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Ind. Eng. Chem. Res. 2002, 41, 5232-5239
Effects of Catalyst and Pigment on Polyester-Melamine in Situ Phosphatizing Coating on a Cold-Rolled Steel System Mary C. Whitten Division of Science and Math, University of the Virgin Islands, St. Thomas, United States Virgin Islands 00802
Yi-Yuan Chuang and Chhiu-Tsu Lin* Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115
A pigmentless polyester-melamine coating catalyzed with p-toluenesulfonic acid (p-TSA) shows high impedance (∼1010 Ω‚cm2) at low frequency (0.1 Hz). After the addition of pigment and additives, the impedance drops to ∼105 Ω‚cm2. However, using an in situ phosphatizing reagent (ISPR) for the dual purpose of catalyzing the polyester-melamine cross-linking reaction and phosphatizing the metal surface does not show this detrimental lowering of the impedance at low frequency. Results of electrochemical impedance spectroscopy measurements and the corresponding electrical equivalent circuits show that the in situ phosphatizing coating (ISPC) applied to both bare and pretreated steel panels provides superior corrosion protection. At low frequency, the panels coated with the ISPC show 1000 times the resistance of the bare and pretreated steel panels coated with the p-TSA-catalyzed polyester-melamine paint. The simultaneous chemical process of the ISPR catalyzing desirable cross-linking reactions and forming the substrate-phosphate layer at the substrate-paint interface is the reason for the superior paint performance of ISPCs. I. Introduction Currently, the elimination of conversion coatings containing toxic elements such as Cr6+ is in progress. In preparation of removing the toxic form of chromium in metal processing, in situ phosphatizing coatings (ISPCs) have been developed1-12 and patented.4 An ISPC works by predispersing an in situ phosphatizing reagent (ISPR) into a stable and compatible paint formulation. Ideally, the ISPR will act to phosphatize the metal surface without the need of a conversion coating for the pretreatment of metal. An ISPC can be applied directly to untreated metal and provides good adhesion to the metal surface. The ISPC of polyestermelamine systems has previously shown good adhesion and corrosion-protective properties on bare 3003 Al11 and 2024 Al alloy.12 In this paper, polyester-melamine coatings are applied to untreated cold-rolled steel (CRS), phosphated steel (B-1000), and phosphated/chromated steel (B-1000 + P60). The first study conducted uses pigmentless paint with the most commonly used acid catalyst for polyestermelamine paints, according to a Cymel 303 brochure (contact Cytec Industries, www.cytec.com), p-toluenesulfonic acid (p-TSA). The pigmentless paint is applied to CRS coupons. The addition of phosphorus-containing acids including phosphoric acid, phosphonic acid, and vinylphosphonic acid to the paint system is studied. These acids are used to produce a metal-phosphate layer at the metal-paint interface to enhance coating adhesion. The resulting coatings are compared using electrochemical impedance spectroscopy (EIS) and saltwater immersion studies. * Corresponding author. Fax: (815) 753-4802. E-mail:
[email protected]. Phone: (815) 753-6861.
