Soybean Meal–Oxidized Lignin as Bio-Hybridized Wood Panel Adhesives with Increased Water Resistance

Abstract

Soybean meal (SM) adhesive is widely acknowledged as a viable substitute for traditional formaldehyde-based adhesives, given its ability to be easily modified, the utilization of renewable sources, and its eco-friendly characteristics. However, the application of SM adhesive in manufacturing has been impeded due to its restricted bonding capacity and inadequate water resistance. Researchers in the wood industry have recognized the significance of creating an SM-based adhesive, which possesses remarkable adhesive strength and resistance to water. This study endeavors to tackle the issue of inadequate water resistance in SM adhesives. Sodium lignosulfonate (L) was oxidized using hydrogen peroxide (HP) to oxidized lignin (OL) with a quinone structure. OL was then used as a modifier, being blended with SM to prepare SM-based biomass (OLS) adhesives with good water resistance, which was found practically through its utilization in the production of plywood. The influence of the HP dosage and OL addition on plywood properties was examined. The changes in the lignin structure before and after oxidation were confirmed using gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The curing behavior and thermal stability of OLS adhesives were analyzed using dynamic mechanical analysis (DMA) and thermogravimetric (TG) analysis. The reaction mechanism was also investigated using FT-IR and XPS. The outcomes indicated a decrease in the molecular weight of L after oxidation using HP, and, at the same time, quinone and aldehyde functionalized structures were produced. As a result of the reaction between the quinone and aldehyde groups in OL with the amino groups in SM, a dense network structure formed, enhancing the water resistance of the adhesive significantly. The adhesive displayed exceptional resistance to water when the HP dosage was set at 10% of L and the OL addition was 10% based on the mass of SM. These specific conditions led to a notable enhancement in the wet bonding strength (63 °C, 3 h) of the plywood prepared using the adhesive, reaching 0.88 ± 0.14 MPa. This value represents a remarkable 125.6% increase when compared to the pure SM adhesive (0.39 ± 0.02 MPa). The findings from this study introduce a novel approach for develo** adhesives that exhibit exceptional water resistance.

1. Introduction

Formaldehyde-based adhesives exhibit superior performance in the field wood-based panel industry [1,2]. Albeit, they establish substantial environmental issues due to their dependence on non-renewable and exhaustible fossil resources like formaldehyde. The continuous usage elevates the possibility of formaldehyde emissions, causing significant health hazards [3,4]. Hence, the scrutiny into bio-based adhesives that can relieve these health and environmental problems constitute a vibrant research area [5,6,7,8]. Soybean is a frequently cultivated cash crop, whereas soybean meal (SM) is derived as a by-product from the extraction of soybean oil that is typically underutilized, in spite of its potential applications in the production of livestock and poultry feed. SM utilization was suggested for incorporation into wood adhesive formulations to enhance its efficient utilization. Its plentiful abundance as a cheap raw material, its ease of handling and modification, and its independence on formaldehyde exposed the importance of this bio-based material [9]. Nonetheless, some inherent drawbacks, including its modest water resistance, high viscosity, and low solid content, limit expanding its use in industrial production [10,11,12,13]. Efforts to overcome these limitations are, therefore, an issue of concern for researchers.

Modification is commonly employed as an efficient technique to boost SM adhesives [14,15,16]. SM protein molecules are rich in hydroxyl, amino, carboxyl, and other reactive groups. Applying the crosslinking agents drives such reactive groups to undertake crosslinking reactions, causing the intermolecular forces to be dominated by covalent bonding rather than hydrogen bonds. Thus, tight crosslinks can be formed, resulting in the enhancement of the bonding properties and water resistance of SM adhesives [17,18]. However, using chemical crosslinkers obtained principally from non-renewable fossil resources in SM adhesives causes an environmental challenge. These environmentally damaging crosslinkers substantially shrink the whole renewability of the adhesives. Bio-based crosslinking agents are assumed to effectively replace the chemical crosslinkers currently in use within the industry, which raises the topic of using non-renewable raw materials as crosslinking agents and their adverse effect [19,20,21,22]. Chen et al. [23] developed a new eco-friendly formulation as a binder considering its origin from soy protein in order to fulfill the required strength by combining it with tannin-based resins. Li et al. [24] enhanced the efficacy of a soy protein binder through incorporating modifications of chitosan, tannic acid, and silver ions. As a result, the hot water shearing strength of the resulting plywood reached 1.29 MPa. However, these adhesives often confront challenges related to complex modification processes or the utilization of a substantial quantity of hazardous reagents during their preparation. As a result, the concern of SM adhesives sustaining excellent adhesion and water resistance via cheap environmentally friendly approaches remains a significant challenge [25].

