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Review

A Narrative Review: Modification of Bio-Based Wood Adhesive for Performance Improvement

by
Caizhi Yu
1,2,
Yi Chen
1,2,
Renjie Li
1,2,
Jun Jiang
1,2,* and
Xiang Wang
3,*
1
Co-Innovation Center of Efficient Processing and Utilization of Forestry Resources, Nanjing Forestry University, Nanjing 210037, China
2
College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, China
3
Guangdong Province Key Laboratory of Durability for Marine Civil Engineering, College of Civil and Transportation Engineering, Shenzhen University, Shenzhen 518060, China
*
Authors to whom correspondence should be addressed.
Coatings 2024, 14(9), 1153; https://doi.org/10.3390/coatings14091153
Submission received: 30 July 2024 / Revised: 4 September 2024 / Accepted: 5 September 2024 / Published: 7 September 2024
(This article belongs to the Special Issue Surface Modification and Strengthening of Bio-Based Materials)
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Abstract

:
Most traditional adhesives applied in the wood industry are synthetic resins obtained from petroleum. However, the production of these resins raises substantial environmental issues because of formaldehyde release, which leads to detrimental impacts on both human health and the environment. In contrast, bio-based adhesives offer an eco-friendly option that is created by renewable biomass resources. These adhesives can effectively overcome the above problems. Hence, it is crucial to pay more attention to bio-based adhesives. However, the inherent characteristics of the raw materials used in the production of bio-based adhesives result in a number of limitations, including weak bond strength, poor water resistance, and susceptibility to mildew, which restrict their further applications. Most researchers have used physical and chemical methods to modify bio-based adhesives in order to improve their overall performance. The defects of bio-based adhesives, including their limited bond strength, inadequate resistance to water, and vulnerability to mildew, are summarized in this paper, and the investigation of potential modification methods on bio-based adhesives is reviewed. Moreover, we encourage the widespread use of bio-based adhesives in various fields to promote sustainable development due to their eco-friendly characters.

1. Introduction

Currently, the wood industry is the main purchaser of traditional adhesives, making up more than 65% of the whole production of these products [1]. Adhesives are mainly based on fossil-based resins, which are urea-formaldehyde (UF), phenol-formaldehyde (PF), and melamine-formaldehyde (MF). We have carefully considered your comments on the price of resin-based adhesives and have revised this section of the manuscript to make it more rigorous. These adhesives are efficient with outstanding performance in terms of bond strength, water resistance, and mildew resistance [2]. Nevertheless, these adhesives are limited by their significant reliance on fossil fuels and the tendency to release formaldehyde, which is harmful to human health [3,4,5]. Furthermore, the resins possess inherent resistance to degradation, which might lead to environmental issues. With the improvement of living conditions and increasing environmental awareness, there is a growing demand for adhesives that are ecologically benign with minimal formaldehyde emissions. Hence, it is interesting to study bio-based adhesives that are generated from agricultural and forestry crops. Commonly available bio-based adhesives can be categorized into four primary groups: protein-based adhesives, lignin-based adhesives, tannin-based adhesives, and polysaccharide adhesives [6,7]. The raw materials of these adhesives are derived from diverse sources, the majority of which can be subject to microbial degradation, thereby preventing the release of free aldehydes during applications [8,9]. Among the various bio-based adhesives, soy protein-based adhesive has garnered significant attention in research due to its abundant raw material resources, cost-effectiveness, renewable nature, strong adhesion properties, and other notable features.
Bio-based adhesives utilize a diverse array of raw resources and possess a certain level of economic viability. Bio-based adhesive, a type of environmentally friendly adhesive, will have a significant impact on energy conservation and emission reduction in the future [10,11]. It has the potential to greatly contribute to the sustainable development of the wood-based panel industry [12]. As a result, bio-based adhesives possess a comparative advantage over conventional adhesives with regard to their environmental sustainability [13]. Bio-based adhesives have the potential for development, but they have several limitations. The presence of an overabundance of reactive groups in the raw materials could cause poor stability of the product, poor water resistance, and susceptibility to mildew of bio-based adhesives [14,15]. Therefore, it is necessary to pay more attention to the physicochemical modification of bio-based adhesives to further enhance their performances for their large-scale applications. To address the aforementioned shortcomings of bio-based adhesives, a variety of methods have been employed, including chemical modification techniques such as cross-linking, oxidation, grafting, and esterification [16], as well as physical modification methods such as co-mixing treatment, ultrasonication, and high-pressure treatment [17]. This review provides a comprehensive analysis of the modification approaches for bio-based adhesives, focusing on modification principles and effects on property enhancement. After that, support is provided for further study on effective modification methods for bio-based adhesives. Furthermore, this study summarizes the issues and drawbacks in the modification methods, aiming to propose a direction for the better utilization of bio-based adhesives.

