Scaling the Long-Term Shear Stability of Aqueous Pigment Dispersions

Sep 1, 2009 - dispersions and provide marginal guidance when developing or testing large-scale paint applications. Scaling the stability against shear...
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MATERIALS AND INTERFACES Scaling the Long-Term Shear Stability of Aqueous Pigment Dispersions Evagelos K. Athanassiou,† Hanspeter Gradischnig,‡ Peter Siemsen,‡ and Wendelin J. Stark*,† †

Institute for Chemical and Bioengineering, ETH Zu¨rich, Zurich CH-8093, Switzerland

and ‡ BMW AG, Munich D-80788, Germany

The industrial application of paints today involves the handling of millions of tons of pigment dispersions. Shear and mechanical stress during pumping, storage, or processing significantly affect color and shelf life. Currently, mainly empirical test series and pilot-scale process models are used to assess the stability of such dispersions and provide marginal guidance when developing or testing large-scale paint applications. Scaling the stability against shear stress is particularly important for water-based metal pigment dispersions where damage to the pigment flakes causes their deterioration. The present work investigates the scaling of shear stress from the production line (>1000 tons/yr) to laboratory scale (100-g scale). We analytically show why the integrated shear stress on a specific dispersion can be deconvoluted into a series of individual shear stress contributions. These individual contributions can be accurately reproduced in the laboratory using a Couettetype shearing setup similar to a classical rheometer. As a representative example, a stable and a shear-sensitive paint from an automotive production line was sampled over 4 months of production. The evolving deterioration, color changes, and dispersion instability could be accurately reproduced using the scaling method outlined here. 1. Introduction Today, aqueous pigment dispersions are often used as coating systems. Metallic water-based paints have been attracting increased attention from the industry, because of the strict environmental regulations.1 In automobile manufacturing, they are commonly used in Europe and favored over other paint systems containing organic solvents or powder pigments. These highly favored aqueous pigment dispersions mainly consist of a metallic flake, often combined with other effect pigments and pigment materials, a film binder, solvent (in this case, mainly water), and other additives. Typical effect pigments are aluminum1-3 or mica flakes.4-7 They normally exhibit a very flat (up to 10 µm) and thin (up to 5-7 nm) mirror-like structure. The electrostatically assisted application of such a paint on a car body favors the parallel orientation of the flakes, whereas pneumatic application leads to a statistical orientation. Particle orientation significantly determines the special optical properties (such as flip-flop effect, brilliance, and color depth) that are highly favored by consumers. To control their interesting optical properties in a paint process, there is an intense interest in the stability of these aqueous metallic pigment dispersions. Aluminum or mica flakes must be surface-modified, to avoid reaction with water (which would result in hydrogen production3,6,8-11) and, finally, agglomeration of the flakes with a loss of optical color quality. Most modifications involve electrostatic stabilization of the surface through organic or inorganic coatings.3 Organic inhibiting agents range from low-molecular-weight substances, such as phenols, aromatic acids,12,13 and surfactants (such as alkyl phosphates14,15) to high-molecular-weight compounds, such as polyelectrolytes.16 It has been shown that even encapsulated aluminumpigmentscanbepreparedbyemulsionpolymerization.5,17 Inorganic coatings have attracted considerable attention as the * To whom correspondence should be addressed. Fax: +41 44 633 10 83. E-mail: [email protected].

thickness of the coating can be used to engineer the optical properties or the high reflectance of the basecoat.18 The most stable inorganic coatings are based on silica9 or titania.7,10 The thickness of a titania coating on mica flakes controls the amount of reflected light18 and results in stable mica flakes with adjustable color appearances. The shear stress stability during processing is of decisive importance, because even minor changes in the flake morphology or dispersion viscosity affect the desired orientation of the flakes and results in color changes2,19 on the varnished product. It has been shown that the applied mechanical stress during processing in the paint system of the automotive manufacturing plants is sufficiently high. The latter, in combination with the frequent flake collisions, highly affect the shear or mechanical stability of such water-based paints.9 As a result, the quality is limited and often entails time-consuming and costly rework or production stops. This creates an urgent demand for intelligent quality control and analysis methods that already indicate early in the development phase, which paints have acceptable shear stress stability. Until now, different stirrer or capillary methods have been applied to simulate the shear stress of the circulation pipelines.9 These methods do not consume much time or paint material; however, it has been shown that they are irreproducible and the conclusions do not necessarily reflect the situation in the automotive manufacturing plant.19 Another approach is based on the manufacture of a pilot circulation pipeline. Although it is a widely used method among the automotive industry, this method faces often scalability problems. In addition, one must consider that every circulation pipeline differs from each other, even in the same automotive company, and the transferability of conclusions is very limited. The present study follows an alternative approach. We show how the particle size distribution of aluminum or mica flakes and measurement of the color parameters are efficient methods to identify the mechanical and shear stress stability of the pigment dispersion in each stage of

