Explosives are often played with as though they were toys. The persons playing with explosives are often too young to have accumulated sufficient formal education in chemistry and physics to understand the hazards involved. One intent here is to supply sufficient information to allow intelligent choices to be made in the handling and use of explosives.
The misuse of explosives causes a knee-jerk reaction against any scientific discussion of the materials. This is unfortunate. They have unique properties that can help us understand materials in general: they "talk to us" with energy. You can build defects into crystals and watch the effects through changes in the explosives' thermal properties and sensitivity. You can easily follow absolute-rate processes. True scientific applications of explosives can be fascinating and rewarding. We hope that the information presented here will suggest methods for using explosives in research.
Three broad categories of hazards can be observed in the use of explosives: sensitivity, stability, and toxicity. Each type of hazard must be considered before explosives are handled.
Safety tests for impact, friction, shock, and sparks generate relative scales for evaluating hazards. Different explosives are compared with one another. It can be stated that TNT is less sensitive than PETN; however, a quantitative statement can not be made about exactly how hard either must be hit to cause initiation under all conditions. Relative scales lack credibility: some small change in experimental conditions can cause errors in hazards interpretations. Many different relative tests must be run to improve the probability that all hazards have been identified.
Safety is estimated from experience with the materials. The experience usually involves an accident and the subsequent accident investigation. If one explosive rarely causes accidents and another is found to be less sensitive to impact, you might jump to the assumption that it will be less hazardous. That is a very dangerous assumption. Each explosive, formulation, and application must be investigated by many different safety tests.
All initiations are thermal in nature. Detonations are characterized by a constant-velocity shock wave; however; the heat generated by chemical reactions supports the shock wave. An understanding of explosives requires an understanding of their thermochemical properties.
Thermochemistry of explosives:
Self-heating accidents have often occurred when small-scale procedures have been scaled up without considering the effects of size. Safe amounts can be estimated in very straightforward ways. Other accidents have occurred when additional materials were added to previously safe operations. Such "compatibility" problems can be observed before they become a hazard.
A large number of tests has been used to observe relative thermal stabilities. In addition, some more recent quantitative methods can be applied to predicting safe temperature ranges for many important explosives. Each approach will be discussed; however, no attempt is being made to make a comprehensive review. All comments and perspectives are based on our experience alone.
Relative Tests for Thermal Stability and Compatibility:
Tests Involving Chemical Product Detection or Analysis:
All of the earliest tests are in this class, and many of the tests are still in use in different parts of the world. Erroneous conclusions will always be drawn from these tests when results are used to compare stabilities of materials that decompose to produce different products.
Abel Test: One of the earliest tests is the Abel Test (1875). It observes the time required to change the color of starch/iodide paper when 1-gram samples are heated to 180 F. It is still used in the quality control of commercial cellulose nitrate (NC), nitroglycerin (NG), and nitroglycol. It can not be used to compare different types of explosives.
Vieille Test: The sample is heated at 110 C with a strip of litmus paper (1896). The sample is opened to the air overnight at room temperature, and the procedure is repeated until the paper turns red within one hour. The overall heating time is the quantity measured. A propellant powder passed by this test self heated to explosion in 1911, and two ships were sunk.
Bergmann-Junk Test: The test involves heating the sample to 132 C and determining extracted nitrites (1904). The test was much modified by Siebert in 1942, who used H2O2 as the trapping liquid, added a better apparatus for trapping the NO, and titrated total acidity. The test works well for quality control of a single explosive, but it can not be used to compare dissimilar materials.
Leclercq's Test: This test (1950) involves the determination of the amount of NO and NO2 liberated at 90 C as a function of time.
Changes in product composition will confuse all of these tests. For example, nitroguanidine decomposes to produce some ammonia. Ammonia changes total acidity and can react with oxides of nitrogen. None of the chemical tests observe the product of decomposition that causes self-heating accidents: heat.
Differential Thermal Analysis (DTA):
When self-heating is the problem, it seems only logical to make heat the quantity measured. DTA observes the production or absorption of heat by a sample.