For many years, polyethylene terephthalate (PET) has enjoyed continuous market development in the packaging industry. For example, in 2002, PET consumption reached 2.1 million tons, or 51% of total plastic consumption in the market in Europe. To sustain this momentum, it is not only necessary to continuously optimize materials and applications, but also to further develop PET recycling routes and new markets for recycled materials. The inactivity of PET to the substances and contaminants slowly mixed into it is high, resulting in the development of many material recovery processes until the so-called ultra-clean process finally allows recycled PET to be used in direct food contact conditions.

Figure 1: Specific sensing signal plots for selected types of contaminants, each with the same concentration.

Discovered mobile interference factors
For several years, the Fraunhofer Packaging Technology Research Institute (IVV) in Germany has maintained a leading position in these developments. By testing and optimizing analytical methods, the quality of PET can be determined in its various “life stages”, from pellets to bottles. Billets and bottles until recycled materials. In this respect, this is also the first online method developed for the monitoring of mobile disturbances in the recycling process. Table 1 lists the respective molar mass ranges of these analytical methods and the impurities that each group can detect.

High temperature gas chromatographic analysis technology was developed to further characterize the volatile interferences. It can detect substances with a molar mass of about 350 g mol-1, with a detection range of less than 0.1 ppm.

Relying on cleanup and capture sampling techniques, this detection range can be reduced to a lower level. The sample was kept at 150°C and the evaporate was absorbed in the cold valve. After all the volatile components have been removed from the material, the sample is thermally desorbed and analyzed by gas chromatography (GC). This method reduces the detection limit by a factor of about 100. Therefore, the method is well-suited for evaluating odoriferous impurities in recycled PET, such as fibers or mats inside the car.

In order to detect general and less volatile substances, expansion and decomposition of PET materials in extremely corrosive solvents, such as hexafluoroisopropanol (HFIP), and advanced automatic extraction methods, such as accelerated solvent extraction (ASE), are Designed and developed. A multi-analyte method based on liquid chromatography (LC) was designed to represent the added state of the extract. The detector uses an ultraviolet (uv) detector or a mass spectrometer (MS). In particular, liquid chromatography analysis provides great potential because of the complex active ingredients in the pure PET resin (UV stabilizers, acetaldehyde barriers, etc., which cause yellowing upon recycling).

The separation of macromolecules from foreign polymers and the determination of inert (usually inorganic) impurities and the determination of the molar mass are mainly based on spectroscopy, which supplements the chromatographic method. Therefore, XRFA is suitable for analyzing the content of catalysts in recycled materials (it varies by region and affects the ease with which the material can be processed). Black spots in the PET preform were analyzed by electron microscopy scanning (SEM) and X-ray microanalysis (XRMA). These methods allow the accurate analysis of possible contaminants generated during use, which helps to make cost-effective judgments about the applications of recycled PET.

Figure 2: This PET recycling facility operated by Erema is equipped with an online analysis system developed by IVV.

(to be continued)

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