A high performance soy protein-based bio-adhesive enhanced with a melamine/epichlorohydrin prepolymer and its application on plywood

Abstract

The aim of this study was to improve the water resistance of a soy protein-based bio-adhesive using a melamine/epichlorohydrin prepolymer (MEP). The multi-epoxy groups in the MEP reacted with active groups on the protein molecule and formed a crosslinked and denser network, which improved the water resistance of the resulting adhesive. In addition, the addition of a MEP increased the thermal ability of the cured adhesive and created a smooth surface with fewer holes and cracks preventing moisture intrusion and further improving the water resistance of the resulting adhesive. Furthermore, the use of MEP introduced a rigid structure into the adhesive formulation, which increased the rigidity of the cured adhesive, and further increased the water resistance of the adhesive. Incorporating 6 wt% MEP effectively improved the water resistance of the adhesive by 10.5%. The wet shear strength of the plywood bonded with the resultant adhesive increased by 281.1% to a maximum value of 1.41 MPa, which was higher than that of the soy beanmeal/PAE adhesive and commercial MUF and UF resin. This improvement in the resultant plywood was attributed to the adhesive water resistance improvement and additional interlock formation between the wood and adhesive resulting from the higher solid content of the adhesive as well as the viscosity reduction. The resultant adhesive has a solid content of 32.65% and viscosity of 499, 400 mPa s, which is acceptable for the industrial application of plywood fabrication.

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1 Introduction

Formaldehyde-based adhesives, especially urea-formaldehyde resin and its modification products, play a dominant role in the wood composites industry because of their advantages, such as good water resistance, ease of use and durability.¹ However, these adhesives have formaldehyde emission issues during the synthesis and use process, which are hazardous to human health. In addition, formaldehyde based adhesives are derived from non-renewable fossil sources.^2,3 Therefore, there is an urgent need to develop adhesives that are based on environmentally friendly and renewable resources.^4–6

In recent years, the use of soy protein-based products as a raw material to develop wood adhesive is one of the researchers' focuses because soybean is abundant, inexpensive, and environmentally friendly.^7,8 However, the low bond strength and water resistance of the soy protein-based adhesive limit their application. The efforts to improve the adhesion properties of the soy protein-based adhesives can be classified into the following three categories: denaturing agent modification, protein molecular modification, and reactive cross-linker and synthetic resin modification. Denaturants, such as urea, sodium dodecyl sulfate, and alkali, can unfold the protein molecule and expose the inside hydrophobicity groups to prevent water intrusion, improving the water resistance of the adhesive.^9–12 However the resulting panel bonded though the adhesive to the denaturants cannot meet the requirements of interior use. Soy protein molecular modification focuses on grafting high activity groups onto soy protein molecules.⁸ These groups react with each other or the active groups in soy protein molecules to form a cross-linked network in the resulting adhesive after curing. This process improves the water resistance of the adhesive, but it is complex and costly, which makes it impractical for plywood fabrication. Active cross-linker and synthetic resin modification include glycidyl methacrylate, polyamidoamine–epichlorohydrin (PAE) resin, polyethylene glycol diacrylate, latex-based adhesive, phenol formaldehyde resin, etc. Based on our previous studies,^13–15 a chemical compound with epoxy groups, such as ethylene glycol diglycidyl ether and 5,5-dimethyl hydantoin polyepoxide, is the most effective cross-linker for improving the water resistance of soy protein-based adhesives and the resulting plywood meets the interior use requirement. However, the reinforcement efficiencies of these chemical compounds are low and requiring a large dosage. In addition, the wet shear strength bonded from the adhesive with these compounds is approximately 20% lower than the commercial MUF resin. This is attributed to the following two reasons: (1) these chemical compounds have few epoxy groups in the chain, resulting in a low crosslinking degree. (2) The straight chain structure of these compounds has a low rigidity, resulting in a low adhesive strength. Therefore, a chemical compound with a rigid structure and multi-active groups may effectively improve of the water resistance and reduce the dosage, reducing the cost of the resulting adhesive.

In this study, a rigid cross-linker with a multi-epoxy group was synthesized using melamine and epichlorohydrin in the lab to modify the soy protein-based (SM) bio-adhesive. Three-ply plywood specimens were fabricated with the resulting adhesives. The solid content and viscosity of the adhesive, functional groups, thermo-stability, and fracture surface of the cured adhesive were characterized and analyzed to understand why the water resistance changed.

