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Oni, O.V.; Lawrence, M.A.; Zappi, M.E.; Chirdon, W.M. Strategies to Improve Water Resistance in Protein Adhesives. Encyclopedia. Available online: https://encyclopedia.pub/entry/50492 (accessed on 08 July 2024).
Oni OV, Lawrence MA, Zappi ME, Chirdon WM. Strategies to Improve Water Resistance in Protein Adhesives. Encyclopedia. Available at: https://encyclopedia.pub/entry/50492. Accessed July 08, 2024.
Oni, Olatunji V., Michael A. Lawrence, Mark E. Zappi, William M. Chirdon. "Strategies to Improve Water Resistance in Protein Adhesives" Encyclopedia, https://encyclopedia.pub/entry/50492 (accessed July 08, 2024).
Oni, O.V., Lawrence, M.A., Zappi, M.E., & Chirdon, W.M. (2023, October 19). Strategies to Improve Water Resistance in Protein Adhesives. In Encyclopedia. https://encyclopedia.pub/entry/50492
Oni, Olatunji V., et al. "Strategies to Improve Water Resistance in Protein Adhesives." Encyclopedia. Web. 19 October, 2023.
Strategies to Improve Water Resistance in Protein Adhesives
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This entry is adapted from the peer-reviewed paper 10.3390/su152014779

Proteins are the most abundant class of bio-based adhesive feedstocks; they are primarily linear polyamides composed of polypeptides linkages of amino acids. The fundamental structure is determined by the polypeptide structure of the protein. The overall mechanism to produce protein-based adhesives is a denaturing process that involves an aqueous reaction matrix. Protein adhesives stand out due to their sustainability, renewable sources, and biodegradability. However, they are limited by poor wet strength and water resistance, which affect their wide acceptability in the marketplace. Researchers have developed multiple strategies to mitigate these issues to advance protein adhesives so they may compete more favorably with their petroleum-based counterparts.

formaldehyde protein adhesives water resistance cross-linking fillers water-soluble constituents sustainable materials protein glue

1. Introduction

Urea–formaldehyde resins are the most utilized synthetic adhesives due to their desirable properties, including good bonding strength, clear color, low cost, fast curing, and resistance to moisture [1]. However, the major challenge associated with synthetic adhesives is the emissions of toxic substances including formaldehyde during manufacture and use [2]. Recent years have witnessed an increase in global public sensitivity and awareness about health and environmental concerns [3]. Over time, stricter laws setting new toxic emissions limits for wood panels have been implemented [4][5]. These are incorporated into standards in the United States, Japan, and Europe [3].
This increasingly tightening of regulations and concerns over pollutant volatilization and the manufacturing ecological footprint (sustainability) of these formaldehyde and phenolics-based adhesives creates an opportunity for the development of renewable and “environmentally-friendly” protein adhesives in the market [6]. However, protein adhesives only constitute a small market portion due to a number of challenges preventing their wider application [7][8]. The primary reason protein adhesives cannot be utilized in more applications is their lack of water resistance [9] and wet strength [10][11], yet there is still a growing interest in protein adhesives as evidenced by the increasing rate of publication, which is shown in Figure 1.
Sustainability 15 14779 g001
Figure 1. Publication of articles indexed by the ScienceDirect database found under the search term “Protein adhesives” since 2010.
Proteins are the most abundant class of bio-based adhesive feedstocks [9]; they are primarily linear polyamides composed of polypeptides linkages of amino acids [12]. The fundamental structure is determined by the polypeptide structure of the protein [13]. Side chains of amino acids can form bonds with several substances, including cellulose and lignocellulose [14]. Yet, certain potentially reactive side chains are not on the surface of the protein. Moreover, the loss of the adhesive bond under moist conditions occurs within the adhesive itself (cohesive failure) and not between the wood and the adhesive [14]. Proteins have to be denatured to expose the functional groups, facilitate stronger bonding to wood, and enable the aggregation of protein molecules [14].
The overall mechanism to produce protein-based adhesives is a denaturing process that involves an aqueous reaction matrix [15]. The goal of the denaturation process for the formation of adhesives is to dissolve the quaternary, tertiary, and secondary structures of the protein [15][16]. The addition of a strong base increases the pH of the proteins above their isoelectric point [17]. Under this condition, the ions in the solution interfere with the hydrogen bonds and the electrostatic dipole–dipole interactions that allow the proteins to retain their shape and higher-order structures [18]. Many of the natural, covalent cross-linking groups (including cysteine and disulfide bonds) that maintain the tertiary structure in these proteins are also vulnerable to disassociation under alkaline conditions [19]. By splitting the cross-linking groups and interfering with the secondary bonding, this denaturation process exposes the hydrophilic portions of proteins, which allows these adhesive groups to adsorb strongly onto substrates (as a glue) and filler materials (as a binder/resin) [20]. The conditions commonly used to denature proteins, especially temperature and pH modification, also catalyze the hydrolysis of the proteins [21]. This results in lower-molecular-weight protein fragments that suffer a significant loss of shear strength, especially when the chains become too short to entangle with each other [22]. Reaction time, temperature, and pH can be optimized and controlled to achieve denaturation without significant hydrolysis resulting in high-quality adhesives and a chemical foundation for other polymeric substances (such as resins and coatings) [23].
Water can degrade protein adhesives by altering the adhesive’s physical and chemical characteristics. Protein adhesives are primarily composed of water-sensitive animal or plant proteins that undergo structural changes when exposed to moisture, even after conversion into adhesives, thus the adhesive also suffers from water-exposure-based structural failures [24].
When water interacts with protein adhesives, it can break the hydrogen bonds and other intermolecular interactions that hold the protein structure together [24]. This can result in the adhesive losing its strength, becoming brittle, and losing its ability to adhere to surfaces [24].
Generally, the applications of protein adhesives are limited by their low water resistance. Several strategies are utilized to mitigate or overcome this challenge, including thermochemical treatment, cross-linking networks, and using modified fillers.

