Anodic electropainting

After rinsing and dry-off, the primer is applied. In most modern plants this means electrodeposition of the primer (Section 14.1). The most widely used primers are cathodic. The body shell is made the cathode and current flows between it and inert anodes in the electropaint bath. The paint is formulated so that the resin is basic and, when neutralised with an acid such as lactic acid, becomes positively charged. The most widely used resins are epoxy-amine adducts ... 

The development of new paint technology has mandated further improvements in the performance of non-chrome rinses. Greater use of a final water rinse in treatment applications has taken place particularly with the advent of anodic and cathodic electropainting which require the use of a deionized rinse to prevent paint contamination. High solids paints which tend to be more... 

Commercial electropainting only dates from the early 1960s and the first processes to be introduced used anodic deposition. Some typical paint formulations would contain (i) polycarboxylic acids based on acrylic acid as monomer solubilized by an organic amine, (ii) alkyds, i.e. branched polyesters based on naturally occurring long-chain carboxylic acids and polyalcohols, e.g. glycerol, and (iii) epoxy resins based on phenols, e.g. 

Fig. 8.24 The principles of cathodic electropainting, (a) A divided cell, (b) Detail of the anode box. (c) A general arrangement, showing the electrical connections. (Courtesy ICI Ltd, Paints Division.)...

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. 

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