Characterization of Glycerin Pitch and Its Application as a Plasticizer in Thermoplastic Starch Films

Glycerin pitch (GP), a low-value biodiesel byproduct, was characterized and evaluated as a potential plasticizer for thermoplastic starch (TPS) films in comparison with crude glycerin, laboratory-grade glycerol, and pharmaceutical-grade glycerol. GP was analyzed for its physical properties, elemental composition, and thermal behavior, and TPS films prepared with different glycerin grades were evaluated for visual appearance, thickness, mass, and morphology. GP exhibited strong alkalinity and a heterogeneous composition, including organic and inorganic constituents, with chloride-rich ash and elevated sodium and trace metals. Elemental analysis indicated that the organic fraction was predominantly glycerol with a relatively high oxygen content. TPS films plasticized with GP showed balanced plasticization, smooth surfaces, moderate elasticity, and the lowest solvent solubility among the samples. In contrast, films plasticized with pharmaceutical-grade glycerol exhibited overplasticization and structural defects at higher loadings. These results demonstrate that GP can function as an effective and low-cost plasticizer for TPS, improving film integrity and reducing solvent solubility while enabling the valorization of an industrial biodiesel byproduct. The findings highlight the potential of GP for use in starch-based biodegradable materials that require enhanced durability and controlled solubility.