Pressure sensitive adhesive composition

Abstract

The present invention relates to a pressure sensitive adhesive composition, a protective film for optical elements, an optical element and a liquid crystal display device. The present invention may provide a pressure sensitive adhesive composition and a protective film for optical elements, having a high peel force at a low rate and a low peel force at a high rate, the balance of which is excellently maintained and showing excellent wettablity to an adherend, durability, repeelability, transparency and antistatic performance.

Claims

1. A protective film for optical elements comprising a pressure sensitive adhesive layer formed by hardening a pressure sensitive adhesive composition comprising an acrylic resin with a weight average molecular weight of 450,000 to 1,100,000, a multi-functional crosslinker, a photopolymerizable compound and a photo initiator, wherein the multi-functional crosslinker having at least three functional groups, and is present in an amount of 0.5 to 2 parts by the weight relative to 100 parts by weight of the acrylic resin, wherein the photopolymerizable compound comprising a multifunctional acrylate having at least 3 (meth)acryloyl groups and is present in an amount of 3 to 20 parts by weight relative to 100 parts by weight of the acrylic resin, wherein the photoinitiator is present in an amount of 0.1 to 10 parts by weight relative to 100 parts by weight of the acrylic resin, wherein after hardening, the composition comprises an interpenetrating polymer network structure having said acrylic resin cross-linked by the multi-functional crosslinker and a polymerized product of the photopolymerizable compound, and has a peel force to a TAC (triacetyl cellulose) sheet of 10 to 40 gf/inch, as measured at a peel angle of 180° and a peel rate of 0.3 m/min, and a peel force to a TAC sheet of 80 to 300 gf/inch, as measured at a peel angle of 180° and a peel rate of 30 m/min, and wherein a ratio (X.sub.2/X.sub.1) of the peel force (X.sub.2) to a TAC sheet, as measured at a peel angle of 180° and a peel rate of 30 m/min, to the peel force (X.sub.1) to a TAC sheet, as measured at a peel angle of 180° and a peel rate of 0.3 m/min, after hardening the composition, is 5 to 13.

2. The protective film for optical elements according to claim 1, wherein the acrylic resin has a weight average molecular weight of 450,000 to 1,000,000.

3. The protective film for optical elements according to claim 1, further comprising an antistatic agent.

4. The protective film for optical elements according to claim 1, wherein a gel content in the state including the interpenetrating polymer network structure is 80 to 99%.

5. The protective film for optical elements according to claim 1, wherein the peel force to a TAC sheet, as measured at a peel angle of 180° and a peel rate of 0.3 m/min, after hardening the composition, is 10 to 30 gf/inch.

6. The protective film for optical elements according to claim 1, wherein the peel force to a TAC sheet, as measured at a peel angle of 180° and a peel rate of 30 m/min, after hardening the composition, is 100 to 280 gf/inch.

7. The protective film according to claim 1, wherein the multi-functional crosslinker has four functional groups.

8. The protective film according to claim 1, wherein the photopolymerizable compound has a molecular weight of less than 1,000.

9. The protective film according claim 1, wherein the multifunctional cross-linker is a tolylenediisocyanate adduct of trimethylolpropane or N,N,N′,N′-tetraglycidyl ethylenediamine.

10. The protective film according to claim 1, wherein after hardening, the composition has a gel fraction of 80%-90%, the gel fraction calculated as: gel fraction =B/A×100, wherein A is a mass of the pressure sensitive adhesive composition comprising the interpenetrating polymer network (IPN) structure, and B is a dry mass of insoluble fractions recovered after depositing the pressure sensitive adhesive composition in ethyl acetate at room temperature for 48 hours.

11. The protective film according to claim 1, wherein the acrylic resin comprises 80-99.8 parts by weight (meth) acrylic acid ester monomers and 0.01-10 parts by weight of cross-linkable monomers, relative to 100 total parts by weight of all acrylic resin monomers.

12. An optical element comprising a substrate; and a protective film according to claim 1 attached to one or both sides of said substrate.

13. The optical element according to claim 12, wherein the substrate is a polarizing plate, a retardation plate, an optical compensation film, a reflective sheet or a brightness enhancing film.

