关键词: 低场核磁共振, 横向弛豫时间, 纤维饱和点, 木材干燥, 水分

Abstract: [Objective] In this work, low field ¹H nuclear magnetic resonance was applied to study the transverse relaxation characteristics of water in wood during drying, aiming at clarifying water changing mechanism on wood seasoning, and providing theoretical basis and data support for wood drying and water treatment.[Method] Southern pine(Pinus spp. )wood with dimension of 20 mm(L)×5 mm(R)×5 mm(T)was used, and the samples were dried from water saturated state to moisture content(MC)around 5% by two steps. MC and transverse relaxation time(T₂)were measured at certain time intervals, and water state and movement were analyzed to discuss the boundary between free water and adsorbed water and its relation with the fiber saturation point(FSP).[Result] 1) There were two relaxation peaks and a shoulder in the saturated samples with their T₂ being 67.65 ms, 1.24 ms and 11-13 ms, corresponding to the free water in cell lumen, adsorbed water in cell wall and capillary water in the microcapillary system,respectively. 2) At initial stage of drying, free water decreased linearly and evaporated over with a total MC of wood reaching 20%, while adsorbed water kept almost constant until a total MC of 40% was achieved, below which water desorption started. 3) With a reduction in total MC of wood during drying, the T₂ of free water decreased, while that for multilayer and monolayer adsorbed water showed slight and notable decrease respectively around 6% MC.[Conclusion] 1) There were three water states in wood, namely free water, adsorbed water and capillary water. 2) Free water could be found even when mean MC was below the FSP defined traditionally(about 30%)due to uneven distributions of water in wood during drying, and a specific MC range(40% to 20% in the present work)existed where loss of adsorbed water at surface layer and free water in center layer of wood took place simultaneously. 3) As drying proceeded, the positions of relaxation peak for free water and adsorbed water shifted left continuously, indicating the average T₂ for both waters shortened gradually, or in other words, the bonds between wood and water became increasingly stronger, and the wood drying became much more difficult accordingly.

Key words: low-field nuclear magnetic resonance(LFNMR), transverse relaxation time, fiber saturation point, wood drying, water