Researchers in India have developed a relatively low-temperature process to convert low-density polyethylene (LDPE)—a common polymer used to make many types of container, medical and laboratory equipment, computer components and plastic bags—into liquid fuel over a kaolin catalyst.

A paper on their work is published in the International Journal of Environment and Waste Management.

In their approach, the team heats the plastic waste to between 400 and 500 Celsius over a kaolin catalyst. This causes the plastic’s long chain polymer chains to break apart in a process known as thermo-catalytic degradation, releasing releases large quantities of much smaller, carbon-rich molecules.

The team used gas chromatography coupled mass spectrometry to characterize these product molecules and found the components of the resulting liquid fuel to be mainly paraffins and olefins 10 to 16 carbon atoms long. This, they explain, makes the liquid fuel very similar chemically to conventional petrochemical fuels.

Kaolin is a clay mineral containing aluminum and silicon. It acts as a catalyst by providing a large reactive surface on which the polymer molecules can sit and so be exposed to high temperature inside the batch reactor, which breaks them apart.

The team optimized the reaction at 450 °C—a temperature with the lowest amount of kaolin at which more than 70% of the liquid fuel is produced.

In other words, for every kilogram of waste plastic they could produce 700 grams of liquid fuel. The byproducts were combustible gases and wax.

They could boost the yield to almost 80% and minimize reaction times, but this required more catalyst: 1 kg of kaolin for every 2 kg of plastic.

The quality and yield of the condensable product has been studied as a function of temperature and amount of catalyst. Both in thermal and catalytic degradation, the condensable fraction was less viscous liquid oil at low temperatures (up to 450°C), whereas with increase of temperature (from 475°C) the fraction became viscous and waxy. The recovery of condensable fraction increased from 30.8 wt.% at 400°C to 71.45% at 450°C and further increased to a maximum of 86.65wt.% at 500°C in absence of catalyst. The catalyst increased the yield of the condensable product and decreased the reaction time. The highest yield of liquid fraction at 450°C was 79.5 wt.% with 1:2 catalyst to plastics ratio.