Effect of delignification on shrinking and swelling of poplar wood assessed using digital image correlation technique

This study investigates lignin's influence on the hygroscopic behavior of poplar wood. Delignification was achieved using an acidic NaClO₂ solution, and digital image correlation (DIC) was employed to measure strain distribution during shrinking and swelling across relative humidity (RH) ranging of 0 % to 97 %. Results showed that lignin removal increased equilibrium moisture content (EMC) by up to 3.6 % in samples with 27 % lignin reduction. Strain analysis revealed significant radial strain differences between earlywood and latewood, with tangential strain suppressed by wood rays. Increased porosity and cellulose slippage were observed following lignin removal, and samples with the highest lignin removal rate exhibited reduction exhibited over 58 % greater ε_xx and over 43 % greater ε_yy compared to untreated wood. These effects were most pronounced in latewood and regions distant from wood rays, where stress concentrations occurred. Delignification-induced cell wall thinning amplified the wood rays' inhibitory effect on anisotropic deformation during shrinking，enhancing the contrast between isotropic deformation in latewood and anisotropic deformation in earlywood during swelling. These findings provide insights into lignin's role in wood-water interactions, supporting the optimization of wood modification techniques.