Flame‐retardant, phosphorous‐based polyurethane triazoles via solvent‐free and catalyst‐free azide–alkyne cycloaddition and their cure kinetics

Abstract

A novel class of phosphorous‐containing polyurethane triazoles (PUTs) with self‐extinguishing property is reported. Initially, a set of new urethane diazide monomers were synthesized by reacting diisocyanates (DI) (isophorone (IPDI), hexamethylene (HDI), and toluene (TDI)) with 2‐azidoethanol and characterized by FTIR, 1H NMR, 13C NMR, and ESI‐MS analysis. Later, the corresponding PUTs were synthesized via azide–alkyne cycloaddition of urethane diazides with triprop‐2‐ynyl phosphate under solvent‐free and catalyst‐free conditions at 80°C via thermal polymerization. Cure kinetic study of the thermally induced polymerization of PUTs was performed to correlate with isocyanate functionality. The activation energies (Ea) of the PUTs derived from nonisothermal multiheating rate DSC tests were fitted to Borchardt–Daniels model. The Ea's were found to be proportional to heating rates for all PUTs and confirmed optimum percentage conversion at lower heating rates. The experimental findings were found to corroborate well with Borchardt–Daniels model. The PUTs were characterized by FTIR, TGA, DSC, MCC, LOI, lab‐scale flame tests, and EDX analysis. All PUTs were self‐extinguishable, but TD‐PUT with aromatic functionality (TDI‐based) demonstrated superior extinguishing performance with lowest total heat release (6.11 kJ/g), peak heat release rate (42.04 W/g), heat release capacity (85.59 J/g K), and 31% LOI comparatively. Lab‐scale flame tests on PUTs confirmed their self‐extinguishing property with little or no smoke evolution. Such PUT resins can be blended with conventional polyurethane coatings for fire‐retardant applications.