2 Experimental

2.1 Materials

Soybean meal (SM) was obtained from Xiangchi Grain and Oil Company in Shandong Province of China, and then milled to 250 mesh flour. Components of the soybean meal flour were tested as follows: 46.85% soy protein, 8.86% moisture, 6.46% ash, and 0.56% fat, and the remainder was polysaccharide. Sodium hydroxide (NaOH), sodium dodecyl sulfate (SDS), melamine, epichlorohydrin, triethylamine, and the other chemicals were AR grade reagents and purchased from Tianjin Chemical Reagent Co. Poplar veneer (40 × 40 × 1.5 cm, 8% of moisture content) was provided from the Hebei Province of China.

2.2 Preparation of the melamine/epichlorohydrin prepolymer (MEP)

The synthesis was performed in a 100 ml glass reactor equipped with magnetic stirring and a reflux condenser in a temperature controlled oil heating bath according to the procedure described in Pedros's research.¹⁶ Melamine (4.8 g) and the solvent DMF (30 ml) were added into the reactor. Stirring and heating were started. Then, ECH (30 ml) and triethylamine (2.8 ml) were added which was followed by the catalyst. After mixing all the compounds, the reactor was heated to a temperature of 100 °C and kept for 48 h. The final products were cooled to room temperature and then filtered. A first distillation was then performed to partially remove the solvent. The obtained products were precipitated in acetone and repeatedly washed with acetone. A viscous liquid was obtained; then, it was dried in an vacuum oven at 45 °C for 24 hours and got a yellow solid. Then the solid was dissolved in methanol, which was followed by the addition of 30% NaOH (30 g) at 60 °C for 2 hours of reaction. Then, the excess NaOH was neutralized by hydrochloric acid (30 wt%). Finally, the mixture was dried and solid MEP was obtained. The products had 4.8–4.9 lateral chains per molecule. The reaction equation is presented in Scheme 1.

Scheme 1 The reaction equation of the MEP.

2.3 Preparation of the soy protein-based bio-adhesive

For the different adhesive samples, soybean meal flour was added into deionized water and stirred for 10 min at 20 °C. Then sodium dodecyl sulfate and MEP were added sequentially and further stirred for 10 min at 20 °C. The adhesive formulations are shown in Table 1.

Table 1 Different adhesive formulations

Sample	Adhesive formulation
0 (SM adhesive)	Soybean meal flour (28 g); deionized water (72 g)
1 (SM/SDS adhesive)	Soybean meal flour (28 g); deionized water (72 g); sodium dodecyl sulfate (1 g)
2 (SM/SDS/MEP-3 adhesive)	Soybean meal flour (28 g); deionized water (72 g); sodium dodecyl sulfate (1 g); MEP (3 wt%)
3 (SM/SDS/MEP-6 adhesive)	Soybean meal flour (28 g); deionized water (72 g); sodium dodecyl sulfate (1 g); MEP (6 wt%)
4 (SM/SDS/MEP-9 adhesive)	Soybean meal flour (28 g); deionized water (72 g); sodium dodecyl sulfate (1 g); MEP (9 wt%)
5 (SM/SDS/MEP-12 adhesive)	Soybean meal flour (28 g); deionized water (72 g); sodium dodecyl sulfate (1 g); MEP (12 wt%)

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As a control, a soy bean meal-based adhesive with a commercial polyamide–epichlorohydrin (PAE) was prepared by the following procedure: soybean meal flour (28 g) was added into deionized water (72 g) and stirred for 10 min at 20 °C. Then, sodium dodecyl sulfate (1 g) and PAE (6 g) were sequentially added and further stirred for 10 min at 20 °C.

A commercial melamine–urea-formaldehyde resin and urea formaldehyde resin were used to fabricate plywood for comparison with the resulting adhesive in this research. The molar ratio of MUF and UF resin was 1.1, and formaldehyde was added at once. Urea was added two times with the first molar ratio of 2.0. For the MUF resin, the melamine content was 8 wt% and added with urea a second time. The solid contents of the resultant MUF resin and UF resin were 54% and 51%, respectively, and the viscosities of the MUF and UF resins were 300 and 120 mPa S, respectively. When bonding plywood, 100 g of MUF/UF resin was mixed with 25 g of wheat flour and 1 g of ammonium chloride to develop a mixture that was then applied on the veneer with 320 g m⁻² for a double side.

