Basic concepts in hazardous chemicals: Explosiveness

EXPLOSION CAUSED BY IMPROPER handling and use of chemicals have had devastating effects on humans. Take the example of the Bright Sparklers incident in Sg Buloh in 1990. The incident has shown the need for proper handling of explosive chemicals during its manufacturing process.

A recent explosive in Shaoyang, China where a massive blast ripped through an unauthorised underground explosives warehouse in a residential area, killed 90 people with 400 injured as well as flattening buildings within a 100-metre radius. This incident demonstrates the need for a proper storage area for explosive chemicals, away from human dwellings and with adequate safety storage conditions.

However, explosives also help us construct tunnels, mine underground, remove obstacles during road construction, extinguish oil well fires, inflate automobile air bags and destroy hazardous wastes. In fact, the application and rigorous study of the development of explosives in the industry has given rise to the field of pyrotechnics which covers the safety aspects of using explosives. Through pyrotechnics, explosives are divided into two categories: detonating and propellant.

Detonating explosives are further classified as initiating (primary) or secondary. Initiating explosives, the more sensitive of the two, must be handled with extreme care. They require only a low energy stimulus, such as being touched with a hot wire or tapped with a hammer, to explode. For that reason, they are put into blasting caps. Secondary explosives are less sensitive and can burn without producing an explosion. They only detonate by means of a severe shock, delivered by another explosive (usually a blasting cap) placed in or near them.

Since they are relatively stable, large amounts can be moved and handled safely. The velocity at which explosives detonate determines their function. Explosives of low detonating velocity supply a slow push or heave. Explosives with a high detonating velocity have a blasting or shattering effect.

Propellant explosives are used for firearms, rockets, general engineering, and demolition work. They differ from detonating explosives in that an avenue of escape is provided for the expanding gases.

Industries rely on small explosive charges for a number of purposes. In the automobile industry, for example, the latest safety feature incorporated to absorb impact that is car airbags, uses an explosive, sodium azide. This compound consists of inter-penetrating lattices of ions of sodium and azide (a group of three chemically- bonded nitrogen atoms). An impact disrupts the lattice structure where the sodium combines with the oxygen while the nitrogen atoms regroup into pairs to form large quantities of nitrogen gas to fill the airbag.

The launching of a satellite also utilises a special kind of explosive, a type of propellant to provide the thrust of taking the satellite into orbit. The solid fuel booster rocket normally contains several kilogrammes of a propellant consisting of energetic pulverized aluminium, ammonium perchlorate oxidizer and a special binder and fuel called PBAN terpolymer.

When this mixture is oxidised , PBAN releases copious quantities of carbon monoxide and carbon dioxide gas and steam that help push the shuttle into space. Ammonium perchlorate is used as a rocket propellant because its decomposition products are all gases and so it enhances the rockets' thrust.

Explosions can normally happen when flammable or reactive vapours and gases are ignited or when chemicals which have the inherent characteristic of explosiveness are subjected to heat, mechanical shock or contact with a catalyst.

Nitroglycerin, for example, is explosive in nature. A small disturbance such as heat or impact causes it to decompose into carbon dioxide, water, nitrogen and oxygen. The result of these conversions will lead to a violent release of energy. As a precaution to prevent an explosion, information on the explosive nature of a chemical can be found in its material safety data sheet (MSDS). It is important to check the compatibility of the chemical being handled with other substances to ensure that no potential explosion hazard may arise.

Should a contact with a chemical or a mixture known to possess an explosive hazard is unavoidable, then precautionary steps are needed to ensure the safety of workers handling it. An explosive material can normally be identified from the precautionary label as shown in the diagram which is attached to the container. For an unknown chemical, the match test is said to provide a qualitative but useful guide to its explosive characteristics.

For this test, approximately 10 mg of the compound is placed on the tip of a wooden handled spatula having a blade about 2.5cm long, and the spatula is balanced over the edge of a suitable surface with the blade hanging over the edge. A small flame from a match held with a pair of tongs is then held below the blade about 2 cm from the sample. If the sample burns with flashing or a detonation occurs, the material should be handled as an explosive. It should always be remembered that never heat any substance in a closed container.

Under normal circumstances, an explosive is a stable material that, upon stimulation, very rapidly changes from a solid or liquid into a hot, expanding gas. The sudden release of energy and the accompanying pressure exerted on the surrounding materials by the expanding gas constitute an explosion. If an explosive is confined, the likelihood of the detonation would be increased. For example, when gunpowder is confined within the paper wrapping of a firecracker, it explodes when ignited.

However, the same powder sprinkled in the open simply burns when ignited. In the confined state, the heat generated at the point of ignition will immediately ignite a large amount of surrounding material. The expanding gases from all the "burning" gun powder will act together, resulting in an explosion.

A clear understanding of the dangerous properties of explosives and due care in the handling of ingredients or finished products is necessary if accidents are to be avoided. In principle, an explosive mixture consists of an oxidizing agent, which usually produces oxygen used to burn the mixture, and a reducing agent, which burns to produce hot gasses. These are not all the possibilities, but they cover most cases of explosive chemicals.

