Preparation and Characterization of Plant Protein Adhesives with Strong Bonding Strength and Water Resistance

Plant protein adhesive has received considerable attention because of their renewable raw material and no harmful substances such as formaldehyde. However, for the plant protein adhesive used in the field of plywood, low cost, strong water resistance, and high bonding strength were the necessary conditions for practical application. In this work, a double-network structure including hydrogen bonds and covalent bonds was built in hot-pressed peanut meal (HPM) protein (HPMP) adhesive, soybean meal (SBM) protein (SBMP) adhesive and cottonseed meal (CSM) protein (CSMP) adhesives. The ether bonds and ester bonds were the most in CSMP adhesive, followed by SBMP adhesive, while the hydrogen bond was the most in HPMP adhesive. The solubility of the HPMP, SBMP, and CSMP adhesives decreased by 14.3%, 24.2%, and 19.4%, the swelling rate decreased by 56.9%, 48.4%, and 78.5%, respectively. The boiling water strength (BWS) of HPMP (0.82 MPa), SBMP (0.92 MPa), and CSMP adhesives reached the bonding strength requirement of China National Standards class I plywood (type I, 0.7 MPa). The wet shear strength (WSS) of HPMP, SBMP, and CSMP adhesives increased by 334.5% (1.26 MPa), 246.3% (1.42 MPa), and 174.1% (1.59 MPa), respectively. This study provided a new theory and method for the development of eco-friendly plant meal protein adhesive and promotes the development of green adhesive.


Introduction
Trialdehyde adhesive (urea formaldehyde resin, melamine formaldehyde resin, and phenolic resin) was widely used in the traditional wood industry [1]. However, trialdehyde adhesive had some problems, such as relying on oil resources, releasing formaldehyde, and polluting the environment [2]. Especially with the rise of global oil prices and the enhancement of people's awareness of environmental protection and safety, the development of renewable biomass aldehyde-free adhesives (starch adhesives [3], tannin adhesives [4], lignin adhesives [5], plant protein adhesives [6]) have become an inevitable trend in the development of adhesive industry.
Soybean meal (SBM) as a widely sourced, environmentally friendly, and renewable biomass adhesive material has attracted much attention in recent years [7]. SBM has been widely studied in the area of plant protein adhesive, with the treatment reagents including enzyme [8], surfactants [9], urea [10], alkali [11], and epoxy resin [12]. The results showed that the dry shear strength (DSS) of SBM protein adhesive was close to that of formaldehyde adhesive, but the water resistance and wet shear strength (WSS) were poor due to large amounts of hydrophilic groups on the surface of soybean protein. Thus, the current research mainly focuses on how to improve the water resistance and bonding strength. At present, the commonly methods include the modification of phytic acid and aminoclay-cellulose HPMP, SBMP, and CSMP adhesives could be prepared by mixing PAE (34 g) w SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives for 30 min.

Fourier-Transform Infrared Spectroscopy (FTIR)
The FTIR spectrometer (Tensor 27, Bruker) of the cured adhesive samples (200 me were used to collect in the wavelength range between 400-4000 cm −1 with a resolution 4 cm −1 with 64 scans [18].

X-ray Photoelectron Spectroscopy (XPS)
The XPS (Thermo Fisher Scientific, USA) was used for monochromatic radiation tection of adhesive samples at ambient temperature [21].

X-ray Diffraction (XRD)
The XRD analysis was performed using a commercial X-ray diffractometer (D8 A vance diffractometer, Bruker) ranging from 10° to 90° [16].  The FTIR spectrometer (Tensor 27, Bruker) of the cured adhesive samples (200 mesh) were used to collect in the wavelength range between 400-4000 cm −1 with a resolution of 4 cm −1 with 64 scans [18].

X-ray Photoelectron Spectroscopy (XPS)
The XPS (Thermo Fisher Scientific, Waltham, MA, USA) was used for monochromatic radiation detection of adhesive samples at ambient temperature [21].

Thermogravimetric (TGA)
The thermal gravimetric (TG) and differential thermogravimetric (DTG) curves of adhesive samples were obtained from TG analysis (Perkin Elmer) with a heating rate of 10 mL/min under nitrogen atmosphere [12]. All adhesive samples on the surface of the study were sprayed with gold, and the micro morphology of the cured adhesive samples was observed by SEM (Hitachi SU8010) [13].

