The Influence of High-Temperature and -Pressure Treatment on Physical Properties of Albizia falcataria Board

Abstract

Albasia (Albizia falcataria), known as sengon wood, is a fast-growing tree species commonly found in Indonesian forests and community plantations. However, the low-density, hardness, and strength significantly restrict its commercial application. The purpose of this study was to determine the influence of densification on the physical properties of Albizia falcataria under high-temperature and -pressure. Different temperatures were applied to the Albizia falcataria board (100 °C, 120 °C, 140 °C, sandwich 140 °C). The densification process influences the density properties, color changes, thickness, compression ratio, equilibrium moisture content, and anatomical properties of the material. With this procedure, the density can be increased to 0.62 kg/L, a gain of approximately 112.78% over untreated wood. The density of wood increases, resulting in the decomposition of its chemical components, especially hemicellulose, which darkens the wood color and stabilizes equilibrium moisture control. As a result, the thermal compression modification treatment under high-temperature and -pressure is a highly effective method for enhancing the physical properties of fast-growing wood species, such as Albizia falcataria.

1. Introduction

Evaluating the environmental impact of material selections is becoming more important as means of addressing sustainability concerns. In terms of ecological advantages, utilizing wood as a construction material is one of the most effective methods to decrease carbon dioxide emissions [1], since wood material contains 50% carbon throughout its growth, collecting carbon dioxide from the air. In brief, the more wood products utilized, the more carbon is stored, thus mitigating the effects of global warming. Unfortunately, trees grow slowly, disrupting the sustainability of the existing wood availability. As a result, developing alternative wood sources is critical, one of which is fast-growing tree species [2]. Due to the plentiful availability of fast-growing tree species, they have been extensively utilized in plantations and community forests, making the sustainability of wood is more promising for the environment.

Albasia (Albizia falcataria), known as sengon wood, is a fast-growing tree species commonly found in Indonesian forests and community plantations. Apart from numerous advantages, such as a short harvest period (4–7 years) and simple location requirements for growth [3,4,5], the development of Albizia falcataria is also consistent with the Indonesian government’s goal of becoming the world’s largest supplier of lightwood, through the potential of Albizia falcataria in the ILCF (Indonesian Lightwood Cooperation Forum) 2018. Fast-growing wood is an alternative solution for replacing the function of broadleaf plants as a material for floor, furniture, and interior elements [6], as well as structural components [7]. However, the low-density, hardness, and strength of fast-growing wood limit its commercial use. Therefore, increasing the density and properties of the wood is critical, as denser wood is frequently preferred for commercial use, particularly in construction [8,9,10,11,12].

To enhance the value of fast-growing wood with low properties, increasing its density in a procedure known as wood densification, is advantageous. Densification of wood is a practical modification technique for improving its properties to produce new materials that do not pose a greater environmental risk than non-modified wood when discarded [13,14]. The first wood densification studies were done in Germany in the 1930s [15]. One well-known method for wood densification is thermal compression under high-temperature and -pressure [16,17,18]. The thermal compression method, under high-temperature and -pressure treatment, consists of two distinct phases: wood softening and compression, followed by a post-treatment phase that minimizes irreversible re-thickness swelling when modified wood makes contact with moist or wet environments. In comparison to other methods of wood densification, the benefits of this method include the absence of chemicals and ability to enhance the properties of wood, regardless of species [19,20,21].

Improving the characteristics of Albizia falcataria in sustainably is one of the most significant challenges facing wood technology. To optimize the process of improving wood properties, it is essential to comprehend how operating parameters affect properties in the wood densification process. Until recently, data on the impact of the wood modification on Albizia falcataria performance were scarce. Furthermore, the purpose of this study was to determine the influence of the thermal compression modification treatment under high-temperature and -pressure on the physical characteristics of Albizia falcataria.

2. Materials and Methods

The wood boards of Albizia falcataria are obtained from one of the building material supplier industries in Kitakyushu City, Fukuoka Prefecture, Japan. The Albizia falcataria used in this study is certified by Indonesian Timber Legality Verification (SVLK) and bears the Indonesian Legal Wood logo for the Japanese market. Under the SVLK rule, regulated by the Indonesian Ministry of Industry, it is regarded permissible in the chain of harvesting, processing, transporting, and trading as an export-oriented destination [22].

