Preparation and Characterization of a Novel Eco-Friendly Acorn-Based Wood Adhesive with High Performance

Abstract

With the concept of sustainable development gaining increasing traction, the high-value utilization of forest biomass has received growing attention. In this study, an acorn-based wood adhesive was developed using Quercus fagaceae, offering a sustainable alternative that not only supports the multifunctional use of acorn shell resources, but also reduces dependence on fossil-based materials in traditional wood adhesives, a development of significant importance to the wood industry. The effects of various crosslinking agents and phenolic resin (PF) additions on the performance of the acorn-based adhesive (AS) were investigated. Among the crosslinking agents tested, isocyanate (MDI), epoxy resin E51, and trimethylolpropane diglycidyl ether (TTE), PF demonstrated the best bonding performance. The modified AS adhesive with a 30% PF addition showed the highest bonding strength (0.93 MPa) and superior water resistance. These improvements are attributed to the formation of a stable, multi-dimensional crosslinked network structure resulting from the interaction between gelatinized starch molecules and PF resin. Moreover, the AS-PF adhesive exhibited a remarkably low formaldehyde emission of 0.14 mg/L, representing a 90.67% reduction compared to the national E1 standard. The incorporation of PF also enhanced the adhesive’s mildew resistance and toughness. These findings highlight the potential of acorn-based adhesives as a sustainable alternative for applications in the wood and bamboo industries.

1. Introduction

Acorns, the general name of Fagaceae, are a kind of biomass resource with great research and development value. As one of the main acorn-producing countries, China is rich in resources, but it has never been effectively used. Acorns contain a lot of starch, which can be used to prepare wood and bamboo adhesives. Starch is a common polysaccharide found in nature, composed of amylose and amylopectin in varying proportions depending on the plant source [1,2,3]. Amylose is a linear polysaccharide consisting of glucose molecules linked by α-1, 4-glucoside bonds, with a straight molecular chain representing approximately 20 to 30% of total starch content [4,5,6]. On the other hand, amylopectin is a polysaccharide where glucose molecules are connected through α-1, 4-glucoside bonds in the main chain and branch out via α-1, 6-glucoside bonds, typically comprising 70 to 80% of the total starch [7,8]. Due to its cost-effectiveness, renewability, natural origin, and biodegradability, starch can serve as a sustainable alternative to traditional petroleum-based adhesives, particularly in the field of wood adhesives [9,10,11]. However, the adhesive strength between starch and wood heavily relies on hydrogen bonds formed by polar groups, such as the alcohol hydroxyl group on starch molecules and hydroxyl groups in wood [12]. These bonds are susceptible to water damage, leading to poor water resistance and weak wet bonding strength. Additionally, gelatinized starch emulsions are prone to instability and aging, resulting in increased viscosity, loss of fluidity, and a gel-like consistency that is unfavorable for both adhesive application and storage. Consequently, unmodified starch-based adhesives often fail to meet the requirements of the wood processing industry.

Various methods, such as starch grafting [13,14], oxidation [15,16,17], enzymatic hydrolysis [18,19], gelatinization [20], and crosslinking [21], have been explored to improve the water resistance of wood adhesives. For instance, Zhang et al. [13] investigated grafting itaconic acid monomer onto cassava starch to develop a biodegradable and environmentally friendly starch-based wood adhesive with improved mechanical properties. Similarly, Amini et al. [20] investigated the use of citric acid in gelatinized corn starch to enhance the properties of starch adhesives for wood composites. Chen et al. [22] studied the impact of sodium dodecyl sulfate on bio-based wood adhesives made from micro-refined starch, demonstrating significant improvements in shear strength and viscosity. Gadhave et al. [21] focused on the crosslinking of a starch–PVA blend with boric acid to produce a starch–PVA film with excellent mechanical and thermal properties. Jimenez Bartolome et al. [18] and Wang et al. [19] also contributed to the field by enhancing the performance and moisture resistance of starch-based adhesives through enzymatic polymerization and hydrolysis, respectively. Oktay et al. [15] utilized hydrogen peroxide to oxidize corn starch, which was then reacted with urea to produce an aldehyde-free green adhesive. This adhesive was further improved by the addition of nano-sized titanium dioxide. However, despite these improvements, the mechanical properties of the adhesive remained unsatisfactory. A literature review revealed that research on using acorn powder to prepare wood adhesives is currently limited. Zhou Lulu et al. [23] developed an eco-friendly wood adhesive using acorn starch, but it lacked sufficient water resistance and bonding strength. In addition, the chemical structure and thermal properties of acorn-based adhesives have not been thoroughly studied. Therefore, developing a high-performance acorn-based wood adhesive with improved water resistance and bonding performance, and systematically analyzing its physical, chemical, and thermal properties, is of great importance for promoting the application of acorns in adhesive materials.

