Abstract
In an attempt to evaluate the effects of thermal treatment on wood cell walls (CWs), Masson pine (Pinus massoniana Lamb.) wood was thermally modified (TM) at 150, 170 and 190°C for 2, 4 and 6 h, respectively. The chemical properties, cellulose crystallinity (CrI) and micromechanics of the control and thermally modified wood (TMW) were analyzed by wet chemical analysis, X-ray diffraction and nanoindentation. The relative lignin content and CrI increased after the TM partly degraded the amorphous wood polymers. The relative lignin content was higher in TMW and the equilibrium moisture content decreased. Moreover, the elastic modulus (Er) and hardness (H) of TMW were lowered along with the creep ratio decrement (CIT) of CWs. However, a severe treatment (e.g. 190°C/6 h) may negatively affect the mechanical properties of CWs caused by the partial degradation of hemicelluloses and also cellulose.
Introduction
Hygroscopicity and low durability are limitations of the outdoor utilization of wood. At the level of chemical composition, amorphous hemicelluloses with their accessible OH groups are mainly responsible for hygroscopicity (Hosseinaei et al. 2012; Özlem et al. 2017). To mitigate the effects of hydrophilicity, several modification methods have been developed, such as acetylation and other chemical modifications (Rowell et al. 1990; Hill and Ormondroyd 2004; Militz and Lande 2009; Ringman et al. 2014; Himmel and Mai 2015, 2016; Moghaddam et al. 2016; Beck et al. 2017a,b; Joffre et al. 2017), chemical impregnation (Xie et al. 2010; Mattos et al. 2015) and thermal modification (TM) (Bourgois et al. 1989; Cademartori et al. 2013; Altgen et al. 2016; Hosseinpourpia et al. 2017; Li et al. 2017a,b). Above all, TM is an eco-friendly and effective approach to increase the resistance toward microorganisms and dimensional stability.
Degradation of hemicelluloses already begins at 120°C (Poncsák et al. 2007), but significant degradation with positive effects occurs above 180°C (Sivonen et al. 2002; Hakkou et al. 2006; Wang et al. 2016; Özlem et al. 2017). However, some mechanical properties of wood are reduced by the disruption of hydrogen bonds during TM at 180°C (Mburu et al. 2008). The mechanical properties of thermally modified wood (TMW) have been intensively analyzed at the macro scale (Gunduz et al. 2009; Kačíková et al. 2013; Tomak et al. 2014). On the other hand, micromechanical parameters of TMW have been less frequently investigated. Nanoindentation (NI) techniques are well suited to this purpose and also in TMW (Oliver and Pharr 1992; Wimmer et al. 1997; Stanzl-Tschegg et al. 2009; Rautkari et al. 2014; Wang et al. 2014).
In the present paper, the effects of TM on the chemical, physical and micromechanical properties of wood were studied. The focus is, in particular, on the interrelationships among the investigated properties, and the changes in cellulose crystallinity (CrI), equilibrium moisture content (EMC) and mechanical behavior at a cell-structure level were observed via NI of the secondary wall as a function of TM parameters.
Materials and methods
Materials
Masson pine (Pinus massoniana Lamb.) wood (35 years old) was obtained from Fujian, China. Defect-free wood samples with the dimensions of 20×20×20 mm3 in tangential (T), radial (R) and longitudinal (L) directions, respectively, were cut around the same latewood growth ring from the sapwood and then were conditioned at 65±3% relative humidity (RH) at 20±2°C until they reached a moisture content (MC) of ≈12%.
Thermal modification (TM)
Samples were oven-dried at 80°C until weight constancy was achieved. Samples were randomly divided into nine treatment groups in addition to one group defined as control samples. Fifteen replicate samples in each treatment group were treated under atmospheric pressure in an oven (Thermo Scientific, Asheville, NC, USA): N2-flow 20 ml min−1 at 150, 170 and 190°C for 2, 4 and 6 h, respectively. The TMWs were then cooled down to room temperature inside the conditioning chamber and further conditioned at 20°C and 65% RH prior to testing.
