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Article

Effects of Aluminum Chloride Impregnating Pretreatment on Physical and Mechanical Properties of Heat-Treated Poplar Wood under Mild Temperature

by
Xujie Wang
,
Cuimei Luo
,
Jun Mu
* and
Chusheng Qi
*
MOE Key Laboratory of Wood Material Science and Application, Beijing Forestry University, Beijing 100083, China
*
Authors to whom correspondence should be addressed.
Forests 2022, 13(8), 1170; https://doi.org/10.3390/f13081170
Submission received: 30 June 2022 / Revised: 17 July 2022 / Accepted: 20 July 2022 / Published: 23 July 2022
(This article belongs to the Section Wood Science and Forest Products)
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Abstract

:
The acid formed by thermal degradation of wood can autocatalyze its heat treatment. In this study, exogenous acid was introduced by impregnation into poplar wood to investigate its effect on the physical and mechanical properties of wood. Equilibrium moisture content (EMC), dimensional stability, mass loss (ML), color, modulus of rupture (MOR), and modulus of elasticity (MOE) of heat-treated poplar were tested under mild temperature (130–160 °C) for different pretreatment concentrations of aluminum chloride (AlCl3). The results show that the EMC of the heat-treated wood diminishes by 2.7%–47.8%, and dimensional stability improves significantly after AlCl3 impregnation pretreatment. The samples impregnated with 0.5 mol/L AlCl3 and heat treated at 160 °C achieved the best dimensional stability, which was better than for the samples only heat-treated at 220 °C. The color changed significantly as the impregnating concentration increased, achieving a color effect similar to that of wood only heat-treated at a high temperature such as 200 or 220 °C. Heat-treatment temperature under the same ML of wood samples was reduced, which also mitigated the reduction of MOR. MOE of heat-treated wood with 0.5 mol/L impregnation pretreatment was 11.4%–30.7% more than for samples heat-treated at 160–220 °C. After exogenous acidic AlCl3 impregnation pretreatment, the cell wall structure of the heat-treated wood was found to remain relatively intact. Thus, AlCl3 impregnating pretreatment exerted a substantial and beneficial effect on the physical and mechanical properties of poplar and achieved good performance of poplar wood treated at a mild temperature.

