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.