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Intumescent material

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Contents
  1. 1How intumescence works
  2. 2Protection of structural steel
  3. 3Seals in fire doors and separations
  4. 4Coatings on combustible materials
  5. 5Limits and uncertainty
  6. 6See also
  7. 7Notes
  8. 8References

An intumescent material is a material that swells when it is heated and turns into a thick, porous layer that insulates whatever lies beneath it.[1][2] NUREG-1805 defines an intumescent coating as a protective chemical coating that, when heated, generates gases internally and expands into a thermally insulating crust containing cavities.[2] Intumescent paints and coatings can swell to many times their installed thickness.[3] The Introduction to Fire Safety Engineering lists this swelling among the ways a material can change in a fire, and notes that it is what gives fire-resistant coatings their insulating properties.[1]

Steel flange with a thin coating before a fire, and the same flange under a thick porous char layer during a fire, with heat arrows reduced.
An intumescent coating before heating (left) and after swelling into an insulating crust with cavities (right). Based on Iqbal and Salley (2004), pp. 17-10 and F-39.

Intumescent materials belong to passive fire protection, and they are activated by the heat of the fire itself.[2] A main use is the protection of structural steel against heating, next to concrete encasement, boards and spray-applied materials.[4] Intumescent materials are also used in hot smoke seals around fire doors.[5] Because a coating changes its thickness and properties during the fire, simple heat transfer calculations with fixed material properties are only approximations for it.[1]

How intumescence works

Many materials change when they are heated. The Introduction to Fire Safety Engineering names four such changes: melting, charring, burning away and swelling.[1] A melting material flows away and may escape further heating, a charring material partly burns and leaves a char layer that may fall off, and a material that burns away disappears from its place.[1] A swelling, or intumescent, material grows thicker and gains insulating properties.[1] The thick, porous layer then slows the flow of heat to the surface below.[2]

This behaviour affects how heat is conducted through a construction. Heat conduction depends on the thermal conductivity, and the speed at which temperature penetrates depends on the thermal diffusivity.[1] Charring, swelling and burning away change both the thickness and the local conductivity of the protected member, so the path for heat conduction keeps changing during the fire.[1] In computer models of structural fire resistance, the thermal conductivity, specific heat and density must therefore be supplemented with phase change data for intumescent materials.[6]

Intumescence also occurs in the decomposition of polymers. Beyler and Hirschler explain that charring materials leave a porous carbonaceous char through which the volatiles from deeper layers must pass.[7] When such chars form appropriately as intumescent layers, they slow further thermal decomposition considerably.[7] In the SFPE Handbook, Milke lists intumescence among the chemical reactions in insulation that reduce the rate of heat transfer to steel. The others are calcination, ablation, thermal hydrogeneration and sublimation.[8]

Protection of structural steel

Unprotected structural steel loses its strength at high temperatures and must be protected against the heat of building fires.[9] NUREG-1805 notes that this protection is often called "fireproofing", a misnomer, because it insulates the steel rather than making it fireproof.[9] More on how steel behaves when heated is given in Steel in fire. The main methods are encasement, surface treatments such as boards or sprayed coatings, and fire-resistant ceilings below floor structures.[9] Intumescent paints and coatings are the most recent of these and form an insulating barrier that increases the fire endurance of the steel.[3]

The International Fire Engineering Guidelines list concrete encasement, board systems, spray-applied protection and intumescent paints as the main protection systems for steel.[4] Most of these systems are proprietary. Their scope of use, approved applicators, application rates, test approvals and limitations come from the manufacturer's specifications.[4] Milke also counts intumescents among the spray-applied materials, next to cementitious plasters, mineral fibres and magnesium oxychloride cements.[10] He notes that enough data exist to estimate the fire endurance of steel protected with cementitious and mineral fibre materials.[10] For intumescent coatings, the SFPE Handbook gives no such estimating method.[10]

How much protection a member needs depends on how fast it heats up. Milke uses the ratio W/D, the weight of the steel per unit length divided by the heated perimeter at the protection interface.[11] For hollow sections, the comparable ratio is the cross-sectional area divided by the heated perimeter.[11] The heated perimeter depends on the profile of the protection. A contour profile is in contact with all steel surfaces, while a box profile forms a rectangular box around the member.[12]

Two sections of an I-shaped steel column: protection following the steel outline and protection as a box, with the heated perimeter marked.
Heated perimeter D for contour and box protection of a steel section; W/D is the weight per unit length divided by D. Based on Milke (2002a), pp. 4-216 to 4-217.

