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Contents:

1. The Milling Process
2. Leafing Pigments
3. Nonleafing Grades
4. Aesthetic Properties
5. Vehicle Selection
6. Uses
7. Metallics

Aesthetic Properties

The hiding (opacity) or tinctorial strength of aluminum pigments shares a direct relationship to particle size distribution. In general, grades with the greatest hiding and highest tinctorial strength have the smallest average particle sizes. The color characteristics can be described as whiter, brighter, or grayer and darker and are all related to particle size distribution.

The lightness or grayness is provided by the amount of light reflected from the surface of the flake. Generally, as the particle size distribution becomes finer, the color becomes darker. However, through particle size separation techniques, it has become possible to provide grades with a small average particle size, while maintaining a high degree of lightness. The characteristics of sparkle are based on a visual assessment, where the relation is dependent on particle size, shape and contour.

Flop, or metallic travel, are terminologies used interchangeably to describe the change in color of a metallic coating when viewed at different angles. It is a characteristic dependent on particle size, particle size distribution and the vehicle system in which the aluminum flake is incorporated.

Orientation of the flake in the coating film will determine the degree of polychromatic effect afforded. Proper orientation parallel to the film surface and substrate will provide a maximum face brightness and angle depth. The cause of these color differences is due to the amount of light reflected back to the eye.

When a metallic coating is viewed head on, the greatest amount of light will be reflected to the eye. As the eye moves to a more oblique angle, the amount of reflected light diminishes, thus a darker color is perceived. Generally, larger flakes produce a higher degree of metallic travel, simply because of the amount of light reflected.

Aluminum pigments by their very nature are rather malleable and somewhat fragile flakes. They are prone to deterioration if subjected to high-speed mixing, or if they undergo continuous or harsh circulation in a coating system. The recoin-mended dispersion technique involves adding sufficient vehicle or solvent to the aluminum paste and, with slow-speed mixing, developing a mixture that has a thick, creamy-like consistency.

Once the thick slurry is uniform, additional solvent and vehicle can be incorporated to complete the let-down. Severe dispersion techniques will destroy the integrity of the flake and result in poor aesthetics.