Wet Shear Strength Of Plywood Research Articles

Study on surface grafting of hydroxyapatite and its influence on the properties of urea-formaldehyde resin

Urea formaldehyde (UF) is the most widely used resin in the wood composite board industry. UF has formaldehyde emission, which is harmful to the environment and human health. In this paper, hydroxyapatite (HAP) was surface modified by silane coupling agent (HAP-KH). And hydroxyapatite/urea formaldehyde resin (HAP-UF) was prepared by adding different amounts of HAP-KH. The optimal amount of HAP-KH was determined by wet shear strength and formaldehyde release. The degradation performance and degradation kinetics of the resin were also studied. The results show that the addition of HAP-KH can improve the wet shear strength of plywood and reduce the release of formaldehyde. The optimal content of HAP-KH was 3%, and the formaldehyde emission of plywood reached the lowest level of 0.35 mg L−1, 52.32% lower than that of the control resin. The wet shear strength was 1.14 MPa, 14.4% higher than that of the control resin. HAP-KH had no significant effect on the thermal stability of UF resins. However, HAP-KH has a significant effect on the thermal degradation mechanism of UF resins. The most likely kinetics mechanism models for UF and HAP-UF are G(a) = [−ln(1−α)]2 and G(a) = [−ln(1−α)]3, respectively.

Effects of Lysine on the Interfacial Bonding of Epoxy Resin Cross-Linked Soy-Based Wood Adhesive.

Soy protein isolate (SPI) is an attractive natural material for preparing wood adhesives that has found broad application. However, poor mechanical properties and unfavorable water resistance of wood composites with SPI adhesive bonds limit its more extensive utilization. The combination of lysine (Lys) with a small molecular structure as a curing agent for modified soy-based wood adhesive allows Lys to penetrate wood pores easily and can result in better mechanical strength of soy protein-based composites, leading to the formation of strong chemical bonds between the amino acid and wood interface. Scanning electron microscopy (SEM) results showed that the degree of penetration of the S/G/L-9% adhesive into the wood was significantly increased, the voids, such as ducts of wood at the bonding interface, were filled, and the interfacial bonding ability of the plywood was enhanced. Compared with the pure SPI adhesive, the corresponding wood breakage rate was boosted to 84%. The wet shear strength of the modified SPI adhesive was 0.64 MPa. When Lys and glycerol epoxy resin (GER) were added, the wet shear strength of plywood prepared by the S/G/L-9% adhesive reached 1.22 MPa, which increased by 29.8% compared with only GER (0.94 MPa). Furthermore, the resultant SPI adhesive displayed excellent thermostability. Water resistance of S/G/L-9% adhesive was further enhanced with respect to pure SPI and S/GER adhesives through curing with 9% Lys. In addition, this work provides a new and feasible strategy for the development and application of manufacturing low-cost, and renewable biobased adhesives with excellent mechanical properties, a promising alternative to traditional formaldehyde-free adhesives in the wood industry.

Improving Bond Performance and Reducing Cross-Linker Dosage of Soy Protein Adhesive via Hyper-Branched and Organic-Inorganic Hybrid Structures.

Eco-friendly soybean protein adhesives could be an ideal substitute for replacing traditional formaldehyde-based adhesives in wood industry. However, a large number of cross-linking agents are required in soy protein adhesive formulations to obtain sufficiently performing properties. Inspired by the high performance of nacre and branched structures, a hyper-branched amine (HBPA) was synthesized and grafted to graphene oxide (GO), generating a hyper-branched amine-functionalized GO (FGO). A novel soy protein-based adhesive was developed by mixing FGO with soy protein (SPI) and a low dose polyamidoamine-epichlorohydrin (PAE). Results showed that the addition of only 0.4 wt% FGO and 0.75 wt% PAE to the SPI adhesive formulation enhanced the wet shear strength of plywood to 1.18 MPa, which was 181% higher than that of the adhesive without enhancement. The enhanced performance is attributed to the denser cross-linking structure and improved toughness of the adhesive layer. Using FGO in the adhesive formulation also greatly reduced the concentration of the additive cross-linker by up to 78.6% when compared with values reported in the literature. Thus, using a hyper-branched functionalized nano-material to form an organic-inorganic hybrid structure is an effective and efficient strategy to reinforce the composites and polymers. It significantly reduces the chemical additive levels, and is a practical way to develop a sustainable product.

