Mechanically Reinforced Rigid Polyimide Foam via Chemically Grafting Isocyanate Acid for Ultrabroad Band Microwave Absorption

Polyimide (PI) foam with excellent microwave absorption(MA) performanceand desirable compressive strength is highly critical and in demandin the structural MA components. Although the satisfactory MA performanceof the present PI-based MA foams has been achieved by employing diversemethods, the relatively low compressive strength (similar to KPa) restrictedthem from use as structural MA foams in practical application. Herein,isocyanate acid was introduced to the backbone of PI resin, whichnot only increased the PI backbone polarity and strength as rigidchain segment, but also served as a self-foaming component. The porousstructure of PI foams was readily regulated by adjusting the waterand carbon nanotube (CNT) filler contents of precursor dispersion.As a result of the improved polarity of the PI backbone resulted fromthe isocyanate group and high dielectric loss of CNT, the high compressivestrength of 7.04 MPa and impressive MA property of the resultant PIfoam with a low CNT loading ratio of 1.5 wt % were achieved, whichwere much higher than those reported previously. Especially, the effectiveabsorption bandwidth (EAB) (RL < -10 dB) was up to 10.7GHz (at the thickness of 3 mm), covering the C, X, and Ku bands simultaneously.Meanwhile, the EAB of the as-prepared PI foam retained 9.3 and 9.7GHz even after being subjected to liquid nitrogen (-196 degrees C)and high temperature (300 degrees C) treatments due to the desirablestability of PI. In addition, the excellent thermal insulation resultedfrom the pores structure and low filler content was achieved, wherethe top surface only presented 60 degrees C after placing on 300 degrees Cplatform for 30 min. The high compressive strength, impressive MAproperty, and thermal insulation endowed the resultant CNT/PI foamwith great potential application as structural MA foam in a harshservice environment.

Automated summary

By introducing isocyanate acid to the backbone of PI resin, the polarity and strength of PI backbone were increased, and it also served as a self-foaming component. The porous structure of PI foams could be easily regulated by adjusting the water and CNT filler contents. The resultant CNT/PI foam exhibited high compressive strength, impressive MA property, and thermal insulation, making it suitable for application as structural MA foam in harsh environments.