Dual Functionality of MFI Zeolite Nanosheets as a PET Depolymerization Catalyst and a Flame Retardant in Repolymerized Polyurethane

Abstract

In recent years, there has been a growing concern about the environmental impact of postconsumer poly(ethylene terephthalate) (PET) waste. To address this issue, significant attention has been directed toward depolymerization techniques that break down PET into its monomeric and oligomeric components. Among these techniques, the direct conversion of PET waste into high-value polymers stands out as it does not require additional separation and purification steps. This research focuses on the utilization of two-dimensional (2D) zeolite socony mobil-5 (MFI) nanosheets, serving a dual purpose in PET upcycling. These nanosheets act as catalysts for PET depolymerization and subsequently function as flame retardants in the production of remanufactured polyurethane foam (PUF). The MFI nanosheets were produced via a hydrothermal synthesis process and were thoroughly examined in terms of their microstructure and acidic properties. The PET depolymerization process was carried out using these MFI nanosheets as catalysts. Remarkably, the nanosheet-based batch exhibited efficient PET depolymerization at relatively low temperatures. Specifically, PET conversion rates exceeded 70% and 80% after 2 h at 140 and 160 °C, respectively. These conversion rates outperformed those achieved with DES-based catalyst systems at the same temperatures, where the conversion remained below 60%. On the other hand, it was observed that the bis(2-hydroxyethyl terephthalate) (BHET) yield was lower in the zeolite nanosheet batch (<50%) compared to the DES batch (>90%) after a 2h reaction at 180 °C. The resulting reaction products were then used for PUF repolymerization. The remanufactured PUF displayed enhanced thermal stability (indicated by a p-heat release rate of <110.8 W/g and a total heat release of <13.2 kJ/g) as well as improved flame resistance (with limiting oxygen values exceeding 29.9%) when compared to conventional samples. This enhancement in thermal properties was attributed to the unique thermally stable sheet-like structure of the zeolite nanosheets, which effectively inhibited heat and flame propagation. These findings highlight the high potential of zeolite nanosheets in advancing PET upcycling methodologies, demonstrating their dual roles in efficient PET depolymerization and the enhancement of thermal properties in remanufactured PUF.

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