Pyrolysis driven pore network evolution and combustion response under slow heating conditions

The structural stability and combustion safety of composite solid propellants under slow thermal stimulation are critical for evaluating their response under abnormal heating conditions. In this study, HTPB/AP/HMX/Al composite propellants with different degrees of thermal damage were prepared by slow-heating pretreatment at room temperature, 100 °C, 120 °C, 180 °C, and 200 °C. TG/DTG-FTIR and DSC were used to characterize pre-ignition thermal decomposition, while SEM, Micro-CT, mercury intrusion porosimetry, and fractal analysis were employed to reveal the evolution of pore and crack networks. The results show that slow heating induces stage-dependent decomposition of HMX, AP, and the HTPB binder. With increasing pretreatment temperature, thermal degradation of the binder, interfacial debonding, and oxidizer particle damage gradually promote the formation of interconnected pore/crack networks. The overall porosity increases from 6.9% for the untreated sample to 37.5% after pretreatment at 200 °C, and the fractal dimension rises from 2.18412 to 2.56522, indicating increased structural complexity. Combustion tests further demonstrate that thermal damage significantly intensifies the combustion response. The peak pressure increases from 1.89 MPa to 4.03 MPa, while the time to peak pressure decreases from 238.3 ms to 90.8 ms. Meanwhile, the laser-ignited burning rate increases from 1.62 mm·s⁻¹ to 6.06 mm·s⁻¹ . These results indicate that slow-heating-induced pore-network evolution provides preferential pathways for gas transport and flame penetration, thereby increasing the effective burning surface area and accelerating combustion. This study establishes a mechanistic relationship among pre-ignition decomposition, pore/crack network evolution, and combustion intensification, providing a basis for thermal safety assessment of composite solid propellants.