J. Phys. Chem. 1995,99, 7613-7620
7613
Chemical Affinity of in-Situ Phosphatizing Reagents on Cold-Rolled Steel T. Yu,L. Li, and C.T. Lin* Department of Chemistry, Northem Illinois University, DeKalb, Illinois 60115-2862 Received: June 7, 1994; In Final Form: August 31, 1994@
Self-phosphating organic coating is a novel finishing technology that can be prepared by predispersing an optimal amount of phosphatizing reagent in the paint formulation. Fourteen phosphate compounds, including different forms of phosphoric acid, phosphonic acid, ester phosphoric acid, or ester phosphonic acid, were tested for their possible use as in-situ phosphatizing reagents (ISPRs). The cold-rolled steel (CRS) panel was soaked in a dilute solution of 0.5-10 wt 5% phosphate compound in a solvent mixture of butyl carbitol and 2-propanol and then baked at 325 OF for 10 min. The nature of chemical bonding, surface/interface morphology, and elemental composition of metal phosphate formed at the CRS surface was probed by 80” grazing angle FTIR and SEM-EDS techniques. The results are summarized as follows: (1) organic phosphatizing reagents are more dispersible in in-situ phosphatizing coatings (ISPCs) and can become very effective ISPRs; (2) the nature of metal phosphate bondings in ISPCs is via an acid-base type interaction, PO--Fe2+, rather than an induced dipole interaction of P=O/Fe complex type; (3) ISPRs containing polymer chains may be good as coating adhesion promoters, but some of them are not very reactive with metal surfaces; they are also not effective as in-situ phosphatizing reagents; (4)water in ISPRs can accelerate the metal surface phosphatization; and (5) the ISPRs/amine complex can provide a long pot life of ISPCs while giving an identical in-situ metal surface phosphatization as the uncomplexed form of ISPRs. The feasibility of using organic versus inorganic ISPRs in the ISPCs will be compared and discussed.
Introduction The protective coating industry, although widely referred to as a “mature” industry, is under enormous pressure for change. Because of environmental, health, economic, and technical factors, there is a continuing and ever-present need for new and more durable coating materials. Recently, the need has shifted to coatings with (1) reduced levels of volatile organic compounds (VOCs),’ (2) “cleaner” alternatives to toxic paint components such as lead, cadmium, chromium, cyanide, and chlorinated solvents,’ and (3) innovative in-situ phosphatization approaches2 to eliminate several process steps, save valuable natural resources, and reduce pollution problems during manufacturing. In the current state-of-the-art multistep coating process, the quality of the metal finish required by an industrial product determines the degree to which the surface pretreatment and phosphating are carried out. Normally, the metal is cleaned by several possible pretreatment steps, phosphated, sealed, dried, and then painted. The phosphating and sealing steps are generally done by a phosphating linebath which is referred to as the conventional zinc and iron phosphate conversion coati n g ~ .Typically, ~ conversion coating processes yield water insoluble crystalline hydrate films such as Zn3(P04)2*4HzOor *2H2O,Zn~Fe(P04)2*4H20, Fe3(P04)2*8H20or -4H20, and Mn3(P04)2*3HzO. To precipitate these hydrate compounds on the metal surfaces, an aqueous medium at high temperature is required. Industrial phosphating is based on the fact! that the metal surface actively corrodes with the formation of anodic and cathodic areas that facilitate the precipitation of phosphate crystals. As soon as 95-99% of the substrate surface is covered with phosphate, the active deposition of phosphate stops and some of the metal surface remains uncoated. The 1-5%
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Absrracts, May 1, 1995.
