•Methodology for ageing characterization based on the phenol index value implemented.•The phenol index allows for assessment of partly oxidized phenolic antioxidants.•Single-, double- and triple-oxidized products of irganox 1330 were monitored.•Embrittlement of the PP-R material was ascertained at a phenol index of 1.0.•A bilinear arrhenius relationship with a transition at 110 °C was obtained. This paper deals with the hot air ageing behaviour of a polypropylene random copolymer (PP-R) model material in a wide temperature range from 65 to 135°C. The PP-R grade was stabilized with a blend of the phenolic antioxidants Irganox 1330 and 1010. To avoid diffusion-limited oxidation effects, micro-specimen with a thickness of 100 µm were investigated. As main ageing indicator the phenol index was evaluated from FTIR-spectra in transmittance mode. The oxidation of the functional phenolic groups was associated with a shift of the absorption peak by 18 and 35 cm-1 of the single- and dual-oxidized phenolic antioxidant Irganox 1330. The implemented FTIR evaluation approach was validated by high-pressure liquid chromatography hyphenated with mass spectroscopy and ultraviolet detection. Gaussian fitting of the triple IR-peak confirmed temperature dependent differences in the degradation pathways of the phenolic antioxidants. The decay of phenol index and the obtained endurance time exhibited a bilinear Arrhenius like behaviour with a transition temperature at 110°C, which was in agreement with the onset of melting of small crystal lamellae. The activation energy values were 94 and 32 kJ mol-1 above and below the transition temperature, respectively. Display omitted