GHS Classification- Flammable Solids

Definition

According to the United Nations GHS (Globally Harmonized System) 10th Revised Edition and China's national standard Chemical Classification and Labeling Specification Part 8: Flammable Solids (GB 30000.8-2013), flammable solids are defined as solids that are readily combustible or may cause or contribute to fire through friction.

Common Examples

In daily life, typical flammable solids include red phosphorus, sulfur, flammable metal powders (e.g., aluminum powder), matches, rosin, camphor, etc.

Key Characteristics

Flammable solids generally exhibit the following basic properties:

1. Low ignition point and high flammability.
2. Intense combustion reactions, often releasing toxic gases or smoke.
3. Some decompose or self-ignite under specific conditions (e.g., heat, moisture, or friction).

Classification Criteria

Both the UN GHS system and the UN TDG (Transport of Dangerous Goods) regulations share the same classification criteria for flammable solids:

1. Powdered, granular, or paste-like substances/mixtures that, when tested under Part III, Subsection 33.2 of the Manual of Tests and Criteria, exhibit one or more ignition times shorter than 45 seconds or a burning rate faster than 2.2 mm/s.
2. Metal powders that can ignite and propagate a reaction across the entire length of the sample (100 mm) within 10 minutes.
3. Friction-sensitive solids (e.g., matches) may be classified analogously under existing categories until specific criteria are established.

Classification Categories

Based on the criteria above, flammable solids are further divided into two hazard categories (additional details would depend on the referenced table).

For flammable solids, the classification and labeling are summarized in the following table:

Classification of Flammable Solids

The classification of flammable solids is primarily based on their combustion characteristics. However, special cases may arise during classification. Below are some typical examples:

Case 1: Classification Differences for the Same Substance in Different States

Example: Magnesium powder, magnesium strips, and magnesium blocks.

• Magnesium powder: Due to its large surface area and increased exposure to air, magnesium powder undergoes rapid oxidation with oxygen, releasing significant heat. This accelerates the reaction, elevating temperatures and increasing the risk of self-heating. Consequently, it is classified as a "Self-reactive substances and mixtures, Category 1". Additionally, magnesium powder reacts exothermically with water to produce hydrogen gas (Mg + 2H₂O → Mg(OH)₂ + H₂↑) and heat. Hydrogen’s low ignition point heightens fire and explosion risks. Thus, magnesium powder is classified as "Self-reactive substances and mixtures, Category 1; Substances and mixtures which, in contact with water, emit flammable gases, Category 2".
• Magnesium strips/blocks: These are classified as "Flammable solids, Category 2", as their bulk form reduces surface reactivity.

Case 2: Confusion with Self-reactive Substances

Example: Sodium dithionite (Na₂S₂O₄, "insurance powder").

Though solid, sodium dithionite is classified as "Self-reactive substances and mixtures, Category 1" rather than a flammable solid. Its intense combustion arises from rapid oxidation in air, which releases heat and flammable gases. The combustion is driven by self-sustaining reactions, not direct burning of the solid itself.

Case 3: Differentiation from Pyrophoric Solids

Example: White phosphorus.

• Pyrophoric solids (e.g., white phosphorus): Ignite spontaneously when exposed to air due to oxidation reactions. No external ignition source is required once the temperature reaches the substance’s autoignition point.
• Flammable solids: Require an external ignition source to burn.
  This distinction highlights the critical difference between pyrophoric and flammable solids.

Case 4: Solids Containing Flammable Liquids (UN3175)

These include:

1. Carrier solids (e.g., alcohol wipes, isopropanol-soaked cleaning cloths): The flammable liquid embedded in a non-combustible solid (regulated under UN3175, Special Provision 216) determines the hazard classification based on the liquid’s flash point and boiling point.
2. Homogeneous mixtures (e.g., paste-like adhesives): Classification depends on physical state:

• If solid: Assess flammability category via burn rate tests.
• If liquid: Classify based on flash point and boiling point per flammable liquid criteria.

Key Challenge in Classification

The primary difficulty lies not in experimental data analysis but in comprehensive judgment for special cases. Practitioners must:

1. Understand GHS classification definitions.
2. Analyze hazard mechanisms under varying conditions.
3. Prioritize hazards based on severity and likelihood.
   This ensures accurate classification and supports effective risk mitigation measures (e.g., preventive actions, emergency response planning).