Hyperbranched catechol biomineralization for preparing super antibacterial and fire-resistant soybean protein adhesives with long-term adhesion

• A hyperbranched catechol polymers (DBA@HBPA) was synthesized to mineralize Ag + . • The reaction among CPBA, SPI, and DBA@HBPA built a water-resistant cross-linking network. • The adhesive with anti-mildew/anti-bacterial properties had a long-term adhesion for 12 d. • The adhesive has high bond strength, good toughness, fire resistance and low VOCs emission. Developing a strong soybean protein adhesive with long-term adhesion and multiple functions to replace petroleum-based adhesives is important but challenging. Inspired by the biomineralization strategy, soybean protein as a matrix, 4-carboxyphenylboronic acid as a crosslinker, a self-synthesized hyperbranched catechol polymer (DBA@HBPA) as a catechol group donor, and AgNO 3 as an Ag + donor were used to develop a strong and multifunctional soybean protein adhesive. The results showed that the coating and prepressing performance of the adhesive was improved by the addition of DBA@HBPA. The prepressing intensity of the adhesive improved by 156% and reached 0.89 MPa. Benefiting from the dynamic balance of quinone-catechol produced by Ag NPs and the super antimildew and antibacterial properties of Ag NPs and boron, the adhesive exhibited long-term adhesion behavior (pot life of 12 d), excellent mildew resistance (30 d), and antibacterial properties (colony count of 0). The wet shear strength of plywood prepared by the resultant adhesive was 1.28 MPa, which was 106% higher than that of the SPI adhesive. In addition, the resultant adhesive exhibited fire retardancy (LOI = 37.4%) and low VOCs emission (4.26*10 6 mg/m 2 ·h). This biomimetic design provides a novel and efficient strategy for the preparation of high-adhesion, multifunctional biomass composite adhesives.

Development of a strong soy protein-based adhesive with excellent antibacterial and antimildew properties via biomineralized silver nanoparticles

Developing eco-friendly soy protein-based adhesives instead of the extensively used aldehyde-based adhesives to fabricate wood-based panels can reduce environmental hazards and benefit human health. However, traditional soy protein-based adhesives lack long-term antibacterial and antimildew properties, reducing their bond strength and durability. Here, 3,4-dihydroxybenzaldehyde-grafted chitosan was prepared as a catechol structure donor and then introduced into the soy protein isolate (SPI) adhesive for the in situ reduction of silver ions to stable silver nanoparticles (Ag NPs); a biomineralization system was thus established. Notably, the catechol–quinone reversible redox reaction in the system endowed the resultant adhesive with excellent antibacterial (more than 99.99% bactericidal rate against Staphylococcus aureus and Escherichia coli , 91.67% bactericidal rate against Candida albicans ) and long-term antimildew properties (40 d). The wet shear strength (1.24 MPa) and prepressing bond strength (0.80 MPa) of the plywood prepared with the adhesive were increased by 63.27% and 129.63%, respectively, relative to the SPI adhesive. The volatile organic compounds release of the adhesive was 1–2 orders of magnitude lower than that of commonly used commercial adhesives. This strategy provides inspiration for reinforcing antibacterial or antimildew bio-based adhesives, hydrogels, and composites. • A system of biomineralized Ag NPs was built in the soy protein-based adhesive. • CS and SPI are used as stabilizers to avoid aggregation and agglomeration of Ag NPs. • The wet shear strength of plywood bonded by the adhesive was improved by 129.63%. • The adhesive has long-term antibacterial, mildew resistance, and low VOCs release.

