P, N-decorated halloysite nanotubes for flame retardancy enhancement of polyamide 6/aluminum diethylphosphinate

•P, N-decorated HNT was prepared via a simple two-step method.•Flame retardancy of PA6/aluminum diethylphosphinate was greatly improved with the addition of modified HNT.•Heat shielding effect by an intumescent protective char layer is the main cause. Flame-retardant functionalized nanomaterials have attracted much research interests due to the combined advantages of organic and inorganic components. In the present study, P, N-decorated halloysite nanotube (MA-PPA@HNT) was prepared via simple steps of phenphosphinic acid intercalation and subsequent electrostatic interaction with melamine. Compared to unmodified HNT, the incorporation of MA-PPA@HNT into polyamide 6 (PA6)/aluminum diethylphosphinic acid (AlPi) composite results in significant improvement of flame retardancy. At equal HNT loading of 2wt%, the UL-94 rating grade of PA6/AlPi/MA-PPA@HNT composite was V-0, whereas that of PA6/AlPi/HNT composite V-2. And compared with PA6, there is a decrease of peak heat release rate (pHRR) and total heat release (THR) by 72.6% and 17.4%, respectively. The flame retardant mechanism was analyzed by cone calorimetry, gas volatility and characterization of char residues. It is considered that MA-PPA@HNT plays the critical role of promoting the formation of P, Si-rich intumescent char in the condensed phase and enhancing the barrier effect for heat and mass transfer.