What should you do if a raw material for a process is explosive? Good question. Just because a material has explosive properties does not automatically disqualify it for use. To use it safely you must accumulate some information on the type and magnitude of stimulus that is required to give a hazardous release of energy.
But first, some comments on the release of hazardous energy. Hazardous energy is that energy which, if released in an uncontrolled way, can result in harm to people or equipment. This energy may be stored in the form of mechanical strain of the sort found in a compressed spring, a tank of compressed gas, the unstable chemical bonds of an explosive material, or as an explosive mixture of air and fuel. A good old fashioned pool fire is a release of hazardous energy as well. The radiant energy from a pool fire can easily and rapidly accelerate past the ignition point of nearby materials.
Accumulating and applying energy in large quantities is common and actually necessary in many essential activities. In chemical processing, heat energy may be applied to chemical reactions. Commonly, heat is also released from chemical reactions at some level ranging from minimal to large. The rate of heat evolution in common chemical condensation or metathesis reactions can be simply and reliably managed by controlling the rate of addition of reactants where two reactants are necessary.
There are explosive materials and there are explosive conditions. If one places the components of the fire triangle into a confined space, what may have been conditions for simple flammability in open air are now the components for an explosion. Heat and increasing pressure will apply PV work to the containment. In confinement, the initiation of combustion may accelerate to deflagration or detonation. The outcome will minimally be an overpressure with containment failure. If the contents are capable of accelerating from deflagration to detonation, then loss of containment may involve catastrophic failure of mechanical components.
Rate control of substances that autodecompose or otherwise break into multiple fragments is a bit more tricky. This is the reaction realm of explosives. The energy output is governed by the mathematics of first order kinetics, at least to some level of approximation. In first order kinetics, the rate of reaction depends on both the rate constant and the intitial concentration of one reactant. Regarding the control of reactions that are approximately first order in nature, some thought should be given to limiting the reaction mass size to that which is controllable with available reactor utilities. A determination of the adiabatic ΔT will give information that will tell you if the reaction will self-heat past the bp of your solvent system.
There is a particular type of explosive behavior called detonation. Detonation is a variety of explosive behavior that is characterized by the generation and propagation of a high velocity shock through a material. A shock is a high velocity compression wave which begins at the point of initiation and propagates throughout the bulk mass. Because it is a wave, it can be manipulated somewhat. This is the basis for explosive lensing and shaped charges.
Detonable materials may be subject to geometry constraints that limit the propagation of the shock. A cylinder of explosive material may or may not propagate a detonation wave depending on the diameter. Some materials are relatively insensitive to the shape and thickness. A film of nitroglycerin will easily propagate as will a slender filling of PETN in detcord. But these compounds are for munitions makers, not custom or fine chemical manufacturers. The point is that explosability and detonability is rather more complex than you might realize. Therefore, it is important to do a variety of tests on a material suspected of explosability.
A characteristic of high order explosives is the ability to propagate a shock across the bulk of the explosive material. However, this ability may depend upon the geometry of the material, the shock velocity, and the purity of the explosive itself. There are other parameters as well. Marginally detonable materials may lose critical energy if the shape of the charge provides enough surface area for loss of energy. The point is that “explosion” and “detonation” are not quite synonymous, and care must be exercised in their use. The word “detonation” confers attributes that are unique to that phenomenon.
Explosive substances have functional groups that are the locus of their explosibility. A functional group related to the onset of explosive behavior, called an explosiphore (or explosaphore), is needed to give a molecule explosability beyond the fuel-air variety. Obvious explosiphores include azide, nitro, nitroesters, nitrate salts, perchlorates, fulminates, diazo compounds, peroxides, picrates and styphnates, and hydrazine moieties. Other explosiphores include hydroxylamino. HOBt, a triazole analog of hydroxyamine, hydroxybenzotriazole, has injured people, destroyed reactors and caused serious damage to facilities. Hydroxylamine has been the source of a few plant explosions as well. It is possible to run a process for years and never cross the line to runaway.
Let’s go back to the original question of this essay. What do you do if you find that a raw material or a product is explosive? The first thing to do is collect all available information on the properties of the substance. In a business organization, upper management must be engaged immediately since the handling of such materials involves the assumption of risk profiles beyond that expected.
