The effect of type and concentration of surfactant on stability and rheological properties of explosive emulsions

Abstract

This study investigated water-in-oil (WIG) 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 usually greater than 0.8. Aqueous phase droplets are deformed by packing and contact with neighbouring droplets. Compounds of this kind are thermodynamically unstable and their instability is related to the coarsening of emulsions (droplet coalescence) and phase transition (crystallisation) in the dispersed phase. However, it was demonstrated that the dominating mechanism is slow crystallisation inside the supercooled droplets. The main goal of this thesis therefore concerned a phenomenological study of the dependence of type and concentration of surfactant, as well as the ageing processes, on the rheological properties of these emulsions. The bulk rheological measurements were carried out using a rotational dynamic rheometer MCR 300 (Paar Physica). Samples of different types of surfactant (Pibsa-MEA, Pibsa-UREA, Pibsa-IMIDE, SMO and SMO/Pibsa-MEA) and different concentrations of surfactants were studied. The results of the measurements include the flow and viscoelastic properties of the materials. The rheological parameters have been correlated with the kinetics of structural changes during ageing as a function of emulsion formulation content. The emulsions under study were non-Newtonian liquids. It was demonstrated that different surfactant types yield different interfacial properties. In fact, both the interfacial tension and the interfacial elastic modulus were found to decrease according to the sequence MEA-UREAMEAlSMG- IMIDE-SMG. It was established that the surfactant type and surfactant concentration affected the bulk rheological properties of explosive emulsions. Indeed, both the elastic modulus and the yield stress as function of surfactant type decreased in the following order: MEA-IMIDE-UREA-MEAlSMG, whereas they also decreased as the surfactant concentration increased. However the sensitivity of the rheological parameters to the type or concentration of surfactant was found to decrease as the droplet size increased. Moreover, the changes in rheological parameters were more strongly expressed than any changes in interfacial tension. This last finding is considered as rather important. It seems reasonable to assume that it provides proof of an active role of a surfactant not only as a compound responsible for the interfacial tension, but also creating additional sources of elasticity.