A Comparative Study on the Effects of Heat Treatment on the Properties of Rubberwood Veneer

Abstract

Heat treatment is a widely employed method for modifying solid wood and has also been extended to veneer-type woods. Owing to the thinness and ease of handling of veneers, the regulation of protective media in heat treatment has not been highly regarded by the industry and is scarcely reported in research. In light of this, in this paper, rubber wood (Hevea brasiliensis) veneer is taken as the research subject to investigate the influences of heat treatment with hot air (HTHA) and heat treatment with superheated steam (HTSS) at different temperatures on the chemical properties, longitudinal tensile strength, color values, hygroscopicity, thermal degradation performance and microstructure of the wood. The results show that heat treatment alters the chemical properties of wood. Both heat treatments reduce the content of hemicellulose and other components in the veneer, and the characteristic peak of lignin in HTSS is slightly enhanced. The crystallinity of the veneer slightly increases after heat treatment, and the increase in HTSS is greater than that in HTHA. Through scanning electron microscopy, it is observed that heat treatment can effectively remove starch granules in rubber wood veneer, with HTSS being superior to HTHA, and the removal effect increases with the rise in temperature. The longitudinal tensile strength of the veneer decreased by 0.69%, 3.87%, and 24.98% respectively at 135~155 °C HTHA, and by 3.25%, 7.00%, and 18.47% respectively at 135~155 °C HTSS. Both heat treatments reduced the lightness of the veneer and increased the chroma index. At 155 °C, the color difference value of the veneer treated by HTSS was smaller than that treated by HTHA. The effects of heat treatment on the moisture absorption performance of the veneer were different. The equilibrium moisture content of the veneer treated at 135 °C HTHA and 135~155 °C HTSS was lower than that of the untreated material, indicating an improvement in moisture absorption stability. The maximum moisture sorption hysteresis of untreated material is 3.39%. The maximum moisture sorption hysteresis of 135 °C HTHA is not much different from that of untreated material. The values of 145 °C and 155 °C HTHA increase by 8.85% and 9.14% respectively. The values of 135 °C, 145 °C, and 155 °C HTSS increase by 22.42%, 25.37%, and 19.47% respectively. The moisture absorption hysteresis of the veneer increases after heat treatment, and the effect of HTSS improvement is more significant. From the TG and DTG curves, it can be seen that the residual mass percentage of the veneer after heat treatment is higher than that of the untreated material. The residual mass percentage of HTHA at 135 °C, 145 °C, and 155 °C increased by 3.13%, 3.07%, and 2.06% respectively, and that of HTSS increased by 5.14%, 7.21%, and 6.08% respectively.

1. Introduction

Rubberwood, as a fast-growing timber species, has a short growth cycle, abundant resources, uniform texture, and clear grain, which is highly suitable for the industrial production of veneer, plywood, sliced thin wood, and finger-jointed materials, etc. [1]. However, due to the inherent properties of rubberwood and the limitations of processing equipment and techniques, problems such as insect infestation, mold growth, and cracking are prone to occur during the veneer production process, seriously affecting product quality and production efficiency [2,3]. Thermal treatment technology, as a green and environmentally friendly wood modification method, can significantly enhance the durability, dimensional stability, and mechanical properties of wood by altering its chemical composition and physical structure [4,5]. Fang et al. [6] densified veneers by mechanical compression combined with heat and steam, reducing the surface roughness and hygroscopicity of the veneers and enhancing their tensile and bending strengths. Ferreira et al. [7] thermally treated the surfaces of loblolly pine veneers at three temperatures of 160, 180, and 200 °C, observing a decrease in surface roughness and changes in the total extractives content of the veneers after treatment. Song et al. [8] modified eucalyptus veneer by oven treatment at 100, 120, 140 and 160 °C. The results indicated that heat treatment could better improve the shear performance. Chen et al. [9] heated white oak logs with steam in a vacuum atmosphere and graded the veneers before and after treatment. The results showed that the performance value of white oak veneer was not adversely affected after 30 min of steam treatment at 56 °C. Altgen et al. [10] treated homogeneous micro-boards under different time, temperature, and humidity conditions and compared the effects of chemical composition changes on the tensile behavior of the treated boards. The results confirmed the adverse effects of preferential removal of hemicellulose on wood strength and toughness. Salca et al. [11] thermally treated poplar and beech boards at 190 °C for different periods of time. The results showed that the brightness of the treated boards decreased, and the color darkened. The analysis of the color difference changes was related to the chemical decomposition of lignin and hemicellulose in the boards. Jiang et al. [12] investigated the influence of heat treatment process parameters (temperature, time, and heating rate) on the surface color of rubber wood. The results indicated that the heat treatment temperature had a significant impact on the colorimetric properties of rubber wood, while the treatment duration and heating rate did not have any effect. Chotikhun et al. [13] conducted heat treatment on rubber wood at three temperature levels of 150 °C, 180 °C, and 220 °C for 20 min each, and tested their bending properties, hardness, and dimensional stability. The results showed that the overall mechanical properties of the samples were adversely affected by heat treatment, but dimensional stability increased. Chotikhun [14] also conducted heat treatment on rubberwood veneer at temperatures of 160, 170, 180, and 190 °C, and then produced five-layer plywood using melamine urea-formaldehyde (MUF) resin as the adhesive. The dimensional stability of the plywood was evaluated. The results indicated that the dimensional stability of the plywood made from heat-treated rubberwood veneer was improved. Heat treatment at 170 °C and 180 °C resulted in the highest modulus of rupture (MOR), modulus of elasticity (MOE), and shear strength of the plywood. However, this study only employed a single heat treatment method and did not explore the impact of heat treatment on the physical and chemical properties of rubberwood veneer. The above-mentioned research indicates that heat treatment has a significant impact on the chemical properties and physical performance of wood veneers, which holds theoretical and applied research value. Many wood species have achieved performance optimization through this technology, but for rubber wood veneer, there are still many aspects of the improvement mechanism of its physical and chemical properties through heat treatment that need to be explored.

