With the push for environmental protection, firms must develop eco-friendly coatings. But, eco-friendly coatings aren’t limited to water-based ones. If enterprises rush to develop water-based coatings, product homogeneity will inevitably occur. Besides water-based coatings, high-solid, solvent-free, and powder coatings are also eco-friendly. They should be the key development focus of the coatings industry. A report said the Asia-Pacific region will have the highest growth in powder coatings from 2017 to 2022. This applies to both value and volume. Powder coating technology is developing rapidly to adapt to the demand-driven market. The market drives the need to study different powders in coatings.
Powder Coating Overview
Introduction to Powder Coatings
Powder coating originated in the 1950s. It is an eco-friendly coating. It has 100% solid content and no VOCs. It saves energy and reduces pollution. It has a simple process, is easy to automate, and has excellent coating performance.
Powder coating is a powder coating composed of polymers, pigments, fillers and additives. Powder coating has no volatilization of its liquid medium. So, it has good ecological and environmental protection. It can form a thicker coating in one application, with very high efficiency. It also has great mechanical properties and chemical resistance. Its performance is excellent, too. Using powder coating saves energy and resources. Its utilization rate can reach 99%. It is safe and very cost-effective. It is a solvent-free coating. It meets the “four E” principles: economy, environmental protection, efficiency, and performance.
Powder Coatings Market Overview
The rise in demand for appliances and light vehicles has boosted the need for powder coatings. A rise in demand from industries in developed and emerging countries has boosted the powder coatings market. A Markets and Markets report says the global powder coatings market will hit US$134.9 billion by 2022. It will grow at 6.75% annually from 2017 to 2022.
China’s powder coatings market has the world’s fastest-growing demand. This is due to many factors. They include rapid urbanization and the growth of housing, construction, and cars. In 2016, China’s powder coatings industry output reached 2.07 million tons.
Powder coatings are 11% of my country’s total coating output. The 13th Five-Year Plan says, “by 2020, the coating industry should produce about 22 million tons.” Of this, 57% should be cost-effective, eco-friendly coatings. In 2020, powder coatings will rise to 18%, with an output of 4 million tons. The rise of powder coatings will boost demand for powder fillers.
Analysis of the application of different powder materials in powder coatings
Fillers in coatings can cut costs. They also improve coating performance.
For example, they can:
- Improve the coating’s wear and scratch resistance.
- Reduce sagging during melt leveling.
- Improve its corrosion and moisture resistance.
When choosing fillers for powder coatings, consider density, dispersion, particle size, and purity. In general, the higher the density, the lower the powder coating’s coverage. The dispersion of large particles is better than that of small ones. The filler is chemically inert. It must not react with some powder formula components, like pigments. The filler should be as white as possible. Powder coatings mainly use these materials: calcium carbonate, barium sulfate, talc, mica, kaolin, silica, and wollastonite.
Application of calcium carbonate in powder coatings
Calcium carbonate has two types: light and heavy. Light is precipitated calcium carbonate. All calcium carbonate types and production methods affect the coating’s gloss. Different particle sizes have a strong effect on it. Calcium carbonate is generally not recommended for outdoor use.
Heavy calcium carbonate is mainly used to replace some other materials. It partially replaces titanium dioxide, color pigments, light calcium, and precipitated barium sulfate. It prevents corrosion and partially replaces anti-rust pigments. It is also used as an increaser.
When using heavy calcium carbonate in indoor paints, it can be used alone or with talcum powder. Calcium carbonate, compared to talcum powder, is better. It can reduce pulverization, improve color retention in light paints, and boost mildew resistance. However, its poor acid resistance hinders its application in exterior coatings.
Light calcium carbonate has a smaller, more uniform particle size than heavy calcium carbonate. It has higher oil absorption and brightness, too. Light calcium carbonate can be used where the maximum matting effect is required.
Application of Barium Sulfate in Powder Coatings
There are two types of barium sulfate used as paint body pigments: natural and synthetic. The natural product is barite powder. The synthetic product is precipitated barium sulfate.
In powder coatings, precipitated barium sulfate can improve leveling and gloss. It is compatible with all colorants. It can make powder coatings achieve ideal thickness and a high coating rate in the spray process.
Barite powder fillers are used in industrial primers and automotive coatings. These coatings need high strength, filling power, and chemical inertness. They are also used in high-gloss topcoats. In latex paints, fine barite powder can replace some titanium dioxide. Its high refractive index (1.637) makes it a translucent white pigment.
Application of Mica Powder in Powder Coatings
Mica powder is composed of complex silicates, and its particles are flaky. It has excellent heat, acid, and alkali resistance. It also affects the melt fluidity of powder coatings. It is used in heat-resistant and insulating powder coatings. It can also be a filler for texture powder.
Of the many types of mica, sericite’s structure is like that of kaolin. It has properties of both mica and clay minerals. Its use in coatings can greatly improve their weather resistance and water permeability. It can also enhance their adhesion, strength, and appearance. At the same time, dye particles can enter the lattice interlayer of sericite powder. This keeps the color from fading for a long time. In addition, sericite powder also has the function of anti-algae and anti-mildew. Therefore, sericite powder is a multifunctional filler with excellent price-performance ratio for coatings.
Application of talc in powder coatings
Talc, also known as hydrated magnesium silicate, is directly crushed from talc ore. The particles are needle-shaped crystals. They feel greasy. They’re soft and not very abrasive. They have good suspension and dispersibility and certain thixotropy. They greatly affect the melt fluidity of powder coatings. So, they are often used for texture powders. At present, they are used in various primers, intermediate coatings, road sign paints, industrial coatings, and interior and exterior architectural coatings.
