Flash Point of Coatings

The flash point of a coating, thinner or solvent is the lowest temperature at which the liquid gives off enough vapor to form an ignitable mixture with air near its surface, so that a small flame or spark causes a brief flash. Flash point is a key safety property. It determines how a product is classified, labeled, stored and shipped, and it guides fire precautions during application.

Flash point and related terms

• Flash point. Vapor ignites momentarily but does not keep burning.
• Fire point. A slightly higher temperature at which the vapor keeps burning after ignition.
• Autoignition temperature. The temperature at which a material ignites on its own without a flame or spark. This is usually much higher than the flash point.
• Flammable limits (LEL and UEL). The range of vapor concentrations in air that can burn. Below the lower explosive limit the mixture is too lean; above the upper limit it is too rich.

All of these values appear on safety data sheets in Section 9 (physical and chemical properties).

Why flash point matters for coatings

Many solventborne paints contain solvents with flash points below normal room temperature. At ordinary working temperatures, they release enough vapor to burn. This affects:

• Spray booth design and ventilation rates.
• Electrical equipment in work areas, which may need to be rated for hazardous locations.
• Storage limits and cabinets.
• Transport classification under dangerous-goods rules.
• Hot work permits and the timing of welding or grinding near fresh coatings.

See fire and explosion hazards and ventilation for coating work for site practices.

How flash point is measured

Closed-cup methods

Closed-cup tests heat the sample in a covered cup, periodically opening a shutter to introduce a small flame. Because vapors are contained, closed-cup results are usually lower than open-cup results, and they are the values most often used for regulatory classification. Common methods include:

• ASTM D56 (Tag closed cup) for low-viscosity liquids with lower flash points.
• ASTM D93 (Pensky-Martens closed cup) for a wide range of liquids, including more viscous ones.
• ASTM D3278 and ISO 3679 (small-scale closed cup, rapid equilibrium) widely used for paints, varnishes and solvents because they use a small sample.
• ISO 1523 and ISO 2719 for closed-cup equilibrium and Pensky-Martens procedures.

Open-cup methods

Open-cup methods such as the Cleveland open cup (ASTM D92) are used mainly for oils and high-flash materials. They are less common for coating classification.

Equilibrium versus non-equilibrium

Some methods hold the sample at each test temperature until vapor and liquid reach equilibrium. These generally give more conservative results for paints, whose film-forming components can slow vapor release.

Classification

Regulatory systems group liquids by flash point and boiling point. Under the Globally Harmonized System (GHS), adopted in the US through OSHA’s Hazard Communication Standard, flammable liquids are placed in categories 1 through 4, with category 1 being the most hazardous (lowest flash point and boiling point). Transport rules, such as US DOT and international dangerous-goods regulations, use similar thresholds to assign Class 3 flammable liquid status and packing groups. Fire codes use their own classes for storage.

Exact thresholds are set in the regulations and codes. Check the current versions or the product’s SDS rather than relying on remembered numbers.

Flash point of common coating types

Flash point depends on the solvents in the formulation:

• Solventborne coatings and thinners often have low flash points because they contain fast-evaporating solvents.
• Waterborne coatings usually have high or no measurable flash point, though some contain co-solvents.
• 100% solids systems, such as many polyureas, epoxies and powder coatings, have little or no volatile solvent. Their component chemicals may still be combustible, and cleaning solvents used with them may be flammable.

Note that powder coatings present a dust explosion hazard instead of a liquid flash point hazard.

Practical guidance

• Read Section 9 of the SDS for every coating, thinner and cleaner on site.
• Use the lowest flash point product on site to set fire precautions.
• Keep containers closed when not in use.
• Control ignition sources, including static electricity. Ground and bond containers during solvent transfer.
• Do not heat solventborne coatings above the manufacturer’s maximum temperature.
• Ventilate enclosed spaces to keep vapor well below the lower explosive limit, and monitor with calibrated instruments in confined space coating work.

Thinning and flash point

Adding thinner can lower a coating’s flash point if the thinner is more volatile than the original solvent blend. Use only thinners approved by the manufacturer, and treat the thinned mixture according to the more hazardous component (see thinning coatings).

Key terms

• Closed-cup test. A flash point method in which the sample is heated in a covered cup; it usually gives lower values than open-cup methods.
• Open-cup test. A method with the cup exposed to air, now used less often for coatings classification.
• Flammable liquid. Under GHS and OSHA hazard communication, a liquid with a flash point at or below 93 °C, divided into categories by flash point and boiling point.
• Lower explosive limit (LEL). The lowest vapor concentration in air that can ignite.
• Autoignition temperature. The temperature at which a material ignites without a spark or flame, normally much higher than the flash point.

Frequently asked questions

Does a high flash point mean a product will not burn?

No. It means ignition from vapor requires a higher temperature. The material may still burn once heated or involved in a fire.

Why do SDS flash points differ between suppliers?

Different test methods and formulation differences give different values. The SDS should name the method.

Is flash point related to VOC content?

Indirectly. Both depend on solvent content, but low-VOC products can still contain low-flash solvents.

Related entries

• Fire and explosion hazards
• Safety data sheets
• Solvents in coatings
• Ventilation for coating work
• Flow cup viscosity testing (new)
• Lacquer coatings (new)