Correlations of the Antioxidant Properties of Softwood Kraft Lignin Fractions with the Thermal Stability of Its Blends with Polyethylene

Since technical lignins are increasingly considered as additives to polyolefins, an effort is made here to understand the fundamental antioxidant properties of softwood kraft lignin and its fractions on the thermal stability of its blends with polyethylene. Lower molecular weight acetone soluble kraft lignin (ASKL) fractions showed better antioxidant properties than unfractionated and acetone insoluble kraft lignin (AIKL). By selectively methylating the phenolic hydroxyl groups of the lignin and its fractions, it was shown that the lignin had no antioxidant ability. The phenolic OH groups in lignin, therefore, play a vital role toward imparting antioxidant characteristics in it. To further understand the role of lignin during the thermal processing of polyethylene, we measured the oxidation induction temperature (OITtemp) of its blends with softwood kraft lignin and its fractions. Once again, the role of the phenolic OH was found to be extremely important toward the thermal oxidative characteristics of kraft lignin and its fractions. Since acetone soluble softwood kraft lignin contains 54% more phenolic units than its acetone insoluble counterpart, its blends (5 wt %) with polyethylene improved its OITtemp by about 50 degrees C with no additional increases at higher lignin contents. At elevated processing temperatures, when polyethylene blends of lignin start to degrade, the aromatic nature of the created char reduces its rate of degradation, concomitantly increasing the thermal degradation temperature of polyethylene. This effect was further investigated and details of the relative contributions of the phenolic OH stabilization mechanism to the charring mechanism are discussed.