Based on the previous investigation, we found that in a wood processing enterprise in Xishuangbanna, China, when using a veneer dryer to dry veneers in actual production, the drying temperature fluctuates between 135 and 155 °C and cannot be controlled stably. This is because the heat is transferred to the rollers of the veneer dryer through hot oil successively, resulting in inconsistent temperatures at different parts and causing differences in the drying quality of the veneers. High-temperature hot air treatment and superheated steam treatment belong to physical modification technologies, which avoid the pollution caused by chemical reagents to the environment and have relatively low requirements for equipment conditions. The heat source comes from the combustion of waste materials, making them green, environmentally friendly, and low-cost modification technologies that are often used in actual production. In addition, according to previous research on the thermal treatment of wood, it is known that a treatment time of 2 h shows good modification effects. Therefore, in this study, rubberwood veneers were subjected to heat treatment with hot air (HTHA) and heat treatment with superheated steam (HTSS) at three temperatures of 135, 145, and 155 °C for 2 h. The differences in the effects of the two heat treatments on the properties of the veneers were compared, providing a reliable theoretical basis for the scientific utilization of rubberwood and the optimization of veneer properties.

2. Materials and Methods

2.1. Materials

Rubberwood sapwood veneer (for Xin Hongxing Wood Industry Co., Ltd., Xishuangbanna, Yunnan, China), measures 1270 mm (length) × 630 mm (width) × 1.7 mm (thickness), with a moisture content of approximately 8% to 12%.

2.2. Methods

2.2.1. Preparation of Heat-Treated Veneer

Defect-free and high-quality rubberwood veneers were selected as test samples and cut into 300 mm (length) × 200 mm (width) × 1.7 mm (thickness) veneer specimens. The specimens were then placed in constant-temperature forced-air drying ovens (A3, A4, A5) and electric steam ovens (S3, S4, S5) at 135, 145, and 155 °C respectively for high-temperature hot air treatment and superheated steam treatment. The constant temperature was maintained for 2 h. After the treatment was completed and the temperature dropped to room temperature, the specimens were taken out for future use.

2.2.2. Fourier-Transform (FTIR) Analysis

Rubberwood veneer samples were crushed by a high-speed universal pulverizer and then passed through a 100-mesh sieve. An appropriate amount of wood powder was taken and mixed with potassium bromide at a mass ratio of 1:100. The mixture was ground and then pressed into semi-transparent sheets. The sheets were placed in the test chamber of the Nicolet iS50 Fourier Transform Infrared Spectrometer (for ThermoFisher Scientific Company, Waltham, MA, USA) for scanning. The thin film was placed in the testing chamber of the Nicolet iS50 Fourier-transform infrared (FTIR) spectrometer for scanning. Chemical structure analysis of the samples was conducted at room temperature, with a wavenumber range of 400 to 4000 cm⁻¹, a resolution of 4 cm⁻¹, and 32 scans.

