Stiffening modification of polyimide composite foam for enhancing fire safety and blast resistance

Sandwich structures with composite foam as the core material exhibit excellent energy absorption capacity, offering broad application potential in protective equipment. However, traditional foam cores suffer from drawbacks such as flammability and low structural strength. Here, a stiffened polyimide composite foam (SSPI) was developed, featuring a density of 0.184 g/cm³, excellent flame retardancy, and high mechanical performance. This SSPI foam was then integrated between steel plates to fabricate a blast-resistant Steel/SSPI/Steel sandwich structure (S-SSPI-S). Compared to polyimide composite foam (PI), the SSPI composite foam exhibits superior fire protection properties. The peak heat release rate decreased by 15.5%, and the corresponding temperature increased by 41.2%. This behavior is ascribed to the barrier performance of ceramicized char. The dynamic compressive properties of SSPI composite foam were investigated using a split Hopkinson pressure bar. Additionally, the relative shear thickening effect of SSPI is increased from 9.1% in PI foam to 69.4%, increased by 662.6%. The incorporation of B-O dynamic covalent crosslinks endows SSPI with tunable shear-stiffening behavior. Furthermore, the synergistic effect of the structure and components endows the S-SSPI-S sandwich structure with excellent blast resistance. After the blast test, the SSPI core material retained its structural integrity with no visible damage. Based on the dynamic response process reproduced through numerical simulation and mesoscale damage features, the energy dissipation mechanisms of the SSPI composite core-based sandwich structure were elucidated. This study offers novel insights into lightweight, flame-retardant, and blast-resistant polymer composite foams, highlighting their strong potential for protective sandwich structures in engineering applications.