Hot Compression of Calcium Chloride and Sodium Carbonate Modifies Wood for Tsoongiodendron odorum

Abstract

To enhance the density and properties of Tsoongiodendron odorum and expand its range of applications, the physical and mechanical characteristics, such as density and hardness, were investigated after impregnating a solution of sodium carbonate and calcium chloride to generate calcium carbonate filler within the wood, followed by thermal compression. The results showed that the surface density of wood was significantly increased by 86.4%. The rate of weight gain gradually increased with sodium carbonate concentration, reaching a maximum of 55.67%. Hardness increased by 85.3%. The optimal treatment process is a compression rate of 5%, a pressing temperature of 170 °C, and a hot-pressing time of 30 min. Microscopic observations revealed that the calcium carbonate was predominantly distributed on the surfaces of wood, while the surface cells exhibited deformation upon compression. Conversely, the cells in the innermost region of the wood remained largely unaltered, thereby achieving a stratified compression effect. Impregnation and compression treatment increases the hardness and bending strength of Tsoongiodendron odorum, but spring-back may occur, and the dimensional stability measures necessitate further investigation and analysis.

1. Introduction

The Tsoongiodendron odorum is a rapidly growing evergreen broad-leaved tree belonging to the Magnoliaceae family [1], which has a fast growth rate and straight trunk. The average height growth of 20-year-old tree is 0.83 m, and the average annual growth of diameter at breast height (DBH) is 1.04 cm [2]. As a unique ancient relic plant in China, Tsoongiodendron odorum is characterized by its fine structure and exquisite pattern, which is one of the main components of forests in Southern China. It was listed as a precious native tree species by the Guangxi Forestry Department in 2010, so the research on its cultivation, processing and utilization has been increased [3]. The material exhibits excellent properties in terms of reduced cracking and ease of processing, making it highly suitable for applications in furniture and other related fields. As a result, it holds promising prospects for development. However, the basic density of Tsoongiodendron odorum is 0.417 g·m⁻³, which is determined as grade 2 light-density grade wood [3] according to the five-point grading standard of wood properties [4], making it difficult to produce high additional products. Through densification, it is possible to increase the density and hardness of Tsoongiodendron odorum wood and apply it to manufacture high-grade solid wood furniture, thereby increasing its added value and promoting plantation wood cultivation of Tsoongiodendron odorum plantation wood and filling the gap in the development and utilization of Tsoongiodendron odorum.

Wood densification is a crucial technique for wood modification. Currently, it is mainly divided into impregnation and compressive densification. Impregnation densification involves filling the internal voids of wood with material by soaking it with an impregnation agent, thereby achieving volume saturation and improved quality. Notably, some studies focused on modifying impregnated wood with impregnation agents. For example, Sun [5] used urea–formaldehyde resin to impregnate poplar. Subsequently, the resin can be cured at high temperatures to achieve a bonding effect, resulting in increased density of the internal structure and improved crystallinity of the wood. Chen [6] utilized low molecular weight phenolic resin to impregnate and modify Masson pine, resulting in a stable wet weight gain rate of approximately 80% and a maximum dry weight gain rate of 39.5%. Raman spectroscopic analysis found that the low molecular weight phenolic resin penetrated at the nanoscale, not only distributed in the cell cavity and pit of wood but also entered the cell wall through the microfilament gap. Lv [7] used epoxy resin to fill Pinus sylvestris, which enhanced its mechanical properties, such as hardness, compressive strength, elastic modulus, and static bending strength while retaining a good wood texture. Liu [8] first employed a mixture of NaOH and Na₂SO₃ to remove lignin and hemicelluloses from the wood with polyethylene glycol, nanosilane, and silane agents. This process resulted in the near-filling of the wood cell lumens with polyethylene glycol. Additionally, the combination of nano-SiO₂ and wood significantly improves its heat storage capacity, offering potential applications to regulate room temperature. These modification methods can enhance the strength and properties of wood to a certain extent. Still, some of them suffer from problems such as insufficient macromolecular impregnation and high raw material costs. Moreover, since resin is a nonrenewable resource, aldehyde resin can release formaldehyde during the wood utilization process, which can negatively impact the environment and human health [9].