Next, two pigmented polyester-melamine coatings are investigated. One system is an ISPC, which is the polyester-melamine paint incorporated with and catalyzed by an ISPR. The ISPR is phenylphosphonic acid. In this system, no p-TSA is added to the paint. The ISPR has the dual purpose of catalyzing the polyestermelamine polymerization reaction and phosphatizing the metal surface in situ for protection.11,12 The second paint is the control, which is the polyester-melamine paint catalyzed by p-TSA. The two different pigmented paints are applied to bare and pretreated steel. EIS is used to evaluate the coating protective performance. The coated panels are subjected to 40 days in a 3% NaCl solution and monitored periodically using EIS after 2, 8, 20, and 40 days. Saltwater immersion is used to evaluate the corrosion inhibition performance of the two different paint systems on the steel coupons. Other methods of evaluating paints were not employed in this study. However, salt fog is one of the hardest tests for a paint to pass. Studies have shown a correlation between performance in EIS and salt fog studies.13 The EIS data are used to find electrical equivalent circuits (EECs) for the coated panels. EECs for the ISPCs and the control paints have been determined in a previous paper on Al.11,12 A comparison of the same paint on steel and Al is made. A physical interpretation is suggested for the differences. II. Experimental Section Materials. The pigmentless paint contains resin, cross-linkers, solvents, and catalyst. A stock paint was made containing the following in percent by weight. AKZO 26-1612 polyester resin (46.7%), Cymel 303 (modified melamine resin, 18.2%), n-butanol (4.0%), 2-butoxyethanol (6.0%), methyl ethyl ketone (13.6%),
10.1021/ie010684z CCC: $22.00 © 2002 American Chemical Society Published on Web 09/19/2002
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xylenes (10.0%), and Cycat 4040 (40% p-TSA in solvent, 1.5% by weight in formulation). In addition to being pigmentless, there were no additives. The stock was separated into four portions. A total of 2% of the formula weight of phosphoric acid was added to one of the four portions (i.e., if the formula weight was 100 g, 2 g of acid was added). The same was done with vinylphosphonic acid and phosphonic acid. The fourth was left alone. The four pigmentless paints created were applied on steel for a study using EIS and saltwater immersion. The coatings containing the phosphorus-containing acids plus p-TSA were compared to the coatings to which only p-TSA was added. In the saltwater immersion studies, the painted steel panels were scribed with an X using a razor. The panels were immersed in a 3% salt solution for 3 days. Upon removal, Duck brand tape was applied to the scribe on the panels and removed to evaluate adhesion. In the pigmented system, a stock paint was made with AKZO 26-1612 polyester resin (31.2%), Rheox MPA2000X (an antisettling agent, 0.8%), Cymel 303 (modified melamine resin, 12.3%), Aerosil R972 (silicon dioxide, 0.2%), Dupont Ti-pure titanium dioxide (40.5%), and solvents [n-butanol (2.3%), 2-butoxyethanol (4.9%), methyl ethyl ketone (1.0%), and xylenes (6.8%)]. This stock formulation was based on a formula given by AKZO for polyester-melamine coatings. The catalysts were added separately to these paints. There was no catalyst in the stock mixtures. The two catalysts chosen were Cycat 4040 and the ISPR, phenylphosphonic acid. Half of the stock solution had 1.5% ISPR by weight of the total formulation, and the other half used 1.5% Cycat 4040 by weight of the total formulation added as the catalyst. The panels used for the substrate were bare CRS, phosphated (B-1000), and phosphated/chromated (B-1000 + P60) panels. The bare steel surface was mechanically deoxidized using a 3 M industrial-strength scouring pad prior to painting. The pretreated panels were purchased from ACT Laboratories, Inc. The pretreated panels were gently washed with soap and water to degrease the surface. Afterward, the pretreated panels were rinsed with deionized water and ethanol. The paint was applied with a spray gun using nitrogen gas as the propellant. The panels were cured at 120 °C for 20 min. The resulting thickness of the paint was approximately 0.8 mils. The painted bare steel panels and the painted pretreated steel panels were used for EIS measurements. After the EIS studies were completed, the soaked area had Duck brand tape applied for an adhesion study. The results from EIS were used in the EEC analysis for the pigmented paint. The differences between the EEC for the paint-coated steel and previously painted Al alloys are discussed in the following. Both the steel and the Al panels had the same ISPCs and control paints applied. Instruments. The ac impedance data for polyestermelamine-coated panels were obtained using a PARC 273 potentiostat/galvanostat and a PARC 5210 lock-in amplifier (EG&G Princeton Applied Research Corp.). The experimental parameters were input, and the data were collected with the aid of EG&G electrochemical impedance software, model 398, installed in an IBMcompatible 486/50 computer. A Ag/AgCl electrode was used for the reference electrode, a platinum electrode was used for the counter electrode, and the coated panel was the working electrode. The coated panel had an area of 10.0 cm2 exposed to the salt solution. The electrolyte
Figure 1. Pigmentless polyester-melamine with (a) p-TSA, (b) p-TSA and 2% phosphoric acid, (c) p-TSA and 2% vinylphosphonic acid, and (d) p-TSA and 2% phosphonic acid.
