Electrostatic Spraying
Nano coatings are composed of extremely tiny droplets. Electrostatic application of a nano coating employs the law of physics describing the electrostatic interaction between electrically charged objects. This law is behind why metal filings are drawn to a magnet, lint is attracted to your clothes or dust clings to the screen of your television. These are all examples of Coulomb's law in action - opposite electrical charges attract and "like" charges repel. (These principles of physics were first published in 1783 by French physicist Charles Augustin de Coulomb.)
In numerous commercial, industrial and agricultural spraying operations, electrostatic forces of attraction significantly improve the deposition (coating) of charged droplets onto target surfaces. Such operations include liquid and powder paint coatings, electrostatic precipitation of air pollutants from stacks, electrostatic spraying of agricultural pesticides onto field crops and orchards, xerographic copying, ink-jet printing, textile flocking, and more recently, disinfection, sanitization and decontamination applications.
Droplet Control & Droplet Size
Common to all these electrostatics based operations are droplet control and droplet size. Droplet control consists of first, imparting an appreciable net electrical charge onto the individual droplets (e.g., 5-15 mC/kg charge – to mass), and secondly, propelling the charged droplets to the target surface.
Droplet size is the result of shattering, or atomizing,the jet of spray coating into much smaller droplets. Droplet sizes are measured in microns. The most common term used to describe droplet size is Volume Median Diameter (VMD). VMD refers to the midpoint droplet size (mean), where half of the volume of spray is in droplets smaller, and half of the volume is in droplets larger than the mean.A VMD of 50, for example, indicates that half of the volume is in droplet sizes smaller than50 microns,and half the volume is in droplet sizes larger than 50 microns.
A micron is 1/1000 millimeter, or about 1/25,000 of an inch.In comparison, a drop of water is about 250 to 300 microns in size, a human hair is about 100 microns in diameter. Droplets 50 microns and less are generally considered to be aerosol droplets. The range and size of droplets in any given spray has tremendous influence on coverage, volume of spray used, retention, and runoff. When droplets greater than 150 microns strike target surfaces, they become flattened, but their kinetic energy is such that they retract and bounce away. Small droplets are better retained as they lack the kinetic energy to overcome surface energy and viscous changes that occur on impact and do not bounce away.
Optimum Droplet Size
The optimum size is generally considered to be between 40 and 100 microns. Droplets in this range cover more surface area, require lower volumes, are less susceptible to bounce back and runoff than those larger than 100 microns. The surface area of a liquid is greatly increased when broken into small droplets. The volume needed to cover target surface decreases proportionally with droplet size. Even small changes in dropletdiameter make big differences in droplet weight. An increase in droplet diameter from 150 microns to about 190 microns doubles the droplet weight. An increase in droplet diameter from 150 microns to about 240 microns increases the weight 4 times. Doubling the diameter to 300 microns increases its weight, andalsoits volume,by 8 times.
Charged vs Uncharged Droplets
When droplets are uncharged, neither the droplets nor the target surface has any influence on the other. Sprayed droplets are principally influenced by forces generated by the spraying device (pressure, air flow) and by external forces of gravity and air drag. Uncharged droplets are more susceptible to air currents in the spraying area while charged droplets are less influenced by air currents.
An electrostatic sprayer induces an electrical charge onto droplets creating an attractive force betweencharged droplets and targets. This electrostatic force overrides gravity and inertia to pull droplets out of their paths – up, down or sideways – to the closest surface.
To compare influence, the electrostatic force on charged droplets is up to 75 times greater than the force of gravity. Electrostatically charged droplets are also influenced by the sprayer’s mechanical forces, however, once they reach the vicinity of the target, electrostatic forces take over to a very large degree.
In water based coatings, the water carries the electrical charge. Charged sprays, unlike uncharged sprays, resist coalescing into larger droplets both in transit and deposition. As each charged droplet is deposited on a surface, electrical charges balance out at that site, making it no longer attractive to other charged droplets. The droplets following are pulled instead to the rest of the surfaces which remain attractive.
Atomization & Induction Charging
A typical droplet of spray from a conventional air-blast applicator is around 250 microns in size. A typical droplet of spray from a electrostatic sprayer is around 40 microns. In the nozzle of a electrostatic sprayer the coating liquid passes in very close proximity (but does not contact) a positively charged electrode, inducing a negative charge (excess electrons) in the grounded liquid stream. The stream of coating liquid, with its excess electrons, is combined in a shearing action with a near sonic velocity jet of air, shattering (atomizing) the coating liquid into droplets 30 to 40 microns in size.
Shattering the liquid stream breaks the droplets free from the grounded liquid stream and traps the excess of electrons. As the electrically charged droplets exit the nozzle, they create a charged field plume that is strongly attracted to the targeted object. The charge on the atomized droplets remains until discharged on a grounded object or the water content of the droplet evaporates.
Induction charging is based on the principle that a field charge, created within the plume, is presented to a grounded target. It is this field charge the plume creates that really makes an electrostatic sprayer so effective. The charge is small, but the force attracting the coating plume to the target is substantial, up to 75 times the force of gravity.
As the target sits with a neutral potential the proximity of the field charge induces a strong attraction. The droplets actually reverse direction and coat the underside, backside and crevasses of the target, creating an "electrostatic wraparound." In comparison, a 250 micron droplet simply runs off a target and is largely wasted.
Additionally there is the second half of Coulomb's Law, that "like" charges repel. Since all of the coating droplets leaving the nozzle have the same charge, they cannot coalesce into large droplets, which again fall off the target. At the same time, the swirling droplets are not attracted to areas already coated and continue to seek out uncovered surface areas until there is uniform coverage (disposition) across the entire target. This results in a consistent, uniform coating, no drips, no runs and no uncoated areas.
What Matters Most
Microscopic organisms are usually hidden in deep cracks and crevasses inside or on the undersides of surface areas, and continue to survive if coverage is spotty. The extraordinary bonding of the coating to the target and the complete, uniform coverage achieved are distinct advantages of electrostatic sprayers. Experience is showing it is not just what is applied but how a coating is applied that matters most.