IMDG Class 5.2 is the hazard subclass for organic peroxides, substances containing the bivalent -O-O- structure derivable from hydrogen peroxide. Organic peroxides decompose exothermically and the decomposition can be self-accelerating: once it starts, it generates heat that drives further decomposition, faster. Many organic peroxides require temperature control during transport to prevent the self-accelerating decomposition starting at all. The class is heavily used as polymerisation initiators (PVC, polystyrene, polyester), as bleaching agents (peracetic acid for food and pharma cleaning), and as crosslinking chemistry across plastics manufacturing.
What defines Class 5.2
A substance is Class 5.2 if it contains the -O-O- structure and meets the IMDG organic peroxide criteria. Each substance is assigned to one of seven types based on test behaviour:
| Type | Behaviour in IMDG tests | Transport allowed? |
|---|---|---|
| A | Detonates or deflagrates rapidly in package | No transport allowed |
| B | Detonates or deflagrates rapidly in package; partially | Limited transport with stringent restrictions |
| C | Does not detonate or deflagrate rapidly; reacts violently to fire | Transport allowed |
| D | Reacts to fire but moderately; some conditions trigger thermal explosion | Transport allowed |
| E | Reacts to fire with low energy release | Transport allowed |
| F | Decomposes slowly; minimal energy in fire | Transport allowed |
| G | Stable; no significant decomposition under fire | Transport allowed; no specific conditions |
Types A and B are essentially restricted to research-quantity shipping. Types C, D, E, and F are the routine commercial shipping range. Type G covers stable formulations (often desensitised peroxides).
Temperature control requirements
Many Class 5.2 entries require temperature-controlled transport. The control temperature is typically 10°C below the substance’s Self-Accelerating Decomposition Temperature (SADT). The emergency temperature is 5°C below SADT. Both must be marked on the package and declared on the shipping papers.
Transport modes for temperature-controlled Class 5.2:
| Cargo size | Transport mode | Cost premium vs ambient |
|---|---|---|
| Small parcels (1-200 kg) | Insulated container with refrigeration packs | Modest premium |
| Drum cargo (200 kg - 5 MT) | Refrigerated container (reefer) set to control temperature | Significant, typically USD 3,000-6,000 per container above standard FCL |
| Bulk (5+ MT) | Specialised reefer trailer with temperature monitoring | Highest, for the rarer bulk shipments |
For a buyer importing organic peroxide polymerisation initiators from China, the temperature-control premium dominates the freight cost. Build the reefer rate into the FOB-to-landed-cost calculation before agreeing volume contracts.
Common Chinese-export Class 5.2 substances
| Substance | UN number | Type | Temp control? | Use |
|---|---|---|---|---|
| Benzoyl peroxide (with phlegmatiser) | UN 3104 / 3105 etc. | C-F depending on concentration | Often yes | PVC and polystyrene polymerisation |
| Dicumyl peroxide | UN 3110 | F | Often no | Polyethylene crosslinking |
| Methyl ethyl ketone peroxide | UN 3105 / 3106 etc. | depends on concentration | Yes | Polyester resin curing |
| tert-Butyl peroxybenzoate | UN 3103 | C-D | Yes | Polymerisation initiator |
| Peracetic acid (with hydrogen peroxide) | UN 3105-3107 | varies | Yes | Bleaching, sanitising |
The exact UN number depends on the formulation, concentration, presence of phlegmatising water content, presence of solvent diluent. The same active peroxide in different formulations can have different UN numbers and different transport requirements.
Packaging requirements
Class 5.2 packaging is designed to prevent decomposition propagation. Specific requirements:
- Plastic and fibreboard combination outer packagings for many parcel-size shipments. UN 4G or 4GU outer boxes
- HDPE inner bottles with vented closures for liquid organic peroxides, vented closures prevent pressure accumulation if slow decomposition occurs
- Quantity limits per outer package, typically 25 kg for type C, lower for more reactive types
- Mandatory phlegmatisation for most types, the active peroxide is diluted with water, organic solvent, or a stabiliser to a percentage that meets the assigned type criteria
The phlegmatisation level is the engineering control. A 75% benzoyl peroxide formulation with 25% water is typically Type D and shippable. The same chemistry at 95% concentration would be Type B and not transportable.
Segregation at sea
Class 5.2 is one of the most segregation-restrictive classes:
- “Separated by a complete compartment or hold from” most other classes including Class 3, Class 4 (all subclasses), Class 5.1
- “Separated from” Class 1 (explosives), Class 6, Class 7, Class 8, Class 9
- Some Class 5.2 substances cannot stow together, type C and type B in proximity is restricted
For temperature-controlled cargo, stowage in a reefer position with active monitoring is operationally non-negotiable. The reefer must run continuously throughout the voyage; a power interruption longer than the reefer’s hold-cold time approaches the emergency temperature and triggers carrier-emergency response.
Documentation chain
Standard DG documentation plus:
- Type code declaration on the DG declaration (e.g. “Type D” alongside the UN number)
- SADT, control temperature, emergency temperature on the DG declaration and on the package
- For temp-controlled cargo: reefer booking confirmation, pre-cooling temperature record, voyage temperature monitoring chart
- For some types: a statement of safe transport conditions signed by the consignor
The reefer booking and pre-cooling steps are the operational fragility. Carriers occasionally ship 5.2 cargo in non-pre-cooled reefers at standard load temperature, accept the cargo, and only after sailing realise the reefer set point was wrong. The cargo arrives compromised.
Operator note: the China-side production licence
Organic peroxide manufacture in China is regulated under the dangerous-chemicals production licence regime. Not every chemical factory can legally produce the higher types (C and D) at scale. The buyer should confirm the factory’s specific peroxide-production licence covers the substance and concentration on order, and that the licence is current. A licence in renewal can suspend export ability mid-contract.
Related terms
IMDG umbrella code. UN number. Class 4.1, covers self-reactive substances with similar SADT-driven temperature-control framework. Class 5.1, strict segregation against 5.2. Segregation table for the full matrix.