Soy protein isolate-based polyamides as wood adhesives

Soy protein isolate was reacted first with maleic anhydride and then hexamethylenediamine to synthesis soy protein isolate-based polyamides (SPIPA). Its bonding properties were measured by preparing laboratory three-layer plywood. Furthermore, a silane coupling agent (KH560), citric acid and maleic anhydride were added as cross-linkers to the SPIPA glue mix before hot pressing the panels. ATR FT-MIR and MALDI-ToF were used to determine the structures of the SPIPA, and thermomechanical analysis was used to analyze its thermomechanical properties. The results show that maleic anhydride reacts with amino acids in the soy protein molecule and then reacts with hexamethylenediamine to form carbamate-structured polyamides. This is confirmed by the ATR FT-MIR and MALDI-ToF results. Moreover, structural analysis indicates that a large number of branched structures are formed and occur in the SPIPA, rendering possible the formation of a network during curing, ensuring the bonding performance of the adhesive. The SPIPA adhesive presented good bonding strength and water resistance above the standard requirements of China National Standard GB/T 9846-2015. In particular, when maleic anhydride was added as a cross-linking agent, namely the SPIPA-MA adhesive, the panels' dry and wet shear strengths were improved so that it can be considered as a suitable soy protein-based wood adhesive.