The particle size effect of MALPB-DEA dispersions on their anticorrosion performances as waterborne coatings on galvanized sheet.
Abstract Maleic anhydride-g-liquid polybutadiene (MALPB) dispersions were prepared by chemical modification with diethanolamine (DEA) during direct polymer emulsification, and their anticorrosion performances on galvanized sheet were found to be strongly dependent on their particle size and distribution with the variation of DEA. The dispersion containing DEA to maleic anhydride at molar ratio of 2:1 exhibited much smaller particle size and narrower size distribution than that containing DEA to maleic anhydride at molar ratio of 1:1, as measured by a laser diffraction particle size analyzer. A more compact organic layer was supposed to be formed by the dispersion composed of smaller size and narrower size distribution particles on the galvanized sheet, which exhibited better anti-corrosive properties. This compact layer slowed down the electrolyte penetration onto the metal surface such that the life span of the metal could be prolonged, which was reflected in the increase in corrosion potential as measured by DC polarization, higher impedance during immersion in 3.5 wt% NaCl solution as measured by electrochemical impedance spectroscopy (EIS), and improved durability in harsh environment as observed by images after salt spray tests.
Keywords MALPB-DEA dispersion, Waterborne coating, Particle size and distribution, Anticorrosion, Galvanized sheet
The buildup of coatings on metal surfaces has been widely used for the corrosion prevention of metals separated from their environment by forming a physical barrier on their surfaces. (1-3) Organic coatings, in general, have the advantage of slowing down water, ions, and other corrosive media penetrating through the organic film. The disadvantage of most organic paints used in the industry, however, is their high volatile organic content (VOC), which is going to become more and more restricted due to environmental and legislation constraints. (4), (5) As such, great efforts have been made to develop paints containing less organic solvents or to pursue total waterborne paints. While preparing waterborne coatings, there are two methods of obtaining polymer dispersions: (1) emulsion polymerization from monomers, and (2) directly dispersing a resin into water with the help of surfactants. Waterborne coatings based on emulsion polymerization methods have been mostly researched (6-9); however, only a few polymers could be dispersed in water even with the addition of surfactants. (10) Thus, it is meaningful to try various ways to develop polymer dispersions for preparing waterborne coatings.
The particle size and distribution of polymer dispersions are considered two of the most crucial factors in determining the performance of waterborne coatings. (11) Winnik (6) proposed that for polymer latexes with narrower particle distribution, their improvements in permeability, anticorrosion, cohesion, and so on were attributed to their closer contact and deformation in forming compact structures as water evaporates, and then polymer entanglement during particle boundary disappearance to form homogeneous solid films. In addition to size distribution, Reyes et al. (8) found that in the particle size ranges of 400-500 nm, mono-dispersed styrene-acrylic latex exhibited better anticorrosive performance than that of those with particle sizes beyond this range. The similar mono-dispersed latexes composed of smaller particle size in particular performed even worse. Deng et al. (10) found that the average particle size and size distribution of the emulsion determined the performance of comb-branched waterborne polyurethane/organo-ontmorillonite nanocomposite coatings, and this nanocomposite emulsion containing 3% organo-ontmorillonite, with a particle size around 63.6 nm, exhibited higher tensile strength and lower water absorption than those with smaller or larger particle sizes. By properly controlling the feeding procedures with good agitation and temperature, Moayed et al. (12) synthesized latex with bimodal particle size distribution for coating applications by using acrylic monomers. The coating exhibited improved quality in gloss, adhesion, UV resistance, and water resistance in comparison with similar commercial resins with broader particle size distribution. These studies reveal that both the particle size and distribution of polymer dispersions have great effect on their performances as waterborne coatings. About their functions during film formation, however, a common knowledge has not been achieved yet, and hence more fundamental studies are necessary to bring new understandings in this field.
In the field of polymer dispersions, much attention has been focused on the ways of preparing various dispersions, the factors in influencing their size and distributions, and the methods in improving the stability of dispersion. There have been only a few reports about the particle size and distribution on the anticorrosion performances of polymer dispersions. In this paper, a polymer dispersion of maleic anhydride-g-liquid polybutadiene (MALPB) modified by dietha-nolamine (DEA) was successfully prepared by directly emulsifying the polymer with the help of surfactants for potential application as a primer on galvanized sheets. The chemical modification was confirmed by Fourier transform infrared spectra (FTIR), and the particle size and distribution of the dispersions were found dependent on the amount of DEA as measured by laser diffraction particle size analyzer. The anticorrosive performances of clear coatings on galvanized sheets were evaluated by electrochemical impedance spectroscopy (EIS), DC polarization measurements in 3.5 wt% NaCl solution, and salt spray tests.
