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. 2024 Jun 18;9(26):28816–28826. doi: 10.1021/acsomega.4c02820

Research on Enhancing the Comprehensive Performance of Fir Wood through Chemical Modification with a Biobased Unsaturated Polyester

Tianle Xu †, Xinran Ju ‡, Hui Tang †,*, Wenli Xiang †
PMCID: PMC11223232  PMID: 38973938

Abstract

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Fir wood was modified using epoxy soybean oil, diethylene glycol, and maleic anhydride as raw materials to enhance its mechanical properties, thermal stability, and water resistance. Diethylene glycol first opens the epoxy ring of the soybean oil and then reacts with maleic anhydride to produce an esterification reaction. The product modifies the fir wood through a chemical impregnation method. A systematic evaluation of the modified wood’s weight gain ratio, density, mechanical properties, thermal stability, water resistance, and microstructural changes was conducted. The results show that the compressive strength increased from 38.1 to 94.9 MPa, the water absorption rate decreased from 158.03 to 6.93%, and the thermal stability was also enhanced. This study provides a simple, low-cost, and green method for improving the comprehensive performance of fast-growing fir wood, offering new insights for achieving sustainable development and green chemical engineering.

1. Introduction

Today, wood has become an incredibly important natural and renewable resource due to its ease of processing and high strength. It is widely used in construction, furniture, and decorative items.1,2 However, natural forests cannot meet the growing demand for wood, and excessive logging can lead to environmental degradation. As a result, artificially cultivated fast-growing wood has caught public attention due to its rapid growth rate, high yield, and short harvesting period, making it an important alternative to natural wood.3,4 Fast-growing wood can meet the market’s increasing demand and is one of the methods to practice sustainable development and protect nature, attracting widespread attention from scholars today.5,6

The modification of fast-growing wood is necessary. Although it has the advantages of high yield and rapid growth, its lower wood strength and susceptibility to corrosion significantly limit its application scope.7−10 Chemical modification is an effective method to improve the mechanical properties, dimensional stability, flame retardancy, and corrosion resistance of fast-growing wood. Among various chemical modification techniques, resin impregnation is considered to be an efficient, low-cost, and simple treatment method that significantly enhances the wood’s multifaceted properties.11,12 The most common chemical modification methods include thermosetting resin treatment,13 acetylation,14 and furfurylation,15 among others. Recently, Peng16 and colleagues have modified fast-growing poplar wood using 1,3-dihydroxymethyl-4,5-dihydroxyethylurea (DM) and alkaline lignin through wood impregnation, effectively improving the wood’s dimensional stability, mechanical properties, and corrosion resistance.

Biomaterials are characterized by their wide availability, low cost, high quantity of functional groups, and high reactivity. A current hot research topic is the production of wood–plastic composites using biomass raw materials such as plant oil polyols, glycerol, maleic anhydride, and lignin. Milena et al.17 produced polyurethane (RPUF) through the cyclization of rapeseed oil and its reaction with diols, which exhibited good mechanical and biological performance, high-dimensional stability, high aging resistance, and low water absorption rate, but their experiment applied polyester in the pumice applications of the cosmetics industry. Maleic anhydride, a common anhydride, is usually prepared from furfural. Furfural can be extracted from various agricultural products, such as corn cobs, wheat bran, and sawdust.18 Therefore, maleic anhydride is a derivative of biomass materials. It is characterized by its strong acidity, low melting point, and the presence of a high content of carboxyl and unsaturated bonds after ring opening. Maleic anhydride, when in contact with hydroxyl-containing compounds at an appropriate temperature, will open its ring and undergo esterification with hydroxyl groups.19

However, there are few published studies that apply biobased unsaturated polyester resin (UPR) to wood reinforcement and modification. For example, Costa et al.20 studied a novel UPR based on soybean oil and coconut oil, which has high thermal stability. Das et al.21 prepared a novel UPR from tung oil, which has higher impact strength, creep resistance, modulus, and hardness. Hence, it is necessary to develop a new method to prepare a biobased unsaturated polyester on the basis of RPUF and apply it to the performance modification of fast-growing wood.

This paper focuses on the preparation of the unsaturated polyester EDM by opening the ring of epoxy soybean oil with diethylene glycol and reacting it with maleic anhydride. A biobased wood-plastic composite material is produced using a chemical impregnation method (Figure 1). This study aims to broaden the application of vegetable oil-based unsaturated polyester in the modification of fast-growing wood, promote a harmless and low-cost method for modifying fast-growing wood, and provide an environmentally friendly and green approach for the production of biobased unsaturated polyester.

Figure 1.

Figure 1

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Schematic diagram of the synthesis of the unsaturated polyester EDM.

2. Experimental Section

2.1. Main Raw Materials and Instruments

2.1.1. Experimental Materials and Reagents

Epoxy soybean oil and fir wood were purchased from the market; diethylene glycol (DEG, A.R.) was acquired from Tianjin Balloon Chemical Reagents Science & Technology Co., Ltd.; p-toluenesulfonic acid (A.R.) was obtained from Xilong Scientific Co., Ltd.; maleic anhydride (MA, A.R.) was sourced from Shanghai Reagent Factory no. 3; benzoyl peroxide (BPO, A.R.) was procured from Sinopharm Chemical Reagent Co., Ltd.; hydroquinone (HQ, A.R.) was provided by Sinopharm Chemical Reagent Co., Ltd.; styrene (St, A.R.) was produced by Shanghai Aladdin Bio-Chem Technology Co., Ltd.; and divinylbenzene (DVB, A.R.) was purchased from Tianjin Chemical Reagents Company.

