Flame retardant

A Flame retardant is a substance or treatment incorporated in or applied to a material to suppress or delay combustion under specified conditions. It can change ignition, decomposition, heat release, flame spread or residue formation. Its effect is conditional and does not automatically make the treated material noncombustible.

Flame retardants act through different physical and chemical mechanisms. These include forming an insulating char, reducing heat feedback, changing decomposition pathways and interfering with flame reactions. The appropriate evaluation considers the complete treated product and its fire exposure.

The treatment can also change smoke and toxic-product yields. Incomplete combustion may produce more carbon monoxide or other hazardous species, while total fuel consumption and fire size may decrease. Fire performance and toxicity therefore require separate assessment.

Purpose and conditions

The purpose of a flame retardant is to reduce a material's contribution to fire under the conditions for which it was evaluated. This can mean delayed ignition, lower heat release, slower spread or a different char structure. The claim should identify which of these outcomes was measured.

Specified conditions include heat flux, orientation, specimen form, backing, oxygen supply and exposure duration. A treatment effective in one arrangement can be much less effective in another. Full-scale deformation, joints, ageing and installation can also change performance.

Additive treatments are incorporated into a material, while reactive treatments are intended to alter its chemistry or response. Surface coatings form another application route. Adhesion and continuity are essential because a damaged or detached layer cannot provide the intended protection.

A flame retardant is not the same as a fire-resistive barrier. It modifies material response, whereas a fire-resistance assessment concerns the performance of a construction in maintaining structural adequacy, integrity or insulation. These measures should not be interchanged.

Char-forming and intumescent mechanisms

Some flame retardants promote a carbonaceous or intumescent residue. Low-density, porous chars can insulate the underlying material and reduce heat feedback from the flame. Continuity, coherence, adherence, oxidation resistance and permeability determine how useful that barrier remains.

An Intumescent material expands or develops a protective layer during heating. This can slow further thermal decomposition considerably and shield virgin material. Volatile products may still have to pass through the char, where secondary reactions can occur.

Intumescent residue can insulate but also oxidise. Based on Beyler and Hirschler (2002), pp. 1-110–1-114 and Ohlemiller (2002), p. 2-200.

Char formation is not simply cessation of combustion. Carbonaceous char can oxidise at its surface at higher temperatures and sustain glowing or smouldering. Protection therefore depends on limiting both heat transfer and subsequent oxidation of the residue.

Inorganic residues can form glassy barriers that reduce volatile escape and protect underlying material. Their effectiveness also depends on continuity and integrity. A cracked or permeable residue can allow heat and oxygen to reach unburnt material.

Chemical inhibition and other mechanisms

Chemical flame retardants can interfere with reactive species in the flame chain reaction. This can increase chain termination relative to branching, reducing flame propagation even when fuel, oxygen and heat remain. The mechanism is represented by the fire tetrahedron rather than by simple fuel removal.

Chemical inhibition interrupts flame chain reactions. Based on Grant (2002), pp. 4-152–4-153 and Simmons (2002), pp. 1-150–1-151.

Other treatments change the material's thermal response. A barrier can reduce heat flux, while altered decomposition can change the quantity or composition of volatile fuel. These effects should be distinguished from direct chemical inhibition in the flame itself.

Diluents and inert additives can reduce the proportion of reactive material or absorb energy during heating. Their effect depends on the treated composition and the resulting temperature and burning velocity. A treatment should not be assumed to act through chemistry merely because it reduces measured heat release.

The mechanism determines which measurements are meaningful. Char thickness and integrity matter for a barrier treatment, while flame chemistry and reaction rates matter for an inhibitor. Heat release rate alone cannot identify which mechanism caused an improvement.

Plastics and treated materials

Plastics in fire include thermoplastics, thermosets and elastomers with different physical and chemical responses. Flame retardants can alter those responses, but polymer type, additives and product geometry still control decomposition. A treatment is therefore one component of the complete material system.

Thermoplastics can soften and melt before decomposing, while thermosets and cellulosics do not pass through the same fluid state. These transformations can expose new surfaces or disrupt a coating. Treatment performance should be evaluated through those changes rather than only at ambient conditions.

