(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 or optical recycle films 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), polycarbonate, polyphenylene oxide, polymethyl methacrylate, and cyclic olefin copolymer, etc. 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 is 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. Although this problem can be solved if non-birefringent

materials are used, these may deteriorate various essential properties. In particular, to adhere to the polarizing film, when using an PSA of which birefringences are systematically controlled, it is very difficult to maintain the intrinsic ability of the adhesive.

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. Patent 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 Hei10-279907 discloses that a mixture of an acryl-based polymer having a high molecular weight and an acryl-based polymer having a molecular weight of 30,000 or less 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 suspected 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. Patent 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, the use of a PSA with the capacity for stress relaxation to relieve the stress build-up in a polarizer unit, generally, 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 precisely cut, 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. Subsequently, it is accomplished by reducing birefringence due to the contraction of a polarizer attached to the liquid crystal display panel.

It is another object of the present invention to provide a polarizer film using the same.

In order to achieve these objects, the present invention provides an acrylic-based PSA composition, which comprises: a) a first PSA layer comprising i) a compound having a positive stress optical coefficient, and ii) an acrylic copolymer ; and b) a second PSA layer comprising an acrylic copolymer.

DETAILED DESCRIPTION OF THE INVENTION The present invention will now be explained in detail.

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 the present 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. With enlargement of liquid crystal displays, the optical characteristics of the PSA are also gaining of importance. Accordingly, the present inventors tried to form an PSA film comprising multi-layers, one of which contains a component that can compensate offset birefringence of the PSA film and another of which contains a component that can improve durability and reworkability.

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 cross-linked, and when the partially cross-linked molecules are stressed, molecular chains of the cross-linked

part are stressed in a specific direction, since the stress of the cross-linked part cannot be sufficiently relieved. The elastic characteristics of PSAs are similar to those of rubber or an elastomer, and the main chains of the PSA are aligned in a direction of applied stress. Therefore, when a material is stressed, it shows positive or negative birefringence, and the polarizer unit under shrinking stress can be characterized by negative stress birefringence in the cross-linked linear molecules of most acrylic PSA compositions.

When the PSA of the present invention is not stressed, the PSA molecules do not change the optical properties of the liquid crystal display, because the PSA remains isotropic, and only when the PSA is stressed due to polarizer shrinkage does it exhibits the birefringence compensation to relieve the display system from the light leakage, that is, the present invention can be distinguished from other inventions by the use of a light compensation technique through the understanding of the stress field of the system.

In addition, the PSA of the present invention and a polarizer film using the same can be prepared in a similar way to the procedure of preparing the conventional PSA layer.

The acryl-based PSA composition of the present invention comprises two layers ; a) the first PSA layer comprises i) a compound having a positive stress optical coefficient and ii) an acrylic copolymer, and further, b) the second PSA comprises an acrylic copolymer.

More particularly, the first PSA composition of the present invention comprises 0.001 to 40 parts by weight of a compound having a positive stress optical coefficient, and 100 parts by weight of an acrylic copolymer comprising 75 to 99.89 parts by weight of a (meth) acrylate ester monomer having Cl to C12 alkyl groups, 0.1 to 20 parts by weight of an a, unsaturated carboxylic monomer, and 0.01 to 5 parts by weight of a functional monomer having a hydroxyl group.

The first PSA may further comprise 0.01 to 10 parts by weight of a multifunctional isocyanate cross-linking agent.

The a) i) component having a positive stress optical coefficient can be used by copolymerization or blending with acrylic copolymer. There are many kinds of materials that can be used as component, but a component having an asymmetric molecular structure in an axial direction is preferred.

In addition to that, it is obvious to use component in a small amount so that the first PSA layer efficiency remains unchanged. For that purpose, a component having an asymmetric structure in an axial direction is used.

The composition of the present invention can be prepared by copolymerization of PSA resin with the component having a positive stress optical coefficient in a suitable ratio in order to form a structure having a side chain branched from a main chain, or by a conventional blending technique, and the prepared composition has similar characteristics to those of the conventional PSA which is used in adhesive-coating and lamination.

