Development of soy protein-based adhesive with high water resistance and bonding strength by waterborne epoxy crosslinking strategy

The introduction of epoxy crosslinking agents is an efficient path to improve the water resistance and adhesion properties of a soy protein-based adhesive. However, it is still a challenge to directly incorporate cheap epoxy resin into a protein system other than by an expensive epoxy diluent. In this work, a waterborne epoxy emulsion (WEU) was prepared by grafting hydrophilic groups onto bisphenol-A (E44) followed by a phase-transformation process for the oil-in-water emulsion, with the aim to modify a soy protein-based adhesive. The effects of different neutralizers on the performance of the resulting adhesive were also analyzed. The prepared WEU presented excellent stability and admirable dispersibility in the adhesive system, as confirmed by digital photos, and served as an active crosslinker, contributing to multiple stable crosslinking networks with soy protein molecules, as characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and thermal gravimetric analysis. The results showed that a soy protein-based adhesive crosslinked by WEU neutralized by triethylamine could endow plywood with 20% higher wet shear strength than that of being crosslinked by WEU neutralized by N, N-dimethyl ethanolamine, which reached 1.14 MPa and 192.5% increment compared to a neat soy protein-based adhesive. Moreover, the residual rate of modified soy protein adhesives increased 4.2% while the moisture uptake of modified soy protein adhesives decreased 8.5% compared to pure SP adhesive. The authors hypothesize that the epoxy crosslinking modification of soy protein with WEU may be an effective method to fabricate high-performance soy protein-based adhesives for practical applications.