Researchers at Fraunhofer LBF aim at obtaining an indication regarding the efficacy of the actual stabilizer content already during the compounding step. Towards this goal the viscosity of the melt is used as a response recorded while varying the recipe. This is realized by incorporating an online rheometer behind the screw tips of a twin screw extruder to measure the flow curves of the shear as well as the elongational viscosity.
First experiments were carried out on a minimally stabilised virgin polypropylene (PP).The amount of stabiliser added was varied at selected screw speeds. The reduced process-related degradation is immediately reflected in an increase in viscosity in the flow curves. Above a certain additive level there is no further increase in viscosity. This means that for the actual processing conditions, the stabiliser concentration has reached the limit above which no further improvement can be achieved.
Thus, online rheology provides the formulation developer with valuable information regarding the efficacy of a processing stabilizer already during compounding.Furthermore, the flow curves of the different polymers are not identical. The information content of a flow curve is therefore much higher than that of a single numerical value from an MVR measurement. In addition, the flow curves of the elongational viscosity can be included in the evaluation. Supported by an appropriate AI based system, online rheology appears to be a very promising tool to implement stabilizing during the production of recyclates with the ability of real-time adjustment to the aging stage of the mill charges.