The ratio explains why some members need more protection than others. A heavy member with a small heated perimeter heats more slowly, so the greater the W/D ratio, the greater the inherent fire resistance.[12] Shapes with a large W/D need less protection material than shapes with a small W/D for the same rating, so this section factor enters the choice of protection thickness.[12]

NUREG-1805 adds a practical limit. Intumescent coatings are primarily used for non-exposed steel that is subject to elevated temperatures, because prolonged exposure to flame can destroy the char.[3] The other insulation types have their own weak points: sprayed mineral fibre can be knocked off during construction or modification, and cementitious materials can spall and lose adhesion.[9][3] Milke lists the properties that steel insulation should preferably have. These include noncombustibility, proven thermal protection in the standard fire test, consistent quality, good bond strength and durability, and resistance to weathering.[8]

Seals in fire doors and separations

Intumescent materials are also used to close gaps. In a series of Australian tests on solid core doors, hot smoke seals that included intumescent materials and medium temperature seals were fitted around the door leaf.[5] The doors were exposed to the standard or the hydrocarbon heating regime, with an instrumented corridor on the non-fire side.[13][5] Under standard heating, the corridor lost all visibility after 21 minutes 30 seconds with seals, but after 6 minutes 15 seconds without them.[5]

Bar chart of times to smoke layer, very low visibility and no visibility in a corridor, with and without intumescent door seals.
Times of three events in the corridor beyond a solid core door under standard heating, with and without hot smoke seals containing intumescent materials. Based on Australian Building Codes Board (2020a), Data Sheet C7, p. 218.

The data sheet draws several conclusions. With seals, smoke spread was substantially reduced, and the time from the start of smoke filling to zero visibility grew to about 15 minutes for standard heating and 7 minutes for hydrocarbon heating.[14] Visibility, not temperature, decided tenability in the corridor.[14] The data sheet warns that seal performance depends on seal design and installation, so the results cannot be applied to generic seal configurations.[14] A closed fire door with such seals keeps the corridor tenable much longer, although a smoke detector in the corridor will then also respond later.[13]

Sealing gaps around doors serves the same purpose as firestopping around penetrations in walls and floors. In New Zealand, C/VM2 (second edition, 2025) states that fire and smoke separations should be fire stopped with proprietary products suitable for the orientation and for the specific separating element.[15] C/VM2 thus ties a fire stop to a specific product and use, much as the door seal results apply only to the tested seal design and installation.[15][14]

Coatings on combustible materials

Intumescent coatings are also applied to wood and other combustible materials, where their role is closer to that of a flame retardant. White explains in the SFPE Handbook that fire-retardant coatings for wood are designed and recognised only to reduce flame spread over the surface.[16] Depending on its thickness and durability, a coating may only delay ignition by a few minutes, or it may form an insulating layer that reduces the charring rate.[16] In full-scale beam tests, an intumescent fire retardant gave less improvement than in earlier small-scale furnace tests, because bending of the beams caused adhesion problems.[16]

The ignition data in the sources give a mixed picture for coated wood. A table in NUREG-1805, taken from an SFPE engineering guide, gives a critical heat flux for ignition of 13 kW/m² for softwood with intumescent paint.[17] For uncoated softwood, the same table gives values between 10 and 13.7 kW/m², depending on the sample.[17] An intumescent coating only forms its insulating crust once it has been heated.[2]

Swelling also complicates fire testing. Babrauskas notes that specimens that intumesce before ignition or during burning are a common difficulty in the cone calorimeter.[18] The simplest solution is a wire grid on top of the specimen, which has a negligible effect on the results if its mass is no more than 0.6 kg/m² of specimen face.[18]

Limits and uncertainty

Intumescent systems are harder to analyse than inert insulation. The Australian data sheet on design fires warns that most empirical formulae of the time-equivalence approach were validated with steel protected by non-reactive insulation that stayed in place.[19] These methods should be applied with caution to protection systems that progressively fall away, such as intumescent protection and exposed timber.[19] Ratings from the standard fire test also have their own limits, because the standard curve is only an agreed temperature course for comparing constructions.[20]

