Recycling multilayer polymers: Unlocking the impact of thermal treatment on rheological properties and mechanical performance
Résumé
The rheology of recycled multilayer polymer materials is a highly significant topic as it provides a deeper understanding of their mechanicalbehavior, incompatibility phenomena, and processability. These multilayer systems, composed of several layers with distinct properties, are widelyutilized across various industries, including food packaging, medical devices, and agriculture. In this study, model multilayer polymer films basedon PP/PP-g-AM/EVOH/PP-g-AM/PE were fabricated using a coextrusion process and subsequently recycled multiple times through thermaltreatment using a twin-screw extruder. This thermal treatment altered not only the materials' color but also their structural behavior and rheologicalproperties. Small-Amplitude Oscillatory Shear (SAOS) rheology measurements were conducted before and after the recycling cycles for themultilayer systems, including neat polymers and multilayers with varying concentrations of the EVOH barrier layer. The study investigated theinfluence of thermal treatment on rheological behavior, focusing on thermal stability, viscoelastic properties, and interfacial phenomena.Additional experiments provided valuable insights into the evolution of molecular weight under thermal exposure and its effects on mechanicalproperties. Particular attention was given to thermo-oxidative degradation mechanisms in PP, characterized by ß-scission, as well as thecrosslinking behavior of EVOH, both of which significantly impact the structural integrity of multilayer systems. Shear rheology proved to behighly sensitive to the structural changes induced by these mechanisms. Furthermore, the morphological properties of the recycled blends werenotably influenced by the pre-shear applied prior to SAOS measurements. Keywords: Rheology, coextrusion, recycling, multilayer, degradation,crosslinking, viscoelasticity.