Sorption and Transport of Physically and Chemically Interacting Penetrants in Kapton® Polyimide

Gravimetric sorption and desorption data are reported for water and anhydrous ammonia in Kapton® at 30°C. The data for water at low activity are described well by Fick’s Law with a concentration dependent diffusion coefficient. At high vapor activity, evidence of small extents of chemical reaction of the water with imide groups was discovered. Even after exposure to a relative humidity of 81.4% for two weeks, however, only 0.31% of the imide structures were affected. Anhydrous ammonia, on the other hand, at rather low relative saturations (0.012 to 0.032) interacted strongly with some of the imide structures of Kapton®. Infrared and gravimetric sorption/desorption measurements were used to characterize the locus and extent of reaction. It was found that after exposure of a 0.5 mil sample to an ammonia relative saturation of 0.0316 for 16 days, roughly 17% of the imide structures were disturbed. Analysis of the coupled diffusion and chemical reaction suggests that only a fraction of the total number of imide groups (~20%) appear to become “activated” or “labilized” during the solid state curing step in which stresses may arise. Stress relief by the limited ammonolysis reaction presumably deactivates the remaining imide groups. The possiblity of using Kapton® as a combined protective barrier and scavenger coating or film is explored briefly.