Effect of Polyethylene Glycol with Different Molecular Weights on the Properties of Mytilaria laosensis Timber

Abstract

Mytilaria laosensis, a common fast-growing tree species in southern China, boasts excellent growth speed and attractive color and texture. However, due to its short growth cycle and high proportion of juvenile wood, it typically exhibits poor dimensional stability and low strength, which significantly limits its practical applications. This study uses vacuum impregnation to modify M. laosensis wood with polyethylene glycol (PEG), focusing on the effects and mechanisms of PEG with different molecular weights on wood properties. The results indicate that PEG enters the wood cell walls through capillary action and diffusion, forming hydrogen bonds with the free hydroxyl groups on cellulose and hemicellulose, which keeps the cell walls swollen and enhances dimensional stability. Post modification, the dimensional stability of M. laosensis wood improved, with an anti-swelling efficiency ranging from 61.43% to 71.22%, showing an initial increase followed by a decrease with increasing PEG molecular weight. The optimal PEG molecular weight for anti-swelling efficiency was 1500 Da, achieving 71.22%. The flexural modulus of elasticity and flexural strength of the treated wood also first decreased and then increased with increasing PEG molecular weight. Among them, the PEG1000-treated material showed the best performance, with the flexural modulus of elasticity increased by about 29% and the flexural strength increased by about 5% compared to untreated wood. Additionally, PEG, having a higher pyrolysis temperature than wood, raised the initial pyrolysis temperature and maximum pyrolysis rate temperature of M. laosensis wood, thus improving its thermal stability. These findings provide scientific evidence and technical support for the efficient utilization and industrialization of M. laosensis wood, promoting its widespread application and industrial development.

1. Introduction

As one of the four major construction materials, wood possesses unique advantages due to its renewable, recyclable, emission-reducing, and reusable characteristics [1], playing a crucial role in the economic and social development of China. China is a major producer and consumer of wood and wood products, with wood widely used in pulp and paper manufacturing, solid wood furniture, solid wood flooring, engineered wood, and construction. In recent years, China has emerged as one of the largest importers and exporters of wood products globally. According to recent data, China accounts for approximately 20% of the world’s total wood imports and about 10% of the global wood exports. This prominent position highlights the country’s reliance on wood resources to support its expansive manufacturing and construction sectors. Recently, the utilization of fast-growing timber has garnered increasing attention to meet the rising demand for wood and sustainability requirements [2]. Mytilaria laosensis, widely distributed in the low-altitude regions of Guangdong, Guangxi, and Yunnan provinces, is noted for its fast growth, beautiful color and pattern, fine texture, strong resistance, and wide adaptability [3].

Wood is a porous polymer material mainly composed of cellulose, hemicellulose, and lignin [4]. The abundant hydrophilic free hydroxyl groups in cellulose and hemicellulose cause wood to absorb or desorb moisture with changes in environmental humidity, leading to changes in the microfibril distance and resulting in the swelling and shrinking of the wood [5,6]. This property makes wood prone to dimensional changes and deformation during actual use, especially in environments with significant humidity fluctuations [7]. Additionally, wood exhibits significant anisotropy, with pronounced differences in the physical and mechanical properties along its longitudinal, radial, and tangential directions [8]. This anisotropy causes varying degrees of dimensional changes during moisture fluctuations, leading to defects such as warping, deformation, and cracking. Due to the short growth and rotation cycles of M. laosensis, the wood typically contains a high proportion of juvenile wood, which inevitably results in issues such as cracking, deformation, and poor dimensional stability, significantly limiting its range of applications. To date, the area of M. laosensis plantations is approximately 6000 hectares, but industrial development has yet to scale up. Common fast-growing tree species in China, such as poplar (Populus spp.), Masson’s pine (Pinus massoniana Lamb.), eucalyptus (Eucalyptus spp.), etc., are widely used to produce various wood products, such as solid wood, plywood, fiberboard and particleboard, etc. These wood products are mainly used in construction, furniture manufacturing, and packaging materials. However, in terms of wood utilization, research on fast-growing M. laosensis has mainly focused on pulping trials and wood properties [9,10]. Due to issues such as decay, mold, high stress, and dimensional instability during solid wood processing, as well as severe cracking and warping during drying, there has been little research on the utilization of M. laosensis for solid wood, plywood, or veneer production.

