Scientia Silvae Sinicae  2017, Vol. 53 Issue (10): 146-153   PDF    
DOI: 10.11707/j.1001-7488.20171016
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文章信息

Guo Xin, Wu Yiqiang, Li Xianjun
郭鑫, 吴义强, 李贤军
Distribution and Existing States of Water in Wood:A Review
木材中水分分布及其存在状态研究进展
Scientia Silvae Sinicae, 2017, 53(10): 146-153.
林业科学, 2017, 53(10): 146-153.
DOI: 10.11707/j.1001-7488.20171016

文章历史

Received date: 2017-04-10
Revised date: 2017-05-19

作者相关文章

Xin Guo
Yiqiang Wu
Xianjun Li

Distribution and Existing States of Water in Wood:A Review
Guo Xin1,2, Wu Yiqiang1 , Li Xianjun1    
1. College of Material Science and Engineering, Central South University of Forestry and Technology Changsha 410004;
2. College of Sciences, Central South University of Forestry and Technology Changsha 410004
Abstract: Wood-water relationship has been studied since the beginning of wood research. With the development of new technology, vast modern analytical method are emerging, and the study of wood-water relationship is changing from macro to micro scale and from cell to molecular level. The main objective of this paper was to review the modern analytical techniques which have been applied to study the wood-water relationship. This paper was divided into four parts. Firstly, the extremely heterogeneity of the wood cell wall structure and chemical composition and the highly complex states of water were discussed, and then the distribution and existing states of water in wood was summarized as an active field of the research about wood-water relationship. Secondly, the application of four kinds of analytical techniques, such as magnetic resonance imaging (MRI), computed tomography (CT), neutron imaging (NI) and vibrational spectroscopic imaging, in the study of water distribution were summarized. Meanwhile, the advantages and limitations of these four techniques were provided. Some of the latest research progress was:Vibrational spectroscopic imaging techniques such as micro-FTIR and confocal Raman could offer visual examination and spectral information of chemical functional groups in situ, and owned high spatial resolution on the micrometer length scale. The spatial resolution of micro-FTIR imaging technique was 6.25 μm by the instrumental parameters, and the spectral changes indicated that the adsorbed water concentration was nonuniform at the cell structure level. Meanwhile, the spatial resolution of confocal Raman imaging technique was higher than 1 μm, and the spectral changes indicated that the amount of water in the cell corner (CC) was less than that in the middle layer of secondary wall (S2) throughout the entire range of relative humidity (RH) levels. Thirdly, the recent advances in the application of four kinds of analytical techniques, such as near infrared (NIR) spectroscopy, nuclear magnetic resonance (NMR), fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, were discussed in the study of existing states of water in wood. The advantages and limitations of these four techniques were also provided. Some of the latest research progress was:The micro-FTIR spectroscopy and a specially designed sample cell were used to examine the molecular association of adsorbed water with wood during adsorption process. It was confirmed that carboxyl C=O, C-O groups as well as OH groups were active sites for water adsorption. Meanwhile, strongly, moderately and weakly hydrogen-bonded water molecules were identified and assigned. What's more, according to the variation trend of these hydrogen-bonded water molecules, three sections were divided for adsorption process. Furthermore, the existing states of water in each section was demonstrated as C=O…(HOH)…OH or OH…(OH2)…OH、WATER…HOH…WATER, and tetrahedral structure. Finally, some future research subjects in the research about wood-water relationship were proposed, such as applying new approaches with greater accuracy and higher resolution, developing the convincing component band analysis.
Key words: wood     water distribution     existing states    
木材中水分分布及其存在状态研究进展
郭鑫1,2, 吴义强1 , 李贤军1    
1. 中南林业科学大学材料科学与工程学院 长沙 410004;
2. 中南林业科学大学理学院 长沙 410004
摘要:木材-水分关系是木材科学领域的传统经典研究内容。随着现代分析技术不断发展,木材-水分关系研究已从宏观进入到微观,从细胞水平上升到分子水平,趋向于系统化和综合性。本文首先论述木材细胞壁壁层结构与化学组分的非均匀性以及木材中水分存在状态的多样性,得出木材-水分关系研究的热点和难点是木材中水分分布以及存在状态。其次,归纳磁共振成像、计算机断层扫描成像、中子成像和振动光谱成像4种现代分析技术应用于木材中水分分布研究的最新进展,并分析这4种技术的优缺点。其中,部分最新研究进展是:属于振动光谱成像的显微红外成像技术具有6.25 μm的空间分辨率,可用于木材细胞壁水分分布研究,结果证实纤维饱和点以下木材细胞壁中水分分布具有不均匀性;而同属于振动光谱成像的显微拉曼成像技术具有1 μm的空间分辨率,适用于木材细胞壁不同形态区水分分布研究,结果证实在纤维饱和点以下木材次生壁中层的含水量高于细胞角隅区。再次,总结近红外光谱、磁共振、红外光谱、拉曼光谱4种分析技术应用于木材中水分存在状态研究的最新进展,并分析这4种技术的优缺点。其中,部分最新研究进展是:木材中水分吸附的主要活性位点是羟基和羰基;纤维饱和点以下木材中吸着水的存在状态为强氢键结合水、中等氢键结合水和弱氢键结合水;根据3种状态水随相对湿度的变化趋势,可将木材中水分吸附过程分为3个阶段,每个阶段所吸附水的分子结构主要为C=O…(HOH)…OH或OH…(OH2)…OH、WATER…HOH…WATER以及四面体结构。最后,本文指出木材中水分分布及其存在状态尚有许多疑问亟待解答,应以研发更高精度、更高灵敏度的分析技术以及更可信的谱图成分分析技术为突破口,在细胞水平、分子水平上深入揭示木材-水分关系。
关键词木材    水分分布    存在状态    

