Recent popularity of thin aluminum flakes for chrome replacement has stimulated development activities to push the limits of color and performance of these pigments in waterborne systems. This paper will explore multiple options for surface treatments of these aluminum flakes and the effect these treatments have on final color, sparkle, opacity, gassing, dispersion, shear, and chemical resistance in both interior and exterior coatings.

Visual assessment and instrumental analysis of coatings containing these pigments will be discussed. Also addressed will be the options available for pigment carrier and delivery form, such as paste, powder and solvent-free granules, designed to perform in the most demanding low VOC coating systems. Proper dispersion and formulating practices will also be explored.

Standard aluminum pigments are made from atomized aluminum powder wet ball milled using the Hall process. Vacuum metallized pigments (VMP) are produced using a physical vapor deposition process that deposits metal on a polymer substrate that is subsequently stripped off to capture, concentrate, and then size the individual aluminum particles. Figures 1 & 2 detail the production processes used to produce standard and vacuum metallized aluminum pigments.

FIGURE 1 | Manufacturing process for standard aluminum pigments.

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FIGURE 2 | Manufacturing process for vacuum metallized aluminum pigments.

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There are four types of aluminum flake morphologies that can exhibit certain benefits and deficiencies when trying to achieve a near chrome effect in a coating. Figure 3 is a snapshot of the four types or aluminum pigments that are well established in the coatings market:

FIGURE 3 | SEM photos of various aluminum pigment morphologies.

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For the purposes of this study, we have selected two morphologies/types of aluminum pigments that are well suited to deliver near chrome effects when properly formulated into a waterborne coating: platinum dollar / thin milled aluminum flake and VMP. Platinum dollar flakes bridge the gap between a traditional silver dollar aluminum flake and a VMP; they are much thinner than a standard aluminum flake but thicker than a standard VMP. While ball-milled flakes have average thickness ranges from 100-300 nanometers (nm), VMP are much thinner with an average range of thickness from 100–500 angstroms (Å). The extremely low thickness of these pigment types helps the flakes to orient themselves parallel to the coated substrate in the applied paint film and allows the flakes to pack and overlap at the film surface, resulting in a smooth, structureless, near chrome finish.

The next essential part of our study is to ensure that only passivated or inhibited aluminum pigments are selected for use in a waterborne binder system. Untreated aluminum flakes can react violently with water generating hydrogen, especially under the alkaline pH levels of a waterborne system. Evolution of hydrogen gas can cause bubbling and foaming in the binder system, and in a sealed container, can cause a dangerous, pressurized, and possibly explosive situation. Figure 4 shows the reaction of aluminum metal with water to create hydrogen gas.

FIGURE 4 | Reaction of aluminum metal with water.

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There are two predominant ways to passivate or inhibit aluminum or vacuum metallized flakes for use in a waterborne system. The first method is by additive stabilization, in which a passivating agent – typically an organic or inorganic phosphate – is adsorbed to the flake surface, slowing the reaction of the aluminum and water. The second method is to encapsulate the aluminum flakes in a layer of silicon dioxide using the sol-gel process. This produces a uniform, thin layer of SiO2 around the flake surface that inhibits the water from reacting with the aluminum. Both methods of post-treatment can stabilize aluminum pigments in a waterborne system and allow for proper in-can stability and safe storage of the finished paint system. Figure 5 shows a graphical representation of the two stabilization techniques.

FIGURE 5 | Stabilization techniques for aluminum pigments in a waterborne medium.

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Despite the added stability that these passivation and inhibition techniques provide, it is always recommended that hydrogen gas evolution is checked in the target waterborne paint system using accelerated storage testing at elevated temperature. A typical test for flake stability is to disperse a known amount of the stabilized aluminum flakes, usually 5-10% of total paint formula by weight, into 300 mL of waterborne binder. The paint system is then adjusted to proper pH range — typically 8-8.5 —  with an amine solution. The finished paint system is then transferred to a glass vessel with a two-chamber top and placed in a heated water bath at temperatures of 40-50 °C. The lower chamber contains water that is displaced to the upper chamber as the insoluble hydrogen gas is generated over a testing period of 7-10 days. After the defined test period is complete, the volume of the water displaced into the upper chamber is weighed; the weight of water displaced is equivalent to the volume of hydrogen gas generated in milliliters (mL). A typical passing criterion for this test is less than 10 mL of hydrogen gas generated at 40 °C over a period of seven days. Figure 6 illustrates the typical equipment and process used to test the gassing stability of aluminum pigments in waterborne systems.

FIGURE 6 | Standard gassing test for stabilized aluminum pigments in a waterborne medium.

