Transfer Efficiency Study

Question:

I’m a student, doing my Bachelor’s Degree project at an automobile plant. I’m to improve the transfer efficiency in one of the automotive painting booths. Searching for information on the Internet, I found several factors that have an effect on the transfer efficiency and that it is recommended to adjust in order to improve the transfer efficiency. Reading through your site, I realized that you have a great knowledge in this area and would like to ask you some questions. Thank you in advance and look forward to your reply. My questions to you are:

1. Paint Resistivity: How come the transfer efficiency increases with decreasing paint resistivity?
2. Applied Voltage: A certain voltage is applied to the paint in an electrostatic gun. It is said that the lower the voltage, the higher the transfer efficiency. Why is this?
3. Solvent Drying Speed: It is said that the slower the drying speed the higher the transfer efficiency. Why is this?

4. Spray distance: Another factor I was wondering about is the spray distance, with this I mean the distance between the gun and the target. Why does the transfer efficiency increase with decreasing distance? P.P.

Answer:

Thanks P.P. for your kind comments. The reason for my success is not that I really have a great knowledge of paints and painting. Instead, it is because, as in a popular song of the 1950’s "I know a little bit about a lot of things.” I will answer your questions by the numbers.

1. Transfer efficiency increases with decreasing coating resistivity because coatings having lower resistivity are more able to pick up electrons. On the other hand, if the resistivity is too low, as in the case of waterborne and metallic paints, the system must be electrically isolated to prevent shorting to ground and there would be no electrostatic charging. It is important to note that there is an optimum range for coating resistivity that differs with the type of electrostatic application equipment.
2. I have never studied the relationship between applied voltage and transfer efficiency. Therefore I am not sure that your statement, “the lower the applied voltage, the higher the transfer efficiency” is true. Actually, you’re asking the wrong Izzo. My Father, Albert Izzo PE, was the Electrical Engineer.
3. Nonpolar solvents have lower resistivity than polar solvents. Coatings reduced with nonpolar solvents apply with higher transfer efficiency. Most nonpolar solvents have lower evaporation rates than polar solvents. This causes coatings reduced with nonpolar solvents to be slower drying. On the other hand polar solvents have faster evaporation rates and coatings reduced with them apply with lower transfer efficiency. Faster drying contributes to “dry spray” which also reduces transfer efficiency. Here again, there is an optimum range for coating resistivity that differs with the type of electrostatic application equipment.
4. One reason transfer efficiency decreases with increased gun to target distance is the effect of gravity on coating particles. Another reason is increased particle velocity. To overcome the effect of gravity, particle velocity must be increased by increasing atomizing pressure. The increased particle velocity will lower transfer efficiency by counteracting electrostatic attraction.