The Impact of Reprocessing with a Quad Screw Extruder on the Degradation of Polypropylene

Abstract

During mechanical recycling, polypropylene typically is reprocessed using a single- or twin-screw extruder. The degradation of polypropylene during this reprocessing reduces the polymer’s molecular weight and, consequently, limits the performance of the recycled resin. This work investigated the impact of a quad screw extruder (QSE), which has greater free volume, on the reprocessing of an impact copolymer polypropylene. To mimic the recycling process, the polypropylene was subjected to three processing cycles using a QSE and a comparable twin-screw extruder (TSE) operated at three screw speeds. The reprocessed materials were characterized for their rheological, morphological, and mechanical properties. For both extruders, increasing the number of reprocessing cycles and the screw speed resulted in higher melt flow indices, decreases in zero-shear viscosity, and shifting of the crossover points for the storage and loss moduli, which indicate reductions in the molecular weight and narrowing of the molecular weight distribution of the polypropylene. The QSE exhibited greater reductions in molecular weight compared to the TSE, probably due to the higher stresses associated with the three intermeshing points along its screws. Reprocessing caused a significant reductions in the Izod impact strength of the reprocessed polypropylene, which correlated with reductions in the particle size and particle size distribution of the dispersed rubbery phase in the polypropylene during reprocessing.

Impact of Multiple Reprocessing on Properties of Polyhydroxybutyrate and Polypropylene

“…In the case of PP, the images (Figure 8a,b) are almost identical too, as also reported by Tochacek et al [47,52], who under increased magnification and after extraction with heptane were able to compare the size of the rubbery phase and noted there was no decrease in particle size with multiple processing. This observation by Tochacek et al was in contradiction to the work performed by Alotaibi et al [49] where the ethylene propylene rubber (EPR) of heterophasic PP was visible and, with each reprocessing step, the EPR steadily decreased in size, which is also similar to the observations of Bouaziz et al [73].…”

“…As a lot of work has been conducted on the reprocessing of PP already and only four papers from Alotaibi et al [49], Hermanova et al [50] and Tochacek et al [51,52] dealt with the recycling and characterization of impact copolymer polypropylene (ICPP), we decided to compare PHB with ICPP. Tochacek et al [47] studied the degradation of ICPP over multiple extrusion cycles and concluded that the degradation behavior is similar to PP as it constitutes around 90% of the mass of the polymer with the effect of the rubbery fraction more pronounced with regard to fracture behavior.…”

“…Alotaibi et al [49] discussed the differences in reprocessing ICPP with a quad screw extruder (QSE) vs. a twin screw extruder (TSE) at three different screw speeds. As the number of cycles progressed and the screw speeds increased, melt temperature had two trends.…”

Applying the Taguchi Method to Improve Key Parameters of Extrusion Vacuum-Forming Quality

“…Examples include beverage cup lids, lunchboxes, frozen liner boxes, and food packaging materials such as plastic cutlery. Extrusion molding [ 9 ], which utilizes an extruder [ 10 ], involves thoroughly mixing and heating plastic fiber or sheet-like polymer raw materials to form sheets. These molten sheets or film materials are then continuously heated and vacuum-formed through aluminum alloy molds [ 11 ] or shaping modules [ 12 ], followed by cooling and cutting to produce various products.…”

Simultaneous Modification of Properties Relevant to the Processing and Application of Virgin and Post-Consumer Polypropylene

“…Various studies in laboratory environments were already carried out to address this topic. The main results are an increasing MFR, a decreasing ductility, and a drop in the oxidation induction temperature, which corresponds to a decreased molecular weight [ 12 , 13 , 14 , 15 , 16 , 17 ].…”