Ind. Eng. Chem. Res. 1992, 31, 424-430
424
radoride, 5623-5; chloroform, 67-66-3; dichlommethane, 7509-2; methanol, 67-56-1; l,l,l-trichloroethane, 7 1 - 6 6 ethane, 74-844 ethanol, 6417-5; propylene, 115-07-1; acetone, 67-64-1; 1bromopropane, 106-94-5; propane, 74984 methyl acetate,79-209; n-propanol, 71-23-8; 2-propanol, 67-63-0; furan, 110-00-9;2-butanone, 78-93-3; ethyl acetate, 141-786; l-chlorobutane, 109-69-3; n-butanol, 71-36-3; sec-butanol, 78-92-2; n-pentane, 109-66-0; benzene, 71-43-2; phenol, 108-952; methylcyclopentane, 96-37-7; n-hexane, 11@54-3;n-peduorohexane, 35542-0; toluene, 108883; n-heptane, 142-82-5.
Literature Cited Boublik, T.; Fried, V.; Hala, E. The Vapor Pressures of Pure Substances, 2nd ed.; Elsevier: Amsterdam, 19W, pp 65-67. Brandani, V.;Di Giacomo, G.; Mucciante, V. A Group Contribution Method for Correlating and Predicting the Second Virial Coefficient of Hydrocarbons, Including Second V i Croea-Coefficients. Chem. Biochem. Eng. Q. 1987,1, 109. Bruin, S.; Prauanitz, J. M. OneParameter Equation for Excess Gibb Energy of Strongly Nonideal Liquid Mixtures. Znd. Eng. Chem. Process Des. Dev.1971, 10, 562. Choliiski, J.; Szafranski,A.; Wyrzykowska-Stankiewicz,D. Computer Aided Second Virial Coefficient Data for Organic Zndividual Compounds and Binary System. PWN-Polish Scientific Publishers: Warszawa, 1986. Dymond, J. H.; Smith, E. B. The Virial Coefficients of Pure Gases and Mixtures. Clarendon Presa: Oxford, 1980. Geller, E. B.; Battino, R.; Wilhelm, E. The Solubility of Gases in Liquids. 9. Solubility of He, Ne, Ar, Kr, Nz, Oz, CO, COz, CHI,
CF, and SFein some Dimethyl-Cyclohexane at 298 to 313 K. J. Chem. Thermodyn. 1976,4197. Hirschfelder, J. 0.; Curtiss, C. F.; Bird, R. B. Molecular Theory of Gases and Liquids; Wiley: New York, 1954. Lobien, G. M.; Prausnitz, J. M. Correlation for the Ratio of Limiting Activity Coefficiente for Binary Liquid Mixtures. Fluid Phase Equilib. 1982,8, 149. Moelwyn-Hughes, E. A. Physical Chemistry, 2nd ed.; Pergamon Press: Elmsford, NY,1961. Pierotti, R.A. A Wed Particle Theory of Aqueous and Nonaqueous Solutions. Chem. Rev. 1976, 76, 717. Prausnitz, J. M.; Grens, E. A.; Anderson,T. F.; Eckert, C. A.; Hsieh, R.; OConnell, J. P. Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria; Prentice-Hak Englewood Cliffs, NJ, 1980. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977. Riddick, J. A; Bunger, W. B. Technique8 of Chemistry; Weesberger, A., Ed.; Organic Solvents, Vol. 11, 3rd ed.; Wiley-Interscience: New York, 1970. Spencer, C. F.; Danner, R. P. Improved Equation for Prediction of Saturated Liquid Density. J. Chem. Eng. Data 1972, 17, 236. Taeeioa, D. SinglaPammeter Equation for Isothermal Vapor-Liquid Equilibrium Correlations. AIChE J. 1971, 17, 1367. Wilson,G. M. Infiiite Dilution Activity Coefficiente Estimation of One Binary Component from the Other. AIChE symp. Ser. 1974, 70, 120. Received for review May 13, 1991 Revised manuscript received Auguet 22,1991 Accepted September 5,1991
Chemistry of a Single-Step Phosphate/Paint System Chhiu-Tsu Lin,* Ping Lin, and Meen-Woon Hsiao Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115-2862
Dean A. Meldrum and Frank L. Martin Finishes Unlimited, Znc., Wheeler Road, P.O. Box 69, Sugar Grove, Illinois 60554
A single-step phosphatelpaint system comprised of polyester-melamine enamels and H3P04was successfully formulated. The system stability, compatibility, and thermal curing behavior as well as chemical mechanism were investigated using the experimental techniques and theoretical modelings. When the unicoat system is applied on a “qpanel, the experimental results indicate that H3P04tends to diffuse to and react with the metal surface, providing a corrosion protective barrier to the substrate and simultaneously making available the proper functionality to form chemical bondings with polymer resins. Electrochemical impedance spectroscopy was employed to measure the resistive properties for the single-step phosphate/paint system. The data show that the unicoat film is a quasi-nonporous coating that has a good quality of the adhesion layer to the substrate and which also has a good corrosion-protective barrier-type property. Introduction The surface treatment of steel prior to the application of a mating or adhesive is a conventional industrial practice to improve the adheaion and inhibit corrosion (Hare, 1978). The phosphate conversion coating consists of a nonconductive layer of crystals/amorphous that insulatethe metal from any subsequently applied film and provides a topography with enhanced “tooth”for holding the film (Hall, 1978). The quality of finish required by an industrial product determines the degree to which the pretreatment and phosphatizing are carried out in the multistep process. The metal surface is normally cleaned by several possible pretreatment steps, phosphated, sealed, dried, and then painted.
