Ind. Eng. Chem. Res. 1999, 38, 3903-3910
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Coating Performance of Polyester-Melamine Enamels Catalyzed by an in Situ Phosphatizing Reagent on Aluminum Mary C. Whitten and Chhiu-Tsu Lin* Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862
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- (pTSA-) 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 (Tg) and the Tg 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. I. Introduction The current practice of applying state-of-the-art organic coatings to metal substrates is a multistep process. Normally, the metal surface is cleaned, phosphated or chromated, possibly sealed (with hot water or carcinogenic chromates),1,2 dried, and finally painted. The pretreatment process is error-prone and costly. However, pretreatment is a necessary feature of the metalfinishing industry.3 Unfortunately, multistep-coating technologies produce wastes including organic solvents, heavy metals, and other toxic and deleterious materials.4 Recently, in situ phosphatizing coatings (ISPCs) have been developed in our laboratory5-14 and patented.8 In an ISPC, an optimum amount of an in situ phosphatizing reagent (ISPR) is predispersed in the paint system to form a stable and compatible coating formulation.5 When a single coat of the in situ self-phosphating paint is applied to a metal substrate, the phosphatizing reagent chemically and/or physically reacts in situ with the metal surface to produce a metal phosphate layer and, simultaneously, forms covalent phosphorusoxygen-carbon (P-O-C) linkages with the polymer resin.7 Sealing the pores of the metal phosphate layer generated at the metal paint interface by covalent P-O-C linkages enhances the coating’s adhesion and suppresses substrate corrosion without the use of the toxic form of chromium (Cr6+). The predispersed ISPRs in ISPCs are acidic by nature. The viability of the use of ISPRs as a catalyst for the thermoset polymerization reactions in polyestermelamine coatings is questioned. Polyester-melamine paints are normally catalyzed by acids. p-Toluenesulfonic acid (p-TSA) is the most widely used catalyst * Corresponding author. Fax: (815) 753-4802. E-mail:
[email protected]. Phone: (815) 753-6861. Present address: Dept. of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115.
for polyester-melamine paint systems since its molecular structure and hydrophobic nature are more compatible with the paint components. The possible reaction pathways that the acid catalyzes in paints are numerous.15 The most popularly studied reaction mechanisms are the co-condensation reaction between polyester polyols and melamine and the self-condensation reaction of melamines. It is possible that the selection of catalyst could have an impact on the resulting coating’s characteristics.15 Therefore, the use of p-TSA as a catalyst could contribute to a less stable paint film which could lead to easier diffusion of electrolytes and corrosion of the metal substrate. So far, research on ISPCs has been conducted on ferrous substrates. In this paper, the focus is on the coating behavior of an ISPR-catalyzed paint applied on an aluminum substrate. The polyester-melamine system chosen for study needs an acid catalyst to cure the paint. The ISPR in the polyester-melamine ISPC system has two functions. First, the acidic nature of the ISPR catalyzes the cross-linking reaction between the polyester polyol and the melamine. Second, the ISPR’s ability to react with the metal surface provides a protective metal-phosphate layer. II. Experimental Section Materials. A 3003 Al panel was polished to a mirror finish. One percent by weight of the ISPR was dissolved in the solvents used in the paint formulation (methyl ethyl ketone, xylenes, butanol, and 2-butoxyethanol). The ISPR employed for the formulation of the ISPC was an aryl phosphonic acid.8 The ISPR solution was applied to the polished Al and baked at 120 °C for 20 min. After the phosphated panel cooled to room temperature, it was rinsed with deionized water, acetone, and ethanol to ensure that the ISPR had chemically reacted with the surface of the polished Al, forming a metal-phosphate layer. The metal-phosphate layer formed on the pol-
10.1021/ie990231i CCC: $18.00 © 1999 American Chemical Society Published on Web 09/15/1999
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ished Al was used to acquire the reflectance Fourier transform infrared (FTIR) data. The polyester-melamine paint system included AKZO resin 26-1612, RheoxsM-P-A 2000X, Cymel 303, Aerosil R972, Dupont Ti-pure titanium dioxide, and solvents (n-butanol, 2-butoxyethanol, methyl ethyl ketone, and xylenes). The amount of ingredients used was governed by the manufacturer’s suggestion supplied with the AKZO resin 26-1612 supply. The two catalysts which were employed were p-TSA (Cycat 4040 catalyst which was 40% p-TSA) and the ISPR. Six paint systems were studied. A stock formula of paint was made without any catalyst added. The stock contained all the ingredients for the paint except for the catalyst. 