2.1. Sample Preparation

Samples were taken in accordance with the requirements and methods specified in the Japanese Industrial Standard (JIS) Z 2101: 2009 [23] for testing the mechanical and physical properties of wood. The defect-free boards had the following dimensions: 13.5 × 150 × 910 mm (thickness × width × length) wrapped in plastic from the manufacturer. All specimens were pre-conditioned to a temperature of 20 °C and humidity of 60% before the experiment. The samples were then cut and prepared for treatment with 13.5 × 120 × 910 mm (thickness × width × length).

2.2. Experiment Procedures

The experiment was carried out in the high-temperature and -pressure apparatus HTP-50/130 (HISAKA Company, Osaka, Japan) at the Graduate School of Environmental Engineering, The University of Kitakyushu, Kitakyushu, Japan. The system consisted of the processing tank and boiler setting machine. The experimental procedure for the Albizia falcataria board at high-temperature and -pressure is visualized in Figure 1. The procedure begins with setting the mold device, inserting the wood specimen into the mold device, adjusting the upper plate on the mold device, inserting the mold device into the processing tank, selecting, and adjusting the experimental order settings, and, finally, performing the consolidation process of high-temperature and -pressure treatment.

As seen in Figure 2, the consolidation step of the high-temperature and -pressure treatment on the Albizia falcataria board begins with a five-minute vacuum phase, followed by a softening phase, involving heating and temperature holding processes. According to the previous study, the minimum temperature required to perform heat treatment is 100 °C [24]. The wooden board specimens were divided into one for control and four categories with varying degrees of high-temperature and -pressure treatment. Six replicated measurements were recorded for each condition. There were four categories used: 100, 120, 140, and a sandwich (treatment formation two samples top as S[1] and bottom as S[2], divided with 3 mm aluminum base) at 140 °C.

The heating process takes ten minutes to reach the desired temperature. This process reduces the defects caused by temperature differences between the wood sample and processing environment. Starting at this point, the temperature holding process can take up to 30 min. After softening, the wood was compressed to 50% thick. During the compressing phase, the two engine cylinders press against the top steel plate.

The recovery in thick timber is one of the method’s main flaws (spring-back). After the experiment, the spring-back phenomenon has been defined as lost compression work [25]. After heating, the temperature difference between the wood and ambient air is high. Thus, to minimize the fault defects in the wood after the modification phase, the cooling phase of the wood is carried out in the processing environment until the temperature is equivalent to that of the external environment. After densification, all specimens were stored for up to 360 H by weighing them periodically until the mass difference was negligible for the test.

2.3. Measurement of Physical Properties

2.3.1. Density Profile Measurement

The wood density was measured using the following Equation (1):

$${\rho }_W=\frac{m_W}{V_W}$$

(1)

where ρ_w denotes the density; m_w denotes the mass; and v_w denotes the volume, all at moisture content w. The mass of the wood specimen was measured using a digital weighing scale, with a readability of 0.01 g (A&D: FZ-5000i). Then, we determined the volume using a caliper (Mitutoyo: ABSOLUTE Digimatic Scale Units-572 Series 0-8in/0–200 mm), with a 0.01 mm accuracy.

Set-recovery for density (SR-D) was determined by Equation (2) [10]:

$$\mathrm{SR}-\mathrm{D} (\%)=\frac{\mathrm{Ds}-\mathrm{Da}}{\mathrm{Do}-\mathrm{Da}} \times 100$$

(2)

Ds = recovery density after equilibrium moisture content was re-reached (kg/L)

Da = actual density after high-temperature and -pressure (kg/L)

Do = original density before high-temperature and -pressure (kg/L)

And the parameter density change (DC) [26] was calculated as listed in Equation (3):

$$\mathrm{DC} (\%)=\frac{\mathrm{Ds}-\mathrm{Do}}{\mathrm{Do}} \times 100$$

(3)

2.3.2. Color Changes

Among the physical properties of wood, color is essential for wood applications because it can determine its market value, regarding aesthetic purposes [27]. Regardless, the original color of the wood may change during processing and after treatment, although the direction of this change varies, according to the wood species.

