A high-performance bio-adhesive derived from soy protein isolate and condensed tannins

Abstract

In this research, condensed tannins (CTs) were modified by hexamine and mixed with soy isolate protein (SPI) under pH 9.0 conditions to form a series of new high-performance bio-adhesives (HCT-SPI). The adhesives had a compact structure and stable thermal performance according to the results of FTIR, SEM and TGA measurements. Their high crosslinking density was attributed to the crosslinking reaction of CT molecules via the imine groups of the hexamine decomposition product; another reason was that the ortho-quinone obtained via pyrocatechol oxidation on the tannins B ring led to covalent interactions and a crosslinking reaction with SPI molecules. The hydrolytic stability of HCT-SPI 5 adhesive (SPI 15 g, CTs 5 g, hexamine 2.5 g) increased by 3.2% compared to the native SPI-based adhesive, and the wet shear strength of plywood bonded by the HCT-SPI 5 adhesive was enhanced by 90.7% compared to plywood bonded by native SPI-based resins, which is acceptable for industrial application in plywood fabrication according to the China National Standard (≥0.7 MPa).