Effects of Thermal Modification on the Physical, Chemical And

Holzforschung 2018; 72(12): 1063–1070

Xinzhou Wang, Xuanzong Chen, Xuqin Xie, Yan Wu, Linguo Zhao, Yanjun Li* and Siqun Wang* Effects of thermal modification on the physical, chemical and micromechanical properties of Masson pine wood (Pinus massoniana Lamb.) https://doi.org/10.1515/hf-2017-0205 Received December 10, 2017; accepted June 12, 2018; previously Introduction published online July 10, 2018 Hygroscopicity and low durability are limitations of the Abstract: In an attempt to evaluate the effects of thermal outdoor utilization of wood. At the level of chemical com- treatment on wood cell walls (CWs), Masson pine (Pinus position, amorphous hemicelluloses with their accessible massoniana Lamb.) wood was thermally modified (TM) at OH groups are mainly responsible for hygroscopicity (Hos- 150, 170 and 190°C for 2, 4 and 6 h, respectively. The chem- seinaei et al. 2012; Özlem et al. 2017). To mitigate the effects ical properties, cellulose crystallinity (CrI) and microme- of hydrophilicity, several modification methods have been chanics of the control and thermally modified wood (TMW) developed, such as acetylation and other chemical modi- were analyzed by wet chemical analysis, X-ray diffraction fications (Rowell et al. 1990; Hill and Ormondroyd 2004; and nanoindentation. The relative lignin content and CrI Militz and Lande 2009; Ringman et al. 2014; Himmel and increased after the TM partly degraded the amorphous Mai 2015, 2016; Moghaddam et al. 2016; Beck et al. 2017a,b; wood polymers. The relative lignin content was higher in Joffre et al. 2017), chemical impregnation (Xie et al. 2010; TMW and the equilibrium moisture content decreased. Mattos et al. 2015) and thermal modification (TM) (Bour-

Moreover, the elastic modulus (Er) and hardness (H) of gois et al. 1989; Cademartori et al. 2013; Altgen et al. 2016; TMW were lowered along with the creep ratio decrement Hosseinpourpia et al. 2017; Li et al. 2017a,b). Above all,

(CIT) of CWs. However, a severe treatment (e.g. 190°C/6 h) TM is an eco-friendly and effective approach to increase may negatively affect the mechanical properties of CWs the resistance toward microorganisms and dimensional caused by the partial degradation of hemicelluloses and stability. also cellulose. Degradation of hemicelluloses already begins at 120°C (Poncsák et al. 2007), but significant degradation Keywords: cell wall, cellulose crystallinity, chemical with positive effects occurs above 180°C (Sivonen et al. components, masson pine wood, nanoindentation (NI), 2002; Hakkou et al. 2006; Wang et al. 2016; Özlem et al. thermal modification 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 *Corresponding authors: Yanjun Li, College of Materials Science suited to this purpose and also in TMW (Oliver and Pharr and Engineering, Nanjing Forestry University, Nanjing 210037, P.R. 1992; Wimmer et al. 1997; Stanzl-Tschegg et al. 2009; China, e-mail: [email protected]; and Siqun Wang, College of Materials Rautkari et al. 2014; Wang et al. 2014). Science and Engineering, Nanjing Forestry University, Nanjing 210037, P.R. China; and Center for Renewable Carbon, University of In the present paper, the effects of TM on the chemi- Tennessee, Knoxville, TN 37996, USA, e-mail: [email protected] cal, physical and micromechanical properties of wood Xinzhou Wang, Xuanzong Chen and Yan Wu: College of Materials were studied. The focus is, in particular, on the interre- Science and Engineering, Nanjing Forestry University, Nanjing lationships among the investigated properties, and the 210037, P.R. China changes in cellulose crystallinity (C I), equilibrium mois- Xuqin Xie: Dehua Tubao New Decoration Material Co., Ltd., Zhejiang r 313200, P.R. China ture content (EMC) and mechanical behavior at a cell- Linguo Zhao: College of Chemistry, Nanjing Forestry University, structure level were observed via NI of the secondary wall Nanjing 210037, P.R. China as a function of TM parameters. 1064 X. Wang et al.: Thermal modification of Masson pine

cellulose. All chemical reagents used were of analytical grade (Jiuyi Materials and methods chemicals, Shanghai, China).