2.3.3. Change in Thickness

Board thickness was determined before and after densification. To assess the densification of spring-back rate, wood specimens were put under predefined relative humidity and temperature conditions for a specified length of time [28,29,30]. In investigating the ‘spring-back’ possibility of the samples, the thickness of each sample was measured with a caliper, under normal atmospheric conditions, on four different sides.

Set-recovery for thickness (SR-T) was determined by Equation (4):

$$\mathrm{SR}-\mathrm{T} (\%)=\frac{\mathrm{Ts}-\mathrm{Ta}}{\mathrm{To}-\mathrm{Ta}} \times 100$$

(4)

Ts = thickness recovery after equilibrium moisture content was re-reached (mm)

Ta = board thickness after high-temperature and -pressure (mm)

To = board thickness before high-temperature and -pressure (mm)

As follows, the final parameter thickness change (TC) was used to analyze the thickness change, in the manner specified in Equation (5).

$$\mathrm{TC} (\%)=\frac{\mathrm{Ts}-\mathrm{To}}{\mathrm{To}} \times 100$$

(5)

2.3.4. Compression Ratio

Compression ratio was used to determine treatment efficiency [31]. The thickness of densified boards was determined prior to and following densification. The level of densification, which also known as the compression ratio (CR), was determined using Equation (6), whereas To denotes the original thickness and Ta denotes the final compressed thickness.

$$\mathrm{CR} (\%)=\frac{\mathrm{To}-\mathrm{Ta}}{\mathrm{To}} \times 100$$

(6)

2.3.5. Equilibrium Moisture Content

The main consequence of high-temperature and -pressure treatment is reducing moisture content equilibrium. The moisture content (MC) of wood changes as the relative humidity (RH) changes. When subjected to constant RH, the wood progressively acquires a steady equilibrium moisture content (EMC) [32]. The moisture level at which the wood is neither accumulating nor losing moisture is known as EMC [33]. It has been well accepted that EMC is a physical property of wood that affects not only the dimensions of wood but also its mechanical qualities [34].

In this study, the moisture content of the wood was determined using a wood moisture meter (Kett: HM-530 20 MHz/2–50% range) at six points on the sample, measured from top to bottom. These measurements leave no trace on the surface of the material being examined, due to the high-frequency measurement format.

2.3.6. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) was used to investigate the anatomical changes caused by the treatment. As seen in Figure 3a, the wood fiber morphology was examined by a JSM-7800F Schottky Field Emission SEM (JEOL Company, Tokyo, Japan). An observation plate was made from fractured 3 × 5 × 5 mm (thickness × width × length) cross-sections of each test sample. Before the anatomical examination, the samples were dried for 24 h at 105 °C in a constant temperature oven DKM600 (Yamato Scientific Co., Ltd., Tokyo, Japan), as shown in Figure 3b, to reduce the moisture content by approximately 3–5%. The outer surface of the wood specimens was examined under SEM (100, 500, and 1000$\mathsf{\mu }\mathrm{m}$ magnification) at 15 kV without metal coating.

2.3.7. Statistical Analysis

For each category, one-way analysis of variance (ANOVA) was used to determine the influence of high-temperature and -pressure treatment on the mass change, density, thickness, and EMC of Albizia falcataria. Statistical analysis was used to elicit useful information about the characteristics of the sample determined in the experiment. To determine the significance and mean separation of the treatments, a homogeneity test was used [35]. Average values were compared using Tukey’s honestly significant difference (HSD) test; this test considers all pairwise comparisons statically significant at p < 0.05 [36]. The significance level in all tests was p < 0.05. All statistical computations and analyses were conducted using SPSS software (Version 28.0, IBM, Armonk, NY, USA).