2.2.3. X-Ray Diffraction Spectroscopy (XRD) Analysis

The crystallinity of cellulose in rubberwood veneer powder before and after treatment was analyzed by using the Rigaku H12 X-ray diffractometer (UltimaN Company, Tokyo, Japan). The sample was 100-mesh rubber wood veneer powder. The selected X-ray tube was a Cu target, with Kα as the radiation source (λ = 0.154060 nm), tube voltage of 40 kV, tube current of 40 mA, 2θ scanning range of 5–90°, scanning rate of 8°/min, and step size of 0.02°. The relative crystallinity of veneer before and after treatment was calculated by the Segal method [15], and the equation is as follows:

$$\mathrm{C}\mathrm{r}\mathrm{I}={\displaystyle \frac{{\mathrm{I}}_{002}-{\mathrm{I}}_{\mathrm{a}\mathrm{m}}}{{\mathrm{I}}_{002}}}\times 100\%$$

(1)

Here CrI represents the relative crystallinity (%); I₀₀₂ is the intensity of the diffraction peak in the cellulose crystalline region at 2θ = 22° for the 002 lattice plane; I_am is the intensity of the diffraction at 2θ = 18° for the amorphous part of cellulose.

2.2.4. Measurement of Tensile Strength Along the Grain Direction

Several rubberwood veneers before and after heat treatment were selected. Samples were processed using the 1325 model CNC (Computer Numerical Control) equipment from Shandong Mulin Sen Machinery Factory, and then tensile strength was measured using the UTM4000 series universal mechanical testing machine from (Shenzhen Sansi Zongheng Co., Ltd, Shenzhen, China). The dimensions of the test specimens are shown in Figure 1. Before the tests, the specimens were placed in a constant temperature and humidity chamber (20 °C, 65%) to balance the moisture content to 8%–12%. The specimens were then removed and the thickness and width at the center of the effective part were measured. The tensile strength tests were carried out using a universal testing machine with a loading speed of 1 mm/min. Data were recorded after the effective part of the specimens broke.

The calculation equation for tensile strength along the grain is as follows:

$$\mathsf{\sigma }={\displaystyle \frac{{\mathrm{P}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}{\mathrm{b}\mathrm{h}}}$$

(2)

Here σ represents the tensile strength along the grain of the specimen, with the unit of megapascal (MPa); P_max is the failure load, with the unit of newton (N); b is the width at the center of the effective part of the specimen, with the unit of millimeter (mm); h is the thickness at the center of the effective part of the specimen, with the unit of millimeter (mm).

2.2.5. Color Measurements

The color values of rubberwood veneers before and after heat treatment were measured using the SC-80c fully automatic colorimeter (for Beijing Kangguang Co., Ltd., Beijing, China). in accordance with the standards of the International Commission on Illumination (CIE). Six color measurement points were evenly selected on the surface of each sample, and the L*, a*, and b* values were measured once at each point before and after heat treatment. The arithmetic mean of the six values was taken as the color parameter value of the sample. The difference in lightness value ∆L*, the difference in red-green chromaticity index ∆a*, the difference in yellow-blue chromaticity index ∆b*, and the color difference value ∆E* of the samples before and after heat treatment were calculated based on the CIE L*a*b* colorimetric system. The calculation equations are as follows [16]:

$$∆{\mathrm{L}}^{*}={\mathrm{L}}_1^{*}-{\mathrm{L}}_0^{*}$$

(3)

$$∆{\mathrm{a}}^{*}={\mathrm{a}}_1^{*}-{\mathrm{a}}_0^{*}$$

(4)

$$∆{\mathrm{b}}^{*}={\mathrm{b}}_1^{*}-{\mathrm{b}}_0^{*}$$

(5)

$${∆\mathrm{E}}^{*}=\sqrt{{\left(∆{\mathrm{L}}^{*}\right)}^2+{\left(∆{\mathrm{a}}^{*}\right)}^2{+\left(∆{\mathrm{b}}^{*}\right)}^2}$$

(6)

Here L₀*, a₀*, and b₀* represent the color values of the wood before heat treatment; L₁*, a₁*, and b₁* represent the color values of the wood after heat treatment; ∆E* represents the color difference value before and after heat treatment.