uncoated metal surface areas exist normally as pores5 of metal phosphate in the conversion coatings that begin to deteriorate shortly after application if not quickly recoated with a suitable primer or finish. Toxic compounds such as cadmium, lead, and chromium and, more recently, nontoxic ones such as phosphate/ molybdate synergism6or organic inhibitor solutions7have been used to provide temporary protection to the uncoated areas. These chemical additives are environmentally threatening as waste products through drag out and inevitable accumulation of waste water. An experienced operator must also solve the added problems of sludge production in the bath and its removal and disposal before deposits occur on the treatment surface. It would be advantageous to have a novel surface conversion process that provides complete coverage of the base metal without the need for a finishing step. This novel surface conversion technique is the in-situ phosphatizing coating (ISPC) which was developed recently in our laboratory.2 In ISPC, an optimum amount of in-situ phosphatizingreagent is predispersed in the paint system to form a stable and compatible coating formulation. When the single-coat, in-situ self-phosphating paint is applied on a metal substrate, the phosphatizing reagent is available to chemically andor physically react in-situ with the metal surface to produce a metal phosphate layer that could cover 100% of the surface and simultaneously to form covalent bonding of P-0-C and/or P-C-C linkages (Le., chemical functionality)with polymer resin. The “simultaneous” reaction of the in-situ phosphatizing reagent with metal substrate and polymer resin should provide the corrosion protective barrier without the need for a chromate or primer finishing step and should enhance the coating adhesion. Both inorganic*and organic9phosphate compoundshave been used commonly in the multistep coating industries, but there are only a couple that have been tested recently in the in-situ phosphatizing coatings.* Some of these phosphate compounds are also used as corrosion inhibitors5and adhesion promotersI0 of metal surfaces. The chemical and physical fundamentals at
0022-3654/95/2099-7613$09.00/0 0 1995 American Chemical Society
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7614 J. Phys. Chem., Vol. 99, No. 19, I995
the molecular level of metal phosphate formed at the coating interface have never been systematically investigated. In this paper, 14 phosphate compounds, selected from the different forms of phosphoric acid, phosphonic acid, ester phosphoric acid, or ester phosphonic acid, were tested for the possible use as in-situ phosphatizing reagents (ISPRs) in solvent-based, water-borne, thermal-cured, UV/electron beam (EB) cured, and air-dried paints. The solution preparation and application, thermal curing, and performance verification of phosphatizing reagents on cold-rolled steel surface were simulated to have exactly the same conditions as those in ISPCs. The chemical bonding and crystalline/amorphous natures of metal phosphates resulting from either a PO--Fe2+ ionic bond or a P=O/Fe complex are examined. The surface coverage and interfacial composition of metal phosphate are interpreted in terms of the chemical reactivity of ISPRs on cold-rolled steel (CRS) panels. The preferred ISPRs for the ISPCs of specific industrial applications will be indicated.
were viewed by scanning electron microscopy (SEM), an IS1 Model DS-130 (InternationalScientific Instruments, Inc.). The elemental composition and chemical reactivity of ISPRs on CRS panels were monitored by energy-dispersive X-ray spectrometry, an EDAX PV9900 system (North American Philips Co.). The relative quantification was done by fixing the acceleration voltages and data acquisition conditions.
Results and Discussion
a. Verification of Phosphate Fundamentals in the Simulated System. H3P04 has been successfully predispersed in the self-phosphating polyester-melamine enamels that were illustrated2to be capable of catalyzing the polymer chemistry and simultaneously activating the phosphate chemistry. The surface morphology of metal phosphate at the polymer/phosphate/ metal interface prepared by using ISPCs (e.g., a unicoat system comprising Cargill 5778 polyester resin, Cyme1 303 melamine cross-linker, and 1.5 wt % H3P04) was viewed by SEM and displayed a very fine uniform but porous thin film.