Phenolic resin (PF resin), synthesized via polycondensation reaction between phenolic compounds (such as phenol, cresol, or xylenol) and aldehyde compounds (such as formaldehyde, acetaldehyde, paraformaldehyde, or furfural) under specific alkaline or acidic catalytic conditions [24], possesses a rich benzene ring structure that imparts unique structural adhesive properties. This resin exhibits strong rigidity and excellent bonding strength and resistance to water, heat, wear, and chemical erosion. As a result, PF resin is widely utilized in the wood and bamboo product processing industry [25,26]. Vázquez et al. [27] successfully developed a pseudoplastic adhesive by copolymerizing tannin with phenolic resin at room temperature. This new adhesive formulation reduces adhesive usage by 40 to 50% compared to pure phenolic adhesive. Epoxy resins and isocyanates (MDIs) are widely used as crosslinkers due to their excellent antistatic properties, strong bonding ability, and stable mechanical properties [28,29]. Previous studies have shown that both can significantly enhance the bonding strength and water resistance of soy-based biomass adhesives [30,31,32,33,34].

In order to improve the performance of acorn-based biomass adhesive, this study investigated the effect of various crosslinking agents using acorn kernels from Quercus acutissima Carruth. The adhesives were characterized using FTIR and TG analyses. The results demonstrated that the modified adhesives exhibited excellent bonding properties, water resistance, and remarkable mildew resistance. These findings not only support the efficient utilization of forestry waste, but also provide a theoretical basis for the high-value application of acorn resources in biomass adhesives development.

2. Materials and Methods

2.1. Experimental Materials

The acorn powder used in this study was derived from acorns in Biyang County, Zhumadian City, Henan Province. The collected acorns were shelled, dried, crushed, and finally passed through a 200-mesh sieve and stored as material for subsequent experiments. The corn starch comes from Jilin Qichen Food Co., Ltd. (Shulang, China). Cassava starch comes from Nanjing Sweet Juice Garden Co., Ltd. (Nanjing, China). The flour comes from Xinxiang Liangrun Whole Grain Food Co., Ltd. (Xinxiang, China). Phenol (purity of 99%) was procured from the Tianjin Damao Chemical Reagent Factory. The Formaldehyde solution (37% concentration) was purchased from Yantai Shuangshuang Chemical Co., Ltd. (Yantai, China). Sodium hydroxide (purity ≥ 96%) was obtained from Sinopod Group Chemical Reagent Co., Ltd. (Chengdu, China). Isocyanate (MDI), epoxy resin (E51), and trimethylolpropane diglycidyl ether (TTE) were purchased from Shanghai McLean Biochemical Technology Co., Ltd. (Shanghai, China). All chemicals used were of analytical grade and did not require further purification.

2.2. Gelatinization of Acorn Powder

A starch suspension with a mass fraction of 30% was prepared in a four-neck flask and mechanically stirred at 600 r/min. The temperature was gradually increased from room temperature to 90 °C using a water bath over a period of 30 min. The resulting gelatinized acorn powder was then denoted as AS.

