Electrostatic spray application, which can be used with conventional, airless or other systems, is preferred in applications to be made on complex shaped metal surfaces where overspray should be less, and in powder paint applications. In electrostatic spray applications, the air around the point where the paint is sprayed is ionized by applying an electrical voltage of 50-125 kV. Electrons released as a result of the ionization of the air attach to the atomized paint particles passing through this region and charge them with a negative (-) charge. Before the application, the surface to be painted is grounded and loaded with a positive (+) charge. Thus, an electrostatic field is created around the surface to be painted. In this case, it is ensured that the negatively charged paint particles reach the surface with the effect of the electrical attraction force on the object (Figure-12). This electrical attraction greatly reduces the spray losses that may occur due to the spray and rebound effect. It minimizes paint losses, especially in painting the back surfaces of objects such as pipes, profiles, which cause spray loss.
In order for the paint particles reaching the surface to transfer the charge on them to the soil over the surface, the electrical conductivity of the paint must be sufficient. In electrostatic applications of solvent paints, highly conductive solvents such as alcohols or additives are added to the product or its thinner. The widest suitable range for wet paint conductivity is between 0.05-20 M. However, for many, a conductivity of 0.5-5 M is preferred.
Spray guns using air as a propellant are commonly used in electrostatic spray applications. In these applications, both the spreading level of the air spray is achieved and the spray losses are minimized. In electrostatic spray applications, disc or bell-shaped rotating head application tools are also used, where the paint is broken into very small droplets with the effect of centrifugal force.
It is usual to apply rotational speeds of 1000 revolutions per minute (1000rpm) and above in rotating discs. In very high speed bells, rotational speeds between 25000 rpm and 60000 rpm are applied. It is possible to encounter problems with low gloss in paints applied at such high cycles. Therefore, it is necessary to take precautions against the tendency of matting caused by high cycles during the design of the paint.
It is possible to apply water paints by electrostatic methods with the introduction of some organic solvent and appropriate electrical insulation on the application equipment.
On the other hand, surfaces can be made conductive by first applying a thin coat of conductive primer to surfaces such as plastics that are not conductive, using the normal air spray method. Subsequent coats of coating can be made by electrostatic spraying.
Both spray losses can be reduced and extremely well spread paint surfaces can be obtained with electrostatic spray applications made with guns, rotating discs or bells. However, this method also has some disadvantages. In particular, the recessed parts on the surface may not receive enough paint due to a problem called the "Faraday cage effect" (Figure-13). High voltage applications cause this effect to be experienced more intensely. As a result, a thicker paint film is obtained on the surfaces where the paint can easily reach, and a thinner paint film is obtained on the surfaces with a Faraday cage.
The second possible source of trouble is the risk of fire. A significant portion of the solvents used in solvent paints can be explosive at room temperature and below. In electrostatic applications, sparks that may occur due to the applied voltage should be adjusted so that the flash point of these solvents is 26 ºC and above.
- The lowest paint loss and minimum paint dust among all spray applications are obtained in electrostatic applications.
- The transfer efficiency is higher.
- In electrostatic applications, angular, round and recessed-protruding metal materials can be coated with equal film thickness.
- It provides a clean working area for the practitioner.
- Electrostatic spray can only be used for painting conductive materials.
- Although it is possible to reach high film thicknesses, since the surface will be isolated after one coat of paint is applied, a second coat cannot be applied electrostatically.
- Equipment cost is high.
- Not useful on large surfaces.
- It can be dangerous at high temperatures (>25-30 ºC).
- In metallic paints, it may give a different appearance compared to a non-electrostatic application.