2.2.6. Dynamic Vapor Sorption (DVS) Measurement

The water vapor adsorption behavior of veneer samples was determined using a European SMS Resolution high-precision dynamic vapor sorption apparatus (for SMS Company, London, UK). The initial sample weight was approximately 40 to 50 mg. The samples were placed on a support connected to a microbalance by a suspension wire. The relative humidity (RH) was controlled by nitrogen and water vapor. The test was conducted at a constant temperature of 25 °C, starting from 0% RH and increasing in increments of 10% RH until 95% RH.

2.2.7. Thermogravimetric (TG) Analysis

The thermal degradation performance of veneers before and after heat treatment was tested using a STA449F3 thermogravimetric analyzer (for Netzsch Company, Selb, Germany). The veneer samples were crushed and sieved through a 100-mesh sieve. Then, 5 to 7 mg of powder was weighed and placed in a high-purity dynamic nitrogen atmosphere with a flow rate of 20 mL/min. The test temperature was raised from 30 °C to 800 °C at a heating rate of 10 °C/min.

2.2.8. Scanning Electron Microscopy (SEM) Observation

The surface microstructure of rubberwood veneers before and after heat treatment was observed using a MIRA LMS scanning electron microscope (for TESCAN Company, Brno, Czech Republic). Before the observation, the samples were fixed on the sample holder with conductive adhesive, and then gold spraying treatment was carried out to improve the conductivity and facilitate the presentation of clear morphology images.

3. Results

3.1. FTIR Analysis

Figure 2 shows the FTIR spectra of rubberwood veneer before and after heat treatment,

Table 1. Wavenumbers and corresponding functional groups.

Wavenumber/cm−1	Functional Group
3350	-OH
2923	-CH3 or -CH2
1753	C=O
1505	Aromatic ring C=C
1245	Lignin S and G units and -OH

shows the Wavenumbers and corresponding functional groups First, the absorption peak at 3350 cm⁻¹ is analyzed, which corresponds to the stretching vibration peak of hydroxyl (-OH). These hydroxyl groups are widely present in cellulose, hemicellulose, and lignin in wood. The intensity of this stretching vibration peak mainly reflects the concentration level of hydroxyl groups. It is clearly seen from the spectra that the untreated rubberwood veneer shows a very strong peak at this wavenumber position, which fully indicates that the rubberwood veneer is rich in hydroxyl groups, reflecting a very high content of cellulose, hemicellulose, and lignin. Compared with the untreated material, the intensity of the absorption peak at this position in HTHA shows a significant attenuation, and the degree of attenuation gradually decreases as the treatment temperature increases, reaching the weakest state at 135 °C. The absorption peak of 135 °C HTSS is significantly weakened, but as the treatment temperature increases to the range of 145–155 °C, the intensity of the absorption peak first increases and then decreases [17]. The absorption peak at 2923 cm⁻¹ represents the asymmetric stretching vibration peak of methyl (-CH₃) or methylene functional groups (-CH₂). As shown in the figure, the intensity of this absorption peak is less affected by the treatment temperature after heat treatment. The intensity of this absorption peak in HTHA slightly weakens, indicating a reduction in the number of methyl or methylene functional groups. The absorption peak intensity at 135 °C HTSS weakens, and when the temperature rises to 145–155 °C, the absorption peak intensity slightly increases. The absorption peak at 1735 cm⁻¹ is the C=O stretching vibration peak, which is generally associated with acetyl groups or carboxylic acid esters in hemicellulose. Compared with untreated wood, the absorption peak of the veneer samples at this position weakens after heat treatment, indicating that heat treatment reduces the content of hemicellulose in the material to a certain extent [18]. During the heat treatment process, carbonyl groups will decrease due to oxidation reactions at high temperatures. However, acetyl groups in hemicellulose will hydrolyze under high-temperature treatment to form acidic conditions, which intensifies the esterification reaction of lignin and generates a certain amount of carbonyl groups [19,20], thus resulting in an insignificant change in the peak value at this position. The characteristic peaks of lignin-like substances are at 1505 and 1245 cm⁻¹. The intensities of the two absorption peaks of HTHA are lower than those of untreated wood, indicating that HTHA reduces the relative content of lignin in the veneer [21,22,23]; while the absorption peak intensities of HTSS increase with the rise of treatment temperature, and at 145–155 °C, the absorption peak intensities are slightly greater than those of untreated wood.