Biomimetic lignin-protein adhesive with dynamic covalent/hydrogen hybrid networks enables high bonding performance and wood-based panel recycling

Exploring the reusability of wood-based panels is imperative in the wood industry for sustainable development and carbon balance. Non-reusable adhesives make wood-based panel recycling difficult. In this study, inspired by the adhesion and de-adhesion behavior of snail slime, we built dynamic covalent/hydrogen hybrid networks into adhesive system for achieving both high bonding performance and reusability. Specifically, the softwood lignin was purified and pretreated by ultrasonication to form a catechol structure (UAL) and then combined with soybean protein to develop a 100 % bio-based wood adhesive. The catechol structure of UAL formed dynamic covalent bonds (CN) with the amino groups of the protein to improve the water resistance and formed multiple hydrogen bonds as a sacrificial network to improve the toughness of the adhesive. Thus, the wet shear strength of plywood bonded by the resultant adhesive improved by 101.4 % to 1.37 MPa. The adhesive also exhibited flame retardancy (LOI = 37.7 %), mildew resistance (60 h), and antibacterial performance (inhibition zone = 8 mm). Notably, owing to the rearrangement of dynamic covalent/hydrogen hybrid networks and the thermoplastic property of UAL, the resultant adhesive was reusable (3 cycles) and degradable (2 months), which provides a potential method for the reuse of wood-based panels.

A high bonding performance and antibacterial soybean meal adhesive with Maillard reaction based cross-linked structure

Environmentally friendly soybean meal-based (SM) adhesive has been developed to replace harmful formaldehyde-based adhesives in the wood-based panel industry. But, the extensive application of SM adhesives is limited by several deficiencies, including high brittleness, poor coating ability, and poor antibacterial/mildew resistance, etc. In this study, a new SM adhesive has been designed by constructing a hyperbranched cross-linked structure via the Maillard reaction instead of traditional epoxide-based crosslinking structure. Specifically, the hyperbranched poly(ethylenimine)-functionalized carboxymethyl chitosan was synthesized via a one-step procedure using carboxymethyl chitosan and polyethyleneimine , and then mixed with soybean meal and a crosslinker (2,3-butanedione) to develop a wood adhesive. The dry and wet shear strength of plywood bonded by the obtained adhesive was 55% and 357% higher than those of the original SM adhesive, respectively, which were attributed to the formation of Maillard reaction-based crosslinked structure and improvement of adhesive toughness. The corresponding wood failure percentage was increased to 70% compared to pure SM adhesive. Notably, the prepared adhesive exhibited mildew resistance for 240 h and had a uniform coating ability on the wood surface. Construction Maillard reaction based crosslinking structure has potential application in the enhancement of other bio-adhesives and protein-based hydrogels.

CaCl2-induced interfacial deposition for the preparation of high-strength and flame-retardant plywood using geopolymer-based adhesive

Widely used formaldehyde-based resins for modern wood-based panels are toxic to humans and the environment. Geopolymer-based adhesives (GA) are abundant environmentally inorganic materials with wide applications in engineering fields. However, the modification and design of GA with excellent properties for practical applications remain a major challenge. Here, calcium chloride (CaCl2) solution was used to coat the wood surface, which quickly polymerized in situ with the geopolymer to form calcified geopolymer during hot-pressing, leading to the construction of a robust geopolymer/wood bonding interface. The formation mechanism of the reinforced interface between wood and geopolymer was characterized, and the wet shear strength and flammability were evaluated based on the binding application of plywood. The calcified geopolymer can achieve the dual effects of blocking the penetration of lye and promoting in situ reinforcement during hot-pressing, which effectively eliminates the impact of the weak interface layer of the alkali/wood and reorganizes the microstructure of the geopolymer/wood bonding interface. The wet shear strength of plywood with a 2.0 mol/L CaCl2 solution loading was increased from 0.45 MPa to 0.87 MPa, an increase of 93% compared with the pristine samples. The inherent flame retardancy of geopolymers contributed to a 42.6% reduction in the first peak of the heat release rate and a decrease of 28.9% in the total heat release compared with industrial urea-formaldehyde resin. This resulting GA, which was prepared by a facile and green method with markedly increased wet shear strength and inherent flammability, may have potential applications in functional wood-based composites and be used as a replacement for formaldehyde-based adhesives.

