Dual inhibition mechanism of core-shell coatings on CL-20 and HMX crystals for suppressing co-crystallization in composite propellants

Abstract

The unintended co-crystallization of hexanitrohexaazaisowurtzitane (CL-20) and cyclotetramethylene tetranitramine (HMX) compromises the structural integrity and safety of composite propellants during long-term storage. Inspired by pharmaceutical crystallization control, this study proposes a colloidal interface engineering strategy that utilizes conformal polyvinylpyrrolidone (PVP) coatings on CL-20 and HMX crystals by freeze-drying. The coatings strategy exhibit “sacrificial dissolution” and “physical barrier” dual inhibition mechanism. PVP preferentially saturates the glycidyl azide polymer (GAP) binder through controlled dissolution, deactivating the capacity to dissolve CL-20 and HMX. Simultaneously, PVP forms a dense barrier against plasticizer dioctyl sebacate (DOS) penetration. Molecular dynamics simulations and density functional theory calculations confirmed superior PVP binding affinity to CL-20 and HMX surfaces, which is validated by X-ray photoelectron spectroscopy and atomic force microscopy. Accelerated aging tests (70 °C/30 days) demonstrated complete co-crystallization suppression in PVP-coated formulations. Thermal analysis revealed a 20-30°C increase in phase transition temperatures and enhanced activation energies, attributed to restricted molecular mobility and optimized heat dissipation. Mechanical sensitivity decreased by 50–100 %, while combustion rates increased by 18–33 %. This work bridges pharmaceutical-inspired interfacial control with energetic materials, establishing a universal methodology for stabilizing high-energy composites through kinetic control.