The exceptional adhesive ability of marine mussel proteins has captured significant attention from researchers [26]. Studies have revealed that these proteins exhibit strong adhesion to wet surfaces such as rocks and boat bottoms. This excellent adhesion is attributed to the secretion of a specific protein known as mussel pedunculated filament protein, which contains a catechol, polyphenol-like structure that plays a vital role in both adhesion and water resistance [27]. This catechol structure enables the formation of robust hydrogen bonds with substrates and undergoes reactions with amino groups upon oxidation to quinone [28,29]. SM is rich in amino groups. Thus, incorporating the catechol structure into SM adhesives can significantly enrich both their dry and wet bonding strength.

Lignin is composed of phenolic hydroxyl groups and aliphatic hydroxyl groups, and the catechol structure can be obtained through various modification methods, including physical and chemical treatments [30,31,32]. Liu et al. [33] synthesized lignin with a catechol structure through demethylation. They developed a bio-based adhesive by combining lignin, copper ions, and soy protein, resulting in enhanced water resistance and the reduced viscosity of the produced adhesive. Consequently, lignin is expected to function as a crosslinking agent in biomass, thus improving the water resistance of SM adhesives.

Drawing upon the structural attributes of lignin and the adhesive properties of mussels, we employed hydrogen peroxide (HP) to oxidize sodium lignosulfonate (L), aiming in the formation of oxidized lignin (OL) with quinone and aldehyde (carboxyl) structures [34]. The presence of quinone and aldehyde functions in OL enables the formation and development of crosslinking networks with the amino groups present in SM, leading to the enhanced water resistance of SM adhesives. The crosslinking in OL-modified SM adhesive systems is exclusively based on renewable lignin, utilizing both covalent and hydrogen bonds instead of relying on non-renewable petroleum-based fossil resources. The objective of this study is to explore the preparation of a fully bio-based OLS adhesive consisting of OL and SM, with a focus on the impact of varying amounts of HP dosages and OL additions on the adhesive properties as applied for plywood fabrication. This study significantly contributes to the advancement of SM adhesives by introducing a new strategy for develo** a simple, non-toxic, and water-resistant biomass-based adhesive using SM.

2. Materials and Methods

2.1. Materials

Populus L. veneer (200 mm × 130 mm × 2 mm) (T × L × R), with a moisture content of 9 ± 1%, was purchased from Linyi Minsheng Wood Co., Ltd. (Linyi, China) The supplier of soybean meal (SM, 200 mesh) was Shandong Yuxin Biotechnology Co. (**an, China) Hydrogen peroxide solution (HP, 30%) was supplied from Tongsheng Chemical Company Limited. (Quzhou, China). Hefei Chisheng Bio-technology Co., Ltd. (Hefei, China) provided analytically pure sodium lignosulfonate (L). Laboratory-prepared distilled water was used.

2.2. Preparation of OL

L and water were combined in a three-necked flask, equipped with mechanical stirring until L was completely dissolved to obtain a 20% mass fraction solution. In a following step, the solution was supplemented with HP, ensuring that the solid content in the reaction system remained at 20%. The reaction was conducted under mechanical stirring, while heating the water bath to 80 °C and kee** the reaction away from light for 3 h. After the oxidation reaction was completed, the reaction was cooled to room temperature, yielding the OL solution.

2.3. OLS Wood Adhesive Preparation

Various OLS adhesives were developed by initially blending 20 g of SM and 80 g of deionized water with a mixer at 1000 r/min for 5 min at room temperature. Then, a specific amount of OL solution was added to SM, adjusting the solid content of the system to 20%. The mixture was thoroughly mixed for 10 min to ensure proper blending. This study explored the impact of HP addition and the OL dosage on the plywood adhesion strength through a one-way experiment, aiming to optimize the adhesive preparation process. In the experiment addressing the influence of HP addition on plywood adhesion properties, the OL dosage was fixed at 10% with respect to the SM dry mass. A range of HOLS adhesives was then created by adjusting the quantity of HP added. The samples with HP additions of 3, 5, 10, 20, and 30% (based on the dry mass of L) were labeled as HOLS-3%, HOLS-5%, HOLS-10%, HOLS-20%, and HOLS-30%, respectively. Additionally, for examining the impact of the OL dosage on the adhesive properties of plywood, the HP dosage was fixed at 10% (based on the dry mass of L). Subsequently, the OL dosage was varied to produce a series of OLS adhesives. The samples with OL dosages of 3%, 5%, 10%, 20%, and 30% (based on the dry mass of SM) were denoted as OLS-3%, OLS-5%, OLS-10%, OLS-20%, and OLS-30%, respectively.