Mass loss (ML)
The % ML following TM was measured using the oven-drying method.
where m0 and m1 are the oven dry sample mass (constant mass after drying at 80°C) before and after TM, respectively.
Chemical composition
The control and TMW powders were prepared for analysis. Holocellulose was determined according to Wise et al. (1946) (sodium chlorite method). Cellulose was determined according to Kürschner-Hoffner (nitric acid method, see Browning 1967), and the lignin content was determined as acid-insoluble Klason lignin (obtained via 72% H2SO4 plus dilute acid hydrolysis), according to the GB/T 2677.8 (1994) standard method. Hemicellulose content was calculated by subtraction, i.e. holocellulose minus cellulose. All chemical reagents used were of analytical grade (Jiuyi chemicals, Shanghai, China).
X-ray diffraction (XRD):
The control and TMWs were ground to obtain 40–60 mesh powders. The crystalline structures were analyzed using a Uitima IV X-ray diffractometer (XRD, Rigaku, Tokyo, Japan); CuKα radiation with a monochromator, 40 kV, electric current 40 mA and diffractograms range 5–40° at a scanning rate of 0.4°·min−1. The relative degree of crystallinity (CrI) was calculated according to Segal et al. (1959):
where I002 represents the intensity of the 200 crystalline peaks and Iam represents the intensity of the diffraction of the amorphous part.
Moisture absorption and dimensional stability
These properties were measured according to GB/T 1931 (2009) and GB/T 1934.2 (2009). Samples were dried at 103°C until a constant mass and volume were achieved. The specimens were kept at 20±2°C and 65±3% RH. The EMC after moisture absorption was calculated:
where m0 is the initial mass of an oven-dried sample and m2 is the mass after equalization. For evaluating the dimensional stability, the anti-swelling efficiency (ASE) was obtained:
where Sc is the volumetric swelling coefficient of control samples and St is the thermal treated volumetric swelling coefficient. The volumetric swelling coefficient was calculated:
where V0 is the initial volume of an oven-dried sample and V is the volume after equalization at 20±2°C and 65±3% RH.
Nanoindentation (NI)
Wood samples were cut into smaller blocks of 5×5×10 mm3 (T×R×L) and the transverse section of the blocks was polished using a diamond knife (Micro Star Inc., TX, USA) in conjunction with ultramicrotomy (Leica Microsystems Inc., Buffalo Grove, IL, USA) until the average roughness of the surface was less than 10 nm. The samples were prepared without embedded epoxy resin as suggested by Meng et al. (2012). NI was performed on a Hysitron TriboIndenter system (Hysitron Inc., Minneapolis, MN, USA), equipped with scanning probe microscopy (SPM) and a Berkovich indenter. All samples were kept for at least 24 h at 20°C and 50±3% RH before testing. The targeted tracheid cell walls (CWs) located at the same annual ring were chosen for NI to measure the elastic modulus (Er) (MOE) and hardness (H), shown in Figure 1. Indentations were performed on the S2 layer in the open-loop mode based on the three-segment load ramp: load application within 5 s hold time 5 s, and unload time 5 s. The peak load was 400 μN for all indents in the experiment (Figure 2a). Finally, the samples were examined by SPM again to evaluate the quality of the indents, shown in Figure 1e–h. About 30 indents were analyzed, which were placed in valid positions (without apparent defects, corner of CW, etc.) on the transverse section of six to 10 CWS for each sample.
Microscope images showing the positioning of indents.
(a–d) Incident light micrograph of the transverse section of wood samples; (e) and (f) SPM images of wood cell walls after NI.
Typical nanoindentation curves of wood cell walls.
(a) Load-depth curves and (b) depth-time curves.
H and reduced MOE (Er)
Based on the method of Oliver and Pharr (1992), the H and Er can be calculated:
where Pmax is the peak load, and A is the projected contact area of the indents at peak load.
where Er is the combined elastic modulus, S is the initial unloading stiffness and β is a correction factor correlated to indenter geometry (β=1.034) (Xu and Li 2008).
