# Soybean protein wood adhesive with enhanced water resistance and low environmental impact through lignin-protein hybridization

Yongli Yang, Haizhu Wu, Jun Zhang, Tianguo Wen, Guanben Du, Bertrand Charrier, Hisham Essawy, Antonio Pizzi, Jiamin Wu, Xiaojian Zhou, et al.

<figure><img src="image_1.png" /></figure>

## ► To cite this version:

Yongli Yang, Haizhu Wu, Jun Zhang, Tianguo Wen, Guanben Du, et al., Soybean protein wood adhesive with enhanced water resistance and low environmental impact through lignin-protein hybridization. Industrial Crops and Products, 2025, 234, pp.121571. 10.1016/j.indcrop.2025.121571. hal-05365177

HAL Id: hal-05365177

https://hal.science/hal-05365177v1

Submitted on 14 Nov 2025

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

<figure><img src="image_2.png" /></figure>
<figure><img src="image_3.png" /></figure>

# Soybean protein wood adhesive with enhanced water resistance and low environmental impact through lignin-protein hybridization

Yongli Yang<sup>a,1</sup>, Haizhu Wu<sup>a,1 D</sup>, Jun Zhang<sup>a</sup>, Tianguo Wen<sup>b</sup>, Guanben Du<sup>a</sup>, Bertrand Charrier<sup>c</sup>, Hisham Essawy<sup>d</sup>, Antonio Pizzi<sup>e D</sup>, Jiamin Wu<sup>a,*</sup>, Xiaojian Zhou<sup>a,*, D</sup>, Xinyi Chen<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

- Soy protein

- Alkaline lignin

- Oxidation

- Water resistance

- Life cycle assessment

## ABSTRACT

Soybean meal, a by-product of soybean oil production, exhibits poor water resistance and is highly susceptible to mold, which are significant limitations hindering its large-scale industrial application. Inspired by the phenolic-amine chemistry, a fully bio-based soy protein wood adhesive was engineered to achieve excellent water resistance and environmental benefits. The process starts with oxidation of alkaline lignin using sodium periodate (NaIO₄) to generate aldehyde/quinone functionality on the expense of some aliphatic/phenolic hydroxyl groups, for promoting its subsequent interaction with soy protein to yield target adhesive by Schiff-base formation and Michael addition reaction. Importantly, residual NaIO₄ was meticulously removed to minimize its impact on the bonded substrate, the cross-linked network, and the environment, as it does not participate in the construction of the cross-linked architecture. The results showed that the dry and wet shear strength of SM-OL1.5 increased by 51.2 % and 80.8 % to 1.83 MPa and 0.94 MPa compared with SM adhesives, surpassing the China National Standard (GB/T 9846-2015, ≥0.7 MPa). Interestingly, the developed adhesive formulation exhibited mildew resistance, which can be attributed to the inherent feature of lignin to disrupt cell membranes and alter permeability, leading to cell lysis. Furthermore, the hybridized adhesive showcased minimal environmental impact, low volatile organic compound (VOC) emissions, and full compliance with global sustainability initiatives, making it a promising candidate for eco-friendly wood adhesive applications.

## 1. Introduction

The adhesives used for fabricating wood-based panels are primarily formaldehyde-based derivatives, which account for over 90 % of the market share in the wood adhesive industry (Xu, 2021; Chen et al., 2023; J. Zhang et al., 2021). However, these adhesives have long been associated with classical issues, including the use of non-sustainable starting materials and the emission of formaldehyde, which have spurred growing concerns and the need to develop bio-sourced wood adhesives. Consequently, there is a strong need to develop eco-friendly and sustainable adhesives derived from regenerative agroforestry resources to

replace petroleum-based synthetic resins and achieve more eco-friendly production processes (Ahire et al., 2024; Alkan et al., 2022; Jin et al., 2020). Soybean meal (SM), a by-product of soybean oil extraction (Jin et al., 2020), is characterized by its sustainability, easy availability, and cost-effectiveness, making it a promising alternative to petroleum-derived sources for the preparation of wood adhesives (Chen et al., 2023; Liu et al., 2022b).

A series of studies have been conducted for enhancing the water resistance and cohesive strength of the adhesives, focusing on either exposing the folded internal active groups of soy protein and subsequently promoting cross-linking, or employing enhanced methods that

<footnote><sup>a</sup> Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China</footnote>

<footnote><sup>b</sup> Yunjing Feilin (Pu'er) New Materials Co., Ltd, Pu'er 665199, China</footnote>

<footnote><sup>c</sup> CNRS/Univ Pau & Pays Adour, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux-Xylomat, UMR5254, Mont-de-Marsan 40004, France</footnote>

<footnote><sup>d</sup> National Research Centre, Department of Polymers and Pigments, Cairo 12622, Egypt</footnote>

<footnote><sup>e</sup> LERMAB, University of Lorraine, 27 rue Philippe Seguin, Epinal 88000, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: wujiamin@swfu.edu.cn (J. Wu), xiaojianzhou@swfu.edu.cn (X. Zhou), chen_xinyi_csuft@126.com (X. Chen).</footnote>

<footnote n="1">¹ These authors contributed equally to this work.</footnote>
<figure><img src="image_4.png" /><figcaption>**Scheme 1.** Schematic illustration on synthesis of SM-OL wood adhesive.</figcaption></figure>

involve chemical cross-linking and the addition of fillers. Examples of such modifications include denatured modification (Liu et al., 2015; Li et al., 2024), chemical cross-linking (Xi et al., 2020; Xu, 2021; Zhang et al., 2018), protein molecular and synthetic resin modification (Luo et al., 2015; Xi et al., 2020; Zhang et al., 2022), and bio-mimetic designs (Gu et al., 2020; Zhang et al., 2022). By contrast, chemical cross-linking modification via some high-reactivity cross-linker was considered an effective method (Hao et al., 2023; Zhang et al., 2022). Typically, the cross-linked structure of plant protein adhesive is built by reacting the cross-linker with the active groups in the protein structure, primarily consuming hydrophilic groups to promote the construction of covalent bonds (Zhang et al., 2022). This enhances the bonding performance and water resistance, and can even improve other functions, such as mildew-proofing, flame-retardancy, and cold-setting properties (Jiang et al., 2024; Li et al., 2024; Zhang et al., 2022). New technologies, such as life cycle assessment (LCA), have also been introduced into the research of soy protein adhesives, primarily to evaluate the environmental impact of adhesive products (Chen et al., 2024; Hu, 2024). Representative strategies for constructing cross-linked networks include: (i) reacting soy protein with epoxy groups (Jin et al., 2020; Xu et al., 2020); (ii) forming phenolamine interactions through Schiff-base and Michael addition (Gu et al., 2020); (iii) constructing amide structures by reacting the -COOH of cross-linkers with the -NH₂ of proteins (Hao et al., 2023; Jiang et al., 2024; Liu et al., 2015; Pradyawong et al., 2019); (iv) leveraging the synergistic effect of combining covalent crosslinking with physical interpenetration or ionic coordination (Jin et al., 2020; Liu et al., 2022b). However, the majority of cross-linkers used in the aforementioned research are typically derived from fossil-based sources, require complex preparation procedures, or involve the use of hazardous reagents and solvents. Identifying a bio-sourced cross-linker that is easy to prepare and cost-effective represents a promising approach, yet it presents significant challenges.