1. Introduction

As a natural renewable forest resource, wood is widely used in construction, furniture making, and the decorative arts, among many applications. Its characteristics include an attractive grain pattern, high strength-to-weight ratio, processability, and versatility [1]. With the increasing depletion of natural forests in the world, especially in China, plantation forests such as poplar, fir, eucalyptus, and masson pine have become reliable sources of industrial wood, mainly because of their wide adaptability and fast growth cycle [2]. The area of plantation forest in China was about 79.54 million hectares, ranking first in the world, and poplar, as one kind of fast-growing wood, accounts for 18.75% of all plantations [3]. However, the use of plantation timber is limited by its poor dimensional stability, light color, low durability, and lack of mechanical strength [4,5,6]. It is therefore important to improve these properties to achieve more efficient utilization and added value of plantation timber.
Heat treatment of wood, as an environmentally friendly method, is valued and is a well-established commercial technology applied on a large scale. Heat-treated wood is widely used in furniture, flooring, decoration, and outdoor landscape among other uses. Heat treatment is typically conducted at 180–240 °C for several hours [1] in a low-oxygen environment. It uses superheated steam, nitrogen, and heat-conducting oil as the heat-transfer medium [7]. Hemicelluloses, as the most thermally labile of the wood polymeric components, degrade through deacetylation, depolymerization, and dehydration [8]. Cellulose is more thermally stable than hemicelluloses [9]; it can be affected by hydrolysis and recrystallization of the amorphous region as temperature increases [10]. Some structural changes occur in lignin due to homolytic cleavage, polycondensation reactions, and crosslinking with cell walls [8,11]. Due to the extensive chemical reactions of the main components, heat treatment can improve wood properties, including dimensional stability, resistance to microbial attack, and durability, as well as aesthetic characteristics.
Heat treatment was traditionally applied at a high temperature, which is highly energy-consuming and increases the cost of the processing as well as causing some loss of mechanical strength of the wood [12]. Excessively high temperatures have higher requirements for equipment quality [13]. Therefore, reducing the temperature required for heat treatment is an important goal in the wood industry. Mitchell (1988) found that heating wood in the presence of steam resulted in the accelerated formation of organic acids (mainly acetic acid) that catalyzed the hydrolysis of hemicelluloses [14]. Garrote et al. (2001) found that hydronium, the catalytic species involved in the degradation of the polymeric fraction, was mainly generated from acetic acid produced by hemicellulose degradation and deacetylation of both hemicelluloses and acetylated oligosaccharides [15]. In other words, the acidic environment formed by hemicellulose degradation can accelerate heat treatment. It means that the energy required for heat treatment can be reduced by acidic pretreatment. Grosse et al. (2019) reported that the dimensional stability and biological resistance of wood impregnated with oligolactic acid (OLA) and heat-treated at 160 °C were significantly higher than those of wood treated only at higher temperature [16]. They suggested that the effect of promoting the further degradation of hemicelluloses and other wood polymers was due to the acidic facilitation of OLA. Hosseinpourpia et al. (2017) studied the application of protic acid pretreatment combined with heat treatment at 180 °C and found that the dimensional stability and steam adsorption characteristics of wood could reach the same levels as with heat treatment at higher temperature (above 250 °C) [17]. Hosseinpourpia et al. (2018) further studied the pretreated wood with Lewis acid (AlCl3) and Lewis acid-protic acid (AlCl3 and H2SO4) and reported that acid catalysis could promote thermal degradation of hemicellulose and part of the cellulose in an amorphous region during the heat treatment and that the equilibrium moisture content (EMC) was significantly lower than that of wood treated only at higher temperature [18]. Qu et al. (2020) showed that samples treated at 15% aluminum sulfate concentration and 160 °C heat treatment achieved the best dimensional and thermal stability [12]. In the past, most studies focused on moisture content, hygroscopicity, and dimensional stability by acidic pretreatment combined with heat treatment, but few focused on physical and mechanical properties, microscopic morphology, or chemical composition.
The objective of this work is to investigate the effects on physical and mechanical properties of poplar heat-treated at a mild temperature with acidic AlCl3 impregnation pretreatment. Changes in microstructure and chemical compositions were also investigated in this work using a scanning electron microscope (SEM), attenuated total reflection flourier transform infrared spectroscopy (ATR-FTIR), and X-ray diffraction (XRD). Together, these provided a knowledge basis for the industrial application of acidic pretreatment combined with mild heat treatment in the future.

2. Materials and Methods

2.1. Materials

Twenty-year-old poplar (Populus tomentosa) was obtained from Linqu County, Shandong Province, China. In order to achieve thorough impregnation with even heating, defect-free sapwood samples were cut into a size of 51 mm (L) × 15 mm (T) × 1.6 mm (R) with a moisture content of 6.5% and a density of 0.46 ± 0.02 g/cm3.
AlCl3 solutions with concentrations of 0.05, 0.1, and 0.5 mol/L (pH0.05M = 3.78, pH0.1M = 3.23, and pH0.5M = 2.43) were prepared using anhydrous aluminum chloride (AlCl3, 99.99%) and deionized water at 20 °C, stirring at 200 rpm for 20 min. Anhydrous aluminum chloride and deionized water were supplied, respectively, by Sahn Chemical Technology Co., LTD. (Shanghai, China) and Beijing Lanyi Chemical Co., LTD. (Beijing, China).

2.2. Treatment

2.2.1. Impregnation Pretreatment

Before impregnation pretreatment, wood samples were oven-dried at 103 ± 2 °C until reaching constant weight, and this weight was recorded as W1.
All samples were placed in a 2 L vessel and secured with a mesh to keep the samples immersed in the solution. Then impregnation pretreatment was conducted in a vacuum drying oven (DZ-1BCII, Tianjin, China) at 20 °C, the vacuum was maintained for 60 min and then released. After impregnation pretreatment, all samples were dried in a drying oven (DUG-9070A, Shanghai, China) at 80 °C until reaching constant weight and weighed as W2.

