(368bm) Towards Circular Use of Thermoplastic Polyurethanes

Polyurethanes are the 6^th most used synthetic polymer in the world with an annual production of 26 MMT in the year 2022. Thermoplastic polyurethanes (TPU), are a class of polyurethanes that distinguishes itself by its ability to be melted and reshaped, unlike thermoset polyurethanes. TPUs consist of 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 plastics, including TPU has resulted in substantial waste in landfills and oceans, necessitating effective recycling solutions to address environmental concerns. Currently, only a small fraction of TPU is recycled, and this recycling is exclusively based on mechanical recycling which leads to degradation of its functional properties. However, while chemical recycling is an attractive alternative, only recovery of a polyol has been demonstrated in the literature to date, and consistent quality remains a challenge. Depolymerization of TPU with the recovery of all constituent components for truly circular reuse has been elusive so far. This work aims to address this gap by proposing a holistic chemical recycling approach to achieve true circularity. The proposed process is comprised of four-steps: 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) hard segments, and 4) repolymerizing new TPU from the recovered hard and soft segments.

Molecular design of capping agents

In the first part of the study, a molecular design approach was employed to select suitable capping agents. A literature review was conducted to survey potential isocyanate “capping agents” based on their decapping (often referred to in literature as deblocking) behaviors. This review guided the selection of capping agents, which were then synthesized into model compounds by reaction with 4,4'-methylene diphenyl diisocyanate (MDI). The deblocking temperatures of the model compounds were analyzed using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA), allowing for correlations between deblocking temperature and physico-chemical features of the capping agents, such as nucleophilicity, pKa, and chain length. The results reveal clear trends, with specific functional groups and molecular characteristics influencing the deblocking temperature.

Catalysts selection for TPU depolymerization

The second part of the study focused on selecting an efficient depolymerization catalyst considering catalyst activity, cost, and toxicity. Tin-based catalysts such as dibutyltin dilaurate (DBTDL) are commonly used in polyurethane synthesis and related exchange reactions. However, organotin-based catalysts show human toxicity, necessitating the exploration of less toxic alternatives. Many other reported catalysts have demonstrated poor performance in depolymerization applications.

Given that depolymerization is the reverse of polymerization, catalysts commonly used in polyurethane synthesis form a suitable basis for catalyst exploration. We began with a thorough literature review to identify catalysts for polyurethane polymerization and related exchange reactions such as transcarbamoylation, transamidation, and transesterification. Potential catalysts were shortlisted based on reported activity, cost, and toxicity, and then tested in a model reaction system involving dibutyl(methylenedi-4,1-phenylene)biscarbamate, with benzyl alcohol as a capping agent. Additionally, this work evaluated the thermal stability of these catalysts and examined which features contributed to their activity. Trends revealed that the electronegativity of the primary metal strongly correlates with catalyst activity.

Selective depolymerization of TPU

Current studies focus on the targeted depolymerization of TPU at the linkage between hard segment (HS) and soft segment (SS) and within the SS in order to separate and recover both structures based on their different physical properties (i.e. solid, crystalline HS, and liquid-phase SS) for repolymerization in a subsequent processing step. This poses a chemical selectivity challenge as the bonds between the chain extender and isocyanate within the HS, and the bond between the polyol and isocyanate at the HS-SS linkage and within the SS are chemically identical. We therefore explored a novel approach which utilizes – usually undesired - mass transport limitations that arise due to the crystallinity of the HS to induce selectivity in the depolymerization reaction. We synthesized model SS, HS, and TPU and performed a thorough solvent screening to identify a solvent that can further enhance selective depolymerization by selectively penetrating the SS. Using this solvent, depolymerization experiments were conducted using an iron acetylacetonate catalyst and benzyl alcohol as capping agent and monitored via H-NMR, GPC, DSC and XRD. Our results indicate successful depolymerization of SS and TPU, while no reaction was observed for the HS, supporting the basic hypothesis of the approach.

Research Interests

1. Plastic Waste Recycling
2. Polymer Chemistry
3. Process Intensification
4. Circularity