2.3. Preparation of Isocyanate and Epoxy Resin Modified Acorn Powder Adhesive

In a four-necked flask, sequentially add 50 g of a 15 wt% polyvinyl alcohol solution, 185 g of deionized water, and 100 g of acorn powder. Stir the mixture at room temperature before heating the reaction system to 60 °C. Adjust the pH of the mixture to between 8.0 and 8.5, maintaining this condition for 50 min. Subsequently, take 45 g of the mixture and transfer it into a plastic cup. Add 4.5 g each of isocyanate (MDI), epoxy resin E51, and trimethylolpropane diglycidyl ether (TTE) separately. Stir the components thoroughly with a glass rod to obtain AS-MDI, AS-E51, and AS-TTE.

2.4. Preparation of Phenolic Resin and Modified Acorn Powder-Based Adhesive

PF resin was synthesized using batch copolymerization technology by reacting phenol and formaldehyde in a molar ratio of 1:2.1 under alkaline conditions. Initially, 50 g of molten phenol, 20 g of 40 wt% sodium hydroxide solution, and 40 g of deionized water were weighed and sequentially added to a four-necked round-bottom flask. The mixture was then heated in a 60 °C water bath and stirred for 5 min to ensure uniform mixing. Subsequently, a formaldehyde solution was gradually added to the flask, constituting 56% of the total formaldehyde. Over a period of 20 to 25 min, the reaction temperature was increased to 90 °C, followed by continued heating and stirring for 40 min to facilitate the reaction. Upon completion of the initial reaction stage, a solution containing 22% of total formaldehyde and 10 g of sodium hydroxide solution was added to the flask. The reaction mixture was then heated and stirred at 90 °C for an additional 40 min to ensure a thorough reaction. Finally, the remaining formaldehyde solution (22% of the total) was added, and the mixture was heated and stirred at 90 °C for another 40 min. The product was rapidly cooled to 40 °C and discharged, yielding phenolic resin (PF). Finally, the gelatinized acorn starch was added with 15%, 20%, 25%, and 30% phenolic resin, and stirred evenly to prepare AS-15% PF, AS-20% PF, AS-25% PF, and AS-30% PF.

2.5. Preparation of Playwood and Lap Samples

Three-layer plywood was prepared using acorn-based adhesives with an adhesive loading of 150 g/m². The veneers were assembled, clamped, and hot-pressed at 1.2 MPa and 140 °C for 8 min. The resulting plywood samples were stored at room temperature for 3 days before testing. Lap shear samples were prepared using basswood veneers cut to 100 mm × 25 mm × 3 mm. The Adhesive was uniformly applied to a 25 mm × 25 mm area on one side of each veneer pair, which were then assembled, clamped, and hot-pressed at 140 °C for 8 min. After cooling at room temperature for 24 h, the bonding strength was measured.

2.6. Adhesive Solid Content and Residual Rate Test

The adhesive was dried to a constant weight in an oven at 120 °C, and the formula for calculating the solid content was based on previous studies [35]. The fully cured adhesive sample was soaked in water at 20 °C for 1 h, and then dried in an oven at 80 °C to a constant weight. The formulas for calculating the residual rate refer to previous studies [35,36].

2.7. Bonding Strength Test

The adhesive bonding strength was tested according to the Chinese national standard GB/T17657-2022 [37,38]. Dry and wet bonding strengths were evaluated separately. For the dry strength, specimens were kept at room temperature (25 °C) for 24 h before testing on an electronic universal testing machine (WDW-20) at a tensile rate of 10 mm/min. For wet strength, specimens were immersed in hot water at 63 ± 3 °C and 93 ± 3 °C for 3 h, then allowed to rest for 10 min before testing under the same conditions. The prepared samples were tested eight times, and the average value was reported. Results were categorized as dry bonding strength, wet bonding strength (63 ± 3 °C), and wet bonding strength (93 ± 3 °C).

2.8. Thermal Stability Test

The thermal stability is tested using a thermogravimetric analyzer of the Q50-IR (TGA Q50) model. First of all, the sample was put in the oven for drying to a constant weight, and the dried sample was placed in the mortar for full grinding. After grinding the powdered sample through the 200-mesh screen, the sample with a mass of 5~10 mg was weighed and placed in the crucible of the thermogravimetric analyzer [39]. Under the protection of an N₂ atmosphere, the temperature was heated up to 600 °C at a rate of 10 °C/min.