2.4. Preparation of Tri-Ply Plywood and Evaluation of Adhesive Bonding Performance

Tri-ply plywood was manufactured utilizing poplar veneer (200 mm × 130 mm × 2 mm) as a raw material. The plywood was prepared through the following procedure: the adhesive was homogeneously stirred and manually applied to the core veneer with an adhesive amount of 180 g/m² on both sides. Subsequently, the two surface veneers were positioned over the core veneer with the orientation perpendicular to the wood grain of the core veneer. The sample was subjected to pre-pressing for a duration of 5 min at room temperature and a pressure of 0.7 MPa; hot pressing was conducted using a single-layer plate vulcanizing hot press. To produce the tri-ply plywood, the hot-press procedure was conducted at a temperature of 160 °C, applying a unit pressure of 1.0 MPa for a duration of 5 min to yield three-layer plywood.

2.5. Measurements and Characterizations

2.5.1. Fourier Transforms Infrared (FT-IR) Spectroscopy

Before testing, OL and OLS were subjected to curing at temperatures of 100 °C and 160 ± 3 °C for 3 h, respectively. L, SM, cured OL, and OLS were pulverized into powder for further FT-IR analysis. For testing, the samples were uniformly blended with KBr powder. The ratio of samples to KBr was mixed at a ratio of 1:100, ground into powder, subsequently pulverized, and then compacted. The infrared spectroscopic testing of the samples was conducted using a Varian 1000 instrument (Palo Alto, CA, USA) for 32 scans over a scanning range of 4000–500 cm⁻¹.

2.5.2. X-ray Photoelectron Spectroscopy (XPS) Analysis

L, SM, cured OL, and OLS samples were analyzed using an Escalab 250 XPS analyzer. (Thermo Fisher Scientific, Waltham, MA, USA). The C1s, O1s, and N1s peaks were fitted and analyzed employing Advantage (v5.9921) XPS software. All bond energies were calibrated based on C1s at 284.8 eV, and the collected data were analyzed utilizing Advantage (v5.9921) software.

2.5.3. Gel permeation Chromatography (GPC) Analysis

The GPC experiments were conducted using a PL-GPC50 gel permeation chromatograph, which was outfitted with an aquagel-OH Mixed m gel column. The column temperature was set at 40 °C. The detection process was performed using an oscillometric refractive detector, with the mobile phase being a 0.1 M NaNO₃ aqueous solution. The flow rate of the mobile phase was maintained at 1.0 mL/min. The calibration curve was established using a range of tightly distributed PEG standards and was employed to compute the relative molecular mass of the samples.

2.5.4. Dynamic Mechanical Analysis (DMA)

The thermo-mechanical properties of SM and OLS adhesives were assessed through the utilization of the DMA-242 analyzer (NETZSCH, Selb, Germany), with the results being logged using Proteus (v8.17) analysis software. To perform the analysis, the samples were prepared from poplar veneers with sizes of 50 mm × 10 mm × 2 mm. The samples were evenly coated with the SM or OLS adhesive, ensuring a uniform adhesive application rate of 180 g/m². During the analysis, the temperature range from 35 °C to 300 °C was investigated, applying a heating rate of 5 K/min and a frequency of 20 Hz. Additionally, a dynamic force of 2 N was sustained.

2.5.5. Thermogravimetry (TG) Analysis

The thermal stability of SM and OLS adhesives was assessed through analytical evaluation using a Netzsch TG 209 F1 instrument from Germany. Approximately 5–6 mg of dried samples were placed in a ceramic crucible and subjected to heating under a nitrogen atmosphere at a flow rate of 50 mL/min. The heating was conducted at a ram** rate of 10 °C/min, within a test temperature range of 35–600 °C. The samples were then heated again at a ram** rate of 10 °C/min up to a desired temperature, with a test temperature of 10 °C/min.

