Class 4 - Division 4.1

Flammable solids include solids that are readily combustible or that may cause or contribute to fire through friction. This division also includes the following:

• Self-reactive substances that are liable to undergo a strongly exothermic, or heat producing, reaction
• Desensitized explosives that can explode if not sufficiently diluted

Readily combustible solids can be powdered, granular, or pasty. They can be easily ignited by brief contact with an ignition source, such as a burning match, and the flame can spread rapidly. In addition to the fire hazard, combustion byproducts can be toxic. Metal powders are particularly dangerous because fires from these powders are difficult to extinguish.

Classification

Powdered, granular, and pasty solid substances are classified in Division 4.1 when the time of burning of one or more test runs is less than 45 seconds, or when the rate of burning is more than 2.2 mm per second. The testing criteria are described in the "UN Manual of Tests and Criteria," Part III, Subsection 33.2.1.

Metal and metal alloy powders are classified in Division 4.1 when they can be ignited and the reaction spreads over the entire length of the sample in 10 minutes or less.

Solids that can cause a fire through friction are also classified in Division 4.1.

Flammable solids, other than metal powders, are assigned to packing groups based on the following criteria:

• If the burning time is less than 45 seconds and the flame passes the wetted zone, then it is assigned to Packing Group II.
• If the burning time is less than 45 seconds and the wetted zone stops the flame propagation for at least 4 minutes, then it is assigned to Packing Group III.

The packing group assignment criteria for metal powders or metal alloys are as follows:

• If the reaction spreads over the whole length of the sample in 5 minutes or less, then it is assigned to Packing Group II.
• If the reaction spreads over the whole length of the sample in more than 5 minutes but less than 10 minutes, then it is assigned to Packing Group III.

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