Acrylic pressure-sensitive adhesive composition and pressure-sensitive adhesive sheet

1. A pressure-sensitive adhesive sheet having an acrylic pressure-sensitive adhesive layer that comprises an acrylic polymer as its base polymer and at least a tackifier resin or a (meth)acrylic oligomer, wherein

the acrylic polymer is a polymer of a starting monomer mixture that comprises an alkyl (meth)acrylate in an amount of 85% by weight or higher, the alkyl (meth)acrylate is a compound represented by the formula (1)

CH 2 ═C(R 1 )COOR 2   (1)

wherein R 1 is a hydrogen atom or a methyl group, and R 2 is an acyclic alkyl group having 1 to 6 carbon atoms,

the acrylic polymer has a weight average molecular weight of 110×10 4 or higher,

the acrylic polymer has a dispersity (Mw/Mn) below 15,

the pressure-sensitive adhesive layer has a gel fraction of 40% by weight or higher, and

the pressure-sensitive adhesive layer has a storage modulus at 85° C., G′(85° C.), of 0.02 MPa or greater.

2. The pressure-sensitive adhesive sheet according to claim 1 , wherein

the pressure-sensitive adhesive layer has a storage modulus at 25° C., G′(25° C.), of 0.15 MPa or greater; and

the pressure-sensitive adhesive sheet satisfies at least one of the following features: having a 180° peel strength of 8 N/20 mm or greater when determined within one minute after press-bonded at 23° C. at a press-bonding load of 0.1 kg or a 180° peel strength of 8 N/20 mm or greater when determined within one minute after press-bonded at 40° C. at 0.05 MPa for 3 seconds.

3. The pressure-sensitive adhesive sheet according to claim 1 , wherein the acrylic polymer includes an acidic group-containing monomer copolymerized therein.

4. The pressure-sensitive adhesive sheet according to claim 3 , wherein the acidic group-containing monomer is acrylic acid.

5. The pressure-sensitive adhesive sheet according to claim 3 , wherein the acidic group-containing monomer has a copolymerization ratio below 10% by weight in the acrylic polymer.

6. The pressure-sensitive adhesive sheet according to claim 1 , wherein the acrylic polymer is crosslinked.

7. The pressure-sensitive adhesive sheet according to claim 1 , used for fixing parts in mobile electronics.

8. The pressure-sensitive adhesive sheet according to claim 1 , used for fixing a flexible printed circuit.

9. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer has a storage modulus at 85° C., G′(85° C.), of 0.03 MPa or greater.

10. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer has a storage modulus at 85° C., G′(85° C.), of 0.04 MPa or greater.

11. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer has a storage modulus at 85° C., G′(85° C.), of 0.05 MPa or greater.

12. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer has the gel fraction of 40% by weight or higher and 70% or lower.

13. The pressure-sensitive adhesive sheet according to claim 1 , wherein the pressure-sensitive adhesive layer comprises the (meth)acrylic oligomer, the (meth)acrylic oligomer has a weight average molecular weight of 1,000 or larger and smaller than 30,000, and the (meth)acrylic oligomer comprises, as a (meth)acrylate monomer, an alicyclic hydrocarbon group-containing (meth)acrylate in an amount higher than 50% by weight.

14. The pressure-sensitive adhesive sheet according to claim 1 , wherein

the pressure-sensitive adhesive sheet consists of the pressure-sensitive adhesive layer, or the pressure-sensitive adhesive sheet comprises a substrate to support the pressure-sensitive adhesive layer,

the substrate is a resin film, a foam film, paper, a fabric, a metal foil, or a composite thereof, and

the foam film is formed from a foam material selected from the group consisting of a polyester-based resin foam, a polyvinyl chloride-based resin foam, a vinyl acetate-based resin foam, a polyphenylene sulfide resin foam, an amide-based resin foam, a polyimide resin foam, a polyether ether ketone foam, a styrene-based resin foam, a urethane-based resin foam and a rubber-based resin foam.

15. The pressure-sensitive adhesive sheet according to claim 1 , wherein

the pressure-sensitive adhesive sheet consists of the pressure-sensitive adhesive layer, or the pressure-sensitive adhesive sheet comprises a substrate to support the pressure-sensitive adhesive layer,

the substrate is a resin film, a foam film, paper, a fabric, a metal foil, or a composite thereof,

the resin film is a polyester-based resin film, a polyvinylic resin film, a vinyl acetate-based resin film, a polyimide-based resin film, a polyamide-based resin film, a fluororesin film, cellophane, or a rubber-based film, and

the foam film is formed from a foam material selected from the group consisting of a polyester-based resin foam, a polyvinyl chloride-based resin foam, a vinyl acetate-based resin foam, a polyphenylene sulfide resin foam, an amide-based resin foam, a polyimide resin foam, a polyether ether ketone foam, a styrene-based resin foam, a urethane-based resin foam and a rubber-based resin foam.