IMDG Class 4 is the most diverse DG class in terms of hazard mechanisms, covering three fundamentally different sub-divisions: solids that ignite through friction (4.1), materials that self-ignite on contact with air (4.2), and substances that react violently with water (4.3). Each sub-division demands specific packaging strategies, and carrier acceptance ranges from broad (4.1) to severely restricted (4.3). This guide walks through each sub-division with practical shipping guidance for China-based exporters.
Class 4.1 — Flammable Solids
Substances that are easily ignited by brief contact with an ignition source (spark, flame, friction) or that are self-reactive and liable to exothermic decomposition. Class 4.1 also includes desensitized explosives.
Product examples:
- Sulfur (UN1350): One of the highest-volume Class 4 exports from China. Used in fertilizer, rubber vulcanization, and chemical manufacturing. Ships in bulk bags, drums, or containers. Must be kept away from oxidizers.
- Matches (UN1944/UN1945): Safety matches (UN1944) and strike-anywhere matches (UN1945). Packaged in inner boxes with friction strips, outer cartons, and DG-labeled pallets.
- Celluloid (UN2000): Used in table tennis balls, film stock, and decorative items. Highly flammable. Requires fire-retardant packaging.
- Naphthalene / Mothballs (UN1334): Sublimes at room temperature; vapor is flammable. Requires sealed packaging and ventilation.
- Charcoal, activated carbon (UN1361/UN1362): Freshly prepared or non-activated forms can self-heat. Must be cooled and ventilated before packing. UN1362 applies to activated carbon from non-animal sources.
- Nitrocellulose (UN2555/UN2556/UN2557): Depending on nitrogen content and wetting agent. Ships wetted with alcohol or water. Desensitized explosive precautions apply.
Class 4.2 — Substances Liable to Spontaneous Combustion
Materials that heat up and ignite within five minutes of contact with air at ambient temperature (pyrophoric), or that self-heat in large quantities and catch fire. These are among the most operationally dangerous DG cargoes.
Product examples:
- Phosphorus, white/yellow (UN1381): Ignites spontaneously in air. Ships under water or inert gas in hermetically sealed containers. Strictly controlled export from China.
- Metal powders — pyrophoric (UN1383, UN3089): Aluminum, magnesium, zirconium powders that ignite in air. Must be shipped under nitrogen blanket in hermetically sealed drums.
- Activated carbon (UN1362): Fresh carbon from steam activation can self-heat. Must be cooled completely, ventilated to dissipate heat, and packed in vented containers. "Freshly prepared" is the key phrase on the MSDS.
- Copra (UN1363): Dried coconut kernel. Can self-heat due to oil oxidation. Requires moisture content monitoring and ventilation during transit.
- Fish meal, unstabilized (UN1374): Protein and oil content can oxidize exothermically. Only stabilized fish meal (with anti-oxidant treatment) may ship as non-DG.
Class 4.3 — Substances Dangerous When Wet
Substances that react with water to release flammable gases (hydrogen, acetylene, methane) in dangerous quantities. Even atmospheric humidity can trigger gas generation. This is the most restricted Class 4 sub-division for carrier acceptance.
Product examples:
- Calcium carbide (UN1402): Reacts with water to produce acetylene gas. China is the world's largest producer and exporter. Ships in hermetically sealed steel drums with nitrogen purge. Absolutely must stay dry.
- Sodium (UN1428): Reacts violently with water producing hydrogen and heat. Ships under mineral oil or inert atmosphere in sealed metal containers.
- Potassium (UN2257): More reactive than sodium. Same shipping requirements — sealed under inert medium.
- Aluminum powder, uncoated (UN1396): Reacts with water to release hydrogen. Must be differentiated from coated/stabilized aluminum powder (UN1309, Class 4.1).
- Zinc powder/dust (UN1436): Reacts with water and acids. Ships in sealed drums with desiccant. Major export from China's zinc industry.
- Magnesium powder (UN1418): Similar to aluminum powder. Particle size determines reactivity.
