Performance of in-Situ Phosphatizing Reagents in Solvent-Borne

The new formulation is then coated and cured on a cold-rolled steel surface. The chemical reactivity of phosphatizing reagents are studied via differe...
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Ind. Eng. Chem. Res. 1997, 36, 368-374

Performance of in-Situ Phosphatizing Reagents in Solvent-Borne Paints Tao Yu and Chhiu-Tsu Lin* Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60115-2862

A novel in-situ phosphating coating system is developed by predispersing an optimum amount of phosphatizing reagents into the paint formulation. Five possible in-situ phosphatizing reagents are used in the formulation of a model solvent-borne polyester-melamine enamel paint system. The new formulation is then coated and cured on a cold-rolled steel surface. The chemical reactivity of phosphatizing reagents are studied via different analytical techniques. The results can be summarized as follows: (i) the phosphatizing reagents, being dispersed into the solventborne paints to form a stable self-phosphating organic coating, perform a successful in-situ phosphatization of the metal substrate; (ii) there is an observable lowering of Tg for the paint films, due to the acceleration of melamine self-condensation by the acidic nature of phosphatizing reagents, which can be corrected by optimizing the coating formulation; (iii) the organic phosphatizing reagents, which are more dispersable in a solvent-borne coating system, are very effective yet have less influence on the cross-linking density of the final film, thus giving a coating with higher AC impedance and providing a better corrosion protective performance. I. Introduction Phosphating is the most widely used metal pretreating process for the surface finishing of ferrous and nonferrous metals (Van Wazer, 1961). The corrosion protection performance of such finished products is determined mainly by the quality of the phosphate coating and its coverage. In today’s metal-finishing industry, a multistage phosphating line (Freeman, 1986), which involves degreasing, hot or cold immersion phosphate conversion, and rinsing, is commonly used. Although thin phosphate film morphology and composition can be varied by manipulating the deposition parameters (Albu-Yaron and Aravot, 1993; Fadrizi et al., 1989), typical conversion coating processes yield crystalline metal phosphate hydrate films such as Zn2Fe(PO4)2‚4H2O, Zn3(PO4)‚2H2O, Fe3(PO4)2‚8H2O, FeHPO4‚H2O, and FePO4‚2H2O. The films thus obtained, however, usually bear a porosity of 0.5%-1.5% of the total substrate surface area. A sealing procedure typically utilizing toxic chromium compounds (Cr6+), or a more recent procedure using a less toxic phosphate/ molybdate synergism (Hegedus et al., 1992, 1991) or some organic inhibitor solutions, has to be employed for the temporary protection of the uncoated area. A novel in-situ phosphatizing coating (ISPC) has been developed in our laboratory (Lin and Li, 1994; Lin, 1994; Lin et al., 1992). It is prepared by predispersing an optimum amount of in-situ phosphatizing reagent (ISPR) in the paint system to form a stable and compatible coating formulation. When applied on a metal substrate, it allows the phosphate conversion process to take place in-situ via the reaction of ISPR with the metal surface. Simultaneously, ISPR reacts with the polymers, forming strong covalent bonds of P-O-C and/ or P-C-C, linking the metal phosphate thin film and the polymer resin matrix. These “simultaneous” reactions provide metal substrates with a corrosion protective barrier without the need for a chromate or primer finishing step. It might also be possible to eventually eliminate the pretreatment and/or posttreatment steps * To whom correspondence should be addressed. E-mail: [email protected]. S0888-5885(96)00257-6 CCC: $14.00