In the field of wood modification research, substantial work has been conducted. Early wood modification research primarily focused on physical and chemical modifications. Physical modifications include methods such as heat treatment, steam treatment, and microwave treatment, which improve wood performance by altering its physical structure [11,12,13,14]. Chemical modifications include treatments such as isocyanate treatment and acetylation, which improve wood performance by introducing new chemical groups or compounds [15,16,17]. However, these treatment methods often have limitations. For example, physical methods involve expensive equipment and costs, while chemical treatments are often harmful to human health and can pollute the environment. Polyethylene glycol (PEG), as an important chemical modifier, has gained wide attention due to its non-toxic, harmless, and easy-to-operate nature and significant modification effects [18]. Studies have shown that PEG can significantly improve the dimensional stability of wood [19,20,21]. Specifically, PEG penetrates the wood cell walls, displacing the water within and forming hydrogen bonds with the hydroxyl groups. This hydrogen bonding allows PEG molecules to swell the wood cell walls, fill the internal micropores and capillaries of the wood, and reduce the wood’s hygroscopicity, thereby enhancing the dimensional stability and reducing shrinkage and swelling [22,23]. Furthermore, PEG with different molecular weights varies in its penetration depth and swelling effect in the wood, impacting the modification effect [24]. Therefore, selecting the appropriate molecular weight of PEG can maximize its modification effect and enhance the performance of the wood.

Currently, there are numerous reports on the PEG modification of various tree species. Studies on species such as poplar, pine, sepetir (Sindora spp.), pisang putih (Mezzettia spp.), and nyatoh (Palaquium spp.) have demonstrated that PEG can significantly improve the dimensional stability and mechanical properties of wood [25,26]. However, research on the PEG modification of M. laosensis wood is relatively scarce. Furthermore, existing studies typically utilize PEG of a single molecular weight, without considering the potential impact of different PEG molecular weights on the modification effects. Given that the interaction between PEG and wood components can vary significantly with molecular weight, it is crucial to investigate whether PEG of different molecular weights could result in differing degrees of improvement in the dimensional stability and mechanical properties of M. laosensis wood. Therefore, this study systematically investigates the modification effects of PEG with different molecular weights on M. laosensis wood, aiming to fill this research gap and provide a comprehensive understanding of the role of PEG molecular weight in wood modification.

2. Materials and Methods

2.1. Materials

Polyethylene glycol (PEG) with molecular weights of 400, 600, 800, 1000, 1500, and 2000 (PEG400, PEG600, PEG800, PEG1000, PEG1500, PEG2000) was of analytical grade and purchased from Anhui Zesheng Technology Co., Ltd, Anqing, China. Fast-growing M. laosensis timber was sourced from Qinzhou, Guangxi. The sapwood-to-heartwood ratio of wood was 50:50. The initial water content of wood was 95%–100% and naturally dried to 25%–30%. Wood samples were selected to be free of defects such as cracks, deformation, mold, decay, insect damage, and knots. The samples were then sawn into specimens with dimensions of A, 20 mm × 20 mm × 20 mm (L × R × T), and B, 300 mm × 20 mm × 20 mm (L × R × T). The specimens were divided into control and treatment groups. The treatment group was further divided into six subgroups, each corresponding to one of the six different PEG molecular weights. Measurement positions were marked at the centers of the transverse, radial, and tangential surfaces of the A-dimension specimens. The specimens were dried to a constant weight in an electric blast drying oven at 103 °C, and the absolute dry mass m₀ was recorded. The longitudinal, tangential, and radial dimensions of the specimens were measured at the center of each face, and the absolute dry volume V₀ was calculated. Deionized water was prepared in the laboratory.

2.2. Preparation of PEG-Modified M. laosensis Wood

First, the specimens were immersed in aqueous solutions of PEG with different molecular weights (20 wt.%). They were then transferred to a vacuum oven. The oven was evacuated to −0.1 MPa and maintained for 2 h before releasing the vacuum. This cycle was repeated 12 times to ensure complete PEG penetration. Afterwards, the A-dimension specimens were placed in a drying oven, dried at 40 °C for 3 h, and then gradually heated to 103 °C at a rate of 10 °C/h until they were absolutely dry. The specimens were then cooled to room temperature in a desiccator. The mass m₁ and dimensions (radial, tangential, and longitudinal) were measured to calculate the volume V₁. The B-dimension treated specimens were first dried in an oven at 60 °C for 36 h, then placed in a constant temperature and humidity chamber at 25 °C and 65% relative humidity to adjust the moisture content to approximately 12%. The entire modification process is illustrated in Figure 1.

2.3. Characterization

2.3.1. Weight Gain Ratio

The weight gain ratio (WPG) is the ratio of the change in the absolute dry mass of the sample after impregnation modification to the absolute dry mass before impregnation, which measures the effectiveness of the impregnation. The WPG is calculated using Equation (1):

$$\mathrm{WPG}={\displaystyle \frac{{\mathrm{m}}_1-{\mathrm{m}}_0}{{\mathrm{m}}_0}}$$

(1)

where m₀ is the absolute dry mass of the sample before impregnation (g), and m₁ is the absolute dry mass of the sample after impregnation (g).