Materials: Masson pine (Pinus massoniana Lamb.) wood (35 years X-ray diffraction (XRD): The control and TMWs were ground to old) was obtained from Fujian, China. Defect-free wood samples with obtain 40–60 mesh powders. The crystalline structures were ana- the dimensions of 20 × 20 × 20 mm3 in tangential (T), radial (R) and lyzed using a Uitima IV X-ray diffractometer (XRD, Rigaku, Tokyo, longitudinal (L) directions, respectively, were cut around the same Japan); CuKα radiation with a monochromator, 40 kV, electric cur- latewood growth ring from the sapwood and then were conditioned rent 40 mA and diffractograms range 5–40° at a scanning rate of at 65 ± 3% relative humidity (RH) at 20 ± 2°C until they reached a 0.4° · min−1. The relative degree of crystallinity (C I) was calculated moisture content (MC) of ≈12%. r according to Segal et al. (1959):

Thermal modification (TM): Samples were oven-dried at 80°C until Cr0I (%)1=×00 (I 02 −I)am /I002 , (2) weight constancy was achieved. Samples were randomly divided into where I represents the intensity of the 200 crystalline peaks and nine treatment groups in addition to one group defined as control 002 I represents the intensity of the diffraction of the amorphous samples. Fifteen replicate samples in each treatment group were am part. treated under atmospheric pressure in an oven (Thermo Scientific, Asheville, NC, USA): N -flow 20 ml min−1 at 150, 170 and 190°C for 2, 4 2 These properties and 6 h, respectively. The TMWs were then cooled down to room tem- Moisture absorption and dimensional stability: were measured according to GB/T 1931 (2009) and GB/T 1934.2 (2009). perature inside the conditioning chamber and further conditioned at Samples were dried at 103°C until a constant mass and volume were 20°C and 65% RH prior to testing. achieved. The specimens were kept at 20 ± 2°C and 65 ± 3% RH. The EMC after moisture absorption was calculated: Mass loss (ML): The % ML following TM was measured using the oven-drying method. EMC (%)1=−00 (mm20)/m0 , (3) =− % ML 100 ()mm01/,m0 (1) where m0 is the initial mass of an oven-dried sample and m2 is the mass after equalization. For evaluating the dimensional stability, the where m and m are the oven dry sample mass (constant mass after 0 1 anti-swelling efficiency (ASE) was obtained: drying at 80°C) before and after TM, respectively.

ASE (%)1=×00 (/SSct− ) Sc , (4) Chemical composition: The control and TMW powders were pre- where S is the volumetric swelling coefficient of control samples and pared for analysis. Holocellulose was determined according to Wise c S is the thermal treated volumetric swelling coefficient. The volumet- et al. (1946) (sodium chlorite method). Cellulose was determined t ric swelling coefficient was calculated: according to Kürschner-Hoffner (nitric acid method, see Brown- ing 1967), and the lignin content was determined as acid-insoluble SV (%) =×100(−VV00)/ , (5) Klason lignin (obtained via 72% H2SO4 plus dilute acid hydrolysis), according to the GB/T 2677.8 (1994) standard method. Hemicellu- where V0 is the initial volume of an oven-dried sample and V is the lose content was calculated by subtraction, i.e. holocellulose minus volume after equalization at 20 ± 2°C and 65 ± 3% RH.

Figure 1: 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. X. Wang et al.: Thermal modification of Masson pine 1065

Nanoindentation (NI): Wood samples were cut into smaller blocks of where h1 and h2 are the initial and final indentation depths at the load 5 × 5 × 10 mm3 (T × R × L) and the transverse section of the blocks was holding stage, respectively. polished using a diamond knife (Micro Star Inc., TX, USA) in conjunc- tion 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 Results and discussion suggested by Meng et al. (2012). NI was performed on a Hysitron Tri- boIndenter system (Hysitron Inc., Minneapolis, MN, USA), equipped with scanning probe microscopy (SPM) and a Berkovich indenter. All Mass loss (ML) 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 The % ML data are presented in Figure 3, which ranged ring were chosen for NI to measure the elastic modulus (Er) (MOE) and hardness (H), shown in Figure 1. Indentations were performed on the between 0.55 and 18.1% and increase as a function of TM

