Plastics have become a deeply worrying environmental issue, but it is unlikely that demand will drop… so are there alternative production methods which could be more environmentally sound? We chat to materials chemist Dr Tom Hasell, whose new catalytic process for making polymers out of sulfur could provide more environmentally friendly production of plastic...
Why have you focused your research on sulfur, which is essentially a waste product?
I was intrigued seeing news reports of huge mountains of unwanted sulfur produced as a by-product of petrochemicals. This would make any materials chemist ask themselves why we are not using such a resource to make things from.
However, polymers made just from neat sulfur are not stable, and decompose back to a powder even at room temperature. In 2013 I saw a research paper from a group in America (led by Jeffrey Pyun), where they reported what they called “inverse vulcanisation”. This technique involves polymerising the sulfur in the presence of an organic small molecule “crosslinker”.
When I secured funding to start my own research group, I knew I was interested in investigating these fascinating polymers made from sulfur – how to make them more easily, how to improve their properties, and what exciting applications could we find for them.
How did this discovery of a new catalytic process come about?
When we started researching sulfur polymers in my group, we tested a lot of alternative crosslinkers. We were looking for alternatives that might improve the physical properties of the polymers, or allow us to make them more cheaply or more sustainably. We managed to find crosslinkers that were also industrial by-products themselves, and some that were renewable being plant based in origin rather than from petrochemicals. However, we found that it was very difficult to get the crosslinkers to react with the sulfur.
We tried a range of catalysts known to work for conventional polymerisations and rubber vulcanisations and tried them on our systems. What we found was that some of the compounds (metal diethyl dithiocarbamates) could increase the rates of reaction, lower the temperatures, and allow us to react crosslinkers that would not work without the catalysts. The lower temperatures required let us avoid the production of hydrogen disulphide gas, and get higher reaction yields.
So, plastics made from sulfur… just a green initiative?
Not only that. It is great to be able to produce polymers from sulfur, simply because it is a waste product of the petrochemicals industry. Most polymers are made directly from the petrochemicals themselves – a limited resource. For every Kg of sulfur we use to make polymers, we save more petrochemicals.
But also, think how many different and wonderful applications there are from the polymers we already use. At the moment almost all polymers are based predominantly on carbon as the main element. Making polymers from sulfur opens up a new frontier of exciting possibilities. Sulfur has very different properties to carbon, and this may lead to many interesting applications we are yet to discover – this field is still in its infancy.
What types of commercial products would sulphur polymers be suited to?
That will depend on the physical properties of the sulfur polymers and how cheaply they can be made. As I say above, these polymers have great potential for use in optics and electronics. For these high end applications, the cost of production will not be such a limiting factor. However, the sulfur content of the polymers should also give them great potential for thermal and electrical insulation materials in fields like construction.
Does plastic still have a future as a mass-produced material or should we stay focused on alternatives?
Plastic absolutely has a future, it is far too useful not too. However, it has become somewhat of a victim of its own success. Not only is it very useful, so we want to use it for almost everything, it has also become so cheap that we use it without thinking about it and throw it away. This has to stop. We should continue to use plastics for all of the wonderful things they can do for humanity, in medicine, wellbeing, communication, and recreation. But we need to be more aware of what we are using and consider alternatives.
How can we overcome the plastic crisis?
Yes, we have to. But we will need to work together. Solving the plastic crisis needs research from scientists, regulation and input from governments, forward thinking and innovation from industry, and demand from the general public – with a willingness for all of us to change our habits. This is already happening, but there is a long way to go. The priority has to be reduce, reuse, recycle. In that order.
Dr Tom Hasell is a Royal Society University Research Fellow in the Department of Chemistry at the University of Liverpool. He has worked in a wide range of areas across materials science, including chemical synthesis, supercritical processing, polymer science, nanocomposites, and porous materials.