* To whom correspondence should be addressed.
The development of a single-step phosphate/paint system technique by incorporating a polymeric resin, cross-linker, and H3P04has never been reported. The simplicity involved in the application of a single-step phosphate/paint over present multistep methods will make this product attractive to many manufacturers of metal products. The success of a single-step phosphate/paint system will increase the quality of finish coatings without the capital and operating expenses of a separate phosphate line. The present state of the art in the principal pretreatment or metal conditioner of the maintenance painting industry is the WP-1Wash Primer (Hare, 1978)or onepack etch primer (Waldie et al., 1984). These primers contains no cross-linker in the formulations. However, this unique material is a two-pack phosphate/vinyl butyl resin system or a single package of poly(viny1 butyral) resin/
0888-5885/92/2631-042~~~3.00 f 0 0 1992 American Chemical Society
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 425 system. In a WP-1 Wash Primer (Hare, 19781, the phosphoric acid component (Le., hydrophilic portion) must be added slowly and carefully to the base component (i.e., hydrophobic portion) under agitation immediately before use. In this way the reaction is initiated. The in-can reaction (even in the absence of metal) will continue for about 8 h after which the reactants will be exhausted and the material becomes useless. The WP-1 Wash Primer daes not gel (as do two-pack thermosets), but nonetheless, the pot life is definitely limited. For singlepackage Wash Primers, the system stability was not mentioned, but it was suggested (Hare, 1978) that the reaction mechanism may be different from that of two-packageWP-1 Wash Primers. Primerless finishing systems for aluminum substrates have been reported recently (Spadafora et al., 1990). This has been accomplished by using either conventional topcoats on a modified pretreatment or a self-primeringtopcoat on conventional pretreatments. Either method eliminates the use of a primer, saving application time, manhours, and materials. In an U.S.patent, Albrecht (1931) described a rust-resisting coating composition and indicated that a suitable amount of solvent, mixable with both phosphoric acid and paint, is required for the coating composition. There are two patented single-step formulations which are similar in composition mixtures to the present study but for completely different purposes. One of these is for synthetic resins (Ring et al., 1985), and the other is for a thermal setting organic coated metallic sheet (Perfetti et al., 1977). The goal of our research is to formulate and demonstrate a compatible and stable phosphate/polymer conversion coating system. This investigation will require characterization of the phosphate mechanism, its controlling chemical factors, and the effect this thin coating produces in enhancing corrosion resistance and substrate protection. Currently, we have achieved various formulations of a single-step phosphate/paint system using different polymer resins, e.g., polyester (aromatic or aliphatic), alkyd, epoxy, phosphoric-modified polyester, and epoxy ester copolymer resins. In this paper, we will describe only the formulation of the polyestermelamine enamels/HBP04 system. The investigation includes the following: (1) chemical stability of H3P04in the system formulated, (2) resin compatibility and thermal curing mechanism and behavior, and (3) physical properties and chemical qualities of the unicoat phosphate/paint system. The results will be used to demonstrate the benefits of a single-step phosphate/ paint coating process over those of multistep techniques.
Experimental Section Water-extendable polyester (Cargill resin no. 5778), cross-linking resin (hexakis(methoxymethyl)melamine, Resimene no. 745, referred to as HMMM), 85% phosphoric acid, water, and organic solvents (e.g., butylcarbitol) were used to formulate a single-step phosphate/paint system (referred to as the 138-67 system). Several additives, Triton X-100 surfactant (octylphenoxypolyethoxyethanol), and 2,Bdimethoxyaniline were used to enhance the thermal-curing process of the unicoat formulation. A AgCl window or "Q" panels were used to cast the thin coatings using a spray gun or a solution-dipping technique. The uncoated mild steel panels ("Q" panel, SAE 1010 C 0.08-0.13%, Mn 0.3-0.6%, P(max) O M % , S(max)0.05%) were mechanically polished to a mirror-finished surface before coating. IR spectra were recorded using either a Sargent Welch 3-200 infrared spectrophotometer or a Mattson Cygnus 25 FTIR spectrophotometer equipped with a Spectra Tech FT-80grazing angle accessory. The
absorbance or transmittance spectra of a thin coating are produced by ratioing the singlebeam spedrum of the same substrate with the thin coating. The interferometer is purged with dry nitrogen. Water, carbon dioxide, and baseline corrections are necessary in most cases. Absorption spectra were recorded at room temperature on a Varian 2290 W-vis spectrophotometer. For emission studies, the sample of thin coating on a "Q" panel was placed inside a liquid nitrogen (77 K) optical Dewar. The 313-nm band of an Oriel 200-W Hg(Xe) lamp was isolated by using a 10-cm light path of an aqueous solution of NiSO4.6H2O(200 g/L) with an UG 11Schott glass filter. This band was used to excite the sample. The emission spectra were detected by an E M 9789 QB photomultiplier tube in conjunction with a 1/2-mJarrel-Ash spectrometer. A Leitz Ortholux II POL-BK polarizing microscope was used to examine the grain structure and surface morphology of the phosphatizing films and single-step phosphate/paint coatings. Silicon graphics (a commercial molecular graphics) equipped with a SYBYL software program was employed to explore phosphate fundamentals at the molecular level based on molecular modeling. To conduct electrochemical impedance spectroscopy measurements, ac impedance data were obtained at the open-circuit potential using a PAR 273 potentiostat/galvanostat and PAR 5210 lock-in amplifier. The data were stored and analyzed using an IBM PS/2-30 computer and an Epson FX-850 plotter. The impedance measurements in all cases were carried out over a frequency range of 100 kHz to 10 mHz using a 5-mV peak-to-peak sinusoidal voltage. In the multisine mode, some higher applied voltages of 5-30 mV were used depending on the thichem of sample coatings. The experimental results were analyzed by means of computerized Bode and Nyquht plotting routines.
