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Article

Effects of Heat Treatment on the Chemical Composition and Microstructure of Cupressus funebris Endl. Wood

by
Jianhua Lyu
1,†,
Jialei Wang
2,† and
Ming Chen
2,*
1
Department of Environmental Design, College of Landscape Architecture, Sichuan Agricultural University, Chengdu 611130, China
2
Department of Product Design, School of Arts and Media, Sichuan Agricultural University, Ya’an 625014, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Forests 2024, 15(8), 1370; https://doi.org/10.3390/f15081370
Submission received: 25 June 2024 / Revised: 26 July 2024 / Accepted: 5 August 2024 / Published: 6 August 2024
(This article belongs to the Special Issue Physical and Mechanical Properties of Wood- and Bamboo-Based Materials)
Browse Figures
Versions Notes

Abstract

:
The effects of heat treatment on Cupressus funebris Endl. wood were examined under different combinations of temperature, time, and pressure. The chemical composition, crystallinity, and microstructure of heat-treated wood flour and specimens were characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD). Vacuum heat treatment led to changes in the functional groups and microstructure of C. funebris wood, and the relative lignin content decreased with increasing treatment temperature, which was significant at lower negative pressures. Cellulose crystallinity showed a change rule of first increasing and then decreasing throughout the heat treatment range, and the relative crystallinity ranged from 102.46% to 116.39%. The cellulose treated at 120 °C for 5 h at 0.02 MPa had the highest crystallinity of 44.65%. These results indicate that although heat treatment can improve cellulose crystallinity, very high temperatures can lead to decreased crystallinity. The morphology and structure of the cell wall remained stable throughout the heat treatment range; however, at elevated temperatures, slight deformation occurred, along with rupture of the intercellular layer.

1. Introduction

Wood is one of the four main building materials, and it has renewable, recyclable, and biodegradable properties [1]. China is rich in Cupressus funebris Endl. resources, widely distributed in the Yangtze River Basin and the southern part of China. It is also an economically and environmentally important species in the hilly areas of central Sichuan, where it is the largest tree species in plantation forests [2], and it is an irreplaceable main tree species in the calcareous purple soil and limestone soil areas of Sichuan Province [3]. There are more than one million hectares of C. funebris in Sichuan Province alone; therefore, it is important to study effective and feasible ways to improve the comprehensive utilization of this tree species [4]. C. funebris wood from Sichuan is also known as cedar wood, and it is commonly used as timber for construction, furniture, shipbuilding, and handicrafts, owing to its hardness, density, fine and uniform structure, and strong texture. The C. funebris wood exhibits a straight or sloping grain, with a medium and uniform structure. The weight and hardness properties are of a medium to large value. The degree of wood shrinkage is slight to moderate, with a medium strength and impact toughness [5], although it contains many knots [6]. In addition, the entire tree can also be utilized to extract rich chemical products. From different parts of C. funebris, the oils, called cedar wood oil, which is an important raw material of synthetic perfume, could be extracted. A total of 33 volatile components were identified from the wood of C. funebris, and the content of ferruginol and cedrol was highest in essential oil [7,8].
Thermal modification or heat treatment is effective in improving the dimensional stability and biological durability of wood, and it has received considerable attention as an environmentally friendly method compared to other methods [9]. Heat-treated modified wood is usually obtained by heating at temperatures ranging from 150 °C to 280 °C under oxygen-deficient conditions using media such as steam, nitrogen, or thermal oil and maintaining such conditions for a certain period. Heat treatment causes the chemical composition and structure of the wood to undergo thermal degradation and crosslinking reactions, thus significantly enhancing the ability of the material to maintain dimensional homeostasis and resistance to microbial attacks [10,11,12]. The magnitude of the color change in heat-treated wood is closely related to its chemical composition, which is the underlying cause of changes in its mechanical properties. In the production process, the technology of heat-treated lumber is determined by elucidating the mechanism of the effect of heat treatment on each strength index and selecting appropriate mechanical indices and treatment media according to the field of application [13,14,15]. A study has found that optimal process parameters for the mid-temperature vacuum heat modification of C. funebris wood were determined based on the mass loss rate and modulus of rupture (MOR), resulting in a modification temperature of 120 °C, holding time of 5 h, and a pressure intensity of 0.1 [16]. However, there is a lack of studies on the changes in functional groups, cellulose crystallinity, and microstructure of C. funebris wood caused by vacuum heat treatment. Heat-treated lumber is widely used in residential decorations, furniture manufacturing, outdoor fencing, and the cladding of building facades [17].
Crystallinity is an important property of wood, having an effect on its physical, mechanical, and chemical properties. In general, a high degree of crystallinity of cellulose results in the high tensile strength, bending strength, and dimensional stability of wood. During heat treatment, the chemical components of the wood cell wall undergo pyrolysis; the pyrolysis of hemicelluloses and cellulose occurs rapidly, with the weight loss of hemicelluloses occurring mainly at 220–315 °C and that of cellulose at 315–400 °C. However, lignin was more difficult to decompose because its weight loss occurred over a wide temperature range, from 160 to 900 °C [18,19].
The increase in temperature and time during heat treatment aggravates the degradation and crosslinking of wood, causing significant changes in the cell wall structure, affecting the physical and mechanical properties of wood and color changes [11,20,21,22,23]. Heat treatment does not damage ray parenchyma pit membranes, bordered pits, large window pit membranes, or Margo fibrils [24]. It is necessary to analyze the effects of heat treatment on the chemical components and microstructure of wood to understand how vacuum heat treatment alters its physical and mechanical properties. The objective of this study was to investigate the effects of the chemical composition and structural characteristics of the cell walls of C. funebris wood that was subjected to vacuum heat treatment. Changes in the functional groups, cellulose crystallinity, and microstructure were investigated under different treatment conditions, e.g., different temperatures, time, and vacuum pressure.

