Metal effect pigments, formerly referred to as bronzes, are pigments used to create an optical metallic effect, commonly also described as a metallic appearance. These pigments typically consist of platelet-shaped particles with particle diameters generally in the two- to three-digit micrometer range, although tetrahedral particle geometries have also been documented, which help prevent inhomogeneities in the metallic effect (flow lines). The optical effect of these effect pigments is based on the alignment of the planar particle surfaces parallel to the surface of the surrounding medium, where directional reflection occurs. Metal effect pigments are listed in the Color Index under C.I. Pigment Metal. Metal effect pigments are used to achieve gloss effects in printing inks, plastics, cosmetics, and paints (see metallic paint). The most economically important materials are aluminum (C.I. Pigment Metal 1, silver bronze, see alumina effect pigment) and brass (C.I. Pigment Metal 2, gold bronze). According to estimates, the global market for metal effect pigments amounts to 25,000 tons annually.
History
Metal effect pigments, alongside pearl luster pigments, are the oldest subgroup of effect pigments. The underlying concept emerged between the fourth and third centuries BC from the gold beater trade in Egypt. Waste generated during the hammering of gold into extremely thin foils was mixed into precursors of modern lacquer systems, imparting a golden appearance. Since the 18th century AD, brass has been used as a substitute for expensive gold. Later, fine aluminum flakes were developed to produce a silver-colored effect. The first process for the large-scale production of aluminum pigments was developed in 1910 in the United States of America (stamping process). As this involved dry grinding, the risk of explosion was very high; consequently, the process was soon replaced by a wet-grinding method known as the "Hall process," named after its inventor and patent holder. Dry grinding is still commonly used for brass pigments.
Classification
Leafing and non-leafing types
A fundamental distinction is made between leafing and non-leafing pigments. In leafing pigments, the platelets orient themselves at the interface after application within the coating film. This produces a strong metallic gloss effect but results in poor smudge and scratch resistance. Typical applications include printing inks, reflective coatings, and aerosol coatings, where optical performance is the primary consideration. Brass pigments are generally leafing types.
Non-leafing pigments are evenly distributed throughout the film matrix after application. This provides protection against abrasion and chemical attack, although the resulting appearance is grayer and less intensely metallic. They can also be combined with colored pigments to produce colored metallic effects. This is only possible with leafing pigments at low concentrations, as their accumulation at the paint surface otherwise obscures the uniformly distributed colored pigments. The main applications of non-leafing pigments include automotive coatings, coatings for cell phones and electrical components, and industrial coatings in general.
Surface properties Standard grades are also referred to as cornflakes due to their shape. These flakes are highly irregular and possess uneven surfaces. Cornflakes represent the original form of metal effect pigments. Since the early 1990s, rounded flakes with smooth surfaces have also been available. Owing to their shape, these are known as "silver dollars" and exhibit greater brightness and brilliance than cornflake types, making them widely used in modern coatings. Silver dollar types are produced from specially prepared semolina with a spherical surface. During grinding, these particles are deformed rather than crushed.
High-purity grades In the past, aluminum grades with very high aluminum content were frequently used for premium coatings, such as automotive finishes. These grades provide significantly improved corrosion resistance compared with standard grades. However, their importance has declined due to the widespread adoption of two-layer coating systems (overcoating with clear lacquer).
Manufacture
The metal to be processed (aluminum or brass) is first melted and, upon reaching the melting temperature, atomized from the melt. This produces irregularly shaped, spherical particles (semolina). The resulting metal powders are then transformed into flakes and comminuted in ball mills. For aluminum pigments, this process takes place in white spirit together with a lubricant (oleic acid for non-leafing pigments and stearic acid for leafing pigments), as otherwise there is a risk of dust explosion. In contrast, dry grinding can be carried out safely for brass pigments, since dust explosions do not occur due to their significantly higher density. After grinding, the white spirit is removed from the resulting mixture (slurry) using a filter press. In the final step, the filter cake is adjusted to the desired commercial form.
Properties
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