2.1.2. Analytical and Testing Instruments

The instruments used are as follows: rotary viscometer: NDJ-1 model, Shanghai Precision & Scientific Instrument Co., Ltd.; Fourier transform infrared (FTIR) spectrometer: Spectrum Two, PerkinElmer, USA; compression testing machine: YAW-100D, Jinan Zhonglu Chang Experimental Machine Manufacturing Co., Ltd.; thermogravimetric analyzer: TGA-50, Shimadzu Corporation, Japan; X-ray photoelectron spectrometer: PHI5000 VersaProbe-II, Ulvac-Phi, Japan; scanning electron microscope: TESCAN VEGA 3, TESCAN, Czech Republic; nuclear magnetic resonance hydrogen (1H NMR) spectrum: AV III HD 400 MHz, Bruker, Germany.

2.2. Preparation of Modified Wood Samples

2.2.1. Synthesis of the Castor Oil-Based Unsaturated Polyester

Epoxy soybean oil and diethylene glycol were mixed in a 1:1.5 molar ratio with 0.9 wt % p-toluenesulfonic acid added to a three-necked flask. The mixture was stirred and heated to 140 °C. After maintaining the temperature for 4 h, the epoxy value of the product was measured according to GB/T 1677–1981.22 Once the epoxy value dropped to zero, the mixture was cooled to 70 °C, and maleic anhydride was added in an amount equal to the molar number of hydroxyl groups in the system. The mixture was stirred for 30 min to ensure thorough mixing, then 1 wt % N,N-dimethylbenzylamine and 0.02 wt % hydroquinone were added, and the temperature was raised to 90 °C. After reacting for 4 h, the product, EDM, a reddish-brown substance, was prepared.

2.2.2. Modification of Wood Samples

To ensure consistency among the fir wood specimens (cut along the grain of the wood to obtain wooden blocks with dimensions of 20 × 20 × 30 mm), they must be hewn from an undamaged, uniform board with a density approximately at the mean value (0.36 ± 0.03 g/cm3). Initially, the specimens underwent vacuum dehydration at 103 ± 2 °C for a duration of 2 h. Subsequently, they were placed in a container designed for impregnation. The samples were subjected to a prevacuum of −0.08 MPa for 2 h, which preceded the introduction of the EDM impregnating solution. This was followed by a 2 h atmospheric pressure immersion to guarantee thorough permeation. Finally, the wood pieces were heat-treated at 100 °C for 2 h to finalize the modification process.

2.3. Test and Analysis Methods

2.3.1. Determination of Gel Content in the Impregnation System

The gel content of the EDM impregnation system was determined according to the method described in GB/T 2576–2005.23 The soluble components in the cured UPR impregnation system were extracted using a Soxhlet extractor with ethyl acetate at a temperature close to its boiling point. The insoluble components were considered as the degree of curing of the resin, α, and calculated using eq 1

2.3.1. 1

  • m is the mass of the sample before extraction, in mg;

  • m’ is the mass of the sample after extraction, in mg.

The degree of curing (β) of the UPR impregnation system in wood: The degree of curing of the UPR impregnation system within the wood was determined by referring to the method for determining the degree of curing of the UPR impregnation system and calculated using eq 2 to find the curing degree (β) of the UPR impregnation regime within the wood.

2.3.1. 2

  • m1 is the mass of the modified wood sample before extraction, in mg;

  • m2 is the mass of the original wood in the modified wood sample before extraction, in mg;

  • m3 is the mass of the modified wood sample after extraction, in mg;

  • c0 is the logarithmic mass loss rate after extraction, in percent, with five samples selected.

2.3.2. Determination of Impregnation Rate and Curing Weight Gain Rate of Modified Wood

The impregnation rate (IY) of wood with the EDM impregnation system is calculated using Formula 3, and the weight gain rate upon curing (WPG) is calculated using Formula 4

2.3.2. 3
2.3.2. 4

W1 is the mass of the log sample, in grams; W2 is the mass of the wood sample after impregnation, in grams; and W3 is the mass of the wood sample after curing, in grams. Five samples were selected.

2.3.3. Determination of Water Absorption of Modified Wood

According to the measurement method in GB/T 1934.1–2009,24 with a measurement period of 7 days, the water absorption rate (A) of the modified wood samples is calculated using the following formula

2.3.3. 5

  • where m0 is the mass of the wood sample when completely dry, in grams;

  • mn is the mass of the wood sample after water absorption, in grams.

The water resistance efficiency of the modified wood samples is calculated using Formula 6

2.3.3. 6

  • where A0 is the water absorption rate of the log sample, in percent;

  • A is the water absorption rate of the modified wood sample, in percent.

  • Five samples were selected.

2.3.4. Determination of Compressive Strength along the Grain of Modified Wood

In accordance with the method specified in GB/T 1935–2009,25 the compressive strength along the grain of wood samples is measured using a compression testing machine. The sample is placed in the center of the spherical movable support of the testing machine in the direction of the grain, the loading speed is set to 0.5 KN/s, and the load at failure is recorded. The compressive strength (σ), in MPa, of the modified wood sample along the grain is calculated using Formula 7

2.3.4. 7

In the formula, Pmax is the load at failure, in Newtons (N); b is the width of the sample, in millimeters (mm); and t is the length of the sample, in millimeters (mm).

2.3.5. Determination of Wood Density and Profile Density of Modified Wood

The densities of the original wood (ρlog) and modified wood (ρm) are determined using the method specified in GB/T 1933–2009.26 The profile density of modified wood (ρs) is determined using the pycnometer method as described in GB/T 15223–200827 and calculated using Formula 8, in g/cm3. In the formula, m0 is the mass of the pycnometer filled with distilled water, in grams (g); m1 is the mass of the modified wood sample, in grams (g); m2 is the total mass of the pycnometer filled with distilled water with the modified wood sample inside, in grams (g); and ρwater is the density of distilled water, in g/cm3.

