Soy-based Adhesives Research Articles

A bio-based soy wood adhesive modified by dual-crosslinking strategy with excellent mechanical strength and water-resistance

Soy-based adhesives have garnered significant attention owing to their distinctive environmentally sustainable characteristics and low cost. However, they exhibit inadequate bonding strength and water-resistance, which can be attributed to the presence of polar groups in soy flours. Herein, a simple and eco-friendly dual-crosslinking strategy was proposed to manufacture a soy-based adhesive (SF/IO4/CA adhesive) with excellent mechanical properties and good water-resistance. Specifically, soy flours were treated with sodium periodate (NaIO4) for oxidization, which facilitated the soy flours' self-crosslinking process by converting carbohydrates into components having the cross-linking action (internal cross-linking agents). Then, incorporating the citric acid (CA) would promote to the carboxyl groups (-COOH) of CA to react with the hydrophilic hydroxyl groups (-OH) present in soy flours by esterification reaction. The findings demonstrated that a high-performance plywood using SF/IO4/CA adhesive was successfully produced, which shear strength of cold-water (20 °C for 24 h), hot-water (63 °C for 3 h), and boiling-water (93 °C for 3 h) reached 1.69 MPa, 1.68 MPa, 1.72 MPa, and 1.47 MPa, respectively. The soy-based adhesive has excellent mechanical strength and water-resistance due to its dual-crosslinking network system, which has significant promise for application use in the wood-based panels industry.

A high wet strength and toughness soy-based adhesive prepared by single cross-linking

Soy-based adhesives are considered to be a substitute for toxic petroleum-based adhesives in wood industry because of its environmentally-friendly property. However, their application range is limited by the lack of high wet bond strength and high toughness. While it is possible to impart these properties to soy-based adhesives by adding a variety of corresponding modifiers, it is difficult to do so by adding a single modifier. In this work, a multifunctional cross-linking agent dodecanedioic acid diglycidyl ester (DADE) with high reactivity, toughening, and hydrophobic effects was synthesized and incorporated into soy-based adhesive to ameliorate the bonding performance of the adhesives. The reaction process between DADE and protein was confirmed by instrumental analyses. Results revealed that the introduction of 4% DADE increased the wet bond strength of the soy protein adhesive by 140% to 1.57 MPa, whereas the wet bond strength (1.11 MPa) of soybean meal adhesive containing 4% DADE was comparable to that of MUF resin. The micrographs of the fracture surface and the force-distance curves of the shear strength test showed that the toughness of the adhesives was significantly improved by DADE. These improvements were attributed to the long hydrophobic aliphatic chains and highly reactive groups from DADE. This modification method avoided using large amounts of chemical additives. This work may provide an efficient and low-consumption method for producing bio-based adhesives with excellent bonding properties.

Preparation of a high bonding performance soybean protein-based adhesive with low crosslinker addition via microwave chemistry

Human health and environmental protection demand wood-based panel industry for innovative soy-based adhesives with high production efficiency, straightforward synthesis processes, non-toxicity, and high bonding performance. A simple and efficient microwave pretreatment process and low addition of bio-derived crosslinking agent was used in this study to prepare a non-toxic and high-bonding performance soybean protein-based adhesive. After 4 min of microwave pretreatment time, the complex quaternary structure of soybean protein molecule unfolds, the soybean protein disperses evenly and stably, and active groups of soybean protein molecules are exposed. After adding 3.85% crosslinking agent, the moisture absorption rate of the soybean protein-based adhesive decreases by 41.77%, the residual rate increases by 3.68%, and the wet shear strength of the resultant plywood increases to 1.12 MPa, which satisfies requirement of interior use plywood. Compared with previously reported soy-based adhesives, this adhesive is dependent on fewer chemical reagents, but has good bonding performance. The 204.41% of relative cell viability indicates the resultant adhesive was non-toxic. The proposed high-efficiency, high-performance, non-toxic biomass adhesive has great prospects for the industrial application.

