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Design and development of a novel high performance emulsion explosive using nanoparticles
Tshilumbu, Nsenda Ngenda
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This study investigated water-in-oil (W/O) super-concentrated emulsions used as pumpable explosives. The aqueous phase of the emulsions is a supersaturated nitrate salt solution (at room temperature), with a volume fraction of approximately 0.9. Instability of such emulsions arises either from crystallization of the dispersed phase in the system during ageing or under high shear conditions. Here, we report an alternative approach to stabilize this highly concentrated W/O emulsion by adding colloidal particles in combination with short amphiphilic molecules. Thus, the primary goal of this research concerned a phenomenological study of the dependence of surfactant-to-particle ratio as well as the particle hydrophobicity index on stability under high shear in the emulsification process, rheological properties and stability against initiation of crystallization of an internal phase both with ageing and under high shear with a view to optimize the time to the start of crystallization of the emulsion both with ageing and under high shear; to elucidate the mechanism of initiation of crystallization of an internal phase (homogeneous or heterogeneous) and shed light in the stabilization mechanism of the emulsion; to determine how the emulsion formulation content affect pumping characteristics as measured by characteristic rheological parameters. A series of five fumed silica nanoparticles, each with a different hydrophobicity index (HI) in the range of 0.60 – 3, were used in the form of single types of particles as well as binary mixtures. These particles were combined with a low molecular weight conventional surfactant, Sorbitan MonoOleate (SMO), into the oil phase prior to emulsification. It has been found that regardless of the particle hydrophobicity, fumed nanosilica alone cannot form highly concentrated W/O emulsion up to 90 vol%. Moreover, Pickering emulsions are unstable under shear conditions and thus it is difficult to make highly concentrated W/O pumpable emulsion explosives using only fumed nanosilica. The correlation between the refinement time and SMO-to-particle ratio showed a deflection point/transitional point in the stabilization mechanism. Below the transitional point the silica content dominates over SMO. Conversely, above the transitional point the particles have little effect and the SMO dominates. A thermodynamic consideration revealed that in this region only SMO is likely to adsorb at the W/O interface and controls the emulsifying process. As with refinement time, the correlation between the shear modulus and SMO/particle ratio shows a deflection /transitional point which, as before, mark the transition point between regions of particle or SMO domination. Interestingly, it was found that for each HI, the initiation of crystallization is the most delayed, both on shelf life and under high shear, when the emulsion is prepared with an SMO-to-particle ratio equaling exactly the value at this transitional point. Moreover, the research demonstrated that a drastic change in mechanism of initiation of crystallization of the dispersed droplets occurs at the transitional point. Homogeneous nucleation within the droplets is the dominating mechanism of initiation of crystallization of an internal phase for SMO/particle ratios below and at the transitional point. In this case, the relationship between the zero modulus of particle dispersions in oil and the SMO-to-particle ratio demonstrated that the most stable emulsions are formed from the most unstable dispersions, indicating that less repulsion between particles is required to delay the onset of crystallization. This was further corroborated by the linear correlation between the time to the onset of crystallization and the shear modulus of the emulsion. On the contrary, it was found that for SMO/particle ratios above the critical point, heterogeneous nucleation catalyzed at the surface of droplets is the dominating mechanism of initiation of crystallization of nitrate salts in the super-cooled droplets This was found to be consistent with SMO-only system. That is the change in the mechanism of initiation of crystallization originates from a drastic change in the emulsion structure due to excess surfactant was highlighted by the drastic change in the linear correlation between the time to the start of crystallization and the strength of the emulsion structure as measured by the shear modulus. The optimum time to the start of crystallization (onset of crystallization associated with optimum SMO-to-particle ratio) is sensitive to the particle HI; increasing with increase of particle HI. A general correlation between the particle HI and optimum time to the onset of crystallization has been identified and formulated for the whole family of single types and mixtures of fumed nanosilica used in this study.