Sulphur finds a way in plastic production
Sulphur feedstock is cheap, available and going to waste. Dr Joseph C Bear of Kingston University, UK, discusses the potential to use sulphur in polymer production.
Sulphur has great potential as a feedstock as it is increasingly available as a low-cost by-product from petroleum refining. As sources of crude oil become evermore difficult to extract, sulphur-rich oil sands such as those in Athabasca, Canada, are being increasingly exploited. As a consequence, large amounts of elemental sulphur are often piled in heaps outside oil refineries. At US$173 per tonne in 2018, according to Bloomberg, sulphur is an inexpensive and highly useful waste product that is not being fully exploited.
Currently, the biggest use of elemental sulphur is the manufacture of sulphuric acid, usually via contact or wet sulphuric acid processes. Nevertheless, a huge surplus of elemental sulphur is produced annually. In 2013, Professor Jeffrey Pyun et al. of the University of Arizona, USA, proposed the idea of ‘inverse vulcanisation’ sulphur polymers in the paper titled, The use of elemental sulfur as an alternative feedstock for polymeric materials, published in Nature Chemistry. This method involved creating polymeric materials consisting of long sulphur chains cross-linked with small organic spacers as a route to offset pressure on traditional polymeric feedstocks, such as crude oil, and to utilise this excess elemental sulphur.
The term inverse vulcanisation stems from its similarity to the process of vulcanisation – the revolutionary process where rubber was heated with sulphur in order to lose the tackiness of the natural material. Later developments saw the introduction of new and better vulcanising agents, such as disulfiram in 1900 and accelerators such as thiocarbanilide, which dramatically reduced curing times and energy consumption in industrial processes. Further advances over the 20th century included ultra-accelerators, used in the manufacture of most modern rubber goods.
This is analogous to the process of inverse vulcanisation insofar as sulphur is used as the main polymer chain as opposed to poly(isoprene) in rubber, and organic molecules such as 1,3-diisopropenyl benzene (DIB) are used as cross-linkers instead of sulphur.
A catalyst for nano-scale
At Kingston University, UK, undergraduate student Annie Rae undertook a project, supervised by myself, as a continuation of her final year work into the synthesis of sulphur polymer-supported catalysts, and the potential for using these and other sulphur polymer-based nanocomposites, for anti-biofouling coatings. Rae was supported in her research with an IOM3 SEPRG grant for students finishing their degree course, which aims to encourage young material scientists to develop their skills and interests.
One area in which we felt that inverse vulcanisation sulphur polymers would excel was as a support material for molecular or nanoscale catalysts. Depending on the cross-linker used, the sulphur polymer can be made almost impervious to any solvent combination, and due to advances in making them porous, they make ideal catalyst supports. Rae had worked in my group on sulphur polymer/gold nanoparticle catalysts, and the SEPRG grant was ideal for a one-month expansion of the themes explored in her project further, in the form of anti-biofouling coatings. As an extra, I also felt it was an excellent opportunity to train Rae in some of the air-sensitive chemistry techniques she would be using in her forthcoming MRes project at the University of Edinburgh, UK.
During the project, porous sulphur polymer supports were synthesised by pouring the polymer (c.a. 200°C) into a mould containing finely ground salt, thus templating the polymer. On cooling, the sulphur polymer was heated in distilled water to dissolve the salt template, leaving the macroporous sulphur polymer support. Gold metal was then deposited from solution onto the porous sulphur polymer support giving the gold nanoparticle-sulphur polymer supported catalyst.
4-nitrophenol is a model organic pollutant that has long been used to test catalyst efficacy as the mechanism and kinetics of its breakdown are well understood. In the presence of sodium borohydride, we found, unsurprisingly, that the reaction rate increases with the gold to polymer ratio. However, if you add too much gold to the support, surface area decreases leading to a lowering of the reaction rate. We also observed no leaching of gold into solution, potentially due to the high affinity sulphur has for gold, making the sulphur polymer the ideal support material for noble metal catalysts.