* The Experimental Section appearance of iron (Fe) and phosphorus (P) peaks in the energyThe selected phosphatizing reagents are as follows: 85% dispersive X-ray (EDS) spectrum has identified the surface layer of metal phosphate product on CRS panels as iron phosphate. phosphoric acid (H3P04) was obtained from Fisher Scientific In FTIR, the degeneracy of both v3 and v4 vibration modes in Co.; monophenylphosphoric acid [(C6&0)(HO)2PO] was obtained from TCI America; phosphonic acid [H(HO)zPO], Po43- for the interfacial iron phosphate layer on CRS panels phenylphosphonic acid [(C6H5)(H0)2PO], diphenylphosphinic becomes united and is observed2 to split into 1195/1147/1105 acid [(C~H~)Z(HO)PO], triphenylphosphine oxide [(C~HS)~PO], and 625/521 cm-I, respectively. The splitting of degeneracy diphenyl phosphate [(C&0)2(HO)PO], triphenyl phosphate that resulted from the symmetry distortion by interaction with [(C6H50)3PO], and 2-aminoethyl dihydrogen phosphate the surrounding crystalline structure" has been used to support the formation of crystalline iron phosphate in ISPCs. A strong [(NHzC~&O)(HO)~PO] were purchased from Aldrich Chemical Co.; and (RO)(H0)2PO, where R is a polymer chain containing covalent bond P-0-C linkage is evidenced at 934 cm-I, an oxirane and hydroxyl functionality(LZ 2061), a hydrocarbon indicating that the phosphate chemistry and the polymer functionality (LZ 2062), and a polyester and hydroxyl funcchemistry in ISPCs can proceed independently and simultationality (LZ 2063), was provided by Lubrizol Performance neously. Products Co. The acid complexing agent triethylamine (TEA) Parts a, b, and c of Figure 1 display the FTIR, EDS, and and solvents 2-propanol and 2-(2-butoxyethoxy)ethanol (butylSEM spectrum, respectively, for the iron phosphate layer carbitol) were obtained from Aldrich Chemical Co. All generated by processing a CRS panel with 10 wt % H3P04 in chemicals were used as received. the simulated system for 1 h. The production of a thick metal To simulate the chemical reactivity of ISPRs on the CRS phosphate layer is expected since a high concentration of surface, the solution of phosphatizing reagent was prepared in phosphatizing reagent and a long processing time for the CRS the same manner as that predispersed in ISPCs. A solvent panel are used. The thick metal phosphate layer seems to mixture of 95 wt % butylcarbitol and 5 wt % 2-propanol was resemble that of the conventional phosphate conversion coating selected to prepare the phosphatizing solution, where the in the multistep coating te~hnique.~ Surface phosphatization concentration of phosphatizing reagent ranges from 0.5 to 10 of CRS panels using low concentration of H3P04 in the wt %. It is noted that the solubility of diphenylphosphinic acid simulated system or a short reaction time produced an interfacial in the designed solvent mixture is quite low, Le., 0.5 wt %. metal phosphate thin film similar to those in the self-phosphating The uncoated CRS panels (SAE 1010: C 0.08-0.13%, Mn 0.3paints. The splitting of degeneracy of both v3 (1 141/1109/1007 0.6%, P(max) 0.04%, S(max) 0.05%) were obtained from cm-' ) and v4 (612/563 cm-' ) vibration modes in P043- is Q-Panel Co. and mechanically polished to a mirror-finished observed for both the thick layer (spectrum la) and thin film2 surface before use. A solution-dipping technique was used to of iron phosphate; that is, they are crystalline in nature. No activate the CRS surface. The polished CRS panel was soaked P-0-C band vibration at 934 cm-' is shown in Figure la, as in the phosphatizing solution for 1 h and removed, and the expected for the surface phosphatization of the simulated system. excessive solution was allowed to drip off before it was The thick layer of iron phosphate is represented by a high EDS thermally cured at 325 "F for 10 min. The surface/interfacial peak intensity of phosphorus (P) at 2.02 keV, as shown in Figure products other than crystalline metal phosphate were removed lb. In Figure IC, the SEM surface morphology of the thick by rinsing with the solvent mixture and water and then soaking iron phosphate layer shows that the layer can be cracked and in tetrahydrofuran solvent for 48 h. peeled off easily. When the EDS spectrum is taken at the peeled-off site, no or only a very small amount of iron phosphate The interfacial metal phosphate layer was rinsed with is detected. The results verified that the phosphate fundamentals deionized water, dried, and characterized by a Mattson Cygnus in the simulated system are the same as those in the in-situ 25 FTIR spectrophotometer equipped with a Spectra Tech FTphosphatizing coatings.2 In ISPCs, the fine uniform thin metal 80 grazing angle accessory. The reflection transmittance spectra phosphate film provides corrosion protection of the metal of a thin coating are produced by ratioing the single-beam substrate and strong adhesion of polymer coatings.2b.Is On the spectrum of the same substrate to that with the thin coating. other hand, the cracked and peeled thick iron phosphate layer The interferometer is purged with dry nitrogen. Water, carbon generated in the simulated system (or in the multistep conversion dioxide, and base-line corrections are necessary in most cases. coatings3)should have an inferior protection of metal surfaces. The surface morphology and metal phosphate layer coverage
J. Phys. Chem., Vol. 99, No. 19, 1995 7615
Chemical Affinity of Phosphatizing Reagents Microns 5.0
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Figure 1. Spectra of the iron phosphate layer on CRS generated by processing with a 10 wt% of phosphoric acid: (a) FTIR, (b) EDS, and (c) SEM.