Creep behavior
The creep behavior is described by the indentation creep ratio (CIT), which is defined as the relative change in indentation depth, when the applied load remained constant during the holding stage (Figure 2b).
where h1 and h2 are the initial and final indentation depths at the load holding stage, respectively.
Results and discussion
Mass loss (ML)
The % ML data are presented in Figure 3, which ranged between 0.55 and 18.1% and increase as a function of TM temperature and duration. Below 150°C, there were no ML changes, which begins to be significant at 170°C. As is known, hemicelluloses degrade first and release acetic acid (AA) (González-peña et al. 2009). The AA concentration increases with increasing temperature and TM duration, while AA as a hydrolysis catalyst accelerates the process considerably (Mosier et al. 2005; Hosseinaei et al. 2011).
Effects of thermal treatment on the mass loss of wood.
Main chemical components
As is visible in Table 1, the cellulose has a better thermal stability than hemicelluloses. TMW190°C,6 h shows a relative hemicellulose content of 9.4%, i.e. it is a decrement of 40% compared to the control. Xylan is especially sensitive to degradation and dehydration reactions. The relative lignin content increment from 28.4 to 33.9% was the highest in TMW190°C,6 h, which was partly due to the gravimetric hemicelluloses degradation, but the conversion products of polysaccharides may also form acid-insoluble residues, which appear as a Klason residue (Boonstra and Tjeerdsma 2006; Wang et al. 2016). Beginning with a TM of 190°C for 6 h, the paracrystalline moiety of cellulose also showed modification tendencies.
Main chemical components of Masson wood as a function of TM parameters temperature and treatment time.
| Temp. | Cellulose (%) | Hemicelluloses (%) | Lignin (%) | ||||||
|---|---|---|---|---|---|---|---|---|---|
| 2 h | 4 h | 6 h | 2 h | 4 h | 6 h | 2 h | 4 h | 6 h | |
| Control | 49.2 | 15.9 | 28.4 | ||||||
| 150°C | 49.3 | 49.3 | 49.2 | 14.8 | 14.0 | 13.6 | 28.4 | 28.5 | 29.0 |
| 170°C | 47.7 | 47.1 | 46.1 | 13.5 | 12.5 | 11.4 | 28.8 | 30.2 | 31.8 |
| 190°C | 45.7 | 43.2 | 42.1 | 11.5 | 10.1 | 9.4 | 29.5 | 31.0 | 33.9 |
X-ray diffraction (XRD)
The peaks in the XRD patterns in Figure 4a show maxima at 14.8°, 16.5°, 22.2° and 34.6°, corresponding to (11̅0), (110), (200) and (040) of the crystallographic plane of cellulose, respectively (French 2014; Wei et al. 2015; Guo et al. 2016). The peak intensities and their broadening effects differ from one sample to another. The most pronounced difference occurs at the maximum of 22.2° corresponding to cellulose I crystalline structure. After TM, there is an intensity increment of the (200) reflection, indicating a higher crystallization degree (CrI). Interestingly, the intensity of diffraction peak at 34.6° is stable, which can be interpreted as that the glucan chains of cellulose are largely unaffected.
XRD analysis of control and TMWs.
(a) XRD spectra and (b) the degree of crystallinity.
The relative degree of crystallinity (CrI) was calculated according to Eq. (2) and the data are presented in Figure 4b. Apparently, TMW (except TMW150°C,2 h) have a higher CrI than that of the control, i.e. CrI increased from 55.5% (control) to 63.3% (TMW190°C,6 h), which is a 14% increment. The literature reports similar observations (Li et al. 2017a,b). CrI increment is partly due to the degradation of hemicelluloses and partly due to the rearrangement of paracrystalline regions of cellulose toward higher crystallinity (Xing et al. 2016; Yin et al. 2017).