Natural polyphenols, such as lignin, constitute the second most abundant biomass resource after cellulose. Industrial lignins, such as alkaline lignin (AL) and lignosulfonate, are by-products of the pulp and paper industry, serves as a feedstock for producing biomass-based wood adhesives (Alkan et al., 2025; Bansode et al., 2023; Huang et al., 2022;

Younesi-Kordkheili and Pizzi, 2023). It has been employed as a cross-linker to enhance the combination performance of soy protein wood adhesives, given its potential to improve the mechanical, thermal, mildew-resistant, and hydrophobic properties of polymers (Chen, 2023; Liu et al., 2022b; Zhang et al., 2024). Current research primarily employs several strategies: (i) the first involves the structural or particle size changes of lignin under physical action to enhance its interaction ability with large soy protein (Pradyawong et al., 2017; Xiao et al., 2013). For instance, integrating lignin with soy protein through the addition of small particle size alkali lignin, achieving a 53.3 % enhancement in the wet bonding strength of plywood (Pradyawong et al., 2017). (ii) the second focuses on constructing covalent cross-linked networks by chemically modifying lignin to enhance its reactivity, as exemplified by modification methods such as lignin epoxidation using ethylene glycol diglycidyl ether (post-modification crosslinking with soy protein achieves 1.02 MPa wet bonding strength) (Chen, 2023; Yu et al., 2025; S. Chen et al., 2021), demethylation to form catechol structures (Liu et al., 2022b), amination (Wang et al., 2024; Xin et al., 2017), hydroxymethylation (Luo et al., 2015) or depolymerization (Pradyawong et al., 2017; Xiao et al., 2013). (iii) additional cross-linking agents, such as epoxy resin, were utilized to construct a covalent crosslinking with the reactive groups of lignin and soy protein (Ibrahim et al., 2013; Liu et al., 2022b). Generally, lignin/activated lignin contributes to the development of a covalent cross-linking matrix in soy protein adhesives by reacting with protein amino groups through mechanisms such as epoxy group reaction, Michael addition, and Schiff-base interaction, etc. The bonding performance and water resistance can be notably enhanced, even along with mildew-proof. Nevertheless, these methods, such as depolymerization and particularly epoxy grafting, typically involve complex preparation processes, the use of toxic chemicals like epichlorohydrin, or the application of petrochemical-based epoxy resins. Oxidation is regarded as a promising strategy capable of selectively breaking certain chemical bonds and creating new active groups (aldehydes and carboxylate groups) (Bansode et al., 2023; Esen and Meier, 2020). Typically, this process can be carried out under mild conditions (room temperature or below 50°C), with no additional chemical reagents required apart from oxidants.
Herein, oxidation strategy is applied to activate the AL using NaIO₄ at room temperature. A bio-sourced soy protein-oxidized lignin (SM-OL) has been designed by involving Michael addition and Schiff-base interactions (as described in Scheme 1). The solvents used, including water and absolute alcohol, can be easily recycled (the total recovery rate is above 88 %) apart from NaIO₄. Oxidized lignin (OL) served as an efficient cross-linker in this work with an addition accounting for less than 11 % based on the dry mass of protein, for obtaining good bonding performance and water-proof characteristics. Moreover, the NaIO₄ has been removed as much as possible to minimize its impact on the bonded substrate and the cross-linked network, even on the environment (it cannot participate in the construction of the cross-linked network). This is an issue overlooked by most studies using NaIO₄ to prepare aldehyde-functionalized biomass, which has consequently led to environmental harm. The resultant adhesive demonstrates excellent environmentally friendly features, as confirmed by GC-MS and LCA evaluations. It has been agreed as having superior environmental advantages and significant industrial application potential.

## 2. Materials and methods

### 2.1. Materials

Defatted soybean meal (SM, 52 % soy protein) was sourced from Shandong Yuwang ecological food industry Co., LTD. Alkaline lignin (AL) was supplied by Shanghai Aladdin Biochemical Technology Co., LTD. Sodium periodate (NaIO₄, AR 99.5 %) and absolute alcohol (98 %) were provided by Shanghai Maclin Biochemical Technology Co., LTD. Poplar veneer (around 10 %-12 % moisture) with the size of 1270 × 840 × 2 mm³ was purchased from Heze Junang wood industry Co., LTD.

### 2.2. Preparation of oxidized lignin

The OL was obtained by a process reported in the literature with slight modifications (Bansode et al., 2023), and compared with previous studies, we further quantified the oxidation time. Briefly, 20 g of alkaline lignin was added to a 1000 ml beaker containing 500 ml of an absolute alcohol/water solution (v/v = 15 %:85 %) with magnetic stirring. Subsequently, 50 g of NaIO₄ was introduced into the beaker and mixed thoroughly. The beaker was covered with aluminum foil to provide a dark environment. The reaction was carried out at room temperature with continuous gentle magnetic stirring for 12 h, 18 h, and 24 h, respectively, to investigate the effect of oxidation time on the bonding performance of adhesives. The supernatant containing OL was transferred to another 1000 ml beaker. The remaining solid product was then washed at least three times with absolute alcohol to collect as much OL as possible. The resulting wash liquor (containing OL) was collected and mixed with the initial supernatant. The OL was separated from the mixture by centrifugation and washed with anhydrous ethanol at least three times. The final OL product was dried at 60°C for 6 h in a vacuum oven, as shown in Figure S1.

### 2.3. Preparation of SM-OL wood adhesives

A series of SM-based wood adhesives was prepared by blending the ingredients at ambient temperature, according to the formulations given in Table S1. The control SM adhesive, without any addition of OL, was prepared by mixing only SM and distilled water. For the preparations of SM-OL adhesives, different amounts of OL (0.5 g, 1.0 g, and 1.5 g) were added to 14 g of SM in a 250 ml flask to form a homogeneous mixture. Subsequently, 76 g of distilled water was introduced into the SM/OL mixture and the mixture was homogenized using a blender (LICHEN, LC-OES-60) at 600 rpm at room temperature for 15 min. The resulting SM-OL adhesives were termed SM-OL0.5, SM-OL1, and SM-OL1.5, respectively, based on the amount of OL added.

## 2.4. Characterizations

Fourier transform infrared (FTIR) spectroscopy was employed as investigation tool using a Thermo Nicolet iS5 spectrometer (Thermo Fisher Scientific Inc., USA). X-ray photoelectron spectroscopy (XPS) investigation was performed using an XPS spectrometer (Thermo Fisher ESCALAB 250Xi, Thermo Fisher Scientific Inc., USA) at 50 eV with a resolution of 0.1 eV. Differential scanning calorimeters (DSC) traces were obtained using DSC ta250 (TA Instruments, USA).

### 2.5. Hydrolysis residue ratio investigation

Liquid SM and SM-OL1.5 adhesives were placed in an oven at 120 ± 3°C to ensure complete curing until a steady mass was achieved. The cured adhesive sample was then ground into a powder and weighed to obtain a mass M. Subsequently, the powder sample was wrapped in filter paper and soaked in hot water at 60°C for 6 h. After soaking, the sample was transferred to an oven at 105°C until a constant dry mass m was obtained. The hydrolysis residue ratio was calculated using the following formula (1). Three replicate experiments were performed.

$$ \text{Hydrolysis residue ratio}(\%) = \frac{m(\text{g})}{M(\text{g})} \qquad (1) $$

### 2.6. Preparation of three-layer plywood

The poplar veneer was prefabricated to a specific size (190 × 120 × 2 mm³). The middle layer was coated with adhesive at a loading rate of 140 g/m² per single face. The three-layer plywood was constructed with the grain orientation of each adjacent layer being perpendicular to one another. The plywood assembly was subsequently placed into a hot press, where it was subjected to hot pressing at a temperature of 120°C for a duration of 5 min, while maintaining a pressure of 1.0 MPa. The resulting specimens, measuring approximately 190 × 120 × 5 mm³, were stored at room temperature for a minimum of 24 h prior to further processing.

