Upcycling Post-Consumer Recycled Polypropylene Using a “Tailor-Made” Dynamic Cross-Linker with Controlled “Cross-Link Distribution”

Abstract

Controlling the distribution of cross-links within a polymeric network is challenging using conventional methods, which often involve random chain scission to achieve a higher gel fraction. Here, we engineer a molecule to facilitate “homo-cross-linking”, enabling precise control over the cross-link distribution and micro phase separation. Establishing a closed-loop circular economy within the plastics or polymer industry is imperative. However, efficiently managing post-consumer recycled (PCR) plastics, including their collection, sorting, and processing, remains a significant challenge. While dynamic cross-linking of virgin polypropylene (PP) has advanced plastic upcycling, its application to PCR PP is limited. This study presents a simple and scalable approach to convert PCR PP into cross-linked PCR PP, enhancing their mechanical strength and rheological properties and enabling circular upcycling. Utilizing a designer dynamic cross-linker, imine installed castor oil (iCO), we establish a dual dynamic covalent adaptable network (CAN) that bridges fragmented maleated-PP chains within the PCR PP matrix besides rendering “homo-cross-linking” in the cross-linked polymer. This local “cross-link distribution” within the “global” matrix (PCR PP) overcomes challenges in upcycling PCR PP, which often undergoes global chain scission during network formation, as observed in other reports. Even at higher cross-linker concentrations (up to 30 wt %), there is minimal impact on percentage crystallinity, promoting amorphous miscibility within the PCR PP and no significant phase separation which has been observed by SAXS and SEM analysis. Cross-linked PCR PP exhibits superior dimensional stability and re-processability, retaining over 90% of their mechanical properties after three rounds of rigorous recycling involving extrusion followed by injection molding techniques. The ability to transform waste PP into a thermoformable material with reprocessing capabilities and favorable thermomechanical properties expands upcycling opportunities, thereby advancing circularity within the industry.