Resonant acoustic mixing of polymer bonded explosives

Current Polymer Bonded Explosive (PBX) formulation is limited by a compromise - optimised final properties against processability. While solid loading (explosive content) would ideally be maximised and plasticiser content would ideally be minimised, this would make the formulation too viscous to cast into its casing and require long and arduous mixing processes using conventional techniques. However, with Resonant Acoustic Mixing (RAM), PBX formulation does not have to be constrained. Instead of traditional mixing blades, mixing is achieved by the use of a vibrating platform to impart acoustic pressure waves (vibrations) into the mixture, agitating it. The added ability to mix in the end use casing (mixing ‘in-situ’) also renders casting obsolete in many scenarios. In order to maximise the benefits of RAM with regards to next generation formulation-optimised PBX manufacture (‘PBneXt’), the underlying mechanisms of how the technique works, how efficiency (time and energy required for homogeneity) can be determined and maximised, and how final material properties may change between casting and ‘in-situ’ processing methods, must be better understood. The research aim of the PhD is therefore to assess how mixing efficiency of RAM can be measured and optimised to maximise its benefits, with a focus on how aspects of machine control and mixing vessel design can be altered to improve the mixing mechanisms on which the technique relies. Areas investigated experimentally include the effects of acceleration and mixer intensity (linked to power draw) setting, mixer model and unit, vessel material (with regards to surface free energy and thermal properties), and vessel surface finish (with regards to roughness). It is found that by modifying these variables, the time and energy required for mixing can be substantially reduced. A comparison between material properties of composites mixed ‘in-situ’ and ‘mixed and cast’ is also undertaken. The findings are then reconciled with wider literature observations and recommendations are made as how to best implement RAM for ‘PBneXt’ manufacture, ultimately allowing for explosive compositions with improved performance, mechanical, safety, and ageing properties.