Modern Coating Approaches to Fire Safety in Wood Structures: A Review of Intumescent and Non-Intumescent Coatings

Abstract

Non-intumescent flame-retardant coatings (NIFRCs) have become effective fire protection solutions for polymeric materials, especially rigid polyurethane foam (RPUF). Unlike intumescent coatings, NIFRCs do not expand when heated; instead, they create stable, inorganic-rich barriers that block heat transfer, delay ignition, and reduce smoke release. This review highlights progress in hydrogel, silica sol, aerogel, and ceramic-based NIFRCs. These developments show remarkable improvements, such as longer time to ignition, lower peak heat release rate, decreased total heat release rate, and less smoke production. The protecting mechanism includes water vapor cooling, char layer formation, and strong barrier effect. A comparison with intumescent coatings highlights the benefits of NIFRCs in durability, thickness, and smoke suppression, making it a promising option for sustainable fire-safe building materials.

Introduction

• Wood has long been used in construction due to its natural properties but is highly flammable, which poses fire safety concerns.

• To address this, fire-retardant coatings—particularly intumescent and non-intumescent types—are increasingly used.

  • Intumescent coatings expand when heated, forming an insulating char.

  • Non-intumescent coatings resist fire via barrier formation and reduced heat/gas transfer.

2. Fire-Retardant Coating Types

Intumescent Coatings

• Swell under heat to form a char barrier.

• Key components: char former, acid source, blowing agent (e.g., melamine), and binder.

• Effective for delaying heat penetration and maintaining structural integrity.

Non-Intumescent Coatings

• Do not expand but protect via:

  • Thermal barrier formation.

  • Reduced pyrolysis gas migration.

  • Insulating carbonaceous layers.

• Types include inorganic, hydrogel, aerogel, and ceramic-based coatings.

3. Key Research Studies (Literature Review)

4. Key Performance Indicators

• LOI (Limiting Oxygen Index): Indicator of flame resistance; higher = better.

• HRR (Heat Release Rate): Lower values indicate better fire performance.

• THR (Total Heat Release): Indicates overall combustion severity.

• Smoke and CO? Emissions: Lower values enhance safety during fires.

• Char Formation: Effective char formation indicates better protection.

• Weatherability: Essential for outdoor wood applications.

5. General Conclusions

• Fire-retardant coatings significantly improve wood’s fire safety without compromising structure or appearance.

• Eco-friendly options using inorganic and bio-based materials are gaining traction.

• Expandable graphite, Mg(OH)?, and phosphorus-nitrogen systems are particularly promising.

• Durability and long-term performance, especially in outdoor settings, require further research and innovation.

? Summary Statement

Modern fire-retardant coatings—particularly those combining organic and inorganic materials—provide highly effective, sustainable, and customizable protection for wood used in construction. Intumescent and non-intumescent systems each offer unique mechanisms to reduce flame spread, heat release, and smoke production, with ongoing research pushing toward safer, greener, and more durable solutions.

Conclusion

Non-intumescent coatings are generally superior for modern wooden applications where maximum thermal insulation is not required but thin, tough, and moisture-resistant protection with excellent smoke suppression ignition delay and environmental durability are needed. Because when heated, the non-intumescent coating undergoes a chemical reaction which hardens and form a glaze-like shell and release water vapors and free radicals which combines with fire to stop the fire and flame spread. And this layer behaves as a physical barrier and resist moisture and heat transfer while maintaining relatively thin coating at 250?C to 500?C. Intumescent coatings remain the best choice for structural steel or where maximum thermal insulation during fire is needed, but they usually require more maintenance and are less suitable for humid environments. Because the intumescent coating expands 50 X to 100 X and forms a physical barrier of expanded char material that can withstand a fire up to its rated time i.e. 60 min, 120 min, 240min. by maintaining the structural temperature below 400?C to 550?C which is useful for steel structure as steel structure losses it’s load bearing capacity by 500?C - 550?C. Thus, for maximum thermal insulation and fire resistance Intumescent coatings are preferred. And for combined flame retardancy, smoke suppression, ignition delay, and durability, non-intumescent coatings provide a modern, superior solution for building wooden structures.

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Copyright

Copyright © 2025 Mohammad Faiz, Varun Sharma, Savyasachi Jha, Mayank , Sangam Singh, Suresh Lade. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.