Sol-Gel Test
The method was carried out according to Gao et al. [22]. The adhesive samples were dried in an oven at 110 • C to constant weight and then ground into powder (100 mesh) with a ball mill. The volume (V a ) of 2.20 g (M a ) of adhesive sample powder was recorded by the graduated cylinder, and then, the sample powder was heated in boiling water for 3 h. After filtration with a filter screen, the volume (V b ) of the solid residue was measured again, and the residue was then dried at 110 • C until constant weight (M b ). The solubility (Equation (1)) and swelling rate (Equation (2)) of the cured adhesive were as follows: (1)

Preparation of Three-Ply Plywood Specimens and Bonding Strength Testing
Three-ply plywood was prepared with prepared HPMP, SBMP, and CSMP adhesives. The adhesive was evenly applied on the surface of poplar veneer with the amount of 220 g/m 2 . We manually assembled the three-layer veneer with vertical grain and then pressed it for 8 min under the conditions of 120 • C and 1.20 MPa. The prepared plywood was cut after being placed at room temperature for 24 h.
The boiling water strength (BWS, type I), WSS (type II), and DSS were tested at the speed of 5 mm/min by using a tensile testing machine according to the description of China National Standards GB/T 9846 (2015) [23]. The plywood panels as shown in Scheme 1. The plywood panels were soaked in boiling water for 4 h, dried at 63 • C for 20 h, then soaked in boiling water for 4 h and soaked in 25 • C water for 1 h, and dried at room temperature for 15 min, and the boiling water strength was tested. For WSS, the plywood panels were soaked in 63 • C water for 3 h, then aged at room temperature for 15 min and tested. All experiments were repeated three times, with 6 samples per group. Scheme 1. Plywood specimen size and tensile force loading diagram.

Physical Property Measurements of Adhesive Sample
Next, we put the fresh adhesive sample (M d ) into an aluminum box and dried it to constant weight at 120 • C and weighed the mass of dry adhesive (M a ). The calculation of solids content (Equation (3)) is presented in the following equation. The viscosity of adhesive samples was evaluated using a Brookfield DV-III rotary viscometer. A surface tension-contact angle meter (SL200KB) was used to test the water contact angle (WCA) and solid surface energy (SSE) for the adhesive samples [24]. (3)

Crack Observations
The different adhesive samples (6 g) were evenly coated on one side of the glass slide, dried at 110 • C for 3 h, and then placed in a dryer to cool to room temperature. The surface morphology of the cured adhesive was observed and photographed with Canon camera (700 d).

Statistical Analysis
The significant difference of bonding strength was analyzed by analysis of variance (ANOVA) test in SPSS 22.0. XRD, FTIR, and XPS spectra were drawn by origin 8.5.