Results and Discussion The single-coat phosphatizing baked enamel formulation includes a polymer coatrforming component, a cross-linking agent for the resin, and phosphoric acid to promote the formation of a metal phosphate film on the metal substrate being protected. It is expected that the phosphoric acid reacts in situ with the metal surface, forming metal phosphates as the paint is being applied. The principles underlying chemical and physical properties of the unicoat paint formulation are (i) stability of phosphoric acid in the baked enamel, (ii) resin compatibility in phosphate/ polymer conversion coating, (iii) diffusion rate of phosphoric acid to, and reaction with, the steel surface being protected, and (iv) the elementary surface chemistry at the molecular level of metal phosphates. A. Chemical Stability of H3P04in the 138-67 System. A marketable single-step phosphate/paint system should have a long pot life. In the absence of a metal surface, the paint formulation should not have gelation, precipitation, or an in-pot reaction at room temperature or some higher temperatures. The 138-67 system is observed to remain as a homogeneous and clear paint formulation at ambient temperature after 6 months storage. To illustrate the stability of H3P04in the 138-67 system, we recorded the IR spectrum of 85% H3P04coated on a AgCl window as shown in Figure 1A. The strong broad absorption, centered at 2900 cm-I (the hydrogen-bonded phosphoric acid O-H stretching) and at 2400 cm-' (the strong hydrogen-bonded O-H stretching vibration), plus the third band, at 1650 cm-' (a sum band of phosphate skeletal vibrations), are all taken to indicate the presence of the P U H group (Carter et al., 1986,1987). Two lower energy peaks at 1015 and 1250 cm-' have been assigned
426 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
20-
(/f-J
--
0
I
1001
01
82 7- , , ,
, , , , , , , , ,, , , , , , , ,, , ,, ,, ,,,, , ,
I , ,
,,
~, , , , , ,,,
, ,
, , , ~,
I
'
3500
2500
1800
1400
1000
Wavenumber (cm .')
Figure 1. IR spectra of thin coating on AgCl window: (A) 85% H3P04 heated at 50 OC for 10 min, (B)138-67 system aged for 6 months, and ( C ) 138-67 system thermally cured at 130 OC for 10 min.
(Carter et al., 1986,1987) to the P-0 stretching mode of the P-0-H group and a P=O stretching vibration, respectively. The time dependence of constant intensity of H3P04 vibrations, in particular the strong hydrogenbonded 0-H stretching vibration at 2400 cm-' is used to probe the H3P04stability in the 138-67 system. It is noted that the 2400-cm-' band is a relatively isolated H3P04 transition from the highly congested polymer bands. Figure 1B and Figure 1C show the IR spectra of the 138-67 system aged for 6 months and then coated on a AgCl window and the newly formulated 138-67 system coated on a AgCl window following a thermal curing at 130 "C for 10 min, respectively. The spectral peak intensity at 2400 cm-' for Figure 1B,C is similar to that of the freshly prepared 138-67 system (not shown), suggesting that H3P04 is quite insensitive to the aging or high-temperature treatment of single-step phosphate/paint system. Some band structure changes are clearly observed in Figure 1C as compared to those in Figure lB, resulting from polymerization in the thermal-curing process. is stable in the 138-67 system only if the system is not in contact with metal surface. When the 138-67 system is applied on a "Q" panel, H3P04is expected to diffuse to and react with the metal surface following the commonly understood iron phosphate reaction (Hare, 1978): Fe
+ 2H3P04
Fe(H2P04)2
-
-
Fe(HZPO& + 2H primary iron phosphate FeHPO, + H3P04 secondary iron phosphate
Recently, there is growing evidence (Gorecki, 1991) that the composition of iron phosphate films is a mixture of ferrous, Fe3(P04)2,and ferric, FeP04, phosphates and oxides with little or no FeHPO, and Fe(H,P04)2 present. The 80" grazing angle FTIR spectra of 85% H3P04coated on a "Q" panel following a heat treatment at 50 "C for 10
I 3500
2500
1500
500
Wavenu rnber Figure 2. FTIR spectra of thin coating on "&" panel: (A) 85% HBP04heated at 50 O C for 10 min, (B)138-67 system (air dry), and (C) 138-67 system thermally cured at 130 "C for 10 min.
