In mechanical recycling, plastics are sorted, washed, re-processed, and eventually introduced into a new life cycle. However, in this new life, they retain characteristics from their previous life and acquire new ones during reprocessing. Extensive research has focused on identifying, understanding, controlling, and mitigating these inherited traits to enhance the engineering properties of recycled plastics. A prime example is the cross-contamination between plastics forming immiscible blends, which is unavoidable due to sorting limitations. Yet, our understanding remains limited regarding how much each polymer retains from its previous life cycle and how significantly this affects its recyclability. A straightforward answer to these questions can be provided by considering the chemistry of degradation occurring during processing and use. These mechanisms often generate reactive or dormant molecular species that can be reactivated during reprocessing or reuse. The complexity increases as these species accumulate and interact, leading to either synergistic or antagonistic effects. Beyond chemical changes, the physical and structural inheritance of polymers is a less-explored but important factor. Even fundamental properties, such as crystallization behavior, can differ significantly between virgin and recycled polymers, with largely unknown consequences for recycling performance.
In this presentation, I will explore some of these challenges and demonstrate how fundamental polymer science can be applied to tackle them. Specifically, I will highlight how rheological methods, combined with computer simulations, provide a powerful approach to understanding and improving the behaviour of recycled polymers.

Multicyclic poly(dimethylsiloxane)s (multicyclic PDMSs) with varying average numbers and molecular weights of cyclic units were synthesized via cyclopolymerization of α,ω-dinorbornenyl-functionalized PDMSs.[1,2] Multicyclic PDMSs were usually found to have a viscous liquid-like appearance at room temperature, similar to linear PDMSs. However, some multicyclic PDMSs exhibited an elastic rubber-like appearance, even though they were soluble in organic solvents. To further investigate this fascinating phenomenon, we studied their viscoelastic properties with frequency sweep measurement. Multicyclic PDMSs exhibited typical terminal relaxation behavior when the absolute weight-average molecular weight of the cyclic units was 44,000 or less. On the other hand, when the molecular weight of the cyclic units exceeded 59,000, multicyclic PDMSs exhibited higher storage modulus than loss modulus within the experimentally accessible frequency range despite the absence of chemical cross-linking. Their loss tangent (tan δ) ranged from 0.10 to 1.0, indicating that they behaved as viscoelastic solids close to critical gels on the experimental time scale. Furthermore, we compared multicyclic PDMSs by fixing the molecular weight of the cyclic units at 59,000 and varying the number of cyclic units from an average of 2 to 26. Consequently, the critical gel-like viscoelastic behavior was observed when the average number of cyclic units reached 4 or more. These results suggested that intermolecular interactions between high-molecular-weight multicyclic units contributed significantly to their specific viscoelastic properties.

Polypropylene (PP) and high density polyethylene (HDPE) are widely used in packaging and are commonly co-present in household waste streams. Due to their similar densities, conventional recycling methods often struggle to separate them, resulting in recycled PP containing HDPE, forming a blend rather than a single pure polymer. These immiscible blends exhibit distinct and poorly understood structure-processing-property relationships compared to virgin PP. The result of this lack of knowledge is an aversion in the market towards such recycled polypropylene.

This study investigates thermal, rheological, and mechanical properties of PP/HDPE blends of various compositions under controlled processing conditions, utilizing extended dilatometry [2] for the first time to imitate industry-relevant, realistic processing conditions. Tensile testing reveals that increased HDPE content reduces strength and elongation under quiescent cooling conditions. However, when cooled under shear flow conditions, the strength of the blends is significantly enhanced, even exceeding that of virgin PP, due to highly oriented crystal structures and rapid solidification as revealed by X-ray diffraction.

These findings indicate that (recycled) PP with high HDPE content is not suitable for replacing virgin PP in weak- or no-flow manufacturing processes, such as compression molding. However, these materials can achieve higher economic value than virgin PP when used in high-flow processes, such as injection molding, where they exhibit greater strength than virgin materials under the same flow conditions.

