Electrodeposition paints applications

Electrodeposition This method of paint application is basically a dipping process. The paint is water-based and is either an emulsion or a stabilised dispersion. The solids of the paint are usually very low and the viscosity lower than that used in conventional dipping. The workpiece is made one electrode, usually the cathode, in a d.c. circuit and the anode can be either the tank itself or suitably sized electrodes sited to give optimum coating conditions. The current is applied for a few minutes and after withdrawal and draining the article is rinsed with de-ionised water to remove the thin layer of dipped paint. The deposited film is firmly adherent and contains a minimum of water and can be stoved without any flash-off period. This process is used for metal fabrications, notably car bodies. Complete coverage of inaccessible areas can be achieved and the corrosion resistance of the coating is excellent (Fig. 14.1). 

The field of application for GDOES is very broad and includes surface treatment studies of samples prepared by different techniques such as galvanization, nitriding, carbonitriding, carburization, diffusion, chemical and thermochemical treatments, thermic treatments, PVD and CVD coating, electrodeposition, painting, and semiconductor multilayer growth. 

Waterborne paints provided the technological basis for electrodeposition paints (Section 3.8), in which negatively charged paint particles (anaphoresis, industrial introduction at the beginning of the 1960s), or positively charged paint particles (cataphoresis, industrial introduction at the end of the 1970s) are deposited from aqueous solution onto metallic substrates by application of an electrical field [3.40]. 

Table 3.11. Typical bath and application data of electrodeposition paints...

Uses. Electrodeposition coatings are used primarily for corrosion protection of steel, galvanized steel, and aluminum (e.g., in automobiles, agricultural machinery, steel furniture, and appliances). New areas of application include coil coating primers [3.127], beverage can coatings [3.128], and photocurable electrodeposition paints as photoresists [3.129]. 

Anticorrosive Primers. Anticorrosive primers are applied in dip tanks so that they reach all parts of the car body dipping is a fast method of application. The standard method for application of primers is electrodeposition. Anodic electrodeposition paints were used when the electrocoating technique was first applied, but cathodic electrodeposition is now predominant because it provides better corrosion protection. 

One of the main differences of electrodeposition paints with conventional water soluble paints is their lower solids and thus solvent content. A typical binder content is around 10%w, the amount of solvent approximately 5%. The rest, apart from pigmentation, is water. The influence of solvent in the early stages of binder/paint formulation is very similar to the effects described for conventional aqueous paints which is also started from an approx. 70% solids binder solution in coupling solvent(s). The choice of the solvent (blend) is, however, less influenced by its evaporation characteristics as the deposited paint film does not contain much water and is stoved after application. Of more importance are paint stability and electrical properties (conductivity, rupture voltage). 

From the above it will be clear that butoxyethanol is often the preferred single solvent, not only in normal aqueous but also in electrodeposition paints. However, for the latter application also other solvent systems are used, for instance blends of ethoxyethanol/butoxyethanol with hexoxyethanol and ethoxyethanolacetate as film coalescent in ratios of approx. 1/1/1. Sometimes propoxyethanol is used instead of ethoxyethanol and in some formulations 10-15% of the solvent blend is isopropanol, which is added because of its volatility and to optimise the partitioning of the solvent blend over the aqueous and organic microphase of the electrodeposition paint. 

Electrodeposition (1882) vt. Method of paint application in which an article to be coated which is an electrical conductor is made one of the electrodes in a tank of water-thinned paint. The other electrode is generally a metal such as copper. The two electrodes are connected to a source of electric power, the polarity of the article to be coated being of the opposite sign to that on the particles in the liquid paint in the tank. The charged particles move towards the articles under the influence of the electric field, and when they give up their charge at the electrode (article) they are deposited and ultimately form a continuous film of paint. Weismantal GF (1981) Paint handbook. McGraw-Hill Corporation Inc., New York. 

The largest industrial use of ultrafiltration is the recovery of paint from water-soluble coat bases (primers) applied by the wet electrodeposition process (electrocoating) in auto and appliance factories. Many installations of this type are operating around the world. The recovery of proteins in cheese whey (a waste from cheese processing) for dairy applications is the second largest application, where a... 

A matter of considerable importance in the selection of an application method is its efficiency. Spray techniques are usually inefficient, since many droplets drift past the target and are lost. Even electrostatic spraying can waste as much as 35% of the paint. There is some loss of paint in most methods, but roller coating, curtain coating and electrodeposition are very efficient. Electrodeposition is also a very useful technique where corrosion resistance is important, since it applies a uniform coating over nearly all surfaces of even the most complex-shaped article. 

Applications Electrodeposition of cationic paint resin on automobiles (connected to the cathode) provides a uniform, defect-free coating with high corrosion resistance, but carries with it about 50 percent excess paint that must be washed off. UF is used to maintain the paint concentration in the paint bath while generating a permeate that is used for washing. The spent wash is fed back into the paint path (Zeman et al., Microjiltration and UltrajUtration, Marcel Dekker, New York, 1996). 

Many of the finishes applied to other types of metal products can also be applied to zinc die castings, although some differences in formulation as well as occasional differences in method of application may be desirable. The types of finishes applicable to zinc die castings include mechanical finishes (buffed, polished, brushed, and tumbled) electrodeposited finishes (copper, nickel, chromium, brass, silver, and black nickel) chemical finishes (chromale, phosphate, molybdate and black nickel) and organic finishes (enamel, lacquer, paint and varnish, and plastic finishes). Electrodeposited coatings of virtually any metal capable of electrodeposition can be applied to zinc die castings. 

A recent development is the use of electrodeposition of paints, which has found wide application in the automobile industry because of the ability to obtain uniform deposition on and in complex geometrical structures. In this, charged particles are caused to move toward the metal surface under the influence of an electric field and to flocculate on meeting the surface. 

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