Can all lignins act as an antioxidant for polypropylene?

Lignin, a high-volume byproduct of the pulp and paper industry, contains sterically hindered phenolic hydroxyl groups that enable antioxidant activity in polymer systems. This study investigates the potential of three eucalyptus-derived lignins, referred to as L1, L2, and L3, extracted from black liquor via acid precipitation under varying conditions, as primary antioxidants in polypropylene (PP). The lignins exhibited distinct pH values: L1 (3.35), L2 (3.68), and L3 (7.97). A comprehensive characterization, combining structural, chemical, and molecular analyses, was performed to correlate intrinsic features with antioxidant efficiency in PP. Although extraction conditions influenced purity and functionality, only detailed characterization allowed reliable prediction of stabilization performance. Lignins with acidic pH values (L1 and L2) showed higher purity and a greater concentration of sterically hindered phenolic groups, correlating with enhanced antioxidant activity. Oxidative induction time (OIT), rotational rheometry, and size exclusion chromatography (SEC) confirmed the superior performance of L1 and L2. At 2000 ppm, PP-lignin formulations containing L1 or L2 exhibited significantly higher OIT values, viscosities, and molar masses compared to PP with conventional synthetic antioxidants. Structural differences between syringyl and guaiacyl units in L1 and L2 further influenced stabilization during high-temperature processing, with L2 demonstrating exceptional performance under elevated temperature, oxygen, and shear. This work highlights the importance of combining extraction strategies with advanced lignin characterization to predict antioxidant efficiency, while introducing methodologies that correlate rheological behavior with molar mass distribution. These findings support lignin’s application as a cost-effective, bio-based, and high-performance antioxidant additive for recycled polypropylene. • Lignin characterization predicts antioxidant efficiency in polypropylene. • High-purity lignins with hindered phenolic groups improve stabilization. • Acidic-pH lignins show different performance due to syringol/guaiacol content. • Methodology links rheology, SEC, and OIT to lignin structure.