This structure-property relationship advances the utilization of recycled PP without the need for complex separation strategies, by just tailoring the material formulation and manufacturing process, thereby promoting sustainable manufacturing.

The increasing accumulation of plastic waste necessitates advancements in recycling methodologies. This study examines the molecular degradation of two blow molding grade high density polyethylenes (HDPE), Ziegler-Natta catalyzed HDPE (ZN-HDPE) and Phillips-catalyzed HDPE (P-HDPE) during mechanical recycling in a twin-screw extruder. Although both polymers exhibit identical melt flow index (MFI), their degradation mechanisms differ significantly. The molecular modifications induced by recycling are analysed using Cross Fractionation Chromatography (1) (CFC), Temperature Rising Elution Fractionation (2) (TREF), small-amplitude oscillatory shear (3,4) (SAOS), and extensional rheology (4). Constitutive rheological modeling is employed to interpret the degradation behaviour and optimise processing parameters such as temperature, recycling time, and screw rotation speed to facilitate HDPE upcycling. The findings indicate that the catalyst type and processing conditions strongly influence molecular degradation, characterized by chain scission, crosslinking, and branching. In P-HDPE, chain scission is predominant at 170 °C, as evidenced by a reduction in zero-shear viscosity. However, at 210 °C, branching and crosslinking occur and lead to an increase in zero-shear viscosity and significant strain hardening in extensional flow. In contrast, ZN-HDPE undergoes a substantial reduction in molecular weight and viscosity at both temperatures, which indicates chain scission at both temperatures. The strain hardening occurs exclusively at high temperatures. The degradation mechanisms are further elucidated through rheological modeling, incorporating CFC data and the Branch-on-Branch constitutive model. These results underscore the critical role of catalyst type in determining the rheological properties of recycled HDPE and provide a framework for optimizing processing conditions to enhance its upcycling.

An optimized extrusion process is desired for both an environmentally friendly and economically sustainable recycling process. The aim of this work is to simulate the melt conveying zone of a single screw extruder when using contaminated polymers instead of commonly used pure materials, to optimize a mechanical recycling process and to reduce the number of measurements needed for rheological input data by using mixing rules. Polypropylene (PP) is blended with a polyamide 12 (PA 12) grade and another PP grade to introduce impurities in the material. The blends are subjected to extrusion experiments in a lab scale single screw extruder with pressure and temperature sensors along the barrel. A simulation method using representative shear rate values is applied to calculate the measured pressure and temperature changes throughout the melt conveying zone. The rheological input data for the simulation is taken from high-pressure capillary rheometry measurements but also substituted with values derived from mixing rules. Results show that the application of the shear viscosity through mixing models yields pressure values similar to those measured in the experiments.

The application of additive manufacturing to the design of inserts in injection molding (IM) application is an advantageous path for the development of new products and for their customization. Material Extrusion (MEX) from filament technique was used to produce samples suitable for a preliminary characterization. Ten different commercial materials were considered for application as IM inserts. Thermal and thermo-mechanical characterizations were used and compared to the temperature and pressure predicted from a simulation software and have allowed the screening of the materials. 9 materials were considered suitable for the injection molding of LDPE and 3 of PP. The inserts were then fabricated and the IM objects were qualitatively evaluated. All selected materials successfully underwent the IM process, producing at least 10 samples without significant issues and the quality of the objects was evaluated based on their weight. The validation of the simulations was also performed by comparing the temperatures recorded by a thermal imaging camera during the molding process with respect to those expected in the simulations. Furthermore, the thermal imaging camera has been used to evaluate the need to increase the cycle time for IM due to the overheating of the inserts. The preliminary tests selected and presented are therefore decisive and sufficient for the choice of materials. The MEX from filament proves to be an effective and promising methodology for the production of inserts suitable for small batches of injection molded products at a reduced cost and in a limited time.