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Characterization of Organofunctional Silane Films on Zinc Substrates
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
185, 197–209 (1997)
CS964604
Characterization of Organofunctional Silane Films on Zinc Substrates WEI YUAN
WIM J.
AND
VAN
OOIJ1
Department of Materials Science & Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012 Received March 15, 1996; accepted September 16, 1996
Organofunctional silane films formed on pure zinc substrates from aqueous solutions were characterized by ellipsometry, contact angle measurements, reflection absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The silanes studied were gamma-aminopropyltriethoxysilane (g-APS) and gamma-ureidopropyltrialkoxysilane (g-UPS). The deposition parameters included solution concentration, solution dipping time, and pH value of the applied solution. Effects of these deposition variables on the thickness of the film, the surface energy of the film, the chemical structure of the film, and the surface topography of the film are discussed. The effect of film aging time on the surface energy of silane-coated pure zinc substrates was also studied. It was found that the silane film thickness obtained depends primarily on the applied solution concentration and is independent of the solution dipping time. The molecular orientations of the applied silane film are determined by the pH value of the applied silane solution and the isoelectric point of the metal substrates. The deposition window in terms of pH value for zinc substrates was between 6 and 9. The total surface energy of silane-coated pure zinc substrate decreases steeply with film aging time for g-APS but not for g-UPS. The stability of both silane films improved after aging in the air. q 1997 Academic Press Key Words: silane; coupling agents; zinc; galvanized steel; characterization.
1. INTRODUCTION
The excellent resistance of zinc to corrosion in many environments and its ability to protect steel galvanically have made galvanized steel a common material used in automotive and construction industries. The zinc coating in galvanized steel, however, still needs to be protected from highly corrosive conditions by painting. One of the problems with the zinc–paint system is that ordinary paints do not adhere well to unweathered zinc surfaces. Thus, the zinc surface has to be pretreated before paints can be applied to it. The currently used pretreatment methods are mainly based on zinc phosphating plus a chromate rinse. These methods are 1 To whom correspondence should be addressed. E-mail: wvanooij@ uceng.uc.edu.
relatively cheap and effective in terms of performance. Chromate, however, has been found to be toxic to the human body and environment. They need to be eliminated from industrial applications. These health and environmental concerns have prompted the development of chromate-free pretreatment methods, e.g., by aqueous organofunctional silane solutions. The ability of organofunctional silanes to improve the adhesion between organic polymers and inorganic materials is well known. Possessing both organic and inorganic properties, these hybrid chemicals have a general structure of R– Si–(OR*)3, where OR* is a hydrolyzable group capable of reacting with inorganic materials and R is a reactive group selected for interacting with organic polymers. Extensive efforts have been made in the past decade to demonstrate the effectiveness of organofunctional silanes as coupling agents between polymers and metals (1–5). Most work, however, has mainly been focused on aluminum and iron substrates. Only few literature references can be found on the subject of galvanized steel or zinc substrates treated with organofunctional silane. Walker (6) showed that silane coupling agents were effective in improving the water resistance of bonds of epoxy and urethane paints to aluminum, steel, cadmium, copper, and zinc. Van Ooij et al. (7, 8) studied various silane coupling agents on polished zinc and phosphated electrogalvanized steel substrates. They developed a two-step treatment consisting of a silicate rinse followed by a silane rinse. The novel aspect of this process was that a very thin film of an inorganic silicate was first deposited onto the surface of the phosphate crystals. This film anchored well to the phosphate crystals and converted the phosphate layer from an acidic surface into a basic surface which is now receptive to a reaction with the acidic –SiOH groups of the hydrolyzed silane, resulting in the formation of –Si–O–Si– linkages at the interface. They found that the silicate/silane treatment modified the paint–phosphate interface to give improved adhesion and corrosion performance. Brockmann and co-workers (9, 10) reported results of a study of silane coupling agents as adhesion promoters for galvanized steel substrates. The main conclusion of their work was that the performance of silane coupling agents used as primers on
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0021-9797/97 $25.00 Copyright q 1997 by Academic Press All rights of reproduction in any form reserved.
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FIG. 1. Effect of solution concentration on the film thickness of g-APS and g-UPS films deposited on zinc substrates; solution dipping time, 1 min; pH value of g-APS solution, 10.2; pH value of g-UPS solution, 8.0.
galvanized steel strongly depended on the surface composition of the metal substrate. Better performance was obtained for electrogalvanized steel substrates rather than hot-dipped galvanized steel substrates. It appears that more work needs
to be done before a special pretreatment method can be obtained for galvanized steel substrates. The results reported in this paper are part of an ongoing program in our laboratory to develop chromate-free pretreat-
FIG. 2. Effect of solution dipping time on the film thickness of g-APS and g-UPS films deposited on pure zinc substrates; solution concentration, 1 vol%; pH value of g-APS solution, 10.2; pH value of g-UPS solution, 8.0.
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FIG. 3. Effect of pH value of silane solution on the surface energy of g-APS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min.
ment methods for metals to replace currently used chromating and/or phosphating pretreatments. Pure zinc was used as a model for galvanized steel to study the interaction between organofunctional silanes and zinc surfaces. It is important to understand the influence of deposition parameters, such as solu-
tion concentration, solution dipping time, and the pH value of the applied solution, on the silane film formation before an optimum pretreatment method can be achieved. The adhesion and corrosion performance results of selected silanes in painted galvanized steel systems will be reported later.
FIG. 4. Effect of pH value of silane solution on the surface energy of g-UPS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min.
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FIG. 5. Effect of film aging time on the surface energy of g-APS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solution, 8.0.
2. EXPERIMENTAL
2.1. Materials The zinc substrates (99.7%, 0.5 mm thickness) were obtained from Goodfellow Inc. The silanes studied were gamma-aminopropyltriethoxysilane (g-APS) and gamma-ureidopropyltrialkoxysilane (g-UPS, 50% in methanol), both supplied by OSI Specialties. The structures of g-APS and g-UPS are H2NCH2CH2CH2Si(OC2H5)3 and H2NC(|O)NHCH2CH2CH2Si(OC2H5)x(OCH3)30x, respectively. Deionized water (DI water) of 17.9 MV cm grade was used thoughout this work. All the chemicals were used as received. 2.2. Sample Preparation The zinc substrate was cut to coupons (25 1 25 mm2) which were ground with silicon carbide paper and polished with 0.05 mm gamma alumina to mirror surfaces. DI water was used as the lubricant in the polishing step. The coupons were then ultrasonically cleaned with acetone and 0.1 wt% sodium hydroxide solution, each for 10 min. The cleaned coupons were subsequently rinsed with DI water before they were dipped into an aqueous silane solution. 2.3. Deposition Conditions The silane solutions were prepared by hydrolyzing the corresponding silane in DI water at the desired pH value for at least 1 h before they were used. The silane was applied
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by dipping the polished and cleaned pure zinc substrate into the silane solution. The deposition parameters included solution concentration, solution dipping time, and pH value of the applied solution. The pH value of the silane solutions was adjusted with 1 M acetic acid solution or 1 M ammonium hydroxide solution. All the coupons after deposition were blown dry with pure nitrogen at a pressure of 172 kPa. 2.4. Characterization Techniques 2.4.1. Ellipsometry. The film thickness of the deposited silane films was determined by a Rudolph Research 43603200E optical ellipsometer equipped with a mercury vapor ˚ ). The polished zinc substrate was cleaned lamp (l Å 5461 A and examined before a silane film was deposited. The measured values of the parameters D and C were used to calculate the refractive index of the substrate with a program written by McCrackin et al. (11). A typical calculated value of the complex refractive index of the polished zinc substrate is about 1.45(1 0 3.46i). The substrate was then cleaned again by the same method before it was dipped into the corresponding silane solution. The silane-coated substrate was reexamined by the ellipsometer after the deposition. The measured values of the parameters D and C, the complex refractive index of the corresponding substrate, and the refractive index of the corresponding silane film were used to calculate the thickness of the silane film with the same program. The values reported in this paper are the average values measured on three substrates. The refractive index
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FIG. 6. Effect of film aging time on the surface energy of g-UPS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solution, 8.0.
used for g-APS is 1.420 and the refractive index used for g-UPS is 1.386. Both values were obtained from the manufacturer’s Material Safety Data Sheets (MSDS).
