Polarizing plate and liquid crystal display comprising the same

Abstract

The present invention relates to a polarizing plate and a liquid crystal display device. In the present invention, the polarizing plate is lightweight and has a small thickness and excellent properties including durability, water resistance, workability, pressure-sensitive adhesion and light leakage prevention ability; and the liquid crystal display device including the same are provided.

Claims

1. A polarizing plate, comprising: a polarizer; an adhesive layer directly attached to at least one surface of the polarizer; and an acrylic pressure-sensitive adhesive layer directly attached to the adhesive layer, wherein the acrylic pressure-sensitive adhesive layer includes a first layer and a second layer, and wherein the first layer is directly adjacent to the adhesive layer and has a tensile modulus at 25° C. of 300 MPa to 850 MPa, and the second layer has a tensile modulus at 25° C. of 0.01 MPa to 1.0 MPa, and wherein the first layer comprises a cross-linking structure comprising an acryl polymer cross-linked by a multifunctional cross-linking agent and a cross-linking structure comprising a polymerized multifunctional acrylate.

2. The polarizing plate according to claim 1, wherein the second layer has a pressure-sensitive adhesive surface for attaching the polarizing plate to a liquid crystal panel.

3. The polarizing plate according to claim 1, wherein the polarizer is a polyvinyl alcohol-based polarizer.

4. The polarizing plate according to claim 1, wherein the acrylic pressure-sensitive adhesive layer has a thickness of 10 to 80 μm.

5. The polarizing plate according to claim 1, wherein the adhesive layer includes at least one selected from the group consisting of a polyvinyl alcohol-based adhesive; an acryl-based adhesive; a vinyl acetate-based adhesive; a urethane-based adhesive; a polyester-based adhesive; a polyolefin-based adhesive; a polyvinylalkylether-based adhesive; a rubber-based adhesive; a vinylchloride-vinylacetate-based adhesive; a styrene-butadiene-styrene adhesive; a hydrogenated styrene-butadiene-styrene-based adhesive; an ethylene-based adhesive; and an acrylic acid ester-based adhesive.

6. The polarizing plate according to claim 1, wherein the adhesive layer includes a thermocurable adhesive composition, a room temperature curable adhesive composition, a moisture curable adhesive composition, or a photocurable adhesive composition in a cured state.

7. The polarizing plate according to claim 1, further comprising: a protective film attached to the surface to which the acrylic pressure-sensitive adhesive layer of the polarizer is not attached.

8. The polarizing plate according to claim 1, wherein the second layer has a tensile modulus at 25° C. of 0.03 to 0.7 MPa.

9. The polarizing plate according to claim 1, wherein the polymerized multifunctional acrylate is present in an amount of 20 to 200 parts by weight with respect to 100 parts by weight of the acryl polymer.

10. A liquid crystal display device, comprising: a liquid crystal panel; and a polarizing plate according to claim 1 attached to one or both surfaces of the liquid crystal panel.

11. The device according to claim 10, wherein the liquid crystal panel is a passive matrix panel, an active matrix panel, an in-plane switching panel or a vertical alignment panel.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic cross-sectional view of a conventional polarizing plate;

(2) FIG. 2 is a cross-sectional view of an exemplary polarizing plate according to the present invention; and

(3) FIGS. 3 and 4 are cross-sectional views of a pressure-sensitive adhesive layer according to the present invention.

MODE FOR INVENTION

(4) Hereinafter, the present invention will be described with reference to examples and comparative examples in detail. However, the present invention is not limited to these examples.

Preparation Example 1. Preparation of Acryl Polymer (A)

(5) 98 parts by weight of n-butyl acrylate (n-BA) and 2 parts by weight of 2-hydroxyethyl acrylate (2-HEA) were put into a 1 L reactor equipped with a cooling device to reflux nitrogen gas and facilitate temperature control. Subsequently, 180 parts by weight of ethyl acetate (EAc) was put into the reactor as a solvent, which was purged with the nitrogen gas for 60 minutes to remove oxygen. Afterwards, the temperature was maintained at 67° C., 0.05 parts by weight of azobisisobutyronitrile (AIBN), as a reaction initiator, was put there into, and the reaction was carried out for 8 hours. After the reaction, the reaction product was diluted with ethyl acetate, and thus an acryl polymer (A) having a solid concentration of 30 wt %, a weight average molecular weight of 1,000,000 and a molecular weight distribution of 4.9 was prepared.

