Strengthening poly (lactic acid): The role of fractionated lignin in the development of sustainable films.

The growing demand for sustainable materials has increased interest in alternatives to petroleum-based polymers. Poly (lactic acid) (PLA) is a promising candidate due to its renewable origin, but its brittleness and low thermal stability limit broader applications. This study investigates the incorporation of fractionated lignin-obtained from eucalyptus kraft lignin via ethyl acetate fractionation-to improve the mechanical, thermal, and functional properties of PLA films. Films with varying lignin contents (0-10 %) were produced by solvent casting and evaluated through mechanical testing, water vapor permeability (WVP), thermal analysis (TGA/DTG), FTIR, SEM, TEM, and color/UV-Vis analysis. Up to 6 % lignin, improvements were observed in tensile strength, elastic modulus, and thermal stability, attributed to better dispersion and interaction within the PLA matrix. WVP decreased at intermediate lignin contents, indicating a barrier effect. FTIR results suggested hydrogen bonding between lignin and PLA, while SEM revealed uniform morphology at low lignin concentrations and aggregation at higher levels. TEM confirmed lignin dispersion within the PLA matrix. Color darkening due to lignin's chromophore groups was accompanied by reduced UV transmittance, indicating potential UV protection. These findings demonstrate that PLA films reinforced with fractionated lignin are promising sustainable materials for applications requiring enhanced mechanical and barrier performance.