PROTECTIVE PROPERTIES OF POLYIMIDE (PMDA-ODA) ON ALUMINUM METALLIC SUBSTRATE

The protective properties of thin polyimide (PI) films on aluminum metallic substrates were studied using the impedance spectroscopy technique. The effects of storage time, film thickness (1.2 and 2.4-mu-m), NaCl concentration (up to 0.5M) and the synergistic effect of temperature (80-degrees-C) and relative humidity (85%) on the lifetime of the PI were investigated. The results obtained can be summarized as follows: (i) early failure was observed for the PI/Al samples tested directly after curing, while extended lifetime was observed for samples stored in a desiccator for two months before testing; (ii) the lifetime increases by increasing the film thickness and/or by decreasing the NaCl concentration; and (iii) temperature and relative humidity dramatically affect time to failure. An attempt was made to relate coating failure to both water and salt uptake by the PI.