Chrome-Free Single-Step In-Situ Phosphatizing ... - ACS Publications

Chapter 5

Chrome-Free Single-Step In-Situ Phosphatizing Coatings 1

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 8, 2016 | http://pubs.acs.org Publication Date: August 15, 2000 | doi: 10.1021/bk-2000-0767.ch005

Tao Yu, MaryC.Whitten, CarmenL.Muñoz, and Chhiu-Tsu Lin

Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, IL 60115-2862

The current process of organic coating on metals involves multiple steps and considerable energy, labor, and process control. It also generates toxic wastes such as chlorinated solvents, cyanide, cadmium, lead, and carcinogenic chromates. The green chemistry technology of in-situ phosphatizing coatings (ISPCs) developed in our laboratory is a one-step self-phosphating process in which the formation of a metal phosphate layer on the substrate surface and the curing of polymer paint film take place independently, but simultaneously. The generation of a metal phosphate layer in-situ essentially eliminates the metal surface pre-treatment step which employs a phosphating line/bath. The use of chemical bonds linked to the paint polymers to seal the pores of the metal phosphate layer in-situ enhances coating adhesion and suppresses metal corrosion without post-treatment final rinses containing chromium (Cr ). The successful application of ISPCs in three types of low-VOC commercial paints on bare and pre-treated cold-rolled steel and 2024 T3 aluminum coupons are presented in this chapter. The protective performance of ISPCs is shown to be superior to that of the current multi-step coating practice. 6+

Introduction The current practice of applying state-of-the-art organic coatings to metal substrates is a multi-step process. Normally, the metal surface is cleaned, phosphated or chromated, possibly sealed (with hot water or carcinogenic chromâtes) (/,2), dried, and finally painted. The surface pre-treatment process is error prone and costly, but it is necessary in the metalfinishingindustry (3) in order to enhance paint film adhesion. Corresponding author. © 2000 American Chemical Society In Green Chemical Syntheses and Processes; Anastas, Paul T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

43


44 Unfortunately, multi-step coating technologies produce wastes including organic solvents, heavy metals, and other toxic and deleterious materials (4). Recently, a novel surface conversion coating technique, namely "in-situ phosphatizing coatings (ISPCs)" has been developed in our laboratory (5-72). Using the ISPC process, phosphate conversion coatings and polymer films can be formed independently and simultaneously in a single-step. The simplicity of applying a singlecoat phosphate/paint over the present multi-step coating practice makes the ISPC system attractive to many manufacturers of metal products. It provides increased quality without the capital and operating expense of a separate phosphating line/bath. In addition, the disposal of toxic wastes produced by the phosphating bath and waste treatment is avoided. In this study, ISPC formulations of three low-VOC (volatile organic compound) commercial paints were made: a high-solids polyester baking enamel, a waterreducible alkyd paint, and a VOC-free thermoset acrylic latex system. The ISPC formulation's room-temperature shelf-storage stability was studied by monitoring its Theological behavior change upon aging. Thermal analysts was used to ensure the coating's successful crosslinking and film formation. The enhanced coating adhesion of ISPCs on cold-rolled steel and aluminum substrates was verified by cathodic delamination measurements and water immersion tests in a 3% NaCi solution. Both electrochemical impedance spectroscopy (EIS) and ASTM corrosion test standards (e.g., salt spray (fog) test) were used to evaluate the coating's corrosion resistance performance. The results are discussed in light of the environmental, economical, and technical advantages of the ISPCs in comparison to current multi-step coating practices.

