Electrophoretic deposition (EPD) is an industrial process in which colloidal particles suspended in a liquid medium migrate under the influence of an electric field and are deposited onto an electrode. Electrophoretic deposition is able to produce coatings of thickness ranging from a few micrometres to a few centimeters.

Process of electrophoretic deposition

In EPD, an electric field is applied to a colloidal suspension of charge particles, causing them to migrate towards an electrode. Such particles include any fine powder like solid less than 30 micrometres across such as polymers, metals and glasses. As the electric field is applied, corresponding pressure is also applied to colloidal particles against the electrode, however, it should be noted that in EPD, particle/electrode reactions are not involved and the coating is formed only due to the pressure exerted by the potential difference between the electrodes. Binders may also be applied to increase the strength of the deposited coating.

The process of EPD is described by Hamaker's equation which relates the instantaneous yield rate with the electric field.

Factors affecting EPD

There are two main solvents used in EPD, water and organic liquids. Organic liquids are usually preferred to water based solutions as in water based solutions, the resulting hydrolysis of water due to the high potential difference causes gas formation within the suspension.

The charge of the colloidal particle also plays a vital role in the electrophoretic mobility of the suspension, and this charge originates from various sources such as adsorbed inorganic ions or from dispersants.

The rate of deposition also varies greatly with the strength of the applied electric field with in turn is dependent on factors such as the resistance of the suspension as well as the resistance of the deposited coating. It is also important to consider that there is a critical limit to the potential difference below which insignificant rates of deposition are observed. Also, addition of conductive electrolytes may also ensure uniform coatings.

It should also be noted that both constant current and constant voltage EPD processes are viable and that high voltages in EPD are usually preferred as they ensure high rates of deposition although it also induces porosity in coatings.

Other factor affecting EPD include the size of the particles and whether the coating achieved is porous.

Uses of EPD

EPD can be used to produce uniform coatings which require high amounts of precision especially in cases where the coating is electrically insulative as one may design conditions in which the growth of the coating is self limiting. EPD is also used in the shaping of free standing objects and coating irregularly shaped objects due to its high throwing power.

See also

• Hamaker constant