UV Resistance Properties of Lignin Influenced by Its Oxygen-Containing Groups Linked to Aromatic Rings

The influence of lignin's primary oxygen-containing functional groups, phenolic OH (Ph–OH) and methoxyl (OMe) groups, especially the Ph–OH/OMe ratio, on its UV absorptivity and long-lasting UV blocking remains unclear. In this study, organosolv lignins (OLs) with varying Ph–OH and OMe contents were prepared to evaluate their UV absorptivity and photostability by EPR (electron paramagnetic resonance). As the Ph–OH contents increased from 0.32 to 6.09 mmol/g, lignin with a higher Ph–OH content displayed better absorptivity with a strong correlation (Pearson's r = 0.95). However, an excessive Ph–OH content (>5.5 mmol/g) reduced the spin content of lignin, negatively affecting its photostability. Taking Ph–OH and OMe groups as the cofactor, OLs with Ph–OH/OMe ratios between 0.44 and 0.64 showed more favorable UV-blocking performance both in the initial state and after UV irradiation. Moreover, lignin-based sunscreens exhibited increasing SPF value even after 8 h of UV exposure, confirming lignin's long-lasting UV-blocking property.