The paper presents and compares two methods for the synthesis of fine particles of the high explosives HNS and HMX by ultrasonic treatment and solvent/antisolvent crystallization. The effect of ultrasonic treatment on the particle size of explosives was studied by varying the amplitude and frequency of ultrasonication for different time periods using an ultrasonic probe and an ultrasonic bath. Solvent/antisolvent recrystallization was performed by varying the process parameters including stirring rate, antisolvent temperature etc. In addition to FT‐IR spectroscopy and thermal analysis by TGA/DSC; the particle size and shape of fine powders of the explosives HMX and HNS were determined using particle size analysis and scanning electron microscopy (SEM). Ultrasonic treatment of the probes resulted in the finer grains of HMX compared to solvent‐antisolvent crystallization. However in the case of HNS, solvent‐antisolvent crystallization produced finer particles compared to ultrasonication.
Solvent–antisolvent recrystallization employed for size reduction of HNS has been described and the effect of various parameters such as stirring rate, effect of antisolvent type, antisolvent temperature, ultrasonication, etc. was investigated. Purified HNS, produced by hot solvent recrystallization of production grade crude HNS, of mean particle size ∼95 μm was used for preparation of ultrafine particles of HNS. Solvent contamination in terms of residual solvent was determined by 1H NMR and GC‐MS analysis. In addition, ultrafine HNS has been characterized for purity (HPLC, 1H NMR), particle size and shape (PSA and SEM), specific surface area (BET analysis), thermal behavior (TGA, DSC), sensitivity (impact, friction), etc. The results have been compared with C‐HNS. UF‐HNS was >99% pure with mean particle size <1 μm. SEM showed submicrometer size rods like particles of HNS as the final material.
The present study was undertaken to evaluate and compare the extent of particle size reduction of HNS and its characterization by applying sequentially the solvent based crystallization, ultrasonication, and ball‐milling. It was found that submicron ultrafine HNS (UF‐HNS) of mean diameter ∼0.5 μm with a size range extending to micron scale (∼3 μm) was produced by the solvent‐antisolvent crystallization process. This HNS was subsequently subjected to long duration ultrasonication up to 28 h and planetary ball milling up to 8 h. The HNS particles were characterized for morphology and particle size by scanning electron microscope (SEM), laser diffraction‐based and dynamic light scattering based particle size analyzer (PSA). The structural analysis was done by FTIR and thermal stability by the thermo‐gravimetric analyzer (TGA). The residual solvent content was estimated by headspace GC‐MS. Impact and friction sensitivity were evaluated by BAM fall hammer and friction sensitivity tester. Compared to ultrasonication varying from 4 h to 28 h, planetary ball milling of the micron‐sized HNS caused the size reduction to maximum extent and resulted in production of nano‐scale HNS (∼300 nm average diameter) with a very narrow size distribution of less than 1 μm which had lower impact sensitivity, thermal stability and lesser residual solvent content compared to micron‐sized HNS (UF‐HNS). Thus, nanoscale HNS with a very narrow size distribution was produced by application of the mechanical method of ball milling sequentially to the solvent based crystallization process.
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