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CLASS 4 - FLAMMABLE SOLIDS
Note: The
following description is not intended to be a definitive technical description of what is
termed a dangerous goods under the New Zealand Standard 5433:1999 - Transport of Dangerous
Goods on Land or The United Nations Recommendations on the Transport of Dangerous Goods.
Flammability is one of the
principle hazard associated with dangerous goods.
There are three classes of flammable materials to
contend with in regards to transporting Dangerous Goods - Gases, Liquids
and Solids as described below.
Class 4.1
- flammable solids;
- self-reactive and related substances;
- desensitised explosives.
Class 4.2
- Substances liable to Spontaneous Combustion
- Pyrophoric liquids
- Pyrophoric solids
- Self-heating substances
Class 4.3
- Substances which, in contact with water,
emit flammable gases
| Each of these separate
classes is identified by a label: Flammable
solids are readily combustible solids and solids which may cause fire through friction.
Combustible solids are certain powdered, granular or pasty substances which become
dangerous if they are exposed to an ignition source, such as a burning match, and if the
flame spreads rapidly. The danger may also come from toxic combustion products. Metal
powders are especially dangerous because of the difficulty of putting the fire out when
the normal extinguishing agents such as carbon dioxide or water can increase the hazard. |
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Self-reactive substances of Division 4.1 are
thermally unstable substances liable to undergo a strongly exothermic decomposition even
without the participation of oxygen (air).
The decomposition of self-reactive substances can
be initiated by heat, contact with catalytic impurities, e.g. acids, heavy-metal
compounds, bases, friction or impact. The rate of decomposition increases with temperature
and varies with the substance. Decomposition can also result in the production of toxic
gases or vapours. For certain self-reactive substances, the temperature must be
controlled. Some self-reactive substances may decompose explosively, particularly if
confined and others burn vigorously. Self-reactive substances include some of the
following types of compounds:
- aliphatic azo compounds (-C-N=N-C-);
- organic azides (-C-N3);
- diazonium salts (-CN2+Z-);
- N-nitroso compounds (-N-N=O); and
- aromatic sulphohydrazides (-SO2-NH-NH2).
Substances with other reactive groups and some
mixtures of substances may have similar properties.
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Self-heating of a
substance leading to spontaneous combustion is caused by a reaction of that substance with
oxygen (in the air) if the heat developed is not conducted away rapidly enough to the
surroundings. Spontaneous combustion occurs when the rate of heat production exceeds the
rate of heat loss and the auto-ignition temperature is reached. Two types of substances
can be distinguished with spontaneous combustion properties:
- Pyrophoric substances -- substances (liquid or
solid) including mixtures and solutions which, even in small quantities, ignite within 5
minutes of coming in contact with air. These substances are the most liable to spontaneous
combustion;
- Self-heating substances -- solid substances which
generate heat when in contact with air without an additional energy supply. These
substances will ignite only in large amounts (kilograms) and after long periods of time
(hours or days).
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| Certain substances in
contact with water emit flammable gases which can form explosive mixtures with air. Such
mixtures are easily ignited by all ordinary sources of ignition, for example naked lights,
sparking handtools or unprotected light bulbs. The resulting blast wave and flames may
endanger people and the environment. |
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