Optimizing the lignin nanoparticles from different pulping by-products in developing cotton-based nanocrystalline cellulose for UV‑light blocking

The development of sustainable and non-toxic ultraviolet (UV) shielding materials is essential to address the limitations of conventional inorganic agents, which often suffer from biotoxicity and limited spectral coverage. In this study, lignin nanoparticles (LNPs) were extracted from rice straw-derived black liquors, a by-product of various pulping processes, using acid precipitation followed by solvent exchange with tetrahydrofuran (THF) and ethylene glycol (EG). These LNPs were incorporated into nanocrystalline cellulose (CNC) matrices to fabricate bio-based UV-shielding nanocomposites. The nanoparticles and their corresponding composites were characterized using transmission electron microscopy (TEM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), atomic force microscopy (AFM), and polarized optical microscopy (POM). The nanocomposites demonstrated enhanced UV-blocking efficiency with increasing LNP loading (1-5 wt%), achieving up to 99.9% shielding in the UVC (200-280 nm) and UVB (280-320 nm) regions. Notably, CNC-LNP composites prepared using LNPs from Kraft pulping [Via tetrahydrofuran (THF); LNPT7)]and KOH/NH₄OH pulping [via Ethylene glycol (EG); LNPE6] exhibited nearly complete UVA protection. LNPE6 was characterized by semi-spherical particles with an intermediate average size of 23.8 ± 7.9 nm, whereas LNPT7 exhibited fully spherical particles with a significantly larger average size of 524.6 ± 233.6 nm. These findings highlight the potential of valorizing industrial lignin waste for the development of environmentally friendly, high-performance UV-protective materials for applications in packaging, personal care, and optical devices.