Fluorescence Lifetime Measurement Enhances Recycling

Sorting plastics using fluorescence lifetime measurement could simplify and speed up the recycling process.

Developed by a team from Ludwig Maximilian University of Munich, the technique takes advantage of the intrinsic fluorescence properties found in different polymers under photoexcitation.

“Plastics emit fluorescent light when exposed to a brief flash of light, and the emission decays with time in a distinctive pattern,” said LMU professor Dr. Heinz Langhals. “Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint.”

The use of fluorescence lifetime measurement permits the identification and sorting of up to 1.5 tons of plastic per hour, according to the study.

“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,” Langhals said. “Our process can make a significant contribution to environmental protection because it makes automated sorting feasible.”

In the study, the researchers exposed particles of plastic to a brief flash of light, which caused the material to fluoresce. Photoelectric sensors then measured the intensity of the light emitted in response to the photoexcitation.

Because different polymers used in the manufacture of consumer plastics have specific fluorescence lifetimes, the form of the fluorescence decay curve can be used to identify their chemical makeup.

“Polymers represent an interesting basis for the sustainable cycling of technological materials,” Langhals said. “The crucial requirement is that the recycled material should be chemically pure.”

Polymers, which are typically processed as thermoplastics and melted at high temperatures, tend to be immiscible, as they are chemically incompatible with one another.

Re-melting polymer mixtures can lead to their partitioning into distinct domains separated by grain boundaries, which often compromises the quality of the final product, the researchers said.

The research was published in Green and Sustainable Chemistry (doi: 10.4236/gsc.2014.43019).

For more information, visit www.en.uni-muenchen.de.