Class 4 - Flammable solids and reactive substances.

Though we have referred to these as reactive substances, the main risk of all these items is flammability.

Class 4 has three subdivisions, and within these are further divisions based on the chemical composition or physical state of the material.

Class 4.1, Flammable solids

This includes:

• Readily flammable solids or articles (can be ignited by friction or the brief introduction of a flame).

• Self-reactive solids or liquids and related substances (items that undergo a rapid exothermic decomposition as a result of heat, contact with impurities, friction or impact).

• Desensitized solid explosives.

Typical examples of class 4.1 commonly encountered include Sulphur, Hexamine (in stoves and fire-sets), Camphor, Naphthalene (Moth Balls), Charcoal, fire-lighters (paraffin wax) and matches.

Class 4.2, Substances liable to spontaneous combustion

• This class is for solids, and some liquids, that can spontaneously ignite in contact with air - pyrophorics.

• Self-heating substances that react slowly and are reasonably unlikely to catch fire.

There are few commonly used materials in this class offered for transport, though some of the more reactive substances such as butyl lithium have laboratory use and may be encountered in small cylinders. Some other items would include Carbon Black pigment, Phosphorus, Seed Cakes and wet wool and textile wastes (though not regulated under ADR).

Class 4.3, Substances which emit flammable gas in contact with water

There are a surprising amount of materials that are classed as 4.3.

The classic example is Calcium Carbide (used in carbide lamps) which emits Acetylene gas when wet.

Commonly, the gas emitted is Hydrogen, the most flammable and potentially explosive material in existence. The gas can either vent itself, build up in a confined space to form a explosive atmosphere or self-ignite due to the temperature increase in the Material forming it.

Typical examples would include many metals when powdered e.g. Aluminium, Magnesium and Zinc, and the reactive metals such as Lithium*, Potassium, Sodium, Calcium etc.

Unsurprisingly, steps must be taken to ensure these materials stay dry and well ventilated during carriage, under IMDG the loading of such products during rain is prohibited.

Recently, the driver of a waste disposal contractor was surprised when the 15 tonnes of aluminium smeltings he was transporting in an open-topped Rollonoff container on a rainy day burst into flames when tipped !

* The effect of accidentally damaging a lithium battery and exposing it's contents to air can been seen in the photo above.

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