Superhydrophobic SiO2–PFOTES Coating on Ultrafine Ammonium Perchlorate: An Effective Strategy for Simultaneously Enhancing Anti-Hygroscopicity and Thermal Decomposition Performance

Abstract

Abstract Ultrafine ammonium perchlorate (UF-AP) is prone to hygroscopic agglomeration, which severely affects the loading uniformity and combustion stability of solid propellants. In this study, hydrophobic modification of nano-silica was conducted using 1H,1H,2H,2H-perfluorooctyltriethoxysilane (PFOTES). SiO2–PFOTES particles with different modifier loadings were prepared and then coated onto the surface of UF-AP via a wet coating process to obtain UF-AP/SiO2–PFOTES composite particles. The microstructure, thermal decomposition behavior, surface wettability, storage stability, and mechanical sensitivity of the composite particles were characterized by SEM, EDS, FTIR, XRD, TG-DSC, contact angle measurement, and hygroscopicity tests. The results show that PFOTES is successfully grafted onto the SiO2 surface. When the modifier loading is 4%, the SiO2–PFOTES particles exhibit superhydrophobicity (water contact angle >150°). At a coating content of 5%, SiO2–PFOTES forms a well-distributed coating layer on the UF-AP surface, as evidenced by SEM, TG, and surface wettability analyses. Consequently, the water contact angle of the composite particles increases from 0° to 79.7°, the moisture absorption rate decreases from 0.26% to 0.08% (a reduction of 69.23%), and the surface energy decreases from 78.72 J/m2 to 31.39 J/m2. Moreover, the composite particles maintained good dispersibility after storage in a laboratory environment for 30 days. Thermal decomposition tests revealed that the low-temperature and high-temperature decomposition peaks of UF-AP merged into a single exothermic peak after coating, with the total heat release increasing from 319.3 J/g to 380.5 J/g (an apparent increase of 18.65% based on composite mass, corresponding to a 25.4% increase when normalized to per gram of AP). Mechanical sensitivity tests showed that the coating did not introduce additional safety hazards under the tested conditions. This study provides an effective strategy for anti-hygroscopic modification of UF-AP, showing good application prospects in the field of solid propellants.