Automotive Coatings Formulation: Chemistry, Physics und Practices

Electrode reactions
As already mentioned, the charged particles move in the direct current towards the anode. At the 
anode, electrolysis splits the water into protons and into molecular oxygen, which escapes from 
the tank. The protons discharge the carboxylate anions from the neutralised resin into carboxyl 
groups. As the carboxyl groups are not polar enough to serve as carrier in aqueous phase, the 
resin coagulates and is deposited on the surface of the object to form a film. There is the additional 
possibility that iron from the object will be oxidised to iron(II) ions, which may react with the ions 
of resins to form insoluble iron salts which become incorporated into the film matrix.
At the cathode, electrolysis yields molecular hydrogen and hydroxyl ions by reduction of water. 
The hydroxyl ions form partner ions for the ammonium ions of the neutralisation agent that 
also move towards the cathode. Iron ions are reduced to iron metal. Without any compensation 
measures, the quantity of amine in the tank would rise continuously. The electrode reactions are 
shown in Figure 3.4.3.
Properties of anodic electro deposition primers
Anodic electro deposition primers have excellent wetting power for pigments and surfaces and 
good levelling properties because the oil molecules have long aliphatic chains. However, their 
corrosion resistance is not optimal. The reason is mainly that iron from the object is oxidised to 
iron(II) ions at the anode, which are incorporated into the film matrix and thus constitute a weak 
point in the corrosion resistance. 
Figure 3.4.3: Electrode reactions in anodic electro deposition
Automotive OEM coatings

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Cathodic electro deposition primers
Cathodic electro deposition primers were introduced in the mid-1970s in an attempt to greatly 
improve the corrosion resistance. These primers offered much better corrosion resistance, 
improved throwing power, and better crosslinking efficiency. The ingredients of cathodic electro 
deposition primers were tailored systematically to the application requirements and conditions 
(see Chapter 3.4.5) 
[58]
: 
• 
Aromatic epoxy resins, the main component, offered optimum corrosion resistance and adhesion
• 
Plasticising compounds guaranteed optimum levelling and adequate film flexibility
• 
Addition of amines to epoxy groups solubilise and functionalise the colloidal dispersion
• 
Evaporable organic acids to neutralise the amines 
• 
Blocked polyisocyanates, whose reaction is catalysed by the basic behaviour of the amines in 
epoxy adducts, to serve as crosslinkers 
• 
Molecular network containing urethane groups after crosslinking to support primer adhesion
Alternative developments aimed at introducing cationic carrier groups into epoxy resins 
[59]
, but 
they failed to gain acceptance in automotive electro deposition primers, although they enjoyed some 
use in general industrial coatings. There were also further developments in the aforementioned 
cathodic electro deposition primers. Hardly any other field of coating chemistry has witnessed 
so many patent applications as that of electro deposition primers. It is all the more remarkable, 
then, that current cathodic electro deposition primers are still based on the same chemistry 
as the first commercial systems dating from the 1970s. The following advances were made:
• 
optimisation of application behaviour
– variation of film layer thickness (e.g. thick coating systems)
– improvement in throughput speed
– optimisation of throwing power
– improvement in edge covering
• 
reduction in emissions of volatile organic compounds (VOCs, co-solvents)
• 
abandonment of harmful pigments (e.g. lead pigments)
Current developments are focusing on:
• 
reducing the stoving temperatures (low-stove systems)
• 
replacing the tin catalysts (by more harmless compounds)
• 
improving the weatherability (particularly for combinations involving effect topcoats but not 
primer surfacers)
In the past, changes to the application process sought to improve the properties of the entire car-
body coating system. The goals were:
• 
an increase in the layer thickness to eliminate the primer surfacer
• 
optimum edge covering
• 
improved throwing power
The methods used to achieve these goals were the:
• 
reverse process
• 
electro deposition of a powder slurry (EPC or electro powder coating)
• 
use of two dipping steps
• 
use of nonionic electrophoresis
• 
studies of autophoresis
Pre-treatment and primers

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