New Zealand's C/VM2 places many fire-retardant-treated timbers in Group Number 2, between limited-combustibility materials and ordinary timber products in its material commentary. This is a classification within the 2025 New Zealand document, not a universal ranking for every treated timber.

Wood treatments may delay ignition or reduce surface flame spread without providing the same fire resistance as a protective membrane. White notes that some coated beams showed less improvement in full-scale tests than in earlier small-scale tests, with bending causing adhesion problems. This illustrates the importance of product-scale evaluation.

Reaction to fire and combustibility

Reaction to fire measurement can show whether a treatment delays ignition, lowers heat release or changes spread. Combustibility remains the underlying ability to burn under specified conditions, so a treatment can reduce fire contribution without eliminating it. Claims should be framed by the measured outcome.

Cone-calorimeter and other controlled tests can compare treated and untreated specimens using consistent exposures. Differences in heat release and mass loss can then be linked to a mechanism where appropriate. Full-scale installation still requires separate evaluation of joints, backing and deformation.

A treatment can improve one metric while leaving another unchanged or worse. Slower ignition does not necessarily lower toxic yield, and a thicker char does not necessarily prevent later oxidation. All relevant criteria should be considered before describing overall performance as better.

Flame retardants are also not a substitute for ignition-source control, compartmentation or suppression. They address material response, while the wider fire scenario still includes ignition, ventilation and occupants. Their contribution should be assessed as part of that system.

Fire toxicity

Fire toxicity can change when a treatment alters combustion efficiency and product composition. Heavily fire-retarded materials can have lower combustion efficiency in Drysdale's cited range, which means more partially oxidised products and less complete energy release. Reduced fire size may offset this, but it does not remove the need to measure yields.

Carbon monoxide, hydrogen cyanide, soot and irritants require separate evaluation. A treatment that suppresses flame can prolong smouldering and alter the dominant exposure period. The resulting atmosphere at an occupied location cannot be inferred from heat release alone.

Small-scale yield data need appropriate equivalence ratios and combustion modes. Full-scale vitiated conditions can produce very different products, and material additives can change them further. Toxic assessment should use conditions that represent the intended scenario.

The treatment itself can also introduce new products into the smoke. Evaluating only the parent polymer may miss additive-related species or interactions. Complete material composition and decomposition chemistry should therefore be considered.

Evaluation and limitations

A useful evaluation compares treated and untreated products under the same relevant exposure. It records ignition, heat release, mass loss, residue, smoke and, where needed, structural or separating performance. These results identify which mechanism and claims are supported.

Treatment can delay but not eliminate combustion. Based on Beyler and Hirschler (2002), p. 1-114 and White (2002), p. 4-260.

The complete product should be tested where practical. Coatings can crack or detach, composites can delaminate and joints can expose untreated regions. White's coated-beam example shows why small-scale success may not persist under full-scale deformation.

Durability and end use also matter. Moisture, ageing, installation and mechanical movement can affect a treatment before any fire begins. Those conditions should be represented or acknowledged when making an application claim.

Finally, a flame retardant should not be described as making a material fireproof. It suppresses or delays combustion under specified conditions, and those conditions define the claim. Fire safety still requires appropriate design, installation and assessment of the complete scenario.

Evidence for treatment performance

A treatment assessment should first identify the intended mechanism. A barrier-forming treatment needs evidence of residue continuity and insulation, while a chemical inhibitor needs evidence of changed flame reactions. Measuring only one outcome can leave the actual mechanism uncertain.

The control specimen should be as similar as possible to the treated one. Differences in polymer formulation, thickness, backing, moisture and ignition exposure can otherwise be mistaken for a treatment effect. A valid comparison isolates the treatment rather than changing several variables at once.

Thermal decomposition should be followed through the test. A treatment can delay the start of volatile production, change its rate or alter the residue, and each has a different effect on heat release. The point at which protection fails is as important as the initial improvement.

Mechanical condition should be included where it can affect the treatment. White's coated-beam example shows that bending can reduce adhesion and diminish full-scale performance. A flat ambient specimen cannot establish durability under those service conditions.