Wood is a hygroscopic material, and its dimensions and density, as well as its mechanical, elastic, and electrical properties are affected by itsmoisture content (Hozjan et al., 2011; Olek et al., 2013; Guindos, 2014). This is the reason why the wood-water relationship has been studied since the beginning of wood research. There are over thousand representative publications available which has provided some useful information about wood-water relationship. However, due to the complexity of chemical constituents and the heterogeneity of anatomical structure of wood, there are still many issues worthy of further study such as the distribution and existing states of water in wood. With the development of new technology, vast modern analytical methods are emerging, and the study of wood-water relationship is changing from macro to micro scale and from cell to molecular level. The main objective of this paper was to review the recent analytical techniques which have been applied to demonstrate the distribution and existing states of water in the wood. Meanwhile, the advantages and limitations of these analytical techniques and the recent progress and the future development tendency of the distribution and existing states of water in the wood were also given. It may help to understand research present situation in this field, choose the appropriate analysis methods and enhance research levels.

1 Characterizing the distribution of water in wood

A number of experimental approaches have been introduced to characterize the water distribution in wood. Although the principles of these approaches are different, all of them have achieved many promising results.

1.1 Magnetic resonance imaging (MRI) technique

Magnetic resonance imaging (MRI)technique is capable of nondestructive investigation of the wood on both microscopic and macroscopic levels, which can provide detailed and quantitative spatial distribution information on water (Brownstein, 1980; Menon et al., 1987; Rosenkilde et al., 2002; Casieri et al., 2004; Merela et al., 2005). Due to the advantage of non-intrusive manner, high spatial resolution and the sensibility of water, MRI technique is generally the preferred method for studying equilibrium states and kinetic changes of water distribution during water uptake or drying. For example, Araujo et al. (1992) obtained one-dimensional images of the bound water and the lumen water in white spruce (Picea glauca) sapwood separately at four different moisture contents ranging from 100% to 17%. Oven et al. (2008) also used this technique to visualize moisture content change in pruned part of a beech (Fagus sylvatica), and provided 3D imaging in which transverse and longitudinal sections were shown separately. Meanwhile, Kekkonen et al. (2014) and Javed et al. (2015) used the MRI to visualize the spatially resolved free water distribution. Passarini et al. (2015) also mapped distribution of liquid and bound water in huayruro (Robinia coccinea), cachimbo (Cariniana domesticate), eucalyptus (Eucalyptus saligna), and red oak (Quercus rubra) under equilibrium moisture contents (EMC) below fiber saturation point (FSP), and interpreted the obtained mapping images with the help of scanning electron microscopy images. Meanwhile, Lamason et al. (2014) used the single point ramped imaging with T1 enhancement to show 2D images of a black spruce sapwood (Picea mariana) sample as it froze, and found that the bound water in this sample was still not frozen even at -60 ℃. In addition, Rosenkilde et al. (2002) obtained moisture content profile in the surface layer of Scots pine (Pinus sylvestris) sapwood with a depth resolution of better than 20 μm. Moreover, Dvinskikh et al. (2011) developed one novel optimized MRI imaging method in which softwood samples adsorbed heavy water (D2O). By this improved method, water distribution and wood density obtained separately at the same time, and the resolution of water distribution was at the sub-millimetre scale.