An environmentally-friendly soybean based resin as an alternative to formaldehyde-based counterpart for biomass composites

Wood based composites are widely used in furniture and other building construction, but the majority of which are bonded with formaldehyde-based resins that may cause serious health concern. This study presents an environmentally-friendly soybean based resin via combination of waste newspaper powder (WNP), soybean flour (DSF), and polyamidoamine-epichlorohydrin for woody composites, and its adhesion properties as investigated via shear strength. The results showed that the developed DSF-WNP resin (or soybean based resin) cured well at currently commercial plywood manufacture temperature (130 °C) and could be cured at a lower temperature of 85 °C for energy saving as well, through chemical cross-linking reactions between hydroxyl-azetidinium ring groups in the polyamidoamine-epichlorohydrin and hydrophilic groups, such as amido, carboxyl and hydroxyl in other components. WNP with particle sizes ranging from 120 μm to 250 μm were suitable for the resin applications. WNP resulted in poor dispersion, but lowered water absorption and increased insoluble fraction content and tensile strength in the cured resin. The wet shear strength of plywood bonded with DSF-WNP resin with a 2/5 dry weight ratio of WNP/DSF was 1.24 MPa, which met the requirements of the Chinese National Standard GB/T 9846–2015 for interior plywood. The resin characteristics including fast curing and excellent wet-cohesion performance are comparable to the mostly used urea-formaldehyde adhesives, which showed great potential in wood composites as an alternative to formaldehyde-derived adhesives.

Modified geopolymer-based wood adhesive using waterborne polyurethane

Geopolymer binders show great potential in the application of eco-friendly wood composite adhesives. The applicability of organic-inorganic hybrid-reinforced geopolymer composites as plywood binder was investigated. In this study, a geopolymer-based wood adhesive was fabricated by mixing a matrix-geopolymeric slurry; a toughening agent, waterborne polyurethane; and a silane coupling agent, that served as the covalent “bridge” between the waterborne polyurethane with a geopolymer matrix. The results showed that the waterborne polyurethane exhibited excellent compatibility with the geopolymer and served as a flexibilizer, which transformed the matrix from a microfractured structure to a denser morphology. Moreover, the shear strength of bonded plywood and the morphology of the fracture surface after the tensile measurement were measured. The resulting geopolymer/wood interface was well bonded, and the interfacial bonding strength was higher than the wood strength matrix after modification. The introduction of waterborne polyurethane and silane coupling agent improved the water resistance of the composites and increased the wet shear strength of plywood from 0 MPa to 0.35 MPa. Notably, a weak wood/alkali interface was formed under alkaline conditions due to the strong diffusion of alkali metal ions between the interfaces.

Improve performance of soy protein adhesives with a low molar ratio melamine-urea-formaldehyde resin

In this study, the low molar ratio melamine-urea-formaldehyde (MUF) resin and sodium hydroxide were used to modify soy protein adhesives, and the addition of MUF was studied as the influencing factor, the properties of soy protein adhesives, formaldehyde emission and bonding strength were studied. The results showed that the solid content and viscosity of the adhesives were suitable for the application of wood adhesives, the formaldehyde emission and wet shear strength of plywood prepared by soybean protein adhesive modified by MUF with 30% addition were 0.38mg/L and 1.25 MPa. Moreover, based on the analysis of the free formaldehyde content of the adhesive and the formaldehyde emission of plywood, it can be inferred that most of the free formaldehyde in the plywood was derived from the unreacted methyl group during the hot pressing process.

An Eco-Effective Soybean Meal-Based Adhesive Enhanced with Diglycidyl Resorcinol Ether.