2.4.2. Surface energy measurements. The surface energy of deposited silane films was determined by a VCA 2000 video contact angle system (Advanced Surface Tech-
FIG. 7. Effect of DI-water rinse on the film thickness of g-APS and g-UPS films deposited on zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solutions, 8.0.
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FIG. 8. FTIR spectra of pure g-APS and pure g-UPS.
nology, Inc.). This system is based on the sessile drop method. DI water and formamide were used as probing liquids. The averaged contact angle of 5 drops for each probing liquid was used as an input to the harmonic mean model to determine the total surface energy of the deposited silane film, as well as its dispersive and polar components (12). All the surface energy data reported in this paper were obtained under the conditions that the concentration of the applied silane solution was 1 vol% and the solution dipping time was 1 min. 2.4.3. Infrared spectroscopy (IR). The chemical structures of pure g-APS and g-UPS monomers were studied by Fourier transform infrared spectroscopy (FTIR). The FTIR spectra were obtained by using a BIO-RAD FTS-40 FTIR spectrometer. The spectra were taken at a resolution of 4 cm01 for 16 scans from 4000 to 450 cm01. The FTIR specimen were prepared by placing the pure g-APS or g-UPS monomer liquids directly on a potassium bromide crystal disk (PKBr 25.0 1 4, OPTOVAC, EM Industries, Inc.), respectively, followed by blow-drying with nitrogen.
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The chemical structures of the deposited silane films were studied by reflection absorption infrared spectroscopy (RAIR). The RAIR spectra were obtained by using the same spectrometer with a variable angle specular reflectance accessory (091-0461) at an incidence angle of 757. The RAIR spectra were taken at a resolution of 4 cm01 for 128 scans from 4000 to 450 cm01. All the RAIR spectra reported in this paper were obtained under the conditions that the concentration of the applied silane solution was 1 vol% and the solution dipping time was 1 min. 2.4.4. X-ray photoelectron spectroscopy (XPS). XPS analysis was carried out using a Perkin–Elmer 5300 ESCA system. The MgKa X ray source was operated at 300 W and 15 kV DC. Pass energies for survey and high resolution spectra were 44.75 and 17.90 eV, respectively. High resolution spectra were corrected correspondingly by assigning a value of 284.6 eV to the C1s peak of saturated hydrocarbons. Atomic concentrations were obtained from the high resolution spectra using sensitivity factors provided with the software. Analysis was done at two different take-off angles,
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TABLE 1 Frequencies and Tentative Band Assignments for Pure g-APS and g-UPS g-APS (cm01)
g-UPS (cm01)
3381 3343 3211 2974 2942
2976 2928 2887 2870
2888 2842
2738 1656 1603 1584 1559 1483 1458 1446 1388 1343
1443 1391 1297
1195 1168
1168 1104 1080 958
1084 956 815 794 779
792 777
Resolution intensity
Assignments
W S M S S S S S S W S S W S M M M M M M S S VS VS S S S S
nas (N–H) nas (N–H) ns (N–H) nas (CH3) nas (O–CH3) nas (CH2) ns (CH3) ns (CH2) ns (O–CH3) ns (C–H) ns (C|O) d (NH2) d (NH2) d (NH) & n (CN) d (CH2) d (CH2) das (CH3) d (CH3) t (CH2) v (CH2) r (O–CH3) r (O–C2H5) nas (Si–O–C) nas (Si–O–C) ns (Si–O–C2H5) ns (Si–O–CH3) ns (Si–O–C2H5) ns (Si–O–C2H5)
Note. Abbreviations: VS, very strong; S, strong; M, medium; W, weak; as, asymmetric; s, symmetric; n, stretching; d, bending; t, twisting; v, wagging; r: rocking.
157 and 757, in order to study the change in the structure of silane films as a function of depth. The take-off angle is the angle between the sample surface and the analyzer. 2.4.5. Atomic force microscopy (AFM). AFM images of polished pure zinc substrates with and without silane films were obtained on a Burleigh Instruments’ ARIS-3500 Personal AFM system. Samples were analyzed on a nanometer scale under ambient conditions. The AFM was operated in the contact mode. During imaging, the micro-machined silicon cantilever was in contact with the surface under a reference force of about 2.11 N. The topography of the surface is a measurement of the adjustments in the vertical direction necessary to keep the reference force constant. 3. RESULTS AND DISCUSSION
3.1. Thickness of the Silane Films Deposited on Zinc Substrates The effects of solution concentration and solution dipping time on the film thickness of g-APS and g-UPS deposited
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on pure zinc substrates are shown in Figs. 1 and 2, respectively. It is observed that the film thickness increases with the solution concentration, but almost remains constant with the solution dipping time. Similar results were also reported by Tutas et al. (13). They found that films deposited on glass from g-APS were the same thickness after a minute of contact as after an hour, while films deposited from 0.1– ˚ thick. These observa5% aqueous solutions were 50–200 A tions are in agreement with the chemistry of organofunctional trialkoxysilanes in aqueous solution. As indicated by Plueddemann, amino-organofunctional trialkoxysilanes at their normal pH in water hydrolyze and condense very rapidly to reach an equilibrium composition in a short time (14). Very dilute aqueous solutions contain monomeric silanetriol, but more concentrated solutions are believed to be an equilibrium mixture of low-molecular-weight (LMW) siloxanols with silanol groups stabilized by hydrogen bonding to amine groups. On the other hand, Plueddemann also indicated that the bonds between silanol groups and most metals were typically inorganic with a high degree of ionic character (14). It is known that ionic, inorganic chemistry is based more on equilibrium constants than on kinetics. Ionic, inorganic bonds are, therefore, formed and broken in reversible reactions determined by concentration and equilibrium constants. Thus, the results obtained in this work indicated that for the silanes studied, the film formation on zinc substrates involves an equilibrium of hydrolysis and condensation in the aqueous solution. Monomeric silanetriols or LMW siloxanols were first formed in the solution and then attached rapidly to the zinc substrates. 3.2. Surface Energy of the Silane Films Deposited on Zinc Substrates The surface energy of g-APS- and g-UPS-coated pure zinc substrates was studied as a function of the pH value of the applied silane solutions. The measurement was conducted after the film was aged for half an hour in ambient air. The results are shown in Figs. 3 and 4, respectively. It is observed that the total surface energy and its polar component of both silane-coated zinc substrates reach a maximum plateau between pH values of 6 and 9, while the dispersive component of the total surface energy of the coated substrates was almost independent of the pH value of the silane solutions. The relatively higher polar components between pH values of 6 and 9, as well as the total surface energy, may be attributed to the fact that the film formed in this range had a better molecular orientation. It has been reported that a silane molecule has better molecular orientation on the metal surface when the pH value of the applied solution is equal to or lower than the isoelectric point (IEP) of the metal oxide (15). Although pure zinc substrates were used in this study,
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FIG. 9. RAIR spectra of g-APS films deposited on zinc substrates at different pH value; solution concentration, 1 vol%; solution dipping time, 1 ˚. min; film thickness, Ç50 A
zinc oxide films on the surface of the substrates were observed in the XPS analysis. Angular dependence measurements of the L2,3M4,5M4,5 Auger transition peaks show the film thickness of ZnO to be about 1–1.5 nm. The IEP for zinc oxide is around 9.0. Therefore, the silane molecules deposited at a pH value below 9.0 might be oriented on the zinc substrates more straight-up with amino or ureido groups directed away from the silane–zinc interface. The decrease in the polar components at high pH value (above 9) was probably attributed to the up-and-down orientation of silanol groups in the film which led to the formation of relatively high molecular weight siloxanes or cyclic structures in the
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film. The decrease in the polar component at a low pH value (below 6) is probably due to the acetic acid left in the film. It has been argued by Plueddemann that when amino functional silanes are neutralized with carboxylic acids in aqueous solution and applied to mineral surfaces, a portion of the acid remains after drying as the acid salt or as the amide (16). Zinc dissolution might also be a factor in the decrease of the polar component at low pH value, since it has been shown that a stable zinc oxide film is only formed in the pH range of 6 to 12.5 (17). The deposition window in terms of pH value for obtaining well-oriented silane films on zinc substrates, therefore, should be between 6 and 9.