S2 layer in the open-loop mode based on the three-segment load ramp: temperature and duration. Below 150°C, there were no load application within 5 s hold time 5 s, and unload time 5 s. The peak ML changes, which begins to be significant at 170°C. As load was 400 μN for all indents in the experiment (Figure 2a). Finally, is known, hemicelluloses degrade first and release acetic the samples were examined by SPM again to evaluate the quality of the acid (AA) (González-peña et al. 2009). The AA concentra- indents, shown in Figure 1e–h. About 30 indents were analyzed, which were placed in valid positions (without apparent defects, corner of CW, tion increases with increasing temperature and TM dura- etc.) on the transverse section of six to 10 CWS for each sample. tion, while AA as a hydrolysis catalyst accelerates the process considerably (Mosier et al. 2005; Hosseinaei et al.

H and reduced MOE (Er): Based on the method of Oliver and Pharr 2011).

(1992), the H and Er can be calculated:

H = PAmax /, (6) where Pmax is the peak load, and A is the projected contact area of the Main chemical components indents at peak load.

π S As is visible in Table 1, the cellulose has a better thermal Er = , (7) 2 β A stability than hemicelluloses. TMW190°C,6 h shows a relative hemicellulose content of 9.4%, i.e. it is a decrement where E is the combined elastic modulus, S is the initial unloading r of 40% compared to the control. Xylan is especially sensi- stiffness and β is a correction factor correlated to indenter geometry (β = 1.034) (Xu and Li 2008). tive to degradation and dehydration reactions. The relative lignin content increment from 28.4 to 33.9% was the

Creep behavior: The creep behavior is described by the indentation highest in TMW190°C,6 h, which was partly due to the gravi- creep ratio (CIT), which is defined as the relative change in indenta- metric hemicelluloses degradation, but the conversion tion depth, when the applied load remained constant during the products of polysaccharides may also form acid-insoluble holding stage (Figure 2b). residues, which appear as a Klason residue (Boonstra and

ChIT = 100 ()21−hh/ 1 , (8) Tjeerdsma 2006; Wang et al. 2016). Beginning with a TM of

Figure 2: Typical nanoindentation curves of wood cell walls. (a) Load-depth curves and (b) depth-time curves. 1066 X. Wang et al.: Thermal modification of Masson pine

Figure 3: Effects of thermal treatment on the mass loss of wood.

Table 1: Main chemical components of Masson wood as a function of TM parameters temperature and treatment time.

Cellulose (%) Hemicelluloses (%) Lignin (%)

Temp. 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 Figure 4: XRD analysis of control and TMWs. (a) XRD spectra and (b) the degree of crystallinity.

190°C for 6 h, the paracrystalline moiety of cellulose also showed modification tendencies. 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.

X-ray diffraction (XRD) 2017a,b). CrI increment is partly due to the degradation of hemicelluloses and partly due to the rearrangement of The peaks in the XRD patterns in Figure 4a show maxima paracrystalline regions of cellulose toward higher crystal- at 14.8°, 16.5°, 22.2° and 34.6°, corresponding to (11̅0), linity (Xing et al. 2016; Yin et al. 2017). (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 EMC and ASE effects differ from one sample to another. The most pro- nounced difference occurs at the maximum of 22.2° cor- Figure 5a shows the EMC of the control and TMWs, responding to cellulose I crystalline structure. After TM, exposed to a humid environment. Expectedly, the EMC there is an intensity increment of the (200) reflection, lowers with increasing severity of TM, while the TM time indicating a higher crystallization degree (CrI). Interest- has a greater impact on the EMC than the TM tempera- ingly, the intensity of diffraction peak at 34.6° is stable, ture. The EMC of the control decreased from 11.3 to 6.6% which can be interpreted as that the glucan chains of cel- in TMW190°C,6 h, and this is, as discussed already, due to the lulose are largely unaffected. elimination of the OH groups of hemicelluloses (Altgen

The relative degree of crystallinity (CrI) was calculated et al. 2016). The ASE values are presented in Figure 5b. according to Eq. (2) and the data are presented in Figure 4b. The improvement of ASE is positively correlated to the TM

Apparently, TMW (except TMW150°C,2 h) have a higher CrI severity. The ASE values of TMW are especially excellent, X. Wang et al.: Thermal modification of Masson pine 1067

Figure 6: Mechanical properties for the control and TMWs.

