Enhance Antioxidant Activity and UV-Barrier Properties of Poly(lactic acid) Packaging Films by Thermally Modified Lignin with Low Molecular Weight Polyethylene Glycol

Single-use plastic waste has emerged as a global environmental crisis, prompting the search for
alternatives such as biodegradable polymers. Poly(lactic acid), PLA, is a promising option derived from
renewable resources that can undergo enzymatic or hydrolytic degradation. PLA possesses good melt
processability and mechanical strength and is approved by Food and Drug Administration (FDA) for
food and beverage packaging. However, it has limitations in terms of low ductility, antioxidant capacity,
and UV-shielding properties. To address these drawbacks while maintaining sustainability, researchers
have explored the incorporation of lignin, an abundant aromatic polymer obtained from industrial paper
and pulping processes. Lignin possesses natural antioxidant and UV-absorbing abilities. In this study,
softwood kraft lignin (S-lignin) was thermally modified with polyethylene glycol (PEG) of low
molecular weight. The resulting Glycol-lignin exhibited a lower glass transition temperature and broader
molecular weight distribution than pristine S-lignin. By blending PLA with Glycol-lignin solutions of
various concentrations, composite films were created using blown film extrusion. The addition of 0.5
wt.% Glycol-lignin to the PLA/PEG composite resulted in brownish films with effective UV-blocking
and excellent visible light transmittance. These films also showed a 33.58 % increase in free radical
scavenging activity compared to PLA/PEG film.