Modified glutine adhesives for structural bonding of glued-in rods in timber engineering

The development of sustainable adhesives is becoming an increasingly important topic in timber engineering, as the industry seeks to replace petroleum-based products with bio-based alternatives. Among the natural candidates, glutine adhesives are attractive due to their renewable origin and long history of use, but their application in structural contexts remains limited by poor resistance to moisture and elevated temperatures. This study addresses these weaknesses by investigating straightforward modifications to raw glutine aimed at enhancing its hygrothermal stability. Several strategies were tested, including reducing water content, incorporating fillers such as chalk or sawdust, and adding phenolic cross-linkers (tannic acid, gallic acid, or combinations). Mechanical performance was evaluated at three scales: lap-shear specimens, small-scale glued-in rods, and large-scale glued-in rods. At room temperature, shear strengths of modified glutine adhesives reached values comparable to epoxy references. Under moderate hygrothermal conditions (45 °C/50%–65% RH), formulations such as M31 (chalk + gallic acid) and M35 (chalk + tannic acid) achieved average shear strengths above 6 MPa, showing substantial improvements over the raw reference adhesive. Severe exposure (60 °C/85% RH), however, led to marked reductions in strength, with failure modes shifting from wood tear-out to adhesive failure. The results show that relatively simple modifications can significantly enhance the hygrothermal stability of glutine adhesives and suggest a promising pathway towards sustainable structural bonding. While further studies are required to clarify degradation mechanisms, assess long-term durability, and evaluate scalability, this work demonstrates the potential of modified glutine adhesives as bio-based alternatives in structural timber applications.