10.1021/ie8017324 CCC: $40.75  2009 American Chemical Society Published on Web 09/01/2009

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the process. This method was validated using a known stable pigment dispersion (referenced as MWS-0701) and a shearsensitive pigment dispersion (referenced as MWU-0702) directly taken out of a running production (circulation) pipeline over a period of four months. In a separate step, a shear stress simulator was developed, to reproduce the shear stress of a production line in a laboratory-scale setup. Based on the fluid mechanics (laminar flow) during the transport process in the pipelines of the plant, the parameters for the simulator could be adjusted to reproduce production conditions in a laboratory, within a single week. The combination of fluid mechanic simulation and corresponding analytical methods offers a time- and costeffective alternative for the development of automotive metallic aqueous paints with high shear stress stability.

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Figure 1. Representative scheme of an automotive manufacturing plant. The paint is being continuously pumped by a supply tank; it is fed through the circulation pipelines in a closed loop to avoid sedimentation of the metallic flakes. Every element (pump, valve) of the plant paint system applies a certain shear stress on the metallic flakes.

2. Experimental Section Waterborne Basecoats. In this study, the stability of two different blue aqueous metallic pigment dispersions manufactured by BASF AG was analyzed. Both investigated paint materials were used in a regular production line of a BMW paint shop. One was highly stable (referenced as MWS-0701) and no color changes on the car bodies have been observed during application over several months; the second paint (referenced as MWU-0702) was less stable and showed considerable color drift during the production process. The present study now compares samples taken regularly from the running production line and simulated shear stress samples. Shear Stress Simulator. From the parameters of the circulation pipelines (tubing, pipeline diameter, 20 mm; length of the pipelines, 150 m; lacquer dynamic viscosity, 0.5 Pa s; total pressure drop, 5 bar) and by assuming that the aqueous dispersion is an incompressible fluid, the volumetric flow rate of the paint was calculated using the Hagen-Poiseuille equation. This resulted in a Reynolds number of Re ) 3.3, indicating that the lacquer flow was laminar in the pipeline. For the reproduction of the shear and mechanical stress during production, we therefore constructed a large cylindrical cuvette setup that consisted of two concentric cylinders (shown in Figure 4, presented later in this paper). To avoid any contamination, the cylinders were manufactured from a solid piece of stainless steel (Type 1.4404-AISI 316 L) and electrostatically polished to achieve a plain surface (Polishing: N4-N3). A sample of the paint (0.75 kg) was then placed between the cylinders and uniformly sheared with a velocity of 180 rpm for seven days. To ensure stable shearing conditions, the inner cylinder was rotated with a motor (Unitec Switzerland AG, 5AZ-90S-8, Code A 337 992) that was externally controlled by a computer. The jacket of the outer cylinder was additionally tempered with cooling water, keeping the temperature of the basecoat constant at 21.4 °C. During shearing, samples were taken after three and seven days and analyzed (see below). Waterborne Basecoat Characterization. The flake morphology of the basecoats was determined by scanning electron microscopy (SEM) (Model FEG 1530, Zeiss Gemini). The hydrodynamic particle size distribution of the flakes of the asprepared or the metallic waterborne basecoats from the production line of BMW AG or the shear stress simulator were measured using an X-ray disk centrifuge particle size analyzer (Brookhaven Instruments; measuring time ) 3 h, disk speed ) 600 rpm). Prior to analysis, the paint was diluted with 2-butoxyethanol (Acros Organics, 99%) to prepare a dispersion (solid content ) 22 wt %). 2-Butoxyethanol was chosen as a solvent because it is one of the main organic components of

Figure 2. Depending on the shear and the mechanical stability of the paint, (a) the metallic flakes can remain stable and exhibit the same optical properties, or (b) their protective coatings can be destroyed, resulting in an agglomeration of the flakes and a diminishing of their optical properties.