2.5.6. Strength of the Plywood

The plywood was subjected to 24 h in a well-ventilated room with a temperature of 25 °C and a relative humidity of 50% ± 10%. Afterward, the plywood bonding strength was tested for dry and wet bonding strength in accordance with the “Test Methods for Physical and Chemical Properties of Wood-based Panels and Veneered Wood-based Panels” (GB/T 17657-2013) [35] and “Plywood for General Use” (GB/T 9846-2015) [36]. The bonding strength of HOLS and OLS plywood was assessed using the WDS-50 KN mechanical testing machine. The dry strength of a specimen is tested directly. When testing the wet strength, the specimens were immersed in hot water at 63 ± 3 °C for 3 h, and the wet bonding strength was determined after cooling at room temperature for 10 min. Ten samples were measured to determine the mean value and standard deviation of the bonding strength.

3. Results

3.1. Characterization of L and OL

3.1.1. GPC Analysis

The GPC profiles of L and OL are presented in Figure 1a. The results demonstrated that the weight average molecular weight (Mw) of L was 8716 Da and the umber average molecular weight (Mn) was 4434 Da. The Mw of OL declined to 7613 Da, and the same trend occurred with Mn which dropped to 4121 Da. In most cases, HP pretreatment led to the breakage of β-O-4 bonds within the lignin chains, resulting in a decrease in the molecular weights [37,38]. However, in some instances, pretreatment could provoke more condensation of lignin chains, causing an increase in the molecular weight [39]. Condensation and fragmentation are opposing reactions that affect the molecular weight of lignin [40]. When the oxidation took place, both condensation and fragmentation reactions proceeded competitively, which induces the change in the lignin molecular weight. If fragmentation reactions are undertaken at a faster rate than condensation reactions, the molecular weight of lignin decreases; conversely, when condensation reactions proceeded at a faster rate than fragmentation reactions, the molecular weight of lignin increases. The term PI denotes the distinct molecular weight fractions of a polymer molecule [41]. When there is a substantial dissimilarity among the molecular weight fractions in a polymer, the polymer will exhibit a higher PI. Conversely, a lower PI will result when the molecular weight fractions are more similar. The PI of a polymer plays a significant role in determining its performance. Generally, a higher PI results in a lower performance. The PI of L was measured to be 1.96174, while the PI of OL was determined to be 1.84737. The decrease in the PI indicates a reduction in the difference in the lignin molecular weight fractions. This decrease primarily arises from the degradation of high molecular weight lignin chains in the presence of the oxidizing agents, leading to a reduction in both the lignin mass and PI. The degradation of lignin during oxidation is depicted in Figure 1b. To further explore the changes in the lignin structure before and after oxidation, we performed the FTIR analysis of L and OL.

3.1.2. FT-IR Analysis

Figure 2 displays the FTIR spectra of the prepared adhesive. Initially, L presented a characteristic peak such as the one near 3434 cm⁻¹, which represents the stretching vibration of the -OH group; the peak near 2933 cm⁻¹ reveals the methyl and methylene groups in the C-H bonds; the peak near 1612 cm⁻¹ indicates the corresponding absorption peak of the aromatic rings; and the peak near 1267 cm⁻¹ indicates the stretching vibration of the methoxy groups [42,43].

Likewise, the stretching vibration peak near 3434 cm⁻¹ in the case of OL refers to -OH, which decreases upon oxidation using HP, suggesting their consumption via oxidation. Two new peaks emerged at 1730 cm⁻¹ and 1631 cm⁻¹, which attribute to C=O and the formation of the quinone structure, respectively [44,45]. This indicates that during the oxidation of lignin, the hydroxyl groups on the branched chains are oxidized, resulting in the formation of aldehyde or carboxylic acid groups, while the methoxy groups on the benzene rings undergo oxidation and are converted to the quinone structure. Consequently, it can be concluded that the oxidation process with hydrogen peroxide leads to the formation of OL with a mostly aldehyde and quinone structure (Figure 3).

3.1.3. XPS Analysis

The findings of XPS analysis can enable the evaluation and analysis of specific performance indicators associated with the adhesives.

To investigate the oxidation process of lignin, XPS scans were conducted on L and OL for the quantitative determination of their carbon and oxygen contents, as depicted in Figure 4 and

Table 1. Elemental contents of the XPS full spectrum of L and OL.

