Thermogravimetric studies, kinetic modeling and product analysis of the pyrolysis of model polymers for technical polyurethane applications

The pyrolysis of four kinds of common polyurethanes comprising a rigid and a flexible foam, a cast elastomer, and a thermoplastic polyurethane (TPU) was investigated by thermogravimetric analysis and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). All samples are based on methylene diphenyl diisocyanate (MDI), with polyether polyols as the soft segment of the foams and polytetrahydrofuran as the soft segment of the cast elastomer and TPU. Each polyurethane degrades in a two-step manner with a high correlation between the polymer structure and the mass loss in each step. Two kinetic modeling approaches based on either parallel or consecutive reactions were successfully applied to describe the pyrolytic degradation. The virgin polymers and pyrolysis intermediates obtained from thermogravimetry with isothermal segments at different temperatures of 350–450 °C were investigated by ATR-FTIR. Both foams exhibit retention of feedstock nitrogen in a carbonaceous intermediate while no significant residue formation and nitrogen retention are observable for cast elastomer and TPU pyrolysis. Aromatic amines in the foam pyrolysis intermediates point to the formation of a secondary polymer originating from the urethane segments in the course of the degradation. In Py-GC/MS, the foams yield nitrogen-carrying fragments of the MDI. MDI and 1,4-butanediol are regenerated from TPU pyrolysis, indicating different degradation pathways. CE pyrolysis also releases 1,4-butanediol but no nitrogen-carrying compounds are detectable. This work demonstrates the individuality of polyurethane materials in terms of their pyrolysis behavior and released products. Potential high-value products are identified in pyrolysis at laboratory scale. The findings of this study underline the need for a comprehensive examination of polyurethane pyrolysis with differentiation of polyurethane composition and morphology to optimize technical scale polyurethane pyrolysis.