Hollow Silica/PUU Composites for Flame Retardancy and Blast Mitigation: From Shock Tube to In Vivo Protection

Abstract Blast shock waves and the resulting thermal burns from secondary fires are among the primary causes of casualties in explosions, underscoring the significance of developing protective materials that simultaneously deliver efficient shock wave attenuation and flame retardancy. Herein, a flame-retardant polyurethane-urea (PUU) elastomer was synthesized via molecular structure engineering and subsequently reinforced with hollow silica microspheres (HSM), yielding PUU composites. The PUU matrix attained a limiting oxygen index of 31.15%, demonstrating self-extinguishing capability. Shock tube tests revealed that the PUU composite with 10 wt % HSM (PUU10S) achieved an attenuation rate of 75%, the excellent attenuation performance originates from continuously distributed impedance mismatch interfaces established by the microspheres, coupled with microsphere debonding and wave path redirection under shock loading. Rat blast protection evaluation directly confirmed the protective efficacy, as the PUU10S-protected group exhibited a reduction in pulmonary injury area from 91.03% to 33.94%, recovery of the lung wet-to-dry ratio from 6.91 to 4.70, and a decrease in 6 h mortality from 50% to 8.3% relative to the unprotected cohort. This study provides a promising strategy for reconciling flame retardancy and blast resistance in a single polyurea-based material: the intrinsically flame-retardant PUU matrix retains its self-extinguishing character upon HSM incorporation, albeit with a moderate reduction in tensile strength and toughness, while the hollow-microsphere architecture delivers a reduction in transmitted peak pressure and a marked reduction of blast-induced pulmonary injury in rats, establishing a foundation spanning materials design to biological protective validation for next-generation high-performance composite protective materials.