Changes in Chemical Composition and Prediction Method for Washing Wastewater Volume during Sodium Hydroxide Treatment of Japanese Cedar and Magnolia Woods to Improve Flexibility

In this study, sodium hydroxide（NaOH）treatments were applied to Japanese cedar（Cryptomeria japonica）and magnolia（Magnolia obovata）woods to impart flexibility. Wood boards larger than the chips typically used in cooking treatments were employed; however, the yield decreased linearly with increasing active alkali charge under identical temperature and time conditions. In both species, the readily degradable hemicellulose fraction（e.g., glucomannan）was removed at an active alkali charge of approximately 35%, whereas hemicellulose less susceptible to alkaline degradation（e.g., xylan）remained even at higher active alkali charges. In addition, the lignin content decreased gradually with increasing active alkali charge. For Japanese cedar and magnolia, the yield could be predicted based on the active alkali charge. The required amount of washing water depended primarily on the residual NaOH in the wood after treatment regardless of the washing method（ultrasonic or displacement）, indicating that the removal of inorganic components（e.g., NaOH）occurred mainly through diffusion driven by water dilution. Furthermore, the NaOH index─defined as the product of the untreated wood weight, yield, and NaOH concentration during treatment─proved to be an effective predictor of wastewater generation during washing of wood after the softening treatment.