Characterization of HPMP, SBMP, and CSMP-Based Adhesives
As shown in the Figure 2, the pure HPMP, SBMP, and CSMP adhesives had obvious characteristic peaks at 1635-1637 cm −1 , 1529-1533 cm −1 , and 1238-1255 cm −1 , representing C=O stretching (amide I, primary amine), N-H bending (amide II, secondary amine), and C-N and N-H stretching (amide III), respectively [22]. As could be seen from Figure 2a-c, compared with pure HPMP, SBMP, and CSMP adhesives, the peak areas of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives at 1403 cm −1 , 1400 cm −1 , and 1407 cm −1 , respectively, increased to varying degrees, indicating that the contents of -COOH increased. This was because SDS unfolds the protein structure and exposed more internal active groups. The results were consistent with the changes of secondary structure. The evidence showed the content of β-sheet increased, the peptide chain was more extended, and the reactivity of the adhesive increased ( Figure 3). The addition of nSiO 2 was an indispensable factor in the construction of the first-level network. The characteristic peak intensities of SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives at 918-921 cm −1 , 1400-1406 cm −1 , and 3282-3298 cm −1 were significantly reduced. Note that the hydrogen bonds formed between nSiO 2 as well as O-H and N-H. After the introduction of PAE, the tertiary structure of protein was expanded by electrostatic and hydrophobic forces, which made it more flexible. The -COOH peak areas of HPMP, SBMP, and CSMP adhesives decreased significantly, and the new absorption peaks were observed at 1743 cm −1 , 1722 cm −1 , and 1720 cm −1 , which was caused by the formation of ester bond between -OH and -COOH [25]. The C-O stretching vibration (ethanol and phenol) of HPMP, SBMP, and CSMP adhesives at 1060 cm −1 , 1055 cm −1 , and 1049 cm −1 was caused by the reaction of azacyclobutane with -OH to formed ether bond. The absorption peak intensities of HPMP, SBMP, and CSMP adhesives at 1529 cm −1 , 1531 cm −1 , and 1533 cm −1 decreased significantly, suggesting that the primary amine was also involved in the crosslinking reaction. In addition, their peaks at 3280-3298 cm −1 were the widest, indicating that the double-network structure formed by covalent bonds (ester bonds and ether bonds) and hydrogen bonds was dense.
To better understand the compositional change and interior interactions in the adhesives by XPS ( Figure S1), the binding energy and characteristic peaks [26] of different adhesive samples are shown in Figures 4 and 5. Compared with pure HPMP, SBMP, and CSMP adhesives, the C 1s and N 1s core-level spectra of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives increased in varying degrees, which proved that SDS contributed to more active group exposure. After the introduction of SiO 2 , the peak area of SDS/nSiO 2 /HPMP adhesive was the largest at 400.38 eV ( Figure 5), indicating that the content of -NH 2 was the most; the findings showed the most hydrogen bond formation in this case, while the peak area of SDS/nSiO 2 /SBMP adhesive was the smallest, and the hydrogen bonds were the fewest. The highest C-O-C content of CSMP adhesive was due to the highest peak strength at 288.09 eV, which was helpful to form more ether bonds and ester bonds. At the same time, from the peak strength of HPM adhesive at 200 eV, it could be seen that ether bond content was much lower than that of SBMP and CSMP adhesives. It was worth noting that CSMP adhesive moved to the left at 398.67 eV, indicating that PAE had more self-crosslinking, further improved its water resistance [27].  To better understand the compositional change and interior interactions in the ad sives by XPS ( Figure S1), the binding energy and characteristic peaks [26] of different hesive samples are shown in Figures 4 and 5. Compared with pure HPMP, SBMP, a CSMP adhesives, the C 1s and N 1s core-level spectra of SDS/HPMP, SDS/SBMP, a SDS/CSMP adhesives increased in varying degrees, which proved that SDS contribu to more active group exposure. After the introduction of SiO2, the peak area   To better understand the compositional change and interior interactions in the adhesives by XPS ( Figure S1), the binding energy and characteristic peaks [26] of different adhesive samples are shown in Figures 4 and 5. Compared with pure HPMP, SBMP, and CSMP adhesives, the C 1s and N 1s core-level spectra of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives increased in varying degrees, which proved that SDS contributed to more active group exposure. After the introduction of SiO2, the peak area of  was worth noting that CSMP adhesive moved to the left at 398.67 eV, indicating that PAE had more self-crosslinking, further improved its water resistance [27].  The XRD patterns of different adhesives were shown in Figure 6a-c. Pure HPMP, SBMP, and CSMP adhesives had an obvious characteristic peak at 21.64°, 20.82°, and 22.68°, respectively (β-sheet) [28]. After the introduction of SDS, the crystallization peak areas of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives increased in varying degrees, and the crystallinity increased by 6.3% (18.2°), 2.6% (19.4°), and 2.2% (10.2°), respectively. This was due to SDS destroying the structure of peanut protein, expanding the molecular structure, and being improved the crystallinity. For SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives, the peak strength of β-sheet decreased by 22.0% (14.2°), 2.1% (19.0°), and 33.3% (6.8°), respectively (Figure 6d-f), which confirmed that nSiO2 interacted with protein to form hydrogen bond. After the introduction of PAE, the  was worth noting that CSMP adhesive moved to the left at 398.67 eV, indicating that PAE had more self-crosslinking, further improved its water resistance [27].  The XRD patterns of different adhesives were shown in Figure 6a-c. Pure HPMP, SBMP, and CSMP adhesives had an obvious characteristic peak at 21.64°, 20.82°, and 22.68°, respectively (β-sheet) [28]. After the introduction of SDS, the crystallization peak areas of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives increased in varying degrees, and the crystallinity increased by 6.3% (18.2°), 2.6% (19.4°), and 2.2% (10.2°), respectively. This was due to SDS destroying the structure of peanut protein, expanding the molecular structure, and being improved the crystallinity. For SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives, the peak strength of β-sheet decreased by 22.0% (14.2°), 2.1% (19.0°), and 33.3% (6.8°), respectively (Figure 6d-f), which confirmed that nSiO2 interacted with protein to form hydrogen bond. After the introduction of PAE, the The XRD patterns of different adhesives were shown in Figure 6a-c. Pure HPMP, SBMP, and CSMP adhesives had an obvious characteristic peak at 21.64 • , 20.82 • , and 22.68 • , respectively (β-sheet) [28]. After the introduction of SDS, the crystallization peak areas of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives increased in varying degrees, and the crystallinity increased by 6.3% (18.2 • ), 2.6% (19.4 • ), and 2.2% (10.2 • ), respectively. This was due to SDS destroying the structure of peanut protein, expanding the molecular structure, and being improved the crystallinity. For SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives, the peak strength of β-sheet decreased by 22.0% (14. , which means that the crystal structure of protein molecules collapsed due to the crosslinking reaction, further improving the crosslinking density of the double net-work [29]. The crystallinity of CSMP adhesive was the lowest, and that of HPMP adhesive was the highest, indicating that the double network formed by CSMP adhesive was the densest. The results could be further proven by the thermal decomposition temperature. All adhesive samples had three thermal degradation stages. Different from HPMP-and SBMP-based adhesives, the CSMP-based adhesive had two degradation stages assigned to protein subunits with different stabilities [25]. The thermal decomposition temperatures of SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives increased by 5.1% (315.3 • C), 14.6% (302.1 • C), and 0.5% (333.1 • C), respectively (Figure 6g-i), showing that the single-network structure formed by hydrogen bond improved the thermal stability of the adhesive. Compared with the single-network, the double-bond network structure contained covalent bonds showed higher thermal stability. The thermal decomposition temperature of HPMP, SBMP, and CSMP adhesives increased by 5.8% (317.2 • C), 17.8% (310.5 • C), and 0.9% (334.5 • C). These results showed that the double-network synergy of hydrogen bonds and covalent bonds could greatly improve the thermal stability of the adhesive. Among them, CSMP adhesive showed the highest thermal decomposition temperature because of the densest double network. The thermal decomposition temperature of SBPM adhesive was lower than that of HPMP adhesive, which might be due to the low protein content. , which means that the crystal structure of protein molecules collapsed due to the crosslinking reaction, further improving the crosslinking density of the double network [29]. The crystallinity of CSMP adhesive was the lowest, and that of HPMP adhesive was the highest, indicating that the double network formed by CSMP adhesive was the densest. The results could be further proven by the thermal decomposition temperature. All adhesive samples had three thermal degradation stages. Different from HPMP-and SBMP-based adhesives, the CSMP-based adhesive had two degradation stages assigned to protein subunits with different stabilities [25]. The thermal decomposition temperatures of SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives increased by 5.1% (315.3 °C), 14.6% (302.1 °C), and 0.5% (333.1 °C), respectively (Figure 6g-i), showing that the single-network structure formed by hydrogen bond improved the thermal stability of the adhesive. Compared with the single-network, the double-bond network structure contained covalent bonds showed higher thermal stability. The thermal decomposition temperature of HPMP, SBMP, and CSMP adhesives increased by 5.8% (317.2 °C), 17.8% (310.5 °C), and 0.9% (334.5 °C). These results showed that the doublenetwork synergy of hydrogen bonds and covalent bonds could greatly improve the thermal stability of the adhesive. Among them, CSMP adhesive showed the highest thermal decomposition temperature because of the densest double network. The thermal decomposition temperature of SBPM adhesive was lower than that of HPMP adhesive, which might be due to the low protein content.  Figure 7a,b showed the solubility and swelling rate of the different adhesives, respectively. Due to the high hydrophilicity of plant proteins, the interaction between proteins and wood (mechanical interlocking and hydrogen bond) was easy to break under wet conditions, resulting in poor water resistance. Therefore, the water resistance of adhesive  Figure 7a,b showed the solubility and swelling rate of the different adhesives, respectively. Due to the high hydrophilicity of plant proteins, the interaction between proteins and wood (mechanical interlocking and hydrogen bond) was easy to break under wet conditions, resulting in poor water resistance. Therefore, the water resistance of adhesive played an important role in the bonding performance of adhesive [30]. The solubility of pure HPMP, SBMP, and CSMP adhesives were 56.7%, 63.2%, and 56.2%, respectively, and the swelling rates were 120.8%, 133.6%, and 131.5%, respectively. After SDS denaturation, the solubility of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives decreased to 54%, 59.2%, and 52.4% due to the exposure of hydrophobic groups in the protein. The swelling rate showed an increasing trend, which was attributed to the changes of viscosity and contact angle. The hydrogen bond network formed by SiO 2 with peanut, soybean, and cottonseed protein im-proved the water resistance, so the solubility of the SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives were reduced by 7.2% (56.7%), 16.8% (63.2%), and 7.5% (56.2%), respectively. The swelling rates of the SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives were further increased to 161.7%, 192.8%, and 157.9%, which was attributed to the improvement of elongation by nSiO 2 . In general, reducing the number of hydrophilic groups and increasing the crosslinking density were conducive to enhance the water resistance of the adhesive [31]. Therefore, the double-network structure formed after the introduction of PAE had better solubility and swelling rate. The results showed that CSMP adhesive had the best water resistance, and the water resistance of SBMP adhesive was slightly better than HPMP adhesive.

