On the stability of PEEK for short processing cycles at high temperatures and oxygen-containing atmosphere

The motivation of this study was to investigate the stability of poly(ether ether ketone) (PEEK) at conditions close to real processing: fast cycles (below 300 s), temperatures higher than 400 °C and oxygen-containing atmosphere. Such conditions significantly affect the consolidation ability of PEEK and its further processability. The overall quality of the polymer after the degradation cycles was investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Significant morphological changes were detected by increasing crosslinking density and decrease in crystallinity with temperature and time. Moreover, the through-thickness polymer degradation was studied in order to understand the local polymer quality. The samples were examined via Fourier transform infrared (FTIR) spectroscopy and light transmission microscopy. The formation of a dark coloured boundary layer by the way of an oxygen diffusion-controlled process on the surface of the polymer was observed. For temperatures below 400 °C, the boundary layer increased constantly with time. In contrast, high processing temperatures resulted in a concentrated barrier layer which acted as protecting cover. •The thermal history in air significantly affects the processing properties of PEEK, espe-cially at the surface.•The boundary layer formed has been studied by DSC and FT-IR.•For temperatures below 400 °C, the boundary layer increased constantly with time.•Above 400 °C, the dark layer is not linearly dependent on t and T and leads to the formation of a protecting cover.