2. Materials and Methods

2.1. Test Material

Thirty-three-year-old Cupressus funebris Endl. trees from pure forest plantations in Yongxin Town, Jingyang District, Deyang City, Sichuan Province, China (31°1′–31°19′ North and 104°15′–104°35′ East), were used in this study. Five C. funebris trees with normal growth, complete trunks, straightness, and no obvious defects were randomly selected from the sample area marked with north–south directions. One log was cut from 1.3 m (diameter at breast height, DBH) to 3.3 m from each sample tree, and processed into 20 × 20 mm × 20 mm (R × T × L) specimens. There was a total of 11 groups of heat-treated specimens, with 10 replicates in each group. To eliminate the influence of wood moisture in the heat treatment process on the test results, the specimens were pre-treated in an electric blast drying oven (101A-3), and the specimens were dried until they reached a moisture content of approximately 12.69%.

2.2. Vacuum Heat Treatment Process

In this experiment, the vacuum heat treatment of C. funebris wood was conducted in an intelligent electric vacuum drying oven (Shanghai Kuntian Laboratory Instruments Co., Ltd., Shanghai, China, DZF-6050AB). The range of heat treatment temperatures commonly used in the industry and the standard technical conditions for modified wood were employed [16]. The heat treatment temperature, time, and vacuum pressure ranges were set to 120–180 °C, 1–5 h, and 0.02 MPa to 0.1 MPa, respectively. A full factorial experimental design was used to generate a balanced orthogonal table, with a total of 11 groups of heat-treated specimens and ten replicates in each group. The control group did not undergo treatment. Minitab software (Version 19, Minitab Inc., State College, PA, USA), an efficient statistical and analysis tool, was used in the experimental design to analyze factor effects and formulate parameter design, and the high and low levels of each factor were systematically planned. The specific experimental factors, levels, and codes are presented in Table 1, and the experimental design is presented in Table 2. The vacuum heat treatment process was divided into the following three steps (Figure 1):
Step 1: The specimen was placed in an electric blast drying oven for pretreatment at room temperature. The temperature was set to 103 °C, and the sample was oven-dried.
Step 2: The pre-treated wood was placed into an intelligent vacuum drying oven and pumped to vacuum. The temperature was increased to the target temperature at a rate of 1.3 °C/min and maintained for the corresponding time with the vacuum negative pressure degree needed accordingly.
Step 3: For the vacuum heat treatment time to meet the requirements of holding time, wait until the intelligent vacuum drying oven box temperature drops to 50 °C, release the pressure to open the oven to remove the specimen, and save it in the desiccator cooled to room temperature for subsequent tests.
Table 1. Encoding and levels of test factors.
Table 1. Encoding and levels of test factors.
Factor EncodingLevel
LowHigh
Heating Temperature (°C)H1A120180
Holding Time (h)T2B15
Vacuum Pressure (MPa)V3C0.100.02
Table 2. Experimental design for heat treatment experiments.
Table 2. Experimental design for heat treatment experiments.
Standard SequenceOperational SequenceHeating Temperature (°C)Holding Time (h)Vacuum Pressure (MPa)
1112010.02
7212050.10
9315030.06
5412010.10
8518050.10
3612050.02
10715030.06
6818010.10
4918050.02
111015030.06
21118010.02
Forests 15 01370 g001
Figure 1. Process flowchart showing negative pressure heat treatment of C. funebris wood samples.
Figure 1. Process flowchart showing negative pressure heat treatment of C. funebris wood samples.
Forests 15 01370 g001

