Electrophoretic painting

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. 

The cathodic process is, as yet, only used by a few US companies. There is no possibility that the electrode reaction will lead to corrosion and the throwing power of this version is even better than the anodic process. However, the paint formulations which are currently employed are complex and require some organic solvent. Moreover, the adhesion of the deposit requires improvement. Hence, although it would seem that in the long term cathodic processes will be expected to predominate, further development is necessary. 

On the other hand, it is obviously only possible to electropaint conducting surfaces and hence the process is limited to a single coat. Also, the colour range is more restricted than with conventional painting. 

As in electroplating, the electrolysis step in electrophoretic painting is only one of a complex sequence which makes up the total surface protection sequence. Thus, for example, the steel in a car body will be treated as follows  

Harrison, J. A. and Thirsk, H. R. (1972) A Guide to the Study of Electrode Kinetics, Academic Press, London. 

Application areas for chromate coatings include the following  

AJuminium or nuip estum-diumtniLjm alloy castings and extrusions are dtromated to provide a decorative and protective finish, 

Zinc or cadmiunMleclroplated steel articles are frequently passivated with a chromate coating in order to enhance corrosion resstance, improve solder-ability and provide a pleasing e.g. iridescent) finish. 

When the substrate is conducting, electrophoretic deposition of paint may be carried out. Indeed, in the case of steel automobile bodies, almost all factory-based primer coating is performed by electrophoretic painting. 

Migration of the charged micelles to the electrode of opposite polarity. 

Aluminium strip is chromated in order to provide an intermediate, adhesion-promoting coating prior to painting and lacquering in the canning industry. 

Paint coatings are perhaps the most versatile and widely applied method of corrosion protection, offering a wide range of decorative colours and textures and the ability to coat complex structures. While the formulation and chemistry of paints is complex, the cured paint film consists, in essence, of a cross-linked organic polymer film (or binder ) containing a dispersed filler (pigment) together with additional solids and inhibitors. A broad spectrum of application methods is used but the vast majority of paint is applied by brush, spray or roller. 

---

Suspensions of oil in water (32), such as lanolin in wool (qv) scouring effluents, are stabilized with emulsifiers to prevent the oil phase from adsorbing onto the membrane. Polymer latices and electrophoretic paint dispersions are stabilized using surface-active agents to reduce particle agglomeration in the gel-polarization layer. 

Electroultrafiltration has been demonstrated on clay suspensions, electrophoretic paints, protein solutions, oil—water emulsions, and a variety of other materials. Flux improvement is proportional to the appHed electric field E up to some field strength E where particle movement away from the membrane is equal to the Hquid flow toward the membrane. There is no gel-polarization layer and (in theory) flux equals the theoretical permeate flux. It... 

Process Objective UF is used for three principle objectives. First, to fractionate, to pass selectively one component through the membrane with the solvent. Second, to concentrate, to pass the solvent. These two, while different, are related and it is common to purify and concentrate a component siiTuiltaneously. The third objective, quite different, is to produce a solvent stream as a product. An example is the operation of an ultrafilter for producing low-cost permeate. An important apphcation of UF is in the automotive industiy where UF is used to remove water and microsolutes from huge electrophoretic paint tanks for use in rinsing excess paint (dragout) from... 

Epoxy based primer systems remain the best suited for the corrosion protection of magnesium. Cathodic epoxy electrophoretic paints , chromate inhibited epoxy-polyamide primers and high temperature stoving epoxy sealers are used to provide protection up to 180°C. For higher temperature applications up to 300°C, epoxy silicone or polyimide based systems can be used. 

Phosphate solutions containing fluorides are used for processing steel, zinc and aluminium when assembled together, but chromate solutions are generally preferred when aluminium is treated alone. The increasing use of cathodic electrophoretic painting on steel, however, has led to a reassessment of the basic processes and formulations that might be most effective. 

In recent years there has been a great increase in the use of demineralised water for rinsing, especially before electrophoretic painting. The demineralised water is generally applied by misting jets at the end of all other pretreatment stages and allowed to flow back into the last rinse tank. 

It is proposed that the apparatus of Figure 2 would be a useful tool in monitoring ultrafiltration of latex and electrophoretic paints and in systems design (11). 

Electroultrafiltration has been demonstrated on clay suspensions, electrophoretic paints, protein solutions, oil-water emulsions, and a variety of other materials. 

Other important metal finishings to protect against corrosion are conversion coatings such as anodization (especially for aluminium), electroless plating, and electrophoretic painting. The first is done to form a passive layer, and is described in greater detail in Section 16.4. 

Much of metal electrodeposition is carried out with the aim of minimizing corrosion, the most common electrodeposits being tin, zinc, nickel and chromium on a cheaper metal substrate, such as iron. Since there is chemical bonding between substrate and electrodeposit, this is better than covering with paint (except electrophoretic painting, see Chapter 15) and additionally the surface generally becomes harder, as it does in nickel electroless plating. 

Cyclic derivatives of type III include cyclic Mannich bases, such as dihydroben-zoxazines 497, employed as detergents for lubricating oils, - and cyclic urcides 498, precursors of crosslinking agents for fabrics, as well as other cyclic derivatives prepared by conversion of Mannich bases. Macromolecular derivatives of type IV are relatively small in size and have branched (star-shaped) structures they are of considerable importance as, for example, corrosion inhibitors 499, plastics stabilizers 500, - pre-polymers for epoxy-based electrophoretic paints, and polyols in polyurethane synthesis. ... 

Surface Finishes. Of almost equal importance are surface finishes and surface coatings. For instance, E-coating (or electrophoretic painting) is an effective contaminant-containment for many machined metal surfaces such as aluminum and magnesium surfaces, and for magnets. Other surface treatments applied in past and current IBM disk drives are listed in Table A1. 

FIG. 1 Steps involved in constructing an SECM/AFM tip (a) Pt microwire inserted in holder (b) electrochemical etching (c) compression to form cantilever component (d) insulation of electrode and cantilever with electrophoretic paint followed by heating to expose tip (e) attachment to commercial AFM chip. (From Ref. 3.)... 

A major problem in automotive vehicles is corrosion control, particularly in the northern areas of the country where salts are used to melt road ice. Protection of exposed steel from corrosion is accomplished by electroplating with copper-nickel-chromium films, particularly on trim and finish moldings. Electrophoretic painting of body panels is used industry-wide, and electrogalvanizing to protect steel under paint is soon to become a standard practice for all vehicles. Electrochemical accelerated tests for the integrity of coatings and for the determination of the concentration of coolants are used extensively. 

Advances in Electrophoretic Painting" Chandler, R. H., Ed. Bl- or Triannual Abstracts since 1966 Braintree Essex, England. 

Electrophoretic paint industrial Bath control, production of rinse... 

There are important applications for which conversion must be low. A good example is the UF of electrophoretic paint. Tbe desired product is permente, which is required for rinsing newly painted objects for recovery of excess pa tat and for removal of electrolytes and contamiannis. Since flux, peini stability, and membrane cleanliness are affected adversely by rising coacentiation, the UF is run al as low a paint concentration as feasible, that is, at low conversion. In such a situation, yield is not a factor, and the absence of lenkage is (he dominant consideration. 

Unwin and coworkers [11] have developed a straightforward approach for fabricating platinum microelectrodes with tip sizes as small as a few nanometers. Their innovation is to use an electrophoretic paint to insulate the electrode walls while leaving an electrochemicaUy active tip exposed. These electrodes have excellent properties for use in scanning electrochemical microscopy (SECM), discussed further in Chapter 3.3. 

---