Developing an antifungal and high-strength soy protein-based adhesive modified by lignin-based polymer

Soy protein adhesive is readily deteriorated by microorganism, which greatly limits its practical application in the plywood industry. In this research, a soybean meal (SM)-based adhesive with excellent antifungal property, thermal stability and bonding strength was successfully prepared using epoxy polymer of lignin (EPL) as a crosslinking agent. The results confirmed that the lignin-based polymer can notably improve the antifungal ability of a soy-based adhesive. The initial microbial attack of SM/EPL adhesive was better than that of pure SM adhesive (on 3rd day), delaying to 22th day under high humidity conditions. Moreover, the fungal growth on the modified SM adhesives was Aspergillus flavus. The mycelia only existed on the surface of the modified SM adhesives, but not observed inside, and the modified SM adhesives had no significant structural change, and still exhibited a good thermal stability. Furthermore, no microbial attack observed on the surface and glueline of plywood bonded by modified SM adhesives. These results may be due to its phenolic hydroxyl groups and dense network structure hindering the erosion of fungi. On the other hand, the wet shear strength of SM/EPL adhesive reached 1.02 MPa, which was a 363.6 % increase compared with the neat SM adhesive (0.22 MPa). Also, the adhesion strength still met the requirements of interior-use plywood (≥ 0.7 MPa) after being treated for a month. This work not only highlights the application potential of lignin-based polymer as an effective antifungal material, but also broadens the application of soy-based adhesives.

Study on Response Surface Optimization of Hot-pressing of Multi-layer Plywood via Soy-based Adhesive

Eucalyptus veneers were applied to fabricate multi-layer plywood with soy-based adhesives as the glue. Prior to optimization, single-factor experiments were performed to select the proper range of each factor. Furthermore, effects of adhesive amount, hot-pressing time, hot-pressing pressure, and hot-pressing temperature were investigated via response surface methodology (RSM). Through RSM optimization, optimum hot-pressing conditions were obtained. To verify the optimized conditions, validation test was carried out. The result presented that the error rate between the predicted value and the measured value was 1.2%, which was reasonable for prediction.

Natural Tannins as New Cross-Linking Materials for Soy-Based Adhesives.

Human health problems and formaldehyde emission from wood-based composites are some of the major drawbacks of the traditional synthetic adhesives such as urea formaldehyde resins. There have been many attempts to decrease formaldehyde emission and replace urea formaldehyde resins with bio-based adhesives for wood-based composites. Because of some weakness in soy-based adhesive, chemicals have been used as modifiers. Modified soy-based adhesives without any formaldehyde have been successfully used to prepare wood panels. To achieve this, different synthetic cross-linking chemicals such as phenol formaldehyde resins and polyamidoamine-epichlorohydrin were used. However, in reality, what we need are totally green adhesives that use natural materials. In our previous research work, the use of tannins in combination with soy-based adhesives to make wood composites was investigated. Thus, in this research work, the feasibility of using three types of natural tannins (quebracho, mimosa and chestnut tannins) as cross-linking materials for soy adhesive was studied. The chemical bond formation and adhesion behaviors of tannin-modified soy adhesives were also investigated by Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-ToF-MS) and thermo-mechanical analysis (TMA). The results showed that at ambient temperature, both ionic and covalent bonds formed between tannin constituents and amino acids; however, at higher temperature, covalent bonds are largely predominate. Based on the results obtained from the thermo-mechanical analysis, the modulus of elasticity (MOE) of soy adhesive is increased by adding tannins to its formulation. In addition, the chemical bond formation was proved by MALDI-ToF-MS.

Environmental benefits of soy-based bio-adhesives as an alternative to formaldehyde-based options.

