Metal powder pigments and pearlescent pigments

With the progress of people's aesthetic requirements, it has become a trend to seek a high aesthetic feeling of paint. The powder coating with metallic or pearlescent effect can show a luxurious decoration effect from crystal metallic luster to satin pearl, so it is widely used for painting furniture, accessories, sports equipment and cars.
This decorative powder is different from pure epoxy, pure polyester, acrylic and other functional powder coatings. Although it can give a certain degree of protection to the substrate, the weatherability, corrosion resistance and heat resistance of the coating film are not as good as the special functional powder.
Metallic powder pigments and pearlescent pigments used for powder coatings are generally scaly particles, and the particle size D50 of commercially available products is 3~75 μ m. It includes aluminum powder (commonly known as silver powder), copper powder (commonly known as gold powder), zinc powder, nickel powder, stainless steel powder, and inorganic mica flake powder (commonly known as pearlescent powder) coated with oxide.
The amount of aluminum powder in powder coatings is Z, followed by copper and gold powder, pearlescent powder, zinc powder and stainless steel powder, which are often used in anti-corrosion coatings.
（1） Aluminium pigment
Aluminum powder pigment is a kind of flaky effect pigment widely used in industry at present. It is divided into floating and non-floating aluminum powder, which accounts for most of the market share of more than 80%;
Non-floating aluminum powder includes those with larger particle size and crystal luster, and those with smaller particle size and satin luster. Due to its excellent pigment wetness through organic color film, it is evenly distributed in the powder coating.
Most of the floating lead powder is placed on the surface of the coating film, showing a bright and even chromium-plated film appearance, with excellent hiding power.
(1) Concealing and shielding effect
The aluminum powder is scaly, and the diameter/thickness ratio of the substrate is about (40:1)~(100:1). The particles are placed in parallel in the coating film to form a continuous multi-channel aluminum powder layer, which hides the substrate and reflects the incident light. The pores between the aluminum powder layers are interwoven, cutting off the pores of the coating film and the path of external water, making it difficult for water vapor to enter the coating film and reach the substrate.
(2) Optical characteristics
Aluminum powder is made of metallic aluminum with high gloss, light color and high purity. After special processing, the surface of aluminum particles is smooth and smooth. The reflectivity of visible light, infrared light and ultraviolet light with a wavelength of 280~400 nm reaches 60%~90%, showing a snow-white and bright appearance.
(3) Angular heterochromatic effect
The aluminum powder has metallic luster and is placed parallel to the surface of the coating. In the coating system with poor visibility or concealment, the glossiness and color depth of aluminum particles change with the change of the incident angle and the observation angle, which is generally called the two-color effect or interference effect. When the aluminum powder is placed in excellent parallel orientation, it is the condition for this optical effect to occur.
During the production process, the flaky aluminum powder is coated with advanced fatty acid. The fatty acid on the surface of the aluminum powder has two adsorption layers. The lipophilic group of the outer fatty acid is connected with the lipophilic group of the inner fatty acid to make the carboxyl group outward. Therefore, the fatty acid coating in the aluminum powder is an oil-repellent powder melting fluid. The aluminum particles move to the surface of the coating under the action of Bernard vortex, so that the aluminum powder floats on the surface of the coating.
When there is no fatty acid (or other organic matter) coating on the surface of aluminum powder, the floating force of aluminum powder is small, and it is difficult to move to the surface of the coating. The factors that affect the floating ability of aluminum powder include the type of fatty acid, the diameter/thickness ratio of aluminum powder scale, aluminum powder processing technology and temperature, particle size of aluminum powder, etc.
The surface treatment process of non-floating aluminum powder is to select unsaturated fatty acid coating or change the grinding temperature of aluminum powder to increase the overall specific gravity of aluminum particles and coating film, showing oil-repellent characteristics.
（2） Copper-gold powder
The general copper and gold powder is a kind of floating flake pigment, which is made of a certain proportion of copper, zinc and aluminum alloy by melting, ball milling and grading. The copper and gold powder produced by different copper content will have different color.
For example, the coarser the particle size of copper and gold powder of series products such as blue gold, red gold and ancient copper and gold, the stronger the metal feeling and the poorer the concealment power. The amount used in the coating formula is more. On the contrary, the smaller the particle size is, the weaker the metal feeling is, the softer the color is, and the stronger the concealing power is.
The powder coating formula designer can select copper and gold powder with different particle size and color according to customer requirements.
The surface of copper-gold powder particles can be coated with a layer of organic materials to reduce the density of copper-gold powder, increase its surface tension, and improve its temperature resistance. Different types of copper-gold powder may change color in different climate and chemical environment, so as to reduce the gloss of the coating film.
（3） Pearlescent pigment
Pearlescent pigment has pearl luster, and is coated with titanium dioxide and other materials on the surface of mica substrate. The coating pigment has multiple reflections and transmission of light to make it pearlescent.
Different coated metal oxide materials have different interference light effects, and the pearlescent color changes greatly and even has color transfer effect.
At present, the color phases of pearlescent pigments are divided into snow-white, mirror-colored and pigmented. Pearlescent pigments are chemically modified and coated to adapt to all natural and synthetic resin systems, as well as thermosetting and thermoplastic powder coatings.