Assessment of antioxidant performance of Silybum marianum extract as a natural antioxidant additive in polyethylene by time-dependent melt rheology tests

In this study, Silybum marianum (SM) extract as a natural primary antioxidant (PAO) additive was introduced into low density polyethylene. This primary antioxidant was supported by a commercial grade secondary antioxidant (SAO, Irgafos® 168). The effect of compositional variations, mainly amount of SM and the weight ratio of PAO: SAO in the composition, on the thermooxidative stability of low density polyethylene (LDPE) was characterized by determining the thermal parameters, oxygen induction time (OIT) and oxidation onset temperature (OOT) using conventional thermal analysis methods performed in a DSC in air atmosphere. Furthermore, time-dependent rheology test procedures were suggested for monitoring the thermooxidative degradation behavior of antioxidant-loaded compounds. It was found that SM loading of 0.2 or 0.5 wt% provided sufficient oxidation stability in polyethylene, at least 5 min at 240 °C under air when supported by introducing two- or three-fold of SAO. It can be concluded that these SM-Irgafos® 168 antioxidant packages can be successfully used in polyethylenes processed in conventional melt processing windows that can be varied in 180–240 °C, in processing devices (e.g., compounder, kneader, extruders, injection molding, rotomolding) because the obtained thermorheooxidative stability times are longer than the residence time of polymer melts, approximately in the range of 1–4 min from the melting zone to die during the extrusion process.