Experimental Three commercial baking enamels obtained from the Sherwin-Williams Company—a gloss ivory high-solids polyester-melamine paint (PERMACLAD® 2500) with a solids content of 82.4%, a water-reducible alkyd paint (KEM AQUA® 1400) with a solids content of 30-33%, and a solvent-free acrylic latex paint (KEM AQUA® 1800T) with a solids content of 50 ± 2% by volume—were used as the control paint formulas. The dry paint film of the commercial baking enamels was achieved by thermal curing at 163°C (325°F) for 15 minutes. An arylphosphonic acid (or arylphosphoric acid) together with a proper pH-adjusting-agent (e.g., an organic amine) were used as in-situ phosphatizing reagents (ISPRs) (7) to formulate the corresponding ISPC systems. The metal coupons tested for the control paints and ISPC systems were bare cold-rolled steel (CRS), iron phosphated (Bonderite 1000, BD), and iron phosphated plus Parcolene 60 chromated (BD+P60) mild steel panels (Q-PANEL Co., Cleveland, OH and ACT Laboratories, Inc., Hillsdale, MI), and bare and chromated 2024 T3 aluminum coupons. The ISPC formulation stability was monitored by Theological measurements. Thermal analysis of cured paint films for the control paints and ISPC systems was conducted by differential scanning calorimetry (DSC) measurements. Electrochemical

In Green Chemical Syntheses and Processes; Anastas, Paul T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.


45 impedance spectroscopy (EIS) was used. This technique reduced the time needed to evaluate the corrosion protective performance of the organic coatings, and provides insight into the chemical nature of the failure mechanisms. The experimental details and instruments used for rheological, DSC, and EIS measurements have been described elsewhere (10-12). All measurements and tests reported here were conducted in duplicate, and the experimental reproducibility was verified. In order to study the formation of interfacial metal phosphate layers, the untreated bare CRS panels were mechanically polished to a mirror finish and used as coating substrates. The ISPC was applied on mirror-finished CRS and cured at 163°C for 15 min. The polymer layer on each panel was removed without damaging the interface by soaking the panel in tetrahydrofuran solvent. The interfacial metal phosphate layer was rinsed with deionized water, dried, and characterized by a Bruker Fourier transform infrared (FTIR) spectrophotometer model Vector 22 equipped with a Spectra Tech FT-80 grazing angle accessory. The disbonding resistance of the cured paints on metal substrates was studied by salt water (3% NaCl) immersion testing and also by cathodic delamination using the same apparatus as for the EIS measurements. In cathodic delamination, a 20.0 cm area of cured paint film on the metal coupon was assembled in a delamination cell and used as the working electrode. The paint film was exposed to a 3% NaCl solution and polarized to -1.1 V versus a saturated calomel electrode (SCE) throughout all testing periods. Initially, a holiday (a hole of ca. 1.0 mm diameter) was drilled through the polymer film to the substrate to observe and measure the coating delamination expanding around the holiday. Dynamic control of the applied potential and data acquisition of the delamination area (or the delamination current at a fixed potential) as a function of time were obtained by using EG&G model 342C corrosion measurement software. 2

Results and Discussion

Rheology and Storage Stability of ISPCs The rheological profile (not shown) of the solventless acrylic latex enamel (KEM AQUA® 1800T) indicated that no significant change in rheology from the original latex control was observed for the latex-ISPC formula. The viscosities of the latexISPC formula after various storage times at room temperature were measured as 82 ± 1 cp (freshly prepared), 110 ± 1 cp (two weeks storage), 175 ± 2 cp (one month storage), and 325 ± 3 cp (two months storage), whereas the corresponding values for the control latex formula were 108 ± I cp, 125 ± 1 cp, 194 ± 2 cp, and 368 ± 4 cp, respectively. Only a very small decrease in the viscosity was seen in the latex-ISPC formula, that may have resulted from relatively smaller latex particles. The reduction of particle radius in the latex-ISPC formula has been known to reduce "effective" hydration by latex particles, leading to a decrease in the apparent viscosity of the