2.3.2. Dimensional Stability

Referring to GB/T 1927.8-2021 [27] “Test Methods for Physical and Mechanical Properties of Wood with Small Clear Specimens—Part 8: Determination of Swelling” and GB/T 1927.6-2021 [28] “Test Methods for Physical and Mechanical Properties of Wood with Small Clear Specimens—Part 6: Determination of Shrinkage”, the swelling and shrinkage rates were calculated. The radial, tangential, and longitudinal dimensions of the PEG-treated material were measured in absolute dry and fully saturated states. The radial (tangential) swelling rate and volume swelling rate were calculated using Formulas (2) and (3):

$${\mathsf{\alpha }}_{\mathrm{s}}={\displaystyle \frac{{\mathrm{L}}_1-{\mathrm{L}}_0}{{\mathrm{L}}_0}}\times 100\%$$

(2)

$${\mathsf{\alpha }}_{\mathrm{v}}={\displaystyle \frac{{\mathrm{V}}_1-{\mathrm{V}}_0}{{\mathrm{V}}_0}}\times 100\%$$

(3)

where α_s is the radial (tangential) swelling rate (%), L₁ is the dimension of the sample after swelling (mm), and L₀ is the dimension of the sample in the absolute dry state (mm). α_v is the volume swelling rate (%), V₁ is the volume of the sample after swelling (mm³), and V₀ is the volume of the sample in the absolute dry state (mm³).

The radial, tangential, and longitudinal dimensions of the PEG-treated material were measured in fully saturated and absolute dry states. The radial (tangential) shrinkage rate and volume shrinkage rate were calculated using Formulas (4) and (5):

$${\mathsf{\beta }}_{\mathrm{s}}={\displaystyle \frac{{\mathrm{L}}_1-{\mathrm{L}}_2}{{\mathrm{L}}_1}}\times 100\%$$

(4)

$${\mathsf{\beta }}_{\mathrm{v}}={\displaystyle \frac{{\mathrm{V}}_1-{\mathrm{V}}_2}{{\mathrm{V}}_1}}\times 100\%$$

(5)

where β_s is the radial (tangential) shrinkage rate (%), L₁ is the dimension of the sample in the saturated state (mm), and L₂ is the dimension of the sample in the absolute dry state (mm). β_v is the volume shrinkage rate (%), V₁ is the volume of the sample in the saturated state (mm³), and V₂ is the volume of the sample in the absolute dry state (mm³).

The anti-swelling efficiency (ASE) is an important criterion for measuring wood dimensional stability, calculated using Formula (6):

$$\mathrm{ASE}={\displaystyle \frac{{\mathsf{\alpha }}_{\mathrm{vu}}-{\mathsf{\alpha }}_{\mathrm{vm}}}{{\mathsf{\alpha }}_{\mathrm{vu}}}}\times 100\%$$

(6)

where ASE is the anti-swelling efficiency (%), α_vu is the volume swelling rate of untreated material (%), and α_vm is the volume swelling rate of treated material (%).

The volume change coefficient (VCC) is calculated using Formulas (7) and (8), which provide a brief assessment of the primary reason for PEG impregnation improving wood dimensional stability:

$$\mathrm{VCC}={\displaystyle \frac{{\mathsf{\alpha }}_{\mathrm{vu}}-{\mathsf{\alpha }}_{\mathrm{vm}}^{\prime }}{{\mathsf{\alpha }}_{\mathrm{vu}}}}$$

(7)

$${\mathsf{\alpha }}_{\mathrm{vm}}^{\prime }={\displaystyle \frac{{\mathrm{V}}_{\mathrm{m}}-{\mathrm{V}}_{\mathrm{m}}^{\prime }}{{\mathrm{V}}_{\mathrm{m}}^{\prime }}}$$

(8)

where α′_vm is the volume swelling rate of treated material before impregnation (%), V_m is the volume of treated material in the saturated state (mm³), and V′_m is the drying volume of treatment material before treatment (mm³).

2.3.3. Water Absorption Rate

Referring to GB/T 1927.7-2021 [29] “Test Methods for Physical and Mechanical Properties of Wood with Small Clear Specimens—Part 7: Determination of Water Absorption”, the water absorption rate was tested and calculated. After testing the shrinkage and swelling rates, the specimens were placed in a container with distilled water for the water absorption test. The weight was measured every 24 h. Each time the specimens were weighed, they were removed from the container, and the surface water was blotted off with absorbent paper before weighing. The water absorption rate for each group of specimens was calculated using Formula (9), and the average value was taken:

$$\mathrm{A}={\displaystyle \frac{\mathrm{m}-{\mathrm{m}}_0}{{\mathrm{m}}_0}}\times 100\%$$

(9)

where A is the water absorption rate (%), m is the mass of the sample after water absorption (g), and m₀ is the absolute dry mass of the sample (g).

