Electrophoretic Deposition - Process of Electrophoretic Painting

Process of Electrophoretic Painting

The overall industrial process of electrophoretic deposition consists of several sub-processes:

1. The object to be coated needs to be prepared for coating. This normally consists of some kind of cleaning process and may include the application of a conversion coating, typically an inorganic phosphate coating.
2. The coating process itself. This normally involves submerging the part into a container or vessel which holds the coating bath or solution and applying direct current electricity through the EPD bath using electrodes. Typically voltages of 25 - 400 volts DC are used in electrocoating or electrophoretic painting applications. The object to be coated is one of the electrodes, and a set of "counter-electrodes" are used to complete the circuit.
3. After deposition, the object is normally rinsed to remove the undeposited bath. The rinsing process may utilize an ultrafilter to dewater a portion of the bath from the coating vessel to be used as rinse material. If an ultrafilter is used, all of the rinsed off materials can be returned to the coating vessel, allowing for high utilization efficiency of the coating materials, as well as reducing the amount of waste discharged into the environment.
4. A baking or curing process is normally used following the rinse. This will crosslink the polymer and allows the coating, which will be porous due to the evolution of gas during the deposition process, to flow out and become smooth and continuous.

During the EPD process itself, direct current is applied to a solution of polymers with ionizable groups or a colloidal suspension of polymers with ionizable groups which may also incorporate solid materials such a pigments and fillers. The ionizable groups incorporated into the polymer are formed by the reaction of an acid and a base to form a salt. The particular charge, positive or negative, which is imparted to the polymer depends on the chemical nature of the ionizable group. If the ionizable groups on the polymer are acids, the polymer will carry a negative charge when salted with a base. If the ionizable groups on the polymer are bases, the polymer will carry a positive charge when salted with an acid.

There are two types of EPD processes, anodic and cathodic. In the anodic process, negatively charged material is deposited on the positively charged electrode, or anode. In the cathodic process, positively charged material is deposited on the negatively charged electrode, or cathode.

When an electric field is applied, all of the charged species migrate by the process of electrophoresis towards the electrode with the opposite charge. There are several mechanisms by which material can be deposited on the electrode:

1. Charge destruction and the resultant decrease in solubility.
2. Concentration coagulation.
3. Salting out.

The primary electrochemical process which occurs during aqueous electrodeposition is the electrolysis of water. This can be shown by the following two half reactions which occur at the two electrodes:

Anode: 2H₂O ---> O₂(gas) + 4H(+) + 4e(-)

Cathode: 4H₂O + 4e(-) ---> 4OH(-) + 2H₂(gas)

In anodic deposition, the material being deposited will have salts of an acid as the charge bearing group. These negatively charged anions react with the positively charged hydrogen ions (protons) which are being produced at the anode by the electrolysis of water to reform the original acid. The fully protonated acid carries no charge (charge destruction) and is less soluble in water, and may precipitate out of the water onto the anode.

The analogous situation occurs in cathodic deposition except that the material being deposited will have salts of a base as the charge bearing group. If the salt of the base has been formed by protonation of the base, the protonated base will react with the hydroxyl ions being formed by electrolysis of water to yield the neutral charged base (again charge destruction) and water. The uncharged polymer is less soluble in water than it was when was charged, and precipitation onto the cathode occurs.

Onium salts, which have been used in the cathodic process, are not protonated bases and do not deposit by the mechanism of charge destruction. These type of materials can be deposited on the cathode by concentration coagulation and salting out. As the colloidal particles reach the solid object to be coated, they become squeezed together, and the water in the interstices is forced out. As the individual micelles are squeezed, they collapse to form increasingly larger micelles. Colloidal stability is inversely proportional to the size of the micelle, so as the micelles get bigger, they become less and less stable until they precipitate from solution onto the object to be coated. As more and more charged groups are concentrated into a smaller volume, this increases the ionic strength of the medium, which also assists in precipitating the materials out of solution. Both of these processes are occurring simultaneously and both contribute to the deposition of material.