b. Chemical Reactivity of Surface Phosphatization. The phosphatizing reagents selected for adding in the ISPCs should be easily dispersible and compatible with polymer film forming resins (binders, cross-linkers, etc.) and reactive with metal surface when applied on the substrate being protected. The binders are intrinsically hydrophobic in nature; thus, the organic phosphates should be the preferred ISPRs if their chemical affinity with the metal surface is equal to or better than the inorganic ones. The Simple Monosubstituted Phosphatizing Reagents: H(HO)zPO, (c6&)(HO)2Po, and ( c ~ H 5 o ) ( H o ) 2 P o . The [P043-] in the H3P04 molecule has a point group of Td, whereas its monosubstituted form of [P032-] in R(H0)2PO belongs to the C3" point group. The v3 (stretching vibration) in [P043-] appears at 1100-1000 cm-l and that in [P032-] occurs at 1030-970 cm-l.12 The v3 and v4 vibration modes in P043for the interfacial iron phosphate layer on the CRS panel produced by the ISPCs of polyester-melamine/H3P04 were observed2 at 1195/1147/1105 and 625/521 cm-', respectively. It is expected12that the surface iron phosphate layer on the CRS
panel prepared by reacting the substrate with simple monosubstituted phosphatizing reagents will have a lower vibration frequency. Figure 2 shows FTIR spectra of metal phosphate products on the CRS panel treated with H(H0)2PO (a), (C6Hs)(H0)2PO(b), and (C6H@)(H0)2PO (c) in the simulated system. As expected, the v3 and v4 vibration modes in spectra 2a, 2b, and 2c appear at 1062/985 and 606/491 cm-', at 1089/ 1069 and 564 cm-l, and at 1132/1116 and 555 cm-I, respectively. The v3 vibration frequency assigned in Figure 2c is lower than that in Figure la, but higher than that in Figure 2a,b. This suggests that the phosphatizing reagent (C6H50)(H0)2POmay be partially hydrolyzed to form [P043-] while reacting with the CRS panel. The P-0-phenyl linkage gives rise to two FTIR bands,12 as shown in Figure 2c. A strong absorption at 1227 cm-' is mainly due to the stretching of the C-0 bond of the phenoxy group, and the medium-intensity band at 940 cm-' can be attributed to the stretching of the P-0 bond. Other bands observed at 1595, 1493, and 779 cm-l in Figure 2c may be assigned to involve "quadrant stretching" of the ring C=C bonds, semicircle stretching of the carbon ring and C-H bending, and out-of-plane C-H wag mode, respectively. When a phenyl group is attached to a phosphorus, as in Figure 2b, the bands, due to the active aromatic C-H in-plane bending and out-of-plane deformation mode, are at 1151 and 749/724/
Yu et al.
7616 J. Phys. Chem., Vol. 99, No. 19, 1995 w=%p-y a r
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Figure 3. SEM of iron phosphate layer on CRS surface produced by phenylphosphonic acid: (a) a macrograph, and (b) a micrograph focused on the thicker parts of the surface. (c) and (d) are the corresponding EDS spectra of (a) and (b), separately.
694 cm-I, respectively. For an 80" grazing angle FTIR measurement, only dipole moments of chemical species that are normal to the metal surface will interact with the radiation and produce IR absorption bands. The intensity of FTIR bands in Figures la and 2 is only a measure of dipole activity and not the film thickness of the metal phosphate on the CRS panel. Both phosphonic acid, H(H0)2PO, and phenylphosphonic acid, (C6Hs)(H0)2PO, react with the metal substrate to form a uniform thin layer of iron phosphate on the CRS surface. On top of the thin iron phosphate layer, a thicker layer grown at some parts of the substrate surface is also noticed. The scanning electron micrograms for the thin and thick layer of iron phosphate on the CRS panel prepared by using (C6&)(H0)2PO in the simulated system are shown in parts a and b of Figure 3, respectively. The corresponding energy-dispersive X-ray spectra are listed in Figure 3c,d, separately. The formation of a smooth thick layer of iron phosphate on top of the thin uniform crystalline layer is evidenced in Figure 3d by the display of a strong sharp peak of phosphorous (P) in the EDS spectrum. The morphology of the thin iron phosphate film in Figure 3a has a dense crystalline-likestructure, and that of the thick layer in Figure 3b shows almost no cracking and no peel-off. These results indicate that the quality of the iron phosphate layer on the CRS surface prepared by phosphonic acid and phenylphosphonic acid is better than that of those treated by phosphoric acid (Figure IC). It is noted that K1 = 1.0 x lo-* for phosphonic acid and 7.1 x for phosphoric acid. If chemical reactivity of metal phosphatization in the in-situ phosphatizing coatings and in the simulated systems is an acidbase type of reaction, H(H0)2PO should be a more effective phosphatizing reagent than H3P04. The scanning electron micrograph of the metal phosphate layer on the CRS panel produced by (C6H50)(H0)2PO is