2.3.4. Mechanical Properties

Referring to GB/T 1927.10-2021 [30] “Test Methods for Physical and Mechanical Properties of Wood with Small Clear Specimens—Part 10: Determination of Modulus of Elasticity in Bending”, the modulus of elasticity in bending was tested for untreated and PEG-treated B-dimension specimens using a tangential four-point bending load test. The distance between loading heads was 80 mm, the span of the supports was 240 mm, and the loading speed was set to 2 mm/min, with lower and upper load limits of 300 N and 700 N, respectively.

Referring to GB/T 1927.9-2021 [31] “Test Methods for Physical and Mechanical Properties of Wood with Small Clear Specimens—Part 9: Determination of Bending Strength”, the bending strength was tested for untreated and PEG-treated B-dimension specimens using a tangential three-point bending load test. The loading speed was set to 5 mm/min, and the span of the supports was 240 mm.

2.3.5. Thermal Stability Analysis

Thermogravimetric analysis was performed on untreated and PEG-treated material using a thermogravimetric analyzer under a nitrogen atmosphere. The temperature range was 30 °C to 800 °C, with a heating rate of 10 °C/min.

2.3.6. Structural Characterization and Performance Testing

Untreated and PEG-treated material powder samples were dried to absolute dryness, mixed with dry potassium bromide powder, ground, and pressed into thin slices for Fourier-transform infrared spectroscopy (FTIR, FTS135, Agilent Technologies, Santa Clara, CA, USA) testing. The FTIR instrument had a resolution of 4 cm⁻¹ and a scan wavenumber range of 400 cm⁻¹ to 4000 cm⁻¹, and 20 scans were performed.

The microstructure of M. laosensis wood was observed using a scanning electron microscope (SEM, SU8010, Hitachi, Tokyo, Japan). Samples were cut from untreated and PEG-treated material, dried to absolute dryness, cooled, and placed on sample stages with double-sided conductive tape, then gold-sputtered before observation.

The crystalline structure of the samples was detected using an X-ray diffractometer (XRD, Bruker D8 ADVANCE, Bruker, Billerica, MA, USA). Untreated and PEG-treated material powder samples were dried to absolute dryness, cooled, and analyzed using XRD with a scan range of 5 ° to 70°. The crystallinity index was calculated using the Segal empirical method to indicate the degree of cellulose crystallinity under different treatment conditions, using the following formula:

$$\mathrm{CI}={\displaystyle \frac{{\mathrm{I}}_{002}-{\mathrm{I}}_{\mathrm{am}}}{{\mathrm{I}}_{002}}}\times 100\%$$

(10)

where CI is the crystallinity index (%), I₀₀₂ is the maximum diffraction intensity of cellulose I near the 002 plane at 2θ = 22°, and I_am is the amorphous reflection intensity of cellulose I at 2θ = 18°, representing the diffraction intensity of the amorphous region [32].

2.3.7. Data Analysis

The normality and homogeneity of variances of the data were confirmed by the Shapiro–Wilk test and Levene’s test, followed by analysis of variance (ANOVA) and post hoc multiple comparisons using Tukey’s HSD test to evaluate the significance of differences between groups. To facilitate illustration and comparison, the bars are labeled with letters. There were no significant differences between groups that shared at least one letter, and there were significant differences between groups that did not share any letters.

3. Results and Discussion

3.1. Physical Performance Analysis

3.1.1. Weight Gain Ratio

The changes in the weight gain ratio (WPG) of M. laosensis wood treated with PEG of different molecular weights are shown in Figure 2. The effect of PEG molecular weight on the WGA of M. laosensis wood was statistically significant (p ≤ 0.05). The results indicate that as the molecular weight of PEG increases, the WPG of the wood shows a decreasing trend, which is consistent with the results of other modification treatments [33,34,35,36]. Specifically, wood treated with PEG 400 and PEG 600 has the highest WPG, approximately 30%. The WPG for PEG 800 is slightly lower but not significantly different from that of PEG 400 and PEG 600. The WPG for PEG 1000 further decreases, showing a significant difference from PEG 400 and PEG 600 but no significant difference from PEG 800, indicating that the permeability starts to be affected within this molecular weight range, but the change is relatively smooth. The WPG for wood treated with PEG 1500 and PEG 2000 is the lowest, with no significant difference between the two. This is because as the molecular weight of PEG increases, it becomes more difficult for PEG to penetrate the wood, leading to a reduced weight gain effect [37,38].

