Aluminum flake is an important component of metallic pigments. In paint compositions, aluminum flake pigments offer a high degree of brightness and specular reflection. Its physical properties depend on the particle sizes. The particle sizes are determined by ultrasonic treatment or grinding.
A small mean particle size is desirable for aluminum flake pigment. If the particles are large, they may protrude from the paint film or have an irregular orientation. They also can cause seeding on the paint film surface.
An inventor has studied the relationship between the mean particle size and the brightness of paint films containing aluminum flake pigments. The brightest paint films are obtained when the mean particle size of aluminum flake pigment is within a certain range.
For this study, the inventors used a variety of preparations that had a mean particle size of less than 0.5 mm. Each preparation had a mean aspect ratio of at least 2.7. These values are based on the major axes D observed from the section of the specimen.
To measure the brightness of paint films containing aluminum flake, a variety of different angles were used. From a specular angle, the L value was measured at a 15 deg. angle. This is an indicator of oxidation.
In addition, the brightness of the paint films was evaluated visually. Brightness increased with increasing the mean particle size. Orientation of the aluminum flake pigments was influenced by formulation and application conditions.
The number of aluminum flake particles with major axes D of less than 10 mm was set to at least 50. As a result, the mean particle sizes of the aluminum preparations were in the same range as those in Example 1.
In addition, the slurry from the grinding step was passed through a vibrating screen of 350 mesh. The slurry was then separated into a solid-liquid through a pan filter.