In the present invention, the component having a positive stress optical coefficient may include one compound or mixtures of two or more selected from the group, consisted of compounds having an asymmetric molecular structure and a positive stress optical coefficient in an axial direction. The compounds having a positive stress optical coefficient, used in this invention, are miscible to the acryl-based PSA in a wide range of composition, and they minimize the change in glass transition temperature of the PSA.

The component having a positive stress optical coefficient may include one compound, or mixtures of two or more, and it exhibits a positive stress optical coefficient when the components are dispersed uniformly in the PSA. In particular, when a main chain of an adhesive is aligned in a constant direction by the applied stress, the component is also aligned in the direction of the main chain, so that the birefringence of the PSA layer is changed. When the composite structure of laminated films exhibits a birefringence due to the stress, the component may compensate the resulting birefringence of each film sheet.

In order to compensate the birefringence of the laminated films, when the component having a positive stress optical coefficient is used in a small amount, large birefringence of the component is preferred. When the component has a rigid molecular structure and asymmetry in the axial direction, the component is capable of aligning effectively according to the applied stress resulting in the change of PSA birefringence. That is, when

molecules of the component have a rigid structure and asymmetry in an axial direction, the molecules are easily aligned in a direction of a main chain of the PSA, resulting in positive birefringence. By considering this individual optical activity of each layer, the complicated birefringence from the laminated polarizer unit comprising protecting film sheets and the PSA layer, which are under specific stress fields, can be controlled effectively.

In addition, when the component having a positive optical coefficient has a low molecular weight, elasticity and resilience of the PSA increase, so the molecular weight of the component is preferably 2000 or less.

The component having a positive stress optical coefficient may include aromatic compounds or alicyclic compounds, wherein the aromatic compounds are classified as aromatic crystalline compounds and aromatic liquid crystalline compounds such as cholesteric and smectic compounds, according to whether the substitute of an aromatic ring exists or according to the kinds of aromatic compounds.

A representative example of the component having a positive stress optical coefficient is preferably an aromatic compound as represented by Chemical Formula 1 or Chemical Formula 2: Chemical Formula 1 Chemical Formula 2

In Chemical Formula 1 and Chemical Formula 2: X, Y, R1, and R2 are, independently or simultaneously, substituents selected from a group consisting of hydrogen, cyano, chlorine, bromine, hydroxy, dimethylamine, cumyl, and Cl to C20 alkyls, alkoxys, and aryls ; A is-CH=N-,-N=N-,-N=N (O)-,-COO-,-CH2O-,-C (R')-CO-,-COO- <BR> <BR> <BR> <BR> <BR> CH2-, -C=C-, -C#C-, -S-, -SO2-, -# (R')-,-CH=N- (R1)-N=CH-,-C=C-¢ (R1)-<BR> <BR> <BR> <BR> <BR> <BR> <BR> N=C-, -C=C-#(R1)-C=C-, -C=C-#(R1)-#(R2)-C=C-, -C=N-#(R1)-#(R2)-N=C-, -C=N-#(R1)-#(R2)-C=C- or a bridge linking each core of naphthalene or anthracene, (wherein R'and R 2 are, independently or simultaneously, substituents selected from a group consisting of Ci to C20 alkyls, alkoxy, and aryls) ; and a and b are integers of 0 to 3 (if a = 0, b is an integer of 1 to 3; otherwise, b is an integer of 0 to 3).

