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A humidified fine powder might cause formations of micro-agglomerations that aggregated by small particles, which will lead to increased coating roughness known as “orange peel”. In addition, micro-agglomerations could also cause coating defects. Therefore, further observations of the pre- and post-humidified powder in a micro scale were necessary.

The SEM (Scanning Electron Microscope) images in Figure 6-12 indicate that the particles of both powders were not aggregated after fluidization with humidified air and the micro-agglomerations were not found. The pre- and post-humidified powders were then sprayed on the sample panels and the coating qualities were inspected. By comparing the cured panels between pre- and post- humidified powders, there were no powder balls, seeds or pin hole observed by the necked eyes.

a. Pre-humidified particles b. Post-humidified particles

Powder C1

c. Pre-humidified particles d. Post-humidified particles

Powder A

The sample panel were also inspected under SEM. The coating surfaces from a high magnification are showed in Figure 6-13 (the cutting line of the coating film was included in the image for better focus). As observed, all the coating surfaces had small bits with size under 10 µm which were normally from fillers or additives but there was no sign of micro-agglomerations. The coatings obtained from the post-humidified powders were smooth and there was no surface defect of the coating.

a. coating surface of the pre-humidified b. coating surface of the post-humidified

Powder C1

c. coating surface of the pre-humidified d. coating surface of the post-humidified

Powder A

Based on the results, it can be concluded that the powders humidified by the fluidized bed would not cause micro-agglomerations and the coating quality of the humidified powders would not be affected.

6.4 Conclusion

This study revealed that the moisture content on the particle surfaces had significant influence on the resistivity of the fine powders. Additional moisture on the particle surfaces could reduce the resistivity of a fine powder. Because of that, the back ionization was delayed during corona charge spraying so that the FPTE could be improved. The resistivity of a regular size powder was much less influenced by the moisture on the particles. Therefore, the increased moisture content on the particle surfaces could not significantly improve the FPTE of a regular size powder.

The study also disclosed that the humidification on the powder would not cause serious deterioration in powder flowability if the recommended protocol was followed and the additional moisture on the particles would not cause adverse effects on coating quality.

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CHAPTER 7: IMPROVEMENT ON THE FARADAY CAGE