Low-temperature curable and strong soy protein/allicin adhesive with excellent mildew resistance via a free-radical-polymerization curing system

Abstract

Epoxide modification effectively improves the water tolerance of soy protein adhesives, showing great potential in the commercial application as alternatives to petroleum-derived formaldehyde-based adhesives. However, epoxide-modified adhesives still present problematic limitations, such as poor mildew resistance and high hot-pressing temperature. In this study, a curing system based on radical polymerization was designed and developed to prepare soy protein adhesives, instead of building a traditional epoxide cross-linking structure. First, allyl glycidyl ether (G) was grafted onto a soy protein (SP) molecule to obtain soy protein with unsaturated double bonds ([email protected]). Allicin (A) was then used as a cross-linker to build a cross-linking structure ([email protected]/A) via free-radical polymerization to enhance the bond performance of the adhesive. Relative to that of pure SP adhesive, the wet shear strength (WSS) and dry shear strength (DSS) of plywood bonded with the [email protected]/A adhesive increased by 108% and 45%, reaching 1.08 MPa and 1.84 MPa, respectively, at 120 °C pressing temperature. Notably, the [email protected]/A adhesive showed superior antibacterial (Escherichia coli and Staphylococcus aureus) activity, high mildew resistance (35 d shelf life), good cytocompatibility, and biodegradability (30 d). Moreover, the WSS of plywood bonded with the [email protected]/A adhesive at 80 °C for 6 min and 20 °C for 72 h reached 1.08 MPa and 1.12 MPa, respectively, indicating the sufficient low-temperature curing ability of the resultant adhesive. Thus, building a cross-linking structure by free-radical polymerization can be extended to improve the performance of film composites, other adhesives, and engineering materials.