MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: III. Anticipating impacts of ageing on identification

Ageing of polymers entails important structural changes and degrades their functional properties, particularly their aspect. Since sorting is a primordial step in recycling to achieve acceptable mechanical properties, the use of promising technologies such as MIR-HSI (Mid-Infrared Hyperspectral Imagery), which could overcome black plastics sorting issue, has to take into account the influence of ageing on identification. As ageing strongly impacts spectra, it can create confusion between materials, especially in an automatized scheme. Based on laboratory FTIR-ATR (Fourier-Transform Infrared Attenuated Total Reflection), this work investigates spectral evolutions of natural and accelerated photodegradation of Waste of Electric and Electrical Equipment plastics (WEEE) as PE, PP, HIPS, ABS and PC to help identifying a polymer despite its ageing degree. Oxidation marks were described and retrieved within a stock of about one hundred of real waste samples, then differentiated from other sources of spectral alteration as formulation. Laboratory ageing data were found to be consistent and often more extreme than real waste samples values. Generally, styrenics showed stronger spectral alteration than polyolefins despite their respective aspects. No significant spectral alteration of PC was obtained here or observed in the waste stock. As an important oxidation marker, the carbonyl peak was also found to often enable fast identification through its wavenumber. If well taken in account, ageing should not induce confusion with other polymers, even formulated, as characteristic signals are different. Finally, the different industrial sub-ranges within MIR are not affected at the same degree, possibly influencing a technological choice for industrial sorting.