Four-In-One Lanthanum-Assisted Core–Shell Intumescent Flame Retardant for Water-Resistant Polypropylene Composites

Polypropylene (PP) is widely used in polymer products, but its high flammability and the poor resistance to water exposure of ammonium polyphosphate (APP)-based flame-retardant systems limit broader applications. Herein, a lanthanum-assisted core–shell flame retardant (LAPP@M) was fabricated by La 3+ modification of APP followed by microencapsulation with an HDI-cross-linked β-cyclodextrin shell. The hybrid structure integrates acid source, gas source, bio-based carbon source, and catalytic functionality within one particle. Structural analyses confirmed the formation of a La-modified APP core and organic shell, which changed the particle surface from hydrophilic to hydrophobic. With only 15 wt% LAPP@M, PP achieved a UL-94 V-0 rating and an LOI of 29.5%, and retained the V-0 rating after 72 h of static water immersion, demonstrating improved short-term resistance of the flame-retardant performance to water exposure. Cone calorimetry showed that the peak heat release rate and total heat release were reduced by 75.2% and 26.3%, respectively, relative to neat PP. Char analysis, Raman spectroscopy, TG-FTIR, and TGA revealed that La promoted condensed-phase charring, while the β-cyclodextrin shell improved water resistance, interfacial compatibility, and char formation. This work provides an effective particle-engineering strategy for flame-retardant PP composites with improved resistance to short-term water exposure.