Effects of heating mediums on microstructure and chemical properties of thermally modified Matoa

doi:10.21203/rs.3.rs-4356788/v1

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Effects of heating mediums on microstructure and chemical properties of thermally modified Matoa

https://doi.org/10.21203/rs.3.rs-4356788/v1

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Thermal modification (TM) is considered to be the most environmentally friendly and effective industrial method to reduce the hygroscopicity of wood. However, different heating mediums of TM often results in various performance. In this study, the changes of microstructure, crystallization, chemical composition and equilibrium moisture content (EMC) of thermally modified wood (TMW) were investigated respectively to explore the effects of heating mediums (saturated steam, superheated steam, air), modification temperature and water leaching post-treatment on TMWs. The results showed the general intensity of TM was in the order of: saturated steam > superheated steam > air. Saturated steam induced severer cell wall destruction than the other two mediums. Although the porosity slightly increased with the increasing TM temperature and leaching treatment, superheated steam and air TM still decreased the porosity compared to that of control, whereas saturated steam TM increased the porosity apparently. Although TM increased both relative crystallinity and crystal size of TMWs. The increasing TM temperature slightly increased the relative crystallinity, but decreased the crystal size. Leaching caused opposite changes in crystal size of TMWs with different heating mediums. The highest crystallinity was observed after saturated steam TM. The increase in relative amount of lignin and cellulose due to the hemicellulose degradation was the main chemical changes of TMWs, further lignin condensation reaction was occurred after saturated steam TM. Although saturated steam TM induced increased porosity, its lowest EMC indicated the decrease of hydroxyl groups was the dominate reason for the decreased hygroscopicity than the changes of microstructure.

thermal modification

heating medium

microstructure

crystallinity

FTIR

Thermal modification (TM) is an effective and commercial technology for improving the physical and chemical properties of wood and its products (Hill et al. 2021). Commercial TM processes usually take place at temperatures between 160 and 240°C with different heating mediums, for example, superheated steam, nitrogen, hot oil, vacuum, etc (Gérardin 2016).

TM results in weight loss (Czajkowski et al. 2020), changes in mechanical strength, and reduction in equilibrium moisture content (EMC) and hygroscopicity, which subsequently improves the dimensional stability (Borůvka et al. 2019) and decay resistance (Cao et al. 2011). These property changes mainly deriving from the degardation of chemical constituent (hemicellulose, cellulose, and lignin) (Wang et al. 2018) and changes of pore structure (Sun et al. 2022). During TM process, the decrease of -OH groups (Xu et al. 2022) due to the degradation of hydrophilic hemicellulose, and the increase of relative crystallinity (Andersson et al. 2005) due to the reduced amorphous cellulose would decrease the hygroscopicity of thermally modified wood (TMW). In addition, the shrinkage and degradation of the wood cell wall at elevated temperature affects the integrity and distribution of pores, which also makes an impact on the sorption property of wood (Zauer et al. 2013; Cai et al. 2020). Thus, the moisture content and the water absorption of TMW are reduced significantly.

Some researches claimed that the changes in properties of TMW are also affected by the moisture conditions (depending on the heating mediums) applied in TM process. TM under a wet condition result in greater mass loss compared with TM under dry, especially when a post-treatment water-leaching is performed (Pockrandt et al. 2018). Because the presence of water accelerates hydrolysis process, and the acetic acid generated from hemicellulose degradation further promotes the degradation of polysaccharides into water soluble degradation products, which could be removed by leaching (Čermák et al. 2016; Marcon et al. 2021). At the same modification temperature, the hygroscopicity of wood decreases greater in wet condition than that in moist and dry condition, which is because the higher amount of moisture in heating medium promotes the degradation of hydrophilic hemicellulose (Liu et al. 2023). TMWs modified in wet and dry conditions also show different behaviors after moistening. Some scholars observed that the volume of TMW manufactured by wet TM process was reversible after undergoing dry-wet cycles, while no similar reversible change was observed in the TMW made under dry conditions (Altgen et al. 2016; Obataya et al. 2017), owing to TMW produced in wet condition showed lower dimensional stability than that in dry condition. On the contrary, Hill et al. (2021) concluded that the reduced EMC due to dry TM was partially reversible, but it did not occurred in TMW produced in wet condition. The explanation was because the stress was remained in amorphous region when wood was heated in dry condition, while the high moisture level helped the stress relaxation during TM in wet condition. Furthermore, Endo et al. (2016) found that the hygroscopicity reversibility was more clear after TM at 60% heating humidity (HRH) than TM at 0% HRH, while TMWs heated at 92% HRH or above had no reversibility, indicating that the TM in dry and moist condition might cause temporary effect on hygroscopicity reduction. Simultaneously, the crystallization state of cellulose is also affected by the moisture content of TM. The crystallinity of TMW treated under dry conditions significantly decreases after the dry-wet cycle, while the sample treated under wet conditions does not show significant changes (Sun et al. 2022).