Name	C1s/%	N1s/%	O1s/%
L	73.24	2.06	24.7
OL	71.69	1.44	26.87

. It is obvious that L and OL contain the highest carbon content. The content of oxygen was the second highest. Following the oxidation using HP, the oxygen content in OL increased from 24.7% to 26.8%, suggesting that the oxidation process led to an elevation in the oxygen content of OL. The nitrogen content was relatively low, possibly present as impurities.

Figure 5 displayed the high-resolution C1s and O1s spectrum of L and OL. The C element in L was primarily present in two bonding states (Figure 5a) as follows: the C1 peak (at 284.8 eV) representing C-C and C-H bonds, and the C2 peak (at 286.4 eV) representing C-O bonds. When compared with the C1s spectra of OL (Figure 5b), it can be observed that a new C3 peak (at 288.78 eV) emerged, indicating the presence of C=O bonding in OL after undergoing oxidation through HP. Similarly, the O element in L showed two binding states (Figure 5c) as follows: the O1 peak (at 531.68 eV) revealing the C-O bond, and the O2 peak (at 532.88 eV) representing the Ar-OH. In line with the C1s spectrum, a new O3 peak (at 530.82 eV) appeared in the O1s spectrum of OL (Figure 5d), signifying the presence of C=O bonds. This illustrates the oxidation of the hydroxyl groups in L to aldehyde and quinone structures in presence of HP, resulting in the generation of C=O bonds.

According to the findings from GPC, XPS, and FT-IR analysis, the L was degraded by oxidation to higher active OL species, in which plenty of quinone and aldehyde groups are formed.

3.2. Characterization of OLS

3.2.1. FT-IR Analysis

Figure 6 depicts the peaks of OL at 1730 and 1631 cm⁻¹, which correspond to the C=O bond and quinone structure, respectively. However, these peaks disappeared after incorporation into the formulation of the OLS adhesive. This disappearance could be ascribed to the consumption of the aldehyde group and quinone structure in the reaction process, leading to the loss of peak signals. Furthermore, the absorption peak of the amino group at 3323 cm⁻¹ in case of the OLS adhesive became narrower. This narrowing could be accounted for by the crosslinking reaction working between the amino group of SM and the aldehyde or quinone structure of OL. As a result, the amino group, aldehyde group, and quinone structure were consumed, leading to a decrease in the relevant peaks’ intensity [33]. Furthermore, the addition of OL caused a shift in the absorption peak of amide II from 1539 cm⁻¹ (SM) to 1543 cm⁻¹ (OLS). This shift indicated a more intense crosslinking of OL compared to SM. Consequently, this result further confirmed the successful occurrence of the crosslinking reaction between SM and OL.

3.2.2. XPS Analysis

SM, OL, and OLS samples were subjected to XPS analysis, where the carbon, nitrogen, and oxygen contents were determined as shown in Figure 7 and

Table 2. Contents of XPS full spectrum elements of SM, OL, and OLS.

Name	C1s/%	N1s/%	O1s/%
SM	65.41	8.06	26.53
OL	71.69	1.44	26.87
OLS	66.38	7.61	26.01

. According to the data presented in Figure 7 and Table 2, C was the most abundant element in SM and OLS, followed by the O element, while the least abundant element was N. The content of the element N in OL was only 1.33%, which may indicate the presence of impurities.

In the XPS spectra of SM, two different peaks were observed for the N1s (Figure 8a) describing the C-N at 400.08 eV and the H-N at 401.28 eV. In the case of OLS, the N element was present as an impurity, and, although a signal was detected, it could not be accurately fitted due to its low content (Figure 8b). The N1s peaks of OLS could be fitted into three different peaks (Figure 8c). In comparison to SM, OLS exhibited a new C=N peak at 399.53 eV. This observation suggested the possibility of a Schiff base reaction between the aldehyde group or quinone structure in OLS and the amino group in SM throughout the curing process, leading to the creation of a C=N structure.

The O1s peaks in SM were fitted into two peaks (Figure 8d) as follows: the C=O peak at 530.54 eV and the C-O peak at 531.88 eV. In OL, the O1s peaks could be fitted into three peaks (Figure 8e) as follows: 530.76 eV, 531.84 eV, and 532.48 eV, corresponding to C=O, C-O, and Ar-OH, respectively. The O1s peaks of OLS could be fitted into three peaks (Figure 8f). In comparison to SM, OLS exhibited a COOR peak at 532.46 eV. This observation could be attributed to the esterification reaction between the hydroxyl group in OLS and the carboxyl group in SM that occurs during the curing process, resulting in the generation of the COOR [46].