46 latex-ISPCs. However, the "up-and-down" shear rate cycle in the rheological profile was an appreciably smooth rheological response, indicating good paint stability for both latex-ISPC and control latex formulas. The rheological profile of water-reducible alkyd ISPC (KEM AQUA* 1400) formula and its corresponding control coating sample was also recorded. The ISPC alkyd paint demonstrated a flow that was close to "Newtonian." There was a small shear thinning effect in the alkyd control formula, suggested by the splitting observed at the low shear end of the "up and down" shear rate cycle. The decreased viscosity value was considered to be a "memory" effect of its high shear history. Both rheological profiles remained smooth throughout the shear rate range tested, indicating that the alkyd control and ISPC-alkyd baking enamels displayed a homogeneous coating with no significant pigment flocculation (13). When an ISPR-amine weak complex was dispersed into the KEM AQUA® 1400 control sample, the resultant water-reducible ISPC formula showed an apparent viscosity increase along with a clear shear thinning effect, while the shear stress-shear rate curve deviated from the straight line of "Newtonian" flow. However, no change in the viscosity of the water-reducible ISPC formula was observed during a long-term shelf-storage. Apparently, the initial viscosity change in the ISPC formula was not related to a chemical reaction of the ISPRs with the backbone polymers or oligomers, but rather was probably due to a physical modification of ionic strength in the waterbased coating system by the incorporation of an ISPR-amine complex. The system's ionic strength in water-based paint normally affects the hydration status of the "hydrophilic" sites or segments of the alkyd resin, altering its molecular conformation, and in turn increasing the apparent viscosity of the paint formula. Incidentally, the ISPC formula high-solids polyester-melamine paint (PERMACLAD® 2500) showed an initial increase in absolute viscosity at room temperature, owing most probably to the acid catalyzed polymer chain extension of polyester molecules in the formula. This is a classic problem with melamine baking enamels, and usually requires the addition of a volatile base to temporarily neutralize the acid (14).

Polymer Chemistry and Paint Film Quality Crosslinking is one of the most important aspects of polymer film formation in coatings (75). It influences many paint film properties such as chemical stability, solvent resistance, network morphology, and mechanical properties (75,76). The degree of crosslinking can be monitored by DSC measurements, wherein the glass transition temperature (T ), T span (T (glass transition end point) - T (glass transition start point)), and specific heat capacity jump (AC in mJ/mg°C) are obtained. A higher T value and a lower A C during the glass transition are indicative of higher crosslinking density in a paint film (17). The thermal properties obtained from DSC analysis of cured paint films, including the polyester control, ISPC polyester, alkyd control, ISPC alkyd, latex control, and ISPC latex baking enamels are listed in Table I. The T values are slightly g

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47 higher for the ISPC formulas as compared to the control samples, indicating a relatively higher average polymer crosslinking density for the ISPC paint films of polyester and alkyd systems. The small increase in T value is consistent with a slightly lower heat capacity change (AC ) associated with the glass transition. For latex systems, on the other hand, the observed thermal properties may be again attributed to the slight ionic strength increase in the latex-ISPC system following the addition of ISPR-amine weak complexes. The effect of ionic strength could promote a reduction in the packing distance of the latex particles in the latex-ISPC system, leading to a minor increase in latex particle coalescence and molecular entanglement. In all cases, the cured coating films of ISPC and control baking enamels had similar T values, and therefore the control and modified coatings were considered comparable. g


p

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Table I. Thermal Chemical Data for the Cured Paint Films of ISPC and Control Enamels T/onset) (°Q

Enamel

17.3 20.6 18.2 17.6 14.0 16.4

Polyester Control ISPC Polyester Alkyd Control ISPC Alkyd Latex Control ISPC Latex

T,(offset) (°C) 40.6 44 54.0 60.8 32.7 33.1

T span (°C)

TJ°C)

ACp (mJ/mg°C)

