Mode of dipentaerythritol stabilization during thermo-oxidative aging of glass fibre reinforced PA66

Thermo-oxidative aging of dipentaerythritol (DPE) stabilized, glass fibre reinforced PA66 compounds was studied at 200°C. The influence of DPE as a thermo-oxidation stabilizer on the retention of tensile properties, molecular weight, crystallinity, oxidized layer thickness as well as the specimen surface and fracture morphology was analysed. Moreover, the impact of DPE on the intrinsic humidity inside the test specimens before aging was examined due to its hygroscopic behaviour. The reduction of the extractable amount of DPE from test granules over the aging time proved the existence of chain extension or crosslinking reactions between the polyamide or its degradation products and DPE. Whereas DPE improves the retention of mechanical characteristics during aging only slightly, the combination of DPE and glass fibres reaches retention of mechanical characteristics above 50 % after 1000 h of aging at 200°C in air. DPE raises the intrinsic humidity during processing and before aging resulting in an increase of PA66 degradation during processing and in the first aging phase due to additional hydrolysis. Both the ensuing shorter chain length and water-induced post-crystallisation give rise to higher crystallinity after processing, increasing the oxygen diffusion limitation which results in slower thermo-oxidation. In addition, DPE accelerates the formation of the oxidized layer on the PA66 surface but slows down its growth during the later aging phase.