Preparation Examples 2 to 5. Preparation of Acryl Polymers (B) to (D)

(6) Except that the monomer composition was changed as shown in Table 1, acryl polymers (B) to (D) were prepared in the same manner as in Preparation Example 1. Weight average molecular weights and molecular weight distributions of the prepared polymers are summarized in Table 1.

(7) TABLE-US-00001 TABLE 1 Acryl Polymers A B C D Monomer n-BA 98 98 98 80 Composition 2-HEA 2 2 — 20 (Part by AA — — 2 — Weight) Weight Average 1000000 1900000 1000000 1000000 Molecular Weight Molecular Weight 4.9 5.5 5.4 4.9 Distribution n-BA: n-butyl acrylate 2-HEA: 2-hydroxyethyl acrylate AA: Acrylic Acid

Example 1

(8) Formation of First Pressure-Sensitive Adhesive Layer

(9) A first pressure-sensitive adhesive composition was prepared by blending 100 parts by weight of an acryl polymer (A), 3 parts by weight of a multifunctional cross-linking agent (TDI-based Isocyanate, Coronate L, Nippon Polyurethane Industry (Japan)), 100 parts by weight of a multifunctional acrylate (trifunctional urethane acrylate, Aronix M-315), 3 parts by weight of a photoinitiator (Irg 184, hydroxycyclohexylphenylketone, Ciba Specialty Chemical (Swiss), and 0.1 parts by weight of a silane coupling agent (M812, a silane coupling agent having a β-cyanoacetyl group, LG Chemical (Korea)) in a solvent to have a solid concentration of 30 wt %. Subsequently, the prepared pressure-sensitive adhesive composition was coated on a releasing treated surface of a PET releasing film (thickness: 38 μm, MRF-38, Mitsubishi) subjected to releasing treatment to have a thickness of 25 μm after drying, and the resulting film was dried in an oven at 110° C. for 3 minutes. Then, a releasing-treated surface of the releasing-treated PET releasing film (thickness: 38 μm, MRF-38, Mitsubishi) was further laminated on the dried coating layer. Afterwards, a UV ray was irradiated under the following conditions, thereby forming a first pressure-sensitive adhesive layer between two PET releasing films. The formed first pressure-sensitive adhesive layer had a tensile modulus (at 25° C.) of 300 Mpa. The tensile modulus in the example was measured according to the following method.

(10) <Conditions for UV Irradiation>

(11) Apparatus for UV Irradiation: High Pressure Mercury Lamp

(12) Irradiation Conditions:

(13) Illuminance: 600 mW/cm.sup.2 Amount of Light: 150 mJ/cm.sup.2
Formation of Second Pressure-Sensitive Adhesive Layer

(14) A second pressure-sensitive adhesive composition was prepared by blending 100 parts by weight of acryl polymer (A), 0.01 parts by weight of a multifunctional cross-linking agent (TDI-based isocyanate, Coronate L, Nippon Polyurethane Industry (Japan)) and 0.1 parts by weight of a silane coupling agent (M812, a silane coupling agent having β-cyanoacetyl group, LG Chemical (Korea)) in a solvent to have a solid concentration of 30 wt %. Subsequently, the prepared pressure-sensitive adhesive composition was coated on a releasing treated surface of a releasing treated PET releasing film (thickness: 38 μm, MRF-38, Mitsubishi) to have a thickness of 25 μm after drying, the resulting film was dried in an oven at 110° C. for 3 minutes to form a second pressure sensitive adhesive later, and the releasing treated PET releasing film (thickness: 38 μm, MRF-38, Mitsubishi) was further laminated. The formed second pressure-sensitive adhesive layer had a tensile modulus (at 25° C.) of 0.06 Mpa.

(15) Subsequently, the first pressure-sensitive adhesive layer and the second pressure-sensitive adhesive layer were laminated on each other, thereby forming a pressure-sensitive adhesive layer having a double-layered structure.