2.4 Preparation of the plywood sample

Three-ply plywood samples were prepared under the following conditions: 240 g m⁻² glue spreading for a single surface, 80 s mm⁻¹ hot press time, 120 °C hot press temperature, and 1.0 MPa hot press pressure. After hot pressing, the plywood samples were stored under ambient conditions for at least 12 h before testing.

2.5 Solid content measurement

The adhesive solid content was determined using an oven-drying method. Approximately 3 g (weight α) of the adhesive was placed into an oven and dried at 105 °C for hours until a constant weight (weight β) was obtained. The value of the solid content was calculated using the following equation. The average value of the solid content was calculated from three parallel samples.

2.6 Dynamic viscoelastic measurement

The apparent viscosity of the different adhesives was determined using a rheometer with a parallel plate fixture (20 mm diameter). The distance was set to 1 mm for all measurements. Experiments were conducted under a steady shear flow at 25 °C. The shear rates ranged from 0.1 to 300 s⁻¹ in 10 s⁻¹ increments. All measurements were conducted in triplicate, and the average value was reported.

2.7 Wet shear strength measurement

The wet shear strength of plywood was determined in accordance with the description in the China National Standards (GB/T 17657-2013). Twelve plywood specimens (2.5 cm × 10 cm) were cut from two plywood panels and submerged into water at 63 ± 2 °C for 3 h; then, they were dried at a room temperature for 10 min before tension testing. The wet shear strength was calculated by the following equation.

2.8 Residual rate test

The adhesive samples were placed in an oven at 120 ± 2 °C until a constant weight (M) was obtained. The cured adhesives were soaked in tap water for 24 h at ambient temperature and then oven-dried at 105 ± 2 °C for 5 h until a constant weight was obtained (m). The residual rate is defined as m divided by M, as shown in eqn (3). The average value of the residual rate was calculated from six parallel samples.

2.9 Fourier transform infrared (FTIR) spectroscopy

The adhesives were cured in an oven at 120 ± 2 °C until a constant weight was obtained and ground into a powder. FTIR spectra of the different cured adhesives were recorded on a Nicolet 7600 spectrometer (Nicolet Instrument Corporation, Madison, WI) from 500 to 4000 cm⁻¹ with a 4 cm⁻¹ resolution using 32 scans.

2.10 Thermogravimetric (TG) measurement

The different adhesives were cured in an oven at 120 ± 2 °C until a constant weight was obtained; then, they were ground into a powder. The thermal stabilities of the cured adhesives were tested using a TGA instrument (TA Q50, WATERS Company, USA). Approximately 5 mg of powdered samples were weighed in a platinum cup and scanned from the room temperature to 600 °C at a heating rate of 10 °C min⁻¹ in a nitrogen environment while recording the weight change.

2.11 Scanning electron microscopy (SEM)

The different samples were poured into a piece of aluminum foil and dried in an oven at 120 ± 2 °C until a constant weight was achieved. A Hitachi S-3400N (Hitachi Science System, Ibaraki, Japan) scanning electron microscope was used to observe fractured surfaces of the cured adhesive. The surface was sputter coated with gold prior to examination under a microscope.

3 Results and discussion

3.1 Solid content measurement

Solid content is an important property for a wood adhesive that influences the performance of the adhesive for bonding plywood. In general, the adhesive performances are improved with solid content. A low solid content of the adhesive indicates that higher water content needs to be removed from the assembled plywood during the hot press process, which easily damages the bond of the resulting plywood.¹⁷ The solid contents of the different adhesives are shown in Fig. 1. The solid content of the SM adhesive was 26.41%, which had a high viscosity and flowed poorly. A further increase in the solid content is determined by the additive in the adhesive formulation. As expected, the solid content of the SM/MEP adhesive increased with the addition of MEP because adding MEP was equivalent to increasing the mass content of the adhesive. Therefore, the solid content of the SM/MEP adhesive increased gradually from 27.81 to 36.27% with increase in the addition of MEP. Moreover, the solid content of the SM/SDS/MEP-6 adhesive was 32.46%, which was increased by 22.9% compared to the SM adhesive.

