Pressure-sensitive adhesive

Abstract

Provided are a pressure-sensitive adhesive, a polarizing plate, and a liquid crystal display device. The pressure-sensitive adhesive may be used to provide a polarizing plate which has a tensile modulus that varies in a thickness direction, and for example, which has a smaller thickness and a lighter weight, and also satisfies all of the physical properties required for the polarizing plate such as durability, water resistance, workability and light leakage preventability, and a liquid crystal display device including the same.

Claims

1. A pressure-sensitive adhesive for a polarizing plate, which is in a shape of a single-layered film having a first surface which is a pressure-sensitive adhesive surface for attaching one surface of a polarizer and a second surface which is a pressure-sensitive adhesive surface for attaching a liquid crystal panel, of which an elastic modulus varies along a thickness direction, of which an average tensile modulus at any room temperature, which is between 10 C. to 40 C., is in the range from 0.065 GPa to 0.25 GPa, wherein a peel strength of the second surface with respect to glass (P2) is in the range from 265 to 660 gf/25 mm, and a peel strength of the first surface with respect to glass (P1) is in the range from 14 to 100 gf/25 mm, at the room temperature, wherein a difference (P2P1) between the peel strength of the second surface with respect to glass (P2) and the peel strength of the first surface with respect to glass (P1) is 250 to 625 gf/25 mm, and wherein the pressure-sensitive adhesive is a UV cured product of a pressure-sensitive adhesive composition comprising an acrylic polymer comprising polymerized units derived from alkyl (meth)acrylate, a compound of Formula 1 and a crosslinkable comonomer; and a multifunctional crosslinking agent: ##STR00002## where R is hydrogen or an alkyl group, A is an alkylene group or an alkylidene group, R.sub.1 is an alkyl group or an aryl group, and n is a number between 1 and 50.

2. The pressure-sensitive adhesive according to claim 1, wherein a ratio (P2/P1) of the peel strength of the second surface with respect to glass (P2) and the peel strength of the first surface with respect to glass (P1) is 5 or more.

3. The pressure-sensitive adhesive according to claim 1, wherein a value (MP2) obtained by multiplying the average tensile modulus (M) at the room temperature and the peel strength of the second surface with respect to glass (P2) at the room temperature is 20 or more, wherein the units of M are GPa, the units of P2 are gf/25 mm and the units of MP2 are GPa.Math.gf/25 mm.

4. The pressure-sensitive adhesive according to claim 1, wherein the pressure-sensitive adhesive composition further comprises an active energy beam-polymerizable compound and a radical initiator.

5. The pressure-sensitive adhesive according to claim 1, wherein the pressure-sensitive adhesive composition further comprises an ultraviolet ray absorbent.

6. A polarizing plate, comprising: a polarizer; and the pressure-sensitive adhesive of claim 1 disposed on a first surface of the polarizer.

7. The polarizing plate according to claim 6, further comprising an adhesive layer between the polarizer and the pressure-sensitive adhesive.

8. The polarizing plate according to claim 7, wherein the adhesive layer is directly attached to the polarizer; and the pressure-sensitive adhesive is directly attached to the adhesive layer.

9. The polarizing plate according to claim 6, further comprising a protective film on a surface of the polarizer opposite to the surface on which the pressure-sensitive adhesive is disposed.

10. A liquid crystal display device, comprising a liquid crystal panel, on one or both sides of which the polarizing plate of claim 6 is attached by the pressure-sensitive adhesive.

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 THE DRAWINGS

(1) FIG. 1 shows a schematic of one illustrative embodiment of the polarizing plate.

(2) FIG. 2 shows a schematic of one illustrative embodiment of the process forming the pressure-sensitive adhesive.

(3) FIG. 3 shows a schematic of one illustrative embodiment of a cross-sectional view of the polarizing plate.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(4) Hereinafter, the present application will be described with reference to Examples according to the present application and Comparative Examples not according to the present application in detail. However, the present application is not limited to the following Examples.

