Extrusion finishing of perfluorinated copolymers

A process for reducing the backbone instability in a raw melt-processible copolymer of 80-99% by weight units of tetrafluorethylene and complementally 20-1% by weight units of hexafluorpropylene which comprises:

• (a) subjecting said raw melt-processible copolymer, which copolymer has a backbone volatiles index greater than 15 and a melt viscosity between 10.sup.2 and 10.sup.6 Pa.s, to a vacuum of less than 10.sup.5 Pa and to a temperature of between 220.degree. C. and 450.degree. C. while moving the copolymer through a twin screw extruder having intermeshing corotating screws that are self-wiping and which have tight clearance,
• and subjecting the copolymer to a shear rate between 500 and 7000 reciprocal seconds for a time sufficient to result in the copolymer having a backbone voltailes index less than 10;
• and
• (b) removing the copolymer from the extruder.