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Once all the necessary safety and formulation requirements were completed to ensure proper pigment choice, we selected the types of pigments for our study, and the physical properties of these pigments were compared. All pigments selected for this study are treated for use in waterborne mediums. Table 1 below shows all relevant data for the selected pigments.

TABLE 1 | Pigment Types and Physical Properties

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All or some of the selected pigments in Table 1 were formulated into both low- and medium-solids waterborne paint systems at an equal solids level. The low solids formulation was targeted for higher pigment loading and less resin solids, like some automotive interior and wheel coating applications. Both one-component and two-component formulations were explored. These paints were applied over black ABS plastic substrate and evaluated for mirror finish both qualitatively and quantitatively, as well as visual assessment with and without a clearcoat layer. Chemical resistance testing was also completed using acidic and caustic spot testing. The higher solids formulation was targeted to mimic an automotive exterior original equipment manufacturer (OEM) finish, which due to application type and solids/VOC limitations can never truly achieve a chrome-like finish – instead, they deliver an exceptionally fine and bright appearance, with extremely low sparkle that gives a structureless metallic appearance. These paints were applied directly to metal substrates by electrostatic rotary bell application and evaluated for color, appearance, flop, and sparkle. Gassing and blender resistance at normal and elevated encapsulation thickness layers were also explored.

Pigments A and D were formulated into the low-solids one-component and two-component waterborne paint systems and applied via air application to ABS molded speed car shapes and flat, textured panels, and evaluated visually for distinctness of image (DOI) and chrome-like finish. Various pigment-to-binder (P:B) ratios were explored to determine the influence of multiple thin coats to film build, as well as the effect of a system with and without a hardener. Finally, the influence of a solvent-based clear layer was explored —  including the best solvent choice for the clear coat diluents to avoid basecoat “strike-in” and maintain the best visual chrome appearance in a two-coat system.

Figure 7 shows the influence of P:B ratio on DOI and mirror-like finish. Pigment D was used for this test, and the results clearly show that increased P:B ratio yields better DOI and chrome finish.

FIGURE 7 | Pigment D formulated at increasing P:B ratios, from left to right (1:5, 1:1, 2:1).

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Similar formulating variations were employed using the platinum dollar flake Pigment A and applied onto ABS speed shapes. In this instance, we also observed improvement with increasing P:B ratio, but the effect was not as pronounced with the VMP Pigment D. Figure 8 details this visual comparison.

FIGURE 8 | Pigment A formulated at decreasing P:B ratios, from left to right (1:1, 1:2, 1:4).

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Pigments A and D were compared to each other at the same P:B ratio and applied over ABS speed shapes. Visual assessment of chrome appearance shows that pigment D, the VMP type pigment, has increased DOI and chrome appearance – this is being driven by the low thickness values for VMP type flakes. Figure 9 shows the comparison of Pigment A to Pigment D in the same formulation and application.

FIGURE 9 | Pigment A & D formulated at same pigment to binder ratio (1:1).

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Using Pigment D, a comparison of visual chrome appearance was made with the same ABS speed shapes – at similar film build – with one trial at 2 coats to film build target and the other at 3 coats to film build target. The results of this test indicate that achieving target film build with multiple thinner coats gives a better chrome-like appearance with improved DOI. Figure 10 shows the results of this test.

FIGURE 10 | Pigment D formulated at same pigment to binder ratio (1:1), 2 coats vs 3 coats to target film build.

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Also investigated was the difference in chrome-like appearance in a one-component versus two-component waterborne binder. Pigment D was used for this comparison testing and was applied in a 1K and 2K system at similar film builds and equal P:B ratio to ABS speed shapes using standard air spray. The results indicate that the presence of a hardener decreases the DOI and chrome-like appearance of the final paint film. Figure 11 shows a visual representation of these results.

FIGURE 11 | Pigment D: one-component vs two-component waterborne binder system.

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The next step in our investigation was to examine the influence of a topcoat or protective clear coat layer over the pigmented base layer. With the high pigment loading needed to achieve a good chrome-like appearance, it can be difficult to maintain this appearance due to “strike in” of the clear coat into the base coat layer. As the topcoat dries, it can disrupt the orientation of the light, thin platinum dollar or VMP flakes, and decrease DOI and mirror finish. To avoid this phenomenon, it is recommended to use very “weak” solvents in the topcoat diluent, such as A100 or DIBK. These solvents have less of a tendency to resolubilize the basecoat layer.

