Wheat Straw Lignin Nanoparticles as Active Filler in Thermoplastic Starch Films

Starch and lignin are promising biopolymers for the production of biodegradable biocomposite materials. The possibility of processing starch into thermoplastic materials qualifies it as a starting material for the preparation of thermoplastic packaging films, and the combination with lignin can even out some inherent weak points of starch, such as moisture and water sensitivity, and can add additional features like antioxidant activity. Lignins from herbaceous biomass carry building blocks that are not found in wood lignins and are known for their bioactivity, such as p-coumaric acid or ferulic acid. In this work, a protocol was developed to initially prepare hybrids of wheat starch granules and lignin nanoparticles, which were then plasticized using glycerol in an extrusion process to produce thin films. The lignin-containing thermoplastic starch films showed higher Young's moduli and less elongation at break compared to neat thermoplastic starch films, while tensile strength remained at the level of the neat films. Thermal stability was slightly increased by lignin addition, and oxygen transmission rates were low for lignin contents as low as 1 wt%. The hydrophobicity of the lignin-containing films increased strongly, and they showed an elevated antioxidant activity over several hours, which was also maintained after 24 h. The preparation of hybrid wheat starch lignin particles was successfully tested for the extrusion of thermoplastic starch films with improved thermomechanical properties, decreased water sensitivity, and prolonged antioxidant activity.