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 [
111,
112]. The incorporation of sodium montmorillonite (Na MMT) into soy protein adhesives at different concentrations was explored by Qi et al. [
113]. 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 [
114]. 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 [
115]. Thus, Qi et al. worked on developing a novel soy protein and clay system with excellent flowability and strong adhesion at high protein content [
113]. Sodium bisulfite (NaHSO
3) was used in these formulations, and hydroxyethyl cellulose was used as a suspension agent [
113]. NaHSO
3 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 [
116]. Another researcher, Zhang and Sun, corroborated this claim by using NaHSO
3 to break the disulfide bonds of soy glycinin to increase the surface hydrophobicity [
117].
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 [
113]. 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 [
113]. 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 [
118]. 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 [
118]. 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 [
118,
119]. The limitation was linked to the increase in the amorphous content of the cellulose after dispersing rice husk in NaOH for a short period [
118]. 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 [
118].
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 [
120]. Previous research has shown that poor water resistance is mainly caused by feedstock water-soluble components [
29]. 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 [
120]. Then, the sample with less water-soluble constituents was separated through a centrifugation [
120]. The application of the modified soy flour adhesive on poplar plywood shows that it had a wet strength of 1.02 MPa [
120]. The remaining multi-level structures of soy protein contributed positively to soy adhesives’ water resistance [
120]. 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 [
120]. 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 [
121]. 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) [
121]. The test result showed that the thermal treatment facilitated the increase in the acetaldehyde value and water-soluble content of T-DSF [
121]. 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 [
121]. 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 [
121].
Qi et al. investigated the effect of liquid 2-octen-1-ylsuccinic anhydride (OSA) on soy protein adhesives [
122]. The OSA possesses a long alkyl chain and oily nature coupled with a succinylation reaction that can help enhance protein adhesion strength [
122]. 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 [
122]. 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 [
122]. 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 [
122]. The researchers stated that this could be the primary factor for the soy protein adhesives’ significantly improved water resistance [
122].
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 [
123]. 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 [
123]. 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 [
123]. 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% [
123]. 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.