min, the 138-67 system sprayed on a "Q" panel and then air-dried, and the 138-67 system sprayed on a "Q" panel and then thermal cured at 120 "C for 10 min are shown in parts A, B, and C of Figure 2, respectively. To illustrate the iron phosphate reaction (Figure 2A), we observed a strong band a t 1092 cm-I corresponding to u3 vibration for the P=O/Fe complex (Carter et al., mode in Po431986) and also a broad band at 623 cm-' associated with u4 vibration mode in Po43-(Sato and Minami, 1989). This observation of vibration modes in P043-provides no indication about the type of iron phosphates in the film composition. The reaction of H3P04with "Q" panel surface in the 138-67 system should show a decrease in the IR spectral intensity at 2400,1250, and 1015 cm-l corresponding to the vibration modes of H3P04 and a possible alteration in the IR spectral intensity at 1200,1092, and 623 cm-I resulting from the basic vibration modes of PO4* (Sato and Minami, 1989). In Figure 2B,C, the spectral peak at 2400 cm-' has disappeared; the spectral transition at 1092 cm-' seems to pick up its intensity relative to the nearby polymer bands at 1100, 1145, and 1170 cm-', whereas that at 1015 cm-'reduces its intensity as compared to those in Figure 1B,C. We have to try and assess the stability and reactivity of H3P04in the 138-67 system by carrying out a difference IR analysis with the organic component of the system (i.e., without H3P04)serving as the baseline control. Unfortunately, the 138-67 formulation without does not cure thermally on AgCl window and on "Q" panels. The phosphatizing of "Q" panel results in the formation of an iron phosphate thin
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 427
Figure 3. Silicon graphics of X-ray erystallagraphical data: (a) HaPo, and (b) Fe(H,PO,),.
film depositing on the substrate surface, where the chemical compositions of FeHPO,, Fe(H2P04),,Fe3(P0,),, or FePO, are not yet clearly characterized. However, the gives superior rust-inhibiting characteristics phosphate fh to the "Q" panel as examined by salt spray and electrochemical impedance techniques. The results will be described in section C. The phosphate film offers also the proper chemid functionality which can then bond to the polyester resin resulting in a strong adhesion as determined by an ASTM D-3359 adhesion testing. We have used a commercial silicon graphics display to view the structural arrangements of H,PO,, Fe(H2P04),, FeHPO,, Fe3(P0,)*, and FePO, with assumed surface structures. This may assist our understanding of chemical fundamentals at the molecular level as to why (i) H3P04 is chemically stable while standing in the formulation mixtures of single-step phosphate/paint system and (ii) when Fe(H,PO,), and FeHPO, (or FePO, and Fe3(P0,),) are produced via the chemical interaction of H3P04and a 'Q" panel, they then become very reactive with polyester resin. Figure 3 shows the silicon graphic displays of X-ray crystallographical data of H,PO, (Blessing, 1988) (graphic a) and Fe(H2P04), (Guse et al., 1985) (graphic b). In H3P0,, we observed a hydrogen-bonding network exists associated with P U H and O=P, and P U H and H U P of H3P04where the bond distance is 1.619 and 2.794 A,respectively. The -OH vibrations for the strong hydrogen-honded P U H group in H,PO, are clearly observed in Figure 1,in particular the IR band at 2400 cm-'. On the other hand, for Fe(H,PO,),, the intermolecular distances of P-0-H and O=P and P-0-H and P-0-Fe are quite long as 5.961 and 4.019 A, respectively. This molecular arrangement at the "Q" panel surface provides four available active bonding sites of P U H in Fe(H,PO& for a possible condensation reaction with polyester resin in the unicoat phosphate/paint system. In the case of FeHPO, (structure similar to CaHPO,, Smith et al. (1955), not shown) only one active bonding site of P U H for the polymer condensation reaction is available. The silicon graphics display in Figure 3 predicts that H3P04is stable while the 138-67 system is stored in a container other than metal while upon application Fe(H,PO,), is chemically
bonded with a 'p panel and should have very strong adhesion with polyester resin. Moreover, FeHPO, should bond very strongly with a 'Q" panel and adhere well with the polymer coating. For FePO, (Ng and Calvo, 1975) and Fe3(P0,), (Calvo, 1968).the silicon graphic pictures (not shown) display a solid network with no active functionality in FePO, and only P=O as a free functional group in Fe3(P04),for the reaction with polyester resin. B. Resin Compatibility and Thermal Curing Behavior in 138-67 System. The possible c a w of overbake softening of the thermal curing of polyestermelamine enamels have been presented recently (Gan et al., 1989). Gan et al. (1989) proposed a broad theoretical model of cross-linking by melamine resins. The complex crosslinking processes were grouped into three reaction categories: (1)the reactions that form new covalent bonds and increase cross-link density; (2) pairs of reversible reactions in which covalent bonds are broken but new ones form, the net effect being a change in network structure but no change in cross-link density; and (3) reactions that break bonds irreversibly, irreversibly changing network structure and perhaps causing a decrease in effective cross-link density. Causes of overbake softening are probably the melamine "molecular aggregation" via reaction category 3. Using p-toluenesulfonic acid as a catalyst to selectively promote desirable reaction categories 1 and 2, but not reaction category 3, it was shown (Gan et al., 1989) that the overhake softening temperature for the thermal curing of polyestermelamine enamels is observed as 150 "C. In this study, phosphoric acid is used as a catalyst instead, for the thermal curing of polyester-melamine enamels in the single-step phosphate/paint system. For HMMM resins, the predominant cocondensation reaction with polyester is proposed as ~-N(cH,ocH~), + P-OH
