Application of metallic pigments and pearlesic pigments in powder coatings

Abstract:
This paper briefly introduces the types, characteristics and some manufacturers' products of metal powder pigments and pearlescent pigments, discusses the application of metal powder pigments in powder coatings, the preparation methods and influence factors of metal flash powders, and puts forward the selection and matching of raw materials, substrate production control, coating points and related use recommendations.
1, the forewordWith the improvement of people's aesthetic requirements, the pursuit of high aesthetic paint has become a trend. Metallic or pearly powder coatings provide a luxurious decorative effect ranging from crystal-like metallic gloss to silk satin pearls, and thus widely used in the spraying of furniture accessories, sports equipment, and

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This decorative powder is different from pure epoxy, pure polyester, acrylic and other functional powder coatings, although the substrate can be given a certain degree of protection, but coating film weather resistance, corrosion resistance and heat resistance is not as special functional powder.
2, metallic powder pigments and pearlesic pigments have a variety ofMetallic powder pigments and pearlescent pigments used in powder coatings are usually scale-like particles, and the particle size D50 of commercially available products is 3 to 75 m, including aluminum powder (commonly known as silver powder), copper powder (commonly known as gold powder), zinc powder, nickel powder, stainless steel powder, and inorganic mica powder (commonly known as pearl powder) coated with oxides.
powder coatings, aluminum powder is used in the largest amount, followed by copper and gold powder and pearl powder, zinc powder, stainless steel powder is often used in anti-corrosion coatings. These pigments also play different roles in powder coatings due to their different properties (see Table 1).
3, application of metal powder pigments and pearlesic pigments
3.1 aluminum pigments aluminum pigments are the most common industrial applications of a flaky effect pigment, divided into floating and non-floating two categories of non-floating aluminum Powder accounts for the vast majority of the market share of more than 80%;
non-floating aluminum powder includes large particle size with crystal gloss varieties and finer particle size silk satin gloss varieties because the surface through the organic color film has good pigment wetting, even distribution in the powder coating.
most of the floating lead powder arranged on the coating surface to present a bright and even chrome-plated coating appearance, excellent cover, aluminum metal powder in the powder coating or coating surface presents the following characteristics.
3.1.1 Masking and shielding
Aluminum powder is scaly, the diameter/thickness ratio of the substing is about (40:1) to (100:1) particles are formed in multiple layers of parallel arrangement in the coating film Continuous multiple layers of aluminum powder cover the substrate, reflecting on the incoming light, and the pores between the layers of aluminum powder interlese each other, cutting off the pores of the coating film and the access to external moisture, making it difficult for water vapor to enter the coating film to reach the substrate (Figure 1).
3.1.2 Optical Features
Aluminum powder is made of high-gloss, light-colored, high-purity aluminum, after a specific process to smooth and flat aluminum particle surface pair wavelength of 280 to 400nm of visible light, infrared light, ultraviolet light reflectivity of 60% to 90%, a silvery white light appearance.
3.1.3 "Coloring with Angle" effect
Aluminum powder has a metallic luster and parallel arrangement on the surface of the coating in the coating system with poor transparency or cover, the gloss and color depth of aluminum particles with the change of incoming angle and observation angle, often referred to as "two-color effect" or "interference effect", of course, the good directional parallel arrangement of aluminum powder is the premise of the optical effect.
3.1.4 Floating
Scale-shaped aluminum powder surface in the production process with advanced fatty acid coating treatment, aluminum powder surface fatty acids have two adsorbent outer fatty acids of the oil-dependent base and inner fatty acids The pro-oil-based connection makes the carboxyl outwards, so the fatty acid envelope in aluminum powder after the oil-absorbing powder molten flow usually aluminum particles in the "Benaud vortex" under the action of the film surface migration, so that aluminum powder floating on the coating surface (Figure 2).
