Reclaiming value in polystyrene waste
Most people are familiar with the polystyrene packaging material that is used to protect everything from electronic appliances to white goods. But, after you’ve unpacked your new TV, what happens to the packaging? Usually it is thrown away and may end up in landfill. More than 1.5 million tonnes of this expanded polystyrene (EPS), as it’s called, is used in packaging each year and therefore becomes waste, with most going to landfill and only a small proportion recycled into low-grade plastic material.
Researchers at Deakin University have developed a simple method that uses acetone spray to break down the polystyrene into a concentrated gel that can then be recycled.
The process avoids many of the disadvantages of current recycling methods,such as contaminants embedded in the end product,high energy usage, dust production and very little reduction in volume.
Rob Nunn developed the acetone process as a spinoff from his PhD studies. Working in the composites group led by Associate Professor Bronwyn Fox at the Institute for Frontier Materials, Rob is investigating whether polystyrene can be used as a precursor for making carbon fibre. A keen environmentalist, he wondered whether he could source this polystyrene from waste material.
So, he collected samples of various different types of EPS – medical containers, general packaging and food containers from the Geelong Recycling Centre and analysed their melting properties and molecular weight range. The results were very encouraging, showing that the packaging samples had very similar molecular weight characteristics to one another, meaning the material would provide a reasonably pure, well defined and predictable source of raw material.
Next, Rob developed a prototype recycling system. Using a discharge pump from an old dishwasher, some old glass doors, a planter tray and some garden misting sprays, he built a spray chamber for treating the polystyrene. The results were dramatic – polystyrene quickly collapses on contact with the acetone, leaving a gel in the bottom of the chamber.
Rob says one of the main advantages of the process is that it does not dissolve many of the usual contaminants associated with EPS packaging (tape, plastic bags, tie wires), allowing these to be separated. The end product is relatively pure (70-75% polystyrene) and so may be used to reconstitute EPS or as a feedstock for other polystyrene manufacturing processes. The process reduces about 1000 L (1 cubic metre) of EPS down to about 20 L of gel!
Because most of the EPS associated with food packaging is heavily contaminated with food wastes, Rob focused his research on recycling moulded packaging EPS, which was also a major component at the Geelong Recycling Centre and proved to be very clean.
While he is still investigating the use of the recovered gel as a precursor for carbon fibre production, this would use only a small fraction of the total EPS capable of being recycled from the waste stream. Further work has shown that the gel could be converted back to polystyrene foam.
More research is planned to better understand the EPS-acetone gel system. The gel could offer potential for value adding - for example polystyrene’s UV stability is a big plus for its potential use as a component in weatherproof protective coatings.