and to obtain a coating providing a better adhesion and corrosion inhibition to the substrate. The development of ISPC relies critically on the selection of an effective ISPR, which forms a stable system with various designed properties when applied and cured on the metal substrate. In our previous work (Yu et al., 1995), we carried out a systematic selection procedure for possible ISPRs which could be used in the thermally cured ISPC system. Fourteen different forms of phosphoric acid, phosphonic acid, ester phosphoric acid, or ester phosphonic acid were tested in a “solventborne simulation system” (referred to as system S) for their effectiveness in forming the metal phosphate film on the cold-rolled steel (CRS) surface. The chemical affinity of the in-situ phosphatizing surface reaction was discussed. For the present experiment, we chose five positively effective ISPRs that resulted from the previous research and extended them into a “thermally cured solvent-borne model coating system”, a polyestermelamine enamel (referred to as system M). The coating system M was applied and cured on a CRS surface. The chemical reactivity of these ISPRs in this model paint was examined and compared to that in the simulation system. The influence of ISPRs on the coating’s polymer chemistry as well as the overall corrosion resistance performance of the coating was evaluated. II. Experimental Section Polyester resin (Cargill resin 5778), cross-linker hexakis(methoxymethyl)melamine (Cymel 303), p-toluenesulfonic acid (p-TSA, cross-linking catalyst, Cycat), in-situ phosphatizing reagents as a concentration of 1.0%-3.0% by weight to the resin, and organic solvents are used to formulate the stable and compatible model solvent-borne ISPC. The five selected ISPRs are 85% phosphoric acid (H3PO4), phenyl phosphoric acid [(C6H5O)(HO)2PO], phosphinic acid [H(HO)2PO], phenylphosphonic acid [(C6H5)(HO)2PO], and diphenyl phosphate [(C6H5O)2(HO)PO]. The major solvent is 2-(2butoxyethoxy)ethanol (butylcarbitol). All chemicals are used as received. For surface analysis, the bare cold-rolled steel (CRS) panels (“Q” panels, from the Q-Panel Company) are © 1997 American Chemical Society

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Figure 1. FTIR results of using phosphoric acid as the ISPR in systems M and S.

mechanically polished to give a mirror-finished surface and are used for ISPC. The coated CRS panels are cured at 325 °F for 10 min. The techniques and procedures of 80° grazing angle Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy combined with energy-dispersive X-ray spectrometry (SEM-EDS), and electrochemical impedance spectroscopy (EIS) measurements have been reported previously (Yu et al., 1995; Li and Lin, 1994; Lin, 1994; Lin et al., 1992). For thermal analysis, the polymer top coating film of the ISPC is peeled mechanically from the sample panel and subjected to glass transition temperature (Tg) measurement using a differential scanning calorimeter (DSC), a Seiko DSC 220C (Seiko Scientific Instruments) equipped with a Seiko SSC5200H Disk Station for scanning program manipulation and data analysis. To release the stress resulting from the mechanical peeling and ISPC film-sampling procedure, as well as to erase its thermal history, the sample is first annealed for 2 min inside the DSC chamber at 100 °C, a temperature above the Tg of the polyester samples and well below the curing temperature used in this system. The DSC scanning is conducted from -60 to 100 °C with the scanning speed of 10 °C per minute. III. Results and Discussion A. Verification of Phosphate Fundamental in in-Situ Phosphatizing Coatings. In our previous work (Yu et al., 1995), we used a solvent-borne simulation system to help investigating the phosphatizing reaction of the ISPCs and the effectiveness of metal phosphatization in various ISPRs. The functional group responsible for the phosphating reactions was shown to result from the acid-base type reaction of P-O- with the metal ion, rather than the weak induced dipole interactions. Five possible in-situ phosphatizing reagents, which successfully displayed their effect in the simulation system (system S), are formulated into the model solvent-borne self-phosphate polyester (system M), to form stable and homogeneous coatings. The reactivities of these in-situ phosphatizing reagents in system M are compared with those in system S. We designed the solvent combination and thermal curing schedule of systems S and M to be similar to those used in the commercial paint system. The in-situ phos-