1%, 2%, and 3% by weight of the ISPR/catalyst was added to the stock to form three of the six paints, and 1%, 2%, and 3% by weight of Cycat 4040 was added to the stock to form three more paint systems, making a total of six paint systems. The manufacturer recommended approximately 1% of the Cycat 4040 by weight in comparison to the total weight of the paint. One gram of the ISPR has approximately 2.7 times the moles of acid when compared to 1 g of Cycat 4040. More moles of the ISPR were required to optimize the catalyzing of the paint, since the ISPR acts as an initiator of the polymer chemistry and also phosphatizes the Al surface. Assuming the wet thickness of the paint applied to a 3 × 5 in. panel was 3 mils (1 mil ) 25 µm), 1% ISPR results in 0.8 g of ISPR/m2; 2% ISPR was 1.6 g of ISPR/m2 and 3 % ISPR was 2.4 g of ISPR/m2. The panels used for the coating substrate were aluminum alloy (The Q panel company, mill finish 3003 H14 aluminum panels, 0.02 × 3 × 5 in). The Al panels were spray painted with the six different paint systems. The painted panels were flashed for approximately 30 min prior to curing at 120 °C for 20 min. The cure temperature and time were recommended by the producers of AKZO resin 26-1612’s recipe. The painted panels were used for electrochemical impedance spectroscopy (EIS), salt water immersion, and pencil hardness tests. The cured paint on these panels was scraped off with a razor and used for differential scanning calorimetry (DSC) analysis. The samples for thermogravimetric (TG) measurements were prepared by placing a drop of the paint in a TG aluminum sample pan. The paint was cured at 120 °C for 20 min in the sample container in a regular oven. Instruments. The FTIR reflectance spectrum of the metal-phosphate on polished 3003 Al generated by the ISPR was taken with a Bruker Vector 22 equipped with a Spectra Tech FT-80 grazing angle accessory. The interferometer was purged with nitrogen. The FTIR was controlled by a computer program, OPUS/IR version 2.2. The background spectrum was bare polished Al with no ISPR added. The frequency characteristics of the source, spectrometer, and unwanted matrix absorptions were eliminated with the background spectrum. The calculation for the elimination is automatically performed after the sample is run by dividing the sample spectrum by the background spectrum. The glass-transition temperatures (Tg) and Tg span (i.e., T2 [glass-transition ending temperature] - T1 [glass-transition starting temperature]) were measured using differential scanning calorimetry (DSC) (Seiko Scientific Instruments, Inc. model DSC220C). The polymer degradation was determined using thermogravimetric analysis (TGA) (Seiko Scientific Instruments,
Inc., model TGA/DTA320). The instrumentation was equipped with a Seiko SSC5200H disc station for scanning program manipulation and data analysis. Colder temperatures were obtained with the aid of a liquid nitrogen autocooling unit. The sample specimens (10 mg of cured paint) were contained in weighed aluminum pans, which were then closed. The sample chambers were purged with nitrogen gas. The specimens were subjected to temperature scans from -50 to 300 °C at a heating rate of 10 °C/min. Each sample was scanned from -50 to 300 °C twice to observe the changes from deterioration of the paint film when subjected to overbaking. To erase the thermal history and release the stress resulting from the paint’s mechanical peeling from the Al coupon, the DSC samples were first annealed at 80 °C, a temperature above the Tg of the film, but well below the curing temperature of 120 °C. The preannealing process allows the polymer backbone to relax and release molecular stress, allowing for a more accurate determination of the Tg.16 It was not necessary to anneal the samples a second time for the sample’s second scan to 300 °C. Alternating current (ac) impedance data for polyestermelamine-coated Al 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 IBM compatible 486/50 computer. A Ag/AgCl electrode was used for the reference, a platinum electrode was used for the counter, and the painted Al panel was the working electrode. The electrolyte used was a 3% NaCl aqueous solution. The impedance measurements were carried out over the frequency range of 100 kHz to 0.01 Hz, with a 5-mV peak to peak sinusoidal voltage. The multisine technique was used at lower frequencies. The coated panels were soaked for 3 days in a 3% NaCl solution prior to the analysis. The equivalent circuits for the impedance data collected were determined with the aid of the computer program EQUIVCRT.PAS version 4.51 written by Bernard A. Boukamp, University of Twente, The Netherlands.17 Saltwater immersion studies were carried out by submerging the painted Al panels in a 3% NaCl solution for 6 days. The panels were scribed with an “X” in the cured paint film using a razor to expose the base metal to aid in the corrosion process. Upon removal from the salt solution, the panels were dried, and a DUCK brand tape was firmly pressed against the scribed area and pulled to remove. The paints were evaluated using the American Society for Testing and Materials (ASTM) method D3359 method A.18 The pencil hardness of the painted Al panels was measured using ASTM method D3363.18 The pencil lead was sanded flat with sand paper. The pencil was held at a 45° angle from the painted metal and pushed on the substrate. After a pencil hardness was reached which cut through the coating to expose the metal surface, the previous pencil hardness was the measured pencil hardness. III. Results and Discussion A. Verification of the ISPR’s Ability To Bond with Al; FTIR of the Aluminum-Phosphate Product. Figure 1 shows the FTIR reflectance spectra of the metal-phosphate layer on polished 3003 Al generated
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3905 Table 1. Tg and Tg Span of the Six Different Polyester-Melamine Paint Systems
Figure 1. FTIR reflectance spectra of the metal-phosphate layer generated by the ISPR on polished 3003 Al.
