In the wood manufacturing industry, wood-based panels (WBP) play an incomparable role in the production of furniture, packaging and building materials because of the economic effectiveness and excellent processibility [1–3]. At the same time, adhesives make a difference in manufacturing of WBP due to their abundance and low cost. Nowadays, up to 90% of the adhesives applied in the WBP industry are formaldehyde-based adhesives that are the primary sources of indoor formaldehyde pollution and severely affect human health, work and living quality [4–6].
To solve this problem, formaldehyde-free adhesives as new type products have aroused people’s strong interest. In recent years, bio-based adhesives show huge potential in replacing formaldehyde-based adhesives owing to their non-toxic and green characteristics such as starch, lignin, tannin, and protein, which are hopeful to reduce the consumption of petrochemical resources and indoor formaldehyde emissions [7–10]. Soy protein comprising about 80% of mass of soy is a residue of soybean oil production. Soy protein adhesives are time honored, which have the advantages of low price, abundance, renewability, and easy processing, showing great promise for serving as formaldehyde-based adhesives substitute [11–13]. However, there are still some disadvantages in soy protein adhesives on account of the restriction of natural hydrophilicity and weak interactions between the protein molecules. Due to the inherent defects of soy protein adhesives that lead to their terrible bonding strength and poor water resistance limiting their practical applications [14–16].
Currently, a large number of strategies have been used to address the intrinsic drawbacks of soy protein adhesives. Numerous ways, including enzymatic degradation, protein molecular design, filler blending, and chemical modification were developed [17–19]. For instance, enzymes can hydrolyze peptide bonds in proteins, which change the structure of protein molecules and promote intermolecular and intramolecular cross-linking. But this method is difficult to apply in industrial production due to the high cost [20–22]. Among these techniques, chemical modification is the most universal approach to enhance properties of the adhesive by constructing a dense cross-linking network via multiple chemical interactions [23, 24]. Aldehydes, magnesium oxide, polyurethane, and phenol formaldehyde resin have been designed as the chemical cross-linking agents to react with the hydrophilic active groups (-NH2, -COOH, -OH) in the soy protein main chains, which could form a dense cross-linked network and increase the hydrophobicity of modified adhesives [25–27]. Nevertheless, most of cross-linking agents are synthesized from non-renewable petroleum hindering the construction of ecological civilization. Excessive addition of fossil-based cross-linking agents is also contrary to the aim of decreasing the consumption of petrochemical resources. Meanwhile, the improvement of soy protein adhesive performance by a single organic crosslinking agent modification is limited [28–30]. Gradually, inorganic substances were popularly used to modify current organic adhesives since the formation of organic-inorganic hybrid structure can greatly improve the properties of adhesives [31–33]. What’s more, with the rapid development of bionics, it is promising to create a green and fully bio-based biomimetic modified soy protein adhesive which is inspired from natural world [34–36].
Natural rosin acid, one of the abundant secondary metabolites, is mainly obtained by distillation from several pine trees and pulp waste. It is mainly composed of various resin acids with hydrogenated phenanthrene ring structure [37, 38]. Nowadays, natural rosin acid is gradually regarded as a novel, environmentally friendly and biodegradable resin substitute [39]. Moreover, the rosin acid derivative has active carboxyl group which makes it have satisfactory reactivity. And hydrogenated phenanthrene ring structure also endows it with excellent hydrophobicity. As a result, the rosin acid derivative has significant potential as a bio-based crosslinker during the progression of soy protein adhesives [40].
In this work, inspired by organic-inorganic hybrid structure, a novel, bionic and green fully bio-based strategy was creatively put forward to prepare high practical performance soy protein adhesives. A unique hybrid reinforcement structure by rosin acid derivative and dopamine hydrochloride (DOPA) was constructed from ring opening reaction. Then the product obtained after the above reaction and calcium sulfoaluminate (CSA), the primary component of a common cement was together added into soy protein adhesive forming a firm crosslinking network through multiple interactions such as covalent bond, hydrogen bond and metal coordination bond. The optimization resultant SPI-RA6-CSA1 adhesive by RSM possessed outstanding bonding strength and water resistance (1.52 MPa and 1.35 MPa). In addition, the adhesive showed incredible coating performance even if coating on the wood after soaking. What’s more, SPI-RA6-CSA1 adhesive also demonstrated prominent antimicrobial, flame retardant and environmentally friendliness properties. This bionic and green fully bio-based soy protein adhesive would take a step forward for further application in the WBP industry.