- freely available
Coatings 2019, 9(4), 254; https://doi.org/10.3390/coatings9040254
2. Materials and Methods
2.1. Method of Epoxy Paint Preparation
2.2. Electrochemical and Thermo-Mechanical Measurements
4. Results and Discussion
4.1. EIS as a Tool for Experimental Detection of PVC-CPVC Transition
4.2. Monitoring of the Coating Film Formation by EIS Measurements (Versus PVC)
4.3. DMTA as a Supporting Technique for Explanation of EIS Results
- We showed that the magnitude (modulus) of impedance at frequencies below circa 100 Hz changes dramatically (for several orders of magnitude) when the pigment volume concentration (PVC) is increased from slightly below to slightly above its critical value (CPVC). The reason is the transformation of coating from a continuous to a discontinuous layer that greatly enhances the penetration of electrolyte to the metallic substrate. Thus, EIS seems a very appropriate technique for accurate detection of practical CPVC value of a given coating, a crucial parameter related to coating protection efficiency.
- Due to reasons similar to those stated above, impedance spectroscopy was also found to be a suitable technique for monitoring the coating film formation in samples with different PVC values. It was shown that—throughout the entire time of film formation—the most indicative impedance parameters of conventional corrosion equivalent circuits (pore resistance, coating capacity, Warburg coefficient) have distinctly different values below and above CPVC. Additionally, temporal evolution of impedance parameters was shown to be different in coatings with PVC above and below the critical value.
- Additional insight into the differences in film formation mechanisms of coatings with different PVC values was obtained using dynamic mechanical thermal analysis (DTMA). The effects of PVC on water evaporation and polymer cross-linking process during film formation were discussed in considerable detail. The effect of curing temperature on these processes was elucidated as well.
- The results indicate that, in order to further increase the coating protection of waterborne epoxy coatings, one needs to find the best compromise between the so-called “barrier effect” and the degree of epoxy-amino cross-linking. Both are decisively affected by the PVC level, however, in different ways. In order to achieve a fine tuning of PVC near the CVPC value, one might want to exploit the very high sensitivity of impedance spectroscopy, as demonstrated in this paper.
Conflicts of Interest
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|Sample PVC [%]|
|p/b Ratio||Dry Film Thickness on Steel Substrates (µm) after 1 Day of Coating Application||Standard Deviation of Dry Film Thickness (µm)|
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