3.1.2. Dimensional Stability

The relationship between PEG molecular weight and the shrinkage and swelling rates of PEG-treated wood is shown in Figure 3. It can be seen that compared to untreated wood, the shrinkage and swelling rates of PEG-treated wood are significantly reduced, indicating that PEG improves the dimensional stability of M. laosensis wood. As the molecular weight of PEG increases, the shrinkage and swelling rates first decrease and then increase. When the molecular weight of PEG is 1000, the tangential shrinkage rate and volume shrinkage rate of the treated wood are 2.2% and 4.0%, respectively, which are lower than those of other treated woods. When the molecular weight of PEG is 1500, the radial shrinkage rate, radial swelling rate, and volume swelling rate of the treated wood are 1.1%, 1.2%, and 4.1%, respectively, which are better than those of other PEG-treated woods. When the molecular weight of PEG is 2000 Da, the tangential swelling rate of the treated wood is 2.3%, the best among all PEG-treated woods. Therefore, higher-molecular-weight PEG provides better modification effects on M. laosensis wood, with lower shrinkage and swelling rates compared to lower-molecular-weight PEG-treated wood.

The relationship between the PEG molecular weight and anti-swelling efficiency (ASE) is shown in Figure 4. The effect of PEG molecular weight on the ASE of M. laosensis wood was statistically significant (p ≤ 0.05). From Figure 4, it can also be seen that the ASE of wood treated with PEG of different molecular weights can reach over 60%, indicating that PEG effectively improves the dimensional stability of M. laosensis wood. As the molecular weight of PEG increases, the ASE initially increases and then decreases, with the ASE ranging from 61.43% to 71.22%. Among them, the ASE value of samples treated with PEG 400 is significantly lower than that of other treated woods, only 61.43%. This is because PEG 400 is a liquid, and its crystallization effect is weaker compared to other solid PEGs. The ASE value of samples treated with PEG 1500 is significantly higher than that of samples treated with PEG of molecular weights 400–1000, reaching as high as 71.22%, and there is no significant difference compared to samples treated with PEG 2000. This indicates that PEG 1500 has the best effect on improving the dimensional stability of the samples, and further increases in molecular weight do not significantly change the modification effect. This is because low-molecular-weight PEG has strong permeability but a smaller swelling effect on the cell walls, making it difficult to achieve a high ASE [39,40,41]. However, although high-molecular-weight PEG has a significant swelling effect on the cell walls, its mobility in the dense cell walls is poor [21]. In addition, compared with other treatment methods, it was found that the efficiency of PEG treatment was significantly higher than other methods (

Table 1. Comparison of similar methods for improving the dimensional stability of wood.

Species	Modification Method	ASE (%)	Refs
Teak wood (Tectona grandis Linn. F.)	Furfurylation treatment	64.17	[42]
Eucalyptus grandis × Eucalyptus urophylla “GLGU9”	Palm oil thermal treatment	56.6	[43]
Silver birch (Betula pendula)	Phenol-formaldehyde resin treatment	65.0	[29]
Ailanthus altissima (Mill.) Swingle	Low point metal alloy treatment	56.0	[44]
Rubberwood (Hevea brasiliensis Müll.Arg)	Isopropenyl acetate treatment	60.6	[45]
Mytilaria laosensis	PEG treatment	71.22	This work

). It is worth noting that compared with other treatment methods, our treatment has the advantages of being relatively simple in terms of the process and environmentally friendly. At the same time, due to the environmental protection properties of PEG, the PEG-treated M. laosensis wood not only maintains environmental protection properties during use but also does not cause secondary pollution to the environment after its service life. PEG treatment is expected to increase the demand for M. laosensis wood in the market, and its application prospects are very broad.

The volume change coefficient (VCC) can be used to simply judge the main reasons for the improvement in wood dimensional stability due to modification. To better understand the difference between the VCC and ASE, a schematic diagram of the VCC and ASE is shown in Figure 5a. The relationship between the VCC and the main reasons for modification is shown in Figure 5b. It is generally believed that there are three mechanisms for improving the dimensional stability of modified wood: swelling, cell wall damage, and cross-linking. A positive VCC indicates that cross-linking with cell wall polymer components is the main reason for the improvement in dimensional stability, while a negative VCC indicates that cell wall damage is the main reason. When the VCC is zero, it indicates that swelling is the main reason for the improvement in dimensional stability [46]. From Figure 5c, it can be seen that the VCC values of PEG-treated wood with different molecular weights are distributed around zero, indicating that the main mechanism for improving the dimensional stability of M. laosensis wood treated with PEG is swelling, and PEG did not form chemical bonds with the chemical components of the wood [47].

