ONE OF the biggest stumbling blocks to the efficient recycling of plastics could soon become a thing of the past thanks to a new fingerprinting technique being pioneered by researchers at the University of Southampton.
Plastic bottles and car bumpers can now be assigned an invisible, tamperproof fingerprint that reveals which plastic they are made from, and even their factory of origin. The fingerprinting system could one day provide a cheap way of pre-screening rubbish for recycling, allowing plastic items to be sorted automatically according to the polymer from which they are made. At the moment, it is extremely difficult to tell which waste items are made from which plastics. Recycled plastics are better quality if they are made from a single polymer.
The Southampton system can inject plastic objects with a unique electric charge as the final product is moulded. 鈥淲e are using sharp needles at a high temperature, when the plastic is molten,鈥� explains John Hughes, the inventor of the fingerprinting system.
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By generating high electric fields at the tip of the needle, Hughes can inject electrons into the molten polymer to create negatively charged ions. Alternatively, the needle can be used to 鈥渟uck鈥� electrons out of the polymer to create positively charged ions.
鈥淭he injection continues while the plastic cools, so the charge is frozen into the polymer structure,鈥� he says. 鈥淭he electrons latch onto, or leave, certain species of molecule and form large immobile ions,鈥� says Hughes.
Through his patented system, Hughes can control the magnitude, pattern and polarity of the charge, creating fingerprints to order. He says that the fingerprints can be read with charge meters that are already widely available.
The hope is that recycling systems can be automated, with the mechanical segregation of plastics controlled by meters preprogrammed to recognise the charge fingerprints. But Hughes says that it will be at least five or six years before such systems are common.
In the shorter term, he hopes that a similar system can be used in tamperproof packaging for consumer items. For example, fingerprints could correspond to sell-by dates, setting off an alarm at supermarket checkouts if a perishable food item is past its deadline.
鈥淭he nice thing is that it鈥檚 invisible and it鈥檚 buried in the bulk of the plastic, so it can鈥檛 be tampered with,鈥� says Hughes. He says that the charge is unaffected by electric or magnetic fields, or exposure of the plastic to water.
