A Design of Experiment (DoE) Approach to Evaluate the Recyclability of a Polypropylene Copolymer in Medical Technology Under the Aspect of Additive Composition

This study evaluates the influence of repeated processing, γ-irradiation (25 kGy), and additive formulation including Irgafos 168 (I168), Tinuvin 622 (T622), and Calcium Stearate (CaSt) on a polypropylene copolymer (PP-C). Motivated by medical technology applications, the study assessed effects on optical properties, yellowing, crystallization, mechanical performance, and viscosity using a full factorial design of experiments (DoE). Results showed γ-irradiation had the most significant impact, especially on zero-shear viscosity, which decreased by 84% after the first irradiation. The Yellowness Index (YI) changed measurably, but discoloration remained imperceptible. Crystallization temperature was influenced mainly by additive interactions, while specific enthalpy was affected by processing and γ-irradiation. Elongation at break and tensile strength were predominantly influenced by γ-irradiation, with elongation at break being a sensitive indicator of degradation. Zero-shear viscosity, correlating with molecular weight, was mainly controlled by γ-irradiation, indicating chain scission without critical embrittlement. Overall, γ-irradiation exerted a stronger effect than processing or additive formulation. Zero-shear viscosity proved a reliable measure of degradation, while elongation at break offered complementary insights. Despite significant viscosity reduction, mechanical properties remained high, confirming the material's suitability for its intended applications.