A new process that will greatly simplify the process of sorting plastics in recycling plants. The method that enables automated identification of polymers, facilitating rapid separation of plastics for re-use, has been developed by researchers from Germany's Ludwig Maximilian University (LMU), Munich. The team has developed a technique which provides for automated recognition of their polymer constituents, thus improving the efficiency of recycling and re-use of the various 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. Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint", as per Prof Langhals. The new 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 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 the professor. Unlike metals, the quality of which often suffers during the recycling process itself, recycled plastics can be processed quite efficiently.
Prof Langhals explains that 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. Contamination levels as low as 5% are sufficient to significantly reduce the quality of the reformed product. 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 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 - never from recycled material. The new method developed by the LMU team could, however, 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, explained Prof Langhals. Indeed, the use of fluorescence lifetime measurements permits the identification and sorting of up to 1.5 tons of plastic per hour. In other words, the method in its present form already meets the specifications required for its application on an industrial scale.