2. Cross-Linking Networks

Cross-linking is a common method for creating or enhancing a network structure, hence improving the protein structural stability and adhesive capabilities of soy protein-based adhesives [25]. These materials are added to protein adhesives during their preparation or before their application [26]. Cross-linking modifications using latex, synthetic resin, polyamide-epichlorohydrin, and isocyanate have been shown to be efficient methods for increasing the water resistance of an adhesive [27][28][29][30].

3. Water Resistance from Modified Fillers

The addition of fillers can significantly impact the thermal and mechanical properties of protein adhesive, and the mechanism involves the interaction between the protein matrix and the filler material [31][32]. The incorporation of sodium montmorillonite (Na MMT) into soy protein adhesives at different concentrations was explored by Qi et al. [33]. Sodium montmorillonite is the most extensively utilized kind of silicate clay in polymer nanocomposites with properties, such as thermal and chemical stability, natural abundance, and non-toxicity [34]. The material is widely employed as a reinforcing and nano-filler material to produce nanocomposites owing to its high aspect ratio and unusual layered and nanoscale structure [35]. Thus, Qi et al. worked on developing a novel soy protein and clay system with excellent flowability and strong adhesion at high protein content [33]. Sodium bisulfite (NaHSO3) was used in these formulations, and hydroxyethyl cellulose was used as a suspension agent [33]. NaHSO3 can be utilized as a reducing agent to break the disulfide bonds in protein molecules, thereby resulting in an increase in surface hydrophobicity, solubility, and flexibility [36]. Another researcher, Zhang and Sun, corroborated this claim by using NaHSO3 to break the disulfide bonds of soy glycinin to increase the surface hydrophobicity [37].
The results showed that the addition of Na MMT significantly improved the adhesion strength of the soy protein adhesives due to the adsorption of the soy protein molecules on the surface of the interlayer of Na MMT via electrostatic interaction and hydrogen bonding [33]. Thus, the water resistance of the soy protein/Na MMT increased to 4.3 MPa compared to the 2.9 MPa of control SP at 8% Na MMT and the dry shear strength from 5.7 MPa to 6.38 MPa [33]. This research shows an innovative way to develop an adhesive with excellent adhesion with the incorporation of silicate clay materials.
Ciannamea et al. prepared soy protein concentrate (SPC)-based adhesives and rice husks (RCs) to produce particleboards with the main goal of upgrading the final water resistance and mechanical properties of RH-SPC particleboards via the alkali treatment of soy protein concentrate and rice husks, coupled with bleaching of rice husks with hydrogen peroxide [38]. This facilitated chemical interactions via hydrogen bonds between the more exposed hydroxyl groups of cellulose from rice husks and the polar groups of the unfolded proteins of soy protein concentrate treated with alkali [38]. The particleboards met the mechanical properties requirements for commercial use consideration but failed to achieve the minimum requirements for water resistance as recommended by US Standard ANSI/A208.1 [38][39]. The limitation was linked to the increase in the amorphous content of the cellulose after dispersing rice husk in NaOH for a short period [38]. This drawback is counterbalanced by the adhesive’s lack of formaldehyde and the use of complete rice husks in particleboard production, which eliminates milling and screening procedures, resulting in cost savings [38].

