Facile synthesis of an efficient phosphonamide flame retardant for simultaneous enhancement of fire safety and crystallization rate of poly (lactic acid)

[Display omitted] •An efficient phosphoramide flame retardant (NTPA) was successfully synthesized through nucleophilic substitution reaction.•NTPA endowed PLA with excellent fire retardancy and high crystallization rate.•NTPA exerted flame-retardant effect both in condensed and vapor phase.•PLA/NTPA composites maintained outstanding mechanical properties and transparency. Polylactic acid (PLA) was regarded as the most prospective sustainable bioplastic in the field of engineering applications, but its slow crystallization and strong sensitivity to fire severely restrict its large-scale applications. In this research, a multi-functional additive N,N′,N″-(nitrilotris(ethane-2,1-diyl))tris(P,P-diphenylphosphinic amide) (NTPA) was successfully synthesized through the substitution reaction of N,N-bis(2-aminoethyl)ethylenediamine and diphenylphosphinyl chloride and its chemical structure was determined. NTPA was introduced into PLA to simultaneously enhance the fire safety and crystallization rate of PLA composites and the corresponding crystallization behaviors, fire retardancy, flame retarded mechanism and mechanical performance of PLA composites were systematically investigated. With the introduction of mere 2.5 wt% NTPA, the PLA/NTPA composites gained UL-94 V-0 grade and its LOI reached 28.5%. The remarkable fire safety of PLA/NTPA composites was mainly assigned to the inhibition function of benzene and phosphooxygen radicals generated from the pyrolysis of NTPA and the dilution effect of inert gases in gas phase. Besides, an uniform and compact char layer exerted a certain barrier effect in condensed phase. Thus, the total heat release and average effective heat combustion of PLA/NTPA composites were evidently declined by 12.2 and 9.8% and the flame retardant index was increased by 38% with only 2.5 wt% NTPA loading. Meanwhile, NTPA was a good nucleating agent for PLA matrix and efficiently enhanced the crystallinity and crystallization rate of PLA. Interestingly, PLA/NTPA composites still retained superior transparency and mechanical properties. This flame-retardant PLA composites exhibited the enormous application prospect in the field of advanced optoelectronic devices.