Protein-based Adhesives Research Articles

A synergistic strategy for formulating a facile and cost-effective soybean protein-based adhesive via co-crosslinking and inorganic hybridization.

Soybean protein-based adhesives (SPAs) are environmentally friendly, sustainable, and renewable, but their wide applications are limited by moderate bonding strength, poor water resistance, undesirable flame retardancy, and high costs. Inspired by the organic-inorganic hybridization of oysters, a facile and cost-effective SPA was developed in this study via co-crosslinking branched melamine-urea-glyoxal (MUG) and polyamidoamine-epichlorohydrin (PAE) resins, as well as inorganic hybridization by using the mineral bentonite (BT). Low-cost branched MUG resins that could efficiently co-crosslink with PAE resin and substitute for 40 % expensive PAE resin were optimally copolymerized from melamine, urea, and glyoxal. This approach provided more co-crosslinkable sites for constructing denser multiple covalently-crosslinked SP-PAE-MUG networks via MUG-protein crosslinking and MUG-PAE co-crosslinking. 30 wt% of low-cost layered mineral BT was intercalated to further reinforce the mechanical properties and flame retardancy of SPA by forming a compact organic-inorganic hybridized structure. The coordination of MUG-PAE co-crosslinking and BT inorganic hybridization resulted in a cost-effective and robust SPA with a 61.2 % higher dry strength, a soaked wet strength of 126.0 %, an aged wet strength of 86.0 %, a residual carbon content after combustion of 94.3 %, and reduced the raw material cost for formulating adhesive by 19.5 % compared with controlled commercial SPA. This novel strategy highlights the potential applications of the SP in bio-adhesives and bio-composites with excellent bonding strength, water resistance, and flame retardancy.

Physical and Chemical Characterization of Sustainable Green Adhesives Derived from Municipal Treatment Plant Sludges

Adhesive formulations derived from sustainable feedstocks, like waste-activated sludge and biosolids from wastewater treatment plants, are developed due to protein-based adhesives receiving attention for their low-cost, resourcefulness, and minimal ecological footprint. The protein composition and associated dynamic changes of the adhesive formulations were studied via gel permeation chromatography, which detailed a molecular size distribution of 8.72 × 105 g/mol for the adhesive formulation and 6.89 × 103 g/mol for the dewatered biosolid base fraction, which confirms the formation of multiple protein functional groups combining to form the larger adhesive molecules. Further analysis determined the types of proteins present in the dewatered biosolids as glutelin, prolamin, globulin, and albumin proteins, with the glutelin proteins as the most prevalent, as thus likely responsible for adhesive formation. The rheological properties of the novel protein adhesive were also studied to interpret the structure of the adhesives, which detailed the findings of viscoelastic properties and flow behaviors of each adhesive in relation to the wastewater treatment plant sample location, which yielded higher flow points, storage moduli, and loss moduli for the dewatered biosolids in comparison to the waste-activated sludge and biosolid adhesives, which correlates with the higher solids content of the dewatered biosolids and potentially cell rupturing when exposed to filtration stress.

Recyclable thermo-responsive elastin-based adhesives with tough underwater adhesion and rapid hemostasis ability

Protein-based underwater adhesives have attracted tremendous attention due to their excellent biocompatibility compared to adhesives prepared with synthetic polymers. However, the fabrication of protein-based adhesives always requires multiple successive steps, and their underwater adhesion is quite limited. Inspired by the human elastic fibers, bone matrix, and marine adhesive proteins, a thermo-responsive underwater adhesive was fabricated through a facial one-step complexation of elastin (ELN), hydroxyapatite (HAP), and tannic acid (TA) based on multiple non-covalent molecular interactions. During underwater adhesion, the adhesive can perform sufficient flowability to adhere seamlessly and strongly to the substrates upon heating at 37 ℃ and then significantly increase its cohesion through cooling down to 25 ℃ to sustain adhesion. The regulation of the balance between adhesion and cohesion in response to temperature change enables the adhesive to exhibit a robust underwater adhesion ranging from 0.3 to 1.1 MPa to the substrates with low, medium, or high surface energy, outperforming most protein-based adhesives. Importantly, the adhesive can be further processed into powders with well-maintained underwater adhesion by a simple drying and grinding method, facilitating adhesion onto irregular-shaped target sites, reusability, and closed-loop recycling. The adhesive powder also possesses a rapid in vivo hemostatic property, suggesting a potential biomedical application.