3.2. XRD Analysis

Cellulose in wood has a crystalline structure, which is formed by the regular connection of β-D-pyran glucose monomers through β-1,4-glycosidic bonds to form linear macromolecules. The crystallinity of cellulose refers to the percentage of the crystal area in cellulose microfibrils, which can reflect the degree of crystallization during the cellulose condensation process [24,25,26]. The ultrafine bone structure of wood cell walls is mainly manifested in the form of cellulose microfibrils and crystalline regions, and the crystallinity is closely related to the elastic modulus, impact toughness, and mechanical strength of wood [27,28]. Figure 3 shows the X-ray diffraction patterns of veneers before and after heat treatment, and Table 1 shows the changes in the relative crystallinity and diffraction peak positions of the veneers.

As shown in Figure 3, a sharp peak appears near 2θ = 22°, representing the (002) plane of cellulose crystals; two broader and weaker peaks are observed near 2θ = 15.5° and 34.8°, corresponding to the (101) and (040) planes. According to the table, the diffraction peak of the (002) crystal plane is within the range of 21.88° to 22.28°. There is no significant change in the positions of the characteristic peaks of the veneers before and after treatment, and no new diffraction peaks have emerged. This indicates that the heat treatment has no obvious effect on the crystalline region of the wood, meaning that the distance between the crystal layers remains unchanged [29].

The relative crystallinity of the veneer before and after heat treatment was calculated by the Segal method and is shown in

Table 2. Relative crystallinity and 2θ of veneer before and after heat treatment.

Sample	CrI/%	Standard Deviation	2θ/°
C	65.46	0.158	22.12
A3	65.97	0.233	22.08
A4	66.65	0.290	22.00
A5	66.83	0.245	22.06
S3	66.26	0.157	22.14
S4	66.79	0.244	22.28
S5	66.95	0.209	21.88

Note: CrI: The relative crystallinity of cellulose; C: Control-Untreated; A3: HTHA at 135 °C; A4: HTHA at 145 °C; A5: HTHA at 155 °C; S3: HTSS at 135 °C; S4: HTSS at 145 °C; S5: HTSS at 155 °C.

. As can be seen from the table, the standard deviations of the relative crystallinity of cellulose under all treatment conditions were all less than 0.3, indicating that the data are relatively concentrated. The crystallinity of the untreated material was 65.46%, and the crystallinity of the veneer samples after both types of heat treatment increased. Compared with the untreated material, the relative crystallinity of 135~155 °C HTHA increased by 0.78%, 1.82%, and 2.09% respectively; that of 135~155 °C HTSS increased by 1.22%, 2.04%, and 2.28% respectively. Under the same temperature conditions, the relative crystallinity of HTSS was slightly higher than that of HTHA, possibly because HTSS is a process where both moisture and heat act on the wood. The increase in temperature and the lubricating effect of moisture enhances the mobility of cellulose molecules, promoting the rearrangement and crystallization of cellulose molecules in the amorphous regions of microfibrils, which is conducive to the increase in the relative crystallinity of cellulose [30]. However, from the perspective of the magnitude of data change, the degree of change within the experimental temperature range was not significant.

3.3. Tensile Strength Along the Grain Direction Analysis

Figure 4 and

Table 3. Longitudinal tensile strength of veneer before and after heat treatment.

Sample	Tensile Strength/MPa	Standard Seviation
C	59.44	9.54
A3	59.03	3.667
A4	57.14	9.627
A5	44.59	6.987
S3	57.51	11.40
S4	55.28	6.79
S5	48.46	7.12

Note: C: Control-Untreated; A3: HTHA at 135 °C; A4: HTHA at 145 °C; A5: HTHA at 155 °C; S3: HTSS at 135 °C; S4: HTSS at 145 °C; S5: HTSS at 155 °C.

