New insights into the influencing mechanisms of hemicellulose removal on dynamic wood-water interactions characterized with low-field NMR

Abstract

Hemicellulose, one of the most hygroscopic components, highly affected wood properties and wood-water interactions. To get more knowledge about the effects of hemicellulose on wood, this study changed hemicellulose content of wood through hydro-thermal removal of about 8 %, 20 % and 70 %. The dynamic cell water state and distribution during 384 h water absorption and air drying were characterized with low-field nuclear magnetic resonance technique (LFNMR). During water absorption, wood with less hemicellulose initially had higher and faster moisture increase, consisting of more free water in cell lumina and some large voids in cell walls than bound water in cell walls. The highest ratio of free water to bound water reached over 4.5, which was higher than that of untreated wood by over 15 %. This was because hemicellulose removal changed wood-water interaction environments suggested by SEM, FTIR and N2 absorption analysis. Specifically, hemicellulose removal enriched porous structures, especially the mesopore in cell walls. Although sorption sites decreased after hemicellulose removal, increased water paths and water accommodations played a more important role during initial water absorption. Besides, hemicellulose removal facilitated wood air drying, and the decreased moisture included more free water than bound water. This was because the enriched water paths accelerated water exchange, and sorption site decrease after hemicellulose loss reduced wood attraction to water molecules. Therein, the bound water leaving was generally slower than free water for hydrogen bond interaction with wood. The T2 s of bound water and free water were both shortened after hemicellulose removal, possibly because mesopore diameter reduction tightened wood-water bonding. This study provided new insights into understanding role of hemicellulose removal in dynamic wood-water interactions and facilitating development of wood processing technology.