Wood has been used for centuries as a bio-based material for indoor–outdoor applications. In recent decades, global interest and use have increased significantly due to several reasons, including its availability, renewability, low price, ease of processing, acceptable mechanical properties, appearance and environmental and health aspects (minimal pollution/emission), among others. However, the use of wood in the construction sector is regulated by fire and environmental safety standards because there are some risks inherent to wood such as its ignitability and the ability to spread fire, which can considerably compromise the structures, as well as human health with the production of smoke and toxic gases, when burned. Consequently, the protection of wood against fire is important for structural, environmental, and safety reasons.

Pyrolysis is the process by which materials break down on exposure to heat, and in the case of wooden structures, can be assumed that it begins in a relatively narrow zone of the wooden elements’ surface, perpendicular to the exposed face of the material. The behavior of wood against fire depends on several factors, such as the proportion of the main wood constituents (cellulose, hemicelluloses and lignin) since they present ranges of decomposition at different temperatures. Hemicellulose and cellulose release a major part of the flammable volatiles, followed by the lignin; therefore, species with a high content of lignin are more thermally stable because it also contributes to creating a greater amount of char. Another important aspect is the moisture content of the material, which could delay the ignition time of wood at higher moisture contents. The type of wood also affects the thermal oxidative decomposition of wood, with softwoods being more susceptible to ignition and oxidation than hardwoods, since their surfaces are more accessible to oxygen, a fact that determines the rate of decomposition.

The most common fire protection treatments applied for wooden structures include low-cost organic and inorganic compounds in their formulations with acceptable performance. However, some of these chemicals, as well as their combustion products, are highly toxic substances, and their use is regulated under the requirements for fire safety of construction sites. These regulate the use of reagents harmful to human health, and raw materials used in the construction industry. It should also be noted that fire-retardant products are applied as coatings or impregnations, and generally present a short lifetime and little resistance to external factors and weathering, as well as altering the structure and natural appearance of wood, so it is necessary to repeat the applications of these products to ensure long-lasting protection.

Consequently, the research in the area of fire and flame retardant products for wood materials is heading towards the production of non-toxic and environmentally friendly ingredients obtained from natural and renewable sources, that will gradually replace classic products and formulations while maintaining similar performance but keeping the wood properties. Since the ignition and burning of wood are surface processes, the most common ways of fire protection are surface treatments using coatings systems (intumescent coatings) and impregnations with fire retardants.

In the case of intumescent coatings, commercial formulations include acid donors, carbon donors, and spumific compounds. The main drawbacks of these products are instability when exposed to exterior conditions, which reduces the service life of the products, and the production of toxic thermolysis products during fire. The impregnation of wood with fire retardant solutions generally consists of inorganic and/or organic substances that slow down the processes of ignition and combustion of the surface layers of wood. The use of boron and formaldehyde systems has been dramatically reduced due to increasing awareness of environmental problems and consumer safety, plus inorganic salts can negatively influence wood characteristics by increasing hygroscopicity and reducing strength and adhesion.

Nowadays, fire retardant formulations containing additives from phosphorus, nitrogen, and silicone, are considered a solution for the problems of flammability and environmental safety. Moreover, the synergetic combination of nitrogen-and-phosphorus-based fire retardants may be a suitable way to improve the formulations. However, new technologies are necessary to solve the lack of fixation of chemical impregnations in the structure of a material. Atmospheric plasma treatments, for example, may be a suitable way to improve the impregnation and service life of the products. Furthermore, nanotechnology has the potential to overcome the main issues regarding impregnation and long-term fixation of products in wood. Due to the small size of the particles that can penetrate deep into the wood structure with a high surface ratio of the desired products, avoiding the elution, weathering, and accretionof fire retardants from the surface.

Finally, the growth of more effective biorefinery processes offers an opportunity to support the production of biomaterials as a viable green option for future flame-retardant additives. Numerous bio-based compounds such as saccharide-based products, bio-based aromatic compounds, proteins, phytic acid and vegetable oils, have been proven that present outstanding char-forming ability. That means that they can act as an insulating barrier protecting the underlying material by reducing the diffusion of oxygen and heat and inhibiting further volatilization of combustible products during material combustion. Combining these bio-based compounds with phosphorus and nitrogen-containing products is an effective way to enhance the action of current flame retardant systems. Research must be conducted to achieve higher grafting yields and better compatibility with the woody matrix to limit the effects of thermal degradation during the decomposition process.

René Herrera Díaz, InnoRenew CoE researcher

REFERENCES

• Costes, L., Laoutid, F., Brohez, S., & Dubois, P. (2017). Bio-based flame retardants: When nature meets fire protection. Materials Science and Engineering: R: Reports, 117, 1-25.
• Mensah, R. A., Jiang, L., Renner, J. S., & Xu, Q. (2022). Characterisation of the fire behaviour of wood: From pyrolysis to fire retardant mechanisms. Journal of Thermal Analysis and Calorimetry, 1-16.
• Montanari, C., Olsén, P., & Berglund, L. A. (2021). Sustainable wood nanotechnologies for wood composites processed by in-situ polymerization. Frontiers in Chemistry, 9, 483.
• Popescu, C. M., & Pfriem, A. (2020). Treatments and modification to improve the reaction to fire of wood and wood based products—An overview. Fire and Materials, 44(1), 100-111.
• Vakhitova, L. N. (2019). Fire retardant nanocoating for wood protection. In Nanotechnology in Eco-efficient Construction (pp. 361-391). Woodhead Publishing.