Fig. 1 Solid content of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

3.2 Dynamic viscoelastic measurement

The apparent viscosity of the different adhesives is shown in Fig. 2. The SM adhesive is a typical non-Newtonian fluid, such as blood and starch solutions, where there is a shear thinning behavior. Additionally the initial apparent viscosities were recorded and are presented in Table 2.

Fig. 2 The apparent viscosity of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

Table 2 The initial viscosity of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive)

MEP addition (%)	0	1	2	3	4	5
Initial viscosity (mPa s)	35 600	158 400	157 000	137 000	121 700	90 270

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The viscosity of the SM adhesive was recorded as 35600 mPa s. After incorporating SDS, the soy protein molecules unfolded and the distance between them decreased, increasing the force between molecules and apparent viscosity of the resulting adhesive by 345.0% to 158, 400 mPa s. The viscosity of the adhesive was decreased with the MEP addition increased. After adding 6% MEP, the apparent viscosity of the adhesive decreased by 13.5% to 137000 mPa s. When MEP content reached 12%, the viscosity of the adhesive further reduced by 43.0% to 90270 mPa s. This indicated that MEP dispersed well and was embedded between the soy protein molecules, reducing the frictional resistance within the liquid and resulting in a low viscosity of the adhesive.¹⁸

3.3 The residual rate measurement

The water resistance of the adhesive is an important property for a wood adhesive, which can be measured by the residual rate.¹⁹ Fig. 3 shows the residual rate of the different adhesives. The residual rate of the SM adhesive was 76.1%. When adding denaturing agent SDS, the residual rate of the SM/SDS adhesive increased to 77.2%, which was attributed to protein molecule unfolding and the exposure of hydrophobic groups in the adhesive. These hydrophobic groups prevent water intrusion and improve the water resistance of the adhesive. After using MEP, the residual rate of the SM/SDS/MEP-3 adhesive increased by 5.8%, and further increased by 4.4% when 6 g of MEP was added to the adhesive system (SM/SDS/MEP-6 adhesive). Compared with the SM/SDS adhesive, the residual rate of the SM/SDS/MEP-6 adhesive was improved by 10.5% and reached to 85.3%. This is because that the MEP possesses an epoxy group and can react with the active groups in the protein molecule, forming a cross-linked network that improved the water resistance of the adhesive. With a further increase in the MEP level, the residual rate decreased to 82.9% of the SM/SDS/MEP-12 adhesive. This reduction in the residual rate might contribute to the excessive MEP dissolved in water.

Fig. 3 The residual rate of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

3.4 FTIR spectroscopic analysis

The FTIR spectra of the different adhesives are presented in Fig. 4. The peak observed at approximately 3305 cm⁻¹ is related to the free and bound O–H and N–H bending vibrations, which can form hydrogen bonds with the carbonyl group of the peptide linkage in the protein and the wood surfaces. The peak observed at approximately 2930 cm⁻¹ is due to the symmetric and asymmetric stretching vibrations of the –CH₂ group in the different adhesives. The main absorption bands of the peptide are related to peaks that are at approximately 1653, 1537, and 1402 cm⁻¹, which are characteristic of amide I (C=O stretching), amide II (N–H bending) and amide III (C–N and N–H stretching), respectively.²⁰ The bands corresponding to C–O bending are located at 1055 cm⁻¹.

Fig. 4 FTIR spectra of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

With increase MEP levels from 0 to 12 wt% in the adhesive, the absorption peaks of amides II and III (1537 and 1402 cm⁻¹) gradually decreased, which was due to the reaction between the epoxy groups of the MEP and the N–H groups on the soy protein molecule. This reaction decreased the number of hydrophilic groups in the adhesive, which increased the water resistance of the adhesive. In addition, MEP cross-linked the soy protein molecule to form a denser network, which increased the crosslinking degree of the adhesive, improving the adhesive water resistance. With MEP increased to 12 wt%, the absorption peak of amide II was negligible, indicating that the addition of MEP was excessive and had reduced the water resistance of adhesive. After incorporating MEP, a strong absorption peak appeared at 925 cm⁻¹ from the characteristic peak of triazine ring, and the peak became stronger as the MEP level increased. This indicated that the MEP was well distributed in the adhesive system. There are supposed to be an epoxy group characteristic peak at 846 cm⁻¹; but, as shown in Fig. 4, there was no peak at 846 cm⁻¹ in the spectrum of the SM/SDS/MEP-6 adhesive. This phenomenon suggests that the epoxy group of MEP reacted with the active hydrogen on the –COOH and –NH₂ groups in the protein molecules during the curing process in a ring-opening reaction (likely the specific reaction discussed in Fig. 5). With increased MEP levels, the peak at 846 cm⁻¹ was observed again, suggesting the MEP was excessive in the formulation of the SM/SDS/MEP-9 and SM/SDS/MEP-12 adhesives.