### 2.7. Shear strength of plywood specimens

The plywood specimens with the final size of 100 × 25 × 5 mm³ was prepared for further investigation. The assessment of shear strength under dry conditions was performed using a universal testing machine, with a shear rate set at 10 mm/min. In accordance with the China National Standard GB/T 17657–2022, the specimens were pre-soaked in hot water at a temperature of 63 ± 3°C for 3 h before testing the shear strength under wet conditions. Then, the pre-soaked sample was stored at room temperature to cool for 10 min. The average value of shear strength was determined based on at least six replicate experiments.

### 2.8. Observation of bonding interphase of plywood

The plywood specimens were cut to a specified size of 25 × 25 mm². The resultant specimens were softened by distilled water at ambient temperature for 24 h. A slice contains bonding interphase with a thickness of 40 µm was obtained by ultra microtome (LEICA SM 2000R, Germany). Any impurities were removed by deionized water, then glycerin was added and the samples were sealed with slides. The thin slices were observed and images were collected using an electron biological microscope (Nikon eclipse 80i DM 2000, Japan).

### 2.9. Life cycle assessment (LCA) analysis

A LCA was conducted to evaluate the comparative environmental impacts of SM-OL1.5 adhesive, urea-formaldehyde resin (UF), melamine-formaldehyde resin (MF), and phenolic resin (PF), in
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Characterizations of OL. (a) Schematic illustration of lignin oxidation; (b) and (c) FTIR spectra of AL and OL, respectively; (d) and (e) are the XPS C1s spectra of AL and OL, respectively.</figcaption></figure>

accordance with the ISO 14040 standard (ISO 14040:2006 Environ-
mental management — Life cycle assessment — Principles and frame-
work). For this analysis, 1 kg of adhesive was used as the functional unit.
The study employed a cradle-to-gate approach to evaluate the produc-
tion of soy protein adhesive, encompassing all inputs such as raw ma-
terials, chemicals, water, and electricity, as well as outputs including
products and emissions within the system boundary. Datasets for the
LCA of adhesive production were sourced from laboratory experiments,
the Ecoinvent database version 3.5, and relevant literature, with
detailed inventories provided in Tables S2, S3 and S4. The environ-
mental impacts associated with adhesive production were quantified
using SimaPro 9.0 software and the TRACI 2.1 methodology.

**3. Results and discussion**

3.1. *Characterizations of OL*

Lignin was oxidized by NaIO₄ to enhance its reactivity through two
mechanisms: (i) converting aliphatic hydroxyl groups to aldehyde
groups; and (ii) promoting the formation of quinone groups after in-situ
demethylation of guaiacyl units (G type) (Bansode et al., 2023; Gosse-
link et al., 2011). The schematic illustration of lignin oxidation is shown
in Fig. 1(a). The color of alkaline lignin was turned from dark brown to
light yellow, indicating the destruction of chromophores in the lignin.
Free phenolic hydroxyl groups are considered a major contributor to the
dark color of alkaline lignin (Qian et al., 2014). This color change il-
lustrates that the transformation of phenolic hydroxyl groups to other
groups does occur. Additionally, the concentration of chromophores
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** (a) The digital photograph of SM-OL1.5 adhesive; (b) Solid content of various adhesives; (c) pH of various adhesives. (d) Viscosity of various adhesives. (e) and (f) Coating performance of the adhesives on wood veneer; (g) DSC curves for SM and SM-OL1.5 adhesives; (h) FTIR spectra for SM and SM-OL1.5 adhesives; (i) and (j) XPS N1s spectra for SM and SM-OL1.5 adhesives, respectively; (k) Possible interactions between SM and OL.</figcaption></figure>

decreased as the aggregation, caused by the oxidative cleavage of β-O-4 linkages, was alleviated (Bansode et al., 2023; H. Zhang et al., 2020).

Credible evidence has been found through the investigation of FTIR and XPS. The FTIR spectra of AL and OL are shown in Fig. 1(b) and (c). The characteristic peak at 2930 cm⁻¹ is assigned to the C-H stretching of the aromatic methoxy group in AL (Boeriu et al., 2004). In the spectrum of OL, the intensity of this bond has decreased, indicating that some methoxy groups have been removed from the lignin aromatic ring after oxidation (Bansode et al., 2023). Furthermore, the peak at 1040 cm⁻¹, which represents C-O vibrations linked to the lignin aromatic ring, is significantly weakened in the spectrum of OL (Jiang et al., 2019). A new

characteristic peak at 1630 cm⁻¹, belonging to the quinone group, appears in the structure of OL (Bansode et al., 2023; Gosselink et al., 2011). The peak at around 1750 cm⁻¹ in OL highlights the carbonyl (C=O) bond in the aldehyde group formed after the oxidation of AL. This is related to the oxidation of aliphatic hydroxyl groups, which are either native to AL or originate from the breakage of β-O-4 linkages during oxidation (Bansode et al., 2023; X. Chen et al., 2021). The peak at around 1114 cm⁻¹ in AL, belonged to C-O-C ether bond stretching vibrations, is weakened in OL, likely due to the breakage of β-O-4 linkages (Liu et al., 2018). The cleavage of β-O-4 linkages promotes the generation of more aldehyde groups in the oxidized lignin. It has been reported
<figure><img src="image_7.png" /><figcaption>Fig. 3. (a) Digital photograph of plywood; (b) Schematic illustration of the shear strength measurement area of the plywood specimen; (c) Test conditions for the shear strength of plywood; (d) Shear strength of adhesives with varying amounts of oxidized lignin addition; (e) Shear strength of adhesives with different lignin oxidation times; (f) Wood failure ratio of plywood specimens; (g) Shear strength of plywood bonded with SM-OL1.5 adhesive before NaIO₄ removal; (h) Shear strength of plywood bonded with SM-OL1.5 adhesive after NaIO₄ removal.</figcaption></figure>

that an increase in aldehyde content from 0.0200 to 0.1116 mmol/g was observed in the lignin structure after oxidation treatment of alkaline lignin under the same preparation conditions (Bansode et al., 2023).

Fig. 1(d) presents the XPS C1s spectrum of AL. Compared with OL in Fig. 1(e), the integral area ratio of C=O to C-OH in AL is 0.9497, which increases to 1.0353 in OL. This result indicates that the transformation of C-OH to C=O has occurred. A significant change is observed by

comparing the C-O-C peak in AL and OL, suggesting that the cleavage of ether bonds, including β-O-4 linkages, occurred during the oxidation process (Bansode et al., 2023). A new peak belonging to O-C=O appears in the spectra of OL, representing carboxyl formed probably during the oxidation. Figure S2 provides the element percentage of carbon (C) and oxygen (O), showing that the relative ratio of O increased from 30 % to 46 %. The main reasons for this change are related to the loss of carbon
<figure><img src="image_8.png" /><figcaption>Fig. 4. (a) Bonding interphase of plywood and the effect of “mechanical interlock”; (b) Possible interaction between soy protein and oxidized lignin for providing the effect of “cohesiveness”.</figcaption></figure>

(C) and hydrogen (H) caused by the demethylation of G-type lignin, the aldehylation of aliphatic hydroxyl groups, and the quinization of aromatic hydroxyl groups. The oxidation of alkaline lignin enhances its reactivity, which is beneficial for promoting cross-linking between OL and soy protein, thereby forming a stable adhesive network.

