RUBBER vehicle tyres are generally strengthened with petroleum-based carbon black, but researchers in the US have found that waste tomato skin and eggshells could be a sustainable alternative.
Around 30% of the average tyre is made up of carbon black, and as demand for tyres grows, producers are looking overseas to ensure there are plentiful supplies of this filler. A team led by Katrina Cornish, endowed chair in biomaterials at Ohio State University, instead decided to investigate whether eggshells and tomato skins, two common food waste materials in the US, would work instead. The US food industry uses around 50bn eggs annually, with the shells generally being landfilled, while the thick, fibrous skins of more than 13m t/y of tomatoes are discarded during canning and other processing operations.
Cornish says that using alternative, sustainable types of filler like these will make rubber products more sustainable, reduce US dependence on imported oil, and reduce the amount of waste being sent to landfill.
To make the fillers, the team dry the eggshells and tomato peel at 55?C and then wet mill them using a ball mill, to give a particle size of 30–100 nm. These fillers were then incorporated into a rubber composite for testing.
The rubbers the team made contained up to 10% of the renewable fillers alongside conventional carbon black. The researchers used standard industry test methods to determine the strength of the rubber, including the elastic modulus (resistance to stretch), elongation to breaking point, and hardness. The rubbers which used the renewable fillers exceeded industrial standards for tyre performance.
The microfillers have a very high surface area compared to their volume, which allows them to interact with the rubber and bind to it. Cornish explained to The Chemical Engineer that the tomato peel is a very lightweight filler. Its chemical composition and structure make it bind particularly to the rubber, while the microstructure – porosity and roughness – of the eggshells makes this suitable as a filler.
Carbon black doesn’t only strengthen rubber, but also adds colour. Tyres strengthened with eggshells and tomato skins are a brown colour rather than black, so Cornish and her team are looking at alternative ways to colour tyres.
“Fillers generally make rubber stronger, but they also make it less flexible. We found that replacing different portions of carbon black with ground eggshells and tomato peels caused synergistic effects – for instance, enabling strong rubber to retain flexibility,” said researcher Cindy Berrera.
As a result, Cornish believes that the new, sustainable fillers could open up new rubber applications not previously possible. Her company, EnergyEne, has licensed the technology from the university and will now look to develop it further.
Rubber Chemistry and Technology doi.org/b2sk
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