DEVELOPMENT OF PRESSING METHODOLOGIES FOR SPRAY DRIED 2,2’,4,4’,6,6’HEXANITROSTILBENE (SPD HNS) USING AN INSTRON 1
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Dr Matthew Andrews , Prof Jackie Akhavan , Dr Daniel McAteer , Dr Adam Hazelwood
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Munitions Safety Information Analysis Centre (MSIAC), Building Z, NATO HQ, Leopold Boulevard III, 1110 Brussels, Belgium.
[email protected] 2
Centre for Defence Chemistry, Cranfield University, Defence Academy of the UK, Shrivenham, Wiltshire. SN6 8LA. UK 3
Explosive Materials, AWE Aldermaston, Reading, Berkshire, RG7 4JS, UK
1. Abstract This study explores the relationship between particle size distribution of spheriodised, spray dried 2,2’,4,4’,6,6’-Hexanitrostilbene (SPD HNS) and pressing parameters using a material testing instrument (Instron) required to form consolidated pellets. Initial characterisation of the Instron has been carried out using inert materials prior to the introduction of energetic materials. Four different particle size distributions (D4,3 = 1.37µm to 6.98µm) of SPD HNS, taken from a parametric study of spray drying parameters, have been consolidated in to pellets 0.2 x 0.2cm (height x diameter) using a range of compression loads and cross-head speeds. Physical properties of the pellets have been determined using metrology, microscopy, SEM and statistical comparison made thereof. For inert pellets crosshead speed had a large effect on pellet density when compared to compression -1
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load with a good correlation at 0.25mm min (r = 0.97) but no correlation existing at 1.00mm min . Analysis of SPD HNS batches showed that the rate of relaxation post pressing was related to particle size distribution with the VSP (1.37µm) showing the greatest relaxation. Statistical analysis showed there was little relationship between particle size distribution and density versus compression load. It has been shown that good reproducibility of pellet dimensions (height = 0.203 ± 0.004cm) and density -3
(1.601 ± 0.007g cm ) can be achieved using an Instron. 2. Introduction The performance of an explosive is related, amongst many interrelated parameters, to the density of the explosive charge (Bailey & Murray, 1989) (Matyas & Pachman, 2013). The density of an explosive is, in turn, dependent upon numerous factors including explosive morphology, particle size and distribution of solid ingredients and composition of the formulation. In order to obtain the greatest performance the explosive density should be close to the theoretical maximum density. For non-melt cast explosive formulations (e.g. Comp A-4) consolidation of the powder is required using uniaxial, diaxial or isostatic compression (Akhavan, 2011). Compression of moulding powders requires large hydraulic press set ups (10 – 500T), remote operation and specific press tooling in order to shape the consolidated energetic material. Generation
of consolidated pellets of the correct density requires the correlation of pellet density versus applied load. The load applied to the press tooling can be recorded via hydraulic gauge pressure or against a calibrated load cell. Final load is recorded and subsequently the ejected pellet density is measured by metrology and/or Archimedes. The process is time/resource intensive with little focussed research understanding the parameters. The produced pellets are subsequently tested (e.g. detonability, mechanical properties) and conclusions drawn without fully understanding the effect of pressing. Small explosive pellets (HNS, PETN) are required for detonator systems such as Exploding Foil Initators (EFI) (Bowden, et al., 2006) but hydraulic pressing of pellets is limited to diameters of 5mm pellet due to low fidelity at low pressing loads and high pressures (>350 MPa) generated by small drift diameters (
