Improved soy protein adhesives by lignin and polyamide‐epichlorohydrin: Adhesion performance and properties

Protein modifications by crosslinkers have been known to be the most effective way to improve the bonding performance of plant-based protein adhesives. However, utilizing a huge amount of chemical crosslinkers in wood industries rises a concern about product's sustainability and environmental pollution. Therefore, the objective of this study was to improve the water resistance of soy protein (SP) adhesives using renewable bio-based lignin with minimal use of polyamide-epichlorohydrin (PAE). The effects of lignin, PAE and combination of lignin-PAE on SP adhesives were investigated. The results showed that the addition of lignin enhanced the thermal properties of SP adhesives with an increment in the denaturing temperature of 11S protein subunit, whereas PAE elevated the denaturing temperature of 7S protein subunit. Moreover, individual addition of either lignin or PAE increased the elastic modulus and wet adhesion performance of SP adhesives. The same trend was also observed on adhesion performance when both lignin and PAE were used in conjunction. Fourier transform infrared spectroscopy spectra indicated interactions of lignin and PAE to carbonyl, hydroxyl, and amino groups of SP, which lead to a stronger SP network. SP adhesive (10%) modified by a combination of 1% lignin and 1% PAE improved wet adhesion strength by 111% (from 1.8 to 3.8 MPa) with no impact on the spreadability. Further addition of either lignin or PAE did not show a positive effect on the adhesion performance, but rather a decrease in adhesive spreadability due to excessive interactions among SP, lignin, and PAE as well as the interference of the excessive lignin/PAE.