2.2.2. Heat Treatment

The AlCl3-impregnation pretreatment samples were individually heat-treated at four different temperatures (130, 140, 150, and 160 °C) for 1 h in a drying oven (BPG-9050 AH, Shanghai, China) with saturated water vapor as the protection and heat-transfer medium. The samples that were only heat treated (the HT groups) were individually heat-treated at six different temperature levels (130, 140, 150, 160, 200, and 220 °C) for 1 h. After heat treatment, the weights (W3) of all samples were measured. Fifteen samples were prepared from each group, and the untreated samples were handled as the control group. The only heat-treated samples were named for the heat-treatment temperature (for example, sample HT-130 °C). The samples that were impregnated with AlCl3 were named from the concentration of AlCl3 and heat-treatment temperature (for example, sample 0.05 M-130 °C).

2.3. Characterization of Physical Properties

The weight percentage gain (WPG) of the samples impregnated at different concentrations was calculated using the following formula [12]:
WPG (%) = [(W2 − W1)/W1] × 100
For the HT groups, the mass loss percentage (ML1) was determined using the formula
ML1 (%) = [(W1 − W3)/W1] × 100
For the groups of heat-treated wood with impregnation pretreatment at different AlCl3 concentrations (0.05, 0.1, and 0.5 M-HT), the mass loss percentage (ML2) was determined using the formula
ML2 (%) = [(W2 − W3)/W2] × 100
Standard color measurement (CIE L*a*b*) was performed on every sample board (3 measurements per sample) with a Spectrophotometer (NR 200, Guangdong, China). The lightness L*, green-red axis a*, and blue-yellow axis b* were determined, and a colorimetric parameter ΔE* was calculated.

2.4. Characterization of Dimensional Stability

The weight and dimensions in tangential and radial directions were measured after samples were conditioned to equilibrium at 30%, 65%, and 95% RH at 20 °C in a climate-controlled chamber (BSC-150, Shanghai, China). After this, samples were oven-dried at 103 °C and then weighed. The following parameters were calculated according to the method described by Ding et al. [19].
The equilibrium moisture content (EMC) was determined from
EMC (%) = [(Wr − W0)/W0] × 100
where Wr is the mass (g) of the samples in a certain equilibrium state and W0 is the oven-dry mass (g) of the samples.
The swelling coefficients (S) both in tangential (St) and radial (Sr) directions were determined from the formula
S (%) = [(L2 − L1)/L1] × 100
where L2 is the dimension (mm) of the samples at RH 95% and L1 is the dimension (mm) of the samples at RH 30%.
The antiswelling efficiency (ASE) was determined from
ASE (%) = [(Snt − St)/St] × 100
where Snt and St are the swellings in the untreated and treated samples, respectively.

2.5. Characterization of Mechanical Properties

All samples were placed in a climatic chamber (BSC-150, Shanghai, China) at 20 °C and 65% relative humidity until reaching constant weight. According to ASTM standards D 790 [20], the modulus of rupture (MOR) and modulus of elasticity (MOE) were determined using a universal test machine (Instron 3366, Shanghai, China) at a loading speed of 5 mm/min with a supporting span of 32 mm and diameter of the supporting roller of 10 mm. Ten replicates were tested for each sample, and their average and standard deviation values were applied for mechanical properties analysis.

2.6. SEM Observation

The surface morphologies of the samples were analyzed using a field emission scanning electron microscope (ZEISS Gemini 300, Oberkochen, Germany) at an accelerating voltage greater than 10 kV. Samples were precoated with gold using a vacuum sputter coater before observation. Aluminum (Al) and chlorine (Cl) were observed in the selected area by X-ray energy spectrometer (EDX) point scanning method.

2.7. ATR-FTIR Analysis

All samples were ball-milled into powders with a mesh size of 100. Spectra were collected by ATR-FTIR (Nicolet 6700, Pittsburgh, PA, USA). Each spectrum was recorded for 32 scans at a spectral resolution of 8 cm−1 over the wavenumbers 500–4000 cm−1. The original spectra were normalized, and the baseline was corrected.

2.8. XRD Analysis

All samples were cut into small pieces and then ground into powders with a particle size between 100–200 mesh and were screened. The powder was analyzed with an X-ray diffractometer (Ultima IV, Tokyo, Japan) with Cu Kα radiation (λ = 0.154 nm). The XRD pattern was obtained with the following parameters: scanning range, 5–40°; voltage, 40 kV; electric current, 40 mA; and scan rate, 5°/min. Each specimen’s crystallinity index (CrI) was calculated according to the Segal method [21].

