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### Cellulose – Hemicelluloses – Lignin – Wood Extractives

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# The molecular properties and carbohydrate content of lignins precipitated from black liquor

Weizhen Zhu
• Forest Products and Chemical Engineering, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Gothenburg, Sweden
• Other articles by this author:
/ Gunnar Westman
• Organic Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Gothenburg, Sweden
• Wallenberg Wood Science Center, Chalmers University of Technology/Royal Institute of Technology Kemigården 4, SE 41296 Gothenburg, Sweden
• Other articles by this author:
/ Hans Theliander
• Corresponding author
• Forest Products and Chemical Engineering, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Gothenburg, Sweden
• Wallenberg Wood Science Center, Chalmers University of Technology/Royal Institute of Technology Kemigården 4, SE 41296 Gothenburg, Sweden
• Email
• Other articles by this author:
Published Online: 2014-06-15 | DOI: https://doi.org/10.1515/hf-2014-0062

## Abstract

Precipitation and utilization of lignin from black liquor (BL) offers many promising advantages to modern kraft pulp mills. A novel process, known as “LignoBoost”, has recently been introduced as a process for separating lignin from BL; it results in lignins with a low ash and high dry solid content. There is a lack of knowledge regarding the influences of process parameters on the behavior of lignin in the precipitation step. In this study, the yield of precipitated lignin and its average molecular weight (MWt) and carbohydrate content were the focus. Nuclear magnetic resonance (NMR) analysis showed that the lignin yield increased at lower pH and temperatures or when the ion strength of BL was elevated. High yield lignins contained more low MWt components and such lignins have more phenolic OH and methoxy groups. Xylan content of the lignins decreased with decreasing pH and increasing temperature, but glucomannan content was virtually unaffected by the conditions of precipitation.

## Introduction

The lignin utilization of material obtained from black liquor (BL) would have many advantages if the energy balance of the kraft mill was not affected. Lignin from BL has been used on a scale of few million tons for many years (Pye 2008). A novel method called LignoBoost (Öhman et al. 2007a,b; Theliander 2008; Tomani 2010) has recently been introduced, and the first commercial LignoBoost plant was started in North America recently (Finaldi 2013). In all processes, lignin is precipitated from BL by lowering the pH. In the LignoBoost process, the lignin filter cake is redispersed in water at pH 2–4 and the resulting slurry is filtrated again before being washed. Such a lignin has a low ash and high dry content. As an alternative option, lignin in BL can be fractionated by membrane filtration prior to precipitation (Wallberg et al. 2003; Holmqvist et al. 2005; Brodin et al. 2009). In this case, the lignin has a more narrow molecular weight (MWt) distribution (MWtD).

According to the Deryagin-Landau and Verwey-Overbeek (DLVO) theory (Evans and Wennerström 1994), the stability of kraft lignin in solution is an interplay of attractive and repulsive forces. If the attractive forces, such as van der Waals force, hydrogen bonding, and other hydrophobic forces, dominate, the aggregation is favored. In the opposite case, the lignin solution is more stable (Rudatin et al. 1989; Norgren et al. 2001). Rudatin et al. (1989) proposed that the balance of these forces is influenced by the kind and number of functional groups, pH, temperature, ion strength, and lignin concentration.

Alén et al. (1979) precipitated lignin from BL of a softwood/hardwood cook (92/8%) at 80°C and found that acidification of BL to pH 2.0 by H2 SO4 gives rise to lignin yields of around 90%. At pH 2.8, the maximum precipitation yield from pine kraft BL was 92% and 94% at BL’s solid content of 30% and 35%, respectively. Uloth and Wearing (1989) compared the methods of acid precipitation and ultrafiltration and demonstrated that the former is more efficient. Norgren and Edlund (2001) observed the influence of temperature on lignin precipitation. The rate of aggregation is higher in the case of high ion strength chemicals in BL (Norgren et al. 2002). Wallmo et al. (2009) precipitated lignin by CO2, and the yield increased with decreasing temperature or increasing lignin concentration in BL. Wang and Chen (2013) fractionated stalk lignin by acid precipitation and reported that high MWt lignin can be obtained at higher pH levels.