The other is compressive densification, a preliminary thermal softening preprocessing of wood. During heating, the lignin increases its network by derivatives, breaks down hemicellulose, and reduces the overall cellulose content, making the modified material more hydrophobic and dimensionally stable and enhancing its compatibility [10]. Commonly used treatments include hydrothermal pretreatment [11,12,13,14], microwave pretreatment [11], and more. For example, Lu explored the spring-back rate of oil palm wood at different hot-pressing temperatures and initial moisture content, and the lowest spring-back rate could be controlled to 2.6% [15]. The moisture content prior to pressing was identified by Han as the primary factor influencing the water absorption thickness expansion rate of modified fast growth. Moreover, increasing the moisture content before pressing can effectively reduce the probability of deformation in compressed wood, thereby enhancing its stability [16]. While compressive densification is a simple and nonpolluting process, it involves physically compressing the entire wood along the radial direction after preprocessing, which increases the density by compressing the wood voids and reduces the volume of the wood to some extent.

Previous studies have examined the characteristics of Tsoongiodendron odorum [3]. However, limited research has been conducted on addressing the issues related to softwood and its limited applications. Therefore, this study employed a novel approach where wood was initially immersed in a calcium chloride solution to ensure complete infiltration of calcium chloride molecules within the cell structure. Subsequently, sodium carbonate solution was introduced under vacuum pressure conditions to form a solid calcium carbonate within the cell cavities. This innovative method effectively resolved the challenge associated with the difficult impregnation of calcium carbonate. The impregnated specimen is rapidly compacted across the wood, resulting in densification of the surface layer and reduced waste of wood resources with support from calcium carbonate. The utilization of calcium carbonate as a material is characterized by its abundant raw resources, significant economic benefits, and extensive applications. Moreover, it exhibits remarkable compatibility with wood and serves as an excellent impregnating filler for enhancing wood densification [17]. The method employed in this study combines the advantages of impregnation and compression densification, yielding favorable outcomes in the densification process of Tsoongiodendron odorum.

2. Materials and Methods

Thirty-two-year-old thinning wood (Tsoongiodendron odorum) was collected in Liangfengjiang National Forest Park in Nanning, Guangxi Zhuang Autonomous Region. Following the removal of heartwood, the sapwood was cut into a 120 × 20 × 20 cm (length × width × thickness) sample using a precision pushing saw table. Subsequently, after 30 days of natural drying at room temperature, it was polished with a planer. The appropriate amount of material was taken to determine fundamental properties, while the remaining portion was classified and numbered. After measuring the water content, one part was utilized to determine bending strength and elastic modulus, whereas the other part underwent impregnation and compression. After being exposed to the equilibrium moisture content (EMC) for 1 day, the physical and mechanical properties were assessed, followed by electron microscope observation and energy spectrum analysis. The drugs used in this study were anhydrous sodium carbonate (analysis of pure AR, purity > 99.8%, Obo Kai Chemical Co., Ltd.,Tianjin, China) and anhydrous calcium chloride (analysis of pure AR, purity > 96%, Damao chemical reagent factory, Tianjin, China) were used. The instruments and equipment are listed in

Table 1. Instruments and equipment.

Device	Model	Manufacturers	Country
Planer (single side planer)	CT508	Nanjing Haiwei Machinery Co., Ltd.	Nanjing, China
Precision push saw bench	F92NT	Odendo (Qinhuangdao) Machinery Manufacturing Co., Ltd.	Qinhuangdao, China
Oven	ED260	Binder GmbH	Tutlingen, Germany
Microcomputer controlled electronic universal testing	CMT5504	Shenzhen New Think Material Testing Co., Ltd.	Shenzhen, China
Profile densimeter	DENSE-LAB Mark 3	Beijing Oulihua Technology Development Co., Ltd.	Beijing, China
Multifunctional hot press	BY102X2	Suzhou Huaxiang Wood Machinery Co., Ltd.	Suzhou, China
Acuum pressure treatment tank	1C0917	Shandong Zhucheng Antai Machinery Co., Ltd.	Shandong, China
Scanning electron microscope	1C0917	Carl Zeiss (Shanghai) Management Co., Ltd.	Shanghai, Germany

.

In situ formation of calcium carbonate: An orthogonal test design with three factors and three levels was employed, wherein the concentration of calcium chloride solution, sodium carbonate solution, and reaction time were designated as the three factors with each factor having equal gradient across three levels. A total of 9 groups were established, with 3 replicates for each group.

Table 2. Design table of impregnation orthogonal test.