Maleic anhydride-g-poly(1,2-butadiene) terminated by p-xyene (MALPB) at Mn = 2200, used as received from Yanshan Petrochemical Corp. (Sinopec, China); ethylene glycol monobutyl ether (EGME), sodium dodecylsulfate (SDS), TritonX-100 (p-(1,1,3,3-tetra-methyl butyl) phenoxy poly (ethylene glycol) [[C.sub.14],[H.sub.21]([CH.sub.2][CH.sub.2]O[).sub.10]OH]), and diethanolamine (DEA) used as received from Shantou Xilong Chemical Co. (China); 1-butanol from Chemical Reagent Beijing Company (China); and tert-butyl hydroperoxide solution (TBHP) from Sinopharm Chemical Reagent Co., Ltd. (China).
Preparation of the coatings
Preparation of MALPB-DEA dispersions
1.0% SDS (wt% of MALPB), 2.0% TritonX-100 (wt% of MALPB), and MALPB were added to a three-neck flask using mechanical stirring at a rate of 500 rpm at room temperature for 20 min. Under magnetic stirring, the stoichiometric amount of DEA (in molar ratio of DEA:MA = 1:1 or DEA:MA = 2:1) was dissolved in deionized [H.sub.2]O in a beaker to get a 30% solution (DEA in water). Then this solution was slowly added to the above flask under continuous mechanical stirring at a rate of 800 rpm for 60 min. After that, deionized [H.sub.2]O was added with continuous stirring so that the content of MALPB-DEA took up about 40% (wt%) in the final dispersion, and an opal, stable dispersion was finally obtained.
Preparation of the clear coating
To improve the flow performance of the dispersion on a metal surface, both 3% 1-butanol (wt% of MALPB-DEA dispersion) and 2% EGME (wt% of MALPB-DEA dispersion) were added into the dispersion as leveling agents. In addition, 1% TBHP (wt% of MALPB-DEA dispersion) was added as a crosslinking agent so that the double bond in MALPB could be effectively cured. (13) Afterwards, the mixture was stirred for 30 min at room temperature to obtain a clear coating.
Film formation and the curing process
The galvanized sheets used for dip coating were freshly degreased in acetone and then ethanol, followed by deoxidization in 1.0 M NaOH solution for 1-2 mill. Then, the sheets were dried at 80[degrees]C for 15 min to remove the water. The galvanized sheets were dip coated in clear coatings at room temperature followed by post-curing at 180[degrees]C for 5 min to obtain testing panels with robust films. The thicknesses of these films were about 15 [micro]m (the thickness of the films was measured by a coating thickness measuring instrument (model DUALSCOPE [R] MPOR from Fisher, Germany) with a test range of 0-50 [micro]m and an accuracy [+ or -]0.5 [micro]m (and the instrument was calibrated by a ferromagnetic standard substrate before measurement).
Fourier transform infrared spectra (FTIR)
FTIR spectra were recorded by using an EOUINOX55 spectrophotometer (Bruker, Germany). The viscous liquid MALPB was diluted by benzene before it was dropped on KBr pellets, while the dispersion was directly dropped on KBr pellets. After the benzene and water were evaporated, these pellets were scanned from 4000 to 500 [cm.sup.-1] with a resolution of 0.1 [cm.sup.-1].
Particle size distribution
The particle size and distribution were measured by the LMS-30 laser diffraction particle size analyzer (Seishin, Japan), and the testing scope was from 0.1 to 1000 [micro]m.
A conventional three-electrode system was used for DC polarization measurements: the sample as a working electrode with an exposed area of 1 [cm.sup.2], a piece of platinum sheet as a counter electrode, and the saturated calomel electrode as a reference electrode. DC polarization was carried out by a CHI 660C electrochemical station (Zhenhua Instrument Corp., China). The DC polarization curves were measured by immersing panels in a 3.5% NaCl solution at the scan rate of 1.0 mV/s at room temperature. The samples were immersed in the electrolyte for 30 min before testing, to get a steady state of the system.
Electrochemical impedance spectroscopy (EIS)
EIS was performed by using the above-mentioned CHI 660C electrochemical station with ZSimpWin software analysis at the frequency range from 1.0 to [10.sup.5] Hz at room temperature by immersing samples in a 3.5% NaCl solution. Similarly, the sample was set as a working electrode with an exposed area of 1 [cm.sup.2], a piece of platinum sheet as a counter electrode, and the saturated calomel electrode as a reference electrode.
Salt spray test
A salt spray test was performed in an YWX/Q-150 salt spray chamber (Suzhou Xinda Instrumental Co., China). The coated samples were scribed by a stainless steel tip to make two cross lines. The depth of the lines were controlled properly so that the depth of the line was about 5 [+ or -] 0.5 [micro]m. The test parameters were set as the following: chamber temperature was at 35[degrees]C with fluctuation range of [+ or -]0.5[degrees]C, the concentration of salt solution was 50 g/L NaCl in water, the amount of salt sedimentation was 1-2 mL/h, and the spray mode was continuous. The surface images of the testing samples were recorded using a digital camera, and they were rinsed by DI water to remove salt before taking the pictures.
