The terms “combustible” and “flammable” are virtually synonymous in normal usage, but if you apply the chemicals law or hazardous substances law to a specific subject, the individual terms mean different things.

In the following, we will look at specific applications of the terms and use basic definitions from the areas of law mentioned.

Application and terms

The two terms are used in different areas of chemicals legislation.

Flammability is mainly used for classification and labeling according to CLP, specifically in the SDS in sections 2 and 3. However, Commission Regulation (EU) 2015/830 [1] amended Annex II of the REACH Regulation so that information on flammability and the flash point are mandatory in section 9. Commission Regulation (EU) 2020/878 [2] amended Annex II again so that further information on flammability and explosive properties must now be provided in Section 9.

If you would like to delve deeper into the topic of fire and explosion hazards associated with hazardous substances, you can find more details in this article on risk analysis.

The flammability can be determined from the SDS on the basis of the classification and the mandatory information just mentioned in section 9.

Flammability is mainly used to determine the storage class; although the CLP classifications are also checked here, there are also flammable substances and mixtures that are not classified as flammable. For these, the additional information, e.g. the flash point, must be used for the assessment. [3]

In GeSi³, a multi-stage automatic system is integrated for determining the storage class, in which the flammability is also determined according to the scheme from the following definition, if not already explicitly specified in Section 9.

Basic definitions

First of all, the aggregate states must be defined; here we use the specifications of the CLP Regulation [4]:

• Gas: substance that
  • has a vapor pressure of more than 300 kPa (absolute) at 50°C
  • or is completely gaseous at 20°C and a standard pressure of 101.3 kPa;
• Liquid: substance or mixture
  • which has a vapor pressure of less than 300 kPa (3 bar) at 50°C,
  • is not completely gaseous at 20°C and a standard pressure of 101.3 kPa
  • and has a melting point or onset of melting of 20 °C or less at a standard pressure of 101.3 kPa;
• Solid: Substance or mixture which does not meet the definition of a liquid or gas.

The physico-chemical properties of substances and mixtures are decisive here. A detailed explanation of the physical-chemical effects and the general GHS hazard classes and dangerous goods classes based on them can be found in the linked articles.

Now we can define what exactly we mean by “combustible” and “flammable”:

General definition [5]: Combustible substances are substances in the form of gas, vapor, liquid, solid or mixtures thereof that can undergo an exothermic reaction with air when ignited.

This definition therefore also implicitly refers to the preservation of combustion. However, there are even more precise definitions that can be traced back to measurable variables:

• Flammable
  • All substances/mixtures which have a classification according to CLP which results in labeling with the symbols GHS01 “Bomb/Explosive” and/or GHS02 “Flame”, or which are also classified in a corresponding hazard category without a pictogram. (Flammable gas 2 without pictogram)
  • Liquids that have a flash point <= 370°C, determined using the closed cup method [6]
  • “Experienced” combustible solids, e.g. paper, wood, polymers. In this case, further measurement methods or the key figures determined with them can help with the classification, e.g. for polymers (and other compact solids) there is the oxygen index from DIN EN ISO 4589, for dusts the calorific value from DIN EN 17077.

For complex mixtures, it makes sense to use the mixture classification in detail in order to correctly determine the flammability.

In the case of flammability, it must first be noted that this always refers to the context of the hazard classification and is therefore separated according to the aggregate states:

• Flammable
  • Gas: Substance or mixture which has an explosive range in air at 20 °C and a standard pressure of 101.3 kPa.
    • Classification also for pyrophoric properties and chemical instability
  • Aerosol: More than 1% of the components classified as flammable and heat of combustion > 20 KJ/g
    • Finer subdivision into spray and foam aerosol
  • Liquid: substances or mixtures with a flash point of maximum 60°C
  • Solids: Substance or mixture which is highly flammable or which may cause or promote fire by friction.
    • “Highly flammable” is defined here as “if the substance/mixture can be easily ignited by brief contact with an ignition source such as a burning match and the flames spread rapidly.”
    • So here, too, it is a question of maintaining combustion; the decisive measured variable is the combustion rate.

For flammable gases, we recommend the blog post Focus on flammable gases in the SDSto understand typical classifications.

In short, flammability always refers to a hazard in the sense of the CLP Regulation; combustibility encompasses this definition and extends it to include categories that are not directly hazardous.

Sources:

[1] Directive 2014/34/EU on equipment and protective systems intended for use in potentially explosive atmospheres (ATEX Product Directive)

[2] Directive 1999/92/EC on minimum requirements for improving the safety and health protection of workers potentially at risk from explosive atmospheres (ATEX Directive)

[3] Technical Rules for Hazardous Substances (TRGS 510): Storage of hazardous substances in portable containers

[4] Guide to the application of the ATEX directives in explosion protection (European Commission)

[5] Flammable substances in explosion protection: Definition and classification (DGUV regulations / BG RCI)

[6] Supplementary guidelines on the practical implementation of the ATEX directives (European Commission)