In 2016 the University of Liverpool posted an article titled “Plastics of the future may be made from sulfur, not oil, putting waste to good use”. Now, in 2020, they have announced their exciting new devlopments for polymers made from waste sulfur. Plastics are ubiquitous to human life so it is important that we find more sustainable ways to manufacture them.
Sulfur is a waste product from refining cure oil and gas in the petrochemicals industry, which generate huge stockpiles of it outside their refineries. It is an abundant chemical element that can be found as a mineral deposit across the world.
Oil companies spend billions of dollars removing sulfurous compounds from petroleum every year to meet environmental regulations. (Sulfur in the air forms sulfuric and nitric acid, which falls as damaging acid rain.)
To be able to produce useful plastic materials from sulfur, could reduce society’s reliance on polymers made from petroleum. In addition, these sulfur polymers may be easier to recycle, which opens up exciting possibilities for reducing current use of plastics
Quoting from articles by the University, let’s start at the beginning:
“Most plastic items are made of chemicals such as polyethylene (PET), polypropylene (PP), polyurethane, or polyvinylchloride (PVC) which are all derived from oil. These monomers are obtained industrially from the fractional distillation of crude oil, and polymerised in great quantities with catalysts in a process developed in the 1950s and 60s.
The industrial feedstocks and methods of manufacturing plastics have not changed significantly for more than 60 years. But the situation has: oil is harder to come by and (usually) more expensive, and environmental pressures are growing.
Under the right conditions, initially discovered by Jeffrey Pyun, sulfur can change from its usual ring-like chemical structure and instead form into long chains. These chains of sulfur can be joined together to create a solid plastic or rubber using other organic molecules to link them together. This process is dubbed inverse vulcanisation, as it is the opposite of the vulcanisation process applied to carbon to make rubber.
Patented by Charles Goodyear in 1844, the vulcanisation process joins long chains of carbon molecules using sulfur, transforming liquid oil into solid rubber. So while rubber is mostly carbon with a small amount of sulfur, conversely sulfur-based plastics are mostly sulfur with a small amount of carbon.”
Inverse vulcanisation is required as sulfer, on it’s own, it will not form a stable polymer therefore organic crosslinker molecules are used to make it stable. Originally this process required high temperatures, long reaction times, and produced harmful by-products. However in 2019 the researches published that:
“…the discovery of a new catalytic process for inverse vulcanization that reduces the required reaction times and temperatures, whilst preventing the production of harmful by-products. It also increases the reaction yields, improves the physical properties of the polymers, and allows a wider range of crosslinkers to be used.
Our research has examined how to change the way sulfur polymers act so as to make them suitable for different uses. For example, we can change the properties and proportion of carbon depending on what is required – hard plastics contain more carbon molecules, soft plastics fewer. We can also add nanoparticles to the mix that have the properties we want the plastic to have, as they donate their characteristics to the end plastic result.
In terms of customisable physical properties, the polymers can be moulded into a variety of shapes, and astonishing detail is possible. By varying the organic content, the polymers can produce hard, glass-like plastics, or tacky, malleable substances which have potential as adhesives. This physical tuneability has led to their use even as a cathode material for a new generation of lithium-sulphur batteries. In addition to this, is the scope for unique new polymers with unprecedented properties. The properties of sulfur are very different to carbon, and this has already opened up a world of possible applications for sulfur polymers including thermal imaging lenses, batteries, water purification and human health.”
That leads us to even more exciting devleopments in 2020:
“In a paper published in Angewandte Chemie, Dr. Hasell and colleagues make an exciting discovery that addresses the weakness of sulfur polymers, a factor that has limited its application.
Led by Ph.D. 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.
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.
In a second paper, published in Chemical Science, 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.”
In conclusion, the use of sulfur to make plastics might solve two world problems: decrease the demand for oil and put to use the huge amounts of waste sulfur that exists.
“Exciting new developments for polymers made from waste sulfur” by University of Liverpool: https://phys.org/news/2020-06-polymers-sulfur.html
“Scientists discover a better way to make plastics out of sulfur” by University of Liverpool : https://phys.org/news/2019-02-scientists-plastics-sulfur.html
“Plastics of the future may be made from sulfur, not oil, putting waste to good use” by Charles Dunnill, University Of Swansea : https://phys.org/news/2016-01-plastics-future-sulfur-oil-good.html