关键词: 木材, 核磁共振, 含水率调控, 加水平衡法, 横向弛豫时间

Abstract:

Objective: Common wood conditioning methods, such as the saturated salt solution method and the saturation-drying method, have limitations such as long equilibrium time and poor control accuracy, which makes it challenging to meet the testing requirements of nuclear magnetic resonance (NMR) technology. A new conditioning method, the water-addition-equilibrium method, was proposed in the present paper to reduce the conditioning time and improve the control accuracy. Method: Poplar wood (Populus spp.) was conditioned to the target moisture contents of 5%, 15% and 25% using the new method. After adding water, the equilibrium conditions were set as follows: an equilibrium temperature of 45 ℃ with equilibrium times of 48 h, 72 h and 96 h; and equilibrium temperatures of 60 and 75 ℃ with times of 24, 48 and 72 h. Two critical parameters in the water-addition-equilibrium method, namely equilibrium temperature and equilibrium time, were optimized by analyzing the moisture content profile along the direction of water addition and transverse relaxation time spectra obtained from the low-field NMR tests. Result: The results demonstrate that at equilibrium temperatures of 45, 60 and 75 ℃, the equilibrium time has no obvious effect on the actual moisture content deviation and standard deviation of the samples. Under similar target moisture content conditions, the actual moisture content deviation of the samples obtained by the water-addition-equilibrium method is generally smaller than that obtained by the saturated salt solution method. For the target moisture contents of 5% and 15%, the samples’ internal moisture content gradient stabilized after conditioning at 45 ℃ for 72 h or 60 ℃ for 48 h; for the target moisture content of 25%, the critical parameters are conditioning at 45 ℃ for 72 h or 60 ℃ for 24 h. Additionally, secondary equilibrium processes conducted at room temperature can further minimize the moisture content gradient within the sample, bringing it closer to a quasi-equilibrium state. Notably, under the conditions of the equilibrium temperature of 75 ℃ and the equilibrium time of 24 h for the target moisture content of 15%, as well as the equilibrium temperature of 60 ℃ and the equilibrium time of 24 h for the target moisture content of 25%, the transverse relaxation time spectra of samples exhibited multiple peaks. In contrast, other equilibrium conditions yielded spectra with only a single bound water peak. The optimal parameters derived from the combined analysis of the internal moisture content gradient and NMR spectra suggest equilibrating at 45 ℃ for 72 h or 60 ℃ for 48 h. Conclusion: Compared to the traditional method, the proposed water-addition-equilibration method is more efficient and more accurate in terms of actual moisture contents. This method offers a new approach to regulating the moisture content during NMR characterization of wood.

Key words: wood, nuclear magnetic resonance (NMR), moisture content regulation, water-addition-equilibrium method, transverse relaxation time