2.3.5. 8

2.3.6. Infrared Spectroscopy Analysis

FTIR spectroscopy was utilized to characterize the hydrolysis products of castor oil, the esterification products, and the modified wood samples. Attenuated total reflection detection was employed, with the number of reflections set to 8. The test wavenumber range covered 4000–400 cm–1.

2.3.7. 1H NMR Spectrum

Deuterated acetone was used as the solvent, and the magnetic field was set to 400 M for analyzing epoxy soybean oil and its ring-opening and esterification products with a Bruker Avance 400 NMR spectrometer.

2.3.8. Scanning Electron Microscopy

Original and modified wood samples were cut into tangential cross-section samples measuring 5 × 5 × 2 mm and examined under a scanning electron microscope to study their surface microstructure. To avoid charge accumulation during characterization, the samples were sputter-coated with gold, and tests were conducted at magnifications of 1000 and 5000× under an accelerating voltage of 20 kV.

2.3.9. X-ray Photoelectron Spectroscopy

Original and modified wood samples were cut into tangential cross sections measuring 5 × 5 × 2 mm, and their surface chemical elements were analyzed using X-ray photoelectron spectroscopy (XPS). An Al anode was used as the X-ray source, with a power of 50 KW, a pass energy of 46.95 eV, and step sizes of 0.8 eV for the full spectrum and 0.2 eV for the high-resolution spectrum. The binding energy for aliphatic carbon’s C 1s (284.8 eV) was used for charge correction.

2.3.10. Thermogravimetric Analysis

The specimens were ground into powder with a maximum particle size of 0.2 mm. Thermogravimetric analysis (TGA) was then utilized to evaluate the thermal stability of the altered wood material. During the analysis, a nitrogen atmosphere was maintained at a flow rate of 20 mL/min, and the temperature of the specimens was incrementally increased from ambient temperature to 800 °C at a rate of 10 °C/min.

3. Results and Discussion

3.1. Study on the Curing Conditions of EDM

To explore the optimal curing conditions and material ratios for EDM, this study conducted a single-round L9(34) orthogonal experiment, diluting with styrene in a 3:1 mass ratio. The factors examined included the amount of cross-linking agent, initiator quantity, curing temperature, and curing time, with the gel content of the cured product as the reference. The results showed that the curing temperature and time significantly affect the resin’s curing outcome.

The optimal conditions selected from the orthogonal experiment yielded a gel content of 89.8%. It was found that although the synthesized UPR oligomer contained other components, they had little effect on the curing of EDM. This is attributed to the complex curing process of unsaturated polyester (UPR), where the C=C bonds on the linear unsaturated polyester molecular chains undergo free radical copolymerization with styrene, forming four types of cross-linked structures: (I) intermolecular cross-linking aided by styrene; (II) intramolecular cross-linking aided by styrene; (III) polyester molecular branching with the help of styrene; and (IV) homopolymerization of styrene.28 Moreover, the steric hindrance effect results in some unreacted unsaturated double bonds within the curing cross-linked network of the unsaturated polyester, making the network more disordered. Thus, other components in the UPR oligomer can also facilitate further curing of EDM.

3.2. FTIR Analysis of the Various Stages of the Biobased Unsaturated Polyester EDM

Characteristic bands of epoxy soybean oil, its ring-opened product, and the synthesized resin are shown in Figure 2. The epoxy ethane ring stretching vibration band at 823 cm–1 and the C–O stretching vibration band at 1156 cm–1 indicate the presence of epoxy groups.29,30 The C=O stretching vibration band near 1750 cm–1 indicates the presence of ester groups; the –CH stretching vibration band near 2900 cm–1 indicates the presence of alkyl groups. The broad and strong –OH stretching vibration band near 3450 cm–1, along with the C=O stretching vibration band near 1750 cm–1, suggests the presence of carboxyl groups, and the broad and strong –OH stretching vibration band near 3450 cm–1, along with the C–O stretching vibration band at 1156 cm–1, indicates the presence of polyols.31 In the FTIR spectrum of ED, the characteristic band of epoxy ethane at 823 cm–1 disappears, and combined with the determination of the epoxy value, it suggests that ESO and DEG have undergone a ring-opening reaction. The absorption bands of alkyl C–H and ester C=O from epoxy soybean oil appear at the same positions in EDM, indicating that the introduction of unsaturated anhydride provides conditions for the cross-linking and curing of EDM.

Figure 2.

Figure 2

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Infrared spectra of the EDM raw material and products at various synthetic stages. In the figure, ESO represents epoxy soybean oil and ED represents the ring-opened product.

3.3. 1H NMR Analysis of Products at Different Stages of EDM

The resonance at 0.88 ppm is identified with the end methyl protons of the fatty acid’s chain, while the signals between 1.30 and 1.33 ppm correspond to the methylene groups inside the fatty acid chain.32 The resonance found between 2.80 and 3.10 ppm in Figure 3 is linked to the presence of epoxy groups.33 The signals at 3.62, 3.72, 3.82, and 4.37 ppm are related to the methylene groups of the DEG component, and a peak at 3.56 ppm verifies the hydroxyl end groups in the product’s structure,33 indicating the successful opening of the epoxy soybean oil ring by DEG. Additionally, the resonances between 6.33 and 6.48 ppm in Figure 3 are attributed to the main chain signals (–CH2=CH–) of maleic anhydride,34,35 implying that maleic anhydride has reacted through esterification with the ring-opened derivative of epoxy soybean oil.