The hygroscopicity of wood is affected by the changes of the chemical composition and microstructure of cell walls. Despite the comprehensive literature on the hygroscopicity of TMW (Hill 2006), there is limited comparison and information concerning the anatomic and chemical changes in TMW modified with different heating mediums. In this study, Matoa wood, a material widely used in flooring for ground with heating system, was thermally modified at two temperatures (140 and 160℃) and three different mediums (e.g. saturated steam, superheated steam and air) with a duration of 2h. The anatomic structure, relative crystallinity, chemical composition and EMC of TMW modified in three different moisture conditions were analyzed and compared respectively. The results of this study would be beneficial for understanding the mechanism of changes in TMW properties under different heating mediums.

2.1 Materials

Matoa (Pometia pinnata J. R. et G. Forst) sawn timbers with a dimension of approximately 930mm (length) × 130mm (width) × 19mm (thickness) provided by Zhejiang Shenghua Yunfeng Greeneo Co.Ltd were used as study materials. All timbers were first kiln dried at 60°C for 8 weeks, and then conditioned at room temperature until the moisture content of wood reached to 11%±0.5% before TM.

2.2 Thermal modification

Timbers were further cutted into 450 mm (length) × 60 mm (width) × 19 mm (thickness), and then thermally modified with different parameters. The modification details are shown in Table 1. Samples were modified at 140 and 160℃, respectively, for 2 hours. Saturated steam, superheated steam and air were used as mediums for different modification groups. For water-leaching process, samples were soaked into warm water (60°C) for 1 hour immediately after TM. Afterwards, the wet samples were mildly dried at 50°C for two weeks and conditioned at 20°C 65% until the EMC achieved. Unmodified boards, originating from the same materials as the thermally modified ones, were used as controls. 15 replicates were prepared for each groups.

Table 1

Thermal modification parameters of samples
Code	Medium	Temperature (°C)	Leaching or not
A4-0	Saturated steam	140	No
A4-1		160	Yes
A6-0		140	No
A6-1		160	Yes
B4-0	Unsaturated steam	140	No
B4-1		160	Yes
B6-0		140	No
B6-1		160	Yes
C4-0	Air	140	No
C4-1		160	Yes
C6-0		140	No
C6-1		160	Yes

2.3 Anatomic structure analysis

2.3.1 Scanning electron microscopy (SEM)

Wood slice of 10 mm (Radial) × 10 mm (tangential) × 2 mm (longitudinal) prepared from each sample was used as specimen for SEM test. The cross section of the specimens were smoothed by sliding microtome to obtain an even surface for observations. Then, the anatomic morphology of specimens with different modifications were captured using SEM (Quanta 200, FEI, Oregon, USA).

2.3.2 Nitrogen adsorption analysis

Powder with ø=0.25 mm were grinded from specimens of each TM group, and oven-dried before nitrogen gas adsorption experiment. The pore structure was analyzed by a specific surface area/porosity analyzer (ASAP2460, Micromeritics, Georgia, USA) at 77.4 K. The specific surface area (S_BET) was calculated according to Brunauer-Emmett-Teller (BET) methods (Brunauer et al. 1938) and the pore size distribution was determined according to Barrett-Joyner-Halenda (BJH) methods (Barrett et al. 1951).

