Polymerizing Substances in the IMDG Code (UN 3531 to 3534)

Polymerizing substances are the fourth type of substance in IMDG Division 4.1 , defined in 2.4.2.5 of the IMDG Code. A substance falls in the type when, without stabilization, it is liable to undergo a strongly exothermic reaction producing larger molecules or polymers, its self-accelerating polymerization temperature (SAPT) is 75 °C or less, and its heat of reaction exceeds 300 J/g.

They are carried under four generic entries, UN 3531 to UN 3534, for solids and liquids, stabilized or temperature controlled. Under Amendment 42-24 (MSC.556(108)), temperature control applies where the SAPT is 50 °C or less in packagings and IBCs, or 45 °C or less in portable tanks (7.3.7.2.3), and a STABILIZED substance may still need it if the transport temperature could exceed 55 °C (7.3.7.2.4).

The full article will cover the classification route, stabilizer practice, the boundary with Class 3 and Class 9 entries for monomers, and the temperature-control requirements on board.