Figure 3.

Figure 3

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1HNMR analysis of products at various stages of EDM.

3.4. Enhanced Modification of Wood

3.4.1. Basic Performance Analysis of Modified Wood

The impact of EDM impregnation on fast-growing wood, enhancing its performance, is mainly reflected in impregnation rate, weight gain rate, and water absorption (Table 1).

Table 1. Performance Table of Fast-Growing Wood Modified by the EDM Impregnation Systema.
UPR impregnation system viscosity (Pa·s) wood type ρlog(g/cm3) ρm(g/cm3) ρs
 IY(%) WPG(%)
          outer middle inner    
EDM 1.24 ± 0.05 S 0.40 ± 0.02 1.02 ± 0.08 1.05 ± 0.07 1.01 ± 0.04 0.97 ± 0.02 219.74 ± 10.26 216.15 ± 9.18
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a

In the table, wood type S represents fir wood.

After impregnation with EDM, the impregnation rate and weight gain rate of fir wood both exceeded 200%. The density differences across the outer, middle, and inner cross sections of the modified fast-growing wood were minimal, showing an increase over the original wood’s density and exhibiting a gradual decrease from the exterior toward the interior. This suggests that the resin effectively penetrated the wood’s interior, with the impregnation impact diminishing from the outer to the inner layers. The EDM impregnation system’s lower viscosity allowed for better filling of the wood’s internal voids, thereby enhancing the modification effect and rendering it more apt for industrial usage.

The application and hardening of EDM within the wood also resulted in enhanced water resistance and mechanical strengths. Water absorption tests revealed that the water absorption rate of untreated spruce stabilized at 158.03 ± 2.17%, whereas that of the EDM-modified fir dropped to below 20% (Figure 4b). Furthermore, the compressive strength of the modified fir saw an increase from 38.1 ± 4.8 to 94.9 ± 4.2 MPa (Figure 4a).

Figure 4.

Figure 4

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(a) Compressive strength–time curve of modified fir; (b) water absorption rate–time curve of modified fir. In these figures, S-E represents modified fir wood.

3.4.2. XPS Analysis of Modified Fast-Growing Wood

The XPS full scan spectra (Figure 5) reveal significant absorption peaks between 283 and 290 and at 532 eV, highlighting that both untreated and treated fir wood surfaces are rich in carbon (C) and oxygen (O) elements. The carbon atoms in wood are categorized into four distinct forms.36,37 Specifically, in the high-resolution C 1s spectrum (Figure 6), the C1 component is linked to carbon atoms found in aliphatic and aromatic carbon chains, interacting solely with carbon or hydrogen atoms, and exhibits a binding energy of approximately 284.8 eV. This is primarily indicative of the lignin, characterized by a phenylpropane structure within the original wood and the main chain structure of UPR. The C2 and C3 components are associated with C–O structures, representing carbon atoms bonded to two noncarbonyl oxygen atoms or one carbonyl oxygen atom, with binding energies around 286.4 and 288 eV, respectively. These states signify the cellulose and hemicellulose structures in wood, both of which comprise numerous carbon atoms linked to hydroxyl groups.38 Lastly, the C4 component, with a binding energy of about 289 eV, relates to carbon atoms connected to one noncarbonyl oxygen atom and one carbonyl oxygen atom. This component is in a higher oxidation state, causing a significant chemical shift, and primarily represents the fatty acids and ester groups present in wood extracts and unsaturated polyester resins.

Figure 5.

Figure 5

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XPS full spectra of fir wood original and modified samples.

Figure 6.

Figure 6

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Fine spectra of C 1s of fir logs and modified fir samples.

In the table, the density of the original fir wood tangential section is the density of the original wood, while the others represent their respective profile densities.

Table 2 outlines the shifts in the relative amounts of different carbon forms within the inner, middle, and outer tangential sections of both the untreated and treated fast-growing wood samples. A notable increase in the relative content of the C4 component is observed, paralleled by a decrease in the C3 component, a trend that aligns with the density data of the profile. Concurrently, there is an overall rise in both the oxygen content and the O/C ratio within the treated fast-growing wood samples. This trend suggests that the biobased unsaturated polyester, EDM, has effectively been absorbed into the fir wood. The findings imply that the EDM impregnation process predominantly operates through lateral infiltration, with the impregnation depth diminishing from the outer layers toward the inner layers.

Table 2. Surface Chemical Element Content of Fir Wood Original and Modified Samples.
sample serial number C(%) C1(%) C2(%) C3(%) C4(%) O(%) O/C(%) ρ(g/cm3)
fir log chord section 68.44 59.04 30.74 9.63 0.59 31.56 46.11 0.40 ± 0.02
modified fir inner chord section 70.82 54.19 36.72 8.48 0.6 20.83 29.41 0.97 ± 0.02
modified fir midchord section 68.70 49.06 39.50 6.71 4.74 31.30 45.56 1.01 ± 0.04
modified fir outer chord section 79.17 73.30 18.20 1.81 6.69 29.18 36.85 1.05 ± 0.07
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3.4.3. SEM Analysis of Modified Fast-Growing Wood

Scanning electron microscopy (SEM) images showcasing the tangential cross sections of the interior surfaces of both untreated and treated fir wood are presented in Figure 7. The image of the untreated wood’s cross-section reveals clear passages without any noticeable residues in the conduits, wood fibers, and ray cell cavities, with the pits remaining open. This demonstrates EDM’s capability to infiltrate the wood’s interior, facilitating its modification. In contrast, the cross-section of the treated wood shows that the conduits, ray cell cavities, and wood fibers are encapsulated and bonded by resin, with the pits also being filled by resin. The control images, Figure 7e,f, distinctly illustrate the resin’s impregnation and curing process within the wood’s pits, affirming EDM’s effective penetration and interaction with the wood’s minor components, all while preserving the wood’s microstructure integrity.