2.4 X-ray diffraction (XRD) analysis

Specimens were grinded into fine powder with diameter of 0.15 mm for the XRD analysis. The crystalline structures of thermally modified and control specimens were analyzed using a XRD Ultima IV (Rigaku Ltd., Tokyo, Japan) spectrometer with Cu Kα radiation from 2θ = 5°-45°, and the scanning speed was 5° min^− 1.

The crystallinity index (CrI) of specimens were calculated according to Segal method (Thygesen et al. 2005) (Eq. 1):

$$CrI\left(\%\right)=\frac{{I}_{002}-{I}_{am}}{{I}_{am}}\times 100$$

Where I₀₀₂ was the intensity of the crystalline peak at 2θ = 22°-23°, and I_am was the minimum intensity at 2θ = 18°-19°.

The width of the crystal obtained from (200) diffraction was obtained based on Scherrer method (Newman 1999), as shown in Eq. 2:

$$D=\frac{k\lambda }{B cos\theta }$$

Where D was the width of crystal, k was the Scherrer constant (0.9), λ was the wavelength of the X-rays, B was the half-bandwidth in radians, and θ was the Bragg angle.

2.5 Fourier transform infrared (FTIR) spectroscopy

The FTIR spectrum of each specimen was recorded with FTIR spectrometer (VERTEX 80V, Bruker, Germany) in the range of 4000 − 400 cm^− 1 with a resolution of 4cm^− 1. Each spectrum was collected from 32 scans in the absorbance mode. Specimens for FTIR analysis were dried, milled and mixed with KBr in a ratio of about 1:25, and compressed under vacuum to form pellets. The spectra were normalized to the band at 2902 cm^− 1 (Peng et al. 2022).

2.6 EMC measurement

The specimens with dimension of 20 mm (radial) × 20 mm (tangential) × 20 mm (longitudinal) were first oven-dried at 105°C until constant mass, and then conditioned at 20°C 65% RH until equilibrium was achieved. The EMC was calculated based on the oven-dry mass and the mass after conditioning.

3.1 SEM analysis

Changes in cell wall structure due to different TM parameters were shown in Fig. 1. As Fig. 1 demonstrated, without water-leaching process, the modification temperature of 140℃ did not affect the microstructure of specimen regardless of the heating mediums. After the specimens were subjected to leaching process, a few cracks were observed in the cell wall of specimens modified in saturated steam and in superheated steam, while the specimens modified in air condition did not show obvious difference in cell wall structure compared with that of controls. At the modification temperature of 160℃, collapse of cells and more cracks in cell wall were observed in the saturated steam modified specimens. Modification in superheated steam and air at 160℃ also lead to a damaged microstructure, for example, more holes near the compound middle lamella were found in those TMW groups. The further water leaching process emphasized on the holes and cracks caused by TM, and led to a greater extent of cell wall degradation, resulting in collapse in cell wall, merging in cell lumens and a more brittle morphological structure.

The holes and cracks in cell wall generated during TM were mainly caused by the degradation of chemical compositions (mainly hemicellulose (Elisabeth and Gerd 2008; Wang and Howard 2018)) and the anisotropic shrinkage of different cell wall layers (Biziks et al. 2013; Wu et al. 2021). The structural destruction degree would affected by the heating mediums. The higher modification temperature (Jang and Kang 2019) and the presence of moisture (Ali et al. 2021; Obataya et al. 2021) in saturated steam and superheated steam would promote the hydrolysis of cell wall components, making the cell wall more degraded and less intact compared with that modified in air condition. Saturated steam led to greater degradation, followed by superheated steam. During the leaching post-treatment, the water removed the degradation products and interacted with the acids formed during TM, which facilitated the cell wall destruction (Lê et al. 2016; Hill et al. 2021). The changes in microstructure would also effect the pore structure of TMW (Obataya et al. ).

3.2 Nitrogen adsorption analysis

The nitrogen adsorption method was applied to study the change of pore structure after different TM. Since quasi-equilibration might be generated during the nitrogen adsorption, which would cause underestimation of micropores (Shi and Avramidis 2018; Broda et al. 2019). However, the results obtained by this method still can be used for making a comparison between modification groups. As illustrated in Fig. 2, the nitrogen adsorption-desorption isotherms of all specimens were similar in shape, with complete and smooth curves, and consistent change trends. According to the classification of International Union of Pure and Applied Chemistry (IUPAC), the isotherms of both modified and unmodified specimens were type Ⅱ, with a H3-type hysteresis loop, indicating the exist of slit-shapes pores in all TWMs and control, which was coincided with the previous research (Yin et al. 2015). TM with saturated steam at 160℃, both with and without water leaching treatment, increased the adsorption amount obviously, while modification with superheated steam and air at 140℃ showed a clear reduction. The curves of the rest modification groups overlapped partly with the control.

