Electrostatic Spray is a common application method for Rilsan® Fine Powders. It allows for pinpoint control of surface thickness and ensures more comprehensive coverage of the workpiece.
What is Electrostatic Spraying?
One of the most convenient coating application methods is electrostatic spray, and the Rilsan® Fine Powder ES and ESY grades are specifically engineered for spray application. The powder particles are positively-charged via electrostatic spray gun or similar devices also referred to as "corona" or "tribo" sprayers. Once the equipment has charged the powder, it is sprayed onto the surface of grounded, metal parts which the powder is statically attracted to. After the spray coating has settled, the parts are moved into an industrial oven to heat cure the final coating, producing a smooth and non-porous surface finish.
Why Use Electrostatic Powder Coating?
Powder coating via electrostatic spray is an ideal operation for facilities needing a thickness below 150 μm. It also allows for coating metal objects where masked or uncoated areas are incorporated into the final design. Line coaters can also fully automate this spraying method optimizing production and increasing finished product consistency. There are a few factors to take into consideration when deciding on an electrostatic spraying method:
Thickness of Metal Part : Ideal for coating parts up to 6mm thick.
Thickness of Coating : Electrostatic gun application allows for variable thickness ranges from 80 to 120μm (Rilsan® ESY Grades) and 100 to 150μm (Rilsan® ES Grades)
Size of Metal Part :Electrostatic spray can be used with any part size, partial coating is also a possible application method.
Substrate Type: This powder spray application can be used on any conductive metal substrate that can withstand a temperature of 220°C (428°F) without deterioration.
Electrostatic Coating Process Steps
1. Surface Treatment, Preparation and Primer
The parts to be coated should be thoroughly cleaned and free of all surface contaminants, grease and oil. For enhanced anticorrosion performance, the use of an Arkema primer is required.
2. Powder Spraying
The Rilsan® ES Grades require the use of a corona type spraying equipment (positive polarity) or a tribo-type spraying system. For corona-style spray systems, a flow rate of 120 g/min, intensity of 20 μA, and a polarity from 30 to 40 kV is recommended for best powder coating finish. For tribo spray application a flow rate of 120 g/min is also recommended.
The Rilsan® ESY Grades should only be applied via a corona process with negative polarity; recommended at -30 to -70 kV with the same flow rate and intensity as the ES Grades (120 g/min, and 20 μA respectively).
It is possible to recycle the over sprayed Rilsan® ES powder when blended with virgin powder in equal proportions. Recycled powder should also be sieved and removed of its electrical charge prior re-use. It is not recommended to recycle Rilsan® ESY powders, however production trials may be held to confirm finish and desired performance.
3. Fusion - Oven Curing
The fusion process for coated parts should occur in a homogenous, temperature controlled oven with good ventilation (air speed <3 m/sec). Tunnel type ovens are recommended for continuous processing.
Rilsan® Fine Powders do not crosslink like thermoset powders, and instead melt at temperatures of 215°C (± 5°C). Once fully melted, the coating properties have been achieved and the surface film is formed.
Hot Spraying - Electrostatic Spray Alternative Application
Rather than oven-curing after spraying the powder on a metal part, the metal part instead can be pre-heated, allowing for the powder spray to melt onto the surface and form the film instantaneously. Rilsan® ES and ESY Grades can be used in hot spraying applications if the desired thickness is lower than 200 μm and the thickness of the metal part is no larger than 6mm.
Powder Coating Defects and Potential Root Causes
Surface Defect
Potential Cause
Bubbles or Bubbling
Too much powder sprayed or primer applied
Excessive humidity in powder or air supply
Degassing of part support
Oven temperature too high
Orange Peeling
Fusion time or oven temperature too low
Distance between part and spray-gun too short
Application voltage or intensity too high
Ambient temperature too low
Pinholes
Coating thickness too thin
Insufficient spraying time
Unclean part surface
Frosting or Unmelted Powder
Fusion time too short or temperature too low
Yellowing
Fusion time too long or temperature too high
Poor Adhesion
Inadequate surface preparation
Too little or too much primer
Inadequate fusion temperature or time
Powder Fall-off
Too much powder applied
Oven ventilation too strong
Too little primer
Application voltage too low
Craters / Inclusions
Cross-contamination from dust/dirt or other coating material in spray equipment or oven
