Water vapour sorption characteristics and surface chemical composition of thermally modified spruce ( Picea abies karst).

Abstract

The objective of this work was to study the hygroscopicity and surface chemical composition of thermally modified (TM) spruce. An effort was also made to study if those features were influenced by a previous exposure to a significant increase in relative humidity (RH). TM and unmodified Norway spruce (Picea abies Karst) samples, both in solid and ground form, were prepared. Water vapour sorption characteristics of the ground samples were obtained by measuring sorption isotherms using a dynamic vapour sorption (DVS). The surface chemical composition of the solid samples, both acetone extracted and non-extracted, were analysed using X-ray photoelectron spectroscopy (XPS). The DVS analysis indicated that the TM wood exposed to the 75% RH revealed a decrease in isotherm hysteresis. The XPS analysis indicated a decrease of acetone extractable or volatile organic components and a relative increase of non-extractable components for the samples exposed to the increased RH condition.

Keywords

Dynamic vapour sorption (DVS)Extractives Norway spruce Surface chemical composition Thermally modified wood X-ray photoelectron spectroscopy (XPS)

This paper is part of a special issue on the Eighth European Conference on Wood Modification

Introduction

There is a growing interest to use new types of biocomposites in various building material applications (Yatim, Nur Hafizah and Mahjoub 2011). In particular, it may be of great benefit from an environmental consequences perspective to reuse byproducts or wood residuals from the construction or wood processing sectors to manufacture biocomposites (Erlandsson and Sundquist 2014). Moreover, the surface energetics of thermally modified (TM) wood residual components (Källbom, Wålinder, Segerholm and Jones 2015), in combination with the influencing effects from water sorption can lead to an increased understanding in how to design and develop efficient material combinations in terms of novel composite materials from recycled sources.

Wood can be considered as a biopolymer composite featuring a pronounced variation in chemical composition, morphology and physical and mechanical properties. Such bio-based materials are also intrinsically hygroscopic in nature. Water and moisture sorption in wood and wood products have crucial effects on their in-service properties and durability in building material applications. For instance, water sorption in wood will often result in problematic swelling and shrinkage as well as a decreased decay resistance. There are various modification methods resulting in decreased hygroscopicity and an increased dimensional stability of wood, also resulting in a decreased risk for biodeterioration. One well established wood modification approach is thermal modification (Seborg, Tarkow and Stamm 1953; Stamm 1956), which has seen a significant industrial development during the last decades (Hill 2006; Navi and Sandberg 2012). Thermal modification in this case means exposure of wood to ca 200°C in a nearly oxygen free environment. This condition induces chemical and morphological changes of the wood substance which also for most wood species results in a darker brownish colour. Some important properties, such as dimensional stability and decay resistance of the wood are thereby improved, although TM wood also shows decreased strength and a more brittle behaviour than the corresponding unmodified (UM) wood (Hill 2006).

Studies of water vapour sorption behaviour of TM wood (Kollmann and Schneider 1963; Chirkova, Andersons, Andersone and Militz 2005; Pfriem, Zauer and Wagenführ 2010 and Guo, Song, Salmén and Yin 2015) show a decrease in equilibrium moisture content (EMC) with increasing treatment temperature. Furthermore, analysis using the dynamic vapour sorption (DVS) method has in recent years become established for studying the sorption behaviour of TM wood (Navi and Girardet 2000; Kamdem, Pizzi and Jermannaus 2002; Kohler, Dueck, Ausperger and Alex 2003; Inari et al. 2006; Boonstra and Blomberg 2007; Jalaludin et al. 2010a and 2010b; Hill et al. 2012; Olek, Majka and Czajkowski 2013). Previous studies on TM wood with exposure to several water vapour sorption cycles has been reported which show differences in later sorption cycles, and a decrease in hysteresis compared with the initial cycle (Hill et al. 2012).

It is also evident that the wettability, surface chemical composition and surface energetics of TM wood is significantly different compared with conventional wood as well as other types of modification approaches, e.g. acetylation and furfurylation of wood (Bryne and Wålinder 2010; Bryne et al. 2010; Källbom et al. 2015). In a parallel study to this work, Källbom et al. (2015) used inverse gas chromatography (IGC) to study the surface energetics of ground TM spruce components indicating significantly higher dispersive surface energy heterogeneity of the modified samples compared with UM samples.

