Our approach for PE is noteworthy because of several reasons: (1) PE can be permanently cross-linked, and is done so commercially, to enhance its properties and expand its applicability, but conventionally cross-linked PE (PEX) is not reprocessable in the melt-state and thus cannot be recycled for high-value use; and (2) our approach can be described just a "drop-in" variation of the commercial process for making PEX, making it well-designed for commercial application. We transform thermoplastic PE into PE CANs via reactive radical-based, melt-state processing with 1 wt% dicumyl peroxide (a radical initiator commonly used in reactive processing of thermoplastic PE to make PEX) and 5 wt% bis(2,2,6,6-tetramethyl-4- piperidyl methacrylate) disulfide (BiTEMPS methacrylate), a dynamic covalent cross-linker. The simple, catalyst-free, one-step reactive process employing dialkylamino disulfide dynamic chemistry has been used to upcycle both commodity and waste thermoplastic PE into thermally stable and reprocessable PE CANs, and the thermomechanical properties of resulting CANs are tunable without sacrificing their recyclability. Low-density PE CANs and high-density PE CANs fully recover cross-link densities and associated properties after multiple reprocessing steps, resist creep deformation at elevated temperature relative to their thermoplastic precursors, and, like PEX but unlike some PE vitrimers, exhibit no phase separation. This novel procedure has also opened the door to the development of CANs based on reactive processing of ethylene-based copolymers as well as cross- linked PE nanocomposites and foams, some of which we shall also describe in our presentation.
Most recently, we have extended a modification of this approach to prepare PP CANs. This is a major advance as permanently cross-linked networks of PP have not been able to be prepared by radical-based reactive processing of thermoplastic PP, because PP preferentially undergoes beta-scission rather than cross-linking when undergoing radical-based processing. As such, a single-step method that produces percolated, dynamic covalent cross-links integrated into PP homopolymer has not been previously demonstrated. We synthesized covalent adaptable networks (CANs) from polypropylene (PP) homopolymers using 180 °C, radical-based, reactive processing with a free-radical initiator, dicumyl peroxide (DCP), and resonance-stabilized, aromatic disulfide cross-linkers, one methacrylate-based and another phenyl acrylate-based. Both cross-linkers yielded networks when reactively processed at 4 wt % with relatively high molecular weight (MW) PP (melt flow index (MFI) = 12) and 4 wt % DCP. The phenyl acrylate-based cross-linker also yielded PP networks at other studied DCP/cross-linker concentrations and with relatively low MW PP (MFI = 35). Notably, our highest cross-link density PP CAN exhibited full recovery of cross-link density after three reprocessing steps by compression molding; that PP CAN also exhibited full cross-link density recovery within experimental uncertainty after reprocessing by melt extrusion.
We have also extended these and related approaches to ethylene-based and propylene-based copolymers and will discuss how the upcycling of both virgin and waste ethylene-based and propylene-based homopolymers and copolymers into reprocessable networks has the potential for both commercial benefit and overcoming recycling and sustainability challenges associated with plastic waste.