Electrostatic Deposition of Powder Coatings

The electrostatic deposition of pol3uner powder coatings includes the following stages  

The dielectric polymer particles can be electrostatically charged in the outer electric field, in the gas discharge, following the contact electrification mechanism, or through triboelectrization. The presence of Cl, whose electric conductivity differs from that of the polymer, in the powder composition considerably affects these mechanisms. 

The formation of a layer of electrically deposited particles on the surface of a cold article includes a number of factors. The charged dielectric particle contacting the substrate experiences (a) the forces of molecular interactions on the sites of actual contacts (b) the forces of electrostatic attraction of the particle to the substrate, and (c) Coulomb forces of interactions with neighboring particles. The formation mechanism of electrically deposited layer can be presented as a two-phase process during which a monolayer of particles whose interactions can be neglected is generated followed by a build-up of this layer. 

The thickness and density of electrically deposited layers are a function of the dielectric properties of the powder composition and the process parameters. Common to all electrostatic deposition techniques of polymer powders is the existence of some critical thickness of the deposited layer. As soon as this is reached, no particles are deposited any more because of the Coulomb repulsion of the approaching unipolar charged particles by the layer. An effect called inverse corona appears in the layer, which means that voltage breakdown occurs in it as a result of raised field intensity, which ionizes the air between the particles, and a flow of ions is generated from the substrate with a charge opposite to that of the particles being deposited. This abruptly slows down the deposition and leads to uneven thickness of the deposited layer, formation of numerous craters and through pores. 

In Fig. 3.33, comparative information is presented on the adhesive strength of polymer powder coatings formed on aluminum substrates by the electrostatic method under an external field of 2.5-3 kV/cm (the charge density in the layer was q/m 3 10 C/kg) and thermal treatment of a similar thickness poured layer of uncharged particles [52]. 

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