AUTONEWS
Turning fly ash waste into greener tire rubber
Tires are essential to modern transport, but their production and use raise environmental concerns. One important issue is zinc oxide, a common activator used in rubber vulcanization. Zinc oxide helps rubber form the crosslinked network that gives tires their strength, elasticity and durability. However, zinc can be released into the environment during manufacturing, recycling and tire wear, where it may affect aquatic ecosystems.
An international collaborative team from Kasetsart University in Thailand and Newcastle University in Singapore investigated whether fly ash, an industrial waste from coal-fired power plants, could partially replace zinc oxide in tire tread rubber. The research is published in the journal Waste Management.
Fly ash contains several metal oxides, including calcium oxide, magnesium oxide, aluminum oxide and iron oxide. These compounds can help activate the vulcanization process in rubber, suggesting that fly ash could do more than act as an inert filler. It could also help reduce reliance on zinc oxide.
Testing fly ash in tread rubber...The researchers prepared tire tread rubber compounds with different zinc oxide-to-fly ash ratios: 3:0 as the control, followed by 2:1, 1:2, 0:3 and 0:5 parts per hundred rubber. They then tested curing behavior, crosslink density, mechanical properties, dynamic mechanical properties, abrasion resistance, zinc release in water and environmental impact using gate-to-gate life cycle assessment.
The results showed that fly ash could support rubber vulcanization even when zinc oxide was reduced. The 2:1 and 1:2 zinc oxide-to-fly ash formulations showed crosslinking behavior comparable to the conventional zinc oxide control. This means zinc oxide could be reduced by around one-third or two-thirds while maintaining effective vulcanization.
Mechanical performance was also largely preserved. The 2:1 and 1:2 formulations showed tensile strength, modulus and crosslink density comparable to the control rubber. Their dynamic mechanical behavior was also similar, suggesting that important tire-related indicators, including road grip and rolling resistance, were not significantly compromised.
Lower zinc release, mixed trade-offs...The environmental benefit was clear in zinc release tests. After three months in water, the 1:2 zinc oxide-to-fly ash formulation reduced zinc release by 63% compared with the control. This is important because zinc leakage from tire materials can contribute to aquatic ecotoxicity.
Life cycle assessment also showed that replacing zinc oxide with fly ash reduced freshwater aquatic ecotoxicity, marine aquatic ecotoxicity and global warming potential. The lowest environmental impacts were observed when zinc oxide was fully replaced at the 0:3 ratio. However, the study also showed that higher fly ash loading can affect material performance, so partial replacement appears to offer the best balance between rubber properties and environmental benefit.
A circular use for fly ash...The work points to a practical circular economy route for tire materials. Instead of treating fly ash only as a waste stream, it can be repurposed as a functional ingredient in rubber compounds. At the same time, tire manufacturers could reduce zinc oxide use, lower zinc release and improve the environmental profile of tire tread formulations.
The findings are relevant to tire manufacturers, rubber compounders, waste management companies and sustainability teams seeking lower-impact materials for transport applications. They also highlight opportunities for countries with both rubber industries and fly ash waste streams to develop more circular industrial supply chains.
Provided by Newcastle University in Singapore
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