Paints ionisation

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). 

Kittelberger and Elm measured the rate of diffusion of sodium chloride through a number of paint films. Calculations based on their results showed clearly that the rate of diffusion of ions was very much smaller than the rate of diffusion of either water or oxygen. Furthermore, they found that there was a linear relationship between the rate of diffusion and the reciprocal of the resistance of the film. This relationship suggests that the sodium chloride diffused through the membrane as ions and not as ion pairs, since the diffusion through the film of un-ionised material would not affect the resistance, because if a current is to flow, either ions of similar charge... 

Apart from paints, electrokinetic separations find limited application for synthetic polymers [905], mainly because of solvent compatibility (CE is mostly an aqueous technique) and competition of SEC (reproducibility). Reasons in favour of the use of CE-like methods for polymer analysis are speed, sample throughput and low solvent consumption. Nevertheless, CE provides some interesting possibilities for polymer separation. Electrokinetic methods have been developed based on differences in ionisation, degree of interaction with solvent constituents, and molecular size and conformation. 

J.J. Boon, and T. Learner, Analytical Mass Spectrometry of Artists Acrylic Emulsion Paints by Direct Temperature Resolved Mass Spectrometry and Laser Desorption Ionisation Mass Spectrometry, J. Anal. Appl. Pyrol., 64, 327 344 (2002). 

The electrocoat bath consists of 80-90% de-ionised water and 10-20% paint solids. The water acts as the carrier for the paint solids which are constantly agitated. Paint solids consist of resin and pigment. The resin, which is an epoxy or acrylic-based thermoset, is the backbone of the final paint film and provides the corrosion protection, durability and toughness of the finish. Pigments are used to provide the colour and gloss. 

Boon, J.J., Learner,T. (2002) Analytical mass spectrometry of artists acryhc emulsion paints by direct temperature resolved mass spectrometry and laser desorption ionisation mass spectrometry. /. Anal. Appl. Pyrolysis, 64, 327. 

Figure 9.12 Gas chromatograms of compounds volatilised during curing of amide paint, (a) flame ionisation detector (b) Nitrogen specific detector. Compounds as tested in Table 9.17. Gas chromatograms of compounds volatilised during curing of anhydride paint (c) Flame ionisation detector (d) Nitrogen specific detector. Compounds are listed in Table 9.18.

---