Water Resistance and Bonding Strength Test
played an important role in the bonding performance of adhesive [30]. The solubility of pure HPMP, SBMP, and CSMP adhesives were 56.7%, 63.2%, and 56.2%, respectively, and the swelling rates were 120.8%, 133.6%, and 131.5%, respectively. After SDS denaturation, the solubility of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives decreased to 54%, 59.2%, and 52.4% due to the exposure of hydrophobic groups in the protein. The swelling rate showed an increasing trend, which was attributed to the changes of viscosity and contact angle. The hydrogen bond network formed by SiO2 with peanut, soybean, and cottonseed protein improved the water resistance, so the solubility of the SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives were reduced by 7.2% (56.7%), 16.8% (63.2%), and 7.5% (56.2%), respectively. The swelling rates of the SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives were further increased to 161.7%, 192.8%, and 157.9%, which was attributed to the improvement of elongation by nSiO2. In general, reducing the number of hydrophilic groups and increasing the crosslinking density were conducive to enhance the water resistance of the adhesive [31]. Therefore, the double-network structure formed after the introduction of PAE had better solubility and swelling rate. The results showed that CSMP adhesive had the best water resistance, and the water resistance of SBMP adhesive was slightly better than HPMP adhesive.    (1) the protein peptide chain was destroyed, and more hydrophobic groups were exposed to enhance the hydrophobicity. (1) PAE optimized the viscosity of the adhesive, which was conducive to the extension and penetration on the wood surface, and formed more mechanical glue nails. (2) The hydrogen bond absorbed and dissipated the force in the fracture process to ensure sufficient toughness of the adhesive [19,32]. (3) The synergistic effect of this double network improved the mechanical properties of the adhesive. The SEM images (Figure 8d) of HPMP, SBMP, and CSMP adhesive plywood samples showed different degrees of failure modes (wood failure and fiber breakage) after bonding strength measurement. There were fibers on surfaces, and it showed that the double-network structure had good effects on three different plant protein adhesives [33]. The CSMP adhesive-bonded plywood had the roughest surface and the most wood fiber, the surface fiber of HPMP adhesive-bonded plywood was the least, which was the same as the BWS of the three adhesives (CSMP adhesive was the highest, SBMP adhesive was the second, and HPMP adhesive was the lowest). The mechanism for improving the bonding strength is shown in Figure 8d.
adhesive, which was conducive to the extension and penetration on the wood surface, and formed more mechanical glue nails. (2) The hydrogen bond absorbed and dissipated the force in the fracture process to ensure sufficient toughness of the adhesive [19,32]. (3) The synergistic effect of this double network improved the mechanical properties of the adhesive. The SEM images (Figure 8d) of HPMP, SBMP, and CSMP adhesive plywood samples showed different degrees of failure modes (wood failure and fiber breakage) after bonding strength measurement. There were fibers on surfaces, and it showed that the doublenetwork structure had good effects on three different plant protein adhesives [33]. The CSMP adhesive-bonded plywood had the roughest surface and the most wood fiber, the surface fiber of HPMP adhesive-bonded plywood was the least, which was the same as the BWS of the three adhesives (CSMP adhesive was the highest, SBMP adhesive was the second, and HPMP adhesive was the lowest). The mechanism for improving the bonding strength is shown in Figure 8d.   Table 2 shows the bonding strength (BWS, WSS, and DSS) comparison between the adhesives developed in this study and the previously reported eco-friendly adhesives. It could be seen that the BWS, WSS, and DSS of HPMP, SBMP, and CSMP adhesives were significantly higher than that of other eco-friendly adhesives. It also proved that this design had good universality, which provides a new theory for preparing protein adhesive with high bonding strength and strong water resistance.