Durability is a second concern. After the World Trade Center investigation, NIST recommended the development of fire-resistive coatings with much better performance and durability after major events.[21] Its examples included improved adhesion, double-layered materials, intumescent coatings and more energy-absorbing sprayed fire-resistive materials.[21] In fire tests of coated timber beams, adhesion also proved to be a weak point.[16]

See also

Notes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Van Rossum (2027), §5.2.
  2. ↑ 2.0 2.1 2.2 2.3 2.4 Iqbal and Salley (2004), p. F-39.
  3. ↑ 3.0 3.1 3.2 3.3 Iqbal and Salley (2004), p. 17-10.
  4. ↑ 4.0 4.1 4.2 National Research Council of Canada et al. (2005), p. 2.6-7.
  5. ↑ 5.0 5.1 5.2 5.3 Australian Building Codes Board (2020a), Data Sheet C7, p. 218.
  6. ↑ Iqbal and Salley (2004), p. B-81.
  7. ↑ 7.0 7.1 Beyler and Hirschler (2002), p. 1-110.
  8. ↑ 8.0 8.1 Milke (2002a), p. 4-212.
  9. ↑ 9.0 9.1 9.2 9.3 Iqbal and Salley (2004), p. 17-8.
  10. ↑ 10.0 10.1 10.2 Milke (2002a), p. 4-214.
  11. ↑ 11.0 11.1 Milke (2002a), p. 4-216.
  12. ↑ 12.0 12.1 12.2 Milke (2002a), p. 4-217.
  13. ↑ 13.0 13.1 Australian Building Codes Board (2020a), Data Sheet C7, p. 214.
  14. ↑ 14.0 14.1 14.2 14.3 Australian Building Codes Board (2020a), Data Sheet C7, p. 219.
  15. ↑ 15.0 15.1 Ministry of Business, Innovation and Employment (2025b), p. 13.
  16. ↑ 16.0 16.1 16.2 16.3 White (2002), p. 4-259.
  17. ↑ 17.0 17.1 SFPE Engineering Guide (2002), as cited in Iqbal and Salley (2004), p. 6-13.
  18. ↑ 18.0 18.1 Babrauskas (2002a), p. 3-74.
  19. ↑ 19.0 19.1 Australian Building Codes Board (2020a), Data Sheet B3, p. 33.
  20. ↑ Van Rossum (2027), §8.6.
  21. ↑ 21.0 21.1 National Institute of Standards and Technology (2005), p. 212.

References

  • Australian Building Codes Board. (2020a). Data sheets: Fire Safety Verification Method (Handbook annex, Print version 1.1).
  • Babrauskas, V. (2002a). The cone calorimeter. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-63–3-81). National Fire Protection Association; Society of Fire Protection Engineers.
  • Beyler, C. L., & Hirschler, M. M. (2002). Thermal decomposition of polymers. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 1-110–1-131). National Fire Protection Association; Society of Fire Protection Engineers.
  • Iqbal, N., & Salley, M. H. (2004). Fire Dynamics Tools (FDTs): Quantitative fire hazard analysis methods for the U.S. Nuclear Regulatory Commission Fire Protection Inspection Program (NUREG-1805). U.S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation.
  • Milke, J. A. (2002a). Analytical methods for determining fire resistance of steel members. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-209–4-238). National Fire Protection Association; Society of Fire Protection Engineers.
  • Ministry of Business, Innovation and Employment. (2025b). Verification Method C/VM2: Framework for fire safety design (2nd ed.).
  • National Institute of Standards and Technology. (2005). Final report on the collapse of the World Trade Center towers (NIST NCSTAR 1).
  • National Research Council of Canada, International Code Council, Department of Building and Housing, & Australian Building Codes Board. (2005). International fire engineering guidelines (2005 ed.). Australian Building Codes Board.
  • Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.
  • White, R. H. (2002). Analytical methods for determining fire resistance of timber members. In P. J. DiNenno, D. Drysdale, C. L. Beyler, W. D. Walton, R. L. P. Custer, J. R. Hall Jr., & J. M. Watts Jr. (Eds.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-257–4-273). National Fire Protection Association; Society of Fire Protection Engineers.
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