Further to this, we sought to synthesise nanocomposite sulphur polymer thin films to exploit the inherent anti-microbial properties of sulphur. By incorporating metal nanoparticles or nanoparticle precursors, we were able to synthesise robust thin films for anti-biofouling, testing of which is on-going. With the intrinsic anti-microbial ability of sulphur combined with metals such as copper, silver or gold with the slippery nature of superhydrophobic surfaces, one can foresee inverse vulcanisation sulphur polymers as the base for the next-generation of anti-biofouling coatings.
One such film, pictured on page 48, shows ‘furry’ copper sulphide nanostructures grown from the decomposition of copper(II) diisopropyldithiocarbamate embedded in a sulphur polymer thin film. The surface roughness imparted by these nanostructures may lead to superhydrophobicity and contact killing of bacteria from the superhydrophobicity, copper sulphide or sulphur polymer.
Since their invention, the applications and improvements in the physical properties of inverse vulcanisation sulphur polymers have increased dramatically. Diversification of the number of cross-linker molecules has been the main driving force for improvements in physical properties, with natural and waste products such as vegetable and cooking oils shown as effective cross-linking agents. Mixtures of linking agents such as 1,3-diisopropenylbenzene mixed with bisphenol A dimethacrylate increases melting temperature and hardness of polymer films. Other vinylic species such as farnesol, divinylbenzene, geraniol, limonene, dicyclopentadiene have found success in this role. Selenium has also been used as an additive, however, as yet there have been no instances of pure inverse vulcanisation selenium or tellurium polymers.
Sulphur polymer applications
Uses for these materials have thus far been somewhat limited due to the fact that the field is in its infancy. However, current examples include cathode materials for lithium-sulphur batteries, infrared lenses, nanocomposite films, water filtration devices, optical filters, fertiliser sorbents, and gas separation materials. Many of these uses rely on the strong affinity of the sulphur polymer to metal ions, particularly heavy metal ions such as mercury (Hg2+) and cadmium (Cd2+) for water filtration or ammonium sulphate, calcium hydrogen phosphate and potassium chloride for fertilisers. However, research in this area is proceeding at a pace.
The synthesis of inverse vulcanisation sulphur polymers is very simple, at least in the laboratory setting. Elemental sulphur melts at around 120°C to form a clear yellow liquid. Raising the temperature above 149°C causes the sulphur S8 rings to open and loosely polymerise into long sulphur chains, turning the liquid red and increasing viscosity. At 185°C, the organic additive is stirred in rapidly for about 10 minutes, all the while maintaining 185°C. After this the mixed polymer is cured further, by placing it in an oven at 200°C. If an organic linker is not added and the sulphur is cooled back to room temperature, S8 rings will form again. The addition of the organic linker molecules prevents this ring re-combination, and cross-links the polymer giving a deep-red to almost black product.
Depending of the cross-linker used, curing time and temperature, the polymer can be tacky to the touch or very hard and glass-like. One notable consideration is that for the most part, inverse vulcanisation sulphur polymers behave as thermoplastics and not thermoset plastics, insofar as they may be melted and re-cast, so are said to be hyper-branched rather than cross-linked. This is very useful when casting the material as it can be easily melted and re-formed. This increases the number of methods one can use to process the polymer into a useful form, so well established and industrially used techniques such as injection moulding or 3D printing come to the fore.
Taking it further
So what does the future hold for such materials? If the 20th century has taught us anything in the field of materials chemistry, it is that we can no longer develop new products and not think about the environmental consequences. Without a detailed and rigorous end-of-life disposal/recycling strategy, we should not rush to commercialise materials and products, plastic pollution in the oceans, ozone-destroying CFC gases and high levels of mercury entering the food chain have shown us that much. However, with recycling strategies such as turning end-of-life sulphur polymers into gas separation materials, sorbents for fertilisers and remediation processes, inverse vulcanisation sulphur polymers may yet have a role to play in offsetting the strain on other polymer feedstocks, such as crude oil.