2.3. Fourier Transform Infrared Spectroscopy Test

Treated wood specimens with a vacuum negative pressure of 0.1 MPa at 180 °C and 120 °C for 5 h and at 180 °C for 1 h and treated wood specimens with a vacuum negative pressure of 0.02 MPa at 180 °C and 120 °C for 5 h, and at 180 °C for 1 h and untreated wood specimens were selected. They were then cut into small pieces and ground into wood powder to a 200-mesh particle size using a mill. The samples were oven-dried at a temperature of (103 ± 2) °C. All analyses were performed using a Scientific Nicolet iS20 Fourier Transform Infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), the scan range was 4000–400 cm−1 with a resolution of 4.00 cm−1, and each spectrum was obtained from 32 scans.
The FTIR spectra were analyzed in terms of spectral band positions to identify the signatures of the main functional groups. An assignment of the main bands was made by analyzing the acquired spectra and comparing them with those available in the scientific literature [25].

2.4. X-ray Diffraction Test

Treated wood specimens with a vacuum negative pressure of 0.1 MPa at 180 °C and 120 °C for 5 h and at 180 °C for 1 h and treated wood specimens with a vacuum negative pressure of 0.02 MPa at 180 °C and 120 °C for 5 h, and at 180 °C for 1 h and untreated wood specimens were selected. They were then cut into small pieces and ground into wood powder to a particle mesh size between 80 and 100 using a mill. The samples were oven-dried at a temperature of (103 ± 2) °C. To analyze the effect of superheated steam treatment on the crystalline structure of cellulose, the powder was examined with an X-ray diffractometer (X’ Pert PRO-30X; Philips, Amsterdam, The Netherlands)) with Cu Kα radiation (λ = 0.154 nm). The XRD pattern was determined using the following parameters: a scanning range of 5°–40°; a voltage of 40 kV; and a scan rate of 2°/min. The crystallinity index (CrI) of each specimen was calculated according to the Segal method [26], as follows:
C r I = I 002 − I a m I a m × 100 %
where CrI is relative crystallinity (%); I002 is the maximum intensity of the lattice diffraction angle of 002; and Iam is the minimum intensity corresponding to amorphous cellulose fraction.

2.5. Scanning Electron Microscopy Test

Treated wood specimens with a vacuum negative pressure of 0.1 MPa at 180 °C, 150 °C, and 120 °C for 5 h and untreated wood specimens were placed in liquid nitrogen for 5–10 min. Cross and tangential section slices (20 µm thick) were then cut using a sliding microtome LM2010R (Leica, Wetzlar, Germany) with a classical microtome blade (Leica). Scanning electron microscopical (SEM) analysis was conducted using a ZEISS Sigma 300 SEM (ZEISS Sigma 300, Zeiss, Germany). After spraying gold on the surface, a 20 kV acceleration voltage was selected for SEM analysis.