3.1.3. Water Absorption

Figure 6 shows the changes in the water content of PEG-treated wood over time. It can be seen that the water absorption rate of untreated wood is higher than that of PEG-treated wood. In the later stages of the experiment, the water absorption rate of untreated wood increases faster than that of PEG-treated wood, indicating that PEG can improve the water absorption properties of M. laosensis wood. According to national standards, when the difference in water absorption rates between two consecutive measurements is less than 5%, the sample is considered to have reached its maximum water absorption rate. The maximum water absorption rates for untreated wood and wood modified with PEG400, 600, 800, 1000, 1500, and 2000 are 109.3%, 91.81%, 90.56%, 90.44%, 90.22%, 88.52%, and 86.05%, respectively, with PEG2000 treated wood having the lowest maximum water absorption rate. It can be seen that as the molecular weight of PEG increases, the water absorption rate of the treated wood decreases over the same soaking time. This is because higher-molecular-weight PEG not only has a better swelling effect on wood cell walls, but also its increased hydrophobicity reduces the ability of wood to bind water [48], thereby reducing the water absorption rate of the treated wood.

3.2. Mechanical Performance Analysis

3.2.1. Flexural Modulus of Elasticity

The flexural modulus of elasticity (MOE) of wood can characterize its stiffness, indicating its ability to resist bending deformation within the proportional limit. The larger the MOE, the smaller the bending deformation and the greater the stiffness. Under the same concentration, the effect of PEG with different molecular weights on the MOE of wood is shown in Figure 7. The effect of PEG molecular weight on the MOE of M. laosensis wood was statistically significant (p ≤ 0.05). The MOE of untreated wood is 3965.1 MPa. Interestingly, treating wood with low-molecular-weight (400–600 Da) PEG does not increase the MOE; instead, it negatively impacts it. The MOE of wood treated with PEG 400 and PEG 600 is significantly lower than that of untreated wood, being 30% and 14% lower, respectively. As the molecular weight of PEG increases, the MOE of the wood gradually increases. The MOE of wood treated with PEG 800 is not significantly different from that of untreated wood. When the molecular weight of PEG is 1000 Da, the MOE of the wood reaches a maximum of 5121.3 MPa, 29% higher than that of untreated wood. This is attributed to the fact that low-molecular-weight PEG, being more flexible and more liquid-like (such as PEG 400), leads to the softening of the wood cell walls, which reduces the stiffness of the wood when subjected to external loads [23]. In contrast, as the molecular weight of PEG increases, its rigidity also increases, providing enhanced structural support within the cell walls. This increased rigidity helps to raise the MOE. When the PEG molecular weight exceeds 1000 Da, the MOE of PEG-treated wood decreases. The MOE of wood treated with PEG 1500 and PEG 2000 is 16% lower than that of wood treated with PEG 1000, with no significant difference between the two. This is because, as the molecular weight of PEG reaches a certain level, its mobility within the wood cell walls decreases, with some PEG filling the cell cavities, thereby reducing the reinforcing effect on the cell walls [41].

3.2.2. Flexural Strength

The flexural strength of wood indicates the maximum load it can bear under bending, which is an important parameter for evaluating the structural strength and stability of wood. The flexural strength of wood treated with PEG of different molecular weights is shown in Figure 8. The effect of PEG molecular weight on the flexural strength of M. laosensis wood was statistically significant (p ≤ 0.05). From Figure 8, it can be observed that within the molecular weight range of 400–1000 Da, the flexural strength of the treated woods gradually increases with the molecular weight and reaches the level of untreated wood at 1000 Da. However, as the molecular weight continues to increase, the flexural strength of the wood decreases to the level of wood treated with PEG400. Among the various groups of PEG-treated wood, the flexural strength of wood treated with PEG 1000 is the highest at 67.7 MPa, and it shows a significant difference compared to the other treated woods, indicating that PEG 1000 provides the best treatment effect. However, there is no significant difference between PEG 1000-treated wood and untreated wood, suggesting that PEG treatment does not significantly enhance the overall flexural strength of the wood. This is because the cell walls of the wood can be softened by PEG, making the microfibrils more prone to deformation under stress [23].

Table 2. Comparison of mechanical properties of similar methods for improving dimensional stability of wood.

Species	Modification Method	Change Ratio of MOE	Change Ratio of MOR	Refs
Teak wood (Tectona grandis Linn. F.)	Furfurylation treatment	−14.0%	−20.0%	[42]
Silver oak (Grevillea robusta)	Vacuum heat treatment	−21%	−60%	[49]
Afrormosia (Pericopsis elata Van Meeuwen)	Air heat treatment	−18%	−24.2%	[50]
Rubber wood (Hevea brasiliensis)	Silica sol and glyoxal melamine urea treatment	+3.4%	+25.2%	[51]
Poplar (Populus)	Dimethylol dihydroxy ethylene urea treatment	+46.6%	−16.9%	[52]
Pinus elliottii Engelm	Raw pine resin treatment	+70.0%	+50.0%	[53]
Mytilaria laosensis	PEG treatment	+29%	+5%	This work

