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

Abstract

Sorting at industrial scale is required to perform mechanical recycling of plastics in order to obtain properties that could be competitive with virgin polymers. As a matter of fact, the most part of the various types of plastic waste are not miscible and even compatible. Mid-Infrared (MIR) HyperSpectral Imagery (HSI) is viewed as one of the solutions to the problem of black plastic sorting. Many Waste of Electrical and Electronic Equipment (WEEE) plastics are black. Nowadays, these materials are difficult to sort at an industrial scale because the main used pigment to produce this color, carbon black, masks the Near-Infrared (NIR) spectra of polymers, the currently most used technology for acute sorting in industrial conditions. In this study, laboratory Fourier-Transform Infrared (FTIR) in Attenuated Total Reflection mode (ATR) has been used as a theoretical toolbox based on physical chemistry to help building an automated HSI discrimination despite its limited conditions, especially shorter wavelengths ranges. Weaker resolution and very short acquisition times are other HSI limitations. Helping fast and exhaustive laboratory characterizations of polymeric waste stocks is the other goal of this study. This study focusses on polyolefins as they represent the second biggest fraction of WEEE plastics (WEEP) after styrenics and since little quantities mixed to styrenics during mechanical recycling can lead to important decrease in mechanical properties. Twelve references were thus evaluated and compared between each other and with real waste samples to highlight spectral elements, which can enable differentiation. Charts compiling the signals of discussed polymers were built aiming to the same objective.

Alterations of plastics spectra in MIR and the potential impacts on identification towards recycling

“…As NIR-HSI, the most used technology to finely discriminate plastics according to their natures, is limited with dark plastics, several alternative sorting technologies are subject to extensive research (Grégoire et al, 2011;Huang et al, 2017;Küter et al, 2018;Langhals et al, 2014;Roh et al, 2017;Wang et al, 2015;. MIR-HSI is one of them (Kassouf et al, 2014;Rozenstein et al, 2017;Signoret et al, 2019aSignoret et al, , 2019b. It was chosen in the present study because this technology begins to be commercially available.…”

Review on the photonic techniques suitable for automatic monitoring of the composition of multi-materials wastes in view of their posterior recycling

“…Degradation of resins coming from post-consumer waste can occur by diverse factors such as temperature, exposure to the environmental elements and microorganisms, among others. This can cause consequent alterations in their polymeric structure, more specifically in the stretching regions of C=C and C–H bonds ( Beigbeder et al, 2013 ; Hadi et al, 2014 ; Moroni et al, 2015 ; Raut et al, 2019 ; Signoret et al, 2019 ). In this regard, vibrational spectroscopies such as NIR or NIR-HSI are nowadays the main photonic techniques implemented at industrial scale for the sorting of plastic waste streams.…”

Handling Mixture Optimisation Problem Using Cautious Predictions and Belief Functions

“…The second source is the optical device which provides spectra of lesser quality. Since small quantity of badly sorted plastics can lead to high decreases of impact resistance [13] and of monetary value, impurities should be limited. Thus, experts have defined tolerance threshold on the proportions of impurities.…”