Talc is cheap, but it has disadvantages, so its usage is limited. First, it has a large oil absorption. For low oil absorption, it must be mixed with fillers and low-oil-absorption barite powder. Also, it has low wear resistance. In high-wear situations, other fillers must be added to compensate. Thirdly, talc with non-metallic minerals is not fit for exterior coatings that need high weather resistance. The impurities can react with acids, like acid rain. Also, talc has matting properties, so it is not used in high-gloss coatings.
Application of Silica in Powder Coatings
Porous powdered quartz belongs to a type of silica system. It is safe and widely used in many coatings, such as: powder, fire retardant, waterproof, and anti-corrosion. Cheap, porous powdered quartz can reduce the cost of powder coatings. It can replace barium sulfate to reduce soluble barium. This will meet environmental standards.
Also, powder coatings often use fumed silica as an anti-caking aid. Fumed silica is a top body pigment and a coating rheology control agent. It has excellent performance. In liquid coatings, it controls rheology by: thickening, thixotropy, anti-sagging, and edge coverage. In solid powder coatings, it improves powder flow and prevents agglomeration and fluidization.
Application of Kaolin in Powder Coatings
Kaolin can improve thixotropy and anti-sedimentation properties. Calcined clay does not affect rheology. But, like untreated clay, it can mat, increase hiding power, and whiteness. These effects are similar to talcum powder.
Kaolin has a high water absorption. It doesn’t help with thixotropy or making hydrophobic coatings. The particle size of kaolin products is between 0.2-1μm. Kaolin with large particles has low water absorption. It has a good matting effect. Kaolin with a small particle size (less than 1μm) can be used for semi-gloss and interior coatings.
Kaolin can be divided into calcined kaolin and washed kaolin. In general, the oil absorption, opacity, porosity, hardness, and brightness of calcined kaolin are higher than those of washed kaolin.
Application of hollow glass microspheres in powder coatings
Hollow glass microspheres are tiny, hollow spherical powders. They are light, have a large volume, and are low in thermal conductivity. They have high compressive strength, are insulated, and resist corrosion. They are non-toxic, well-dispersed, and stable.
When used in powder coatings, hollow glass microspheres can play the following roles:
- (1) Heat insulation, insulation, and low water absorption. Hollow glass microspheres have a vacuum or rarefied gas inside. This creates a density and thermal conductivity difference with epoxy resin. So, they insulate heat well. They are an excellent filler for high-temperature-resistant powder coatings.
- (2) It can improve the physical and mechanical properties of powder coatings. Hollow glass microsphere fillers can increase the hardness and stiffness of powder coatings. However, impact resistance decreases. The degree of decrease depends on the surface treatment of hollow glass microspheres. Using the right coupling agent for pretreatment can reduce its impact resistance.
- (3) Low oil absorption. The oil absorption rate of hollow glass microspheres varies. It is between 7mg and 50mg per 100g. This low oil absorption filler increases the filling amount in disguise during production. It reduces the overall cost.
Application of wollastonite in powder coatings
Wollastonite is mainly calcium silicate. It has a density of 2.9g/cm3, a refractive index of 1.63, and an oil absorption of 30-50%. It has a needle-like structure and good brightness.
Powder coatings generally use natural wollastonite powder, which is processed from natural wollastonite. Wollastonite can be a body pigment. It can replace some white pigment in the coating. It will improve coverage and reduce paint costs. Its good conductivity makes it a common choice for epoxy insulating powder coatings. Wollastonite is a white, needle-like material. It can improve the bending and tensile properties of powder coatings.
Development trend of powder fillers for powder coatings
Surface treatment of powder fillers
Powder coating fillers are polar. Powder coating resins are extremely polar. This will cause poor compatibility between them and harm the coating’s performance. So, it is usually necessary to treat powder fillers. Use physical methods (surface coating and adsorption) or chemical methods (substitution, hydrolysis, polymerization, and grafting) to reduce the particle size of the aggregates or improve the system’s fluidity. This improves the coating’s processing, surface quality (gloss, color brightness), and mechanical strength.
Micronization of powder fillers
A constant resin-to-filler ratio in powder coating is key. Theoretically, smaller filler particles yield better surface and mechanical properties. If the filler particle size is reduced to that of titanium dioxide (0.2-0.5μm), the agglomerated titanium in the formula can be isolated. This will create better dispersion centers. It will improve the hiding power of titanium dioxide. This is the spatial separation principle of micronized fillers. Similarly, micronized fillers can also reduce the amount of pigment used.
Powder Filler Nanotechnology
The most commonly used nanomaterials are nano-silicon dioxide, nano-titanium dioxide and nano-calcium carbonate. Nano-titanium dioxide is reported to be transparent. It can improve the mechanical properties and UV absorption of coatings. It also can greatly enhance the weather resistance of powder coatings in automotive topcoat varnish. Nanomaterials are very fine particles with high surface activity. They easily agglomerate and flocculate. Therefore, the surface treatment of nanofillers, the addition method and the selection of dispersing equipment, the amount of nanomaterials added to powder coatings, and how to disperse them into the base resin are the key to the application of nanomaterials in powder coatings. In designing powder coatings, choose fillers based on performance requirements. This will achieve the best effect.
Functionalization of powder coating fillers
The goal for functional powder coatings is to improve their properties. This includes their physical, chemical, and mechanical aspects. Or, to add new functions to them. Kaolin and wollastonite powders make electrical insulating powder coatings. They can reduce costs and improve insulation. Aluminum and magnesium hydroxides can retard flames. They can make flame-retardant powder coatings. They can also control rheology, improve adhesion, control gloss, and improve hiding power . Thus, fillers for powder coatings should shift from just cutting costs. They should focus on research to develop new, high-performance, low-cost fillers. This is to meet the growing demands of powder coatings.