Correlations of the Antioxidant Properties of Softwood Kraft Lignin Fractions with the Thermal Stability of Its Blends with Polyethylene Hasan Sadeghifar †,‡ and Dimitris S. Argyropoulos* ,†,§ † Departments of Chemistry and Forest Biomaterials, Organic Chemistry of Wood Components Laboratory, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ Department of Wood and Paper Science, Islamic Azad University, P.O. Box 48161-19318, Sari Branch, Iran § Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia ABSTRACT: Since technical lignins are increasingly consid- ered as additives to polyolefins, an effort is made here to understand the fundamental antioxidant properties of soft- wood 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 anti- oxidant 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 (OIT temp ) 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 OIT temp by about 50 °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. KEYWORDS: Softwood kraft, Lignin, Methylation, Polyethylene, Oxidative induction temperature, Lignin antioxidant, Thermal stability, DSC, Oxidation, Fractionation, Fractions, Phenolic hydroxyl, Antioxidant, Lignin-polyethylene blends, Thermal stability, OIT temp ■ INTRODUCTION Lignin as the second main component of wood is an aromatic biopolymer composed of phenyl propane units of p- hydroxyphenyl (H), Guaiacyl (G), and Syringyl (S). These precursor units are linked to each other in the lignin structure by various ether and C-C bonds. 1 The structure of kraft lignin, derived from the pulping process, is more heterogeneous than native lignin in the wood. Lignin structure and functional groups change during the delignification process. 2,3 The use of lignin as a filler or reinforcements in thermoplastic materials is not new and dates back to 1960s. 4 In recent years, lignin has been examined for use in many materials such as resins, adhesives, and polymer blends. 4-10 Overall, there are significant economic and environmental factors that contribute to lignin’s use as a component in synthetic polymer blends. As such, significant efforts are apparent in the literature over the span of the past decade. The availability of technical lignins such as kraft lignin offers more compelling reasons for its use as an alternative to fillers or as a reinforcement or extender component within a variety of synthetic polymer systems. In general, the presence of the phenolic OH groups within the lignin structure has been documented as being responsible for beneficial effects toward oxidative, thermal, and light stability characteristics when present in polymer blends. 11,12 These inhibition and UV stabilization effects of lignin have been attributed to its free radical scavenger abilities reducing oxygen radicals and stabilizing oxidation reactions. 13-18 It is not surprising that such effects vary with the type and origin of the raw lignin, and the isolation and purification protocol as well as the molecular weight distribution, functional group content, its conjugation, heterogeneity, and presence of carbohydrates. 16,19 Many synthetic polymers are highly combustible and as such safety requirements are becoming increasingly stringent in terms of their fire resistance performance. For this reason a variety of diverse and efficient additives have been introduced for improving such characteristics. Typical classes of materials Received: November 26, 2014 Revised: December 30, 2014 Research Article pubs.acs.org/journal/ascecg © XXXX American Chemical Society A DOI: 10.1021/sc500756n ACS Sustainable Chem. Eng. XXXX, XXX, XXX-XXX