To date, designing efficient bio-based flame retardants for polypropylene (PP) and exploring a green and scalable strategy have remained a hot topic of research. Here, we present a green and convenient preparation of a bio-based flame retardant (LDH-LS@CS@PAMn) for layer-by-layer assembly by utilizing water as an assembly medium. By electrostatic interactions, LDH-LS@CS@PAMn was prepared by the sequential assembly of sodium lignosulfonate-modified layered double hydroxide (LDH-LS) with positively charged chitosan (CS) followed by negatively charged sodium phytate (PAMn). With increasing LDH-LS@CS@PAMn loading in PP composites, the corresponding total heat release rate (THR) and peak heat release rate (PHRR) values were reduced remarkably. With loading 20 wt % LDH-LS@CS@PAMn, the THR and PHRR of PP/LDH-LS@ CS@PAMn decreased by 37.6 and 59.1%, respectively, while the limiting oxygen index significantly improved to 31.6% and the V-0 rating was successfully passed by the UL-94. Especially for PP/LDH-LS@CS@PAMn composites, the total smoke production (TSP) was reduced by 47.4%. More importantly, the residual PP/LDH-LS@CS@PAMn char changed from gray to brown-black compared to PP/LDH, indicating a stronger residual char formation. The outstanding fire safety, smoke suppression, and char formation capabilities of PP composites were ascribed to the synergistic LDH-LS@CS@PAMn hybrid's action. This work provided a feasible and reasonable modification idea to improve smoke suppression and flame retardancy for LDH.

Owing to the poor dispersion of layered double hydroxide (LDH) in a polypropylene (PP) matrix and its own flameretardant efficiency being finite, the development of high-performance PP/LDH-based composites is still a tremendous challenge. To improve the dispersion of LDH and its inherent flame retardancy, phosphonitrilic chloride trimer (HCCP) was grafted onto the surface of LDH through the bridging effect of (3-aminopropyl)triethoxysilane and then further modified with aniline to replace the chlorine element in HCCP, so a flame-retardant (LDH@HA) was successfully synthesized. The flame retardancy and smoke suppression of LDH@HA with PP composites (PP/LDH@HA) were tested with the limit oxygen index (LOI), UL-94, and cone calorimetry tests. With a loading of 20 wt % LDH@HA, the peak heat release rate, total heat release, and total smoke rate of PP/ LDH@HA composites were 300.1 kW•m −2 , 57.7 MJ•m −2 , and 697.5 m 2 •m −2 , respectively, which were decreased by 64.4, 46.4, and 49.1% in comparison to those of PP, and the LOI value of PP/LDH@HA reached 30.6% and achieved the UL-94 V-0 rating. The improvements in flame retardancy and smoke suppression were due mainly to the physical barrier effect of LDH and the catalytic carbonization effect of HCCP together with the nonflammable gases formed by the decomposition of aniline, which diluted the combustible gas concentration. Compared with PP/LDH, the optimization of the mechanical properties of composites was due mainly to the introduction of HCCP and aniline, which significantly enhanced the compatibility between LDH and PP and promoted the uniform distribution of LDH in the PP matrix. In conclusion, LDH@HA has low cost, easy fabrication, and an excellent smoke suppression effect, and the strategy provides a specific idea for the preparation of fire-resistant PP composites.

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