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Article

Changes in Temperature and Vapor-Pressure Behavior of Bamboo Scrimber in Response to Hot-Pressing Parameters

by
Yanglin Ge
1,
Tong Lu
1,2,
Xingong Li
1,
Xiaofeng Hao
1,
Shoulu Yang
3,
Tonghua Lu
2,
Kang Xu
1,* and
Xianjun Li
1,*
1
College of Material Science and Engineering, Central South University of Forestry and Technology, Changsha 410004, China
2
Treezo Research Department, Treezo New Material Technology Group Co., Ltd., Hangzhou 311100, China
3
Forestry Industry Research Department, Guizhou Academy of Forestry, Guiyang 550005, China
*
Authors to whom correspondence should be addressed.
Forests 2024, 15(4), 620; https://doi.org/10.3390/f15040620
Submission received: 11 March 2024 / Revised: 25 March 2024 / Accepted: 27 March 2024 / Published: 28 March 2024
(This article belongs to the Section Wood Science and Forest Products)
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Abstract

:
This study investigated the heat-transfer behavior of heat-treated and phenolic resin-impregnated bamboo bundle slabs during the hot-pressing process. The significance of these findings lies in their potential to drive advancements in hot-pressing technology, contribute to energy-conservation efforts, and facilitate emission reduction within the bamboo scrimber industry. In this study, the variations in temperature and vapor pressure were investigated during the hot-pressing of bamboo slabs under various conditions, including hot-pressing temperatures (140 °C, 150 °C, 160 °C, and 170 °C), hot-pressing holding times (15 min, 20 min, 25 min, and 30 min), and hot-pressing pressures (4 MPa, 5 MPa, 6 MPa, and 7 MPa). This was achieved using thermocouple sensors and a self-made vapor pressure-monitoring system. The results indicated that higher hot-pressing temperatures significantly increased the heating rate, peak temperature, and core-layer vapor peak pressure of the bamboo bundle slab, with the vapor peak pressure at 170 °C being twice that at 140 °C. Furthermore, extending the holding time had a lesser effect on increasing the peak temperature of the slab but significantly increased the peak vapor pressure in the core layer. Thus, increasing the hot-pressing pressure proved beneficial for slab heating but had a lesser effect on the surface and core-layer peak temperatures. The core-layer vapor pressure of the slab subjected to a hot-press pressure of 7 MPa was 1.8 times higher than that at 4 MPa.