Additionally, to ensure the environmental sustainability of the pro-
duction process, we have implemented the recycling of water and
ethanol. The recovery rates for wastewater and waste ethanol were
85.65 % and 93.58 % (in Table S5), respectively. These recycled mate-
rials have been successfully reintegrated into the preparation of new
oxidized lignin, and the performance of the resulting oxidized lignin-
modified soy protein adhesive has not been compromised, as shown in
Figure S3.

## 3.2. *The appearance, coating performance, and curing behavior of SM-OL adhesive*

The soy protein-oxidized lignin wood adhesive can be prepared by simply mixing the ingredients (SM and OL) at room temperature. The appearance of the resultant SM-OL1.5 adhesive is shown in Fig. 2(a). The solid content of the adhesives is depicted in Fig. 2(b), indicating a range of approximately 14 %-16 %. The pH of adhesives are approximately 7 (Fig. 2(c)), demonstrating a friendly characteristic towards the bonding substrate. Moreover, the viscosity of various adhesives prepared is shown in Fig. 2(d). Despite its high initial viscosity, the SM-OL1.5 adhesive can be easily applied to the wood veneer surface using a rubber roller, suggesting that it meets the requirements for industrial application. The high initial viscosity is a typical characteristic of soy protein wood adhesives (Liu et al., 2022b; Xu, 2021). The adhesive can be evenly applied to the surface of the wooden veneer, as shown in Fig. 2 (e) and (f).

Fig. 2 (g) displays the DSC curves of neat SM and SM-OL1.5 adhesives. The initial curing temperature of the SM-OL1.5 specimen is approximately 69°C, which is slightly lower than that of the control SM adhesive (approximately 72°C). Furthermore, the peak temperature for the SM-based adhesive decreases slightly from 124°C to 122°C with the addition of OL. This phenomenon indicates that OL can promote cross-linking. Notably, the integral area for the main curing peak of SM-OL1.5 is greater than that of the control SM adhesive. This suggests

that the heat release during the curing reaction of the SM-OL1.5 adhe-
sive is greater, indicating a more complete cross-linking reaction.
Therefore, the experiments for plywood preparation in this work were
conducted at 120°C, which aligns with the energy conservation re-
quirements of actual industrial production. Furthermore, the addition of
oxidized lignin can increase the thermal stability of the adhesive, and
related analysis is demonstrated in Supplementary Information and
Figure S4.

3.3. Proposed cross-linking between OL and soy protein

Fig. 2 (h) presents the FTIR spectra of control SM and SM-OL1.5 adhesives to further investigate the cross-linking reaction between SM and OL. Characteristic peaks attributed to soy protein are observed in both spectra. Typically, the sharp peak at 1635 cm⁻¹ is assigned to the C=O stretching in amide I (Xu et al., 2020; Zhang et al., 2018). A peak at 1516 cm⁻¹ belongs to N-H bending in amide II (Zhang et al., 2018). Yet, the intensity of this peak in the SM adhesive decreases upon the introduction of OL (in the spectrum of SM-OL1.5). This indicates SM and OL interacted efficiently to form integrated structure, which likely encountered consumption of the free amino groups in the soy protein structure (Xu et al., 2020). Furthermore, the peak at 1235 cm⁻¹ is related to N-H stretching/C-N stretching vibrations in amide III (Zhang et al., 2018). Fig. 2 (i) and (j) show the XPS N1s spectra for SM and SM-OL1.5 adhesives, respectively. In comparison, the integral area for C-N increases from 34.24 % in the SM adhesive to 57.13 % in the SM-OL1.5 adhesive. This suggests that the cross-linking between SM and OL is primarily due to the reaction of amino groups in soy protein with aldehyde groups and/or quinone groups in OL (Fig. 2 (k)). Conversely, the integral area for C-N decreases from 66 % in the SM adhesive to 43 % in the SM-OL1.5 adhesive. This indicates the depletion of amino groups of protein structure, which in turn forms a C=N bonds with oxidized lignin. Nevertheless, the contribution of both Michael addition and Maillard reaction, involving amino groups and quinone units, can be also considered.
<figure><img src="image_9.png" /><figcaption>Fig. 5. (a) Digital photos of cured SM and SM-OL1.5 adhesives in hot water; (b) Hydrolyzed residue ratios of SM and SM-OL1.5 adhesives; (c) Digital photos of plywood bonded with SM and SM-OL1.5 adhesives in hot water; (d) Proposed mechanism of water repellency for SM-OL1.5 adhesive.</figcaption></figure>

## 3.4. Mechanical performance and suggested bonding mechanism of SM-based wood adhesives

The SM-OL1.5 adhesive was used to prepare plywood, and a plywood specimen measuring 20 × 20 cm² was obtained, as shown in Fig. 3(a). Subsequently, plywood specimens were prepared in accordance with the China National Standard GB/T 17657–2022 to assess the dry and wet shear strength. The bonding area of the plywood specimen was 25 × 25 mm², as shown in Fig. 3(b). The specimen was then mounted on the testing equipment as illustrated in Fig. 3(c).

The dry and wet shear strengths of three-layer plywood samples bonded with SM and SM-OL adhesives are presented in Fig. 3(d). The specimen bonded with the control SM adhesive exhibits a dry shear

strength of 1.21 MPa and a wet shear strength of 0.52 MPa after pre-soaking in 63°C hot water. The introduction of OL enhances the shear strength of the plywood samples, indicating that effective cross-linking is promoted by OL. For example, the plywood specimen bonded with 1.5 g of OL shows the best performance, with a dry shear strength of 1.83 MPa and a wet shear strength of 0.94 MPa after pre-soaking in 63°C hot water. These values exceed the requirements of the China National Standard GB/T 9846–2015 for interior applications (Type II, ≥0.7 MPa). Compared to the neat SM adhesive, the dry shear strength and wet shear strength (after pre-soaking in 63°C hot water) increased by 51% and 81%, respectively.

Additionally, the effect of lignin oxidation time was further investigated, with the corresponding results shown in Fig. 3(e). The amount of
<figure><img src="image_10.png" /><figcaption>Fig. 6. The digital photographs of mildew resistance of SM and SM-OL1.5 adhesives (in Supplementary Information).</figcaption></figure>

lignin added was set at 1.5 g based on previous evaluations. AL without oxidation was added to the pure SM adhesive, and the wet shear strength of the SM-alkaline lignin adhesive showed a slight increase compared to the pure SM adhesive (as shown in Fig. 3(d)). This may be attributed to the interaction between AL and soy protein (Pradyawong et al., 2017). The pH of the soybean meal and distilled water mixture is typically neutral or slightly alkaline, which can facilitate the air oxidation of AL during hot-pressing (Schutyser et al., 2018). This is consistent with the theory of alkaline aerobic oxidation of lignin (Schutyser et al., 2018). However, this interaction and/or oxidation is limited and insufficient to support the formation of strong cross-linked network structures. The shear strength of plywood specimens can be significantly improved by increasing the oxidation time to 12 h, satisfying the requirements of the China National Standard GB/T 9846-2015 (Type II, ≥0.7 MPa). Furthermore, the best performance for the plywood is achieved when the AL is oxidized for 18 h, as shown in Fig. 3(e). The bonding performance of the plywood decreases when the oxidation time is extended to 24 h, which is probably related to the over-oxidation of AL that promotes the conversion of aldehyde groups to carboxyl products (-COOH). Therefore, an oxidation time of 18 h is selected for further investigation, which is beneficial for improving production efficiency compared to the reported work (Bansode et al., 2023). In addition, Figure S5 displays the shear strength of plywood prepared at various hot-pressing temperatures.

Fig. 3(f) presents the wood breakage ratios of plywood bonded with control SM and SM-OL1.5 adhesives, respectively. By comparison, the wood failure ratio of specimens bonded with SM-OL1.5 adhesive is significantly higher than 50 %, confirming the high bonding performance of the OL-modified SM adhesive indirectly.