Figure 2. DSC scans for the six paint systems showing Tg for the first time the samples were ramped to 300 °C. (a) 1% ISPR, (b) 2% ISPR, (c) 3% ISPR, (d) 1% Cycat 4040, (e) 2% Cycat 4040, and (f) 3% Cycat 4040.
by the ISPR. The peaks at 1230/1196/1166 cm-1 are attributed to the PdO absorption band19 or υ3.10 The peaks at 603/565 cm-1 are due to O-P-O absorption bands19 or υ4.10 The splitting of the degeneracy of υ3 and υ4 absorption bands is an indication that the metalphosphate layer produced on the Al surface is possibly crystalline.20 Confirmation of a crystalline surface requires further analysis using XRF/XRD. The other peaks are from C-H out-of-plane and in-plane bending.19 When the FTIR spectra of the metal-phosphate layer produced on a polished steel substrate are compared to the spectra on the polished Al substrate, the PdO absorption bands on the Al substrate are higher in frequency. The steel-phosphate layer shows υ3 absorption bands at 1195/1147/1105 cm-1,10 while the Al-phosphate layer’s absorption band is at 1230/1196/ 1166 cm-1. This shift to higher frequencies can be attributed to a higher bond order of the P-O bond due to the larger ionicity of the Al-O bond compared to the Fe-O bonds.20 The appearance of the peaks in the FTIR spectrum of the ISPR on the polished 3003 Al shows that a phosphate layer has been produced on the surface of the Al metal. Furthermore, since the phosphate layer was not removed when the sample was rinsed with deionized water, acetone, and ethanol, the ISPR has bonded with the surface. B. Thermal Analysis of Cured Paint Films. The paint samples were run in duplicate. Figure 2 shows one of the six paint system’s DSC scans for the first time the previously cured paint film was ramped to 300 °C.
% catalyst
Tg (scan 1)
Tg span (1)
Tg (scan 2)
Tg span (2)
1% ISPR 2% ISPR 3% ISPR 1% Cycat 4040 2% Cycat 4040 3% Cycat 4040
22.2 37.5 41.5 39.5 34.3 41.1
16.5 23.3 31.0 26.0 25.2 28.7
64.9 53.6 34.7 33.9 21.5 20.3
34.7 48.2 26.8 25.8 23.9 16.8
The onset and offset temperatures for the determination of the glass-transition temperature were determined by crossing points on the DSC curve of the base lines for the glassy and elastic states and the tangential line for the transition state. The Tg was designated as the midtemperature between the two. An average for the duplicate values measured for each paint system for Tgs and Tg spans are reported in Table 1 in °C. The markedly different behavioral trends that the different catalysts exhibit should be noted. Under the heading “Tg (scan 1)” is the average Tg for the cured paint. It is obvious that on increasing the amount of the ISPR/catalyst, the Tg increases (1% ISPR [22.2 °C], 2% ISPR [37.5 °C], and 3% ISPR [41.5 °C]). A higher Tg indicates a higher average molecular weight or cross-linking density of the cured coating’s polymers. The Tg span also increases with an increasing amount of the ISPR, as can be seen under the heading “Tg span (1)” (1% ISPR [16.5 °C], 2% ISPR [23.3 °C], and 3% ISPR [31.0 °C]). A larger value for the Tg span suggests a broader distribution of cross-linkings in the polymer network. A trend in the amount of p-TSA (as Cycat 4040) added is not as easily observable. The Tgs are 1% Cycat 4040 [39.5 °C], 2% Cycat 4040 [34.3 °C], and 3% Cycat 4040 [41.1 °C]. The Tg spans are 1% Cycat 4040 [26.0 °C], 2% Cycat 4040 [25.2 °C], and 3% Cycat 4040 [28.7 °C]. The manufacturer’s recommended amount of Cycat 4040 results in a Tg of 39.5 °C. When the Tg of the paint systems cured with the ISPR is observed, the ISPR as a catalyst provides a cured paint film with an appropriate cross-linking density that is suitable for the polyester-melamine paint. More interesting results are given under the headings “Tg (scan 2)” and “Tg span (2)”. These values are obtained when the same samples which were used to determine Tg (scan1) and Tg span (1) are re-scanned to a temperature of 300 °C. Therefore, the sample has previously been exposed to extreme temperatures after already having been to 300 °C in the first scan. It is extremely interesting to note that the Tg increases in the paints with 1% and 2% ISPR as the catalyst. This means that upon severe heating of the coating the average molecular weight of the polymers increased. The molecular weight of the polymers in a paint film on an Al coupon is derived from the behavior of the Tg measurement. The Tg value is generally related to the cross-linking density of a paint film, and thus correlated to the molecular weight of the resultant polymer film.21 In the 1% ISPR coating, the Tg increased from 22.2 to 64.9 °C, and from 37.5 to 53.6 °C in the 2% ISPR coating. In the 3% ISPR paint formula, the Tg decreases from 41.5 to 34.7 °C, indicating that 3% ISPR is enough catalyst to end the trend of an increased Tg after exposure to high heat. The observed thermal trends using the various percentages of ISPRs as the catalyst were reproducible. The Tg in the p-TSA-catalyzed paint
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Figure 3. TGA scans for (a) 3% ISPR and (b) 3% Cycat 4040.
