Acrylic pressure-sensitive adhesive composition

1. An acrylic pressure sensitive adhesive composition comprising:

acrylic polymers including an alkyl(meth)acrylic acid ester monomer having 2 to 14 carbon atoms of an alkyl group, and

a multifunctional crosslinking agent,

wherein the acrylic polymers comprise an acrylic polymer for crosslinking structure that includes an alkyl(meth)acrylic acid ester monomer having 2 to 14 carbon atoms of an alkyl group and vinyl and/or acrylic crosslinkable monomer having carboxylic group or hydroxyl group crosslinkable with the multifunctional crosslinking agent, and an acrylic polymer for non-crosslinking structure that includes an alkyl(meth)acrylic acid ester monomer having 2 to 14 carbon atoms of an alkyl group and has no crosslinkable functional group,

the acrylic polymer for non-crosslinking structure having a molecular weight greater than 300,000

wherein, the acrylic polymer for crosslinking structure, the acrylic polymer for non-crosslinking structure and the multifunctional crosslinking agent are comprised such that they can form a crosslinked product of the acrylic pressure sensitive adhesive composition by a crosslinking reaction of the acrylic polymers and the multifunctional crosslinking agent,

wherein the crosslinked product has a gel fraction represented by the following formula (1) in a range of from 45 to 75%, and a swelling ratio represented by the following formula (2) in a range of from 5 to 20, and

wherein the crosslinked product comprises sol that can be eluted from the crosslinked product by ethyl acetate and said sol has a weight average molecular weight of at least 300,000:

gel fraction(%)= B/A× 100  (1)

swelling ratio= C/B   (2)

wherein A represents the weight of the crosslinked product, B represents the dry weight of insoluble content after immersing the crosslinked product in ethyl acetate at ambient temperature for 48 hours, and C represents the sum of the weight of the insoluble content and the weight of solvent permeated into the insoluble content.

2. The acrylic pressure sensitive adhesive composition of claim 1 , wherein the gel fraction and swelling ratio satisfy the following formula (3):

−0.15 x+ 25 ≦y≦− 0.05 x+ 10  (3)

wherein x represents the gel fraction and y represents the swelling ratio.

3. The acrylic pressure sensitive adhesive composition of claim 1 , wherein the acrylic polymer for crosslinking structure comprises 80 to 99.8 parts by weight of (meth)acrylic acid ester monomers having 2 to 14 carbon atoms of an alkyl group and 0.01 to 10 parts by weight of vinyl and/or acrylic cross-linkable monomers having a carboxyl group or hydroxy group which is cross-linkable with a multifunctional crosslinking agent.

4. The acrylic pressure sensitive adhesive composition of claim 3 , wherein the (meth)acrylic acid ester monomers are one or more selected from the group consisting of 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, n-octyl(meth)acrylate, isooctyl(meth)acrylate, isononyl(meth)acrylate, lauryl(meth)acrylate, and tetradecyl (meth)acrylate.

5. The acrylic pressure sensitive adhesive composition of claim 3 , wherein the cross-linkable monomers are one or more selected from the group consisting of 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 4 -hydroxybutyl(meth)acrylate, 2-hydroxyethyleneglycol(meth)acrylate, 2-hydroxypropyleneglycol(meth)acrylate, acrylic acid, methacrylic acid, acrylic acid dimer, itaconic acid, maleic acid, and maleic anhydride.

6. The acrylic pressure sensitive adhesive composition of claim 3 , wherein the acrylic polymer for cros slinking structure further comprises 20 or less parts by weight of copolymerizable vinyl monomers.

7. The acrylic pressure sensitive adhesive composition of claim 1 , wherein the multifunctional cros slinking agent is one or more selected from the group consisting of isocyanate, epoxy, aziridine and metal chelate compounds.

8. The acrylic pressure sensitive adhesive composition of claim 1 , further comprising one or more additives selected from the group consisting of plasticizer, epoxy resins, curing agent, UV stabilizers, antioxidants, coloring agents, reinforcing agents, fillers, antifoaming agents, and surfactants.

9. A process for preparing the acrylic pressure sensitive adhesive composition of claim 1 , comprising the steps of:

reacting the alkyl(meth)acrylic acid ester monomer having 1 to 12 carbon atoms of the alkyl group with vinyl and/or acrylic cross-linkable monomers having the carboxyl group or hydroxyl group which is cross-linkable with the multifunctional crosslinking agent to prepare the acrylic polymer for a crosslinking structure;

reacting the alkyl(meth)acrylic acid ester monomer having 1 to 12 carbon atoms of the alkyl group to prepare the acrylic polymer for a non-crosslinking structure, the acrylic polymer for the non-crosslinking structure having no crosslinkable functional group;

and said acrylic polymer for the non-crosslinking structure having a weight average molecular weight greater than 300,000

mixing the acrylic polymer for a crosslinking structure and the acrylic polymer for non-crosslinking structure so as to form a polymer mixture;

mixing the polymer mixture with the multifunctional crosslinking agent such that they can form the crosslinked product as recited in claim 1 after the crosslinking reaction of them; and

obtaining the acrylic pressure sensitive adhesive composition of claim 1 .

10. A process for preparing the acrylic pressure sensitive adhesive composition of claim 1 , comprising:

a first step of reacting the alkyl(meth)acrylic acid ester monomer having 1 to 12 carbon atoms of the alkyl group with vinyl and/or acrylic cross-linkable monomers having the carboxyl group or hydroxyl group which is cross-linkable with the multifunctional crosslinking agent to prepare the acrylic polymer for a crosslinking structure;

a second step of further reacting the alkyl(meth)acrylic acid ester monomer having 1 to 12 carbon atoms of the alkyl group to prepare the acrylic polymer for non-crosslinking structure in the presence of the acrylic polymer for crosslinking structure prepared in the first step, the acrylic polymer for the non-crosslinking structure having no crosslinkable functional group;

and said acrylic polymer for the non-crosslinking structure having a weight average molecular weight greater than 300,000

mixing and reacting the mixture obtained in the second step with the multifunctional crosslinking agent such that they can form the crosslinked product as recited in claim 1 after the crosslinking reaction of them; and

obtaining the acrylic pressure sensitive adhesive composition of claim 1 .

11. The process for preparing the acrylic pressure sensitive adhesive composition of claims 9 or 10 , wherein the reaction for preparing the acrylic polymer is carried out by a polymerization method selected from the group consisting of solution polymerization, photo-polymerization, bulk polymerization, suspension polymerization, and emulsion polymerization.

12. The process for preparing the acrylic pressure sensitive adhesive composition of claims 9 or 10 , wherein the vinyl and/or acrylic cross-linkable monomers comprise a hydroxy group.

13. A protective film comprising the acrylic pressure sensitive adhesive composition of claim 1 as adhesive layer.

14. The protective film of claim 13 , wherein the adhesive layer is coated on one side or both sides of transparent substrate film having a single layer, or two or more laminated layers.

15. A polarizer comprising the protective film of claim 13 or 14 .

16. A liquid crystal display comprising liquid crystal panel in which the polarizer of claim 15 is adhered to one side or both sides of liquid crystal cell.