Detection of Black Plastics in the Middle Infrared Spectrum (MIR) Using Photon Up-Conversion Technique for Polymer Recycling Purposes

Abstract

The identification of black polymers which contain about 0.5 to 3 mass percent soot or black master batch is still an essential problem in recycling sorting processes. Near infrared spectroscopy (NIRS) of non-black polymers offers a reliable and fast identification, and is therefore suitable for industrial application. NIRS is consequently widely used in polymer sorting plants. However, this method cannot be used for black polymers because small amounts of carbon black or soot absorb all light in the NIR spectral region. Spectroscopy in the mid infrared spectral region (MIR) offers a possibility to identify black polymers. MIR spectral measurements carried out with Fourier-transform infrared spectrometers (FTIR) are not fast enough to meet economic requirements in sorting plants. By contrast, spectrometer systems based on the photon up-conversion technique are fast and sensitive enough and can be applied to sort black polymer parts. Such a system is able to measure several thousand spectra per second hence is suitable for industrial applications. The results of spectral measurements of black polymers are presented.

Black plastics: Linear and circular economies, hazardous additives and marine pollution

“…Efficient recovery and recycling of non-EEE plastics relies on sorting into monopolymeric fractions (and according to resin identification codes) that can be performed cheaply, reliably, safely and automatically (Bezati et al, 2011). Currently available technology is based on spectral signatures derived from near infra-red (NIR) reflectance spectroscopy (0.8 to 2.5 m) where plastics are identified according to stretching vibration modes of CH, CH2 and CH3 groups (Becker et al, 2017).…”

“…Plastics coloured with carbon black and other black pigments, however, exhibit very low reflectance of light in the NIR spectral region and the signal-to-noise ratio of present sensors is insufficient to allow classification according to polymer type (Rozenstein et al, 2017); identification may be hampered further by the presence of additional additives and lacquer films (Becker et al, 2017).…”

“…Alternative technologies to identify black plastics have recently been investigated that are based on mid-wave infra-red spectroscopy (3 to 12 m) but thus far these have not proved to be feasible on a commercial scale (Becker et al, 2017;Rozenstein et al, 2017). A review into the problem by the UK government-funded recycling group, WRAP (Waste Resources Action Programme), concluded that, in combination with existing NIR technology, either alternative colourants or the addition of fluorescent markers would be the most suitable option to achieve a sufficient throughput of materials at a recovery or reprocessing facility (Dvorak et al, 2011).…”

Quantum cascade laser-based reflectance spectroscopy: a robust approach for the classification of plastic type

“…dark grey) plastics due to their low reflectance in the NIR spectral range. In the NIR region, other materials such as carbon black and soot both absorb completely [9]. For polymer sorting, Fourier transform infrared (FTIR) spectroscopy is typically too slow [9].…”

MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: II. Specific case of polyolefins

“…A few papers also focused on its application to plastics identification. Becker et al (2017) studied the application of an internally developed converter that could transform MIR (Mid-Infrared) signals into NIR, analyzable with well-developed NIR-HSI cameras. However, the relevant wavelengths range is already reachable with current commercial MIR-HSI cameras.…”