The restrictions imposed on the use of formaldehyde in wood panel adhesives have been the driving force behind the development of formaldehyde-free resins for the manufacture of wood products. Considering as a boundary condition the idea that the use of fossil-based raw materials should be replaced by biological options, there is growing interest in the environmental assessment of different alternatives for soy-based adhesives, as possible options to replace commonly used synthetic resins. This report includes the environmental profiles of soy-based adhesives taking into account the life cycle assessment (LCA) methodology. In addition, in order to increase their potential to replace synthetic resins, a sensitivity analysis of the main contributors to environmental damage was performed, thus giving an open guide for further research and improvement. This study aims to provide innovative alternatives and new trends in the field of environmentally friendly bio-adhesives for the wood panel industry.

Soy-Based Adhesives Functionalized with Pressure-Responsive Crosslinker Microcapsules for Enhanced Wet Adhesion

With excellent bonding performance, formaldehyde-based adhesives have been widely used for the production of woody materials. However, these adhesives could potentially cause harm to human health a...

Soy Protein Adhesives for Particleboard Production – A Review

Formaldehyde-based resins are commonly used in the particleboard industry due to their high bonding strength and excellent water resistance. Nowadays, the healthy and sustainable issues lead to the development of bio-based wood adhesives. In the last past decade, soybeans were the most prominent crop for the production of environmentally friendly adhesive with no formaldehyde in its composition. This review covers the strengths, weaknesses, opportunities and threats of soy-based adhesives as a replacement of conventional resins. Moreover, this work details the different techniques used to improve the performance of soy protein adhesives in order to encourage their use in the wood industry.

Improving Bond Performance and Reducing Cross-linker Dosage for Soy Flour Adhesives Inspired by Spider Silk

Soy-based adhesives are environmentally friendly alternatives to the harmful formaldehyde-based resins in manufacturing wood-based panels. However, almost all of the modification methods use large ...

Evaluating Volatile Organic Compound Emissions from Cross-Laminated Timber Bonded with a Soy-Based Adhesive

Volatile organic compound (VOC) emissions from indoor sources are large determinants of the indoor air quality (IAQ) and occupant health. Cross-laminated timber (CLT) is a panelized engineered wood product often left exposed as an interior surface finish. As a certified structural building product, CLT is currently exempt from meeting VOC emission limits for composite wood products and confirming emissions through California Department of Public Health (CDPH) Standard Method testing. In this study, small chamber testing was conducted to evaluate VOC emissions from three laboratory-produced CLT samples: One bonded with a new soy-based cold-set adhesive; a second bonded with a commercially available polyurethane (PUR) adhesive; and the third assembled without adhesive using dowels. A fourth commercially-produced eight-month-old sample bonded with melamine formaldehyde (MF) adhesive was also tested. All four samples were produced with Douglas-fir. The test results for the three laboratory-produced samples demonstrated VOC emissions compliance with the reference standard. The commercially-produced and aged CLT sample bonded with MF adhesive did not meet the acceptance criterion for formaldehyde of ≤9.0 µg/m3. The estimated indoor air concentration of formaldehyde in an office with the MF sample was 54.4 µg/m3; the results for the soy, PUR, and dowel samples were all at or below 2.5 µg/m3.

Synthesis and performance evaluation of nanocomposite soy protein isolate/carbon nanotube (SPI/CNTs) adhesive for wood applications

In this article, effect of the incorporation of carbon nanotubes (CNTs) into soy-based adhesive, obtained from soy protein isolate (SPI), on the performance of the nanocomposite adhesive is presented. Multifunctional and exceptional properties of carbon nanotubes (CNTs) in nanofiller in polymer matrix were explored in this study to develop nanocomposite soy-based adhesive with improved shear strength and water resistance. Consequently, nanocomposite SPI/CNT adhesives having different CNT loading were synthesized and evaluated. Morphology, thermal stability and chemical functionalities of the synthesized nanocomposite adhesives were checked using scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy, respectively. The bond strength test of the adhesives was carried out according to European standards, EN-204 and EN-205. The results showed that CNTs improved the shear strength of the nanocomposite SPI/CNT adhesive at reduced percentage loading. About 100% increase in the shear strength and water resistance was recorded for the nanocomposite SPI/CNTs adhesive when compared with an alkaline modified adhesive without carbon nanotubes. The shear strength of the nanocomposite adhesive increased from 3.48 MPa (adhesive without CNTs) to 6.91 MPa (adhesive with 0.3 wt% CNT loading) and then decreased afterwards as the CNT loading was increased beyond 0.3%. Enhanced shear strength of soy-based adhesives using different fillers has been reported in the literature, but at relatively high loading of the filler. In this study, relatively small amount of CNTs (0.3%) has shown a substantial enhancement in shear strength as compared to other fillers. It is expected that the newly developed adhesive could pave the way to solving the challenge of low shear bond strength and poor water resistance of soy-based adhesives for wood applications in the industry.

