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Moisture in untreated, acetylated, and furfurylated Norway spruce studied during drying using time domain NMR
Abstract
Using time domain NMR, the moisture in Norway spruce (Picea abies (L.) Karst.) sapwood subjected to four different treatments (never-dried, dried and remoistened, acetylated, and furfurylated) was studied during drying at 40°C, at sample average moisture contents above fiber saturation. Spin-spin relaxation time distributions were derived from CPMG relaxation curves using multiexponential fitting (CONTIN), and the resulting water populations were assigned according to the literature and their behavior during drying. It was found that both acetylation and furfurylation increased the average spin- spin relaxation time of the lumen water in earlywood tracheids from about 80 - 100 ms to 200 and 300 ms, respectively. The average spin-spin relaxation time of the cell wall water was reduced from about 1.4 to 0.65 ms by furfurylation, while acetylation had less effect on this water. The relaxation times of both the earlywood lumen water and of the cell wall water were slightly longer for the never-dried samples than for the dried and remoistened samples.
... Low-field nuclear magnetic resonance (LFNMR) spectroscopy has been used to study different states of water in wood [9,10]. It can assess the spin relaxation of hydrogen nuclei in water molecules in a magnetic field after excitation by a radio frequency pulse and characterise how water is bound in wood by the spin-lattice relaxation time (T 1 ) and the spin-spin relaxation time (T 2 ) [11,12]. ...
... The peak with the shortest T 2 relaxation time (below 3 ms) was assigned to the cell wall water. Later peaks, usually observed at the ranges of 9-80 ms and 30-400 ms, were assigned to the capillary water [9,11,[14][15][16][17][18]. Of these two capillary water peaks, the peak with the shorter T 2 relaxation time has been determined as the water in the lumen of latewood and ray cell and the peak with the longer T 2 relaxation time has been determined as the water in earlywood lumen, based on size difference of lumen of earlywood and latewood [11,19]. ...
... Acetylated wood showed a reduced peak area of cell wall water at the water-saturated condition compared to that of unmodified wood [20], which indicated a reduction in the maximum amount of water absorbed in cell walls due to acetylation. An increased T 2 relaxation time was observed for various modifications and was concluded to be due to a decreased hydrophilicity of the cell wall [9,18,20,31]. ...
In this study, the wood–water interactions in Scots pine sapwood modified with maleic anhydride (MA) and sodium hypophosphite (SHP) was studied in the water-saturated state. The water in wood was studied with low field nuclear magnetic resonance (LFNMR) and the hydrophilicity of cell walls was studied by infrared spectroscopy after deuteration using liquid D2O. The results of LFNMR showed that the spin–spin relaxation (T2) time of cell wall water decreased by modification, while T2 of capillary water increased. Furthermore, the moisture content and the amount of water in cell walls of modified wood were lower than for unmodified samples at the water-saturated state. Although the amount of accessible hydroxyl groups in modified wood did not show any significant difference compared with unmodified wood, the increase in T2 of capillary water indicates a decreased affinity of the wood cell wall to water. However, for the cell wall water, the physical confinement within the cell walls seemed to overrule the weaker wood–water interactions.
... The FID signal reflects both T 2 effect and effect of field inhomogeneity. In the case of wood, FID provides information on both the solid material and the water inside it, while CPMG only provides information about the water molecules [18]. The signal of hydrogen nuclei in wood polymers rapidly decays, which is distinguishable from the water signal. ...
... The FID signal reflects both T2 effect and effect of field inhomogeneity. In the case of wood, FID provides information on both the solid material and the water inside it, while CPMG only provides information about the water molecules [18]. The signal of hydrogen nuclei in wood polymers rapidly decays, which is distinguishable from the water signal. ...
... By measuring the relaxation times and deconvoluting them, the water components in wood, i.e., cell wall water, cell lumen water in pores with various sizes, can be evaluated. Since the late 1970s, some studies have carried out 1D NMR relaxation analysis, including the establishment of the quantitative relationships between MC and FID signals [19,[26][27][28][29]. Later works mainly used CPMG relaxation [18,22,28,[30][31][32][33][34][35][36][37][38][39][40][41]. Further understanding of the water components was presented with the assistance of T 1 -T 2 TD-NMR [13,[42][43][44][45]. ...
This review summarizes the development of the experimental technique and analytical method for using TD-NMR to study wood-water interactions in recent years. We briefly introduce the general concept of TD-NMR and magnetic resonance imaging (MRI), and demonstrate their applications for characterizing the following aspects of wood-water interactions: water state, fiber saturation state, water distribution at the cellular scale, and water migration in wood. The aim of this review is to provide an overview of the utilizations and future research opportunities of TD-NMR in wood-water relations. It should be noted that this review does not cover the NMR methods that provide chemical resolution of wood macromolecules, such as solid-state NMR.
... [8,9] It has also emerged as a powerful tool with which to study porous media, such as wood and wood products. In previous work, NMR techniques have extensively been applied to both the quantitative and qualitative analysis of wood-water relations, [10][11][12][13][14][15][16] to elucidate the effect of chemical/thermal modification on water absorption properties, [17][18][19][20][21][22] to measure the moisture content (MC) profiles within wood during drying, [23][24][25][26] and to determine the pore size distribution of wood. [27,28] The spin-spin relaxation time (T 2 ) of LF-NMR affords more detailed insight into the moisture behavior in wood compared with the spin-lattice relaxation time (T 1 ), and a considerable amount of work concerning the wood-water relationship that has been undertaken has relied on the T 2 relaxation time. ...
... The total amount, as well as the distribution, of water through the cell walls and cell lumens of wood can be appraised from T 2 distributions, as the cell wall substance, free water, and bound water yield a clearly distinguishable T 2 value. [17,25] In addition, some researchers [12,29] have also used the LF-NMR approach to investigate the influence of resinous extractives in maritime pine heartwood on the MC determination. According to their studies, it is possible to determine the contribution of the aqueous and organic phase extractives to the NMR signal or T 2 distribution. ...
... Several studies have shown that the wood cell wall and the water in wood can be readily distinguished due to their differences in T 2 behavior as related to molecular mobility. [10,17,30,31] The cell wall components, consisting of stiff cellulose fibrils embedded in a lignin and hemicellulose matrix, have very short T 2 values-around tens of microseconds-which is too short to be detected due to the dead time of the NMR instrument used. The bound water, with hindered local motion, yields T 2 values ranging from hundreds of microseconds to several milliseconds, whereas the free water principally located in the cell lumen exhibits a much longer relaxation time, which typically ranges from tens to hundreds of milliseconds. ...
In this study, the spin–spin relaxation time (T2) distributions of free water and bound water, as well as the moisture content (MC) profiles of MUF resin-impregnated poplar wood (Populous tomentosa) sample (RI) were investigated by low-field nuclear magnetic resonance, with the aim of providing insights into how MUF resin impregnation affects the moisture states and moisture transport in modified poplar wood during drying. The T2 curves demonstrated that the resin treatment did not increase the number of peaks in the T2 distributions, but affected the T2 value as compared to the control. Above the fiber saturation point (FSP), the T22 (corresponds to the free water in wood rays and wood fibers) of the RI sample exhibited an increase compared to the control, while the T23 (corresponds to the free water in the vessels) was almost unchanged. Below the FSP, a shorter T21 (corresponding to the bound water) of the RI sample was observed compared to the control. The drying curves and MC profiles indicated a significant difference in the moisture transport in the RI sample as compared to the control. The gradually cured resin system in the wood surface layer during drying provided a barrier for the transfer of water in the center layer toward to the surface, causing the resin curing reactions in the surface and core layers to be out of sync. Therefore, a more significant MC gradient was observed for the resin-impregnated sample.
... To differentiate between different pools of water within the wood structure, low-field nuclear magnetic resonance (LFNMR) can be used [54][55][56][57][58][59][60][61][62][63][64] (for a theoretical background, see Rutledge [65]). Here, spin-spin relaxation time (T 2 ) distributions are evaluated from exponential decay functions. ...
... This lowered cell-wall moisture content is a possible reason for the enhanced durability, since a low cell-wall moisture content could hinder the transport of substances necessary for the fungi [87]. There are, however, indications that acetylation not only changes the cell-wall moisture content, but also influences the interactions between wood and capillary water [62,67]. Wood-degrading fungi need capillary water [12], and acetylation lowers the over-hygroscopic desorption isotherm [43]. ...
... Wood-degrading fungi need capillary water [12], and acetylation lowers the over-hygroscopic desorption isotherm [43]. Thygesen and Elder [62] and Beck et al. [67] studied the influence of acetylation on interactions between wood and water using LFNMR and found that acetylation increased T 2 for the peak corresponding to lumen water. Beck et al. [67] found that this increase was linked to the weight percentage gain of the acetylated wood; a higher weight percentage gain achieved from the acetylation led to a longer T 2 . ...
Wood is a hygroscopic material that absorbs and desorbs water to equilibrate to the ambient climate. Within material science, the moisture range from 0 to about 95–98% relative humidity is generally called the hygroscopic moisture range, while the exceeding moisture range is called the over-hygroscopic moisture range. For wood, the dominating mechanisms of moisture sorption are different in these two moisture ranges; in the hygroscopic range, water is primarily bound by hydrogen bonding in cell walls, and, in the over-hygroscopic range, water uptake mainly occurs via capillary condensation outside cell walls in macro voids such as cell lumina and pit chambers. Since large volumes of water can be taken up here, the moisture content in the over-hygroscopic range increases extensively in a very narrow relative humidity range. The over-hygroscopic range is particularly relevant for durability applications since fungal degradation occurs primarily in this moisture range. This review describes the mechanisms behind moisture sorption in the over-hygroscopic moisture range, methods that can be used to study the interactions between wood and water at these high humidity levels, and the current state of knowledge on interactions between modified wood and water. A lack of studies on interactions between modified wood and water in the over-hygroscopic range was identified, and the possibility of combining different methods to acquire information on amount, state, and location of water in modified wood at several well-defined high moisture states was pointed out. Since water potential is an important parameter for fungal growth, such studies could possibly give important clues concerning the mechanisms behind the increased resistance to degradation obtained by wood modification.
... 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. ...
... However, below FSP, both the T 2 value (Menon et al. 1987;Araujo et al. 1994;Almeida et al. 2007) and signal amplitude (Araujo et al. 1992) of this population decrease with decreasing MC. In addition to the W CW peak, previous CPMG studies on softwoods identified two to three other peaks in water saturated or green samples, which represent free water in various macro-void spaces of wood (Menon et al. 1987;Flibotte et al. 1990;Araujo et al. 1992Araujo et al. , 1994Labbé et al. 2002Labbé et al. , 2006Thygesen and Elder 2008;Telkki et al. 2013;Kekkonen et al. 2014;Fredriksson and Thygesen 2017). Void size has a strong effect on T 2 relaxation, causing liquid water in smaller voids to have a shorter T 2 (Brownstein and Tarr 1979;Menon et al. 1987;Almeida et al. 2007). ...