X-ray photoelectron spectroscopy (XPS, also called electron spectroscopy for chemical analysis, ESCA) yields information on the chemical composition of surfaces, with analysis depths of only few nanometres. It has been utilised in the analysis of paper and refined lignocelluloses for more than three decades (Dorris and Gray 1978a and 1978b). However, all XPS experiments are conducted in ultra high vacuum (UHV) using soft X-ray irradiation while exposures to very dry vacuum conditions and irradiation are known to affect sensitive surfaces of biopolymers like wood. This may be the reason why only recently the method has been successfully applied on wood (Shchukarev, Sundberg, Mellerowicz and Persson 2002; Nzokou and Kamdem 2005; Inari et al. 2006; Johansson et al. 2012; Muguet et al. 2013) as well as on chemically or TM wood (Bryne et al. 2010; Tuong and Li 2011; Wang et al. 2011; Rautkari, Hänninen, Johansson and Hughes 2012; Sedighi Moghaddam, Wålinder, Claesson and Swerin 2013; Wang, Zhu, Cao and Sun 2015). In analysis of lignocelluloses and wood, acetone extraction is often used in order to differentiate between the extractable species (comparably small mobile molecules such as resin and fatty acids), non-extractable large lignin macromolecules and the cellulosic backbone of wood (cellulose and hemicelluloses) (Dorris and Gray 1978a and 1978b).

Owing to various modification methods or exposure to changes in the surrounding environment, the extractive compounds migrate within the wood and influence surface chemical analysis. This migration phenomenon has for example been observed during drying of wood, where an increased content of nitrogenous compounds was observed at the surface (Boutelje 1990). An increased amount of extractives at the wood surface leads to an increased hydrophobic character of the surface (Nussbaum 1999; Nussbaum and Sterley 2002). Based on both wetting and spectroscopic studies, Bryne and Wålinder (2010) and Bryne et al. (2010) indicated that aging of TM spruce resulted in a pronounced hydrophobisation process due to migration or redistribution, and possible chemical changes, of low-oxygenated species (presumably extractives) to the wood surface. Nuopponen, Vuorinen, Jämsä and Viitaniemi (2003) studied the distribution of extractives within the bulk and on the surface after thermal treatment of wood and showed that resin acids were totally removed at temperatures above 200°C. Furthermore, acetone or dichloromethane extraction also helps in passivation of hydroxyl groups at cellulose surfaces in water-free conditions, lowering the effect of surface contamination (Johansson et al. 2011).

The aim of this work is to further increase the insight about the alterations of the surface characteristics and moisture sorption behaviour of wood due to the thermal treatment. The specific objective was to study the water vapour sorption behaviour and the elemental surface chemical composition of TM and UM spruce and if these features are influenced by a previous exposure to a significant increase in relative humidity (RH).

Materials and methods

Preparation of wood samples

UM and TM Norway spruce (Picea abies Karst) samples were first prepared as solid wood planks. The thermal modification was executed according to the ThermoWood^® D process (Anonymous 2003). The ThermoWood^® D process is performed in three phases, from drying, thermal treatment (above 200°C) to cooling and moisture exposure, giving a moisture content of about 4–7% (Anonymous 2003).

For the DVS analysis, ground wood component samples were prepared in a two-step grinding procedure, described in Segerholm, Vellekoop and Wålinder (2012), sifted through a 120 mesh (<0.125 mm), and then stored and aged for several weeks in indoor room environment (20°C and 20–40% RH). Two replicates of both the UM and TM wood components were exposed to three dry-moist cycles at 0 and 75% RH conditions at 30°C, following procedures as described in a parallel IGC study (Källbom et al. 2015). The TM wood components were also exposed to an additional three cycles at 0 and 25% RH conditions at 30°C. After the cycling in the IGC, these wood component samples were exposed to a 75% RH condition at 30°C in a climate chamber for 4 weeks (wood component samples denoted as ‘exposed’). In addition, another set of the ground wood component samples were stored and conditioned in an indoor room environment (20°C and 20–40% RH) (wood component samples in some cases denoted as ‘non-exposed’).

Smaller samples of solid wood were cut from the planks for UM wood of dimensions 90 × 50 × 17 mm³, and for the TM wood of dimensions 140 × 50 × 20 mm³. One set of these solid wood samples, both TM and UM, were exposed to 75% RH at 30°C in a climate chamber during 4 weeks until EMC was reached (samples denoted as ‘exposed’ or with the index ‘exp’). Another set of the corresponding solid wood samples were stored and conditioned in an indoor room environment (20°C and 20–40% RH) (samples in some cases denoted as ‘non-exposed’).