Soybean meal-based adhesive is a good wood adhesive mainly due to its renewable, degradable, and environmentally friendly features. To improve the enhancement efficiency for adhesives, diglycidyl resorcinol ether (DRE) containing a benzene ring and flexible chain structure was used as an efficient cross-linker to enhance the adhesive in the study. The physicochemical properties of adhesives, the dry shear strength, and wet shear strength of plywood were measured. Results suggested that DRE reacted with the functional groups of soybean meal adhesive and formed a cross-linking network during hot press process in a ring-opening reaction through a covalent bond. As expected, compared to adhesive control, the soybean meal adhesive with 4 wt% DRE incorporation showed a significant increment in wet shear strength by 227.8% and in dry shear strength by 82.7%. In short, soybean meal adhesive enhanced with DRE showed considerable potential as a wood adhesive for industrial applications.

Soybean Meal-Based Wood Adhesive Enhanced by Phenol Hydroxymethylated Tannin Oligomer for Exterior Use.

Bio-based adhesives have low water resistance and they are less durable than synthetic adhesives, which limits their exterior applications. In this study, a bio adhesive was developed from soybean meal and larch tannin that was designed for exterior use. Phenol hydroxymethylated tannin oligomer (PHTO) was synthesized and then mixed with soybean meal flour in order to obtain a soybean meal-based adhesive (SPA). The results showed that the moisture absorption rate, residual rate, and solid content of SPA with 10 wt % PHTO (mass ratio with respect to the entire adhesive) were improved by 22.8%, 11.6%, and 6.8%, respectively, as compared with that of pure SPA. The wet shear strength of plywood with SPA with 10 wt % PHTO (boiling in 100 °C water for 3 h) was 1.04 MPa when compared with 0 MPa of pure SPA. This met the bond strength requirement of exterior-use plywood (GB/T 9846.3-2004). This improved adhesive performance was mainly due to the formation of a crosslinked structure between the PHTO and the protein and also PHTO self-crosslinking. The formaldehyde emission of the resulting plywood was the same as that of solid wood. The PHTO-modified SPA can potentially extend the applications of SPAs from interior to exterior plywood.

A High‐Performance Bio‐Adhesive Using Hyperbranched Aminated Soybean Polysaccharide and Bio‐Based Epoxide

AbstractWood adhesives fabricated from biomass can substitute formaldehyde‐based ones, reducing the consumption of fossil resources and eliminating formaldehyde pollution. In this study, a novel and sustainable water‐soluble soybean polysaccharide (WSSPS)‐based adhesive is designed. Specifically, a hyperbranched aminated polysaccharide (HBPA–g–WSSPS), as an adhesive matrix, is synthesized via grafting reaction between oxidized WSSPS and hyperbranched polyamide (HBPA). Triglycidylamine, a bio‐based epoxide cross‐linker, reacted with the HBPA–g–WSSPS to afford a cross‐linked hyperbranched network, providing strong cohesion for the adhesive. Additionally, polyacrylamide imparted an adjustable viscosity to the adhesive. The resultant WSSPS‐based adhesive enhanced the wet shear strength of plywood (1.24 MPa) by 49% compared to the E0‐level industrial‐use melamine‐urea‐formaldehyde (MUF) resin and by 26% compared to the industrial‐use soybean meal/polyamidoamine‐epichlorohydrin (PAE) adhesive. In addition, the adhesive has characteristics similar to those of the MUF resin, including viscosity, coating performance, hot‐pressing conditions, and dry bond strength, which are superior to those of the soybean meal/PAE adhesive. This developed polysaccharide‐based adhesive can potentially enhance various composites and can be used in industrial sectors.