... Therefore, the void water (W void ) peaks are highly variable for different wood species because they are affected by the wood cell anatomy. Two peaks were found in sapwood of Western red cedar (Thuja plicata Donn ex D. Don) at relaxation times of 20-100 ms and 100-300 ms (Menon et al. 1987;Flibotte et al. 1990), two to three peaks for maritime pine (Pinus pinaster Aiton) in the range of 10-500 ms (Labbé et al. 2002, three peaks for Scots pine (Pinus sylvestris L.) in the range of 10-100 ms (Kekkonen et al. 2014), and three peaks for Norway spruce (Picea abies (L) H. Karst) in the range of 10-1000 ms (Thygesen and Elder 2008;Fredriksson and Thygesen 2017). In studies, where three W void peaks were observed, the samples were water saturated Thygesen and Elder 2008;Kekkonen et al. 2014). ...
Moisture in radiata pine (Pinus radiata D. 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. T2 values 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 increasing T2 values 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 WCWT2 data was complicated by peak splitting in the relaxation spectrum. Constant T2 values 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.
... Time domain nuclear magnetic resonance (TD-NMR), also called low field-time domain-nuclear magnetic resonance (LF-TD-NMR), is a powerful tool for charac- terization of the static water states and their dynamics in wood. TD-NMR was applied for MC determination and identification of the water states ( Thygesen et al. 2008Thygesen et al. , 2009Telkki et al. 2013;Zhang et al. 2013;Fredriksson et al. 2017). Briefly, the transverse or spin-spin relaxation time (T 2 ) determined by TD-NMR via the Carr-Purcell-Mei- boom-Gill (CPMG) sequence is one of the most important parameters in water characterization. ...
... The bW and fW contents are often studied separately because their simultaneous detection by traditional methods is challenging ( Zhang et al. 2013). The TD-NMR method is in principle suitable to this task ( Topgaard et al. 2002;Fantazzini et al. 2006;Thygesen et al. 2008;Elder et al. 2013;Zhang et al. 2013), while the separation of bW peaks is based on the typical T 2 range for both water states. However, Telkki et al. (2013) suggested that T 2 distribution should have two components, i.e. a large one and a smaller one, while the latter is observed in traditional TD-NMR tests, and the small and longer T 2 components are hidden by the fW peak in the traditional T 2 spectrum. ...
... The relative amount of bW becomes higher during the drying process (red lines in Figure 5a, c, e and g) ( Meder et al. 2015). This was also observed by Thygesen et al. (2008) in the initial phase of drying. The water bound by hydrogen bridges is seen as peak 2 ( Figure 5) and in micropores as peak 3 ( Figure 5) at the specific MC range (indication by black arrow in Figure 5). ...
The time domain nuclear magnetic resonance (TD-NMR) technique was applied to observe the water states, water migration and the T2cutoff of sapwood (sW) and heartwood (hW) of Xinjiang poplar and Mongolian Scotch pine wood during drying. Three components in the T2 spectrum could be observed for poplar and pine at room temperature, and a large, short time component and a small, long time component of free water was visible at the melting point of water. During drying, the long time T2 component diminished rapidly above the fiber saturation point (FSP), and almost disappeared close to or below the FSP. The short time components of the T2 distributions slightly enlarged with decreasing moisture content (MC) above the FSP but rapidly diminished and shifted to the left side below the FSP. The bound water content estimated based on the T2cutoff is in agreement with previous findings. Free and bound water have been found to be in two primary states both in poplar and pine. The loss of free water was rapid and profound but the bound water slightly increased with decreasing MC above the FSP, and the free water evaporated completely close to the FSP, where the bound water began to disappear significantly.
... Each curve represents one specimen at the EMC R listed space within the cell wall by filling the available volume. NMR studies carried out by Thygesen and Elder (2008) on acetylated Norway spruce sapwood corroborated these findings. In this case, cell wall water relaxation times appeared to be lowered as result of a possible reduction in the sizes of the space available to water within cell wall due to bulking. ...
... Considering that the second peak was observed only in wood that was ball-milled (Zelinka et al. 2012) or submitted to high WPG (present work), the appearance of two freezing peaks might be related to the extreme physical changes that occur to wood at high mechanical disintegration or high cell wall chemical modification. Ball-milling completely disrupts the cell wall organization and chemical bonds between cellulose (Fukazawa et al. 1982), whereas acetylation at high WPG alters the thermodynamics of the liquid water interacting with internal lumen walls (Thygesen and Elder 2008), changing contact angle and adhesion. In extreme cases, under high WPG, water droplets, rather than a film, might be formed on the lumen wall (Thygesen and Elder 2008). ...
... Ball-milling completely disrupts the cell wall organization and chemical bonds between cellulose (Fukazawa et al. 1982), whereas acetylation at high WPG alters the thermodynamics of the liquid water interacting with internal lumen walls (Thygesen and Elder 2008), changing contact angle and adhesion. In extreme cases, under high WPG, water droplets, rather than a film, might be formed on the lumen wall (Thygesen and Elder 2008). The second peak was observed only in samples under a short range of EMC R above the estimated FSP R , i.e., only 10-15% MC attributed to free water. ...
This paper evaluates the effects of acetylation level and experimental method on the observed fiber saturation point (FSP) of loblolly pine (Pinus taeda) wood measured using differential scanning calorimetry. To achieve this goal, 1-mm-thick latewood samples were tested over a wide range of equilibrium moisture content (EMC). In this work, the FSP was defined as the non-freezable portion of water of the samples. Two experimental methods were used: the extrapolation of the melting enthalpy to zero and the direct calculation of the non-freezable water amount. For both methods, the FSP decreased with increasing acetylation, varying from about 27% reduced EMC (EMCR) for control to about 9% EMCR for the highest level of acetylation. For unmodified samples, the measured FSP was higher with faster scan rates. Moreover, under a specific range of EMCR, freezing curves revealed the occurrence of two water phase transitions for samples at the highest level of acetylation. Based on previous studies and in present findings, there is strong evidence that the lower temperature freezing peak may result from the homogenous nucleation of water, which is physically separated from water that freezes heterogeneously.
... In LFNMR studies on softwoods, three or four peaks representing different water populations have been identified in water-saturated or green specimens (Menon et al. 1987(Menon et al. , 1989Flibotte et al. 1990;Araujo et al. 1992;Labbé et al. 2002Labbé et al. , 2006Thygesen and Elder 2008). The 1 st peak, generally at <3 ms (Menon et al. 1987;Araujo et al. 1992Araujo et al. , 1994Labbé et al. 2002Labbé et al. , 2006Thygesen and Elder 2008;Telkki et al. 2013) is assigned to the water bound in the cell wall, while peaks with a longer T 2 are assigned to free water. ...
... In LFNMR studies on softwoods, three or four peaks representing different water populations have been identified in water-saturated or green specimens (Menon et al. 1987(Menon et al. , 1989Flibotte et al. 1990;Araujo et al. 1992;Labbé et al. 2002Labbé et al. , 2006Thygesen and Elder 2008). The 1 st peak, generally at <3 ms (Menon et al. 1987;Araujo et al. 1992Araujo et al. , 1994Labbé et al. 2002Labbé et al. , 2006Thygesen and Elder 2008;Telkki et al. 2013) is assigned to the water bound in the cell wall, while peaks with a longer T 2 are assigned to free water. This differentiation between bound water and free water is made because the former has a T 2 which is constant and independent of MC changes above the fibre saturation point (FSP) (Araujo et al. 1992). ...
... Peaks with longer T 2 are attributed to free water, because their amplitudes decrease with decreasing MC above the FSP (Araujo et al. 1992). In addition, a 4 th peak with a longer T 2 was detected in two studies (Labbé et al. 2006;Thygesen and Elder 2008). Labbé et al. (2006) attributed this peak to organic compounds. ...
Low-field nuclear magnetic resonance (LFNMR) relaxometry was applied to determine the spin-spin relaxation time (T2) of water-saturated Norway spruce (Picea abies (L.) Karst.) specimens cut from mature sapwood (sW) and mature and juvenile heartwood (hW), where earlywood (EW) and latewood (LW) were separated. In combination with quantitative wood anatomy data focusing on the void volumes in various morphological regions, the NMR data served for a more reliable assignment of free-water populations found in water-saturated solid wood. Two free-water populations were identified within most sample types. One was assigned to water in the tracheid lumen and the other to water inside bordered pits. Whether water in the ray cell lumina was included in one or the other of these two populations depends on the curve-fit method applied (continuous or discrete). In addition, T2 differences between the different tissue types were studied and, for comparison, sorption isotherms were measured by means of a sorption balance. There was a significant difference between EW and LW as well as between juvenile wood and mature wood in terms of T2 related to the cell wall water. However, no differences were seen between the sorption isotherms, which indicates that the observed T2 differences were not due to differences in cell wall moisture content (MC).
... La RMN est une technique non destructive utilisée de plus en plus pour étudier les relations eau-bois. Elle permet d'évaluer les différentes phases de l'eau dans le bois (Araujo et al. 1992;Araujo et al. 1993;Thygesen et Elder 2008Telkki et al. 2013;Zhang et al. 2013), en fournissant des informations qui représentent une moyenne sur l'ensemble de l'échantillon (MacMillan et al. 2011). La séquence Carr-Purcell Meiboom-Gill (CPMG), basée sur la séquence de base d'écho de spin, est normalement utilisée pour étudier les états de l'eau dans le bois (l'eau liée et l'eau liquide), tandis que le signal de FID se montre utile à déterminer la teneur en humidité du matériel (Labbé et al. 2006). ...
... Plus tard, Zhang et al. (2013) Zhang et al. (2013), les résultats obtenus à l'aide de la RMN ont montré que la perte de l'eau liée du bois de tulipier de Virginie a débuté à partir de 46 % de teneur en humidité, bien au-dessus du PSF. Menon et al. 1987 Les valeurs de T2 dépendent de l'environnement local des protons 1 H des molécules de l'eau et reflètent la mobilité de ces protons (Thygesen et Elder 2008Cox et al. 2010), de sorte que la dynamique des molécules de l'eau liée est plus lente que celle de l'eau liquide (Telkki 2012). Ainsi, à mesure que le bois perd de l'humidité, les molécules d'eau liée deviennent attachées aux sites de sorption par des forces de plus en plus fortes. ...
... Ainsi, à mesure que le bois perd de l'humidité, les molécules d'eau liée deviennent attachées aux sites de sorption par des forces de plus en plus fortes. Cela fait que la mobilité des protons de l'eau soit plus restreinte, réduisant les valeurs de relaxation T2 (Thygesen et Elder 2008;Elder et Houtman 2013;Zhang et al. 2013). La réduction du T2 de l'eau liquide à mesure que le bois sèche a été attribuée à la diminution de la taille des réservoirs d'eau dans le bois, tel que les lumens des cellules (Brownstein et Tarr 1979;Menon et al. 1989). ...