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FIG. 10. RAIR spectra of g-UPS films deposited on zinc substrates at different pH value; solution concentration, 1 vol%; solution dipping time, 1 ˚. min; film thickness, Ç50 A
The effect of film aging time on the surface energy of gAPS- and g-UPS-coated pure zinc substrates is shown in Figs. 5 and 6, respectively. It is observed that the polar component of the total surface energy of both silane-coated substrates decreased with the film aging time, while the dispersive component remained almost constant during the film aging period. As a result, the total surface energy of both silane-coated substrates shows the same trend as its polar component. The decrease rate, however, is quite different for g-APS and g-UPS. As indicated in Fig. 5, the total surface energy of g-APS-coated zinc substrate decreased rapidly during the first 24 h of aging in the air and had only
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about 59% of initial surface energy left after the first 24 h aging. The total surface energy of the g-APS-coated substrate then continued to decrease at a much slower rate and the surface energy retention was about 53% after 168 h aging in the air. In contrast, the total surface energy of g-UPScoated zinc substrate, as shown in Fig. 6, decreased very slowly with film aging time and the surface energy retention was about 93% after 168 h aging in the air. It should be noted that, right after the formation of the silane film, the drop of DI water completely wetted the coated zinc surface to form a 07 contact angle, which indicated that the total surface energy of the silane film studied
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TABLE 2 XPS Results for Polished Zinc Substrates with and without gUPS Films (solution concentration, 1 vol%; solution dipping time, 1 min) g-UPS coated zinc substrate (%)
Take-off angle
Element detected
Blank zinc substrate (%)
pH 4.3
pH 7.3
pH 10.3
157
C N O Si Zn N/Si C N O Si Zn N/Si
33.5 — 48.1 — 18.4 — 17.4 — 52.3 — 30.3 —
44.0 11.0 36.5 6.7 1.9 1.6 39.3 8.9 40.0 5.9 5.9 1.5
40.3 11.0 36.6 7.9 4.2 1.4 30.4 7.6 41.0 7.3 13.7 1.1
46.4 13.6 31.2 8.0 0.8 1.7 42.1 11.9 35.6 8.3 2.1 1.5
757
is above or at least equal to the surface tension of the DI water. Therefore, the surface tension of DI water was taken as the initial surface energy of the silane film without aging. The different decrease rate for g-APS and g-UPS suggests that the decrease of surface energy with film aging time is due not only to the adsorption of hydrocarbons from the air. It may also involve molecular rearrangement in the film and the condensation and dehydration reactions after the film has been formed. The different behavior, in terms of the surface energy with film aging time, between g-APS film and g-UPS film might be due to the different nature of the films. Ellipsometry was used to measure the film thickness of both silane before and after DI water rinsing. Two sets of samples were analyzed. One set was freshly deposited films, and another set was the deposited films aged for 2 days in the air. The results are shown in Fig. 7. For the freshly deposited films, the thickness retention of g-UPS film was about 8% after 30 s DI water rinsing, while the thickness retention of g-APS was about 42% after 30 s DI water rinsing. However, the sample aged for 2 days showed improved stability as compared with the freshly deposited sample. The thickness retention of the gUPS film increased to about 68% after 30 s DI water rinsing, while the thickness retention of the g-APS film increased to about 90% after 30 s DI water rinsing. These results indicate that the silane film was initially hydrogen bonded to the zinc oxide. During aging, the silane film became covalently bonded to the zinc oxide and crosslinking occurred due to the condensation of the silanol groups and dehydration, which increased the stability of the silane film. The better stability of the g-APS film over the g-UPS film is probably because the amine group in the g-APS is more easily coordinated to
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the silicon or metal substrates through hydrogen bonding, which will be discussed in the following section. 3.3. Chemical Structure of the Silane Films Deposited on Zinc Substrates The FTIR spectra of pure g-APS and g-UPS monomers are shown in Fig. 8. Tentative band assignments for these two compounds are listed in Table 1. The band assignments for g-APS have been discussed in the literature before (18). The band assignments for g-UPS, however, are new. The bands near 1559, 1603, 1656, and 3347 cm01 are assigned to the vibration modes of the ureido group, while the bands near 815, 956, 1084, 1195, 2842, and 2942 cm01 are assigned to the vibration modes of ester groups on silicon. The RAIR spectra of the g-APS film, deposited from the silane solution onto zinc substrates at different pH values, are compiled in Fig. 9. It can be noted that the g-APS film deposited at pH value of 4.5 had a completely different chemical structure from those deposited at pH of 6.0, 8.0, and 10.2. It shows relatively strong bands at 1588 and 927 cm01. The band at 1588 cm01 was assigned to the vibration mode of free NH2 groups, while the band at 927 cm01 was assigned to the vibration mode of silanol groups. These results suggested that g-APS forms monomeric silanetriols in its acid aqueous solution. As the pH value of the applied silane solution increased, the band at 927 cm01 disappeared and very strong bands appeared at 1117 and 1018 cm01, which were assigned to the vibration mode of siloxane groups. These results indicated that the silanol groups formed are less stable and condense to polysiloxanes as the pH value of the applied silane solution increased. As shown in Fig. 9, the higher the pH TABLE 3 XPS Results for Polished Pure Zinc Substrate with g-APS Films (solution concentration, 1 vol%; solution dipping time, 1 min) g-APS coated zinc substrate (%)
Take-off angle
Element detected
pH 4.5
pH 8.0
pH 10.3
pH 8.0, Aged for 46 h
157
C N O Si Zn N/Si C N O Si Zn N/Si
56.7 2.8 29.1 4.4 7.0 0.6 42.8 3.8 33.7 6.3 13.4 0.6
44.2 5.2 34.7 7.1 8.8 0.7 36.2 5.0 39.3 6.8 12.8 0.7
68.7 3.9 19.5 6.7 1.2 0.6 53.5 3.7 29.9 7.8 5.1 0.5
61.1 6.7 23.3 6.7 2.2 1.0 46.9 5.5 33.1 7.6 6.9 0.7
757
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FIG. 11. AFM three dimensional image of the polished pure zinc substrate.