The aromatic compounds represented by Chemical Formula 1 or Chemical Formula 2 include biphenyl, trans-stilbene, azobenzene, p- terphenyl, m-terphenyl, cumylphenylbenzoate, diphenylacetylene, 4- ethylbiphenyl, 4'-pentyl-4-biphenyl carbonitrile, 4-biphenyl carbonitrile, 4'- <BR> <BR> <BR> pentylbiphenyl, 4'-penthoxy-4-biphenyl carbonitrile, 4'-hexyl-4-<BR> <BR> <BR> <BR> <BR> <BR> <BR> biphenylcarbonitrile, 4'-octyl-4-biphenylcarbonitrile, trans-4-octyl-4'-

ethoxystilbene, naphthalene, anthracene, 4'- methoxybenzylideneaminostilbene, 4'- methoxybenzylideneaminoazobenzene, a derivatives thereof, and a mixture thereof, and the aromatic compounds are not limited to the aforementioned.

In addition, aromatic compounds having a positive stress optical coefficient includes crystalline material such as trans-stilbene, terphenyl, diphenyl acetylene, or biphenyl, when the aromatic compounds do not have substituents of X or Y, or they are small in size. When the X or Y derivatives have a suitable combination of polar-nonpolar or chiral groups, the material having the X derivatives or Y derivatives are smectic or cholesteric liquid crystalline compounds, and exemplary aromatic compounds and alicyclic compounds are described. (Polymer Liquid Crystals, A. C. Cifferri, W. R. Krigbaum, R. B. Meyer, Academic Press (1982)).

Since a molecular structure and a miscibility of the composition are critical to the optical properties and the PSA performance, the compound should have a positive stress optical coefficient such as with aromatic compounds and alicyclic compounds. That is, any compound with an anisotropic electron molecular structure can be used without limitation.

The compound having a positive stress optical coefficient can be selected variously, depending on the characteristics such as birefringence and adhesion properties. In order to maintain adhesion characteristics, this composition preferably ranges from 0.001 to 40 parts by weight, more

preferably from 0.01 to 30 parts by weight, and most preferably from 0.05 to 25 parts by weight. The composition may be blended with a PSA by a conventional way. In addition, this composition can be incorporated into the PSA as a multi-layer PSA structure.

Viscoelastic properties of the PSA mainly depend on molecular weight of polymer, its distribution, and molecular structure thereof. In particular, these properties are more preferably determined by its molecular weight. Therefore, the molecular weight of the a) i) acryl-based copolymer ranges from 200,000 to 2,000, 000, and it preferably ranges from 600,000 to 1,500, 000, and it comprises i) 75 to 99.89 parts by weight of a (meth) acrylic acid ester monomer comprising alkyl ester of C, toC, 2, preferably an alkyl ester of C2 to C8, based on 100 parts by weight of the acrylic copolymer, and ii) a functional monomer which is reactive with a cross-linking agent.

The content of the (meth) acrylic acid ester monomer preferably ranges from 75 to 99.89 parts by weight, and more preferably from 80 to 98 parts by weight.

When the alkyl (meth) acrylate has an alkyl group having a long chain, the adhesive has a low cohesive force, so an alkyl group of C2 to Cg is preferably used in order to prevent lowering of the cohesive force.

When the alkyl (meth) acrylate is excessive, its cohesive force lowers.

When the content of the alkyl (meth) acrylate is below 75 parts by weight, the adhesive force of the adhesive decreases, and its production cost may be increased.

The (meth) acrylic acid monomers comprises butyl (meth) acrylate, 2- ethylhexyl (meth) acrylate, ethyl (meth) acrylate, methyl (meth) acrylate, n- propyl (meth) acrylate, isopropyl (meth) acrylate, t-butyl (meth) acrylate, pentyl (meth) acrylate, n-octyl (meth) acrylate, isononyl (meth) acrylate and a mixture thereof, but it is not limited to the aforementioned. The monomers may further include various acryl-based monomers that are not mentioned here and vinyl-based monomers for the specific purpose.

The functional monomers of the ii) acrylic copolymer may include 0.1 to 20 parts by weight of unsaturated a, 15 carboxylic monomers, or 0.01 to 5 parts by weight of a monomer having a hydroxy group, and a mixture thereof, based on 100 parts by weight of the acrylic copolymer.