show the longitudinal tensile strength of the veneer before and after heat treatment. The tensile strength of the veneer without heat treatment is 59.44 MPa. When the temperature rises from 135 °C to 155 °C, the longitudinal tensile strength of HTHA decreases to 59.03 MPa, 57.14 MPa, and 44.59 MPa respectively, which are 0.69%, 3.87%, and 24.98% lower than that of the untreated material. The longitudinal tensile strength of HTSS is 57.51 MPa, 55.28 MPa, and 48.46 MPa respectively, which are 3.25%, 7.00%, and 18.47% lower than that of the untreated material. The decrease in the longitudinal tensile strength of HTSS at 155 °C is slightly less than that of HTHA. It can be seen from this that the tensile strength of rubber wood veneer after heat treatment has decreased. This may be due to the fact that heat treatment causes the hemicellulose within the veneer material to undergo thermal decomposition, making the wood more brittle and reducing its impact toughness. At the same time, since hemicellulose is interwoven among microfibrils and coexists with lignin, it plays a role in bonding. After heat treatment, part of the hemicellulose degrades, weakening this bonding, which leads to a decrease in the tensile strength of the veneer along the grain [31,32,33].

3.4. Color Analysis

The L*, a*, and b* values of the veneers before and after heat treatment were all greater than 0, and the overall color tended to be red-yellow. Observe the color difference of veneer before and after heat treatment (Figure 5 and

Table 4. Color difference values of veneer.

Sample	∆L*	∆a*	∆b*	∆E*	Standard Deviation
A3	−4.26	2.84	5.82	7.75	0.053
A4	−9.72	4.09	6.10	12.18	0.088
A5	−15.15	6.35	7.02	17.86	0.103
S3	−11.91	4.03	5.89	13.88	0.064
S4	−11.95	2.89	6.02	13.69	0.067
S5	−14.80	4.18	4.89	16.13	0.095

Note: A3: HTHA at 135 °C; A4: HTHA at 145 °C; A5: HTHA at 155 °C; S3: HTSS at 135 °C; S4: HTSS at 145 °C; S5: HTSS at 155 °C.

), as the treatment temperature increased from 135 °C to 155 °C, the lightness L* of HTHA veneer decreased significantly, while the chromaticity indices a* and b* were greater than those of the untreated material and showed an increasing trend with the rise in treatment temperature. The color difference ΔE* increased with the increase in treatment temperature. For HTSS, L* decreased, and both a* and b* were higher than those of the untreated material. Among them, a* first decreased and then increased with the increase in temperature, while b* first increased and then decreased. ΔE* decreased first and then increased with the increase in treatment temperature. Overall, both types of heat treatment reduced the lightness L* and increased the chromaticity indices a* and b* of the veneer. The reduction in the lightness L* of the veneer surface may be due to the formation of thermal degradation products of hemicellulose and extractives, or it may be attributed to the polymerization reaction of lignin during the treatment process [34]. At 135 °C and 145 °C, the lightness of HTSS veneer was lower than that of HTHA, while at 145 °C and 155 °C, the chromaticity indices a* and b* of HTSS veneer were lower than those of HTHA. The ΔE* of 135 °C HTHA was the smallest, indicating that the color change of the veneer was the least under this treatment condition. At 155 °C, the ΔE* of HTSS was smaller than that of HTHA, suggesting that HTSS had a smaller impact on the color of the veneer at high temperatures.

3.5. DVS Analysis

As depicted in Figure 6, the moisture absorption-desorption isothermal curves of untreated and heat-treated rubberwood veneers are presented. Within the entire moisture absorption range, the equilibrium moisture content of the veneer treated at 135 °C HTHA is lower than that of the untreated veneer, indicating enhanced moisture absorption stability. In the relative humidity range of 0%–50%, the equilibrium moisture content of the veneer treated at 145 °C and 155 °C HTHA is slightly higher than that of the untreated veneer, but as the humidity rises, it gradually approaches that of the untreated veneer. The equilibrium moisture content of the veneer treated at 135–155 °C HTSS is lower than that of the untreated veneer. The reduction in equilibrium moisture content is related to the quality loss and degradation of strongly hygroscopic hemicellulose that occurs during the heat treatment process, which weakens the hygroscopicity of the veneer [34].