(a) Reduced elastic modulus (Er) and (b) hardness (H).

of lignin condensation via cross-linking reactions with furfural set free from hemicelluloses (Gindl and Schoberl Figure 5: Moisture absorption and dimensional stability of the 2004; Wang et al. 2014). The increased crystallinity index control and TMWs determined at 65% RH and 20°C. of cellulose may have also contributed to this effect. (a) Equilibrium moisture content (EMC) and (b) anti-swelling efficiency However, a decreasing trend shows Er of CWs TMW190°C for (ASE). 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. if TM temperature exceeds 170°C. The ASEmax of 56% was A severe treatment at 190°C for 6 h not only induces the observed in TMW190°C,6 h. degradation of hemicelluloses, but also damages the crys-

talline cellulose. The Er decrement is obvious in view of the importance of cellulose on the MOE in the L direction NI H and reduced MOE (Bergander and Salmen 2000; Wang et al. 2016).

The average data concerning the reduced MOE (Er) and H in the secondary CWs are summarized in Figure 6, which Indentation CIT show the positive effects of TM. It is well known that MC has a negative effect on the stiffness of CWs below the The creep behavior of wood is an important performance fiber saturation point (FSP) (Yu et al. 2011; Li et al. 2013; index, when large wooden structures are submitted to long-

Xing et al. 2016). The CWs of TMW150°C exhibited a slight term loading. The effects of TM on the creep behavior of increase in Er and H compared to the control with average CWs were analyzed with a constant load for 5 s after ending values of 11.2 and 0.37 GPa, respectively, which is attribut- the loading stage. As is illustrated in Figure 7, the inden- able to the reduced EMC. TMW170°C shows significant stiff- tation CIT of the control CWs was 8.4%, which decreased ness increment as a function of treatment severity. For after TM, especially above 170°C. The CWs in TMW190°C,6 h instance, the Er and H values of CWS TMW170°C,6 h increased showed the lowest CIT, which is equivalent to a 16% decre- by about 77 and 58%, respectively, probably as a result ment against the control, which means that the short-term 1068 X. Wang et al.: Thermal modification of Masson pine

Employment or leadership: None declared. Honorarium: None declared.

