The trials could improve the way various types of plastic are re-used and recycled by taking advantage of the polymer-specific nature of the fluorescence induced by photoexcitation.
'Plastic Planet'
Dr Heinz Langhals, professor, Department of Chemistry, LMU, led the study and released a report called ‘High Performance Recycling of Polymers by Means of Their Fluorescence Lifetimes’, on the findings.
The report states ‘there is a necessity for the development of efficient processes because of increasing environmental pollution by polymers (‘plastic planet’).
“Plastics emit fluorescent light when exposed to a brief flash of light, and the emission decays with time in a distinctive pattern,” he said.
“Their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint.”
Details of the method, which tested the technical polymers Luran, Delrin and Ultramid, are subject to a patent application and appear in the latest issue of the journal Green and Sustainable Chemistry.
Photoexcitation
The technique 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 identify their chemical nature.
“Polymers represent an interesting basis for the sustainable cycling of technological materials. The crucial requirement is the recycled material should be chemically pure,” added Langhals.
Most technical polymers are processed as thermoplastics, i.e., they are melted at high temperature and the finished article is produced by injecting the molten material into an appropriate mold, where it allowed to set.
However, reheating recycled plastic can lead to alterations in the properties of the material unless the sorted material is of high purity.
'Down-cycling'
Contamination levels as low as 5% can significantly reduce the quality of the reformed product.
Langhals believes the reason for this “down-cycling” effect is that, as a general rule, polymers tend to be immiscible, as they are chemically incompatible with one another.
“Remelting of polymer mixtures often leads to partitioning of the different polymers into distinct domains separated by grain boundaries, which compromises the quality of the final product,” he said.
“For this reason, high-quality plastics are always manufactured exclusively from pristine precursors – never from recycled material.
“The method developed by the LMU team could change this. The waste problem can only be solved by chemical means, and our process can make a significant contribution to environmental protection, because it makes automated sorting feasible.”
He added the use of fluorescence lifetime measurements permits the identification and sorting of up to 1.5 tons of plastic per hour. Therefore, the method in its present form already meets the specifications required for its application on an industrial scale.
Source: Green and Sustainable Chemistry
Published: August 5, 2014 Volume 4, Number 3 DOI 10.4236/gsc.2014.43019
Title: ‘High Performance Recycling of Polymers by Means of Their Fluorescence Lifetimes’
Author(s): Heinz Langhals, Dominik Zgela, Thorben Schlücker