Development of an online rheometer for simultaneous measurement of shear and extensional viscosity during the polymer extrusion process

The demand for valid and reliable online process surveillance control of modern production plants in extrusion and compounding has considerably increased in recent years. We present an online rheometer for simultaneously measuring the extensional and shear viscosity of polymer melts and filled compounds during processing. The unique die design features a hyperbolic slit contraction that enables constant extensional rates along the die. Further, pressure transducers can be incorporated directly into the two slit sections, which prevents material from accumulating in pressure holes. The rheometer was implemented to a compounding line containing a Leistritz ZSE27 MAXX co-rotating twin-screw extruder and strand pelletizing. We demonstrated the applicability for condition monitoring based on several examples. The rheological behavior of a polypropylene random copolymer is characterized during twelve successive extrusion cycles. We compared our results to oscillatory shear measurements with a plate-plate rheometer. Decrease of extensional viscosity were less severe than the decrease of shear viscosity during multiple extrusion runs. Blending a high and a low viscous polypropylene produced similar behavior. The Trouton ratio which is based on both the extensional and shear viscosity was introduced as an additional quality parameter. For distinguishing between different PP/PS compounds or PP glass fiber compounds, the Trouton ratio is a valid and beneficial parameter. Additionally, different methods for evaluating the melt flow rate (MFR) from shear viscosity data were compared.