The fluorescent fingerprint of plastic
29 Dec 2014 by Evoluted New Media
A new technique based on the fluorescence of materials could greatly simplify the sorting of plastics in recycling plants. The method, developed by researchers at Ludwig-Maximilians-Universutaet (LMU), will allow automated identification of polymers by taking advantage of the polymer-specific nature of the intrinsic fluorescence induced by photoexcitation. “Plastics emit fluorescent light when exposed to a brief flash of light, and the emission decays with time into a distinctive pattern,” said Professor Heinz Langhals from the Department of Chemistry. “Thus their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint.” Photoelectric sensors measure the intensity of the light emitted to determine the dynamics of its decay. Different polymer materials used in manufacture of plastics display specific fluorescence lifetimes, the form of the decay curve can be used to identify their chemical nature. “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,” said Langhals. Using this technique, the researchers, who published their work in Green and Sustainable Chemistry, believe recycled plastics can be processed quite efficiently. “Polymers represent an interesting basis for the sustainable cycling of technological materials,” said Langhals. “The crucial requirement is that the recycled material should be chemically pure. In that case bottles made of PET, for example, can be relatively easily turned into synthetic fibre for use in waterproof windcheaters.” The new method could also solve the problem of down-cycled plastics – that is contaminated plastics with a lower grade of perfection which have been reheated. “The waste problem can only be solved by chemical means and out processes can make a significant contribution to environmental protection, because it makes automated sorting feasible,” Langhals concluded. High Performance Recycling of Polymers by Means of Their Fluorescence Lifetimes*