3. Results and Discussion

3.1. Fourier Transform Infrared Spectroscopy Analysis

The identified changes in the functional groups before and after heat treatment reflected the chemical changes in C. funebris wood during the heat treatment process. Figure 2 shows that the localized spectra of vacuum thermally modified C. funebris wood changed significantly compared to those of untreated wood, indicating differences in the chemical components of the thermally modified wood. Although various functional group changes were observed in the spectra of the heat-treated wood, only the functional groups that directly affected the mechanical and dimensional stability were analyzed in this study. During negative vacuum heat treatment, various chemical reactions occurred within the wood, including the degradation of cellulose, pyrolysis of lignin, and extensive decomposition of hemicelluloses. These chemical changes led to variations in the contents of different functional groups in the wood, such as the hydroxyl, methyl, and carboxyl groups, which affected the physical and chemical properties.
As illustrated in Figure 2, Figure 3 and Figure 4, 3431 cm−1 is the O-H group stretching vibration peak, and the absorption peak here was enhanced with negative pressure and temperature increases in the range of 120–180 °C during intermediate temperature heat treatment. This mainly occurred via changes in the internal structure of the material in relation to the heat treatment and the increase in the crystallinity of the wood fibers [27]. Essentially, all cellulose hydroxyl groups function as hydrogen bonds. The increase in crystallinity was attributed to the formation of strong hydrogen bonds, resulting in a more ordered fiber structure and the absence of free hydroxyl groups in cellulose, either crystalline or amorphous cellulose. In the spectrogram, 2925 cm−1 characterizes the strength of the stretching vibrations of C-H in methyl and methylene groups. This indicates that when the polysaccharides in hemicelluloses were thermally decomposed, the number of groups decreased; therefore, the absorption peaks diminished with increasing temperature and time, which agrees with the findings of Liang et al. based on poplar hemicelluloses [28]. The intermolecular interaction forces in hemicelluloses were enhanced at higher pressures, which may have slightly enhanced the lightness of the methyl group peaks; this result is consistent with that of Chang et al. in relation to interactions between cellulose at high pressures [29,30]. In the spectrum, 1732 cm−1 is mainly the C=O bond stretching vibration peak, and it represents the xylan in hemicelluloses, as well as the acetyl, carboxyl, and alcohols in lignin, and the peak value of the absorption peaks at this time does not change much, which may be due to the oxidative decomposition of some unstable compounds caused by the elevated temperature of the heat treatment and the reduction of the carbonyl group and the content of the other groups [21].
However, the higher heat treatment temperature conditions allowed the hydrolysis of the acetyl group of hemicelluloses to generate acetic acid, which created an acidic environment and promoted the esterification reaction of lignin. As this subsequently increased the number of carbonyl groups to a certain degree [31], the amplitude of the wave peak change was weaker. The wave peak at 1509 cm−1 was related to the vibration of the aromatic skeleton in lignin, and a significant change in the peak only occurred when the heat treatment was elevated to 180 °C [32]. This was because the structure of lignin began to change at 180 °C, and the peak was weakened by the decreased lignin content under the low-pressure environment. In contrast, the thermal decomposition of hemicelluloses was promoted by the higher pressure so that the relative content of lignin increased, and the peak was enhanced [33]. Similar to the findings of other studies, lignin polycondensation reactions with other cell wall components, resulting in further crosslinking, contribute to an apparent increase in lignin content [9]. Despite the chemical changes in the cell walls of the wood induced by heat treatment under vacuum conditions, the FTIR spectra showed few overall changes.