Device	Model
	1	2	3
Reaction time/h	24	36	48
CaCl2 concentration/%	35	40	45
Na2CO3 concentration/%	20	25	30
Vacuum degree 0.08 MPa, vacuum time 20 min, vacuum pressure 0.8 MPa

shows the orthogonal experimental design of impregnation treatment. Physical and mechanical measurement was performed after treatment. First, Tsoongiodendron odorum boards were immersed in a calcium chloride solution with three concentration gradients of 35%, 40% and 45% for five days and then dried at low temperature (60 °C) after being removed from the solutions for 1 day. Then, it was pumped and pressurized in a vacuum treatment tank. Figure 1a shows the impregnation of calcium chloride in a vacuum treatment tank. The sodium carbonate solution with a concentration of 20%, 25% and 30% was impregnated, respectively, so it quickly entered the interior of the boards. It prevented a large amount of calcium carbonate from being generated on the surface of the boards to block the intercellular canal. After being removed from the solution for one day, it was dried again at a low temperature (60 °C) until the EMC is reached. This impregnation process enabled calcium carbonate production within the wood, which was essential for the subsequent compression and densification process. Figure 1b shows the image of calcium carbonate produced inside the wood.

Compression: The orthogonal test design of three factors and three levels was set with press temperature, hot-pressing time and compression ratio as factors (

Table 3. Compression orthogonal test design.

Factor	Level
	1	2	3
Compression ratio/%	5	10	15
Pressurization temperature/°C	160	170	180
Hot-pressing time/min	15	30	45
Holding pressure 14.0 Bar

). Each experiment was repeated 3 times. Put the impregnated boards and untreated boards into the oven for low-temperature drying, measure their dimensions, and then put them into the multifunction hot press. Set the parameters of the hot press according to Table 3. After hot pressing, the size of the sample wood was measured again, and then it was heated in the oven at 180 °C for 3 h to fix. Figure 2 shows the comparison between impregnated compression treatment and uncompressed and normal compression.

Profile density: To obtain the density data of each profile and generate a scatter diagram, we utilized a plate saw to produce a 50 (tangential) × 50 mm (transverse) specimen. The transverse section was placed horizontally, while the tangential section was placed in a unified direction in the profile densimeter, with the upper tangential section serving as the starting profile. The instrument was then operated to obtain density data for each profile and create a scatter plot for analysis.

Hardness: According to ISO 13061-12:2014, the hardness test was carried out on the microcomputer-controlled electronic universal test machine (Figure 3). The steel ball of the testing machine pressed the sample to 5.64 mm at a uniform speed of 5 mm/min. The sample was measured once on each of the two chord surfaces, and the chord surface size was 50 × 50 mm. The hardness formula is as follows:

$${\mathrm{H}}_{\mathrm{W}}=\mathrm{K}\mathrm{P}$$

(1)

$${\mathrm{H}}_{12}={\mathrm{H}}_{\mathrm{W}}\left[1+0.03\left(\mathrm{W}-12\right)\right]$$

(2)

• ${\mathrm{H}}_{\mathrm{W}}$—Hardness when the moisture content is W (N);
• H₁₂—Hardness when the moisture content is 12% (N);
• K—Coefficient (i.e., 1 and 4/3 when the depth of the specimen is 5.64 and 2.82 mm, respectively);
• P—Loading on the specimen (N).

Spring-back ratio: The thicknesses of the nine sets of samples were determined according to the compression orthogonal test design. Before hygroscopic treatment was performed, the size and quality of the samples were measured, and then, the samples were put into a constant temperature and humidity box for hygroscopic treatment, the temperature was 40 °C and the relative humidity was 90% (simulating the summer in Southern China) [18].

R = (T_r − T_c)/(T₀ − T_c) × 100%

(3)

• R—Spring-back ratio, %;
• T_r—Specimen thickness after hygroscopic drying, mm;
• T_c—Specimen thickness after compression, mm;
• T₀—Specimen thickness before compression, m.

Modulus of elasticity and bending strength: Referring to ISO 3349, an electronic universal testing machine was used to test the bending elastic modulus. The span of the support was set to 240 mm, and the pressure direction was run at a constant speed of 5 mm/min so that the inpressure head of the intermediate test machine was constantly pressurized until the specimen was damaged. The calculation formula [19] is as follows:

$${\mathsf{\sigma }}_{\mathrm{b}\mathrm{w}}=\frac{3{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\mathrm{L}}{2\mathrm{b}{\mathrm{h}}^2}$$

(4)

• P_max—Maximum load (N);
• L—Support distance (mm);
• b—Specimen width, i.e., radial (mm);
• h—Specimen height, i.e., chord (mm).

