Mechanical Performance Related to Chemical Variability in Differently Pretreated and Densified Birch Wood

Birch wood veneer specimens were subjected to alkaline delignification with varying degrees of severity, in terms of concentration, temperature, and duration, resulting in up to 50% mass loss. Upon densification, substantial improvements in wood mechanical performance beyond the expected scaling with an increased density were obtained. With values of 50 GPa (elastic modulus) and 450 MPa (tensile strength), the elastic modulus increased by more than a factor of 2, and the strength roughly tripled. The mechanical performance first increased with increasing mass loss, peaked at intermediate levels, and then decreased again. Fourier-transform infrared (FT-IR) spectroscopy revealed that the initial mass loss was primarily related to the removal of hemicellulose, while with increasing mass loss, bands assigned to lignin were also affected. A correlation analysis between IR bands showing changes due to pretreatment and the mechanical performance of the respective wood specimens showed positive correlations for bands associated with cellulose and lignin, whereas bands assigned to hemicellulose showed a negative correlation with mechanics. In total, 71% of the variability of the mechanical performance was explained by IR spectra. Within the setting of the present study dealing with alkaline treatment of birch wood, it is concluded that the initial removal of hemicellulose under very mild alkaline extraction conditions had a positive effect on the mechanical performance of specimens. The progressive removal of lignin components with more severe treatment did not lead to further improvements but ultimately led to reduced improvements compared to the untreated reference.