Pressure-sensitive adhesive composition for optical film

Abstract

A pressure-sensitive adhesive composition for an optical film, a method of preparing a pressure-sensitive adhesive for an optical film, a polarizing plate and a liquid crystal display are provided. The pressure-sensitive adhesive composition may be effectively coated even when a coating solid content is controlled to be high. Therefore, productivity in formation of a pressure-sensitive adhesive or manufacture of an optical film such as a polarizing plate may be significantly increased, and excellent durability and reliability and re-workability may be exhibited after the pressure-sensitive adhesive is prepared.

Claims

1. A pressure-sensitive adhesive composition for an optical film, comprising: an acrylic polymer which comprises 80 to 96 parts by weight of (meth)acrylic acid ester monomer, 3.5 to 5.5 parts by weight of monomer having hydroxyl group, 0.05 to 0.3 parts by weight of monomer having carboxyl group, the acrylic polymer prepared in the presence of 0.05 to 0.1 parts by weight of n-dodecyl mercaptan relative to 100 parts by weight of total monomers forming the acrylic polymer and of which a weight average molecular weight of 700,000 to 950,000; a multifunctional crosslinking agent; and a silane coupling agent represented by Formula 1, wherein a coating solid content is 20 to 22 weight %, and a gel fraction is 55 weight % to 85 weight % after being crosslinked and wherein the (meth)acrylic acid ester monomer is methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, t-butyl (meth)acrylate, sec-butyl (meth)acrylate, pentyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, 2-ethylbutyl (meth)acrylate, n-octyl (meth)acrylate, isobornyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate or lauryl (meth)acrylate, wherein the pressure-sensitive adhesive composition has a viscosity at 23° C. of 900 to 2,300 cP in the state having a solids content of 20 to 22 weight %:
(R.sub.1).sub.nSi(R.sub.2)(.sub.4−n)  [Formula 1] wherein R.sub.1 is a beta-cyanoacetyl group or beta-cyanoacetylalkyl group, R.sub.2 is an alkoxyl group, and n is a number between 1 and 3.

2. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the monomer having hydroxyl group is hydroxyalkyl (meth)acrylate or hydroxyalkyleneglycol (meth)acrylate.

3. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the monomer having carboxyl group is (meth)acrylic acid, acrylic acid dimer, itaconic acid, maleic acid or maleic acid anhydride.

4. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the multifunctional crosslinking agent is an isocyanate crosslinking agent.

5. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the multifunctional crosslinking agent is comprised in an amount of 0.01 to 5 parts by weight, relative to 100 parts by weight of the acrylic polymer.

6. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the silane coupling agent is comprised in an amount of 0.01 to 5 parts by weight relative to 100 parts by weight of the acrylic polymer.

7. The pressure-sensitive adhesive composition for an optical film of claim 1, wherein the gel fraction after being crosslinked is 60 to 80 weight %.

Description

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(1) Hereinafter, a pressure-sensitive adhesive composition will be described in detail through Example and Comparative Example, but a range of the pressure-sensitive adhesive composition is not limited by the following Example.

(2) In the following Examples and Comparative Examples, respective physical properties were evaluated by the following methods:

(3) 1. Weight Average Molecular Weight of Polymer

(4) A weight average molecular weight and molecular weight distribution of acrylic polymers were measured using GPC according to the following condition. To plot a calibration curve, standard polystyrene produced by Agilent System was used, and the measurement result was converted.

(5) <Condition for Measuring Weight Average Molecular Weight>

(6) Measurer: Gel Permeation Chromatography (Waters Alliance System)

(7) Column: PL Mixed B type

(8) Detector: Refractive Index Detector

(9) Column Flow Rate and Solvent: 1 mL/min, tetrahydrofuran (THF)

(10) Analysis Temperature and Measuring Amount: 40° C., 200 μl

(11) 2. Evaluation of the Coating Solid Content

(12) The coating solid content was measured by the following method:

(13) <Order of Measuring the Coating Solid Content>

(14) 1) Weight (A) of an aluminum dish was measured.