3. Results and Discussion

Normally, during the thermal densification method under high-temperature and -pressure, changes in the wood structure occur based on many treatment circumstances, wood species, and, especially, the treatment intensity (temperature and duration) [37]. As a result, the wood specimen was transformed at elevated temperatures (in our study, 100, 120, 140, and a sandwich at 140 °C). These alterations include the removal of hydrophilic OH groups from wood and their replacement with O-acetyl groups [38,39], which inhibits water reabsorption and formation of hydrogen bonds between water and wood polymers. After all, hemicellulose is the first component of wood to decay, due to its lower molecular weight and branching structure, which leads to an increase in lignin concentration [40,41,42].

The analysis of variance for mass, density, thickness, and EMC value is shown in

Table 1. ANOVA test results on mass, density, thickness, and EMC.

Dependent Variable	SS	MS	F Ratio	p Value
Mass (g)	41,106.233 a	8221.247	6.371	0.001
Density (kg/L)	0.435 b	0.087	3.130	0.026
Thickness (mm)	146.507 c	29.301	3.511	0.016
EMC (%)	90.005 d	18.001	0.409	0.837

SS, sum of squares; MS, means square; ^a, R squared = 0.570 (adjusted R squared = 0.481); ^b, R squared = 0.395 (adjusted R squared = 0.269); ^c, R squared = 0.422 (adjusted R squared = 0.302); ^d, R squared = 0.079 (adjusted R squared = −0.113).

, indicating that a significant difference exists, when the computed F value exceeds the F table (F count > F table) or the p value is less than 0.05 (p value 0.05). The significant variables were then subjected to a further analysis using Tukey’s HSD test (

Table 2. Tukey’s HSD test results for main effects.

Category	Mass (g)	Density (kg/L)	Thickness (mm)	EMC (%)
100	452.058 abc	0.558 ab	8.052 ab	10.718 a
120	391.034 ab	0.57 ab	7.178 a	12.884 a
140	441.212 ab	0.606 b	7.504 a	10.1 a
140 S[1]	490.298 c	0.618 b	7.972 ab	14.784 a
140 S[2]	455.898 bc	0.568 ab	7.994 ab	12.65 a
Control	385.304 a	0.266 a	13.61 b	14.432 a

Different letters ^a, ^b, and ^c, given next to the mean value, show statistically significant differences between the treatments for each sample categories at p < 0.05.

). Significant differences were marked by different lowercase letters, and the significance level was marked in alphabetical order from maximum to minimum.

3.1. Effect on the Density Profile

Figure 4 shows the specimen masses measured. The mass of the wood increases rapidly after treatment; after 120 H, it decreases and stabilizes. However, the final mass of the specimen was higher than the original mass. Treatment at high-temperature and -pressure resulted in the greatest increase in wood mass (30% at 100 °C) and smallest increase (7% at 120 °C) immediately following treatment. After 120 H, the highest change in wood mass occurred in the 140 °C S [1] category, while the lowest occurred at 120 °C, at 4 and 2%, respectively. After 240 H, the largest changes occurred in the 140 °C S [1] and S[2] categories, while the smallest changes occurred in the 120 °C, at 7 and 1%. Furthermore, after 360 H, the highest changes occurred at temperatures of 100 and 140 S [1] and S [2], totaling 7%, while the lowest changes occurred at 120 °C and 140 °C, totaling 5%. This result can be confirmed from the mass data shown in Table 2, where the highest value is found in the 140 S [1] and S[2] category. The ANOVA showed that there were significant differences (F = 6.371; p < 0.05) in the percentage change in mass found in treated wood, with respect to the control sample (Table 1). Thermal deterioration of thermally treated wood was determined to be the cause of mass loss [43]. Furthermore, Tenorio and Moya [44] investigated mass loss during heat treatment and discovered that it is species- and treatment-intensity dependent.

As shown in

Table 3. The density of Albizia falcataria before and after densification treatment.

