Effect of temperature on wood modification with citric acid

Citric acid (CA) has emerged as a promising biosourced reagent for wood modification, improving the wood's dimensional stability and durability. However, CA treatments are often associated with reduced mechanical strength, and the specific role of curing temperature in this balance remains insufficiently explored. This study investigates the effect of curing temperature on the chemical, physical, and mechanical behavior of CA-modified wood. Scots pine (Pinus sylvestris L.) sapwood samples were impregnated with 30 % CA solution and cured at 100, 120, 140, and 160 °C for 24 h. Chemical changes were analyzed by FTIR and solid-state ¹³C NMR, while physical and mechanical performance was assessed through weight gain, cell wall bulking, anti-swelling efficiency, moisture exclusion efficiency, Brinell hardness, and three-point bending strength. Results demonstrated that esterification between CA and wood hydroxy groups increased with temperature, enhancing dimensional stability, especially at 140-160 °C. However, curing at 160 °C also promoted acid-catalyzed hydrolysis of cell wall components, leading to significant reductions in bending strength. The best compromise between stability and strength was achieved at 140 °C. These findings highlight curing temperature as a critical factor for optimizing CA treatments, providing a pathway for industrially viable wood modification strategies that balance performance with sustainability.