New process simplifies recycling of plastics

Researchers at Ludwig-Maximilians-Universitaet (LMU) in Munich have developed a new process claimed to simplify the process of sorting plastics in recycling plants.

A team of researchers led by Prof Heinz Langhals of LMU’s Department of Chemistry have developed a technique that provides for automated recognition of their polymer constituents, thereby improving the efficiency of recycling and re-use of the various types of plastic.

The technique 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. Thus, their fluorescence lifetimes are highly characteristic for the different types of polymers, and can serve as an identifying fingerprint,’ Langhals said in a statement. Details of the new method appear in Green and Sustainable Chemistry.

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,’ said Langhals.

Unlike metals, the quality of which often suffers during recycling, recycled plastics can be processed quite efficiently.

‘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,’ said Langhals.

The vast majority of technical polymers are processed as thermoplastics; they are melted at high temperature and the finished article is produced by injecting the molten material into an appropriate mould, where it allowed to set.

Reheating of recycled plastic can, however, lead to deleterious alterations in its properties of the material unless the sorted material is of high purity. Contamination levels as low as five per cent are sufficient to significantly reduce the quality of the reformed product.

The reason for this so-called 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 from pristine precursors and not recycled material.

The new 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 Langhals.