Dual Modification of Layered Double Hydroxide by Phosphonitrilic Chloride Trimer and Aniline for Enhancing the Flame Retardancy of Polypropylene

Owing to the poor dispersion of layered double hydroxide (LDH) in a polypropylene (PP) matrix and its own flame-retardant 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 m2·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.