(15) 2) Approximately 0.3 to 0.5 g of a pressure-sensitive adhesive composition (sample before being dried) prepared in Example or Comparative Example was taken and put in the aluminum dish whose weight was measured in advance.

(16) 3) A small amount of a polymerization inhibitor solution (hydroquinone) (concentration: 0.5 weight %) dissolved in ethyl acetate was added to the pressure-sensitive adhesive using a pipette.

(17) 4) The resulting solution was dried in a 150° C. oven for approximately 30 minutes so as to remove solvent.

(18) 5) The solution was cooled at room temperature for approximately 15 to 30 minutes, and the weight of the remaining component (weight of the sample after being dried) was measured.

(19) 6) The coating solid content was measured according to the following Equation:
Coating TSC (solid content, unit:%)=(DS−A)/(S+E)×100

(20) DS: Weight of Aluminum Dish+Weight of Sample After being Dried (unit: g)

(21) A: Weight of Aluminum Dish (unit: g)

(22) S: Weight of Sample before being Dried (unit: g)

(23) E: Weight of Removed Component (ex. Solvent) (unit: g)

(24) 3. Evaluation of Coatability

(25) Coatability exhibited during coating process of a pressure-sensitive adhesive composition prepared in Example or Comparative Example was evaluated by observing the state of a coating layer with the naked eyes according to the following criteria:

(26) <Criteria for Evaluating Coatability>

(27) ο: Neither bubbles nor stripes on a coating layer were observed with the naked eyes.

(28) Δ: Fine bubbles and/or stripes on a coating layer were observed with the naked eyes.

(29) ×: Bubbles and/or stripes on a coating layer were clearly observed with the naked eyes.

(30) 4. Measurement of Gel Fraction

(31) A pressure-sensitive adhesive layer prepared in Example or Comparative Example was maintained in a constant temperature and humidity chamber (23° C., relative humidity: 60%) for 10 days. Then 0.3 g of the pressure-sensitive adhesive layer was taken and put in a #200 stainless wire mesh. The mesh was then put into 100 mL of ethyl acetate so as for the pressure-sensitive adhesive layer to be completely submerged in the ethyl acetate, and then maintained in a dark room at room temperature for 3 days. Then, portions (non-dissolved parts) of the pressure-sensitive adhesive layer, which were not dissolved in the ethyl acetate, was taken, and then dried at 70° C. for 4 hours so as to measure the weight (dry weight of the non-dissolved parts) of the non-dissolved parts.

(32) Then the gel fraction (unit: %) was measured by substituting the measured results to the following Equation:

(33) [Equation for Measuring Gel Fraction]
Gel Fraction=B/A×100

(34) A: Weight of the Pressure-Sensitive Adhesive (0.3 g)

(35) B: Dry Weight of the Non-dissolved Parts (unit: g)

(36) 5. Evaluation of Re-Workability

(37) A specimen was manufactured by cutting a pressure-sensitive polarizing plate formed in Example or Comparative Example so as to have a width of 90 nm and a length of 170 mm. Subsequently, a releasable PET film attached to a pressure-sensitive adhesive layer was peeled, and then the pressure-sensitive adhesive polarizing plate was attached to a non-alkali glass (Corning) using a 2 kg roller according to the JIS Z 0237. The non-alkali glass to which the polarizing plate was attached was left in a constant temperature and humidity chamber (23° C., relative humidity: 60%) for approximately 1 hour, heated at 50° C. for 4 hours, and then left at room temperature for 1 hour. Afterward, the polarizing plate was peeled from the non-alkali glass at a peel rate of 300 mm/min and a peel angle of 180 degrees using a texture analyzer (Stable Micro Systems, UK) to evaluate re-workability according to the following criteria:

(38) <Criteria for Evaluating Re-Workability>

(39) ο: when the polarizing plate was easily peeled and thus no transfer residue remained

(40) Δ: when peeling was not easy, or some transfer residue of the pressure-sensitive adhesive remained on the glass after peeling

(41) ×: when peeling was very difficult, enough to destroy the polarizing plate or glass, or a large amount of a transfer residue of the pressure-sensitive adhesive remained on the glass

