A Complementary DSC-NMR Methodology for Elucidating Isocyanurate Formation Pathways in Polyurethanes

Abstract

Polyurethanes (PU) are a versatile class of polymers whose properties can be tuned through chemical composition and network structure. Among these materials, rigid PU foams, formed from polyols and isocyanates in the presence of blowing agents, are widely used in construction and refrigeration applications due to their excellent insulating properties. Their fire resistance is enhanced through incorporation of isocyanurate linkages formed via cyclotrimerization of excess isocyanate catalyzed by potassium carboxylates, yielding polyisocyanurate (PIR) foams. A detailed understanding of the reaction sequence and associated heat release during PIR formation is essential for optimizing foam performance. Herein, we present an integrated approach combining differential scanning calorimetry (DSC) with nuclear magnetic resonance (NMR) to correlate enthalpic changes with reaction progress during isocyanurate formation. By quenching reactions at selected temperatures and analyzing mixtures by NMR, we constructed a detailed reaction profile for a mono-functional, low-molar-mass model isocyanate-alcohol system catalyzed by potassium acetate (KOAc) and potassium 2-ethylhexanoate (K-2-EH). Rapid carbamate formation occurs at low temperatures, followed by allophanate and isocyanurate formation. Allophanate acts as a key intermediate in the formation of isocyanurate till ~70 °C, above which it undergoes catalytic degradation to yield isocyanurate and carbamate. Two exothermic events observed in DSC coincided with changes in the reaction mechanism: first at 60-70 °C, arising from allophanate accumulation and concurrent isocyanurate formation; second at 80 °C, from catalytic allophanate degradation. We envisage that the combined DSC-NMR approach can provide a practical platform for studying polymer-forming systems under bulk conditions, providing insights into reaction and catalysis mechanisms.