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 a 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 anti-aging 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 crosslink density by 28% (v, 4.107 × 10−4 mol/cm3), consequently reducing ozone vulnerable areas of the matrix, and further improving 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 the future eco-friendly rubber compounds.