The concentration of the unsaturated a, carboxylic monomer preferably ranges from 0.5 to 15 parts by weight. The functional monomer of ii) gives a PSA of the present invention adhesion strength or cohesive force. When the content of the unsaturated a, f3 carboxylic monomer is below 0.1 parts by weight, the effect of improvement in adhesion strength may be decreased, and when it is greater than 20 parts by weight, the adhesion strength may be deteriorated due to a decrease in mobility resulting from increasing its cohesive force. Unsaturated a, j3 carboxylic monomers include acrylic acid, methacrylic acid, acrylic acid dimer, itaconic acid, maleic acid, maleic anhydride, and a mixture thereof, but they are not limited to the aforementioned.

The functional monomers having hydroxy groups that are reacted

with a cross-linking agent provides the PSA with cohesive force due to a chemical bond which is sufficient to bear a cohesive failure of the PSA at high temperature, and the amount of the functional monomer having a hydroxy group preferably ranges from 0.01 to 5 parts by weight, based on 100 parts by weight of the acrylic copolymer. When the amount of the functional monomers having hydroxy groups is below 0.01 parts by weight, a cohesive failure of the PSA at high temperature occurs, and when it is greater than 5 parts by weight, the softness may be deteriorated at high temperature. The functional monomers include 2- hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 2- hydroxyethyleneglycol (meth) acrylate, 2- hydroxypropyleneglycol (meth) acrylate, and a mixture thereof, but they are not limited to the aforementioned. The vinyl-based monomer having a hydroxy group may also be used in the present invention as long as the monomers are suitable for the present invention. Each of the aforementioned monomers may be used, and a mixture thereof may also be used.

The unsaturated a, f3 carboxylic monomers and the functional monomers having hydroxyl groups can be used or replaced by other functional monomers as long as the objects of the present invention are met. For example, the PSA can be cross-linked by reacting the multi- functional epoxy-based compound with the functional group of a carboxylic acid, reacting the multi-functional aziridine with a functional group of

carboxylic acid, or by using a UV curing agent. In addition, in order to provide the PSA of the present invention with the adhesion force, a polar material may be used to replace the unsaturated a, f3 carboxylic monomer. The multi-functional compounds can be used to produce the cross-linked structure and the adhesion reliability of the PSA also increases.

The a) ii) acryl-based copolymer may be prepared by solution polymerization, photopolymerization, bulk polymerization, suspension polymerization, or emulsion polymerization. In particular, the acryl-based polymer is preferably prepared by solution polymerization at a temperature ranging from 50 to 140 °C, and it is preferable that the initiator is added when the monomers are uniformly mixed.

The multi-functional cross-linking compound used in this invention enables the polymer main chains and side chains to orient, and when it is used for a viscoelastic PSA, it helps to orient main chains and side chains.

The multi-functional cross-linking compound includes isocyanate cross-linking agents, epoxy-based cross-linking agents, aziridine-based cross-linking agents, and metal chelate-based cross-liking agents. The isocyanate cross-linking agent includes tolylene diisocyanate, diphenylmethanediisocyanate, hexamethylenediisocyanate, and their adducts with trimethylolpropane, but it is not limited to the aforementioned.

The above compound may effectively provide the cross-linked structure that is necessary in this invention, and it may be selected from a combination of itself and functional groups having hydroxy groups.

During the mixing step, the cross-linking reaction of compound with functional groups having hydroxyl groups does not occur severely and it may be coated uniformly. After the coating of the PSA layer, drying, and the aging step, the resulting PSA layer having elasticity and strong cohesive strength due to its cross-linked structure is formed. The strong cohesive force of the PSA enhances durability and cuttability. The PSA may also be cross-linked by using open techniques of UV or EB.

