Morpho-structural and thermo-mechanical characterization of recycled polypropylene and polystyrene from mixed post-consumer plastic waste

In this work, a complete sorting and characterization of dirty non-recyclable mixed plastic (plasmix fraction) from municipal solid waste was performed. The procedure comprised a visual inspection and identification of the materials presented in the mixed plastic bales, and subsequent moisture and dirt content determination. Afterwards, both polypropylene and polystyrene fractions obtained from the sorting process were recycled and physico-mechanically and thermally characterized for the assessment of recycled material quality. The dirty non-recyclable mixed plastic from municipal solid waste was composed of polypropylene (28.89 %), polyethylene terephthalate (22.02 %), polystyrene (9.65 %) and rigid polyethylene (4.68 %). Regarding the moisture and dirt content of the mixed plastic fraction, the results were highly variable, mainly due to the heterogeneity of the material, its origin, atmospheric conditions, etc. The average sum of both parameters (moisture and dirt) was around 15 %. The mechanical properties were within the expected range for each type of polymer, noting that polystyrene recycled materials are more brittle than recycled polypropylene. The recycling of non-food polypropylene wastes yielded a material with higher impact resistance (70 kJ m−2) and Young´s modulus (1934 MPa) as compared to that obtained from the recycling of food packages. Concerning the polystyrene recyclates, it is worth mentioning that recycled materials obtained from foamed residues (expanded polystyrene and extruded polystyrene showed approximately twice the tensile strength with respect to the recycled materials from high impact polystyrene and general-purpose polystyrene residues. In some cases, infrared and thermal analyses revealed cross-contamination with traces of other polymers.