3. Results

3.1. EMC and Dimensional Stability

The basic and important effects of heat treatment are the reduction of hygroscopicity and the improvement of dimensional stability, which improve the overall performance of wood in application [22].
Table 1 shows the EMC of samples in different humidity at 20 °C, which can be used to describe hygroscopicity. The EMC of the heat-treated wood was lower than that of the untreated wood when exposed to the same climate condition. It was worth noting that the EMC of the heat-treated wood under mild temperature with AlCl3-impregnation pretreatment can be significantly reduced by 2.7%–47.8%. Conditioned to equilibrium at 20 °C and RH of 30%, 65%, and 95%, EMC of the 0.05 M-130 °C samples (5.6%, 7.9%, and 12.1%) were nearly identical to those of the HT-200 °C samples (5.2%, 7.7%, and 12.0%).
The decrease in hygroscopicity implies an increase in dimensional stability. Usually, S and ASE in the tangential and radial directions are the significant factors for estimating the dimensional stability [23]. Table 1 shows that compared with the HT groups, the S values of heat-treated wood under mild temperature with impregnation pretreatment can be markedly reduced, and ASE can clearly be improved. For example, the 0.1 M-150 °C samples (ST: 1.4%, SR: 0.3%; ASET: 53.0%, ASER: 64.0%) have values close to the those of the HT-220 °C samples (ST: 1.4%, SR: 0.3%; ASET: 54.2%, ASER: 61.0%). The samples subjected to the 0.5 M-160 °C treatment achieved the best dimensional stability. Moreover, compared with the HT-220 °C samples, ST and SR were reduced by 42.9% and 66.7%, respectively, and ASET and ASER were significantly improved by 34.3% and 48.5%, respectively. These results indicate that the dimensional stability of wood heat-treated at mild temperature with AlCl3-impregnation pretreatment is far better than that of wood heat-treated at high temperature without impregnation.
Hosseinpourpia et al. (2017) proposed that the improvement in dimensional stability of heat-treated wood with pre-treated AlCl3 is mainly attributable to the reduction of available sorption sites and also the slight enhancement of the matrix stiffness due to cross-linking [17]. Himmel and Mai (2015) demonstrated a reduction in EMC of Lewis-acid-treated wood due to an increase in matrix stiffness [24]. In our study, acidic AlCl3, which will increase the acidity of the heat treatment environment, may promote thermal degradation of hemicellulose and amorphous regions of cellulose at mild heat treatment temperature, leading to a reduction in hydroxyl groups.

3.2. Mass Change

WPG values of the samples impregnated with different AlCl3 concentrations are presented in Figure 1a, which shows that the WPG of the 0.05 M, 0.1 M, and 0.5 M groups were, respectively, 0.7%, 1.9%, and 15.0%. The WPG rate increased with an increase in impregnating concentration. Figure 1b shows the ML of the samples under different heat treatment temperatures; it increased as the temperature and concentration of AlCl3 increased. Heat treatment below 160 °C had been thought to affect only some volatile extracts and have little influence on the degradation of hemicelluloses [25,26]. After impregnating, ML significantly increased. Moreover, compared with heat-treated temperature, the impregnation concentration had a more significant effect on ML. Thus, the acid AlCl3 impregnation pretreatment perhaps exerts a catalytic effect on the heat treatment, and markedly reduces heat-treated temperature for the same ML.

