An advanced coating that combines the fast cure and mechanical properties of a coil coating with the environmental benefits of powder can provide a new option for coating metal.
Coil coating is a continuous industrial process in which multiple layers of organic film are applied and cured on a moving metal strip (see Figure 1). The paints used are liquid (solvent-based) and are generally composed of polyesters with acid- or hydroxy- endgroups able to crosslink with melamines or isocyanates to form a complete network with film properties that are tailored to the final application of the coated metal panel (building products, beverage cans, domestic appliances, etc.). The total film thickness is around 5 to 25 µm, which allows for perfect color matching, surface hardness and transformation of the flat panel through bending or shaping without damage. The paints that are used for this application are usually based on a thermoset system involving catalyzed chemical reactions at temperatures around 240°C. The main advantages of coil coating are its fast cure time - approximately 25 seconds - and its ability to create an already-painted substrate that is flexible enough to fabricate parts.
Powder coating typically involves application to a metallic substrate using spray guns. The chemistry relies on the reactions that occur between resins and hardeners, such as epoxy/polyester resins, amines/epoxy, isocyanates/polyesters, hydroxylalkylamides (Primid®)/polyesters and polyepoxides/polyesters. Powder coatings are flexible but are generally applied to metallic pieces that are already formed into their final shape. The coating is typically applied at 60 to 120 µm thicknesses, and curing takes longer than coil coating - approximately 10-15 minutes at temperatures of 150 to 200°C. However, powder coating is widely known as a more environmentally friendly process because the coatings are solvent-free.
What if the advantages of both techniques could be combined? Would it be possible to achieve a powder system that offers both hardness and flexibility and can be cured in seconds rather than minutes?
Researchers at SigmaKalon Group developed a powder coating technology with enhanced reactivity. The resulting heat-crosslinked film can be cured within seconds, like liquid paints used in coil coating applications, and exhibits the same finished properties.
The researchers determined that when the powder coil coating is heated and cured to approximately 242°C, the catalyst is deblocked. The reaction begins at about 150°C after 10 seconds, along with the first crosslinking. At 30 seconds and 242°C, the reaction is complete, and the coating is densely crosslinked.
Table 1 shows the different components of the powder coil coating. These components can be adjusted to optimize the coating for the application. Table 2 (p. xx) shows the different gel times measured at various temperatures for different chemical systems, including an epoxy (EPE), polyurethane (PUR) and polyester (PE). All of these systems can be adapted to coil coating conditions.
During testing, the powder coil coating achieved a complete cure within 3 to 20 seconds. This cure behavior is substantially different from conventional powder coatings, which typically require cure times of 10-15 minutes at temperatures of 150 to 200°C.
Particle size plays an important role in the process. As mentioned previously, coil coatings typically require a film thickness of 5 to 25 µm to achieve the optimum finished properties, while powder coatings are generally applied at thicknesses of 60 to 120 µm. For the new powder coil coating to be competitive from an economic standpoint, the researchers knew that the applied thickness would need to be reduced substantially. Furthermore, thicker films can present an inhomogeneous thermal gradient inside the film during curing, leading to incomplete curing and defects in the final film.
The new powder coil coating has a very low particle size (D50) and can be applied at around 30-35 µm. Additional testing has indicated that a thickness as low as 10 µm is achievable, although further work in this area is needed.
Achieving low viscosity during the melt phase was also a major aspect of the project. The flexibilizers used in the formulation can act on the friction between the macromolecular chains and allow better mobility. In general, the viscosity of a coating incorporating flexibilizer is similar to the same coating without flexibilizer at temperatures 10 to 20°C higher. This slight difference allows enhanced flexibility, better film formation and a better surface appearance, as well as improved mechanical properties of the cured film.
The new coating was tested on several different substrates, including primed chrome (Cr) and Cr-free hot-dip galvanized (HDG) panels, pretreated Cr and Cr-free HDG panels and naked HDG panels, and it exhibited good adhesion in all cases. The coating’s finished properties are similar to those of conventional coil coatings, and the technology can be applied on all substrates. Generally, powders applied on metals without primers show better curing at lower temperatures, better flow and better mechanical properties. Curing can be achieved using ultraviolet, electron-beam or near-infrared technologies and is completed within seconds, compared to the much longer curing times required with conventional powder coatings. Like other powder coating systems, the new coating is solvent-free.
With the development of this novel powder coil coating, metal finishers have a new option for meeting environmental regulations and improving productivity, without compromising quality.
Authors’ note: The work discussed in this article has been financially supported by The Region Wallonne, Belgium.