The production of conventional crosslinked polymer networks and their composites, i.e., thermosets and thermoset composites, occurs at a rate of many billions of kilograms of polymer annually. Unfortunately, thermosets cannot be melt-reprocessed into moderate- to high-value products because permanent crosslinks prevent melt flow. We have developed seven simple one-step or two-step chemistries to produce networks and their composites with dynamic covalent crosslinks that are robust at use conditions but allow for chemical recycling involving melt-state reprocessing. Importantly, we have developed four approaches for melt-state processing or reprocessing of polymer networks and network composites made by step-growth polymerization, including those for polyurethane, polythiorurethane, polyhydroxyurethane or non-isocyanate polyurethane (NIPU), and non-isocyanate polythiourethane. The non-isocyanate cases are of particular interest for sustainability as isocyanates are toxic. These four approaches are the subject of my talk. As developed, these approaches allow for full crosslink density recovery of networks after multiple reprocessing steps. An "Achilles' heel" has been identified regarding the application of dynamic covalent networks, i.e., such networks are subject to creep at elevated or sometimes even room temperature, which is often highly undesirable. We have successfully addressed this issue via some of these dynamic chemistries. Additionally, because conventional polyurethane networks are commonly used as foams, we have explored ways to make self-blowing foams from non-isocyanate foams and have obtained NIPU foams in as little as 20 min of reaction/processing time. Implications will also be discussed for the potential benefits of such chemistry for improving performance independent of sustainability concerns.

Dynamic covalent step-growth chemistries for sustainable chemical recycling of isocyanate-based and non-isocyanate polyurethane networks and their nanocomposites: Transforming thermosets into thermoplastics

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ACS Fall 2022

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