Strong solvents like MEK should be avoided. For this test, Pigment D was chosen, and a one-component waterborne basecoat binder was used. The base layer was allowed to sufficiently cure, and a topcoat layer was applied in multiple thin coats. We used both low-OH and medium-OH polyol clear in combination with strong and weak solvents for the topcoat. As seen in Figure 12, the use of a medium -OH polyol with medium to weak solvent diluents gave the best final appearance and DOI, maintaining the chrome effect without flake disruption.

FIGURE 12 | Pigment D: Influence of topcoat and diluent chemistry on final appearance.

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In certain applications, especially monocoat automotive interior coatings, resistance of the basecoat to attack from chemicals like acids, bases, sunscreen, and cleaning solvents is a requirement. To meet the minimum color change requirements of these applications, an encapsulation layer on the aluminum flake is essential. Untreated aluminum flakes and even additive-stabilized flakes leave the flake surface vulnerable to chemical attack – especially the small amount of iron that is present even in the highest purity aluminum alloy. A typical test for this application is to apply the pigmented basecoat over a black ABS substrate and do acid and caustic spot testing at multiple time intervals. For this test, Pigments C & D along with the base untreated VMP flake were tested. The results of this testing show improved chemical resistance when the flake surface is modified with additive stabilization or encapsulation as compared to the untreated VMP flake. Figure 13 shows the results of this testing.

FIGURE 13 | Pigment C & D: Influence of surface treatment on chemical resistance in a monocoat system.

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Visual assessment of all our results in a low-solids waterborne paint system indicated that Pigment D, the VMP-type pigment, is the best overall choice for chrome replacement coatings. Optimization of the paint formulation is essential, and we recommend a high P:B ratio, in a one-component system, and applied in multiple thin coats to target film build in a monocoat system to achieve the best results. These tips can deliver a better mirror-like finish that can be useful for chrome replacement efforts. However, care should be taken at these lower solids and higher pigment levels to ensure that other physical properties of the paint film such as adhesion, humidity resistance, scratch and mar resistance are not negatively affected. If a topcoat is necessary for the specific application, then we recommend the use of a medium -OH polyol clear with medium to weak diluent solvents to avoid basecoat “strike in” and maintain the desired effect.

The next part of the study focused on the performance of the platinum dollar and VMP type pigments in a high solids, automotive waterborne OEM type basecoat. In these types of waterborne systems, specifically with the electrostatic rotary bell application, it is extremely difficult to achieve a near chrome appearance from either a platinum dollar or even the thinnest VMP flake. Due to these formulation and application difficulties, most of the development has been geared towards achieving the finest, brightest, and most structureless appearance possible. For the first part of the study in this OEM waterborne medium, we investigated the gassing performance of Pigments A, C, and D. The gassing protocol used was 10 days at 40 °C using a gassing rig set up as discussed and shown previously in Figure 5. The P:B ratio used for these studies was kept constant for all pigments at 9%. The first gassing test included pigments A, C, & D to compare an encapsulated platinum dollar, and encapsulated VMP and an additive-stabilized VMP pigment. Figure 14 shows the results of this gassing test. It is clear from the results that encapsulated pigments have better gassing stability than additive-stabilized flakes.

FIGURE 14 | Pigments A, C, and D: gassing performance in a medium solids, OEM-type waterborne system.

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The next gassing study was done with Pigment B, a 20-micron encapsulated platinum dollar pigment. The thickness of the silicon dioxide layer was varied to show the effect of encapsulation layer thickness on the gassing stability in a medium solids OEM type, waterborne binder system. The same gassing equipment was used as above, however the time duration of the test at 40°C was increased to 38 days to evaluate long-term gassing stability. The results of this testing show that increasing the thickness of the encapsulation layer results in better gassing stability to a point of near diminishing return. Color, opacity, and sparkle are generally negatively affected as the thickness of the encapsulation layer is increased, so it is necessary to perform ladder studies during product development to create an encapsulation layer that is thick enough to provide sufficient gassing resistance, but not too thick to change the appearance away from that of the base aluminum flake. Figure 15 details the results of this testing.

FIGURE 15 | Pigment B: gassing performance at various encapsulation thickness.