the basecoat and exhibits a similar dielectric constant as the fresh water-based paint. For the measurement of the color parameters of the paint, one kilogram of the paint was sprayed on a metallic sheet. The first layers were applied electrostatically (ESTA Eco Bell; cone diameter, 0.9 mm; M6 × 0.75 ASV VA6; Glockenteller D55 GR 0.4 VS-D38 Ecobell-A) and the final layers were applied pneumatically (cone AV-4915-FF-1.4 055; air flap C NO 797; devilbiss type AV 651 FF), similar to that in the BMW AG production line. The color parameters were measured by a multiangle spectrometer (X-RITE MA 68II; type MA 68.B). 3. Results and Discussion Original equipment manufacturers (OEMs) in the automotive industry today apply aqueous, metallic pigment paints on car body parts using a paint feed system with a central mixing room and circulation pipelines (see Figure 1). The paint is continuously pumped from the feed tank through the circulation pipelines (see Figure 1) in a closed loop to ensure continuous mixing and homogeneity and to avoid sedimentation of the pigment and metal flakes. Although the flow through the main part of the pipelines is laminar, the valves, pumps, and bifurcations can exert a considerable shear stress on the pigment material. Taking into consideration the frequent collisions between the flakes, the total cumulative stress increases with circulation time. Aluminum or mica flakes (see Figure 2a) are the most widely used metallic pigment materials. Although there are many studies on the encapsulation or inhibition of flakes, their shear stability is often insufficient. Therefore, paint producers extensively use many stabilizing additives in the paint formulations. Figure 2 illustrates the importance of the shear and mechanical stability of a typical car paint formulation using

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two examples. Mica flakes from a “stable” (MWS-0701) and a “sensitive” aqueous paint (MWU-0702) were isolated and analyzed by scanning electron microscopy (SEM). The stable formulation (MWS-0701) contained very thin flakes (d ) 5 nm) with a high surface (flake width ) 7 µm), even after prolonged shearing. In contrast, Figure 2b shows that the applied stress protocol had resulted in the total destruction and delamination of flakes of a sensitive formulation (MWU-0702). The thin protective coating of the latter flakes was partially removed and the different silicate layers on the mica core became exposed to the paint medium. Such damaged flakes exhibit a different surface charge than original flakes and have a tendency to agglomerate (clustering and formation of larger sediment). Arbitrary and inhomogeneous deposition or misorientation of flakes on a car body cause an undesired change in color, a decrease in reflected light, and a diminished flip-flop effect (angle-dependent reflection and sparkling). Agglomeration of flakes can also change the viscosity or pH of a formulation. However, a study by Bosch et al.19 showed that a change in paint viscosity alone did not affect the color stability of an aqueous metallic paint. Therefore, color changes are directly linked to the deformation or agglomeration of pigment and flakes. A reliable quality control system and analytical methods are needed to evaluate early changes in morphology or the hydrodynamic diameter of the flakes. This would enable to recognize unstable formulations at an early stage and significantly reduce downtime in production. Within this study, we identified the hydrodynamic particle diameter of flakes as an indicator to evaluate the shear and mechanical stress stability of waterborne basecoats. To validate this concept for shear stress and color stability, two water-based paint dispersions with different stability (MWS0701 and MWU-0702) were analyzed from a major production plant of BMW AG in Munich. The pigment dispersions were in the circulation pipeline for a total of four months and sprayed daily on the produced cars. Samples of paint were collected over that period and analyzed regarding to particle size distribution and color parameters. Figure 3a showed that the stable MWS-0701 basecoat exhibited a relative narrow particle size distribution. The mean hydrodynamic particle diameter was ∼1.6 µm and did not increase during processing for four months. It could be concluded that MWS-0701 exhibited a high shear stress stability as the particle size distribution of the flakes did not grow. In contrast, the unstable formulation MWU-0702 displayed a wider particle size distribution with a considerable tailing (see Figure 3b). Over the first three months, the tailing increased up to 15 µm, and in the final month, the entire distribution became broader. The latter confirmed that MWU0702 was considerably more unstable than MWS-0701. The flow through the pipelines and the other items (valves, pump) of the plant (see Figure 1) resulted in agglomeration of the flakes (see Figures 2 and 3b) and destabilization of the paint. The destabilization was also reflected in the color parameters of the basecoats (see Table 1). Parameter D is a factor defined by BMW AG and is being derived from the standard CIELAB System (L*a*b system), according to DIN 6174. The term D25 indicates that the color parameters were measured under radiation of an incident light at an angle of 25°. The broadening of the particle size distribution of MWU-0702 resulted in an absolute difference of ∆D25 ) 1.3 (see Table 1) over four months. This absolute change was large and resulted in a clear color change. The same observations were conducted in the plant during the application of the paints (see Table 1). The ∆D25 of the stable formulation MWS-0701 was much lower and further