Lignin precipitation from an alkaline solution is closely related to the protonation of ionized phenolic groups (OHphen) of lignin. This leads to a decrease in the electrostatic repulsion forces between the molecules (Gilardi and Cass 1993; Sundin 2000; Vainio et al. 2004), which in turn become less hydrophilic and thereby the solubility is decreasing. The equilibrium of dissociation of OHphen in lignin can be written as

$L-OH↔L-O-+H+ (1)$(1)

where L is lignin and -OH is the OHphen. The dissociation constant (Ka) of OHphen is written as a quotient of the concentrations of {L-OH}, {L-O-}, and {H+}:

$Ka={L-O-}{H+}{L-OH} (2)$(2)

if it is assumed that the protonized lignin is in a solid phase and the concentration (activity) of solid lignin is in unity (Eq. (1)). The activity coefficients of ions (L-O- and H+) are also assumed to be in unity (Eq. (1)). Eq. (2) can be simplified as

$Ka=[L-O-][H+] (3)$(3)

The logarithmic constant, pKa, which is equal to -log10 Ka, is used more widely to describe the dissociation of OHphen. It has been found that the pKa value is correlated to temperature, ion strength, and the solvent used (Ragnar et al. 2000). The apparent pKa of kraft lignin is also influenced by the MWt of lignin (Rudatin et al. 1989; Norgren and Lindström 2000a).

The main purpose of this study was to investigate the parameters of lignin precipitation from BL, and the lignins obtained should be characterized with the MWt in focus. The OHphen and methoxy groups (OMe) will be studied by nuclear magnetic resonance (NMR) analysis. The carbohydrates present in BL filtrates and filter cakes should be analyzed by total hydrolysis and high-performance liquid chromatography (HPLC) of the sugars obtained.

## Lignin isolation

The BL was obtained from a batch kraft pulp mill that produces bleachable-grade pulp in two fiber lines. The cooked wood was composed of approximately 1/3 softwood (a mixture of Scots pine and Norway spruce) and 2/3 hardwood (mainly birch). The BLs from the two fiber lines were mixed prior to entering the chemical recovery system. The precipitation experiments were carried out on laboratory scale according to Theliander (2010) and Zhu et al. (2013). In this study, 100 g BL was weighed and placed in a plastic bottle with a magnetic stirrer. A determined amount of sodium sulfate (Fisher Scientific; 99.5%, Leicester, UK) was added to the BL to increase the Na+ and K+ contents by 5%, 10%, 15%, or 20%. The bottle (closed with a lid) was placed in a water bath for 1 h and was shaken every 10 min. When the target temperature (45–80°C) was reached, 6 M H2 SO4 was added to the sample to reach the target pH. The pH measurement was performed at room temperature by JENWAY Model 370 pH/mV Meter (Dunmow, UK) with temperature correction. A three-point calibration at pH 7.0, 10.0, and 12.0 was performed before the measurements were made. The sample was then shaken every 10 min for 1 h to obtain an apparent equilibrium. When the precipitation was complete, the sample was filtrated through a Büchner funnel equipped with a filter paper (Munktell, 70 mm, Gryckso, Sweden); the filtrate was collected and stored in a gas tight bottle, ready for further analysis. The filter cake (as a dark-colored solid) was then washed with deionized water adjusted to the same pH (by H2 SO4) and ion strength gradients as the precipitation to minimize the changes in pH during washing. Finally, the filter cake was dried at 105°C for 8 h. For detailed experimental parameters, see Table 1.

Table 1

Experimental parameters of lignin precipitation.

The total dry solid (TDS) content of BL was determined according to Tappi T650 om-09, in which the sample is dried at 105°C for 24 h. The lignin concentration in the original BL was determined by ultraviolet (UV) spectroscopy (Specord 205, Analytik Jena, Jena, Germany) at 280 nm. The samples were adjusted to pH 11 prior to measurement. The absorption coefficient for calculations was 24.6 l g-1 cm-1 (Fengel et al. 1981). The contents of NaOH and Na2 S were measured according to Tappi T625. After wet combustion in a microwave oven, the Na and K concentrations of BL were measured by AAS (Thermo Scientific iCE 3000, Cambridge, UK).