For smoke assessment, both yield and total mass burned matter. Reduced fuel consumption can lower total effluent, while lower combustion efficiency can increase partially oxidised products per kilogram. The receiver's concentration and exposure time determine which effect dominates.

The residue should be examined after exposure as well as during it. Char can protect underlying material but can also oxidise, crack or detach, exposing the untreated core. A post-fire assessment should therefore include reignition potential and the condition of the remaining barrier.

Treatment claims should distinguish test evidence from end-use assumptions. A result obtained for one formulation, orientation or exposure does not prove the same performance in every installation. Where those conditions differ, the assessment needs a suitable model or additional testing.

A final report should state which outcomes improved and which did not. Delayed ignition, reduced heat release, lower toxicity and maintained structural performance are different claims. Presenting only a general flame-retardant label would hide the conditions and trade-offs that determine actual fire safety.

A treatment should also be reviewed for the point at which its protection is exhausted. Continued heating can consume the char, change its permeability or expose fresh material, so the later heat-release curve may differ sharply from the initial one. The time at which that transition occurs belongs in the assessment.

Alternative outcomes should be reported explicitly when a treatment changes the fire mode. Suppressed flaming can give way to smouldering, which has different particle and toxic-product behaviour and can persist longer. Judging the treatment only by the absence of flame would miss that different exposure pathway.

For toxic assessment, the treatment's complete formulation and combustion history are necessary. Additives can change product composition, while changing oxygen supply can change it again. A constant yield measured under one condition should not be used for the whole fire.

For separating or structural applications, the treatment should not be credited with fire resistance without assembly evidence. Fire resistance requires structural adequacy, integrity or insulation under defined conditions, which is a different performance claim from delaying combustion. The relevant test and endpoint must be shown.

The final claim should therefore be specific about the treatment, the measured improvement and its limits. A statement that combustion is suppressed or delayed can be supported only with the relevant exposure, product and endpoint. Broader claims about noncombustibility, toxicity or structural safety require separate evidence.

An application should also consider whether the treatment changes the meaning of the acceptance criterion. A lower peak heat release can be beneficial for one scenario, while a longer smouldering period can increase exposure duration for another. The evaluation should therefore use the complete time history and the actual performance objective.

For readers, the treatment should be presented as one part of the material's fire response, not as a guarantee. Its mechanism, test evidence, installation requirements and unresolved effects belong together. That allows a flame-retardant claim to be assessed without implying more protection than the evidence supports.

A final evaluation should connect the treatment's intended mechanism with the measured endpoint. If the mechanism is unclear, alternative explanations such as changed heat transfer, altered decomposition and flame inhibition should be considered. This prevents a generic performance improvement from being assigned to the wrong mechanism.

The report should also identify any conditions under which the treatment was not effective. Those limits can be more informative than the initial improvement, especially when heat, deformation or oxygen access changes. Preserving them makes the flame-retardant claim technically useful rather than promotional.

See also

• Intumescent material
• Plastics in fire
• Combustibility

References

• Australian Building Codes Board, National Research Council of Canada, International Code Council, & Department of Building and Housing New Zealand. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
• Babrauskas, V. (2002). Heat release rates. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Drysdale, D. D. (2002). Thermochemistry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Gottuk, D. T., & Lattimer, B. Y. (2002). Effect of combustion conditions on species production. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Grant, C. C. (2002). Halon design calculations. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Janssens, M. (2002). Calorimetry. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Kanury, A. M. (2002). Flaming ignition of solid fuels. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Kodur, V. K. R., & Harmathy, T. Z. (2002). Properties of building materials. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Ministry of Business, Innovation and Employment. (2025). C protection from fire: Verification method C/VM2 (2nd ed.). New Zealand Government.
• Ohlemiller, T. J. (2002). Smoldering combustion. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Purser, D. A. (2002). Toxicity assessment of combustion products. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Quintiere, J. G. (2002). Surface flame spread. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• Simmons, R. F. (2002). Premixed burning. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.
• White, R. H. (2002). Analytical methods for determining fire resistance of timber members. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed.). National Fire Protection Association.