2. Improvement of Bond Strength Performance

2.1. Protein-Based Adhesives

The bonding mechanism of pure soy protein-based adhesives is based on the entanglement of protein molecules and the formation of intermolecular hydrogen bonds with molecules in the wood under high temperature and pressure conditions [18,19]. However, due to the weak bonding of hydrogen bonds and their affinity for water, they can be easily disrupted in a moist condition, leading to a diminished strength of bonding [20]. To overcome this limitation, various methods have been employed to enhance the bond strength, including physical modification, chemical modification, and biomass polymer blend modification. The weak intermolecular interactions are converted into stable chemical bonds through chemical reactions between reactive groups in protein molecules and other molecules, thereby enhancing the bond strength of protein-based adhesives [21].
Li et al. [22] constructed a multifunctional adhesive based on high-performance soybean meal (SM) using chitosan-functionalized boron nitride nanosheets (CS@BNNSs) instead of β-sheet nanocrystals to mimic the structural properties of natural spider silk. The prepared MSM (TGA-modified SM adhesive)/CS@BNNSs adhesive showed a significant improvement in mechanical properties with increased dry shear strength (1.96 MPa vs. 1.42 MPa) compared to pure cross-linker-modified soy protein adhesive. A significant factor contributing to the exceptional performance is the formation of numerous covalent and non-covalent intermolecular contacts between the CS@BNNSs and the protein chains. And the good compatibility between the CS@BNNSs and the SM matrix favored energy dissipation and transfer during the loading process. Zhang et al. [23] employed high-pressure homogenization to process soy protein isolate, which led to a 62% reduction in the particle size of soybean proteins. Consequently, the particle size distribution uniformity increased. In addition, high-pressure homogenization exposed the active functional groups within the soy protein, which increased the reactivity of the functional groups with the cross-linking agent (triglycidylamine). The exposed functional groups facilitated the formation of covalent complexes with tighter structures in the adhesive system. The results showed that the bond strength of the treated adhesive was increased by 212% compared to the unmodified adhesive. Pang et al. [24] examined the alteration of adhesive properties in high-temperature defatted soy flour protein (containing 43%–48% of proteins) using cationic non-covalent bonding forces inspired by the strong cationic contacts found in mussels. The adhesive system was enhanced by incorporating highly active polyamide epichlorohydrin (PAE) and folic acid (FA), resulting in the formation of a dual network system stabilized by strong cationic interactions. This modification successfully enhanced the adhesive’s pre-compression and bond strength. When employing this method for modification, it is crucial to take precautions because applying liquid PAE could irritate the skin. Pang et al. [25] discovered that dopamine-decorated silk fiber (PSF) inspired by mussels and waterborne epoxy emulsions (WEU) worked in concert to enhance the mechanical characteristics of SPI-based film. Using various physical and chemical combinations of WEU, PSF, and soy protein matrices, a robust crosslinking network was successfully established in films based on SPI.

