The results of MFR and DSC allow the conclusion that molecular weight of the polymer decreases as a result of multiple extrusions, i.e. thermo-mechanical stress. The resulting degradation of PP has been described as a series of chemical reactions by several authors [75e81]. Summarizing one may say that several reaction paths lead to the formation of unsaturated bonds and, in the presence of ox- ygen, the formation of intermediates or products that contain carbonyl groups.