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End-of-life rubber tyres from motor vehicles are a huge waste problem in South Africa. The recycling challenge is to devulcanise (soften) the rubber in these tyres in an eco-friendly manner so that it can be recovered and re-used in new tyres and other products.

This is according to Dr Jabulani Mnyango who graduated with his PhD in Chemistry during Nelson Mandela University’s April graduation ceremony. Rubber has to be vulcanised (hardened) to make it usable in tyres. To retrieve and re-use the rubber it needs to be devulcanised. “The problem is that the current methods used for devulcanisation include chemical agents that are toxic to the environment and expensive,” he said.
“What I did for my PhD research, starting in 2019, was to focus on a South African indigenous plant, Tulbaghia violacea, as a readily available and non-toxic agent to devulcanise waste tyre rubber in an eco-friendly manner.” Findings from his research showed that the Tulbaghia crude extracts can significantly devulcanise rubber to produce high-quality rubber that can be re-used.
From over 22 000 indigenous plants in South Africa, how did he hone in on Tulbaghia violacea? “My co-supervisor Dr Buyiswa Hlangothi, who is the Acting Director of the School of Biomolecular and Chemical Sciences at Nelson Mandela University, was already working with a number of plants from the Eastern Cape region. Her research interest is medicinal plant chemistry and she put me onto Tulbaghia violacea, specifically because of its sulphur compounds. Sulphur compounds are required for devulcanisation.”
Dr Mnyango was familiar with the plant: “I’m originally from a town called Mtubatuba in KwaZulu-Natal where my grandmother grew this plant (called isihaga in isiZulu) around our homestead to chase away snakes. It was a common practise in the deep rural areas. The plant exudes a strong garlic smell and its leaves are used for cooking. Its English name is wild garlic.”
So began his PhD, co-supervised by Dr Hlangothi, Professor Christopher Woolard (a chief researcher in renewable polymers and fuel and rubber chemistry at Stellenbosch University) and Professor Percy Hlangothi (Director of the Centre for Rubber Science and Technology at Nelson Mandela University).
For the first year of his PhD he focused on understanding the chemical makeup of the plant to determine how stable its sulphur compounds are when subjected to heat in the thermo-chemical devulcanisation process. He also needed to work out how to treat these sulphur compounds so that they don’t degrade with heat before they could be used for devulcanisation.
“In my second year, I started smearing the extract onto the rubber, and then put it in a reactor with CO₂ in the form of dry ice. I then had to work out the devulcanisation optimal conditions from the super critical point of CO₂, looking at the combination of temperature, pressure, time and amount of extract. This required testing and re-testing many times over. It took me a full three years to get it right and to achieve the product - devulcanised rubber.”
Asked how he felt once he had worked it out - that ‘aha!’ moment - he says: “I was very pleased with what I had achieved with the plant as it is a green or eco-friendly process, which is wonderful. However, while I have shown that there is an alternative form of devulcanisation that can be utilised in place of the commonly used toxic chemicals, the process still requires energy in the form of electricity to devulcanise. The gain is that the plant process requires far lower temperatures to achieve this than the common chemical process but to completely green the process would require addressing the electricity issue.”
He says the plan is to possibly commercialise his process down the line but before this can be considered, it requires checking Tulbaghia violacea from other provinces to see if they work as well as the Tulbaghia they used from the Eastern Cape. 
Dr Mnyango now has his sights set on doing postdoctoral research overseas. “It’s been a very interesting journey so far, as I never envisaged myself as an academic scientist, but here I am, and I have my supervisors to thank for encouraging me to further my postgraduate studies.
“My ultimate goal is to be in both academia and industry, as I would like to own or co-own a company that deals with chemistry and polymer science for troubleshooting and fault-finding of industry samples when they are having problems with their polymer or chemical products.” – ENDS

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