The total pore volume and average mesopore diameter were estimated based on the cylindrical pore model using the BJH method. The parameters describing N₂ adsorption capacity, the surface area (S_BET), N₂ volume between 1.7-300nm (V_total) and average diameter (D_BJH) of the mesopores were listed in Table 2. The results showed that the higher the N₂ adsorption capacity, the bigger the S_BET and V_total. Compared to the control, the nitrogen adsorption capacity, S_BET and V_total of the saturated steam modified TMW increased, while that of the superheated steam and air modified wood mainly decreased. This was partly inconsistent with the results reported by Liu et al. (2023), who claimed that the S_BET and V_total were both decreased no matter in saturated steam or in air condition at 180 and 210°C. However, Sun et al. (2022) found that the porosity of thermally modified pine first increased and then decreased with the increasing modification temperature, indicating the change of porosity was related with the intensity of TM. Higher modification temperature and leaching post-treatment mostly induced an increase in porosity, however, the higher TM temperature also resulted in a decrease in the average mesopore diameter.

Since the number of pores cannot reflected directly by the pore volume due to the various morphology and depth of different pores, so the pore size distribution of all specimens was represent by the pore volume in different ranges as shown in Table 3. TM in saturated steam at 140°C decreased the volume of micropores, but increase the volume of meso- and macropores. Whereas TM in saturated steam at 160°C generally increased the volume of pores in all scale. On the contrary, TM in superheated steam and air conditions normally resulted in a reduction of all kinds of pores.

The changes in pore structure of TMW was an overall outcome of degradation and removal of wood components, as well as the changes of the microstructure. The shrinkage of cell wall, closer binding between the microfibers (Hill 2006), increase in crystallinity, and filling in the intermicellar pores by softening and flowing of lignin during TM (Cai et al. 2020) were probably the main reasons of the reduced porosity in the superheated steam and air modification groups. However, the further degradation and volatilization of cell wall chemical composition facilitated by the moisture in heating medium, especially the saturated steam (Jang and Kang 2019; Rahimi et al. 2019), would be the dominate reason for the increased micro- and mesopores in saturated steam TMWs. On the other hand, the increase in the macropores could also due to the collapse of cell wall and merging in cell lumens as shown in Fig. 1. The removal of the degradation products by water leaching post-treatment explained its increased TMW’s porosity (Kymäläinen et al. 2018; Willems et al. 2020). The intenser TM temperature resulted in a decrease in the average mesopore diameter, which probably indicated the generation of smaller pores or increased proportion of smaller pores due to the physical and chemical degradation of wood occurred during TM (Junghans et al. 2005).

Table 2

The N₂ adsorption capacity, surface area, total pore volume and average pore diameter of specimens.
Code	Max adsorption capacity of N₂/(cm³·g^− 1)	S_BET/(m²·g^− 1)	V_total/(mm³·g^− 1)	D_BJH/(nm)
Control	1.312	0.673	1.911	18.867
A4-0	1.398	0.557	2.010	20.270
A4-1	1.729	0.803	2.503	17.448
A6-0	2.428	1.294	3.688	15.407
A6-1	3.353	1.307	5.068	16.871
B4-0	0.685	0.319	0.947	23.286
B4-1	0.870	0.414	1.214	21.879
B6-0	1.198	0.586	1.722	18.087
B6-1	1.111	0.675	1.641	12.265
C4-0	0.696	0.350	0.960	16.159
C4-1	1.100	0.551	1.585	16.482
C6-0	0.892	0.518	1.279	12.687
C6-1	1.393	0.836	2.099	13.502