Flexural strength was tested, referring to ISO 3133. Set the span of the support to 240 mm, pressurize at 1/3 and 2/3, and install a deflection dial indicator at 1/2 below the specimen. The calculation formula is as follows:

$${\mathrm{E}}_{\mathrm{w}}=\frac{23\mathrm{P}{\mathrm{L}}^3}{108\mathrm{b}{\mathrm{h}}^3\mathrm{f}}$$

(5)

• P—Load difference between upper and lower limits (N) (upper limit 700 N, lower limit 300 N);
• L—Support distance (mm);
• b—Specimen width, i.e., radial (mm);
• f—Center deflection (mm).

In order to reduce the difference caused by the different water content of each specimen, the formula of 12% water content is uniformly converted:

$${\mathsf{\sigma }}_{\mathrm{b}12}={\mathsf{\sigma }}_{\mathrm{b}\mathrm{w}}\left[1+0.004\left(\mathrm{W}-12\right)\right]$$

(6)

$${\mathrm{E}}_{\mathrm{b}12}={\mathrm{E}}_{\mathrm{b}\mathrm{w}}\left[1+0.015\left(\mathrm{W}-12\right)\right]$$

(7)

W—The moisture content of the specimen is measured.

Scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS): The test material with a compression ratio of 15%, press temperature of 170 °C and hot-pressing time of 15 min and the test material with impregnation of 40% calcium chloride and 30% sodium carbonate and reaction time of 24 h were 3 pieces for each group. The width of the sample was less than 10 mm. The thickness was approximately 60 μm. The voltage was 15.0 kV. Scanning electron microscopy was utilized to observe the samples’ cell wall morphology, and then the energy spectrum was obtained by “point” scanning method for multiple imaging, and then the element distribution in the surface and middle of the wood was analyzed.

Data treating: Excel was utilized for data sorting and statistical analysis. SPSS 26.0 was used for significance analysis of variance, and multiple comparison tables were used to mark the significance of different treatments. At the same time, the range analysis is used to analyze the experiment, and the optimal test conditions are obtained.

3. Results and Discussion

3.1. The Effect of Wood Weight Gain

The impenetrable weight gain of the Tsoongiodendron odorum not only enhances its strength but also improves the stability of the wood. As shown in

Table 4. Analysis of weight gain effect and range of Tsoongiodendron odorum.

Numbering	Calcium Chloride/%	Sodium/%	Reaction Time/h	Weight Gain Rate/%
1	35	20	24	41.31 ± 2.57 abc (0.0621)
2	35	25	36	31.52 ± 0.48 cd (0.0151)
3	35	30	48	51.32 ± 1.22 a (0.0238)
4	40	20	36	48.19 ± 2.27 ab (0.0471)
5	40	25	48	34.02 ± 4.53 bcd (0.1454)
6	40	30	24	55.67 ± 4.10 a (0.0736)
7	45	20	48	48.50 ± 3.14 ab (0.0647)
8	45	25	24	21.81 ± 3.41 d (0.5164)
9	45	30	36	55.20 ± 3.05 a (0.0553)
X1	41.38%	46.00%	39.60%
X2	43.68%	29.12%	44.97%
X3	49.76%	54.06%	44.61%
R	8.37%	24.95%	5.37%

X1, X2, X3: three repetitions of the experimental design. R: range value representing the degree of influence of the factor. a–d: the significance of differences between different treatments. The coefficient of variation is in parentheses.

, different impregnation processes had varying effects on the rate of weight gain. However, the increase ranged from 21.81% to 55.67%, with an average of more than 40%, indicating a favorable weight gain effect. Notably, the weight gain rate was significantly higher when the concentration of sodium carbonate was at the third concentration gradient (30%) compared to other concentrations. Conversely, the rate of weight gain was relatively low when the sodium carbonate concentration was 25%. These findings suggest that impregnation with sodium carbonate at a concentration of 30% is optimal for achieving the desired weight gain in Tsoongiodendron odorum.