Physical Property
The solids content was an important physical property of wood adhesive. Low solids content means high moisture content; the adhesive would penetrate laterally, and bubbles would appear during hot pressing. If the solid content is too high, it would not be conducive to the longitudinal penetration of the adhesive to the wood board and thus affect the plywood quality [15]. Therefore, the proper solids content range (20-35%) of adhesive is vital to improve the mechanical properties and plywood quality. As shown in Figure 9a-c, the solids content of the three pure adhesives (pure HPMP, SBMP, and CSMP adhesives) was very low (18.0%, 19.6%, and, 18.4%). After the addition of nSiO 2 , the solids content increased significantly by 48.9%, 60.7% and 44.6%, which was attributed to physical filling. The solids content of the three adhesives (HPMP, SBMP, and CSMP adhesive) showed the same change trend, increasing by 65.0%, 60.7%, and 76.6% respectively, and remained in the normal range.
Viscosity is an important parameter referring to handling and surface wetting ability of adhesives [37]. The viscosity of pure HPMP and CSMP adhesives were lower than the normal range, while the viscosity of pure soybean meal adhesive was within the normal range (Figure 9d-f), but the water resistance was poor. The SDS could destroy the secondary structure of protein and reduce the axial spacing between molecules so as to increase the friction [38]. Therefore, the viscosity of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives was increased by 96.1%, 17.8%, and 67.0% respectively. After the addition of nSiO 2 , the viscosity of SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives showed different degrees of improvement again because nSiO 2 could combine with protein molecules to form hydrogen bonds and inhibit its fluidity [39], but the viscosity was too large to be applied. After PAE crosslinking, the viscosity of the three adhesives (HPMP, SBMP, and CSMP adhesives) decreased significantly and remained in the normal range. There were two reasons: (1) The positively charged groups of PAE formed electrostatic interaction with charged protein chains, resulting in the reduction of attraction and repulsion between surrounding molecules, thus reducing the viscosity [25]. (2) The viscosity of PAE itself was lower than adhesive. In the process of crosslinking, PAE could be embedded into molecules and act as a "lubricant" [40]. Thus, the coatability and permeability of the adhesive were improved. In the process of practical application, we found that SBMP adhesive had the best applicating performance, and the applicated effect of HPMP adhesive was slightly better than that of CSMP adhesive; it was related to the natural structure of protein. This was also an important reason why SBMP adhesive could be sold in plywood enterprises, and the viscosity of HPMP adhesive and CSMP adhesive needs to be further optimized.
Water contact angle (WCA) is a significant parameter of an adhesive's affinity for an adherent, where smaller WCA indicates better wettability [41]. There was a negative correlation between the solid surface energy and the WCA (Figure 9g-i). In fact, adhesives with small WCA and high solid surface energy are easy to spread onto a substrate surface. The pure SBMP adhesive exhibited high WCA (64.77 • ) and poor solid surface energy (37.03 J/m 2 ) owing to its high molecular mass. On the contrary, the solid surface energy of HPM adhesive was the highest (57.12 J/m 2 ), while CSMP adhesive was the second (41.93 J/m 2 ). The WCA of three adhesives increased after adding SDS and nSiO 2 , while the solid surface energy decreased, which was related to their increasing viscosity. The WCA of HPMP, SBMP, and CSMP adhesives decreased significantly, and the solid surface energy increased significantly, indicating that PAE effectively improved the wettability of adhesives and was conducive to transverse spreading on the wood surface. Viscosity is an important parameter referring to handling and surface wetting ability of adhesives [37]. The viscosity of pure HPMP and CSMP adhesives were lower than the normal range, while the viscosity of pure soybean meal adhesive was within the normal range (Figure 9d-f), but the water resistance was poor. The SDS could destroy the secondary structure of protein and reduce the axial spacing between molecules so as to increase the friction [38]. Therefore, the viscosity of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives was increased by 96.1%, 17.8%, and 67.0% respectively. After the addition of nSiO2, the viscosity of SDS/nSiO2/HPMP, SDS/nSiO2/SBMP, and SDS/nSiO2/CSMP adhesives showed different degrees of improvement again because nSiO2 could combine with protein molecules to form hydrogen bonds and inhibit its fluidity [39], but the viscosity was too large to be applied. After PAE crosslinking, the viscosity of the three adhesives (HPMP, SBMP, and CSMP adhesives) decreased significantly and remained in the normal range. There were two reasons: (1) The positively charged groups of PAE formed electrostatic interaction with charged protein chains, resulting in the reduction of attraction and repulsion between surrounding molecules, thus reducing the viscosity [25]. (2) The viscosity of PAE itself was lower than adhesive. In the process of crosslinking, PAE could be embedded into molecules and act as a "lubricant" [40]. Thus, the coatability and permea-  Figure 9 provides the microcosmic SEM images of different cured adhesives. There were many irregular holes and cracks on the surface of the three adhesives (pure HPMP, SBMP, and CSMP adhesives), which means that water was easy to invade (Figure 10a1,a2,e1,e2,i1,i2), resulting in poor water resistance [42]. However, more cracks appeared on the surface of SDS/HPMP, SDS/SBMP, and SDS/CSMP adhesives (Figure 10b1,b2,f1,f2,j1,j2) because SDS destroyed the secondary structure of protein. Although the cracks were more regular, they showed brittle fracture. The number of large holes in SDS/nSiO 2 /HPMP, SDS/nSiO 2 /SBMP, and SDS/nSiO 2 /CSMP adhesives decreased significantly (Figure 10c1,g1,k1). After 2000× magnification (Figure 10c2,g2,k2), it could be seen that the number of cracks on the surface was significantly reduced, but there were ups and downs of folds. It was attributed to the formation of the first level of the network structure by hydrogen bonding between nSiO 2 and the active group of the protein. The holes and cracks of HPMP, SBMP, and CSMP adhesives were further reduced (Figure 10d1,h1,l1). It could be clearly observed that the surface became denser, which was due to the synergistic effect of the double network of hydrogen bonds and covalent bonds to form a certain network structure. The above results showed that the double-network structure could significantly improve the water resistance of the three different adhesives. After the comparison of the three adhesives (HPMP, SBMP, and CSMP adhesives), it was found that the surface of CSMP adhesive was the densest, indicating that water resistance was the best, while the surface of HPMP adhesive was the most wrinkled, which also supported the test results of bonding strength and water resistance. . SEM images of (a1,a2) A1-pure HPMP, (b1,b2) B1-SDS/HPMP, (c1,c2) C1-SDS/nSiO2/HPMP, (d1,d2) D1-HPMP adhesives; SEM images of (e1,e2) B1-pure SBMP, (f1,f2) B2-SDS/SBMP, (g1,g2) C2-SDS/nSiO2/SBMP, (h1,h2) and D2-SBMP adhesives; SEM images of (i1,i2) A3pure CSMP, (j1,j2) B3-SDS/CSMP, (k1,k2) C3-SDS/nSiO2/CSMP, (l1,l2) and D3-CSMP adhesives.

