Mechanical recycling of post-consumer HIPS yogurt cups blended with virgin HIPS for food contact applications

Blends of recycled high impact polystyrene (HIPS), made from post-consumer yogurt cups, and virgin HIPS were investigated. Blends containing 30, 40, and 50 wt% of mechanically recycled HIPS, made from yogurt cups collected at a university canteen, were prepared and characterised. Recycled HIPS contains substantially less residual styrene as compared to virgin HIPS (∼60%), suggesting volatilisation and thermo-oxidative reactions during processing. Non-intentionally added substances (NIAS) screening revealed greater chemical complexity in yogurt cups, linked to polymer degradation and food-related residues. Detected compounds included organic volatile, semi-volatile, and non-volatile substances, with the latter being exclusively present in the post-consumer samples. A marked difference in chemical complexity was observed, with 32 distinct compounds identified in the commercial material compared to 68 in the recycled samples. Several substances exceeded the threshold of toxicological concern, particularly aldehydes and styrene oligomers. Overall migration increased with recycled content but remained within European regulatory limits. Thermal, structural, and mechanical analyses indicated moderate changes due to reprocessing, including signs of thermo-oxidative ageing and slight embrittlement at higher recycled contents, without substantial loss of performance compared to commercial HIPS. These effects were consistent with colour variations and FTIR results, indicating the formation of oxidised species. Altogether, the results demonstrate that mechanically recycled HIPS from a defined post-consumer source can meet functional requirements, while highlighting the need for substance-specific migration testing and robust risk assessment. These findings support the potential of such recycling strategies to enable the safe integration of recycled polystyrene into circular food packaging systems.