Soy protein adhesive with bio-based epoxidized daidzein for high strength and mildew resistance

Soy protein based adhesives showed great potential for replacing petroleum derived formaldehyde adhesives in the commercial wood industry. However, soy protein adhesives have several problematic limitations, particularly with respect to low water and mildew resistence. As such, in order for soy protein-based adhesives to be practically applicable on an industrial level, a multi-function bio-based crosslinker that resolves both low water and mildew resistance must be designed. In this study, we synthesized a multifunctional crosslink agent (DDE) by reacting soybean-derived daidzein with epichlorohydrin (ECH) and incorporating it into soy protein to parpare a 100% bio wood adhesive with significantly improved water and mildew resistances. The results showed that by using 6 wt% DDE in the adhesive formulation, the dry and wet shear strength of plywood bonded with the SPI/DDE adhesive increased by 52.3% and 164.4%, respectively, compared with that of SPI adhesive; and was shown to be 22.3% and 69.6% higher than that of the industrially used SPI/PAE adhesive. These improvements were attributed to formation of double cross-linked network and elevated adhesive toughness. Furthermore, the resultant adhesive’s mildew resistance property were also improved with the addition of DDE, which extended the shelf life of adhesive and durability of the resultant plywood. Thus, this resultant DDE can be used to improve the mechanical properties and mildew resistance for bio-hydrogels or bio-based composites and polymers.

Investigation of Synthesis Mechanism, Optimal Hot-Pressing Conditions, and Curing Behavior of Sucrose and Ammonium Dihydrogen Phosphate Adhesive.

In this study, a further investigation was carried out on the synthesis mechanism, optimal manufacturing conditions, and curing behavior of a sucrose-ammonium dihydrogen phosphate (SADP) adhesive. The results of 13C nuclear magnetic resonance (NMR) spectroscopy confirmed that SADP was composed of 5-hydroxymethylfurfural (5-HMF), deoxyfructosazine (DOF), amino compounds, Schiff base, monosaccharides, and oligosaccharide. The optimal hot-pressing conditions were a hot-pressing temperature of 170 °C, a hot-pressing time of 7 min, and a spread rate of 120 g/m2. The wet shear strength of plywood bonded at optimal manufacturing conditions met the requirements of China National Standard (GB/T 9846-2015). Thermal analysis and insoluble mass proportion measurements showed that the main curing behavior of the SADP adhesive occurred at curing temperatures higher than 145 °C, and more than 50% insoluble mass was formed when the heating time was longer than 5 min. Fourier-transform infrared spectroscopy (FT-IR) indicated that cross-linking of the cured adhesive was promoted by prolonging the heating time. In addition, pyrolysis gas chromatography and mass spectrometry (Py-GC/MS) confirmed that the cured SADP adhesive was composed of furan and nitrogen-containing compounds.

Enhanced performance of urea–glyoxal polymer with oxidized cassava starch as wood adhesive

To eliminate the hazard of formaldehyde from wood-based products to human and environment, formaldehyde was replaced by glyoxal to produce wood adhesive. Urea–glyoxal (UG) resin was environmental friendly, while its bonding strength was very poor, especially its water resistance. The object of this work was to improve the mechanical properties of UG resin by oxidized cassava starch addition. Hence, the urea–glyoxal (UG) resin was synthesized and the oxidized cassava starch was added through mechanical mixing. The bonding strength, structure distributions, and the morphology features of the cured UG resin system were investigated by producing a three-layer plywood, FTIR, and SEM analysis. The results of dry and wet shear strength of plywood indicated that there was a positive effect of oxidized cassava starch on bonding strength of a three-layer plywood, and when the oxidized cassava starch content was increased to 45%, the dry strength could reach 1.21 MPa, and the wet strength was 0.72 MPa. The FTIR results showed that chemical reaction between UG resin and oxidized cassava starch was beneficial to the branched structure formation and higher cohesion strength of UG resin. Meanwhile, the tightness structure of enhanced UG resin system was observed by SEM analysis as well. These improved properties were contributed to water resistance improvement of UG resin.

Development of a High-Performance Adhesive with a Microphase, Separation Crosslinking Structure Using Wheat Flour and a Hydroxymethyl Melamine Prepolymer.

