Supercritical CO2 extraction of contaminants from polypropylene intended for food contact: Effects of contaminant molecular structure and processing parameters

• Extraction efficiency is inversely proportional to the contaminant molecular weight.
• Complete extraction can be achieved, even with the heaviest contaminant (807 g/mol).
• Pressure and SCCO2 flow rate have positive effects on the extraction efficiency.
• Temperature effect is complex due to the competition between diffusion and solubility.
• The extraction process may be controlled by diffusion, solubility or both phenomena.

Supercritical CO2 (SCCO2) extraction has been employed to remove model molecules (surrogates) which simulate the real and potential contaminants which can be present into recycled post-consumer polypropylene, limiting the recyclability for food contact applications. Different substances covering a wide range of molecular weights were extracted and evaluated. The effects of contaminant molecular structure and several processing parameters (pressure, temperature, extraction time, solvent flow rate, and matrix shape and size) upon extraction rate were investigated. The operating conditions studied were: pressure ranges of 100–300 bar, temperature ranges of 50–90 °C, indicating CO2 densities varying from 0.20 to 0.87 g/mL, and solvent flow rate ranges of 60–160 mL/min. Two shapes of contaminated material were studied: pellets and films (thickness ranges of 100–300 μm). Quantitative results for extraction kinetics have been obtained by gas chromatography. High extraction yields (100%) were achieved with films at restricted extraction times and, likewise, with pellets at reasonable extraction times. Operating conditions of 200 bar, 90 °C and 7.5 h of extraction were sufficient to achieve a complete extraction from pellet matrix (2.5 mm of diameter), even for the heaviest contaminant (807 g/mol).

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