Another advantage of the study is that the oxidant was removed as much as possible. This prevents the oxidant from causing oxidative degradation of the glued substrate during plywood storage, although high temperatures during plywood preparation may render the oxidant ineffective. Additionally, the presence of an oxidizer reduces the relative content of effective lignin at the same addition amount of cross-linker, thereby decreasing bonding strength. This can be verified by comparing the strength of plywood in Fig. 3(g) and (h). Moreover, the shear strength of plywood prepared with SM-OL containing NaIO₄ decreases significantly with extended storage time. Furthermore, oxidants do not participate in the construction of the adhesive network but increase the risk of their loss, thereby creating environmental risks. The strength of plywood bonded with the SM-OL1.5 after oxidant removal does not decrease but increases even after 30 days of storage. This observation confirms that oxidants can further disrupt the adhesive bonding network, thereby validating the effectiveness of the oxidant removal strategy. Furthermore, we fabricated seven-layer plywood and achieved desired bonding strength, as illustrated in Figures S6 and S7.

Therefore, the proposed bonding mechanism is illustrated in Fig. 4. The OL-modified adhesive can penetrate the wood veneer surface

(verified by the results shown in Figure S8), forming interactions between the adhesive and the cell walls, as confirmed by the optical microscopy results shown in Fig. 4(a), which is in compliance with a previous study (Yang et al., 2023). This confirms the presence of “mechanical interlock” effects. Additionally, AL is activated by oxidation, introducing groups such as aldehyde and quinone groups (verified by the results shown in Fig. 1), which can react with soy protein. For example, (i) Schiff-base formation occurs through the reaction of amino groups in SM with aldehyde groups or quinone groups (Yu et al., 2023). (ii) A Michael addition reaction can occur between the amino groups in the internal structure of the protein and the quinone groups of OL. These reactions not only reduce hydrophilic groups in the adhesive system but also form stable covalent bonds, collectively enhancing cohesion and water resistance of the adhesive (Fig. 4(b)). Moreover, the abundant hydroxyl groups on lignin can form hydrogen bonds with soy protein, and those formed between the polymer chains further contribute to a dense adhesive network. The synergistic effect of hydrogen bond interactions, covalent bonds, and mechanical interlocking ensures the overall performance of the SM-OL adhesive.

## 3.5. Water-resistance of SM-OL adhesive

In order to more clearly demonstrate the water resistance of the SM-OL adhesive, pure SM and SM-OL1.5 adhesives were applied to filter paper, subsequently cured in an oven, and then immersed in hot water at 60°C for a duration of 3 h, as shown in Fig. 5(a). Clearly, the leaching solution for the pure SM adhesive is light brown, indicating that some components were leached out from the adhesive network. In contrast, this was not particularly evident in the leaching solution of the SM-OL1.5 adhesive. Fig. 5(b) presents the hydrolyzed residue ratios of the pure SM and SM-OL1.5 adhesives. This phenomenon is attributed to the weak intermolecular cross-linking of soy protein, which is a typical characteristic of soy protein wood adhesives (Xu et al., 2020). The hydrolyzed residue ratio for the pure SM adhesive is 74 %, which increased to 86 % for the SM-OL1.5 adhesive, representing an increase of 11 %. The findings highlight the enhanced water resistance of the SM-OL1.5 adhesive. Plywood bonded with the control SM adhesive and the SM-OL1.5 adhesive was immersed in hot water (63°C) to evaluate the water resistance of the adhesive bond. The plywood maintained good integrity with no damage, indicating that the soy protein adhesive provided sufficient bonding strength and water resistance, even with the SM adhesive (as shown in Fig. 5(c)). The probable mechanism of water resistance for the SM-OL1.5 adhesive network is proposed in Fig. 5(d). A stable adhesive network is formed through covalent bonds formed by the co-reaction of soy protein and OL during hot-pressing. Moreover, hydrogen bonds inevitably formed between the components of the adhesive matrix promote dense cross-linked network (Wu et al., 2025). The synergistic effect between the cross-linked network formed by the primary and secondary bonds mentioned above enable high cohesive
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 7. (a) Life cycle assessment contributions for 1 kg soy protein adhesive production; (b) Impact values associated with UF, MF, PF, and SM-OL1.5 adhesives synthesis, cradle-to-gate. Functional unit: 1 kg adhesives.</figcaption></figure>

strength of the SM-OL1.5 adhesive, even achieving the desired water-
proof performance.

3.6. *Mildew resistance of the adhesives*

Soy protein wood adhesives often suffer from mildew due to their
nutrient-rich nature, which can compromise bonding strength and shelf
life (Xu et al., 2020). Fig. 7 illustrates that unmodified SM adhesive
degraded quickly, showing significant mold growth already at the third
day. In contrast, SM-OL1.5 adhesive remained mold-free for the first
three days, with only minor mold spots appearing on the fourth day. This
suggests that adding lignin, even in small amounts (about 11 % of the
dry protein mass), enhances the mildew resistance of soybean meal

adhesives. This resistance is inherent to lignin, which can disrupt cell
membranes and alter permeability, causing cell lysis (Liu et al., 2022b).
Moreover, lignin can interact with cellular components, potentially
inhibiting key metabolic processes (Reyes et al., 2024). Utilizing the
natural properties of lignin to improve adhesive performance is an
environmentally friendly and sustainable approach, aligning with
cleaner production goals.

3.7. *Life cycle assessment*

The fabrication of SM-OL1.5 adhesive in this work involves the use of
electricity consumption, OL production, water production, and soybean
meal production. To assess the environmental impacts of these inputs, a
<figure><img src="image_13.png" /><figcaption>**Fig. 8.** (a) and (b) GC-MS chromatograms of VOCs emission of SM and SM-OL1.5 adhesives; (c) A preliminary comparison of cross-linker dosage and wet shear strength between the literature and this study (Chen et al., 2024, 2023; Li et al., 2017; Liu et al. 2023; Luo et al., 2016; Yuan et al., 2017, 2016; J. Zhang et al., 2020; Zhang, 2024; Zhang et al., 2018); (d) and (e) Comparative analysis of the current study with UF, PF, and previously reported bio-based adhesives, focusing on key performance indicators such as dry shear strength, wet shear strength, raw material sustainability, non-formaldehyde emissions, and simple preparation.</figcaption></figure>

life cycle assessment (LCA) contribution analysis was conducted across 10 impact categories (Fig. 7(a) and Table S6). Fig. 7(a) shows that OL has the highest environmental impact, accounting for 68–95 % of the total impact values. Specifically, OL contributes the most significant impact in the fossil fuel depletion category, with a value of 3.42 MJ surplus, representing 95 % of the total impact (Table S6). Electrical energy consumption is a key factor in producing SM-OL1.5 adhesive. The production of oxidized lignin (especially centrifugation and drying) requires significant electrical energy. Despite its low additive proportion (1.5 g), this step accounts for 34 % of the total electrical energy consumed in adhesive production.

