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

ABSTRACT 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.