Crack Observation
The size and thickness of surface cracks of different cured adhesives could be compared through crack observation (Figure 11), evaluating the toughness and heat resistance of different adhesives [43]. There were many large holes and cracks on the surface of cured pure HPMP, SBMP, and CSMP adhesive, and the adhesives were loose and fragile, which was reflected their inherent brittleness. Although the holes of SDS/SBMP adhesive decreased, they showed larger cracks. The SDS/HPMP and SDS/CSMP adhesives also showed larger cracks because SDS destroyed the secondary structure of protein, reduced their cohesion, and increased the brittleness of the adhesive [44]. After the introduction of SiO2, there were only a few cracks in the surface of the cured SDS/nSiO2/SBMP adhesive, and the surface of cured SDS/nSiO2/HPMP and SDS/nSiO2/CSMP adhesives were closer, but there were still many uneven cracks, indicating that the cohesion provided by hydrogen bond was not enough to maintain toughness. The surface of the cured SBMP adhesive looked smoother because the soybean protein adhesive had high viscosity and covered the holes. There were still a few holes on the surface of cured HPMP adhesive, while the cracks and holes on the surface of cured CSMP adhesive were further reduced and dispersed. This was due to the dissipation of the internal stress by the double-network structure composed of covalent bonds and hydrogen bonds before it played its role. Figure 10. SEM images of (a1,a2) A1-pure HPMP, (b1,b2) B1-SDS/HPMP, (c1,c2) C1-SDS/nSiO 2 /HPMP, (d1,d2) D1-HPMP adhesives; SEM images of (e1,e2) B1-pure SBMP, (f1,f2) B2-SDS/SBMP, (g1,g2) C2-SDS/nSiO 2 /SBMP, (h1,h2) and D2-SBMP adhesives; SEM images of (i1,i2) A3-pure CSMP, (j1,j2) B3-SDS/CSMP, (k1,k2) C3-SDS/nSiO 2 /CSMP, (l1,l2) and D3-CSMP adhesives.