value, the broader the bands at 1117 and 1018 cm01. The band at 1117 cm01 developed into two bands (1117 and 1044 cm01) as the pH value of the applied silane solution increased to 10.2, which indicated that a relatively long siloxane chain was formed (19). It was also noted that when the pH value of the applied silane solution was above 6, the NH2 vibration mode shifted from 1588 to 1561 cm01. This shifting was attributed by Boerio et al. to the formation of acceptor amine groups as a result of hydrogen bonding or coordination of the amine nitrogen to the silicon or metal substrates (20). The RAIR spectra of g-UPS films deposited from the silane solution onto zinc substrates at different pH values are compiled in Fig. 10. Unlike the results obtained for the g-APS films, the g-UPS films show similar spectra after deposition from the silane solution at pH values of 4.5, 8.1, and 10.0. The band at 1139 and 1059 cm01 was assigned to the vibration mode of siloxane groups, while the band at 913 cm01 was assigned to the vibration mode of silanol groups. These observations suggest that the g-UPS films were comprised of LMW siloxanol molecules and the chemical structures of the films were not strongly affected by the pH value of the applied solution. XPS was used to study the effect of pH value of both silane solutions on the molecular orientation of the silane film deposited on a pure zinc substrate. The results are compiled in Table 2 for g-UPS and Table 3 for g-APS. Zinc was detected in all circumstances which indicated that the ˚ , which is lower silane film thickness was not more than 50 A than the results obtained from ellipsometry measurement. However, the dissolution of zinc oxide film and its diffusion into the silane film, especially at low pH value, may give a rational interpretation to the discrepancy. It was observed that the carbon content around a pH value of 8.0 was lower than that obtained at a pH value of 4.5 and 10.3 for both g-APS- and g-UPS-coated zinc substrates. The oxygen content around a pH value of 8.0, however, was higher
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than that obtained at a pH value of 4.5 and 10.3 in both silane films. These data might explain the results obtained from surface energy measurement that the surface energy reaches a high plateau between the pH values 6 and 9. It is shown in Table 2, for g-UPS at pH value of 4.3, that the N/Si ratio decreased from 1.6 to 1.5 when the take-off angle changed from 157 to 757. At a pH value of 8.0, the N/Si ratio decreased from 1.4 to 1.1 when the take-off angle changed from 157 to 757. At a pH value of 10.3, the N/Si ratio decreased from 1.7 to 1.5 when the take-off angle changed from 157 to 757. The N/Si ratio was below the theoretical value of 2.0 in all circumstances, even though ammonium hydroxide was used to adjust the pH value of g-UPS solutions. Table 3 shows the results for g-APS. At pH 4.5 and 8.0, the N/Si ratio remained unchanged at 0.6 and 0.7, respectively, when the take-off angle changed from 157 to 757. At pH 10.3, the N/Si ratio decreased from 0.6 to 0.5 when the take-off angle changed from 157 to 757. Thus, it is impetuous, based on these results, to make a conclusion that the silane film had a better molecular orientation at certain pH value. XPS was also used to study the aging effect on the molecular orientation of the g-APS film deposited on a pure zinc substrate with the aim to explain the big drop observed in the surface energy after 24 h aging in the air. The g-APS film was applied on the pure zinc substrate from the 1 vol% g-APS solution at a pH value of 8.0. The film was then aged for almost 2 days before conducting the XPS analysis. It was found that after 2 days aging, the carbon and nitrogen contents in the film increased, while the contents of other elements, such as Si, O, and Zn, decreased. The increase in carbon content is from 44.2 to 61.1 at 157 take-off angle, and from 36.2 to 46.9 at 757 take-off angle. These observations might indicate that in addition to the hydrocarbon adsorption, molecular rearrangement also occurred during the aging process. This explains why the total surface energy of
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FIG. 12. AFM three dimensional image of the polished pure zinc substrate coated (a) with g-APS and (b) with g-UPS; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solutions, 8.0.
g-APS-coated zinc substrate decreased steeply during the first 24 h aging.
3.4. Surface Topography of the Silane Films Deposited on Zinc Substrates The AFM images of polished pure zinc substrates with and without silane film are shown in the Figs. 11 and 12, respectively. Figure 11 shows large gradual undulations across the surface for polished pure zinc substrate without silane coating. The surface roughness is about 39 nm, which is close to the scale of final polishing step of 0.05 mm. However, the surface roughness decreases to about 23.5 nm when the polished pure zinc substrate was coated with the silane films, as shown in Fig. 12. It is interesting to note the topographic difference between the g-APS and g-UPS films. As shown in Fig. 12a, a relatively smooth and uniform surface was observed for the g-APS film. Figure 12b, however, shows the small and sharp undulations across the surface for the polished zinc substrate with the g-UPS film.
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4. CONCLUSIONS
Organofunctional silanes (g-APS and g-UPS) have been deposited on pure zinc substrates from aqueous solutions. The silane film formation appears to involve a true equilibrium of hydrolysis and condensation in the aqueous solution. The silane film thickness obtained depends primarily on the solution concentration and is almost independent of the solution dipping time. The molecular orientations of the applied silane film are determined by the pH value of the applied silane solution and the isoelectric point of metal substrates. The deposition window in terms of pH value for zinc substrates is between 6 and 9. The total surface energy of silanecoated pure zinc substrate decreases with film aging time. The decrease rate, however, is determined by the nature of the silane film. The stability of both silane films improved after aging in air. AFM images show that the g-APS film deposited on the zinc substrate was smoother than the gUPS film. The performance results of selected silanes in painted galvanized steel systems will be reported later.
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ACKNOWLEDGMENTS We thank Rohit Poppat for conducting the XPS analysis and Burleigh Instruments, Inc. for conducting the AFM analysis included in this paper. The U.S. Environmental Protection Agency through its Office of Research and Development funded the research described here under assistance agreement CR822989 to the University of Cincinnati. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
REFERENCES 1. Van Ooij, W. J., and Sabata, A., Mat. Res. Soc. Symp. Proc. 304, 155 (1993). 2. Van Ooij, W. J., Edwards, R. A., and Sabata, A., To Armco Steel Co., L.P., USP 5,292,549 (March 8, 1994). 3. Van Ooij, W. J., and Sabata, A., To Armco Steel Co., L.P., USP 5,108,793 (April 28, 1992). 4. Walker, P., in ‘‘Materials and Processing-Move into the 90’s’’ (S. Benson, T. Cook, E. Trewin, and R. M. Turner, Eds.), p. 227. Elsevier, Amsterdam, 1989. 5. Kerr, C., and Walker, P., in ‘‘Adhesion 11’’ (K. W. Allen, Ed.), p. 17. Elsevier, London, 1987. 6. Walker, P., J. Coatings Technol. 52, 33 (1980).
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7. Van Ooij, W. J., and Sabata, A., J. Adhesion Sci. Technol. 5, 843 (1991). 8. Sabata, A., Van Ooij, W. J., and Koch, R. J., J. Adhesion Sci. Technol. 7, 1153 (1993). 9. Brockmann, W., and Bremont, M., Presented at EUR ADH 94 Adhesion, Mulhouse, France, Sept. 12–15, 1994; Societe Francaise du Vide, France, 574 (1994). 10. Bremont, M., Brockmann, W., Guimon, M. F., and Pfister-Guillouzo, G., J. Adhesion 41, 147 (1993). 11. McCrackin, F. L., and Colson, J., ‘‘Nati. Bur. Stds. Tech. Note 242.’’ U.S. Govt. Print. Off., Washington, DC, 1964. 12. Wu, S. J., Polym. Sci. 34, 19 (1971). 13. Tutas, D. J., Stromberg, R., and Passaglia, E., SPE Trans. 4, 256 (1964). 14. Plueddeman, E. P., ‘‘Silane Coupling Agents,’’ 2nd ed. Plenum Press, New York, 1991. 15. Dow Corning Form No. 23-012B-85, ‘‘A Guide to Dow Corning Silane Coupling Agents.’’ Dow Corning Corporation, Midland, MI, 1985. 16. Plueddemann, E. P., and Stark, G. L., Mod. Plast. 54, 76 (1977). 17. Roetheli, B., Cox, G., and Littreal, W., Metals Alloys 3, 73 (1963). 18. Chiang, C., Ishida, H., and Koenig, J. L., J. Colloid Interface Sci. 74, 396 (1980). 19. Socrates, G., ‘‘Infrared Characteristic Group Frequencies,’’ 2nd ed. Wiley, New York, 1994. 20. Boerio, F. J., Schoenlein, L. H., and Grievenkamp, J. E., Appl. Spectrosc. 22, 203 (1978).
