An Introduction to Ultrafiltration Electrodeposition Paint

WHAT:
In the context of electrodeposition paint, ultrafiltration is a process for separating a portion of the low molecular weight component of the paint bath from the main paint stream. This low molecular weight component - referred to as PERMEATE - generally consists of low molecular weight resin, water, solvent, solubilizer and dissolved salts that accumulate as a result of pretreatment.

WHY:
The main purpose for the process is to provide a medium for rinsing parts after electrocoating that will:

1. Allow for the recovery of dragged out paint without affecting the chemistry of the paint bath.
2. Provide an adequate rinse so that the surface finish is acceptable either "as is", or for finish painting*.
3. Provide a cost effective method to remove dissolved salts from the paint bath for conductivity control.

Using ultrafilter permeate for rinsing, all three of the objectives can be met. By rinsing the part with permeate and counterflowing the rinse back to the paint tank, rinsed paint solids migrate back to the paint tank. Since the rinse medium is an actual component of the paint tank, no compromise in paint bath chemistry occurs. The paint recovery effectiveness and the finish on the rinsed part are related to the concentration of paint solids in the rinse tank(s). The concentration of paint solids in the rinse tank(s) is directly related to the volume of permeate produced. Therefore, the more permeate produced, the higher the paint recovery and the better the quality of the surface finish on the rinsed part. A cost effective method of removing dissolved salts from the paint bath is to put a portion of the permeate to drain. By doing this, salt is removed, conductivity is controlled and the only losses are 0.2% - 0.3% of low molecular weight resins.

NOTE:
Although some electrocoat paint systems use ultrafilter permeate as a final rinse, it is still an accepted procedure to follow the permeate rinse stage(s) with one or more deionized water rinses.

HOW:
Ultrafiltration is, in its simplest terms, a filtration method that utilizes the concept, of cross-flow as opposed to conventional through-flow (bag & cartridge type) filtration methods. The heart of the system is a thin, semi-permeable, polymeric membrane that selectively retains paint solids and large molecular weight materials. Simultaneously it allows water, solvent, dissolved salts and other low molecular weight materials to pass. Also, since the feed flow is across rather than through the membrane, velocity and turbulence of the feed stream aids in keeping its surface clean.