Stabilization of Fish Protein-Based Adhesive by Reduction of Its Hygroscopicity.

Protein-based fish adhesives have historically been used in various bonding applications; however, due to the protein's high affinity for water absorption, these adhesives become destabilized in high-moisture environments, resulting in reduced bondline strength and early failure. This limitation makes them unsuitable for industrial applications with higher demands. To address this issue, water-insoluble raw powder materials such as iron, copper, or zeolite were incorporated into natural fish adhesives. In this study, the hygroscopicity, dry matter content, thermal analysis (TGA/DSC), FT-IR spectroscopy, surface tension measurements, vapour permeability, and scanning electron microscope (SEM) of the modified adhesives were determined. In addition, the bonding properties of the modified adhesives were evaluated by the tensile shear strength of the lap joints, and mould growth was visually inspected. The resulting modified protein-based adhesives demonstrated improved stability in high humidity environments. Enhancing the hygroscopic properties of protein-based fish adhesives has the potential to unlock new opportunities and applications, providing a healthier and more environmentally sustainable alternative to petroleum-based adhesives.

Cold-set, crosslinker-free wood adhesives: A comparative study of ultrasound-extracted duck feather keratin and traditional food proteins

This study investigates the potential of keratin, a waste from duck feathers, as a sustainable alternative for wood adhesive applications. Keratin was extracted by sonochemical hydrolysis, a green and innovative method. Keratin was compared in its properties with casein, soybean, and pea proteins, three of the most studied proteins, as alternative eco-friendly materials to petroleum-derived products. However, these proteins remain used in the food industry. Protein-based adhesives were activated with sodium and calcium hydroxide. A comprehensive analysis was also conducted to study the influence of solid content, alkali, pH, viscosity, polarity, and structural composition of adhesives on their mechanical properties. The thermal properties of proteins and adhesives and the effects of post-alkaline treatment on protein secondary structure, crosslinking, and water resistance were studied. The key finding is that keratin shows potential as an eco-friendly adhesive, with tensile shear strength tests on beech wood meeting European standards. In addition, it showed superior water resistance due to its effective crosslinking ability compared to other food-derived proteins. The results position keratin as a feasible replacement for food-derived and formaldehyde-based adhesives, offering a sustainable option for interior wood applications and presenting an environmentally friendly solution with appropriate performance.

Carboxymethylated Lignin Reinforcement of SPI Adhesive: Enhancing Strength, Antimicrobial, and Flame-Retardant Properties without Excessive Alkali Introduction.

To address the challenges associated with formaldehyde emissions in engineered wood adhesives and simultaneously enhance adhesive properties related to water resistance, fire resistance, and mold resistance, a novel environmentally sustainable biomass-based adhesive was formulated. In this work, kraft lignin was carboxymethylated and then blended with the soy protein isolate (SPI)-based adhesive, the dry and wet shear strength of the plywood bonded by the resultant adhesive was enhanced from 1.10 and 0.63 MPa to 1.73 and 1.23 MPa, respectively, resulting in improvements of 157% and 195%. Carboxymethylated lignin (CML) significantly improved the mold resistance and flame-resistance residual rate of the adhesive and decreased the water absorption rate from 190% to 108%. Furthermore, the adhesive exhibits outstanding flame-retardancy, with self-extinguishing capability rendering it suitable for industrial production. In addition, we also evaluated the performances of resulting adhesives cured with different diepoxides and triepoxides, and the comparisons of the adhesive in this work to commercial urea glue and soy protein-based adhesives were conducted. To our delight, the SPI-10CML adhesive presented comparable or even improved performances, showing its promising practical applications such as for fire doors.