Results and discussion
MALPB reacting with DEA confirmed by FTIR
In this study, the modification reaction was simultaneously preceded by the emulsification since DEA was initially dissolved in water before it was added to the flask containing MALPB and surfactants. The schematic plot of DEA reacting with MA is shown in Fig. 1.
[FIGURE 1 OMITTED]
The FTIR spectra of MALPB and MALPB with DEA (DEA:MA = 1:1) are shown in Fig. 2 and band assignments corresponding to their groups are listed in Table 1. The new peak of OH vibration appearing at around 3410 [cm.sup.-1] suggests the existence of a hydroxyl group in MALPB-DEA. The new peak that obviously appeared at around 1160 [cm.sup.-1] is ascribed to C-N bending of amide, indicating the reaction of the amine group from DEA with the anhydride group from MA to form an amide group. (14), (15) The symmetric triple peaks appeared at around 1865, 1791, and 1700 [cm.sup.-1], the characteristic O=C stretching of anhydride group changing to double peaks appeared at 1737 [cm.sup.-1] (for O=C stretching of carboxyl groups) and 1702 [cm.sup.-1] (for O=C stretching of amide groups), (15-17) providing further evidence that MA reacted with DEA to form amide to achieve chemical modification during emulsification.
[FIGURE 2 OMITTED]
Table 1: Band Assignments of MALPB and MALPB-DEA (DEA:MA = 2:1) Bands ([cm.sup.-1]) Assignments ~3410 O-H stretching (MALPB-DEA) ~3070 C-H stretching of benzene(MALPB terminated by p-xylene) ~2910 C-H stretching of CH, [CH.sub.2] ~2860 C-H stretching ([-CH.sub.3] end group of p-xylene) ~1865, 1791, 1700 C=O stretching of anhydride group ~1737, 1702 C=O stretching of amide group ~1635 C=C stretching ~1390, 1435 CH, [CH.sub.2] bending ~1235, 1060 C-O-C bending of anhydride group ~1160 C-N bending of amide group ~990 C=O bending of carbonyl group ~910 C=C bending of p-benzene (MALPB terminated by p-xylene) ~690 [CH.sub.2] multiple split from ([CH.sub.2]-)n (n > 4)
Particle size and distributions of the dispersions
The particle size distributions of the MALPB-DEA dispersion with different additions of DEA at molar ratio of DEA:MA = 1:1 and 2:1 are shown in Figs. 3a and 3b, respectively. In Fig. 3a, all the particle diameters were larger than 1.0 [micro]m and 85% of the particle sizes were in the range of 2.0-7.0 [micro]m; while in Fig. 3b, almost all of the particles' diameters were smaller than 2.0 [micro]m and 85% of particle's sizes were distributed in the range of 0.5-1.5 [micro]m. With the addition of more DEA, the particle size became much smaller and the particle size distribution got narrower. During emulsification, the anhydride of MALPB reacted with the amine of DEA to form an amide so that more and more of the hydrophilic hydroxyl group could be introduced. With the increase in added water, the mixed solution involved a phase transition from water-in-oil (W/O) to oil-in-water (O/W), and a stable dispersion was finally formed. In the meantime, when an amine group reacted with an anhydride in MALPB at molar ratio of 1:1, an acid group from the ring opening of maleic anhydride was formed. With the addition of more DEA, this acid group could react with the amine of DEA such that more hydrophilic groups could be introduced in MALPB during emulsification (refer to Fig. 1). This was similar to the preparation of acrylate emulsion as reported by Zhang et al., (18) in which an emulsion with smaller particle size and narrower size distribution was obtained if higher hydroxyl monomer content was added. With more DEA added, much more hydrophilic hydroxyl groups available in MALPB made it easier to be dispersed, and hence dispersions composed of smaller particle size and narrower size distribution were obtained.
[FIGURE 3 OMITTED]
Figure 4 shows DC polarization curves of the galvanized sheets with clear waterborne coatings prepared by MALPB-DEA dispersions with different particle size and distribution in an aqueous solution of 3.5% NaCl. (19), (20) The corrosion current density of the samples coated with clear coating at a molar ratio of DEA:MA = 1:1 or 2:1 showed roughly the same value of around [10.sup.-6] mA/[cm.sup.2], which is much higher than the bare sheet with a value of around [10.sup.-2] mA/[cm.sup.2], (13) indicating that these MALPB-DEA coatings performed well in protecting galvanized sheets. The corrosion voltage was about 40 mV higher if the addition of DEA was doubled, and this difference could be attributed to the variations of particle size and distribution. Particles with smaller size and narrower distribution have a tendency to contact closer to form a more compact layer than that of relatively bigger sizes and broader distributions, as such to provide better protection. This supports the mechanism of solid film formation from polymer dispersions as proposed by Winnik. (6) This more compact layer acted as a barrier, providing superior protection on galvanized sheets. The barring mechanism of these films which slowed down the penetrating speed of electrolyte onto the surface of galvanized sheets is further elaborated upon in the following EIS results.
