New sorting technique gives recycled plastics a ‘fluorescent fingerprint’

A team of researchers claim to have developed a new process that will “greatly simplify” and “markedly expedite” the process of sorting plastics in recycling plants.

The method, which has been devised by a team from Ludwig-Maximillians-Universitaet (LMU) in Germany, enables automated identification of polymers, facilitating rapid separation of plastics for re-use.

Led by Professor Heinz Langhals of LMU’s Department of Chemistry, the new technique - which features in the latest issue of the Green and Sustainable Chemistry journal - takes 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 in a distinctive pattern,” explained Langhals. ‘Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint.”

The new technique, which is the subject of a patent application, involves exposing particles of plastic to a brief flash of light, which causes the material to fluoresce. Photoelectric sensors then measure the intensity of the light emitted in response to the inducing photoexcitation to determine the dynamics of its decay. Because the different polymer materials used in the 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,” added Langhals.

The LMU researchers say that the effective new technique makes automated sorting of plastics feasible, as well as eliminating cross-polymer