subject: Apparatus And Process For Forming A Powder Coating [print this page] An apparatus and process for forming a coating on a substrate, including establishing a fluidised-bed having a fluidising gas and a powder coating composition, immersing the substrate wholly or partly in the fluidised bed, the substrate being either electrically isolated or earthed, providing an electrically conductive electrode, to which a voltage is applied, positioned to influence the extent to which charged particles adhere to a region of the substrate, withdrawing the substrate from the fluidised-bed and forming the adherent particles into a continuous coating over at least part of the substrates. The apparatus is arranged and the process is conducted such that the maximum potential gradient existing in the fluidised bed is below the ionisation potential of the fluidising gas.
. A process for forming a coating on a substrate, including the steps of: establishing a fluidised bed comprising a fluidising gas and a powder coating composition, thereby effecting tribostatic charging of the powder coating composition,immersing the substrate wholly or partly in the fluidised bed, whereby tribostatically charged particles of the powder coating composition adhere to the substrate, the substrate being either electrically isolated or earthed, providing an electricallyconductive electrode in the fluidised bed, applying a voltage to the electrically conductive electrode, the electrode being positioned, in relation to the substrate, where the extent to which charged particles adhere to regions of the substrate isinfluenced by the electrode, withdrawing the substrate from the fluidised-bed and forming the adherent particles into a continuous coating over at least part of the substrate, the process being conducted such that the maximum potential gradient existingin the fluidised bed is below the ionisation potential of the fluidising gas.
Powder coatings are solid compositions which are usually applied by an electrostatic application process in which the powder coating particles are electrostatically charged and caused to adhere to a substrate which is usually metallic andelectrically earthed. The charging of the powder coating particles is usually achieved by interaction of the particles with ionised air (corona charging) or by friction (triboelectric, tribostatic or "tribo" charging) employing a spray gun. The chargedparticles are transported in air towards the substrate and their final deposition is influenced, inter alia, by the electric field lines that are generated between the spray gun and the substrate.