2.2. Lignin-Based Adhesives

Lignin, to partially replace phenolic resin and urea-formaldehyde resin adhesives, can also be used directly as a raw material to produce adhesive [26,27]. To expand the utilization of lignin, it is crucial to enhance the bond strength of lignin-based adhesives. Typical techniques for enhancing the strength of lignin-based adhesives involve incorporating functional groups like phenolic hydroxyl and methoxy groups into lignin, creating various network structures through cross-linking processes [28].
Yang et al. [29] synthesized phenol-lignin by phenol modification, and super-strong trimer adhesives were prepared from phenol-lignin and commercially available tris (ethylene glycol) divinyl ether (DVE-3). Phenol-lignin shows better stiffness due to the presence of phenolic hydroxyl groups. As shown in Figure 1, the combination of phenol-lignin and DVE-3 improved the microphase separation of the rigid segment of the phenol-lignin, which was conducive to improving the cohesive strength of the adhesive. The wood shear strength produced by phenol-lignin-based adhesive was 12.87 MPa. Gong et al. [30] prepared a high-performance formaldehyde-free lignin adhesive by mixing phenol-modified lignin with a biocompatible aqueous solution of polyvinylpyrrolidone. It took advantage of the phenolic hydroxyl and methoxy groups on the benzene ring, which can undergo further cross-linking reactions. Another important reason is the presence of dipole forces between the polyvinylpyrrolidone units. The bond strength of lignin-based adhesives treated by this method is much higher than that of commercially available phenolic resin adhesives. Another efficient modification method is to use lignin as a substrate to improve the adhesive properties of lignin-based adhesives by constructing multiple network structures. Zhang et al. [31] enhanced the existing lignin adhesive by incorporating furfuryl alcohol, epoxy resin, and glyoxal for cross-linking. This modification resulted in the formation of a multiple network structure, leading to stronger bonding performance. In comparison with other adhesives, the adhesive produced by mixing lignin adhesive with 9% epoxy resin had a better bond strength than that of lignin-furfuryl alcohol adhesive. Also, the modulus of elasticity was higher than that of lignin-furfuryl alcohol and phenol-formaldehyde adhesives. However, the high prices of furfuryl alcohol and epoxy resin make this modification method not economical enough. Liu et al. [32] successfully augmented the phenolic hydroxyl content of lignin obtained from corn kernels through aldolization and phenolization. This resulted in a notable enhancement of the hydrogen bonding force between pure poly (vinyl alcohol) (PVA) adhesives and wood. Consequently, the bond strength of the PVA adhesives was significantly improved. The wood lap shear strength is greatly enhanced using PVA/resorcinol-lignin, with a measured strength of 6.27 MPa, showing a remarkable improvement of 77.6%.

2.3. Tannin-Based Adhesives

Li et al. [33] broke down acacia tannins (AMT) into smaller molecules in an acidic environment by employing 2-methylfuran as a reactive substance. As shown in Figure 2, the depolymerized tannins (DAMT) were combined with polyethyleneimine (PEI) to prepare tannin-based phenolic resin adhesives. Due to the high reactivity of DAMT, the prepared PEI and DAMT-modified phenolic resin adhesives (DTPF-PEI) showed a high degree of cross-linking and three-dimensional network structure. The results showed that the bond strength of the modified adhesives increased by 63.6%. The high performance of the adhesives can be attributed to the highly cross-linked interactions and the stable three-dimensional network structure through the hydrogen bonds and cation-π between the catechol structure and the adherent wood surface. However, the incorrect use of 2-methylfuran in the modification process of this method can cause harm to the human body. Chen et al. [34] prepared a high-performance adhesive using condensed tannin-functionalized boron nitride nanosheets, amino chitosan, and soybean meal as the main raw materials. The resulting adhesive showed a significant improvement in mechanical strength and toughness, including a dry shear strength of 1.67 MPa. This study provided a green and low-cost strategy for the preparation of high-performance bio-based adhesives.

2.4. Polysaccharide-Based Adhesives

Polysaccharides are a class of multifunctional macromolecules in which monosaccharides are combined by glycosidic bonds, and they are characterized by wide sources and degradability [35]. Polysaccharides have promising potential for creating bio-based adhesives. Currently, the primary focus is on addressing the limitations of polysaccharide-based adhesives during application, such as inadequate sizing performance and low strength [36]. Starch is a plentiful and inexpensive polymer that occurs naturally. It is renewable, meaning it can be replenished, and it is also biodegradable. Starch is one of the most prevalent natural polymers, second only to cellulose, in terms of its concentration. Several researchers have solved the limitations by altering and pre-treating adhesives that are based on starch [37,38].
Grylewicz et al. [39] created thermoplastic starch (TPS) composites using potato starch, wood fibers (WF) with a maximum content of 41.5 wt%, and deep eutectic solvents (DES) consisting of choline chloride with either urea or glycerol (Gl), as well as imidazole (Im) with Gl. DES can serve as a starch plasticizer, a WF surface modification, and an enhancer of interfacial adhesion in composite components. The composites with Im/Gl had the highest TPS/WF performance, with a tensile strength of up to 11 MPa. Antinio et al. [40] conducted a study on the utilization of wood pulp as a means of strengthening thermoplastic starch. The composites were fabricated using conventional corn flour plasticized with glycerol in the presence of fiber. The matrix compositions consisted of starch and glycerol in weight ratios of 70/30, 80/20, and 90/10. The weight percentage of wood pulp fiber was adjusted between 5% and 15%. The use of fibers resulted in a significant enhancement of the tensile strength. The specific strength modification effects are shown in Table 1.