The details of the Klason method applied were described by Theander and Westerlund (1986). Shortly, either 0.2 g of an oven-dried precipitated sample of lignin or 1.2 g BL (filtrated or original) was weighed and 3 ml of 72% H2 SO4 were added to the sample. The sample was then evacuated for 15 min and placed in a water bath at 30°C for 1 h. Then, 84 g deionized water was added to the sample and heated to 125°C in an autoclave for 1 h. After hydrolysis, the sample was filtrated and the insoluble solid residue (Klason lignin) was measured gravimetrically according to Tappi T222 cm-00. The filtrate was then diluted to 100 ml in a volumetric flask. A solution that is 100 times diluted is suited for UV determination of acid-soluble lignin (205 nm, absorption coefficient 110 l g-1 cm-1 according to Dence 1992). Fucose served as an internal standard for the following high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). The solution was filtered through a 0.45 μm polyvinylidene difluoride (PVDF) filter prior to being injected into the HPLC instrument.

The analysis of monomeric sugars was performed in a Dionex ICS-5000 HPLC system (electrochemical detector, Sunnyvale, CA, USA) equipped with CarboPac PA1 columns with NaOH and NaOH/NaAc (0.2 M) as eluents. Software: Chromeleon 7, Chromatography Data System, version 7.1.0.898 (Sunnyvale, CA, USA). The amounts of sugars analyzed were corrected for the acid hydrolysis loss (Janson 1974) and as reported by Wigell et al. (2007). The amounts of xylan and (galacto)glucomannan (GGM) were calculated based on the algorithm described in the Appendix.

For MWt determination, the gel permeation chromatography (GPC) method was applied. The dried precipitated lignin sample was dissolved in dimethyl sulfoxide (DMSO) (Lab-Scan Analytical Sciences; HPLC grade, Gliwice, Poland), with the addition of 10 mM LiBr, to a concentration of 0.25 g l-1. The resulting solution was then analyzed with the instrument PL-GPC 50 Plus, Integrated GPC System from Polymer Laboratories (a Varian, Inc. company, Shropshire, UK) equipped with refractive index (RI) and UV detectors. The UV detection was performed at 280 nm (indicating lignin), while the RI responses were both for lignin and carbohydrates. The system was equipped with two PolarGel-M (300×7.5 mm) columns and a PolarGel-MGuard column (50×7.5 mm). The mobile phase was DMSO with the addition of 10 mM LiBr. The sample was injected via a PL-AS RT GPC Autosampler at a flow rate of 0.5 ml min-1, and the data were evaluated by the Cirrus GPC Version 3.2 software, Shropshire, UK. Pullulan of nine different MWt (708, 375, 200, 107, 47.1, 21.1, 11.1, 5.9, and 0.667 kDa) was employed for calibration (Polysaccharide Calibration Kit, PL2090-0100, Varian, Cheshire, UK). The estimated error of determination was about 5% based on the calibration curve of standard samples. All the results obtained were baseline corrected.

For NMR examination, the samples were acetylated according to Lundquist (1992b), in which the precipitated lignin (∼100 mg) was acetylated with 1 to 2 ml Ac2 O (EMSURE, ACS, ISO, Reag. Ph Eur grade, Darmstadt, Germany)/pyridine (EMSURE, ACS. Reag. Ph Eur grade, Darmstadt, Germany) (1:1, v/v) at room temperature overnight in a 50 ml flask. After acetylation, 25 ml EtOH (SOLVECO, 99.5%, Rosersberg, Sweden) was added for 30 min, and the solvents were removed via rotary evaporation. The repeated addition and removal (rotary evaporation) of EtOH (5–10 times) resulted in the removal of acetic acid and pyridine from the sample. Finally, the acetylated lignin was dried in a desiccator over KOH and P2 O5. A 50 mg acetylated lignin was dissolved in 0.5 ml DMSO-d6 (ARMAR Chemicals, 99.8%, Döttingen, Switzerland). The 1H and 13C NMR spectra were recorded at 25°C on a Bruker Avance III HD 18.8 T NMR spectrometer (Rheinstetten, Germany) equipped with a 5 mm TCI Cryoprobe (cold 1H and 13C channels) operating at a frequency of 800 MHz for 1H and 201 MHz for 13C detection. The 1H spectra were recorded with a 90° pulse angle, 5 s pulse delay, 1024 scans, and 2.56 s acquisition time. The 13C spectra were recorded with an inverse-gated decoupling sequence, 90° pulse angle, 12 s pulse delay, 3200 scans, and 1.36 s acquisition time. The resulting spectra were baseline corrected and the data were processed by MestreNove (Mestrelab Research, Santiago de Compostela, Spain).

