In Situ Fabricating Rigid Polyurethane Foams With Lightweight yet High‐Strength, Passive Flame‐Retardant, and Active Fire‐Fighting Properties

ABSTRACT In situ fabricating polyurethane foams (PUFs) have gained increasing attention as lightweight, efficient filling materials for emergency repair and construction. However, they usually exhibit the drawbacks of low mechanical properties and poor fire safety owing to their inherently porous structure. In this study, we developed novel fire‐extinguishing microcapsules (FMs) using microfluidic technology, employing poly(ethoxylated trimethylolpropane triacrylate) as the shell material to encapsulate perfluorohexanone. The FMs, serving as rigid functional additives, together with polyurethane containing a rigid‐structured phosphorus flame retardant (DM), were used to in situ fabricate unique foam composites (FM30/DPF7.5). This strategy simultaneously achieves lightweight yet high‐strength, passive flame‐retardant, and active fire‐fighting capabilities. The FM30/DPF7.5 composite exhibited outstanding compressive and flexural strengths at an expansion ratio of ≈4, showing increases of 153.2% and 294.5%, respectively, compared with pure PUFs. Moreover, FM30/DPF7.5 integrated both passive and active fire protection owing to the synergistic effects of FMs and DM. The composite could extinguish flames in a small enclosed space within 8 s. For a large space, it effectively prevented flame propagation after releasing perfluorohexanone, achieving a UL‐94 V‐0 rating. This study presents a novel approach to in situ fabricating rigid PUFs with excellent mechanical strength and outstanding fire resistance.