1. Introduction

Global energy-related carbon dioxide (CO2) emissions experienced a 0.9% growth, totaling 321 Mt, in 2022, and surpassed 36.8 Gt, marking a new record (IEA 2023) [1]. The escalating concern for reducing CO2 emissions has increased globally, with a growing interest in forest ecosystems due to their pivotal role in carbon sequestration and storage [2]. Bamboo forests, known for their rapid growth, excellent regeneration ability, and high yield in just a few years, have garnered significant attention [3]. Bamboo can reach maturity within 3–5 years, regenerating new shoots shortly after harvesting [4]. Moreover, its strength and elasticity properties, similar to specific tropical hardwoods, make bamboo attractive [5], leading to the development of a diverse range of bamboo products such as bamboo scrimber [6].
Moso bamboo (Phyllostachys edulis), a globally significant bamboo species, accounts for ~70% of the total bamboo-growing area in China [7]. Notable products derived from Moso bamboo include bamboo scrimber, glue-laminated bamboo lumber, and others [5,8]. Bamboo scrimber, resembling products developed in Australia, undergoes a process involving drying, heat treatment (at temperatures ranging from ~150 °C to 210 °C), water-soluble phenolic (PF) resin impregnation (with a resin content of ~15%), secondary drying (at ~60 °C), assembling, and hot-pressing, resulting in the desired specific gravity and thickness [9,10]. Yu et al. [11] found that after hot-pressing, the ground tissue in bamboo scrimber, such as parenchyma cells and vessel cells, were deformed or even crushed under high pressure (at ~5 MPa), and the volume ratio decreased from 48.5% and 9.5% to 14.7% and 0.4%, respectively. The fiber cells remained relatively intact, but their volume proportion doubled. Therefore, the densified bamboo scrimber could have desirable weather resistance, with the thickness swelling rate after 24 h of water absorption as low as 0.4%, far lower than the European standard EN13329 (20%) [12,13]. Additionally, compared with glue-laminated bamboo products, bamboo scrimber exhibits a higher material utilization rate (over 90%), excellent mechanical strength, and remarkable hardness, with the modulus of rupture (MOR) increasing from 178.5 MPa to 398.0 MPa [4,6], thus making bamboo scrimber suitable for diverse applications such as building construction, furniture manufacturing, courtyards, and outdoor areas [14,15].
The dominant “cold in-cold out” hot-pressing method in bamboo industrial production exhibited drawbacks such as a low production efficiency and high consumption of cooling water and energy [16]. The need to improve bamboo scrimber hot-pressing technology, reduce energy consumption, and minimize carbon emissions has attracted considerable attention from both academia and the business sector. Recent attention from academia and the business sector has focused on the bamboo scrimber hot-pressing process, a critical factor influencing the physical–mechanical properties, energy consumption, and carbon emissions of bamboo scrimber [10,15,17]. Furthermore, using a single-daylight hydraulic plate press and multi-stage hot-pressing process, Cheng [18] increased the hot-pressing temperature from 140 °C to 180 °C, resulting in a 23.3% increase in the bamboo-scrimber modulus of rupture and a 221.4% increase in internal bond strength (IB). These improvements were attributable to the increased core-layer temperature of the bamboo bundle slab (BBS), enhancing PF resin-curing and bonding performance. Cui [19] focused on core-layer temperature and PF resin-curing behavior, discovering PF resin cured at 142.9 °C. Furthermore, addressing heat dissipation during the hot-pressing process, Mao [20] suggested a hot-pressing temperature of 155 °C. Pineda [21] developed a novel three-dimensional simulator for bamboo-scrimber mat formation, and Chen et al. [10] studied compaction behavior and heat transfer in bamboo scrimber using lab and industrial pressing methods. These studies indicate the intricate nature of the hot-pressing process, involving coupled heat and mass transfer, non-linear mat deformation, and resin curing. As the hot-pressing process initiates, the contact area between bamboo bundles and hot-pressing plates increases, transferring heat and increasing the BBS core layer temperature [5]. Subsequently, the heat is transferred from the hot-pressing plate to the BBS, and the temperature of the BBS core layer gradually approaches the desired value. Thus, with the hot-pressing duration extended, the moisture inside the bamboo bundles is vaporized. Consequently, the bamboo bundles are gradually softened and plasticized under the combined effect of thermo-hydro-loading forces [22]. At the same time, the PF resin also undergoes a condensation reaction and eventually solidifies in the presence of heat [23]. Previous studies [17,24,25,26], have highlighted the predictive capabilities of analyzing temperature and vapor-pressure variations within the BBS during hot-pressing processes. These analyses prove instrumental in forecasting the degree of bamboo bundle softening and adhesive curing, aiding in the optimization of hot-pressing parameters.
Extensive research has been conducted on heat and mass transmission during the hot-pressing processes of wood-based panels. Humphrey et al. [27] investigated the interaction between heat transfer and moisture movement in particleboard hot-pressing processes. Yu et al. [28] examined the differences in heating transfer processes within core layers between the wet slabs with an initial moisture content of 5.5% and oven-dry slabs, uncovering that heat and mass transfer involved heat conduction, convection involving gas flow, and phase change. Early in the hot-pressing process, the rapid increase in slab internal temperature correlated with heat convection. However, as the hot pressing progressed, heat conduction dominated the temperature change. Thoemen et al. [29] noted a pronounced cross-sectional moisture profile during wood-based composite hot-pressing, while Rofii et al. [30] discovered that temperature behaviors inside the mat were influenced by the initial moisture content, wood particles, and press temperature, resulting in varying plateau temperatures. In our previous investigations [17], the profound influence of physical parameters such as moisture content, slab density, and thickness on internal temperature as well as vapor-pressure changes during the BBS hot-pressing process was identified.
In our previous research, we investigated the impact of physical parameters such as moisture content, target density, and target thickness on the temperature and vapor-pressure changes of the BBS. This exploration provided theoretical support for the innovation of “hot in-cold out” bamboo-scrimber hot-pressing technology. However, the absence of discussion on hot-pressing parameters prompted this study to investigate the effects of key hot-pressing parameters (temperature, pressure, and holding time) on the internal temperature and vapor pressure of bamboo scrimber during the hot-pressing process. To accomplish this, thermocouples and vapor-pressure sensors embedded within the BBS were utilized to precisely measure temperature changes in the surface and core layers, as well as the distribution of vapor pressure in the core layer during the hot-pressing process. The objective of this study was to establish solid theoretical support for optimizing the “hot in-cold out” hot-pressing process of bamboo scrimber, with the potential goal of shortening the hot-pressing cycle and reducing energy consumption.