2.9. Fourier Transform Infrared Spectroscopy

The microstructure and functional groups of the adhesive sample were analyzed using an infrared spectrometer. Initially, the adhesive sample was frozen in liquid nitrogen for 10 min and then dried in a freeze-dryer (FD-2A) until all the moisture was removed. Subsequently, the dried sample was ground with a mortar and sifted to obtain a 200-mesh adhesive sample powder. The sample for testing was prepared using the tablet pressing method. Approximately 2 mg of adhesive sample powder was weighed in an agate mortar along with about 200 mg of potassium bromide powder in a 1:100 ratio. The mixture was thoroughly ground, loaded into a tablet-pressing mold, and pressed into sheets using a tablet press at 20 MPa. Finally, the sheets were analyzed using Nicolete 6700 spectrometer (Nicolet Instrument Corporation, Waltham, MA, USA) with a scanning wavelength range of 600 to 4000 cm⁻¹.

2.10. Formaldehyde Emission, Crack Observation, and Mildew Control Test

The formaldehyde emission of plywood was evaluated according to the Chinese national standard GB/T17657-2022 using the dryer method [37,38]. Ten plywood specimens measuring 150 mm × 50 mm were cut and dried in an oven. Subsequently, these specimens were vertically fixed in a glass dryer with a volume of 10 L. A crystallizing dish with 300 mL of deionized water was placed at the bottom of the dryer, which was then sealed for 24 h at 20 °C. The deionized water from the dish was collected and analyzed for formaldehyde content using the acetyl acetone method. Surface cracks of the adhesive sample were also examined. The adhesive sample was evenly applied to a slide, cured at 120 ± 2 °C for 2 h in an oven, and then cooled in a dryer for 15 min. The resulting film was photographed to document any surface cracks. Additionally, the mold resistance of the adhesive sample was assessed by placing it in a round plastic petri dish with an 80 mm diameter. The sample was observed at 25~30 °C and photographed daily.

2.11. Statistical Analysis

A statistical analysis was performed using analysis of variance (ANOVA) in SPSS version 24.0. Duncan’s multiple range test was used for pairwise comparisons, with significance set at p < 0.05.

3. Results and Discussion

3.1. Effects of Different Types of Crosslinking Agents on Water Resistance of Acorn Powder-Based Biomass Adhesive

Solid content refers to the proportion of solids in the adhesive and plays a key role in bonding performance, durability, and molding quality [40]. The residual rate is closely linked to the safety and environmental impact of the adhesive during application [41,42]. Figure 1 shows the solid content and residual rates of starch-based adhesives (AS-PF, AS-E51, AS-TTE, and AS-MDI) modified with different crosslinkers. The solid content of the pure acorn powder-based adhesive was 25.4%. Upon the addition of crosslinkers, the solid content increased significantly, primarily due to the inherently high solid content of the crosslinkers, indicating enhanced adhesive stability [43,44]. As shown in Figure 1b, only AS-PF exhibited a higher residual rate compared to the pure AS adhesive. The residual rates for AS-E51, AS-TTE, and AS-MDI were lower, suggesting weaker crosslinking and reduced water resistance. In contrast, the strong crosslinking reaction between AS and PF resulted in an increased residual rate, indicating superior water resistance in the AS-PF adhesive. The pure AS adhesive exhibited a residual rate of 80%, reflecting its inherent resistance to water solubility. The residual rates for AS-E51, AS-TTE, and AS-MDI were 65.75%, 70.81%, and 65.91%, respectively, implying that these crosslinkers did not form dense crosslinked network structures with AS, leading to partial dissolution during testing. AS constituted over 62% of the total mass in these formulations, the residual rate is expected to reach 50%, even in the absence of strong crosslinking. The remainder of the residue primarily consists of undissolved E51, TTE, and MDI components. Consequently, the overall residual rates of AS-E51, AS-TTE, and AS-MDI were all above 60%.