Crack Observation
The size and thickness of surface cracks of different cured adhesives could be compared through crack observation (Figure 11), evaluating the toughness and heat resistance of different adhesives [43]. There were many large holes and cracks on the surface of cured pure HPMP, SBMP, and CSMP adhesive, and the adhesives were loose and fragile, which was reflected their inherent brittleness. Although the holes of SDS/SBMP adhesive decreased, they showed larger cracks. The SDS/HPMP and SDS/CSMP adhesives also showed larger cracks because SDS destroyed the secondary structure of protein, reduced their cohesion, and increased the brittleness of the adhesive [44]. After the introduction of SiO 2 , there were only a few cracks in the surface of the cured SDS/nSiO 2 /SBMP adhesive, and the surface of cured SDS/nSiO 2 /HPMP and SDS/nSiO 2 /CSMP adhesives were closer, but there were still many uneven cracks, indicating that the cohesion provided by hydrogen bond was not enough to maintain toughness. The surface of the cured SBMP adhesive looked smoother because the soybean protein adhesive had high viscosity and covered the holes. There were still a few holes on the surface of cured HPMP adhesive, while the cracks and holes on the surface of cured CSMP adhesive were further reduced and dispersed. This was due to the dissipation of the internal stress by the double-network structure composed of covalent bonds and hydrogen bonds before it played its role.