3. Improvement of Water Resistance

The qualities of a polymer are determined by its structure. Specifically, the ability to resist water is generally linked to the polymer’s three-dimensional arrangement and the amount of hydrophilic functional groups it contains [41]. When organic adhesives are cured, the molecules form irreversible crystals or cross-link into a three-dimensional network structure, which is necessary for their excellent wet strength [42].

3.1. Protein-Based Adhesives

Reactive groups in protein molecules include carboxyl, amino, and hydroxyl groups. These groups are hydrophilic and easily react with polar molecules to form hydrophilic regions [43]. Therefore, protein-based adhesives have the disadvantage of poor water resistance due to their structure [44]. The improvement in water resistance of protein-based adhesives is achieved by building a multiple network structure or forming stable and water-resistant chemically interacting bonds through chemical cross-linking. The loose structure inside the protein becomes tight and prevents water molecules from penetrating. During the reaction process, amino acid residues and peptide chains are modified, which increases the bond strength of protein-based adhesives and improves their water resistance at the same time [45].
At the beginning of the 20th century, the concept of combining mussel adhesive proteins with green soy proteins was proposed as a promising method to enhance the water resistance of the adhesive at a lower cost. Zhao et al. [46] synthesized dopamine-functionalized polyurethane (D-PU) elastomers as a bio-inspired crosslinking unit to improve the performance of soy protein (SP) adhesives, in Figure 3. The catechol group in D-PU not only promoted the interaction between the polyurethane and SP matrix to form a stable cross-linked network with excellent load-bearing capacity but also acted as a hydrophobic barrier to reduce moisture erosion of the resin. The catechol group in D-PU played a dual role in improving the performance of the adhesives. Firstly, it facilitated the interaction between the polyurethane and the SP matrix. Secondly, it acted as a hydrophobic barrier, preventing the attack of moisture on the resin. Furthermore, the introduction of D-PU resulted in the formation of a micro-phase separation morphology within the continuous protein phase. This micro-phase separation structure simultaneously enhanced the strength and toughness of the adhesive layer, facilitating efficient energy consumption and stress transfer during the loading process and expediting the formation of the physical interactions. In comparison to unmodified SP adhesives, SP-D-PU adhesives demonstrated a 133.9% enhancement in wet bond strength. Wang et al. [47] developed a mussel-inspired co-deposition process to synthesize soy protein (SPI)/CN composite adhesive nanostructured layers with good water repellency on the surface of cellulose nanofibers (CNF). This study extended the application of renewable functional biopolymers in plant protein-based binder systems to improve the bonding performance and water-repellent interface. Averina et al. [43] prepared protein-based adhesives from natural corn gluten protein, potato protein, and pea isolate protein using glyoxal and polyethyleneimine (PEI) as crosslinkers. The results showed that the cross-linking of glyoxal and polyethyleneimine could effectively improve the wet strength of protein-based adhesive. Averina et al. [48] developed proteins obtained as side-products from starch production (potato and corn proteins) for wood adhesive application. Glyoxal was introduced as a crosslinking agent to enhance the wet strength of protein-based adhesives. This study revealed that the pH level has a more pronounced impact on wet strength than solid content and protein-to-crosslinker ratio. The wet strength of potato and maize proteins, when crosslinked with glyoxal, reached its maximum at an acidic pH range. Pang et al. [49] successfully created a bio-based carboxymethylated wood fibers (CMWFs) soy meal-based adhesive with exceptional performance characteristics. Carboxymethylation was employed as a pre-treatment method to impart wood fibers (WFs) with desired properties. CMWFs containing abundant carboxyl groups served as a multiple “crosslinking core” and effectively cross-linked with soy protein sidechain, resulting in the construction of a stable adhesive system. The adhesive formulated with modified soy meal demonstrated exceptional adhesive strength and efficacy in resisting water. Specifically, the tension tests revealed that the adhesive modified with CMWFs exhibited a water-resistant bond strength of 1.69 MPa, which was 160% greater than that of the original adhesive.

