Amine-Functionalized Lignin as an Eco-Friendly Antioxidant for Rubber Compounds

Although the typical antioxidant, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD), ensures high durability and long lifespan for rubber compounds, it generates a highly toxic quinone in water, causing serious environmental pollution. Herein, as an alternative material of 6PPD, we newly introduce eco-friendly amine-functionalized lignin (AL) to be incorporated in rubber, which can provide excellent combinatorial antiaging properties of thermal stability and ozone/fatigue resistances through radical scavenging effect. The heterolytic ring-opening reaction of AL and sulfur can accelerate curing and improve the cross-link density by 28% (v, 4.107 × 10^-4 mol/cm³), consequently reducing the ozone vulnerable areas of the matrix and further improving the aging resistance. Notably, AL allows its rubber compound to exhibit superior anti-ozone performance after ozone aging, with the arithmetic surface roughness (Sa) of 2.077 μm, which should be compared to that of 6PPD (4.737 μm). The developed chemically modified lignin and the methodology have enormous potential as a promising additive for future eco-friendly rubber compounds. The eco-friendly lignin-based antioxidant manufactured by amination reaction has the potential to reduce environmental pollution for the future rubber industry.