3.3. Macro-Color of Wood Surface

Color is an important property of wood since it influences the aesthetic of products. It can be seen in Figure 2a that the color of heat-treated wood gradually changed from light yellowish white to dark brown as the temperature increased. The color of heat-treated wood under mild temperature with AlCl3 impregnation pretreatment changed significantly as the impregnating concentration increased, achieving a color similar to that of wood that was only heat-treated at high temperatures such as 200 or 220 °C. Figure 2b shows that the lightness value (L*) was reduced as impregnating concentration increased, and the lightness difference (ΔL*) increased significantly as temperature increased. This was attributed mainly to a self-condensation reaction of chemical components in the acidic environment formed by heat treatment [27]. The introduction of AlCl3 can increase the concentration of acidity in the environment and accelerate a chain reaction. The green–red axis in the figure (a*) is related to quinones, which can absorb light complementary to red light and make the wood appear red [28]. In Figure 2c, a* of impregnated samples is shown to increase at a mild heat-treated temperature, and the Δa* values were all positive, which indicates that acid might promote thermal condensation and degradation, increasing quinones and other products, and finally transform the wood color to red. Figure 2d shows that the blue–yellow axis (b*) and Δb* fluctuated slightly.
The colorimetric parameter (ΔE*) is an important indicator of color change. Figure 2e shows that impregnation pretreatment had a strong influence on ΔE* after heating. ΔE* of heat-treated wood under mild temperature with impregnation pretreatment exhibited nearly identical values to those of wood only heat-treated at high temperature. Qu et al. (2021) came to the same conclusion using a method that combined soaking aluminum sulfate pretreatment with mild-temperature heat treatment [29]. AlCl3 might promote the degradation of hemicelluloses under heat treatment in a mild temperature environment, and part of the hemicelluloses can be degraded into colored extractives. In addition, deacetylation of hemicelluloses produces acetic acid, which can promote lignin degradation [30]. AlCl3, as one of the Lewis acids, has a good catalytic depolymerization effect on lignin, such as promoting ether bond fracture and depolymerization into phenolic compounds and catalyzing the repolymerization of depolymerization products [31]. Lignin might react into chromophore compounds and quinones that are rich in carbonyl by oxidation, condensation, β-O-4 bond breaking, or demethoxidation with phenolic compounds, aromatic amine compounds, and inorganic compounds, further deepening the color of the wood [32].

3.4. Microstructure of Wood Samples

Figure 3a shows that the cell wall of an untreated sample was smooth and intact. Figure 3b,c show a small number of burrs and delamination of partial structures on cell wall in the HT-160 °C sample, while there were a large number of delamination structures on cell walls in the HT-220 °C sample. These cracks appeared due to the destruction of the S1 and S2 layers during heat treatment at high temperatures [33]. Figure 3g shows that AlCl3 uniformly covered the cell wall through impregnating. Figure 3d–f clearly show the region of cell wall impregnated by 0.5 mol/L. The AlCl3 compound and the Al and Cl elements could be detected at the corresponding positions. Figure 3h,i show that the cell wall structure of the 0.05 M-160 °C sample remained intact, and that of the 0.5 M-160 °C sample remained relatively intact, only partially delaminated and cracked.

3.5. Mechanical Properties

Figure 4a shows that MOR decreased significantly during heat treatment, consistent with previous research results [34,35,36]. Compared with the untreated samples (75.5 MPa), MOR of wood samples heat treated at 160 °C, 200 °C, and 220 °C decreased by 31.9%, 46.4%, and 59.2%, respectively. After impregnating, the MOR of a sample decreased by 29.1%–41.3% during heat treatment at 130 °C, and increased slightly at 140 °C, then decreased substantially at a high heat-treatment temperature. Boonstra et al. (2006) found that binding force is weakened between wood microfibril and the matrix due to thermal degradation of polysaccharides [37]. Slip between the microfibers might be caused by the weakening of the binding force, leading to a decrease of the shear strength, resulting in a marked decrease in MOR. Hemicelluloses hydrolysis could lead to the destruction of hydrogen bonds, which act mainly between hemicelluloses and cellulose [38]. Depolymerization in polysaccharides autohydrolysis has been found to accelerate under acidic conditions [8,11,39].
Figure 4b shows that the MOE of the samples heat treated at 160 °C, 200 °C, and 220 °C decreased by 13.2%, 16.1%, and 13.0%, respectively, relative to untreated samples (5622.3 MPa). As impregnation concentration and heat-treatment temperature increase, the MOE of the heat-treated samples subjected to 0.5 M impregnation pretreatment improved significantly, by 11.4%–30.7% over the samples that had heat treatment only at 160–220 °C. The introduction of Lewis acid could improve the hydrolysis of polysaccharides during heat treatment [40]. The aggregated microfibrils might become larger by gathering together between cellulose microfibrils [41], and crystallization formed by molecular rearrangement could cause the sample to become more rigid [42], yet hydrolysis of polysaccharides could increase the relative content of lignin [43], which acts as a hardening agent, giving wood its hardness and rigidity. Wang (2020) suggested that Lewis acid has a good catalytic effect on ether bonds in lignin and can catalyze the repolymerization of lignin depolymerization products [31]. The crosslinking reactions between degradation products of hemicelluloses and lignin molecules probably improve the stiffness of the cell wall matrix, leading to an increase in MOE.

