Newly published method converts polypropylene, found in containers, carpets and cars, into petrol- and diesel-like fuels that could be suitable for motorists.

Scientists at Purdue University in Indiana have discovered a way to turn polypropylene plastic waste into oil, which can then be turned into petrol or diesel fuel.

As part of a paper published in Sustainable Chemistry and Engineering, a team led by professor Linda Wang created the oil by adding pellets of purified polypropylene waste to supercritical water, an extremely hot and highly-pressurised form of the liquid.

In this case, the supercritical water is between 380°C and 500°C, and is subject to pressures around 225 times greater than atmospheric pressure at sea level.

Depending on the temperature of the water, the process takes between one and six hours, and converts around 90 per cent of the polypropylene waste into oil. The oil can be distilled into petrol- and diesel-like substances.

According to the team's initial analysis, this method is energy positive, and "potentially has a higher energy efficiency and lower greenhouse gas emissions than incineration and mechanical recycling".

If a method can be found to scale up this process to commercial levels, newly generated polypropylene waste could used to satisfy around four per cent of the world's annual demand for the petrol and diesel fuels.

The researchers say roughly five billion tonnes of plastic waste has been dumped into landfill and the natural environment over the past 50 years, with polypropylene accounting for around 23 per cent of that figure.

Only about 12 per cent of the world's lifetime plastic waste has been incinerated, and roughly nine per cent recycled.

Due to its high heat resistance and strength, polypropylene is used in food packaging, crockery, carpets, lab equipment, plastic hinges, and automotive parts.