Based on XPS and FT-IR analyses, it can be admitted that Schiff base and esterification reactions occurred between OL and SM, leading to the creation of a densely interconnected network structure. The potential bonding mechanism is illustrated in Figure 9.

3.3. Bonding Performance of HOLS and OLS Adhesive

The adhesive strength of plywood is essential in establishing its physical and mechanical properties. The wet strength of an adhesive is a highly significant property to consider [47]. It reflects its water resistance, serving as an indicator of its adhesive quality. To demonstrate the viability of OLS as a wood adhesive, the hot-pressing technique (160 °C, 1.0 MPa, 5 min) was employed to fabricate the three-layer plywood. The resulting plywood was then subjected to a bonding strength evaluation. Figure 10a displays the specimen samples.

Based on Figure 10b, the wet strength of the HOLS adhesive exhibited an initial enhancement followed by a decrease with an increasing HP dosage. When the HP dosage was adjusted at 3% of the L mass, the wet strength of the HOLS-3% adhesive was around 0.63 ± 0.11 MPa, and the wet strength of the HOLS adhesive continued to improve when the HP dosage increased. The highest wet strength of the HOLS-10% adhesive was reached for HP at the level of 10% of L mass, 0.88 ± 0.14 MPa, this value is 125% higher compared to that of the SM adhesive. When the HP dosage exceeded 10% of L mass, the wet strength of the HOLS adhesive showed a decreasing trend. The reason for that drop may be because the amount of HP determines the extent of L oxidation. Interestingly, the amount of added HP is critical at around 10% of the L mass; with the increasing HP dosage, the molecular weight of L decreased, and more and more aldehyde groups and quinone structures were oxidized, which provided more reaction sites for the crosslinking of OL with SM. As a result, more crosslinking networks were formed, which enhanced the wet strength of HOLS adhesives. When the HP dosage was 10% of the L mass, the system structure was most encouraging for the crosslinking of OL and SM. As the HP dosage continues to increase, L undergoes over-oxidation, which causes the severe degradation of lignin chains, weakening the intermolecular hydrogen bonds. At the same time, lignin undergoes a ring-opening reaction to form muconic functional groups [42]. This over-oxidation reaction is unfavorable for crosslinking and results in a decrease in the bond strength.

The influence of the addition of OL on the bonding strength of OLS adhesives is presented in Figure 10c. As shown in Figure 10c, the wet strength of the OLS adhesive showed a tendency to enhance, and then decreased with the increasing addition of OL. When the OL addition was 3% of the dry weight of SM, the wet strength of the OLS-3% adhesive reached 0.68 ± 0.07 MPa, which does not reach the standards specified in (GB/T9846-2015, ≥0.7 MPa), likely because there were fewer crosslinking points and the reticulated crosslinking structure was not dense enough following the curing process, thus resulting in the lower wet strength of the plywood. Nevertheless, with the increasing addition of OL, the wet strength of plywood improved. Specifically, when the OL addition was 10% of the dry weight of SM, the wet strength of OLS-10% adhesive reached 0.88 ± 0.14 MPa, signifying a 125% improvement when compared to the SM adhesive. The enhancement could be attributed to the interaction between the aldehyde, carboxyl, and quinone structures of OL and the hydroxyl and amino groups in SM, resulting in the formation of a robust interconnected network that amplifies the adhesive’s wet strength. However, additional increments in the amount of OL did not result in any improvement in the wet strength of the adhesive, whereas a declining trend in the hot water bonding strength was observed when the OL addition reached 20% of the dry weight of SM. This can be translated into excessive amounts of OL, provoking self-crosslinking in the quinone structure, and limiting the critical crosslinking level between OL and SM. Based on the experimental results of the bonding strength, the optimal preparation process of the adhesive involved a HP dosage of 10% with respect to the mass of L, while, for OL, a dosage around 10% of the mass of SM is preferential. Under these conditions, the plywood prepared with the OLS-10% adhesive exhibited a good bonding strength and wet strength.

