Charge to Mass Distributions in Electrostatic Sprays

The dependence of the charge to mass ratio or specific charge of the spray stream on fluid and automization parameters in electrostatic spraying of various solvents and paints has been investigated. In addition, atomized paint particle sizes have been analyzed. It has been found that an optimum value of fluid conductivity exists at 2 ?ho/cm where the specific charge of the spray stream exhibits a maximum. From the particle size distribution and from the dependence of the specific charge of the spray stream on conductivity it has been concluded that the basic spray droplet charging mechanism is due to the breakup of a charged liquid surface, while corona discharge does not appear to play a significant role in the charging of the particles.     

Electrostatic Generation of Fine Paint Droplets

Small vibrating capillaries have been used with good success to generate uniform liquid droplets as small as 20 ?m in diameter. However, this technique has not been successful for viscous highly pigmented fluids such as paints. Since paint tends to clog fine capillaries, the smallest droplet that can be reproducibly obtained with conventional methods is about 400 ?m. A method is described for electrostatically generating paint droplets down to 30 ?m. A 250 ?m diameter paint jet is vibrated by an electromechanical transducer at a frequency of 2200 Hz in the presence of an electric field. Fine ligaments are formed between the large drops. Under the action of the electric field the ligaments break up into small highly charged droplets which can be spatially segregated according to size. Calculations of droplet charge based on the equations of motion for the system indicate that the charge on the finer droplets approaches the Rayleigh limit.     

Charge Separation During Splashing of Liquid Droplets in the Presence of an Electric Field

When conductive liquid droplets splash on solid or liquid surfaces, fragments are ejected from the surface that in the presence of an electric field can be inductively charged to the opposite polarity. This phenomenon can reduce the deposition efficiency of electrostatically applied conductive coatings such as water base paints. The charge separation process was investigated by collecting and measuring the size and charge of splashed fragments ejected from surfaces in the presence of an electric field.     

Surface Charge Buildup During Electrostatic Spraying

In industrial electrostatic spray coating operations, liquid paint is applied to substrates of varying electrical characteristics. In many cases a metal substrate is chemically pretreated and/or primed prior to electrostatic spraying. A previously rejected part may have already received a number of thoroughly cured topcoats prior to the final spray application. Typical primers and topcoats have large surface resistivities (108 to 1010 ohm.cm-2) and exhibit slow surface potential decay rates. A surface potential measurement system has been developed to determine the time dependent electrical properties of various substrates when coated with commercial water base and organic paint systems. It has been observed that during some electrostatic recoat operations, the charged paint particles appear to be repelled from the surface to be coated. It is found that this phenomenon can be predicted when conditions are such that adequate charge leakage paths to ground are not available due to high surface and bulk resistivities of the film resulting in a repelling charge and potential buildup on the surface. It has been found that the charge removal process cannot be described as a simple exponential decay, but as a superposition of three exponential decay processes each with its own decay constant ?.