Process for treating polyester film

Claim 1

A PROCESS FOR IMPARTING DRY-STRIPPING PROPERTIES TO AT LEAST ONE SURFACE OF A HYDROPHOBIC FILM ESSENTIALLY COMPOSED OF A MACROMOLECULAR POLYESTER PREPARED BY REACTING (1) AT LEAST ONE DIHYDRIC ALCOHOL OF THE FORMULA HOCH2WN-CH2OH WHERE W IS A DIVALENT HYDROCARBON RADICAL SELECTED FROM THE GROUP CONSISTING OF METHYLENE, UNSUBSTITUTED AND ALKYL-SUBSTITUTED POLYMETHYLENE RADICALS OF 1 TO 8 CARBON ATOMS AND A CYCLOALKYLENE RADICAL OF 5 TO 6 CARBON ATOMS, AND N IS ONE OF THE CARDINAL NUMBERS 0 TO 1 WITH (2) AT LEAST ONE COMPOUND SELECTED FROM THE GROUP CONSISTING OF AN AROMATIC DICARBOXYLIC ACID AND AN ESTER-FORMING DERIVATIVE OF SAID DICARBOXYLIC ACID OF WHICH AT LEAST 15 MOLE PERCENT IS SELECTED FROM THE GROUP CONSISTING OF TEREPHTHALIC ACID AND AN ESTERFORMING DERIVATIVE OF TEREPHTHALIC ACID AND UP TO 20 MOLE PERCENT CAN BE SELECTED FROM THE GROUP CONSISTING OF AT LEAST ONE ALIPHATIC DICARBOXYLIC ACID AND AN ESTER-FORMING DERIVATIVE OF SAID ACID, CHARACTERIZED BY TREATING SAID SURFACE (A) BY CONTACTING IT WITH A SOLUTION COMPRISING 77.0 TO 83.0 PARTS BY WEIGHT OF H2SO4, 14.6 TO 22.8 PARTS BY WEIGHT OF WATER AND AN AMOUNT OF A CHROMIC ACIDYIELDING COMPOUND EQUIVALENT TO 0.1 TO 3.25 PARTS BY WEIGHT OF CR2O3, SAID COMPOUND YIELDING CHROMIC ACID IN WATER CONTAINING SULFURIC ACID, FOR A PERIOD UP TO 60 SECONDS AND (B) BY CONTACTING THE TREATED SURFACE OF THE FILM WITH AN EXCESS OF WATER.