The chemical composition within the veneers undergoes alterations subsequent to heat treatment, potentially exerting an influence on water adsorption and the extent of hysteresis. The moisture adsorption hysteresis value represents the disparity in the equilibrium moisture content during the processes of moisture adsorption and desorption of wood under a specific relative humidity. As shown in Figure 7, throughout the entire moisture adsorption procedure, the moisture adsorption hysteresis values of rubberwood veneers exhibit a tendency to initially increase and subsequently decrease. The maximum value emerges within the relative humidity range of 60%–70%. The moisture adsorption hysteresis value of the veneers treated at 155 °C HTSS attains its maximum at 70% relative humidity, while for other heat treatment conditions, the relative humidity at which the maximum hysteresis value is reached is 60%. The maximum moisture adsorption hysteresis value of the untreated material is 3.39%. The maximum moisture adsorption hysteresis values of the veneers treated at 135–155 °C HTHA are 3.33%, 3.69%, and 3.7%, respectively. Compared with the untreated material at 145–155 °C, they increased by 8.85% and 9.14%, respectively. The maximum moisture adsorption hysteresis values of the veneers treated at 135–155 °C HTSS are 4.15%, 4.25%, and 4.05%, respectively, representing increases of 22.42%, 25.37%, and 19.47% compared to the untreated material. This is attributed to the changes in the chemical composition and microstructure of the cell wall induced by heat treatment [35]. On the whole, the moisture absorption hysteresis of the veneer increases after heat treatment, and it increases with the rise of the treatment temperature. Particularly within the humidity range of 50%–80%, the disparity compared to the untreated material is more conspicuous. Furthermore, the moisture adsorption hysteresis of HTSS is higher than that of HTHA. Analyzed by FTIR, the lignin content in the veneers of HTSS is higher than that of HTHA. Within the molecular structure of lignin, there exist numerous unsaturated groups, which reduce the flexibility of lignin molecules and thereby enhance the hysteresis [36,37].

3.6. TG Analysis

Through the combination of TG (Figure 8) and DTG (Figure 9) analyses, the process of the veneer’s mass variation with temperature can be comprehended, thereby enabling an understanding of its pyrolysis characteristics [38]. It can be seen that the pyrolysis patterns of HTHA and HTSS rubberwood veneers are similar, and the pyrolysis process can be divided into three stages: drying, rapid pyrolysis, and carbonization. During the drying stage, the rubberwood veneers absorb heat, and the free water, physically adsorbed water, crystalline water within the molecules, and some extractives in the veneers evaporate. Simultaneously, a small amount of hemicellulose undergoes pyrolysis [39]. As shown in

Table 5. TG measurement parameters of veneer before and after heat treatment.

Sample	Phase 1			Phase 2			Phase 3
	MLP/%	MLR/%/°C	Peak Temp/°C	MLP/%	MLR/%/°C	Peak Temp/°C	MRP/%
C	1.89	0.35	67.6	82.16	9.19	363.5	15.95
A3	2.89	0.57	54	80.67	9.45	360.0	16.45
A4	2.30	0.49	58.6	81.27	10.38	362.5	16.44
A5	2.90	0.56	62.4	80.81	9.52	359.1	16.28
S3	4.18	1.00	56.24	79.04	9.31	361.3	16.77
S4	2.63	0.78	34.92	79.26	7.87	359.9	17.10
S5	3.52	0.91	34.74	79.55	8.74	362.3	16.92

Note: MLP: Mass loss percentage; MLR: Mass loss rate; Peak Temp: Peak temperature; C: Control-Untreated; A3: HTHA at 135 °C; A4: HTHA at 145 °C; A5: HTHA at 155 °C; S3: HTSS at 135 °C; S4: HTSS at 145 °C; S5: HTSS at 155 °C.

, the mass loss percentage of the veneer samples in this stage is less than 5%, and the mass loss rate of the veneers after heat treatment is marginally higher than that of the untreated ones. In the rapid pyrolysis stage, the lignin, cellulose, and hemicellulose in the veneers undergo rapid pyrolysis [40], resulting in a substantial mass loss. The maximum thermal degradation temperature of the veneer after heat treatment decreases. This is because heat treatment first degrades some of the hemicellulose, weakening the interaction among lignin, cellulose, and hemicellulose in the veneer [41,42]. The mass loss percentage and loss rate of the untreated veneers are 82.16% and 9.19% respectively. The mass loss rate of the veneers treated with HTHA at 135–155 °C is lower than that of the untreated veneers, and it demonstrates a trend of initially increasing and subsequently decreasing with the rise in temperature. At 135 °C, the mass loss percentage is the lowest at 80.67%. The mass loss rate of HTHA is higher than that of the untreated veneers and shows a trend of initially increasing and subsequently decreasing with the increase in treatment temperature. The mass loss percentage and loss rate of the veneers treated with HTSS at 135–155 °C are slightly lower than those of HTHA. The mass loss percentage and loss rate of HTSS were slightly lower than those of the hot air treatment group. The mass loss percentage increased with the increase of treatment temperature, while the loss rate showed a trend of first decreasing and then increasing. After the carbonization stage, the mass residue percentage of the untreated veneers is 15.95%. Compared with the untreated veneers, the mass residue percentage of the veneers after both types of heat treatment increases. The mass residue percentage of the veneers treated with HTHA increased by 3.13%, 3.07%, and 2.06% respectively, and those of HTSS increased by 5.14%, 7.21%, and 6.08% respectively. It can be observed that HTSS is slightly higher than HTHA, and the veneers treated with HTSS at 145 °C have the highest mass residue percentage.