The main purpose of this work was to study the changes in the physical properties and in the wood-water relations of four hardwood species below and above the fiber saturation point (FSP). Two tropical, one temperate and one plantation grown species with high variable anatomical structure were investigated. In the first part of this work, samples of all species were destined to three desorption conditions at 21°C from full saturated state or from green condition until reach equilibrium moisture contents (EMC) below the FSP. The water state and the distribution of liquid and bound water were analysed by nuclear magnetic resonance and magnetic resonance (MR) microimaging. Transverse relaxation times T2 showed entrapped liquid water below the FSP. The MR microimaging study allowed identifying in which anatomical elements this water could be located. Also, bound water was not uniformly distributed in wood structure even at a global EMC, being the denser tissues more hygroscopic than the lighter ones. Both techniques revealed that wood anatomy plays a major role in water drainage and diffusion. The second part of this work was dedicated to study the effect of four desorption rates on the dimensional behaviour of Eucalyptus saligna wood under a range of moisture contents covering the whole hygroscopic domain. Slower desorption rates implied higher shrinkages, but these different rates have not affected the slope of shrinkage-EMC curves. The collapse of Eucalyptus saligna wood was more important for slower desorption rates and it was greater in the tangential direction. The FSP determined by the extended linear portion of the shrinkage-EMC curves revealed to be not adequate for collapse-prone species. Finally, the results indicated the phenomenon of the beginning of the loss of bound water before all liquid water has been drained.
... However, the degradation of wood by decay fungi requires moisture conditions where capillary water is present in the porous wood structure (Ringman et al. 2014;Schmidt 2006;Zabel and Morell 1992). Acetylation of cell wall surfaces weakens the interaction energy between capillary water and wood (Beck et al. 2018b;Thygesen and Elder 2008;Thygesen et al. 2010;Yang et al. 2020) and changes the contact angle (Bryne and Wålinder 2010;Englund et al. 2009;Moghaddam et al. 2015). These changes will affect how moisture is taken up by acetylated wood through capillary action or capillary condensation at high air humidity. ...
... Based on these continuous T 2 distributions, the influence of the modifications on water in different locations within the wood structure was evaluated. Since the T 2 is related to the physical environment (surface-to-volume ratio of pores) (Menon et al. 1987) water within cell walls can be separated from water outside of cell walls and capillary water in differently sized voids within the wood structure can be distinguished (Almeida et al. 2007;Araujo et al. 1992;Fredriksson and Thygesen 2017;Labbé et al. 2006;Thygesen and Elder 2008). The T 2 at the maximum intensity of each peak was evaluated, and in those cases where a certain water population was represented by more than one peak (e.g. ...
Wood is an increasingly important material in the sustainable transition of societies worldwide. The performance of wood in structures is intimately tied to the presence of moisture in the material, which directly affects important characteristics such as dimensions and mechanical properties, and indirectly its susceptibility to fungal decomposition. By chemical modification, the durability of wood in outdoor environments can be improved by reducing the amount of moisture present. In this study, we refined a well-known chemical modification with acetic anhydride and showed how the spatial distribution of the modification of Norway spruce ( Picea abies (L.) Karst.) could be controlled with the aim of altering the wood-water interactions differently in different parts of the wood structure. By controlling the reaction conditions of the acetylation it was possible to acetylate only the cell wall-lumen interface, or uniformly modify the whole cell wall to different degrees. The spatial distribution of the acetylation was visualised by confocal Raman microspectroscopy. The results showed that by this targeted acetylation procedure it was possible to independently alter the wood-water interactions in and outside of cell walls. The cell wall-lumen interface modification altered the interaction between the wood and the water in cell lumina without affecting the interaction with water in cell walls while the uniform modification affected both. This opens up a novel path for studying wood-water interactions in very moist environments and how moisture distribution within the wood affects its susceptibility towards fungal decomposition.
Graphic abstract
... Low-field nuclear magnetic resonance spectroscopy and magnetic resonance imaging Low-field (LF) nuclear magnetic resonance (NMR) spectroscopy, also known as time-domain NMR (Thygesen and Elder 2008) or NMR relaxometry (Kekkonen et al. 2014), can distinguish between water-bound hydrogen located in different physical and chemical environments. Consequently, it is a good technique for determining the distribution of water within different parts of the wood structure (Araujo et al. 1992). ...
... The amount of moisture (MC R ) found in lumina remains the same, while the amounts in pits, small cavities, and cell walls are reduced. Original LFNMR data from Thygesen and Elder (2008) radiofrequency pulses. Although the amount of energy put into the sample by each pulse is tiny, the accumulated number of pulses can heat up the sample considerably in extreme cases. ...
Water plays a central role in wood research, since it affects all material properties relevant to the performance of wood materials. Therefore, experimental techniques for characterising water within wood are an essential part of nearly all scientific investigations of wood materials. This review focuses on selected experimental techniques that can give deeper insights into various aspects of water in wood in the entire moisture domain from dry to fully water-saturated. These techniques fall into three broad categories: (1) gravimetric techniques that determine how much water is absorbed, (2) fibre saturation techniques that determine the amount of water within cell walls, and (3) spectroscopic techniques that provide insights into chemical wood–water interactions as well as yield information on water distribution in the macro-void wood structure. For all techniques, the general measurement concept is explained, its history in wood science as well as advantages and limitations.
... Hence, another nondestructive, low-field nuclear magnetic resonance (LF-NMR) approach has been extensively used in food science, petroleum industry, chemical, medical research, etc. [14][15][16][17][18] It emerged as a powerful tool for determining not only the mass fraction but also elucidating the nature of water present in wood. [19][20][21][22][23][24] The Spin-lattice (or longitudinal) relaxation times (T 1 ) or spin-spin (or transverse) relaxation times (T 2 ) could be obtained from the NMR signal according to the experiment implemented, [25,26] and the T 2 , as compared to the T 1 , affords more detailed insight into moisture in wood. [27,28] Moreover, although the wood-water system is complicated, the researchers have found that the solid wood (cell wall) and the water distributed within it, including bound water and free water, can be readily separated due to the distinguishable T 2 behavior. ...
... The interpretation of the T 2 for wood-water system has been extensively discussed, and a good summary for the assignment of water in softwood has been provided in the literature. [22] Poplar wood, as a diffuse-porous hardwood, is mainly composed of rays, fibers, and vessels. Water molecules distribute in those elements which have various sizes or cell dimensions, and process different mobility, thus diverse relaxation times. ...
In this article, the low field nuclear magnetic resonance (LF-NMR) was used to measure the moisture content (MC) and MC profiles in poplar wood during real-time drying. The T2 distribution curve at each drying stage measured using the Carr-Purcell-Meiboom-Gill pulse sequence provided detailed information in the dynamic change of free water as well as bound water of the whole wood sample. In addition, a new developed SE-SPI pulse sequence was firstly used to evaluate the spatially resolved T2 distribution of the successive non-destructive sliced layer of wood. Combined with the area integration method, the moisture content in each layer was calculated, and the change of MC profiles within wood at the MC above and below FSP was well reflected.
... temperature and humidity [1][2][3]. Wood-water interactions were studied for many years by numerous methods including proton nuclear magnetic resonance ( 1 H NMR) [2][3][4][5][6]. 1 H-NMR is unique in the sense that information can be extracted about both the wood and water components. With the aid of NMR it is possible to provide insight into the interactions between wood and water in materials [4][5][6][7]. ...
... Wood-water interactions were studied for many years by numerous methods including proton nuclear magnetic resonance ( 1 H NMR) [2][3][4][5][6]. 1 H-NMR is unique in the sense that information can be extracted about both the wood and water components. With the aid of NMR it is possible to provide insight into the interactions between wood and water in materials [4][5][6][7]. The NMR methods lead to determine the evolution of the water in the porous network of materials by relaxation times [7][8][9][10]. ...
The aim of this work was to use 2-dimensional pulsed field gradient (PFG) NMR-spectroscopy to explore the anisotropic diffusion of water in wood. Simulations based on the theory of 2D diffusion-diffusion correlations with following application of Inverse Laplace transformation (ILT) have been carried out for orthogonal and collinear pairs of gradients with parameters of 2D experiment on wood. The results showed how these are reflected in 2D (D 1 , D 2) ILT maps for different ratios D 1 /D 2 in two orthogonal directions. When two pairs of gradients were applied orthogonally in 2D experiment on wood the features of diffusion anisotropy appeared as off-diagonal " wings " in 2D diffusion map whereas in the experiment with two collinear pairs of gradients anisotropic diffusion of water in wood cells has been observed as two diagonal peaks in 2D map. Keywords—NMR PFG, water, wood, anisotropy, ILT 2D (D1, D2) maps (spectra) obtained from 2D DDCOSY experiment on wood samples with HL=51% (left) and HL=59% (right) for orthogonal pairs of gradient pulses. Diffusion anisotropy is characterised by off-diagonal " wings " ., D2) spectra obtained from the data set of 2D DDCOSY experiment on wood sample with HL=60% carried out for collinear pairs of gradient pulses. Diffusion anisotropy is characterised by two extended spots on the diagonal.
https://issuu.com/dropcap/docs/____________________________________1dbea5eaba223f
... In this manner, the solidwood signal disappears in tens of milliseconds, allowing an easy way to separate from the cellwater signal, whose T 2 values ranges from one to a few milliseconds. The third component, the liquid water located in cell lumina, is characterized by a T 2 value in order of tens to hundreds of milliseconds (Hsi et al 1977;Menon et al 1987;Araujo et al 1992;Araujo et al 1993;Thygesen and Elder 2008). used the NMR technique to separate the components of water in wood of two temperate and one tropical species at different EMCs. ...
... This smaller area allows for a more efficient relaxation (less mobile) and consequently a shorter T 2 time for Eucalyptus saligna compared with red oak (Menon et al 1987;Araujo et al 1992). According to Thygesen and Elder (2008), during the T 2 relaxation measurement, if cell lumen is small enough to allow a considerable portion of water molecules to collide with the cell wall, these molecules will exhibit a faster T 2 time than unconfined water. ...
Nuclear magnetic resonance (NMR) is a useful, powerful, and noninvasive technique to study the dynamics of wood-water relations, both quantitatively and qualitatively. The main objective of this study was to use NMR to characterize the state of water below the FSP. Two tropical hardwood species, huayruro (Robinia coccinea Aublet) and cachimbo (Cariniana domesticata [C. Martius] Miers), a plantation-grown eucalyptus species (Eucalyptus saligna Smith), and a temperate species, red oak (Quercus rubra L.), were studied. These species were chosen for their diversity in terms of anatomical and physical properties. Desorption tests were carried out at 21°C in a single-step procedure from full saturation state for huayruro, cachimbo, and red oak and from green condition for E. saligna. Discrete T2 times were obtained for each species and equilibrium moisture content (EMC). The results showed that even under EMC, there was a region in the hygroscopic range in which the loss of bound water takes place before all liquid water was drained. This region varies according to wood species. Furthermore, variation in the fast T2 values among the different wood species gives an indication of how bound water is distributed and arranged in sorption sites.
... When water is absorbed by the cell wall, the wood swells, and when water In contrast, other methods focused on measuring the saturation moisture content of the wood cell wall, which was invoked in Tiemann's speculative explanation. These methods include DSC [124][125][126][127], low-field NMR relaxometry [128][129][130][131][132], and solute exclusion [126,[133][134][135]. In these methods, a fully water-saturated sample at its maximum moisture content is normally used. ...