14. A liquid crystal display device comprising a liquid crystal panel; and an optical element according to claim 13 attached to one or both sides of said liquid crystal panel.

Description

MODE FOR INVENTION

(1) The present invention is explained in more detail through examples according to the present invention and comparative examples not according to the present invention below, but the scope of the present invention is not restricted by the following examples.

PREPARATION EXAMPLE 1

Preparation of Acrylic Resin (A1)

(2) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 98 parts by weight of 2-ethylhexyl acrylate (2-EHA) and 2 parts by weight of 2-hydroxybutyl acrylate (2-HBA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour to remove oxygen, and the temperature was maintained at 62° C. Then, 0.03 parts by weight of a reaction initiator, azobisisobutyronitrile (AIBN), and 0.05 parts by weight of a molecular weight regulator, n-dodecylmercaptan (n-DDM), were poured, and reacted for 8 hours. Following the reaction, the reactant was diluted with ethyl acetate (EAc) to prepare an acrylic resin (A1) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 450,000.

PREPARATION EXAMPLE 2

Preparation of Acrylic Resin (A2)

(3) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 97.5 parts by weight of butyl acrylate (BA) and 1.5 parts by weight of 2-hydroxybutyl acrylate (2-HBA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour to remove oxygen, and the temperature was maintained at 62° C. Then, 0.03 parts by weight of azobisisobutyronitrile (AIBN) and 0.05 parts by weight of n-dodecylmercaptan (n-DDM) were poured, reacted for 8 hours and diluted with ethyl acetate (EAc) to prepare an acrylic resin (A2) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 600,000.

PREPARATION EXAMPLE 3

Preparation of Acrylic Resin (A3)

(4) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 99 parts by weight of 2-ethylhexyl acrylate (2-EHA) and 1 part by weight of hydroxybutyl acrylate (2-HBA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour to remove oxygen. With maintaining the temperature in the reactor at 60° C., 0.01 parts by weight of azobisisobutyronitrile (AIBN) was poured, reacted for 8 hours and diluted with ethyl acetate (EAc) to prepare an acrylic resin (A3) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 1,200,000.

PREPARATION EXAMPLE 4

Preparation of Acrylic Resin (A4)

(5) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 99 parts by weight of 2-ethylhexyl acrylate (2-EHA) and 1 part by weight of acrylic acid (AA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour, and the temperature was maintained at 62° C. Then, 0.03 parts by weight of azobisisobutyronitrile (AIBN) and 0.05 parts by weight of n-dodecylmercaptan (n-DDM) were poured, reacted for 8 hours and diluted with ethyl acetate (EAc) to prepare an acrylic resin (A4) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 600,000.

PREPARATION EXAMPLE 5

Preparation of Acrylic Resin (A5)

(6) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 98 parts by weight of 2-ethylhexyl acrylate (2-EHA) and 2 parts by weight of 2-hydroxybutyl acrylate (2-HBA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour to remove oxygen, and the temperature was maintained at 60° C. Then, 0.03 parts by weight of a reaction initiator, azobisisobutyronitrile (AIBN), and 0.01 parts by weight of a molecular weight regulator, n-dodecylmercaptan (n-DDM), were poured, and reacted for 8 hours. Following the reaction, the reactant was diluted with ethyl acetate (EAc) to prepare an acrylic resin (A5) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 800,000.

PREPARATION EXAMPLE 6

Preparation of Acrylic Resin (A6)

(7) In a 1 L reactor with nitrogen gas refluxed and an installed refrigerator to easily regulate a temperature, 99 parts by weight of 2-ethylhexyl acrylate (2-EHA) and 1 parts by weight of 2-hydroxybutyl acrylate (2-HBA), and 100 parts by weight of ethyl acetate (EAc) as a solvent were poured. Subsequently, nitrogen gas was purged for 1 hour to remove oxygen, and the temperature was maintained at 60° C. Then, 0.01 parts by weight of a reaction initiator, azobisisobutyronitrile (AIBN), and 0.02 parts by weight of a molecular weight regulator, n-dodecylmercaptan (n-DDM), were poured, and reacted for 8 hours. Following the reaction, the reactant was diluted with ethyl acetate (EAc) to prepare an acrylic resin (A6) having a solid content concentration of 44% by weight and a weight average molecular weight (M.sub.w) of 1,000,000.