phatizing reagents are thus expected to behave similarly in the surface phosphatizing reactions in all systems. As it can be noted that there is no polymer resin in system S, the only difference lies in their ability to diffuse to and react with the metal substrate surface. Phosphoric acid ([(HO)3PO]) is the most commonly used phosphatizing media in today’s commercial multistep phosphate lines; it is also employed as an in-situ phosphatizing reagent in the ISPC. Its effectiveness in forming the metal phosphate thin film on the CRS surface is thoroughly characterized by FTIR and SEMEDS. Figure 1 shows the IR spectra of metal phosphate thin film resulting in-situ from system M (a) and system S (b). In spectrum 1a, the degeneracy of both ν3 and ν4 vibrational modes in PO43- for the interfacial iron phosphate layer on the CRS panel becomes united and is observed splitting into the peaks at 1195/1147/1105 and 625/521 cm-1, respectively. The splitting of degeneracy, which results from the symmetry distortion by interaction with the surrounding crystalline structure (Osaka et al., 1984), has been used to support the formation of crystalline iron phosphate in the ISPC. The formation of a strong covalent bond P-O-C linkage is evidenced at 934 cm-1, confirming the reaction of phosphatizing reagents with the polymer matrix of the coating. This indicates that the phosphate chemistry and the polymer chemistry in the ISPC can proceed independently and simultaneously. In spectrum 1b, the splitting of degeneracy of both ν3 (1146/1119/1107 cm-1) and ν4 (622/529 cm-1) vibrational modes in PO43- is observed; i.e., they are crystalline in nature. On the other hand, as expected for the surface phosphatization of system S, no P-O-C band vibration at 934 cm-1 is detected in Figure 1b. The formation of metal phosphate thin film on the CRS substrate for both systems is further verified by the appearance of phosphorus (P) peak at 2.02 keV in the EDS spectra, as displayed in Figure 2c (System M) and d (System S), both using H3PO4 as the in-situ phosphatizing reagent. This confirms that phosphoric acid is an effective ISPR in both system S and system M. The morphology of the metal phosphate layer is revealed by SEM characterization, as shown in Figure 2a (system M) and (system S). Figure 2a shows the formation of a very fine uniform thin film (presumably the amorphous/crystalline iron phosphate) on the metal surface. In comparison, Figure 2b shows

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Figure 2. SEM-EDS spectra of the metal phosphate layer generated by using H3PO4 in system M (a, c) and system S (b, d).

a layer with similar surface coverage, except for some thicker spots (a clustering of bead like crystals) which might have resulted from the localized surface reaction of some high-density phosphatizing reagents in system S. Similar to H3PO4, phosphinic acid [H(OH)2PO] is also tested and shown to perform as an in-situ phosphating reagent in systems M and S. The FTIR and SEM-EDS characterizations indicate that the chemical nature of surface phosphatization is essentially the same as that of H3PO4, i.e. forming a metal phosphate thin film on the CRS surface, with almost exactly the same IR spectrum and quite similar morphology under the SEM characterization. Being hydrophilic itself, phosphoric acid or phosphinic acid is not a preferred phosphatizing reagent in the hydrophobic solvent-borne organic coating systems. This is especially true in a solvent-borne paint with pigments, where long-term stability for such coating is desired for satisfactory paint shelflife. In these systems, it is more suitable to use the organic phosphatizing reagents. Starting from the standard of phosphoric acid, we tested two of its organic monosubstituted derivatives, phenylphosphonic acid and phenyl phosphoric acid, as insitu phosphatizing reagents in both systems S and M. Figure 3 shows the IR spectra of metal phosphate formed on the CRS surface using phenylphosphonic acid as the ISPR in System M (Figure 3a) as compared to that obtained from the solvent-borne system S (Figure 3b). The IR spectra of the metal phosphate layer generated by using phenylphosphoric acid in these two systems are shown in the spectra 3c,d. For the set of spectra using phenyl phosphonic acid as the ISPR, there is an apparent difference in the intensity of the corresponding peaks, presumably due to the difference in the effective diffusion of the phosphatizing reagents that react with the metal surface. The peaks are shown to appear at the same frequencies in the spectra, suggest-

