31st August 2014

New method automates plastic sorting for recycling

By measuring a plastic item's fluorescence half-life, it's possible to identify the exact kind of plastic it's made from.

A team of researchers at the University of Munich in Germany (LMU) has developed a new process that will greatly simplify the sorting of plastics in recycling plants. This method enables the automated identification of polymers, facilitating rapid separation of plastics for re-use.

The new technique, which is being patented soon, involves exposing particles of plastic to a brief flash of light which causes their material to fluoresce. Photoelectric sensors then measure the intensity of the light emitted in response to the induced photoexcitation to determine the dynamics of its decay. Because the different polymer materials used in the manufacture of plastics display specific fluorescence lifetimes, the pattern of the decay curve can be used to identify their chemical nature.

Prof. Heinz Langhals, of the LMU's Department of Chemistry: “With this process, errors in measurement are practically ruled out; for any given material, one will always obtain the same value for the fluorescence half-life, just as in the case of radioactive decay.”

Even in its current prototype form, the machine can sort up to 1.5 tons (1.4 tonnes) of plastic per hour. According to the researchers, this figure meets the specifications required for application on an industrial scale.

A paper on the research is published in the journal Green and Sustainable Chemistry.

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