Rheological characterization of PET resins for flows with significant amount of extension

In film casting or extrusion coating of polyethylene terephthalate (PET) resins, thickness and film-width nonuniformities arise due to edge neck-in or to flow instabilities such as draw resonance. Flow modelling can identify the parameter range of stable operation but requires accurate knowledge of melt rheology/elasticity in both shear and extension. Rheological characterization of a number of linear and branched PETs was carried out by means off a parallel-plate and capillary rheometers. The rheological data obtained were used to fit by nonlinear optimization, the linear relaxation spectrum and nonlinear parameters of a multimode Phan-Thien and Tanner (PTT) constitutive relation. The fitted PTT relation was used in a finite-element model of the stability of film casting flow based on the thin membrane approximation.