Figure 7.

Figure 7

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SEM images of fir wood and modified fir wood samples: images (a,b) are SEM images of the tangential section of fir wood, images (c,d) are SEM images of the tangential section of modified fir wood, and images (e,f) are for the control group. Images (a,c) are magnified 1000 times and images (d–f) are magnified 5000 times.

Pit channels act as conduits, allowing the entry and movement of small molecules like water between various microstructural cells within the wood. By filling the majority of these internal pits, the EDM resin acts as a barrier against the ingress of water and other small molecules, significantly bolstering the wood’s resistance to water. Furthermore, the resin that fills the wood’s structure engages in reactions, cross-linking, or forms hydrogen bonds with the hydroxyl groups present in the wood cells, which contributes to the enhancement of the wood’s compressive strength.39

The diagram in Figure 8 delineates the process of impregnation and curing of EDM components within the wood’s structure. The wood’s intercellular spaces and pit channels are identified as the main routes for the penetration of substances into the wood cells. Utilizing vacuum and pressure techniques enables the infusion of EDM, which contains carboxyl and hydroxyl groups, as well as DVB that is characterized by its double bonds, deep into the wood’s structure. When heated to a specific temperature, EDM undergoes cross-linking through its double bonds. Concurrently, some of the carboxyl groups may react with the hydroxyl groups present in the wood, and hydroxyl groups from both EDM and the wood may form hydrogen bonds. This process, as highlighted not only in our findings but also in studies by other researchers,36,37,40,41 results in the EDM resin network being securely anchored within the wood, which is anticipated to significantly enhance wood modification. This method of chemical impregnation was selected for wood modification in our study due to its efficacy.

Figure 8.

Figure 8

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Schematic diagram of the impregnation and curing reaction of the EDM system inside wood.

3.4.4. TGA of Modified Fast-Growing Wood

The TG and derivative thermogravimetry (DTG) spectra depicted in Figure 9, alongside specific data points such as T10% (the temperature at which there’s a 10% mass loss), T50% (the temperature at which a 50% mass loss occurs), and Tmax (the temperature of maximum decomposition rate), facilitate the evaluation of the thermal stability of wood samples. Initially, the untreated wood exhibits minor weight loss from room temperature to 100 °C, predominantly due to moisture evaporation from the wood’s cell walls. Following this, the thermal breakdown of wood constituents starts, with hemicellulose being the least thermally stable, decomposing first within the 210–300 °C range. A notable mass reduction is observed around 360 °C on the DTG curve, chiefly attributed to cellulose breakdown, while lignin undergoes decomposition across a broader spectrum, from 300 °C up to 500 °C.42 As temperatures escalate further, decomposition of C–C and C–O bonds occurs, leading to the formation of small molecules like H2O, CO2, and hydrocarbons. By 800 °C, the wood’s components have fully decomposed, leaving a char residue of approximately 17.33% in fir wood. The DTG curve also reveals that the peak associated with moisture evaporation in treated wood is substantially reduced compared to that of untreated wood, indicating that the cross-linked UPR significantly diminishes the wood’s ability to absorb water.

Figure 9.

Figure 9

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TG (a) and DTG (b) spectra of fir wood and its modified wood.

Additionally, Table 3 indicates that the protective impact of unsaturated polyester on wood at T10% is relatively minor. In the 210 to 300 °C range, not only does hemicellulose decomposition occur in the treated wood but also a minor breakdown of UPR. The interval from 300 to 500 °C is characterized by the thermal decomposition of cellulose, lignin, and any remaining UPR, with the T50% and Tmax values for treated wood being marginally elevated by about 10 °C compared to those of untreated samples.43 This minor increase suggests that the cured UPR offers protection that delays the thermal degradation of wood components. Thus, it is deduced that the infusion and curing of EDM within the wood structure notably enhance the wood’s thermal stability.

Table 3. TGA Data for Original and Modified Wood Samples.
sample T10% (°C) T50% (°C) Tmax (°C) residue mass at 800 °C (%)
S 274.87 350.87 356.87 17.33
S-E 279.08 368.08 370.08 10.42
EDMc 278.25 398.25 405.25 4.41
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The thermal decomposition of cured EDM resin can be explained in three main steps. Initial decomposition is observed before 220 °C, where approximately 6 wt % weight loss is due to the evaporation of moisture and uncured materials from the resin surface.44 Then, a sharp weight loss is observed between 260 and 410 °C, with the DTG peak maximum at 405.25 °C. This is mainly due to the rapid decomposition and volatilization of cured UPR at high temperatures, until the TGA and DTG curves return to their original baseline. Above 500 °C, a gradual weight reduction is observed as the thermally stable char produced during the decomposition process slowly oxidizes. From Table 3, it can also be seen that the cured EDM resin has higher T10%, T50%, and Tmax stages than both the original and modified wood, indicating that the cured EDM resin itself possesses good thermal stability performance.

In the table, S-E represents modified fir wood and EDMc represents the cured EDM resin.

4. Conclusions

This study synthesized a biobased unsaturated polyester (EDM) using epoxy soybean oil, diethylene glycol, and maleic anhydride and modified fir wood through a chemical impregnation method, enhancing the wood’s water resistance, compressive strength, and thermal stability. The results indicate that EDM can not only penetrate into the wood’s microstructure but also react with the wood, forming a stable cross-linked network within. Compared to the original wood, the modified fir wood’s density increased from 0.40 g/cm3 to 1.02 g/cm3, its compressive strength increased from 38.1 to 94.9 MPa, its water absorption rate decreased from 158.03 to 6.93%, and its thermal stability was also improved. This experiment organically combines biobased unsaturated polyester with fast-growing wood. It aims to broaden the application scope of fast-growing wood and promote a harmless, low-cost modification method. Additionally, it provides an environmentally friendly, green new pathway for the production of unsaturated polyester, aligning with the principles of “green chemical engineering” and sustainable development.

