Self-extinguishable, intumescent poly(urethane-triazole)s as halogen-free flame retardant materials via bulk polymerization of azide-alkynes

In the present study, an unexplored class of non-halogenated self-extinguishable and intumescent phosphorous-based poly(urethane-triazole)s (P-PUTs) were achieved through the bulk polymerization of azide-alkynes notably in absence of catalyst. A series of azide-functionalized urethane monomers (tetra azides) were synthesized by the reaction of 1,3-diazido-propan-2-ol with commonly used, but three structurally different diisocyanates, viz, aliphatic, alicyclic, and aromatic. Concurrently, a dialkyne, phenyl di-prop-2-ynyl phosphonate was synthesized and co-polymerized with these urethane tetrazides through bulk polymerization at 80 °C. The curing kinetics of the polymerization was monitored by differential scanning calorimetry (DSC) under non-isothermal conditions with different heating ramps. Thermal behaviour, elemental and morphological properties of the synthesized P-PUTs were comprehensively investigated employing thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), and energy-dispersive X-ray spectroscopy (EDX), respectively. The flame retardant properties of P-PUTs were assessed by UL–94 vertical burning test, limiting oxygen index (LOI) and lab-scale flammability test. All the P-PUT compositions were found to self-extinguish immediately after the removal of flame with a V-0 rating of UL-94 V, the LOI value (vol%) are 26.3 %, 28.2 % and 29.0 % for H2P-PUT, I2P-PUT and T2P-PUT, respectively and the corresponding char yields were 26.03 wt%, 25.76 wt% and 39.58 wt% along with significant intumescence. This apart, the films with a transparency of 96.5 %, 96.7 % and 94.3 % in the visible wavelength for H2P-PUT, I2P-PUT and T2P-PUT, respectively, can possibly make these P-PUTs materials attractive as fire retardant top coats on wood substrates.