Durability of tannin-citric acid modified Scots pine against weathering and fungal exposures

Abstract Wood modification is essential to enhance the performance and longevity of wood as a material while maintaining its natural and sustainable characteristics. By modifying wood, its properties can be significantly improved without the need for toxic preservatives. Thus bio-based wood modification is vital for enhancing wood properties sustainably, reducing environmental impact, and meeting the demand for eco-friendly, non-toxic, and renewable material solutions. In this study, bio-based formulae were used on pine sapwood for enhanced decay and weathering resistance. Quebracho tannin solutions (20 %) with different concentrations of citric acid (CA) as a cross-linker were prepared and then vacuum-pressure impregnated in Scots pine sapwood before curing them at 140 °C. The durability performance of untreated and tannin-impregnated wood samples was assessed against accelerated weathering, mould growth and brown rot decay fungus Coniophora puteana (Schumach.) P. Karst. Experimental results showed that the weathering resistance was apparently improved by tannin modification with higher levels of CA, as reflected by color stability, crack formation and contact angle. While tannin modification increased mould susceptibility, the decay resistance was notably enhanced in all modified wood samples. The acidity of the formulas at high CA levels may increase the risks of hydrolytic degradation of wood, and thus, low CA concentrations (e.g. 2 %) appear optimal for balancing performance and chemical stability. These findings underscore the potential of 100 % bio-based tannin–CA systems to enhance wood durability, offering a promising pathway for sustainable wood protection strategies.