Printing, pre-press, publishing and photography services – yesterday, today and tomorrow

Contents:

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

Aluminum pigments have a versatile use in printing inks. They can produce a variety of shades, including bright metallic colors.

Aluminum metal is one of the most widely used metallic elements. It is also one of the most abundant. The basic raw material is bauxite ore which is melted and refined, then remelted into products that range in applications from aerospace to home-use products.

Aluminum pigments are manufactured from the pure metal and possess many of the metal's characteristics. The typical purity of the aluminum used in manufacturing flakes ranges from 99.3% to 99.7% depending on the grade being made.

Upon milling, the pigment's most striking feature is the physical appearance of the particle itself. Aluminum pigments are actually aluminum flakes, rather than spheres or granules like most other pigments. This flake shape is responsible for many of the pigments' characteristics, including high reflectance to visible light, infrared and ultraviolet radiation.

If you were to look at aluminum pigments under a microscope, you would see them either as irregularly shaped particles or smooth, uniform particles, depending on the grade. They are all flake-like in geometry, which produces a variety of aesthetics.

When certain flakes are incorporated into a suitable vehicle system, they orient themselves in a multilayered position parallel to the substrate and in or near the surface of the film. Other flakes are distributed evenly throughout the entire film. The type of flake orientation depends on the milling lubricant used during the manufacturing process.

The Milling Process

Aluminum pigments are produced by the ball milling process (see Figure 1). This process carries out the particle size reduction in the presence of a suitable lubricant and an aliphatic hydrocarbon solvent. Three raw materials -- aluminum metal, mineral spirits and a suitable fatty acid, generally stearic or oleic -- are charged into the ball mill containing steel balls.

The mill is operated at a speed that permits the balls to cascade onto the aluminum metal, thus flattening it and eventually breaking it into tiny flakes. The lubricant prevents cold welding of the metal, and a sufficient amount of mineral spirits is charged to the formulation to maintain a wet slurry.

The length of time for ball mill operation depends on the grade being manufactured and the particle size distribution desired. Generally, however, a range from 5 to 40 hours is typical. When the grinding operation is completed, the slurry is passed over an appropriate mesh screen that separates out undesirable flakes.

That process, along with the length of milling, differentiates one grade from another. The oversized flakes are returned to the ball mill for further grinding, while the remaining aluminum flakes pass into a filter press. Excess solvent is removed in the process, which results in a filter cake having a volatile content of approximately 20%. Products designed for the ink industry are processed further by removing the mineral spirits solvent portion and replacing it with other types of solvents, or with polymer carriers.

The actual size of the flakes varies depending on the grade. The average diameter of commercially produced flake pigments ranges from 2 to 12 microns for fine grades, 13 to 32 microns for medium and over 60 microns for the coarsest grades. The particle size on all grades of aluminum pigments is stringently controlled. Variations in particle size or distribution can result in changes in color, hiding and sparkle.