Oxidizing agents such as nitrates, chlorates, peroxides, oxides, chromates and perchlorates provide the oxygen. They usually consist of a metal ion and the actual oxidising radical. For example, potassium nitrate contains a metal ion (potassium) and the oxidizing radical (the nitrate). As a comparison, the nitrates compounds are stingy with the oxygen that they give up. They only give one third of what they have. Chlorates are very generous, on the other hand. They give up all the oxygen they have.

Furthermore, they give it up more easily. It takes less heat, or less shock to get that oxygen loose. Mixtures using chlorates burn more spectacularly, because a smaller volume of the mix needs to be wasted on the oxidizer, and the ease with which the oxygen is supplied makes it burn faster. But the mixture is also much more sensitive to shock.

Perchlorates round out our usual set of oxidizing tools.

Perchlorates contain even more oxygen than chlorates, and also give it all up. However, they are not as sensitive as the chlorates, so they make mixtures that are "safer". That is, they're less likely to explode if you drop or strike them. Reducing agents such as sulphur and charcoal (carbon) simply burn the oxygen to produce sulphur dioxide and carbon dioxide. Examples of potentially explosive chemical mixtures are shown in the illustrated table.

With the potential harm posed by the hazard of explosion from chemicals, always observe all possible precautions particularly the following:

On an industrial scale, operations are commonly carried out by remote control and the considerations of quantity-distance and barrier design apply. On a lab scale, remote weighing, mixing and pressing are not practical, and the protection of the personnel must take the form of shielding for eyes, face and hands, as well as through protection for the hair and the choice of suitable clothing.

Complete eye shields, while desirable, are often foregone in order to make the minimum protection of ordinary safety glasses enforceable. Rubber or plastic gloves are recommended for work with acidic or caustic reagents, but these must not be worn when handling flammable materials because they melt and stick to the damaged skin.

In certain cases, suede leather gloves, which must be washed after use to prevent the impregnation of flammable or toxic matter, are preferred. Wear face shields when handling hazardous liquids, and always wear a suitable cover for expansive hair styles. Stationary shields are preferred when work is performed while seated at a work bench or when the equipment is situated in a hood.

Experimental or hazardous work should only be performed when two persons are present, although each person should have a separate work area assigned in order to avoid the danger of both being injured by the same accident.

Quite properly, the prohibition of matches and open flames is a universal one in any explosive-handling settings. Most facilities have separate rest areas for their personnel which feature oversized cigarette lighters which discourage their being pocketed. Moreover, smoking should not be permitted when personnel is wearing protective clothing which is contaminated with flammable dust.

Special fire-fighting agents must be provided when working with hazardous chemicals. Water and aqueous cupric sulphate solutions are recommended for phosphorus fires. Reactive metals must not be mixed with chlorinated hydrocarbons, instead hydrocarbons should be used for storage and degreasing.

Special carbonaceous powders are marketed for smothering burning alkali metals. Propellant and explosives fires can be drenched with water provided the material is not confined.

To minimize the risk of electrostatic discharge, materials, personnel and facilities should be grounded. The source of the static charge is often the packing material unless special precautions are taken in its selection.

Glass bottles and jars as well as polyethylene containers are to be avoided for the pouring and shaking of dielectric liquids and powders. Pointed spatulas, particularly when these are used to scrape or dislodge dry components, are frequently found to be the cause of mishaps. Some powdered materials are best handled when moistened with alcohol.

Always wear a face shield or at least shatter proof safety glasses. If there is need to handle any chemical in dust form, always wear a dust respirator. These small particles gather in the lungs and may cause serious illness later in life.

It is a safer procedure to avoid the conditions which promote static charge accumulation as a matter of routine. Low ambient humidity is the chief contributing factor in the accumulation of hazardous static charges. For this reason, air conditioned air, in the absence of artificial humidification, should be strictly controlled. In order to maintain a safe humidity above 50% relative humidity, the use of evaporative coolers is helpful.

Advance planning for the possibility of an accident will greatly minimize the consequences. A common hazard is found when a bench scale operation is scaled up and when this scale-up occurs in an over-utilized laboratory area. Solvents in small and large quantities may be found in the immediate vicinity and batches of oxidizers, explosives and similar hazardous ingredients may be present where they may be exposed to ignition by one or more mechanisms.

Often, other personnel are present within the structure, not knowing of the potentially hazardous operations which are conducted in their immediate vicinity. Exits and walk-ways may be blocked by materials, equipment or personnel in transit.

Explosives have an abundance of toxicological hazards, the extent of which is being increasingly identified. Isocyanate curing agents can cause serious allergenic reactions in sensitive individuals. Nitrate esters cause severe headaches in low concentrations, although personnel have been known to become addicted to these vapours, causing severe withdrawal symptoms when they are denied access to them.

Organic dyes and chromic oxides as used in pyrotechnic smokes and signals, as well as in gasless reaction mixtures, are rightly suspected of being carcinogenic. Metal chlorides, found in combustion products, are known to cause pulmonary edema upon hydrolysis in the lung tissue. Heavy metal ions of barium and lead can bring on chronic impairment of liver and kidney functions.

As a conclusion, under normal circumstances, we must therefore assume that hazardous operating conditions are in fact common and that continued vigilance by the worker himself coupled with unrelenting surveillance will remain the best prevention in chemical accidents be it explosions or fires.