systems decreases in all the paint systems, as seen in Table 1 (1% Cycat 4040 [39.5-33.9 °C], 2% Cycat 4040 [34.3-21.5 °C], 3% Cycat 4040 [41.1-20.3 °C]). A decrease in the Tg temperature upon heating shows that thermal decomposition occurred with the use of p-TSA as the catalyst since the average molecular weight of the polymer decreased. The greater thermal decomposition or the forming of smaller molecular weight segments of the paints with p-TSA as the catalyst was confirmed using TGA. On the average, the cured paints with the ISPR as the catalyst maintain approximately 94% of their weight when brought to 300 °C. The p-TSAcatalyzed paints maintain approximately 90% of their original weight. Figure 3 shows an example of TGA scans for the cured paint films with 3% ISPR-catalyzed paint and 3% Cycat 4040-catalyzed paint. More thermal decomposition occurs when using p-TSA as the catalyst. After thermal analysis, all the paint systems displayed a color change from ivory to beige, owing to some thermally activated chemical processes. There are many components (resins, cross-linkers, solvents, pigments, catalysts, additives, etc.) which are added to a paint system to optimize the final product. Each component could affect the polymer chemistry of the cured paint film. Until the detailed chemical mechanism that occurs during the entire baking process is understood, the reason why the 1% and 2% ISPRcatalyzed paint is more thermally stable will not be fully known. However, three categories of polymerization reactions have been proposed.15 They are (1) reactions that form new covalent bonds and increase cross-link densities, (2) pairs of reversible reactions in which covalent bonds are broken, but new ones formed, and (3) reactions which break bonds irreversibly.15 In the first category, co-condensation reactions between the polyester polyol and the melamine and self-condensation of the melamines would be considered reactions that form new covalent bonds that increase the cross-linking density.15 In the second category, reversible bond breakage reactions would be alcoholysis of ether groups, hydrolysis of ether groups, and trans-etherification.15 These reactions are reversible, so a reaction from the first category could follow. The third category contains reactions that are irreversible or not readily reversible, such as alcoholysis of ester groups, hydrolysis of ester groups, and melamine decomposition.15 The cross-linking density would decrease as a result of any of the
Figure 4. For 1% ISPR-catalyzed paint. (a) Nyquist plot: experimental data (×); simulated data (0). (b) Bode plot, y-axis scale on the left: experimental data (×); simulated data (0). Phase angle diagram, y-axis on the right: experimental data (+); simulated data (4). (c) Equivalent circuit.
reactions in the third category. For example, the crosslinking density decreased upon severe heat when Cycat 4040 was used as the catalyst. Therefore, it is suggested that the ISPR can selectively promote the reactions involved in the first and second categories and suppress the undesirable reactions involved in the third category. C. Electrochemical Impedance Spectroscopy/ Equivalent Circuit Analysis. The electrochemical impedance spectra of the coated aluminum coupons are obtained through EIS. The observed spectra with the equivalent circuit that overlapped for three of the paint systems are shown in Figures 4-6. The Nyquist plot, Bode-magnitude plot, Bode-phase diagram, and equivalent circuit for the 1% ISPR-catalyzed paint are shown in Figure 4; 3% ISPR-catalyzed paint plots and the equivalent circuit are shown in Figure 5; and 3% Cycat 4040-catalyzed paint plots and the equivalent circuit are shown in Figure 6. The three chosen spectra are examples of the three different types of circuits which the polyester-melamine paints on aluminum substrate exhibited. For the Nyquist plots (imaginary impedance vs real impedance; Figures 4a, 5a, and 6a), the actual data is the “×” and the simulation using the EQUIVCRT.PAS program is the “0”. The Bode-magnitude plot (frequency vs impedance; Figures 4b, 5b, and 6b) and the Bodephase diagram are shown in the same plot. The actual data for the Bode-magnitude plot are represented by the “×” while the simulation by the 0. The y-axis scale for the Bode-magnitude plot is on the left side. The actual data for the Bode-phase diagram (frequency vs phase angle) is represented by the “+” while the simulation by the “4 ”. The y-axis scale for the Bode-phase diagram is on the right.
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3907
Figure 5. For 3% ISPR-catalyzed paint. (a) Nyquist plot: experimental data (×); simulated data (0). (b) Bode plot, y-axis scale on the left: experimental data (×); simulated data (0). Phase angle diagram, y-axis on the right: experimental data (+); simulated data (4). (c) Equivalent circuit.
