Study on the Hygroscopic Properties and Mechanism of Novel Melt-Cast Matrix 3,4-Dinitropyrazole (DNP)

3,4-Dinitropyrazole (DNP) is a promising candidate as a next-generation matrix for melt-cast explosives. However, its hygroscopicity severely limits the application of DNP. In this work, the macroscopic hygroscopic properties of DNP powder and charge were determined through moisture absorption tests under varying temperature and relative humidity (RH) conditions. At the micrometer scale, the morphological evolution after moisture absorption was observed by scanning electron microscopy (SEM). The moisture absorption mechanism of DNP at the molecular level was elucidated using Raman spectroscopy. The results demonstrate that the hygroscopicity of DNP intensifies with rising temperature and RH. The critical relative humidity (CRH) was determined to be 85% at 25 °C, 62% at 40 °C, and 42% at 55 °C. The surface of dried DNP particles exhibits a highly developed porous structure conducive to moisture adsorption from the environment. The moisture absorption mechanism of DNP involves water molecules forming hydrogen bonds with both the N–H bonds and nitro groups of DNP molecules. The hydrogen bonds between water and DNP molecules replace the original N-H···O/N hydrogen-bond network within the DNP crystal and disrupt the intermolecular π-π stacking interactions.