Class 3 (Flammable Liquids) & Class 4 (Flammable Solids, Spontaneous Combustibles, Dangerous When Wet)

Class 3 (Flammable Liquids) & Class 4 (Flammable Solids, Spontaneous Combustibles, Dangerous When Wet)

Liquid and solid substances capable of igniting rapidly present severe risks during air transport due to operational factors such as cabin pressure reductions, elevated temperatures in cargo holds, and structural vibrations. The IATA DGR establishes clear quantitative thresholds to categorize liquids (Class 3) and solids or water-reactive substances (Class 4) into standardized hazard groups.

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Class 3: Flammable Liquids Classification Criteria

Class 3 (Flammable Liquids) comprises liquids, mixtures of liquids, or liquids containing solids in solution or suspension (e.g., paints, varnishes, lacquers, thinners) that release flammable vapors below specific temperature limits.

Definitional Parameters

Under IATA DGR Section 3.3, a liquid is classified as Class 3 if it satisfies either of the following criteria:

1. Has a closed-cup flash point of not more than 60°C (140°F), or an open-cup flash point of not more than 65.6°C (150°F);
2. Is offered for transport at temperatures at or above its flash point; or
3. Is transported as an elevated temperature liquid (e.g., liquid asphalt) at or above 100°C and below its flash point.

   Flash Point Thresholds (Closed-Cup)
   |------------------------|------------------------|------------------------>
   0°C                     23°C                     60°C                    Temp (°C)
            PG II                   PG III                   Non-Regulated
   (if Boiling Pt > 35°C)  (if Boiling Pt > 35°C)   (Flash Pt > 60°C)

Important Regulatory Exemption: Liquids with a flash point exceeding 35°C (95°F) that do not sustain combustion (as verified by standard combustibility tests in the UN Manual of Tests and Criteria, Part III, section 32.5.2) are not considered Class 3 Flammable Liquids for air transport purposes.

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Class 3 Packing Group Assignment Matrix

The degree of danger presented by a Class 3 liquid dictates its Packing Group (PG) assignment. The assignment is determined using two primary thermodynamic parameters: Flash Point and Initial Boiling Point.

Operational Examples

• Diethyl Ether (UN 1155): Flash point $-45^\circ\text{C}$, Initial boiling point $35^\circ\text{C} \rightarrow$ Class 3, PG I (High volatility and high flash potential).
• Acetone (UN 1090): Flash point $-20^\circ\text{C}$, Initial boiling point $56^\circ\text{C} \rightarrow$ Class 3, PG II.
• Fuel, aviation, turbine engine / Jet A-1 (UN 1863): Flash point around $38^\circ\text{C}$, initial boiling point about $175^\circ\text{C} \rightarrow$ Class 3, PG III. (Kerosene shipped as such is the separate entry UN 1223.)

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Class 4: Flammable Solids, Spontaneous Combustibles & Water-Reactive Substances

Class 4 covers substances other than explosives that are readily combustible, liable to cause or contribute to fire through friction, liable to spontaneous heating, or liable to emit flammable gases when in contact with water. Class 4 is divided into three sub-divisions:

                                ┌─────────────────────────────────────────┐
                                │       CLASS 4: FLAMMABLE SOLIDS         │
                                └────────────────────┬────────────────────┘
                                                     │
         ┌───────────────────────────────────────────┼───────────────────────────────────────────┐
         ▼                                           ▼                                           ▼
┌─────────────────┐                         ┌─────────────────┐                         ┌─────────────────┐
│  Division 4.1   │                         │  Division 4.2   │                         │  Division 4.3   │
│Flammable Solids,│                         │  Spontaneously  │                         │ Dangerous When  │
│ Self-Reactive,  │                         │   Combustible   │                         │       Wet       │
│ Polymerizing    │                         │ (Pyrophoric/Self│                         │(Emits Flammable │
└─────────────────┘                         │     Heating)    │                         │     Gases)      │
                                            └─────────────────┘                         └─────────────────┘

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Division 4.1: Flammable Solids, Self-Reactive & Polymerizing Substances

Division 4.1 encompasses four distinct groups of materials:

1. Readily Combustible Solids: Powders, granules, or pasty substances that ignite easily through brief contact with an ignition source (such as a burning match) and spread fire rapidly. Under UN test methods (burning rate test), if the burning time is less than 45 seconds over a 100 mm distance, it is classified under Division 4.1.
2. Self-Reactive Substances: Thermally unstable substances liable to undergo a strongly exothermic decomposition even without oxygen (air). If the Self-Accelerating Decomposition Temperature (SADT) is $\le 55^\circ\text{C}$, special temperature control during transport may be required. Crucial Rule: Any self-reactive substance requiring temperature control is FORBIDDEN FOR AIR TRANSPORT under regular provisions.
3. Solid Desensitized Explosives: Explosive substances wetted with water, alcohol, or diluted with other substances to form a homogeneous solid mixture to suppress explosive properties (e.g., Wetted Picric Acid, UN 1344).
4. Polymerizing Substances: Substances that undergo strong exothermic reactions leading to larger molecules; evaluated via SAPT (Self-Accelerating Polymerization Temperature).

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Division 4.2: Substances Liable to Spontaneous Combustion

Division 4.2 materials ignite without external spark or flame due to exothermic reactions with atmospheric oxygen. They are divided into two categories:

• Pyrophoric Substances: Liquids or solids that, even in small quantities, ignite within 5 minutes of coming into contact with air. Pyrophoric substances present extreme danger and are ALWAYS assigned to Packing Group I. Examples: White or Yellow Phosphorus (UN 1381), Organometallic compounds.
• Self-Heating Substances: Substances (other than pyrophoric) that self-heat in contact with air without energy supply. They ignite only in large quantities (kilograms) and after long periods of time (hours or days). Assigned to Packing Group II or Packing Group III based on standardized 25 mm and 100 mm cube oven testing at $100^\circ\text{C}, 120^\circ\text{C}$, and $140^\circ\text{C}$. Examples: Activated carbon (UN 1362), Copra (UN 1363).

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Division 4.3: Substances Which, in Contact with Water, Emit Flammable Gases (Dangerous When Wet)

Division 4.3 covers substances that interact with water or moisture to produce flammable gases capable of forming explosive mixtures with air.

Quantitative Criteria

A substance is assigned to Division 4.3 if, during standardized laboratory immersion testing, the evolution rate of flammable gas exceeds 1 liter per kilogram of substance per hour (> 1 L/kg/h).

Division 4.3 Packing Group Criteria

• Packing Group I: Ignites spontaneously upon contact with water at ambient temperature, or evolves flammable gas at a rate equal to or greater than 10 liters per kilogram per minute (\ge 10 L/kg/min) over any one-minute period. Examples: Sodium metal (UN 1428), Potassium metal (UN 2257).
• Packing Group II: Evolves flammable gas at a rate equal to or greater than 20 liters per kilogram per hour (\ge 20 L/kg/h), and does not meet PG I criteria. Examples: Calcium carbide (UN 1402, listed in both PG I and PG II), Aluminium powder, uncoated (UN 1396). Note that lithium hydride (UN 1414) is PG I, not PG II.
• Packing Group III: Evolves flammable gas at a rate greater than 1 liter per kilogram per hour (> 1 L/kg/h), and does not meet PG I or PG II criteria. Examples: Ferrosilicon (UN 1408).