film on a metal panel.23 One of the more popular models has been chosen for the analysis of the organic paint film which was catalyzed by 1% ISPR and applied to an aluminum substrate. The equivalent electrical circuit model22 is displayed in Figure 4c where the circuit elements are as follows: RΩ is the solution resistance, the resistance of the electrolyte solution between the coated panel and the reference electrode and the electrical leads; Cc is the coating capacitance, the capacitance at the electrolyte-coating interface; Cd is the doublelayer capacitance or pseudo-capacitance at the coating metal-interface; Rp is the resistance of the polymer film;24 and Rt is the transfer resistance at the coatingmetal interface.22 The EIS curve for the ISPR-catalyzed paint with 2% ISPR is similar to the 3% ISPR-catalyzed paint which is shown in Figure 5a,b. The equivalent circuit for 2% and 3% ISPR-catalyzed paint is seen in Figure 5c. When the equivalent circuit in Figure 5c is compared to that in Figure 4c, the Rp and Cd have been replaced by Warburg impedance, which has been described.25 The Warburg impedance in Figure 5c is attributed to charge-transfer activity through the paint film instead of corrosion at the metal surface since no corrosion was evident on the panel and the adhesion was acceptable as will be illustrated later. The best fit computer-simulated equivalent circuit for EIS spectra of polyester-melamine coatings with p-TSA as the catalyst could not conform to the popular circuit displayed in Figure 4c either. Therefore, the circuits for the p-TSA-catalyzed paints have an extra resistor and an extra capacitor as shown in Figure 6c. Cd and Rt in Figure 4c are generally attributed to the electrochemical reactions and impedance at the metal/coating interface. It is speculated that the aluminum oxide barrier on the aluminum substrate could have a large enough electrochemical effect to cause the extra capacitor and resistor on the equivalent circuit for the p-TSA-catalyzed coatings (Figure 6c). This extra resistor and capacitor could also be present in the ISPR-catalyzed paint, but the value is so small in comparison to the other resistors and capacitors that it is undetectable. The larger values for the resistors and capacitors in the ISPR-catalyzed paints is attributed to the in situ phosphatization of the metal surface and better film properties achieved with the ISPR as the catalyst which cannot be done with p-TSA as the catalyst. The extra resistor and capacitor in Figure 6c are labeled as Rom and Com for the resistance and capacitance at the oxide/metal interface. All the values for resistance are in ohms, and the values for capacitance are in farads. The values for the electrical components of the equivalent circuits for all six paint systems are listed in Table 2. In the equivalent circuit model, the frequency dependence of a constant phase element (CPE) has a value Q which can be described mathematically17 as
Q ) j/[Y(ω)n] Figure 6. For 3% Cycat 4040-catalyzed paint. (a) Nyquist plot: experimental data (×); simulated data (0). (b) Bode plot, y-axis scale on the left: experimental data (×); simulated data (0). Phase angle diagram, y-axis on the right: experimental data (+); simulated data (4). (c) Equivalent circuit.
The circuit elements in an equivalent circuit represent the various macroscopic processes of an electrically active system.22 There are several models which have been accepted as a feasible circuit for an organic paint
where ω (frequency in radians per second) ) 2πf (f is the frequency in hertz). When n ) 0, the value of the CPE remains the same, regardless of the frequency. Therefore, the CPE is a resistor with R ) 1/Y, and Y is the admittance. The imaginary unit j (which is the square root of -1) can be removed since it is only a bookkeeping element for balancing the mathematical manipulations. A resistor is a “real” value so the j is unnecessary. When n ) 1, the value of Q changes with
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Table 2. Values of Resistors and Capacitors for the Equivalent Circuits of the Six Paint Systems Studied RΩ Rp Rt Rom Rtot Cc Cd W Com
1% ISPR
2% ISPR
3% ISPR
1% Cycat
2% Cycat
3% Cycat
-859.2 1.676 × 108 2.561 × 1010 N/A 2.578 × 1010 4.971 × 10-10 1.535 × 10-10 N/A N/A
-961.5 N/A 2.508 × 1010 N/A 2.508 × 1010 3.673 × 10-10 N/A 2.521 × 10-10 N/A
-897.6 N/A 2.470 × 109 N/A 2.470 × 109 6.681 × 10-10 N/A 1.036 × 10-9 N/A
-912.7 3.919 × 105 5.473 × 105 3.377 × 106 4.315 × 106 2.273 × 10-9 1.757 × 10-8 N/A 3.235 × 10-6
-1743 3.421 × 105 5.118 × 105 6.399 × 106 7.251 × 106 4.411 × 10-9 1.823 × 10-8 N/A 3.519 × 10-6
-1415 8.217 × 104 8.953 × 104 2.853 × 106 3.023 × 106 7.138 × 10-9 2.289 × 10-8 N/A 3.924 × 10-6
frequency. The impedance contributed from a capacitor is defined as j/(ωC), where Q is the value resulting from a capacitor with C ) Y. When n ) 0.5, the resulting resistance is from Warburg impedance.17 The values of n which are in between the above values listed are simply called CPEs. The results for the best fit using the EQUIVCRT.PAS program did not give values of n ) 1 and n ) 0.5. However, for simplicity, if n was >0.75, then the value is referred to as a capacitor in Table 2. The majority of the capacitor values listed in Table 2 have a value of n greater than 0.9. The resistances reported are true resistors. The value of n for the two Warburg impedances are 0.51 and 0.43 for 2% and 3% ISPR-catalyzed paints, respectively. The program does allow a value of n to be forced. The resulting values for the capacitors when n is forced to be 1 have the same magnitude. However, the simulation plots do not fit the experimental data points as well. Viewing