Sustainable Alternatives to 6-PPD? Assessing the Viability of Lignin and Propyl Gallate as Green Substitutes for 6-PPD Used as Anti-Ozonant and Antioxidant in the Rubber Industry

The tire antioxidant, N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6-PPD), poses a severe environmental threat due to its conversion to 6-PPD-quinone, a compound highly toxic to aquatic life. Propyl gallate and lignin are bio-derived compounds advocated as alternatives for 6-PPD. This study presents a comprehensive side-by-side evaluation of propyl gallate and lignin against 6-PPD, assessing their material performance and leaching potential. Rheological and mechanical testing reveal significant trade-offs: while propyl gallate offered superior processing safety by increasing scorch time by over 40%, it drastically altered the vulcanizate's properties, reducing tensile strength by 20% and modulus by 46% while increasing elongation by 35%. Lignin's performance is closer to 6-PPD, maintaining mechanical properties but increasing compound viscosity. Crucially, leaching studies quantified via LC–MS reveal that propyl gallate migrated substantially from the rubber, with 4.1 ppm detected in water after 216 h. Under the same conditions, leaching of the 6-PPD is negligible, though its environmental transformation pathway differs. Lignin leaching is confirmed, but its complex polymeric nature prevented quantification, a key analytical challenge. The results demonstrate that replacing 6-PPD is fraught with compromise; no drop-in alternative currently satisfies the dual mandate of technical performance and environmental safety, underpinning the need for further innovation and ecotoxicological assessment of new compounds.