Improved Manufacture of a Melt-Cast Explosive

Furthermore, when doped with 5 mole% dinitrotoluene, pellets of doped DNAN showed no sign of irreversible expansion when repeatedly cycled (40 cycles) in the temperature range from 260-274 K. In contrast, the undoped sample showed significant radial expansion when cycled under the same heating and cooling regime.

Our most recent exciting result shows that a new additive present at a level of 5 mole% completely inhibits the II-III transition over the entire operational temperature range. This has significant implications for the manufacture and processing of DNAN, as well as for the ultimate safety of DNAN-containing formulations. These studies have also provided valuable insight into nucleation and crystallisation processes encountered in melt systems.

Academic:
Prof Colin R. Pulham (Edinburgh)

Researchers:
Paul L. Coster (Edinburgh)
Daniel Ward (Edinburgh)

The development of insensitive munitions that are less susceptible to accidental initiation and hence increase safety is an area of major interest. 2,4-dinitroanisole (DNAN) is a candidate material that is attracting significant interest as a replacement for trinitrotoluene (TNT) in melt-cast formulations on account of the dramatic sensitivity improvements demonstrated during qualification testing. Several DNAN-based formulations are already in use, but there remain several issues associated with the replacement of TNT by DNAN. These include the thermal behaviour of pure DNAN and DNAN-based formulations. In particular, temperature-cycling experiments on DNAN-containing compositions have demonstrated irreversible volume increases of up to 15% with potentially deleterious consequences. One potential cause of this volume increase is the polymorphic transition from form-II to form-III that occurs at 266 K in pure DNAN. Repeated cycling across this transition is believed to disrupt the packing of microcrystallites, either as a result of the anisotropic expansion and contraction of the unit cell, or through some form of “ratchet” mechanism.

The processing of DNAN and its formulations involves crystallisation from the melt (mpt. 95 °C) and so research has focussed on controlling nucleation and crystallisation processes of both the melt and solid solutions. Previous work by the Edinburgh team has demonstrated that in the absence of any seeds of form-I, crystallisation from the melt results in the formation of the metastable form-II and that form-II is indefinitely metastable in the absence of form-I. The effects on crystallisation of dopant compounds in the range 5-10 mole% have been studied and certain compounds such as 2,4-dinitrotoluene have been shown to form solid solutions that suppress the temperature of the II-III transition to below 240 K when present at a level of 5 mole% and down to 210 K when present at a level of 10 mole%. Our current working hypothesis is that substitution of DNAN molecules with slightly smaller molecules results in slightly more space within the crystal structure of the solid solution thereby suppressing the disorder-order transition associated with the II-III conversion.