Current Problems and Future Prospects
Compared with trialdehyde adhesive, plant protein adhesive could solve the problem of formaldehyde release. However, due to the limitation of raw materials and preparation technology, there are still the following four problems: (1) Plant protein adhesive is prone to mildew. (2) The bonding strength is unstable. (3) The initial adhesion of plant protein adhesives is poor. (4) Plant protein adhesives need crosslinking agents, so they are costly. This is also the direction of future research. It is believed that with the in-depth research of plant protein adhesives and the gradual solution of practical application problems, as one of the most environmentally friendly natural biomass adhesives with development potential, plant protein adhesives will be widely applied in the production of wood-based panels.
China is a big consumer of grain and oil. For agri-food business operators, hotpressed peanut meal containing Aspergillus flavus and cottonseed meal containing gossypol are wastes. Their annual output exceeds 10 million tons, which is an endless stream of green, raw materials. However, they cannot be processed into food or feed, so they can provide raw materials to adhesives or plywood companies to make protein adhesives, solving the waste recycling problem and generating additional income.

Conclusions
This study reported a low-cost, double-network construction theory, which was applied to high-performance HPMP, SBMP and CSMP adhesives. Under the synergistic

Current Problems and Future Prospects
Compared with trialdehyde adhesive, plant protein adhesive could solve the problem of formaldehyde release. However, due to the limitation of raw materials and preparation technology, there are still the following four problems: (1) Plant protein adhesive is prone to mildew. (2) The bonding strength is unstable. (3) The initial adhesion of plant protein adhesives is poor. (4) Plant protein adhesives need crosslinking agents, so they are costly. This is also the direction of future research. It is believed that with the in-depth research of plant protein adhesives and the gradual solution of practical application problems, as one of the most environmentally friendly natural biomass adhesives with development potential, plant protein adhesives will be widely applied in the production of wood-based panels.
China is a big consumer of grain and oil. For agri-food business operators, hot-pressed peanut meal containing Aspergillus flavus and cottonseed meal containing gossypol are wastes. Their annual output exceeds 10 million tons, which is an endless stream of green, raw materials. However, they cannot be processed into food or feed, so they can provide raw materials to adhesives or plywood companies to make protein adhesives, solving the waste recycling problem and generating additional income.

Conclusions
This study reported a low-cost, double-network construction theory, which was applied to high-performance HPMP, SBMP and CSMP adhesives. Under the synergistic action of hydrogen bonds and covalent bonds, the BWS of HPMP, SBMP, and CSMP adhesives exceeded the bonding strength requirement of type I plywood (0.7 MPa) by 17.1%, 35.7%, and 41.4%. The CSMP adhesive showed better water resistance and bonding strength than SBMP adhesive, and HPMP adhesive was also close to SBMP adhesive. Importantly, the cost of HPM and CSM was far lower than that of SBM, which helped to solve the problem of high dependence on soybean meal and provided an important theory basis for the high-value-added utilization of different kinds of plant meal.