Fig. 5 The curing process of the SM/SDS/MEP adhesive.

3.5 TGA analysis

Fig. 6 shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the different adhesives. The thermal degradation process of the adhesives could be divided into three stages. The first stage (I) is a post-reaction stage, at a temperature of 50–210 °C, which is attributed to the possible reaction of the system under thermal action. The second stage (II) is an initial degradation stage from 210 °C to 290 °C, which results from the weight loss of the small molecule subject degradation and break of some unstable chemical bonds. The third stage (III) is a skeleton structure of the sample degradation stage, at a temperature of 290–380 °C, which is attributed to the cross-linking network structure degradation. Before the first degradation stage, the small weight loss is attributable to the evaporation of the residual moisture. After the third degradation stage, further heating causes breakages of the C–C, C–N, and C–O linkages, and the soy protein backbone peptide bonds are decomposed, producing gases like CO, CO₂, NH₃, and H₂S.²¹ The decomposition of the residual modifier is also considered and belongs to this degradation stage.

Fig. 6 The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

After using MEP in the adhesive formulation, the peaks of the DTG curve in the second stage were stronger than that in the SM adhesive, indicating there was a reaction between the MEP and soy protein molecule. In this process, the cross-linked protein molecule network played a major role in the adhesive system, which improved the water resistance of the adhesive. As the MEP addition further increased, the reactive groups on the protein deceased (FTIR analysis); as a result, the MEP was residual in the adhesive system, which caused the adhesive water resistance to decrease.²² Additionally, the weight loss at a temperature of 350–400 °C was from the decomposition temperature of thiotriazinone. As MEP level increased, the peak of weight loss increased, demonstrating that MEP was well distributed in the adhesive. In the third stage, the thermal degradation behavior was different for samples with and without MEP. The SM adhesive had the highest degradation rate, which then decreased significantly with MEP incorporation. After MEP was added to the adhesive formulation, the degradation rate decreased, suggesting there was better thermal stability of the SM/MEP adhesive. These results indicated the MEP addition increased the thermal stability of the cured soy protein-based bio-adhesives via crosslinking reaction.

3.6 SEM analysis

The fracture surface micrographs of the different cured adhesives are shown in Fig. 7. Many holes and cracks were observed on the fracture surface of the SM adhesive. In addition, the entire fracture surface appeared very loose and disordered. These holes and cracks were formed by water evaporation in the adhesive during the hot press process. Moisture could intrude into those cracks, resulting in swelling to breaking the bond, which reduced the water resistance of the adhesive.²³ After adding SDS, no holes or cracks were observed and the fracture surface became smooth. This was attributed to the unfolded protein molecules following rearrangement with denaturation and formation of a more crystalline domain, which smoothened the section.

Fig. 7 The fracture surface micrograph of the different cured adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), and 5 (SM/SDS/MEP-12 adhesive).

After introducing the MEP, the fracture surface of the cured adhesive became more compact due to the reaction between the MEP and soy protein to, which resulted in a strong cross-linked network that could effectively prevent moisture intrusion and improve the water resistance of the adhesive. With further increases in MEP level, holes, cracks, and disordered surfaces were observed again on the facture surface, which may be from the excessive MEP, which then act as a filler, filling in the adhesive system and affecting the water gasification. Additionally, the holes and cracks helped moisture attach and swell, which decreased the water resistance of the adhesive.