2. Materials and Methods

2.1. Materials

Bamboo bundles were purchased from Hunan Taohuajiang Bamboo Technology Co., Ltd., Yiyang, China. Freshly harvested Moso bamboo (Phyllostachys edulis) was cut into 400 mm long bamboo tubes and segmented into bamboo strips along the longitudinal direction. Subsequently, these bamboo strips were defibered into bamboo bundles and dried, achieving a moisture content of ~10%. Finally, the bamboo bundles underwent heat treatment at 150 °C for 5 h.
The PF resin with a solid content of 50.4% was purchased from Beijing Taier Chemical Co., Ltd., Beijing, China. It was then diluted with water to achieve a 25% concentration. Heat-treated bamboo bundles were immersed in the PF resin solution for 20 min, air-dried for an additional 20 min, and then placed in an oven at 60 °C until achieving a moisture content of 10%. The PF resin-treated, heat-treated bamboo bundles were carefully stored for further use.

2.2. Bamboo-Scrimber Preparation

A specific mass of PF resin-treated heat-treated bamboo bundles was weighed according to the bamboo scrimber target density (ρ = 1.1 g/cm3) and arranged longitudinally in a mold to form a slab. As shown in Figure 1, the “hot in-cold out” method was employed for the bamboo-scrimber hot-pressing process. Specifically, the process involved placing the BBS into the hot-press device once the press-plate temperature reached the preset hot-pressing temperature, followed by the immediate closure of the upper and lower press plates. After the center of the BBS core layer reached 120 °C, the hot-pressing pressure was maintained for a specified duration as per the predetermined hot-pressing holding time. Subsequently, the heating was stopped, and the hot-pressing plates were cooled with water circulated through the internal pipe of the upper and lower press plates until the core layer of the bamboo-scrimber board reached a temperature of 80 °C. Then, the hot-pressing pressure was released, yielding a board with 400 mm (length) × 400 mm (width) × 30 mm (thickness) dimensions.
The experimental design employed a single-factor test (Table 1). The hot-pressing temperatures were set at 130 °C, 140 °C, 150 °C, and 160 °C, hot-pressing holding times varied from 15 min to 30 min, and hot-pressing pressures ranged from 4 MPa to 7 MPa.

2.3. Measurement of Vapor Pressure and Temperature

In this study, during the bamboo bundle paving process, six K-type thermocouple temperature sensors were placed at different locations: the center region of the slab surface layer (S1), the end region of the surface layer along the longitudinal direction (S2), the edge region of the surface layer along the transverse direction (S3), the center region of the slab core layer (C1), the end region of the core layer along the longitudinal direction (C2), and the edge region of the core layer along the transverse direction (C3). Additionally, a self-made press-monitor system was positioned at point C1 to measure the vapor pressure during the hot-pressing process [17]. The arrangements of the temperature sensors and vapor-pressure-measuring device are shown in Figure 2.

2.4. The Physical Properties Test of Bamboo Scrimber

Vertical density profile (VDP) scanning (DENSE-LAB X, EWS, Nuremberg, Germany) was conducted on bamboo scrimber samples with dimensions of 50 mm (length) × 50 mm (width) × 30 mm (thickness). The measurement accuracy was ±0.5% with a scanning speed of 0.5 mm/s.
The thickness sorption rate (TSR) of each bamboo-scrimber sample was measured according to the Chinese national standard GB/T 17657-2013 [31]. Each sample, with dimensions of 50 mm (length) × 50 mm (width) × 30 mm (thickness), was submerged in the water at a controlled temperature of (20 ± 1) °C for 120 h, with measurements taken every 12 h to track thickness changes.

