Mechanical Wood Properties and Color of Combined PLA- and Thermal-Treated Beech Wood (Fagus sylvatica L.)

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Abstrakt

European beech (Fagus sylvatica L.) was modified by thermal treatment at 180 °C, oligomeric lactic acid (OLA) impregnation followed by catalyst-free polymerization (120 or 160 °C), and their combined application to evaluate changes in mechanical performance and color. The results for untreated beech wood showed low variability; in a four-point bending test, it exhibited an average modulus of rupture of 120.4 MPa and a bending modulus of elasticity of 8.7 GPa. Its compression strength and compression modulus of elasticity were 45.4 MPa and 2.48 GPa, respectively, while the corresponding Brinell–Mörath hardness values were 29.0 MPa on the side and 64.0 MPa on the end grain. Thermal treatment alone maintained bending strength of untreated beech, while highly increasing strength properties. OLA-based treatments generally produced significant reductions in modulus of rupture, bending modulus of elasticity, and deflection at maximum load. OLA-treated specimens exhibited increased brittleness and variability. Color measurements revealed substantial treatment-dependent darkening, with overall color differences ranging from 19.5 to 69.0. The results demonstrate that moderate thermal modification can enhance selected mechanical properties of beech wood, whereas OLA impregnation, especially under severe curing conditions, causes pronounced mechanical deterioration with the curing performed.

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