Layered withdrawal coating

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 IRE of the Prism Cell was coated with a layer of hydrocarbon "model soil" in the same way as was done in earlier work with the CIRCLE (5.12). The IRE is withdrawn mechanically from the solution of the hydrocarbon of interest (2-4 wt.% in hexane) at a slow, controlled rate. The hexane flashes off, leaving a layer of hydrocarbon behind. By varying the withdrawal rate and the concentration of the hydrocarbon, layers of varying thicknesses, as judged by the intensity of the bands of the hydrocarbon spectrum, can be obtained. 

You are now to apply the thin-layer technique to a group of colorless compounds. The spots may be visualized under an ultraviolet light if the plates have been coated with a fluorescent indicator, or chromatograms may be developed in a 4-oz bottle containing crystals of iodine. During development, spots appear rapidly, but remember that they cilso disappear rapidly. Therefore, outline each spot with a pencil immediately on withdrawal of the plate from the iodine chamber. Solvents suggested are as follows ... 

Film-coating is the formation of an adhering dispersion layer as a result of the drag force exerted by the substrate during withdrawal from a dispersion. To achieve pure fihn-coating, the capillary suction of the porous substrate must be suppressed. The thickness of the entrained dispersion layer increases with increasing withdrawal speed and increasing dispersion viscosity. The contact... 

Figure 6.10 shows the layer thickness as a function of withdrawal speed for film-coating and capillary filtration as the main coating mechanisms. The decrease for capillary filtration is caused by the decreasing contact time with increasing withdrawal speed. 

Fig. 6.52. Log-log plot of mean layer thickness versus withdrawal speed for film-coating of strongly agglomerated (pH = pH (i.e.p.) = 8) and stable alumina suspensions (pH = 4).

Fig. 8.1, The film-coating process. (A) stages of the process. (B) Detail of the liquid flow patterns. U is the withdrawal speed, h is the thickness of the fluid film, S is the boimdaiy layer. From Scriven [8].

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