(16) Formation of Polarizing Plate

(17) A polarizer was prepared by stretching a polyvinyl alcohol-based resin film, staining the film with iodine, and treating the resulting film with a boric acid aqueous solution. Subsequently, a 60 μm-thick triacetyl cellulose (TAC) film was attached to one surface of the polarizer using a water-based polyvinyl alcohol-based adhesive. Afterwards, the formed pressure-sensitive adhesive layer having the double-layered structure was laminated to a surface of the polyvinyl alcohol-based polarizer to which a TAC film was not attached using the water-based polyvinyl alcohol-based adhesive. In this step, the first pressure-sensitive adhesive layer was disposed on the polyvinyl alcohol-based polarizer (the structure of the polarizing plate: TAC film.fwdarw.water-based polyvinyl alcohol-based adhesive.fwdarw. polarizer.fwdarw.water-based polyvinyl alcohol-based adhesive.fwdarw.first pressure-sensitive adhesive layer.fwdarw.second pressure-sensitive adhesive layer.fwdarw.PET releasing film).

Examples 2 to 7 and Comparative Examples 1 to 5

(18) Except that components of first and second pressure-sensitive adhesive layers were changed as shown in Tables 2 and 3, a polarizing plate was formed by the same method as described in Example 1. However, Comparative Examples used a single-layered structure of a single pressure-sensitive adhesive, not a double-layered pressure-sensitive adhesive.

(19) TABLE-US-00002 TABLE 2 Examples 1 2 3 4 5 6 7 First Pressure- Kind of Acryl A B C D A A A Pressure-Sensitive Sensitive Polymer Adhesive Adhesive Content of Acryl 100 100 100 100 100 100 100 Composition Polymer Content of 3 3 3 3 3 3 3 Multifunctional Cross-linking Agent Content of 100 100 100 100 100 — 100 MFA1 Content of — — — — — 50 — MFA2 Content of 3 3 3 3 3 3 3 Photoinitiator Content of 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Silane Coupling Agent UV Curing ◯ ◯ ◯ ◯ ◯ ◯ ◯ Method Pressure- Thickness (μm) 25 25 25 25 25 25 25 Sensitive Tensile Modulus 300 300 350 850 300 400 300 Adhesive Second Pressure- Kind of Acryl A A A A A A C Pressure-Sensitive Sensitive Polymer Adhesive Adhesive Content of Acryl 100 100 100 100 100 100 100 Composition Polymer Multifunctional 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Cross-linking Agent MFA1 — — — — 10 — — MFA2 — — — — — — — Photoinitiator — — — — 1 — — Silane Coupling 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Agent UV Curing X X X X ◯ X X Agent Adhesive Thickness (μm) 25 25 25 25 25 25 25 Tensile Modulus 0.06 0.06 0.06 0.06 0.6 0.06 0.06 Content Unit: Part(s) by Weight Multifunctional Cross-linking Agent: TDI-based Isocyanate Cross-linking Agent (Coronate L, Nippon Polyurethane Industry) MFA1: 3-functional Urethane Acrylate (Aronix M-315, Dongwoo Corporation) MFA2: 6-functional Urethane Acrylate (UA 306I, Kyoeisha Chemical) Photoinitiator: Hydroxycyclohexylphenyl Ketone (Irg 184, Ciba Specialty Chemical) Silane Coupling Agent: silane coupling agent having a β-cyanoacetyl group (M812, LG Chemical) Tensile Modulus: tensile modulus at 25° C., unit: MPa

(20) TABLE-US-00003 TABLE 3 Comparative Examples 1 2 3 4 First Pressure- Kind of Acryl A A A A Pressure- Sensitive Polymer Sensitive Adhesive Content of Acryl 100 100 100 100 Adhesive Composition Polymer Content of 3 0.01 0.01 0.01 Multifunctional Crosslinking Agent Content of MFA1 100 — 10 10 Content of MFA2 3 — — — Content of 3 — 1 1 Photoinitiator Content of Silane 0.1 0.1 0.1 0.1 Coupling Agent UV Curing ◯ X ◯ ◯ Method Pressure- Thickness (μm) 25 25 25 50 Sensitive Tensile Modulus 300 0.06 0.6 0.6 Adhesive (MPa) (25° C.) Content Unit: Part(s) by Weight Multifunctional Crosslinking Agent: TDI-based Isocyanate Crosslinking Agent (Coronate L, Nippon Polyurethane Industry) MFA1: 3-functional Urethane Acrylate (Aronix M-315, Dongwoo Corporation) MFA2: 6-functional Urethane Acrylate (UA 3061, Kyoeisha Chemical) Photoinitiator: Hydroxycyclohexylphenyl ketone (Irg 184, Ciba Specialty Chemical) Silane Coupling Agent: silane coupling agent having a β-cyanoacetyl group (M812, LG Chemical)
<Evaluation of Physical Properties>
1. Evaluation of Tensile Modulus