3.2. Analysis of Cross-Sectional Density

In general, wood with higher density exhibits better physical and mechanical properties. Tsoongiodendron odorum wood exhibited a significant increase in density upon impregnation and compression. Figure 4 shows the density profile of the wood, with impregnated and normal compressed wood, indicating approximately symmetric density profiles, with a significant increase in density observed in the surface section. The maximum profile density of the impregnated compressed wood was observed at 3.7 mm, reaching 1540 kg/m³, followed by a decrease in the form of small waves, which stabilized at approximately 700 kg/m³ at 4.2 mm. Two reasons could explain these observations. First, calcium carbonate is primarily formed on the wood surface during impregnation, significantly increasing surface density. Second, during the hot-pressing process, the wood surface reaches a high-temperature state, leading to a transitional hemicellulose and lignin in the cell wall from a glassy to a highly elastic state. Consequently, under the pressure exerted by the hot-pressing plate, deformation occurs within the cell wall [20]. The middle part of the wood has a low temperature and is not softened, so it is not easy to compress and deform. The density of the impregnated compressed material was approximately 40% higher than that of the untreated material, with the impregnated compressed material exhibiting a density 86.4% higher than that of the untreated material.

3.3. Analysis of Spring-Back Ratio

The spring-back of wood has a significant impact on its processing and utilization. Generally, wood with smaller spring-back deformation is more conducive to maintaining the stability of its physical and mechanical properties. As depicted in Figure 5 and

Table 5. Range analysis of spring-back ratio.

Level	Impregnated Compression Material
	Compression Ratio	Pressurization Temperature	Hot-Pressing Time
X1	43.4%	33.9%	33.4%
X2	35.5%	34.6%	32.4%
X3	30.6%	27.0%	29.8%
R	12.8%	7.6%	3.7%

X1, X2, X3: three repetitions of the experimental design. R: range value representing the degree of influence of the factor.

, the average spring-back ratio of impregnated compressed materials ranges from 10.1% to 34.1%, the compression rate having the most significant influence on the spring-back ratio. The increase in the compression rate results in a downward trend in the spring-back ratio. The lowest spring-back ratio was observed at a compression rate of 15%, press temperature of 180 °C, and hot-pressing time of 30 min. This is attributed to the filling of internal voids in the impregnated compressed wood with calcium carbonate, creating support stress during compression. Calcium carbonate has low plasticity, enabling larger compression rates to stabilize wood size and enhance stability. High-temperature hot pressing softens the wood, improving plasticity. However, low temperatures have little compression effect, while high temperatures deepen the color of wood panels. Therefore, it is advisable to control press temperature within the range of 160 °C to 180 °C. Hot-pressing time has little effect on the spring-back ratio due to the relatively loose internal structure of Tsoongiodendron odorum. After calcium carbonate impregnation and hot pressing, some filling materials completely solidify in the wood.

3.4. SEM Analysis

Figure 6 shows a scanning electron microscopy image of the cell wall cross section of a Tsoongiodendron odorum impregnated with a compressive material. The cell wall on the surface of the wood was significantly deformed compared to the cell wall in the middle of the sample, and the volume of the cell cavity became smaller after rapid compression (Figure 6a). In contrast, the cells in the middle remain intact and do not deform (Figure 6b). This was attributed to a stronger support stress in the cell cavity due to calcium carbonate impregnation. Rapid compression squeezes the surface cells, and the pressure weakens as it is transferred to the interior of the wood, resulting in insufficient external pressure to alter cell morphology. Rapid compression after calcium carbonate impregnation leads to the surface layer’s densification and maintains the internal cells’ original shape without deformation. This improves the hardness and strength of the wood surface layer, achieving the goal of enhancing the physical and mechanical properties of the wood and conserving volume.

3.5. EDX Analysis

Figure 7 and Figure 8 present the energy spectrum analysis of the transverse cross-section of compressed wood impregnated with Tsoongiodendron odorum. The highest levels of elements in the test material were C and O, primarily from the wood itself, with a small amount from calcium carbonate in the impregnation. The immersion in sodium carbonate and calcium chloride results in the corresponding Na, Cl, and Ca peaks. Although the concentration of calcium chloride is higher than sodium carbonate in the impregnation agent, the Na content of the impregnated wood is higher, indicating that the vacuum pressure impregnation method allows more material to enter the wood for better impregnation. Comparing the cross section energy spectrum analysis of the surface layer and middle part of impregnated compressed wood, the atomic mass fraction of calcium in the surface layer of the test material is 0.231%, while that in the middle part of the test material is 0.054%, indicating the sufficient in situ formation of calcium carbonate for wood impregnation, thus filling the interior of the wood with calcium carbonate.

3.6. Hardening Effect

Hardness and bending resistance are crucial mechanical properties of wood. Improving wood hardness enhances product durability and corrosion resistance, reducing cracking and deformations.

