IMDG Class 4.1 is the hazard subclass for flammable solids, self-reactive substances, and solid desensitised explosives. The class groups three different hazard mechanisms under one heading, and the specific entries within each behave very differently in transport. Common Chinese-export examples include sulphur (UN 1350), naphthalene (UN 1334), AIBN (UN 3234, a self-reactive polymerisation initiator), and matches.
What defines Class 4.1
Three sub-categories sit inside Class 4.1:
| Sub-category | Hazard | Examples |
|---|---|---|
| Flammable solids | Solids that ignite readily and burn vigorously | Sulphur, naphthalene, hexamine |
| Self-reactive substances | Solids and liquids that undergo strongly exothermic decomposition without oxygen involvement | AIBN, certain organic peroxide formulations, some monomers |
| Solid desensitised explosives | Explosives wetted, dissolved, or suspended to suppress sensitivity | Wetted nitrocellulose, picric acid wetted with at least 30% water |
The packing group assignment depends on burn rate (for flammable solids) and self-accelerating decomposition temperature, or SADT (for self-reactives). PG I is reserved for highly dangerous substances with very fast burn rates or low SADTs. PG II is the bulk of routine shipments. PG III is rare in Class 4.1.
Common Class 4.1 chemicals exported from China
Three substances come up often in Chinese chemical sourcing:
- Sulphur (UN 1350, PG III), bulk industrial sulphur from refining or mining; used in fertiliser, vulcanisation, and chemical synthesis. Largest volume Class 4.1 export from China by mass.
- Naphthalene (UN 1334, PG III), coal-tar derivative used in dye, plasticiser, and PNS concrete admixture chemistry. Crystalline solid, ships in 25kg bags or 1MT super sacks.
- AIBN. Azobisisobutyronitrile (UN 3234, self-reactive type C, PG II), polymerisation initiator for PVC, polystyrene, and other polymers. Self-reactive: requires temperature control during transport.
For sulphur, the issue at Chinese load ports is rarely the substance itself but the marine pollutant designation if the sulphur is in molten form (UN 2448. Sulphur, molten, has different requirements). Confirm the UN number with the factory before booking.
The temperature-control trap
Self-reactive substances in Class 4.1 are assigned to one of seven types (A through G) based on their behaviour in IMDG test methods. Types A and B are too dangerous to transport by sea at any quantity. Types C, D, E, and F are transportable, with types D, E, and F requiring temperature control during transport when the SADT is at or below 55°C.
AIBN is type C and does not require temperature control under standard transport. But many other azo compounds and peroxide-based polymerisation initiators do. The control temperature is typically 10°C below SADT, the emergency temperature 5°C below SADT. Both must be marked on the package and declared on the shipping papers.
For a buyer importing temperature-controlled Class 4.1 cargo, the carrier requirement is a refrigerated container (reefer) set to the control temperature, plus a temperature monitoring chart for the voyage. The freight rate premium versus a standard FCL is significant, typically USD 3,000 to USD 6,000 extra per container on the China-to-US lane. Build this into the FOB-to-landed-cost calculation before agreeing to a quote.
Packaging requirements
Class 4.1 packaging must be UN-certified to the assigned packing group. For sulphur and naphthalene in 25kg multi-wall paper bags, the bag itself is UN-marked. For super-sack shipments the super-sack must be UN 13H1, 13H2, 13H3, or 13H4 depending on lift type. AIBN typically ships in fibreboard boxes containing UN-certified plastic inner packagings, often with a small ice pack or refrigerant pack for additional thermal buffering.
Compatibility considerations matter inside the container. Class 4.1 cargo loaded with cargo that releases heat or ignition sources is a poor stowage. Mixed loads should be reviewed against the segregation table before booking.
Segregation at sea
Class 4.1 must be stowed:
- “Away from” most other DG classes (Class 1, 2.1, 3, 4.2, 4.3, 5.1, 5.2, 6.1, 8)
- “Separated from” Class 5.1 (oxidisers) and Class 5.2 (organic peroxides), these classes can supply oxygen to a Class 4.1 fire
- “Segregated from” Class 7 (radioactives)
For self-reactive substances specifically, segregation is stricter. They cannot stow with Class 5.1 oxidisers under any condition, and stow restrictions on heat sources (engine room bulkheads, fuel tank bulkheads) apply.
Documentation chain
Same as Class 3 but with extras for self-reactives:
- DG declaration with UN number, PSN, class, sub-class type code (e.g. “Type C” for AIBN), PG, quantity
- SDS with section 14 carrying full transport classification including SADT and control/emergency temperatures for self-reactives
- UN-certified packaging with correct marking
- Container placards, orange diamond with the “4” Class 4.1 designation. Self-reactives may carry a secondary subsidiary risk placard
- For temperature-controlled cargo: a reefer booking confirmation, control/emergency temperatures on the package and B/L, and a voyage temperature chart
Operator note: Chinese export licence requirements
Some Class 4.1 substances require a specific Chinese export licence beyond the standard MEE declaration. AIBN, for example, has been on the China dual-use export control list at various points. Before contracting a new factory for an AIBN or self-reactive purchase, confirm the factory has current export-licence eligibility and that the substance is not on the current dual-use list. The licence-check failure mode is the cargo being held at the export port pending licence issue, with no clear timeline for release.
Related terms
IMDG umbrella code. Class 4.2 covers spontaneously combustible substances. Class 4.3 covers substances that emit flammable gases on contact with water. Segregation table for stow compatibility.