Figure 3. X-ray particle size measurements of (a) MWS-0701 and (b) MWU-0702. The measurements show that MWS-0701 exhibited a high shear stress stability. In contrast, MWU-0702 exhibited a wide particle size distribution with a large tailing, indicating its poor shear stress stability. Table 1. Color Parameter Measurements of the Water-Based Metallic Pigment Dispersions from the BMW Manufacturing Plant and the Shear Stress Simulator ∆D25 sample

BMW line (exposure ) 4 months)a

stress simulator (exposure ) 1 week)b

MWS-0701 MWU-0702

0.6 1.3

0.4 1.1

a Absolute difference of the color parameter D25 between the as-prepared sample and the samples taken from the BMW plant in Munich after 4 months. b Absolute difference of the color parameter D25 between the as-prepared sample and samples taken from shear stress simulator after 7 days.

corroborated its improved shear stress stability. From that observation, it could be concluded that a change in the particle morphology or aggregation of the metallic flakes could result in color drifts and destabilization of the aqueous paint. Furthermore, it was shown that X-ray disk centrifugation is an adequate method for particle size analysis. Although it is less established than classical light scattering,20 or turbidimetry,21 the centrifugation method has the advantage of being more robust, because it is not dependent on the shape of the particles. X-ray disk centrifugation works at high concentration (typically 0.2-1.5 wt %) and yields a hydrodynamic particle size distribution in the micrometer-to-nanometer range.22,23 Theoretical Considerations. By assuming that the water-based metallic pigment dispersion is an incompressible fluid, the volumetric flow rate (Q ) 2.6 × 10-5 m3/s) in the pipeline system of the manufacturing plant was calculated by the Hagen-Poiseuille equation. This results in a Reynolds number of Re ) 3.3, which indicates that the aqueous paint dispersions had a laminar flow through the circulation pipelines in the plant. Every element of

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Table 2. Collision Opportunity of the Different Elements of the BMW Manufacturing Plant element

collision opportunity, Gta

pipelineb piston pumpc valvec

40000 2000 1600

a The factor corresponds to one run through the BMW pipeline system. b The mean shear rate of a pipe with a laminar flow can be calculated by G′ ) 8Q/3πR3, where Q is the volumetric flow rate and R is the radius of the pipe.25 c Mean shear rate as given by the manufacturer.

the plant (Figure 1) applied a certain shear stress on the metallic flakes. This stress accumulated depending on the total residence time of the basecoat in the circulation pipeline system. As every applied shear stress on the flake results in a certain collision rate with neighbor flakes, the Smoluchowski equation can be used to approximate the orthokinetic flocculation of the flakes in every element (pipes, pump, valve) of the plant: -

16 dN ) RN2Ga3 dt 3

(1)

where N is the number of particles, a the particle radius, G the shear rate, and R the collision efficiency. Despite the nonspherical shape of the flakes, eq 1 is adequate for semiquantitative description of the flocculation kinetics. As the volume fraction of the flakes, φ remains constant during flocculation, eq 1 can be written in a pseudo-first-order24 form: -

GN dN ) 4Rφ dt π

( )

(2)

This expression can be integrated to give Gt N ) exp -4Rφ N0 π

[

( )]

(3)

where N0 is the initial concentration and N is the concentration remaining at time t. From eq 3, it can be concluded that the actual parameters that vary within every element of the pipeline system and influence the collision and the aggregation process is the dimensionless number Gt, which is also known as the collision opportunity.25 By comparing the collision opportunity numbers of every element of the manufacturing plant (Table 2), it can be concluded that the pipelines are mainly responsible for the possible destabilization of a paint. Although the mean shear rate in the pipeline (G′ ) 8Q/3πR3 ) 22.2 s-1) is low, compared to a piston pump (G′ ≈ 500 s-1) or a valve (G′ ≈ 100 s-1), the decisive longer residence time of the flakes of the aqueous dispersion in the pipeline induce a much higher collision rate than in the other parts. With the continuous circulation of the paint in the pipeline system, this effect is being accumulated, inducing aggregation of the flakes. The corresponding Peclet number of Pe ) 7.9 × 106 (the critical Peclet number is given as Pecritical ) 160) verifies that the destabilization of the paints is being controlled by a shear-induced aggregation.26 The latter means that the total stress applied on the metallic flakes could be described as i

Σstress )

j

∑ ∑f

i,j(a, b, c)

0

Figure 4. Schematic of the shear stress simulator. Its parameters (distance gap, d; angle velocity, ω; and applied temperature, T) allowed to experimentally simulate, on a laboratory scale, the shear stress of the BMW manufacturing plant.