(42) 6. Durability and Reliability and Durability and Reliability after Long-Term Storage

(43) A specimen was manufactured by cutting a polarizing plate formed in Example or Comparative Example so as to have a width of 90 mm and a length of 170 mm, and two sheets of the specimen manufactured as described above were attached to both surfaces of a glass having a width of 110 mm, a length of 190 mm and a thickness of 0.7 mm so as for light-absorption axes of the polarizing plate to be crossed with each other, thereby preparing a sample. A pressure applied in the attachment was approximately 5 kg/cm.sup.2, and the process was performed in a clean room to prevent entering of impurities or bubbles.

(44) The humidity and heat resistance properties were evaluated by observing whether bubbles were generated or peeling occurred after the sample was left under conditions of a temperature of 60° C. and relative humidity of 90% for 1,000 hours.

(45) In addition, heat resistance was evaluated by observing whether bubbles were generated or peeling occurred after the sample was left at 80° C. for 1,000 hours.

(46) Evaluation of the humidity and heat resistance or heat resistance properties was performed after the specimen obtained after being left under the humidity and heat resistance or heat resistance condition was maintained at room temperature for 24 hours.

(47) In addition, the durability and reliability after long-term storage was evaluated by examining the humidity and heat resistance and heat resistance properties in the same manner as described above after the sample was maintained for 5 months or more under conventional storage conditions.

(48) Criteria for evaluating the durability and reliability were as follows:

(49) <Criteria for Evaluating Durability>

(50) ∘: No bubbles were generated and no peeling occurred.

(51) Δ: Some bubbles were generated and/or peeling somewhat occurred.

(52) ×: A large amount of bubbles were generated and/or peeling considerably occurred.

PREPARATION EXAMPLE 1

(53) 96 parts by weight of n-butyl acrylate (n-BA), 3.9 parts by weight of hydroxybutyl acrylate, and 0.1 parts by weight of acrylic acid were poured into a 1L reactor in which a nitrogen gas is refluxed and which has a cooling apparatus to facilitate temperature control, and a suitable amount of n-dodecyl mercaptan (n-DDM) was added. After 150 parts by weight of ethyl acetate was poured as a solvent, the reactor was purged with nitrogen gas for 60 minutes to remove oxygen. Afterward, the temperature was maintained at 66° C., 0.03 parts by weight of azobisisobutyronitrile (AIBN) was added as a reaction initiator, and the reaction product was diluted with ethyl acetate after a 16 hours reaction, thereby preparing an acrylic polymer solution (A1) having a weight average molecular weight of 950,000 and a solid content of 25.7 weight %.

PREPARATION EXAMPLES 2 to 10

(54) Acrylic polymer solutions (A2 to A9) were prepared in the same manner as described in Example 1, except that components were controlled as shown in the following Table 1:

(55) TABLE-US-00001 TABLE 1 Preparation Example 1 2 3 4 5 6 7 8 9 10 Acrylic polymer A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 Solution Composition n-BA 96 96 96 98 93 96 96 96 99 96 of Monomer HBA 3.9 3.9 3.7 2 6.7 3.9 3.9 3.9 1 3.9 AA 0.1 0.1 0.3 0.3 0.3 0.03 0.5 0.1 — 0.1 AIBN 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03 nDDM 0.05 0.1 0.05 0.05 0.05 0.05 0.05 0.2 — — EAc 120 120 120 120 120 120 120 120 120 120 Mw (unit: ×10,000) 95 75 93 94 94 93 94 60 150 110 Content unit: part by weight n-BA: n-butyl acrylate HBA: hydroxybutyl acrylate AA: acrylic acid AIBN: azobisisobutyronitril nDDN: n-dodecyl mercaptan EAc: ethyl acetate Mw: weight average molecular weight

EXAMPLE 1

(56) Preparation of Pressure-Sensitive Adhesive Composition (Coating Solution)

(57) A coating solution (pressure-sensitive adhesive composition) was prepared by blending 0.1 parts by weight of a multifunctional crosslinking agent (a tolylenediisocyanate addition product of trimethylol propane, TDI-1) and 0.1 parts by weight of beta-cyanoacetylpropyl trimethoxy silane (LG Chemical Ltd., M-812) relative to 100 parts by weight of a solid content of the acrylic polymer solution (A1) of Preparation Example 1, and diluting the resulting product to have the coating solid content in an amount of approximately 22 weight %.