A facile strategy to fabricate low viscosity and high cold tack soy protein-based adhesive for particleboard production

The utilization of soy protein as a wood adhesive has been successfully implemented in the industrial production of plywood. However, it is still a significant challenge to directly utilize soy protein-based adhesive for the fabrication of particleboard owing to its high viscosity and low cold tack. In this study, polyvinyl acetate (PVAc) emulsion was used to modify soy protein-based adhesive to reduce its viscosity and enhance its cold tack. By incorporating a content of 30% PVAc emulsion, the viscosity of the adhesive was reduced to 880 mPa·s, which was much lower than the pure soy protein-based adhesive value. Simultaneously, the cold tack was significantly increased by nearly 500 times after the addition of PVAc. The obtained adhesives met the requirements for particleboard production. In addition, the internal bonding strength of the produced particleboard reached 0.7 MPa, significantly exceeding the requirements for P6-grade particleboards (≥ 0.40 MPa, GB/T4897–2015). Moreover, it exhibited no formaldehyde emissions, making it a high-quality particleboard suitable for furniture applications. This study successfully presents a soy protein-based adhesive suitable for particleboard production using a simple blending modification approach, thereby expanding the application scope of soy protein-based adhesives.

Handling-friendly, waterproof, and mildew-resistant all-bio-based soybean protein adhesives with high-bonding performance via bio-inspired hydrophobic-enhanced crosslinking network

Eco-friendly and formaldehyde-free soybean protein adhesives are highly attractive to the wood-based panel industry but typically suffer from low water-resistant bonding strength, high viscosity, and susceptibility to mildew. Inspired by adaptive hydrophobic and hydrophilic interactions of mussel foot proteins, herein, a high-performance and versatile all-bio-based adhesive (STF) was synthesized through employing bio-derived furfural and natural tannic acid to assist soybean protein in developing a biomimetic physical/chemical cross-linking network. The collaboration of the hydrophobic furan ring and the hydrophilic catecholic moiety endowed the as-prepared STF adhesives with exceptional water-resistant bonding strength, easy-coating performance, and mildew resistance. Compared to the original soybean protein, the optimal STF demonstrated dry and wet shear strengths of up to 2.52 MPa and 1.71 MPa, respectively, representing an increase of 65.8 % and 263.8 %. Notably, the aged bonding strength of STF surpassed that of previously reported soybean protein-based adhesives with a value as high as 1.19 MPa. The viscosity of the STF decreased by ≈ 98.1 % compared to the unmodified adhesive. Additionally, the STF exhibited excellent mildew resistance (no mildew for 7 days) and storage stability (> 72 h). Combined with life cycle assessment, the biomimetic STF adhesive holds great potential for large-scale production of environment-friendly wood panels.

Performance of eco-friendly soy protein adhesive reinforced by aldehyde sodium alginate

Faced with many challenges such as energy crisis and environmental protection issues, biomass soy protein-based adhesives for replacing aldehyde-based adhesives have gradually become one of the research hotspots for wood industry upgrading. However, soy protein-based adhesives still have many shortcomings. Low bonding strength, poor water resistance and high viscosity restrict their widespread industrial application. In this work, a modified soy protein isolate (SPI) based adhesive with strong water-resistant bonding strength and low viscosity was prepared. Oxidized sodium alginate (OSA) and calcium ions were employed in SPI adhesive system. Results showed that cross-linking between OSA, calcium ions and SPI occurred successfully. The mechanical properties of SPI were significantly improved. The dry and wet bond strengths were 2.05 MPa and 0.97 MPa, respectively, in 65.3 % and 49.2 % increments for comparison. The adhesive samples also showed a reduction in viscosity and an increase in thermal stability. It can be developed as a facile method for preparing bio-adhesives.

Upcycling of protein concentrates from industrial byproducts into polyurethane wood adhesives

Wood structures generally rely on synthetic adhesives for their strength and versatility. However, environmental concerns linked to the chemical composition of these adhesives have stimulated the search for more environmentally friendly adhesives. Researchers have explored replacing petroleum-based constituents with natural raw materials such as lignins, tannins, and proteins. Of these alternatives, proteins, being biological macromolecules, are recognized for their capacity to enhance adhesion to wood substrates. This study considered the development of protein-based adhesives derived from diverse sources, including soybean meal, microbrewery spent grains, shrimp shells, and skim milk powder. These raw materials were subjected to mild alkaline conditions to yield protein concentrates. The resulting adhesives were formulated at various protein content levels: 5%, 10%, 15%, and 20%. The study’s findings showed that the incorporation of proteins into the polyurethane adhesive system not only can preserve but also augment adhesive performance. This enhancement encompasses deeper penetration into wood substrates and an overall improvement in mechanical strength. These results underscore the promise of proteins as a sustainable alternative to petroleum-based polyols in adhesive formulations.