23.3 23.4 35.8 43.2 18.7 16.7

28.9 32.5 36.1 39.2 23.9 24.9

0.088 0.085 0.173 0.163 0.070 0.069

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In-Situ Phosphatization of Metal Surface The dispersion of phosphatizing reagents in ISPCs was designed especially for synthesizing a metal phosphate thin layer in-situ at the interface of the metal substrate and polymer coating. The nature of metal phosphate bonds in ISPCs was found to have an acid-base type interaction, P-O* - M , rather than an induced dipole interaction of the P=0/metal complex type (9). The metal phosphate products synthesized by thermal-cured solvent-borne polyester ISPC and acrylic latex ISPC on polished bare CRS coupons were investigated by means of reflectance FTIR spectra (not shown). The formation of a metal phosphate product was confirmed by peaks at 1073 cm' and 574 cm" for the solvent-borne system and at 1063 cm' and 561 cm' for the water-borne system, corresponding respectively to the υ (stretching type) and υ (deformation type) vibration modes of the phosphate group being distorted by the crystal field. For the water-borne system, the band intensity of the υ mode was higher than that of the υ vibration which is in contrast to the band intensity observed in the spectra of the solvent-based system. It is not known at this time why the metal phosphate layer synthesized by water-borne ISPCs has a different chemical nature from that produced by solvent-borne ISPCs. In principle, the higher spectral band n +

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48 intensity in the reflective mode of FTIR spectrum should correspond to a larger projection of electric dipole transition perpendicular to the substrate surface. The ISPRs in latex-ISPCs are situated in the hydrophilic phase of the latex emulsion system, whereas those in solvent-borne ISPCs are incorporated into a slightly polar solvent/polymer environment. Thus, the acid-base nature of ISPRs' interaction with the metal surface in latex-ISPCs is different from that in solvent-borne ISPCs. This may cause the differing bond dipoles of the metal phosphate layer seen in the FTIR spectra. Besides the metal surface phosphatization, the pre-dispersed ISPRs have the function of reacting with backbone polymers in the coating to form covalent linkages with the polymer. This was confirmed spectroscopically by the FTIR peak around 944 cm" , corresponding to the P-0 bond distortion caused by formation of the P-O-C linkage. In addition, by comparing the FTIR spectra of the metal-phosphate layer produced on a polished CRS substrate to those on the polished 2024 Al substrate, the P=0 absorption band on the Al substrate was higher in frequency by about 50 cm' . This spectral shift to higher frequency was attributed to a higher bond order of the P-0 bond due to the larger ionicity of the Al-0 bond compared to the Fe-0 bond (18). 1

1

Coating Protective Performance Electrochemical impedance spectroscopy (EIS) has proven to be a powerful tool for the determination of coating performance and underfilm metallic corrosion (19,20). To show the effect of substrate pre-treatments on the coating protective performance, the control formula of solvent-free acrylic latex enamel was applied on three different types of cold-rolled steel panels: (i) untreated bare CRS panel, (ii) iron phosphated (B-1000) panel, and (iii) iron phosphated plus Parcolene 60 chromated (BD+P60) panel. A dry film thickness of about 1.1 mil cured at 163°C for 30 min was prepared. The coated panels were soaked in a 3% NaCl solution for 72 hours before the EIS measurements. Figure 1 shows the Bode-magnitude plots for latex control coating on bare CRS (curve la), B-1000 (curve lb) and BD+P60 (curve lc) panels. In the high frequency region, all three curves la, lb, and lc, were practically unified, as the impedance in this region is dominated by the pure dielectric properties of the nonconductive organic paint film. This indicated that the control latex dry film coated on three different types of substrates had similar dielectric properties. In the low frequency region of Figure 1, the three Bode-magnitude curves split, as the impedance becomes more and more affected by the degree of pre-treatment at the coating/substrate interface while responding to the AC signal. In general, all curves deviated from the linear relationship of log IZI vs. log f (f = frequency in hertz), and became more frequency-independent. A frequency-independent horizontal line in the Bode-magnitude diagram is characteristic of a pure resistor. At f = l.OxlO" Hz, the IZI values were measured as 7.2 χ 10 Ω-cm , 2.6 χ 10 Ω-cm , and 5.3 χ 10* Ω •cm for curves la, lb, and lc, respectively. The results followed nicely the expectation that pre-treatment of iron phosphate on CRS tripled the IZI value, and that the additional chromate rinse of B-1000 substrate further doubled the impedance value of the control 2