(21) Herein, the tensile modulus of the pressure-sensitive adhesive was measured by a tensile stress-strain test according to a method defined in ASTM D638, or when it was difficult to directly measure the tensile modulus, a storage modulus was measured by the following method and then converted by the following formula. Specifically, a sample having a stacked structure of a PET releasing film (thickness: 38 μm, MRF-38), a pressure-sensitive adhesive layer and a PET releasing film (thickness: 38 μm, MRF-38), prepared in Examples or Comparative Examples, was cut into a dog bone-type specimen to a size of 7 cm (length)×1 cm (width). Both ends of the specimen were fixed with tensile test jigs, and a tensile modulus was measured according to ASTM D638. The conditions for measuring the tensile modulus were as follows.

(22) <Conditions for Measuring Tensile Modulus>

(23) Measuring Apparatus: Universal Test Machine (UTM)

(24) Equipment Model: Zwick Roell Z010, Instron

(25) Measurement Conditions:

(26) Load Cell: 500 N Tensile Speed: 3 mm/sec
<Measurement of Storage Modulus and Conversion into Tensile Modulus>

(27) A pressure-sensitive adhesive layer was cut to a size of 15 cm×25 cm×25 μm (width×length×thickness), and 5 pressure-sensitive adhesive layers cut as described above were stacked. Subsequently, the stacked pressure-sensitive adhesive layers were cut in a circle having a diameter of 8 mm, and pressed using glasses overnight to improve a wetting property at an interface between layers, thereby removing air bubbles generated during stacking. As a result, a specimen was prepared. Subsequently, the specimen was placed on a parallel plate, and a gap was adjusted. Then, a zero point of Normal & Torque was adjusted, the stability of a normal force was checked, the storage modulus was measured under the following conditions, and the tensile modulus was calculated according to the following conversion formula.

(28) Measurement Apparatus and Measuring Conditions

(29) Measurement Apparatus: ARES-RDA, TA Instruments Inc. with forced convection oven

(30) Measuring Conditions: Geometry: 8 mm parallel plate Gap: around 1 mm Test Type: dynamic strain frequency sweep Strain=10.0 [%], temperature: 30° C. Initial Frequency: 0.4 rad/s, final frequency: 100 rad/s
E=3G  <Conversion Formula>

(31) In the above formula, E is a tensile modulus, and G is a storage modulus.

(32) 2. Evaluation of Peeling Force and Repeelable Characteristic

(33) A polarizing plate was cut to a size of 25 mm×100 mm (width×length), and thus a specimen was prepared. Subsequently, a PET releasing film was peeled from the specimen, and the polarizing plate specimen was attached to a non-alkali glass by means of a pressure-sensitive adhesive layer using a laminator. Afterwards, the resulting product was pressed in an autoclave (50° C., pressure of 0.5 atm) for approximately 20 minutes, and stored under a constant temperature and humidity (23° C., 50% relative humidity) for 25 hours. Then, while the polarizing plate was peeled from the non-alkali glass using a texture analyzer (TA) apparatus (Stable Microsystem (UK)) at a peel rate of 300 mm/min and a peel angle of 180 degrees, a peeling force was measured. Furthermore, the repeelable characteristic was evaluated according to the following criteria.

(34) <Criteria for Evaluation of Repeelable Characteristic>

(35) ∘: Peeling force of 800 N/25 mm or less as measured one day after the attachment Δ: Peeling force of 1,000 N/25 mm or more as measured one day after the attachment x: Peeling force of 2,000 N/25 mm or more as measured one day after the attachment
3. Evaluation of Durability and Reliability

(36) The polarizing plates formed in Examples and Comparative Examples were cut to a size of 90 mm×170 mm (width×length), thereby preparing two specimens for each Example or Comparative Example. Subsequently, two specimens were attached to both surfaces of a glass substrate (110 mm×190 mm×0.7 mm=width×length×thickness). However, the specimens were attached with their optical absorbance axes crossing each other, thereby forming a sample. A pressure applied to the substrate during the attachment was approximately 5 kg/cm.sup.2, and works were performed in a clean room to avoid the generation of air bubbles or foreign materials to interfaces.

(37) The humidity and heat resistance of the sample was determined by observing whether air bubbles were generated or peeling occurred at a pressure-sensitive adhesive interface after the sample was maintained for 1000 hours under conditions including a temperature of 60° C. and a relative humidity of 90%.