3.6. ATR-FTIR and XRD Results

The distribution and binding pattern of cellulose, hemicelluloses, and lignin are important factors affecting macro- and microproperties.
In Figure 5a,b, the band near 1737 cm−1 (corresponding to C=O stretching vibrations) is the characteristic peak of hemicelluloses. It decreased significantly as the impregnation concentration and heat-treatment temperature increased. Luo et al. (2022) [44] confirmed that AlCl3 can significantly reduce the pyrolysis activation energy of hemicelluloses and accelerate the thermal decomposition rate. It should be remarked that hemicelluloses play a partial bonding role between the cellulose skeleton and lignin filler, endowing wood with shear strength, highly correlated with the MOR [45]. When exogenous acid is employed to catalyze deacetylation groups of hemicelluloses, it can decrease the number of nonconjugated carbonyl groups [11]. The binding force between wood microfibril and matrix would then be weakened, leading to a decrease in MOR. The peak at 2899 cm−1, which is ascribed to methylene (CH2) and methyl (CH3), also decreased significantly. This might explain why the transition-metal ion Al3+ could activate the glycosidic bond during heat treatment [46], further promoting the degradation of polysaccharides. Absorption at 3340 cm−1 (corresponding to the hydroxyl group) decreased significantly, while absorption at 1048 cm−1 (corresponding to the C-O stretching vibration) increased. This suggests that exogenous acidic AlCl3 might promote thermal degradation of the hemicelluloses and cellulose amorphous region of wood, leading to the loss of a large number of hydroxyl group [47].
Peaks at 1323, 1422, and 1455 cm−1 (corresponding to the shear vibration of CH2 in the benzene ring and the C-H bending vibration of lignin) decreased significantly as the impregnating concentration increased under heat treatment at 160 °C, which indicates that heat treatment in an acidic environment can accelerate the degradation of lignin to release groups of the side chain. The band near 1230 cm−1 (representing phenolic ether bonds in lignin molecules) decreased significantly, indicating that acidic pretreatment combined with heat treatment promotes the breaking of the ether bond in lignin when it is heated. The peaks at 1153, 1110, and 1608 cm−1 arise from phenolic hydroxyl vibration, the characteristic absorption peak of the benzene ring, and the stretching vibration peak of the benzene ring in lignin, respectively. All of these peaks increase as the heat-treatment temperature increases for 0.5 M-HT groups. A possible explanation is that acidic AlCl3 accelerated the repolymerization of lignin depolymerization products. At the same time, the condensation reaction of the benzene ring increased the number of conjugated structures and prolonged the conjugated system [47]. Based on the above analysis, Figure 5c illustrates the effect of acidic AlCl3 on the relevant functional groups of the main chemical components of wood.
Figure 5d,e show the X-ray diffraction (XRD) patterns for different treatment conditions. The diffraction peaks at the 2θ angles of 16°, 22°, and 35° correspond to (101), (002), and (040) crystals of cellulose, respectively. The positions of these diffraction peaks were not shifted after treatment, indicating that AlCl3 impregnation and heat treatment did little damage to the crystal structure but did have some influence on the CrI. When impregnating with 0.5 M AlCl3, the CrI of the samples increased first and then decreased with increasing heat-treatment temperature. The CrI of the sample reached its highest value of 54.64% at 140 °C. Combining the results displayed in Figure 5 with those in Figure 4a, it is found that the trend of CrI and MOR are consistent as the heat-treatment temperature increases after the impregnating pretreatment. That is probably because AlCl3 can cause initial-step degradation of hemicelluloses, while xylan and mannan in hemicelluloses also have the ability to crystallize after the removal of acetyl groups [48]. The acidic environment during the heat treatment not only promoted the hydrolysis of hemicelluloses but also accelerated the fracture of the cellulose amorphous region, so that the distance between some molecules of microfibers was reduced close to the crystallization region. With increasing temperature, the transition-metal Al3+ might promote the activation of glycosidic bonds [46]. A high-temperature acidic environment further promoted the hydrolysis and oxidative cracking of cellulose chains [49], so finally the CrI decreased.

