Novel evaluation of environmental aging resistance in commercial fire-retardant coatings for wood applications

The durability of fire-retardant (FR)-treated wood products is significantly affected by aging. Ensuring the long-term fire performance of these materials is a major concern across the construction, transportation, marine, and energy infrastructure industries. This study evaluates the environmental durability and sustainability of four commercially available FR coatings: a ceramic-based formulation (FR A), an intumescent lacquer (FR B), a phosphorus compound (FR C), and a hybrid ceramic–intumescent coating (FR D), all applied to Finnish spruce wood. To the best of the authors’ knowledge, this is the first study to systematically evaluate the environmental aging and durability performance of multiple commercially available FR coatings under standardized simulated exposure conditions. Durability testing included artificial weathering (500 h UV–condensation per ISO 4892-2), water absorption and thickness swelling (EN 317), and cyclic freeze–thaw testing (EN 15534-1). Additional assessments of color stability and surface roughness were conducted to evaluate long-term visual and mechanical changes. Results showed that FR D demonstrated superior resistance to weathering and physical degradation, with minimal color change (ΔE = 0.5), no roughness increase, and excellent freeze–thaw integrity. In contrast, FR C experienced the most degradation. Sustainability evaluation indicates that FR D has the potential to reduce material consumption due to improved durability and longer maintenance intervals compared to lower-performing coatings. These findings emphasize the importance of combining environmental durability with fire protection performance to support the long-term, sustainable use of wood in demanding applications.