Paints, electrodeposition

Dip coating is one of the simplest and oldest coating methods. In addition to dipping in solvent- or waterborne paints, electrodeposition has become important for large-scale series production (Table 8.5). 

Anodic deposition is controlled by either fluid shear (cross-flow filtration) (48), similar to gel-polarization control, or by continual anode replacement (electrodeposited paints) (46). High fluid shear rates can cause deviations from theory when E > (49). The EUF efficiency drops rapidly... 

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... 

Recovery of paint from watersoluble coat bases (primers) applied by the wet electrodeposition process (electrocoating) in auto and appliance factories. 

Paints are specially formulated for one of two processes. In the first, anodic electrodeposition, the article to be painted is made the anode in a d.c. circuit. Cathodic electrodeposition is the reverse operation. The process takes place under carefully controlled conditions in a tank. Current passes through the paint causing it to deposit uniformly over the article in a uniform film whose thickness can be accurately controlled. The paint film is insoluble in water and exhibits remarkable adhesion after stoving. Electrodeposition is widely used to apply primers, e.g. by car manufacturers, and one coat finishes. 

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). 

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. 

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 ... 

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). 

Folonari, C. V. and Garlasco, R., Ion-paired HPLC characterization of cathodic electrodeposition paint polymers, /. Chromatogr. Sci., 19, 639, 1981. 

Beck, F "Fundamental Aspects of Electrodeposition of Paint", in "Progress in Organic Coatings", 1976,4,1 Elsevier Sequiola S. A., Lasagna. 

All paints were electrodeposited on SAE 1010 bare steel paint test panels to yield 20fim dry film thickness, and were baked to yield fully crosslinked, solvent resistant films. 

ELECTRODEPOSITION. The precipitation of a material at an electrode as the result of the passage of an electric current through a solution or suspension of the material, for example, alkaline-earth carbonates, rubber from latex, paint films on metal. A technique for electrodepositing refractory carbide coatings on metal has been reported. The electrode is in ihe shape of the desired article. An important advantage or electrodeposition is its ability lo coat complex shapes having small and irregular cavities with exact thickness control. 

PROTECTIVE COATING. A film or thin layer of metal glass of paint applied to a substrate primarily to inhibit corrosion, and secondarily for decorative purposes, Metals such as nickel, chromium, copper, and tin are electrodeposited on the base metal paints may be sprayed or brushed on. Vitreous enamel coatings are also used these require baking. Zinc coating are applied by continuous bath process in which a strip of ferrous metal is passed through molten zinc. 

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. 

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. 

Resource recovery—recovery of inks, textile sizing agents, and electrodeposition paints. 

The use of electrodeposition to apply paint is a comparatively recent innovation. The history of this development has been detailed by Brewer (2, ). The electrodeposition of paint, or electrocoating as it is commonly labeled, is derived from the old rubber latex technology and has many features in common but it differs in one important respect-- it provides rapid and complete current cutoff. 

Reiter, S. Ruhlig, D. Ngounou, B. Neugebauer, S. Janiak, S. Vilkanauskyte, A. Erichsen, T. Schuhmann, W., An electrochemical robotic system for the optimization of amperometric glucose biosensors based on a library of cathodic electrodeposition paints, Macromol. Rapid Commun. 2004, 25, 348-354... 

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