Thermo-Physical Characterization of Kraft Lignin Mixed with Bio-Plasticizers: A Valorization Approach

Kraft lignin, a by-product of the paper industry, is well known for its binding properties, enabling its use in the production of pellets and briquettes from biomass. Different bio-plasticizers, by-products from the vegetable oil processing industry, could serve as plasticizers for biomass briquettes. The properties of three bio-plasticizers (glycerol, fatty acid, and biodiesel), when mixed with isolated Kraft lignin, were studied to identify their potential application as efficient binders for biomass briquettes. The phase transition characteristics (glass transition, pre-melting crystallization, and melting) of two types of isolated lignin samples (acidic and alkaline) and lignin-plasticizer mixtures were determined using a differential scanning calorimeter. The mechanical (tensile strength) and physical (density) characteristics of the compacted cylindrical lignin pellets were tested. The spatial distribution of micropores in the lignin pellets was studied using x-ray tomography. Even though an overlap was observed in the glass transition temperatures of the mixtures, a significant depression in the pre-melting crystallization and melting temperatures was observed for both lignin samples containing elevated concentrations of the bio-plasticizers, and the highest tensile strength was obtained for pellets with 10% bio-plasticizer. Among the three bio-plasticizers, crude glycerol showed the greatest depression in melting point with increasing proportions of both acidic lignin (60.7 degrees C +/- 2 degrees C) and alkaline lignin (85.1 degrees C +/- 2 degrees C). In general, alkaline lignin showed some limitations over acidic lignin in the tensile strength of the pellets as well as their fusion temperature, even though the addition of a bioplasticizer improved the strength and depressed the melting point in both lignin-based samples.