Patent ID: 8900676

Claim:
A polarizing plate protective film comprising: a cellulose ester film containing Polymer X and Polymer Y which have no aromatic ring in their molecule and exhibits negative birefringence; and a hard coat layer on the cellulose ester film; the cellulose ester film exhibits a retardation value Ro, defined by Expression (I) of 0 to 10 nm and a retardation value Rt, defined by Expression (II), of −20 to 20 nm: Ro =( nx−ny )× d Expression (I): Rt ={( nx+ny )/2 −nz}×d Expression (II): wherein nx is a refractive index in a slow axis direction in a plane of the polarizing plate protective film, ny is a refractive index in a direction orthogonal to the slow axis in the plane, nz is a refractive index in a film thickness direction, and d is a film thickness (nm), and wherein a difference value ΔRt, defined by Expression (III), is less than 15 nm n, Δ Rt (nm)=Rt(23° C.,80% RH )− Rt (23° C.,20% RH ) Expression (III) where Rt (23° C., 80% RH) represents a retardation value Rt measured after the cellulose ester film was placed under an environment of 23° C. and 80% RH for 10 hours, and Rt (23° C., 20% RH) represents a retardation value Rt measured after the cellulose ester film was placed under an environment of 23° C. and 20% RH for 10 hours, wherein Polymer X has a weight average molecular weight of 5000-30000 and is obtained via copolymerization of ethylenically unsaturated monomer Xa having neither an aromatic ring nor a hydrophilic group in the molecule, and ethylenically unsaturated monomer Xb having no aromatic ring but having a hydrophilic group in the molecule; and wherein the Polymer X is represented by Formula (1), and Polymer Y is represented by Formula (2): —([CH 2 —C(—R 1 )(—CO 2 R 2 )] m [CH 2 —C(—R 3 )(—CO 2 R 4 —OH)-] n -[Xc] p - Formula (1): Ry-[CH 2 —C(—R 5 )(—CO 2 R 6 —OH)-] k -[Yb] q - Formula (2) wherein R 1 , R 3 , and R 5 are H or CH 3 , R 2 is an alkyl group of a carbon number of 1-12 or a cycloalkyl group, R 4 and R 6 are —CH 2 —, —C 2 H 4 —, or —C 3 H 6 —, Ry is OH, H, or an alkyl group of a carbon number of at most 3, Xc is a monomer unit polymerizable with Xa or Xb, Yb is a monomer unit copolymerizable with Ya; and m, n, k, p, and q are molar composition ratios wherein m≠0, n≠0, k≠0, m+n+p=100, and k+q=100.