Table 2, the film resistance of the cured paint is largest for the 1% ISPR-catalyzed paint which is 1.676 × 108. The resistance of the 1% ISPR paint film is 2-3 orders of magnitude more resistant than any of the p-TSA-catalyzed paint systems. The Rp for the p-TSAcatalyzed paint is 3.919 × 105, 3.421 × 105, and 8.217 × 104 for 1%, 2%, and 3% Cycat 4040, respectively. The results suggest that 1% ISPR-catalyzed paints have less surface defects and offer better metal surface protection. Inhibition of oxygen and water transport by organic coatings is not the only key to corrosion protection. A decrease in the electrical transport between anodic and cathodic sites on the metal surface is more important.26 The transfer resistance for 1%, 2%, and 3% ISPRcatalyzed polyester-melamine paints coated on a 3003 Al panel are 2.561 × 1010, 2.508 × 1010, and 2.470 × 109, respectively. The resistance to transfer of charge from the coating to the aluminum substrate is greatest for the 1% ISPR-catalyzed paint and decreases with increasing amounts of ISPR dispersed in the paint system. The Rt for the p-TSA-catalyzed paint systems is much lower in comparison to that of the ISPRcatalyzed paints. The 1%, 2%, and 3% Cycat 4040catalyzed paints have a resistance to transfer of charge to the oxide layer on the aluminum panel of 5.473 × 105, 5.118 × 105, and 8.953 × 104, respectively. The transfer of charge is contributed to a coating/oxide transfer since the Cycat 4040-catalyzed paint systems have an extra resistor and capacitor which is presumed to be a result of the oxide/metal interface. The observed Rt in Table 2 is approximately 4 orders of magnitude higher in the ISPR-catalyzed paints than in the Cycat 4040-catalyzed paints. Rt is also referred to as the Faradaic corrosion resistance.22 Therefore, the magnitude of the higher Rt for the ISPR-catalyzed paint is extremely significant since it pertains to the corrosion resistance of the aluminum panel. This high value of resistance suggests that the ISPR-catalyzed paint is
10 000 times more resistant to metal surface corrosion than the p-TSA-catalyzed paints. In ISPR-catalyzed paints, the in situ phosphatization of the metal surface and the supposed suppression of undesirable curing reactions is presumably responsible for the increase to resistance from charge transfer. Rom is the presumed resistance at the oxide/metal interface. The value for Rom is measured only for the p-TSA-catalyzed paints. The value is similar in all three paint systems. For 1%, 2%, and 3% Cycat 4040, Rom is 3.377 × 106, 6.399 × 106, and 2.853 × 106, respectively. The total resistance for 1%, 2%, and 3% ISPRcatalyzed coatings is 2.578 × 1010, 2.508 × 1010, and 2.470 × 109, respectively. The total resistance is approximately 1000 times more than the total resistance for the p-TSA-catalyzed paint systems. 1%, 2%, and 3% Cycat 4040 has a total resistance of 4.315 × 106, 7.251 × 106, and 3.023 × 106, respectively. It is commonly acknowledged that a coating with a total resistance above 108 will have good protective properties, a total resistance between 106 and 108 will have moderate protection, and a total resistance less than 106 will have poor protection.22 Clearly, ISPR-catalyzed polyestermelamine paints on aluminum show excellent protection while the Cycat 4040-catalyzed paints on aluminum are borderline poor. The coating capacitance (Cc) for 1%, 2%, and 3% ISPR in the paint is 4.971 × 10-10, 3.673 × 10-10, and 6.681 × 10-10, respectively. The values are approximately 10 times smaller than the Cc values for the p-TSAcatalyzed paint systems. Therefore, the ISPR-catalyzed coatings can hold 10 times more charge at the surface of the coating before allowing it to pass through. Similarly, the double-layer capacitance (Cd) for the 1% ISPR-catalyzed paint system is approximately 100 times smaller than the Cd of the p-TSA-catalyzed paints. The 1% ISPR-catalyzed coating can hold 100 times the charge before allowing the charge to pass on to the substrate. The 2% and 3% ISPR-catalyzed paints do not have Rp and Cd as readily noticeable as in the other paint systems. However, Warburg impedance has been described as a resistor and capacitor in series which are both frequency-dependent. Furthermore, they depend on the frequency symbiotically, resulting in a constant phase angle of -π/4 between the current and the applied potential.24 Possibly, the excess amount of the ISPR entrapped in the paint film of the 2% and 3% ISPRcatalyzed paints aide diffusion through the coating layer. The 2% and 3% ISPR-catalyzed paint have Warburg impedances of 2.521 × 10-10 and 1.036 × 10-9. The entrapped ISPR seems to reduce the charge-holding power of the ISPR-catalyzed paint films. It is suspected that the portion of the p-TSA which is not used for the acid catalysis remains in the paint film after curing. The remaining p-TSA in the cured coating
Ind. Eng. Chem. Res., Vol. 38, No. 10, 1999 3909 Table 3. Pencil Hardness Results for the Six Paint Systems Catalyst
Pencil Hardness
1% ISPR 2% ISPR 3% ISPR 1% Cycat 4040 2% Cycat 4040 3% Cycat 4040
F 2H H-2H F H-2H 2H