(38) Furthermore, the heat resistance was determined by observing whether air bubbles were generated or peeling occurred at a pressure-sensitive adhesive interface after the sample was maintained for 1000 hours at a temperature of 80° C.

(39) Samples formed just before measurements of the humidity or heat durability and reliability were left for 24 hours at room temperature, and then evaluated. Evaluation conditions were as follows.

(40) <Criteria for Evaluation of Durability and Reliability>

(41) ∘: No air bubbles or peeling was generated. Δ: Air bubbles and/or peeling was somewhat generated. x: Air bubbles and/or peeling was considerably generated.
4. Evaluation of Water Resistance

(42) Polarizing plates prepared in Examples and Comparative Examples were cut to a size of 90 mm×170 mm (width×length), thereby forming specimens. Each specimen was attached to one surface of a glass substrate (110 mm×190 mm×0.7 mm=width×length×thickness), thereby forming a sample. A pressure applied during the attachment was approximately 5 kg/cm.sup.2, and works were performed in a clean room to avoid the generation of air bubbles or foreign materials to interfaces. Subsequently, the sample was put into water at 60° C., kept for 24 hours, and then taken out to observe whether air bubbles or peeling was generated. Thereby, the water resistance was evaluated according to the following criteria.

(43) <Criteria for Evaluation of Water Resistance>

(44) ∘: No air bubbles or peeling was generated. Δ: Air bubbles and/or peeling was somewhat generated. x: Air bubbles and/or peeling was considerably generated.
5. Evaluation of Uniformity in Light Transmission

(45) The polarizing plates prepared in Examples and Comparative Examples were attached to a 22-inch LCD monitor (LG Philips LCD) in a state in which optical absorption axes crossed each other, stored for 24 hours under constant temperature and humidity (23° C., 50% relative humidity), and left at 80° C. for 200 hours. Afterwards, light was irradiated to the monitor using a back light in a dark room, and the uniformity in light transmission was evaluated according to the following criteria.

(46) <Criteria for Evaluation of Uniformity in Light Transmission> ⊚: Non-uniformity in light transmission was not observed with the naked eye at four peripheral parts of the monitor ∘: Non-uniformity in light transmission was slightly observed with the naked eye at four peripheral parts of the monitor Δ: Non-uniformity in light transmission was somewhat observed with the naked eye at four peripheral parts of the monitor x: Non-uniformity in light transmission was significantly observed with the naked eye at four peripheral parts of the monitor
6. Evaluation for Weight Average Molecular Weight and Molecular Weight Distribution

(47) The weight average molecular weight and the molecular weight distribution of an acryl polymer were measured using a GPC under the following conditions. To draw a calibration curve, standard polystyrene of an Agilent system was used, and measurement results were converted.

(48) <Conditions for Measuring Weight Average Molecular Weight> Measuring Apparatus Agilent GPC (Agilent 1200 series, USA) Column: two connected PL mixed B Column Temperature: 40° C. Eluent: Tetrahydrofuran Flow Rate: 1.0 mL/min Concentration: ˜2 mg/mL (100 μL injection)

(49) The measurement results are summarized and shown in Table 4.

(50) TABLE-US-00004 TABLE 4 Re- Uni- Peeling peel- Humi- formity force able Heat dity Water in Light (N/25 charac- Dura- Dura- Resis- Trans- mm) teristic bility bility tance mission Examples 1 500 ◯ ◯ ◯ ◯ ⊚ 2 500 ◯ ◯ ◯ ◯ ⊚ 3 500 ◯ ◯ ◯ ◯ ⊚ 4 500 ◯ ◯ ◯ ◯ ⊚ 5 400 ◯ ◯ ◯ ◯ ⊚ 6 500 ◯ ◯ ◯ ◯ ◯ 7 800 ◯ ◯ ◯ ◯ ◯ Comparative 1 30 ◯ X X X X Examples 2 500 Δ X X X X 3 500 ◯ Δ X X X 4 500 ◯ Δ Δ Δ Δ

DESCRIPTION OF REFERENCES

(51) 1,2: a polarizing plate 3, 4, 13, 22: a pressure sensitive adhesive layer 11, 21: a polarizer 12a, 12b, 23: a protecting film 31: a first surface 32: a second surface 41: a first pressure sensitive adhesive layer 42: a second pressure sensitive adhesive layer