Pigments and pigment types
Aluminium pigments are platelet-shaped particles with a diameter ranging from 2-70 µm and a thickness of around µm/10. Typically, they are milled in a suspension of white spirit using a ball mill. Depending on the raw material (aluminium granulates) and the process used, either irregular Cornflakes or round Silverdollar particles are formed.
The metallic effect is caused by light reflected off the flat surfaces of the pigment particles. The various properties, such as brilliance, flop, color intensity, distinction of image, hiding power etc., depend on the particle size, particle-size distribution and particle morphology. In conventional systems, solvent-based pigment pastes are used. In the case of water-based systems, different processes either lead to pastes stabilized in water or solvents that can be mixed with water.
For powder coatings, aluminium powder is used that has received the appropriate post treatment to meet the desired end-use requirements.
Gold-bronze pigments consist of copper or a copper-zinc alloy (bronze) and, in contrast to their aluminium equivalents, are reduced to size in the dry state. The Zn/Cu ratio determines the color: copper (100% Cu), pale gold (approximately 10% Zn), rich-pale gold (approximately 20% Zn) or rich-gold (approximately 30% Zn). Further colors can be obtained by controlled oxidation. The intended end use determines the selection of particle size.
Vacuum Metallized Pigments (VMPs)
VMPs are produced in a different way to classic metallic pigments. Aluminium is deposited on a supporting substrate by a physical vapor deposition (PVD) process. The resulting film is then separated from the support and reduced in size. Due to the extremely smooth surface, it is possible to achieve a high-gloss effect similar to that of a film. The pigment may be used in both solvent- and water-based systems.
The milling agent used in the production of metallic pigments has a characteristic influence on their wetting properties.
Leafing pigments particles float to the top of the medium and create a high-gloss metallic surface. Aside from this “chrome effect”, which is used in both industrial and decorative coatings, the reflective properties of leafing aluminium pigments are employed in functional coatings (for example wall and roof coatings).
This family of pigments has the disadvantage of poor abrasion resistance.
The milling agent used in the production of metallic pigments has a characteristics influence on their wetting properties.
Non-leafing pigments are thoroughly wetted and therefore “sink” into the paint film. The result is a coating with good abrasion resistance which can be used in combination with transparent, colored pigments to create polychromatic effects.