The tackifiers may be further added in order to control the PSA properties, and the amount of the tackifiers ranges from 1 to 100 parts by weight. When the amount of the tackifiers is excessive, the cohesive force of the PSA may decrease. The tackifiers include (hydrogenated) hydrocarbon-based resin, (hydrogenated) rosin-based resin, (hydrogenated) rosin ester-based resin, (hydrogenated) terpene resin, (hydrogenated) terpene phenolic resin, polymerized rosin resin, and polymerized rosin ester resin. The tackifiers may also include a mixture thereof. In addition, the PSA composition of the present invention may comprise mixtures of epoxy resin and cross-linking agents, silane-based coupling agents, plasticizers, antioxidants, UV stabilizers, reinforcements, fillers, and colorants depending on the final use of the present invention.

For the acrylic copolymer of the b) second PSA layer, the same acrylic copolymer as that of the a) first PSA layer can be used. It is preferable that the second PSA layer contains an acrylic adhesive with superior durability and reworkability.

Thickness of the first and the second PSA layers can be determined by their respective functions. In general, a thickness of 10 to 50um is preferable. Since the first PSA layer is optically active, its thickness can be controlled, depending on the optical property of the second PSA layer.

Based upon requirement of the durability and reworkability characteristics, the thickness of the second PSA layer is, preferably, 10 to 20pm. It is desirable to design the molecular structure of the PSA so that the first and the second PSA layers are joined sufficiently, resulting in the prevention of peeling caused by interfacial stress.

The cross-linking density of the acrylic PSA composition of the present invention is 1 to 95%, preferably 30 to 90%, and more preferably 40 to 80%.

The PSA composition can be prepared by various methods such as random copolymerization, graft copolymerization, and block copolymerization. The PSA composition can also be prepared by photopolymerization, wherein a suitable and conventional photo-initiator is used.

The PSA composition of the present invention is not limited to the aforementioned uses, and the concept of the present invention is applicable to silicon-based, rubber-based, urethane-based, polyester-based, and epoxy-based PSA and adhesives, heat-activated PSAs, and hot-melt adhesives, regardless of the type of materials. That is, the described composition is applicable to all types of adhesive laminations that are used

as optical materials providing the birefringence compensation for the light leakage problem.

An acrylic PSA composition of the present invention can effectively offset complex birefringence under stress, while maintaining PSA properties and durability at high temperature and humidity, as well as cuttability.

The present invention also provides a polarizer comprising the aforementioned acrylic PSA composition. The polarizer of the present invention comprises the PSA layer (s) that is formed on either surface of the polarizer. Conventional components can be used for a polarizer film and a polarizer element that is used in the present invention. For example, the conventional polarizer includes a film prepared by adding iodine or dichloric dye to polyvinylalcohol-based films such as polyvinylalcohol, polyvinylformal, polyvinylacetal, ethylene, and a vinyl acetate copolymer, stretching thereof, and laminating the film with protecting films such as triacetyl cellulose, polycarbonate film, and polyethersulfone film.

The PSA layer is formed on the polarizing film by coating the PSA solution on the surface of a polarizing film using a bar-coater, and drying it; or by coating the PSA solution on the surface of strippable material, drying it in order to form a PSA layer, transferring the PSA layer to the surface of the polarizer, and aging it.

The PSA layer of the present invention can be formed on one side or both sides of the polarizing film. In addition, the polarizer of the present invention may be coated with a protecting layer, a reflection layer, an anti-

glare layer, a retardation plate, a film for a wide view angle, or films for brightness enhancement.

The PSA composition of the present invention is not limited to the aforementioned uses, and the concept of the present invention is applicable to silicon-based, rubber-based, urethane-based, polyester-based, and epoxy-based PSA and adhesives, heat-activated PSAs, and hot-melt adhesives, regardless of the type of materials. That is, the described composition is applicable to all types of adhesive laminations that are used as optical materials providing the birefringence compensation for the light leakage problem.

The present invention provides a polarizer comprising the aforementioned multi-layered PSA composition.

The polarizer of the present invention comprises the PSA layer (s) that is formed on either surface of the polarizing film. Conventional components can be used for a polarizing film and a polarizer element that is used in the present invention. For example, the conventional polarizer includes a film prepared by adding iodine or dichloric dye to polyvinylalcohol-based films such as polyvinylalcohol, polyvinylformal, polyvinylacetal, ethylene, and a vinyl acetate copolymer, stretching thereof, and laminating the film with protecting films such as triacetyl cellulose, polycarbonate film, and polyethersulfone film.