## Characterization of the BL

Table 2 summarizes some important characteristics of the original BL. Obviously, the contents of carbohydrates and acid-soluble lignin in the original BL are rather high. This is most likely due to the high hardwood content of the raw material before cook; similar observations were made by Musha (1974) and Gellerstedt et al. (2012). The buffering capacity of the BL was already described (Zhu et al. 2013) and it followed the same profile as in earlier investigation (Wallmo et al. 2007).

Table 2

Some important characteristics of the BL sample.

## Precipitation yield of lignin versus MWt of precipitated lignin

The relationship between yield and MWt and Mn data of precipitated lignin is listed in Table 3 and the plots MWt versus yields obtained under different conditions are presented in Figure 1. As visible in Table 3, the MWt and Mn data detected by RI response are, in general, higher than those detected by UV response, which is in agreement with findings of Zhu et al. (2013). Interestingly, the opposite is true for samples 1 and 2 (high pH precipitation). Table 4 reveals that the concentration of carbohydrates in samples 1 and 2 is rather high (especially in sample 1) and the lignin yield is low. Accordingly, these samples are rich in carbohydrates; probably, the carbohydrate moiety has lower MWt than that of the lignin.

Table 3

Experimental data of the lignin precipitation yield and the weight average and number average MWt (Mw and Mn) of the precipitated lignin.

Figure 1

Weight average MWt of precipitated lignin at different precipitation yields.

(If pH varied: 45°C without sodium addition; if temperature varied: pH 10.5 without sodium addition; if sodium addition varied: 65°C and pH 10.5.)

Table 4

Concentrations of the carbohydrates (xylan and GGM) in the filter cakes (precipitated lignin) and filtrates.

## Effects of pH, temperature, and ion strength

The lignin yield was calculated as follows:

$Yield=LBL-LFLBL×100% (4)$(4)

where LBL is the lignin concentration in original BL and LF is the lignin concentration of lignin-lean filtrate after precipitation. The input of lignin concentration in Eq. (4) was determined as Klason lignin.

Expectedly, Figure 1 reveals that the highest yields are obtained at the lowest pH because hydrogen ions protonize the negatively charged lignin (L-O-) and neutralize the charges on the molecular surface. The repulsive forces are reduced; eventually, precipitation of lignin occurs. A higher concentration of hydrogen ions (lower pH) will promote the protonation of phenolic groups and thus increase the precipitation yield of lignin.

At a constant pH and ion strength of BL, the lignin yield decreases with increasing temperature (Figure 1). This is understandable because the solubility of lignin increases with increasing temperature (Evstigneev 2011). This could be due to the fact that the pKa value of kraft lignin is lower at a higher temperature. It indicates an increased dissociation of the OHphen groups, which leads to a better solubility. At higher temperatures, the electrostatic repulsive forces are also higher, because more thermal energy is provided to the system (Lee et al. 2012). On the contrary, at higher temperatures, the logarithm of the autoprotolysis constant of water, pKw, is lower. This is shown in Eq. (5) (Norgren 2001):

$[A-][HA]=α1-α=KaKw[OH-] (5)$(5)

where [HA] is the concentration of a protonated OHphen group and [A-] is the concentration of its corresponding base. At a certain pH, the net dissociation of lignin (α) is correlated to the quotient of Ka and Kw, and Kw may prevent the dissociation of OHphen groups at a higher temperature (higher Kw).

In the experiments, where the ion strength of BL was regulated by addition of sodium sulfate in amounts of 5–20% while the pH and temperature were constant (Figure 1), it is obvious that the lignin yield increases with increasing ion strength of the BL. The salt may influence the system in at least two different ways: the weak acid has a lower pKa value in the case of higher ionic strength than in the opposite case. On the contrary, there is also a charge screen of lignin, both in solid and dissolved state, in the presence of added salt, which decreases the electrostatic repulsion forces between lignin molecules. Our results can be interpreted that the latter has the greatest influence on the system as was done by Rudatin et al. (1989) and Norgren et al. (2001). Clearly, the precipitation pH has a stronger influence than the temperature and ion strength on the precipitation yield.

