Recycling of Polyurethane Waste: Facile Hydrothermal Conversion Using Acidic and Basic Additives

Polyurethane (PU) is a widely utilised plastic material due to its versatile properties. The chemical recycling, especially by hydrothermal treatment, is an effective way to achieve the circular use of PU. This article reports the results of hydrothermal treatment of PU with and without the use of acidic and basic catalysts. Both non-catalytic and catalytic approaches showed that PU could be depolymerised to the monomers using hydrothermal treatment. The use of a catalyst improved PU conversion and 2,4-toluenediamine (TDA) yield. An organic amine showed better catalytic activity than inorganic base NaOH, inorganic acid H₂SO₄, and organic acid acetic acid. Among the catalysts tested, the organic amine ethylenediamine exhibited the highest activity, achieving a TDA yield of 13.6 wt% and a PU conversion of 28.2% at 180°C. Organic bases outperformed inorganic acids and bases, such as H₂SO₄, acetic acid, and NaOH, which is attributed to their ability to form ionic interactions with PU-derived zwitterions and their uniform distribution across vapour and liquid phases under vapour-liquid equilibrium.