Radical–quinone chain suppression in Catechyl lignin via isocyanate modification for permanently whitened and dispersible optical additives

Abstract

Lignin is an attractive bio-based feedstock for optical plastics, yet its use is limited by the intrinsic dark color arising from chromophores, radical-driven quinone chemistry, and aggregation-induced scattering. Here, structurally uniform catechyl lignin (C-lignin) is used to develop a mild, universal isocyanate modification strategy (50 °C, 5 h) that forms urethane linkages without breaking the benzodioxane backbone. The benzyl isocyanate modified C-lignin (BI-CL) achieves high lightness (L* = 92.87) with long-term color stability (>360 days). Quantitative phosphorus-31 nuclear magnetic resonance spectroscopy (31P NMR) confirms high hydroxyl consumption (up to 97.3%), while UV–vis and electron paramagnetic resonance (EPR) collectively show suppression of the visible absorption tail and phenoxy radicals. As a low-absorption, dispersible filler, BI-CL enables matrix-dependent optical engineering in low-density polyethylene with high transmittance and preserved thermal processability, providing an ideal route to sustainable optical films and packaging.