## Average MWt and MWtD

In agreement with previous findings, the average MWt of precipitated lignin decreases with increasing yield (Figure 1) (Zhu et al. 2013). The apparent pKa of lignin seems to change significantly with MWt, probably because the higher MWt fraction (without split β-O-4 linkages) contains less OHphen groups. Another reason could be the increased electrostatic attraction of hydrogen ions by the larger lignin molecule, which could also lead to the higher pKa (Norgren and Lindström 2000a,b). A combined explanation of these effects is most plausible.

There is a good general correlation between the MWt of lignin and the parameters investigated, while pH and temperature seem to have a rather similar effect, although the ion strength shows a slightly different behavior. The change in ion strength was not very large and experimental variations may have superimposed the small effects.

The MWtD of lignin obtained at various conditions are shown in Figure 2. The profiles based on RI and UV responses are similar, which may be indicative for lignin-carbohydrate complexes (LCC). Based on Figure 2a and c, there is a clear trend with respect to the influence of pH on the changes of lignin MWt. A weak influence of temperature is visible in Figure 2b and d. However, the influence of ion strength is not perceptible; thus, the curves are not shown.

Figure 2

MWtD of precipitated lignin at different levels of pH and temperature (T).

The variation of ion strength is not shown because no effect is visible. (See also Table 1.)

## Carbohydrate analysis

The concentrations of xylan and GGM in the filter cakes of lignin and filtrates are listed in Table 4 and illustrated in Figure 3. The concentration of xylan is higher than that of GGM because of the presence of hardwood xylan in the BL. Moreover, the majority of GGM is degraded during kraft pulping and only a trace amount is connected to the lignin after degradation. It can be observed (Figure 3a) that the amount of both carbohydrates in the lignin decreases with decreasing pH. At higher pH, the lignin is less degraded and has a high MWt. It can be assumed that, in the larger nondegraded lignin molecules, the carbohydrates are not degraded either.

Figure 3

Concentrations of xylan and GGM in the filter cakes and filtrates.

(a) 45°C without sodium added, (b) pH 10.5 without sodium added, and (c) 65°C and pH 10.5.

At a constant pH of 10.5, the concentration of xylan increases slightly with decreasing temperature (Figure 3b). These results are contradictory to the results presented in Figure 3a. This may be related to the different pKa values of the carboxyl groups in xylan and those of OHphen groups in lignin, but the explanation of these effects is waiting for investigation. As seen in Figure 3b and c, the ratio lignin/GGM remains at a constant low level both in the filter cakes and the filtrates. Accordingly, at a precipitation pH of 10.5, the amount of GGM linked to lignin is not affected strongly by changing temperature and ion strength.

## Quantitative NMR spectroscopy

The 13C NMR spectra of acetylated lignin precipitated are presented in Figure 4a. The chemical shift assignments are based on the literature (Mörck and Kringstad 1985; Pu and Ragauskas 2005) and summarized in Table 5, along with the integrated peak areas of the lignin moieties. The integral of aromatic region (102–160 ppm) was calibrated to six, which represents six aromatic carbons (Landucci et al. 1998; Ralph and Landucci 2010; Min et al. 2013; Wells et al. 2013). Quantitative evaluation of functional groups (OHphen and OMe) and carbohydrates was thus integrated related to this value (Robert 1992; Faix et al. 1994; Ralph and Landucci 2010; Choi and Faix 2011; Min et al. 2013).

Figure 4

13C NMR and 1H NMR spectra of the precipitated kraft lignins.

Table 5

Quantitative comparison of kraft lignin based on 13C-NMR (left) and 1H-NMR (right) spectra.

The signal of carbonyl groups within the acetoxy group (acetylated form of OHphen) is visible in the region δC 166–169.5 ppm. As seen in Table 5, the amount of OHphen increases from samples 1 to 5 and from samples 10 to 13, that is, in the order as the precipitation yield increases. The OHphen content decreases from samples 6 to 9 with decreasing precipitation yield. Table 3 shows that more low MWt lignin is precipitated in the case of higher lignin yields. This confirms the theoretical consideration that small lignin molecules have more OHphen groups as a result of split β-O-4 linkages (Gellerstedt and Lindfors 1984; Gellerstedt 2008).

