Tough and functional cross-linked bioplastics from sheep wool keratin

Abstract

Novel bioplastic flms derived from wool keratins were prepared by protein solution in an alkaline mild oxidative method that splits disulphide (-S-S-) bonds. The native structure of the keratin macromolecules was partially modifed upon extraction as revealed by the decrease of the β-sheet to α-helices/coils ratio but high molecular weight fractions (31, 22 and 13 KDa) was retained permitting flm formation and plastic behaviour of flms. Keratin flms were plasticised with glycerol and sodium dodecyl sulphonic acid (SDS), which provided diferent hydrophobic character to bioplastics. Water content in the flms depend on the relative humidity (RH), being able to absorb up to 35 wt% H₂O at an ambient of 80% RH. Films were mechanically, thermally and optically analysed. The spectroscopic analyses revelled that these bioplastic flms absorb UV light, what is interesting for packaging applications. Thermogravimetric and thermomechanical analysis revealed high stability of keratin macromolecules up to 200°C with no inherent thermal transitions. Tough bioplastics (19±4 MJ∙ m−3) were obtained after thermal cross-linking with glycerol and formaldehyde outperforming mechanical properties previously reported for protein films.