Highly effective flame retarded TPU introduced by a new phosphorus-containing Schiff base derivative at low addition

Flame retardant thermoplastic polyurethane elastomer (TPU) applied in the field of wires and cables often need a large amount of flame retardant additive. To improve the efficiency, this work prepared a highly effective novel phosphorus-containing Schiff base derivative flame retardant N, N′-bis[1-(salicylidene) methanephenylphosphonic acid]-1,2- ethylene diamine (SMAE), which was successfully synthesized via the addition reaction between Phenylphosphinic acid and N, N'-bis(salicylidene)ethylenediamine. It had great thermal stability ( T 5% , 271 ℃) and char forming ability (29.6 wt. % at 700 °C), which was added to TPU at different additions to improve the flame retardant performance. Results showed that only 4% SMAE increased the limiting oxygen index value of TPU from 22.3% to 32%, passing UL 94 V-0 rating. Meanwhile, the peak heat release rate, average heat release rate and total heat release of TPU also decreased by 3%, 19% and 16%, respectively. The constituent and microscopic morphology of the residual char of TPU samples were observed by Fourier transform infrared spectroscopy and scanning electron microscopy, which indicated SMAE produced phosphinic acid to promote the formation of a more complete and dense char layer for TPU. The pyrolysis gas phase products were recorded by thermogravimetric Fourier transform infrared spectroscopy. The results showed that SMAE could produced PO∙ terminated the chain reaction and released inert gases (carbon dioxide and ammonia) to dilute combustible gases during combustion. Therefore, SMAE can endow TPU with excellent flame retardancy.