4. Removal of Hydrophilic Content

Gui et al. centered their research on the preparation of water-resistant soy flour adhesives through the reduction in the water-soluble constituents [40]. Previous research has shown that poor water resistance is mainly caused by feedstock water-soluble components [41]. The design approach involved suspending the defatted soy flour in water followed by the adjustment of the dispersion pH at different temperatures and time points to 4.5 by adding NaOH and HCl solution, respectively [40]. Then, the sample with less water-soluble constituents was separated through a centrifugation [40]. The application of the modified soy flour adhesive on poplar plywood shows that it had a wet strength of 1.02 MPa [40]. The remaining multi-level structures of soy protein contributed positively to soy adhesives’ water resistance [40]. Although this is a simple and novel way of developing water-resistant soy adhesives, the product is still limited by low solid content and fluidity compared to formaldehyde adhesives [40]. Thus, further research is required to enhance these properties.
Zhang et al. subjected defatted soybean flour (DSF) adhesive to thermal treatment at different test temperatures to improve its water resistance and investigate the effects of the thermal pretreatment on increasing the water-insoluble content, crystalline degree, and chemical structure [42]. The team also tested the thermal stabilities and bonding qualities of soy adhesives made from thermal treatment DSF (T-DSF) and cross-linker epichlorohydrin-modified polyamide (EMPA) [42]. The test result showed that the thermal treatment facilitated the increase in the acetaldehyde value and water-soluble content of T-DSF [42]. Thus, thermal treatment can enhance protein–carbohydrate Maillard reactions, protein–protein self-cross-linking, and protein–EMPA cross-linking by unfolding the globular form of soy protein and releasing hidden functional groups [42]. Uncertain are the quantitative contributions of protein–protein self-cross-linking, protein–carbohydrate Maillard processes, and protein–EMPA cross-linking, as well as their impact on the water resistance of T-DSF-based adhesives [42].
Qi et al. investigated the effect of liquid 2-octen-1-ylsuccinic anhydride (OSA) on soy protein adhesives [43]. The OSA possesses a long alkyl chain and oily nature coupled with a succinylation reaction that can help enhance protein adhesion strength [43]. Thus, the team studied the adhesive properties of soy protein adhesives modified by OSA at different concentrations as well as characterized its physicochemical properties like morphological, rheological, thermal, and turbidity properties [43]. The OSA modification increased the wet shear strength in plywood samples up to 3.2 MPa with up to 60% wood cohesive failure in comparison to 1.8 MPa of wet strength for the control. They found that the modification of the soy protein adhesives with OSA facilitated the introduction of hydrophobic materials to the protein structures [43]. Due to OSA’s hydrophobic properties, hollow cavities could not form since water could not penetrate the interface between the wood surface and the adhesive [43]. The researchers stated that this could be the primary factor for the soy protein adhesives’ significantly improved water resistance [43].
Zhang et al. worked on the modification of soy protein adhesives using epoxidized oleic acid and prepared the chemically modified adhesive soy protein and rice straw formulation to produce a fiberboard that might serve as a viable substitute for wood-based fiberboard [44]. The utilized epoxidized oleic acid contains many free epoxy groups that could have a reaction with the functional amino groups in the SPI molecule. Epoxidized oleic acid was utilized to react with the amino groups in SPI molecules to increase the mechanical and water resistance qualities of the adhesive [44]. The team investigated modified-SPI adhesive addition, the effects of NaOH concentration, and fiberboard density on the water resistance and mechanical properties of rice straw fiberboards [44]. The results showed that the fiberboards have optimal water resistance and mechanical performance, which is due to the reaction between the soy protein and the epoxidized oleic acid, and the removal of the wax layer using NaOH with a water resistance value of around 64% [44]. The advantage is that the raw materials are low-cost and easily biodegradable; thus, these fiberboards are an excellent substitute for petroleum-based panels and can be utilized in indoor furniture and decoration.

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
Olatunji V. Oni
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Michael A. Lawrence
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Mark E. Zappi
,
William M. Chirdon
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Update Date: 19 Oct 2023
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