the paint’s adhesiveness to the substrate. After the painted Al panels were immersed in a 3% NaCl solution for 6 days, a pressure-sensitive tape test was performed, and the picture is shown in Figure 7. The 1% ISPR-catalyzed paint exhibits excellent adhesion to the aluminum substrate. The test results conform to 5A according to ASTM method D3359 test method A.18 5A classifies paints which have no peeling or removal of the paint from the substrate. The 2% and 3% ISPR-catalyzed paints have approximately 0.5 mm removed from each side of the incision. They are classified as 4A, trace peeling or removal along incision. After the tape removal from the 1% Cycat 4040catalyzed paint, paint detaches beyond the area of the tape placement. Therefore, 1% Cycat 4040-catalyzed paint is classified as 0A, removal of paint beyond taped area. The 2% and 3% Cycat 4040-catalyzed paints detached from the substrate only where the tape was applied. Therefore, these paints are classified as 1A, removal of paint from the area under the tape. The ISPR-catalyzed paints indisputably outperform the p-TSA-catalyzed paints with their adhesive strength toward the substrate. This is attributed to the ISPR’s ability to phosphatize the metal substrate in situ. The acid nature of the ISPR is able to catalyze the thermal curing of the paint film, while the phosphatizing functionality of the ISPR can migrate to the surface of the substrate, produce a metal phosphate layer, and chemically bond to the metal, sealing up pores and pits which generally allow electrolytes through to corrode the metal. E. Pencil Hardness. The manufacturer’s paint description provided by AKZO Resins includes a pencil hardness of F-H for AKZO resin 26-1612. Table 3 shows the results of the pencil hardness test conducted on the six different paints studied. The pencil hardness in both systems with 1% catalyst is F, which is softer than the paints with the higher levels of catalyst. The 2% and 3% catalyst paints are harder with pencil hardness results of H and 2H. Overall, the differences in pencil hardness are insignificant. IV. Conclusion Figure 7. Painted Al panels subjected to the saltwater immersion test and followed by the tape test. The paint used the following catalyst: (a) 1% Cycat 4040, (b) 1% ISPR, (c) 2% Cycat 4040, (d) 2% ISPR, (e) 3% Cycat 4040, and (f) 3% ISPR.
provides no useful purpose. Therefore, a flaw in the coating is more easily attained since the coating is not as stable. However, the ISPR plays an active role during the curing process. The ISPR can react with the metal surface and bond with the polymer, creating a more stable film. D. Saltwater Immersion Test. The saltwater immersion test is a very good qualitative test to determine
Further studies to examine the shelf life of the ISPC are in order along with more extensive corrosion studies including condensing humidity, blister box, exterior exposures, etc. However, ISPRs can selectively catalyze the cross-linking reactions necessary for curing polyester-melamine paint systems and create a more corrosion-resistant film. Simultaneously, ISPRs in ISPCs can effectively diffuse to and react with the metal surface to produce a metal-phosphate layer at the metal-coating interface. From the FTIR data, the ISPR is confirmed to react with the metal surface. From the DSC and TGA results, the ISPR-catalyzed paints are more stable than the p-TSA-catalyzed paints, possibly
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a result of more co-condensation reactions with the use of the ISPR and less irreversible bond-breaking reactions. In the DSC scans, the ISPR-catalyzed paints are able to cross-link more upon infliction of exceedingly high temperatures. The p-TSA-catalyzed paint decomposes upon being subjected to high temperatures which is evident from the lower average molecular weight of the polymer after subjection to overbaking. The equivalent circuit analysis obtained from EIS results shows that the ISPR-catalyzed paints are overall 1000 times more resistant to corrosion than the p-TSA-catalyzed paints. Saltwater immersion tests show that the ISPC is able to adhere to the metal surface without the use of a conversion coating. From the pencil hardness tests, the durability of the different catalyzed paints to mechanical strain is shown to be similar. Using an ISPR as a catalyst for polyester-melamine paint systems shows promising results since the ISPR provides a dual purpose of catalyzing the polymer curing reactions and providing in situ metal phosphatization for 3003 Al surface protection. Acknowledgment Financial support from the National Science Foundation, Grant CTS-9312875, is acknowledged. The authors thank AKZO Resins and Dupont Ti-Pure for their donations of some paint materials. Literature Cited (1) Freeman, D. B. Phosphating and Metal Pretreatment; Industrial Press, Inc.: New York, 1986. (2) Spadafora, S. J.; Hegedus, C. R.; Hirst, D. J.; Eng, A. T. Primerless Finishing Systems for Aluminum Substrates. Mod. Paint Coat. Sept. 1990, 36. (3) Rausch, W. The Phosphating of Metal; ASM International: Metal Park; Finishing Publications Ltd.: Teddington, Middlesex, England, 1990. (4) Paint Waste Reduction and Disposal Options; Hazardous Waste Research and Information Center: Champaign, IL, June 1992; Vol. I, HWRIC RR-060; Feb 1993; Vol. II, HWRIC TR-008. (5) Lin, C. T.; Lin, P.; Hsiao, M. W.; Meldrum, D. A.; Martin, F. L. Chemistry of Single-Step Phosphate/Paint System. Ind. Eng. Chem. Res. 1992, 31, 424. (6) Meldrum, D. A.; Lin, C. T. AC Impedance Analysis and Factorial Designs of an In Situ Phosphatizing Coating. J. Coat. Technol. 1993, 65, 47. (7) Lin, C. T.; Lin, P.; Quitian-Puello, F. Interfacial Chemistry of a Single-Step Phosphate/Paint System. Ind. Eng. Chem. Res. 1993, 32, 818. (8) Lin, C. T.; Additive Package for In Situ Phosphatizing Paint, Paint and Method. U.S. Patent, 5,322,870, 1994. (9) Li, L.; Lin, C. T. SEM-EDS Investigation of Self-Phosphating Coatings. Ind. Eng. Chem. Res. 1994, 33, 3241. (10) Yu, T.; Li, L.; Lin, C. T. Chemical Affinity of In Situ Phosphatizing Reagents on Cold-Rolled Steel. J. Phys. Chem. 1995, 99, 7613. (11) Yu, T.; Lin, C. T. The Performance of In Situ Phosphatizing Reagents in Solvent-Borne Paints. Ind. Eng. Chem. Res. 1997, 36, 368.
