Upcycling Virgin and Waste Thermoplastic Polyethylene, Polypropylene, and Their Copolymers into Recyclable, Reprocessable Networks That Fully Recover Cross-link Density and Provide Sustainability Advantages

Polyethylene (PE), polypropylene (PP) and their copolymers account for >50% of global polymer production. Although thermoplastic PE and PP can be melt-state reprocessed, less than 10% of waste PE and PP is effectively recycled. PE can be permanently cross-linked to enhance its properties, leading to a commercial cross-linked PE, often called PEX, that is produced at levels comparable to those of polystyrene and poly(vinyl chloride); however, PEX is not reprocessable. When the reactive process used to make PEX from thermoplastic PE is applied to thermoplastic PP, it results in chain scission instead of cross-linking and thus no cross-linked PP product. We have developed a simple catalyst-free, reactive melt-process that upcycles virgin and waste PE, PP, and related copolymers into reprocessable covalent adaptable networks (CANs). Our CAN process resembles that of the commercial PEX process but with a “drop in” of several weight percent of well-designed dynamic covalent cross-linkers allow for CAN formation from thermoplastic PE, thermoplastic PP, and related thermoplastic copolymers. At high temperatures above the melt transition temperature of PE, the covalent cross-links are dynamic, allowing for melt-state reprocessing. Upon cooling the CANS toward ambient conditions, the cross-link densities and associated properties are fully restored. Compared to their thermoplastic precursors, our CANs made from PE, PP, and their copolymers strongly suppress high-temperature creep, which is weak point of many CAN systems. We will also describe how these polyolefin CANs can address a major environmental Achilles’ heel associated with plastics.