Development of biobased mildew- and bacterial resistant adhesives through keratin molecule recombination

The adhesion forces between wood substrates of keratin-based adhesives modified by hydrogen peroxide reached up to approximately 2.4 and 1.3 MPa under dry and wet conditions, respectively.

Novel adhesive based on black soldier fly larvae flour for particleboard production

Protein-based adhesives are receiving considerable attention due to their potential to contribute to the development of environmentally friendly materials. Black soldier fly larvae (BSFL) can convert organic waste into high value-added products, such as proteins and fatty acids. The objective of this study is to formulate adhesives from BSFL flour and evaluate their suitability in the production of particleboards. To expose the functional groups of the proteins for reaction with the wood, denaturation strategies involving alkaline treatments were assessed. The effectiveness of these treatments was evaluated through Bradford solubility, SDS-PAGE, and FTIR analysis. Bradford analysis confirmed that the alkali treatment increased the protein solubility from 10.1 g/l for 0 M to 89.1 g/l for 1 M. FTIR analysis revealed changes in the relative intensity of bands attributable to the Amide I, II, and III regions, which are associated with the α-helix and β-sheet structures. The adhesive was applied in the manufacturing process via spraying, requiring an evaluation of the rheological properties of different formulations. Based on the rheological properties, the viscosities of 0.25 M and 1 M were measured at 0.082 Pa·s and 0.113 Pa·s, respectively, falling within the suggested range for particleboard manufacture. Subsequently, eucalyptus wood panels were manufactured using the BSFL-based adhesives, and their mechanical properties as well as water resistance were evaluated. Static bending and tensile strength perpendicular to the surface testing revealed that boards produced with BSFL adhesive exhibited mechanical values comparable to those of commercial resins. The performance of the manufactured materials was compared with the minimum standards outlined in international regulations.

Preparation of Strong and Thermally Conductive, Spider Silk-Inspired, Soybean Protein-Based Adhesive for Thermally Conductive Wood-Based Composites.

The development of formaldehyde-free functional wood composite materials through the preparation of strong and multifunctional soybean protein adhesives to replace formaldehyde-based resins is an important research area. However, ensuring the bonding performance of soybean protein adhesive while simultaneously developing thermally conductive adhesive and its corresponding wood composites is challenging. Taking inspiration from the microphase separation structure of spider silk, boron nitride (BN) and soy protein isolate (SPI) were mixed by ball milling to obtain a BN@SPI matrix and combined with the self-synthesized hyperbranched reactive substrates as amorphous region reinforcer and cross-linker triglycidylamine to prepare strong and thermally conductive soybean protein adhesive with cross-linked microphase separation structure. These findings indicate that mechanical ball milling can be employed to strip BN followed by combination with SPI, resulting in a tight bonded interface connection. Subsequently, the adhesive's dry and wet shear strengths increased by 14.3% and 90.5% to 1.83 and 1.05 MPa, respectively. The resultant adhesive also possesses a good thermal conductivity (0.363 W/mK). Impressively, because hot-pressing helps the resultant adhesive to establish a thermal conduction pathway, the thermal conductivity of the resulting wood-based composite is 10 times higher than that of the SPI adhesive, which shows a thermal conductivity similar to that of ceramic tile and has excellent potential for developing biothermal conductivity materials, geothermal floors, and energy storage materials. Moreover, the adhesive possessed effective flame retardancy (limit oxygen index = 36.5%) and mildew resistance (>50 days). This bionic design represents an efficient technique for developing multifunctional biomass adhesives and composites.

Biomimetic Structural Proteins: Modular Assembly and High Mechanical Performance.

