The ultimate guide to working with explosives: from hazard classification to primary & secondary effects

What are explosives?

 Explosives are solid or liquid substances (or mixtures) with an intrinsic ability to undergo rapid chemical reactions that produce high temperature and pressure gases which travel at high enough speeds to cause damage to the surroundings.

Many explosions involve oxidizers since the oxygen in air is not enough to produce the rate of combustion necessary for an explosion.¹ Trinitrotoluene (TNT) for instance contains both an organic portion (toluene) and an oxidizer portion (nitro groups).

Pyrotechnic substances are considered explosives even though they do not produce gases. Instead, their non-detonative self-sustaining exothermic chemical reactions produce light effects, sound, gas or smoke or a combination of these.

Note that explosions can occur with chemicals that are not considered GHS explosives. For instance: (i) the large-scale exothermic polymerization of a monomer may trigger an explosion, (ii) sodium azide in the presence of a proper initiator may violently decompose into nitrogen gas and sodium ions, and (iii) an explosive reaction may occur only when mixing certain organic substances with strong oxidizers as in the case of mixing combustible fuel oil with ammonium nitrate fertilizer.¹

The ‘exploding bomb’ pictogram is used to represent three physical GHS hazard classes

Deflagration vs detonation

There are two types of explosions: deflagration and detonation with detonations being more destructive. Deflagration is the rapid combustion of explosive particles causing a rapid “burn” through the material at speeds below the speed of sound (e.g. burning of wood, fireworks, and other pyrotechnics, etc.).  Some chemicals that deflagrate, such as black powder need confinement in order to be explosive. Detonation on the other hand is an almost instantaneous (supersonic) combustion of an explosive that produces high temperature-pressure shock waves. Note that chemicals that detonate are dangerous without confinement.

Primary and secondary effects

Primary effects always accompany an explosion, while secondary effects may not always occur. There are three types of primary effects, which are (almost always) more destructive than secondary effects:

(i) Blast pressure: the destructive force, which travels outward from the origin of the explosion.

(ii) Fragmentation: when broken items including the explosive’s container and nearby materials are propelled.

(iii) Thermal: when (intense) temperatures are generated for short periods of time, characterized by flashes and fireballs.

Secondary effects include shock wave modification, shock wave transfer, and fire.

Residual hazards after an explosion

After an explosion, scattered bits and pieces of (sensitive) unexploded material will likely remain, requiring very little force to initiate smaller secondary explosions. The scene of an explosion must be assessed by authorized individuals before entering the area for cleanup activities.

How to mitigate the risks of working with explosive chemicals

1. Receiving:

• Maintain a detailed record of all explosive chemicals including:
• Date of opening and expiration
• The lab member responsible for the sample
• Date used and by which lab member
• Compound identification (structure, chemical name)
• Concentration if applicable
• Warning information (explosive, potentially explosive, explosive when concentrated)

 2. Storage:

• Include an “explosive” sign on all explosive containers and areas storing explosive chemicals. 
• When transferring explosives from one container to another such as for dilutions, include an “explosive” label on the new bottle.
• Unless an inhibitor is added, unopened containers of potentially explosive or shock-sensitive materials should be discarded within 6 months of opening.
• When not in use, close and return explosives to storage areas as soon as possible.
• If you are synthesizing an explosive or a chemical with an explosive intermediate, avoid storing them. If storage is required, do so in solutions under 1M and reduced temperatures.
• Avoid storing explosives in areas exposed to heat, friction, light, catalyst, or impact.

 3. Handling:

• Designate an area of the lab for working with explosives (e.g. fume hood or a glove box). Consider using a blast shield.
• Review reagent SDS prior to handling. Additional training may be required.
• Avoid using metal spatulas or needles when working with compounds where metals may catalyze an explosive decomposition.
• Avoid using glassware with ground glass joints to reduce the risk of friction and mechanical shock.
• In addition to standard PPE, additional protection may be required including chemical-resistant gloves, disposable lab coats, face shields, heavy duty lab coats, leather outer garments, or garments for high temperature blasts.
• Vacuum pumps should be equipped with cold traps and filters to prevent particulate release, and the exhaust must be circulated to an approved exhaust duct or fume hood.
• Be aware of reactions that may cause explosions even if the reagents being used are not considered explosives. Improper preparation of Grignard reagents for instance may result in heat and violent reactions.
  

 4. Emergencies and spills:

• Have a Hazard Control Plan (HCP) to determine the steps that should be taken in case of an explosion.
• Ensure emergency irrigation including showers and eyewash stations are within a 10 second walk of the area where work with explosives is being conducted.
• Have appropriate emergency equipment including fire extinguishers and first aid kits readily accessible.
• If an explosion occurs, immediately leave the area and contact lab security and emergency services.
• Clean the area after an explosion only if you have received approval from emergency services. Explosive particles may still be in the area, which can easily ignite with heat or pressure. Do not attempt to move explosive containers or clean without approval. When cleaning, use non-static wipes or wet cleaning methods.

 5. Awareness of surroundings:

• Place an “explosive” label on your fume hood to warn your colleagues of the hazard.
• Be aware of other chemicals, experiments, and conditions around you, such as colleagues working with flammables or using an open flame. Chemicals not directly used with the explosive reagent should be put away.
• Do not work alone. Ensure that someone is nearby in case of an emergency.
• Eliminate all sources of heat and fire such as burners and static from clothing, plastic, or transferring reagents. Note that low humidity environments increase the potential for static.

 6. Alternatives:

• Review your SOP to determine if there are less sensitive explosive chemicals you can use. If it is not possible, determine if other forms are available. Use powdered solids for instance rather than pellets to avoid grinding, which may cause an explosion via friction.
• Before attempting to substitute an explosive chemical, determine by what means it is explosive. Certain chemicals may explode when heated but are less sensitive to shock for instance.
• Remove sources of confinement for chemicals that deflagrate.
• When working with explosives, perform small-scale work. Avoid using or creating large batches of explosives.

 7. Waste:

• After working with explosives, wash your hands thoroughly with soap and water.
• Clean off all lab counters and fume hood surfaces to prevent accidental cross contamination.
• If a reactive chemical or explosive chemical container is damaged, budding, or past its expiration date, leave the area and contact lab security or waste management services.
• Avoid mixing explosive waste: some reagents such as organics and strong oxidizers are not explosive but may result in an explosion if combined.
• Arrange to have explosive waste removed from your laboratory regularly.

Reference:

1.  Safe Operating Procedure (unl.edu)