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

Controlling\nthe distribution of cross-links within a polymeric\nnetwork is challenging using conventional methods, which often involve\nrandom chain scission to achieve a higher gel fraction. Here, we engineer\na molecule to facilitate “homo-cross-linking”, enabling\nprecise control over the cross-link distribution and micro phase separation.\nEstablishing a closed-loop circular economy within the plastics or\npolymer industry is imperative. However, efficiently managing post-consumer\nrecycled (PCR) plastics, including their collection, sorting, and\nprocessing, remains a significant challenge. While dynamic cross-linking\nof virgin polypropylene (PP) has advanced plastic upcycling, its application\nto PCR PP is limited. This study presents a simple and scalable approach\nto convert PCR PP into cross-linked PCR PP, enhancing their mechanical\nstrength and rheological properties and enabling circular upcycling.\nUtilizing a designer dynamic cross-linker, imine installed castor\noil (iCO), we establish a dual dynamic covalent adaptable network\n(CAN) that bridges fragmented maleated-PP chains within the PCR PP\nmatrix besides rendering “homo-cross-linking” in the\ncross-linked polymer. This local “cross-link distribution”\nwithin the “global” matrix (PCR PP) overcomes challenges\nin upcycling PCR PP, which often undergoes global chain scission during\nnetwork formation, as observed in other reports. Even at higher cross-linker\nconcentrations (up to 30 wt %), there is minimal impact on percentage\ncrystallinity, promoting amorphous miscibility within the PCR PP and\nno significant phase separation which has been observed by SAXS and\nSEM analysis. Cross-linked PCR PP exhibits superior dimensional stability\nand re-processability, retaining over 90% of their mechanical properties\nafter three rounds of rigorous recycling involving extrusion followed\nby injection molding techniques. The ability to transform waste PP\ninto a thermoformable material with reprocessing capabilities and\nfavorable thermomechanical properties expands upcycling opportunities,\nthereby advancing circularity within the industry.