarizer unit, which is called the unexpected birefringence phenomenon or the light leakage problem. In order to prevent the birefringence problems with the films, the inventors of the present invention investigated various components of a liquid crystal display such as the polarizer unit, the liquid display panel, and the PSA layer. As a result, it was discovered that the light leakage is resulted from the birefringence that occurs by stress build-up in composing films when materials having different dimensional stability are bonded together. In this invention, in order to provide a solution for the light leakage problem, a PSA composition comprising an optically active component that is capable of automatically compensating the birefringence by the same stress field of the system was suggested.
Since the PSA layer needs a strong cohesive force at high temperatures, properly cross-linked viscoelastic materials may be used so that the adhesive layer has excellent durability. The molecular structure of cross-linked PSAs is usually partially cr
Chang Suk-Ky
Han In-Cheon
Han Sang-Eon
Hwang In-Seok
Nikolay Malimonenko
Buttner David J.
Keehan Christopher
LG Chem Ltd.
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