(58) Preparation of Pressure-Sensitive Polarizing Plate

(59) A pressure-sensitive adhesive layer was formed by coating the prepared coating solution on a release-treated surface of a poly(ethyleneterephthalate) (PET; MRF-38, Mitsubishi) film to have a thickness of 30 μm after drying, and drying the coated film under a suitable condition to have a gel fraction of approximately 70%. A pressure-sensitive polarizing plate was prepared by laminating the formed pressure-sensitive adhesive layer on one surface of an iodine-based polarizing plate having a thickness of 185 μm.

EXAMPLES 2 and 3 and COMPARATIVE EXAMPLES 1 to 7

(60) A polarizing plate was prepared in the same manner as described in Example 1, except that the composition of a pressure-sensitive adhesive composition, the gel fraction of a pressure-sensitive adhesive and the coating solid content were as shown in the following Table 2. Though Comparative Example 7 uses a pressure-sensitive adhesive composition containing an acrylic polymer having a high molecular weight to have a coating solid content of 20%, it was impossible to perform coating and form a pressure-sensitive adhesive layer. Thus, a gel fraction could not be measured.

(61) TABLE-US-00002 TABLE 2 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Kind of Polymer Solution A1 A2 A3 A10 A1 A4 A5 A6 A7 A8 A9 Solid Content in Polymer 100 100 100 100 100 100 100 100 100 100 100 Solution Content of Crosslinking 0.1 0.1 0.1 0.1 0.05 0.1 0.1 0.1 0.07 0.2 0.1 Agent Content of Coupling 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Agent Gel Fraction (unit: %) 70 66 75 70 35 20 90 45 75 65 — Coating solid content 22 25 22 20 23 22 22 22 22 27 20 (unit: %) Content unit: part by weight Crosslinking agent: tolylene diisocyante addition product of trimethylolpropane (TDI-1) Coupling agent: beta-cyanoacetylpropyl trimethoxy silane (LG Chemical Ltd., M-812)

(62) Evaluation results for physical properties with respect to Examples and Comparative Examples were summarized in Table 3.

(63) TABLE-US-00003 TABLE 3 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 7 Coatability ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X Re-workability ◯ ◯ ◯ ◯ X X ◯ X ◯ X — Durability & Heat ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ Δ — Reliability resistance Humidity ◯ ◯ ◯ ◯ Δ ◯ Δ ◯ Δ Δ — and heat resistance Durability & Heat ◯ ◯ ◯ ◯ Δ Δ ◯ Δ ◯ X — Reliability resistance After Humidity ◯ ◯ ◯ ◯ X X X X X X — Long- and heat term resistance Storage

(64) As seen from the results in Table 3, Examples 1 to 4 showed excellent re-workability, durability and reliability and coatability, and also showed excellent durability and reliability even after long-term storage.

(65) Meanwhile, in Comparative Examples 1 to 4, coating was possible due to a large solid content (coating solid content), but the re-workability and the durability and reliability were degraded. In addition, in Comparative Example 5, it was impossible to ensure the durability and reliability during long-term storage, and Comparative Example 6 was degraded in re-workability and durability and reliability. In addition, in Comparative Example 7, it was impossible to evaluate the physical properties since it was impossible to perform coating and form a pressure-sensitive adhesive layer from the beginning when the coating solid content was set to 20%.

(66) A pressure-sensitive adhesive composition of the present application can be effectively coated even when the coating solid content of the composition is high. Thus, the pressure-sensitive adhesive composition can have considerably increased productivity in formation of a pressure-sensitive adhesive or manufacture of an optical film such as a polarizing plate, and excellent durability and reliability and re-workability even after being formed into a pressure-sensitive adhesive.