Waterborne polyurethane conjugated with novel diol chain-extender bearing cyclic phosphoramidate lateral group: synthesis, flammability and thermal degradation mechanism

A diol bearing a cyclic phosphoramidate pendant group, namely 2-(5,5-dimethyl-2-oxo-2 lambda(5)-1,3,2-dioxaphosphinan-2-ylamino)-2-methyl-propane-1,3-diol (PNMPD) was synthesized and characterized by FTIR and NMR. Subsequently, PNMPD was applied as a flame-retardant chain extender to prepare phosphorus-nitrogen containing waterborne polyurethane (PNWPU), and the influence of PNMPD on the hydrolysis, thermal and flame-retardant properties of PNWPU was investigated. The fabricated PNMPD-based PNWPU shows controllable mechanical properties and maintains the good hydrolysis-resistance property of polyurethane. Thermal stability and flammability analysis demonstrate that though the covalent conjugation of PNMPD induces a slight thermal destabilization effect, it efficiently promotes char formation in PNWPU, as a result, a 27.2% limiting oxygen index (LOI) value and a UL-94 V-0 rating can be achieved with only 12 wt% PNMPD incorporated. Compared with WPU, the peak heat release rate (PHRR), total heat release (THR), peak smoke production rate (PSPR) and total smoke release (TSR) of PNWPU-12, evaluated with cone calorimetry, are decreased by 44.7%, 39.0%, 42.9% and 36.1%, respectively. In addition, the flame retarding mechanism of PNWPU was comprehensively investigated by SEM microscopy, EDX analysis, real time FTIR and TG-FTIR, and results elucidate that these notable reductions in fire hazards are probably attributed to the formation of polyphosphoric acid-rich rugged and intumescent char in the condensed phase, which behaves as a labyrinth effect to effectively inhibit the transmission of heat, oxygen and volatile fragments from entering into the flame zone and shield the underlying PNWPU matrix against flame.