References

Altgen, M., Hofmann, T., Militz, H. (2016) Wood moisture content during the thermal modification process affects the improvement in hygroscopicity of Scots pine sapwood. Wood Sci. Technol. 50:1–15. Beck, G., Strohbusch, S., Larnøy, E., Militz, H., Hill, C. (2017a) Accessibility of hydroxyl groups in anhydride modified wood Figure 7: Nanoindentation creep ratio for the control and thermal as measured by deuterium exchange and saponification. Holz- treated wood samples. forschung 72:17–23. Beck, G., Thybring, E.E., Thygesen, L.G., Hill, C. (2017b) Characteri- zation of moisture in acetylated and propionylatedradiata pine creep resistance of the CWs improved. The increased mois- using low-field nuclear magnetic resonance (LFNMR) relaxom- ture resistant capability, relative lignin content and crystal- etry. Holzforschung 72:225–233. linity increment due to the degradation of hemicelluloses Bergander, A., Salmén, L. (2000) The transverse elastic modulus of the native wood fibre wall. J. Pulp Pap. Sci. 26:234–238. are the main reasons for CIT increment (Keckes et al. 2003; Inari et al. 2007; Wang et al. 2016; Zhan et al. 2018a,b). Boonstra, M., Tjeerdsma, B. (2006) Chemical analysis of heat treated softwoods. Holz Roh Werkst. 64:204–211. ° Above 170 C, the temperature and modification time have Bourgois, J., Bartholin, M.C., Guyonnet, R. (1989) Thermal treatment little effect on the creep resistance of CWs. of wood: analysis of the obtained product. Wood Sci. Technol. 23:303–310. Browning, B.L. Method of Wood Chemistry, Vol. II., Conclusions Interscience/Wiley, New York, USA, 1967. DOI: 10.1002/ pol.1968.160061112. Cademartori, P.H.G., Santos, P.S.B.D., Serrano, L., Labidi, J., Gatto, The results confirm the literature data concerning the D.A. (2013) Effect of thermal treatment on physicochemi- effects of TM above 170°C, which leads to the lowering of cal properties of Gympie messmate wood. Ind. Crops Prod. free hydroxyl groups via degradation of hemicelluloses. 45:360–366. As a consequence, the EMC is decreased and the dimen- French, A.D. (2014) Idealized powder diffraction patterns for cel- sional stability of wood is improved. NI led to new insights lulose polymorphs. Cellulose 21:885–896. GB/T 2667.8 (1994) Fibrous raw material-Determination of acid- into the changes at the CWs level caused by TM. Increased insoluble lignin. Standardization Administration of the Peo- crystallinity and higher condensation degree of lignin are ple’s Republic of China, Beijing. other influential factors measured by XRD and NI, respec- GB/T 1931 (2009) Method for determination of moisture content of tively. Severe TM conditions at 190°C during 6 h show a wood. Standardization Administration of the People’s Republic negative effect on the mechanical properties of CWs’ TMW. of China, Beijing. GB/T 1934.2 (2009) Method for determination of swelling of wood. Standardization Administration of the People’s Republic of Acknowledgments: The authors are grateful for the project China, Beijing. funded by Natural Science Foundation of China (Funder Id: Gindl, W., Schoberl, T. (2004) The significance of the elastic modu- 10.13039/501100001809, No. 31570552), China Postdoctoral lus of wood cell walls obtained from nanoindentation measure- Science Foundation (Funder Id: 10.13039/501100002858, ments. Compos. Part A. 35:1345–1349. 2016M600418) and the Key University Science Research González-peña, M.M., Curling, S.F., Hale, M.D.C. (2009) On the effect of heat on the chemical composition and dimensions Project of Jiangsu Province (17KJA220004), Innovation of thermally-modified wood. Polym. Degrad. Stabil. 94: Project of Jiangsu Province (CX173028) and Tennessee 2184–2193. Experimental Station Project #TEN00422. The authors Gunduz, G., Aydemir, D., Karakas, G. (2009) The effects of thermal would also like to thank Mrs. Suyong Huang for assisting treatment on the mechanical properties of wild pear (Pyruse- with the TriboIndenter instrument. laeagnifolia, Pall.) wood and changes in physical properties. Mater. Des. 30:4391–4395. Guo, J., Guo, X., Wang, S., Yin, Y. (2016) Effects of ultrasonic Author contributions: All the authors have accepted treatment during acid hydrolysis on the yield, particle size responsibility for the entire content of this submitted and structure of cellulose nanocrystals. Carbohydr. Polym. manuscript and approved submission. 135:248–255. X. Wang et al.: Thermal modification of Masson pine 1069

