Identifying polymers enables rapid separation of plastics

Researchers from Ludwig-Maximilians-Universitaet München (LMU) have developed a process which will greatly simplify the process of sorting plastics in recycling plants. Their method, published in the journal Green and Sustainable Chemistry, enables the automated identification of polymers.

The technique takes advantage of the polymer-specific nature of the intrinsic fluorescence induced by photoexcitation. As explained by team leader Professor Heinz Langhals, “Plastics emit fluorescent light when exposed to a brief flash of light, and the emission decays with time in a distinctive pattern. Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint.”

As the plastic particles fluoresce, photoelectric sensors 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.

The use of fluorescence lifetime measurements permits the identification and sorting of up to 1.5 tonnes of plastic per hour, greatly increasing efficiency. Professor Langhals said, “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.”

Unlike metals, the quality of which often suffer during the recycling process itself, recycled plastics can be processed quite efficiently. Professor Langhals said, “Polymers represent an interesting basis for the sustainable cycling of technological materials. 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 vast majority of technical polymers are processed as thermoplastics, ie, they are melted at high temperature and the finished article is produced by injecting the molten material into an appropriate mould. Reheating of recycled plastic can, however, lead to deleterious alterations in its properties of the material unless the sorted material is of high purity.

Polymers tend to be immiscible, as they are chemically incompatible with one another. Remelting of polymer mixtures therefore often leads to partitioning of the different polymers into distinct domains separated by grain boundaries, which compromises the quality of the final product. For this reason, high-quality plastics are always manufactured exclusively from pristine precursors, as opposed to recycled material; but 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,” said Professor Langhals.

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