illustrates the impacts of various wood dimensional stability improvement methods on mechanical strength. It is evident from the table that modification using PEG, which enhances dimensional stability through a swelling mechanism, does not improve the mechanical properties of wood as effectively as chemical cross-linking methods based on sol–gels or resins. However, it significantly outperforms methods that primarily cause cell wall damage, such as heat treatment or Furfurylation treatment. Although chemical cross-linking methods can substantially enhance the mechanical properties of wood, the residual harmful substances from these processes and their potential for causing long-term environmental pollution during the wood disposal stage cannot be overlooked. As a highly biocompatible and biodegradable substance, the use of PEG offers distinct environmental advantages. Moreover, wood treated with PEG exhibits superior dimensional stability compared to the aforementioned methods, providing a more significant improvement, as illustrated in Table 1. This method effectively balances the needs for environmental protection with industrial applications.

3.3. Thermal Stability Analysis

Figure 9 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves for untreated and PEG-treated wood. During the pyrolysis of wood, the stage below 150 °C is the drying phase, dominated by the endothermic evaporation of free and bound water in the cell walls, with almost no change in the wood’s chemical components [54]. The phase from 150 °C to 200 °C is the endothermic degradation phase, where the chemical components of the wood begin to change, and unstable components like hemicellulose start to pyrolyze, although the changes are still relatively slow. The phase from 220 °C to 450 °C is the thermal decomposition and carbonization phase, where the pyrolysis rate accelerates, forming a combustion chain reaction, resulting in significant mass loss due to rapid pyrolysis and substantial charcoal formation [55,56]. On the DTG curve, untreated wood shows a slight hemicellulose pyrolysis peak around 300 °C and a major mass loss peak at 360 °C due to the vigorous degradation of cellulose, while lignin pyrolysis occurs over a broader temperature range of 250 °C–500 °C. Above 450 °C is the calcination phase, where the primary process is the expulsion of residual volatiles and an increase in fixed carbon content in the charcoal.

Compared to untreated wood, the initial pyrolysis temperature and the temperature at the maximum pyrolysis rate for PEG-treated wood are higher. PEG impregnation covers the wood components’ surfaces, inhibiting heat transfer and slowing the thermal decomposition rate, thereby increasing the initial pyrolysis temperature and the temperature at the maximum pyrolysis rate, thus enhancing the thermal stability. As the PEG molecular weight increases, the derivative thermogravimetric curves of the modified wood exhibit two peaks, indicating that the pyrolysis stage of PEG-impregnated modified wood is divided into two parts, determined by both the M. laosensis wood and PEG. The first peak around 360 °C originates from the pyrolysis of wood components, while the second peak around 400 °C is due to the pyrolysis of PEG [57]. Under high-temperature conditions, PEG molecular chains depolymerize and decompose into gases like methane, while wood pyrolysis produces combustible gases such as carbon monoxide, hydrogen, methane, and ethane. After pyrolysis, the residual mass proportion of PEG-modified wood is lower than that of untreated wood. The residual mass proportions for PEG1500- and PEG2000-modified wood are 2.38% and 2.34% lower than those of untreated wood, respectively. For PEG400, PEG600, and PEG1000, the residual mass proportions are 4.59%, 4.16%, and 4.25% lower, respectively, and the lowest is for PEG800, which is 6.70% lower than untreated wood. This indicates that PEG reduces the char yield during the pyrolysis process. In conclusion, PEG treatment can improve the thermal stability of M. laosensis wood.

3.4. Mechanism of Modification

3.4.1. FTIR Analysis

Figure 10 shows the infrared spectra of PEG-treated wood. The spectra use absorbance as the vertical axis, making them more suitable for quantitative analysis. As seen in the spectra, the presence of a significant number of -CH₂ groups in PEG results in an increased C-H stretching vibration at 2900 cm⁻¹ compared to untreated wood. At 1110 cm⁻¹, the C-O stretching vibration is enhanced in PEG-treated wood, indicating an increased PEG content in the treated wood due to successful impregnation via vacuum treatment. Hydrogen bond formation typically leads to a shift of the stretching vibration absorption peak towards a lower wavenumber because the formation of hydrogen bonds reduces the bond force constant between hydrogen atoms and their connected atoms, thereby lowering the vibration frequency. The untreated wood shows an -OH stretching vibration peak at 3423 cm⁻¹, which shifts to 3400 cm⁻¹ after PEG treatment, indicating the formation of hydrogen bonds between the hydroxyl groups in PEG and the wood cell walls [58,59]. The infrared spectra of untreated wood and PEG-treated wood with different molecular weights show similar patterns without new peaks, suggesting that PEG does not chemically bond with the wood but is primarily physically adsorbed or deposited within the wood structure [60,61].