2.5. Statistical Analysis

The analysis of variance (ANOVA) was performed on TSR values of bamboo-scrimber samples with different hot-pressing parameters using the statistic software SPSS (Version 18.0, IBM, Armonk, NY, USA).

3. Results and Discussion

3.1. Effect of Hot-Pressing Temperature on the BBS Temperature and Vapor Pressure

Figure 3a–f depict the effect of various hot-pressing temperatures on the temperature change of the BBS surface layer and core layer during the hot-pressing process. The results indicated that the hot-pressing time was significantly reduced with a higher hot-pressing temperature, enhancing the hot-pressing efficiency of the bamboo scrimber. At higher hot-pressing temperatures, the increased heat flux in the contact between the BBS surface layer and the hot-pressing plate led to this outcome. Before the BBS core-layer temperature reached 100 °C, the moisture-evaporation rate within the surface layer increased and the vapor diffusion coefficient was enhanced [32], facilitating the convective heat transfer, leading to a significant increase in the heating rate of the BBS surface and core layers. However, as the BSS core layer temperature approached the evaporation temperature, thermal conduction superseded thermal convection as the dominant heat-transfer mode [28]. The higher hot-pressing temperature increased the rate of the BBS thermal conduction heat transfer, owing to the intensive molecular thermal motion [33]. Interestingly, when the core layer of BBS reached 120 °C, the occurrence of the BBS core temperature surpassing the surface temperature correlated with the findings of Tian et al. [34]. This phenomenon was attributable to the gradual compression and densification of the slab as the surface temperature was transferred to the core layer. This process resulted in a reduction in airflow or pore channels. Consequently, water vapor in the core layer encountered obstacles in smooth discharge, leading to a continuous increase in water-vapor pressure. This transformation evolved from a saturated-steam state to a superheated-steam state [30,35]. Additionally, at the same hot-pressing temperature, the horizontal temperature distribution both in the surface layer and core layer was uneven. This was attributable to the heat loss, owing to the temperature transfer from the BBS edge region to the metal thickness gauge.
As shown in Figure 3g, there was no vapor pressure at the beginning of the hot-pressing process. As the hot-pressing process progressed, the moisture within the BBS surface layer gradually vaporized and the water vapor moved to the BBS core layer at the vapor-pressure gradient. Furthermore, when the temperature of the C1 point reached 90 °C, the vapor pressure gradually increased, reaching a peak value. This indicated that the higher the hot-pressing temperature, the shorter the time for the core layer to reach the critical temperature of water evaporation, increasing the peak value of the vapor pressure. This result was attributable to the higher hot-pressing temperature, facilitating more efficient heat flux transfer from the hot-pressing plate to both the surface layer and core layer of the BBS simultaneously. Consequently, this shortened the time for the BBS core layer to reach the vaporization temperature. Additionally, under the same conditions, the higher the temperature, the greater the steam pressure, improving the vapor pressure [36]. These findings were elucidated by observing that the rate of increase in vapor pressure for the BBS core layer reached 8.1 kPa/min at a hot-pressing temperature of 170 °C, making a substantial 3.6-fold increase compared with the 140 °C hot-pressing temperature. The decrease in vapor pressure occurred after the initiation of the cooling phase, and it was noted that a higher hot-pressing temperature led to a more rapid decrease in vapor pressure. This was attributable to the decreased external temperature. Water molecules in the saturated or overheated state were difficult to maintain, and the vapor pressure of the high hot-pressing temperature decreased faster compared with the low hot-pressing temperature [17]. Additionally, when the cooling time was sufficient, some residual vapors existed within the BBS core layer. Furthermore, as the hot-pressing process ended, it adversely affected the quality of the bamboo-scrimber product [34].
As shown in Figure 3h, the VDP of the bamboo scrimber sample became more uniform with increasing hot-pressing temperature. This was attributable to the more synchronous curing behavior of the PF resin due to a reduced temperature gradient within the BBS under the higher hot-pressing temperature [37,38]. Additionally, the bamboo-scrimber sample TSR after 120 h of water absorption was evaluated. As shown in Figure 3i, the results indicated that there was a slight interaction between the water resistance of the bamboo scrimber and the hot-pressing temperature, resulting in a difference of <1% between 140 °C and 170 °C. ANOVA indicated that the hot-pressing temperature exhibited no significant impact on the TSR (Table 2). This result was attributable to the fact that, under hot-pressing temperature conditions ranging from 140 °C to 170 °C, the surface-layer and core-layer temperatures of the BBS reached the curing temperature of the PF resin. Thus, the difference in bonding properties and the degree of resin occurrence was negligible [39].