Table 3

Pore size distribution of both modified and control specimens (mm³/g).
Code	1.7-2 nm	2–50 nm	> 50nm
Control	0.069	1.015	0.827
A4-0	0.021	1.132	0.856
A4-1	0.056	1.463	0.984
A6-0	0.116	2.192	1.380
A6-1	0.137	3.011	1.920
B4-0	0.008	0.451	0.488
B4-1	0.022	0.603	0.589
B6-0	0.055	0.908	0.759
B6-1	0.082	0.952	0.607
C4-0	0.020	0.461	0.479
C4-1	0.067	0.832	0.686
C6-0	0.061	0.661	0.557
C6-1	0.096	1.226	0.777

3.3 Crystalline analysis

The X-ray diffraction patterns and the crystallization performance of all specimens were shown in Fig. 3 and Table 4. The higher crystallization not only contributes to the higher rigidity, but also reduces the hygroscopicity of wood. Within the scanning interval of 0–40°, two prominent refection peaks appeared on the curve. The one located at about 2θ = 18° denoted the scattering intensity of the diffraction angle in the amorphous area, whereas the other one appeared around 2θ = 22° represented the maximum strength (I002) of the diffraction angle in the crystalline area. The almost unchanged location and numbers of refection peak of all groups indicated that all TM methods did not transform the crystalline type of wood (Bhuiyan et al. 2000).

According to Table 4, all TM methods increased the relative crystallinity of TMW, regardless of different heating mediums, which was attributed to the degradation of amorphous carbohydrates (Kim et al. 2010) and the rearrangement and reorientation of the cellulose and hemicellulose molecule chains (Olek and Bonarski 2014). Among three mediums, saturated steam had the greatest improvement of relative crystallinity, followed by superheated steam and air, respectively. This result was consistent with Bhuiyan et al. (2000), who found that the high humidity TM conditions would promote the crystallization in both pure and wood cellulose. The relative crystallinity of TMWs was slightly increased by increasing the modification temperature from 140 to 160°C. This was also consistent with the previous researches (Zheng et al. 2016; Durmaz et al. 2019), and the reason for the increased relative crystallinity was because of the degradation of hemicellulose and the amorphous cellulose at elevated temperature (Birinci et al. 2022; Wang et al. 2022). The result showed the water leaching post-treatment only led to negligible decrease in relative crystallinity. However, Altgen and Militz (2016) claimed that water-leaching can restore the mobility of the structure and rearrange the molecular structure within the cell wall, which could promote the increase of relative crystallinity.

The crystal size also reflect the crystallization performance of wood. As shown in Table 4, all TM methods, except C4-1, increased the crystal size of wood to a certain extent. Among all mediums, saturated steam had the highest increase in crystal size, followed by superheated steam and air, indicating that high moisture condition promoted crystallization (Hill et al. 2021). Regardless of heating mediums, although the relative crystallinity slightly increased with the increasing modification temperature, the crystal size of TMWs decreased. This result was partly inconsistent with Dwianto et al. (1996), who found the crystal size was increased in air TM condition, while it was decreased in saturated steam condition with the increasing modification temperature. However, other researchers (Andersson et al. 2005; Cheng et al. 2017) discovered that the crystal size would firstly increased and then decreased, and then increased again with the rising temperature. Water leaching resulted in a decrease in crystal size of TMWs modified in superheated steam and air conditions. But it was surprised to found the crystal size of saturated steam groups increased after leaching. There was no certain reason to explain the increase in saturated steam group after leaching, however, the reorganization of cellulose molecules during water leaching and oven-dry process before XRD analysis might partly explain this phenomenon (Altgen and Militz 2016).

Table 4

Crystallization properties of control and TMWs.
Code	Relative Degree of Crystallinity (%)	Crystal size (nm)
Control	46.69%	0.23
A4-0	54.99%	0.36
A4-1	53.41%	0.51
A6-0	54.52%	0.25
A6-1	54.03%	0.42
B4-0	53.56%	0.33
B4-1	53.45%	0.23
B6-0	54.48%	0.31
B6-1	52.75%	0.24
C4-0	49.03%	0.30
C4-1	51.24%	0.18
C6-0	54.64%	0.24
C6-1	53.89%	0.23

3.4 Chemical composition analysis

The characteristic FTIR spectra of wood with different modification methods were shown in Fig. 4. The assignment of signals were shown in Table 5.

