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SORTING | INNOVATION


A recent research paper takes a deep dive into the issue of NIR identifiability of black and coloured plastics used for packaging


Clariant research looks at black sorting problem


How best to detect black plastics on sorting lines is an issue that companies in the plastics recycling sector are working hard to solve. Some, including one major carbon black producer and at least one supplier of sensor sorting technologies, say products loaded with carbon black can be reliably sorted with the right equipment. But that has not stopped numerous colour companies coming up with “almost blacks” that enable detection on regular near-infrared (NIR) units. Researchers at Clariant recently wrote a paper,


“Coloured plastics with positive NIR identifiability,” looking at the issue. Marc Zwart and Olivia Li from Clariant Coatings (Shanghai), together with Lijuan Tian from Thermo Fisher Scientific, were the authors. They say Clariant has established a range of achromatic black colorants that have positive NIR identifiability. The positive identifiability was confirmed using a Thermo Scientific microPHAZIR PC handheld plastics analyser. NIR spectroscopy provides fast and non-destruc- tive identification of the plastic part by measuring the distinct reflection of NIR energy and comparing it with those in a database. Accurate results are obtainable since plastics have unique NIR spectral fingerprints. NIR spectroscopy however requires a minimum


www.plasticsrecyclingworld.com


amount of NIR energy reflectance back to the sensor for the technology to work. This becomes problematic if the plastic part absorbs instead of reflects NIR energy, for example due to the presence of NIR absorbing colorants. Carbon black pigment absorbs visible light but


also near infrared light. So, when added to a plastic, it results in a part that cannot, or less successfully, be identified by plastic type when using NIR technology and this therefore hinders recyclability. A project was started by Clariant STAR Center to determine the NIR identifiability of black and coloured plastics used for packaging, as well as select alternative colorants with positive NIR identifiability. Tests were carried out on injection moulded coloured chips of six types of plastics (PET, HDPE, PVC, LDPE, PP and PS). Evaluation of the injection moulded chips with


chromatic pigments that do not absorb in the whole NIR range show that while the percentage of NIR reflectance varies by colour, the unique characteris- tics in the curve of the plastic remain visible. This was also confirmed using the microPHAZIR PC analyzer. The plastic chip remains identifiable and the sorting for recycling remains possible. Evaluation of the injection moulded chips with the achromatic NIR absorbing carbon black


September 2021 | PLASTICS RECYCLING WORLD 25


Main image: A selection of the consumer packing materials evaluated by Clariant and Thermo Fisher


IMAGE: CLARIANT


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