== D-NCH,O-P I
+
nom,
CH,0CH3
where Tr represents the triazine ring of HMMM and P represents the polymer being cross-linked. It was anticipated that in the cocondensation of melamine with polyester resin, the IR absorbances at 901 and 3546 cm-' should drop sharply, indicating rapid consumption of -CH20CH3and -OH groups. On the other hand, selfcondensation of melamine resins should result in an intensity reduction of IR abaorbances at 2936 cn-' (aeaipned as C-H stretching) and at 912 cm-' (assigned as methoxymethyl group) (Gan et al., 1989). Moreover, spectral changes during overbake including an increase of IR absorption in the 3300-3500-cn~'region have been reported (Gan et al., 1989). Based on this information, infrared spectra showed that the overbake softening temperature in the single-stepphosphate/paint system is 200 "C. This indicates that the H3P0, catalyst selectively channeled the desirable reactions and prevented the premature overbake stage of melamine molecular aggregation. The surfactant Triton X-100 is a highly effective emulsifier. As the concentration of a surfactant increases at or higher than cmc values (critical micellar concentration), a micellar emulsion is thus established in the single-step phosphate/paint system. Since the polymer resin is relatively hydrophobic in nature and the phosphoric acid is a hydrophilic species, the coexistence of polymer resins and H3P04depends on the polymer vehicle or solvent used. It is expected that within a micellar configuration the polyestermelamine enamels should reside in the hydrocarbon core as monomer droplets and that the phosphoric acid should exist in an aqueous phase, thus forming a stable and compatible single-step phosphate/paint system.
428 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
It is also expected that when the emulsified unicoat system is applied to a metal "Q" panel, the evaporation of water should lead to the collapse of micellar structure forming an instantaneous mixing of polymer resins and H3P04. This allows the diffusion of H3P04to the metal surface and upon temperature elevation promotes Fe(H,PO,), and FeHP04 (or FeP04 and Fe3(P04),) to react with the polyester-melamine enamels. Infrared spectra illustrated that the overbake softening temperature for the Triton X-100 emulsified single-step phosphate/paint system is 220 "C. Upon the addition of Triton X-100 in a unicoat system, a 20 "C increase in the overbake softening temperature indicated that the monomer droplets of polyestermelamine enamels were stabilized by the surfactant, Triton X-100. In addition to aiding the overbake softening problem, other desirable properties may be influenced by a surfactant addition and the resultant homogeneous effects. It is believed (Mcewan, 1979) that polymer particle size is controlled by the surfactant concentration. Small particle size is believed to be desirable for gloss; Le., a smooth surface is essential and discontinuities larger in size than half the wavelength of visible light (i.e., -0.3-0.5 pm) will cause scattering and hence a disturbed reflected image if the polymer particles do not completely coalesce. Melamine molecular aggregation has been proposed (Gan et al., 1989) as a cause of overbake softening in the chemistry of melaminepolyester diol reactions. However, Ferlauto (1988) and Hill et al. (1988) suggested that the decomposition of the triazine rings of HMMM cross-linker plays a role in overbake softening. Electronic spectroscopy, in particular the emission spectroscopy of symmetric triazine and its derivatives, is well-known (McGlynn et al., 1969). At 77 K, triazine displays a well-resolved phosphorescence emission spectrum maximum at 379 nm which resulted (McGlynn et al., 1969) from a triplet n,r*-type gave a considtransition, while 2,4,6-triphenyl-s-triazine erable vibrational structure phosphorescence emission spectrum peak at 406 nm which was assigned (McGlynn et al., 1969) to the triplet r,?r*-type transition. It is expected that molecular aggregation of HMMM cross-linker would lead to a red-shift in the emission spectrum. On the other hand, decomposition of the triazine rings of HMMM cross-linker during the thermal-curing process would reduce the phosphorescence emission intensity. Figure 4 shows emission spectra a t 77 K of polyester-melamine/ H3P04 (left-portion spectra) and melamine/H3P04 (right-portion spectra) coated on a "Q" panel, where spectra A and D are unbaked, B and E are cured at 130 "C for 10 min, and C and F are overbaked at 200 "C for 10 min. Spectrum A resembles that of triazine at 77 K (McGlynn et al., 1969) with a band maximum at 375 nm. After thermal baking a t 130 and 200 "C for 10 min, as shown in spectra B and C, the emission spectrum is redshifted to 420 and 463 nm, respectively. Moreover, the emission intensities in Figure 4B,C are much weaker than that of Figure 4A. These results suggest that self-condensation of HMMM cross-linkers produce molecular aggregation. The reduction of the emission intensity further suggests the decomposition of triazine rings occurs as a result of high curing temperature. When HMMM cross-linker is admixed in the 138-67 system, polyester/melamine/phosphoricacid, the emission spectrum (Figure 4D) is similar to that of triazine derivative (e.g., 2,4,6-triphenyl-s-triazine)(McGlynn et d,1969) with a spectral peak maximum at 422.5 nm. Upon thermal curing, at 130 "C (spectrum E) and at 200 "C (overbaked, spectrum F), the emission spectra show only slight redshifting to 425 and 428 nm, respectively. The spectral
\
350
450
550
350
450
550
Wavelength (nm) Figure 4. 77 K emission spectra of thin coating on 'Q" panel: left, 138-67 system; right, 138-67 system less polyester (i.e., HMMM/ HIP04/solvent). Top two coatings were air-dried,middle two were cured at 130 "C for 10 min, and bottom two were overbaked at 200 O C for 10 min.