When the aluminum powder surface does not have fatty acids (or other organic matter) coating film, aluminum powder floating force is small difficult to migrate to the coating surface to affect the floating capacity of aluminum powder factors also include fatty acid species, aluminum powder scale diameter/thickness ratio, aluminum powder processing process and temperature aluminum powder particle size.
non-floating aluminum powder surface treatment process is to choose unsaturated fatty acid coating or change the abrasive temperature of aluminum powder, so that the overall proportion of aluminum particles and cladding film increased. It shows the dredging characteristics.
3.2 Copper and gold powderOrdinary copper and gold powder is a floating flaky pigment, by a certain proportion of copper, zinc and aluminum alloys by smelting, ball grinding grading to make the required products, copper content will vary from copper to gold pink phase.
such as green gold, red gold, bronze gold and other products copper and gold powder particle size is thicker, the stronger the metal sense, poor cover. In the paint formula, the more dosed, on the contrary, the smaller the particle size, the weaker the metal sense, the softer the color, the stronger the covering force.
powder coating formula designers can choose different particle sizes and colors of copper and gold powder varieties according to customer requirements.
Copper gold powder particle surface can be coated with a layer of organic material to reduce the density of copper gold powder, and increase its surface strain, improve temperature resistance, different types of copper and gold powder in different climates and chemical environments may change color, so that the coating film gloss is reduced.
3.3 Pearl pigments Pearl pigment has pearl luster, is in the mica substrate table bread covered with titanium dioxide and other materials. The coating pigment produces multiple reflections and transmissions of light to give it a pearly effect.
different metal oxide materials, the interference light effect is also different, the presentation of pearly color changes greatly or even produces a "color transfer effect."
At present, the color of pearlescent pigments is divided into silver and white, magic color and coloring categories, pearlescent pigments through the surface chemical correction and coating modified, adapted to all natural and synthetic resin systems, but also suitable for thermosystic and thermoplastic powder coatings.
4, the preparation method of the effect powder coating Effect pigments give powder coatings unique and dazzling visual effects for a variety of resin systems. There are three main methods for the preparation of effect powder coatings.
4.1 Melt extrusion method Conventional powder coating production methods are to squeeze all raw materials in the formula into a crushed sieve, and most flaky effect pigments are often destroyed during processing due to the high shear force of the screw.
structure of the effect pigment will be severely damaged during the high-speed crushing process, resulting in poor pigment orientation and reduced metal or pearlescent effect of the powder coating. At present, only metal hammer powder can be achieved by melting and extruding processing methods.
Aika has developed a line of dust-free metallic powder pigments that can be extruded (internally) directly. It is very convenient to prepare powder coatings with molten extrusion process, while the commonly used postmix process and Bonding process require additional processing steps.
In addition, the molten extruded metal powder coating in the coating and over-spraying powder recovery process there is no metal pigment separation phenomenon, the deficiency of this pigment is a large amount of use, high cost, can not get a complete cover and delicate effect of the coating film. Powdersafe has a particle size range of D5015 to 60 m.
4.2 Dry Mix The effect pigment in the "dry" state will be added to the powder coating substrate that has been finely ground, the commonly used aftermix equipment for "V" shape tank or conical tank is the disadvantage of mixing effect and the density and performance of the base powder, particle size and particle shape, in the coating and over-spraying powder recovery prone to metal powder separation from the bottom powder phenomenon, poor stability.
4.3 State Law That is, flaky metal pigments or pearlescing pigments at a certain temperature to bond to the surface of the powder particles, Bonding machine under the protection of inert gas N2 so that the base powder and effect pigment fluid suspension, stirring between the paddle and material particles and material particles between the different moving speed to produce frictional heat, so that the material temperature rises, the bottom powder softens and the pigment sticks to it.
Bonding metal powder has effect pigments, less dosage, not easy to occur effect pigment and substrate separation, recycling is convenient, but Bonding bonding process on the softening point of the base powder, Bonding temperature, Bonding time control is very strict, Bonding effect and equipment design and process operation is also closely related.
