Injection moulding of post-consumer recycled polyolefins

Closing the material loop in plastics processing is becoming an increasingly important challenge throughout the plastics industry. Currently, only a small proportion of plastic waste generated is disposed of properly and circularly reused. Polyolefin recyclates from household waste, known as post-consumer recycled (PCR) materials, exhibit a high degree of material inhomogeneity due to the variety of products in the respective waste stream. This can lead to instabilities in the injection moulding process and inconsistent part properties. The main causes are (in)organic contamination, as well as foreign polymer contents, the composition of which varies seasonally and regionally. These variations in material properties make it difficult to transfer conventional processing and development strategies directly onto PCR materials. Given the regulatory requirement to increase the use of recycled materials, particularly within the European Union, it is necessary to conduct an in-depth analysis of the interactions between material composition, process control and part properties. This study identifies three key areas of research differentiating PCR from virgin material. Firstly, PCR polyolefins exhibit atypical hygroscopic behaviour. An increase in residual moisture content is detectable with increasing storage time, affecting viscosity and processing behaviour. Drying processes such as hot air or infrared drying significantly reduce residual moisture, but the results show that hot air drying can cause thermo-oxidative ageing of the PCR material. Secondly, the varying composition of HDPE PCR material is analysed in the context of batch-to-batch fluctuations. The presented results highlight that particularly the foreign polymer content (PP in HDPE and vice versa) has a significant influence on the rheological and crystalline behaviour of the recyclate. One approach to compensate for seasonal fluctuations is to add virgin material to the PCR material. For this reason, the applicability of mixing rules to process and component key figures is evaluated. Finally, the influence of the immiscibility of PP and PE on the resulting phase morphology after injection moulding is analysed. Based on design recyclates, a methodology for morphological evaluation is developed and applied to real recyclates. In this context, the geometry-dependent influence of compatibilisers is also determined. The gained insights can be used as a basis for adapted product development strategies and allow the derivation of design guidelines for targeted integration of PCR material into injection moulding processes.