Packaging Requirements by Sub-Division
4.1 Flammable Solids
- UN-certified fiberboard boxes with inner plastic liners (for small volumes)
- UN-approved steel or plastic drums for bulk shipments
- Must be protected from friction and impact during transport
- Segregation from oxidizers (Class 5.1) is mandatory — minimum 3 meters or separate compartment
4.2 Spontaneously Combustible
- Hermetically sealed packaging with nitrogen or inert gas blanket
- For substances shipped under water (phosphorus): sealed metal drums with sufficient water cover
- Temperature monitoring recommended for bulk shipments
- Container must be stowed on deck with ventilation
4.3 Dangerous When Wet
- Hermetically sealed steel drums with desiccant packs inside
- Nitrogen purge recommended for calcium carbide
- Moisture indicator strips should be included inside packaging
- Container must be absolutely dry before loading; check container floor for condensation
- On-deck stowage mandatory — never below deck where bilge water could be present
Carrier Acceptance Table
| Carrier | Class 4.1 | Class 4.2 | Class 4.3 |
|---|---|---|---|
| MSK | Accepted | Accepted (restricted) | Accepted (case-by-case) |
| HPL | Accepted | Case-by-case | Not accepted |
| MSC | Accepted | Accepted (restricted) | Limited acceptance |
| COSCO | Accepted | Case-by-case | Case-by-case |
| HMM | Accepted | Not accepted | Not accepted |
| OOCL | Accepted | Not accepted | Not accepted |
| EMC | Accepted | Not accepted | Not accepted |
| YML | Accepted | Case-by-case | Not accepted |
| CMA CGM | Accepted | Case-by-case | Case-by-case |
Port Recommendations
Qingdao Port is the recommended loading port for all Class 4 sub-divisions. It has dedicated DG storage areas with fire suppression systems, experienced stevedores for 4.2/4.3 cargo, and the highest carrier acceptance rate among Chinese ports for Class 4. Shanghai is a viable alternative for 4.1 shipments to Europe and North America. For 4.3 shipments, Qingdao is strongly preferred — its dry climate (relative to Shanghai) reduces ambient moisture risk during port handling.
Documentation Checklist
- MSDS (Material Safety Data Sheet) — must specify sub-division, particle size (for powders), and coating status (for metals)
- DG Packaging Certificate (危包证) — with hermetic seal or moisture-barrier notation for 4.2/4.3
- Maritime DG Declaration
- Port Filing Approval
- Packing declaration stating moisture-barrier measures taken (for 4.3)
- Nitrogen purge certificate (if applicable for 4.2/4.3)
- Commercial Invoice, Packing List, B/L with DG endorsement
- Carrier DG Booking Confirmation
Frequently Asked Questions
What is the difference between coated and uncoated aluminum powder for DG classification?
Coated (stabilized) aluminum powder is typically Class 4.1 UN1309 — the coating (often stearic acid) prevents reaction with moisture. Uncoated aluminum powder that can react with water to produce hydrogen is Class 4.3 UN1396. The MSDS must explicitly state the coating material and percentage. This distinction is critical because 4.3 shipments face much stricter carrier acceptance, packaging requirements, and higher freight rates than 4.1 shipments.
Can calcium carbide ship in containers from China to Europe?
Yes, calcium carbide (UN1402, Class 4.3) is routinely shipped from China to Europe in 20-foot containers with hermetically sealed steel drums. China exports approximately 1 million tons of calcium carbide annually. However, carrier acceptance is limited — we typically book calcium carbide on MSK or MSC vessels. Key requirements: nitrogen-purged packaging, on-deck stowage, desiccant packs inside each drum, and advance DG booking at least 7-10 working days before vessel cut-off.
Can Class 4 solids ship by air freight?
Almost never. Class 4 substances are generally prohibited on both passenger and cargo-only aircraft under IATA DGR. There are very limited exceptions for small quantities of certain 4.1 solids in excepted quantities, but volume is so restricted (typically grams, not kilograms) that air freight is impractical for commercial shipments. Sea freight is the standard mode for all Class 4 exports from China.
What happens if water enters a container with 4.3 cargo?
Water ingress into a container carrying Class 4.3 substances can trigger an immediate hazardous reaction: generation of flammable gas (hydrogen or acetylene), heat buildup, and potential explosion or fire. This is why container inspection before loading is critical — we check every container floor for condensation, seal integrity, and any signs of water ingress. Containers carrying 4.3 must be stowed on deck so any accumulated gas can vent. Below-deck stowage is prohibited because gas could accumulate in enclosed spaces.