EXAMPLE 1

Preparation of Pressure Sensitive Adhesive Composition

(8) 100 g of the acrylic resin (A1) of Preparation Example 1, 10 g of trimethylolpropane triacrylate, 1.5 g of a photo-initiator (2,2-dimethoxy-1,2-diphenylethanone), 1.5 g of a cross-linker (tolylenediisocyanate adduct of trimethylolpropane), 0.3 g of LiTFSi (lithium bis(trifluoromethanesulfonyl)imide) and 1.2 g of polyethyleneglycol bis(2-hexanoate) were homogeneously formulated and diluted to a proper concentration, considering coating performance, to prepare a pressure sensitive adhesive composition.

Preparation of Protective Film for Optical Elements

(9) The above pressure sensitive adhesive composition was coated on one side of a biaxially-stretched PET (poly(ethylene terephthalate)) film (thickness: 38 μm)) and dried to form a uniform coating layer having a thickness of 20 μm. Subsequently, a release film was laminated on the coating layer, followed by carrying out the ultraviolet treatment under the following condition with a high pressure mercury lamp. Then, it was aged at 50° C. for 3 days to prepare a protective film.

(10) <Ultraviolet Treatment Condition>

(11) Illuminance: about 700 mW/cm.sup.2 to 750 mW/cm.sup.2,

(12) Light amount: about 150 mJ/cm.sup.2 to 200 mJ/cm.sup.2

EXAMPLES 2 TO 9 AND COMPARATIVE EXAMPLES 1 TO 7

(13) Pressure sensitive adhesive compositions and protective films were prepared by the same method as that of Example 1, except for changing the composition ratios of the pressure sensitive adhesive compositions as represented in the following table 1 or 2 and regulating the irradiation conditions considering their composition ratios, if the ultraviolet irradiation needed, such that the composition can be sufficiently hardened.

(14) TABLE-US-00001 TABLE 1 Examples 1 2 3 4 5 6 7 8 9 Resin (A1) 100 100 — — — 100 — — — Resin (A2) — — 100 — — — 100 — — Resin (A4) — — — 100 100 — — — — Resin (A5) — — — — — — — 100 — Resin (A6) — — — — — — — — 100 Crosslinker 1.5 0.5 2 0.5 0.5 0.5 1.0 0.5 2.0 A Crosslinker — — — 1.5 1.5 — — — — B MFA 10 20 7.5 15 7.5 3.0 7.5 5.0 5.0 Photo- 1.5 3.0 0.5 1.0 0.5 0.5 0.5 1.0 1.0 initiator Plasticizer 1.2 1.2 1.2 1.2 1.2 2.0 1.2 1.2 1.2 Metal salt 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Content Unit: Part by weight Crosslinker A: Tolylenediisocianate adduct of trimethylolpropane Crosslinker B: N,N,N′,N′-tetraglycidyl ethylenediammine MFA: Trimethylolpropane triacrylate Photoinitiator: 2,2-dimethoxy-1,2-diphenylethanone (Irgacure 651) Plasticizer: Polyethyleneglycol bis(2-hexanoate) Metal salt: Lithium bis(trifluoromethanesulfonyl)imide

(15) TABLE-US-00002 TABLE 2 Comparative Examples 1 2 3 4 5 6 7 Resin(A1) 100 100 — — — 100 — Resin(A2) — — — — — — 100 Resin(A3) — — 100 — — — — Resin(A4) — — — — — — — Crosslinker A 3 12 5 — — 0.5 2 Crosslinker B — — — — — — — MFA — — — 100 100 35 35 Photoinitiator — — — 15 1 3.5 3 Plasticizer 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Metal salt 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Content Unit: Part by weight Crosslinker A: Tolylenediisocianate adduct of trimethylolpropane Crosslinker B: N,N,N′,N′-tetraglycidyl ethylenediammine MFA: Trimethylolpropane triacrylate Photoinitiator: 2,2-dimethoxy-1,2-diphenylethanone (Irgacure 651) Plasticizer: Polyethyleneglycol bis(2-hexanoate) Metal salt: Lithium bis(trifluoromethanesulfonyl)imide

(16) For pressure sensitive adhesives or pressure sensitive adhesive films (protective films) prepared in Examples and Comparative Examples, their physical properties were evaluated by the following manner.

