Drop in Film Thickness


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Q. We recently found that film thickness across the board at our e-coat plant has massively decreased, from 35 microns to around 27 microns. To combat this, we first tried the obvious: testing the paint bath for solvents, PB and solids, but the results were as they should be. We also confirmed that the voltage and current through the bath are correct. We replaced nearly all of the anode tubes within the tank, and we finally tried increasing the paint temperature and increasing time in the bath, but this showed very little improvement. I have done a lot of research into the problem and have found that a buildup of bacteria could cause less film build. Is this possible? Thanks for your help.              K.Y.S.
A. There are many variables that can contribute to loss of film build in an e-coat bath. You mention several items that have been checked and found to be okay or not contributing to the loss of film build, including anodes. You do not mention grounding, but I am sure you checked to make sure this is correct (including the racks’ contacts points) and the continuity through the entire system—rectifier, anodes, racks, grounding. 
I don’t think the issue is bacteria, or you would mention other important things: foul odors, high pH, poor film appearance, low ultrafilter output, etc. 
Although it seems you have covered all primary variables or potential root causes, there maybe a couple of others that could play a significant role. One is the pH of the permeates; if you have a cathodic system and you have a very low pH in your permeate rinsing tanks, you could be depositing e-coat paint in the e-coat bath at the desired film for the voltage applied and at the same time removing part of the film at the rinsing stages. The high acid concentration in the permeates solubilizes the uncured e-coat film, resulting in low film thickness. 
This is the opposite of what happens when you have bacteria in the system, which makes your pH go up.
Another potential cause is the low-molecular-weight resin materials that exist in an e-coat system. These materials are difficult and expensive to test in a typical finishing facility, so it just isn’t done, and many in the business have not even heard about them. These materials come from the resin component and are the resin fraction that has the most impact on film build. Because they are small molecules, they pass through the ultrafilter system and distribute through the permeates until finding a concentration of equilibrium in the system. Because of line upsets, maintenance schedules or other operational control issues, e-coat systems sometimes drain large quantities of permeate. While doing so, they also are draining those valuable low-molecular-weight resin materials.
If your system ratio of volume permeates to volume e-coat bath is high, draining an entire permeate tank can really deplete the e-coat bath of low-molecular-weight resin materials and interfere dramatically with the bath’s ability to coat.
I observed this in a start-up of a large e-coat bath with three immersion permeates of equal volumes as the e-coat bath and a system volume ratio of three to one. When we drained the last permeate tank a few days after paint fill due to mechanical issues and re-filled it with DI water, the e-coat bath could not deposit enough film. Draining that 25 percent of permeate with the low-molecular-weight materials almost stopped the bath’s ability to coat. The only way to recover a bath that has been depleted of low-molecular-weight resin is to add more resin and operate temporarily with a higher percent of solids until the bath returns to expected film thickness and voltage. Adding new resin incorporates additional low-molecular-weight materials and gradually increases the ability to deposit film.