Additionally, Fig. 7(b) and Table S7 compare the impact values associated with UF, MF, PF, and SM-OL1.5 adhesives synthesis, cradle-to-gate. SM-OL1.5 adhesive has a lower environmental impact than the other three resins across all categories (Table S7), making it a promising commercial alternative to traditional adhesives. Notably, the global warming (GW) impact of soy protein adhesive (1.61/kg CO₂-eq.)

is significantly lower, reduced by 37 %, 64 %, and 52 % compared to urea formaldehyde resin (2.54/kg CO₂-eq.), melamine formaldehyde resin (4.44/kg CO₂-eq.), and phenolic resin (3.32/kg CO₂-eq.), respectively. Conversely, some commonly used biomass adhesives exhibit a higher GW impact than those in this study (Ahire et al., 2024; Arias et al., 2020). Furthermore, regarding the energy consumption in the production process, SM-OL1.5 adhesives demonstrate significant advantages. The production of these adhesives is straightforward, requiring only a suitable mixer to ensure uniform blending of the raw materials. For instance, the fossil fuel depletion impact of producing 1 kg of SM-OL1.5 adhesive is 3.58 MJ surplus, which is lower than values reported in previous studies (Ahire et al., 2024). These findings underscore the significant advantage of SM-OL1.5 adhesive in reducing overall environmental impacts.
## 3.8. VOCs emission and comparison research of SM-OL1.5 adhesive

Fig. 8(a) and (b) display the VOCs emissions of the control cured SM and SM-OL1.5 adhesives, as assessed by GC-MS chromatography using a method reported in the literature (Liu et al., 2022a) (Supplementary Information). The VOCs emissions of the SM-OL1.5 adhesive are only 18 % higher than those of the SM adhesive, primarily due to the minimal addition of OL. Compared to other protein-based adhesives, the VOCs emissions reported in this study are significantly lower (Liu et al., 2022a; Y. Zhang et al., 2021). Table S8 show SM-OL1.5 adhesive main VOCs have been detected. These results confirm the environmental benefits of the SM-OL1.5 adhesive and indirectly validate the effectiveness of the low cross-linker addition strategy employed in this study.

Fig. 8(c) summarizes a comparative study between SM-OL1.5 and other soy protein-based adhesives, focusing on cross-linker dosage and wet shear strength. The results show that the SM-OL1.5 adhesive offers a significant advantage. Even with a lower cross-linker dosage (11 %) compared to other studies, it achieves ideal bonding properties and water resistance (Table S9). In addition to the environmental benefits highlighted in Fig. 8(a) and (b), the cross-linker also offers a significant cost advantage.

We conducted a comprehensive comparison of SM-OL1.5 adhesives with both petrochemical-based adhesives and biomass adhesives across five key aspects to highlight the production convenience, environmental benefits, and sustainability of this adhesive (Bansode et al., 2023; Shao et al., 2024). Fig. 8(d) and (e) demonstrate that, like other biomass adhesives (soy-based and tannin-based adhesives), the SM-OL1.5 adhesive exhibits non-formaldehyde release characteristics. The bio-sourced lignin used as a cross-linker in this study aligns more closely with the idea of green adhesives compared to others. Moreover, it achieves the desired bonding strength and water resistance.

## 4. Conclusions

In summary, an environmentally friendly soy protein wood-adhesive was successfully developed using soybean meal and oxidized alkaline lignin as key components that inspired by the concept of phenolic-amine chemistry. The oxidation of lignin with NaIO₄ introduced aldehyde/ quinone functionalities, enabling strong interactions with soy protein by Schiff-base formation and Michael addition reaction to form a hybridized adhesive with enhanced water resistance and mechanical performance. The meticulous removal of residual NaIO₄ minimized its environmental impact and ensured the safety of the bonded substrates, addressing potential hazards associated with chemical residues. The results showed that the dry and wet shear strength of SM-OL1.5 increased by 51.2 % and 80.8 % to 1.83 MPa and 0.94 MPa compared with unmodified SM adhesives. Furthermore, the adhesive exhibited low VOC emissions and low environmental impact. However, the SM-OL1.5 displayed an enhanced mildew-resistance performance (from 2 d to 4 d), but there is still considerable room for improvement. This straightforward and efficient adhesive preparation method not only establishes a viable eco-friendly strategy for developing multifunctional soy-based adhesives but also demonstrates considerable potential for scalable industrial adoption.

### CRediT authorship contribution statement

**Xiaojian Zhou:** Writing – review & editing, Funding acquisition.
**Xinyi Chen:** Writing – review & editing, Funding acquisition.
**Jiamin Wu:** Validation, Software.
**Jun Zhang:** Investigation, Formal analysis.
**Tianguo Wen:** Validation, Methodology.
**Yongli Yang:** Writing – original draft, Investigation.
**Haizhu Wu:** Methodology, Investigation.
**Hisham Essawy:** Writing – review & editing, Data curation.
**Antonio Pizzi:** Writing – review & editing, Supervision.
**Guanben Du:** Writing – review & editing, Supervision.
**Bertrand Charrier:** Writing – review & editing, Validation.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgement

This work was financially supported by the Major Science and Technology Project of Yunnan Province (Grant No. 202402AE090027) and the National Natural Science Foundation of China (NSFC 32471797). This research was funded by the “Xingdian Talents Support Plan” Yunling Scholar and Youth Talent Programs, the 111 Project (D21027), National High-end Foreign Expert Project (Grant No. G2023039003L), Yunnan Provincial Expert Workstation (202305AF150006), and Yunnan Foreign Experts Project (202505A0120006).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.indcrop.2025.121571.

### Data availability

Data will be made available on request.

### References

Ahire, J.P., Mousavi-Avval, S.H., Rajendran, N., Bergman, R., Runge, T., Jiang, C., Hu, J., 2024. Techno-economic and life cycle analyses of bio-adhesives production from isolated soy protein and kraft lignin. J. Clean. Prod. 447, 141474. https://doi.org/10.1016/j.jclepro.2024.141474.

Alkan, Ü.B., Kızılcan, N., Bengü, B., 2022. Urea glyoxal and urea melamine glyoxal wood adhesives hardened with acid ionic liquid for particleboard pressing. Eur. J. Wood Prod. 80, 961–973. https://doi.org/10.1007/s00107-022-01811-9.

Alkan, Ü.B., Kızılcan, N., Bengü, B., 2025. Lignosulfonate and glycidyl ether modified urea formaldehyde wood adhesives for interior particleboard production. PRT 54, 116–124. https://doi.org/10.1108/PRT-08-2023-0076.

Arias, A., González-García, S., González-Rodríguez, S., Feijoo, G., Moreira, M.T., 2020. Cradle-to-gate Life Cycle Assessment of bio-adhesives for the wood panel industry. A comparison with petrochemical alternatives. Sci. Total Environ. 738, 140357. https://doi.org/10.1016/j.scitotenv.2020.140357.

Bansode, A., Villarreal, L.A.P., Wang, Y., Asafu-Adjaye, O., Via, B.K., Farag, R., Erramuspe, I.B.V., Auad, M.L., 2023. Kraft Lignin Periodate Oxidation for Biobased Wood Panel Resins. ACS Appl. Polym. Mater. 5, 4118–4126. https://doi.org/10.1021/acsapm.3c00324.

Boeriu, C.G., Bravo, D., Gosselink, R.J.A., Van Dam, J.E.G., 2004. Characterisation of structure-dependent functional properties of lignin with infrared spectroscopy. Ind. Crops Prod. 20, 205–218. https://doi.org/10.1016/j.indcrop.2004.04.022.

Chen, H., Wu, Q., Ren, X., Zhu, X., Fan, D., 2024. A fully bio-based adhesive with high bonding strength, low environmental impact, and competitive economic performance. Chem. Eng. J. 494, 153198. https://doi.org/10.1016/j.cej.2024.153198.

Chen, S., 2023. Investigation of humidity-heat aging resistance of a soy protein adhesive fabricated by soybean meal and lignin-based polymer. Polym. Test. 120, 107971. https://doi.org/10.1016/j.polymtesting.2023.107971.

