Anode Membranes
How are anode membranes made and what are basic characteristics a shop should look for?
Q. We have very old, flat anodes, and we have replaced the membranes many times because of holes and tears. The last time they were replaced, our maintenance people changed them with materials that we can no longer find a replacement for. Can you tell us how membranes are made, and what are the basic membrane specifications or characteristics we should ask other vendors for? —G.T.
A. Anolyte system membranes used in the electrocoat industry are typically thin, backing nets of thermoplastic materials like polyethylene or polypropylene that are impregnated with binders and ion-selective resin beads, creating a cloth-type composite material or membrane. These membranes can be classified as anodic or cathodic depending on the type of ionic selectivity of the resin beads. The membranes are typically made by the roll or the press processes with very similar chemical properties, but with different physical properties and durability.
The type of anode membranes needed depends on the type of electrocoat system you have. For cathodic electrocoat systems, the ion-exchange membranes typically used are anodic. For anodic electrocoat systems, the membranes typically used are cathodic.
Regardless of the ionic selectivity, all electrocoat-compatible membranes should have a minimum thickness of 18 mils or provide a minimum tensile strength of 14 Kg/cm2 to avoid easy tearing under operating and maintenance operations. Additionally, the membranes must have a maximum electrical resistivity of 20 ohms/cm to provide minimal resistance to current flow and provide lower voltage drops from anodes to electrocoat paint.
The ionic membranes must also have a minimum of 96 percent permeate ionic selectivity to allow the acids released from the cathodic electrocoat paint during coating to be separated efficiently from the electrocoat bath through the anolyte system. By dumping controlled amounts of anolyte solution to drain, the electrocoat system can maintain a stable pH.
Lastly, the membrane must be temperature-resistant to 200°F to endure potential high anode temperatures during intense and prolonged periods of operation.
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