used was a 3% (w/w) NaCl aqueous solution. The impedance measurements were carried out over the frequency range of 100 kHz to 10 mHz, with a 5 mV peak-to-peak sinusoidal voltage in the high-frequency range. A multisine technique was used at lower frequencies down to 0.01 Hz with an applied voltage of (10 mV. The coated panels were soaked for 2 days in the 3% NaCl solution, allowing for complete swelling of the polymer film prior to taking the EIS spectra. The spectra were further monitored at 8, 20, and 40 days. EEC data were acquired with the aid of the computer program EQUIVCRT.PAS, version 4.51, written by Bernard A. Boukamp, University of Twente, Twente, The Netherlands. III. Results and Discussion A. Investigation of Pigmentless Paint. Figure 1 shows the Bode plot for the pigmentless polyestermelamine coating catalyzed with p-TSA (curve a) and with an additional 2% of the following acids: phosphoric acid (curve b), vinylphosphonic acid (curve c), and phosphonic acid (curve d). The spectra were taken after being immersed in a 3% salt solution for 3 days in order to allow swelling of the polymer film. The higher frequency portion of the curve is attributed to the dielectric properties of the coating.14 All of the coatings in the figure have a slope of -1 at high frequency, indicative of a pure capacitor. Therefore, all of the coatings showed good dielectric properties. At low frequencies (0.01 Hz), the maximum impedance is an indication of the protective barrier the coating can provide. A coating with a maximum impedance at low frequency of 109 Ω‚cm2 or higher is indicative of a good protective barrier.15 A coating that gives a maximum impedance between 107 and 109 Ω‚cm2 is a mediocre barrier, while a maximum impedance of less than 107 Ω‚cm2 is suggestive of a poor protective barrier.15 The coatings made without pigment using p-TSA only and with additional phosphoric acid and vinylphosphonic acid were considered to provide good protective barriers because the impedance at low frequency was on the order of 109 Ω‚cm2 or higher. The paint with p-TSA and 2% phosphonic acid was on the order of 108 Ω‚cm2, which indicates a mediocre barrier. The impedance at low frequency increased in the order of 2% phosphonic acid < 2% vinylphosphonic acid < 2% phosphoric acid < p-TSA alone with no phosphorus-containing acids added. The addition of 2% phosphorus-containing acids
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Figure 2. Bode magnitude plots for the ISPC and the control on steel after 2 days of immersion in a 3% NaCl solution: (a) ISPC/bare steel; (b) ISPC/phosphated steel; (c) ISPC/phosphated-chromated steel; (d) control/phosphated steel; (e) control/bare steel; (f) control/ phosphated-chromated steel.
in this optimal formulation resulted in an excessive amount of acids. The results of Figure 1 indicate that the polyester-melamine clear paint with p-TSA is an optimal formulation. The excess acids may have become entrapped in the paint film and caused a negative effect in protective performance. The pigmentless polyestermelamine paint without any additives showed a very impressive impedance spectrum (Figure 1a). This coating practically behaved as a pure capacitor throughout the entire frequency range studied because it maintained a slope of -1. The painted panels were scribed with an X and placed in a salt solution for 3 days. The saltwater immersion studies show that the control paint catalyzed by p-TSA does not perform as well as the polyester-melamine paints that contained the phosphorus-containing acids. Tape applied to the “X” scribe after 3 days of immersion in a salt solution shows that all of the paint is removed from the paint when p-TSA alone is used. The paint systems with 2% vinylphosphonic acid and 2% phosphonic acid has
