Turning around waste plastics
An upcycling process is said to turn mixed waste plastics into perfectly spherical carbon microspheres (CMSs) for use in printer ink, anode materials in rechargeable batteries and heat-dispersing agents in car tyres.
While efforts to recycle polyethylene bags, polystyrene cups and other packaging materials are a high priority, it is usually imperative to sort the different types of plastic prior to recycling.
Researcher Vilas Pol, working within the Electrochemical Energy Storage Department at Argonne National Laboratory, has developed a technique to upcycle plastics without separation. He says, ‘The CMSs produced by [the] upcycling process have a unique spherical morphology that has not been achieved by previous methods’.
The solvent-free autogenic proc ess converts various waste plastics such as low and high density polyethylene and polystyrene (and mixtures thereof) into CMSs. The key piece of equipment is a specially designed closed reactor, which withstands autogenic pressures in the region of 500 to 600psi and temperatures in excess of 700°C.
To demonstrate the technique, Pol fed one gramme of WP into a 5cc reactor filled with air at room temperature. This was then closed tightly and heated uniformly up to 700°C in a furnace. After an optimised time, the reactor was cooled to room temperature and opened to reveal a dry black powder containing only CMSs. According to Pol, a yield of 40% is typical and no further process is required to separate the CMSs.
‘We had to optimise the furnace to allow the reactor to be heated uniformly’. He adds, ‘Initially there are certain nuclei and extra carbon sheets inside the reactor and, together with the self-generated pressure this helps to achieve the spherical morphology of the microspheres. Polymers start to degrade at about 500°C in a closed container. But to achieve graphitic character, and make the end product useful, the optimum temperature was found to be 700°C.’
Various techniques have subsequently been used to scrutinise the composition, structure and conducting properties of the CMSs, such as transmission and scanning electron microscopy and energy-dispersive X-ray analysis. Pol explains, ‘The measured electrical conductivity of the single carbon sphere is around 45S/m...and the purity of the carbon [was] high’. The reliability of the process has also been verified by putting commercial feedstock through the identical upcycling procedure.
Colin Williamson, Director at recycling company Smile Plastics, UK, has a few reservations on the environmental credentials. He says, ‘The research is interesting. Of course, this may happen with clean plastics but waste plastics are dirty and contaminated and [so] I doubt this raw material would produce ‘pure’ carbon. Secondly, the process involves temperatures of 700°C which carries an environmental footprint. If waste plastics [need] to be cleaned, then [it is] much better to recycle it [into] a plastics material with a smaller carbon footprint.’
Pol is now optimising the reaction conditions and pursuing applications. ‘The rubber in car tyres contains around 20% carbon. ‘Friction with the road creates heat and to dissipate that heat we use carbon. We are in contact with companies to find out if this is a potential application for our CMSs. They could also be used as the anode material in lithium-ion batteries, thanks to their graphitic character.’