Water vapor sorption characteristics and hysteresis of earlywood and latewood within the same growth ring of Catalpa bungei

Catalpa bungei is an economically important native hardwood in China whose hygroscopic behavior is vital for industrial applications as it influences the final product’s dimensional stability and mechanical properties. In this study, the adsorption and desorption behavior of earlywood and latewood in the same growth ring of C. bungei wood samples was documented using a dynamic vapor sorption resolution and analyzed using the Guggenheim–Andersen–de Boer (GAB) model. The earlywood and latewood exhibited varying sorption isotherms and hysteresis degrees, and the reasons were analyzed in terms of structure and chemical composition (mainly hemicellulose and lignin). The influence of benzene–alcohol extracts and vessels on sorption characteristics was also examined. The GAB model perfectly fits the experimental data (R² ≥ 99.7%) over the full relative humidity range. Specifically, parameters such as the internal specific surface area of wood can be obtained from the GAB model to help explain the differences in sorption properties between earlywood and latewood. The maximum water content bound to the primary sites for earlywood and latewood is 6.87% and 7.47%, respectively. Correspondingly, two internal specific surface areas are 261 m²/g and 284 m²/g, respectively. The adsorption isotherms of earlywood and latewood in C. bungei cannot be fully classified as type II.