3.2. Crystallinity Analysis of Heat-Treated Cellulose

Figure 5 demonstrates the changes in the cellulose crystallinity of C. funebris under different heat treatment conditions. The results show that the heat treatment had a limited effect on the crystalline region, and the diffraction position was basically stable. Although there was no significant change in the crystalline structure, the heat treatment adjusted the cellulose crystallinity of C. funebris.
Table 3 illustrates relatively limited changes in the relative crystallinity of C. funebris wood under different vacuum heat treatment conditions. Under 0.02 MPa, the cellulose crystallinity at 180 °C for 1 h and 5 h was 39.58% and 39.30%, respectively, and the cellulose crystallinity at 120 °C for 5 h was 44.65%. Under 0.1 MPa, the cellulose crystallinity at 180 °C for 1 h and 5 h was 42.52%, respectively, 41.95% at 0.1 MPa, and 41.24% at 120 °C for 5 h. Compared with the untreated specimens, with a crystallinity of 38.36, the crystallinity increased by 3.19%, 2.46%, and 16.39% with a vacuum negative pressure of 0.02 MPa at 180 °C for 5 h, respectively. Furthermore, compared with the untreated specimens, the crystallinity increased by 10.84%, 9.36%, and 7.50% with a vacuum negative pressure of 0.1 MPa at 180 °C for 1 h and 5 h and at 120 °C for 5 h, respectively. The crystallinity of cellulose increased by 10.84%, 9.36%, and 7.50% in the 1 h and 5 h treatments at 180 °C and in the 5 h treatment at 120 °C under 0.1 MPa negative pressure. It can be seen that negative pressure had the most significant effect on the crystallinity of cellulose, while at lower temperatures, the crystallinity decreased with an increase in negative pressure. At higher temperatures, the crystallinity increased with an increase in negative pressure. Some studies have found that because the high crystallinity of cellulose is usually positively correlated with the strength of wood, this finding indicated that the degree of negative vacuum pressure was the most significant factor affecting the flexural strength of wood in mechanical property tests and that treating wood at 120 °C and 0.1 MPa maximized its flexural strength [16,34]. The microfilament angle can influence the density, dimensional stability, and modulus of elasticity, strength, and creep properties of wood. Additionally, the microcrystalline morphology affects the wettability and fiber strength, as well as other properties, of wood. It can be observed that the cell wall microfilament angle and cellulose crystallinity serve as crucial parameters for assessing the quality and performance of wood. This may be because the increased pressure weakened the intramolecular interatomic repulsive forces, and the cellulose molecules were closer together. Through a dynamic simulation, Jiang et al. also determined that pressurized heat treatment increased the stiffness of wood [29].
The relative crystallinity of cellulose in C. funebris wood tended to increase first and then decrease with increasing heat treatment temperature. At the beginning of the heat treatment, the increase in cellulose crystallinity may have originated from hemicellulose degradation, leading to an increase in the proportion of crystallization. Meanwhile, the bridging reaction in the amorphous region of cellulose contributed to a more orderly arrangement of microfibrils, shortening the molecular spacing and tending toward the crystalline region, which increased the possibility of hydrogen bond formation, thus increasing the degree of crystallinity. Increased crystallinity reduces the hygroscopicity and swelling of the cell walls, thus enhancing the dimensional stability of C. funebris wood [35]. When the heat treatment was increased to 180 °C, the crystallinity decreased, mainly due to hemicelluloses degrading to produce acetic acid. This led to an acidic environment and promoted the degradation of trace microfilaments in the amorphous regions of cellulose, resulting in glycosidic bond breaking and depolymerization decreasing the degree of crystallinity. However, the crystallinity decreased during the heat treatment. This was mainly due to the acetyl groups on hemicelluloses falling off and acetic acids forming, which resulted in the partial acidolysis of the cellulose molecular chain at high temperatures and further destroyed the cellulose aggregation to reduce the polymerization degree of cellulose and, accordingly, led to the crystallinity reduction and thus to a decrease in the physical and mechanical properties of the wood [36]. Changes in cellulose crystallinity directly affect the physicomechanical properties of wood, such as its dimensional stability, hardness, and stiffness, and Birinci et al. obtained similar conclusions in their study on camphor pine and beech [37]. Understanding these changes is important for optimizing the wood heat treatment process and its performance in various applications.