Improving Mildew Resistance of Soy Meal by Nano-Ag/TiO2, Zinc Pyrithione and 4-Cumylphenol.

As a byproduct from the soybean oil industry, soy meal can be reproduced into value-added products to replace formaldehyde as a plywood adhesive. However, the use of soy meal has been limited by its poor antifungal and antiseptic properties. In this work, three kinds of material, namely nano-Ag/TiO2, zinc pyrithione, and 4-cumylphenol were applied to enhance the mildew resistance of soy meal via breakdown of the cellular structure of mildew. The fungi and mold resistance, morphology, thermal properties, and mechanism of the modified soy meal were evaluated. The success of the antifungal and antiseptic properties was confirmed by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy. The results indicated that all three kinds of material improved the fungi and mold resistance of soy meal, and sample B, which was modified with a compound of nano-Ag/TiO2 and zinc pyrithione, was the effective antifungal raw material for the soy-based adhesives. FTIR indicated that the great improvement of antifungal properties of soy meal modified with 4-cumylphenol might be caused by the reaction between COO– groups of soy protein. This research can help understand the effects of the chemical modification of nano-Ag/TiO2, zinc pyrithione, and 4-cumylphenol on soy meal, and the modified soy meal exhibits potential for utilization in the plywood adhesive industry.

Response Surface Methodology as an Approach to Optimize the Preparation of Maize Straw Panels Applying Soy-based Adhesives

In this study, maize straw was used to prepare formaldehyde-free panels with soy-based adhesives. Influences of adhesive amount, hot-pressing time, temperature and pressure on the internal bonding strength were optimized by applying response surface methodology (RSM). The results showed that based on experimental mathematical model, the optimum hot-pressing conditions were obtained. Adhesive amount was 14%, hot-pressing pressure was 3.0 MPa, hot-pressing temperature was 126 °C, hot-pressing time was 28 s/mm. The actual internal bonding strength was 0.52 MPa while the predicted value was 0.51 MPa, indicating that the experimental mathematical model was accurate and reliable.

Study on Soy-Based Adhesives Enhanced by Phenol Formaldehyde Cross-Linker.

To find the effects of cross-linker phenol-formaldehyde (PF) resin on the performance of soy-based adhesives, the reaction between model compounds hydroxymethyl phenol (HPF) and glutamic acid were studied in this paper. HPF prepared in laboratory conditions showed higher content of hydroxymethyl groups than normal PF resin, which was proved by the results of Electrospray Ionization Mass Spectrometry (ESI-MS) and 13C Nuclear Magnetic Resonance (13C-NMR). The results of ESI-MS, Fourier transform infrared spectroscopy (FT-IR), and 13C-NMR based on resultant products obtained from model compounds showed better water resistance of the soy protein-based adhesive modified by PF-based resin, which indicated the reaction between PF resin and soy protein. However, it seemed that the soy-based adhesive cross-linked by HPF with the maximum content of hydroxymethyl groups did not show the best water resistance.