Hakkou, M., Pétrissans, M., Gérardin, P., Zoulalian, A. (2006) Inves- Militz, H., Lande, S. (2009) Challenges in wood modification tigations of the reasons for fungal durability of heat-treated technology on the way to practical applications. Wood Mater. beech wood. Polym. Degrad. Stab. 91:393–397. Sci. Eng. 4:23–29. Hill, C.A.S., Ormondroyd, G.A. (2004) Dimensional changes in Moghaddam, M., Wålinder, M.E.P., Claesson, P.M., Swerin, A. (2016) Corsican pine (Pinus nigra Arnold) modified with acetic anhy- Wettability and swelling of acetylated and furfurylated wood dride measured using a helium pycnometer. Holzforschung analyzed by multicycle Wilhelmy plate method. Holzforschung 51:493–547. 70:69–77. Himmel, S., Mai, C. (2015) Effects of acetylation and formalization Mosier, N., Wyman, C., Dale, B., Elander, R., Lee, Y.Y., Holtzapple, on the dynamic water vapor sorption behavior of wood. Holz- M., Ladisch, M. (2005) Features of promising technologies for forschung 69:633–643. pretreatment of lignocellulosic biomass. Bioresour. Technol. Himmel, S., Mai, C. (2016) Water vapour sorption of wood modi- 96:673–686. fied by acetylation and formalisation – analysed by a sorption Oliver, W.C., Pharr, G.M. (1992) An improved technique for determin- kinetics model and thermodynamic considerations. Holz- ing hardness and elastic modulus using load and displacement forschung 70:203–213. sensing indentation experiments. J. Mater. Res. 7:1564–1583. Hosseinaei, O., Wang, S., Rials, T.G., Xing, C., Zhang, Y. (2011) Özlem, Ö., Durmaz, S., Boyaci, I.H., Eksi-Kocak, H. (2017) Determina- Effects of decreasing carbohydrate content on properties of tion of chemical changes in heat-treated wood using ATR-FTIR wood strands. Cellulose 18:841–850. and FT Raman spectrometry. Spectrochim. Acta A 171:395–400. Hosseinaei, O., Wang, S., Enayati, A.A., Rials, T.G. (2012) Effects of Poncsák, S., Shi, S.Q., Kocaefe, D., Miller, G. (2007) Effect of thermal hemicellulose extraction on properties of wood flour and wood- treatment of wood lumbers on their adhesive bond strength plastic composites. Compos. Part A. 43:686–694. and durability. J. Adhes. Sci. Technol. 21:745–754. Hosseinpourpia, R., Adamopoulos, S., Holstein, N., Mai, C. (2017) Rautkari, L., Honkanen, J., Hill, C.A.S., Ridley-Ellis, D., Hughes, M. Dynamic vapour sorption and water-related properties of (2014) Mechanical and physical properties of thermally modi- thermally modified Scots pine (Pinus sylvestris, L.) wood pre- fied scots pine wood in high pressure reactor under satu- treated with proton acid. Polym. Degrad. Stab. 138:161–168. rated steam at 120, 150 and 180°C. Eur. J. Wood Wood Prod. Inari, G.N., Petrissans, M., Gerardin, P. (2007) Chemical reactivity of 72:33–41. heat-treated wood. Wood Sci. Technol. 41:157–168. Ringman, R., Pilgård, A., Brischke, C., Richter, K. (2014) Mode Joffre, T., Segerholm, K., Persson, C., Bardage, S.L., Hendriks, C.L.L., of action of brown rot decay resistance in modified wood: a Isaksson, P. (2017) Characterization of interfacial stress trans- review. Holzforschung 68:239–246. fer ability in acetylation-treated wood fibre composites using Rowell, R.M., Simonson, R., Tillman, A.M. (1990) Acetyl balance for x-ray microtomography. Ind. Crops Prod. 95:43–49. the acetylation of wood particles by a simplified procedure. Kačíková, D., Kačík, F., Cabalová, I., Durkovič, J. (2013) Effects of Holzforschung 44:263–270. thermal treatment on chemical, mechanical and colour traits in Segal, L., Creely, L., Martin, A.E., Conrad, C.M. (1959) An empirical Norway spruce wood. Bioresour. Technol. 144:669. method for estimating the degree of crystallinity of native cel- Keckes, J., Burgert, I., Frühmann, K., Müller, M., Kölln, K., Hamilton, lulose using X-ray diffractometer. Text. Res. J. 29:786–794. M., Burghammer, M., Roth, S.V., Stanzl-Tschegg, S., Fratzl, Sivonen, H., Maunu, S.L., Sundholm, F., Jämsä, S., Viitaniemi, P. P. (2003) Cell-wall recovery after irreversible deformation of (2002) Magnetic resonance studies of thermally modified wood. Nat. Mater. 2:810. wood. Holzforschung 56:648–654. Li, X., Wang, S., Du, G., Wu, Z., Meng, Y. (2013) Variation in physical Stanzl-Tschegg, S., Beikircher, W., Loidl, D. (2009) Comparison of and mechanical properties of hemp stalk fibers along height of mechanical properties of thermally modified wood at growth stem. Ind. Crops Prod. 42:344–348. ring and cell wall level by means of instrumented indentation Li, T., Cheng, D.L., Avramidis, S., Wålinder, M.E.P., Zhou, D.G. tests. Holzforschung 63:443–448. (2017a) Response of hygroscopicity to heat treatment and its Tomak, E.D., Ustaomer, D., Yildiz, S., Pesman, E. (2014) Changes in relation to durability of thermally modified wood. Constr. Build. surface and mechanical properties of heat treated wood during Mater. 144:671–676. natural weathering. Measurement 53:30–39. Li, Y., Huang, C., Wang, L., Wang, S., Wang, X. (2017b) The effects Wang, X., Deng, Y., Wang, S., Min, C., Meng, Y., Pham, T., Ying, Y. of thermal treatment on the nanomechanical behavior of (2014) Evaluation of the effects of compression combined with bamboo (Phyllostachys pubescens Mazel ex H. De Lehaie) cell heat treatment by nanoindentation (NI) of poplar cell walls. walls observed by nanoindentation, XRD, and wet chemistry. Holzforschung 68:167–173. Holzforschung 71:129–135. Wang, X., Li, Y., Deng, Y., Yu, W., Xie, X., Wang, S. (2016) Contribu- Mattos, B.D., Lourençon, T.V., Serrano, L., Labidi, J., Gatto, D.A. tions of basic chemical components to the mechanical behavior (2015) Chemical modification of fast-growing eucalyptus wood. of wood fiber cell walls as evaluated by nanoindentation. Wood Sci. Technol. 49:273–288. BioResources 11:6026–6039. Mburu, F., Dumarçay, S., Bocquet, J.F., Petrissans, M., Gérardin, P. Wei, L., Liang, S., Mcdonald, A.G. (2015) Thermophysical proper- (2008) Effect of chemical modifications caused by heat treat- ties and biodegradation behavior of green composites made ment on mechanical properties of Grevillea robusta wood. from polyhydroxybutyrate and potato peel waste fermentation Polym. Degrad. Stab. 93:401–405. residue. Ind. Crops Prod. 69:91–103. Meng, Y., Wang, S., Cai, Z., Young, T.M., Du, G., Li, Y. (2012) Wimmer, R., Lucas, B.N., Tsui, T.Y., Oliver, W.C. (1997) Longitudinal A novel sample preparation method to avoid influence of hardness and Young’s modulus of spruce tracheid secondary embedding medium during nano-indentation. Appl. Phys. A walls using nanoindentation technique. Wood Sci. Technol. 110:361–369. 31:131–141. 1070 X. Wang et al.: Thermal modification of Masson pine

