Title: Towards Circular Use of Thermoplastic Polyurethanes

Abstract: Thermoplastic polyurethane (TPU) is the 6th most used synthetic plastic in the world with an annual production of 26 million metric ton in the year 2022. It is segmented into crystalline hard segments, composed of diisocyanates and chain extenders (short-chain diols), and flexible, amorphous soft segments that consist of a polymeric alcohol (polyol). The widespread use of TPU has resulted in substantial waste in landfills and oceans, necessitating effective recycling solutions to address environmental concerns. The majority of TPU that gets recycled, is recycled physically, which degrades its physical properties. While chemical recycling is an attractive alternative, only recovery of the polyol has been demonstrated in the literature to-date. Depolymerization of TPU with recovery of all constituent components for truly circular reuse has been elusive so far. This PhD project aims to address this gap by proposing a holistic chemical recycling approach to achieve true circularity.

We are proposing a four-step process involving: 1) catalytic depolymerization of TPU using capping agents, 2) separation of the capped hard and soft segments, 3) thermal dissociation to recover the capping agent and (uncapped) segments, and 4) repolymerizing new TPU from the recovered hard and soft segments. The project is divided into five aims, with the first four focusing on each step of the proposed mechanism and the fifth aim focusing on a comparative life cycle assessment (LCA) of the combined process versus a traditional physical recycling process.

Aim 1 focuses on catalytic depolymerization of TPU. First, we screen for efficient catalysts using a model system. Next, we target depolymerization of TPU at the linkage between hard and soft segments to separate and recover both structures based on their different physical properties. This poses a chemical selectivity challenge as the bonds between the chain extender and isocyanate within the hard segment, and the bond between the polyol and isocyanate at the hard segment-soft segment linkage are chemically identical. We therefore explore a novel approach that utilizes – usually undesired - mass transport limitations that arise due to the crystallinity of the hard segment to induce selectivity in the depolymerization reaction. Aim 2 addresses the phase separation of capped hard segment and polyol. Due to the selective depolymerization, the capped hard segments are solids, and therefore can be filtered out. The separation of the liquid phase (i.e. polyol, catalyst, solvent, and excess capping agent) will be studied as part of the proposed work. Aim 3 focuses on the thermal decapping of the capped hard segment, specifically, on identification of trends in deblocking temperatures to enable targeted “design” of suitable capping agents. Aim 4 aims to repolymerize new TPU from recovered hard segment and polyol, which will be studied as part of the proposed work. Lastly, Aim 5 will perform a comparative LCA to find the environmental impacts associated with a traditional physical recycling process and the proposed chemical recycling process. We also test for changes in the global warming potential as a function of changing solvents, changes in temperature, etc. as a guide for our proposed process.

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

Dr. Daylan Sheppard

Senior Urethane Synthesis Chemist, The Lubrizol Corporation