| The polymer melting process of linear low-density Polyethylene (LLDPE) in pellet form and Polyvinyl Chloride (PVC) in powder form was investigated by ultrasound in-line monitoring technique in an intermeshing counter-rotating twin-screw extruder. The melting behavior was revealed by ultrasound signal patterns. More specifically, the melting level of PVC in C-chambers was characterized by the analysis of wave attenuation. Based on the experimental observations, a dispersed melting model in calender gaps was developed to predict the melting length, and computational results were compared with experiments.; Two homemade ultrasound probes for the in-line monitoring were developed based on delay line technique. The ultrasound probes could resist harsh processing environments during polymer extrusion. Thus material properties or states in melting zone, mixing zone and die were monitored along the extrusion direction, respectively.; First, model studies of PVC compounds were performed in a chamber at both dispersed mode and film/bed mode to investigate the sensitivity of ultrasound signals to material thickness and morphology at conductive heating conditions.; Second, the melt pressure effect on ultrasound signals (wave velocity and attenuation) was investigated in low and high pressure range, respectively. Experiments at low pressure range were conducted with an ultrasound probe in a mixing section, and experiments at high pressure range were conducted with an ultrasound probe in a die. The insensitivity of wave attenuation to the melt pressure within a broad range provided an advantage for the analysis of melting process by ultrasound signals.; Third, ultrasound signal patterns gleaned from the in-line monitoring experiments revealed a variety of melting behavior depending on material and processing conditions. Effects of PVC additives and processing conditions were discussed by using normalized amplitude value. Furthermore, ultrasound wave scattering theory for a dilute suspension system was applied to predict the concentration of unmelted PVC particles in C-chambers by measuring scattering coefficient.; Finally, a dispersed melting model in calender gaps was developed to predict the melting length in terms of number of calender gaps. In this model, the rheological model of the mixture of unmelted PVC particles and melt was considered as a filled-polymer melt system. The computational results were compared with the experimental observations by both "screw pulling out" and "ultrasound in-line monitoring" methods. |