Wise, L.E., Murphy, M., D’Addieco, A.A. (1946) Chlorite Yin, J., Yuan, T., Lu, Y., Song, K., Li, H., Zhao, G., Yin, Y. (2017) Effect holocellulose, its fractionation and bearing on summative of compression combined with steam treatment on the poros- wood analysis and on studies on the hemicelluloses. Pap. ity, chemical composition and cellulose crystalline structure of Trade J. 122:35–43. wood cell walls. Carbohydr. Polym. 155:163–172. Xie, Y., Xiao, Z., Grüneberg, T., Militz, H., Hill, C.A.S., Steuernagel, Yu, Y., Fei, B., Wang, H., Tian, G. (2011) Longitudinal mechanical L., Mai, C. (2010) Effects of chemical modification of wood properties of cell wall of Masson pine (Pinus massoniana particles with glutaraldehyde and 1,3-dimethylol-4,5-dihydrox- Lamb) as related to moisture content: a nanoindentation study. yethyleneurea on properties of the resulting polypropylene Holzforschung 65:121–126. composites. Compos. Sci. Technol. 70:2003–2011. Zhan, T., Jiang, J., Lu, J., Zhang, Y., Chang, J. (2018a) Influence of Xing, D., Li, J., Wang, X., Wang, S. (2016) In situ measurement of hygrothermal condition on dynamic viscoelasticity of Chinese heat-treated wood cell wall at elevated temperature by nanoin- fir (Cunninghamia lanceolata). Part 1: moisture adsorption. dentation. Ind. Crops Prod. 87:142–149. Holzforschung 72:567–578. Xu, Z., Li, X. (2008) Effects of indenter geometry and material prop- Zhan, T., Jiang, J., Lu, J., Zhang, Y., Chang, J. (2018b) Influence of erties on the correction factor of Sneddon’s relationship for hygrothermal condition on dynamic viscoelasticity of Chinese nanoindentation of elastic and elastic-plastic materials. Acta fir (Cunninghamia lanceolata). Part 2: moisture desorption. Mater. 56:1399–1405. Holzforschung 72:579–588.