EMC and ASE
Figure 5a shows the EMC of the control and TMWs, exposed to a humid environment. Expectedly, the EMC lowers with increasing severity of TM, while the TM time has a greater impact on the EMC than the TM temperature. The EMC of the control decreased from 11.3 to 6.6% in TMW190°C,6 h, and this is, as discussed already, due to the elimination of the OH groups of hemicelluloses (Altgen et al. 2016). The ASE values are presented in Figure 5b. The improvement of ASE is positively correlated to the TM severity. The ASE values of TMW are especially excellent, if TM temperature exceeds 170°C. The ASEmax of 56% was observed in TMW190°C,6 h.
Moisture absorption and dimensional stability of the control and TMWs determined at 65% RH and 20°C.
(a) Equilibrium moisture content (EMC) and (b) anti-swelling efficiency (ASE).
NI H and reduced MOE
The average data concerning the reduced MOE (Er) and H in the secondary CWs are summarized in Figure 6, which show the positive effects of TM. It is well known that MC has a negative effect on the stiffness of CWs below the fiber saturation point (FSP) (Yu et al. 2011; Li et al. 2013; Xing et al. 2016). The CWs of TMW150°C exhibited a slight increase in Er and H compared to the control with average values of 11.2 and 0.37 GPa, respectively, which is attributable to the reduced EMC. TMW170°C shows significant stiffness increment as a function of treatment severity. For instance, the Er and H values of CWS TMW170°C,6 h increased by about 77 and 58%, respectively, probably as a result of lignin condensation via cross-linking reactions with furfural set free from hemicelluloses (Gindl and Schoberl 2004; Wang et al. 2014). The increased crystallinity index of cellulose may have also contributed to this effect. However, a decreasing trend shows Er of CWs TMW190°C for long periods. For example, in the case of CWs TMW190°C,6 h, Er decreased by about 6% as compared to that of TMW170°C. A severe treatment at 190°C for 6 h not only induces the degradation of hemicelluloses, but also damages the crystalline cellulose. The Er decrement is obvious in view of the importance of cellulose on the MOE in the L direction (Bergander and Salmen 2000; Wang et al. 2016).
Mechanical properties for the control and TMWs.
(a) Reduced elastic modulus (Er) and (b) hardness (H).
Indentation CIT
The creep behavior of wood is an important performance index, when large wooden structures are submitted to long-term loading. The effects of TM on the creep behavior of CWs were analyzed with a constant load for 5 s after ending the loading stage. As is illustrated in Figure 7, the indentation CIT of the control CWs was 8.4%, which decreased after TM, especially above 170°C. The CWs in TMW190°C,6 h showed the lowest CIT, which is equivalent to a 16% decrement against the control, which means that the short-term creep resistance of the CWs improved. The increased moisture resistant capability, relative lignin content and crystallinity increment due to the degradation of hemicelluloses are the main reasons for CIT increment (Keckes et al. 2003; Inari et al. 2007; Wang et al. 2016; Zhan et al. 2018a,b). Above 170°C, the temperature and modification time have little effect on the creep resistance of CWs.
Nanoindentation creep ratio for the control and thermal treated wood samples.
Conclusions
The results confirm the literature data concerning the effects of TM above 170°C, which leads to the lowering of free hydroxyl groups via degradation of hemicelluloses. As a consequence, the EMC is decreased and the dimensional stability of wood is improved. NI led to new insights into the changes at the CWs level caused by TM. Increased crystallinity and higher condensation degree of lignin are other influential factors measured by XRD and NI, respectively. Severe TM conditions at 190°C during 6 h show a negative effect on the mechanical properties of CWs’ TMW.
Funding source: China Postdoctoral Science Foundation
Award Identifier / Grant number: 2016M600418
Funding statement: The authors are grateful for the project funded by Natural Science Foundation of China (Funder Id: 10.13039/501100001809, No. 31570552), China Postdoctoral Science Foundation (Funder Id: 10.13039/501100002858, 2016M600418) and the Key University Science Research Project of Jiangsu Province (17KJA220004), Innovation Project of Jiangsu Province (CX173028) and Tennessee Experimental Station Project #TEN00422. The authors would also like to thank Mrs. Suyong Huang for assisting with the TriboIndenter instrument.
Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Employment or leadership: None declared.
Honorarium: None declared.
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