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CS964604
Characterization of Organofunctional Silane Films on Zinc Substrates WEI YUAN
WIM J.
AND
VAN
OOIJ1
Department of Materials Science & Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0012 Received March 15, 1996; accepted September 16, 1996
Organofunctional silane films formed on pure zinc substrates from aqueous solutions were characterized by ellipsometry, contact angle measurements, reflection absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The silanes studied were gamma-aminopropyltriethoxysilane (g-APS) and gamma-ureidopropyltrialkoxysilane (g-UPS). The deposition parameters included solution concentration, solution dipping time, and pH value of the applied solution. Effects of these deposition variables on the thickness of the film, the surface energy of the film, the chemical structure of the film, and the surface topography of the film are discussed. The effect of film aging time on the surface energy of silane-coated pure zinc substrates was also studied. It was found that the silane film thickness obtained depends primarily on the applied solution concentration and is independent of the solution dipping time. The molecular orientations of the applied silane film are determined by the pH value of the applied silane solution and the isoelectric point of the metal substrates. The deposition window in terms of pH value for zinc substrates was between 6 and 9. The total surface energy of silane-coated pure zinc substrate decreases steeply with film aging time for g-APS but not for g-UPS. The stability of both silane films improved after aging in the air. q 1997 Academic Press Key Words: silane; coupling agents; zinc; galvanized steel; characterization.
1. INTRODUCTION
The excellent resistance of zinc to corrosion in many environments and its ability to protect steel galvanically have made galvanized steel a common material used in automotive and construction industries. The zinc coating in galvanized steel, however, still needs to be protected from highly corrosive conditions by painting. One of the problems with the zinc–paint system is that ordinary paints do not adhere well to unweathered zinc surfaces. Thus, the zinc surface has to be pretreated before paints can be applied to it. The currently used pretreatment methods are mainly based on zinc phosphating plus a chromate rinse. These methods are 1 To whom correspondence should be addressed. E-mail: wvanooij@ uceng.uc.edu.
relatively cheap and effective in terms of performance. Chromate, however, has been found to be toxic to the human body and environment. They need to be eliminated from industrial applications. These health and environmental concerns have prompted the development of chromate-free pretreatment methods, e.g., by aqueous organofunctional silane solutions. The ability of organofunctional silanes to improve the adhesion between organic polymers and inorganic materials is well known. Possessing both organic and inorganic properties, these hybrid chemicals have a general structure of R– Si–(OR*)3, where OR* is a hydrolyzable group capable of reacting with inorganic materials and R is a reactive group selected for interacting with organic polymers. Extensive efforts have been made in the past decade to demonstrate the effectiveness of organofunctional silanes as coupling agents between polymers and metals (1–5). Most work, however, has mainly been focused on aluminum and iron substrates. Only few literature references can be found on the subject of galvanized steel or zinc substrates treated with organofunctional silane. Walker (6) showed that silane coupling agents were effective in improving the water resistance of bonds of epoxy and urethane paints to aluminum, steel, cadmium, copper, and zinc. Van Ooij et al. (7, 8) studied various silane coupling agents on polished zinc and phosphated electrogalvanized steel substrates. They developed a two-step treatment consisting of a silicate rinse followed by a silane rinse. The novel aspect of this process was that a very thin film of an inorganic silicate was first deposited onto the surface of the phosphate crystals. This film anchored well to the phosphate crystals and converted the phosphate layer from an acidic surface into a basic surface which is now receptive to a reaction with the acidic –SiOH groups of the hydrolyzed silane, resulting in the formation of –Si–O–Si– linkages at the interface. They found that the silicate/silane treatment modified the paint–phosphate interface to give improved adhesion and corrosion performance. Brockmann and co-workers (9, 10) reported results of a study of silane coupling agents as adhesion promoters for galvanized steel substrates. The main conclusion of their work was that the performance of silane coupling agents used as primers on
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FIG. 1. Effect of solution concentration on the film thickness of g-APS and g-UPS films deposited on zinc substrates; solution dipping time, 1 min; pH value of g-APS solution, 10.2; pH value of g-UPS solution, 8.0.
galvanized steel strongly depended on the surface composition of the metal substrate. Better performance was obtained for electrogalvanized steel substrates rather than hot-dipped galvanized steel substrates. It appears that more work needs
to be done before a special pretreatment method can be obtained for galvanized steel substrates. The results reported in this paper are part of an ongoing program in our laboratory to develop chromate-free pretreat-
FIG. 2. Effect of solution dipping time on the film thickness of g-APS and g-UPS films deposited on pure zinc substrates; solution concentration, 1 vol%; pH value of g-APS solution, 10.2; pH value of g-UPS solution, 8.0.
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FIG. 3. Effect of pH value of silane solution on the surface energy of g-APS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min.
ment methods for metals to replace currently used chromating and/or phosphating pretreatments. Pure zinc was used as a model for galvanized steel to study the interaction between organofunctional silanes and zinc surfaces. It is important to understand the influence of deposition parameters, such as solu-
tion concentration, solution dipping time, and the pH value of the applied solution, on the silane film formation before an optimum pretreatment method can be achieved. The adhesion and corrosion performance results of selected silanes in painted galvanized steel systems will be reported later.
FIG. 4. Effect of pH value of silane solution on the surface energy of g-UPS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min.
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FIG. 5. Effect of film aging time on the surface energy of g-APS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solution, 8.0.
2. EXPERIMENTAL
2.1. Materials The zinc substrates (99.7%, 0.5 mm thickness) were obtained from Goodfellow Inc. The silanes studied were gamma-aminopropyltriethoxysilane (g-APS) and gamma-ureidopropyltrialkoxysilane (g-UPS, 50% in methanol), both supplied by OSI Specialties. The structures of g-APS and g-UPS are H2NCH2CH2CH2Si(OC2H5)3 and H2NC(|O)NHCH2CH2CH2Si(OC2H5)x(OCH3)30x, respectively. Deionized water (DI water) of 17.9 MV cm grade was used thoughout this work. All the chemicals were used as received. 2.2. Sample Preparation The zinc substrate was cut to coupons (25 1 25 mm2) which were ground with silicon carbide paper and polished with 0.05 mm gamma alumina to mirror surfaces. DI water was used as the lubricant in the polishing step. The coupons were then ultrasonically cleaned with acetone and 0.1 wt% sodium hydroxide solution, each for 10 min. The cleaned coupons were subsequently rinsed with DI water before they were dipped into an aqueous silane solution. 2.3. Deposition Conditions The silane solutions were prepared by hydrolyzing the corresponding silane in DI water at the desired pH value for at least 1 h before they were used. The silane was applied
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by dipping the polished and cleaned pure zinc substrate into the silane solution. The deposition parameters included solution concentration, solution dipping time, and pH value of the applied solution. The pH value of the silane solutions was adjusted with 1 M acetic acid solution or 1 M ammonium hydroxide solution. All the coupons after deposition were blown dry with pure nitrogen at a pressure of 172 kPa. 2.4. Characterization Techniques 2.4.1. Ellipsometry. The film thickness of the deposited silane films was determined by a Rudolph Research 43603200E optical ellipsometer equipped with a mercury vapor ˚ ). The polished zinc substrate was cleaned lamp (l Å 5461 A and examined before a silane film was deposited. The measured values of the parameters D and C were used to calculate the refractive index of the substrate with a program written by McCrackin et al. (11). A typical calculated value of the complex refractive index of the polished zinc substrate is about 1.45(1 0 3.46i). The substrate was then cleaned again by the same method before it was dipped into the corresponding silane solution. The silane-coated substrate was reexamined by the ellipsometer after the deposition. The measured values of the parameters D and C, the complex refractive index of the corresponding substrate, and the refractive index of the corresponding silane film were used to calculate the thickness of the silane film with the same program. The values reported in this paper are the average values measured on three substrates. The refractive index
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FIG. 6. Effect of film aging time on the surface energy of g-UPS-coated zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solution, 8.0.
used for g-APS is 1.420 and the refractive index used for g-UPS is 1.386. Both values were obtained from the manufacturer’s Material Safety Data Sheets (MSDS).
