BIAXIALLY ORIENTED COPOLYESTER FILM FOR CAPACITOR DIELECTRIC USE OR THERMAL TRANSFER RIBBON USE

摘要

The present invention comprises a biaxially oriented copolyester film useful for capacitor dielectrics or thermal transfer printing ribbons. Such films have good winding characteristics, slipperiness, good mechanical properties (stiffness and strength), and excellent thermal stability. The biaxially oriented copolyester film has at least 25 mole percent of the dicarboxylic acid or its ester equivalent content in the copolyester of 4,4'-bibenzoic acid. The remainder of the copolyester is such that the melting point is about 260 DEG C such that the copolyester is capable of withstanding a thermal treatment of 260 DEG C for about 10 seconds. The biaxially oriented copolyester film also includes an effective amount of fillers having a bimodal particle distribution sufficient to provide slip and good winding characteristics such as those that are known in the art. The bimodal particle distribution generally should comprise: (a) fine particles having an average particle diameter of 3 to 50 nm and employed in the range of from about 0.01 to about 3 percent by weight based on weight of the copolyester, and (b) a large particle distribution having an average particle diameter of 0.05 to 4 mu m and an aspect ratio of from 1.0 to 1.2 and being employed in the range of 0.005 to 1 percent by weight based on the weight of the copolyester.