3.7. SEM Analysis

The microscopic morphology of the tight side of the rubberwood veneer before and after heat treatment was observed. As shown in Figure 10, the number of starch granules in the ray parenchyma cells of untreated and heat-treated rubberwood veneer was observed. The changes in starch content in untreated rubberwood veneer and rubberwood veneer under different heat treatment conditions were analyzed. The electron microscope images clearly showed that the cell cavities of untreated rubberwood veneer were filled with a large number of complete circular starch granules (about 5–15 μm in diameter), with smooth surfaces and intact structures, indicating that natural rubberwood contains abundant starch reserves. These starch granules were closely arranged and filled the cell cavities, which was the main factor leading to the easy decay of rubberwood. After heat treatment, the starch granules gradually dissolved until they almost disappeared. By comparing the differences in the effects of two heat treatment methods at the same temperature, it was found that the removal rate of 135 °C HTSS was higher than that of HTHA, and the removal effect of 145 °C HTSS was equivalent to that of 155 °C HTHA; the starch removal rate of 155 °C HTSS was the highest. This indicates that superheated steam treatment has a significant advantage in starch dissolution, possibly because the heat conduction efficiency of steam is higher, and water molecules are brought into the veneer by water vapor, increasing the moisture content in the wood. Under high temperatures, it promotes the gelatinization and dissolution of starch. Steam treatment can better maintain the cell wall structure, which is conducive to the exudation of starch [43,44].

4. Discussion

This study comparatively analyzed the effects of heat treatment with hot air (HTHA) and heat treatment with superheated steam (HTSS) on the physical, chemical, and mechanical properties of rubberwood veneer, aiming to enhance its quality and performance. The research results indicated that heat treatment significantly altered the chemical composition of the veneer, improving its durability, especially in addressing issues such as the veneer’s susceptibility to decay, cracking, and fungal attack. Consistent with previous studies, heat treatment reduced the content of hemicellulose and starch in the veneer. HTSS not only slightly increased the relative content of lignin but also performed better in removing starch granules, which contributed to enhancing the dimensional stability and anti-corrosion properties of the wood. Regarding moisture absorption, the study found that heat treatment effectively improved the moisture absorption stability and hysteresis of rubberwood veneer, with HTSS showing a better improvement effect than HTHA. Specifically, HTSS significantly enhanced the stability of the veneer in high-humidity environments by more efficiently removing starch granules and altering the internal structure of the wood. Additionally, heat treatment led to color changes in the wood, characterized by a decrease in brightness and an increase in color difference, which might be more favored in decorative applications, especially in scenarios requiring a darker wood appearance. Moreover, the residual mass rate of the veneer increased after heat treatment, and it exhibited higher thermal stability, with HTSS showing a better improvement effect.

Future research could further explore the roles of potential mechanisms during the heat treatment process, such as the combined effects of temperature and pressure, as well as the application effects of protective media. Additionally, long-term performance evaluations will help understand the practical application performance of heat-treated rubberwood veneers under different environmental conditions, including outdoor exposure and high-humidity environments. In conclusion, this study demonstrated that heat treatment, especially HTHA and HTSS, is an effective method to enhance the performance of rubberwood veneer, with broad application prospects.

5. Conclusions

Heat treatment alters the chemical properties of rubberwood veneer, reducing the content of hemicellulose, increasing the relative crystallinity of cellulose, and effectively lowering the starch content. Heat treatment also affects the physical properties of rubberwood veneer, reducing the longitudinal tensile strength and brightness index of the veneer, increasing the chromaticity index, and enhancing the moisture absorption stability, moisture absorption hysteresis, and thermal stability of the veneer. Overall, the improvement of HTSS is greater than that of HTHA.