Class 4 Flammable Solids, Spontaneously Combustible and Dangerous When Wet

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Class 4.1 Flammable solids

 

These burn more easily than ordinary substances, and may reach high temperatures quickly and be difficult to put out.  Examples include fire lighters and aluminium powder.  Desensitised explosives are included in this class – for example, TNT wetted to 30% with water.

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Class 4.1 also includes self reactive products that either break down (decompose) or polymerise.  If these are allowed to become too warm, a cycle starts where heat is generated by the reaction that causes even more reaction and heat, until the product burns or explodes.  This is known as the SADT (self accelerating decomposition temperature) and the SAPT (self accelerating polymerisation temperature).  To avoid this, products of class 4.1 may be transported under temperature control (fridge or insulation).  If this is the case, it will be made clear on the transport document.  The temperature must be monitored by the driver throughout the journey, with a readout in the cab and in the load compartment.

 

There are three temperatures that are important:

• Control temperature – this is what the fridge is set to, the highest safe working temperature.

• Emergency temperature – if this is reached, emergency services must be called.

• SADT / SAPT – the product will burn or explode.

 

Class 4.2 - spontaneously combustible

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These products will react with oxygen in the air and release heat – sometimes enough to cause fires or explosions.  These substances are kept in air tight containers, or are transported in oxygen free environments.  Examples include carbon, fish guts, fresh rubber granules, freshly shelled pistachios and phosphorus.

Phosphorus ignites on contact with air, it is pyrophoric.

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Class 4.3 – dangerous when wet

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These products react with water or water vapour to release flammable gases.  The reaction may release enough heat to make the product and gases burn or explode.  These are kept in hermetically sealed containers, sometimes under oil in the containers.

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Examples of these substances include sodium, lithium, potassium, aluminium powder and calcium carbide.

The easy way to remember the order of class 4 substances is HOW – heat, oxygen and water, relevant for 4.1, 4.2 and 4.3.  However, all class 4 substances burn, so should be kept away from heat and sources of ignition (including static if powders are present).

 

Class 4 substances may have subsidiary hazards, including being toxic and corrosive.

The minimum PPE required is a high visibility vest, goggles and gloves.  If the product is a powder, or if it is toxic, a respirator will be required.

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