Retardation film, polarizing plate and liquid crystal display device

1. A retardation film comprising:

(a) at least one cellulose derivative selected from the group consisting of triacetyl cellulose, diacetyl cellulose, cellulose acetate propionate, cellulose acetate butyrate, cellulose acetate phthalate and cellulose acetate trimellitate, wherein the cellulose derivative has a total acyl group substitution degree of 2.0 to 2.8; and

(b) an esterified compound in which all or a part of the OH groups in a compound (B) are esterified by at least one aromatic monocarboxylic acid, wherein the compound (B) comprises two to twelve of at least one of a furanose structure and a pyranose structure, which are bonded in the compound (B), and

an ester plasticizer represented by the following Formula (4):

B-(G-A) n -G-B  Formula (4),

wherein B represents a residue of a benzene monocarboxylic acid group, G represents an alkylene glycol group having 2 to 12 carbon atoms, an aryl glycol group having 6 to 12 carbon atoms or an oxyalkylene glycol group having 4 to 12 carbon atoms, A represents a residue of an aryl dicarboxylic acid group having 6 to 12 carbon atoms, and n represents an integer of 1 or more,

wherein the retardation film has the following retardation values for light having a wavelength of 590 nm under a condition of a temperature of 23° C. and a humidity of 55% RH: an in-plane retardation value R 0 represented by a Formula (i) is 20 to 80 nm, a thickness-direction retardation value R t represented by a Formula (ii) is 100 to 250 nm and a ratio of R t /R 0 is 2.0 to 5.0,

R 0 =( nx−ny )× d   Formula (i):

R t =(( nx+ny )/2 −nz )× d   Formula (ii),

wherein nx represents a refractive index in a film in-plane slow axis direction, ny represents a refractive index in a direction perpendicular to the slow axis, nz represents a refractive index in a film thickness-direction, and d represents a film thickness in nm,

wherein the retardation film has a variation of 3 to 20 nm in the thickness-direction retardation value R t due to a humidity change of 20% RH to 80% RH at a temperature of 23° C.,

and wherein the variation is a reversible variation.

2. The retardation film of claim 1 , wherein the total acyl group substitution degree is 2.2 to 2.55.

3. The retardation film of claim 1 , wherein the retardation film contains 5 to 30% by weight of the esterified compound.

4. The retardation film of claim 3 , wherein the compound (B) comprises a compound having the furanose structure and the pyranose structure.

5. The retardation film of claim 3 , wherein the compound (B) comprises at least one saccharide selected from the group consisting of lactose, sucrose, nystose, 1F-fructosylnystose, stachyose, maltitol, lactitol, lactulose, cellobiose, maltose, cellotriose, maltotriose, raffinose, kestose, gentiobiose, gentiotriose, gentiotetraose, xylotriose and galactosyl-sucrose.

6. The retardation film of claim 5 , wherein the compound (B) comprises sucrose.

7. The retardation film of claim 6 , wherein the cellulose derivative has a number average molecular weight (Mn) of 40000 to 200000.

8. The retardation film of claim 7 , further comprising a UV absorber selected from the group consisting of a triazine compound, an oxi-benzophenone compound, a benzotriazole compound, a salicylate compound, a benzophenone compound, a cyanoacrylate compound and a nickel complex salt compound.

9. A polarizing plate, comprising:

a polarizer; and

the retardation film of claim 1 which is pasted on at least one surface of the polarizer.

10. A liquid crystal display device, comprising

a liquid crystal cell; and

the retardation film of claim 1 .

11. The liquid crystal display device of claim 10 , wherein the liquid crystal cell is a VA liquid crystal cell.