Acknowledgments

T.X. would like to extend his deepest gratitude to all those who have supported and collaborated with him throughout this research journey and beyond. The members of the author’s research team have been incredibly generous with their time and expertise, offering both personal and professional support that has been crucial in navigating the complexities of scientific research and life. The author owes a great deal of appreciation to his graduate advisor, H.T.. As a teacher and mentor, Professor Tang has been instrumental in guiding him through the intricacies of scientific exploration while also imparting the importance of maintaining a healthy balance between work and personal life. His mentorship has been invaluable, and for that, the author is profoundly thankful. Furthermore, the author would like to dedicate a special acknowledgment to his family and friends, whose unwavering support and encouragement have played a pivotal role in his journey. The sacrifices they have made, the confidence they have instilled in the author, and the endless encouragement have fueled the author’s ambition and resilience. Their love and belief in author’s potential have been the bedrock of his personal and academic achievements. It is with a full heart that the author expresses his sincerest gratitude for their endless positivity and the strength they have provided him. Their presence in author’s life is a gift he treasures immensely.

The authors declare no competing financial interest.

References

  1. Dong Y.; Yan Y.; Zhang Y.; Zhang S.; Li J. Combined Treatment for Conversion of Fast-Growing Poplar Wood to Magnetic Wood with High Dimensional Stability. Wood Sci. Technol. 2016, 50 (3), 503–517. 10.1007/s00226-015-0789-6. [DOI] [Google Scholar]
  2. Qin Y.; Dong Y.; Li J. Effect of Modification with Melamine–Urea–Formaldehyde Resin on the Properties of Eucalyptus and Poplar. J. Wood Chem. Technol. 2019, 39 (5), 360–371. 10.1080/02773813.2019.1636821. [DOI] [Google Scholar]
  3. Yang H.; Gao M.; Wang J.; Mu H.; Qi D. Fast Preparation of High-Performance Wood Materials Assisted by Ultrasonic and Vacuum Impregnation. Forests 2021, 12 (5), 567. 10.3390/f12050567. [DOI] [Google Scholar]
  4. Wang J.; Yao Y.; Huang Y.; Ma Y.; Xi J.; Wang X.; Li H.; Yang Z. Effects of the Combination of Compression and Impregnation with Phenolic Resin on the Dimensional Stability in the Multiscale Wood Structure of Chinese Fir. Constr. Build. Mater. 2022, 327, 126960. 10.1016/j.conbuildmat.2022.126960. [DOI] [Google Scholar]
  5. Li J.; Zhang A.; Zhang S.; Gao Q.; Chen H.; Zhang W.; Li J. High-Performance Imitation Precious Wood from Low-Cost Poplar Wood via High-Rate Permeability of Phenolic Resins. Polym. Compos. 2018, 39 (7), 2431–2440. 10.1002/pc.24226. [DOI] [Google Scholar]
  6. Gao R.; Huang Y.; Gan W.; Xiao S.; Gao Y.; Fang B.; Zhang X.; Lyu B.; Huang R.; Li J.; Wei X.; Deng Y.; Lu Y. Superhydrophobic Elastomer with Leaf-Spring Microstructure Made from Natural Wood without Any Modification Chemicals. Chem. Eng. J. 2022, 442, 136338. 10.1016/j.cej.2022.136338. [DOI] [Google Scholar]
  7. Ouakdi E. H.; Louahdi R.; Khirani D. Laurent Tabourot. Evaluation of Springback under the Effect of Holding Force and Die Radius in a Stretch Bending Test. Mater. Des. 2011, 35, 43. 10.1016/j.matdes.2011.09.003. [DOI] [Google Scholar]
  8. Priadi T.; Hiziroglu S. Characterization of Heat Treated Wood Species. Mater. Des. 2013, 49, 575–582. 10.1016/j.matdes.2012.12.067. [DOI] [Google Scholar]
  9. Salca E.-A.; Hiziroglu S. Evaluation of Hardness and Surface Quality of Different Wood Species as Function of Heat Treatment. Mater. Des. (1980–2015) 2014, 62, 416–423. 10.1016/j.matdes.2014.05.029. [DOI] [Google Scholar]
  10. Tjeerdsma B. F.; Militz H. Chemical Changes in Hydrothermal Treated Wood: FTIR Analysis of Combined Hydrothermal and Dry Heat-Treated Wood. Holz Roh- Werkst. 2005, 63 (2), 102–111. 10.1007/s00107-004-0532-8. [DOI] [Google Scholar]
  11. Xu B.-H.; Yu K.-B.; Wu H.-C.; Bouchaïr A. Mechanical properties and engineering application potential of the densified poplar. Wood Mater. Sci. Eng. 2021, 17 (6), 659–667. 10.1080/17480272.2021.1924857. [DOI] [Google Scholar]
  12. Yue K.; Wu J.; Xu L.; Tang Z.; Chen Z.; Liu W.; Wang L. Use Impregnation and Densification to Improve Mechanical Properties and Combustion Performance of Chinese Fir. Constr. Build. Mater. 2020, 241, 118101. 10.1016/j.conbuildmat.2020.118101. [DOI] [Google Scholar]