4. Conclusions

In this study, acidic AlCl3 was employed to catalyze wood thermal degradation and reduced the temperature required for its heat treatment process with desirable dimensional stability. With impregnation pretreatment of AlCl3 (0.05–0.5 mol/L), the poplar wood thermally treated under mild temperature (130–160 °C) could achieve better equilibrium moisture content, swelling coefficient, and antiswelling efficiency than that thermal treated under high temperature. Hydrolysis of polysaccharides was accelerated with the exogenous acidic AlCl3, and depolymerization and repolymerization of lignin in the AlCl3 environment during heat treatment might be promoted. Poplar wood first treated by 0.5 mol/L AlCl3 for 1 h and then heated at 160 °C for 1 h could achieve optimal properties, and its EMC, ST, SR, ASET, and ASER are 8.6%, 0.8%, 0.1%, 72.8%, and 90.6%. In the future, the quantitative relationship between hygroscopicity and dimensional stability of acid and heat-treated poplar wood needs to be further investigated.

Author Contributions

Writing—original draft preparation, formal analysis, X.W.; investigation, C.L.; resources, X.W.; conceptualization, methodology, supervision, and writing—review and editing, J.M. and C.Q.; All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation of China (No. 31971589 and No. 31870536).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this research are available on request from the corresponding author.

Acknowledgments

The authors are grateful for the financial support of the National Science Foundation of China (No. 31971589 and No. 31870536).

Conflicts of Interest

The authors declare no conflict of interest.

