Rheological properties and instabilities in polymer processing of polyethylene terephthalates and polyethylenes

Molecularly well-characterized polyethylene terephthalates (PET) including linear and branched PETs and polyethylenes (PE) including high density polyethylene (HOPE), low density polyethylene (LDPE) and linear low density polyethylene (LLDPE) are studied in a series of rheological and processing experiments.

The materials were rheologically characterized in steady shear measurements which include steady shear viscosity measurements and the first normal stress difference measurements. The results of steady shear measurements indicate that broadening of molecular weight dis tribution and introduction of long chain branching enhance the non-Newtonian character.

Uniaxial elongational flow measurements were carried out on the elongational rheometer which was built at the University of Akron. It was found that the deformation rate softening characteristics in elongational flow cannot always be predicted from those in steady shear flow. This was especially observed for LDPEs.

Based on the rheological properties obtained from the steady shear measurements and the elongational flow measurements, a new material parameter was introduced for the expression for the deformation rate dependence of the relaxation time.

Flow instabilities which occur in continuous polymer processing operations such as extrusion, melt spinning and tubular blown film extrusion all of which involves elongational flow or the combination of elongational and shear flow. The experimental results were well interpreted in terms of the rheological properties obtained from the elongational flow measurements and the activation energy of viscous flow.

Mathematical modelling of the nonisothermal tubular blown film extrusion process for non-Newtonian fluids was carried out. The results of computer simulation indicates that both non-Newtonian characteristics and activation energy of viscous flow affect the flow behavior of the tubular film. However, when the cooling is very efficient, the effect of thermal properties dominates.