Solid-State Reprocessing of Thermoset Polyurethanes: Top-Down vs Bottom-Up Approaches

Abstract

Polyurethane (PU) is the sixth largest class of polymers by utilization, and thermoset PUs (including foams and elastomers) account for over 97% of all PUs. Unfortunately, traditional thermoset PUs are non-recyclable due to their cross-linked structure, leading to over 90% of these plastics being incinerated or deposited in landfills. To tackle end-of-life challenges of thermoset PUs, researchers have investigated thermo-mechanical solid-state reprocessing techniques to enable cost-efficient short-loop recycling, which directly converts waste plastics into final or semi-finished products. In this review, we analyze literature studies on solid-state recycling of thermoset PUs and categorize these studies as top-down or bottom-up approaches. In the top-down approach, additives are incorporated into thermoset PU waste to promote dynamic carbamate bond exchange and enable thermo-mechanical reprocessing. In the bottom-up approach, new thermosets are designed from novel monomers with dynamic linkages that make these polymers inherently reprocessable through thermo-mechanical processes. We analyze these approaches based on key parameters, such as reprocessing technique and temperature, the robustness of the recycling process, the toxicity of reagents, and the ability to maintain the mechanical properties of PUs after reprocessing. Future studies should place a collective emphasis on all these parameters as they dictate the practicality and scalability of the recycling approaches and their potential to address thermoset PU recycling challenges. Overall, we suggest that the top-down approach is better suited for the short to medium term (<10 years) since it provides an end-of-life solution for existing PU waste. On the other hand, the bottom-up approach provides the ultimate solution as it introduces innovative PU materials that are recyclable by design. However, we believe bottom-up approaches are better suited for the long term (>10 years), given the limitations in the current infrastructure for collecting, sorting, and recycling specific streams of plastics.