Physical Properties and Durability Aspects of Combined PLA and Thermal Treatment of Beech Wood (Fagus sylvatica L.)

Abstract

The effects of thermal modification, lactic acid (LA) impregnation, and their combined application on the physical properties and biological durability of European beech (Fagus sylvatica L.) were investigated to identify potential synergistic interactions between these environmentally friendly wood-modification methods. Beech lamellae were subjected to thermal treatment at 180 °C, LA impregnation followed by polymerization at 120 or 160 °C, and two different treatment sequences combining both modifications. Density, moisture sorption, swelling–shrinking behavior, anti-swelling efficiency, anatomical changes, and fungal durability were evaluated. Thermal treatment caused a moderate mass loss of 3.2–3.7%, whereas LA-treatment produced weight gains of 39.3–56.5%. All treatments reduced moisture uptake and equilibrium moisture content, with the lowest value (5.6%) obtained after impregnation followed by thermal treatment. The combined treatment also provided the greatest dimensional stabilization, reducing volumetric swelling by 67.6% compared with untreated wood. SEM observations confirmed enhanced polymer fixation after higher-temperature curing. Overall, combining LA-treatment with thermal modification substantially improved the dimensional stability and durability-related properties of beech wood, while the treatment sequence proved critical for maximizing modification efficiency.