What Does Self-Heating Chemical Mean?
Self-heating chemicals are chemicals that emit heat when they react with oxygen. If the heat produced by exposure to oxygen exceeds the rate of heat loss, it can lead to combustion or self-ignition.
These chemicals are known as self-heating because they don't require an external source of energy.
They are distinct from pyrophoric chemicals. While pyrophoric chemicals ignite spontaneously (within five minutes of coming in contact with oxygen), self-heating chemicals only ignite after hours or days and only when present in large amounts.
Safeopedia Explains Self-Heating Chemical
OSHA’s Hazard Communication Standard (HCS 2012) defines self-heating chemicals as:
"a solid or liquid substance or mixture, other than a pyrophoric liquid or solid, which, by reaction with air and without energy supply, is liable to self-heat; this chemical differs from a pyrophoric liquid or solid in that it will ignite only when in large amounts (kilograms) and after long periods of time (hours or days)."
Self-Heating Chemical Classifications
There are two categories of self-heating chemicals, based on tests performed n accordance with test method N.4 in Part III, sub-section 188.8.131.52 of the UN ST/SG/AC.
Criteria: A positive result is obtained in a test using a 25 mm sample cube at 140°C (284°F)
Criteria: A negative result is obtained in a test using a 25 mm cube sample 140°C (284°F), a positive result is obtained in a test using a 100 mm sample cube at 140°C (284°F), and:
- The unit volume of the chemical is 3 m3; or
- A positive result is obtained in a test using a 100 mm cube sample at 120°C (248°F), and the unit volume of the chemical is more than 450 liters; or
- A positive result is obtained in a test using a 100 mm cube sample at 100°C (212°F)
Self-Heating Chemical Fire Hazards
Self-heating chemicals gradually heat up in the presence of oxygen in the air and can self-ignite or combust. This happens when the rate of heat production exceeds the rate of heat loss, and the temperature of the substance rises. In other words, the substance must generate more heat than it loses to the environment.
Since the volume of a material increases by the cube, the larger the volume of material, the more it is susceptible to self-heating. Larger volumes also have lesser surface area over which to lose heat.
The Globally Harmonized System (GHS) criteria for defining a self-heating substance states that if for a volume of 27 m3, the spontaneous combustion temperature of a chemical is higher than 50°C or 122°F, it shall not be classified as a self-heating chemical.
The following interrelated factors also contribute to a fire hazard by self-heating substances:
- The volume of chemicals present
- The rate at which the material self-reacts
- Ambient temperature and insulating factors
- Availability of oxygen/moisture
Mechanisms of Self-heating
Self-heating can result from exothermic chemical reaction or by exothermic decomposition
. In the exothermic (or heat releasing) mechanism, the chemical reaction is an oxidation reaction with air. In exothermic decomposition, for unstable materials, while releasing heat, it results in less complex molecules and sometimes gases. However, it does not need additional reactants and is mostly independent of its environment.