2.4.2. Surface energy measurements. The surface energy of deposited silane films was determined by a VCA 2000 video contact angle system (Advanced Surface Tech-
FIG. 7. Effect of DI-water rinse on the film thickness of g-APS and g-UPS films deposited on zinc substrates; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solutions, 8.0.
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FIG. 8. FTIR spectra of pure g-APS and pure g-UPS.
nology, Inc.). This system is based on the sessile drop method. DI water and formamide were used as probing liquids. The averaged contact angle of 5 drops for each probing liquid was used as an input to the harmonic mean model to determine the total surface energy of the deposited silane film, as well as its dispersive and polar components (12). All the surface energy data reported in this paper were obtained under the conditions that the concentration of the applied silane solution was 1 vol% and the solution dipping time was 1 min. 2.4.3. Infrared spectroscopy (IR). The chemical structures of pure g-APS and g-UPS monomers were studied by Fourier transform infrared spectroscopy (FTIR). The FTIR spectra were obtained by using a BIO-RAD FTS-40 FTIR spectrometer. The spectra were taken at a resolution of 4 cm01 for 16 scans from 4000 to 450 cm01. The FTIR specimen were prepared by placing the pure g-APS or g-UPS monomer liquids directly on a potassium bromide crystal disk (PKBr 25.0 1 4, OPTOVAC, EM Industries, Inc.), respectively, followed by blow-drying with nitrogen.
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The chemical structures of the deposited silane films were studied by reflection absorption infrared spectroscopy (RAIR). The RAIR spectra were obtained by using the same spectrometer with a variable angle specular reflectance accessory (091-0461) at an incidence angle of 757. The RAIR spectra were taken at a resolution of 4 cm01 for 128 scans from 4000 to 450 cm01. All the RAIR spectra reported in this paper were obtained under the conditions that the concentration of the applied silane solution was 1 vol% and the solution dipping time was 1 min. 2.4.4. X-ray photoelectron spectroscopy (XPS). XPS analysis was carried out using a Perkin–Elmer 5300 ESCA system. The MgKa X ray source was operated at 300 W and 15 kV DC. Pass energies for survey and high resolution spectra were 44.75 and 17.90 eV, respectively. High resolution spectra were corrected correspondingly by assigning a value of 284.6 eV to the C1s peak of saturated hydrocarbons. Atomic concentrations were obtained from the high resolution spectra using sensitivity factors provided with the software. Analysis was done at two different take-off angles,
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TABLE 1 Frequencies and Tentative Band Assignments for Pure g-APS and g-UPS g-APS (cm01)
g-UPS (cm01)
3381 3343 3211 2974 2942
2976 2928 2887 2870
2888 2842
2738 1656 1603 1584 1559 1483 1458 1446 1388 1343
1443 1391 1297
1195 1168
1168 1104 1080 958
1084 956 815 794 779
792 777
Resolution intensity
Assignments
W S M S S S S S S W S S W S M M M M M M S S VS VS S S S S
nas (N–H) nas (N–H) ns (N–H) nas (CH3) nas (O–CH3) nas (CH2) ns (CH3) ns (CH2) ns (O–CH3) ns (C–H) ns (C|O) d (NH2) d (NH2) d (NH) & n (CN) d (CH2) d (CH2) das (CH3) d (CH3) t (CH2) v (CH2) r (O–CH3) r (O–C2H5) nas (Si–O–C) nas (Si–O–C) ns (Si–O–C2H5) ns (Si–O–CH3) ns (Si–O–C2H5) ns (Si–O–C2H5)
Note. Abbreviations: VS, very strong; S, strong; M, medium; W, weak; as, asymmetric; s, symmetric; n, stretching; d, bending; t, twisting; v, wagging; r: rocking.
157 and 757, in order to study the change in the structure of silane films as a function of depth. The take-off angle is the angle between the sample surface and the analyzer. 2.4.5. Atomic force microscopy (AFM). AFM images of polished pure zinc substrates with and without silane films were obtained on a Burleigh Instruments’ ARIS-3500 Personal AFM system. Samples were analyzed on a nanometer scale under ambient conditions. The AFM was operated in the contact mode. During imaging, the micro-machined silicon cantilever was in contact with the surface under a reference force of about 2.11 N. The topography of the surface is a measurement of the adjustments in the vertical direction necessary to keep the reference force constant. 3. RESULTS AND DISCUSSION
3.1. Thickness of the Silane Films Deposited on Zinc Substrates The effects of solution concentration and solution dipping time on the film thickness of g-APS and g-UPS deposited
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on pure zinc substrates are shown in Figs. 1 and 2, respectively. It is observed that the film thickness increases with the solution concentration, but almost remains constant with the solution dipping time. Similar results were also reported by Tutas et al. (13). They found that films deposited on glass from g-APS were the same thickness after a minute of contact as after an hour, while films deposited from 0.1– ˚ thick. These observa5% aqueous solutions were 50–200 A tions are in agreement with the chemistry of organofunctional trialkoxysilanes in aqueous solution. As indicated by Plueddemann, amino-organofunctional trialkoxysilanes at their normal pH in water hydrolyze and condense very rapidly to reach an equilibrium composition in a short time (14). Very dilute aqueous solutions contain monomeric silanetriol, but more concentrated solutions are believed to be an equilibrium mixture of low-molecular-weight (LMW) siloxanols with silanol groups stabilized by hydrogen bonding to amine groups. On the other hand, Plueddemann also indicated that the bonds between silanol groups and most metals were typically inorganic with a high degree of ionic character (14). It is known that ionic, inorganic chemistry is based more on equilibrium constants than on kinetics. Ionic, inorganic bonds are, therefore, formed and broken in reversible reactions determined by concentration and equilibrium constants. Thus, the results obtained in this work indicated that for the silanes studied, the film formation on zinc substrates involves an equilibrium of hydrolysis and condensation in the aqueous solution. Monomeric silanetriols or LMW siloxanols were first formed in the solution and then attached rapidly to the zinc substrates. 3.2. Surface Energy of the Silane Films Deposited on Zinc Substrates The surface energy of g-APS- and g-UPS-coated pure zinc substrates was studied as a function of the pH value of the applied silane solutions. The measurement was conducted after the film was aged for half an hour in ambient air. The results are shown in Figs. 3 and 4, respectively. It is observed that the total surface energy and its polar component of both silane-coated zinc substrates reach a maximum plateau between pH values of 6 and 9, while the dispersive component of the total surface energy of the coated substrates was almost independent of the pH value of the silane solutions. The relatively higher polar components between pH values of 6 and 9, as well as the total surface energy, may be attributed to the fact that the film formed in this range had a better molecular orientation. It has been reported that a silane molecule has better molecular orientation on the metal surface when the pH value of the applied solution is equal to or lower than the isoelectric point (IEP) of the metal oxide (15). Although pure zinc substrates were used in this study,
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FIG. 9. RAIR spectra of g-APS films deposited on zinc substrates at different pH value; solution concentration, 1 vol%; solution dipping time, 1 ˚. min; film thickness, Ç50 A
zinc oxide films on the surface of the substrates were observed in the XPS analysis. Angular dependence measurements of the L2,3M4,5M4,5 Auger transition peaks show the film thickness of ZnO to be about 1–1.5 nm. The IEP for zinc oxide is around 9.0. Therefore, the silane molecules deposited at a pH value below 9.0 might be oriented on the zinc substrates more straight-up with amino or ureido groups directed away from the silane–zinc interface. The decrease in the polar components at high pH value (above 9) was probably attributed to the up-and-down orientation of silanol groups in the film which led to the formation of relatively high molecular weight siloxanes or cyclic structures in the
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film. The decrease in the polar component at a low pH value (below 6) is probably due to the acetic acid left in the film. It has been argued by Plueddemann that when amino functional silanes are neutralized with carboxylic acids in aqueous solution and applied to mineral surfaces, a portion of the acid remains after drying as the acid salt or as the amide (16). Zinc dissolution might also be a factor in the decrease of the polar component at low pH value, since it has been shown that a stable zinc oxide film is only formed in the pH range of 6 to 12.5 (17). The deposition window in terms of pH value for obtaining well-oriented silane films on zinc substrates, therefore, should be between 6 and 9.