displayed in Figure 4a, which shows the formation of a localized growth of iron phosphate structure. The localized metal phosphate layer shows a cracked feature but a crystalline-like morphology under high magnification, as displayed in Figure 4b. The EDS spectrum gives a phosphorous (P) peak at 2.02 keV only on the localized structures (Figure 4d) and not on the other blank areas (Figure 4c) of the CRS surface. The surface phosphatization of the CRS substrate using monophenylphosphoric acid is not as effective as that employing phosphoric acid, suggesting that the acidity of former species is probably less than the latter ones. H3P04 produces a porous iron phosphate layer with a 1-5% uncovered surface area,5whereas (C6H50)(H0)2PO generates a localized iron phosphate layer with large void structures. For the thick iron phosphate layer, both (C6HsO)(H0)2PO and H3P04 produce a cracked surface morphology. This similarity may be due to the fact that monophenylphosphoric acid can be hydrolyzed easily to form [P043-], as in the case of H3P04. The Disubstituted Phosphatizing Reagents: (C6H5)2(H 0 ) P O and (C&O)2(HO)PO. (C6Hs)2(HO)PO is the only phosphatizing reagent in this study that shows low solubility in both polar and nonpolar solvents. The maximum amount soluble in the simulated solvent system (a mixture of 95% butylcarbitol and 5% 2-propanol) is only 0.5 wt %. (C,jH5)2(HO)PO is clearly not a suitable phosphatizingreagent for the in-situ phosphatizing coatings. Under the described procedure in the Experimental Section, diphenylphosphinic acid is not reactive with the CRS surface and does not produce iron phosphate film, as verified by FTIR and SEM-EDS techniques. To ensure that the unreactive nature of (C6H&(HO)PO on the CRS surface is not due to its low solubility, 0.5 wt % of (C6Hs0)2(HO)PO in the simulated system was prepared and used as surface phosphatizing reagent. Unlike diphenylphos-
Chemical Affinity of Phosphatizing Reagents
J. Phys. Chem., Vol. 99, No. 19, 1995 7617
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Figure 4. SEM of iron phosphate film on CRS treated with phenylphosphoric acid: (a) a macrograph, and (b) a micrograph focused on the localized iron phosphate region. (c) and (d) are the EDS spectra focused on the blank and localized iron phosphate region, respectively.
phinic acid, the surface phosphatization of the CRS substrate using diphenyl phosphate is very fast. The formation of a crystalline iron phosphate layer is shown in the SEM plates of Figure 5a (for a 0.5 wt % of diphenyl phosphate used) and 5b (for a 10 wt % of diphenyl phosphate used). Using 10 wt % of (C6H50)2(HO)PO in the simulated system, the iron phosphate layer displayed in Figure 5b gives a very strong FTIR spectrum (Figure 5c) and a sharp EDS peak of phosphorous at 2.02 keV (Figure 5d). The FTIR spectrum of Figure 5c shows the usual v3 and v4 modes of iron phosphate on the CRS panel at 1109 and 6251555 cm-’, respectively. The spectral bands at 1208 and 963 cm-I are related separately to the C-0 and 0-P stretching modes of the P-0-C (phenyl) functionality. As has been assigned in Figure 2c, other bands observed at 1592, 1493, and 77717541689 cm- in Figure 5c should involve “quadrant stretching’’ of the ring C=C bonds, semicircle stretching of the carbon ring and C-H bending, and an out-of-plane C-H wag mode, respectively. In view of the chemical reactivity and surface morphology of crystalline iron phosphate produced on CRS panels, (C&0)2(HO)PO is probably the most suitable phosphatizing reagent for the formulation of in-situ phosphatizing coatings. Aside from the effectiveness of forming a corrosion protective barrier, the success of using diphenyl phosphate as a phosphatizing reagent depends also on how easily the cocondensation of (C6H50)2(HO)PO with polymer film forming resins can occur. This would ensure a strong coating adhesion by the formation of P-0-C covalent bond between the metal phosphate layer and polymer topcoat. The H3PO4-Modijied Polymer as Phosphatizing Reagents: Lubrizol LZ 2061, 2062, and 2063. The technology of in-situ phosphatizing coatings has established a unique chemical principle2 that the phosphate chemistry and the polymer chemistry proceed independently and simultaneously. In phos-