  13. Lin C.-F.; Karlsson O.; Martinka J.; Rantuch P.; GarskaiteMantanis E. G. I.; Jones D.; Sandberg D.; Sandberg D. Approaching Highly Leaching-Resistant Fire-Retardant Wood by in Situ Polymerization with Melamine Formaldehyde Resin. ACS Omega 2021, 6 (19), 12733–12745. 10.1021/acsomega.1c01044. [DOI] [PMC free article] [PubMed] [Google Scholar]
  14. Rabe S.; Klack P.; Bahr H.; Schartel B. Assessing the Fire Behavior of Woods Modified by N-Methylol Crosslinking, Thermal Treatment, and Acetylation. Fire Mater. 2020, 44 (4), 530–539. 10.1002/fam.2809. [DOI] [Google Scholar]
  15. Liu M.; Guo F.; Wang H.; Ren W.; Cao M.; Yu Y. Highly Stable Wood Material with Low Resin Consumption via Vapor Phase Furfurylation in Cell Walls. ACS Sustainable Chem. Eng. 2020, 8 (37), 13924–13933. 10.1021/acssuschemeng.0c03172. [DOI] [Google Scholar]
  16. Peng Y.; Fan Q.; Ou R.; Hao X.; Guo C.; Liu Z.; Liu T.; Sun L.; Wang Q. Modification of Poplar Wood Cells Using 1,3-Dihydroxymethyl-4,5-Dihydroxyethylideneurea/Alkaline Lignin for Enhanced Mechanical Properties and Decay Resistance. Constr. Build. Mater. 2023, 368, 130354. 10.1016/j.conbuildmat.2023.130354. [DOI] [Google Scholar]
  17. Zieleniewska M.; Leszczyński M. K.; Kurańska M.; Prociak A.; Szczepkowski L.; Krzyżowska M.; Ryszkowska J. Preparation and Characterisation of Rigid Polyurethane Foams Using a Rapeseed Oil-Based Polyol. Ind. Crops Prod. 2015, 74, 887–897. 10.1016/j.indcrop.2015.05.081. [DOI] [Google Scholar]
  18. Li C.; Lei H.; Wu Z.; XI X.; Du G.; Pizzi A. Fully Biobased Adhesive from Glucose and Citric Acid for Plywood with High Performance. ACS Appl. Mater. Interfaces 2022, 14 (20), 23859–23867. 10.1021/acsami.2c02859. [DOI] [PubMed] [Google Scholar]
  19. Gu W.; Liu X.; Li F.; Shi S. Q.; Xia C.; Zhou W.; Zhang D.; Gong S.; Li J. Tough, Strong, and Biodegradable Composite Film with Excellent UV Barrier Performance Comprising Soy Protein Isolate, Hyperbranched Polyester, and Cardanol Derivative. Green Chem. 2019, 21 (13), 3651–3665. 10.1039/c9gc01081e. [DOI] [Google Scholar]
  20. Costa C. S.; Fonseca A. C.; Moniz J.; Godinho M.; Serra A. C.; Coelho J. F. Soybean and Coconut Oil Based Unsaturated Polyester Resins: Thermomechanical Characterization. Ind. Crops Prod. 2016, 85, 403–411. 10.1016/j.indcrop.2016.01.030. [DOI] [Google Scholar]
  21. Das K.; Ray D.; Banerjee C.; Bandyopadhyay N. R.; Mohanty A. K.; Misra M. Novel Materials from Unsaturated Polyester Resin/Styrene/Tung Oil Blends with High Impact Strengths and Enhanced Mechanical Properties. J. Appl. Polym. Sci. 2010, 119 (4), 2174–2182. 10.1002/app.32957. [DOI] [Google Scholar]
  22. Shanxi Institute of Chemical Industry (Co., Ltd.); Zhejiang Jiaao Environmental Protection Technology Co., Ltd.; Xinjinlong Bio-based Materials (Hubei) Co., Ltd; Shandong Kexing Chemical Co., Ltd.; Nantong Haierma Technology Co., Ltd.; Xiamen University Determinating the Epoxy Value of Plasticizers; State Administration for Market Regulation, National Standardization Administration, 1982. [Google Scholar]
  23. China Building Materials Industry Association . Test Method for Insoluble Matter Content of Resin Used in Fiber Reinforced Plastics; General Administration of Quality Supervision, Inspection and Quarantine of the People Republic of China, China National Standardization Administration Committee, 2005. [Google Scholar]
  24. Method for Determination of the Water Absorption of Wood; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China, 2009. [Google Scholar]
  25. Method of Testing in Compressive Strength Parallel to Grain of Wood; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China, 2009. [Google Scholar]
  26. Institute of Chinese Academy of Forestry Timber Industry . Method for Determination of the Density of Wood; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China, 2009. [Google Scholar]
  27. National Quality Supervision and Inspection Center of synthetic resin . Plastics – Liquid Resins – Determination of Density by the Pycnometer Method; Administration of Quality Supervision, Inspection and Quarantine of People’s Republic of China; Standardization Administration of China, 2008. [Google Scholar]
  28. Le Guen M.-J.; Newman R. H.; Fernyhough A.; Staiger M. P. Tailoring the Vibration Damping Behaviour of Flax Fibre-Reinforced Epoxy Composite Laminates via Polyol Additions. Composites, Part A 2014, 67, 37–43. 10.1016/j.compositesa.2014.08.018. [DOI] [Google Scholar]
