The Electrodeposition Process in Practice

Polymers for electrodeposition are formulated as polyelectrolytes. That is to say that they are made containing a proportion of functional groups which, when reacted with an acid or a base, as appropriate, will have a multiplicity of ionic groups along the chain. These ionic groups make the polymer compatible with water. In some cases the polymer is essentially dissolved in the water, and the solution is clear. In other cases a proportion of a suitable organic solvent is needed to produce a clear solution, and in still other cases the polymer is there essentially as a dispersion and the solution is opaque. These opaque solutions perform perfectly well and a clear solution is not necessary for electrodeposition (or for any other aqueous paint system). 

The polymers can be divided into two types, depending on which electrode they will deposit onto. Thus resins which deposit on the anode are known as anodic resins, and those which deposit on the cathode are known as cathodic resins. 

Anodic resins are those which contain acid groups along the chain and which are made water dispersible by the addition of a base. The acid-base reaction produces carboxyl ions (COO ) along the chain, stabilising the aqueous dispersion. 

As can be deduced from the discussion in the previous section, the processes that occur during electropainting give rise to an accumulation of hydrogen ions at the anode and hydroxyl ions at the cathode. If a polymer which is stabilised by the existence of carboxyl ions along the chain approached the anode, the carboxyl ions will be neutralised and the polymer will precipitate out of solution onto the anode. Similarly, if a polymer stabilised by acid-neutralised amino groups approaches the cathode, then this will be precipitated out by the hydroxyl ions. 

A certain minimum concentration of the hydrogen of hydroxyl ions is necessary to actually precipitate the resin, which means that it does not begin to precipitate until a certain minimum amount of electricity has passed. All the time the current is passing the ions are diMising away from the electrode by normal diffusion processes, so if the current passing is not high enough to overcome this, the resin does not precipitate - it has been found experimentally that there is a minimum current below which no film is formed. 

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