3.2. Effects of Different Types of Crosslinking Agents on the Bonding Strength of Acorn Powder-Based Adhesives

Due to the poor water resistance and difficulty in modifying starch-based biomass adhesives, it is essential to accurately assess the degree of modification of acorn powder-based adhesives using various crosslinking agents. Therefore, the dry strength, the strength after 24 h of cold water immersion, the bond strength after 3 h of hot water immersion at 63 °C, and the strength after 3 h of boiling water immersion for specimens made from different adhesive combinations were measured [45]. Figure 2 shows the bonding strength of acorn powder-based adhesives with different crosslinking agents. It is evident from the figure that the pure acorn powder-based adhesive has high dry strength but poor water resistance. The 24-h cold water immersion test shows that AS-TTE also lacks sufficient water resistance. The cold water immersion strength of AS-E51 is merely 0.18 MPa, showing minimal improvement. As shown in Figure 2, the wet bonding strengths of AS-E51 and AS-TTE at both 63 and 93 °C were 0 MPa, indicating that epoxy resin E51 and TTE did not effectively crosslink with AS under 140 °C hot pressing, resulting in poor bonding strength and water resistance. AS-MDI showed a wet strength of only 0.16 MPa at 93 °C, much lower than AS-PF (0.56 MPa), though it performed well at 63 ± 3 °C (1.21 MPa), exceeding the Class II plywood requirement of 0.70 MPa [38]. In comparison, AS-PF demonstrated consistently excellent bonding performance and water resistance, with a wet bonding strength of 0.76 MPa at 63 °C (p < 0.05), meeting tClass II plywood standards, and 0.53 MPa at 93 °C–far superior to unmodified AS and AS-MDI, both of which showed nearly no bonding strength after boiling water immersion. This suggests that the hydrogen bonds, along with certain amino and carboxyl groups in acorn powder, interact with phenolic resin to form a dense crosslinked structure, thereby enhancing water resistance and further improving bonding strength [46,47].

3.3. FTIR Analysis of Different Arcon-Based Biomass Adhesives

The FTIR spectra of the adhesives are shown in Figure 3. The spectra of modified acorn powder-based adhesives were generally similar to those of the unmodified adhesive. In the unmodified acorn powder-based adhesive, a broad band at 3500–3200 cm⁻¹ corresponds to -OH stretching vibrations, indicating the presence of hydroxyl groups in starch [48,49,50]. Other characteristic absorption peaks of starch appeared at 2925 cm⁻¹, 1621 cm⁻¹, and 1025 cm⁻¹, which correspond to C-H stretching, H₂O bending vibration, and the O-C stretching of anhydrous glucose rings, respectively [51]. The band at 1157 cm⁻¹ is attributed to the C-O, C-C, and O-H vibrations and asymmetric stretching of the C-O-C glycosidic bond [52]. In the modified adhesives, the sharper peak at 2925 cm⁻¹ suggests the cleavage of the starch macromolecular chains, increasing -CH₂ groups [53]. Figure 3a shows that AS and AS-MDI had nearly identical spectra, showing no new peaks, indicating weak crosslinking under the hot pressing condition (140 °C). The reduced intensity at 1741 cm⁻¹ in AS-MDI suggests some interaction between MDI and AS. Notably, a new peak at 1240 cm⁻¹ appeared in AS-PF, confirming chemical bonding between AS and PF, indicating successful crosslinking. This crosslinking reaction likely contributes to the improved bonding strength of the AS-PF adhesive [54].

3.4. Effect of Different Phenolic Resin Dosages on Water Resistance of Acorn Powder-Based Biomass Adhesive

The solid content in an adhesive is crucial as it indicates the proportion of active substances to water. A higher solid content increases bonding strength in plywood by optimizing the adhesive’s effectiveness [55]. Figure 4 shows that pure acorn powder adhesive had a solid content of 25.40%, which significantly increased with the addition of PF resin. For instance, when PF resin reached 30%, the solid content of acorn powder adhesive increased to 33.40%. This higher solid content signifies reduced water content in the adhesive, enhancing its viscosity. Consequently, during plywood pressing, the shortened evaporation time could improve dimensional stability, adhesive performance, and production efficiency.

