Electrocoat paint

The electrocoat paint market constitutes the largest single application of UF in the world. Virtually every automobile plant in the world uses electrocoat for the undercoat and virtually every installation utilizes UF. There are over 1,000 UF units in the automotive industry alone-not to mention appliance manufacturing, metal furniture, coil coating, and other metal electrocoat operations. 

The electrocoat process for primer coating involves the electrophoretic deposition of charged colloidal resinous particles in aqueous dispersion onto a conductive substrate such as an automobile body. The process is universally favored because once the paint particles are deposited, they insulate the body from further deposition at that point. The impressed electric field thereby causes the migration of paint particles to uncoated areas. The result is an extremely uniform, coherent and defect free coating-even on sharp edges and in recessed areas inaccessible to other methods. 

The paint savings alone typically pay for the UF unit is less than six months. Additional savings result from reduced deionized water use, lowered waste treatment costs, and better control of bath composition. 

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A typical plot illustrating the slow decrease in flux that can result from consolidation of the secondary layer is shown in Figure 6.5 [14], The pure water flux of these membranes is approximately 50 gal/min but, on contact with an electrocoat paint solution containing 10-20% latex, the flux immediately falls to about 10-12 gal/min. This first drop in flux is due to the formation of the gel layer of latex particles on the membrane surface, as shown in Figure 6.4. Thereafter, the flux declines steadily over a 2-week period. This second drop in flux is caused by slow densification of the gel layer under the pressure of the... 

Figure 6.5 Ultrafiltration flux as a function of time of an electrocoat paint latex solution. Because of fouling, the flux declines over a period of days. Periodic cleaning is required to maintain high fluxes [14]. Reprinted from R. Walker, Recent Developments in Ultrafiltration of Electrocoat Paint, Electrocoat 82, 16 (1982) with permission from Gardner Publications, Inc., Cincinnati, OH...

Because of the challenging environment in which ultrafiltration membranes are operated and the regular cleaning cycles, membrane lifetime is significantly shorter than that of reverse osmosis membranes. Ultrafiltration module lifetimes are rarely more than 2-3 years, and modules may be replaced annually in cheese whey or electrocoat paint applications. In contrast, reverse osmosis membranes are normally not cleaned more than once or twice per year and can last 4-5 years. 

Figure 6.15 Effect of membrane surface charge on ultrafiltration flux decline. These membranes were used to ultrafilter cathodic electrocoat paint, which has a net negative charge. Electrostatic repulsion made the negatively charged membrane significantly more resistant to fouling than the similar positively charged membrane [13]...

The importance of membrane surface characteristics on performance is illustrated by Figure 6.15. The feed solution in this example was a cathodic electrocoat paint solution in which the paint particulates had a net positive charge. As a result, membrane flux declined rapidly with the positively charged membranes but much more slowly with essentially identical membranes that had been treated to give the surface a net negative charge [13]. 

Figure 6.21 Flow schematic of an electrocoat paint ultrafiltration system. The ultra-filtration system removes ionic impurities from the paint tank carried over from the chromate/phosphate cleaning steps and provides clean rinse water for the counter-current rinsing operation...

R. Walker, Recent Developments in Ultrafiltration of Electrocoat Paint, Electrocoat 82, 1 (1982). 

FIGURE 31 An ultrafiltration electrocoat paint recovery system. [Warashina et al. (1985, September). Chemtech, pp. 558-561.]... 

In other applications such as oil-water separation and electrocoat paint recovery, it may be necessary to adjust the feed pH to prevent precipitation or adsorption of certain species on the membrane surface or into the pores. 

Cross-flow filtration (CFF) also known as tangential flow filtration is not of recent origin. It began with the development of reverse osmosis (RO) more than three decades ago. Industrial RO processes include desalting of sea water and brackish water, and recovery and purification of some fermentation products. The cross-flow membrane filtration technique was next applied to the concentration and fractionation of macromolecules commonly recognized as ultrafiltration (UF) in the late 1960 s. Major UF applications include electrocoat paint recovery, enzyme and protein recovery and pyrogen removal. 

The largest current applications for UF (in terms of installed membrane area) were not envisioned in the early sixties (e.g., UF of electrocoat paint). Further, many of the projected applications have not yet materialized (e.g., fractionation of polymers and proteins). 

Figure 3.84 UF of electrocoat paint, single-rinse system.

Figure 3.86 UF of electrocoat paint-stability of anodic paint flux with XM-50 membrane.

Abcor (now a division of Koch Industries) installed the first industrial ultrafiltration plant to recover electrocoat paint from automobile paint shop rinse water in 1969. Shortly afterwards, systems were installed in the food industry for protein separation from milk whey and for apple juice clarification. The separation of oil emulsions from effluent wastewaters has also become a significant application. The current ultrafiltration market is approximately US 200 million/ year, but because the market is very fragmented no individual end-use segment is more than US 10-30 million/year. In the chemical and refining industries, the principal application of ultrafiltration is the treatment of oily wastewater. 

Fig. 31. Ultrafiltration flux as a function of time of an electrocoat paint latex solution. Because of fouling, the flux declines over a period of days. Periodic cleaning is required to maintain high fluxes.

Interweld gaps and narrow capillaries which are, by design, hardly accessible to electrocoat paint ... 

Basic lead silicochromate is composed of basic lead chromate and basic lead silicate firmly bound to a silica core. It was used in coatings for metal protection in structural steel, and finer grades were used in electrocoat paints. Because these pigments contain both lead and hexavalent chromium, due to toxicity and human health reasons, they have lost their commercial importance. 

LP Haack, JW Holubka. Bake oven induced variation of surface chemistry on electrocoat paint Effect on primer-electrocoat intercoat adhesion. J Coatings Technology 72(903) 35-44, 2000. 

Spiral-wound membrane modules, with hydrophilic membranes, are suited for water-based paint recovery in spray paint installations, and also for electrocoat paint filtration. Figure 4.32 shows a schematic for an ultrafiltration system for treating spray booth waste. 

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