intensities of Figure 4E,F are only slightly different from that of Figure 4D. This suggests that in the 13867 system, the cocondensation between polyester and melamine is more important or proceeds at a faster reaction rate than that of the self-condensation between melamines. The melamine molecular aggregation or triazine ring opening is not observed in the 138-67 system below the overbaked temperature of 200 "C. During the thermal-curing process of a single-step phoaphate/paint system, we noticed that the H,P04-catalyzed polyester-melamine enamels are not always completely cured. The paint coating surface of a film which has been cured at 130 "C for 10 min displays a slightly wet appearance. We discovered that the thermal curing of the polyestel-melamine enamels/H3P04 system would reach its completion resulting in a completely dry and pencilhardness of 2H paint coating, when a small amount of 2,5-dimethoxyaniline is added to the paint composition. The exact molecular function of 2,5-dimethoxyaniline as to how it promotes the thermal-curing process in the 138-67 system is not yet completely understood. Our preliminary experimental results show the following: (1) the ultraviolet-visible absorption spectrum of the 138-67 system displays a spectral band at 370 nm which might be assigned (McGlynn et al., 1969) to the electronic transition of the aromatic carbonyl group of a polyester resin. The addition of 2,5-dimethoxyaniline in the 13867 system leads to a spectral intensity reduction of this W band. (2) While the UV band at 370 nm is disappearing, a strong, new, visible band a t 617 nm and a shoulder band at 570 nm appear which are tentatively assigned as *charge transfer" in nature. This suggests that in the presence of H3P04in the 138-67 system, a charge-transfer complexation between the additive, 2,5-dimethoxyaniline, and the aromatic carbonyl group of the polyester resin is operative. Perhaps, the "charge-transfer" complex protects the aro-
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 429 matic carbonyl groups and prevents them from absorbing H20 via hydrogen bonding; Le., a completely dry coating may result from the thermal-curing process. C. Physical Properties and Chemical Qualities of the Unicoat Phosphate/Paint System. An optical microscope was used to view the surface morphologies of a bare "Q" panel, an ASTM prepared phosphate "Q" panel (Chemfos 1581, a commercially prepared bonderite panel (Parker-Amchem bonderite lOOO), and the 138-67 system applied to a "Q" panel. The micrographs of the surface structure of the bare "Q" panel and the iron phosphate layer (FeHP04,Fe(H,PO,),, FeP04, or Fe3(P04)2)of the ASTM panel were employed to monitor and verify the proper formation of a metal-phosphate layer in the 13867-coated "&" panel. It was observed that a single coat of the 138-67 paint system on "Q" panel resulted in such a smooth, uniform, and transparent film that we could compare the optical micrograph of the unicoat substrate surface to that of the ASTM prepared phosphate panel and see that they were identical. This implies not only excellent chemical and physical coating qualities of the single-step phosphate/ paint system, but also that proper control of H3P04migration to and then the reaction with the surface of a "Q" panel results in the formation of metal phosphates as the paint is being applied. The evaluation of the 138-67 system performance from an industrial application perspective was also conducted. A dry film thickness of 1.1 mil of a 138-67 system coated on a "Q" panel and baked at 135 "C for 10 min achieved 470 salt spray h, 1800 h of humidity testing, and a 2H pencil hardness. Repetitive duplication of these industrially applied tests indicates wide ranges in interpretation and reproducibility; however, as an indication to those experienced in the paint industry we concluded that these are fairly consistent values. Since corrosion is an electrochemical process, it makes sense to try to examine the effectiveness of a single-step phosphate/paint system on a "Q" panel by finding ita response to an applied electrical potential (Scully, 1986). In this we are helped by the fact that a paint coating, unless it is very thick (>500 pm, 20 mil), does not isolate the metal "Q" panel from the environment. Shortly after the coating gets wet (humidity testing, salt spray testing, etc.), it is possible to measure a potential (voltage) generated by the metal or metal oxide interface. The aim of an electrochemical testing method is to find out something about what is happening at that interface. However, the data obtained often include a major contribution from the paint. The properties of the paint itself are important in controlling ita ability to prevent corrosion, so this information should be valuable. Electrochemical impedance spectroscopy (EIS) can provide accurate, error-free kinetic and mechanistic information using a variety of techniques and output formats. If corrosion is occurring underneath a paint, then there will be small voltage fluctuations (a few tens of millivolts) on a time scale from 1Hz to 10 m H z , i.e., from 1to 100 s. If these are monitored, using a high-impedance voltmeter, then the degree to which corrosion is occurring might be deduced from the degree of the noisiness of the signal. Information about the dielectric properties of the polymer is gathered from the high-frequency end, e.g., polymer-pigment interaction and water uptake. A fall in the ac resistance with time is a sign that the paint is saponifiable, and a rise with time is an indication of pores becoming plugged, probably with corrosion products. The simple dc measurement of resistance has been used in the past with success, so it is commonly expected that in most
1-90 -2
I
,
I
I
I
I
lor Frequency (Hz)
I
x.1
5
5WE6
I
2'
(ohm)
Figure 5. Electrochemical impedance spectroscopy data of 138-67 system: (A) Bode magnitude, (B) Bode phase diagram, and (C) Nyquist complex plane plot. An area of exposed surface, 1.13 om2, should be used to compute the electrochemical impedance results.