The PSA layer is formed on the polarizing film by coating the PSA solution on the surface of a polarizing film using a bar-coater, and drying it;

or by coating the PSA solution on the surface of strippable material, drying it in order to form a PSA layer, transferring the PSA layer to the surface of the polarizer, and aging it.

The PSA layer of the present invention can be formed on one side or both sides of the polarizer. In addition, the polarizer of the present invention may be coated with a protecting layer, a reflection layer, an anti- glare layer, a retardation plate, a film for a wide view angle, or films for brightness enhancement.

The following Examples and Comparative Examples illustrate the present invention in further detail, but the present invention is not limited by these Examples.

[Example] [Example 1] (Preparation of acrylic copolymers) A mixture of 94.5 parts by weight of n-butyl acrylate (BA), 5 parts by weight of acrylic acid (AA), and 0.5 parts by weight of 2- hydroxyethyl (meth) acrylate (2-HEMA) was placed in a 1000 cc reactor equipped with a temperature controller and a condenser with nitrogen gas reflux. 100 parts by weight of ethylacetate (EAc) was added thereinto.

The reactor was purged with nitrogen gas for 20 minutes in order to remove oxygen from the reactor, 0.03 parts by weight of 50% azobisisobutyronitrile (AIBN) which was diluted with ethyl acetate was added, they were reacted together for 10 hours at 65 C, and thereby an acrylic polymer (PA-1) was

obtained. The molecular weight of the acrylic polymer ranging from 600,000 to 1,000, 000 was measured by using polystyrene standard samples.

(Preparation of acrylic PSA) Preparation of first PSA laver The acryl-based polymer solution (PA-1,50% of solid content) was mixed with 7 parts by weight of diphenylacetylene, based on 100 parts by weight of the acryl-based polymer solution. Then, 1.2 parts by weight of tolylendiisocyanate adduct (TDI-1), which was diluted to 10% with ethyl acetate, was added thereto with high speed mixing, and it was diluted to a desired concentration for good quality of coating on a release film, and dried in order to prepare the first uniform PSA layer with a thickness of 20 fini.

Preparation of second PSA layer The acryl-based polymer solution (PA-1,50% of solid content) was mixed with 1.5 parts by weight of tolylendiisocyanate adduct (TDI-1), based on 100 parts by weight of the acryl-based polymer solution. This was diluted to 10% with ethyl acetate, was added thereto with high speed mixing, and it was diluted to a desired concentration for good quality of coating on a release film, and dried in order to prepare the second uniform PSA layer with a thickness of 10 fziii.

Preparation of polarizer The adhesive layer was laminated on a iodine-based polarizer with

a thickness of 185 M, with a laminator, so that a polarizer with the first and second PSA layers was prepared.

Example 2 The procedure was performed in the same way as in Example 1, except for using 4 parts by weight of diphenylacetylene and 1.8 parts by weight of toluylen diisocyanate adduct (TDI-1) of trimethylolpropane in preparing the first PSA layer.

Example 3 The procedure was performed in the same way as in Example 1, except for using 94.3 parts by weight of n-butyl acrylate (BA), 5 parts by weight of acrylic acid (AA), and 0.7 parts by weight of 2-hydroxyethyl (meth) acrylate (2-HEMA) in preparing the acrylic copolymer, and using 6 parts by weight of diphenylacetylene in preparing the first PSA layer.

Example 4 The procedure was performed in the same way as in Example 1, except for using 86. 5 parts by weight of n-butyl acrylate (BA), 10 parts by weight of methylacrylate (MA), 3 parts by weight of acrylic acid (AA), and 0.5 parts by weight of 2-hydroxyethyl (meth) acrylate (2-HEMA) in preparing the acrylic copolymer, and using 7 parts by weight of diphenylacetylene in preparing the first PSA layer.