Preparation Example 1. Preparation of Acryl Polymer (A)

(5) 79 parts by weight of n-butyl acrylate (n-BA), 20 parts by weight of methoxy ethyleneglycol acrylate (MEA) and 1.0 part by weight of hydroxyethyl acrylate (HEA) were put into a 1 L reactor refluxing nitrogen gas and equipped with a cooling device to facilitate temperature control. Subsequently, 120 parts by weight of ethyl acetate (EAc) was put as a solvent into the reactor, which was purged with the nitrogen gas for 60 minutes to remove oxygen. Afterward, the temperature was maintained at 60 C., 0.03 parts by weight of azobisisobutyronitrile (AIBN) was put into the reactor as a reaction initiator, and the reaction was carried out for 8 hours. After the reaction, the reaction product was diluted with ethyl acetate (EAc), and thus an acrylic polymer (A) having a solid content concentration of 15 weight %, a weight average molecular weight of 1,800,000, and a molecular weight distribution of 4.5 was prepared.

Preparation Examples 2 to 8. Preparation of Acryl Polymers (B) to (H)

(6) Acryl polymers were prepared by the method as described in Preparation Example 1, except that monomer compositions were changed as shown in Table 1 (in Table 1, the acrylic polymer (A) is the same as the polymer prepared in Preparation Example 1).

(7) TABLE-US-00001 TABLE 1 Copolymer A B C D E n-BA 79 94 79 79 98.7 MEA 20 EEA 15 30 PEA 25 2-HEA 1 1 1 1 1 AA 0.5 0.3 M.sub.w (in 180 175 165 150 175 tens of thousands) PDI 4.5 5.2 5.1 4.7 3.8 Content Unit: parts by weight n-BA: n-butyl acrylate MEA: methoxy ethyleneglycol acrylate EEA: ethoxy diethyleneglycol acrylate PEA: phenoxy ethyleneglycol acrylate 2-HEA: 2-hydroxyethyl acrylate AA: acrylic acid Mw: weight average molecular weight PDI: molecular weight distribution

Example 1

(8) Formation of Pressure-Sensitive Adhesive Layer

(9) A pressure-sensitive adhesive composition was prepared by blending 100 parts by weight of the acrylic polymer (A), 3 parts by weight of a multifunctional crosslinking agent (TDI-based isocyanate, Coronate L, Nippon Polyurethane Industry (Japan)), 100 parts by weight of a multifunctional acrylate (trifunctional urethane acrylate, Aronix M-315, To a Gosei K.K.), 2 parts by weight of hydroxycyclohexylphenylketone as a photoinitiator (Irgacure 184, Ciba Specialty Chemicals (Switzerland)), 2 parts by weight of a triazine-based UV absorbent (Tinuvin 400, Ciba Specialty Chemicals (Switzerland)) and 0.2 parts by weight of a silane coupling agent having a -cyanoacetyl group (M812, LG Chem (Korea)) in a solvent to have a solid concentration of 30 weight %. Subsequently, the prepared pressure-sensitive adhesive composition was coated on a releasing-treated surface of a PET film (thickness: 38 m, MRF-38, Mitsubishi) subjected to releasing treatment to have a dry thickness of 25 m, and the resulting film was dried in an oven at 110 C. for 3 minutes. Then, a releasing-treated surface of another releasing-treated PET film (thickness: 38 m, MRF-38, Mitsubishi) was further laminated on the dried coating layer, thereby forming a stacked structure having a structure shown in FIG. 2, and UV rays (illumination: 250 mW/cm.sup.2, luminous energy: 300 mJ/cm.sup.2) were irradiated using a high pressure mercury lamp, thereby forming a pressure-sensitive adhesive layer (a cured layer of the layer 201 of the pressure-sensitive adhesive composition) between two of the released PET films 202A and 202B. Hereinafter, for the convenience of description, a surface of the pressure-sensitive adhesive layer on which the UV rays were incident is referred to a first surface 201a, and the opposite surface is referred to a second surface 201b.