Wood-water interactions are central to the utilization of wood in our society since water affects many important characteristics of wood. This topic has been investigated for more than a century, but new knowledge continues to be generated as a result of improved experimental and computational methods. This review summarizes our current understanding of the fundamentals of water in wood and highlights significant knowledge gaps. Thus, the focus is not only on what is currently known but equally important, what is yet unknown. The review covers locations of water in wood; phase changes and equilibrium states of water in wood; thermodynamics of sorption; terminology including cell wall water (bound water), capillary water (free water), fiber saturation point, and maximum cell wall moisture content; shrinkage and swelling; sorption hysteresis; transport of water in wood; and kinetics of water vapor sorption in the cell wall.
... Peaks 3 and 4, in the range of 60 ms to 150 ms, were attributed to free water in the tracheid lumina of latewood and earlywood, respectively. (Thygesen and Elder 2008;Telkki et al. 2013;Fredriksson and Thygesen 2017;Gezici-Koç et al. 2017). ...
The vaporized heat of bound water in radiata pine wood sapwood, which was thermally treated at 200 °C, via N2 protection for 24 h, were studied using low-field nuclear magnetic resonance (LFNMR) and differential scanning calorimetry (DSC) analysis. The bound water was divided into two states using LFNMR, which were absorbed water bonded with cell wall polymers and condensed bound water in the micro-pores of cell wall, respectively. The mass of the two states of bound water vaporized during DSC test was calculated based on the total mass of bound water vaporized and the moisture content of different water states in the water-saturated sapwood obtained, respectively. The reduction of moisture during DSC test was monitored synchronously using thermogravimetric analysis. The results showed thermal treatment decreased the spin-spin time (T2) of absorbed bound water and increased T2 of condensed bound water. The moisture content of the two states of bound water was reduced by thermal treatment. The vaporized heat of the bonded bound water was increased and that of the condensed bound water was reduced, which agreed with the change of T2 in the LFNMR experiment. The results suggest that bound water in the thermal treated wood is easier to vaporize when the relative humidity condition is more than 60%.
... At the state of equilibrium moisture content, Fig. 9c indicates that the predominant state of water present in bamboo was bound water (< 10 ms). The T 2 of bound water in MA-bamboo (0.455 ms) was shorter than that of the control bamboo (3.181 ms), indicating that the water in the modified bamboo was more tightly bound to the bamboo structure, which was caused by the filling and esterification between maleic anhydride and the bamboo cell walls (Thygesen and Elder 2008). Additionally, TO-bamboo and MT-bamboo also had a shorter T 2 (< 1 ms), demonstrating that the hydrophilic structure of bamboo could be partially changed by tung oil heat treatment (Gao et al. 2019), which was consistent with the FTIR results. ...
Bamboo is known to be a sustainable material with fast growth and high mechanical properties, while the high hygroscopicity and low-dimensional stability limit its further utilization in the construction sector. Thus, a novel dual modification method combining maleic anhydride and tung oil heat treatment was firstly applied to bamboo, and an orthogonal experiment was designed to investigate the influence of the processing parameter on the physical properties of bamboo, including weight change rate (WCR), moisture excluding efficiency (MEE) and anti-swelling efficiency (ASE). Furthermore, the morphological characteristics, chemical structure and water status of modified bamboo were also studied. The results showed that a parameter combination of maleic anhydride pretreatment at 80 °C for 3 h and tung oil heat treatment at 140 °C for 3 h was considered as an optimized process. The MEE and ASE (volume) of the modified bamboo could reach approximately 80 and 60%, respectively. Maleic anhydride was deposited on the cell wall and the reaction sites were grafted onto the bamboo by esterification. A portion of tung oil could be impregnated into the bamboo and occluded the water channel, such as vessels and pits, and the chemical linkage between the modified bamboo and tung oil was then established during heat treatment through the Diels–Alder reaction. Additionally, the dual modification method not only changed the water distribution of bamboo, but also shortened the spin–spin relaxation time (T2) of the bound water and free water.
... The change in the T 2 value of the bound water may have been due to the increase in density and the decrease in moisture content after FA treatment. On the other hand, the effects of FA on free and bound water were different because FA resin is a hydrophobic substance that enlarged the cell wall contact angle more than that of the control wood, resulting in increasing T 2 values between the wood and free water [42,43]. In addition, the proportions of the different water populations of furfurylated and control wood showed that although furfurylation reduced the moisture content of the wood, it increased the proportion of bound water and decreased the proportion of free water in the poplar and Chinese fir wood following water-saturation treatment, during which the proportion of free water in the vessel increased and that in the wood fiber decreased. ...
Furfurylation with a low concentration of furfuryl alcohol (FA) promotes the improvement of the properties and the effectiveness of FA on cell–wall action without darkening the furfurylated wood to the point that it affects its applications. In this paper, the effects of furfurylation on the hygroscopicity and water uptake dimensional stability of poplar (Populus sp.) and Chinese fir (Cunninghamia lanceolata) were analyzed. Meanwhile, the distribution of FA resin, the relationship between wood and water, the change in pore size distribution, and the weight percentage gain and cell wall bulking coefficient of wood were also investigated. The results were as follows: (1) A low concentration of FA could better enter the cell walls of the Chinese fir than the poplar, as FA resin was almost cured in the secondary walls, cell corners, and compound middle lamellae when a 10% concentration of FA was applied to the Chinese fir and poplar. When the FA concentration was increased to 30%, there were no significant increases in the amount of FA entering the cell walls and the amounts of FA cured in the cell lumen of the poplar were greater than those of the Chinese fir. Meanwhile, the modification of cell walls was more suitable in poplar than in Chinese fir. (2) The pointed ends of the pit chambers and the pit apertures (800–1000 nm) in the poplar and the small pores of the pit membranes and the pit apertures (1–6 μm) in the Chinese fir were partially deposited by the FA resin, which formed new pores in the size ranges of 80–600 nm and 15–100 nm, respectively. The porosity of the poplar was greater than that of the Chinese fir, and the bulk density of the poplar was less than that of the Chinese fir before and after modification. (3) Furfurylation with a low concentration of FA was able to better reduce the equilibrium moisture content, improve the anti-swelling efficiency, and enhance the dimensional stability of the poplar wood compared to the Chinese fir. Furfurylation effectively reduced water uptake due to the hydrophobic property of the FA resin. The water uptake of the Chinese fir increased by 17%–19% in second cyclic water soaking when treated with FA with various concentrations, which indicated the loss and leaching of FA resin during the test. Low-field NMR was used to demonstrate that the furfurylation not only reduced the amount of water but also affected the combination state of bound and free water with wood. Thus, furfurylation at a low concentration is a feasible method by which to extend applications of furfurylated wood.
... Acetylation weakens the interaction between liquid water and wood as seen with lowfield NMR spectroscopy [49,50] and from the decrease in contact angle with water [7][8][9][10]. For this reason, acetylation is expected to decrease the rate of liquid water uptake by capillary suction which has also been observed for acetylated alder [51]. ...
Wood modifications are becoming popular as a way to enhance the performance of wood, either to make it more durable, improve the performance of wood, or give it new functionality as a multifunctional or smart material. While wood modifications have been examined since the early 1900s, the topic has become a dominant area of study in wood science over the past decade. This review summarizes recent advances and provides future perspective on a selection of wood modifications, i.e., the methods that are currently commercialized (acetylation, furfurylation, and thermal modification), a rediscovered ancient practice (charring), a family of polymerization modifications that have so far made it to the pilot scale, and examples of novel wood-based functional materials explored at laboratory scale.
... In the past, chemical treatment of wood is a common method for enhancing water repellency and durability of wood. The chemical treatment of wood includes acetylation treatment [11][12][13], graft modification treatment [14][15][16][17][18], etc. These chemical treatments can effectively slow down the wood's moisture absorption and extend its service. ...
Hydrophobic surfaces have aroused considerable attention because of their extensive potential applications. In this work, we developed a facile strategy for fabricating hydrophobic and anti-fouling surfaces on wood substrates. The modification was accomplished simply by immerging wood into the tetramethylcyclotetrasiloxane (D4H) modifier solution for 5 min. The D4H modified wood was characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope, and energy dispersive spectrometer. The result shows that the D4H modified wood had good hydrophobicity, and the water contact angle of wood in the radial and cross sections reached 140.1° and 152°. In addition, the obtained hydrophobic wood surface also showed anti-fouling properties, UV resistance and could withstand the tape peel test and finger wiping.
... Modification does not only affect moisture within cell walls. For instance, acetylation and furfurylation have been shown to affect the interaction between wood and capillary water [54,119,127]. This could be caused by a change in contact angle, which has been observed for both acetylated and furfurylated wood [128]. ...
Moisture plays a central role in the performance of wood products because it affects important material properties such as the resistance to decomposition, the mechanical properties, and the dimensions. To improve wood performance, a wide range of wood modification techniques that alter the wood chemistry in various ways have been described in the literature. Typically, these modifications aim to improve resistance to decomposition, dimensional stability, or, to introduce novel functionalities in the wood. However, wood modification techniques can also be an important tool to improve our understanding of the interactions between wood and moisture. In this review, we describe current knowledge gaps in our understanding of moisture in wood and how modification has been and could be used to clarify some of these gaps. This review shows that introducing specific chemical changes, and even controlling the distribution of these, in combination with the variety of experimental methods available for characterization of moisture in wood, could give novel insights into the interaction between moisture and wood. Such insights could further contribute to applications in several related fields of research such as how to enhance the resistance to decomposition, how to improve the performance of moisture-induced wooden actuators, or how to improve the utilization of wood biomass with challenging swelling anisotropy.
... These populations of differently constrained water pools represent three physically and chemically distinct environments within the biomass, and have previously been attributed to water present in the lumen, in capillaries, and within the cell wall, respectively, where shorter relaxation times correspond to increased water constraint. This distribution of water in the biomass is in accordance with previous studies (Thygesen and Elder 2008). ...
Processing of lignocellulosic biomass is complex due to the heterogeneity of the substrate, but also due to lengthy unit operations, which complicates process control including for enzymatic saccharification. Methods for predicting enzymatic saccharification yield based on the properties of the pretreated biomass would be advantageous to process optimization and control. Biomass-water interaction measurements provide a method for quickly predicting biomass recalcitrance. Correlating water retention value (WRV) and enzymatic saccharification yield (ESY) on pretreated biomass has shown promise, especially when assessing only single biomass types pretreated with one specific chemistry. However, with comparisons between different types of biomasses, predictive powers have been low. We investigate the effect of pretreatment chemistry on the predictive power of WRV, when keeping the biomass static. Wheat straw was pretreated with dilute acid, hydrothermal, or alkaline chemistries at five different temperatures. Furthermore, low field nuclear magnetic resonance was used to measure water constraint in the pretreated materials, to better understand how biomass-water interactions change with pretreatment severity and chemistry. We show that the correlation of WRV and ESY is highly pretreatment dependent, while WRV strongly predicts ESY within each pretreatment chemistry. While ESY and WRV correlated under all chemistries, the direction of the correlations were divergent, suggesting a more complex interplay between recalcitrance and biomass-water interactions. Using T2 relaxation profiles, reductions in hemicellulose composition was related to the decrease in size of the most constrained water population present in the pretreated biomasses for all chemistries, suggesting a new identification of this population of constrained water.
