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

The influence of lignin’s\nprimary oxygen-containing functional\ngroups, phenolic OH (Ph–OH) and methoxyl (OMe) groups, especially\nthe Ph–OH/OMe ratio, on its UV absorptivity and long-lasting\nUV blocking remains unclear. In this study, organosolv lignins (OLs)\nwith varying Ph–OH and OMe contents were prepared to evaluate\ntheir UV absorptivity and photostability by EPR (electron paramagnetic\nresonance). As the Ph–OH contents increased from 0.32 to 6.09\nmmol/g, lignin with a higher Ph–OH content displayed better\nabsorptivity with a strong correlation (Pearson’s <i>r</i> = 0.95). However, an excessive Ph–OH content (>5.5 mmol/g)\nreduced the spin content of lignin, negatively affecting its photostability.\nTaking Ph–OH and OMe groups as the cofactor, OLs with Ph–OH/OMe\nratios between 0.44 and 0.64 showed more favorable UV-blocking performance\nboth in the initial state and after UV irradiation. Moreover, lignin-based\nsunscreens exhibited increasing SPF value even after 8 h of UV exposure,\nconfirming lignin’s long-lasting UV-blocking property.