Characterization of stable and unstable states of moisture in wood during sorption by low-field NMR

Wood is a hygroscopic material that often undergoes dynamic moisture exchange with surroundings. Exploring the moisture state during moisture exchanges is crucial for understanding wood-moisture and bamboo-moisture relations. This study utilized low-field NMR (LF NMR) to analyze the T2 relaxation behavior of wood, rattan, and bamboo during sorption processes. We focused on characterizing the variation in T2 relaxation peak time in both unstable and stable moisture states. The T2 peak time exhibited clear moisture content and species dependences. Notably, when moisture content was consistent, the T2 peak time of wood in stable moisture state was shorter than that in unstable state, indicating that left movement of T2 spectrum occurred. At high moisture contents, the degree of left movement was more significant. Furthermore, The T2 peak time was associated with sorption history, i.e., the T2 peak time of the wood stabilized through adsorption was less than that stabilized through desorption. Based on the correlation analysis after normalization, the T2 peak area and peak height were significantly correlated with the cell wall ratio of wood. The degree of the left movement was primarily influenced by the effective adsorption area of the wood. The findings in this work effectively characterized the stable and unstable states of moisture during sorption, providing valuable insights into the wood-moisture relationship in the wood, clarifying the moisture sorption kinetics.