Example 5 The procedure of Example 1 was performed in the same way as in Example 1, except for using 6 parts by weight of terphenyl instead of 7

parts by weight of diphenylacetylene in preparing the first PSA layer.

Comparative Example 1 The procedure was performed in the same way as in Example 1, without using the compound having a positive stress optical coefficient.

Comparative Example 2 The procedure was performed in the same way as in Example 1, except that 95 parts by weight of n-butyl acrylate (BA) was used in preparing the acrylic copolymer, and the compound having a positive stress optical coefficient and the multifunctional cross-linking agent was not used in preparing the first PSA layer.

Experiment For polarizers prepared with the acrylic PSA compositions prepared in Examples 1 to 5 and Comparative Examples 1 and 2, durability, uniformity of light transmittance, cuttability, and reworkability were evaluated by the following methods. The results are shown in the following Table1. a) Durability Polarizer of 90 mm x 170 mm was attached on both sides of a glass of 110 mm x 190 mm x 0.7 mm using a laminator at about 5 kg/cm"pressure.

The optical axis of each polarizer was crossed each other to obtain a dark state. The lamination step was carried out in a clean room in order to prevent the. panel from taking up bubbles or contamination. The test samples were put in a humidity chamber of 60 °C, 90% RH for 1000 hours

in order to examine the formation of bubbles or edge lifting by the wet-heat condition. Also the heat-resistance of the samples was tested in an oven at 80 °C for 1000 hours in the same way as the wet-heat test. The test samples were further conditioned for 24 hours at room temperature before the evaluation was performed. The durability was evaluated as follows : O : No bubbles and no edge lifting was observed.

, L : a few small bubbles and a little edge lifting were observed.

X: a large amount of bubbles and edge lifting were observed. b) Light leakage The light leakage of the samples that were prepared as above (crossed polarizer state) was evaluated by observing them using a backlight system in a dark room. The measurement of uniformity of their light transmittance is following.

O : No light leaking was observed by the naked eye.

: a little non-uniform light transmission was observed.

X: a severe light leaking from the edge of polarizers was observed. c) Cuttability The polarizer comprising the PSA was cut with a Thomson cutter.

The cross section of the cut polarizer was observed and evaluated as follows : O : The degree of adhesive pull out after cutting was less than 0.2 mm.

A : The degree of adhesive pull out from the edge was from 0.2 to 0. 5 X: The degree of adhesive pull out from the edge was greater than 0. 5 mm. d) Reworkability Polarizer of 90mmx170mm, coated with the certain amount of the PSA, was aged at room temperature (23°C, 60% R. H. ) for 7 days, and then attached on each side of a glass substrate of 110mmx190mmxO. 7mm.

After this polarizer was held for 1 hour at R. T. , it was heated at 50°C for 4 hours, and subsequently allowed to hold at R. T. for 1 hour. Further, the polarizer was peeled off from the glass substrate. The evaluation of the reworkability is as follows : O : it is easy to peel.

A : it is slightly difficult to peel.

X: it is difficult to peel, for example, the fractured glass or the broken substrate.

Table 1 Example Example Example Example Example Comp. Comp. Classification 1 2 3 4 5 Example 1 Example 2 Durability O O O O O O X Uniformity of light O A-0 O O O X O transmittance Cuttability O O O O O O X Reworkability # # # # # # X

As shown in Table 1, PSAs prepared according to the present invention (Examples 1 to 5), which comprise the first PSA layer comprising a compound with a positive stress optical coefficient and the second PSA layer comprising an adhesive film with superior durability and reworkability, have superior durability, uniformity of light transmittance, cuttability, and reworkability, compared to the resins of Comparative Examples 1 and 2.

As described above, an acrylic PSA composition of the present invention can solve the light leakage problem of a liquid crystal display panel by reducing birefringence due to contraction of a polarizer attached to the liquid crystal display panel, without sacrificing the key properties, such as PSA properties, durability and cuttability.