In the study of water distribution, MRI technique has submillimeter spatial resolution. However, this technique still has some limitations, one is this equipment is relatively expensive, the other is slightly difficult to quantify the water when the moisture content of wood is below the FSP. Below the FSP, water molecule has properties similar to the solid, for the adsorbed water molecules are tightly bounded to the wood, so the relaxation time T2 becomes shorter which leads to difficulties in quantifying the moisture content.

1.2 Computed tomography (CT) technique

Another non-destructive technique to study the water distribution in wood is the computed tomography (CT) scanning based on X-ray or gamma-ray. The X-ray and gamma-ray were used to estimate moisture content of wood with high accuracy (Kim et al., 2015; Tanaka, 2015), and CT scanning based on X-ray or gamma-ray showed it had the ability to detect the water distribution in wood. For example, Davis et al. (1993) measured moisture distribution of mountain ash (Eucalyptus regnans) using the CT scanner based on gamma-ray, similar method was also applied to southern pine (Pinus palustris). Fromm et al. (2001) also used this technique to map the water distribution within the sapwood and heartwood of green spruce (Picea abies) and oak (Quercus robur) with a spatial resolution of 0.122 5 mm3. Meanwhile, the results from CT experiments showed that water distribution was strongly uneven in longitudinal uptake. Recently, a high resolution CT technique such as micro-CT has been applied to obtain the 3D structure of wood and 3D response toward adsorbed water changes (Mannes et al., 2010). Due to the high spatial resolution, micro-CT can get information only from a few cell layers. For example, Trtik et al. (2007) applied micro-CT to the 3D imaging of spruce (Picea abies), and obtained the resolution of 1.5 μm. Derome et al. (2011) used the micro-CT to investigate the 3D changes of Norway spruce wood sample during water adsorption and desorption process.

In the study of water distribution, CT technique has been improved in spatial resolution (0.5-1.5 μm). However, it still has some limitations. One is the measurement result from the combination of both wood and water, the other is that the density of the dry wood sample has to be known in order to calculate the moisture content of subsequent samples.

1.3 Neutron imaging (NI) technique

Neutron imaging (NI) technique is also a very efficient technique for investigating the water distribution of wood (Islam et al., 2003). The high accuracy of this technique is due to the large scattering and absorption cross section for thermal neutrons which interact quite well with hydrogen nuclei, a component of water. What's more, NI can provide the time-dependent observation of water diffusion processes. Mannes et al. (2009) demonstrated that even small amounts of water absorbed from air moisture in the order of 0.2% for European beech (Fagus sylvatica) and 0.4% for Norway spruce could be detected by this technique, and the moisture-dependent diffusion coefficients for beech and spruce in the longitudinal direction could be also identified. Sedighi-Gilani et al. (2012) observed the process of water uptake and visualize the spatially resolved water distribution with a resolution of 0.07 kg·m-3 by the neutron radiographs. Lanvermann et al. (2013) used this technique to determine moisture variation in Norway spruce and showed that the moisture content in the wood cell wall was constant regardless of the particular growth ring position. Recently, NI technique has been improved in spatial resolution up to 30-50 μm which is on between the mesoscopic and microscopic level.

In the study of water distribution, (NI) technique is also a very efficient tool, and has been improved in spatial resolution up to 30-50 μm. However, it still has some limitations. One of the main limitations is radiation safety issue.