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Using the sample variations of Pigment B, another study was performed to evaluate the shear stability of an encapsulated platinum dollar flake, with varying encapsulation thickness layers, using a standard automotive OEM “blender” test. In this type of Waring blender test, a pigmented paint is sheared in a water-cooled blender and compared to the control paint to observe the degree of L* change at low viewing angles. In doing this test, we are trying to mimic the shear that an aluminum flake may be subjected to in the recirculation systems of automotive OEM assembly and paint plants. Sufficient blender resistance is a crucial key performance indicator (KPI) for pigments selected for exterior OEM car body finishes to ensure good color harmony between the first and the last cars painted on an automotive OEM paint line. Three variations of Pigment B were used for this study: the base platinum dollar feed flake with no encapsulation, another sample with standard encapsulation thickness for gassing resistance, and a third variation with increased encapsulation layer thickness to add additional shear stability along with better gassing stability. Each variation of Pigment B was loaded into the paint system at equal solids level, and were applied to metal panels (basecoat + clearcoat), using rotary bell application. The results of this testing show that increased encapsulation thickness results in better shear stability with platinum dollar aluminum flakes. Without an encapsulation layer, as in some in situ passivated OEM paint systems, platinum dollar flakes could not be used because the amount of color and lightness change after shear would lead to loss of color harmony on an automotive OEM paint line. In this case, the encapsulation layer offers not only gassing resistance but also offers further functional advantages that allow brighter, finer, and thinner aluminum flakes to be an essential part of an automotive OEM pigment toolbox. Figures 16, 17, and 18 show the results of this study.

FIGURE 16 | Pigment B: shear resistance with no encapsulation

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FIGURE 17 | Pigment B: shear resistance with standard encapsulation.

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FIGURE 18 | Pigment B: shear resistance with increased encapsulation layer thickness.

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The results of our studies in medium solids OEM waterborne binder systems indicate that while it is difficult to achieve a true mirror finish using platinum dollar or VMP type aluminum pigments, it is possible to push the limits of paint formulations (lower the total solids content) and to modify the applications parameters of the rotary bell applicator to realize a fine, bright, near structureless metallic finish that can decrease the color and appearance differences when you compare air spray versus rotary bell spray applications. For our final study, Pigment D – an encapsulated VMP type pigment – was applied by both bell and air spray application in the same waterborne OEM binder system, however with a lower total solids content (increased butyl glycol solvent and water as further diluent) and decreased resin solids. Figure 19 details the specifics of the paint formulation.

FIGURE 19 | Modified paint formulation using Pigment D.

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The paints were then sprayed by both air and bell application. The bell application parameters are defined in Figure 20.

FIGURE 20 | Electrostatic bell parameters for application of Pigment D.

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The resulting spray panels from bell, robot air, and hand air spray containing Pigment D were analyzed with a goniospectrophotomer also capable of measuring sparkle. Table 2 contains L* data at five viewing angles using D65/10 illumination. The sparkle area at two angles was also compared. The results indicate that at decreased resin solids and low viscosity, it is possible to get quantitative measurements in bell application that closely mimic those from a more traditional air spray application.

TABLE 2 | Comparison of Application Methods in Low Solids Waterborne Binder System with Pigment D.

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Future development work for the pigment types used in this study are focused on reducing the carbon footprint of the products and the processes used to apply them in automotive and industrial waterborne coatings. All four pigments evaluated in this study are supplied as a paste or a dispersion, and contain varying amounts of VOC carriers (isopropanol or glycol ether) that can hinder the formulation latitude when faced with ever increasing VOC and sustainability targets. R&D efforts have been targeted on delivering at least some of these pigment types in a low or zero VOC carrier that will enhance the sustainability of the product delivered.

One such technology is to employ a low VOC carrier package that can be added to the paste product (namely Product A & B) and then extruded into pellet form. The pellets are then dried to remove the residual VOC solvent, producing a super low, easy to handle, platinum dollar encapsulated pigment that can be added to waterborne formulations in the same way as conventional pastes, but without the worry of bringing additional, unwanted VOCs to the paint formulation. Research work continues to this day to find a carrier package that performs equally well across the various waterborne paint systems in the automotive and industrial coatings markets. Figure 21 shows Pigment A platinum dollar type encapsulated pigment in a new low VOC granule delivery form. Figure 22 shows the gassing performance comparison of the new granule type carrier and the standard paste form of Pigment A. Results are encouraging and future work will continue to improve the performance and sustainability of these pigment types to impart greater formulation latitude to meet the increasing demands of a chrome-like metallic finish in low VOC waterborne paint systems.

FIGURE 21 | Pigment A in a low VOC, easy-to-handle delivery form.

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FIGURE 22 | Pigment A gassing test – 10 days at 40 °C – paste versus granule delivery form.

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In summary, to achieve a near chrome metallic finish in today’s waterborne paint systems is not an easy task. But with the combination of the correct products, paint formulations, and application parameters, it is possible to get remarkably close to the target of chrome replacement in a waterborne coating system. Future developments will continue in the areas of delivery form and sustainability to allow formulators to meet their challenging targets without the additional worry of VOC constraints and carbon footprint.