Melting of Heterogeneous Polymer Bulk Solids

Single screw extruders are the most important machines in polymer processing. The melting or plasticating process is often the limiting factor of these machines. Especially in the recycling of polymers, knowledge of the melting behavior is crucial, as mixtures of different materials are processed. This work shows the results of model experiments, comparable to Tadmors “drag induced melting” model, which were carried out using Polypropylene-Homopolymer (PP-H), Polymethylmethacrylate (PMMA) and different mixtures of both bulk materials. The experiments show, that the melting rate of pure PMMA is roughly 30 % lower than that of pure PP-H, while the shear stress is four to six times higher. The experiments with the mixture of 80 % PMMA and 20 % PP-H results in the lowest melting rate, less than half the value of pure PMMA and less than one third of that of pure PP. The measured shear stress in this experiment is about as low as that of pure PP. The mixture of 80 % PP-H and 20 % PMMA shows a lower melting rate than pure PP-H, while the shear stress increases slightly. The results reveal that mixtures of the materials melt much slower than the pure materials. Due to this behavior, the modeling of this process is difficult, as mixing rules cannot be applied.