THE sheer variety of plastics we use creates a massive headache for recycling companies. Separating mountains of plastic waste into compatible types 鈥� from tough transparent cola bottles to soft, light polystyrene packaging 鈥� currently costs an uneconomic 拢125 per tonne. But a new recycling technique might end the need for this expensive sorting process.
When an assortment of different polymers is melted, the different components don鈥檛 mix and then they solidify into a material that contains all sorts of different types of plastic. This makes the material unstable and weak. 鈥淒ifferent polymers don鈥檛 give a homogeneous mix 鈥� you just get rubbish,鈥� says Alan Davey, general manager of Linpac, Britain鈥檚 largest plastics recycling company.
But now Francesca Cavalieri and Franco Perella at the Italian National Agency for New Technology, Energy and the Environment (ENEA) in Rome have come up with a technique that produces a strong, high-value polymer from a mixture of waste plastics. It works by smashing up the molecules of the waste polymers, then allowing them to recombine to produce hybrid molecules that yield a homogenous melt.
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ENEA鈥檚 polymer recovery process uses a laboratory-scale version of an industrial pulverising machine called a ball mill 鈥� a chamber partially filled with tough tungsten carbide balls 8 to 15 millimetres in diameter. When the chamber is vibrated vigorously, the balls smash anything else in the chamber to a fine powder.
But this alone isn鈥檛 enough to break apart the polymer molecules. Cavalieri and Perella鈥檚 new twist is to add dry ice 鈥� frozen carbon dioxide 鈥� into the mix. Under pressure, the CO2 becomes a liquid that generates tiny explosions as the hot colliding balls turn it to vapour in a fraction of a second. This releases enough energy to snap nearby polymer chains.
The result is an assortment of molecular fragments with highly reactive unpaired electrons at the end points where they broke off the chain. The fragments quickly recombine, but this happens at random, producing new hybrid polymer molecules that include sections from the different kinds of molecules in the original mix. The whole process takes about 10 minutes, and the result is a hybrid polymer that yields a homogenous melt that can be formed into tough new components.
So far, the process has only been proven with two common polymers, but others are being investigated.
The recovered polymer has also been mixed with polyester fibres from recycled tyres, making a selection of composite materials. These composites can be made as strong as freshly made polyethylene or as rigid and durable as PVC.