Table 6. Hardness measurement results.

No.	Compression Ratio(%)	Press Temperature (°C)	Hot-Pressing Time (min)	Hardness (MPa)
				Ordinary Compression Material	Impregnated Compression Material
1	5	160	15	2445.31 ± 104.92 cd (0.04)	3027.41 ± 61.39 abcd (0.02)
2	5	170	30	2554.32 ± 147.23 cd (0.06)	3885.57 ± 40.11 a (0.01)
3	5	180	45	2673.79 ± 129.58 ab (0.05)	2878.31 ± 76.70 cd (0.03)
4	10	160	30	2356.74 ± 114.45 cd (0.05)	2682.65 ± 52.9 d (0.02)
5	10	170	45	2763.09 ± 84.56 bc (0.03)	3045.15 ± 40.32 abcd (0.01)
6	10	180	15	2576.13 ± 28.45 cd (0.01)	2783.97 ± 61.15 bcd (0.02)
7	15	160	45	3202.46 ± 93.68 a (0.03)	3405.76 ± 70.97 abc (0.02)
8	15	170	15	2621.91 ± 93.21 bcd (0.04)	3458.30 ± 21.93 ab (0.01)
9	15	180	30	2621.67 ± 82.04 d (0.03)	3198.82 ± 88.21 bcd (0.03)

a–d: the significance of differences between different treatments. The coefficient of variation is in parentheses.

shows that the hardness of ordinary compressed materials ranges from 2356.74 to 3202.46 MPa, while impregnated consolidated materials reach a hardness of 2682.65 to 3885.57 MPa, and the average hardness is 21.3% higher than that of ordinary compressed materials.

Table 7. Hardness determination range result.

Level	Impregnated Compression Material
	Compression Ratio	Press Temperature	Hot-Pressing Time
X1	3195.59 MPa	3038.60 MPa	3089.89 MPa
X2	2837.26 MPa	3463.01 MPa	3255.68 MPa
X3	3354.29 MPa	2885.52 MPa	3041.56 MPa
R	517.03 MPa	577.49 MPa	214.11 MPa

X1, X2, X3: three repetitions of the experimental design. R: range value representing the degree of influence of the factor.

shows that the press temperature range is the largest (R = 577.49), indicating that it has the most significant effect on the hardening of Tsoongiodendron odorum, followed by the compression ratio (R = 517.03). The press temperature has a significant effect on the hardening effect. At a temperature of 170 °C, the test material’s hardness increases significantly, surpassing those at press temperatures of 160 °C and 180 °C. With a compression ratio of 5% and a hot-pressing time of 30 min, the hardness is maximum at 385.57 MPa. This is attributed to the reduced moisture content of the wood during the hot-pressing process. High temperature weakens the transverse bonding strength between wood fibers, leading to a decrease in mechanical properties [20]. Low temperatures result in insufficient softening of the lignin.

3.7. Bending Performance

Bending strength measures the ability of wood to resist bending without breaking. Enhancing the bending strength improves the mechanical properties of Tsoongiodendron odorum and expands its usefulness.

Table 8. Bending resistance results.

No.	Compression Ratio (%)	Press Temperature (°C)	Hot-Pressing Time (min)	Ordinary Compression Material		Impregnated Compression Material
				Bending Strength (MPa)	Modulus of Elasticity(MPa)	Bending Strength (MPa)	Modulus of Elasticity (MPa)
1	5	160	15	81.50 ± 7.13 ab (0.09)	9732.33 ± 194.42 ab (0.02)	66.18 ± 7.98 a (0.12)	8422.89 ± 428.23 a (0.05)
2	5	170	30	83.41 ± 4.67 a (0.06)	9837.45 ± 574.08 a (0.06)	78.82 ± 7.66 a (0.10)	9392.62 ± 652.94 a (0.07)
3	5	180	45	68.26 ± 0.70 bcd (0.01)	8529.05 ± 309.27 bc (0.04)	74.72 ± 7.97 a (0.11)	7513.35 ± 248.31 a (0.03)
4	10	160	30	75.43 ± 5.13 abc (0.07)	9689.30 ± 334.64 abc (0.03)	82.73 ± 7.98 a (0.10)	9293.48 ± 684.75 a (0.07)
5	10	170	45	80.99 ± 8.34 ab (0.10)	9688.31 ± 370.96 a (0.04)	80.78 ± 7.98 a (0.10)	8561.52 ± 247.17 a (0.03)
6	10	180	15	63.23 ± 10.33 d (0.16)	9509.16 ± 567.91 c (0.06)	85.59 ± 9.52 a (0.11)	9865.58 ± 368.72 a (0.04)
7	15	160	45	79.83 ± 3.52 ab (0.04)	9362.55 ± 81.68 abc (0.01)	81.15 ± 8.60 a (0.11)	9548.67 ± 282.34 a (0.03)
8	15	170	15	65.87 ± 9.96 cd (0.15)	9270.19 ± 288.89 abc (0.03)	75.57 ± 7.98 a (0.11)	9476.65 ± 467.59 a (0.05)
9	15	180	30	80.12 ± 7.94 ab (0.10)	9689.72 ± 652.27 ab (0.07)	66.99 ± 7.98 a (0.12)	8595.16 ± 430.40 a (0.05)