Table 5

Assignment of the FTIR signals for wood constituents.
Wavenumber(cm^− 1)	Assignments	References
1739	C = O of non-conjugated aldehyde	Kotilainen et al. (2000) and Özgenç et al. (2017)
1625	Absorbed water	Marcon et al. (2021)
1592	Aromatic skeletal vibrations and the C = O stretch (lignin)	Zylka et al. (2009) and Hillis and Rozsa (1978)
1510	C = C stretching of the aromatic skeletal vibrations (lignin)	Colom et al. (2003) and Temiz et al. (2007)
1330	OH in plane bending (cellulose)	Kotilainen et al. (2000)
1266	Guaiacyl-ring (lignin)	Kotilainen et al. (2000)
1158	C-O-C symmetric stretching (cellulose)	Kotilainen et al. (2000) and Özgenç et al. (2017)
1110	C = C stretching (cellulose)	Kotilainen et al. (2000) and Nuopponen (2005)
1058	C-O stretching (cellulose)	Özgenç et al. (2017)
1033	C-O and C = O stretching in cellulose, symmetric C-O-C stretching of dialkyl ethers, aromatic C-H deformation in lignin	Hakkou et al. (2005) and Nuopponen (2005)

The peaks at 1739cm^− 1 referred to the vibration of carboxylic groups (C = O) of the non-conjugated acetyl group in hemicelluloses. Compared to control, the saturated steam modified specimens showed a clearer decrease than that of superheated steam and air modified groups, indicating severer broken of acetyl side chain and degradation of hemicelluloses during saturated steam TM (Guo et al. 2015; Li et al. 2015). Such decrease was more evident with the increasing modification temperature. The acetic acid produced by the degradation of hemicellulose would further promoted the depolymerization of amorphous polysaccharides and catalysis the degradation and condensation reactions of lignin, which will lead to a decrease in the number of carbon matrices (Hill et al. 2021).

The signal at 1625 cm^− 1 corresponded to the H-O-H deformation vibration of absorbed water. The decrease at this band after all TM groups might be attributed to the decreased water accessibility caused by the dehydration and deacetylation reaction during TM process (Bryne et al. 2010). An increase at this band was observed after water leaching post-treatment in superheated steam and air TM groups, indicating the increase of water affinity after leaching (Marcon et al. 2021; Endo et al. 2016). However, it was surprised to find that saturated steam TM showed a remarkable reduction at this band after leaching, implying decrease of water affinity.

The lignin peaks at 1592, 1510 and 1266 cm^− 1 increased in all TM groups, and the intensity of these peaks increased slightly with the raising modification temperature. This result might owing to the increase of the relative amount of lignin due to the degradation of polysaccharides during TM (Kučerová et al. 2019). In addition, the peak at 1510 cm^− 1 shift to 1514 cm^− 1 after modification at 160°C in saturated steam condition, suggesting the condensation reaction caused by the cleavage of the aliphatic side chain and the cleavage of the β-O-4 connection in the lignin structure (Jang and Kang 2019; Sikora et al. 2018). However, this kind of change was not observed in the other two mediums, indicating the superheated steam TM and air TM did not affect lignin, while the saturated steam TM could cause lignin reaction at the same modification temperature. Different changes of the peak at 1510 cm^− 1 were reported previous, for example, Li et al. (2015) found such peak intensity of thermally modified teak wood with steam increased slightly with increasing temperature. Özgenç et al. (2017) found the intensity of this peak of air TM pine and spruce decreased, while that of beech increased. Moreover, Windeisen et al. (2007) found this peak increased in softwood, but remain unchanged in hardwood. The reason for the differences between those studies might be due to the different tree species and TM methods.

The increased intensity at 1330cm^− 1 was detected in all TM groups, particularly in saturated steam groups. This signal was primarily assigned to the cellulose and was related to the content of crystallized cellulose I (Colom et al. 2003), indicating the increase in crystallinity after TM. This change was consistent with the results of the relative crystallinity obtained by X-ray diffraction. In addition, the changes in the TMWs at 1033, 1058, 1158 and 1110 cm^− 1 were mainly related to cellulose and, to a lesser extent, lignin. The increase in the intensity of these peaks indicated the increased concentrations of the alcohol and/or carboxyl groups in cellulose (Bhuiyan and Hirai 2005).