Protein-based biomaterials attract growing interests due to their encoded and programmable robust mechanical properties, superelasticity, plasticity, shape adaptability, excellent interfacial behavior, etc., derived from sequence-guided backbone structures, particularly compared to chemically synthetic counterparts in materials science and biomedical engineering. For example, protein materials have been successfully fabricated as (1) artificial implants (man-made tendons, cartilages, or dental tissues), due to programmable chemistry and biocompatibility; (2) smart biodevices with temperature/light-response and self-healing effects; and (3) impact resistance materials having great mechanical performance due to biomimetics. However, the existing method of regenerating protein materials from natural sources has two critical issues, low yield and structural damage, making it unable to meet demands. Therefore, it is crucial to develop an alternative strategy for fabricating protein materials. Heterologous expression of natural proteins with a modular assembly approach is an effective strategy for material preparation. Standardized, easy-to-assemble protein modules with specific structures and functions are developed through experimental and computational tools based on natural functional protein sequences. Through recombination and heterologous expression, these artificial protein modules become keys to material fabrication. Undergoing an assembly process similar to supramolecular self-assembly of proteins in cells, biomimetic modules can be fabricated for formation of macroscopic materials such as fibers and adhesives. This strategy inspired by synthetic biology and supramolecular chemistry is important for improving target protein yields and assembly integrity. It also preserves and optimizes the mechanical functions of structural proteins, accelerating the design and fabrication of artificial protein materials.In this Account, we overview recent studies on fabricating biomimetic protein materials to elucidate the concept of modular assembly. We discuss the design of biomimetic structural proteins at the molecular level, providing a wealth of details determining the bulk properties of materials. Additinally, we describe the modular self-assembly and assembly driven by inducing molecules, and mechanical properties and applications of resulting fibers. We used these strategies to develop fiber materials with high tensile strength, high toughness, and properties such as anti-icing and high-temperature resistance. We also extended this approach to design protein-based adhesives with ultra-strong adhesion, biocompatibility, and biodegradability for surgical applications such as wound sealing and healing. Other protein materials, including films and hydrogels, have been developed through chemical assembly routes. Finally, we describe exploiting synthetic biology and chemistry to overcome bottlenecks in structural protein modular design, biosynthesis, and material assembly and our perspectives for future development in structural biomaterials.

A strong soy protein-based adhesive with excellent water retention

Traditional synthetic resin adhesives release harmful substances and can be replaced with more sustainable, eco-friendly, and healthier soy protein-based adhesives. However, the low water retention of these adhesives leads to an unstable bonding performance, which hampers their widespread application. Enhancing the water retention of the adhesive while maintaining its bonding and coating performance poses a significant challenge. In this work, a “bond-stitch” topology was designed and constructed by free radical copolymerization between acrylic acid and zwitterionic sulfobetaine methacrylate. This topology was combined with lithium chloride and a self-made epoxy crosslinking agent to construct a water-retaining and enhancing structure in soy protein adhesive systems, achieving simultaneous improvement in water retention, bonding strength, and coating performance. The adhesive exhibited an average shear strength of 1.77 MPa for wood and 1.82 MPa for stainless steel. Additionally, the coating performance was significantly improved, allowing uniform coating on wet poplar veneers (up to 120% moisture content) and low surface free energy substrates. Importantly, the adhesive allowing over 75% of the mass to remain after 24 h of storage in an open environment. This strong water-retaining and enhancing structure construction strategy offers a promising way to develop bio-based adhesives and hydrogels, promoting their widely application.

Thermal-driven cationic waterborne polyurethane crosslinker with oxime-based and catechol structure for the preparation of soybean protein-based adhesive with excellent adhesion properties

Recently, the research of waterborne polyurethane (WPU) as toughening matrix of soybean protein-based adhesives has attracted much attention. However, the intermolecular interaction between polyurethane and protein molecules is too weak to construct a compact crosslinking system to achieve strengthening and toughening adhesive systems. In this work, a thermal-driven cationic waterborne polyurethane (CWPOU) with oxime-isocyanate groups and catechol groups was synthesized. The obtained CWPOU emulsion was introduced into soybean meal (SM) matrix to fabricate a SM/CWPOU bio-adhesive. Thermal-driven elements in polyurethane molecules released isocyanate groups, increasing covalent crosslinking between crosslinker and protein molecules, synergies with non-covalent forces formed between catechol structures to improve the comprehensive bonding properties of the adhesives. As a result, the dry/wet bonding strengths of the modified adhesives were noticeably enhanced with the highest values observed (SM/CWPOU-4) reaching 169.6% and 191.4% that of pure SM adhesives, respectively. Furthermore, the hydrogen bonding provided by hard segments of polyurethane and noncovalent forces between catechol groups and wood substrates contributed to excellent cold-pressing performance of the bio-adhesives with a maximum value pleasantly reaching 298% that of pure SM. The mildew resistances of SM/CWPOU adhesives were attributed to bactericidal performance of the cationic structures and catechol groups in CWPOUs. This work provides an effective method to prepare the “green” and high-performance protein-based adhesives.

