Sustainable use of fly ash waste in tire tread rubber: Characterization of physical properties and environmental impact assessment

Abstract

This study explores the use of fly ash (FA), a waste material, to partially replace zinc oxide (ZnO) as an activator in tire tread processing. Reducing ZnO addresses its environmental risks, particularly the impact of ZnO leakage into aquatic ecosystems throughout the tire’s life cycle. The FA was altered by including rubber compound with and without ZnO, using ZnO-to-FA ratios of 3:0 (control), 2:1, 1:2, 0:3, or 0:5 parts per hundred of rubber (phr). The results show that crosslinking of the rubber compound occurred with FA, even in the absence of ZnO. Notably, sample recipes with ZnO-to-FA ratios of 2:1 and 1:2 phr had similar Δ torque values to the control (3:0), allowing for ZnO reductions of 33.7% and 67.0%, respectively. This effectiveness is likely due to metal oxides in FA, such as CaO, MgO, Al₂O₃, and Fe₂O₃, which support the vulcanization process. Additionally, tensile strength and modulus remained unchanged. Elemental analysis further indicated that a ZnO-to-FA ratio of 1:2 reduced zinc release by 63.0% compared to the control recipe. A gate-to-gate life cycle assessment revealed that replacing ZnO with FA in vulcanized rubber formulations reduces environmental impacts, with the lowest effects observed at the 0:3 ZnO:FA ratio, though higher FA content may increase impacts. Using FA as a partial replacement for ZnO in tire tread processing shows promise for reducing environmental impact in tire manufacturing by lowering zinc release, decreasing ecotoxicity, and promoting waste reduction through the recycling of fly ash.