Protein adhesives: Alkaline hydrolysis of different crop proteins as modification for improved wood bonding performance

•Alkaline treatment can facilitate the increment of solid content of protein adhesives.•Potato protein from starch production can be used as a raw material for adhesives.•Increased proteins solubility results in increased homogeneity of system.•Excessive reduction of protein molecular size can result in a bond strength decrease. Wheat gluten, soy, potato and pea protein concentrates were studied to evaluate their wood bonding performance. The aim of comparing different plant proteins was to find alternatives sufficiently available in Europe with comparable bonding performance to soy. Proteins were subjected to alkaline hydrolysis under different conditions. The relation between protein molecular weight, viscosity, solids content, solubility and bonding performance of alkaline protein dispersions was studied using plant proteins, which differ in properties. Viscosity and pH change of dispersions were tracked during hydrolysis indicating protein structure alteration and size reduction. Tensile shear strength measurements were performed using beech wood substrates bonded with protein dispersions to investigate bonding performance. It was found that potato protein was able to form dispersions with a high solid content at comparably low viscosity, which is one of the key issues in protein adhesives production for particle boards. Soy and potato protein showed improved dry bonding performance after treatment with 1 M NaOH alkaline condition and 60 °C temperature, while for pea protein better values of tensile shear strength were obtained after the treatment under 0.1 M NaOH. Wheat protein hydrolysis also resulted in increased tensile shear strength under intensified alkaline conditions. However, the values were still lower than for soy and potato protein.