change of the CI [48] or they did not measure significant carbonyl signals [34e36,40,45,52]. Latter state that there is not enough ox- ygen in the polymer melt during extrusion to cause detectable oxidation. On the other hand Hinsken [49] measured a well detectable increase of the carbonyl signal as a function of the number of extrusions during the analysis of pure PP without any stabilizers or antioxidants. The commercial polymers used in most other studies contain, with a high certainty, antioxidants. These hinder oxidation of the polymer by the small amount of oxygen available in the extruder barrel. Regarding the iron content other studies measured a heightened CI compared to pure PP by means of FTIR at polymers subjected to thermal aging, photo-oxidation and artificial weathering [58,82e84]. These results can be transferred to degradation mechanisms during extrusion.