(4)

0

whereas i denotes the process stage, j is the elements (valves, pumps, pipeline) of the circulation pipeline, and a, b, and c all the parameters of the elements such as diameter, length, and

angle of a pipeline that influence the velocity of the laminar flow of the basecoat. To simulate the shear stress of the entire manufacturing process, the complex problem needed to be deconvoluted and the parameters (a, b, and c) had to be transformed into controllable parameters for flow and shear stress (see Figure 4) of a simulator. The shear stress simulator consisted of two concentric cylinders, where the inner one was rotating. The paints were uniformly sheared between the two cylinders. The rotation velocity of the inner cylinder and the gap between the two cylinders (see Figure 4, Ra/Ri ) 1.05) followed all requirements for a Couette flow (ISO Norm: Ra/Ri ) 1.20). In addition, the jacket of the outer cylinder was tempered to keep the temperature of the pigment dispersion constant. Thus, the total shear stress (τ) on the metallic flake of every process stage i and every process element j could be reproduced as i

Σstress )

j

∑ ∑f 0

i,j(τ, ∆t, T))

0

∫

t

0,i

f(d, ω, t, T) dt

(5)

whereas d is the distance gap, ω the rotation velocity, t the time of the stress application, and T the temperature of the basecoat. Because the collision opportunity number indicated that the pipeline contributes most to paint destabilization, the rotation speed of the simulator has been adjusted to induce the same total collision opportunity number as in the BMW manufacturing plant over a period of 4 months. Therefore, the angular velocity (ω ) 18.8 s-1) of the inner cylinder has been chosen accordingly, so that the flow of the aqueous metal dispersion could be described by a uniform Couette flow. The concentric cylinder geometry with a rotating inner cylinder has been studied extensively, both theoretically and experimentally. In such a system, there exists a critical angular velocity, above which a new secondary axisymmetric flow in the form of regularly spaced horizontal toroidal vortices occurs.27 This effect can be described by the dimensionless Ta number:26 Ta )

4Ra2(Ra - Ri)4 ω ν R 2 - R2 a

i

2

()

(6)

where ν is the kinematic viscosity of the paint. Our flow conditions exhibited a Ta value of 1270, which is smaller than the critical Ta value (Tac ) 1708). The mean shear rate for the

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suggested here has the advantage that it does not exhibit scalability problems. The parameters of the shear stress simulator (d, ω, T) can be further adjusted to describe the shear stress of different manufacturing plant paint systems. The present system could evaluate the aqueous pigment dispersion stability in a laboratory scale after seven days. It reproduced stress and flow conditions of several months in the plant. From that, it could be concluded that the shear stress simulator, in combination with particle size distribution and color parameters measurements, could be a valuable tool for determining the shear stress stability of water-based metallic pigment paints. 4. Conclusions

Figure 5. X-ray particle size measurements of samples derived from the shear stress simulator resulted in the same observations as Figure 3, indicating that the shear stress simulator could successfully reproduce the shear stress of the BMW manufacturing plant.

laminar flow (Res ) 130) in the gap between the cylinders of the simulator was calculated as Gs ) 2ωRaRi/(Ra2 - Ri2) ) 372 s-1.28 The operation of the simulator for seven days induced a collision opportunity number of Gst ) 2.2 × 108. Samples of MWS-0701 after three and seven days showed that the particle size distribution slightly broadened, when compared to a fresh (untreated) batch of paint (see Figure 5a). Nevertheless, it was similar to the samples taken from the plant, indicating that the pigment dispersion remained stable. In contrast, MWU-0702 destabilized after three days (see Figure 5b). Its particle size distribution was very broad and exhibited a long tailing (up to 15 µm). The applied stress on the metallic pigments resulted in the broadening particle size distribution and in a change of the color parameters. The stable formulation MWS-0701 exhibited a value of ∆D25 ) 0.4 (see Table 1), confirming its high shear and mechanical stress stability. The broadening of the particle size distribution of unstable MWU0702 resulted in an absolute difference of ∆D25 ) 1.1 (see Table 1), indicating that the basecoat exhibited a critical stability. From this and numerous similar experiments on other formulations, it could be concluded that the shear stress simulator successfully reflected the paint flow in the automotive circulation pipeline system. By comparing the collision opportunity number of the simulator (Gst ) 2.2 × 108) with the mean share rate of the pipeline system (G′ ) 22.2 s-1), we can calculate a conditioning time of t′ ) 117 days. This means that the simulator flow conditions of one week correspond to ∼4 months of circulation of the paint in the BMW manufacturing plant. This implies that the flow conditions were similar in both systems and allow a successful evaluation of paint stability. The latter proved that the assumptions made here for transforming the manufacturing plant parameters to a simulator were correct. The solution