The residual rate is a crucial metric for assessing the water resistance of adhesives. The water resistance of plywood largely depends on the adhesive’s ability to repel water, which is influenced by its crosslinking density [56]. Adhesives with low crosslinking are more susceptible to water penetration when submerged. On the other hand, highly crosslinked adhesives form a tight network structure that effectively blocks water molecules, reducing solubility and enhancing water resistance. Pure acorn powder-based adhesive had a residual rate of 80.00% due to the presence of gelatinized starch containing numerous active groups that easily bind with water molecules. In contrast, the addition of PF resin significantly increased the residual rate, reaching a peak of 88.00% at 30% PF resin content [57,58]. This indicates that PF resin enhances the crosslinking of the acorn powder adhesive, forming a stable network structure that improves water resistance by preventing water infiltration. Hence, the interaction between active groups in the acorn meal-based adhesive and PF resin resulted in a more robust adhesive with improved water resistance [59].

3.5. Effect of Different Phenolic Resin Dosages on the Bonding Strength of Acorn Powder-Based Biomass Adhesive

Figure 5a shows the effect of different PF resin dosages on the bonding strength of acorn powder-based biomass adhesives. Although unmodified acorn powder-based adhesive demonstrated excellent dry bonding strength, its wet shear strength was 0 MPa. However, the addition of PF resin gradually improved wet strength. At 63 °C and 93 °C, the wet bonding strengths of CS-30% PF, TS-30% PF, WS-30% PF, and AS-30% PF were 0.51, 0.70, 1.15, and 1.12 MPa and 0.37, 0.44, 0.70, and 0.93 MPa, respectively. Compared to CS-30% PF and TS-30% PF, the WS-30% PF and AS-30% PF exhibited better bonding strength and water resistance. This enhancement is mainly due to the presence of protein in wheat starch and acorn powder, whose amino groups can crosslink with PF resin to enhance adhesive performance. At a 30% PF resin content, the wet bonding strength peaked at 0.93 MPa (p < 0.05), satisfying the national standard for Class I plywood. Figure 5b shows that the dry and wet bonding strength of corn starch and cassava starch adhesives was lower than that of the acorn-based adhesives. All starch-based adhesives showed similar dry strength. Overall, acorn-based adhesives demonstrated superior performance compared to the other three starch-based adhesives [60].

The high dry strength of the acorn-based adhesive is due to starch gelatinization, which disrupts hydrogen bonds and exposes more bonding sites. This allows water to penetrate the starch’s microcrystalline structure, increasing molecular disorder and viscosity. However, its low wet strength may result from hydrophilic groups reacting with water, weakening adhesion. The addition of PF resin significantly enhanced the water resistance due to the high reactivity of aldehyde and carboxyl groups in the acorn roux. These groups interact with the benzene ring and aldehyde groups in PF resin to form a stable, robust multidimensional network structure, fostering strong cohesion and significantly improving both bonding strength and water resistance (Figure 5c) [61].

3.6. Analysis of Formaldehyde Emissions

Formaldehyde, a potent protoplasmic poison that binds easily with proteins, can lead to protein abnormalities and poses a significant health risk [62]. In newly renovated homes in China, formaldehyde is identified as the primary indoor pollutant and classified as a top-tier carcinogen by the World Health Organization [63,64]. With plywood widely used in interior decoration, effective control of formaldehyde emissions is crucial, which serves as a key metric for adhesive performance and maintaining indoor environmental health [65]. The AS-30% PF adhesive sample emitted only 0.14 mg/L of formaldehyde, significantly below the national E1 standard limit (≤1.5 mg/L), representing a 90.67% reduction. This makes the adhesive suitable for interior decoration without concerns about excessive formaldehyde. However, it is important to note that during plywood production, adhesive spillage may occur, potentially leading to slightly higher formaldehyde release levels. Nevertheless, the formaldehyde emissions from the newly developed acorn flour-based biomass adhesive still meet interior decoration standards, underscoring its exceptional environmental performance (Figure 6).