3.2. Effect of Hot-Pressing Holding Time on the BBS Temperature and Vapor Pressure

Figure 4a–f show the effect of hot-pressing holding time on the internal temperature variations on the BBS during the hot-pressing process. In this study, the hot-pressing holding time was defined as the duration during which the slab continued to be held under high pressure and temperature after reaching point C1 of the BBS, which reached 120 °C. The BBS temperature reached 120 °C, following the attainment of 120 °C at point C1. The temperatures of the BBS increased gradually, correlating with the extension of the set hot-pressing holding time. The peak temperature of the surface and core layers of the BBS were 124.8 °C and 130.8 °C, respectively. Furthermore, for the 15 min holding time, the temperature was 128.7 °C, and for the 30 min holding time, it was 136.8. This indicated a difference of 3.9 °C and 6 °C between the peak temperatures of the surface and middle layers of the BBS. Moreover, with the increase in hot-pressing holding time, the temperature coefficient of variation of the BBS surface-core layer did not exceed 0.8. The results estimated that the extended hot-pressing holding time exhibited a limited effect on peak BBS temperatures, and exhibited little effect on BBS core- and surface-layer temperatures. Additionally, when compared with the commonly employed “cold in-cold out” hot-pressing method prevalent in the bamboo scrimber industry in China, the “hot in-cold out” hot-pressing process exhibited significantly shorter durations, thereby enhancing the efficiency of the bamboo scrimber production [16].
Notably, it was observed that when the other hot-pressing parameters of the BBS were identical, there were differences in the time taken for the core layer to reach 120 °C. The variations in the evacuation degree of the bamboo bundles were inevitable during the defibering process. This led to an uneven distribution of the PF resin and moisture after soaking and drying, along with uneven heat transfer during the hot-pressing process [40]. Additionally, as a kind of biomass heterogeneous material, the density of each bamboo varies, which influences the BBS heat capacity and the thermal diffusivity during the hot-pressing process. These variations in heat capacity and thermal diffusivity might result in different internal temperature distributions of BBS [41].
Figure 4g shows that the time of initial steam pressure in different plates was the same. It was observed that extending the holding time exhibited little effect on the initial rise rate vapor pressure in the BBS. The differences in the rise rate with various holding times were small, all ranging from 3.3 kPa/min to 3.9 kPa/min. This result was attributable to the varying degrees of bamboo bundle evacuation, differences in the uniformity of the slab lay-up, and the uneven distribution of adhesive and moisture content [29,42,43,44]. Additionally, the peak vapor pressure increased significantly with increasing holding time, and the appearance time was delayed with the holding time. The rate of pressure reduction of the BBS was the same for different holding times, with only 30 min of the BBS exhibiting residual vapor pressure at the end of the hot-pressing.
As shown in Figure 4h, the VDP results of the bamboo scrimber sample with different hot-pressing holding-time conditions irregularly fluctuated, and the standard deviations of the density curves were 2.6%, 2.4%, 3.4%, and 2.3%, respectively. The standard deviation data were similarly dispersed, indicating that the duration of calibration pressure did not exhibit a significant effect on the VDP of the bamboo scrimber. Furthermore, as shown in Figure 4i, the TSR results of bamboo scrimber samples were 0.84%, 1.53%, 1.63%, and 1.48% for hot-pressing holding times of 15 min, 20 min, 25 min, and 30 min, respectively. The results indicated that there was no significant difference in the IB of the bamboo scrimber sample under different holding times, owing to their TSR results exhibiting a slight difference between 15 min to 30 min holding times. ANOVA results on the effect of holding time on the TSR of bamboo scrimber samples indicated that there was no significant effect of holding time on TSR and that there was no significant correlation between them (Table 3). Conclusively, it was recommended to increase the hot-pressing holding time of the bamboo-scrimber production from 15 min to 20 min, improving production efficiency and reducing production costs.