3.5 EMC analysis

Figure 5 showed the EMC of the specimens determined after conditioned at 20°C 65% RH. The results showed that TM significantly reduced the EMC of all specimens. The decrease of the wood EMC depended on the heating medium and modification temperature. The saturated steam TM reduced the EMC to a greater extent than the superheated steam TM and air TM. No significant difference was observed between the later two heating medium groups. According to the pore structure and FTIR results in this paper, compared to superheated steam TM and air TM, saturated steam TM tended to promote hemicellulose degradation and lignin condensation, and to improve the crystallinity, but also to increase the porosity. The lowest EMC achieved in saturated steam group indicated the changes in wood components was the predominate reason for the decreased hygroscopicity than the changes of cell wall pore structure. Higher modification temperature resulted in lower EMC because of the elimination of hemicellulose hydroxyl groups (Phuong et al. 2007) and increase of the crystallinity by elevated temperature (Zhao et al. 2023), which was corresponds with previous researches (Cai et al. 2020; Bayani et al. 2019).

Surprisingly, the water leaching post-treatment did not affect the EMC of different groups in a consistent way. Leaching decreased the EMC of superheated steam TMWs, whereas it increased the EMC of superheated and air TMWs. However, this finding was also in accordance with the above XRD and FTIR results, which showed leaching increased the crystal size and decrease the absorbed water band of the saturated steam modified specimens.

This study showed that the moisture content in the heating medium played an important role in the cell wall structure and chemical changes of TMWs. TM induced cracks and holes in cell wall, such destruction were more prone to occur after saturated steam modification than superheated steam or air modification. The porosity of cell wall decreased after superheated steam TM and air TM, whereas it surprisingly increased after TM in saturated steam condition. TM generally increased the relative crystallinity and crystal size, and the greatest increase was observed in saturated steam groups. The increasing modification temperature slightly increased the relative crystallinity of TMWs, but decreased the crystal size. TM normally resulted in hemicellulose degradation and subsequent increase in relative amount of lignin and cellulose. The lignin structure were not affect by superheated steam or air TM at temperature below 160°C. However, the lignin condensation reaction was only observed after saturated steam TM at 160°C, indicating the saturated steam could offer greater reaction energy during TM process.

Increasing in TM temperature decreased the EMC of TMWs. Compared to the other two heating mediums, saturated steam not only induced the greater degradation of hydroxyl groups, but also increased the porosity, however, its lowest EMC implied the changes in wood components was the dominate reason for the decreased hygroscopicity than the changes of cell wall pore structure. The water leaching post-treatment tended to increase the porosity, decrease the relative crystallinity and subsequently increased the EMC of TMWs by superheated steam and air TM. However, the increased crystal size of saturated steam modified wood observed after leaching could probably explain its decreased EMC.

Conflict of interest statement:

The authors declare no conflicts of interest regarding this article.

Author Contribution

Ling Caishan (First Author): Investigation, Data Curation, Methodology, Formal Analysis, Visualization, Writing - Original Draft.Cai Chenyang(Corresponding Author): Conceptualization, Funding Acquisition, Formal Analysis, Writing - Review & Editing.Shen Yunfang: Project Administration, Resources, Supervision.Xiong Xianqing (Primary Corresponding Author): Conceptualization, Resources, Supervision, Writing - Review & Editing.

Acknowledgements

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (No. 32301521).

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Effects of heating mediums on microstructure and chemical properties of thermally modified Matoa

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Abstract

Figures

1 Introduction

2 Experimental

2.1 Materials

2.2 Thermal modification

2.3 Anatomic structure analysis

2.3.1 Scanning electron microscopy (SEM)

2.3.2 Nitrogen adsorption analysis

2.4 X-ray diffraction (XRD) analysis

2.5 Fourier transform infrared (FTIR) spectroscopy

2.6 EMC measurement

3 Results and discussion

3.1 SEM analysis

3.2 Nitrogen adsorption analysis

3.3 Crystalline analysis

3.4 Chemical composition analysis

3.5 EMC analysis

4. Conclusions

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