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The coating behavior of a polyester−melamine paint catalyzed by an in situ phosphatizing reagent (ISPR) was investigated, and the results were compared to p-toluenesulfonic acid- (p-TSA-) catalyzed paints. Fourier transform infrared spectroscopy was used to ascertain that the reaction between the ISPR and the Al surface takes place. Differential scanning calorimetry was used to measure the glass-transition temperature (T g) and the T g thus providing an indication that the ISPR catalyzed the paint film formation. The deterioration of the coatings by overbaking was studied using thermogravimetric analysis. The corrosion resistance of the coatings on aluminum coupons was examined using electrochemical impedance spectroscopy. Equivalent electrical circuits were utilized to give numerical values to the resistors and capacitors in the coated panel. The paint film adhesion was tested by saltwater immersion. The high-quality performance of the ISPR-catalyzed paint can be attributed to the simultaneous reactions of the paint curing with the acid provided by the ISPR and the in situ phosphatization of the metal surface. Using the ISPR as the catalyst instead of p-TSA gives rise to a more corrosion-resistant film and a more thermally stable coating.

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Six different phosphorus-containing reagents were used as self-phosphating agents and acid catalysts in the formulation of a polyester-melamine coating. The phosphorus compounds chosen for the study were Cycat 4040 (4040), Cycat 296-9 (296), Albright & Wilson PA-75 (PA-75), phosphonosuccinic acid (PPSA), and two in situ phosphatizing reagents (ISPR-1 and ISPR-2, phenylphosphonic acid and fosfosal, respectively). Each individual phosphorus-containing compound was evaluated separately using a varying percentage of agent in the paint formulation. The different paints made using the different modifiers were applied to bare 3003 or 3105 aluminum panels. After the individual phosphorus-containing reagents were optimized, the optimized additives were compared to each other utilizing electrochemical impedance spectroscopy (EIS), differential scanning calorimetry (DSC), saltwater immersion, and pencil hardness tests. The most commonly used acid catalyst (4040) did not perform as well as the other acid catalysts. Using 1% phenylphosphonic acid as the acid catalyst and self-phosphating agent in a polyester-melamine coating produced the best protective barrier for the Al substrate of the coatings studied here. Using different acid catalysts in a polyester-melamine paint can dramatically alter the performance of the coating.

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