3.4.2. XRD Analysis

The swelling of cellulose can be categorized into two types: inter-crystalline swelling and intra-crystalline swelling. Inter-crystalline swelling occurs when the swelling agent penetrates only the amorphous regions of cellulose, while intra-crystalline swelling involves penetration into both the amorphous and crystalline regions of cellulose. To distinguish between these types, one can observe the XRD patterns after swelling. If the position of the XRD peaks remains unchanged and only the peak intensity changes, it indicates inter-crystalline swelling. Figure 11 shows diffraction peaks of cellulose at 15°, 22°, and 35° corresponding to the (101), (002), and (040) crystallographic planes, respectively [62]. The positions of these peaks are similar for both untreated and PEG-treated wood with various molecular weights, indicating that PEG treatment does not disrupt the crystalline structure of cellulose and that the swelling is inter-crystalline. The calculated crystallinity of untreated wood and PEG-treated wood with different molecular weights is 30.45%, 41.85%, 44.71%, 38.10%, 40.84%, 44.84%, and 49.03%, respectively. The higher crystallinity of PEG-treated wood compared to untreated wood is due to the crystalline nature of PEG itself, which influences the overall crystallinity of the treated wood, further confirming that PEG does not chemically bond with the wood [63].

3.4.3. SEM Analysis

Using scanning electron microscopy, the morphological characteristics of the cell walls of M. laosensis wood treated with different molecular weights of PEG were observed and compared with those of untreated wood. From Figure 12, it can be seen that M. laosensis wood has numerous vessels that are uniform in size and distribution, indicative of a diffuse-porous wood type. The perforation plates within the vessels are scalariform, suggesting good permeability. Compared to untreated wood, the cell walls of the modified wood samples treated with different molecular weights of PEG showed varying degrees of thickening. This is because PEG can diffuse into the wood cell walls, replacing the water within, thereby supporting and swelling the cell walls, which contributes positively to the dimensional stability of M. laosensis wood [64]. From the radial section, it is evident that the cell lumen surfaces of the treated wood are coated with a layer of material. This is the PEG that did not penetrate the cell walls but instead adsorbed onto the cell lumen surfaces, reducing the contact between the wood and moisture. As the molecular weight of PEG increases, more PEG is observed on the cell lumen surfaces, indicating that higher-molecular-weight PEG is more difficult to penetrate into the cell walls.

In summary, the mechanism by which PEG enhances the performance of M. laosensis wood primarily involves physical swelling and penetration rather than chemical modification. PEG molecules, depending on their molecular weight, infiltrate the wood’s cell lumens and penetrate the cell walls through diffusion, filling in pores and displacing water. This interaction is facilitated by the formation of hydrogen bonds between the hydroxyl groups of PEG and the wood components, leading to the physical swelling of the cell walls without altering the chemical structure of the cellulose. Low-molecular-weight PEG, such as PEG400 and PEG600, can penetrate the cell wall more effectively, while higher-molecular-weight PEG, such as PEG1500 and PEG2000, can play a better swelling role. Overall, the primary mechanism is physical adsorption and swelling, as PEG does not chemically bond with the wood but improves its properties through physical interactions, as shown in Figure 13.

4. Conclusions

The primary mechanism of the PEG modification of M. laosensis wood is physical swelling. PEG monomers penetrate wood cell cavities and cell walls through pores, forming hydrogen bonds with wood components, causing cell walls to swell without disrupting the crystalline structure of cellulose. Post modification, the dimensional stability of M. laosensis wood improved, with an anti-swelling efficiency ranging from 61.43% to 71.22%, showing an initial increase followed by a decrease with increasing PEG molecular weight. The optimal PEG molecular weight for anti-swelling efficiency was 1500 Da, achieving 71.22%. The flexural modulus of elasticity and flexural strength of the treated wood also first decreased and then increased with increasing PEG molecular weight. Among them, the PEG1000-treated material showed the best performance, with the flexural modulus of elasticity increased by about 29% and the flexural strength increased by about 5% compared to untreated wood. In addition, PEG treatment also improved the thermal stability of M. laosensis wood. Compared with other methods for improving the dimensional stability of wood, the PEG treatment in this study appears to be more environmentally friendly because it uses low-toxic and biodegradable materials. This study provides a scientific basis and technical support for the efficient utilization and industrialization of M. laosensis wood, promoting its broader application and industrial development. Although the current PEG treatment method can effectively prevent wood cracking during the wood drying process, it is still water-soluble after contact with moisture and may be lost over time. Therefore, developing more durable improved methods or combining other technologies to enhance the durability of PEG treatment will be an important direction to further improve the dimensional stability of wood.