The melt fracture behavior of two Teflon® resins was studied in capillary extrusion in order to identify the critical conditions for the onset of melt fracture and wall slip. These resins were copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) and TFE/HFP/PAVE (perfluoro (alkyl vinyl ether)) (PAVE) respectively. The incorporation of the third monomer in the molecules of the resin was found to improve the processability of polymer without substantially changing its rheology. Surface melt fracture (sharkskin) appeared at wall shear stresses greater than about 0.18 MPa, practically independent of temperature in the range of 300 to 350°C. At higher apparent shear rates oscillating melt fracture was observed due to the presence of wall slip and compressibility of the melt. Furthermore, a superextrusion region was identified at apparent rates greater than about 700 s−1, beyond those where oscillating melt fracture was obtained. In this region, the extrudate appears again smooth. Finally, it was found that the addition of 0.1 % of polyethylene in the resins, reduces dramatically the pressure drop along the capillary die and eliminates extrudate distortion over the whole range of apparent shear rates up to the superextrusion region.
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Rosenbaum et al. (1995) studied this question.