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Forests 13 01170 g001 550
Figure 1. (a) WPG of samples and (b) ML of samples for different treatment conditions.
Figure 1. (a) WPG of samples and (b) ML of samples for different treatment conditions.
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Figure 2. (a) Colors of samples. (b) Lightness L* and ΔL*. (c) Red and green index a* and Δa*. (d) Yellow and blue index b* and Δb*. (e) Color difference ΔE * for different treatment conditions.
Figure 2. (a) Colors of samples. (b) Lightness L* and ΔL*. (c) Red and green index a* and Δa*. (d) Yellow and blue index b* and Δb*. (e) Color difference ΔE * for different treatment conditions.
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Figure 3. (a) Untreated sample. (b) HT-160 °C sample. (c) HT-220 °C sample. (d) Sample with only impregnation pretreatment with 0.05 mol/L AlCl3. (e) Al element in panel (d). (f) Cl element in panel (d). (g) Sample impregnated only with 0.05 mol/L AlCl3. (h) 0.05 M-160 °C sample. (i) 0.5 M-160 °C sample.
Figure 3. (a) Untreated sample. (b) HT-160 °C sample. (c) HT-220 °C sample. (d) Sample with only impregnation pretreatment with 0.05 mol/L AlCl3. (e) Al element in panel (d). (f) Cl element in panel (d). (g) Sample impregnated only with 0.05 mol/L AlCl3. (h) 0.05 M-160 °C sample. (i) 0.5 M-160 °C sample.
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Figure 4. (a) MOR and (b) MOE of samples for different treatment conditions.
Figure 4. (a) MOR and (b) MOE of samples for different treatment conditions.
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Figure 5. (a) ATR-FTIR spectra and (d) XRD analysis of the 0.5 M-HT groups. (c) Hypothetical diagram of changes in related functional groups. (b) ATR-FTIR spectra and (e) XRD analysis of samples during heat treatment at 160 °C with different impregnating concentrations.
Figure 5. (a) ATR-FTIR spectra and (d) XRD analysis of the 0.5 M-HT groups. (c) Hypothetical diagram of changes in related functional groups. (b) ATR-FTIR spectra and (e) XRD analysis of samples during heat treatment at 160 °C with different impregnating concentrations.
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Table
Table 1. EMC, S values, and ASE values in tangential and radial directions of untreated and treated samples in different conditions.
Table 1. EMC, S values, and ASE values in tangential and radial directions of untreated and treated samples in different conditions.
Sample NameEquilibrium
Moisture Contents (%)		Dimensional Stability during
RH30% to RH95% (%)
RH30%RH65%RH95%STASETSRASER
Control6.3 (0.09) a9.4 (0.25)16.5 (0.43)3.0 (0.77)—0.9 (0.21)—
HT-130 °C6.3 (0.11)9.6 (0.27)16.2 (0.43)2.6 (0.69)13.00.8 (0.16)2.0
HT-140 °C6.2 (0.18)9.7 (0.09)16.1 (0.33)2.6 (0.41)14.50.8 (0.23)1.9
HT-150 °C6.2 (0.09)9.5 (0.27)16.1 (0.24)2.5 (0.44)16.00.8 (0.11)7.6
HT-160 °C6.0 (0.16)9.3 (0.02)14.8 (0.80)2.5 (0.32)17.30.7 (0.27)13.1
HT-200 °C5.2 (0.13)7.7 (0.22)12.0 (0.35)1.8 (0.43)40.60.4 (0.03)53.4
HT-220 °C4.5 (0.15)6.6 (0.24)9.8 (0.61)1.4 (0.57)54.20.3 (0.12)61.0
0.05 M-130 °C5.6 (0.10)7.9 (0.11)12.1 (0.85)2.0 (0.30)34.80.4 (0.11)51.0
0.05 M-140 °C5.5 (0.15)7.9 (0.10)11.8 (0.64)1.7 (0.39)41.70.3 (0.07)68.3
0.05 M-150 °C5.4 (0.20)7.6 (0.14)10.9 (0.21)1.5 (0.48)51.20.3 (0.13)65.0
0.05 M-160 °C5.1 (0.11)7.3 (0.11)10.8 (0.12)1.4 (0.24)53.00.2 (0.12)77.8
0.1 M-130 °C5.8 (0.18)7.9 (0.10)12.1 (0.18)1.7 (0.41)44.80.5 (0.16)44.1
0.1 M-140 °C5.5 (0.17)7.5 (0.15)10.8 (0.60)1.5 (0.39)50.60.4 (0.14)48.1
0.1 M-150 °C5.3 (0.14)7.1 (0.08)10.5 (0.30)1.4 (0.26)53.00.3 (0.16)64.0
0.1 M-160 °C5.1 (0.24)7.1 (0.18)10.5 (0.28)1.2 (0.47)61.30.3 (0.13)70.5
0.5 M-130 °C6.1 (0.30)7.8 (0.29)11.1 (0.71)1.1 (0.59)62.00.5 (0.19)41.8
0.5 M-140 °C5.9 (0.35)7.7 (0.26)10.2 (0.50)1.1 (0.23)64.60.3 (0.15)62.9
0.5 M-150 °C5.8 (0.25)7.4 (0.25)9.7 (0.31)0.9 (0.45)69.80.2 (0.17)72.1
0.5 M-160 °C5.7 (0.04)7.1 (0.04)8.6 (0.27)0.8 (0.10)72.80.1 (0.07)90.6
a Each value is the mean of 10 replicates, and the standard deviation is indicated in parentheses.
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Wang, X.; Luo, C.; Mu, J.; Qi, C. Effects of Aluminum Chloride Impregnating Pretreatment on Physical and Mechanical Properties of Heat-Treated Poplar Wood under Mild Temperature. Forests 2022, 13, 1170. https://doi.org/10.3390/f13081170

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Wang X, Luo C, Mu J, Qi C. Effects of Aluminum Chloride Impregnating Pretreatment on Physical and Mechanical Properties of Heat-Treated Poplar Wood under Mild Temperature. Forests. 2022; 13(8):1170. https://doi.org/10.3390/f13081170

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Wang, Xujie, Cuimei Luo, Jun Mu, and Chusheng Qi. 2022. "Effects of Aluminum Chloride Impregnating Pretreatment on Physical and Mechanical Properties of Heat-Treated Poplar Wood under Mild Temperature" Forests 13, no. 8: 1170. https://doi.org/10.3390/f13081170

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