3.2. Lignin-Based Adhesives

Compared to adhesives modified with lignin, there are relatively few adhesives that directly utilize lignin as an adhesive material. This is mainly because adhesives synthesized directly from lignin have poor water resistance and do not meet the required standard for use [50]. In addition, the dark color of lignin-based adhesives affects their popularity and application [51]. The common methods used to improve the water resistance of lignin include chemical treatment, phenolization, and the construction of multiple network structures.
Huo et al. [52] blended the synthesized waterborne lignin-based epoxy resin emulsion with polyamide to obtain a high-performance formaldehyde-free adhesive. The plywood prepared with the resulting adhesive exhibited good bonding properties, especially water resistance, which was much higher than the requirements of the Chinese national standard for Class I plywood for outdoor use. Wei et al. [53] found that lignosulfonates could be used as a cheaper raw material instead of catechol, based on previous work on catechol’s enhanced adhesion properties. They utilized polyamide epichlorohydrin to electrostatically complex with lignosulfonate to produce a lignin adhesive with high wet strength, which was fluid but insoluble in water; thus, the modified lignin adhesive was more widely used. Wang et al. [54] prepared a lignin-based epoxy adhesive using lignosulfonate as the raw material, and the tensile shear strength of the lignin-based epoxy adhesive was still as high as 9.30 MPa after immersion in boiling water for 12 h. The high temperature and high humidity environment did not have much effect on the performance of the lignin-based epoxy adhesive. Also, this study provides useful insights into the selection of raw materials for high-performance lignin-based adhesive preparation. Ibrahim et al. [55] used laccase and sodium borohydride to pretreat lignin and then mixed it with soy protein to prepare adhesives, respectively. The results showed that the chemical pretreatment significantly enhanced the water resistance strength of the adhesive, but the introduction of excess lignin caused self-polymerization on its own, which led to a decrease in the gluing properties of the adhesive. Wang et al. [54] synthesized lignin-based epoxy resin adhesives by substituting bisphenol with lignosulfonate as the primary ingredient. The lignosulfonates underwent chemical modification and were then mixed with the adhesive to create lignin-based epoxy adhesives featuring a dual interpenetrating network structure. The experimental results demonstrate that the adhesive’s tensile shear strength is 10.13 MPa after being submerged in water at 20 °C for 12 h. Additionally, the adhesive’s tensile shear strength is 9.30 MPa after being submerged in boiling water at 100 °C for 12 h. Thus, the epoxy adhesives derived from lignin can maintain excellent performance even in conditions of elevated temperature and humidity.

3.3. Tannin-Based Adhesives

Tannins are characterized by high molecular weight and high local resistance [55]. This leads to the problem of poor water resistance of adhesives prepared from tannins [56]. Therefore, it is possible to improve the overall performance and water resistance of tannin-based adhesives by constructing multi-layer network structures and cross-linked structures.
Arias et al. [43] added sodium hydroxide to tannins to create ionic connections and establish a complex network structure, which can enhance the water resistance and mechanical robustness of tannin-based adhesives. Liu et al. [57] exploited the catechol structure in condensed tannins (CTs) by mixing condensed tannins with SPI. In an acidic environment, the catechol structure on the B-ring was protected, and the active sites (C6 and C8) of the A-ring underwent a functional reaction with the SPI, forming a cross-linked network. This effectively introduced catechol into the SPI. Conversely, in an alkaline environment, the catechol structure on the B-ring underwent oxidation, forming an o-quinone cross-linked SPI, which resulted in the formation of a dense structure. The formation of a cross-linked structure in two ways enhanced the water resistance of the resulting adhesive. In addition, the adhesive’s cross-linked structure provided it with excellent thermal stability and a consistent fracture surface, which enhanced its resistance to water. Ghahri et al. [58] utilized chestnut shell hydrolyzed tannins and mimosa condensed tannins, in combination with hexanedial and glyoxal, to enhance the characteristics of soy protein adhesives. The condensed tannins possessing a catechol structure exhibited superior water resistance owing to a bionic bonding mechanism. Furthermore, the presence of the catechol structure was found to enhance the water resistance of the adhesive system.