The objective of this study is to use wheat flour (WF) and hydroxymethyl melamine prepolymer (HMP) to develop a low cost, highly water-resistant, starch-based bio-adhesive for plywood fabrication. Three-layer plywood was fabricated using the resultant adhesive, and the wet shear strength of the plywood samples was measured under various conditions. After determining that water resistance was significantly improved with the addition of HMP, we evaluated the physical characteristics of the starch-based adhesive and functional groups and analyzed the thermal stability and fracture surface of the cured adhesive samples. Results showed that by adding 20 wt.% HMP into WF adhesive, the sedimentation volume in the resultant adhesive decreased by 11.3%, indicating that the increase of crosslinking in the structure of the adhesives increased the bond strength, and the wet shear strength of the resultant plywood in 63 °C water improved by 375% when compared with the WF adhesive. After increasing the addition of HMP to 40 wt.%, the wet shear strength of the resultant plywood in 100 °C water changed from 0 MPa to 0.71 MPa, which meets the exterior use plywood requirement. This water resistance and bond strength improvement resulted from (1) HMP reacting with functions in WF and forming a crosslinking structure to prevent moisture intrusion; and (2) HMP self-crosslinking and combining with crosslinked WF to form a microphase separation crosslinking structure, which improved both the crosslinking density and the toughness of the adhesive, and subsequently, the adhesive’s bond performance. In addition, the microphase separation crosslinking structure had better thermostability and created a compact ductile fracture surface, which further improved the bond performance of the adhesive. Thus, using a prepolymer to form a microphase separation crosslinking structure within the adhesive improves the rigidity, toughness, and water resistance of the material in a practical and cost-effective manner.

Characterization and Performance of Soy-Based Adhesives Cured with Epoxy Resin.

Soy-based adhesives have attracted much attention recently because they are environmentally safe, low cost, and readily available. To improve the gluability and water resistance of soy-based adhesives, we prepared an enzyme-treated soy-based adhesive modified with an epoxy resin. We investigated the wet shear strength of plywood bonded with the modified adhesive using the boiling-water test. Fourier transformed infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance analysis were used to characterize the reaction between epoxy groups and –NH2 groups in the modified soy-based adhesives. FTIR analysis confirmed the cross-linking structure in the cured adhesives. Viscosity and the solid content of soy-based adhesives gradually increased with the increasing amount of epoxy resin, but had little effect on its operability. Wet shear strength of plywood samples increased as the amount of epoxy resin was increased, whereas the inverse trend was observed regarding the water absorption of cured adhesives. Compared to an unmodified adhesive, the addition of 30% of epoxy resin increased the wet shear strength of plywood samples by 58.3% (0.95 MPa), meeting the requirement of the Chinese National Standard for exterior plywood. Differential scanning calorimetry and thermogravimetric analysis showed the improved thermostability of the cured adhesives after curing at 160 °C. These results suggest that epoxy resin could effectively improve the performance of enzyme-treated soy-based adhesives, which might provide a new option for the preparation of soy-based adhesives with high gluability and water resistance.

A novel water-based process produces eco-friendly bio-adhesive made from green cross-linked soybean soluble polysaccharide and soy protein

In this study, an eco-friendly soy protein adhesive was developed that utilized two components from soybean meal without addition of any toxic material. A plant-based, water-soluble and inexpensive soybean soluble polysaccharide was used as the novel renewable material to combine with soy protein to produce a soy protein adhesive. Three-plywood was fabricated with the resulting adhesive, and its wet shear strength was measured. The results showed the wet shear strength of plywood bonded by the adhesive reached 0.99MPa, meeting the water resistance requirement for interior use panels. This improvement was attributed to the following reasons: (1) Combination of cross-linked soybean soluble polysaccharide and soy protein formed an interpenetrating network structure, improving the thermal stability and water resistance of the cured adhesive. (2) Adding CL-SSPS decreased the adhesive viscosity to 15.14Pas, which increased the amount of the adhesive that penetrate the wood’s surface and formed more interlocks.