Research on the application of ultrafine powder coating in the field of automotive coating

As powder coating technology continues to evolve, so does its use in the automotive sector. In recent years, powder coatings have been widely used to coat engine hoods and body parts.

However, due to its thick coating and poor appearance flatness, powder coating has not been widely used in automotive exterior coating with high appearance requirements.

In order to save costs and improve the quality of the coating film, powder coating manufacturers are continuously reducing the particle size of the powder coating to improve the flatness of the powder coating film.

However, the reduction of particle size also leads to poor fluidity of powder coatings, which seriously affects the use. Therefore, a new technology is needed to improve the flow characteristics of ultrafine powders, such as the use of nano-sized additives as spacers (fluidization additives), to enable the use of ultrafine powder coatings in existing spraying equipment.

In addition, more and more heat-sensitive plastic or composite parts are used in the automotive industry, and the development of low-temperature curing powder coatings, especially low-temperature curing ultra-fine powder coatings, has also received wider attention.

Accordingly, this paper first reviews the development and challenges of ultrafine powder coatings in the field of automotive coating, and examines the flow properties and coating properties of ultrafine powder coatings after adding fluidizing additives.

Application of ultra-fine powder coating in the field of automotive coating

1.1 Ultrafine powder coatings

Ultrafine powder coating usually refers to powder coating with a medium particle size of less than 25 μm with a particle size of less than 50. Compared with ordinary powder coatings (medium particle size D50 greater than 30μm) and fine powder coatings (medium particle size D50 between 25~30μm), ultra-fine powder coatings have unique excellent properties. As the particle size decreases, the thickness of the coating film also decreases, so that the coating film has better leveling and good decorative effect. In addition, the economic costs are reduced due to the reduction in material consumption. According to the Geldart powder classification, ordinary powder coatings belong to Class A particles and are easily fluidized; However, when the particle size is reduced to ultra-fine powder coating (Class C particles), the force between the particles (van der Waals force) increases, and the powder particles are prone to agglomeration and cannot fluidize normally, resulting in a series of problems such as air flow transportation and spraying difficulties. In order to improve the fluidity of ultrafine powder coatings, an appropriate amount of fluidizing additives (guest particles) must be added to the coating, and these nano-sized particles adhere to the surface of ultrafine powder particles (host particles) in the form of small agglomerates, increasing the distance between the particles. Because the bulk density or apparent particle density and size of these added guest particles are lower than those of ultrafine powder particles, the interparticle force between the adhered guest particles and the host particles is significantly smaller than that between the host particles, thus reducing the van der Waals force between the ultrafine powder particles.

In order to promote the depolymerization of the main particles, the fluidizing additives particles usually need to be evenly dispersed in the ultrafine powder and evenly attached to the surface of the ultrafine powder particles, but they do not need to be attached in the form of single particles, and the form of small agglomeration is more helpful to reduce the inter-particle force of the main particles. Additives can significantly improve the flow of ultra-fine powder coatings, but they can also cause other problems, such as loss of gloss on the surface of the coating film and defects such as craters and "particles". In addition, nano-sized additives are inherently prone to cohesion and tend to form strong and large agglomerates that cannot be dispersed by ordinary dry blending processes (even with high-shear mixers), resulting in unavoidable defects such as "particles" in the final coating. Overcoming these problems requires new technologies, including specially designed additive formulations and special mixing processes, to improve the dispersion and compatibility of additives with powder coatings, ensuring that excellent coating properties are maintained while reducing inter-particle forces and improving flowability. The Particle Technology Research Center of the University of Western Ontario, Canada, has successfully developed a high-efficiency ultra-fine (HEUF) powder coating technology; According to the various resin systems of the coating, the technology adopts the corresponding fluidization additive formulation and optimized addition and dispersion methods to ensure the excellent fluidity, sprayability and coating quality of the ultra-fine powder coating, and its minimum coating film thickness is only 20~30μm (as shown in Table 1). Figure 1 shows the coating surface profile (measured by Dektak contact surface profiler) with an average particle size of 16.4 μm ultra-fine powder coating and 38.8 μm ordinary powder coating. The greater the fluctuation of the ordinate value, the rougher the coating surface.

As can be seen from Table 1 and Figure 1, compared with ordinary powder coatings, the high-efficiency ultra-fine HEUF powder coating significantly reduces the surface roughness of the coating film and improves the visual effect of the coating film. The reduction in film thickness results in a significant reduction in cost, while the film also offers the same durability, recyclability, and powder loading rate as standard powder coatings.

1.2 Overview of the application of ultrafine powder coatings in the automotive field

With the rapid growth of demand for high-end vehicles in the Asia-Pacific region, as well as the increasing environmental requirements of regulatory agencies such as the Ministry of Environmental Protection of China, the European REACH (Registration, Evaluation, Authorization and Restriction of Chemicals), and the U.S. EPA (Environmental Protection Agency), the automotive coating market is increasingly shifting from solvent-based coatings to low-VOC coatings. As an environmentally friendly product with zero VOC emissions, powder coating has been widely used in wheels, car covers, decorative trims, bumpers, hubcaps, door handles, truck bases, radiators, filters and a large number of engine parts. But powder coatings have their own problems. Ordinary powder coatings typically have higher film thicknesses, lower flatness, and a relatively poor visual appearance, which limit their application in automotive exterior coatings. In order to achieve better coating results, powder coating manufacturers have begun to reduce the film thickness and improve the flatness of the coating film by reducing the particle size of the powder coating. In recent years, fine powder coatings have begun to be applied to car body coatings. General Motors and Chrysler have achieved mass production of midcoats in the United States and several other countries. The German company BMW has successfully applied fine powder varnish (topcoat). Although fine powder coatings have made great progress in the field of automotive coating, so far, ultra-fine powder coatings with a particle size of less than 25 μm have not been applied to the field of automotive coating. Although fine powder coatings have made great progress in the field of automotive coating, so far, ultra-fine powder coatings with a particle size of less than 25 μm have not been applied to the field of automotive coating. Powder coatings have not yet made a major technological breakthrough in the application of this high-end market, so their application in automotive body coating is still limited, and more than 95% of body coatings are still liquid coatings. The flow problem of ultrafine powder coatings is an important factor limiting their application in the field of automotive coatings. The development of new fluidization additive technology will be the main way to solve this problem, and will also open a new door for the application of ultrafine powder coatings in the field of automotive coatings.