a–d: the significance of differences between different treatments. The coefficient of variation is in parentheses.

and

Table 9. Range analysis of bending strength and modulus of elasticity of impregnated compressed materials.

Level	Compression Ratio		Press Temperature		Hot-Pressing Time
	Bending Strength (MPa)	Modulus of Elasticity (Mpa)	Bending Strength (Mpa)	Modulus of Elasticity (Mpa)	Bending Strength (Mpa)	Modulus of Elasticity (Mpa)
X1	73.24	8442.95	76.69	9088.35	75.78	9255.04
X2	83.03	9240.19	78.39	9143.60	76.18	9093.75
X3	74.57	9206.83	75.77	8658.03	78.88	8541.18
R	9.79	797.24	2.62	485.57	3.10	713.86

X1, X2, X3: three repetitions of the experimental design. R: range value representing the degree of influence of the factor.

present the bending resistance of the Tsoongiodendron odorum impregnated compression material. The bending strength of ordinary compressed materials ranges from 63.23 to 83.41 MPa. In comparison, that of impregnated compressed materials ranges from 66.18 to 85.59 MPa, slightly higher than that of ordinary compressed materials. As can be seen in Table 9, the compression rate (bending strength: R = 9.79, modulus of elasticity: R = 797.24) is the most prominent factor affecting the bending resistance, while the compression temperature and the hot-pressing time do not have a regular effect on the bending resistance. Generally, the bending strength and bending modulus of impregnated compression material are higher when the compression temperature is 180 °C and the hot-pressing time is 15 min, contributing to the best bending resistance. Since the mechanical properties are primarily determined by the internal structure of the wood, press temperature and hot-pressing time within a certain range do not significantly affect the internal structure and surface density of the wood. Also, calcium carbonate fills the primary voids within the wood. After rapid compression, the difference in elasticity between the interior and exterior of the wood is small, and the compression ratio shows a more significant effect. When the compression ratio is 10%, the average bending strength and bending modulus of the impregnated compressed material are relatively higher. Therefore, the bending resistance is the best when compression temperature and hot-pressing time is 180 °C and 15 min.

The properties of ordinary and impregnated compressed wood with those of the material regarding bending properties were compared. The results in Figure 9 indicate that Tsoongiodendron odorum exhibits optimal bending strength and bending modulus after impregnation compression treatment. As shown in

Table 10. Analysis of bending resistance of impregnated and untreated materials and ordinary compressed materials.

Treatment	Modulus of Elasticity (MPa)	Improved Value (%)	Bending Strength (MPa)	Improved Value (%)
Impregnated compressed timber	9865.58	33.2	85.59	55.7
Unseasoned timber	7406.04		54.97
Impregnated compressed timber	9865.58	0.3	85.59	2.6
Ordinary compression	9837.45		83.41

, the bending modulus of the impregnated compressed material is 0.3% and 33.2% higher than that of the ordinary compressed and impregnated materials, respectively. The bending strength was also increased by 2.6% and 55.7%, respectively, demonstrating that the compression treatment significantly enhanced the bending resistance of Tsoongiodendron odorum. Moreover, the impregnation compression treatment yields superior results compared to the ordinary compression treatment.

