Physico-chemical properties of fractionated softwood kraft lignin and its potential use as a bio-based component in blends with polyethylene

This study describes a systematic characterization of lignin samples fractionated from industrial black liquor and an evaluation of their suitability as a component (50 wt.%) in thermoplastic blends with polyethylene with a special emphasis on tensile and impact properties. Industrial softwood kraft lignin was isolated from three different cooking stages and subsequently fractionated by sequential acid precipitation. Altogether, nine lignin fractions were subjected to several chemical/thermal analyses to compare their structural features and thermal decomposition properties. Lignin samples precipitated at pH 10.5 exhibited the highest molecular weight (M w) and purity, demonstrated by the lowest content of sulfur and polysaccharides. In contrast, samples precipitated at a low pH in general exhibited higher amount of impurities and low methoxyl group content. It was found that lignin precipitated at low pH contained the biggest share of sulfur present in kraft lignin. However, about 70 % of sulfur in these samples is present in non-bounded form and could be extracted with CS2. Additionally, low M w lignin exhibited a significantly lower T g value, which could favor material processing. A notable decrease in the thermal stability of the tested lignin samples was observed with a decrease in the molecular weight. In addition, lignin with a low M w, high phenolic hydroxyl groups, and lower number of double bonds seems to be favorable for increased tensile strength and elastic modulus of the polyethylene–lignin blend materials.