(12) Yu, T.; Lin, C. T. In Situ Phosphatizing Coatings: A HighSolids Polyester Baking Enamel, Proceedings of the 24th Annual International Symposium on Waterborne, Higher Solids, and Powder Coatings, New Orleans, LA, Feb 5-7, 1997; Storey, R. F., Thames, S. F., Eds.; University of Southern Mississippi: Hattiesburg, MS, 1997; pp 56-69. (13) Wang, C. H.; Chuang, Y. Y.; Lin, C. T. In Situ Phosphatizing Coatings I: An Air-Dried Lacquer System. J. Coat. Technol. 1999, 892, 61. (14) Li, L.; Cao, Y.; Lin, C. T. Environmentally Safe In Situ Phosphatizing Coatings for Corrosion Protection. Environmentally Acceptable Inhibitors and Coatings; Taylor, S. R., Isaacs, H. S., Brooman, E. W., Eds.; The Electrochemical Society, Inc.: Pennington, NJ, 1997; proceedings Vol. 95-16, pp 118-132. (15) Gan, S.; Solimeno, R. D.; Jones, F. N.; Hill, L. W. Recent Studies of the Curing of Polyester-Melamine: Possible Causes for Overbake Softening. Proceedings of Sixteenth Water-Borne and Higher-Solid Coatings Symposium, New Orleans, LA; The University of Southern Mississippi: Mattiesburg, MS, 1989; pp 87103. (16) Bershtein, V. A. Differential Scanning Calorimetry of Polymers: Physics, Chemistry, Analysis and Technology; Kemp, T. J., Translation Editor; Ellis Horwood: NewYork, 1994. (17) Boukamp, B. A. Equivalent Circuit (EQUIVCRT.PAS) Users Manual; Netherland Report CT88/265/128, CT89/214/128; University of Twente, Department of Chemical Technology: The Netherlands, May 1989. (18) Paint and Coating Testing Manual, 14th Edition of the Gardner-Sward Handbook; Koleske, J. V., Eds.; ASTM Manual Series: MNL 17, ASTM PCN 28-017095-14; ASTM: Philadelphia, PA, 1995. (19) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; John Wiley and Sons, Inc: New York, 1981. (20) Akiyoshi, O.; Takahashi, K.; Ikeda, M. Infrared Study of Trivalent cations B and Fe in Amorphous and Crystalline Phosphates. J. Mater. Sci. Lett. 1984, 3, 36. (21) McKenna, G. B. Glass Formation and Glassy Behavior. In Comprehensive Polymer Science; Allen, G., Berington, J., Eds.; Pergamon Press: London, 1989; Vol. II. (22) Boukamp, B. A Nonlinear Least Squares Fit Procedure for Analysis of Immittance Data of Electrochemical Systems. Solid State Ionics 1986, 20, 31. (23) Scully, J. R. Electrochemical Impedance Spectroscopy for Evaluation of Organic Coating Deterioration and Under Film Corrosion: A State of the Art Review; Report DTNSRDC/SME86/ 006; David W. Taylor Naval Ship Research and Development Center: Bethesda, MD, Sept. 1986. (24) Haruyama, S.; Asari, M.; Tsuru, T. Impedance Characteristics during Degradation of Coating Steel. In Corrosion Protection by Organic Coatings; Kendig, M., Leidheiser, H., Eds.; The Electrochemical Society: Pennington, NJ, 1987; proceedings vol. 87-2. (25) Taylor, S. R.; Gileadi, E. Physical Interpretation of the Warburg Impedance. Corros. Sci. 1995, 51, 664. (26) Kendig, M.; Scully, J. Basic Aspects of Electrochemical Impedance Application for the Life Prediction of Organic Coatings on Metal. Corrosion 1990, 46, 22.
Received for review March 29, 1999 Revised manuscript received June 15, 1999 Accepted July 28, 1999 IE990231I