phate conversion coatings, the water insoluble crystalline metal phosphate hydrate films (Le., the phosphate chemistry) on metal substrates were finished before the application of film-forming polymer coatings. On the other hand, the use of H3P04modified polymer resins in the paint system guarantees a strong covalent bonding of the phosphorus-oxygen-carbon linkage (i.e., the polymer chemistry) and provides excellent miscibility with polymer binders. However, the chemical reactivity of a H3P04-modified polymer with the metal surface would be hindered greatly by the slow rate of diffusion and its polymer chain conformation specific for the phosphatizing reagent. The H3P04-modified polymers, polyester acid phosphate,2 epoxy acid phosphate13 (e.g., Dow epoxy phosphate esters XU8096.07, XP7 1739, and XU71899.00), and LubrizolIo LZ 20611206212063have been introduced. The adhesion promoters LZ 2061,2062, and 2063 were donated by Lubrizol Corp., and LZ 2062 was shown to be unstable with a short pot life. Only the chemical affinities of LZ 2061 and 2063 with CRS surfaces are reported. As expected, the formation of the iron phosphate layer is very slow when the CRS substrate is soaked in the simulated system of LZ 2061 and 2063. Both LZ 2061 and 2063 produced thin iron phosphate layers that covered only some parts of the CRS surface, as viewed in SEM plates and monitored by EDS spectra. The observed weak and broad FTIR bands (not shown) at 1175 and 105011110 cm-I, respectively indicate the production of iron phosphate on the CRS panel when treated with LZ 2061 and 2063. c. Chemical Functionality Responsible for the Metal Phosphating. There are three functional groups, POH, PO-, and P=O, in organic phosphatiiing reagents that can interact with metal surfaces and produce an iron phosphate layer on CRS panels. When POH in ISPRs dissociates to give PO-, the resonance effect makes the functionalities PO- and P=O
7618 J. Phys. Chem., Vol. 99, No. 19, 1995
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Figure 5. SEM macrograph of iron phosphate layer on CRS treated with 0.5 wt% (a) and 10 wt% (b) of diphenylphosphate. The iron phosphate on macrograph (b) gives the corresponding FTIR (c) and EDS (d) spectra.
indistinguishable. For example, H3P04 becomes [P043-] and H(HO)2PO produces [P032-]. The bonding strength of PO-/ Mn+ (metal surface) is ionic in nature and is stronger than that of P=O/metal (an induced dipole or charge transfer type interaction) and POH/H20 absorbed on a metal surface (a hydrogen-bonding interaction). For a strong coating adhesion, the acid-base type interaction of PO-/Mn+ is preferred. As has been described in the sections above, the chemical bonding of iron phosphate on CRS panels, produced by phosphoric acid, phosphonic acid, phenyl phosphoric acid, phenylphosphonic acid, and diphenyl phosphate, is probably ionic in nature. To search for the possible chemical interactions of P=O/ metal and POH/H20 absorbed on metal surfaces, (C&)3PO, (C6H@)3PO, and (NH2C2H40)(H0)2PO were selected as surface phosphatizing reagents in the simulated systems. The triphenylphosphineoxide and triphenylphosphate possess a P=O functionality, whereas 2-aminoethyl dihydrogen phosphate provides not only P=O but also POH functional groups for interacting with metal surfaces. Due to the neutral/basic nature of the (NH2C2I&O)(H0)2PO molecule, the molecular functional group of POH should remain intact. No formation of PO- from the dissociation of POH and no generation of (P+O-) from the resonance effect of P=O are expected. The surface chemical reactivity of (CsH&PO, (C~HSO)~PO, and (NH2C2H40)(H0)2PO in the simulated systems with the CRS panel is practically not observable. The spectroscopic evidence shows no iron phosphate vibrations in the FTIR spectrum, no phosphorous peaks at 2.02 keV in the EDS scan, and no iron phosphate layer. However, a blank CRS surface in the SEM picture is observed. No or low reactivity of POH and P=O on the metal surface is definitely not due to the bulky substituents in (C6H5)3PO and
(C6H50)3PO, since no substituting group would give a steric effect in (NH2C2&0)(H0)2PO. The results indicate that POis the chemical functionality responsible for the phosphatizing reagent to form a metal phosphate layer. The metal phosphatization on the CRS panel is an acid-base type of reaction. d. Phosphatizing Reagents for Water-Borne Pigmented ISPCs. In the water-borne coating system, the polymer film forming binder is normally neutralized by organic amines, e.g., dimethylethanol amine (DMEA), which become completely soluble in water for the formulation of coatings. In pigmented paints, a pH adjustment of the coating formulation is often necessary to prevent pigment aggregations. For this reason, it is important to investigate how the addition of water and amine into the simulated system of phosphatizingreagents would affect the chemical reactivity and surface morphology of iron phosphate produced on CRS panels. Since the phosphatizing reagent is acidic in nature, the addition of water in the simulated system would promote the dissociation of POH in ISPRs to form PO- and increase the rate of