... However, pyridine is an excellent swelling agent [9], so the observed disappearance of one cell wall water peak in the LFNMR results might be due to the porosity changes after the swelling of the cell walls. The longer T2 for cell wall water, void water, and cell lumen water in acetylated wood is consistent with previous research [9,44]. These changes are most probably related to a change in chemical environment rather than physical environment, since acetylation does not change the size of the macro voids in the wood structure. ...
To investigate the effects of changes in biopolymer composition on moisture in acetylated poplar wood (Populus euramericana Cv.), the acetylation of control wood was compared to the acetylation of wood with reduced hemicellulose or lignin content (about 9% reduction of total specimen dry weight in both cases). Time-domain nuclear magnetic resonance relaxometry of water-saturated wood gave spin–spin relaxation times (T2) of water populations, while deuteration in a sorption balance was used to characterize the hydroxyl accessibility of the wood cell walls. As expected, the acetylation of pyridine-swelled wood reduced hydroxyl accessibility and made the cell wall less accessible to water, resulting in a reduction of cell wall moisture content by about 24% compared with control wood. Hemicellulose loss per se increased the spin–spin relaxation time of cell wall water, while delignification had the opposite effect. The combined effect of hemicellulose removal and acetylation caused more than a 30% decrease of cell wall moisture content when compared with control wood. The acetylated and partially delignified wood cell walls contained higher cell wall moisture content than acetylated wood. An approximate theoretical calculation of hydroxyl accessibility for acetylated wood was in the low range, but it agreed rather well with the measured accessibility, while acetylated and partially hemicellulose-depleted and partially delignified wood for unknown reasons resulted in substantially lower hydroxyl accessibilities than the theoretical estimate.
... Solidstate NMR techniques reveal the details of the chemical composition of wood (Sivonen et al. 2002;Koenig et al. 2010). Time-domain NMR has also been exploited in the investigation of drying and water absorption of (modified) wood (Hoffmeyer et al. 2011;Thygesen and Elder 2009) as well as in the investigation of natural and artificial aging of wood (Rostom et al. 2019). Twodimensional (2D) T 1 -T 2 correlation experiments significantly improve the resolution and information content of the analysis; for example, they enable the resolution of two types of bound water in cell walls (Cox et al. 2010;Bonnet et al. 2017;Rostom et al. 2019). ...
Wood decay is an economically significant process, as it is one of the major causes of wood deterioration in buildings. In this study, the decay process of Scots pine ( Pinus sylvestris ) samples caused by cellar fungus ( Coniophora puteana ) was followed by nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) methods. Altogether, 30 wood sample pieces were exposed to fungus for 10 weeks. Based on the decrease of the dry mass, the samples were categorized into three classes: decomposed (mass decrease 50–70%), slightly decomposed (10–50%), and nondecomposed (<10%). MRI made it possible to identify the active regions of fungus inside the wood samples based on the signal of free water brought by the fungus and arisen from the decomposition of wood carbohydrates. MRI implies that free water is not only created by the decay process, but fungal hyphae also transports a significant amount of water into the sample. Two-dimensional ¹ H T1 - T2 relaxation correlation NMR measurements provided detailed information about the changes in the microstructure of wood due to fungal decomposition. Overall, this study paves the way for noninvasive NMR and MRI detection of fungal decay at early stages as well as the related structural changes.
... Typically, one or several fastrelaxing components (1-10 ms) are assigned to the water content in the cell wall. The medium-speed or slow-relaxing components (~100 ms) are assigned to the water in cell lumina with different sizes (Menon et al. 1987;Almeida et al. 2007;Thygesen and Elder 2008;Passarini et al. 2014Passarini et al. , 2015. Moreover, the water contents within different water reservoirs can be quantitatively analyzed via the T 2 spectrum on the basis of the proportionality of the peak integrals to the MC of each component (Telkki et al. 2013). ...
Low-field nuclear magnetic resonance (LF-NMR) was used to clarify the difference between the fiber saturation point (FSP) of Southern pine ( Pinus spp.) determined at the hygroscopicity limit (HL) vs. cell wall saturation (CWS). The HL samples were conditioned at 100% relative humidity (RH) while the CWS samples were fully saturated with distilled water. Cell wall water was found to increase by around 10% in the CWS state. A possible explanation may be that the cell wall was further swollen under liquid saturation. The increase in water content located in the cell wall was determined using freezing-point depression. About 11% of the moisture content (MC) was found in cell wall nanopores with sizes ranging from 1.73 nm to 13.80 nm. The discrepancy between the HL and CWS values of the samples was attributed to freezable water in the nanopores of the matrix components and inter-microfibrils. In the drying process, wood shrinkage was further evaluated in the MC region corresponding to the discrepancy, where the tangential and radial dimensions changed slightly.
... In previous studies, the signal value from the free induction decay (FID) curve, the first echo amplitude value of the decay curve obtained by the Carr-Purcell-Meiboom-Gill (CPMG) sequence and the integral peak areas of NMR relaxation curves according to the CPMG experiment were applied to describe the MC changes during the wood drying and moisture adsorption processes [12][13][14][15][16][17][18][19][20]. Since the decay rate of the NMR signal is relevant to the mobility and local environment of molecules, the moisture components (bound water and free water) can easily be separated by the different spin-spin (or transverse) relaxation time (T 2 ) values for either hardwood or softwood [15,[21][22][23][24][25][26][27][28]. Moreover, bound water that is hydrogen bonded to the wood cell wall components, or in the void spaces between the cellulose chains and in the cell wall micropores, and free water from earlywood or latewood could be distinguished [28]. ...
Knowledge of the dynamic changes in the water absorption process of heat-treated wood is important for providing a scientific basis for the reasonable application of heat-treated wood, especially for outdoor applications. Nuclear magnetic resonance (NMR) techniques provide detailed information about the moisture components and moisture transport processes in wood, which are not available with other methods. In this work, water absorption of untreated and heat treated Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) heartwood was investigated using various NMR methods. The heat treatment temperatures were varied between 160 °C and 220 °C. According to the spin-spin relaxation time (T2), there were two components of water in the samples heat-treated at 160 °C and 180 °C as well as the untreated sample, while three components of water were found in the samples heat-treated at 200 °C and 220 °C, and the mass of each component was calculated by the integral peak areas of the T2 curve. The amount of bound water and free water in heat-treated samples were less compared to the untreated ones, and the water absorption decreased correspondingly, due to the increasing heat-treated temperature. The results obtained by one dimensional frequency coding indicated that the heat treatment made wood difficult to be accessed by moisture. Besides, NMR images revealed that the free water absorption in latewood was faster than in earlywood, but earlywood could absorb more water than latewood.
... Nuclear magnetic resonance (NMR) enables instantaneous determination of the proton density in liquids and is thus convenient for determining the MC of wood. It is a non-destructive, non-invasive and noncontact technique already being successfully applied in wood science (Callaghan 1991;Contreras et al. 2002;Bucur 2003a, b;Morales et al. 2004;Labbé et al. 2006;Oven et al. 2008Oven et al. , 2011Thygesen and Elder 2008;Dvinskikh et al. 2011;Merela et al. 2009a, b;Cox et al. 2010;Hernandez and Caceres 2010;Kekkonen et al. 2014;Javed et al. 2015;Passarini et al. 2015;Ž lahtič et al. 2017;Mikac et al. 2018;Gezici-Koç et al. 2017). In addition, an NMR modality, magnetic resonance imaging (MRI), is a versatile tool widely used for investigating the spatial distribution of moisture in various specimens, including modified wood and other porous materials (Kanazawa et al. 2017;Thybring et al. 2018). ...
Water has a major influence on wood properties, especially dynamic moisture cycles, which affect the wood in outdoor applications. It is thus important to understand the penetration and distribution of water in wood. In this study, rainfall events were simulated to correspond to water immersion periods of 1 h. Specimens were imaged by magnetic resonance imaging (MRI) after 1 h of immersion. These measurements were used to determine the water distribution in the wood and to elucidate changes during the drying of specimens of five wood species: sweet chestnut heartwood (Castanea sativa), European larch heartwood (Larix decidua), Scots pine heartwood and sapwood (Pinus sylvestris) and Norway spruce (Picea abies). Both gravimetric and MRI analysis showed that after 1 h of immersion, pine sapwood took up the highest amount of water, followed by spruce wood. Considerably lower moisture contents were determined in pine heartwood, chestnut and larch, which correlated with a lower signal intensity. The outer parts of the specimens exhibited similar patterns with all of the specimens. The most variable results were the moisture content time profiles in the middle part of the specimens. Comparison of the MRI measurements and gravimetrically determined moisture contents during drying validated the MRI measurements and confirmed the method to be suitable for giving comprehensive information about the water drying kinetic.
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... Acetylation in un-decayed samples increases relaxation times for cell wall water (peak 1) and both void water populations (peaks 2-3) (Fig. 5). This agrees with previous results where increased relaxation times for water populations in acetylated wood were explained by the reduced affinity of water for the acetylated cell wall (Thygesen and Elder, 2008;. ...
Earlywood samples of unmodified and acetylated radiata pine were exposed to the brown-rot fungus Rhodonia placenta for 1, 2, 3 and 4 weeks for unmodified samples and 10, 16, 24 and 28 weeks for acetylated samples. Longer incubation periods were used for acetylated samples based on the hypothesis that given enough time under favourable conditions the fungus would eventually degrade the wood. After exposure, samples were weighed and chemically characterized by ATR-FTIR analysis, acetyl content by saponification, and hydroxyl (OH) accessibility by deuterium exchange. Longer incubation times for acetylated samples led to comparable levels of mass loss between unmodified and acetylated wood. Initial brown-rot decay in acetylated wood exhibited a different trend compared to unmodified wood, with an increased OH accessibility and a significant reduction in acetyl content. This was followed by a stable, low OH accessibility and plateau in acetyl content above 10% mass loss in acetylated wood. In unmodified wood, the OH accessibility was nearly constant throughout decay, while the initially low acetyl content decreased linearly with mass loss. ATR-FTIR analysis confirmed the differences in acetyl removal between unmodified and acetylated wood. Wood-water relations before and after brown-rot decay were determined with low-field nuclear magnetic resonance (LFNMR) relaxometry on water saturated samples. For the decayed acetylated wood, the behaviour of the water corresponded well with de-acetylation observed by chemical characterization. The results show that after removal of acetyl groups, degradation of acetylated wood by R. placenta occurred at a similar rate to that of unmodified wood.
... NMR spectroscopy has been used to investigate the states of water in lignocellulose, and isolated cell wall components. One-dimensional (1D) 1 H NMR resolves a slowly relaxing component that is characteristic of lumen water and a second component that is assigned to water in the cell walls [14][15][16] , and using two-dimensional (2D) T 2 -T 1 NMR it is possible to resolve two different states of bound water in spruce wood 17 . In addition, a solid state NMR study of water dynamics in lignocellulose revealed that pectin significantly affects water mobility in plant cell walls 18 . ...
