Size Reduction of HNS to Nanoscale by in Tandem Application of Chemo-mechanical methods

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.