DIFFERENCES AND SIMILARITIES IN THE THERMOOXIDATIVE DEGRADATION OF POLYAMIDE-46 AND POLYAMIDE-66

A comparison is made between the thermooxidative degradation of polyamide 46 (PA46) and polyamide 66 (PA66), using oven ageing, oxygen uptake and viscosity measurements on the two polymers. Model experiments were used to determine the degradation mechanism of PA46. Although on a chemical basis it is expected that PA46 would be less stable than PA66, the opposite is found. The time until the tensile strength becomes 50% of its initial value is twice as long for unstabilized PA46 compared with PA66 at 145 degrees C. It is shown that the degradation of PA46 is a surface phenomenon and thus oxygen diffusion limited. PA46 is more stable than PA66 due to its lower oxygen permeability, probably caused by its higher crystallinity and density of the amorphous phase.