Explosives may be classified according to a wide variety of parameters, some of which are related to their properties, while others – to the applications they serve. The classification is usually performed on the basis of on one or more of the following characteristics:
- Chemical structure
- Chemical and physical properties: state, detonation speed, brisance (the shattering effect of a high explosive)
- The material’s sensitivity to initiation (classification based on safety considerations)
- The explosives’ civilian and military applications
Liquid explosives may be classified into the following categories:
Explosives can be manufactured from either one or a number of substances. Liquid explosives, similarly to all other explosives, are found in a metastable state, enabling (under suitable conditions) the occurrence of a quick chemical reaction without requiring the presence of an environmental reactant, such as oxygen. Liquid explosives can be initiated through mechanical shock, friction or heat.
The first liquid explosive, nitroglycerin, was invented in 1846 by an Italian chemist named Ascanio Sobrero, who nitrated glycerin with a solution containing nitric acid and sulfuric acid. This substance, belonging to the nitro esters family, significantly expanded the range of applications in which explosives could be used, to include new applications previously served only by black gunpowder.
The safe manufacture and transport of nitroglycerin was problematic; over the years, it took the lives of many. When transporting nitroglycerin, small bubbles are created, which are then compressed when the substance is in motion. This compression results in a local, momentary rise in temperature and pressure, causing a detonation even when the motion is slight.
Many potential ways of overcoming this problem were examined before a solution was found in 1860 by Alfred Nobel: mixing nitroglycerin with a solid substance, which absorbs it. This mixture, which contains nitroglycerin and a Kaisal Gohr-type clay, is called dynamite.
In a terror attack perpetrated on December 11, 1994, a bomb made of nitroglycerin was planted on Philippine Airlines flight 434. It was concealed in a bottle purportedly containing contact lens cleaning solution, with a delay detonation mechanism. The blast caused a rupture in the fuselage, resulting in the death of one passenger and the injury of several others.
Another terror attack with the involvement of liquid explosive was used in 1987, where an explosive device combining C4 and PLX was used to blow up Korean Airlines flight 858. The device was carried onboard by North Korean agents.
Liquid explosives exist in a variety of colors and densities, as well as in configurations other than the liquid phase – in mixtures of solids, gels and emulsions. The performance level of most liquid explosives, when comparing detonation speed and brisance, is as high as those of solid explosives; however, the use of liquid explosives in the liquid phase is limited, mainly due to their relatively high volatility and high toxicity – limitations that demand uniquely cautious storage conditions and usage.
Whenever attempting to determine which liquid explosives are suitable for particular applications, one must remember that in contrast to pure liquid explosives, the existing variety of mixtures and the ratios of their various components require in-depth knowledge.