3.3. Effect of Hot-Pressing Pressure on the BBS Temperature and Vapor Pressure

Figure 5a–f show that with increasing hot-pressing pressure, the heating rate of the surface and core layers of the BBS increased significantly. Furthermore, when the hot-pressing pressure was 4 MPa, the total hot-pressing time required for the plates was 83 min; when the hot-pressing pressures were 5 MPa, 6 MPa, and 7 MPa, the whole hot-pressing time required for the plates was reduced by 27.7%, 34.9%, and 35.1%, respectively, compared with the hot-pressing pressure of 4 MPa. These results indicated that the air and moisture between the bamboo bundles was expelled more rapidly under higher pressing pressure, and the BBS was compressed to the target thickness in a shorter time. Additionally, the densified BBS benefited, improving its internal heat transfer efficiency [45]. However, when the hot-pressing pressure exceeded 5 MPa, the increase in hot-pressing pressure exhibited a limited effect on the heating rate of the BBS. It was observed that the loose slab required more time to compress to the target thickness when the hot-pressing pressure was 4 MPa. Subsequently, increased hot-pressing pressure resulted in decreased compression times, increasing the convective heat-transfer and heat-conduction efficiency [46,47]. However, after the hot-pressing pressure reached 6 MPa, it had a slight effect on the increase in BBS heating rates. The observed phenomenon was attributable to the fact that, under the given hot-pressing pressure conditions, the slab was compressed to densification. At this point, the predominant form of heat transfer shifted from convection to conduction. Furthermore, increasing the pressure potentially reduced pores and enhanced heat-transfer efficiency. However, the impact was limited in terms of the overall improvement of the slab [48,49]. The effect of hot-pressing pressure on the peak temperature of the BBS was relatively small, with the peak temperatures of the surface and core layer of the slab increasing by 4.2 °C and 3.6 °C, respectively. Under a hot-pressing pressure of 7 MPa compared with 4 MPa, there was no significant increase. Additionally, the cooling rate of BBS with different hot-pressing pressures exhibited no significant difference under the cooling section of the hot-pressing processes, which were all between 3.6 °C/min to 4 °C/min.
Figure 5g depicts that the emergence time of vapor pressure increased with increasing hot-pressing pressure. This occurred because, under increased hot-pressing pressure, the BBS was compressed to a compacted state earlier, leading to the blockage of vapor flow paths in all directions within the slab. As a result, vapor did not accumulate in the core layer [50]. Similarly, the ramp-up rate of the vapor-pressure curve also increased significantly with the increase in hot-pressing pressure. The ramp-up rates were 1.4 kPa/min, 2.7 kPa/min, 3.5 kPa/min, and 4 kPa/min for the BBS with hot-pressing pressures ranging from 4 MPa to 7 MPa, respectively. The phenomenon was attributable to the reduced porosity inside the BBSs at higher hot-pressing pressure. Furthermore, based on the ideal gas law, the vapor pressure inside the slab increased significantly when the hot-pressing temperature was unchanged, decreasing the pore volume [30,35]. Additionally, the premature compaction of the slab under the high hot-pressing loading made it difficult for vapor to escape from the BBS during the hot-pressing process. Thus, the peak vapor pressure increases as the hot-pressing pressure increases. This led to the BBS exhibiting some vapor pressures of 6 MPa and 7 MPa at the end of the hot-pressing process.
Figure 5h,i show the effect of the hot-pressing pressure on the VDP and the expansion rate of the water absorption thickness of bamboo scrimber. As shown in Figure 5h, the VDP curves indicated that the surface-layer density of the bamboo scrimber was higher compared with its core layer. The uniformity of bamboo scrimber with hot-pressing pressure from 4 MPa to 6 MPa was close, while the VDP of the bamboo scrimber with 7 MPa exhibited less uniformity. This was attributable to the fact that, at the beginning of the hot-pressing process, the heat from the press plates was not fully transferred to the core layer of the BBS. Simultaneously, the amorphous region of bamboo had not reached the softening temperature, limiting the ductility of the BBS [33]. Figure 5i shows that the overall TSR of the bamboo scrimber tended to increase with hot-pressing pressure under current test conditions. However, the difference between 4 MPa and 7 MPa did not exceed 1%. At this stage, the BBS was subjected to a substantial external force, the bamboo cell walls were easily crushed, and the bundles were brittle, negatively impacting the physical properties of the bamboo scrimber [39,51]. ANOVA results indicated that the hot-press pressure exhibited a highly significant effect on the expansion rate of the water absorption thickness of bamboo scrimber (Table 4). This was attributable to the fact that the excessive hot-pressing pressure collapsed some of the cell walls of the bamboo fibers, facilitating the invasion of the bamboo-scrimber interior by water molecules. This resulted in an increased TSR of the bamboo scrimber [52].

