Have we found a successor for TNT?

Materials World magazine
,
1 Aug 2018

Khai Trung Le talks to David Chavez about a melt-cast bis-oxadiazole compound that could be the ideal replacement for the longstanding and toxic explosive, TNT.

Not much lasts 116 years. But trinitrotoluene, better known as TNT, has been the ubiquitous explosive for over a century. However, its high toxicity meant a replacement has long been sought after, and finally, after 116 years of services, TNT may be ready for pasture.

TNT is known for its stability – the UK 1875 Explosives Act didn’t class it as an explosive in terms of storage and handling – and is most commonly used in military applications, including shells, bombs, and grenades, but still sees industrial use such as underwater blasting. However, TNT is also highly toxic. The USA Environmental Protection Agency lists the explosive as a possible carcinogen, and the Centers for Disease Control and Prevention, USA, asserts exposure has been linked to disorders of the liver, blood, and spleen.

Through the Joint Munitions Program, the US Army Research Laboratory and the Los Alamos National Laboratory (LANL) have pursued a non- or low-toxic alternative with the correct melting point to allow it to be liquefied and cast for use in munitions, and believe they have found one in a nitrogen-containing compound, bis(1,2,4oxadiazole)bis(methylene)dinitrate.

The compound has a calculated detonation temperature 50% higher than that of TNT, a relatively high decomposition temperature irregular for nitrate-based compounds, and lower sensitivity to impact and friction, making it a suitable alternative to TNT.

Yielding to pressure

David Chavez, Principle Investigator at LANL, said, ‘One major challenge was coming up with a formula that would significantly surpass the explosive energy of TNT, but still have melt-casting capability. When designing a molecule to be melt-cast yet with high-performance, you encounter many challenges due to numerous obstacles,’ including getting a high yield out of the synthesis process.

The team believes the 24-atom molecule compound is a likely candidate to replace TNT. When exploded, it has a yield 1.5 times greater than TNT, with a similar melting point and insensitivity. It is also environmentally friendly, with lower toxicity.

The joint team has been investigating the bis-oxadiazole compound for two years. The intramolecular hydrogen bonding is believed to be responsible for the insensitivity that provides the desired difficulty in igniting the TNT replacement. Chavez told Materials World, ‘The nitrate ester functional groups are capable of bonding to the hydrogen atoms in the molecule. This happens in both intra- and intermolecular fashion. This extra stabilisation improves the overall thermal stability of the bis-oxadiazole, and is the likely cause of its reduced sensitivity to impact and friction.’

One dominating issue was increasing the synthesis yield, which started at around 4%, to around 44%. Chavez said, ‘Firstly, we improved the synthesis of diaminoglyoxime, which resulted in an increased safety profile, while doubling the yield of this intermediate, from 40% to 80%.

‘We also saw a significant improvement in the step that forms the bis-oxadiazole core. The old method was very inefficient, but by changing our approach and reagents, we were able to optimise the cyclisation. Yield improved around sixfold, from 11% to 66%,’ he continued.

However, the LANL was unable to provide further information on the speed and cost of the synthesis process, with Chavez simply stating, ‘Our goals are to make this in bulk scales in a cost efficient manner, so that experimental testing can occur.’