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The healing partnership of silicone and sulphur - Annual Report 2024
14 June, 2025
The versatility of the group of materials known as silicones is something to marvel at. Silicones are used for everything from kitchen implements, to industrial adhesives, to life-saving medical equipment. Based on chains of alternating silicon and oxygen, silicones can range in consistency from thick oils to flexible rubber, and they are resistant to changes in temperature and chemistry.
But the exact qualities that make silicones so useful, also make them very hard to recycle. The global silicone market was valued at 18.5 billion US dollars in 2022 and is growing - silicones are used extensively in the construction industry. Currently, most used or expired silicone goes to landfill, rendering a potentially useful material into a waste product.
The MacDiarmid Institute's Reconfigurable Systems research programme is working to reduce waste by developing new materials that are recyclable or reconfigurable. This includes designing materials that can repair themselves at a molecular level.
Principal Investigator Associate Professor Erin Leitao of Waipapa Taumata Rau University of Auckland first became intrigued by chemistry through cooking as a young child. The inventive activities and excellent teachers she encountered in high school physics and first year chemistry captured her interest. Combined with a love of logic and problem solving, chemistry has since taken her from a PhD in Canada to an academic position in Aotearoa New Zealand.
Erin Leitao and Mahsaalsadat Rokni are experimenting with adding sulphur to silicone materials to give them re-healing properties.
Erin's research focuses on making new materials from earth-abundant elements such as silicon, nitrogen, phosphorus, sulphur and oxygen to reduce reliance on carbon, eliminate the creation of toxic byproducts and enable recycling. She first started working on silicones and sulphur with MacDiarmid Institute-affiliated PhD student Kun Woo Park in 2019 and the research continues as part of her student Mahsaalsadat (Mahsa) Rokni's PhD. Erin explains, "We basically just wanted to find an easy way to repair existing silicone."
Currently, most used or expired silicone goes to landfill.
The challenge in changing any material to make it more reuseable is finding a way of doing so that doesn't change the properties that make it so useful in the first place. "The reason we started working with sulphur is because it has similar properties to silicone plus an ability to 're-heal', that is to be able to repair itself. We thought about bringing them together to enhance those properties rather than adding anything that would detract from the original silicone material." And, in line with the environmentally friendly goals of the overall research, sulphur is an obvious choice because it's widely available as a waste byproduct.
Mahsa carried out the process to introduce elemental sulphur into the long chain polymers of a commercial silicone sample.
Typically, when silicone gets damaged, it is sent to the landfill (left panel). MacDiarmid Institute researchers are experimenting with adding sulphur bonds into the predominantly silicon-oxygen structure to enable the material to self-heal (right panel). Reproduced from Rokni et al., 2024.
In their 2024 paper, the research team describe cutting discs of unmodified silicone as well as their sulphur-infused silicone with a knife, then heating them to 120 degrees Celsius and applying pressure with a heat press for a day. The unmodified silicone discs remained damaged. The sulphur-infused samples healed - not just visibly but also at a molecular level. To Erin and Mahsa's delight, the synthesis had worked - they had created a repairable silicone.
Already, there are others excited about these findings. Within the MacDiarmid Institute itself, the researchers are testing the silicone used by Principal Investigator Professor Volker Nock of Te Whare Wānanga o Waitaha University of Canterbury. He uses silicone replicas of body parts to trial medical applications, and he would be thrilled if he could remould the silicone instead of throwing it away when it's worn out.
Erin explains that existing research on reusing silicone is based on analogues, not on commercially available materials. "No one has taken the commercial product itself and played with it the way we have." She hopes to run trials with New Zealand companies who use silicone.
Silicone discs synthesized by the researchers (top row) with corresponding scanning electron microscope images (bottom row). Panels show damaged discs on the left and "healed" discs on the right for a) silicone without sulphur (no healing observed); b) silicone with some sulphur; c) silicone with high amounts of sulphur. Reproduced from Rokni et al., 2024.
The researchers found that their re-healing technique worked for a few different types of silicones but it's not a 'one size fits all'. There are many formulations, with slightly different properties, so there's further work underway to enable healing properties for a variety of formulations. "In our initial proof of concept, we tried marrying the two components and we were pleasantly surprised that it worked with the first formulation we attempted. Now we need to expand the portfolio."
I want full circularity of silicone. That’s the ultimate goal for this research.
Associate Professor Erin Leitao MacDiarmid Institute Principal Investigator
The team are awaiting materials to scale-up the processing of repairable silicone and carry out extensive testing. Further testing will help to determine the materials' suitability for different applications. For example, there is confidence that this material will have properties that are well suited for some areas in healthcare. Ultimately though, Erin is focused on the waste reduction possibilities: "I want full circularity of silicone. That's the ultimate goal for this research."
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