Interactions of water with cellulose are of both fundamental and technological importance. Here, we characterize the properties of water associated with cellulose using deuterium labeling, neutron scattering and molecular dynamics simulation. Quasi-elastic neutron scattering provided quantitative details about the dynamical relaxation processes that occur and was supported by structural characterization using small-angle neutron scattering and X-ray diffraction. We can unambiguously detect two populations of water associated with cellulose. The first is “non-freezing bound” water that gradually becomes mobile with increasing temperature and can be related to surface water. The second population is consistent with confined water that abruptly becomes mobile at ~260 K, and can be attributed to water that accumulates in the narrow spaces between the microfibrils. Quantitative analysis of the QENS data showed that, at 250 K, the water diffusion coefficient was 0.85 ± 0.04 × 10⁻¹⁰ m²sec⁻¹ and increased to 1.77 ± 0.09 × 10⁻¹⁰ m²sec⁻¹ at 265 K. MD simulations are in excellent agreement with the experiments and support the interpretation that water associated with cellulose exists in two dynamical populations. Our results provide clarity to previous work investigating the states of bound water and provide a new approach for probing water interactions with lignocellulose materials.
... It could then be hypothesised that these rearrangements might be more intense at the end of the desorption step. From T 2 times obtained by NMR analyses of water in wood, bound water is more tightly attached to wood at lower MC Thygesen and Elder 2008;Cox et al. 2010;Elder and Houtman 2013;Zhang et al. 2013;Passarini et al. 2014), which could hinder water loss and reduce desorption rate. ...
Understanding the fundamental process involved in wood sorption and
wood dimensional changes is essential to improve drying and to obtain high-quality
end products at a reasonable price. The main objective of this study was to examine
the dimensional behaviour and the wood–water relationships of never-dried Eucalyptus
saligna wood under a wide range of moisture contents below and above the
fibre saturation point (FSP). Four desorption conditions from full saturated state
were carried out at 21 �C in several steps until 58 % relative humidity (RH) was
reached. Slower desorption rates produced higher radial, tangential, and volumetric
shrinkages. The estimated collapse was also higher for milder desorption rates,
being greater in the tangential direction. However, desorption rates did not change
the slope of shrinkage–equilibrium moisture content (EMC) curves in the 0–58 %
RH range. Further, the FSP established by extending the straight linear portion of
these curves to 0 % shrinkage revealed to be not reliable for a collapse-prone
species like E. saligna. Finally, shrinkage–EMC curves suggest the presence of
entrapped liquid water at RH higher than about 76 %, confirming the occurrence of
a region in the hygroscopic range in which the loss of bound water takes place
before all liquid water has been drained.
... The components with longer relaxation times, from dozens of ms to hundreds of ms, are associated with free water, according to the size of lumens (Hartley et al. 1994;Labbé et al. 2002;Almeida et al. 2007;Telkki et al. 2013). By comparing the change in relaxation time between modified and unprocessed specimens, the modified effects can also be evaluated (Thygesen and Elder 2008). ...
Nuclear magnetic resonance (NMR) relaxation time distributions can provide detailed information about the moisture in wood. In this paper, the bound water content and pore size distributions in swollen cell wall of two kinds of softwoods (Pinus sylvestris and Cunninghamia lanceolata) and three kinds of hardwoods (Populus sp., Fraxinus excelsior L., and Ochroma lagopus) were determined by NMR cryoporometry. The total bound water content of swollen cell wall almost exceeds 35%, based on dry mass, which is obviously higher than the fiber saturation point (FSP) (appr. 30%) measured by the extrapolation method. The bound water content of different species is consistent with the hypothesis that with the decrease of basic density, the more bound water could be contained in wood. The proportion of the pore diameter smaller than 1.59 nm is higher than 70%, and the proportion of the pore diameter larger than 4 nm is no more than 10%.
... In this manner, the solid wood signal disappears in tens of microseconds, allowing an easy way to separate it from the cell-water signal, whose T 2 values range from one to a few milliseconds. The third component, the liquid water located in cell lumina, is characterized by T 2 values in the order of tens to hundreds of milliseconds (Hsi et al. 1977;Menon et al. 1987;Araujo et al. 1992Araujo et al. , 1993Thygesen and Elder 2008). used the NMR technique to separate the components of water in wood of two temperate and one tropical species at different EMCs. ...
The distribution of liquid and bound water in wood samples under equilibrium moisture contents (EMC) below fiber saturation point (FSP) was assessed by magnetic resonance (MR) microimaging. Two Amazonian hardwoods, huayruro (Robinia coccinea) and cachimbo [Cariniana domesticata], a plantation grown eucalyptus (Eucalyptus
saligna), and a temperate species red oak (Quercus rubra) were studied. Desorption tests were performed at 21 °C from full saturation state for huayruro, cachimbo, and red oak, and from green condition for eucalyptus. The EMC was reached under three desorption conditions [58, 76, and 90 % relative humidity (RH)]. MR microimages were obtained based on T
2 times and on 1H concentration. Scanning electron microscopy images helped us to interpret MR microimages. The results showed that wood structure plays a major role in liquid water drainage and in water diffusion. Eucalyptus saligna and red oak showed liquid water entrapped in parenchyma tissues, even below FSP (90 % RH). At this same RH level, all liquid water was, however, drained for cachimbo and huayruro. For these woods, bound water was not uniformly distributed in wood structure, concentrating it more in fibers for both species. Huayruro showed the highest heterogeneity in hygroscopicity, which is explained by its particular wood anatomy.
... Nuclear magnetic resonance (NMR) spectroscopy is not only a versatile analytical tool in chemistry ( Keeler 2010 ), but it also provides detailed information about moisture in wood as the amplitude of the 1 H NMR signal is directly proportional to the moisture content (MC) ( Sharp et al. 1978 ;Menon et al. 1987 ;Hartley et al. 1994 ;Merela et al. 2009 ). Although the NMR spectrum in this context typically shows only a single and broad peak, bound water and free water can be distinguished by means of T 2 or T 1 relaxation time distributions ( Riggin et al. 1979 ;Araujo et al. 1992 ;Hartley et al. 1994 ;Xu et al. 1996 ;Labb é et al. 2002 ;Fantazzini et al. 2006 ;Almeida et al. 2007 ;Thygesen and Elder 2008 ;Cox et al. 2010 ;Hoffmeyer et al. 2011 ;Elder and Houtman 2013 ;Telkki et al. 2013 ;Zhang et al. 2013 ;Kekkonen et al. 2014 ). NMR self-diffusion measurements enable the observation of moisture transport in wood ( Wycoff et al. 2000 ;Hietala et al. 2002 ;Topgaard and S ö derman 2002 ;Meder et al. 2003 ;Johannessen et al. 2006 ;Kekkonen et al. 2009 ;Telkki and Jokisaari 2009 ;Kekkonen et al. 2010 ). ...
Thermal modification is an environmentally friendly process that enhances the lifetime and properties of timber. In this work, the absorption of water in pine wood (Pinus sylvestris) samples, which were modified by the ThermoWood process, was studied by magnetic resonance imaging (MRI) and gravimetric analysis. The modification temperatures were varied between 180 ° C and 240 ° C. The data shows that the modification at 240 ° C and at 230 ° C decreases the water absorption rate significantly and slightly, respectively, while lower temperatures do not have a noticeable effect. MR images reveal that free water absorption in latewood (LW) is faster than in earlywood (EW), but in the saturated sample, the amount of water is greater in EW. Individual resin channels can be resolved in the high-resolution images, especially in LW regions of the modified samples, and their density was estimated to be (2.7 ± 0.6) mm-2. The T 2 relaxation time of water is longer in the modified wood than in the reference samples due to the removal of resin and extractives in the course of the modification process.
... Beyond its application as a building material, wood is also used in paper production and as an energy source. The water mobility and cellular structure of wood have been probed by laboratory NMR using relaxation time [426][427][428] and diffusion measurements [429,430]. Transport of water between cells has been monitored using 2D T 2 -T 2 exchange measurements [431]. ...
In this review we focus on the technology associated with low-field NMR. We present the current state-of-the-art in low-field NMR hardware and experiments, considering general magnet designs, rf performance, data processing and interpretation. We provide guidance on obtaining the optimum results from these instruments, along with an introduction for those new to low-field NMR. The applications of lowfield NMR are now many and diverse. Furthermore, niche applications have spawned unique magnet designs to accommodate the extremes of operating environment or sample geometry. Trying to capture all the applications, methods, and hardware encompassed by low-field NMR would be a daunting task and likely of little interest to researchers or industrialists working in specific subject areas. Instead we discuss only a few applications to highlight uses of the hardware and experiments in an industrial environment. For details on more particular methods and applications, we provide citations to specialized review articles.
... The percolation threshold is consistent with previous nuclear magnetic resonance (NMR) and differential scanning calorimetry (DSC) measurements of water in wood and cellulose that exhibit multiple types of water. Using NMR, researchers have observed water with three different relaxation times which they have classified as free water, bound water, and water with a relaxation time between bound water and free water [9] [10] [11] [12]. Examining hardwoods, Almeida et al. observed this intermediate water appeared between 12–16% MC for sugar maple (Acer saccharum) and beech (Fagus grandifolia) [10]. ...
This paper examines phase transformations of water in wood and isolated wood cell wall components using differential scanning calorimetry with the purpose of better understanding “Type II water” or “freezable bound water” that has been reported for cellulose and other hydrophilic polymers. Solid loblolly pine (Pinus taeda) milled pine, and holocellulose, cellulose, and lignin isolated from the same parent board were tested. For each sample preparation, the freezing and melting of water was examined at 10–20 different moisture contents. Only one freezing peak was observed for most sample preparations; in these cases, the phase transformation corresponded to the “Type I” or “free water” peak. The freezing and melting temperatures of this Type I peak depended on moisture content, and appreciable undercooling (−30 °C from bulk water) was observed at the lowest moisture contents. The isolated ball-milled cellulose exhibited a Type II peak, yet the Wiley-milled cellulose, from which the ball-milled cellulose was made, did not exhibit a Type II peak. When present, the Type II peak is consistent with the homogeneous nucleation temperature of bulk water. The results suggest that the detection of Type II or freezable bound water in wood may depend more on sample preparation than the chemical nature of the cell wall component in question; a point not discussed by previous researchers describing Type II water in hydrophilic polymers.