Application of a self-crosslinking strategy based on oxidization modification for the preparation of soy-based adhesive

Crosslinking modification is a key to acquiring soy protein-based adhesives with super performances, for which external crosslinkers have been adopted in most of the present studies. Carbohydrates are the important causes restricting the water resistance of soy protein-based adhesives. In this study, defatted soy flour was treated using different oxidizing agents, which mainly acted upon carbohydrates, specifically, converted these side-effect carbohydrates into components with the crosslinking action (internal crosslinking agents) to facilitate the crosslinking reaction with soy proteins, thereby preparing self-crosslinking soy protein-based adhesives. The bonding strength and water resistance of the prepared plywood were mainly analyzed, and the performances of adhesives were also analyzed via differential scanning calorimetry (DSC), Fourier transform-infrared spectrometry (FT-IR), thermogravimetry (TG) and X-ray diffraction (XRD). The results showed that: (1) The oxidizing agents presented not strong oxidizing properties, resulting by the low content of nonvolatile aldehyde and large molecular weight and steric hindrance of reactions for the oxidation products. The bonding performance of soy protein-based adhesives prepared through the oxidization of H2O2, (NH4)2S2O8 and NH4NO3 was not obviously improved. (2) Both the bonding strength and water resistance of soy protein-based adhesives prepared through the oxidization of NaIO4 and KMnO4 met requirements for Class II plywood specified in GB/T 17657–2013. (3) The oxidization of KMnO4 is a nonselective oxidization, with great variability in the degree of oxidization and degradation. The oxidization of NaIO4 was partially selective, and the products were mostly poly-aldehydes, which were more beneficial to the construction of the spatial network structure for adhesives, with the better corresponding bonding strength and water resistance.

Sestec adhesive based on animal proteins for the production of particleboards

The development of a biodegradable adhesive that uses waste from industry will bring enormous economic, social, environmental, and health benefits. Many laboratories around the world conduct research on the development of natural adhesives that can replace those based on toxic formaldehyde. Works in this area are also carried out by Sestec Polska Sp. z o. o, an example of which is the Sestec B0021G adhesive. This product belongs to the family of animal protein-based adhesives and is dedicated to particle board (PB) production (classes P1–P5 according to EN 312). This work presents the results obtained during the preparation of boards based on Sestec B0021G protein adhesive, manufactured with the use of variable production parameters, and assesses the influence of these parameters on the properties of the obtained products. The emphasis was especially on presenting the impact of production parameters on the PB strength and water resistance. The tests were carried out using different densities (560–800 kg/m3), pressing temperatures (150–230 °C), amounts of adhesive (7–15% in the surface layer and 5–13% in the core layer) and specific press times (5–13 s/mm). The boards showed excellent mechanical properties and water resistance. Higher PB classes were obtained under standard production conditions. Eventually, the obtained results showed that the binder can be used to produce all classes of PB products P1 to P7, and not only P1-P5 as initially assumed.

Preparation of desirable plant protein-based adhesive via bioinspired Laponite-assisted organic-inorganic copolymerization

The preparation of plant protein-based adhesives is one of the potential strategies to mitigate environmental pollution. In this work, the organic-inorganic copolymerization of calcium phosphate oligomer and soy protein was performed by adopting the concept of organic-inorganic polymerization and the physics of polymer adsorption to eliminate the interphase boundary, preparing an adhesive layer with continuous and uniform network structure without using chemical cross-linkers. The participation of ionic and hydrogen bonds improved the mechanical strength. In addition, the Laponite with anisotropic charge distribution was introduced into the cross-linking network to increase the density and cohesion of cross-linking and further improve the bonding performance without complex chemical synthesis. The modified adhesive had an optimum wet shear strength of 1.01 MPa, and excellent water resistance (85.7 % residue rate). The modified adhesive shows good cell compatibility plus flame retardancy. Overall, this biomimetic study has excellent promises for the design of protein-based adhesives with desirable properties for engineering applications.

Crosslinking Mechanism of Soy Protein-based Adhesives based on Glyoxal and a Compound of Protein Model

Crosslinking Mechanism of Soy Protein-based Adhesives based on Glyoxal and a Compound of Protein Model