For the XPS analysis, samples in the form of cubes of dimensions 10 × 10 × 10 mm³ were cut from the sapwood of each solid wood sample using a wood chisel, creating new surfaces on all sides two days prior to the XPS analysis. Just before the XPS experiments, new surfaces were prepared by cleaving the previously prepared cubes. The sample surfaces were thereafter measured either as such or after acetone extraction (6 hours in Soxhlet), in order to differentiate between the extractable species (small mobile molecules and non-bonded contamination) from non-extractables (mainly large lignin macromolecules).

DVS analysis

The water vapour sorption isotherms, that is the EMC plotted versus the RH, of UM and TM spruce wood components were determined at 30°C using a DVS apparatus (DVS ET1, Surface Measurement Systems Ltd., London, UK). Four ground wood component samples, prepared as described above, were investigated: UM, TM, UM exposed and TM exposed. The amount of wood components used for each sample was about 20 mg. The temperature was kept at 30°C during the whole analysis for all the tests and started with sorption measurements at 0% RH going up to 95% RH with steps of 5% RH. After reaching the level of 95% RH, desorption measurements were performed using the same procedure, reversing back to 0% RH. A further description of the DVS methodology can be found in (Hill et al. 2012).

XPS analysis

The elemental surface chemical composition of the freshly cleaved wood samples was analysed using XPS analysis (AXIS Ultra electron spectrometer, Kratos Analytical, UK).

Eight solid wood samples, prepared as described above, were investigated: TM samples, non-exposed and exposed (samples denoted TM and TM exp); UM, non-exposed and exposed (samples denoted UM and UM exp); TM, non-exposed and exposed, acetone extracted (extr), (samples denoted TM extr and TM exp extr); and UM, acetone extracted, non-exposed and exposed, (UM extr and UM exp extr). The freshly cleaved/extracted samples were pre-evacuated overnight in order to stabilise analysis conditions in UHV. Data from the analysis was recorded using monochromatic A1 Kα X-rays, at 100 W. Survey scans were recorded with 160 eV pass energy with 1 eV step, and the high resolution regions were taken with 20 eV pass energy with 0.1 eV step. Pure cellulose filter paper was used as an in-situ reference (Johansson and Campbell, 2004).

Elemental surface concentrations were determined from low resolution survey data (example in Fig. 3) and carbon chemistry was evaluated from high resolution C1s components, fitted with four Gaussian components (C1–4), classified as carbon atoms with 0, 1, 2 or 3 oxygen neighbours (Table 1). Nominal surface coverages for lignins, extractives and celluloses were calculated (Koljonen, Österberg, Johansson and Stenius 2003), and the O/C atomic ratio was also depicted against non-cellulosic C1 component of the high resolution C1s data (Rautkari et al. 2012).

Table 1

The C1s peak, deconvoluted into four carbon peaks (C1–4) together with the approximate binding energy, functional groups and the percentage distribution of wood constituents from a total of 100%, related to different carbon peaks

			Percentage distribution/%**
C1s	Approximate binding energy/eV*	Functional groups	Cellulose	Lignin	Hemicellulose	Extractives
C1	285.0	C–C, C=C and/or C–H		49		94
C2	286.7	C–O and/or C–O–C	83	49	78
C3	288	C=O and/or O–C–O	17	2	19
C4	289.4	O–C=O and/or C(=O)OH			3	6

*Binding energies are given according to the database by Beamson and Briggs (1992).

**Approximate percentage distribution of wood constituents based on theoretical values from Laine, Stenius, Carlsson and Ström (1994) where arabinoglucuronoxylan was used for hemicellulose and oleic acid for extractives.

Results and discussion

DVS analysis

The sorption isotherms of the four different ground wood samples are shown in Fig. 1, where each sample's upper curve represents the desorption isotherm and the lower curve represents the adsorption isotherm. As shown in Fig. 1, there is as expected a distinct reduction in EMC for the TM wood components, compared with the UM wood components. The maximum EMC value obtained for TM and UM wood was 11 and 22%, respectively. This is valid for both the sorption and desorption isotherms, and is furthermore also in agreement with previous studies on TM wood (Jalaludin et al. 2010a; Hill et al. 2012; Olek et al. 2013) and TM spruce (Yin, Berglund and Salmén 2011; Guo et al. 2015), using DVS.

