Material Chemistry Control for the Additive Manufacture of Composite Propellants

Abstract

This thesis has sought to aid the additive manufacture of propellants using a novel dry powder printing system developed at Cranfield. The energetic performance and hazard safety of crystalline energetic materials is intrinsically linked to crystal properties such as size, morphology, and crystalline phase. By optimisation of cooling, antisolvent, sonocrystallisation, spray drying and microencapsulation, the properties of cyclotrimethylenetrinitramine (RDX) and ammonium perchlorate (AP) have been engineered towards better performance within our printing system. Crystallisation of AP from solution in water has been assessed as a means of producing particles with a controllable particle size and morphology. Slow cooling processes (−7.5 °C hr-1 ) failed to produce material suited for use in propellant formulations. However, by significantly increasing the nucleation rate using rapid cooling crystallisation processes (~ −5 °C min-1 ) the size of generated crystals was greatly reduced, with a d50 range of 79.1 - 152.3 µm, compared to ~ 500 – 2000 µm. The application of ultrasonic radiation via a horn to the rapid cooling crystallisation gave promising results – leading to particle size reduction (d50 range: 33.5 – 43.4 µm) and a reduced frequency of secondary nucleation. Moreover, the average particle size distribution width was reduced from 245 µm to 75 µm by the application of sonication. Flow character, as assessed by angle of repose measurements, was good for these sonicated materials (31.0° to 34.1°). Spray drying and micro encapsulation was assessed as a means of RDX particle size reduction. Initial studies using paracetamol as an inert simulant demonstrated that modifications to spray drying process parameters (flow, atomisation pressure, nozzle diameter and feed concentration) produced measurable changes in particle size and size distribution. However, attempts to rationalise these effects using a multifactorial design of experiment were inhibited with the significant errors retrieved from the model. Attempts to understand how particle properties impact the flow character of a powder led to the observation that increased particle size gave decreased angle of repose. However, the magnitude of change was negligible when compared to the effect of reformulation in the presence of known glidant nanomaterials. Microencapsulation of RDX with cellulose acetate butyrate (CAB) was conducted at a range of operating temperatures between 55 and 100 °C. Both particle morphology and impact Figure of insensitiveness were demonstrably affected by drying temperature, and both were minimised by the use of lower drying temperatures (d50 = 2.60 µm, FoI = 102.0). FoI values for RDX/CAB microparticles correlated negatively with drying temperature, suggesting that the strain imparted by this rapid crystallisation process may be retained in the material thereby acting to influence its hazardous nature. Crystallisation of RDX by antisolvent precipitation and spray drying was assessed with the inclusion of five different tailor-made additives (TMAs). Of the assessed TMAs, 2,4-dimethyl-1nitrobenzene and 1,2-diemthyl-3- nitrobenzene were noteworthy for causing significant particle size reduction of antisolvent precipitated RDX. Crystal size enlargement and aspect ratio elongation was most pronounced when 1,3,5-triazine-2,4-diamine impurity was present. A novel application of the Scherrer equation was employed, to study the effect of TMA inclusion on the constituent crystallites within spray dried microparticles. The investigation revealed reduced coherence length of the (002) plane and extension of the (210) plane when RDX was spray dried in the presence of TMAs.