(10) Formation of Polarizing Plate

(11) A polarizer was formed by extending a polyvinylalcohol-based resin film, dying the film with iodine and treating the film with a boric acid aqueous solution. Subsequently, a 60 m-thick triacetyl cellulose (TAC) film was attached to one surface of the polarizer using a water-based polyvinylalcohol-based adhesive conventionally used to attach a protective film to a polarizer. Afterward, the first surface of the previously formed pressure-sensitive adhesive layer was laminated using the same water-based polyvinylalcohol-based adhesive as that used previously on a surface of the polyvinylalcohol-based polarizer to which the TAC film was not attached, thereby forming a polarizing plate.

Examples 2 to 7 and Comparative Examples 1 and 2

(12) Preparation of Pressure-Sensitive Adhesive Composition

(13) A pressure-sensitive adhesive composition was prepared by the same method described in Example 1, except that compositions were changed as shown in Tables 2 and 3, and a polarizing plate was formed by the same method as described in Example 1, except that the radiated luminous energy and illumination of UV rays were suitably changed in consideration of desired characteristics such as an elastic modulus and peel strength.

(14) TABLE-US-00002 TABLE 2 Example 1 2 3 4 5 6 7 8 9 Polymer(A) 100 100 100 100 100 Polymer(B) 100 100 Polymer(C) 100 Polymer(D) 100 MFA1 100 100 100 100 20 100 MFA2 80 80 80 80 Crosslinking 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Agent Irg184 2.0 2.0 2.0 2.0 3.0 3.0 3.0 2.0 3.0 UVA 2.0 2.0 2.0 2.0 2.0 3.0 3.0 2.0 2.0 M812 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 T-789J 0.4 0.4 Content Unit: Parts by Weight MFA1: Trifunctional urethane acrylate (Aronix M-315, Toa Gosei K.K) MFA2: Hexafunctional urethane acrylate (UA 306I, Kyoeisha) Crosslinking Agent: TDI-based isocyanate Crosslinking Agent (Coronate L, Nippon Polyurethane Industry) Irgacure 184: Hydroxy cyclohexylphenyl ketone (Ciba Specialty Chemicals, Switzerland) UV Absorbent: Triazine-based UV Absorbent (Tinuvin 400, Ciba Specialty Chemicals, Switzerland) M812: a silane coupling agent having a -cyanoacetyl group (LG Chem (Korea)) T-789J: Silane Coupling Agent containing acetoacetate group (Soken Co. (Japan)))

(15) TABLE-US-00003 TABLE 3 Comparative Example 2 5 Polymer(A) 100 Polymer(E) 100 MFA1 20 MFA2 80 Crosslinking 3.0 3.0 Agent Irg184 2.0 UVA 2.0 2.0 T-789J 0.2- 0.2- Content Unit: Parts by Weight MFA1: Trifunctional urethane acrylate (Aronix M-315, Toa Gosei K.K) MFA2: Hexafunctional urethane acrylate (UA 306I, Kyoeisha) Crosslinking Agent: TDI-based isocyanate Crosslinking Agent (Coronate L, Nippon Polyurethane Industry) Irgacure 184: Hydroxy cyclohexylphenyl ketone (Ciba Specialty Chemicals, Switzerland) UV Absorbent: Triazine-based UV Absorbent (Tinuvin 400, Ciba Specialty Chemicals, Switzerland) T-789J: Silane Coupling Agent containing acetoacetate group (Soken Co. (Japan))

(16) <Evaluation of Physical Properties>

(17) 1. Evaluation of Tensile Modulus

(18) In this document, a tensile modulus of a pressure-sensitive adhesive layer was measured by a stress-strain test due to tension according to a method defined in ASTM D638, or when it was difficult to directly measure a tensile modulus, a storage modulus was measured and then converted into the tensile modulus by the following Conversion Formula. In detail, a stacked structure having a structure of FIG. 2 (a stacked structure of a released PET film 202A, a cured product of a layer 201 of a pressure-sensitive adhesive composition and a released PET film 202B) as prepared in Examples and Comparative Examples was cut into a dog bone-type specimen in a size of 7 cm (length)1 cm (width), both ends of the specimen were fixed with jigs for a tensile test, and a tensile modulus was measured. The conditions for measuring the tensile modulus were as follows.

