Title: Towards Circular Reuse of Thermoplastic Polyurethanes

 Abstract: Widespread use and growing global demand for polyurethanes (PU) are raising environmental concerns and driving the development of sustainable recycling strategies. Physical recycling is cost-effective but degrades functional properties of the polymer. Chemical recycling offers a more sustainable alternative, but efforts to-date have focused largely on polyol recovery. While this addresses the carbon-intensive nature of polyol production, it overlooks substantial environmental and health hazards associated with isocyanates. A broader approach that targets recovery of the entire molecule is therefore essential for advancing sustainable and circular solutions for PU recycling.

This PhD project addresses this gap by proposing a chemical recycling approach to achieve true circularity. It focuses on selective depolymerization of thermoplastic polyurethane (TPU) using transcarbamoylation and blocked isocyanate chemistry. The proposed process involves: 1) catalytic selective depolymerization of TPU using capping agents, 2) separation of the capped hard and soft segments, 3) thermal dissociation to recover capping agent and (uncapped) segments, and 4) repolymerizing new TPU from the recovered hard and soft segments.

A key novelty of this work lies in exploiting typically undesired mass transport limitations arising from structural differences between hard and soft segment for selective depolymerization. Paired with preferential solvent selection, this enables selective cleavage of soft segment urethane linkages while preserving the hard segments. Furthermore, we developed a tool for inverse molecular design of capping agents, using a small experimental dataset of deblocking temperatures combined with computational methods rooted in chemical physics and machine learning. The developed equation gives insight into structure-property relationships and enables prediction of deblocking temperatures. For transcarbamoylation chemistry, catalysts were screened and tested for thermal stability. We also provided some initial insight into molecular features that may influence catalyst performance and could inform future design efforts. Finally, a comparative life cycle assessment quantifies environmental benefits of the proposed approach relative to conventional physical recycling. Altogether, this dissertation advances circular use of TPUs by combining molecular design, process intensification, and sustainability assessment.

Dissertation Chair:

Dr. Götz Veser

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Committee Members:

Dr. John Keith

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Dr. Susan Fullerton

Department of Chemical and Petroleum Engineering, University of Pittsburgh

Dr. Michael Bockstaller

Department of Material Science and Engineering and Department of Chemistry, Carnegie Mellon University

Dr. Daylan Sheppard

Senior Urethane Synthesis Chemist, The Lubrizol Corporation