Melting of Mixtures of PP‑H/ HDPE and PP‑H/PMMA Bulk Solids Related to Single Screw Extrusion

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 deals with model experiments, comparable to Tadmors “drag induced melting” model and with the adaption of the mathematical modelling considering mixtures of different polymers. The experiments were carried out using Polypropylene-Homopolymer (PP-H), High Density Polyethylene (HDPE), Polymethylmethacrylate (PMMA) and different mixtures of PP-H/HDPE and PP-H/PMMA. Furthermore, different approaches for the modelling of the melting behavior of mixtures are presented and discussed. The experiments show, that the volumetric melting rate of pure HDPE is much higher than that of pure PP-H, while the shear stress is about 50 % higher. Mixtures of PP-H and HDPE only melt a little bit faster than pure PP-H. Mixtures of PP H and PMMA showed a completely different behavior than mixtures of PP H and HDPE. The volumetric 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, but all analyzed mixtures melt slower than the pure bulk materials. 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 materials melt much slower than expected, in some cases even slower than the pure materials. Due to this behavior, the modeling of this process is difficult, as mixing rules cannot be applied. The adaption of different existing models delivers promising results.