(17) Measurement Method 1: Measuring Weight Average Molecular Weight

(18) The weight average molecular weight of the acrylic resin was measured under the following conditions using GPC. Using standard polystyrene on Agilent system for preparing calibration curves, the measuring results were converted.

(19) <Conditions of Measuring Weight Average Molecular Weight>

(20) Measuring Instrument Agilent GPC (Agilent 1200 series, USA)

(21) Column: linked two PL Mixed B

(22) Column temperature: 40° C.

(23) Eluent: Tetrahydrofuran

(24) Flow rate: 1.0 mL/min

(25) Concentration: ˜2 mg/mL (100 μL injection)

(26) Measurement Method 2: Measuring Peel Forces at Low and High Rates to TAC Sheet

(27) The peel force (X.sub.1) at a low rate and the peel force (X.sub.2) at a high rate were measured in accordance with the following procedures. The prepared protective film for optical elements was cut to a size of 10 inch×1 inch (width×length). Subsequently, the cut film was attached to a TAC sheet (Trade name: UZ-TAC, manufactured by Fuji Film Corporation (Japan)) using a 2 Kg roller in accordance with JIS Z 0237. Then, the TAC sheet, to which the protective film was attached, was stored at a temperature of 23° C. and a relative humidity of 65% for 24 hours. Subsequently, using a tensile tester at room temperature, the protective film was peeled off at a peel angle of 180° at a peel rate of 0.3 m/min to measure the peel force (X.sub.1) at a low rate, and also peeled off at a peel angle of 180° and a peel rate of 30 m/min to measure the peel force (X.sub.2) at a high rate.

(28) Measurement Method 3: Measuring Peel Forces at Low and High Rates to AG Sheet

(29) The peel force (X.sub.1) at a low rate and the peel force (X.sub.2) at a high rate were measured in accordance with the following procedures. The prepared protective film for optical elements was cut to a size of 10 inch×1 inch (width×length). Subsequently, the cut film was attached to an antiglare (AG) sheet (Trade name: AGL25, manufactured by DNP (Japan)) using a 2 Kg roller in accordance with JIS Z 0237. Then, the AG sheet, to which the protective film was attached, was stored at a temperature of 23° C. and a relative humidity of 65% for 24 hours. Subsequently, using a tensile tester at room temperature, the protective film was peeled off at a peel angle of 180° and a peel rate of 0.3 m/min to measure the peel force (X.sub.1) at a low rate, and also peeled off at a peel angle of 180° and a peel rate of 30 m/min to measure the peel force (X.sub.2) at high rate.

(30) Measurement Method 4: Measuring Wettability

(31) Each protective film prepared in Examples and Comparative Examples was attached to a polarizing plate and cut to a size of 2.5 cm×25 cm (width×length) to prepare a sample. The prepared sample was attached to a glass substrate using double-sided tape. Then, the protective film was peeled off from the sample, the peeled off protective film was again placed on a polarizing plate. A certain pressure was applied to points which uniformly trisected the longitudinal direction of the protective film, in order to measure the time that the protective film was thoroughly wet to a surface of the polarizing plate and evaluate wettability according to the following criteria.

(32) <Criteria for Evaluating Wettability>

(33) ⊚: less than 10 seconds to thoroughly wet a surface of the polarizing plate

(34) ◯: at least 10 seconds and less than 15 seconds to thoroughly wet a surface of the polarizing plate

(35) Δ: at least 15 seconds and less than 20 seconds to thoroughly wet a surface of the polarizing plate

(36) ×: at least 30 seconds to thoroughly wet a surface of the polarizing plate

(37) Measurement Method 5: Measuring Heat Resistance

(38) Each protective film prepared in Examples and Comparative Examples was attached to each side of a TAC sheet (UZ-TAC, manufactured by Fuji Film Corporation (Japan)) and a AG sheet (Trade name: AGL25, manufactured by DNP (Japan)) using a 2 Kg roller, and held in an oven at 50° C. for 7 days, followed by measuring peel forces at low and high rates at room temperature by the same method as described above to evaluate heat resistance according to the following criteria.