3.3. Microstructural Analysis of Heat-Treated Wood

The morphology and microstructure of wood at different heat treatment temperatures were investigated with scanning electron microscopy ( ZEISS Sigma 300, Zeiss, Germany). Figure 6 shows the cross-section of wood after heat treatment at 150 °C, in which the surface is seen to be relatively flat and smooth, and it can be seen that all cell walls were cut clear at 4000× magnification. However, slight shrinkage and the deformation of the cell wall were observed, indicating that the temperature began to affect the wood fiber. Despite the slight compression of the cell wall, the wood microstructure remained relatively stable, with a high degree of surface smoothness.
Changes in the cross-section were evident after heat treatment at 180 °C. The surface of the cell wall exhibited a rougher texture, cracks in the intercellular layer were pronounced, and the wall became thin. These cracks may have resulted from lignin and cellulose degradation. Some studies have found that it is known that hemicelluloses were the wood cell components most degraded by the heat treatment due to their amorphous nature, low-molecular weight, and branched structure. Zauer et al. also found that cell wall compression produced cracks after the thermal modification of spruce [38]. Radial cracks occurred mainly in impermeable wood, such as Norway spruce, caused by large stresses in the wood structure during treatment [24]. Such structural changes suggest that chemical–physical changes occurred within the wood at 180 °C, which may have led to a decrease in strength and stability. In summary, when heat treatment was conducted at negative pressure above 180 °C, significant changes in the cell wall structure began to occur, which affected the properties of heat-treated timber.
Figure 7 shows the microstructure of C. funebris wood in a tangential section after heat treatment obtained using SEM at 2000× magnification (Figure 7a,b) and 10,000× magnification (Figure 7c,d). The morphology of the wood rays is shown in detail. In the samples treated at 150 °C, the wood rays have a smooth and flat appearance, and the integrity of the cellular structure is basically maintained, indicating that the microstructure of the wood was not obviously damaged (Figure 7a,c). However, after heat treatment at 180 °C, the structural integrity of the cell walls of the wood rays is maintained, but the cell lumens are narrowed owing to thermal stress (Figure 7b,d).
This change reflects a significant increase in the vapor pressure within the wood, owing to the conversion of internal moisture to vapor during heat treatment. When the vapor pressure exceeded the structural stability of the cell walls composed of cellulose and lignin in wood, the cell walls ruptured, and the connections between the original fibers broke, resulting in cracks. These cracks may also have been caused by the high vapor pressure generated by the resistance to moisture movement when the wood was heated, as the stomata of such wood were clogged with gum and methylcellulose. This further restricted the escape of moisture, thus exacerbating the internal pressure [39].
In addition, as the heat treatment temperature increased, the components of the wood cell wall, such as lignin, hemicelluloses, and cellulose, degraded to produce volatile organic compounds, further increasing the pressure inside the wood. When the pressure inside the cell lumen exceeded the tolerance limit of the cell wall, the wood ray membrane deformed, leading to crack formation, cell wall destruction, and tissue detachment. The effect of temperature led to changes in the chemical composition of heat-treated C. funebris wood, which was mainly attributed to the degradation and volatilization of carbohydrates in hemicelluloses (pentoses and hexoses). Some studies have found that the extractives content increased significantly as a function of the temperature of treatment [40]. At temperatures above 150 °C, heat treatment led to an increase in the total extractive content because of the presence of low-molecular weight substances, whereas heat treatment promoted the degradation of cellulose when the temperature was increased to 180 °C. However, most of the extractives disappear or degrade during the heat treatment, especially the most volatile ones [41].

4. Conclusions

This study explored the changes in functional groups of heat-treated C. funebris wood using Fourier transform infrared spectroscopy analysis. At the same time, the crystallinity of cellulose and the microstructure of the wood cell wall were characterized using X-ray diffraction and scanning electron microscopy. These analyses revealed the major changes in the microstructure of C. funebris wood induced by vacuum heat treatment. The principal findings can be summarized as follows:
The cellulose crystallinity of the wood increased, owing to the decomposition of hemicelluloses. It was not easy to form hydrogen bonds under negative pressure, and the number of free hydroxyl groups increased. Cellulose contains crystalline and amorphous regions, and crystallinity is a measure of the weight fraction of the crystalline regions. The decrease in the amorphous region in wood after heat treatment results in increased crystallinity [42]. The decomposition of polysaccharides in the cell wall caused a decrease in the absorption peak of the hydrocarbon groups. At the same time, the degradation products of hemicelluloses promoted the reaction of other chemical components. The relative content of lignin decreased with an increase in treatment temperature, and the performance was significant under an environment of lower negative pressure. The higher the heat treatment temperature, the greater the degradation of chemical components and the greater the impact of vacuum heat-treated C. funebris wood physics and mechanics. Heat treatment changed the chemical composition of the wood cell wall, but only minimal changes were observed via FTIR spectroscopy. After the vacuum heat treatment of C. funebris wood between 120 and 180 °C, the crystallinity showed the change rule of increasing first and then decreasing and was higher than the untreated wood specimens; the relative crystallinity range is 102.46–116.39. The negative pressure is a significant factor affecting the changes in cellulose crystallinity, thus affecting the mechanical strength and dimensional stability of wood. At lower temperatures, crystallinity decreased with increasing negative pressure, whereas at higher temperatures, crystallinity increased with increasing negative pressure. The SEM results showed that the overall morphology and structure of the cell wall remained stable under the medium-temperature vacuum heat treatment conditions; when the temperature rose to 150 °C, the structure of the cell wall was slightly deformed by the extrusion of the cell wall, and the intercellular layer produced cracks. When the temperature reached 180 °C, the cell lumen appeared to be a significantly narrowing phenomenon.
Future research could further deepen the influence of cell wall chemical components and microstructural changes on the physical and mechanical properties of C. funebris wood to reveal the specific mechanism of C. funebris properties changes at the cell wall level and provide more scientific guidance for C. funebris wood modification and application.