Category	Density (kg/L)			SR-D(%)	DC(%)
	Do	Da	Ds
100 °C	0.29	0.75	0.58	36.96	100.00
120 °C	0.26	0.84	0.59	43.10	126.92
140 °C	0.29	0.85	0.62	41.07	113.79
140 °C S[1]	0.32	0.78	0.67	23.91	109.38
140 °C S[2]	0.29	0.71	0.62	21.43	113.79
Control	0.26	-	-	-	-

, treated Albizia falcataria (0.62 kg/L) had a higher average density than untreated (0.29 kg/L). The treatment increased the density by an average of 112.78%. The densities of Albizia falcataria board, after treatment under various temperature conditions, are presented in Figure 5. The density increase was accomplished by reducing the volume of the wood, since the compression process was dependent on the viscoelastic nature of the wood [45]. The densities of the wood samples were significantly enhanced by high-temperature and -pressure treatment at different category levels, with an increase of 100 (at 100 °C), 127 (at 120 °C), 114 (at 140 °C), 109 (at 140 °C S [1]), and 114% (at 140 °C S [2]), compared with the control density. The highest density was achieved with high-temperature and -pressure treatment at 120 °C (0.59 kg/L). This is most likely related to the decomposition of wood chemical components, particularly hemicellulose, due to wood mass loss [46].

The density profile affects the physical and mechanical properties of the wood and is defined by its thickness. Additionally, moisture content and thickness also have a great effect on the density. Untreated wood had nearly equal density, due to its thickness not being compressed. Immediately after treatment, treated wood gained significant density. However, after 120 H, the density decreased and remained steady. This consistent density correlated with the material used in this experiment [45]. The selection of defect-free wooden boards contributes to the establishment of consistent density distribution.

As expected, the values for the density profile increased significantly following treatment. The ANOVA demonstrated that, when compared to control board, high-temperature and -pressure treatment had a significant effect (F = 3.130; p < 0.05) on the density profile (Table 1). The control density or initial density has a significant influence on the magnitude of the densified board density [47]. This is confirmed by the results of Tukey’s HSD tests (Table 2). The initial density category, 140 S [1], has a high density, which leads in a high final density value. The rise in density as the temperature of the treatment increases may also be related to the softening of solid wood at higher temperatures [48,49] and decrease in the volume of the lumen cavity in the wood, as well as a rise in the amount of cell walls per unit volume [50].

3.2. Color Changes

Wood can change color during and after processing under high-temperatures and -pressure. Albizia falcataria from Indonesia has gained industrial interest in the global market. This species becomes a value-added product in international trade because of its color. The worldwide demand for Albizia falcataria wood board by supply from the Indonesian processing industry, specifically the Japanese market, is very high, owing to the light color and rough texture of the wood [51].

In this study, there was a difference in wood color between untreated wood (control) and treated wood (100, 120, 140, and sandwich 140 °C), as shown in Figure 6. Wood may be modified mechanically and physically by high-temperature and high-pressure treatment [52]. Moreover, the color parameter, particularly the overall color difference, can be used to quantify and predict the strength of wood.

The specimens showed that the treated wood had a darker color than the control. In untreated wood, cellulose and hemiocellulose do not absorb visible light or contribute to wood discoloration. In color change wood treated studies, when the wood is thermally processed, the decrease in lightness and increase in color difference were attributed to the presence of formation by-products [53], caused by a drop in hemicellulose content, particularly pentosan [54]. Polysaccharides and lignin are oxidized to form phenolic chemicals, which may potentially cause color changes [49]. Additionally, the color difference generated by steaming increased as the treatment pressure and duration increased [55]. The deeper color in treated Albizia falcataria board might be an aesthetic advantage for some applications.

3.3. Change in Thickness

Table 4. The thickness of Albizia falcataria before and after densification treatment.

