Performance of kraft lignin as a filler in PLA composites via the acetic acid casting technique

Poly(lactic acid) (PLA) is a bio-based polymer considered a sustainable alternative to petrochemical-derived plastics. This study explores the solubilization of PLA in acetic acid, aiming to enhance eco-friendly production methods. PLA was exposed to acetic acid concentrations of 50%, 60%, 70%, and 100% at temperatures ranging from 20 to 100 °C, with dissolution times extending up to 24 h. Solubilization was categorized as “dissolved” or “not dissolved” based on visual inspection. Selected samples underwent Fourier transform infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction to evaluate chemical structure, thermal stability, and crystallinity. PLA dissolved within 6 h at temperatures above 80 °C with at least 60% acetic acid, and higher acid concentrations further reduced dissolution times. While the PLA chemical structure showed no significant alterations, thermal stability was slightly reduced, and crystallinity was significantly affected by both solvent concentration and temperature. Films were successfully formed only at 120 °C with 100% acetic acid. Additionally, kraft lignin powder (KLP) derived from Pinus was incorporated into PLA composites through casting in acetic acid, with KLP concentrations of 5%, 10%, and 25%. At 5% KLP, dispersion was good, while higher KLP contents led to particle aggregation. The inclusion of 5% KLP increased hydrophilicity (~ 11%) and reduced tensile strength (~ 40%), likely due to KLP trimodal particle size distribution (0.1 to nearly 500 μm), which made the film matrix more heterogeneous. However, the addition of 5% lignin had minimal impact on composite properties, making it a promising option for cost reduction as lignin is a by-product.