Patent ID: 7494611

Claim:
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.