Anisotropic Transparency of Alkali‐Treated Wood

ABSTRACT This study elucidates the mechanism by which alkali treatment enhances the transparency of delignified wood, with a focus on the cellulose microfibril skeleton. Following delignification, the resulting material remains translucent due to light scattering from preserved lumens. Subsequent potassium hydroxide (KOH) treatment further removes hemicellulose and exchanges carboxyl‐group counterions, which collectively soften the cell walls. This process allows the cellulose microfibril skeleton to undergo greater densification during drying, thereby reducing light scattering and yielding a highly transparent material without the need for polymer impregnation. We discovered that the inherent anisotropic structure of the wood's skeleton causes differential swelling between tangential and radial sections. The tangential sections, with their lower swelling ratio, undergo a more complete collapse of cell lumens, leading to higher density and superior transparency compared to the radial sections. This optical anisotropy, a direct consequence of the cellulose microfibril arrangement, was also evident in transparent wood‐polymer composites. These findings highlight the fundamental role of the wood's underlying structure in determining its optical properties.