[FIGURE 4 OMITTED]
Electrochemical impedance spectroscopy (EIS)
The Bode plots of galvanized sheets coated with clear coatings prepared by different MALPB-DEA dispersions in 3.5% NaCl solution at 1, 24, 48, 72, and 100 h immersion times are presented in Fig. 5. At the initial immersion stage, the impedance of samples coated with dispersion composed of DEA:MA = 2:1 (1650 Ohm [cm.sup.2]) was higher than that of the sample coated with dispersion composed of DEA:MA = 1:1 (1480 Ohm [cm.sup.2]). After a longer immersion time, all their impedance values exhibited a gradual decrease with immersion times. In all the testing time, however, the impedances of the sample coated with dispersion composed of DEA:MA = 2:1 were always higher than those of the one coated with dispersion composed of DEA:MA = 1:1, suggesting that the enhanced barrier capability of the particle sizes of the dispersions was smaller. The decrease of impedance resulted from the penetration and saturation of water inside the coating layer, and after 100 h of testing, all the samples still had impedance of more than 700 Ohm [cm.sup.2]. This means that they were in the middle immersing stage during which the ions and water were saturating inside the coating layer and the coatings were still protecting the sheet. (21) These results provided the fact that MALPB-DEA waterborne coatings prepared by the smaller emulsion particle size could form a more compact protective layer on the surface of galvanized sheets, and in turn could effectively slow down the diffusion of the electrolyte onto the surface of galvanized sheets. It could be inferred that after a longer immersion time, the impedances would be lower and lower; the electrolyte would infiltrate through the MALPB-DEA layer and gradually reach the surface of the sheet.
[FIGURE 5 OMITTED]
Salt spray test
By observing the extent of the damage on their surfaces, the overall anticorrosion performances of different coatings could be compared by doing a salt spray test. (13), (22) Figure 6 presents the digital camera images of samples coated with MALPB-DEA clear coatings prepared by dispersions of different particle size and distribution after a salt spray test for 240, 480, and 600 h, respectively. After 240 h of testing in the salt spray chamber, some small blisters were observed on both of the samples. Compared to the image in Fig. 6a1, many more small blisters and some tiny stripes appeared along the mark lines in Fig. 6b1, while these stripes were not clearly observed in Fig. 6a1. (23) After testing for 480 h, however, the blisters in Fig. 6a1 grew bigger, and tiny stripes appeared along the marked lines in Fig. 6a2; while in Fig. 6b2, the stripes along the mark lines in Fig. 6b1 expanded to large spoiled areas, and much bigger blisters appeared on the surface. After 600 h of testing, the spoiled areas in both Figs. 6a3 and 6b3 were much larger. Furthermore, because their outer layers were predamaged by a scriber, the ions and water would gradually penetrate inside the coatings along the lines. As a result, the coating became looser in Fig. 6b3 than that in Fig. 6b2, and some coatings on the outer surface even began peeling off along marked lines. Even under this situation, these two coatings did not de-bond from the sheets and rusty phenomena were not observed, suggesting their strong adherence to the sheets. (24) In summary, during all the testing times, the coatings composed of dispersion with smaller size and narrower distribution always exhibited better anticorrosive performances, and the more compact organic layers provided much more effective barriers to protect the sheet from water and ion penetrations so that its life span could be prolonged.
[FIGURE 6 OMITTED]
MALPB-DEA dispersions were successfully prepared by direct polymer emulsification and simultaneous chemical modification with DEA. Dispersion composed of smaller particle size and narrower distribution was obtained with DEA addition at a molar ratio of DEA: MA = 2:1 because more hydrophilic groups were introduced during emulsification. The formation of compact film by the closer contact of smaller particle size and narrower distribution resulted in an improvement in anticorrosive performances on the galvanized sheets. This enhancement was reflected by its higher corrosion prevention potential and higher impedance in all the immersion times. This more compact film slows down the diffusion speed of water and ions so that its protective capability in harsh environment is improved.
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J. Wang ([??]), J. Tang, Y. He
Beijing Key Laboratory for Corrosion, Erosion and Surface Technology, Institute of Advanced Materials, University of Science and Technology Beijing, Beijing 100083, China
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|Author:||Wang, Jinwei; Tang, Jinwei; He, Yedong|
|Date:||Jan 1, 2011|
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