Comparative Analysis on Fire Performance of Wood Coated with Intumescent and Modified Intumescent Coating

Abstract

This study presents a comprehensive evaluation of the fire performance of teak wood (Tectona grandis) coated with intumescent and modified intumescent coatings under controlled cone calorimeter testing at 50 kW/m² heat flux. Modified formulations incorporating synergistic additives—magnesium hydroxide (1 wt.%), boric acid (2 wt.%), and calcium carbonate (5 and 10 wt.%)—were systematically compared against uncoated wood and commercially available intumescent coatings. The modified coatings exhibited superior fire-retardant performance through multi-phase char formation comprising magnesium oxide, boron oxide, calcium oxide, and ceramic borate-phosphate networks. Key findings demonstrate substantial improvements: time to ignition increased from 10 seconds (uncoated) to 59 seconds (5% CaCO? formulation), representing a 490% enhancement. Total heat release decreased from 33.15 MJ/m² to 26.71 MJ/m² (10% CaCO?), while smoke production rate and carbon monoxide emission were reduced by 37% and 35%, respectively. Thesynergistic interaction between inorganic fillers promoted endothermic decomposition, blowing gas generation, and formation of compact, thermally stable char structures with enhanced mechanical integrity and oxidation resistance. The study reveals critical formulation dependencies: 5% CaCO? optimizes ignition delay, while 10% loading provides superior heat suppression during sustained combustion. These findings advance the development of environmentally sustainable, halogen-free fire protection systems for structural wood applications, demonstrating practical applicability for timber construction where rapid fire spread poses significant life safety risks.

Introduction

This study investigates the development of environmentally sustainable intumescent flame-retardant coatings for structural wood by incorporating magnesium hydroxide (Mg(OH)?), calcium carbonate (CaCO?), and boric acid as synergistic inorganic fillers. Flame-retardant coatings play a vital role in fire safety by delaying ignition, reducing heat release, and limiting flame spread. Conventional intumescent coatings form a protective char layer but are often sensitive to environmental and mechanical conditions, creating the need for improved formulations.

Mg(OH)? and CaCO? were selected due to their high thermal stability, non-toxicity, and complementary flame-retardant mechanisms. Mg(OH)? decomposes endothermically at moderate temperatures, releasing water vapor that cools the substrate and dilutes flammable gases, while CaCO? decomposes at higher temperatures to release CO? and strengthen char structure. Boric acid acts as a synergistic additive by promoting char formation and forming a glassy borate layer; however, its acidity was neutralized with Mg(OH)? to form magnesium borates, improving coating stability and char integrity.

Two modified coating formulations containing 5% and 10% CaCO? were prepared and applied to teak (Sagwan) wood substrates. Fire performance was evaluated using cone calorimetry, measuring heat release rate (HRR), total heat release (THR), mass loss rate (MLR), smoke production, CO generation, and time to ignition (TTI). Chemical and structural changes in the char residue were analyzed using FTIR.

Results showed that uncoated wood had the poorest fire resistance, while the commercial intumescent coating significantly improved fire performance. The modified coatings demonstrated further enhancement due to synergistic effects. The 5% CaCO? formulation provided the longest ignition delay, indicating optimal intumescence, whereas the 10% CaCO? formulation achieved the lowest heat release and smoke production during sustained burning. These findings highlight a trade-off between ignition delay and long-term fire suppression at higher filler loadings.

Overall, the combined use of Mg(OH)?, CaCO?, and boric acid creates a dense, thermally stable, ceramic-reinforced char that effectively insulates the wood substrate, reduces heat feedback, and suppresses smoke and toxic gas release. The study demonstrates a sustainable pathway for optimizing multifunctional intumescent coatings by balancing filler content to meet specific fire-performance objectives.

Conclusion

This study successfully demonstrated the enhanced fire-retardant performance of modified intumescent coatings incorporating magnesium hydroxide, boric acid, and calcium carbonate on wood substrates under cone calorimeter testing at 50 kW/m² heat flux. The systematic investigation of four sample configurations uncoated wood (W), conventional intumescent coating (IC), and modified coatings with 5% (S5) and 10% (S10) calcium carbonate revealed significant improvements in fire safety parameters through synergistic chemical interactions. The pre-reaction of magnesium hydroxide and boric acid at 40°C generated a reactive precursor system that, upon thermal exposure, provided multiple fire-retardant mechanisms including endothermic dehydration, blowing gas generation, and formation of protective ceramic-glass char structures. The integration of calcium carbonate as a mineral filler further enhanced performance through additional endothermic decomposition and char reinforcement, while creating complex interactions with phosphate species that improved char integrity and thermal stability. The superior performance of the modified coatings is attributed to the formation of a multi-phase char structure comprising magnesium oxide, boron oxide, calcium oxide, and complex borate-phosphate ceramics that collectively provide thermal insulation, mechanical strength, and oxidation resistance. The synergistic interactions among these components create a compact, continuous protective barrier that effectively shields the wood substrate from thermal attack while suppressing combustible volatile release. The observed trade-off between ignition delay and sustained fire suppression at higher filler loadings underscores the critical importance of component balance in intumescent formulations. While 5% calcium carbonate provides optimal ignition resistance, 10% loading offers superior performance during prolonged fire exposure. These findings contribute significantly to the development of environmentally sustainable fire protection systems for wood-based construction materials. The use of inorganic mineral fillers and the elimination of halogenated compounds align with contemporary environmental regulations while providing enhanced fire safety performance. The demonstrated effectiveness under high heat flux conditions suggests potential applications in structural fire protection, interior finishing systems, and other building applications where rapid fire spread poses significant life safety risks. Future research should focus on long-term durability testing, weatherability assessments, and scaling up for commercial applications. Additionally, investigation of the coating system\'s performance under different heat flux conditions and fire exposure scenarios would provide valuable insights for developing comprehensive fire protection standards and application guidelines.

References

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Copyright

Copyright © 2025 Savyasachi Jha, Mohammad Faiz, Varun Sharma, 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.