1.4 Vibrational spectroscopic imaging techniques

Vibrational spectroscopic imaging techniques such as micro-FTIR and confocal Raman spectrometer equipped with additional visible-light microscope could offer visual examination and spectral information of chemical functional groups in situ, and thus they are considered to be promising tools for studying water distribution in wood. The latest progress was reported by Guo et al.(2016), which proved that the confocal Raman imaging technique could provide useful information about the water distribution at the morphologically distinct cell wall region level and the obtained result showed that the amount of water adsorbed in the CC region was less than that in the S2 region throughout the entire range of RH levels (Guo et al., 2017). Meanwhile, Guo et al. (2017) also showed that the micro-FTIR imaging technique could provide useful information regarding the water distribution at the cell structure level, and proved that the water concentrations in the three randomly selected cell walls differed throughout the range of RH level.

These two vibrational spectroscopic imaging techniques play complementary roles in the physical selection rules and spatial resolutions. For the physical selection rules, the peaks of micro-FTIR are caused by the change of electric dipole moment of sample molecules, and the peaks of Raman are determined by electric dipole-electric dipole polarizability. For the spatial resolutions, the spatial resolution of micro-FTIR imaging technique is 6.25 μm by the instrumental parameters, and the spatial resolution of confocal Raman imaging technique is higher than 1 μm. Due to the limited resolution, micro-FTIR imaging technique can be used to observe the water in different wood cell walls and confocal Raman imaging technique is appropriate for investigating water in morphologically distinct regions of the wood cell wall. However, these methods also have some limitations. Due to the strong absorption bands, only thin samples can be measured in transmission mode of micro-FTIR spectroscopy. Meanwhile, due to the fluorescence of wood, only one point within samples can be measured in confocal mode of Raman spectroscopy.

2 Determining the existing states of water in wood

There are many kinds of classifications of water in wood (Berthold et al., 1996; Weise et al., 1996). One classification divides the water into two categories: 1) "water of constitution"; 2) "other water". The "water of constitution" is water molecule combined with the chemical components of cell wall, which cannot be left out without changing the chemical structure of wood cell wall. The "other water" exists in forms of three states: "bound water", "free water" and "vapor water". The "bound water" is water molecule absorbed by sorption sites exist in chemical components of wood cell wall; the "free water" is liquid water molecule exist in cell lumen and voids; and the "vapor water" is vaporous water molecule exist only in completely saturated wood. As differing chemical sites for water adsorption exist and there are different structural forms of adsorbed water that deviate from ordinary water, the structural changes of adsorbed water during the water adsorption process are highly complex. Although there are many kinds of classifications of water in wood, characterizing the states of water in wood remains an active field of research with relevant questions still lacking answers. Many methods such as Near-infrared spectroscopy, nuclear magnetic resonance(NMR), FTIR spectroscopy and Raman spectroscopy have been employed to get a better understanding about the existing states of water in wood.

2.1 Near-infrared (NIR) spectroscopy

Near-infrared (NIR) spectroscopy is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum (from about 700 nm to 2 500 nm), and based on molecular overtone and combination vibrations of C-H, O-H, N-H, and S-H function groups (Sehwanninger et al., 2004), so it can be used to estimate moisture content of wood and determine molecular interactions between water and wood. For example, Berthold et al. (1998) showed that NIR spectroscopy was sensitive enough to distinguish between water molecules adsorbed onto different hydrophilic groups. Moreover, Tsuchikawa et al. (2003) used this spectroscopy to analyze the different states of OH groups in wood. Afterwards, Koumbi-Mounanga et al. (2015) used this technique to determine moisture content (MC) of trembling aspen (Populus tremuloides) flakes.

NIR spectroscopyhas been proved to be useful tool for determining the moisture content in wood. Moreover, the existing states of water in wood can be identified in further analysis of the NIR spectroscopy obtained. However, this method has some limitations which are due to the overlapping absorption bands and undeniable light scattering effects, which offer limited precise information and give rise to errors. These errors are mainly from sample and sampling procedures (Azzouz et al., 2003).