Wood is a porous, hygroscopic material with engineering properties that depend significantly on the amount of water (moisture) in the material. Water in wood can be present in both cell walls and the porous void-structure of the material, but it is only water in cell walls that affects the engineering properties. An important characteristic of wood is therefore the capacity for water of its solid cell walls, i.e. the maximum cell wall moisture content. However , this quantity is not straightforward to determine experimentally, and the measured value may depend on the experimental technique used. In this study, we used a triangula-tion approach to determine the maximum cell wall moisture content by using three experimental techniques based on different measurement principles: low-field nuclear magnetic resonance (LFNMR) relaxometry, differential scanning calorimetry (DSC), and the solute exclusion technique (SET). The LFNMR data were furthermore analysed by two varieties of exponential decay analysis. These techniques were used to determine the maximum cell wall moisture contents of nine different wood species, covering a wide range of densities. The results from statistical analysis showed that LFNMR yielded lower cell wall moisture contents than DSC and SET, which were fairly similar. Both of the latter methods include factors that could either underestimate or overestimate the measured cell wall moisture content. Because of this and the fact that the DSC and SET methods are based on different measurement principles, it is likely that they provide realistic values of the cell wall moisture content in the water-saturated state.
Radiata pine wood (W) was modified with acetic anhydride and glutaraldehyde (GA) resulting in W Ac and W GA to various weight percent gains (WPGs), whereas in W Ac the effect is due to grafting and in W GA , crosslinking. The heat of vaporization of bound water (BW) of the modified woods was studied by differential scanning calorimetry (DSC) and the mass loss (due to water loss) of the samples by thermogravimetry (TG). The temperature program was in both cases from 25 to 40°C with 10°C min ⁻¹ . The adsorbed or condensed water in wood were observed via low-field nuclear magnetic resonance (LFNMR). At a comparable WPG level, the LFNMR analysis showed that the interaction of water with W GA was stronger than that with W Ac . In both modified woods, a considerable reduction in the vaporization heat of BW was visible due to cell wall hydrophobization and bulking. The reduction of condensed water in micropores was lower for W GA than W Ac , probably because BW needs more energy to evaporate from the crosslinked stiff W GA cell walls.
Introducing catalytically induced polyolefin polymerization methods into the field of
wood science may open up a vast number of novel wood modification possibilities, i.e.
tuning the hydrophobic properties and with that creating the option of new
functionalization to enhance the property profile further. The scope of our ongoing
research is focused on polymerizing ethylene within the wood structure by in situ
polymerization techniques. This is achieved by a highly specialized catalytic system,
consisting of a metallocene catalyst and an aluminum alkyl co-catalyst. This system
exhibits promising features in the fields of polyolefin nanocomposite production, and it
has attracted interest in the macromolecular science community as well as in the
industry. The approach followed in this study comprises three steps. In the first step,
small solid wood samples of pine sapwood are pre-treated with the co-catalyst trimethyl
aluminum (TMA), which is adsorbed by the wood surface and adsorbed within the
pores. In the second step the metallocene catalyst is introduced, which is binding to the
immobilized co-catalyst. Hence, catalytically active sites are foremost formed on the
wood surface and within the pores. In the third step, ethylene is introduced under low
pressure. Upon initiation of the ethylene polymerization, polyethylene (PE) is formed
on the wood surface and inner pores. Field emission scanning electron microscopy
(FESEM) and energy-dispersive X-ray spectroscopy (EDX) were utilized to analyze
cross sections of the treated wood samples. The FESEM images did show filled,
partially filled as well as empty cell lumen. The EDX analysis of the same cross
sections displayed high shares of oxygen distributed along the cell walls, whereas
negligible shares were distributed in the empty and filled cell lumen. The aluminum
distribution, which is attributed to the co-catalyst, cumulates within the voids. This
finding makes us assume that the cell lumen contain or even are filled with PE.
Acetylated wood (WAc) shows improved properties largely due to the reduced amount of water in its cell wall, but the exact mechanism of water reduction remains unclear. Acetylation reduces hydroxyl (OH) content by acetyl (Ac) substitution but may also limit water access to unmodified OH groups by steric hindrance. In the present work, the accessibility of OH groups in acetylated or propionylated Radiata pine (Pinus radiata D. Don) wood (WAc and WPr) was investigated by deuterium exchange, saponification in sodium hydroxide followed by high-performance liquid chromatography (HPLC) analysis and weight percentage gain determination of the modified samples. Acetylation reduced OH accessibility (OHA) to a greater extent than would be predicted, if OH substitution were the only responsible mechanism for accessibility reduction. The combination of deuterium exchange and saponification results provides strong evidence that steric hindrance plays a key role in reduction of water accessibility to unmodified OH groups in WAc. The supramolecular architecture of WPr samples seems to be modified by the propionylation reaction, which leads to increased OHA at low levels of substitution. This suggests that molecular restructuring within the cell wall exposes new OH groups after propionylation. At higher levels of substitution, however, the WPr exhibited less OHA than expected indicating steric hindrance from the propionyl groups.
Magnetic resonance (MR) microimaging was used to determine the distribution of liquid water in sugar maple wood (Acer saccharum Marsh.). Two moisture desorption tests were applied using saturated salt solutions at 21°C. Desorptions were accomplished between 58 and 96% RH starting from the full saturation state and from the FSP. Each moisture sorption condition at equilibrium was associated with a MR microimaging scan. Signal intensity (represented by false colors in the MR images) allowed visualization of the concentration of liquid water distributed into the wood structure. In most cases, the presence of liquid water was noticed in samples coming from the full saturation state at moisture contents below FSP. This result shows the coexistence of liquid and bound water even at moisture contents below the FSP. The remaining liquid water in the wood appears to be located principally in the lumina of the least accessible libriform fibers.
The fiber saturation point (FSP) is an important concept in wood–moisture relations that differentiates between the states of water in wood and has been discussed in the literature for over 100 years. Despite its importance and extensive study, the exact theoretical definition of the FSP and the operational definition (the correct way to measure the FSP) are still debated because different methods give a wide range of values. In this paper, a theoretical definition of the FSP is presented based on solution thermodynamics that treats the FSP as a phase boundary. This thermodynamic interpretation allows FSP to be calculated from the chemical potentials of bound and free water as a function of moisture content, assuming that they are both known. Treating FSP as a phase boundary naturally lends itself to the construction of a phase diagram of water in wood. A preliminary phase diagram is constructed with previously published data, and the phase diagram is extended to a state diagram by adding data on the glass transition temperatures of the wood components. The thermodynamic interpretation and resulting state diagram represent a potential framework for understanding how wood modification may affect wood–moisture relations.
The major purpose of this study is to improve hygrothermal simulation of wood responses to environmental vapor and moisture conditions under high relative humidity conditions. The article first reviews moisture property-related wood microstructures, sorption behavior, the concept of fiber saturation point, the potential for vapor to condense in wood under high relative humidity conditions, the measurement of equilibrium moisture content using traditional sorption methods, and the use of pressure plate test method at relative humidities above 95%. It then summarizes the results of equilibrium moisture content measurements for red pine sapwood at high relative humidity conditions using both sorption and pressure plate methods, with capillary saturation as maximum moisture content. It also discusses a number of wood microstructure and end-use-related factors that could influence the moisture content in service and the measurement of equilibrium moisture content in laboratory. Inconsistencies were found with other equilibrium moisture content data using the pressure plate test method.
A comparison of moisture loss from Pinus radiata sapwood by conventional forced air-drying and a novel supercritical carbon dioxide (scCO2) dewatering process has been examined in situ using magnetic resonance microimaging. Air-drying results in the nonuniform removal of moisture within the wood volume, leading to a dry core and wet perimeter where water evaporated, whereas the scCO2 dewatering process resulted in moisture expulsion more uniformly throughout the volume of the specimen, especially so within the earlywood.
Thermal modification is an environmentally friendly method to increase the lifetime and improve the properties of timber. In this work, we investigate absorption of moisture in thermally modified pine wood (Pinus sylvestris) immersed in water using various nuclear magnetic resonance (NMR) methods. Magnetic resonance images (MRI) visualize the spatial distribution of absorbed free water. Spin–echo spectra measured both below and above 0 °C reveal that thermal modification partially blocks the access of water to cell walls; even modification at 180 °C slightly reduces the amount of bound water, and the amount decreases about 80% in the case of the sample modified at 240 °C. The spectra and MRI show that, above the modification temperature of 200 °C, the amount of free water decreases, indicating that high modification temperature tends to close the pits connecting the wood cells. T2 relaxation time distributions measured using the Carr–Purcell—Meiboom–Gill sequence show four components, two associated with bound water and two with free water. NMR cryoporometry measurements indicate that the bound water sites are mostly below 2.5 nm in size. A unique combined NMR cryoporometry and relaxometry analysis showed that the size of cell wall micropores is between 1.5 and 4.5 nm, and thermal modification significantly hinders the access of water to the pores.
Distributions of nuclear magnetic resonance (NMR) relaxation times provide detailed information about the moisture absorbed in wood. In this work, T2*, T2, and T1 distributions were recorded from fresh sapwood and heartwood samples of pine (Pinus sylvestris) and spruce (Picea abies) at various temperatures. Below the melting point of bulk water, free water is frozen and its signal disappears from the distributions. Then, the low-temperature distributions of the unfrozen bound water contain more information about its components, because the large free water peaks hiding some smaller bound water peaks are absent and the exchange between free and bound water is prevented. Comparison of the total moisture signal integrals above and below the bulk melting point enables the determination of fiber saturation point (FSP), which, in this context, denotes the total water capacity of cell wall. T2*, T2, and T1 distributions offer different kinds of information about moisture components. All the peaks in the distributions were assigned, and it was demonstrated that the accessible hydroxyl site content and the amount of micropores can be estimated based on the peak integrals.
This paper reviews recent findings on wood–water interaction and puts them into context of established knowledge in the field. Several new findings challenge prevalent theories and are critically discussed in an attempt to advance current knowledge and highlight gaps. The focus of this review is put on water in the broadest concept of wood products, that is, the living tree is not considered. Moreover, the review covers the basic wood–water relation, states and transitions. Secondary effects such as the ability of water to alter physical properties of wood are only discussed in cases where there is an influence on state and/or transition.
Two-dimensional T(1)-T(2) (1)H NMR relaxation correlation spectra and T(2)-T(2) relaxation exchange spectra for samples of spruce wood across the full moisture content range from 0% to > 100% are presented. The T(1)-T(2) spectra unambiguously identify water in different environments within the wood that overlap in the corresponding and more traditional one-dimensional T(1) or T(2) experiment. The T(2)-T(2) spectra unambiguously reveal the presence of water exchange between the lumen and cell wall above the fibre saturation point in sapwood. An estimate is made of the exchange rate: 1/30 to 1/3 ms(-1).
The low field H-1 NMR relaxometry technique has been shown to be effective in determining the qualitative and quantitative moisture content and the water state distribution in maritime pine wood (Pinus pinaster Ait). Further investigations on extractive materials in resin-rich samples evinced the presence of oleoresin components, which may disturb adhesion on pine wood boards.
Nuclear magnetic resonance lineshape second moments of the solid wood protons and spin-spin relaxation times, T2, of the bound water in lodgepole pine heartwood have been measured at 30-degrees-C for a range of moisture contents, mainly in the hygroscopic range. The second moment of the solid wood protons for oven dry wood was found to be about 23% lower than the rigid lattice calculation, indicative of a rigid structure with some anisotropic molecular motion of the polymeric constituents. Above 5% M, the second moment decreased by a further 13 to 16% implying a ''loosening'' of the molecules in the solid with increased moisture content. The T2 of the bound water increased with moisture content, and provided no evidence of separate hydration and solution water fractions as predicted by isotherm theories. The NMR measured fibre saturation point of 27% agreed with the value calculated by the Hailwood-Horrobin isotherm model.