ing that the metal phosphate thin films formed in the two systems have extremely similar chemical structure. For system M (spectrum 3a), a peak at 943 cm-1 is clearly observed that corresponds to the formation of the P-O-C linkage, i.e. the covalent bonding of phenylphosphonic acid with the polymeric coating matrix, the polyester-melamine enamel. For system S (spectra 3b), no such IR peak is resolved. Instead, it shows an intense band at 1178 cm-1, resulting from the C-H bending on the phenyl ring. Figure 4 shows the SEM and EDS spectra in system S for the metal phosphate layer on the CRS surface, generated by using phenylphosphonic acid and phenyl phosphoric acid as the in-situ phosphatizing reagents. Using phenylphosphonic acid, a uniform formation of metal phosphate thin layer on the substrate surface with almost 100% coverage is evidenced in Figure 4a. Some thicker iron phosphate layers are also noticed at certain locations on the substrate surface. The EDS spectrum in Figure 4b shows the formation of a metal phosphate thin layer with a sharp peak at 2.02 keV for the element phosphorus. The thicker iron phosphate layer gives a larger ratio of phosphorus to iron in the EDS spectrum, proving the presence of more phosphorus. For the system using phenyl phosphoric acid, the formation of a localized growth of iron phosphate structure is observed in its SEM spectrum, shown in Figure 4b, while the actual phosphating effect is confirmed by the EDS spectrum in Figure 4d. The difference in the metal phosphate coverage suggests the difference in the surface reactivity of these two organic phosphatizing reagents. More importantly, an uniform metal phosphate coverage on the substrate is very critical to a good corrosion resistance performance of the surface phosphatization. Since phenyl phosphoric acid can only provide an extensively localized metal phosphate protection, phenylphosphonic acid, which shows

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Figure 3. FTIR results for using monosubstituted organic ISPRs in systems M and S: phenylphosphonic acid (a, b); phenyl phosphoric acid (c, d).

much more uniform products, should be preferred as an ISPR in the solvent-borne systems. The disubstituted organic phosphate derivatives were also considered as the organic phosphatizing reagents. While diphenylphosphonic acid shows a poor solubility in both aqueous and solvent systems, diphenyl phosphate [(C6H5O)2(HO)PO] displays a metal surface phosphatization in both systems S and M. Figure 5 is a comparison of FTIR spectra for the metal phosphate layer obtained by using diphenyl phosphate in system M (5a) and system S (5b). Except for the variation of spectral peak intensities, the two spectra are almost superimposable. This observation suggests that diphenyl phosphate can also be an in-situ phosphatizing reagent for the ISPC system. Since diphenyl phosphate already has a P-O-C bonding in its molecular structure, around 964 cm-1 in the IR spectrum, it is hard to analyze its possible linkage with the coating polymer matrix. B. Influence of the Phosphatizing Reagents on the Polymer Chemistry of the ISPC. The ISPC system under investigation is a polyester-melamine enamel. The polymer coating film forms via a crosslinking reaction. The polymerization is a cocondensation of the polyester backbone and the melamine crosslinker through the hydroxyl groups of the polyester and the amino groups in the melamine. This reaction is strongly acid catalyzed, while the acid-catalyzing mechanism and efficiency rely mainly on the structure and the acidity of the catalyst. It is reported (Wicks et al., 1992; Gan et al., 1989; Blank, 1982; Berge et al., 1970) that only certain types of acids, and in their optimum