Curing Dynamics of Soy Flour-Based Adhesives Enhanced by Waterborne Polyurethane

In this paper, thermogravimetric (TG) analysis was carried out to make clear the curing properties of soy flour-based adhesives (SFAs) enhanced by waterborne polyurethane (WPU) with different addition levels. The kinetic parameters were evaluated by a thermal dynamics method, including activation energy and preexponential factor. In addition, the structure characteristics of both soy flour and modified soy flour-based adhesives were tested by Fourier transform infrared spectroscopy (FTIR). The results revealed that the FTIR spectra of pristine soy flour-based adhesives were different from those of soy flour after alkali treatment and waterborne polyurethane modification. Furthermore, there were four main degradation phases in the derivative thermogravimetric (DTG) curves of modified soy-based adhesives while there were two phases of a defatted soy flour sample. The kinetics analysis demonstrated that the curing process could be described as a consecutive first-order curing process. Moreover, with the addition level of WPU growing, the apparent activation energy of each phase of the curing process was increasing compared with that in pristine soy-based adhesives.

Preparation of Soy‐Based Adhesive Enhanced by Waterborne Polyurethane: Optimization by Response Surface Methodology

Response surface methodology was used to optimize the preparation conditions of soy‐based adhesives (SBAs) in this work. The parameters such as the effects and interactions of waterborne polyurethane (WPU) addition level (X1), temperature (X2), and time (X3) on wet shear strength (Y) were investigated. The regression model for SBA preparation was significant (p = 0.0034 < 0.05). The coefficient of determination (R2) of this model was to be 0.9256. According to the results, WPU addition level (X1) had a significant influence on the wet shear strength, whereas reaction temperature (X2) and reaction time (X3) were not significant. The optimal preparation conditions of SBA were 12 wt.% WPU addition level for 101 min at 76°C. Under the optimal conditions, the wet shear strength was 1.07 ± 0.08 MPa, which was in good agreement with the model predicted value. An analysis of FTIR spectra of WPU, soy flour, and soy‐based adhesive further confirmed the validity of the model.

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.

Improving soy-based adhesives for wood particleboard by tannins addition

Two different types of commercial tannins, namely chestnut (a hydrolysable tannin) and mimosa (a condensed flavonoid tannin), were used with two different types of hardener, namely hexamine and glyoxal, to upgrade soy-based adhesives for bonding of wood particleboard. Different soy-based adhesive formulations were tested by differential scanning calorimetry (DSC) and by thermomechanical analysis (TMA). The physical properties of different adhesive formulations such as viscosity, solid contents, acidity and density were also measured. One-layer laboratory particleboards bonded with the experimental adhesive formulations were prepared and tested. Based on the results obtained, tannins appear to decrease effectively soy-based adhesives’ viscosity. The TMA analysis showed that chestnut tannin extract appeared to react well with soy flour and to improve the soy flour adhesion properties. The TMA results also showed that both tannin types shorten the adhesives gel time and gel temperature. The DSC analysis also showed that tannin changes the thermal properties of soy-based adhesive and decreases both its glass transition temperature (Tg) and its denaturation temperature (Td). Moreover, particleboard bonded with tannin-modified adhesives showed internal bond strength results higher than particleboard bonded with unmodified soy adhesive. Thickness swelling and water absorption of the boards decreased when using tannins in soy-based adhesive formulations.

Improving Water Resistance of Soy-Based Adhesive by Vegetable Tannin

In this research tannic acid was used to prepare soy-based adhesives for making plywood and fiber board. The different resin formulations were analyzed by Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and its derivative as a function of temperature (DTG) and Fourier Transform Infra-red (FTIR) spectroscopy. The results showed that the addition of tannic acid to soy-based adhesive decreased soy-based adhesive viscosity and its pH. The DSC analysis showed that the denaturation temperature of soy-based adhesives decrease by adding tannic acid. The TGA and DTG curves showed that the thermal degradation of soy flour starts above 146 °C. The FTIR spectroscopy results also showed that the soy flour amino acids appeared to react well with tannic acid. Furthermore, delamination and shear strength test results showed the good water resistance of plywood bonded with soy-based tannic acid-modified adhesive. The mechanical and physical properties such as MOR, MOE, IB, and water resistance of fiberboard were improved, by adding tannic acid to the soy-based adhesive.