Salen based Schiff bases to flame retard thermoplastic polyurethane mimicking operational strategies of thermosetting resin

摘要

A classical Schiff base N,N'-bis(4-hydroxysalicylidene) ethylenediamine (L2), a member of the Salen group that has been introduced as a non-phosphorus and non-halogen flame retardant in thermoplastic polyurethane (TPU) is found to operate in an intriguing way to fire retard the material. L2 blended with TPU significantly improves the inherent flammability of TPU without a synergist. A plausible mechanism including intermediates in the reinforcement of this elastomer towards flame retardancy is elaborated in line with the char forming abilities of this beta-resorcylaldehyde based Schiff base. Conclusions were drawn based on the input from thermal, spectroscopic, microscopic and operando spectroscopic techniques. While its un-substituted counterpart, N,N'-bis(salicylidene) ethylenediamine (L1) lags behind in performance, its structural isomer N,N'-bis(5-hydroxysalicylidene) ethylenediamine (L3) is equally efficient but exhibits another mechanism of action. The performance of L2-TPU is found to be mainly in a condensed phase and is due to decoding of intrinsic cross-linking ability of the key building block of the additive i.e. resorcinol via structural transformation over a range of temperature. This results in the methylene bridged phenolic resin type material having high temperature stability. The process is also found to interfere with TPU unzipping process delaying its thermal degradation and favoring extensive cross-linking promoting char formation with insulative properties.