Influence of the material history on the properties of recycled glass fiber reinforced polypropylene - impact of screw speed during injection molding

The changing properties of thermoplastic injection molding compounds during mechanical recycling complicate their use in structural lightweight engineering applications. Above that, an impact of the often unknown processing history on the recycled materials can be assumed, since it influences the material structure and degradation. In the present work it is investigated, how the shear load in plastification affects the properties of recycled glass fiber reinforced polypropylene during multiple processing cycles. Based on a design of experiments five processing cycles with different screw speeds are performed resulting in 47 different processing variants (screw speed, processing cycle). The samples undergo mechanical (tensile, impact), rheological (Melt Volume Rate) and morphological (fiber length distribution) tests. It was determined that the influence of the processing cycle is crucial for material degradation and properties. Also the screw speed setting directly affects the processing behaviour since it influences the melt temperature and viscosity. However, for the chosen process parameters, no clear influence of the screw speed on the resulting mechanical, rheological and morphological properties is found. The study suggests that knowledge of the screw speed setting is not essential for circular economy scenarios, as long it does not exceed the common processing recommendations.