5, process control points
5.1 effect pigment and color pigment selection when preparing the effect powder coating, the cover force of the residual film depends on the metal powder pigment, rather than the color pigment, should choose poor coloring force or transparent pigment and metal pigment preparation, do not use or less cover strong pigment.
should select the effect pigment suitable for outdoor powder and indoor powder as needed, and some suppliers and products of the effect pigment should be found in Table 2.
5.2 resin selection Effect pigments are mostly treated with epi bread film (e.g. fatty acid metals and non-metallic oxides, etc.), and the orientation of effect pigments has a great relationship with the reaction active monosome structure of resin system.
At present, powder coatings with epoxy, polyaeth mixed outdoor pure polyester metal powder has gradually been developed and applied, the choice of polyester is the key, different manufacturers of polyester with the same proportion of formula, coating appearance of the metal sense and performance is different.
powder coating resins are polymer polymers, resin glassing problem Tg, molecular weight and molecular weight distribution and other indicators have a great impact on resin performance, without affecting the fluidity of the melting level of the coating, the slightly larger molecular weight resin is more conducive to the direction of metal pigments.
5.3 Pigments and Additives are selected Powder coating fillers are mainly nihil sulfate, calcium carbonate, silicon powder, kaolin, talcum powder, mica powder, etc., particle size 2 to 10 m, the key to control the particle size distribution when using fillers, large particles (D50 ≥10 m) on the film extension of the negative impact is greater than small particles.
with the improvement of coating technology research and raw material application technology, nanofillers and surface bread film fillers gradually introduced powder coatings.
preparation effect powder coating when the application of additives is also very important, such as the manufacture of shiny film, leveling agent dosage can be slightly more (regulating system flow), control system adhesion time of 2.5 to 6min, to ensure that flaky pigment adjusted to the most effective flash angle.
, directional arrangers, hydrophobic gas phase silicon dioxide, etc. are helpful to the realization of special optical effects. In order to increase the scratch resistance and anti-fouling resistance of the coating surface, the corresponding wax powder can also be used to achieve the purpose through internal agents or mixing.
control of the 5.4 substrate particle size Whether it is "dry mixing" or "bonding" (Bonding process), the charge of the substrate particles, the oscic gravity, the average particle size, the particle shape and the ultra-fine powder (D10≤10 m) content have an effect on the larger process.
metal powder pigment use control will bring a series of coating construction problems, such as poor powder, easy to produce powder, blocking the gun head and so on. The relationship between powder particle size and charge is shown in Figure 3, the average particle size of metal powder coating is 40 to 50 m, and the paint is most charged when the ultra-fine powder content is less than 12%.
5.5 effect powder coating construction Powder coatings are generally sprayed with corona guns or friction guns. Friction gun spraying is not suitable for metal powder coating, it is recommended to use corona spray gun spray effect powder coating.
spray gun construction coating system must have a good ground, and set a lower static voltage and powder output, to prevent spray fire, long-term spraying should pay attention to clean the gun head and electrode needle.
in order to prevent the difference in coating effect between batches of products, it is recommended that the proportion of recycled powder mixed with new powder is controlled to 1:4, as far as possible to maintain the stability of spray conditions, static voltage is not greater than 80kv.
in order to ensure spray stability and cost needs, pigment particle size should be controlled at D50≤15 m of aluminum powder content ≤1.5%, particle size D50≥15 m content of less than 4%.
6, Conclusions Although the effect pigment gives the powder coating rich appearance effect, but the use effect is greatly influenced by the coating system, the application technology of the effect pigment in the powder coating is far less mature than the application in the liquid pigment.
With the continuous improvement of powder coating technology, the key technologies applied by effect pigments in powder coatings will be solved one by one, and the widespread use of effect pigments will greatly improve the added value of powder coatings and coating appearance, giving coating products a stronger competitiveness.
source: Suzhou Yanlian Powder Coatings Co., Ltd