4. Antimicrobial Performance

Microbial growth can be facilitated by the presence of moisture and nutrients in natural polymers. Consequently, bio-based adhesives are susceptible to microbial contamination, which causes a reduction in the lifespan of the product [59]. It is essential to take measures to prevent the growth of mold to expand the application of bio-based adhesives. The traditional methods for these treatments involve the incorporation of natural anti-mold or antibacterial compounds or chemicals into the adhesive [60].

4.1. Protein-Based Adhesives

As an organic biomass material, soy protein-based adhesives are susceptible to bacterial attack, which can shorten their life and cause deterioration [61]. Additionally, as time passes, their adhesion qualities decrease, which impacts their further applications.
Inspired by biological cartilage and mussels, Zhou et al. [62] developed a tough, mold-resistant, and recyclable soy protein isolate (SPI)-based adhesive, SPI/BP@mica, to co-assemble mica and SPI via dynamic boron nitrogen (B—N) liganded catechol-derivatized borate (B—O) interfacial bonding. From Figure 4a, there was no visible mold damage on the fresh SPI/BP@mica16 adhesive after being stored for 50 days (16 represented the mass fraction of BP@mica based on SPI). The remarkable ability to resist mold is due to the inherent antibacterial characteristics of the borate and phenolic hydroxyl groups. As shown in Figure 4c, the borate group forms hydrogen bonds with phospholipid bilayers, lipopolysaccharides, and glycoproteins in mold cell membranes, thereby inhibiting enzymes in metabolic pathways. The phenolic hydroxyl group modifies the functional activity of SPI and the physiology of the bacteria, resulting in the inhibition of metabolic pathway enzymes and the physiological function of the bacteria. Furthermore, Figure 4b demonstrates the remarkable mildew resistance property of the cured SPI-based adhesive, as evidenced by its antifungal activities and growth zone test against Aspergillus flavus. Aladejana et al. [63] explored a feasible, versatile, green, and sustainable strategy to develop superior SP-based adhesives using furfuryl alcohol (FA) and 2,5-diamino-1,4-benzenedithiol hydrochloride (DBH). FA/DBH was conjugated via carbodiimide-mediated DBH addition under light-avoiding conditions. The acidity was controlled by NaOH, which provided a weakly acidic environment for FA/DBH to form a highly cross-linked network. FA and DBH were hybridized through amide bonds, followed by SP integration through dynamic covalent bonds and amide bonds. The resulting organic network improved the mold resistance and shelf life of the soy protein adhesive. The remarkable mold resistance reveals that FA/DBH could penetrate the mildew membrane and hamper its cell formation. Gu et al. [64] used boric acid ions to cross-link proteins and polysaccharides in soy protein-based adhesives to synergistically enhance the antimicrobial properties of soy protein-based adhesives with hyperbranched polyesters. It was shown that the adhesive without sodium tetraborate began to mold and deteriorate within 12 h, whereas this phenomenon did not occur in the adhesive containing sodium tetraborate within 48 h. This indicates that the boric acid group can significantly improve the antimicrobial properties and durability of the soy protein adhesive. Further investigation revealed that this adhesive displayed excellent antibacterial properties against S. aureus. The adhesive supplemented with borate did not exhibit significant antibacterial action against Escherichia coli, suggesting that its antibacterial characteristics were selective. This phenomenon may be associated with a biological component known as lipopolysaccharide, which is exclusively found in the outer membrane of Gram-negative bacteria and has been documented to provide a protective barrier against boron chemicals. Bai et al. [44] developed a soy protein adhesive with anti-mildew properties by utilizing the reinforcing mechanism of the micro-nano micelle structure. The micelles of benzyl dodecyl dimethylammonium bromide (BDAB) formed in their original location connected with protein chains through dynamic bonds, whereas 1,6-hexanediol diglycidyl ether (HDE) served as the flexible cross-linker. Micelles with many amino cations demonstrated highly effective antibacterial activities. The adhesive produced demonstrates exceptional mechanical characteristics and a stable network structure. The addition of HDE and BDAB enhanced the antifungal and antibacterial (E. coli and S. aureus) properties of the modified SM adhesive, hence greatly prolonging its storage life. This was attributed to the fact that the BDAB possesses the hydrophobic hydrocarbon chain and positively charged amino groups, which can destroy the cell wall structure of bacteria and fungi.