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FIG. 10. RAIR spectra of g-UPS films deposited on zinc substrates at different pH value; solution concentration, 1 vol%; solution dipping time, 1 ˚. min; film thickness, Ç50 A
The effect of film aging time on the surface energy of gAPS- and g-UPS-coated pure zinc substrates is shown in Figs. 5 and 6, respectively. It is observed that the polar component of the total surface energy of both silane-coated substrates decreased with the film aging time, while the dispersive component remained almost constant during the film aging period. As a result, the total surface energy of both silane-coated substrates shows the same trend as its polar component. The decrease rate, however, is quite different for g-APS and g-UPS. As indicated in Fig. 5, the total surface energy of g-APS-coated zinc substrate decreased rapidly during the first 24 h of aging in the air and had only
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about 59% of initial surface energy left after the first 24 h aging. The total surface energy of the g-APS-coated substrate then continued to decrease at a much slower rate and the surface energy retention was about 53% after 168 h aging in the air. In contrast, the total surface energy of g-UPScoated zinc substrate, as shown in Fig. 6, decreased very slowly with film aging time and the surface energy retention was about 93% after 168 h aging in the air. It should be noted that, right after the formation of the silane film, the drop of DI water completely wetted the coated zinc surface to form a 07 contact angle, which indicated that the total surface energy of the silane film studied
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TABLE 2 XPS Results for Polished Zinc Substrates with and without gUPS Films (solution concentration, 1 vol%; solution dipping time, 1 min) g-UPS coated zinc substrate (%)
Take-off angle
Element detected
Blank zinc substrate (%)
pH 4.3
pH 7.3
pH 10.3
157
C N O Si Zn N/Si C N O Si Zn N/Si
33.5 — 48.1 — 18.4 — 17.4 — 52.3 — 30.3 —
44.0 11.0 36.5 6.7 1.9 1.6 39.3 8.9 40.0 5.9 5.9 1.5
40.3 11.0 36.6 7.9 4.2 1.4 30.4 7.6 41.0 7.3 13.7 1.1
46.4 13.6 31.2 8.0 0.8 1.7 42.1 11.9 35.6 8.3 2.1 1.5
757
is above or at least equal to the surface tension of the DI water. Therefore, the surface tension of DI water was taken as the initial surface energy of the silane film without aging. The different decrease rate for g-APS and g-UPS suggests that the decrease of surface energy with film aging time is due not only to the adsorption of hydrocarbons from the air. It may also involve molecular rearrangement in the film and the condensation and dehydration reactions after the film has been formed. The different behavior, in terms of the surface energy with film aging time, between g-APS film and g-UPS film might be due to the different nature of the films. Ellipsometry was used to measure the film thickness of both silane before and after DI water rinsing. Two sets of samples were analyzed. One set was freshly deposited films, and another set was the deposited films aged for 2 days in the air. The results are shown in Fig. 7. For the freshly deposited films, the thickness retention of g-UPS film was about 8% after 30 s DI water rinsing, while the thickness retention of g-APS was about 42% after 30 s DI water rinsing. However, the sample aged for 2 days showed improved stability as compared with the freshly deposited sample. The thickness retention of the gUPS film increased to about 68% after 30 s DI water rinsing, while the thickness retention of the g-APS film increased to about 90% after 30 s DI water rinsing. These results indicate that the silane film was initially hydrogen bonded to the zinc oxide. During aging, the silane film became covalently bonded to the zinc oxide and crosslinking occurred due to the condensation of the silanol groups and dehydration, which increased the stability of the silane film. The better stability of the g-APS film over the g-UPS film is probably because the amine group in the g-APS is more easily coordinated to
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the silicon or metal substrates through hydrogen bonding, which will be discussed in the following section. 3.3. Chemical Structure of the Silane Films Deposited on Zinc Substrates The FTIR spectra of pure g-APS and g-UPS monomers are shown in Fig. 8. Tentative band assignments for these two compounds are listed in Table 1. The band assignments for g-APS have been discussed in the literature before (18). The band assignments for g-UPS, however, are new. The bands near 1559, 1603, 1656, and 3347 cm01 are assigned to the vibration modes of the ureido group, while the bands near 815, 956, 1084, 1195, 2842, and 2942 cm01 are assigned to the vibration modes of ester groups on silicon. The RAIR spectra of the g-APS film, deposited from the silane solution onto zinc substrates at different pH values, are compiled in Fig. 9. It can be noted that the g-APS film deposited at pH value of 4.5 had a completely different chemical structure from those deposited at pH of 6.0, 8.0, and 10.2. It shows relatively strong bands at 1588 and 927 cm01. The band at 1588 cm01 was assigned to the vibration mode of free NH2 groups, while the band at 927 cm01 was assigned to the vibration mode of silanol groups. These results suggested that g-APS forms monomeric silanetriols in its acid aqueous solution. As the pH value of the applied silane solution increased, the band at 927 cm01 disappeared and very strong bands appeared at 1117 and 1018 cm01, which were assigned to the vibration mode of siloxane groups. These results indicated that the silanol groups formed are less stable and condense to polysiloxanes as the pH value of the applied silane solution increased. As shown in Fig. 9, the higher the pH TABLE 3 XPS Results for Polished Pure Zinc Substrate with g-APS Films (solution concentration, 1 vol%; solution dipping time, 1 min) g-APS coated zinc substrate (%)
Take-off angle
Element detected
pH 4.5
pH 8.0
pH 10.3
pH 8.0, Aged for 46 h
157
C N O Si Zn N/Si C N O Si Zn N/Si
56.7 2.8 29.1 4.4 7.0 0.6 42.8 3.8 33.7 6.3 13.4 0.6
44.2 5.2 34.7 7.1 8.8 0.7 36.2 5.0 39.3 6.8 12.8 0.7
68.7 3.9 19.5 6.7 1.2 0.6 53.5 3.7 29.9 7.8 5.1 0.5
61.1 6.7 23.3 6.7 2.2 1.0 46.9 5.5 33.1 7.6 6.9 0.7
757
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FIG. 11. AFM three dimensional image of the polished pure zinc substrate.
