US researchers say they have developed “green” chemicals that could replace the lead-based primary explosives that are used to detonate everything from blasting caps to ballistic missiles. They also claim their process may make the manufacturing of such energetic compounds safer.
Primary explosives are the relatively weak yet highly sensitive materials used to set off powerful explosions. Lead-based chemicals came into use for this purpose one hundred years ago to replace the even more toxic mercury fulminate.
But studies show that toxic plumes are released as lead-based explosives are discharged. A 1991 survey revealed that maintenance workers at an FBI gun range had nearly 10 times as much lead in their blood as is permitted under government safety regulations.
Stable but sensitive
However, chemists have struggled to find replacement chemicals with the right stability and sensitivity, says My Hang Huynh, the explosives expert at Los Alamos National Laboratory in New Mexico who engineered the green explosives.
Her team has developed primary explosives based on a chemical called nitrotetrazole. In fact, nitrotetrazole itself has been studied for the past few decades as a next-generation explosive. However, it only makes a good primary explosive when partnered with toxic perchlorate-based chemicals.
Huynh’s team overcame this by tweaking the distribution of charge on the nitrotetrazole molecule. Switching charge groups on the molecule also changed the substance’s overall properties, such as its sensitivity to ignition through physical contact or heat. This could allow the substance to be tailored for specific applications, says Huynh.
However, one potential stumbling block for the new class of chemicals is that they lose sensitivity when they get wet. This has been one of the major hurdles in the race for other potential primary explosives, says Thomas Klapotke at the University of Munich, Germany, who has also worked on developing greener explosives.
Keeping it wet
Huynh sees this effect as a benefit, however, since the chemical returns to its explosive state after drying out. This property can be exploited to make the manufacturing and storage processes safer.
Producing conventional lead-based primary explosives is so risky that the US and UK import the chemicals rather than produce them. However, Huynh’s chemicals, which are produced in solution, do not become active until they are carefully dried out.
In addition, since they do not degrade in their wet form, they can be safely stored indefinitely, she says.