Chen, S., Chen, H., Yang, S., Fan, D., 2021. Developing an antifungal and high-strength soy protein-based adhesive modified by lignin-based polymer. Ind. Crops Prod. 170, 113795. https://doi.org/10.1016/j.indcrop.2021.113795.

Chen, S., Aladejana, J.T., Li, X., Bai, M., Shi, S.Q., Kang, H., Cao, J., Li, J., 2023. A strong, antimildew, and fully bio-based adhesive fabricated by soybean meal and dialdehyde chitosan. Ind. Crops Prod. 194, 116277. https://doi.org/10.1016/j.indcrop.2023.116277.

Chen, X., Xi, X., Pizzi, A., Fredon, E., Du, G., Gerardin, C., Amirou, S., 2021. Oxidized demethylated lignin as a bio-based adhesive for wood bonding. J. Adhes. 97, 873–890. https://doi.org/10.1080/00218464.2019.1710830.

Esen, E., Meier, M.A.R., 2020. Sustainable functionalization of 2,3-dialdehyde cellulose via the passerini three-component reaction. ACS Sustain. Chem. Eng. 8, 15755–15760. https://doi.org/10.1021/acssuschemeng.0c06153.

Gosselink, R.J.A., Van Dam, J.E.G., De Jong, E., Gellerstedt, G., Scott, E.L., Sanders, J.P. M., 2011. Effect of periodate on lignin for wood adhesive application. Holzforschung 65, 155–162. https://doi.org/10.1515/hf.2011.025.

Gu, W., Liu, X., Ye, Q., Gao, Q., Gong, S., Li, J., Shi, SheldonQ., 2020. Bio-inspired deposition strategy of aramid fibers to improve performance of soy protein isolate-based adhesive. Ind. Crops Prod. 150, 112424. https://doi.org/10.1016/j.indcrop.2020.112424.
Hao, Z., Xi, X., Hou, D., Lei, H., Li, C., Xu, G., Du, G., 2023. A fully bio-based soy protein wood adhesive modified by citric acid with high water tolerance. Int. J. Biol. Macromol. 253, 127135. https://doi.org/10.1016/j.ijbiomac.2023.127135.

Hu, Y., 2024. An environmentally friendly multifunctional soy protein adhesive composed of rosin acid derivative inspired from organic-inorganic hybrid structure. J. Clean. Prod. 449, 141847. https://doi.org/10.1016/j.jclepro.2024.141847.

Huang, C., Peng, Z., Li, J., Li, X., Jiang, X., Dong, Y., 2022. Unlocking the role of lignin for preparing the lignin-based wood adhesive: a review. Ind. Crops Prod. 187, 115388. https://doi.org/10.1016/j.indcrop.2022.115388.

Ibrahim, V., Mamo, G., Gustafsson, P.-J., Hatti-Kaul, R., 2013. Production and properties of adhesives formulated from laccase modified Kraft lignin. Ind. Crops Prod. 45, 343-348. https://doi.org/10.1016/j.indcrop.2012.12.051.

Jiang, K., Dong, X., Chen, Y., Fan, D., Chu, F., 2024. A room-temperature curing plant protein-based adhesive with high strength and flame retardancy for heat-free adhesion. Adv. Funct. Mater. 34, 2403490. https://doi.org/10.1002/adfm.202403490.

Jiang, P., Li, Q., Gao, C., Lu, J., Cheng, Y., Zhai, S., An, Q., Wang, H., 2019. Fractionation of alkali lignin by organic solvents for biodegradable microsphere through self-assembly. Bioresour. Technol. 289, 121640. https://doi.org/10.1016/j.biortech.2019.121640.

Jin, S., Li, K., Gao, Q., Zhang, W., Chen, H., Li, J., Shi, S.Q., 2020. Multiple crosslinking strategy to achieve high bonding strength and antibacterial properties of double-network soy adhesive. J. Clean. Prod. 254, 120143. https://doi.org/10.1016/j.jclepro.2020.120143.

Li, K., Li, X., Luo, J., Li, Jingjing, Gao, Q., Li, Jianzhang, 2017. A highly water-resistant soy-based bioadhesive with 1,4-butanediol diglycidyl ether and its application on plywood, 5, 31-38. https://doi.org/10.7569/JRM.2017.634131.

Li, X., Shao, J., Chen, S., Bian, Y., Chen, H., Cao, J., Li, Jiongjiong, Li, Jianzhang, 2024. Enzyme-induced "Sea-Island" structure reinforced bio-based adhesive: toward high bonding strength, toughness, and bacterial resistance. ACS Mater. Lett. 6, 3794-3803. https://doi.org/10.1021/acsmaterialslett.4c00811.

Liu, H., Li, C., Sun, X.S., 2015. Improved water resistance in undecylenic acid (UA)-modified soy protein isolate (SPI)-based adhesives. Ind. Crops Prod. 74, 577-584. https://doi.org/10.1016/j.indcrop.2015.05.043.

Liu, X., Zhao, G., Ren, S., Fang, G., Chen, Z., Li, S., 2018. Zirconia-supported 12-tung-stophosphoric acid: characterization and catalytic behavior in degradation of alkali lignin. Ind. Crops Prod. 125, 496-504. https://doi.org/10.1016/j.indcrop.2018.09.035.

Liu, X., Bian, Y., Zhang, X., Liu, Z., Weng, T., Wang, G., Li, J., Chen, H., Gao, Q., 2023. Preparation of strong and mildew-resistant soybean meal adhesives with self-assembled core-shell structured nanospheres. Ind. Crops Prod. 205, 117446. https://doi.org/10.1016/j.indcrop.2023.117446.

Liu, Z., Chen, M., Xu, Y., Zhang, J., Huang, X., Luo, J., Li, J., Shi, S.Q., Gao, Q., 2022a. Preparation of a strong and multiple-function soybean flour adhesive via the construction of tannin microspheres with a core-shell structure. Composites Part B Engineering 242, 110114. https://doi.org/10.1016/j.compositesb.2022.110114.

Liu, Z., Liu, T., Li, Y., Zhang, X., Xu, Y., Li, J., Gao, Q., 2022b. Performance of soybean protein adhesive cross-linked by lignin and cuprum. J. Clean. Prod. 366, 132906. https://doi.org/10.1016/j.jclepro.2022.132906.

Luo, Jing, Luo, Jianlin, Yuan, C., Zhang, W., Li, J., Gao, Q., Chen, H., 2015. An eco-friendly wood adhesive from soy protein and lignin: performance properties. RSC Adv. 5, 100849-100855. https://doi.org/10.1039/C5RA19232C.

Luo, Jing, Luo, Jianlin, Zhang, J., Bai, Y., Gao, Q., Li, J., Li, L., 2016. A new flexible soy-based adhesive enhanced with neopentyl glycol diglycidyl ether: properties and application. Polymers 8, 346. https://doi.org/10.3390/polym8090346.

Pradyawong, S., Qi, G., Li, N., Sun, X.S., Wang, D., 2017. Adhesion properties of soy protein adhesives enhanced by biomass lignin. Int. J. Adhes. Adhes. 75, 66-73. https://doi.org/10.1016/j.ijiadhadh.2017.02.017.

Pradyawong, S., Qi, G., Sun, X.S., Wang, D., 2019. Laccase/TEMPO-modified lignin improved soy-protein-based adhesives: adhesion performance and properties. Int. J. Adhes. Adhes. 91, 116-122. https://doi.org/10.1016/j.ijiadhadh.2019.03.005.

Qian, Y., Deng, Y., Li, H., Qiu, X., 2014. Reaction-free lignin whitening via a self-assembly of acetylated lignin. Ind. Eng. Chem. Res. 53, 10024-10028. https://doi.org/10.1021/ie5010338.