cases an in situ resistance of 1 X lo8 ohms.cm2will have good protective properties, a resistance of 1x 106ohms-cm2 will be poor, and paints with resistance between 1 X lo6 and 1 X lo8 ohms-cm2will be borderline. Figure 5 presents the Bode magnitude (A), phase diagrams (B), and Nyquist complex plane plots (C)for the EIS test results obtained at two exposure intervals ( 0 and X are 24- and 72-h exposure to a 3% NaCl solution) for 138-67 system coated on a "Q" panel (thickness 0.9 mil). The calculated values for 2, Z'or 2"should have unit of ohmsan2; thus the experimentally measured results in Figure 5 must be multiplied by the area of exposed surface (area = 1.13 cm2). Note that the shape of total impedance, 121,and phase angle, 8, as a function of frequency do not change over the 3-day exposure to 3% NaCl solution. For polyester-melamine enamels/H3P04 coatings, the lowfrequency-impedance values at 0.16 Hz were on the order of 107-108ohms*cm2depending on the film thickness (8.1 X lo7 ohms.cm2 at 0.9-mil f ilm thickness). This provides the coating capacitance, C, = 100-10 nF. This result pointa to a good corrosion barrier type property and confirms the results of industrial salt spray testing. The phase angle data shows a gradual transition from capacitive (-90') to resistive (0') behavior with decreasing frequency, suggesting a quasi-nonporous thin coating for the single-step phosphate/paint system. Again,the high-frequency phase angles were between -80' and -90', indicating good barrier properties, where -90' would be a perfect capacitor barrier. In the Nyquist complex plane diagram of Figure 5C, the pore resistance, R,, can be calculated from the low-frequency intercept of the semicircle with the real axis. Figure 5C seems to be made up by more than one semicircle, but R, at the lower frequency intercept has a relatively high value of 1.0 X lo7 ohms.cm2. This value is at least a 2-order magnitude larger than that determined for the cationic primers (MSkoviE-StankoviEet al., 1991). The high pore resistance values indicate the quality of the paint
430 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
layer adhesion to the substrate, which is one of the most important features for the quality of a coating, ita porosity and microdefecta, water uptake, and degradation of the coating. Thus, greater pore resistance values point to better protective properties of coatings. The pore resistance of the ionic paths correlates also with the development of corrosion. It has been experimentally proven (Hubrecht et al., 1984) that a decrease in pore resistance is related to the increase of the corrosion rate which suggests that the pore resistance can be considered as the criterion for estimating the protective properties of organic film against corrosion. Conclusion A single-step phosphatizing paint composition, the 138-67 system, has been formulated. The shelf life of the formulation in the absence of contact with metals is shown to be longer than 6 months. The phosphoric acid in the 138-67 system is stable and nonreactive at temperatures as high as 130 "C, the polyestel-melamine enamel's curing temperature. Upon the application of the 138-67 system on a "&" panel, H3P04is shown to migrate to and react with the surface of the "Q" panel. This results in the formation of metal phosphates, Fe(H2P04)2and FeHP04 (or FeP04 and Fe,(PO,),), that can protect and inhibit surface corrosion and can provide the proper functionalities for the condensation reactions with polymer resins. This covalent-bond formation enhances the adhesion of polymer coatings to the "Q"panel surfaces. The details of surface phosphate film composition are currently under investigation in our laboratory. The emission spectroscopy studies at 77 K show that the thermal curing of melamine resin can result in an irreversible molecular aggregation and a ring opening of triazine in HMMM cross-linker. However, the thermal curing behavior of the 138-67 system indicates that the rate of cocondensation between polyester and melamine is much faster than the self-condensation between melamine cross-linkers. An excellent coating quality of the singlestep phosphate/paint system is obtained. EIS measurementa of the 138-67 formulation provide evidence that polyester-melamine enamels/H3P04 display a good corrosion protective barrier as evidenced by the results of industrial salt spray testing. Moreover, high values of pore resistance indicate a good quality of coating layer adhesion to the "Q" panel as determined by an ASTM D-3359 adhesion testing. The present state-of-the-art paint coating involves a multistep process and considerable energy, labor, and control. The success in the combination of the phosphate and top-coat application into a single-step process through the balancing and control of chemical reactions during the application and thermal-curing process should lead to a significant reduction of time and energy in this energy-intensive manufacturing process. Acknowledgment Financial support from the Illinois Department of Commerce and Community Affairs, a State of Illinois Technology Challenge Grant, Finishes Unlimited, Inc., Cargill and Northern Illinois University Graduate School, College of Liberal Arts and Sciences, and Technology Commercialization Center is acknowledged. Registry No. HMMM, 137571-96-1; phosphoric acid, 766438-2.
