Wheat straw lignin as active filler in thermoplastic starch packaging

Abstract

Lignin is an interesting candidate as an additive in biopolymer composites in order to confer anti-oxidant, UV-shielding, and antimicrobial properties in packaging films made from biopolymers such as thermoplastic starch (TPS). As the second most abundant biopolymer after cellulose, lignin is produced as a side stream in biorefineries or the pulp and paper industry in vast amounts. In contrast to lignin deriving from wood, lignin from herbaceous biomass such as wheat straw is characterized by additional structural features like p-hydroxycinnamic acids, which can be present either incorporated into the lignin macromolecule or as pendant groups on lignin side-chains. These structural features could promote interactions between the active filler and the biopolymer matrix during film formation providing improved thermal and mechanical properties. Two different non-wood lignins were utilized as active fillers in TPS films. Organosolv wheat straw lignin was prepared using ethanol-water for Organosolv extraction and separated by precipitation after evaporation of ethanol, followed by repeated washing with acidified water (pH 2), centrifugation and freeze-drying. For comparison, commercially available wheat straw lignin from alkaline pulping (Protobind 1000, PLT Innovations AG, Rüschlikon, Switzerland) was used. Biocomposite films were realized by using a twin-screw microextruder (DSM Xplore 5&15 CC Micro Compounder/Film Device). During the mixing/plasticization, the screw speed was set at 60 rpm, with a temperature profile of 135-140-145 °C for 180 s. The wheat starch matrix was amended with lignin nanoparticles before mixing with glycerol and eventual film extrusion. SEM analysis showed two distinct particle size fractions of wheat starch granules. Wheat starch granules were covered and held together by phases of agglomerated lignin nanoparticles. Extruded films were analysed regarding their UV-light blocking capacity, antioxidant activity and their degradation behaviour in composting conditions. Mechanical properties were investigated using tensile tests. Acknowledgments: This publication is based upon work from COST Action WIRE, CA20127, supported by COST (European Cooperation in Science and Technology).