This is a review of the poorly known story of TNT. Few people realize that TNT, though developed at the same time as dynamite, is actually a different compound entirely. The history of TNT has an interesting starting point: it was originally developed as a dye, not an explosive!
TNT revolutionized civil engineering and has cemented itself in the public consciousness as the definition of explosives. TNT is actually an acronym, representing Trinitrotoluene. Its chemical formula is C6H2(NO2)3CH3. Contrary to popular belief, TNT is a completely different beast from dynamite. TNT was invented in 1863, just four years before dynamite, by German chemist Joseph Wilbrand.
Interestingly enough, TNT's intended purpose was as a yellow dye, not as an explosive. TNT's explosive capabilities weren't even realized until years after its invention. Its explosiveness went unnoticed because TNT has a fairly high activation energy. In addition, it was actually less powerful than alternative explosive compounds of the day. However, TNT has a number of advantages, which will be explained below, that make it far superior to those compounds. Because of these advantages, TNT is still one of the most commonly used industrial and military explosives.
Advantages of TNT
Prior to the development of TNT, explosives were highly instable and prone to near-spontaneous combustion. Compounds like black powder and nitroglycerine were often used, sometimes in volatile multi-compound mixtures. These compounds were highly explosive but had one severe drawback: they had low activation energies. In practice, this meant that explosives could accidentally combust without ignition. In fact, the first shipment of liquid nitroglycerine killed fifteen people when a crate exploded en route to a construction site.
TNT's stable character made it a good choice for construction operations, such as rock-blasting. Another advantage of TNT is that its melting point is well below the temperature at which it spontaneously combusts. This allows manufacturers to safely pour the compound in liquid form. Additionally, TNT does not dissolve in or absorb water, making it safe to use in wet environments.
TNT has a very high detonation velocity of 6,640 m/s, with an energy content of 4.6 megajoules per kilogram. This energy-density figure is still used as a reference figure for modern bombs; nuclear bomb yields are typically given in Megatons of TNT equivalency.
Military forces around the world quickly realized TNT's possibilities as a weaponized explosive. In 1902, German forces began using TNT in their artillery shells. These shells were able to pass through British ships' armor prior to exploding, while the British shells exploded on contact. Thus, TNT-containing shells were able to wreak havoc on contemporary ships by dumping a huge portion of their energy inside of the target.
TNT has had many applications outside of warfare. It was widely used alongside dynamite in the construction of the American railroad system. During this construction, thousands of miles of rail were laid across the country. The railroads' paths often required passage through a whole mountain, and so TNT and dynamite were brought in by the ton to clear passageways. There were still many deaths during this process, some of which were no doubt related to unexpected combustion of explosives, but without TNT this construction would likely have been impossible. TNT was also used in the construction of highway systems in America and elsewhere.
Development of TNT Blends
TNT's safety and usefulness have been improved greatly over the years through the development of various explosive mixtures. Many of these blends make use of TNT's excess of carbon. Unmixed, detonation of each molecule of TNT produces seven atoms of carbon, which presents as soot. Scientists were able to increase TNT's explosive yield by adding in oxygen-containing compounds. One example is amatol, a mixture of TNT and ammonium nitrate. This mixture was a common military explosive in the 20th century, particularly during World Wars 1 and 2, due to limited supply of explosives. Amatol is still widely used in mining and quarrying operations, though it is now called Ammonite and is less powerful than the military-strength amatol mixes.
Many other mixtures with TNT have been developed, such as Composition B, Composition H6, Cyclotol, Octol, and Pentolite.
Though TNT is relatively stable, it does have some drawbacks. Explosives containing TNT can degrade when stored at high temperatures. This in turn may create cracks in the explosive, increasing shock sensitivity. In addition, TNT's energy density is somewhat low for an explosive compound. The US army has replaced TNT with a compound called IMX-101 in 155 mm artillery shells, and engineers familiar with the matter say IMX-101 may completely replace TNT within ten years. IMX-101 is less shock-sensitive than TNT as well as more stable at high temperatures, while possessing a similar explosive yield.
For years to come, though, TNT and mixtures thereof will remain the explosive of choice for civil engineering applications.