Simple way to make graphite anodes from waste plastic

Turning plastic into solid carbon is not easy. Heat the polymers to rearrange their carbon atoms, and they decompose into light hydrocarbons and gases. But researchers at the US National Energy Technology Laboratory (NETL) have devised a simple process to make crystalline graphite from waste polyethylene (PE).

Researchers have known that adding oxygen when heating PE keeps the polymer from decomposing. The oxygen coaxes the carbon chains to cross-link and form a stable material. But oxygen does not diffuse into the bulk of the viscous PE melt no matter how much you mix, NETL research scientist Christopher Matranga says.

So Matranga and his colleagues put table salt in the mixture. This produces a granular, sticky material in which each salt particle is coated with a thin layer of polymer melt that is easily oxidized. “We increase the effective surface area of the polymer melt using a really low-tech solution,” he says.

After removing the salt, the team added an iron-based catalyst and heated the samples at 1,500 °C for a few hours. The catalytic reaction converts the stable PE into highly crystalline graphite particles. Traditional graphite processing requires temperatures twice as high and takes several days. The team can process 0.25 kg of PE at a time and is now developing a 4 kg reactor, enough to make roughly 1 kg of graphite, Matranga says. “We’re excited about this and continue to figure out how to do it bigger and better.”