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Preparing flame-retardant polyamide 66 (PA66) fibers through melt spinning remains one of the biggest challenges nowadays. In this work, dipentaerythritol (Di−PE), an eco-friendly flame retardant, was blended into PA66 to prepare PA66/Di−PE composites and fibers. It was confirmed that Di−PE could significantly improve the flame-retardant properties of PA66 by blocking the terminal carboxyl groups, which was conducive to the formation of a continuous and compact char layer and the reduced production of combustible gas. The combustion results of the composites showed that the limiting oxygen index (LOI) increased from 23.5% to 29.4%, and underwriter laboratories 94 (UL-94) passed the V-0 grade. The peak of heat release rate (PHRR), total heat release (THR), and total smoke production (TSP) decreased by 47.3%, 47.8%, and 44.8%, respectively, for the PA66/6 wt% Di−PE composite compared to those recorded for pure PA66. More importantly, the PA66/Di−PE composites possessed excellent spinnability. The prepared fibers still had good mechanical properties (tensile strength: 5.7 ± 0.2 cN/dtex), while maintaining good flame-retardant properties (LOI: 28.6%). This study provides an outstanding industrial production strategy for fabricating flame-retardant PA66 plastics and fibers. [ABSTRACT FROM AUTHOR]

• A reactive S- & P-containing flame retardant (PDDDS) was designed and synthesized. • The LOI of PA66 with 2% PDDDS increased to 31.6% from 22.7%. • The PHRR, THR, and TSP of PA66-2%PDDDS were depressed by 47%, 45.7% and 65.1%, respectively. • PA66-PDDDS has excellent flame retardant both in gas phase and condensed phase. A sulfur- and phosphorus-containing flame retardant monomer phosphonic diamide, P-phenyl-N, N′-bis(p-sulfanilylphenyl) (PDDDS) was designed, synthesized, and incorporated into polyamide 66 (PA66) through polycondensation to suppress smoke generation upon combustion and achieve satisfactory mechanical properties, a good UL-94 rating (V-0), and a high limiting oxygen index (31.6%). The increased flame retardancy of the PA66-PDDDS copolymer was ascribed to its decomposition into (i) gas-phase P–O and SO 2 species scavenging free radicals such as H• and OH• and (ii) condensed-phase products catalyzing the dehydration of PA66 to form char layer. Thus, our work presents a strategy for improving the fire safety of PA66 to significantly expand its practical application value. [ABSTRACT FROM AUTHOR]

• A liner charring agent (ETDPCs) containing benzene and triazene is synthesized. • ETDPCs exhibited excellent thermostability and char-forming ability. • LOI value of 30.4% for PP is reached with optimal ratio of FR and ETDPCs (3:1). • Synergistic effect of FR and ETDPCs suppresses the heat transfer and smoke release. • The continuous and dense graphitic char layer endows it with a high barrier effect. A novel linear triazine-based charring agent, N-Ethanolamine Triazine-4,4-diaminodiphenylsulfone-piperazine copolymer hybrids (ETDPCs), was successfully synthesized and was combined with fire-retardant (FR) (mass ratio 2:1 of ammonium polyphosphate (APP) and melamine pyrophosphate (MPP)) to fabricate intumescent flame-retard PP composites at a 20 wt% loading, which exhibited excellent char-forming capacity and thermostability. The value of the limiting oxygen index (LOI) reached 30.4% and the vertical burning test (UL-94) passed V-0 grade for the PP2 sample with a mass ratio of FR to ETDPCs being 3:1. The cone calorimeter test revealed that the total heat release (THR) and total smoke release (TSR) of PP2 were significantly decreased by 61.8% and 87.9% compared with that of pure PP. The flame retardancy index (FRI) of PP2 was 12.2, showing an "Excellent" grade of flame retardancy. The thermogravimetric results showed that ETDPCs presented good char formation ability, and it would greatly increase the charring yield of PP when combined with FR due to the synergistic effect. The investigation of the structures and properties of char residues indicated that the continuous and compact char layer with high mechanical stability and low thermal conductivity, containing benzene and triazine derivatives, was formed for PP2 during combustion, which acted as a good barrier against the transmission of heat and volatiles and movement of high-temperature melt. This work expanded the avenue in the synthesis of high-efficiency charring agents and improved the intumescent flame retardancy of PP. [ABSTRACT FROM AUTHOR]