3.7. Crack Observation and Mildew Test Analysis of Adhesive

In order to evaluate the anti-mold performance of the adhesive, the samples were stored at 30 °C and 90% RH to observe the mold growth process. The resistance of adhesives to mold is crucial for their storage and industrial applications, as mold can damage their structure, resulting in reduced stability and bond strength. As shown in Figure 7, adhesives made from seed powder are particularly susceptible to mold due to their starch content. Therefore, maintaining the mildew resistance of starch adhesive is of great significance for extending the storage period and ensuring the long-term effectiveness of bonding. The pure acorn powder adhesive developed mold within 3 days, and over time, the affected area expanded and gave off a strong odor. Because the main component in AS is starch, it is easy to be eroded by mold under high temperature and humid conditions. Over time, by day 6, the adhesive surface was covered with a lot of mold mycelium and completely deteriorated. In contrast, the acorn-based adhesives added with PF resin showed no mycelia within 14 days, which highlighted that PF resin has obvious anti-fungal activity and can be stably fixed in the starch adhesive system for a long time, effectively preventing the attack of mold on acorn-based adhesives.

The toughness of the solidified adhesive was assessed by identifying cracks. Figure 7b shows numerous pronounced cracks in the pure acorn powder-based adhesive during the curing process. These cracks could be attributed to intense contraction forces within the adhesive, causing the film to rupture, which indicates a high level of brittleness and insufficient toughness. Upon the addition of PF resin, these significant cracks disappeared entirely, and no surface bubbles formed. This suggests that the formation of a crosslinked network structure within the adhesive substantially improved its toughness. Consequently, the newly developed acorn flour-based biomass adhesive demonstrates outstanding toughness performance.

The cost of adhesives is one of the important factors in industrial applications. According to the market price, the cost of an acorn-based biomass adhesive is about 1340 RMB/ton, which is lower than the phenolic resin (about 3400 RMB/ton) [66] and soybean protein adhesive (about 3400 RMB/ton) [67] commonly used in plywood production. In addition, the amount of sizing of plywood is also roughly the same, so it has great potential as a biomass-based adhesive. Most importantly, the adhesive makes full use of forestry waste and is of great significance for the production of high-quality wood-based panels.

4. Conclusions and Prospect

In this study, acorn starch was utilized as a raw material for adhesive production to promote the high-value use of forestry waste. A starch-based biomass adhesive was developed to address issues of insufficient bonding strength and poor water resistance, thereby enhancing the utilization of acorn resources. The modification of acorn-based adhesives with various crosslinking agents and different proportions of phenolic resin (PF) revealed that the PF addition significantly improved both the solid content and residue rate. The synergistic interaction between gelatinized acorn powder and PF resin formed a robust, multi-dimensional crosslinked network, greatly enhancing the bonding performance of the adhesive. Notably, the addition of 30% PF resin increased the water-resistant bonding strength from 0 MPa to 0.93 MPa. Moreover, the adhesive exhibited low formaldehyde emissions, with a maximum of just 0.14 mg/L, well below the E1 national standard limit. The inclusion of PF resin also improved anti-mildew performance and toughness, contributing to the overall performance enhancement. An FTIR analysis confirmed that gelatinized starch molecules interacted effectively with PF resin, forming a stable multi-dimensional spatial network structure. This research supports sustainable development by advancing the high-value utilization of biomass resources and reducing reliance on fossil-based adhesives in the wood industry, offering significant environmental and economic benefits.

This study also revealed that AS-PF adhesive, prepared using PF resin as a crosslinking agent, exhibited the best overall performance. However, there is currently no systematic research on the curing behavior of acorn-based wood adhesive. Given that curing temperatures can vary widely among different adhesive systems, understanding the curing characteristics of AS-PF is essential for optimizing its application. Current findings indicate that AS-PF resin offers superior bonding strength and water resistance. To further elucidate these properties, advanced analytical techniques such as XRD, XPS, and ¹³C-NMR can be employed to provide a more comprehensive understanding of its structural and chemical behavior. In addition, further in-depth studies are needed to explore the feasibility of applying AS-PF adhesives in the manufacturing of particleboard and fiberboard, representing a promising direction for future research and industrial application.