Effects of Lignin Fractionation on the Mechanical and Thermo-Oxidation Properties of SSBR/Silica Composites

Lignin is a polyphenolic compound extracted from plant pulps in the paper industry. It has attracted significant interest for the rubber industry due to its inherent antioxidant function and reinforcement effect. However, the structural heterogeneity and poor solubility of lignin have limited its applications. This study systematically investigated the chemical composition, molecular weight, and radical scavenging capability of four commercial lignin products and their fractionated portions, derived through sequential fractionation processes. The first fraction (F1) of hardwood lignin (UPM) has the lowest molecular weight (<i>M</i> _w = 2700) and yield (6%) but the highest radical scavenging activity (RSA = 38.45%). In contrast, F1 of enzymatic hydrolysis lignin (EHL) obtained the highest yield (44%) with a moderate RSA (27.60%), slightly exceeding that of raw EHL (EHL F0). EHL F0 and F1 were further compounded with SSBR/silica for evaluating their effects on the rubber composites through mechanical, rheological, and thermo-oxidant characterization. The addition of 2∼10 phr EHL F0 or F1 shows a semi-reinforcement effect without deteriorating the nonlinear dynamic behavior of the rubber composites. The small loadings of EHL F0 or F1 showed faster oxidation induction than the commercial antioxidant <i>N</i>-(1,3-dimethylbutyl)-<i>N</i>'-phenyl-<i>p</i>-phenylenediamine (6PPD) in an oxygen environment but better long-term thermo-oxidant resistance in hot air. The dispersion and compatibility of lignin with SSBR play decisive roles in performance enhancement.