This study showed that color instability of aqueous metallic pigment dispersions could be related to agglomeration and delamination of metallic pigments. The agglomeration resulted in color drifts and in diminishing of the flip-flop and other optical effects. X-ray disk centrifuge appeared as an adequate method for the determination of the particle size distribution of the metallic flakes. The combination of color measurements and size distributions allow a straightforward stability control of water-based paints. An analysis along flow conditions, collision numbers, and residence time gave access to the design of a shear stress simulator that successfully reproduced the flow conditions, shear and mechanical stress in a real scale manufacturing plant. The shear stress simulator reproduced the color changes and particle growth of a four-month manufacturing plant run on a laboratory scale within a single week. The latter showed the potential application of the aforementioned system in controlling and developing aqueous pigment dispersions with a required high shear stress and color stability. Literature Cited (1) Niemann, J. Waterborne coatings for the automotive industry. Prog. Org. Coat. 1992, 21, 189. (2) Karbasi, A.; Moradian, S.; Tahmassebi, N.; Ghodsi, P. Achievement of optimal aluminum flake orientation by the use of special cubic experimental design. Prog. Org. Coat. 2006, 57, 175. (3) Karlsson, P.; Palmqvist, A. E. C.; Holmberg, K. Surface modification for aluminium pigment inhibition. AdV. Colloid Interface Sci. 2006, 128, 121. (4) Cavalcante, P. M. T.; Dondi, M.; Guarini, G.; Barros, F. M.; da Luz, A. B. Ceramic application of mica titania pearlescent pigments. Dyes Pigments 2007, 74, 1. (5) Ghannam, L.; Garay, H.; Shanahan, M. E. R.; Francois, J.; Billon, L. A new pigment type: Colored diblock copolymer-mica composites. Chem. Mater. 2005, 17, 3837. (6) Kalenda, P.; Kalendova, A.; Stengl, V.; Antos, P.; Subrt, J.; Kvaca, Z.; Bakardjieva, S. Properties of surface-treated mica in anticorrosive coatings. Prog. Org. Coat. 2004, 49, 137. (7) Tan, J. R.; Fu, X. S.; Hou, W. X.; Chen, X. Z.; Wang, L. The preparation and characteristics of a multi-cover-layer type, blue mica titania, pearlescent pigment. Dyes Pigments 2003, 56, 93. (8) Creutz, S.; Jerome, R.; Kaptijn, G. M. P.; van der Werf, A. W.; Akkerman, J. M. Design of polymeric dispersants for waterborne coatings. J. Coat. Technol. 1998, 70, 41. (9) Kiehl, A.; Greiwe, K. Encapsulated aluminium pigments. Prog. Org. Coat. 1999, 37, 179. (10) Ren, M.; Yin, H. B.; Wang, A. L.; Jiang, T. S.; Wada, Y. J. Mica coated by direct deposition of rutile TiO2 nanoparticles and the optical properties. Mater. Chem. Phys. 2007, 103, 230. (11) Stengl, V.; Subrt, J.; Bakardjieva, S.; Kalendova, A.; Kalenda, P. The preparation and characteristics of pigments based on mica coated with metal oxides. Dyes Pigments 2003, 58, 239. (12) Kummert, R.; Stumm, W. The Surface Complexation of OrganicAcids on Hydrous Gamma-Al2O3. J. Colloid Interface Sci. 1980, 75, 373. (13) Bommarito, G. M.; Pocius, A. V. An electrochemical study of the changes in the passivation of an aluminum alloy surface induced by the presence of a self-assembled monolayer. Thin Solid Films 1998, 329, 481.

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ReceiVed for reView November 13, 2008 ReVised manuscript receiVed June 8, 2009 Accepted August 17, 2009 IE8017324