value, the broader the bands at 1117 and 1018 cm01. The band at 1117 cm01 developed into two bands (1117 and 1044 cm01) as the pH value of the applied silane solution increased to 10.2, which indicated that a relatively long siloxane chain was formed (19). It was also noted that when the pH value of the applied silane solution was above 6, the NH2 vibration mode shifted from 1588 to 1561 cm01. This shifting was attributed by Boerio et al. to the formation of acceptor amine groups as a result of hydrogen bonding or coordination of the amine nitrogen to the silicon or metal substrates (20). The RAIR spectra of g-UPS films deposited from the silane solution onto zinc substrates at different pH values are compiled in Fig. 10. Unlike the results obtained for the g-APS films, the g-UPS films show similar spectra after deposition from the silane solution at pH values of 4.5, 8.1, and 10.0. The band at 1139 and 1059 cm01 was assigned to the vibration mode of siloxane groups, while the band at 913 cm01 was assigned to the vibration mode of silanol groups. These observations suggest that the g-UPS films were comprised of LMW siloxanol molecules and the chemical structures of the films were not strongly affected by the pH value of the applied solution. XPS was used to study the effect of pH value of both silane solutions on the molecular orientation of the silane film deposited on a pure zinc substrate. The results are compiled in Table 2 for g-UPS and Table 3 for g-APS. Zinc was detected in all circumstances which indicated that the ˚ , which is lower silane film thickness was not more than 50 A than the results obtained from ellipsometry measurement. However, the dissolution of zinc oxide film and its diffusion into the silane film, especially at low pH value, may give a rational interpretation to the discrepancy. It was observed that the carbon content around a pH value of 8.0 was lower than that obtained at a pH value of 4.5 and 10.3 for both g-APS- and g-UPS-coated zinc substrates. The oxygen content around a pH value of 8.0, however, was higher
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than that obtained at a pH value of 4.5 and 10.3 in both silane films. These data might explain the results obtained from surface energy measurement that the surface energy reaches a high plateau between the pH values 6 and 9. It is shown in Table 2, for g-UPS at pH value of 4.3, that the N/Si ratio decreased from 1.6 to 1.5 when the take-off angle changed from 157 to 757. At a pH value of 8.0, the N/Si ratio decreased from 1.4 to 1.1 when the take-off angle changed from 157 to 757. At a pH value of 10.3, the N/Si ratio decreased from 1.7 to 1.5 when the take-off angle changed from 157 to 757. The N/Si ratio was below the theoretical value of 2.0 in all circumstances, even though ammonium hydroxide was used to adjust the pH value of g-UPS solutions. Table 3 shows the results for g-APS. At pH 4.5 and 8.0, the N/Si ratio remained unchanged at 0.6 and 0.7, respectively, when the take-off angle changed from 157 to 757. At pH 10.3, the N/Si ratio decreased from 0.6 to 0.5 when the take-off angle changed from 157 to 757. Thus, it is impetuous, based on these results, to make a conclusion that the silane film had a better molecular orientation at certain pH value. XPS was also used to study the aging effect on the molecular orientation of the g-APS film deposited on a pure zinc substrate with the aim to explain the big drop observed in the surface energy after 24 h aging in the air. The g-APS film was applied on the pure zinc substrate from the 1 vol% g-APS solution at a pH value of 8.0. The film was then aged for almost 2 days before conducting the XPS analysis. It was found that after 2 days aging, the carbon and nitrogen contents in the film increased, while the contents of other elements, such as Si, O, and Zn, decreased. The increase in carbon content is from 44.2 to 61.1 at 157 take-off angle, and from 36.2 to 46.9 at 757 take-off angle. These observations might indicate that in addition to the hydrocarbon adsorption, molecular rearrangement also occurred during the aging process. This explains why the total surface energy of
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YUAN AND VAN OOIJ
FIG. 12. AFM three dimensional image of the polished pure zinc substrate coated (a) with g-APS and (b) with g-UPS; solution concentration, 1 vol%; solution dipping time, 1 min; pH value of the silane solutions, 8.0.
g-APS-coated zinc substrate decreased steeply during the first 24 h aging.
3.4. Surface Topography of the Silane Films Deposited on Zinc Substrates The AFM images of polished pure zinc substrates with and without silane film are shown in the Figs. 11 and 12, respectively. Figure 11 shows large gradual undulations across the surface for polished pure zinc substrate without silane coating. The surface roughness is about 39 nm, which is close to the scale of final polishing step of 0.05 mm. However, the surface roughness decreases to about 23.5 nm when the polished pure zinc substrate was coated with the silane films, as shown in Fig. 12. It is interesting to note the topographic difference between the g-APS and g-UPS films. As shown in Fig. 12a, a relatively smooth and uniform surface was observed for the g-APS film. Figure 12b, however, shows the small and sharp undulations across the surface for the polished zinc substrate with the g-UPS film.
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4. CONCLUSIONS
Organofunctional silanes (g-APS and g-UPS) have been deposited on pure zinc substrates from aqueous solutions. The silane film formation appears to involve a true equilibrium of hydrolysis and condensation in the aqueous solution. The silane film thickness obtained depends primarily on the solution concentration and is almost independent of the solution dipping time. The molecular orientations of the applied silane film are determined by the pH value of the applied silane solution and the isoelectric point of metal substrates. The deposition window in terms of pH value for zinc substrates is between 6 and 9. The total surface energy of silanecoated pure zinc substrate decreases with film aging time. The decrease rate, however, is determined by the nature of the silane film. The stability of both silane films improved after aging in air. AFM images show that the g-APS film deposited on the zinc substrate was smoother than the gUPS film. The performance results of selected silanes in painted galvanized steel systems will be reported later.
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ORGANOFUNCTIONAL SILANE FILMS ON Zn SUBSTRATES
ACKNOWLEDGMENTS We thank Rohit Poppat for conducting the XPS analysis and Burleigh Instruments, Inc. for conducting the AFM analysis included in this paper. The U.S. Environmental Protection Agency through its Office of Research and Development funded the research described here under assistance agreement CR822989 to the University of Cincinnati. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
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7. Van Ooij, W. J., and Sabata, A., J. Adhesion Sci. Technol. 5, 843 (1991). 8. Sabata, A., Van Ooij, W. J., and Koch, R. J., J. Adhesion Sci. Technol. 7, 1153 (1993). 9. Brockmann, W., and Bremont, M., Presented at EUR ADH 94 Adhesion, Mulhouse, France, Sept. 12–15, 1994; Societe Francaise du Vide, France, 574 (1994). 10. Bremont, M., Brockmann, W., Guimon, M. F., and Pfister-Guillouzo, G., J. Adhesion 41, 147 (1993). 11. McCrackin, F. L., and Colson, J., ‘‘Nati. Bur. Stds. Tech. Note 242.’’ U.S. Govt. Print. Off., Washington, DC, 1964. 12. Wu, S. J., Polym. Sci. 34, 19 (1971). 13. Tutas, D. J., Stromberg, R., and Passaglia, E., SPE Trans. 4, 256 (1964). 14. Plueddeman, E. P., ‘‘Silane Coupling Agents,’’ 2nd ed. Plenum Press, New York, 1991. 15. Dow Corning Form No. 23-012B-85, ‘‘A Guide to Dow Corning Silane Coupling Agents.’’ Dow Corning Corporation, Midland, MI, 1985. 16. Plueddemann, E. P., and Stark, G. L., Mod. Plast. 54, 76 (1977). 17. Roetheli, B., Cox, G., and Littreal, W., Metals Alloys 3, 73 (1963). 18. Chiang, C., Ishida, H., and Koenig, J. L., J. Colloid Interface Sci. 74, 396 (1980). 19. Socrates, G., ‘‘Infrared Characteristic Group Frequencies,’’ 2nd ed. Wiley, New York, 1994. 20. Boerio, F. J., Schoenlein, L. H., and Grievenkamp, J. E., Appl. Spectrosc. 22, 203 (1978).
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