One-sided surface charring of beech wood (Fagus sylvatica L.) was analyzed. Specimens were charred on one surface using contact heating system with a hot plate, at a temperature of 220 °C and atmospheric pressure for 15 or 40 min. Surface charring was applied on the radial or tangential surface. Spectrophotometric chemical analysis was carried out to determine the total amount of soluble carbohydrates and phenolic compounds beneath the surface in order to evaluate the depth (within 1, 2, 3 and 4 mm) of the degradation affected by the charring. Specimens of dimensions 14 × 14 × 14 mm were used to determine the moisture behavior of charred specimens. The equilibrium moisture content (EMC) of specimens was calculated at a temperature of 20 °C and 65% relative humidity. Water absorption (WA) was recorded during the water immersion for 240 h at various intervals. Finally, the three-point bending tests were undertaken and modulus of rupture (MOR) and modulus of elasticity were calculated. Degradation of the main wood compounds due to chemical changes occurred during the surface charring is closely associated with a mass loss (3.5–5.5%) and further related to the severity of process and orientation of specimens to heat flow. Application of surface charring resulted in the significant reduction in EMC (~ 20%) as a result of a decrease in hydroxyl groups, an increase in cellulose crystallinity as well as further cross-linking of lignin. Furthermore, the WA of one-sided surface-charred wood considerably decreased (15%) when compared to that of the control specimens. A significant increase in soluble carbohydrates as well as phenolic compounds was found, and it can be stated that one-sided charring affected wood properties to depth 2–3 mm from the surface. Significant differences (14–24%) of bending strength MOR results are most likely due to improved sorption behavior of wood. The results show that beech wood charring improved moisture-related characteristics and consequently led to better mechanical behavior, but more studies are needed to exploit the potential of surface charring method for future use.
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This work was financially supported by Grant Agency of the Czech Republic (GAČR), Project No. 19-25171Y “Multi-scale Hygro-thermo-mechanical Behavior of Modified Wood-based Materials”.
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Čermák, P., Dejmal, A., Paschová, Z. et al. One-sided surface charring of beech wood. J Mater Sci 54, 9497–9506 (2019). https://doi.org/10.1007/s10853-019-03589-3