4.2. Tannin-Based Adhesives

Li et al. [65] analyzed the catechol and amino moieties of mussel proteins, which played an important role in adhesion. They then combined condensed catechol tannins with a catechol structure and polyethyleneimine (PEI) to create an adhesive that does not contain aldehyde. The product effectively improved the antibacterial property of bio-based adhesives. This was attributed to the fact that amino cations can adsorb bacteria and bind to anions on the bacterial cell wall surface, impeding bacterial growth synthesis and causing denaturation. Efhamisisi et al. [66] used boric acid to increase the self-shrinkage rate of the tannin-based adhesives to produce poplar plywood. In this study, tannin, sodium hydroxide, hexamethylenetetramine, boric acid, and isocyanate were used as raw materials to prepare the adhesive. The addition of boric acid reduced the curing time and temperature, and the modulus of elasticity of the adhesive was increased. The addition of boric acid not only improved the bonding quality of plywood but also enhanced the antifungal properties (strain: CTB 863A; white rot), which can prevent fungi from attacking the adhesive. Borate groups induce hydrogen bonding with different constituents of fungi cell membranes, including phospholipid bilayers, lipopolysaccharides, and glycoproteins, therefore suppressing enzymes implicated in metabolic pathways. Jin et al. [67] conducted a study where they added CuS04 to a soy protein adhesive modified with tannin (TA) and hyperbranched silicone (HBSi). The purpose was to examine the antimicrobial properties of Escherichia coli and Staphylococcus aureus. The results showed that the combination of covalent and noncovalent bonds greatly improved the overall performance of the adhesive. Additionally, the combined effect of TA and Cu2+ effectively inhibited the growth of Staphylococcus aureus and Escherichia coli, leading to enhanced resistance against bacteria in the adhesive.

5. Conclusions

(1)
Bio-based adhesives offer a diverse array of raw material origins and possess the advantages of being renewable and degradable. Their development and application can address the problems of organic volatile emissions and reliance on fossil resources inherent in synthetic resin adhesives. The primary challenge lies in overcoming the bonding performance, water resistance, and mold resistance of bio-based adhesives during their use.
(2)
This paper presents a review of recent developments in the functional modification of bio-based adhesives. It provides a solution to the issues of formaldehyde release and excessive reliance on fossil resources for synthetic resin-based adhesives. The modification of bio-based adhesives leads to significant improvement in bonding performance, waterproofing performance, and mold resistance, which could be achieved by constructing multiple network structures, adding chemical reagents, and introducing functional groups. However, some of these modification methods have limitations, such as the corrosive nature of the modifying reagents, the high price of the modifying reagents, and the complexity of the modification steps. More research and development can be conducted in the future on the topic of modification methods for bio-based adhesives that can solve these problems.
(3)
The functional modification of bio-based adhesive could significantly expand the utilization scope of this adhesive in various areas. It can be employed as an alternative to traditional aldehyde-based resin adhesive. Furthermore, some of these bio-based adhesives have been employed in wood-based composite preparation, thereby facilitating the development of eco-friendly material manufacture. However, in the wood processing area, bio-based adhesives still have some problems, such as high viscosity and poor stability, which urgently need to be solved to improve their comprehensive performances. Fortunately, many researchers have an interest in the research of bio-based adhesives, which could be used in various fields. This will promote the green development of the wood industry.