phosphatization on metal substrates. On the other hand, the formation of metal phosphate in its crystalline form requires a source for its water of crystallization. The introduction of water to the phosphatizing reagents should enhance the production of a crystalline iron phosphate layer on the CRS panel. For the purpose of illustration, the surface phosphatization of the CRS substrate using (C&)(H0)2PO in the simulated system with and without 1 wt % of water added was investigated. Under the same experimental conditions, the growth of an iron phosphate layer using phenylphosphonicacid with and without the addition of water as phosphatizing reagents shows a similar SEM morphology. However, the chemical reactivity of the
J. Phys. Chem., Vol. 99, No. 19, 1995 7619
Chemical Affinity of Phosphatizing Reagents
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phosphatizing reagent with water on the CRS panel is about twice as fast as that without water. This is illustrated in the FTIR spectra of Figure 6a (without water) and b (with water), and in the EDS scans of Figure 6, parts c (without water) and d (with water). The band intensity for the v3 and v4 modes of iron phosphate at 1080 and 570 cm-’, respectively, in spectrum 6b is stronger than that in spectrum 6a. The mass fraction of phosphorus (P) at 2.02 keV in the EDS scan of Figure 6c (without water) is 672 counts and that of Figure 6d (with water) is 1138 counts. The results indicate that the PO- functional group is responsible for the metal phosphatization reaction and supports the success of the formulation of water-bome in-situ phosphatizing coatings.2 A proper dispersion of polymers and pigments in solvents and surfactants is essential in coating formulation^.'^ Small pigment particle size is desirable for gloss of dry paint films, because polymer and pigment particles residing at the coating surface with discontinuities larger in size than half the wavelength of visible light (Le., 0.3-0.5 pm) will cause scattering and lead to a disturbed reflected image. Thus, the stability and compatibility of phosphatizing reagents in the in-situ phosphatizing coatings often depend on the exact pH adjustment of the paint formula. Since phosphatizing reagents in ISPCs are acidic in nature, some in-pot reactions are possible if they are not properly protected and dispersed in the coating formulations. Four technical approaches have been adapted2 individually or in combination to stabilize the phosphatizing reagents as they are incorporated in the unicoat system. In one, an amine stabilizer (e.g., triethylamine, triethanolamine, 2-amino-2-methyl 2-propanol) can be added to form weak complexes with phosphatizing reagents and to achieve unicoat system stability. Upon the application of the ISPCs, the thermal dissociation of weak complexes would lead to the initiation of “simultaneous” phosphate chemistry and polymer chemistry on metal substrates.
The phosphatizing reagents (C6H50)(H0)2PO free acid and
(C6H50)(H0)2PO/triethyiamineweak complex (in 1:1 molar ratio) were prepared in the simulated system and used to phosphate a CRS panel. For both free acid and its neutralized complex, a localized growth of iron phosphate covering only some parts of the CRS surface is observed in the SEM plates. The iron phosphate layer on the CRS panel produced by both phosphatizing reagents gives similar FTIR spectra and EDS scans. However, the rate of phosphatization using the weak complex form is slightly slower than that using the free acid species. This could be due to the small difference in the pH value of both simulated systems. The results indicate that the weak complex (C&I5O)(HO)2PO/triethylaminedissociates readily at the thermal-cuing schedule and allows the free (C&O)(HO)2PO to diffuse to and react with the metal surface. The same observation has also been reported for ISPCs of a Ti02 pigmented water-borne alkyd paint.2
Conclusion We have illustrated that both inorganic and organic phosphate compounds in the simulated coating systems are capable of reacting with metal substrates and producing metal phosphate layers of different morphology and thickness. The chemical reactivity, bonding nature, and surface morphology revealed that phosphonic acid, phenylphosphonic acid, and diphenyl phosphate are better phosphatizing reagents than phosphoric acid, which in turn is better than monophenyl phosphoric acid. The trisubstituted phosphine oxide and phosphate are commonly used as plasticizers, but are not reactive as phosphatizing reagents. The chemical functionality of phosphatizing reagents responsible for reacting with the metal surface has been deduced to be the PO- group. The preferred surface bonding of metal phosphate is the PO-/Mfl+ via an acid-base type of interaction.