2.2 Nuclear magnetic resonance (NMR) technique

Nuclear magnetic resonance (NMR) techniqueis base on a physical phenomenon in which nuclei can absorb or send out electromagnetic radiation. Because the wood and water can be clearly demonstrated by NMR signals, and the bound water and lumen water can also be easily separated on the basis of spin-spin relaxation times (T2), the NMR has become a particularly convenient tool for the study of existing types of water and the interactions between water and wood (Fredriksson et al., 2017). For example, Froix et al. (1975) obtained spin-lattice and spin-spin relaxation times of the cellulose at seven different moisture contents ranging from 96% to 17%, discriminated four types of water incorporated in cellulose (primary bound water, secondary bound water, free water and bulk water), and showed the transition between primary bound and secondary bound water at the point of plasticization as well as the increasing of free water above the point of plasticization. Menon et al. (1987) also used the technique to study water in Douglas-fir (Pseudotsuga menziesii) and western red cedar (Thuja plicata), and assigned the three distinct spin-spin relaxation times to water on the cell wall, water in the ray and latewood tracheid lumens and water in the earlywood tracheid lumens with the help of anatomical data. Moreover, Araujo et al. (1992) measured the spectra of spin-spin relaxation time (T2) for white spruce (Picea glauca) which contained three peaks corresponding to the different types of water, and demonstrated that the T2 time of about 1 ms was due to bound water, the T2 time in the range of 10 to 100 ms was from lumen water, and the T2 time from lumen water was a function of wood cell radius. Merela et al. (2008) confirmed that the moisture content of wood can be determined instantaneously according to mass and the amplitude of NMR free-induction-decay (FID) signal. Furthermore, Zhang et al. (2013) used the NMR technique to distinguish water states according to spin-spin relaxation time, and the results of this study showed that yellow poplar (Liriodendron tulipifera) had five components in water states (bound and free water) according to spin-spin relaxation time at moisture contents greater than 100%, and the number of different water states decreased with decreasing moisture content. Afterwards, Kekkonen et al. (2014) investigated moisture absorption of thermally modified and unmodified pine wood (Pinus sylvestris), and confirmed that T2 relaxation time was attributed to four types of water (water hydrogen bonded to the hydroxyl groups and between the cellulose chains, water from cell wall micropores, water from early wood tracheid lumens and water from late wood tracheid lumens as well as ray lumens) both in the case of thermally modified and unmodified samples. Passarini et al. (2014) used the variation of the fast T2 values among the different wood species to indicate the distribution of bound water, and confirmed that there was a region in which the loss of bound water took place before all liquid water was drained.

The research progress has proved the benefit of NMR in the study of existing states of water. However, NMR technique has some limitations just like MRI technique. The most serious limitation is that the equipment is relatively expensive.

2.3 Fourier transform infrared (FTIR) spectroscopy

Fourier transform infrared (FTIR) spectroscopy has been exploited previously to study the moisture adsorption in cellulosic materials. Hofstetter et al. (2006) investigated the interaction between cellulose and moisture by the combination of deuteration and FTIR spectroscopy, and showed the role of different hydrogen bonds in the moisture uptake. Olsson et al. (2004) examined the adsorption characteristics of some wood polymers by FTIR spectroscopy at nine levels of relative humidity ranging from 0% to 80%, determined one important relationship between the weight gain and the increase of O-H stretching band, and indicated the chemical sites for water adsorption. Laity et al. (2000) demonstrated that it was possible to reproduce water sorption kinetics by recording FTIR spectra. Célino et al. (2014) showed that the spectral information of FTIR spectra allowed both qualitative and quantitative analyses of the moisture absorption mechanisms of natural fibres. Furthermore, the idea that moisture is adsorbed to specific sites on the lignocellulosic material has also been advocated by FTIR spectroscopy. Haxaire et al. (2003) showed that infrared spectroscopy can help to identify whether the hydroxyl groups are hydrogen bonded or not with water molecules. The latest progress is reported by Guo et al., and they used micro-FTIR spectroscopy and a specially designed sample cell to examine the molecular association of adsorbed water with wood during adsorption process, confirmed that carboxyl C=O and C-O groups as well as OH groups were active sites for water adsorption. Meanwhile, strongly, moderately and weakly hydrogen-bonded water molecules were identified and assigned. What's more, according to the variation trend of these hydrogen-bonded water molecules, three sections were divided for adsorption process. Furthermore, the existing states of water in each section was demonstrated as C=O…(HOH)…OH or OH…(OH2)…OH、WATER …HOH…WATER, and five-molecule tetrahedral structure of water (Carey et al., 1998; Guo et al., 2016).