The OMe signal is at δC 54–57.5 ppm. In Table 5, the amount of OMe groups seems to increase from samples 1 to 5 and from samples 10 to 13, that is, in the order of increasing precipitation yield. For samples 6–9, an OMe content with decreasing precipitation yield is observed. According to Table 3, the amount of OMe groups increases with decreasing MWt of the precipitated lignin, confirming the findings of Zhu et al. (2013), who also stated that precipitated hardwood lignins have lower MWt than softwood lignin. In this study, the S/G ratio was determined according to the literature (Mörck et al. 1988; Pu and Ragauskas 2005; Samuel et al. 2010) based on the peaks in the regions δC 102–109 and 110–121 ppm, which represent C2/C6 in syringyl and guaiacyl units in the precipitated lignin. In agreement with the literature, the S/G ratios of precipitated lignin with lower MWts are higher (Mörck et al. 1988; Toledano et al. 2010).

The xylan signals in the 13C NMR spectra are assigned (Kringstad and Roland 1983; Mörck et al. 1986); see Table 5. The results of the carbohydrate analysis by 13C NMR spectra are in accordance with those by HPAEC-PAD presented above, that is, the lignins precipitated at a higher pH have higher MWt and a higher xylan content.

A segment of 1H NMR spectra of lignins are depicted in Figure 4b, where phenolic acetate (δH≈2.3) and OMe (δH≈3.8) groups are in focus (Lundquist 1991, 1992a,b). The numbers of proton per aromatic ring (δH≈7) is assumed to be 2.5 (Li and Lundquist 1994) for kraft lignin; thus, the quantitative integration of acetylated phenolic and OMe groups can be calculated related to this value. A quantitative evaluation (Table 5) shows that the content of OHphen and OMe groups increases with increasing precipitation yield, a finding that is in agreement with the results obtained from 13C NMR spectra.

## Conclusions

The precipitation yield of lignin increases with decreasing precipitation pH and temperature and with increasing ion strength of the BL. The amount of low MWt lignin increases with a higher precipitation yield. The pH of precipitation is the most relevant parameter, while the influence of temperature is less important and the influence of ion strength was hardly observable. The content of xylan in the precipitated lignin decreases with decreasing pH or increasing temperature. The GGM content in the lignin precipitated at pH 10.5 is not affected by temperature or ion strength. According to 13C and 1H NMR analysis, the amount of phenolic groups is higher in samples with increasing yield. The same is true for the OMe content of the lignins because their S/G ratio is also increasing, that is, the syringyl moiety of low MWt hardwood lignins is precipitated mainly under more harsh conditions.

## Acknowledgments

1H and 13C NMR spectroscopy was performed by Mr. Göran Karlsson and Mr. Maxim Mayzel at The Swedish NMR Centre, Gothenburg, Sweden. Chalmers Energy Initiative is gratefully acknowledged for their financial support.

## Appendix

The contents of cellulose, GGM, and xylan were calculated after carbohydrate analysis by means of the following assumptions/corrections: anhydro sugars were calculated from sugar monomers by the withdrawal of water (multiplication by 0.88 in the case of pentosans and 0.90 in the case of hexosans). Glucomannan was calculated as the sum of galactan, mannan, and part of the glucan. The molar ratio between the mannose and the glucose in GGM was assumed to be 3.5:1 (Meier 1958). All galactan measured was included in GGM. Acetyl groups were, however, not included. Xylan was calculated as the sum of xylan and arabinan. All arabinan measured was included in the xylan. Cellulose was calculated as the content of glucan after withdrawal for the contribution of glucan to GGM. Cellulose=Glc-(1/3.5)×Man; GGM=Gal+[1+(1/3.5)]×Man; Xylan=Xyl+Ara. The analyses were summed up in a mass balance under the assumption that the carbohydrates consist of cellulose, GGM, and xylan, which were calculated as described above.

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Corresponding author: Hans Theliander, Forest Products and Chemical Engineering, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE 41296 Gothenburg, Sweden; and Wallenberg Wood Science Center, Chalmers University of Technology/Royal Institute of Technology Kemigården 4, SE 41296 Gothenburg, Sweden e-mail:

Accepted: 2014-05-15

Published Online: 2014-06-15

Published in Print: 2015-02-01

Citation Information: Holzforschung, Volume 69, Issue 2, Pages 143–152, ISSN (Online) 1437-434X, ISSN (Print) 0018-3830,

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