Author Contributions

Conceptualization, C.Y. and J.J.; methodology, R.L. and X.W.; validation, Y.C., C.Y. and X.W.; formal analysis, Y.C.; writing—original draft preparation, C.Y. and R.L.; project administration, J.J.; funding acquisition, J.J. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the Student Practice Innovation and Training Program of Nanjing Forestry University (2021NFUSPITP0773, 202410298099Z).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable. No new data were created or analyzed in this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Schematic illustration of (a) TALD-assisted extraction and phenolation of lignin and (b) preparation of phenol-lignin-based vitrimer by catalyst-free dynamic acetal exchange reaction, reproduced with permission from Elsevier [29].
Figure 1. Schematic illustration of (a) TALD-assisted extraction and phenolation of lignin and (b) preparation of phenol-lignin-based vitrimer by catalyst-free dynamic acetal exchange reaction, reproduced with permission from Elsevier [29].
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Figure 2. Adhesion mechanisms of the DTPF-PEI adhesives, reproduced with permission from Elsevier [33].
Figure 2. Adhesion mechanisms of the DTPF-PEI adhesives, reproduced with permission from Elsevier [33].
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Figure 3. Description of the synthetic process of SP-D-PU and the internal microstructure and interactions within the SP-D-PU resin, reproduced with permission from Wiley [46].
Figure 3. Description of the synthetic process of SP-D-PU and the internal microstructure and interactions within the SP-D-PU resin, reproduced with permission from Wiley [46].
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Figure 4. (a) Mildew resistance of adhesives after storage for 0, 3, 10, and 50 days: 1 (SPI), 2 (SPI/mica), 3 (SPI/BP@mica4), 4 (SPI/BP@mica8), 5 (SPI/BP@mica12), 6 (SPI/BP@mica16); (b) anti-fungal properties and growth zone test of cured SPI-based adhesive towards Aspergillus flavus; (c) antibacterial mechanism of SPI/BP@mica adhesive; (d) Wet shear strength of adhesive after different storage times, reproduced with permission from Elsevier [62].
Figure 4. (a) Mildew resistance of adhesives after storage for 0, 3, 10, and 50 days: 1 (SPI), 2 (SPI/mica), 3 (SPI/BP@mica4), 4 (SPI/BP@mica8), 5 (SPI/BP@mica12), 6 (SPI/BP@mica16); (b) anti-fungal properties and growth zone test of cured SPI-based adhesive towards Aspergillus flavus; (c) antibacterial mechanism of SPI/BP@mica adhesive; (d) Wet shear strength of adhesive after different storage times, reproduced with permission from Elsevier [62].
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Table 1. Comparison of the effects of different adhesive strength enhancement modification methods.
Table 1. Comparison of the effects of different adhesive strength enhancement modification methods.
AdhesivesModification MethodsTest MethodsSubstratesBond strength (MPa)Ref.
Protein-basedDual network
by cationic interactions		Three-ply
GB/T 17657-2013		Poplar 1.50[24]
Increasing
intermolecular interactions		Three-ply
GB/T 17657-2013		Poplar 1.96[22]
High-pressure
homogenization to process 		Three-ply
GB/T 17657-2013		Poplar1.03[23]
Lignin-basedPhenol modification
and combination of DVE-3 		Two-ply
Not mentioned		Walnut lumber12.87[29]
Ghaldolization
and phenolization		Two-ply
Not mentioned		Walnut lumber6.27[32]
Phenol-modified
and cross-linking reactions		Two-ply
GB/T 14732-2017		Not mentioned6.27[30]
Tannin-basedDepolymerized
tannins combine with PEI 		Three-ply
Not mentioned		Poplar1.44[33]
Condense tannin-
functionalized boronnitride 		Three-ply
GB/T 17657–2013		Poplar1.67[34]
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Yu, C.; Chen, Y.; Li, R.; Jiang, J.; Wang, X. A Narrative Review: Modification of Bio-Based Wood Adhesive for Performance Improvement. Coatings 2024, 14, 1153. https://doi.org/10.3390/coatings14091153

AMA Style

Yu C, Chen Y, Li R, Jiang J, Wang X. A Narrative Review: Modification of Bio-Based Wood Adhesive for Performance Improvement. Coatings. 2024; 14(9):1153. https://doi.org/10.3390/coatings14091153

Chicago/Turabian Style

Yu, Caizhi, Yi Chen, Renjie Li, Jun Jiang, and Xiang Wang. 2024. "A Narrative Review: Modification of Bio-Based Wood Adhesive for Performance Improvement" Coatings 14, no. 9: 1153. https://doi.org/10.3390/coatings14091153

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