Southern pine and aspen flakes were acetylated with acetic anhydride alone without cosolvent or catalyst by a simple dip procedure. The new procedure greatly shortens reaction time and simplifies chemical recovery. Acetylation weight gains of 15% to 20% can be achieved in 1 to 3 hours with southern pine flakes and in 2 to 4 hours with aspen flakes.Flakeboards made from acetylated southern pine or aspen flakes absorbed much less water, both in water-soaking tests and when subjected to humid air, and swelled at a lower rate and to a lower extent than did control boards.Hygroscopicity of the resulting flakeboards decreased with increased level of wood acetylation. The equilibrium moisture content (EMC) for flakeboards made from acetylated flakes was lower at each relative humidity tested than that of control boards.
A comprehensive investigation into the effect of molecular size of the substituent group of softwood modified with linear chain carboxylic acid anhydrides, namely acetic, propionic, butyric, valeric, hexanoic, upon the sorption of water vapour has been performed. The sorption isotherms for untreated and chemically modified wood were analysed using the Hailwood–Horrobin model. The experimental analysis of the sorption isotherms showed that esterification affects the total, polymolecular and monomolecular sorption. The effect of molecular size of the substituent group on site accessibility was addressed by comparing the effect on water vapour sorption produced by adducts with differences in molecular size. Similar levels of cell-wall bulking were produced at different levels of hydroxyl substitution. Analysis of the sorption isotherms at comparable weight percentage gain revealed that the five anhydrides used show similar effectiveness in both total, polymolecular and monomolecular sorption, despite the substantial difference in the proportion of hydroxyl groups reacted. It is concluded that the reduction in total, polymolecular and monomolecular sorption produced by the linear chain anhydrides is primarily determined by the volume of adduct deposited in the cell wall (bulking) rather than by the number of hydroxyl groups that have been substituted. The validity of the Hailwood–Horrobin model is questioned.
The moisture content-water potential relationship was determined at 40°C and 60°C for sugar maple (Acer saccharum Marsh.) sapwood and at 60°C for white spruce (Picea glauca (Moench.) Voss.) heartwood from green to dry conditions. The pressure membrane technique was used for high moisture contents and equilibration over salt solutions for low moisture contents. The results show that at high moisture contents, the equilibrium moisture contents obtained from the green condition are lower than those obtained from full saturation (boundary desorption). It is recommended that the sorption history must be taken into account when modeling wood drying. Water potential at a given moisture content increases with temperature. There is a characteristic plateau in the green moisture content-water potential relationship obtained for sugar maple at water potentials between -2,000 and -6,000 J kg-1, which can be attributed to its heterogeneous capillary structure. The maximum concentration of effective pore radius occurs at 0.02 μm in the case of sugar maple, corresponding to the size of the pit membrane openings.
The time domain of H-1 NMR spectroscopy allows straightforward editing of the T-2 relaxation profiles in maritime pine wood. A new method from the Carr-Purcell-Meiboom-Gill sequence is proposed to measure the amount and distribution of water in wood, as well as the location of major dissolved organic materials. A general calibration model giving reliable and precise identification of these parameters is described. The method presented for editing T2 relaxation profiles (obtained by the Contin program) may be helpful in solving practical drying and gluing problems in the wood industry. It can be used for monitoring chemical modifications of wood fibers involved in the design of wood composite materials.
A diffusion model for spin-spin relaxation of compartmentalized water with a surface relaxation was verified for lumen water in wood. Spin-spin relaxation measurements were carried out on water in redwood sapwood, spruce sapwood, and spruce compression-wood samples, which possessed different cell-lumen radius distributions as measured by scanning electron microscopy. For the redwood sample, NMR measurements were made for seven temperatures between 4 and 55°C over which the average lumen-water T2 decreased from 177 to 103 ms. The lumen-water theory and experiment were in agreement, and evidence of higher-order relaxation modes, theoretically predicted for low temperatures, was found. This model was extended to two water regions to characterize the surface relaxation in terms of the spin-spin relaxation and diffusion coefficient of the cell-wall water and the partition coefficient. Using the extension and measurements of the spin-spin relaxation times and relative populations of lumen and cell-wall-water, estimates for cell-wall-water diffusion in a maximally hydrated redwood varied from 0.92 × 10−6 cm2/s at 4°C to 5.89 × 10−6 cm2/s at 55°C. The activation energy for cell-wall-water diffusion in redwood sap-wood in this temperature range was 6700 cal/mol, about 40% higher than the free-water value of 4767 cal/mol.
Least-squares and linear programming algorithms for the interpretation of NMR relaxation data in terms of a spectrum of relaxation times are presented. These stable methods are noniterative and avoid all of the difficulties associated with nonlinear optimization schemes. Moreover, they are not restricted by any assumptions which limit the number of possible relaxation times. The inherent nonuniqueness due to the finite number of inaccurate measurements means that a variety of discrete and continuous spectra exist which fit the data equally well. For a complete interpretation the range of possible solutions must be examined. The algorithms discussed explore the space of acceptable solutions by constructing relaxation spectra from a continuum of models ranging from a few isolated delta functions to a continuous distribution. Alternatively, the interpreter can choose that class of spectrum (discrete or continuous) which best represents the physical system under investigation and so enhance the reliability of the interpretation. This flexibility is essential in order to avoid the pitfalls of interpreting a single spectrum returned by a single algorithm. In addition, the linear programming approach reduces the non-uniqueness by constructing unique information in the form of bounds on all spectra which satisfy the observations. These bounds and the corresponding extreme models are necessary for any quantitative determination of how spectral peaks change in amplitude or position with a change in the physical system.0 1989 Academic Press, Inc.
The sorption of moisture by ligno-cellulosic materials has a significant influence on their physical properties since it affects both swelling and softening. In order to estimate the adsorption behaviour, an attempt is made to calculate the number and type of water molecules (whether non-freezing bound water or freezing bound water) adsorbed by the polar groups (OH, COO−, SO3−) with counterions (H+, Na+, Ca2+) common in paper products. The analysis is based on sorption experiments using differential scanning calorimetry and thermogravimetric analysis. The main contributors to the adsorption on wood are the hydroxyl groups of the carbohydrates and the lignin. Any adsorption of freezing bound water in the studied moisture range is attributed to ionic groups present in the sample. Differences in adsorption between weak carboxylic acid and strong sulphonic acid systems are explained as being a function of their different degrees of dissociation.The values calculated for the polar groups using model substances are used in an attempt to predict the adsorption for two xylans, a lignin preparation and a sulphonated wood meal of which the composition and the degree of crystallinity are known, assuming a simple law of additivity. The calculated results agree well with the experimental results.
Wood modification with furfuryl alcohol is a non-toxic alternative to conventional preservation treatments. A process in which furfuryl alcohol polymerises in situ was previously proposed for chemical modification of wood. In the present work, liquid model systems were investigated using compounds that resemble repeating units of lignin to verify whether chemical bonds form between the furfuryl alcohol polymer and wood. Using different NMR spectroscopic techniques we confirmed that these model compounds do form covalent bonds with the polymerising polymer. The results indicate that the furan polymer grafts to lignin, supporting observations in similar studies performed with genuine wood materials.
Proton magnetic resonance has been used to study water in Douglas fir and western red cedar. The free induction decay, when combined with a knowledge of chemical composition of the wood, gives an accurate measure of the absolute moisture content. Spin-lattice relaxation was found to be significantly different for the two species. In sapwood, three distinct spin—spin relaxation times, T2, were measured and assigned, with the help of anatomical data, to water in and on the cell wall, water in the ray and latewood tracheid lumens, and water in the earlywood tracheid lumens. This T2 behavior was explained by a model in which free water in a void exchanges with a small fraction of bound water on the lumen surface. The three T2's were almost independent of moisture content, suggesting physically separate compartments. The behavior of the three water components during drying was studied. The fiber saturation point could be determined from a single T2 measurement on a green sapwood sample. Magnetic resonance imaging of logs was investigated.
A review of water interaction in cellulosic-systems, particularly wood, is presented. Discussed are the different states of water in these systems according to Nuclear Magnetic Resonance results, the BET, Dent, and Hailwood and Horrobin sorption isotherm models. The discussion includes details of water structure, and, conformational analysis of cellulose crystals and amorphous cellulose. The water cluster theory is used to more adequately explain the sigmoid curve of the wood isotherm.
Two new proton magnetic resonance techniques, relaxation spectra and relaxation selective imaging, have been used to investigate the distribution of water in samples of normal white spruce sapwood, heartwood, and juvenile wood as well as two rehydrated heartwood samples containing incipient decay and compression wood respectively. It is demonstrated that the spin-spin (T2) relaxation behavior in wood is best presented as a continuous spectrum of relaxation times. Spectra of T2 for white spruce show separate peaks corresponding to the different water environments. Bound water gives a peak with an T2 time of about 1 ms and lumen water gives a distribution of T2 times in the range of 10 to 100 ms. The lumen water T2 time is a function of the wood cell radius. Consequently, the different cell lumen radii distributions for spruce sapwood, juvenile wood, and compression wood are readily distinguishable by the shape of their T2 spectra. Water environments which are separable on a T2 spectrum may be imaged separately. Imaging has been carried out in one dimension for bound water and lumen water of a spruce sapwood sample at four different moisture contents ranging from 100% to 17%. For the first time, we demonstrate that above the fibre saturation point the moisture density profile of the bound water is largely independent of moisture content. The feasibility and utility of using these techniques for internal scanning of logs and lumber is discussed. These techniques should provide new insights into the wood drying process.
Corsican pine (Pinus nigra) sapwood was chemically modified with acetic, or hexanoic anhydride to a variety of weight percentage gains. The cell wall microporosity of the wood before and after chemical modification was determined using the technique of solute exclusion. The results showed that the cell wall microporosity decreased as the level of substitution increased, but the cell wall remained accessible at high levels of substitution. Values of the fibre saturation point (FSP) calculated from solute exclusion data ranged from c. 40% (for unmodified wood) to c. 20% at approx. 25% weight percentage gain, but were dependent to some degree upon the calculation method. Evidence is presented suggesting that the reduction in FSP may be attributable to bulking of the cell wall by bonded acyl adduct. It is concluded that the level of hydroxyl substitution in the cell wall is not the primary mechanism for giving decay protection in anhydride-modified wood.
Chars from the thermal degradation of silver maple (Acer saccharinum), red maple (Acer rubrum), sugar maple (Acer saccharum), and white oak (Quercus spp.), performed at temperatures from 250 to 350 °C, were examined using time domain-nuclear magnetic resonance spectroscopy. Prior to analysis, the chars were equilibrated under conditions insuring the presence of bound water only and both bound water and free water. Transverse relaxation times were found to be related to the moisture content of the chars, which varied with temperature. At elevated temperatures the number of signals assigned to free water decreased, indicative of an increase in pore size within the chars.
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