Author Contributions

Conceptualization, J.L. and M.C.; methodology, J.L. and M.C.; writing—original draft preparation, J.W. and J.L.; writing—review and editing, J.L. and M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Sichuan Science and Technology Program (grant no. 2023YFS0462), the Double Support Plan of Sichuan Agricultural University (grant no. 2022SYZD06).

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 2. Infrared spectra of C. funebris wood samples under different heat treatment conditions (CK indicates control check group).
Figure 2. Infrared spectra of C. funebris wood samples under different heat treatment conditions (CK indicates control check group).
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Figure 3. Effect of heat treatment and pressure (CK indicates control check group).
Figure 3. Effect of heat treatment and pressure (CK indicates control check group).
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Figure 4. Effects of (a) heat treatment time and (b) heat treatment temperature (CK indicates control check group).
Figure 4. Effects of (a) heat treatment time and (b) heat treatment temperature (CK indicates control check group).
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Figure 5. Effect of heat treatment on crystallinity of cellulose.
Figure 5. Effect of heat treatment on crystallinity of cellulose.
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Figure 6. SEM images of cross-section of C. funebris wood after heat treatment at (a) 150 °C and (b) 180 °C.
Figure 6. SEM images of cross-section of C. funebris wood after heat treatment at (a) 150 °C and (b) 180 °C.
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Figure 7. SEM images of tangential section of C. funebris wood after heat treatment (a,c) 150 °C and (b,d) 180 °C.
Figure 7. SEM images of tangential section of C. funebris wood after heat treatment (a,c) 150 °C and (b,d) 180 °C.
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Table 3. Relative crystallinity of C. funebris wood under different heat treatment conditions.
Table 3. Relative crystallinity of C. funebris wood under different heat treatment conditions.
Process ConditionsI002IamCrystallization IndexRelative Crystallization Index
Untreated group19,793.0612,200.6838.36100
120 °C/5 h/0.1 MPa20,227.4411,886.341.24107.50
180 °C/5 h/0.1 M Pa22,941.4913,317.841.95109.36
120 °C/5 h/0.02 MPa23,425.5912,966.8444.65116.39
180 °C/1 h/0.1 MPa20,144.4611,579.8642.52110.84
180 °C/5 h/0.02 MPa18,621.1111,302.8839.30102.46
180 °C/1 h/0.02 MPa18,318.1211,067.1739.58103.19
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Lyu, J.; Wang, J.; Chen, M. Effects of Heat Treatment on the Chemical Composition and Microstructure of Cupressus funebris Endl. Wood. Forests 2024, 15, 1370. https://doi.org/10.3390/f15081370

AMA Style

Lyu J, Wang J, Chen M. Effects of Heat Treatment on the Chemical Composition and Microstructure of Cupressus funebris Endl. Wood. Forests. 2024; 15(8):1370. https://doi.org/10.3390/f15081370

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Lyu, Jianhua, Jialei Wang, and Ming Chen. 2024. "Effects of Heat Treatment on the Chemical Composition and Microstructure of Cupressus funebris Endl. Wood" Forests 15, no. 8: 1370. https://doi.org/10.3390/f15081370

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