Lichen‐Derived Mineral Residue as a Renewable Particulate Filler in Polylactic Acid Biocomposites: Structure, Melt Rheology, Viscoelastic, and Flexural Behavior

Abstract

A lichen‐derived natural material, known as “höllük” in southeastern Turkey, is introduced as a renewable filler for polylactic acid (PLA). X‐ray diffraction and scanning electron microscope analysis of höllük identified quartz and calcite as the major crystalline phases within a silicate‐organic matrix, and thermogravimetric analysis indicated that the decomposition of filler began at 325°C, well above the melt‐compounding window of PLA. PLA‐lichen composites, containing 5, 10, and 15 wt% of filler were prepared in a twin‐screw extruder with 3 phr of triacetin. Rheological measurements in the melt state revealed that höllük addition slightly improves storage modulus ( G ′) and viscosity of PLA and acted as antioxidant. In dynamic mechanical analysis, 5 wt% filler increased the storage modulus ( E ′) from 3.37 to 3.54 GPa and the flexural strength from 114 to 160 MPa, whereas 15 wt% lowered E ′ to 2.19 GPa, consistent with filler agglomeration and interfacial micro‐cavities observed by scanning electron microscopy. Differential scanning calorimetry analysis implied that filler addition restricted cold crystallization and yielded slightly higher crystallinity. Oxygen permeability improved about 37% with introducing 15 wt% of filler, and a dose‐dependent inhibition of Escherichia coli was recorded. The results identify that 5 wt% of loading balances stiffness, melt processability, and thermal stability in this biocomposite system.