Transforming sulfur into an alternative to plastics

Could a super material to help the world overcome its over-reliance on plastics be hiding in plain sight?

Sulfur, the yellow stuff with a whiff of the bad egg smell, is mentioned 13 times in the Bible and at present lacks the glamorous image of newer ‘meta’ materials. But bolt on ultra-modern materials chemistry and sulfur has been rebooted into new compounds with incredible new properties that could be used to clean our air and water, and provide components for everything from batteries to optics, all while being far more recyclable than plastics.

Dr Tom Hasell at the University of Liverpool is at the forefront of industrially useful sulfur chemistry. He’s interested in porous materials with high surface areas that could have large-scale applications in gas storage, catalysis and filtration. But many trendy new materials are limited by the high cost of production. “We are developing new porous materials from inorganic waste and other low cost or renewable resources,” says Hasell. “A good example is sulfur-polymers, and we recently showed that polymers made from elemental sulfur can be used to filter mercury from water.”

Improving on existing processes

More than 70 million tonnes of sulfur is made each year as an industrial by-product of oil refining. That means its available for just $100 per ton and it’s a win-win to use a waste material from an industrial process that can foul soils by changing its pH. However, polymers made just from neat sulfur are not stable, and decompose back to a powder, even at room temperature.

The game-changer was rediscovering and improving on a process called ‘inverse vulcanisation’ by which sulfur polymers are made – a combination of sulfur and other organic ‘crosslinker’ compounds that tether sulfur polymers together, making them more stable and preventing them from decomposing. Hasell and colleagues found that adding a small amount of a catalyst (metal diethyl dithiocarbamates) would increase reaction rates as well as chemical yield, and lower the temperature needed, thus avoiding the production of highly undesirable hydrogen disulphide gas.

“This makes inverse vulcanization more widely applicable, efficient, eco-friendly and productive than previous techniques,” says Hasell. He adds that this not only broadens knowledge of the fundamental chemistry itself, but opens the door to wider industrialisation because sulfur is a very different element to the carbon found in all plastics, so sulfur polymers have some really interesting properties.

For example, unlike in carbon polymers, infra-red light goes through sulfur polymer lenses providing optical applications like thermal imaging lenses. Then there are novel antimicrobials, more stable lithium batteries, and porous sulfur compounds that can be used to capture harmful (and valuable) heavy metals like gold and mercury, benefitting human health and the environment. Some combinations contain up to a whopping 90% sulfur by weight.

International collaboration

Hasell has utilised collaborations across the world to work towards realising these applications. Closer to home, the Materials Innovation Factory at the University of Liverpool is a world-leading centre were Hasell can characterise the compounds he makes in the chemistry department. “We have great access to the expertise of the people there and the most advanced shared equipment,” he says. “We use techniques like gel permeation chromatography, infrared spectroscopy and powder diffraction to analyse our polymers."

"We’ve also collaborated with Dr John Griffin, University of Lancaster, who used solid-state nuclear magnetic resonance (NMR) to help us understand the structures of the polymers that formed.”

Hasell adds that he thinks Liverpool is at the forefront in the UK in terms of materials chemistry and discovery, a forward-looking place with drive and momentum that includes a collegiate feel and support among young academics.

New developments

June 2020

Dr Tom Hasell and his team, have published two papers which demonstrate practical and exciting developments for sulfur polymer technologies and application.

This new research builds on their game changing discovery (detailed above) in 2019 when they reported a new catalytic process to make polymers out of sulfur.

The first paper, 'Inverse Vulcanized Polymers with Shape Memory, Enhanced Mechanical Properties, and Vitrimer Behavior' (doi.org/10.1002/anie.202004311) is published in Angewandte Chemie.

Led by PhD student Peiyao Yan, the paper demonstrates that adding a second type of bonding, urethane bonds, to the materials increases the strength of sulfur polymers by up to 135 times. The way this second type of bonding is introduced means that its amount can be controlled, and in turn controls the physical properties of the polymers.

The strengthened sulfur polymers were found to have shape-memory effects – they can be set in one shape, before being temporarily deformed into another. When heated a little, they ‘remember’ the previous shape and go back to it. This setting process and temporary deformation can be repeated multiple times.

This is a first for sulfur polymers, and despite these unusual properties, the sulfur bonds of the polymers mean they are still easy to recycle and opens up potential applications in areas such as soft robotics, medicine, and self-repairing objects. This paper is also featured as an article in Chemistry Views.

The second paper, 'Chemically induced repair, adhesion, and recycling of polymers made by inverse vulcanization' (doi.org/10.1039/D0SC00855A) is published in Chemical Science.

For the second paper, Dr Hasell’s group teamed up with researchers at Flinders University in Australia to show that sulfur polymers could form rubber like materials that could be easily self-repaired to their original strength within minutes, just by applying an amine catalyst that helps the bonds in the broken surfaces heal back together.

This new kind of rubber and catalyst can be used with low energy consumption to make flexible, repairable, sustainable objects - providing a very real and useful application for these new sulfur polymers.