Amine-quinone polyimides – High temperature polymers that protect iron against corrosion - Polyimides and Other High Temperature Polymers - page 490

poor mechanical properties. For the other amine-quinone polyimides the tensile
moduli were between 0.9 and 1.5 GPa and the tensile strengths ranged from 92 to
161 MPa. All polymer films behaved as ductile materials with good modulus,
high strength, and moderate elongation at break.
The moisture absorption was determined by exposing the dried polyimide films
to 85°C and 85% relative humidity for one week and measuring the percent in-
crease in weight. The amine-quinone polyimides had a moisture absorption in the
range of 0.7 to 1.0%, Table 1. The conventional polyimide absorbed 1.5% water.
Clearly, the amine-quinone functional groups had very little impact on the affinity
of the amine-quinone polyimides for water.
Samples consisting of the polyimide coatings on iron were exposed to NaCl
electrolyte and the degradation of the coatings monitored by electrochemical im-
pedance spectroscopy (EIS). In Figures 4 and 5 are shown the Bode plots, i.e.
plots of impedance and phase angle as a function of frequency. The initial plot
was taken after five minutes exposure. The initial Bode plot for the BTDA/MDA
film on iron showed an impedance of 3.9 x 10 5 ·cm 2 at low frequency, Fig. 4.

The values of the phase angle were less than the -90° as one would expect if the
polymer film were acting as a dielectric. After 72 hour exposure to NaCl electro-

Ω

Figure 4. Bode plots of impedance and phase angle as a function of frequency for the conventional
polyimide (PI) film on iron. The circles are the initial curves, while the squares are the curves
obtained after 72 hours exposure to 0.1 M NaCl eletrolyte. The filled circles and squares are the
impedance curves. The open circles and squares are the phase angle curves.