(19) <Conditions for Measuring Tensile Modulus>

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

(21) Equipment Model: Zwick Roell Z010, Instron

(22) Measurement Conditions: Load Cell: 500 N Tensile Rate: 3 mm/sec

(23) <Measurement of Storage Modulus and Conversion into Tensile Modulus>

(24) A pressure-sensitive adhesive layer was cut in a size of 15 cm25 cm25 m (widthlengththickness), and then the cut pressure-sensitive adhesive layers were stacked in five layers. Subsequently, the stacked adhesive layers were cut into circles having diameters of 8 mm, and pressed using glasses overnight to enhance wettability at an interface between the 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, after Normal & Torque was adjusted to zero, and the stabilization of normal force was checked, the storage modulus was measured under the following conditions, and a tensile modulus was calculated according to the following Conversion Formula.

(25) Measurement Apparatus and Measuring Conditions

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

(27) 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>

(28) In the above Formula, E is a tensile modulus, and G is a storage modulus.

(29) 2. Evaluation of Peel Strength and Reworkability

(30) A polarizing plate was formed by the method as described in Example 1 using each of the pressure-sensitive adhesive layers formed in Examples or Comparative Examples, except that a direction of the pressure-sensitive adhesive layer was changed along a surface of the pressure-sensitive adhesive layer whose peel strength was to be measured. That is, when the peel strength of a first surface was measured in the formation of the polarizing plate disclosed in Example 1, a second surface was attached to the side of the polarizer, and when the peel strength of the second surface was measured, the first surface was attached to the side of the polarizer, thereby forming a polarizing plate. Afterward, a specimen was formed by cutting the polarizing plate in a size of 25 mm100 mm (widthlength). Subsequently, a released PET film attached to the pressure-sensitive adhesive layer was peeled off, and the surface of the pressure-sensitive adhesive layer was attached to alkali-free glass using a 2 kg roller according to the specification of JIS Z 0237. Subsequently, the alkali-free glass to which the pressure-sensitive adhesive layer was attached was compressed in an autoclave (50 C., 0.5 atm) for approximately 20 minutes, and stored under constant temperature and humidity conditions (23 C., relative humidity: 50%) for 25 hours. Then, using a texture analyzer (TA) (Stable Micro System (United Kingdom)), the polarizing plate was peeled off of the alkali-free glass at a peel rate of 300 mm/min and a peel angle of 180 degrees to measure a peel strength. In addition, reattachment was evaluated under the following criteria:

(31) <Criteria for Evaluation of Reattachment> : A day after attachment, the peel strength was 800 N/25 mm or less. : A day after attachment, the peel strength was 1,000 N/25 mm or more. x: A day after attachment, the peel strength was 2,000 N/25 mm or more.

(32) 3. Evaluation of Durability

(33) 2 specimens were prepared by cutting a specimen prepared by cutting a polarizing plate in a size of 90 mm170 mm (widthlength) per Example or Comparative Example. Subsequently, the two specimens were attached to both surfaces of a glass substrate (110 mm190 mm0.7 mm=widthlengththickness) such that optical absorption axes were crossed, thereby preparing a sample. A pressure applied during attachment was approximately 5 kg/cm.sup.2, and the attachment was carried out in a clean room to avoid air bubbles or extraneous materials at an interface. Afterward, the humidity and thermal resistance of the sample was determined by observing whether air bubbles or peeling were generated at a pressure-sensitive adhesive interface after the sample was left for 1,000 hours under conditions including a temperature of 60 C. and a relative humidity of 90%, and the thermal resistance was determined by observing whether air bubbles or peeling were generated at a pressure-sensitive adhesive interface after the sample was left for 1,000 hours at a temperature of 80 C. The formed samples were left at room temperature for 24 hours, followed by the evaluation of the humidity and thermal resistance or thermal resistance. Evaluation conditions were as follows:

(34) <Criteria for Evaluation of Durability> : No air bubbles and/or peeling were generated. : Air bubbles and/or peeling were generated somewhat. x: Air bubbles and/or peeling were generated considerably.