At this point, an evaluation must be made in relation to the value of the product in your business model vs the magnitude of the risk. Dow’s Fire and Explosion Index is one place to start. This methodology attempts to quantify and weight the risks of a particular scenario. A range of numbers are possible and a ranking of risk magnitude can be obtained therein. It is then possible to compare the risk ranking to a risk policy schedule generated beforehand by management. The intent is to quantify the risk against a scale already settled upon for easier decision making.
But even before such a risk ranking can be made, it is necessary to understand the type and magnitude of stimulus needed to elicit a release of hazardous energy. A good place to start is with a DSC thermogram and a TGA profile. These are easy and relatively inexpensive. A DSC thermogram will indicate onset temperature and energy release data as a first pass. Low onset temperature and high energy release is least desirable. High onset temperature and low exothermocity is most desirable.
What is more difficult to come to a decision point on is the scenario where there is relatively high temperature onset and high exothermicity. Inevitably, the argument will be made that operating temperatures will be far below the onset temp and that a hazardous condition may be avoided by simply putting controls on processing temperatures. While there is some truth to this, here is where we find that simple DSC data is inadequate for validating safe operating conditions.
Onset temperatures are not inherent physical properties. Onset temperatures are kinetic epiphenomena that are dependent on sample quality, the Cp of both the the sample and the crucible, and the rate of temperature rise. What is needed once an indication of high energy release is indicated by the DSC is a determination of time to maximum rate (TMS) determination. While this can be done with special techniques in the DSC (i.e., AKTS). TMR data may be calculated from 4 DSC scans at different rates, or it may be determined from Accelerated Rate Calorimetry, or ARC testing. Arc testing gives time, temp, and pressure profiles that DSC cannot give and in my mind, is the more information-rich choice of the two approaches. ARC also gives an indication of non-classical liquid/vapour behavior that is useful. ARC testing can indicate the generation of non-condensable gases in the decomposition profile which is good to know.
Other tests that indicate sensitivity to stimulus is the standard test protocol for DOT classification. Several companies do this testing and rating. There are levels of testing applied based on the result of what the lower series tests show. Series 1 and 2 are minimally what can be done to flesh out the effects of basic stimuli. What you get from the results of Series 1, 2, and 3 are a general indication of explosabilty and detonability, as well as sensitivity to impact and friction. In addition, tests for sensitivity to electric discharge and dust explosability should be performed as well.
The Gap test, Konen test, and time-pressure test will give a good picture of the ability to detonate, and whether or not any explosability requires confinement. The Konen test indicates whether or not extreme heating can cause decomposition to accelerate into an explosability sufficient to fragment a container with a hole in it.
BOM or BAM impact testing will indicate sensitivity to impact stimulus. Friction testing gives threshold data for friction sensitivity.
ESD sensitivity testing gives threshold data for visible effects of static discharge on the test material. Positive results include discoloration, smoking, flame, explosive report, etc.
Once the data is in hand, it is necessary to sift through it and make some determinations. There is rarely a clear line on the ground to indicate what to do. The real question for the company is whether or not the risk processing with the material is worth the reward. Everyone will have an opinion.
The key activity is to consider where in the process an unsafe stimulus may be applied to the material. If it is thermally sensitive in the raneg of heating utilities, then layers of protection guarding against overheating must be put in place. Layers of protection should include multiple engineering and administrative layers. Every layer is like a piece of Swiss cheese. The idea is to prevent the holes in the cheese from aligning.
If the material is impact or friction sensitive, then measures to guard against these stimuli must be put in place. For solids handling, this can be problematic. It might be that preparing the material as a solution is needed for minimum solids handling.
If the material is detonable, then all forms of stimulus must be guarded against unless you have specific knowledge that indicates otherwise. Furthermore, a safety study on storage should be performed. Segregation of explosable or detonable materials in storage will work towards decoupling of energy transfer during an incident. By segregating such materials, it is possible to minimize the adverse effects of fire and explosion to the rest of the facility.
With explosive materials, electrostatic safety is very important. All solids handling of explosable solids should involve provisions for suppression of static energy. A discharge of static energy in bulk solid material is a good way to initiate runaway decomposition in an energetic material. This is how a material with a high decomposition temperature by DSC can find sufficient stimulus for an explosion.
Safe practices involving energetic materials require an understanding the cause and effect of stimulus on the materials themselves. This is of necessity a data and knowledge driven activity. Along with ESD energy, handwaving arguments should also be suppressed.