Giving plastic bags a second life (Image: Jeff J Mitchell/Getty)

Waste plastic from “throwaway” carrier bags can be readily converted into carbon nanotubes. The chemist who developed the technique has even used the nanotubes to make lithium-ion batteries.

This is called “upcycling” – converting a waste product into something more valuable. Finding ways to upcycle waste could encourage more recycling: for instance, bacteria can convert plastic drinks bottles into a more expensive plastic.

The carrier-bag-to-nanotube technique was developed by Vilas Ganpat Pol at the Argonne National Laboratory in Illinois and converts high or low-density polyethylene (HDPE and LDPE) into valuable multiwalled carbon nanotubes.


Bag baker

Pol made the nanotubes by cooking 1-gram pieces of HDPE or LDPE at 700 °C for 2 hours in the presence of a cobalt acetate catalyst and then letting the mixture cool gradually. Above 600 °C the chemical bonds within the plastic completely break down and multiwalled carbon nanotubes grow on the surface of the catalytic particles.

A lot of catalyst is needed to get good results – about a fifth of the weight of the plastic being converted – and it cannot easily be recovered afterwards. But Pol says this is still one of the cheapest and environmentally friendly ways yet found to grow nanotubes.

“Other methods generally require a vacuum to avoid oxygen interaction with the catalyst as well as with the system,” he says. “In my new reaction there is no vacuum – the formation of oxide is inhibited due to the presence of a continuous reducing hydrocarbon atmosphere at 700 °C.”

Nanotube nuggets

Individual pieces of the catalyst become trapped inside forests of newly grown nanotubes. But Pol has shown the nanotubes can be used as is without further processing to cut them free.

“I have used the as-prepared cobalt-encapsulated nanotubes as an anode material for lithium-ion batteries and they work fantastically,” he says. “The specific capacity of my carbon nanotubes is higher than commercial nanotubes.” He thinks that might be down to slight imperfections in the usually-regular structure of the nanotubes, created by the reducing atmosphere during fabrication.

The cobalt impurities also make the nanotubes suitable for use in lithium-air batteries, because the cobalt is converted to cobalt oxides that perform as catalysts to help the reactions of ions in the battery that let current flow, says Pol. He has patented the use of the cobalt-containing nanotubes in both lithium-ion and lithium-air batteries: “The cobalt is not an impurity, it is an asset,” he says.

Recycling jigsaw

Geoffrey Mitchell at the University of Reading in the UK is an expert in recycling plastic. He thinks the new technique is an “interesting part of the jigsaw” of recycling plastic waste to make high-value electronic materials.

But he thinks the use of relatively expensive cobalt as a non-recoverable catalyst might be problematic if the system is ever to be scaled up. Pol agrees, but adds that the type of batteries he proposes using the nanotubes for are already recycled for their cobalt, so the metal would ultimately be recovered.

Leaving the catalyst out of the process altogether yields another carbon product of potential value, though: carbon spheres between 2 and 10 micrometres across that can be used in printer ink, says Pol.

Journal reference: Journal of Environmental Monitoring, DOI: 10.1039/b914648b