amounts, can be effective in catalyzing this major crosslinking of polyester and melamine. In general, acids also catalyze the self-condensation of melamine molecules, producing a polyester network with a significantly lowered cross-linking density along with some melamine clustering defects. Because the phosphatizing reagents discussed in this paper are essentially acidic in nature, a study concerning their influence on the cross-linking reaction in the polyester system M is necessary. Theoretically, a higher Tg value for the cured paint film usually corresponds to a network with higher crosslinking density (Chang, 1992; Hale et al., 1991). For the polymer network formed in the ISPC system, the glass transition temperature (Tg) can be an index for the cross-linking conversion, although the relationship is rather complicated and cannot be expected to be linear. We studied the cross-linking density change in the resulting polymeric film of the self-phosphate coatings using different ISPRs, by measuring their Tg values. The results are listed in Table 1. From Table 1, it is apparent that all the coating films formed with the existence of acidic phosphatizing reagents show a lower Tg value than the control sample, which is void of any acid of ISPRs. This observation suggests a decrease in degree of overall cross-linking in the paint film polymer network, when ISPR is present in the coating system. However, this lowering on Tg is of different degrees, depending on the phosphatizing reagent species being used. This result is consistent with the theory (Wicks et al., 1992; Samaraweera et al., 1992; Gan et al., 1989; Blank, 1982) that states only

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Figure 4. SEM-EDS spectra of the metal phosphate layer generated by using phenylphosphonic acid (a, c) and phenyl phosphoric acid (b, d) in system S.

Figure 5. FTIR results for using diphenyl phosphate as the ISPR in systems M and S. Table 1. Glass Transition Temperatures of Various ISPCs in-situ phosphatizing reagent

Tg (°C)

control sample [none] phosphoric acid [H3PO4] phosphinic acid [H(HO)2PO] phenylphosphonic acid [(C6H5)(HO)2PO] phenyl phosphoric acid [(C6H5O)(HO)2PO] diphenyl phosphate [(C6H5O)2(HO)PO]

31.1 11.0 8.4 24.1 20.8 28.4

specific acids in their optimum concentrations can effectively catalyze the polymer cross-linking reaction in the polyester-melamine enamel, while all others will cause melamine self-condensation and lower the polymer network cross-linking density. Obviously, among

the ISPRs we have tested, the organic reagents (diphenyl phosphate, phenylphosphonic acid, and phenyl phosphoric acid) result in a higher cross-linking density as compared to the inorganic ones (phosphoric acid and phosphinic acid) and thus are preferred in the ISPCs. On the basis of the polymer chemistry behind the apparent lowering of Tg, we adjusted the coating formulation by incorporating a slightly larger amount of melamine into the coating. This leads to a higher Tg (36.0 °C) for the control sample, while the corresponding system M using H3PO4 gives a Tg of 20.3 °C. This observation suggests to us that, by adjusting the backbone polymer to cross-linker ratio in the formulation, the side effect of lowering cross-linking density can be reduced.

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Figure 6. Bode plot of EIS results for using various ISPRs in system M: the control sample, no ISPRs (a); using phosphoric acid (b); phosphinic acid (c), phenylphosphonic acid (d), phenyl phosphoric acid (e), and diphenyl phosphate (f) as ISPRs.

C. The Corrosion Resistance Evaluation of the ISPCs. Various industrial standard methods can be used to evaluate the corrosion resistance of the coatings, while most of these methods take a considerably long time and extrapolation of these accelerated testing results to the real life situation is questionable. In recent years, electrochemical impedance spectroscopy (EIS) has been used in coating evaluation. It provides a rapid, nondestructive means for characterizing not only the corrosion rate but also the corrosion mechanism of metals in a variety of environments. The EIS method is used to evaluate the quality of the industrial phosphate conversion on the bare metal surface (Sankara Narayanan and Subbaiyan, 1994, 1992; Bretherton et al., 1993), as well as phosphated metal surfaces with an organic top coating (Kendig et al., 1990; Vijayan, 1986). We utilize this method to evaluate the corrosion resistance of model system M, containing various ISPRs. Two major factors contribute to the overall corrosion resistance performance of self-phosphate coatings. First, the formation of the nonconductive interfacial metal phosphate thin film under the organic coating layer considerably enhances the corrosion resistance. Secondly, the permeability degradation of the paint film, which depends on the lowered degree of cross-linking in the polymer network, is less favored for corrosion protection. Figure 6 shows a Bode impedance plot for various ISPCs. The plots reveal the AC impedance response of the coating (including the interfacial metal phosphate thin film) to the frequency changes of the external field. According to electrochemical principles, the logarithm of the AC impedance modulus should have a linear relationship with log frequency in the Bode impedance plot. Deviations from the linearity at the lower frequency region reflects the onset of conducting pathways through the coating, which is related to the corrosion of the metal surface (Meldrum and Lin, 1993; Hepburn et al., 1986). The higher impedance in the lowfrequency region of the Bode plot is often indicative of better corrosion resistance by the coating. Figure 6 shows a better corrosion resistive performance for system M which contains different ISPRs as compared to the control sample which is void of ISPRs. The organic phosphatizing reagent, phenylphosphonic acid, shows the best corrosion inhibition performance. This result is consistent with the fact that a uniform thin layer of metal phosphate is obtained interfacially, as well as the fact that phenylphosphonic acid does not hinder the formation of the polymer network as do the rest of the ISPRs tested. Although structurally similar,

