Investigation of humidity-heat aging resistance of a soy protein adhesive fabricated by soybean meal and lignin-based polymer

Abstract

The excellent bonding strength and durability of bio-based protein adhesives are key to expand their practical applications in wood industry. The durability of adhesive is affected by environmental exposure such as temperature and humidity, but there are limited reports about the durability of the protein adhesive. Herein, a soybean meal (SM)-based adhesive modified by epoxy polymer of lignin (EPL) was prepared and then treated under high humidity-heat aging conditions (90 ± 5% RH and 50 ± 2 °C) for 1200 h to assess the durability of adhesive. The results showed that the main structure and thermal stability properties of SM/EPL adhesive were affected by the humidity-heat aging treatment, and the thermal stability of adhesive remained good. The wet shear strength of the adhesive decreased with the aging time, which was because that the hydrolysis, oxidation, and hydrogen bonds dissociation of the adhesive reduced the cross-linking density and intermolecular force in the adhesive system. Furthermore, there was no mildew occurred on the SM/EPL adhesive, and the bonding strength of SM/EPL adhesive still met the interior-use plywood requirement (≥0.7 MPa) after being treated for 1200 h. This study demonstrates that the wood products fabricated by SM/EPL adhesive are durable in high humidity-heat environment.