Patent ID: 8545978

Claim:
A composite film comprising an acrylic polymer and a urethane polymer, wherein said acrylic polymer comprises an acryl component containing at least an acrylic acid-based monomer and a monofunctional acrylic monomer whose homopolymer has a glass transition temperature (Tg) of 273 K or more; a content of said acrylic acid-based monomer in said composite film is from 0.5 to 15 wt %; a glass transition temperature (Tg ac) of said acrylic polymer, represented by the following formula (1), is 273 K or more; a glass transition temperature (Tg ur) of said urethane polymer is 273 K or less; and a glass transition temperature (Tg) of the composite film, represented by the following formula (2), is 269 K or less: [ Math . ⁢ 1 ] ⁢ 1 Tg ⁢ ⁢ ac = ∑ n = 1 n ⁢ W ⁢ ⁢ n Tg ⁢ ⁢ n ⁢ ( 1 ) Tg ac: the glass transition temperature (temperature unit: K) of the acrylic polymer, Tg n: the glass transition temperature (temperature unit: K) when the acrylic monomer becomes a homopolymer, and Wn: the weight fraction of the monomer components based on the entire acrylic polymer (here, the total sum of monomer components is ΣWn=1); [ Math . ⁢ 2 ] ⁢ 1 Tg ⁢ ⁢ com = W ⁢ ⁢ ac Tg ⁢ ⁢ ac + W ⁢ ⁢ ur Tg ⁢ ⁢ ur ( 2 ) Tg com: the glass transition temperature (temperature unit: K) of the composite film Tg ac: the glass transition temperature (temperature unit: K) of the acrylic polymer calculated according to formula (1), Tg ur: the glass transition temperature (temperature unit: K) of the urethane polymer, W ac: the weight fraction of the acrylic polymer in the composite film, and W ur: the weight fraction of the urethane-based polymer in the composite film (here, Wac+Wur=1).