FLOW IMPLICATIONS IN THE PROCESSING OF TETRAFLUOROETHYLENE HEXAFLUOROPROPYLENE COPOLYMERS/

The melt fracture behavior of two Teflon(R) resins was studied in capi llary extrusion in order to identify the critical conditions for the o nset of melt fracture and wall slip. These resins were copolymers of t etrafluoroethylene (TFE) and hexafluoropropylene (HFP) and TFE/HFP/PAV E (perfluoro (alkyl vinyl ether)) (PAVE) respectively. The incorporati on of the third monomer in the molecules of the resin was found to imp rove the processability of polymer without substantially changing its theology. Surface melt fracture (sharkskin) appeared at wall shear str esses greater than about 0.18 MPa, practically independent of temperat ure in the range of 300 to 350 degrees C. At higher apparent shear rat es oscillating melt fracture was observed due to the presence of wall slip and compressibility of the melt. Furthermore, a superextrusion re gion was identified at apparent rates greater than about 700 s(-1), be yond those where oscillating melt fracture was obtained In this region , the extrudate appears again smooth. Finally, it was found that the a ddition of 0.1% of polyethylene in the resins, reduces dramatically th e pressure drop along the capillary die and eliminates extrudate disto rtion over the whole range of apparent shear rates lip to the superext rusion region.