3.4. Thermal Properties of OLS Adhesives

3.4.1. DMA Analysis

To investigate the curing behavior of OLS adhesives in more detail, DMA tests were conducted on different OLS adhesives, and SM, as a comparison group, was also subjected to DMA testing. The findings are depicted in Figure 11. The energy storage modulus of both samples demonstrated an increasing trend, followed by stability, eventually decreasing with rising temperatures. Specifically, in Figure 11, the energy storage modulus of OLS-10% began to increase at approximately 80 °C, while the other four OLS adhesives showed an initial increase at temperatures of 100 °C or higher, indicating a lower curing temperature for the OLS-10% adhesive. Between 120 °C and 150 °C, the energy storage modulus of all samples exhibited a substantial increase. This implies the existence of crosslinking reactions between SM and OL, leading to the formation of a network structure and subsequent enhancement in the energy storage modulus. The slope of the energy storage modulus curve frequently signifies the rate of curing for the adhesive being tested [48], with the OLS-10% adhesive exhibiting the steepest slope, indicating higher reactivity. At 160 °C, the temperature at which the maximum storage modulus was reached for all samples, sufficient crosslinking reactions between OL and SM were indicated, validating the selection of a hot-pressing temperature of 160 °C to perform adequate curing. As the temperature continued to rise, the energy storage modulus of the samples reached a plateau. To some extent, the peak energy storage modulus values observed in the DMA curves correspond to the mechanical characteristics of the cured adhesives [49]. Notably, the OLS-10% adhesives exhibited higher energy storage modulus values, confirming their excellent bonding strength. Additionally, the energy storage modulus of all five OLS adhesives exceeded that of SM adhesives, implicating the enhancement of the bonding strength through the incorporation of OL. This finding aligns with the results obtained for the plywood bonding strength. Above 214 °C, the energy storage modulus values of the OLS adhesives rapidly declined, predominantly due to the softening and deterioration of the adhesive network. This thermal breakdown led to a reduction in the energy storage modulus. Overall, the DMA analysis demonstrated the excellent curing properties and bond strength of the OLS-10% adhesive.

3.4.2. TG Analysis

Figure 12 illustrates the thermal decomposition processes of the SM adhesive and OLS adhesive within the temperature range from 30 to 600 °C. Both adhesives primarily underwent thermal decomposition between 170 and 450 °C. It should be noted that the weight loss before 170 °C was irregular, primarily due to moisture evaporation. Minimal thermal degradation occurred within this temperature range. As the temperature increased, both the SM and OLS adhesives experienced varying degrees of thermal degradation. Between 170 and 310 °C, mass loss was mainly attributed to the rupture of peptide chains between protein molecules and the presence of volatile or degradable small molecules within the adhesive. Non-covalent bonds, including hydrogen bonds between molecules and within peptide backbones, are broken within this temperature range. Subsequently, beyond 310 °C, mass loss primarily occurred due to the degradation of the adhesive’s crosslinked network, a fracture in the skeletal structure, and polymer degradation. The thermogravimetric derivative (DTG) profile provided insights into the speed of mass change at different temperatures. The DTG curve of the SM adhesive exhibited a peak at 301 °C. However, upon the incorporation of OL, the functional groups in OL formed extensive interactions with the SM molecules, bringing about the formation of a compacted, interconnected crosslinked network. Consequently, the peak value of the DTG curve of the OLS adhesive increased, and the maximum heat loss temperature was elevated to 307 °C. Therefore, the combination of thermogravimetric analyses indicated that OL acts as an effective modifier to retard the thermal degradation rate of SM adhesives and enhance the thermal stability of the adhesives.

4. Conclusions

In this study, OL and SM were combined to create a novel SM-based wood adhesive for water-resistant biomass. The idea stems from the fact that the reactive functional groups in SM can create a crosslinked network with the aldehyde group and quinone structure in OL, thus enhancing the water resistance of the adhesive. The plywood test findings revealed that the OLS adhesive performed best when the HP was 10% L, and the OL was 10% SM. The dry and wet (63 °C, 3 h) bonding strengths of the plywood made with this adhesive were 1.86 ± 0.07 MPa and 0.88 ± 0.14 MPa, respectively, which exceeded the GB/T9846-2015 national norm (0.7 MPa) and which are anticipated to fulfill the demands of the wood industry. Therefore, the inclusion of OL can be considered a novel approach to enhancing the bonding properties and water resistance of SM-based adhesives. This expands the potential value of SM-based adhesives in industrial production and application.

The current hot press temperature is 160 °C, and the next step will involve studying low-temperature curing. At the same time, this study did not test the adhesive for mildew resistance. In the near-future, we will modify the soybean meal adhesive to enhance its mildew resistance, aiming to achieve a soybean meal-based adhesive with a low curing temperature, mildew resistance, and good water resistance.