Antioxidant and Thermal Stabilization of Polypropylene by Addition of Butylated Lignin at Low Loadings

The effect of phenolic hydroxyl group content and molecular mass of a technical lignin on the thermo-oxidation resistance and compatibility with polypropylene (PP) was studied by a combination of approaches, including 31P NMR, gel permeation chromatography (GPC), thermogravimetry (TG), dynamic scanning calorimetry (DSC), accelerated aging tests, scanning electron microscopy (SEM), and dynamic mechanical analysis (DMA). The compatibility of lignin with PP upon melt compounding in a screw system was improved after modification of lignin with n-butyric anhydride. The thermal oxidation stability of films of PP blends, as measured in an oxygen atmosphere, was increased by low addition levels of the butylated lignins while maintaining, and in some cases improving, their thermo-mechanical performance. Notably, the oxidation induction time (OIT) and induction aging time of PP blended with butylated lignins (<5% loadings) reached maximum values of 16 min and 576 h, respectively, indicating an effect equivalent to that of commercial antioxidants. It is concluded that an increased compatibility with PP can be achieved by more extensive lignin butylation (higher degree of OH substitution), but this also limits the OIT and thermal-oxidative stability of the blend, indicating a trade-off between compatibility and antioxidant capability.