Characterization of the secondary structure, renaturation and physical ageing of gelatine adhesives

Animal glues have been used since antiquity, but their popularity decreased in the twentieth century with the rise of synthetic adhesives. Currently they are primarily used in restoration of works of art. This study focuses on animal glue samples derived from bone and hide tissues used mainly for veiling and carpentry applications, examining their secondary structure, thermal and rheological properties, to shed light into their adhesive behaviour. Thermogravimetric analysis and differential scanning calorimetry highlight differences between hide and bone glues, showing that the latter are more hydrolysed. The calorimetric curves show varying values of denaturation enthalpy thus indicating a varying degree of gelatine renaturation. Additionally, the calorimetric analysis demonstrates the physical ageing of the glue samples, which is known to play a key role in maintaining their adhesive properties under specific storage conditions. The rheological data provide fundamental information on properties which are relevant for the use of animal glues as adhesives in various applications. XRD data suggest a low structural order in the samples investigated, where the amorphous component complicates the univocal interpretation of the spectra. FTIR reveal a relatively high content of beta-structures in all commercial glue samples, as well as gelatine and collagen standards. Moreover, spectra of the glue films dried for 1 week show that the beta-structures are recovered following dissolution in water and application. Therefore, beta-structures seem crucial to explain the structure-dependent mechanical properties observed.