How Does Electrostatic Spraying Work?
The main crux of electrostatic spray painting involves the concepts of charge and electric fields. We are all familiar with these phenomena as most will have witnessed plastic wrap attaching itself to surfaces or clothes sticking to each other as they are removed from a tumble dryer. In depth analysis of particle physics is outside the realm of this article, but basically the electrostatic technique of applying a coating to a metal surface involves the law of attraction between positively and negatively charged particles.
With an aerosol, compressed air forces the paint through the end of the spray gun which atomises the liquid into a fine spray. Atomisation essentially breaks up the paint into small droplets and this process is also utilised in electrostatic spraying, but with one unique difference regarding the methodology. The paint is atomised in a static field which is formed at the end of the electrostatic spray gun.
Just before the fog of paint leaves the nozzle, it is given a positive charge. The charged paint droplets are sprayed through a strong electric field which is a term used to describe patterns of forces. The negatively charged, grounded metal item attracts the positively charged liquid to its surface very much like a magnet. Here, a basic law of electricity is inherent in the electrostatic system of spray applying paint, namely that opposite polarities attract. Scientifically, this relatively simple concept is known as Coulomb’s Law which states that the same electrical charges repel each other whereas opposite charges attract as in iron filings to a magnet.
A Corona Discharge is crucial for attraction within the electrostatic spraying technique to take place.
An electrical discharge occurs through the ionisation of the paint within the electric field surrounding the conductor (in this case the metal). This is known as ‘Corona Discharge’. Even though the corona is gaseous, it is still conductive because it contains free electrons and therefore enables the current to flow from the charged gun through the air. The process involved in the paint particles becoming charged within the electric field of corona discharge is dictated by Pauthenier’s Theorum. Pauthenier’s equation encompasses the law that the charge on a particle will increase until it is equal to its surrounding electric field. Consequently, charging is influenced by the strength of the electric field as well as the length of time particles spend in this field, but also the size and shape of the individual particles.
The electric field also influences the force of attraction. After the electric field has been created, the ions follow the field lines towards the negatively charged surface. The force attracting each of these positively charged particles to the grounded metal substrate is equal to the charge of the particle itself multiplied by the electric field strength. Even though the electrical charge given to the paint is small, it is strong enough to counteract the effects of aerodynamics and gravity. In fact, the force of attraction is up to 75 times greater than the force of gravity alone and when the particle has reached the metal the force of attraction is strong enough to keep it there. This is partly because the chemicals generally used in electrostatic painting are strong dielectrics in that once the particles become charged, the charge remains for a sufficient amount of time for the attraction to be sustained.
As the attraction of the electrically charged particles to the grounded surface is so strong, paint drift is reduced. This means there is a high transference rate of paint to surface averaging at 98%. Furthermore, the paint droplets are pulled towards surfaces in all directions, so that all sides of target areas are coated. Some particles actually change direction as they are pulled up, down and sideways towards all angles of a façade. The underside of substrates are coated and paint is renowned for reaching around the back of surfaces if an electrostatic system is used. The new coating will even get into crevasses and this ‘electrostatic wraparound’ makes it an ideal technique for coating hard to reach areas. The wraparound effect that electrostatic painting is well-known for allows this technique to be used in enclosed spaces such as offices and retail outlets. Less paint is released into the air because it is so strongly attracted to grounded surfaces, so fumes are reduced and the environment is safer than when other painting methods are used.
The aspect of Coulomb’s Law that testifies to ‘like’ charges repelling others applies to the electrostatic paint spraying technique and adds to the success of this method. The particles leaving the spray gun have the same positive charge and therefore repel each other. This means that they do not conglomerate into large droplets, so the paint mist remains fine and evenly sized. Likewise, the attraction of spray to uncoated surfaces is stronger than those that have already been coated. It is as if the charged paint droplets seek out the uncoated areas of substrate resulting in a more uniform coverage than other more traditional methods of painting.
The lack of overspray and even coverage results in a finish that is as close to powder coating as is possible with the added advantage of this electrostatic paint spraying system being portable. This means it can be administered onsite whereas powder coating can only take place in factories where large ovens are needed to cure coatings. Large architectural metal items do not need to be dismantled for re-spraying. They can remain where they are and recoated electrostatically in situ. So not only is the finish as close to perfect as possible, it can be administered anywhere.