4. Conclusions

This study evaluated the impact of hot-pressing temperature, holding time, and pressure on the surface–core layer temperature, as well as the vapor-pressure behavior of the bamboo scrimber during the hot-pressing process. Higher hot-pressing temperatures resulted in a faster heating rate for the BBS, significantly shorter hot-pressing times, and higher maximum vapor pressures within the BBS core layer. The maximum vapor pressure at 170 °C increased by a factor of one compared with 140 °C. Additionally, extending the holding time exhibited a lesser effect on increasing the maximum temperature of the surface and core layers of the slab, but significantly increased the maximum vapor pressure of the core layer. The increase in hot-pressing pressure enhanced slab heating, exhibiting a slight effect on the maximum surface- and core-layer temperatures. Thus, hot-press pressure exhibited a significant effect on the maximum vapor pressure of the slab core layer, increasing by 1.8 times for 7 MPa compared with 4 MPa. The former exhibited a residual vapor pressure of 19.1 kPa at the end of the hot-pressing process.

Author Contributions

Methodology, X.H. and X.L. (Xianjun Li); formal analysis, Y.G.; investigation, Y.G. and T.L. (Tong Lu); resources, T.L. (Tonghua Lu); writing—original draft, Y.G. and T.L. (Tong Lu); writing—review & editing, K.X.; supervision, K.X. and X.L. (Xianjun Li); funding acquisition, X.L. (Xingong Li), X.H., S.Y., K.X. and X.L. (Xianjun Li). All authors have read and agreed to the published version of the manuscript.

Funding

Y.G. and T.L. (Tong Lu) contributed equally to this study. This study received financial support from the National Natural Science Foundation of China (No. 32071852, No. 32371981), the Science and Technology Innovation Program of Hunan Province, China (No. 2023RC3161, No. 2021RC4062), the General Program of the National Natural Science Foundation of Hunan Province, China (No. 2023JJ30993), and the Top-quality Innovative Talents Program in Guizhou Province, China (No. GCC[2023]066).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the privacy of the participants.