Category	Thickness (mm)			SR-T (%)	TC (%)
	To	Ta	Ts
100 °C	13.5	6.53	6.88	5.02	−49.04
120 °C	13.41	4.36	6.02	18.34	−55.11
140 °C	13.23	5.04	6.61	19.17	−50.04
140 °C S[1]	13.24	6.74	6.6	−2.15	−50.15
140 °C S[2]	13.29	6.9	6.59	−4.85	−50.41
Control	13.52	-		-	-

summarized the thickness change of the experiment. The effects of high-temperature and -pressure on the thickness of Albizia falcataria are substantial. Previous research established that the specimen thickness decreases when the densification temperature and pressure increase [26]. At the same densification temperature, the higher the densification pressure, the higher the TC. Yet, it should be mentioned that this analysis discovered no discernible difference between 100 and 140 °C. This study reduced the average initial thickness from 13.4 mm to 6.5 mm. Densification of wood, up to 50% of its original thickness, can increase the density up to two times denser [56]. The average density increase of 113% in this study was in the direction of wood densification, up to 51% of its original thickness.

The greater the final density of the treated wood, the more TC is produced. A negative value indicates that the thickness of the Albizia falcataria board has been reduced. The 120 °C category exhibited substantial increases in thickness throughout the treatment process, while the 100 °C category exhibited very minor alterations. In terms of the final thickness change parameter, a substantial change in thickness happens at a temperature of 120 °C of 55.11%, while a modest change occurs at a temperature of 100 °C of 49.04%. This result is also explained by the density value of the 120 °C category, which is higher than the lower density value of the 100 °C category (Table 2). In addition, the ANOVA results revealed that there were significant differences (F = 3.511; p < 0.05) in the high-temperature and -pressure treatment of wood thickness (Table 1).

Due to the fact that wood is a viscoelastic material, the cell deformations generated by compression may result in internal stresses being stored in the microfibrils and matrix of the wood, which explains why densified wood springs back [45]. The integrity of the compressed Albizia falcataria thickness over time is also critical for subsequent applications of the wood board. After the cooling phase and storing the board under predefined relative humidity and temperature conditions, the surfaces of the control and treated boards were compared (see Figure 7). The two tangential surfaces of the treated board are not as flat as the surface of the control board. Then, low, medium, and high treatment flatnesses were, thus, markedly more stable, when compared to the sandwich treatment. It is conceivable that the surface of the wood board treated with the sandwich result is not entirely flat following treatment, due to compression employing a steel plate separator. It is essential to further investigate the configuration of the steel separator plates used in the sandwich category in future studies.

3.4. Compression Ratio

Table 5. Correlation between Albizia falcataria density and compression ratio.

Category	Density (kg/L)		CR (%)
	Before	After
100 °C	0.29	0.58	103.63
120 °C	0.26	0.59	104.11
140 °C	0.29	0.62	103.78
140 °C S[1]	0.32	0.67	103.79
140 °C S[2]	0.29	0.62	103.79
Control	0.26	-	-

represents data on Albizia falcataria compression ratio densification. The compression ratio dictates the change in density; a higher compression ratio results in a denser surface layer with a higher wood density [57,58]. At a compression ratio of 104.11%, the wood density rose by 127% to 0.59 kg/L. When the compression ratio is 104%, the thickness of the layer with the largest densification is roughly uniform in thickness of the layer with the lowest densification. Among the wood specimens investigated, the 120 °C category had the highest compression ratio, and the 100 °C category had the lowest compression ratio. These findings were consistent with the rise in density following treatment, in that the percentage increase in density was greater in the sample with a high compression ratio. Along with density, the compression ratio determines the decrease in thickness of compressed wood (Table 4). The observed increase in the compression ratio, as densification increased, was attributable to a reduction in the volume of the lumen’s cavity in the wood. The density of densified wood is highly dependent on the compression ratio, densification method, and wood species properties [59].

3.5. Equilibrium Moisture Content

The equilibrium moisture content (EMC) of the treated and untreated Albizia falcataria board samples of various categories are presented as bar graphs in Figure 8. The results of this study indicated that the densified wood had a more stable EMC than the control. Immediately after treatment, the EMC increased from 8% to 23.25%. The treatment atmosphere is quite humid during the cooling process, resulting in a very moist wood surface when measured. Furthermore, the results decreased from 120 H to 360 H and reached a stable EMC level. In comparison to untreated wood, the yield varies; it does not remain constant.

