Cellulose ester film, cellulose ester dope, protective film of polarizing plate and polarizing plate

1. A method for preparing a cellulose ester film comprising a cellulose ester and a polymer prepared by polymerizing at least one ethylenically unsaturated monomer selected from the group consisting of vinyl esters and acrylic esters, the method comprising the steps of:

casting a cellulose ester dope containing the monomer and a photopolymerization initiator on a metal support to form a cellulose ester web;

applying light to the web to initiate photopolymerization, whereby the polymer is produced in the web;

peeling the web from the metal support; and

drying the web in a drying apparatus to obtain a cellulose ester film,

wherein the polymer has a weight average molecular weight of not more than 5,000, and wherein the rate of mass chance of the cellulose ester film is not more than 2%, the rate of mass change being represented by the following formula:

Rate of mass change (%)=( ly−zl/y )×100

wherein y is the weight of the cellulose ester film measured at 23±3° C. and at 55±3% RH, and z is the weight of the cellulose ester film measured at 23±3° C. and at 55±3% RH after the film has been stored at 80±3° C. and at 90±3% RH for 48 hours, and then stored at 23±3° C. and at 55±3% RH for 24 hours.

2. The method of claim 1 , wherein the polymer has a weight average molecular weight of not more than 5,000.

3. The method of claim 1 , wherein the cellulose ester dope further contains fine particles having an average particle diameter of 0.01 to 1.0 μm.

4. The method of claim 1 , wherein the drying apparatus comprises a tenter.

5. The method of claim 1 , wherein the polymer contains an alkyl acrylate monomer in an amount of not less then 30 weight % or an alkyl methacrylate monomer in an amount of not less than 30 weight %.

6. The method of claim 5 , wherein the polymer contains a methyl acrylate monomer in an amount of not less than 30 weight %.

7. The method of claim 1 , wherein the polymer has a water solubilizing group.

8. The method of claim 7 , wherein the water solubilizing group is a hydroxyl group.

9. The method of claim 1 , wherein the content of said polymer in the cellulose ester film is 0.5 to 30 weight % based on the cellulose ester film.

10. The method of claim 1 , wherein the thickness of the cellulose ester film is 30 to 150 μm.

11. The method of claim 3 , wherein the fine particle content of the film is 0.005 to 0.3 weight %.

12. The method of claim 3 , wherein the fine particles are silicon dioxide particles.