Conflicts of Interest

Author Tonghua Lu is employed by the company Treezo New Material Technology Group Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The Treezo New Material Technology Group Co., Ltd. had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. The “hot in-cold out” hot-pressing process of bamboo scrimber.
Figure 1. The “hot in-cold out” hot-pressing process of bamboo scrimber.
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Figure 2. Sensor arrangement schematic.
Figure 2. Sensor arrangement schematic.
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Figure 3. (a–f): The surface-layer and core-layer temperature changes of BBS treated with different hot-pressing temperatures; (g) vapor-pressure changes in the core layer of BBS treated with different hot-pressing temperatures; and (h,i) vertical density profile (VDP) and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing temperatures.
Figure 3. (a–f): The surface-layer and core-layer temperature changes of BBS treated with different hot-pressing temperatures; (g) vapor-pressure changes in the core layer of BBS treated with different hot-pressing temperatures; and (h,i) vertical density profile (VDP) and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing temperatures.
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Figure 4. (a–f): The surface-layer and core-layer temperature changes of BBS treated with different hot-pressing holding times; (g) vapor-pressure changes in the core layer of BBS treated with different hot-pressing holding times; (h,i) and the VDP and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing holding times.
Figure 4. (a–f): The surface-layer and core-layer temperature changes of BBS treated with different hot-pressing holding times; (g) vapor-pressure changes in the core layer of BBS treated with different hot-pressing holding times; (h,i) and the VDP and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing holding times.
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Figure 5. (a–f): The surface-layer and core-layer temperature changes of treated BBS with different hot-pressing pressures; (g) vapor-pressure changes in the core layer of treated BBS with different hot-pressing pressures; and (h,i) the VDP and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing pressures.
Figure 5. (a–f): The surface-layer and core-layer temperature changes of treated BBS with different hot-pressing pressures; (g) vapor-pressure changes in the core layer of treated BBS with different hot-pressing pressures; and (h,i) the VDP and 120 h water-absorption TSR of bamboo scrimber treated with different hot-pressing pressures.
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Table 1. Hot-pressing parameters of BBS.
Table 1. Hot-pressing parameters of BBS.
NumberHot-Pressing
Temperature/°C		Hot-Pressing
Pressure/MPa		Hot-Pressing
Holding Time/min
1130520
2140
3150
4160
5150420
65
76
87
9150515
1020
1125
1230
Table 2. ANOVA for TSR of bamboo scrimber made with different hot-pressing temperatures.
Table 2. ANOVA for TSR of bamboo scrimber made with different hot-pressing temperatures.
ParametersSource
of Difference		Degrees
of Freedom		SquareMean SquaredF-ValueSignificance
Hot-pressing temperatureInter-group37.4602.4872.633-
Within-group2018.8900.945
Total2326.350
Note: “-” indicates that the level difference was not significant.
Table 3. ANOVA for TSR of bamboo scrimber made with different holding times.
Table 3. ANOVA for TSR of bamboo scrimber made with different holding times.
ParametersSource
of Difference		Degrees
of Freedom		SquareMean SquaredF-ValueSignificance
Holding timeInter-group32.2990.7661.208-
Within-group2012.6910.635
Total2314.990
Note: “-” indicates that the level difference was not significant.
Table 4. ANOVA for TSR of bamboo scrimber made with different hot-press pressures.
Table 4. ANOVA for TSR of bamboo scrimber made with different hot-press pressures.
ParametersSource
of Difference		Degrees
of Freedom		SquareMean SquaredF-ValueSignificance
Hot-press pressuresinter-group35.3421.7815.753**
within-group206.1900.310
total2311.532
Note: “**” indicates that the level difference was highly significant at the 0.01 level.
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Ge, Y.; Lu, T.; Li, X.; Hao, X.; Yang, S.; Lu, T.; Xu, K.; Li, X. Changes in Temperature and Vapor-Pressure Behavior of Bamboo Scrimber in Response to Hot-Pressing Parameters. Forests 2024, 15, 620. https://doi.org/10.3390/f15040620

AMA Style

Ge Y, Lu T, Li X, Hao X, Yang S, Lu T, Xu K, Li X. Changes in Temperature and Vapor-Pressure Behavior of Bamboo Scrimber in Response to Hot-Pressing Parameters. Forests. 2024; 15(4):620. https://doi.org/10.3390/f15040620

Chicago/Turabian Style

Ge, Yanglin, Tong Lu, Xingong Li, Xiaofeng Hao, Shoulu Yang, Tonghua Lu, Kang Xu, and Xianjun Li. 2024. "Changes in Temperature and Vapor-Pressure Behavior of Bamboo Scrimber in Response to Hot-Pressing Parameters" Forests 15, no. 4: 620. https://doi.org/10.3390/f15040620

APA Style

Ge, Y., Lu, T., Li, X., Hao, X., Yang, S., Lu, T., Xu, K., & Li, X. (2024). Changes in Temperature and Vapor-Pressure Behavior of Bamboo Scrimber in Response to Hot-Pressing Parameters. Forests, 15(4), 620. https://doi.org/10.3390/f15040620

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