Bonding Performance of Natural Protein-Based Adhesive Systems on European Beech Wood Under Ambient and Moderate Thermal Exposure

The growing demand for sustainable materials has renewed interest in replacing conventional petroleum-based wood adhesives with natural, environmentally friendly alternatives. This study evaluated the bonding performance of a conventional polyvinyl acetate (PVAc) adhesive and three natural adhesives—bone glue, fish glue and casein—for bonding European beech (Fagus sylvatica L.) wood of Greek origin. Bond quality was assessed according to ISO 6238:2018 by measuring shear strength and wood failure under ambient laboratory conditions (23 ± 2 °C) and after exposure to 50 °C for 15 days, simulating elevated temperatures that may occur in indoor environments. PVAc exhibited the most consistent bonding performance, whereas casein achieved shear strength comparable to that of PVAc, demonstrating its potential as a sustainable alternative for interior wood bonding. In contrast, bone glue and fish glue exhibited lower shear strength, greater variability in bond performance, and practical limitations associated with their shorter working and setting times. The percentage of wood failure generally followed the same trend as shear strength, confirming the relationship between bond quality and adhesive performance. Moderate thermal exposure did not significantly affect shear strength but resulted in lower wood failure percentages for the natural adhesives, whereas PVAc maintained, and slightly improved, its bond strength after thermal exposure. These findings demonstrate the promising performance of casein as a natural wood adhesive while highlighting the influence of moderate thermal exposure on the durability of natural adhesive systems intended for indoor applications.