Electrostatic spray coating consists of depositing electrically charged powder particles on the metal surface at ambient temperature. The electrostatic charge is obtained by conveying the powder through a special spraying device, such as “corona” or “tribo” spray-gun. The charged powder particles are attracted to the grounded part to be coated. The powder coated part is then moved into an oven where the Rilsan® powder fuses to produce a smooth and non-porous coating. Two series of Rilsan® grades are suitable for this application process: the Rilsan® ES and Rilsan® ESY ranges.
Electrostatic Spray Process
Criteria For Choosing This Process
Thickness of the part:
The process is recommended for coating parts that are up to 6 mm thick.
Thickness of coating desired:
The process allows controlled thickness ranging from 80 to 120 μm (Rilsan® ESY grades), and from 100 to 150 μm (Rilsan® ES grades).
Size of the part to be coated:
There are no limits to the dimensions of the part to be coated; partial coating is possible.
Nature of part - surface treatment:
The process is suitable for all types of conductive metal substrates capable of withstanding a temperature of 220°C without deterioration.
The parts to be coated should be clean, and free of grease or oil. The primer promotes the adhesion of Rilsan® onto the surface of the metal to be protected. For both Rilsan® ES and Rilsan® ESY grades, the primer is required to achieve anticorrosion performance as well as excellent resistance to hot water. For Rilsan® ES grades, it is also recommended to apply a primer also to prevent electrostatic discharge and the powder falling off during fusion. For further information, please refer to the section on Surface pre-treatments and primers.
Powder Application - Spraying
Rilsan® ES grades require the use of spraying equipment either of the corona type (positive or negative polarity) or the tribo type. For corona spraying, polarity from 30 to 40 kV, intensity of 20 μA and a flow rate of 120 g/min is advised. The same flow rate should be targeted with tribo type spraying equipment. Note that these parameters should be optimized on specific equipment. Rilsan® ESY grades should only be applied using the corona process with negative polarity. Advised polarity is -30 to -70 kV, while keeping the intensity and flow rate the same as for ES grades. It is possible to recycle overspray Rilsan® ES powder and to blend it with virgin powder in proportions which should be held constant after sieving and removing its electrical charge. It is not recommended to recycle Rilsan® ESY powders. Ideal conditions for the electrostatic spray process are a temperature of 20°C ± 5°C and air humidity close to 50%.
Fusion should occur in a temperature-controlled oven (homogeneous temperature throughout) with good ventilation (air speed typically below 3 m/sec) in order to prevent air blowing too fast around the coated parts. Tunnel type ovens are most suitable for continuous processing. Unlike thermoset powders, Rilsan® powders do not crosslink: surface temperatures of 215°C +/- 5°C allow the powder to melt, and the coating properties are reached as soon as the film formation is achieved. The time needed to melt the powder and cool the coating depends on the thickness and configuration of the part. For a smooth, cold-rolled steel part, the residence time in an oven at 215°C required to melt a Rilsan® coating is shown on the following graph, as an indication.
Hot spraying process consists of spraying a pre-heated part with Rilsan® powder. As soon as the powder impacts the preheated article it melts to form a film on the surface. When hot spraying, the Rilsan® T FB range is recommended for coatings of 250 μm to 600 μm, but only for parts with a sufficient thickness (min 3 mm). Rilsan® ES and Rilsan® ESY ranges can be used if the desired thickness is lower than 200 μm and if the thickness of the part is not too high (6 mm max).
The surface preparation and pre-heating of the part is equivalent to the process used for fluidized bed dipping: degreasing, sand or grit-blasting, priming, preheating and coating. The powder can be sprayed using conventional electrostatic powder guns as long as the charge is turned off. Note that it is critical to adjust oven time and temperature to ensure proper curing of the primer and maintain sufficient pre-heating of the part. For more information, please refer to the sections on Fluidized bed dipping process and Surface preparation and primers.
Advantages of Electrostatic Spraying
- Good control over powder deposition
- Partial coating of the part can be achieved
- Lower temperature to fuse powder
- Shorter time to fuse powder
- No crosslinking risk compared to thermoset materials
- Smaller powder stock required
- Process may be automated