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50 latex coating. It is commonly expected when measuring paint resistance (21) that, in most cases, a measured resistance of 1 χ 10 Ω-cm predicts good corrosion protection properties, a resistance of 1 χ 10 Ω-cm is poor, and paints with resistance between 1 χ I0 and l χ ΙΟ Ω •cm are borderline. In short, the latex control coated on pretreated substrates (curves lb and lc) had good protective properties, whereas that on the untreated panel (curve la) was borderline. 8

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Upon application of the single-step latex-ISPC system to a metal substrate, the pre-dispersed phosphatizing reagent reacted effectively with the metal substrate. Figure 2 shows the Bode-magnitude plots of latex-ISPCs coated on bare CRS (curve 2a), on iron phosphated B-1000 (curve 2b) and on iron phosphated and chromated BD+P60 (curve 2c) coupons. All three curves appeared to overlap throughout the experimental frequency region, and all behaved as pure capacitors (i.e., the curve of log IZI versus log f gives a straight line with a slope of -1). At f = 1.0 χ 10" Hz, the IZI value was measured as 1.0 χ 10 Ω-cm , 1.2 χ 10 Ω-cm , and 1.4 χ 10 Ω-cm for curves 2a, 2b, and 2c, respectively. This observation clearly indicated that the cured latex-ISPC film on metal substrates (both untreated and pre-treated) could be classified as excellent in terms of corrosion protective performance. An increase in coating resistance (IZI) of more than two orders of magnitude was observed for the latex-ISPC (curve 2a) as compared to the control latex formula (curve la) on bare CRS. This supported the conclusion that the in-situ metal phosphate layer was successfully generated at the metal/paint interface by the ISPC coating. Moreover, the corrosion inhibition of latex-ISPC on bare CRS (curve 2a, IZI = 1.0 χ 10 Ω-cm ) is equivalent to or better than that of the control latex formula on BD+P60 panel (curve lc, IZI = 5.3 χ 10 Ω-cm ). Similar results were also observed for the ISPC polyester and ISPC alkyd systems. The results provided strong evidence that a successful latexISPC could effectively replace the traditional phosphate/chromate bath/line. The metal phosphate layer produced on 2024 T3 Al substrate had a higher bond order of the P-0 bond than that on the CRS substrate. It is expected that the coating protection of ISPCs on 2024 T3 Al panel would be equal to or better than that on the CRS coupon. The Bode-magnitude plots for a cured paint film (about 1.1 mil dry film thickness) of polyester-melamine white paint (AKZO resin 26-1612) on bare and chromated 2024 T3 Al substrates recorded after soaking in a 3% NaCl solution for 72 hours and 2500 hours, are shown in Figures 3 and 4, respectively. The ISPCs on both bare and chromated 2024 T3 AI substrates displayed a pure capacitive behavior in Figure 3 and their polymer coatings remained intact even after 2500 hours of soaking in 3% NaCl solution, as evidenced in Figure 4. The polyester ISPCs painted on both the bare and the chromated aluminum panels displayed very high impedance values of >10 Ω-cm at f = 1.0 χ 10* Hz. Under the same processing conditions, the IZI values obtained for the ISPC-polyester system were at least 1000 times higher than those of the polyester-control system. In Figure 3, the polyester control painted on bare Al gave a maximum impedance of less than 10 Ω-cm , which is considered indicative of a poor protective barrier (27). The polyester control painted on chromated Al substrate increased the impedance at low frequencies by only an order of magnitude. 2

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Chrome-Free Single-Step In-Situ Phosphatizing ... - ACS Publications

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