  29. Sawicz-Kryniger K.; Niezgoda P.; Stalmach P.; Starzak K.; Wysocka A.; Świergosz T.; Popielarz R. Performance of FPT, FTIR and DSC Methods in Cure Monitoring of Epoxy Resins. Eur. Polym. J. 2022, 162, 110933. 10.1016/j.eurpolymj.2021.110933. [DOI] [Google Scholar]
  30. Liu Y.-Y.; He J.; Li Y.-D.; Zhao X.-L.; Zeng J.-B. Biobased, Reprocessable and Weldable Epoxy Vitrimers from Epoxidized Soybean Oil. Ind. Crops Prod. 2020, 153, 112576. 10.1016/j.indcrop.2020.112576. [DOI] [Google Scholar]
  31. Rajalakshmi P.; Marie J. M.; Maria Xavier A. J. Castor Oil-Derived Monomer Ricinoleic Acid Based Biodegradable Unsaturated Polyesters. Polym. Degrad. Stab. 2019, 170, 109016. 10.1016/j.polymdegradstab.2019.109016. [DOI] [Google Scholar]
  32. Orozco L. M.; Cardona S.; Gomez C.; Inciarte H.; Villada Y.; Rios L. Evaluation of KHSO4 as a Recyclable Catalyst in the Production of Dehydrated Castor Oil to Be Applied in Alkyd Resins. Prog. Org. Coat. 2021, 161, 106467. 10.1016/j.porgcoat.2021.106467. [DOI] [Google Scholar]
  33. Favero D.; Marcon V. R. R.; Barcellos T.; Gómez C. M.; Sanchis M. J.; Carsí M.; Figueroa C. A.; Bianchi O. Renewable Polyol Obtained by Microwave-Assisted Alcoholysis of Epoxidized Soybean Oil: Preparation, Thermal Properties and Relaxation Process. J. Mol. Liq. 2019, 285, 136–145. 10.1016/j.molliq.2019.04.078. [DOI] [Google Scholar]
  34. Abele S.; Zicmanis A.; Graillat C.; Monnet C.; Guyot A. Cationic and Zwitterionic Polymerizable Surfactants: Quaternary Ammonium Dialkyl Maleates. 1. Synthesis and Characterization. Langmuir 1999, 15 (4), 1033–1044. 10.1021/la980562k. [DOI] [Google Scholar]
  35. PappuruChakraborty S. D.; Ramkumar V.; Chand D. K. Ring-opening copolymerization of maleic anhydride or L-Lactide with tert-butyl glycidyl ether by using efficient Ti and Zr benzoxazole-substituted 8-Hydroxyquinolinate catalysts. Polymer 2017, 123, 267–281. 10.1016/j.polymer.2017.06.073. [DOI] [Google Scholar]
  36. Wang P.; Wang Y.; Hong P.; Zhou C. Di-Aldehyde Starch Crystal: A Novel Bio-Crosslinker for Strengthening the Structure and Physio-Chemical Properties of Gelatin-Based Films. Food Biosci. 2021, 43, 101308. 10.1016/j.fbio.2021.101308. [DOI] [Google Scholar]
  37. Frihart C.; Brandon R.; Beecher J.; Ibach R. Adhesives for Achieving Durable Bonds with Acetylated Wood. Polymers 2017, 9 (12), 731. 10.3390/polym9120731. [DOI] [PMC free article] [PubMed] [Google Scholar]
  38. Le Guen M.-J.; Newman R. H.; Fernyhough A.; Staiger M. P. Tailoring the Vibration Damping Behaviour of Flax Fibre-Reinforced Epoxy Composite Laminates via Polyol Additions. Composites, Part A 2014, 67, 37–43. 10.1016/j.compositesa.2014.08.018. [DOI] [Google Scholar]
  39. Lu M.; He W.; Li Z.; Qiang H.; Cao J.; Guo F.; Wang R.; Guo Z. Effect of Lignin Content on Properties of Flexible Transparent Poplar Veneer Fabricated by Impregnation with Epoxy Resin. Polymers 2020, 12 (11), 2602. 10.3390/polym12112602. [DOI] [PMC free article] [PubMed] [Google Scholar]
  40. Xia C.; Wu Y.; Qiu Y.; Cai L.; Smith L. M.; Tu M.; Zhao W.; Shao D.; Mei C.; Nie X.; Shi S. Q. Processing High-Performance Woody Materials by Means of Vacuum-Assisted Resin Infusion Technology. J. Cleaner Prod. 2019, 241, 118340. 10.1016/j.jclepro.2019.118340. [DOI] [Google Scholar]
  41. Weigenand O.; Militz H.; Tingaut P.; Sèbe G.; de Jeso B.; Mai C. Penetration of Amino-Silicone Micro- and Macro-Emulsions into Scots Pine Sapwood and the Effect on Water-Related Properties. Holzforschung 2007, 61 (1), 51–59. 10.1515/HF.2007.009. [DOI] [Google Scholar]
  42. Bañuls-Ciscar J.; Pratelli D.; Abel M.-L.; Watts J. F. Surface Characterisation of Pine Wood by XPS. Surf. Interface Anal. 2016, 48 (7), 589–592. 10.1002/sia.5960. [DOI] [Google Scholar]
  43. Kong L.; Guan H.; Wang X. In Situ Polymerization of Furfuryl Alcohol with Ammonium Dihydrogen Phosphate in Poplar Wood for Improved Dimensional Stability and Flame Retardancy. ACS Sustainable Chem. Eng. 2018, 6 (3), 3349–3357. 10.1021/acssuschemeng.7b03518. [DOI] [Google Scholar]
  44. Salasinska K.; Celiński M.; Barczewski M.; Leszczyński M. K.; Borucka M.; Kozikowski P. Fire Behavior of Flame Retarded Unsaturated Polyester Resin with High Nitrogen Content Additives. Polym. Test. 2020, 84, 106379. 10.1016/j.polymertesting.2020.106379. [DOI] [Google Scholar]

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