Recently, micro-FTIR and a specially designed sample cell have been developed to determine the existing states of water in wood. However, this method also has some limitations. Due to the strong absorption bands, only thin samples (0-35 mm) can be measured in transmission mode of FTIR spectroscopy. However, in the attenuated total reflection mode, the size of wood sample is allowed to be enlarged (Célino et al., 2014).

2.4 Raman spectroscopy

Raman spectroscopy has been exploited previously to study the moisture adsorption or evaporation in cellulosic and lignocellulosic materials. Agarwal et al. (2005) demonstrated that the water evaporation in the cellulose filter paper and black spruce (Picea mariana) thermomechanical pulp has a linear dependence relation with the declining Raman intensity of the O-H stretching envelope. Scherer et al. (1985) observed cellulose acetate film which was exposed to water vapor from 0% to 100% relative humidity, and characterized the water adsorption by the increasing O-H stretching envelope as a function of RH. Furthermore, confocal mode has been introduced into Raman spectroscopy which can reduce the fluorescence of sample by only allowing the signal originating from the focus point. Thanks to the high spatial resolution of confocal Raman spectroscopy, it has been successfully used to study the water adsorbed by micron-sized samples (Li et al., 2006; Shou et al., 2011).

One trend is the development of in situ use of confocal Raman spectroscopy. The latest progress showed the confocal Raman technique could provide useful information about the molecular interactions between water and wood (Guo et al., 2017). In order to widely spread this application, sample heating by Raman laser should be addressed clearly. The heating problem involved three objects: laser, wood and water. Hopkins et al. (2004) used the confocal laser to control water droplets, 1-14 μm in diameter, over time frames of hours at trapping laser powers of less than 10 mW. Over time frames of hours for stable trapping proved that the evaporation of water droplets could be negligible. The consistent band shapes of the OH stretching vibration from the water droplet further proved (Mitchem et al., 2006). So it can conclude that the evaporation of water can be negligible when the laser power is less than 10 mW. Agarwal (2006) used a states-of-the-art 633-nm laser-based confocal Raman microscope to determine the distribution of cell wall components in the cross section of black spruce (Picea mariana) wood in situ, and confirmed that the intensity of the band at 1 600 cm-1 remained stable. Meanwhile, in order to test the effects of different laser powers, Heraud et al. (2007) obtained a range of spectra corresponding to the laser powers from 0.1 to 3 mW. The obtained stable spectra confirmed that there was little or no degradation of chlorophyll α or β-carotene caused by laser exposure during the measurement time of 10 s. Chen et al. (2007) showed that laser's heating effect on the sample could be ignored. All these experiments proved that the laser's heating effect under certain conditions (laser power was less than 3 mW) is negligible. In conclusion, the wood sample and water heating by Raman laser can be ignored under certain conditions. Therefore, the confocal Raman spectroscopy could be one promising tool for determining the existing states of water in wood. The latest progress furthermore supports this conclusion (Guo et al., 2017).

Recently, Raman spectroscopy has been developed to determine the existing states of water in wood. However, this method also has some limitations. Due to the fluorescence of wood which may give rise to errors into the spectroscopy, confocal mode or complex sample handling should be introduced.

3 Future directions

As is well known, all these mentioned techniques still suffer from resolution-related questions, the size of the measured area, the range of measurable moisture, the size of wood sample, and measurement time. Therefore, some future research subjects should be proposed.

Firstly, there exists limited knowledge about water distributionin wood, such as the water distribution in wood cell walls with different moisture content, the axial and radial distributions of water in wood with the same moisture content and so on. A better understanding of water distribution in wood is urgently needed for developing more approaches with greater accuracy and higher resolution.

Secondly, water in wood includes many different states, such as primary bound water, secondary bound water, free water, bulk water and so on. However, there is no common idea about the existing states of water with different moisture content. I think that the convincing component band analysis which can be used to distinguish the existing states of water should be further developed.

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