Different lignin fraction extraction by pH fractionation for lignin microspheres preparation as UV-blocking PVA film.

Kraft lignin (KL), a major by-product of the pulp and paper industry, has garnered significant research attention due to its abundant availability and distinctive chemical structure, making it a promising candidate for applications such as biofuels, carbon materials, and polymer composites. Despite these advantages, its utilization in fine chemical industries, including nanomaterials, is constrained by its inherent dark color and broad molecular weight distribution. Addressing these limitations has become a central focus in ongoing research efforts aimed at enhancing the functionality and commercial viability of KL. In this study, we fractionated KL from a paper-making enterprise using pH fractionation, and obtained four kinds of lignin with different molecular weights. Following that, lignin microspheres (LMPs) were formed by self-assembly from four sizes of KL. Analyzing the KL and LMP properties, the low molecular weight lignin, such as KL-2, exhibited good antioxidant properties (84.27 %, 5 mg/mL), high brightness (ISO% = 7.6), high L value (L = 86.2) and low polydispersity index (PDI = 1.19). The LMP showed a narrow size distribution (1.0-1.4 μm). When the LMP content is 5 %, the 5 % LMP/PVA film maintains high transparency and exhibits excellent UV protection, achieving a UV shielding rate of up to 94.6 %. Therefore, this study offered an effective way for the comprehensive utilization of pulping waste KL.