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The chemical reactivity increases when a certain amount of water is formulated in the coating system. No change in the rate of surface phosphatization is observed when a weak amine complex of phosphatizing reagent is used as compared to the corresponding free acid species. This provides the opportunity for the success of formulating stable and compatible water-bome pigmented ISPCS.~ Currently, the chemical affinity of ISPRs is being tested in our laboratory for aluminum, magnesium, and lead substrates. The results would provide the opportunity for a technical advancement in the following coating areas: (i) chromium-free in-situ phosphatizing paints, (ii) low VOC water-bome selfphosphating paints, and (iii) the chemistry of encapsulatingleadbased paints.
Acknowledgment. Financial support from the National Science Foundation, Grant CTS-93 12875, and the Illinois Hazardous Waste Research and Information Center, ENR Contract No. RRT 19, is acknowledged. The authors would like to thank Lubrizol Corporation and G. J. Nikolas Co., Inc., for their donations of some paint materials, and Professor Alan P. Genis for the use of the EDAX PV9900 system. References and Notes (1) Paint Waste Reduction and Disposal Options; Hazardous Waste Research and Information Center: Champaign, IL,June 1992 (Vol. I, HWRIC RR-060) and February 1993 (VOL. 11, HWRIC TR-008). (2) (a) Lin, C. T.; Lin, P.; Hsiao, M. W.; Meldrum, D. A.; Martin, F. L. Ind. Eng. Chem. Res. 1992, 31, 424. (b) Meldrum, D. A.; Lin, C. T. J. Coat. Technol. 1993, 65, 47. (c) Lin, C. T.; Quitian-Puello, F. Ind. Eng. Chem. Res. 1993, 32, 818. (d) Lin, C. T.; Qvyjt, F. Chemistry of In-Situ
Phosphatizing Coatings. Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering, Spring 1994, San Diego, CA, March 13-18, 1994 (Polymn. Mater. Sci. Eng. 1993, 70, 76). (e) Lin, C. T. Additive Package for In-Sifu Phosphatizing Paint, Paint and Method. U.S.Patent 5,322,870, June 21, 1994. (0 Li, L.; Lin, C. T. SEM-EDS Investigations of Self-Phosphating Coatings. Ind. Eng. Chem. Res., accepted for publication. (3) Kuehner, M. A. Met. Finish. 1985, August, 15. (4) Funke, W.; Leidheiser, H., Jr.; Dickie, R. A.; Dinger, H.; Fischer, W.; Haagen, H.; Henmann, K.; Mosle, H. G.; Oechsner, W. P.; Ruf, J.; Scantlebury, J. S.; Svoboda, M.: Sykes, J. M. J. Coat Technol. 1986, 58, 79. ( 5 ) Rausch, W. The Phosphating of Metal; ASM International: Metal Park; Finishing Publications LTD: Teddington, Middlesex, England, 1990. (6) (a) Hegedus, C. R.; Hirst, D. J.; Eng, A. T.; Green, W. J. HighGloss Corrosion Resistant Coatings. U.S. Patent 5,043,373, August 27, 1991. (b) Hegedus, C. R.; Hirst, D. J.; Green, W. J.; Eng, A. T. Corrosion-Resistant Acrylic Coatings. US.Patent 5,100,942, March 31, 1992. (7) Lindert, A. E.; Troy, Mich. U.S. Patent 4,433,015, February 21, 1984. (8) Albrecht, H. 0. Rust-Resisting Coating Composition U.S. Patent 1,995,954, June 10, 1931. (9) Howell, G.D.; Lange. D. A. Process for Sealing Phosphatized Metal Components U.S. Patent 4,220,485, September 2, 1980. (10) Higgins, W. A. Organic Phosphate Substrate Treatments and Coating Additives Paint Additives Technical Paper Reprint; The Lubrizol Corporation, 1986. (1 1) Osaka, A.; Takahashi, K.; Ikeda, M. J. Mater. Sci. Lerf. 1984, 3, 36. (12) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1975. (13) (a) Carter, R. 0..III; Gierczak, C. A,; Dickie, R. A. Appl. Spectrosc. 1986, 40, 649. (b) Carter, R. O., 111; Parsons, J. L.; Holubka, J. W. Ind. Eng. Chem. Res. 1987, 26, 1518-1523. (14) Mcewan, I. H. J. Water Borne Coat 1979, November 3. (15) Massingill, J. L.; et al. J. Coat. Technol. 1990, 62 (781), 31; 1991, 63 (797), 47; 1993, 65 (824), 65. JP941387T
