Water migration in poplar wood during microwave drying studied by time domain nuclear magnetic resonance (TD-NMR)

Li, Xinyu; Gao, Yulei; Zhang, Minghui; Wang, Ximing; Wei, Xinyue

doi:10.1515/hf-2017-0040

Cited by 11 publications

(8 citation statements)

References 25 publications

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“…As a result of this process, in this case, the drying rate is limited by the diffusion process so that further increasing the air flux intensity does not change it. These results allow to rationalize and explain various previous observations ( 1 , 4 , 32 ) on the macroscopic evolutions of bound and free water during drying.…”

Section: Introductionsupporting

confidence: 85%

“…For drying, different phenomena were observed: in some cases, a homogeneous decrease of the water content throughout the medium and, in other cases, a dry front developing from the free surface ( 29 – 32 ). In addition, it was observed that during slow drying, the bound water concentration decreases when most free water has disappeared ( 31 – 33 ), while for fast drying in hardwood, the bound water content decreases proportionally to the free water content ( 4 ). To explain the latter effect, it was suggested that the free water is extracted by absorption in the cell walls and then transported toward the free surface ( 1 ).…”

Section: Introductionmentioning

confidence: 99%

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Two-step diffusion in cellular hygroscopic (vascular plant-like) materials

et al. 2022

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Vascular plants, a vast group including conifers, flowering plants, etc., are made of a cellular hygroscopic structure containing water in the form of either free (i.e., in a standard liquid state) or bound (i.e., absorbed in the cell walls) water. From nuclear magnetic resonance techniques, we distinguish the dynamics of bound water and free water in a typical material (softwood) with such a structure, under convective drying. We show that water extraction relies on two mechanisms of diffusion in two contiguous regions of the sample, in which respectively the material still contains free water or only contains bound water. However, in any case, the transport is ensured by bound water. This makes it possible to prolong free water storage despite dry external conditions and shows that it is possible to extract free water in depth (or from large heights) without continuity of the free water network.

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Section: Introductionsupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

Two-step diffusion in cellular hygroscopic (vascular plant-like) materials

et al. 2022

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“…A homogeneous decrease of the water content throughout the medium was for example observed in some cases whereas a dry front developed from the free surface in other cases [23][24][25][26]. Also, it was observed that during slow drying, bound water concentration decreases when most free water has disappeared [26][27][28] while for fast drying bound water content decreases proportionally to free water content [29].…”

Section: Introductionmentioning

confidence: 99%

How Bound Water Regulates Wood Drying

et al. 2020

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For most wood-based product uses it is essential to remove a large part of the water content from wet or green (fresh-cut) wood, to reduce further dimensional variations under varying humidity conditions, improve its mechanical characteristics and protect it from biological attacks. However, the internal mechanisms of drying are not fully described. Here we observe drying at different scales using macroscopic measurements (weighing), NMR (Nuclear Magnetic Resonance) measurements allowing to distinguish bound and free water contents, and XRCT (X-ray computed tomography) images of air-liquid interfaces at the smallest pore scale (wood lumens). We show that during wood drying, even well above the Fiber Saturation Point, bound water diffusion in cell walls (instead of capillary effects) ensures the extraction of liquid water from pores and its transport towards the surface of evaporation, and thus controls the drying rate. The distribution of bound water content (uniform or heterogeneous) along the main sample axis and the drying rate evolution depend on the competition between the external conditions and a characteristic rate of transport due to bound water diffusion. For sufficiently slow drying this distribution remains homogenous until free water is fully extracted. An original physical phenomenon is thus at work which plays a major role of regulation of water extraction, in that it maintains a constant drying rate and a homogeneous distribution of the (mean) water content throughout the material. These results provide sound concepts for modeling and controlling drying properties of wood materials. They open the way to the understanding or control of the properties of many other materials containing two water types in food or civil engineering applications. Our results complete recent observations that bound water diffusion also controls imbibition in hardwood and finally show that transfers between bound and free water play a major role in the interaction of plant-like systems with water.

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“…The relaxation times are differentiated as T 1 (spin-lattice) and T 2 (spin-spin) relaxation. With respect to wood-water interactions, previous LFNMR research has primarily focused on T 2 relaxation (Flibotte et al 1990;Araujo et al 1992;Labbé et al 2002Labbé et al , 2006Elder et al 2006;Thygesen and Elder 2008;Thygesen et al 2010;Telkki et al 2013;Fredriksson and Thygesen 2017;Li et al 2017), while a few studies have employed two-dimensional (2D) T 1 T 2 LFNMR correlation spectroscopy (Cox et al 2010;Bonnet et al 2017), which appears to be an even more powerful method for moisture characterization.…”

Section: Introductionmentioning

confidence: 99%

Characterization of moisture in acetylated and propionylated radiata pine using low-field nuclear magnetic resonance (LFNMR) relaxometry

et al. 2018

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Moisture in radiata pine (Pinus radiataD. Don) earlywood (EW), which was acetylated or propionylated to various degrees, was measured by low-field nuclear magnetic resonance (LFNMR) relaxometry. Spin-spin relaxation times (T2) were determined for fully saturated samples at 22 and −18°C.T2values for EW lumen water increased with increasing acetylation weight percentage gain (WPG), perhaps caused by the less hydrophilic acetylated wood (AcW) surface. Cell wall water (WCW) and the water in pits and small voids also showed increasingT2values as a function of WPG but with a weaker tendency. A possible explanation is the counteracting effects of decreased hydrophilicity and reduced moisture content (MC) of these water populations at higher levels of acetylation. The evaluation of propionylation on WCWT2data was complicated by peak splitting in the relaxation spectrum. ConstantT2values for void water populations at various WPG levels for propionylated samples indicate a modification gradient in the cell wall. Fiber saturation point (FSP) was significantly reduced by both modifications. Slightly higher FSP values for propionylated samples suggest that physical bulking is not the only factor causing moisture exclusion in AcW. But this interpretation is tentative because of the possibility of cell wall damage caused by propionylation.

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Water migration in poplar wood during microwave drying studied by time domain nuclear magnetic resonance (TD-NMR)

Cited by 11 publications

References 25 publications

Two-step diffusion in cellular hygroscopic (vascular plant-like) materials

Two-step diffusion in cellular hygroscopic (vascular plant-like) materials

How Bound Water Regulates Wood Drying

Characterization of moisture in acetylated and propionylated radiata pine using low-field nuclear magnetic resonance (LFNMR) relaxometry

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