the system using phenyl phosphoric acid as the ISPR gives a less corrosion resistive coating, due to the localized nature of the interfacial metal phosphate. The disubstituted diphenyl phosphate, even though it forms a uniform layer of metal phosphate and the polymer cross-linking is not hindered, still cannot provide a corrosion resistance as good as that of phenylphosphonic acid. This may be explained by its monofunctionality of P-O- and the stereohindrance stemming from the additional phenyl group, so that the disubstituted diphenyl phosphate cannot provide a strong covalent bonding to the polymer matrix to enhance coating adhesion. As compared to the organic phosphatizing reagents, the ISPC systems utilizing the inorganic ISPRs, namely phosphoric and phosphinic acids, show a reasonable corrosion resistance performance, although they significantly reduce the degree of cross-linking of the polymer network. This result again proves the significant role that metal phosphating plays in the corrosion resistance performance of ISPCs. Also, it should be noted that due to their high hydrophilicity and incompatibility with pigments, these inorganic reagents are not good candidates for the solvent-borne ISPRs. IV. Conclusion Five possible in-situ phosphatizing reagents employed in simulation system S have been used successfully in the formulation of model paint system M, a stable ISPC of solvent-borne polyester-melamine enamel. The effectiveness in forming the interfacial metal phosphate thin film on the CRS surface has been evidenced by the FTIR and SEM-EDS characterizations. Aside from the possible differences in their diffusions to the metal surface, the phosphatizing reagents show very similar reactivity in both systems S and M. This confirms the applicability of the formulation for commercial selfphosphate-coatings. The results also confirm that the organic phosphatizing reagents, which are compatible to the solvent-borne coating systems and are preferred to those inorganic ones in the ISPCs, can be effective as phosphatizing reagents. The study on the bulk polymeric coating film suggests that the acidic in-situ phosphatizing reagents will also influence the filmforming reactions in the polymer matrix, lowering the cross-linking density in the paint film. This influence on the polymer cross-linking can be compensated by adjusting the coating formulation, namely the backbone polymer to cross-linker ratio. The organic phosphatizing reagents, which apparently show less impact on the cross-linking, are also preferred in the polyestermelamine system. The in-situ phosphatizing coatings, no matter which phosphatizing reagent is used, show a better corrosion resistance performance than the control sample as predicted by the EIS data. Among the five phosphatizing reagents tested, phenylphosphonic acid shows the most ideal result in forming a self-phosphate polyester-melamine enamel system, and it provides a model coating with the best corrosion inhibition result. Acknowledgment Financial support from the National Science Foundation, Grant CTS-9312875, and the Illinois Hazardous Waste Research and Information Center, ENR Contract No. RRT19, is acknowledged. The authors also thank Dr. Li Li and Mr. Yi-Yuan Chuang for their technical assistance and Prof. Alan P. Genis for the use of SEMEDS system.

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Received for review May 8, 1996 Revised manuscript received October 21, 1996 Accepted October 25, 1996X IE960257Z

X Abstract published in Advance ACS Abstracts, December 15, 1996.