The rise in temperature, associated with heat treatment, results in a reduction in the EMC of the wood [60]. Meanwhile, the findings of this study, based on ANOVA results, revealed that there was no significant influence of temperature treatment on the EMC of compressed wood (F = 0.409, p > 0.05) when subjected to elevated temperatures and pressures (Table 1). Nandika et al. [8] also stated that there was no significant difference in the results of temperature control treatment, when using the impregnation method on sengon wood. There was no correlation between temperature levels in many categories and EMC. This may be due to damage or disruption of the cellular structure during the treatment process at high-temperature and -pressure, allowing for easier absorption/evaporation of moisture [61].

Several studies demonstrate that the minimum temperature required to modify wood is 100 °C, with water absorption decreasing as the temperature increases above 100 °C [62,63]. Compared with control board, the treated wood has a lower EMC after 120 H. The decrease in the EMC of wood results from a decline in the quality of the hemicellulose, induced by high-temperature and -pressure treatment, which impairs the wood’s ability to bind OH. The lower the EMC value of wood, which should be significantly less than the fiber saturation limit of between 21 and 32%, the better the wood’s physical and mechanical properties [64], meaning these mechanical and physical properties will increase as the moisture content decreases [65].

3.6. Scanning Electron Microscopy (SEM)

SEM micrographs of cross-sections from control and treated specimens, at various magnifications, are shown in Figure 9. The fracture surface of the control is not smooth at 500 $\mathsf{\mu }\mathrm{m}$ magnification. At 1000$\mathsf{\mu }\mathrm{m}$ magnification, fiber pulling occurs more frequently, resulting in large, regular voids on the control wood surface. When Albizia falcataria is modified at 140 °C, the polarity of the fibers decreases, resulting in an increase in the interfacial bonds between the fibers in the wood. The interface bonding is superior to that of the control sample. The pulling phenomena are practically invisible on the surface of the modified wood; nevertheless, tensile fractures occur instead. The tensile fractures occur in this modified wood, resulting in increased voids, smaller voids, and a rougher, denser surface than the control wood. The cavity (empty slot) is relatively small and uneven, so the contact between the wood fibers is not apparent.

The morphology of the wood was improved by modifying it with high-temperature and -pressure. Even though the tensile fracture is not entirely controlled, and voids remain on the fractured wood’s changed surface, the interfacial binding is better than that of untreated fiber composites [66]. Consequently, the improved morphological properties under high-temperature and -pressure are compatible with earlier work, regarding the optimized thermal compression morphology [67,68,69]. This investigation demonstrated that treated Albizia falcataria showed characteristic enhancement, as indicated by the magnification of 1000$\mathsf{\mu }\mathrm{m}$.

4. Conclusions

The densification method under high-temperature and -pressure in this study affect the physical properties of the Albizia falcataria board. The main findings of this study are as follows:

• The Albizia falcataria board that has been densified to a density of 0.62 kg/L is denser than the control board that has not been densified to 0.29 kg/L.
• The treated specimens were darker than the control board. As the density of the wood increases, its chemical components, particularly hemicellulose, decompose, darkening the color of the wood.
• Board thickness can be modified to 50.95%, nearly twice its pre-treatment thickness. However, upon treatment and storage, the tangential surface of the board is not as flat as the control.
• The increased density and hemicellulose degradation decreased moisture, with treated specimens exhibiting a more stable EMC than the control board. These mechanical and physical properties increase as the moisture level declines.
• SEM analysis revealed that the treatment increased wood morphology. Notably, the observations demonstrated an improvement in mechanical properties. Although the interfacial bonding was not perfectly controlled, the results were better than the untreated board.
• The ANOVA showed that the high-temperature and -pressure treatment had no significant effect on the EMC of compressed wood (p > 0.05) but had a significant effect on the mass, density, and thickness changes of the wood (p < 0.05), compared to the control board.

Finally, the enhanced performance properties of wood board create new possibilities for its use as a flooring material. The study results provide a guide for advancing wood densification to increase the value of fast-growing wood species, such as Albizia falcataria, throughout the wood industry.