It can also be seen in Fig. 1 that the desorption isotherms are fairly similar for the non-exposed and exposed samples, both in the case of UM and TM wood. In contrast to this, the adsorption isotherms slightly differ between the non-exposed and the exposed case for both the TM and UM samples. For the exposed UM samples a decrease in EMC was detected, which was more pronounced above 70% RH, compared with the non-exposed samples. For the exposed TM samples, an increase in EMC was detected, which was more pronounced from 50% RH and above. These measurements of the sorption isotherms were performed at 30°C, which is a slightly higher temperature than what has normally been used in other studies. Sorption tests performed at higher temperature conditions will lead to a lower EMC at a given RH. However, it can be concluded that the EMC at 75% RH for the sorption isotherm is in accordance with the moisture content that was obtained from the IGC measurements on the same samples, at 30°C, 75% RH for both TM and UM wood (Källbom et al. 2015).

The hysteresis between the adsorption and desorption isotherms for each sample is shown in Fig. 2. In agreement with the results presented above related to Fig. 1, the UM wood shows an increase in hysteresis as a result of the previous exposure to the 75% RH condition, while the TM wood shows a decrease. Similar results have been observed for TM wood exposed to sorption cycling (Hill et al. 2012) and also for aged wood (Popescu and Hill 2013). From Hill et al. (2012) a more pronounced difference was observed between the adsorption and desorption curves of the sorption isotherms for the second and third cycle after multiple cycle analyses. However, in that study the maximum RH level was reached at 95% RH, which is higher than the pre-exposure of 75% RH condition in this study. According to Lu and Pignatello (2002 and 2004) and Hill, Norton and Newman (2009), the hysteresis between the moisture sorption and desorption curve for wood can be described as the sorption process taking place in a glassy polymer matrix. Within the matrix of the wood substance there are microvoids created as a result of the sorption. Thereafter, during desorption these microvoids collapse, which leads to a difference between the adsorption and desorption curve, related to changes in the cell morphology.

When water molecules enter the wood cell walls, some of the internal hydrogen bonds between wood polymers are replaced with bonds to water molecules. Hereby, the material is weakened yielding a lower wood stiffness. However, presumably water does not enter the aggregated cellulose microfibrils (Matthews et al. 2006) but may adsorb on microfibril surfaces or to the surrounding matrix of hemicelluloses and lignin (Hill et al. 2009).

It is well known that thermal modification of wood results in changes in the wood chemistry, including chemical degradation and mass loss (Stamm 1956), leading to an altered wood structure and composition and a reduced hygroscopicity. This has earlier been explained by a reduced amount of available hydroxyl groups in wood, primarily related to the hemicelluloses (Esteves and Pereira 2009). It has furthermore been shown that high temperature modification of wood leads to an enhanced mobility of the molecular chains in the cell wall (Fahlén and Salmén 2003 and Hakkou, Pétrissans, Zoulalian and Gérardin 2005). It has been suggested by Obataya and Tomita (2002) that irreversible changes (chemical) in the matrix of lignin and cellulose is a result from thermal treatment. According to Hoffmeyer et al. (2003), the microvoids left behind due to the loss of material may cause an increased hygroscopicity at higher RH due to capillary condensation in micropores, whereas the hygroscopicity at lower RH is decreased due to loss of sorption sites in the cell wall.

XPS analysis

Figure 3 shows an example of an obtained XPS survey spectrum, in this case for the extracted TM wood sample (TM extr). Similar survey spectra were also obtained for the other samples. Figure 4 shows the high resolution spectra for the non-exposed extracted TM wood (TM extr) and for the exposed extracted TM wood (TM exp extr), respectively. Differences in C1 between the non-exposed and exposed samples indicate that there was a change in lignin or extractive content. Furthermore, the extracted samples all showed a pronounced lowering of the amount of C1 compared with the non-extracted samples, which is likely due to the removal of some of the wood extractives during the preceding acetone extraction.

Figure 5 shows the correlation graph of C1 (C–C/C–H) of total C% versus the O/C ratio. In this figure, all the samples are displayed in relation to theoretical values of wood constituents (cellulose, lignin and extractives). The non-extracted samples that contained more extractives can be found closer to the theoretical point of extractives (oleic acid). The extracted samples can be found closer towards the theoretical point of cellulose, and the non-exposed extracted samples were all farthest from the theoretical extractive value of oleic acid, which seems reasonable. The data for 2–3 replicates for each sample are presented in Fig. 5, and as can be seen, the data points for the replicates of the non-extracted TM samples, both non-exposed and exposed are less scattered than the data points of the UM samples. This indicates a less stable surface of the non-extracted UM samples under XPS conditions, or a more chemically heterogeneous surface compared with the TM samples. In contrast to this, it was indicated (Källbom et al. 2015) that similar samples of TM wood as used in this study, exhibited a more pronounced heterogeneity compared with UM samples in terms of the dispersive (non-polar) component of the surface energy. As the acetone extraction was performed to stabilise the chemical changes occurring at the surface during the XPS analysis, this is likely explaining the scattered results of the non-extracted UM samples. The unstable surface of non-extracted UM wood has been observed in an earlier study (Johansson et al. 2012). The TM samples had due to the modification, already been exposed to conditions that would have led to chemical changes at the surface and thus resulting in more stable surface conditions.