Tsoongiodendron odorum is a species of broad-leaved tree with soft wood and low density, which limits its range. In this study, we aimed to enhance the properties of the wood by impregnating it with a solution of calcium chloride and pressing sodium carbonate into the test material using a vacuum pressure method. The collaborative treatment of impregnation and compression synergistically integrates the merits of both methods while mitigating their respective limitations, thereby yielding superior outcomes in modification. A precipitate reaction takes place in the cell cavity, resulting in the formation of calcium carbonate and weight gain. Sodium carbonate and calcium chloride are cost-effective, readily available and environmentally friendly. Furthermore, they penetrate deeper into the interior of the wood after vacuum pressure impregnation, making them ideal impregnation materials. The calcium carbonate produced by this reaction exhibits a good affinity for wood and enhances weight gain. The in situ generation method overcomes the problems of blockage of the surface channels caused by the fillers and insufficient impregnation.

The rate of weight gain is a key indicator of the effectiveness of impregnation treatment. Range analysis revealed that the concentration of sodium carbonate is the primary factor affecting the rate of weight gain. This is because the test material was initially impregnated with a sodium carbonate solution as the base liquid for the precipitate reaction, and the concentration plays a crucial role in the amount of precipitate. When the sodium carbonate solution is in the third concentration gradient (30%), the overall weight gain exceeds 50%, and the average weight gain rate is 54.06%. By comparison, traditional impregnation treatments, such as phenolic resin, achieved an impregnation rate of 49.6% with a weight gain rate of only 14.56% [21]. Similarly, the highest weight gain rate of Masson pine impregnated with tea polyphenols was 14.45% [22], and the highest weight gain rate of Chinese fir modified with PF resin was 9.2% [23]. Compared to conventional polymerization methods, in situ formation of calcium carbonate yields superior polymerization effects and partially resolves the challenging polymerization problem.

Further verification tests involved energy spectrum analysis of the test material’s surface and middle (Figure 7 and Figure 8). The results indicated that the impregnating agent had fully penetrated the wood and reached the material’s center, as demonstrated by the presence of Ca. This is similar to Chen Gongzhe’s conclusion [24]. Although the addition of sodium carbonate under vacuum pressure initially reacted with calcium chloride on the surface, pressure impregnation enabled the impregnating agent to enter the wood interior and form calcium carbonate filler, resulting in a nearly 40% higher density in the middle of the impregnated compression material than that of the material (Figure 4).

The scanning electron microscope (SEM) comparison of the surface and middle parts (Figure 5) revealed that the surface cell wall was compressed and deformed, while the middle cell wall was almost intact and not compressed. Wang’s periodic hot pressing densification SEM images of Chinese fir showed that the early and late wood cell walls and wood rays experienced varying degrees of deformation and distortion [25], indicating that traditional physical compression compressed the entire plate and significantly reduced the volume. The profile density distribution in our study (Figure 4) showed that the surface density was significantly higher than the middle density, demonstrating a high outside and low inside’ density distribution resulting from the formation of calcium carbonate that increased the test material’s density. Due to the support of calcium carbonate, the rapid compression after impregnation increases the surface density of the wood and achieves the effect of stratified compression. This is an ideal state of dense wood surface [26], with a hardened surface and minimal increase in internal density, which can enhance wood’s physical and mechanical properties and reduce volume loss. Meanwhile, the filling and rapid compression of the calcium carbonate significantly improved the hardness of the wood, which was nearly 30% higher than that of ordinary compressed wood. Surface hardening elevates Tsoongiodendron odorum to the standard of furniture materials, addressing the problems of narrow application range and low product-added value. In addition, during heat treatment, with the increase of temperature, lignin increases its network through ramification, hemicellulose decomposition, and holocellulose content decreases, thus making wood materials more hydrophobic and dimensional stable. The lower thermal degradability enhances the application range of wood as enhanced solid wood or wood–polymer composite materials, and also improves their material properties and compatibility [10].

4. Conclusions

With the method of generating calcium carbonate inside wood, the concentration of calcium carbonate exerts a significant influence on the weight gain of wood, and within a specific range, increasing the concentration of calcium carbonate can enhance the weight gain rate. At the same time, the vacuum impregnation process makes the wood completely filled with calcium carbonate, which solves the problem of conventional polymer impregnation difficulty. After rapid compression, the surface cells of wood were highly compressed and dense, while the middle cells remained almost intact, which saves the volume of the wood, increases the surface density, and significantly enhances the hardness of the Tsoongiodendron odorum, achieving the effect of improving the performance of the wood without increasing the loss of wood. This modification method can be used for more species with low density and soft materials and provide a theoretical basis for the high-quality utilization of future planted forests. The further research plan is to further explore the dimensional stability of wood in view of the problem of wood resilience and optimize the wood modification technology of generating calcium carbonate inside wood.