Reyes, D.C., Ma, Z., Romero, J.J., 2024. The antimicrobial properties of technical lignins and their derivatives—a review. Polymers 16, 2181. https://doi.org/10.3390/polym16152181.

Schutyser, W., Kruger, J.S., Robinson, A.M., Katahira, R., Brandner, D.G., Cleveland, N. S., Mittal, A., Peterson, D.J., Meilan, R., Román-Leshkov, Y., Beckham, G.T., 2018. Revisiting alkaline aerobic lignin oxidation. Green. Chem. 20, 3828-3844. https://doi.org/10.1039/C8GC00502H.

Shao, J., Li, X., Liu, T., Chen, S., Gong, S., Cao, J., Zhou, W., Li, C., Li, J., 2024. Multiple function hyperbranched polysiloxane nanoclusters for controlling a cross-linking structure to convert soy meal into a strong, tough, and multifunctional adhesive. ACS

Sustain. Chem. Eng. 12, 3966-3976. https://doi.org/10.1021/acssuschemeng.3c06212.

Wang, H., Zhou, J., Sun, L., Lai, J., Zheng, X., Lu, B., Zhang, L.-M., Zhang, W., 2024. Constructing multiple bonding mechanisms with a novel lignin-activated modified polymer core for the fabrication of strong, water-resistant, and mildew-resistant adhesives. Ind. Crops Prod. 222, 119536. https://doi.org/10.1016/j.indcrop.2024.119536.

Wu, H., Duan, Z., Huang, H., Yang, R., Hong, M., Du, G., Charrier, B., Essawy, H., Pizzi, A., Zhou, X., Chen, X., 2025. Tannin-based non-isocyanate polyurethane wood adhesive rich in complicated cross-linked networks with ultra-strong bonding property and excellent water resistance by introducing poly-urea structure. Ind. Crops Prod. 223, 120177. https://doi.org/10.1016/j.indcrop.2024.120177.

Xi, X., Pizzi, A., Gerardin, C., Chen, X., Amirou, S., 2020. Soy protein isolate-based polyamides as wood adhesives. Wood Sci. Technol. 54, 89-102. https://doi.org/10.1007/s00226-019-01141-9.

Xiao, Z., Li, Y., Wu, X., Qi, G., Li, N., Zhang, K., Wang, D., Sun, X.S., 2013. Utilization of sorghum lignin to improve adhesion strength of soy protein adhesives on wood veneer. Ind. Crops Prod. 50, 501-509. https://doi.org/10.1016/j.indcrop.2013.07.057.

Xin, J., Zhang, P., Wolcott, M.P., Zhang, J., Hiscox, W.C., Zhang, X., 2017. A novel and formaldehyde-free preparation method for lignin amine and its enhancement for soy protein adhesive. J. Polym. Environ. 25, 599-605. https://doi.org/10.1007/s10924-016-0844-x.

Xu, C., Xu, Y., Chen, M., Zhang, Y., Li, J., Gao, Q., Shi, S.Q., 2020. Soy protein adhesive with bio-based epoxidized daidzein for high strength and mildew resistance. Chem. Eng. J. 390, 124622. https://doi.org/10.1016/j.cej.2020.124622.

Xu, Y., 2021. Constructing a triple network structure to prepare strong, tough, and mildew resistant soy protein adhesive. Compos. Part B 211, 108677. https://doi.org/10.1016/j.compositesb.2021.108677.

Yang, G., Gong, Z., Luo, X., Chen, L., Shuai, L., 2023. Bonding wood with uncondensed lignins as adhesives. Nature 621, 511-515. https://doi.org/10.1038/s41586-023-06507-5.

Younesi-Kordkheili, H., Pizzi, A., 2023. Lignin-based wood adhesives: a comparison between the influence of soda and Kraft lignin. Int. J. Adhes. Adhes. 121, 103312. https://doi.org/10.1016/j.ijiadhadh.2022.103312.

Yu, C., Wang, M., Zhang, D., Liang, K., Zhou, M., Fu, J., Dong, Y., Chen, C., Cai, L., Xia, C., Wang, Q., 2025. Mussel-inspired robust and waterproof soybean protein adhesives enhanced with phenolated lignosulfonate for wood bonding. Int. J. Biol. Macromol. 314, 144419. https://doi.org/10.1016/j.ijbiomac.2025.144419.

Yu, Z., Li, Q., He, X., Wang, X., Wen, Y., Zeng, L., Yu, W., Hu, P., Chen, H., 2023. A multifunctional hydrogel based on nature polysaccharide fabricated by Schiff base reaction. Eur. Polym. J. 197, 112330. https://doi.org/10.1016/j.eurpolymj.2023.112330.

Yuan, C., Luo, Jing, Luo, Jianlin, Gao, Q., Li, J., 2016. A soybean meal-based wood adhesive improved by a diethylene glycol diglycidyl ether: properties and performance. RSC Adv. 6, 74186-74194. https://doi.org/10.1039/C6RA07844C.

Yuan, C., Chen, M., Luo, J., Li, X., Gao, Q., Li, J., 2017. A novel water-based process produces eco-friendly bio-adhesive made from green cross-linked soybean soluble polysaccharide and soy protein. Carbohydr. Polym. 169, 417-425. https://doi.org/10.1016/j.carpol.2017.04.058.

Zhang, H., Fu, S., Chen, Y., 2020. Basic understanding of the color distinction of lignin and the proper selection of lignin in color-dependent utilizations. Int. J. Biol. Macromol. 147, 607-615. https://doi.org/10.1016/j.ijbiomac.2020.01.105.

Zhang, J., Zhang, M., Zhang, Y., Shi, S.Q., Zhan, X., Li, J., Luo, J., Gao, Q., 2021. Improving bond performance and reducing cross-linker dosage for soy flour adhesives inspired by spider silk. ACS Sustain. Chem. Eng. 9, 168-179. https://doi.org/10.1021/acssuschemeng.0c06333.

Zhang, J., Long, C., Zhang, Xin, Liu, Z., Zhang, Xilin, Liu, T., Li, J., Gao, Q., 2022. An easy-coating, versatile, and strong soy flour adhesive via a biomimetic structured combined with a biomimetic brush-like polymer. Chem. Eng. J. 450, 138387. https://doi.org/10.1016/j.cej.2022.138387.

Zhang, W., Liu, C., Du, Z., Wang, H., Du, G., Essawy, H., Lei, H., Xi, X., Zhou, X., Cao, M., 2024. Soybean meal-oxidized lignin as bio-hybridized wood panel adhesives with increased water resistance. Forests 15, 1036. https://doi.org/10.3390/f15061036.

Zhang, X., 2024. A bionic strong nanostructured soy protein-based adhesive enabled antistatic and self-extinguishing wood-based composites. Sustain. Mater. Technol. 40, e00979. https://doi.org/10.1016/j.susmat.2024.e00979.

Zhang, Y., Zhang, M., Chen, M., Luo, J., Li, X., Gao, Q., Li, J., 2018. Preparation and characterization of a soy protein-based high-performance adhesive with a hyperbranched cross-linked structure. Chem. Eng. J. 354, 1032-1041. https://doi.org/10.1016/j.cej.2018.08.072.

Zhang, Y., Liu, Z., Xu, Y., Li, Jingchao, Shi, S.Q., Li, Jianzhang, Gao, Q., 2021. High performance and multifunctional protein-based adhesive produced via phenol-amine chemistry and mineral reinforcement strategy inspired by arthropod cuticles. Chem. Eng. J. 426, 130852. https://doi.org/10.1016/j.cej.2021.130852.