(39) <Criteria for Evaluating Heat Resistance>

(40) ◯: peel forces at low and high rates represent 1.2 times or less than those in the first stage

(41) Δ: peel forces at low and high rates represent more than 1.2 times and 1.5 times or less than those in the first stage

(42) ×: peel forces at low and high rates represent 2 times or more than those in the first stage

(43) Measurement Method 6: Measuring Peel-Off Constant Voltage

(44) Each protective film prepared in Examples and Comparative Examples was attached to each side of a TAC sheet (UZ-TAC, manufactured by Fuji Film Corporation (Japan)) and a AG sheet (Trade name: AGL25, manufactured by DNP (Japan)) using a 2 Kg roller, and stored at a temperature of 23° C. and a relative humidity of 50% for 24 hours to prepare a sample (sample shape: rectangular, sample size: ratio of width and length (width: length)=3:4, diagonal line=15 inch). A constant voltage occurred during the peeling-off of the film from each sample at a peel rate of 40 m/min was measured at a 1 cm height from the sample surface with a constant voltage measuring instrument (STATIRON-M2). In each case, the constant voltage was measured 3 times and averaged to evaluate the peel-off constant voltage according to the following criteria.

(45) <Evaluation Criteria>

(46) ◯: peel-off constant voltage of 0.5 kV or less

(47) ×: peel-off constant voltage of more than 0.5 kV

(48) Measurement Method 7: Heat Resistant Peel-Off Constant Voltage

(49) The sample which was prepared by the same method as that in the above measurement method 6 was held in an oven at 50° C. for 7 days, and a peel-off constant voltage was measured at room temperature by the same method as the above measurement method 6 to evaluate the heat resistant peel-off constant voltage according to the following criteria.

(50) <Evaluation Criteria>

(51) ◯: peel-off constant voltage of 0.5 kV or less

(52) ×: peel-off constant voltage of more than 0.5 kV

(53) Physical properties measured by the above methods were organized and described in the following tables 3 to 5.

(54) TABLE-US-00003 TABLE 3 Examples 1 2 3 4 5 Peel Force AG Low Rate 12.3 10.5 14.2 10.8 12.3 (gf/inch) sheet (X.sub.1) High Rate 105 100 125 105 114 (X.sub.2) TAC Low Rate 14.5 11.2 16.5 10.7 13.7 sheet (X.sub.1) High Rate 117 120 130 110 121 (X.sub.2) Wettability ⊚ ⊚ ⊚ ⊚ ⊚ Heat Resistance ◯ ◯ ◯ ◯ ◯ Peel-off Constant ◯ ◯ ◯ ◯ ◯ Voltage Heat Resistant ◯ ◯ ◯ ◯ ◯ Peel-off Constant Voltage

(55) TABLE-US-00004 TABLE 4 Examples 6 7 8 9 Peel AG Low Rate 33.2 19.0 15.5 8.1 Force sheet (X.sub.1) (gf/inch) High Rate 248 140 221 92 (X.sub.2) TAC Low Rate 39.2 22.5 19.4 8.5 sheet (X.sub.1) High Rate 270 141 232 97 (X.sub.2) Wettability ⊚ ⊚ ⊚ ⊚ Heat Resistance ◯ ◯ ◯ ◯ Peel-off Constant ◯ ◯ ◯ ◯ Voltage Heat Resistant ◯ ◯ ◯ ◯ Peel-off Constant Voltage

(56) TABLE-US-00005 TABLE 5 Comparative Examples 1 2 3 4 5 6 7 Peel AG Low 6.7 3.1 2.4 2.4 7.1 5.3 4.7 Force sheet Rate (gf/ (X.sub.1) inch) High 350 90 75 85 210 110 120 Rate (X.sub.2) TAC Low 9.2 4.0 2.5 2.4 8.3 7.0 4.4 sheet Rate (X.sub.1) High Rate (X.sub.2) 370 101 92 87 216 121 124 Wettability ⊚ Δ Δ X X Δ Δ Heat Resistance X Δ Δ ◯ ◯ Δ Δ Peel-off Constant ◯ ◯ ◯ ◯ ◯ ◯ ◯ Voltage Heat Resistant X ◯ ◯ ◯ ◯ ◯ ◯ Peel-off Constant Voltage