Literature Cited Albrecht, H. 0. Rust-Resisting Coating Composition. U.S. Patent 1,995,954,1931,June 10. Blessing, R. H. New Analysis of the Neutron Diffraction Data for Anhydrous Orthophosphoric Acid and the Structure of HSP0, Molecules in Crystals. Acta Crystallogr. 1988,B44, 334-340. Calvo, C. The Crystal Structure of Graftonite. Am. Mineral. 1968, 53,742-750. Carter 111, R. 0.; Gierczak, C. A.; Dickie, R. A. The Chemical Interaction of Organic Materials with Metal Substrates. Part 11: FT-IR Studies of Organic Phosphate Films on Steel. Appl. Spectrosc. 1986,40,649-655. Carter 111, R. 0.; Parsons, J. L.; Holubka, J. W. Synthesis and Characterization of Epoxy Phosphate Steel Surface Modifiers. Znd. Eng. Chem. Res. 1987,26,1518-1523. Ferlauto, E. C. Comparison of Volatile Organic Content (VOC) and Volatiles Released from a Melamine Crosslinked High-Solids Polyester. J. Coat. Technol. 1988,60,51-59. Gan, S.;Solimeno, R. D.; Jones, F. N.; Hill, L. W. Recent Studies of the Curing of Polyester-Melamine Enamels. Possible Causes of Overbake Softening. Proceedings of the Sixteenth Water-Borne and Higher-Solid Coatings Symposium, New Orleans; University of Southern Mississippi: Hattiesburg, MS, 1989;pp 87-108. Gorecki, H. Corrosion 91; National Aesociation of Corrosion Engineers, Inc.: Houston, TX, 1991;conference paper 381. Guse, W.; Klaska, K. H.; Saalfeld, H.; Adiwidjaja, G. The Crystal Structure of Iron (11)Dihydrogen Phosphate Dihydrate [Fe(H2P0,)22H20]. Neues Jb. Mineral., Monutsh. Mh. 1985,10,433-438. Hall, W. S.Theory and Practice of Coating by Autodeposition. J. Water Borne Coat. 1978,Aug, 2-10. Hare, C. H. Corrosion and the Preparation of Metallic Surfaces for Painting, Federation Series on Coating Technology; Federation of Societies for Paint Technology: Philadelphia, PA, 1978;Unit 26,pp 5-50. Hill, L. W.; Kaul, A.; Kozlowski, K.; Santer, J. 0. Accelerated Weathering (QUV) of Acrylic Clearcoats Crosslinked with Etherified Melamine Formaldehyde Resina. Polym. Mater. Sci. Eng. 1988,59,283-288. Hubrecht, J.; Vereecken, J.; Piens, M. Corrosion Monitoring of Iron, Protected by an Organic Coating, with the Aid of Impedance Measurements. J. Electrochem. SOC.1984,131,201+2015. Mcewan, 1. H. Aqueous Dispersion Enamels a New Automotive Topcoat Technology. J. Water Borne Coat. 1979, Nov, 3-10. McGlynn, S . P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy of the Triplet State; Prentice-Hall: Englewood Cliffs, NJ, 1969. MiBkoviE-StankoviE, V. B.;DraziE, D. M.; AEamovi, N. M. Determination of the Protective Properties of Cationic Primers with A.C. Impedance Measurements. J. Coat. Technol. 1991, 63, 25-29. Ng, H. N.; Calvo, C. Refinement of the Crystal Structure of the Low-Quartz Modification of Ferric Phosphate. Can. J. Chem. 1975,53,2064-2067. Perfetti, G. A.; Darlington, H. Coated Sheet Metal and Method of Forming Products Therefrom. U.S. Patent 4,032,678,1977,June 28. Ring, J.; French, D.; Hickling, M.; Sturgess, M. G. Synthetic Resina and Coating Compositions Containing Them. U.S. Patent 4,508,765,1985,April 2. Sato, N.; Minami, T. Influence of Metal Components in Hopeite Films on IR and Laser Raman Spectra. J. Mater. Sci. 1989,24, 3375-3379. Scully, J. 'Electrochemical Impedance Spedroscopy for Evaluation of Organic Coating Deterioration and under Film Corrosion A State of the Art Technical Review"; David W. Taylor Research Center Report SME-86/ooS,September 1986. Smith, J. P.; Lehr, J. R.; Brown, W. E. Crystallography of Monocalcium and Dicalcium Phosphates. Am. Mineral. 1955, 40, 893-899. Spadafora, S. J.; Hegedus, C. R.; Hirst, D. J.; Eng, A. T. Primerless Finishing Systems for Aluminum Substrates. Mod. Paint Coat. 1990,Sept, 36-48. Waldie, J. M. et al., Eds.Surface Coatings; Tafe Educational Books. Randwick, Australia, 19U;Vol. 2,p 459. Received for review May 15, 1991 Accepted September 26,1991