(35) 4. Evaluation of Water Resistance

(36) Samples were prepared by attaching specimens formed by cutting the polarizing plates formed in Examples and Comparative Examples to a size of 90 mm170 mm (widthlength) to one surface of a glass substrate (110 mm190 mm0.7 mm=widthlengththickness). A pressure applied during attachment was approximately 5 kg/cm.sup.2, and the attachment was executed in a clean room to prevent bubbles or extraneous materials at an interface. Subsequently, the formed sample was put into water at 60 C., left for 24 hours, and it was observed whether bubbles or peeling were generated. The water resistance was evaluated according to the following criteria.

(37) <Criteria for Evaluation of Water Resistance> : No air bubbles and/or peeling were generated. : Some bubbles and/or peeling were generated at an interface. x: Bubbles and/or peeling were generated considerably.

(38) 5. Evaluation of Haze

(39) A sample (thickness of pressure-sensitive adhesive layer: 23 m) having the structure of FIG. 2 was prepared using a pressure-sensitive adhesive composition in Example or Comparative Example, and a haze of the pressure-sensitive adhesive layer in the sample was measured using a haze meter (HR-100, Murakami Color Research Laboratory, Japan) according to the specification of JIS K 7105-1.

(40) 6. Evaluation of Uniformity of Light Transmission

(41) The polarizing plates formed in Examples and Comparative Examples were attached to both surfaces of a 22-inch LCD monitor (LG Philips LCD) in a state in which optical axes crossed, stored under constant temperature and humidity conditions (23 C., relative humidity: 50%) for 24 hours, and left at 80 C. for 200 hours. Subsequently, light was radiated to the monitor using a back light in a dark room, and the uniformity of light transmission was evaluated according to the following criteria:

(42) <Criteria for Evaluation of Uniformity of Light Transmission> : non-uniformity of light transmission was not observed in four peripheral regions of a monitor with the naked eye : non-uniformity of light transmission was observed slightly in four peripheral regions of a monitor with the naked eye : non-uniformity of light transmission was observed somewhat in four peripheral regions of a monitor with the naked eye x: non-uniformity of light transmission was observed considerably in four peripheral regions of a monitor with the naked eye

(43) 7. Evaluation of Weight Average Molecular Weight and Distribution of Molecular Weight

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

(45) <Conditions for Measuring Weight Average Molecular Weight>

(46) Measuring Apparatus: Agilent GPC (Agilent 1200 series, USA)

(47) Column: Two connected PL mixed B

(48) Column Temperature: 40 C.

(49) Eluent: Tetrahydrofuran

(50) Flow Rate: 1.0 mL/min

(51) Concentration: 2 mg/mL (100 L injection)

(52) The measurement results are summarized and listed in Tables 4 and 5.

(53) TABLE-US-00004 TABLE 4 Example 1 2 3 4 5 6 7 8 9 Tensile Modulus (Room 95 65 130 175 207 250 215 65 207 Temperature, MPa) Peel Strength on First 25 35 20 18 14 15 25 35 14 Surface (gf/25 mm) Peel Strength on Second 500 650 400 350 335 265 520 660 350 Surface (gf/25 mm) reworkability Thermal Resistance Humidity and Thermal Resistance Water Resistance Haze (%) 7.2 8.4 6.5 6.8 5.2 5.3 5.9 8.4 5.2 Uniformity of Light Transmission

(54) TABLE-US-00005 TABLE 5 Comparative Example 1 2 Tensile Modulus (Room 9 0.04 Temperature, MPa) Peel Strength on First Surface 120 500 (gf/25 mm) Peel Strength on Second Surface 850 850 (gf/25 mm) reworkability Thermal Resistance x x Humidity and Thermal Resistance x Water Resistance x x Haze (%) 15.0 6.4 Uniformity of Light Transmission x