3.7 Wet shear strength measurement

The wet shear strength of plywood was determined by both the resistance of the adhesive and the solid content and viscosity of the adhesive. The wet shear strengths of the plywood bonded to the different adhesives are shown in Fig. 8. The wet shear strength of the plywood bonded to the SM adhesive was 0.37 MPa, which failed to meet the interior use plywood requirement (≥0.7 MPa). The wet shear strength of the plywood bonded to the SM/SDS/MEP-3 adhesive increased by 195% to 1.09 MPa, which met the interior use plywood requirement. Further increasing the MEP level, the wet shear strength of the plywood bonded to SM/SDS/MEP-6 adhesive increased by 281.1% compared with the SM adhesive and reached a maximum value of 1.41 MPa. This was because the MEP could react with the active groups on the soy protein molecule and a cross-linking network could form between the MEP and soy protein molecule, introducing a solid structure to that improved the water resistance. In addition, the MEP had a thiotriazinone in its structure, which could introduce rigidity to the structure in the adhesive system and further increase the water resistance of the adhesive. An adhesive with elevated water resistance led to an increase in the wet shear strength of the resulting plywood. From another perspective, after adding MEP to the adhesive system, the solid content of the adhesive was improved and the viscosity of the adhesive was reduced, which helped the adhesive penetrate into the wood surface and form more interlocks. This further increased the wet shear strength of the plywood.²⁴ By increasing the MEP content, the wet shear strength of the resulting plywood bonded with SM/SDS/MEP-12 adhesive decreased to 1.02 MPa, contributing to the water resistance reduction of the adhesive. This was also in accordance with result of the residual rate measurement.

Fig. 8 The wet shear strength of the different adhesives: 0 (SM adhesive), 1 (SM/SDS adhesive), 2 (SM/SDS/MEP-3 adhesive), 3 (SM/SDS/MEP-6 adhesive), 4 (SM/SDS/MEP-9 adhesive), 5 (SM/SDS/MEP-12 adhesive), 6 (SM/PAE adhesive), 7 (MUF resin), and 8 (UF resin).

Fig. 8 also shows the wet shear strength comparison of the plywood bonded with different adhesives. The wet shear strength of plywood bonded with 6% MEP was 57% higher than that bonded with 6% PAE, demonstrating that a chemical compound with multi-epoxy groups and a rigid structure has better efficiency for improving the water resistance of the resulting plywood. Compared with the commercial formaldehyde based adhesive, the wet shear strength of the plywood bonded with the soy bean meal-based adhesive with 6% MEP was 26% higher than that of MUF resin and 3.4 times higher than that of UF resin. Also, because of the rigid structure introduction and the denser crosslinked network formation, the aging resistance of the SM/SDS/MEP-6 adhesive was also greatly improved and even better than the MUF resin (Fig. S1†). But a bio-based adhesive is easy to mildew and thus decreases the bond strength. After modification with MEP, the mildew resistance property was much lower than that of MUF resin (Fig. S2†).

4 Conclusions

Introducing MEP as a cross-linker effectively increased the water resistance of a soy bean meal-based adhesive due to the multi-epoxy groups and rigid structure of MEP. The use of 6 wt% MEP improved the water resistance of the resulting adhesive by 10.5%, and the residual rate reached a maximum value of 85.3%, which was attributed to the formation of a solid crosslinking network from the reaction between the epoxy group of MEP and active groups on the soy protein molecule. This denser network also increased the thermal stability of the adhesive and created a smooth, denser fracture surface of the cured adhesive to prevent moisture intrusion. This further increased the water resistance of the adhesive. Using the resulting adhesive to bond plywood, the wet shear strength was increased by 281.1% to 1.41 MPa, which met the interior use plywood requirement. This improvement was due to the adhesive water resistance improvement, solid content of the adhesive improvement, and viscosity of the adhesive reduction. An elevated water resistance prevented water intrusion, improving the wet shear strength of the plywood. Additionally, the elevated solid content and reduced viscosity resulted in a higher level of interlocking between the wood and adhesive, which further increased the wet shear strength of the resulting plywood. Compared with other adhesives, soy bean meal-based adhesive with MEP was 57% stronger than that of soy bean meal-based adhesive with 6% PAE, and also was 26% and 340% stronger than commercial MUF and UF resins, respectively.

Acknowledgements

The authors are grateful for financial support from the Fundamental Research Funds for the Central Universities (BLYJ201625) and the Beijing Natural Science Foundation (2151003).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15597a

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