Patent ID: 6913714

Claim:
A method of preparing a thermoplastic polycarbonate film comprising: (a) forming a molten extrudate of a thermoplastic polycarbonate polymer in an extruder having a terminal die through which said molten extrudate is passed, said molten extrudate having a temperature of 150° C. to 400° C. upon emerging from said terminal die; and (b) passing the molten extrudate, from said die, between two counter-rotating rolls each having an elastomeric polymer surface, said counter-rotating rolls having a roll speed ratio and a nip, the distance between said terminal die and said nip being a die-nip distance; wherein the extrudate temperature, the roll speed ratio and the die-nip distance are each selected such that a thermoplastic polycarbonate film having an optical retardation value of less than or equal to 20 nm is formed.