Figure 6 presents an alternative overview and more condensed version of the results presented in Fig. 5, also with some clarifying trends pointed out by the added arrows. As can be seen in Fig. 6, the previous exposure to the 75% RH condition influenced the chemical composition of the surface of both the TM and the UM wood samples. This was also indicated, and as pointed out by the arrows in Fig. 6, by a smaller difference between exposed non-extracted and the exposed extracted sample. A theoretical composition of the surface can be suggested, combining the results from the non-extracted and extracted samples, constituted of three main groups of compounds representing non-extractable (such as lignin), extractable (such as hydrophilic and hydrophobic wood extractives) and cellulose.

As a result of the exposure to the 75% RH condition, the concentration of acetone extractable compounds decreased both for TM and UM wood. Furthermore, the relative concentration of non-extractable compounds increases. An explanation for this could be that hydrophilic or low-molecular-weight compounds, non-extractable with acetone, had been redistributed at the surface layer of the wood samples, or migrated to the surface, caused by water diffusion during drying of these exposed samples.

As concluded from Denes, Cruz-Barba and Manolache (2005), migration of low-molecular-weight wood compounds and water vapour from the lignocellulosic matrix will occur during exposure to low-pressure environments. This could explain the decreased relative concentration of actual cellulose at the surface, and moreover the increased concentration of non-extractable components. The exposure to the 75% RH condition seemed to affect the UM samples more than the TM samples, in terms of changes in the surface chemical composition. This could be due to, as mentioned earlier, the fact that changes in chemical composition already had taken place during the thermal modification. TM wood is furthermore known to be more dimensionally stable, compared to UM wood, which is a result from a reduction of available hydroxyl groups due to the modification process. Comparing the TM and UM samples in Fig. 6, it could furthermore be noted that the TM samples in general possessed a lower percentage of extractives in relation to the UM samples, with the exception of the extracted UM sample. However, this is not in agreement with previous studies (Inari et al. 2006).

Conclusions

Both the DVS and XPS analyses appear to be valuable tools to further increase the insight about the alterations of the moisture sorption behaviour and surface characteristics of wood due to thermal treatment. Subtle influences on these features indicated that the TM wood exposed to a 75% RH condition revealed a slight decrease in the sorption isotherm hysteresis for the 50–95% RH range compared with non-exposed sample. The opposite trend was observed for the exposed UM wood, i.e. a slight increase in sorption isotherm hysteresis for the 70–95% RH range. This indicates that for both the thermally and the UM wood such a moisture exposure history influences their moisture sorption behaviour.

The XPS analysis of the surface chemical composition indicated an increase of non-extractable compounds along with a decrease of extractable compounds due to exposure to the 75% RH condition, valid for both TM and UM wood. Furthermore, the relative cellulose concentration decreased as a result of changes in the amount of extractables. This is suggested to be related to accumulation of migrated compounds, migrated to or redistributed at the surface as a result of water diffusion to the surface during the exposure. Changes in the surface chemistry of wood would naturally lead to an altered structure and thus the amount of accessible hydroxyl groups and changes in sorption properties.

In addition, it was indicated that the non-extracted TM samples in general possessed a lower percentage of extractives in relation to the corresponding UM samples.

Footnotes

Acknowledgements

Financial support from Stiftelsen Nils och Dorthi Troëdssons forskningsfond (Projektnr 793/12 Hydro-termo-mekanisk modifiering av trä) and EcoBuild Institute Excellence Centre (established by VINNOVA, The Knowledge Foundation and the Swedish Foundation for Strategic Research) are greatly acknowledged. Many thanks also to the COST Action FP0904 for enabling the Short-Term Scientific Mission which was the foundation of this study. Furthermore, financial support from the Swedish Research Council Formas (project EnWoBio 2014-172) is acknowledged. The XPS experiments were performed using Aalto University Bioeconomy Facilities.

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