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Ultrafine powder coatings have superior film performance but poor flowability due to the high interaction between particles, which can be improved by the addition of inorganic flow additives. However, the improvement is limited and more importantly, the film performance deteriorates because of their incompatibility with the organic coating resin. In this study, a resin encapsulation approach was proposed to modify the commercial inorganic flow additives for the improvement of powder flowability and maintaining film quality. The effects of resin content and additive dosage on the flowability characterized by angle of repose (AOR) and avalanche angle (AVA), and film performance characterized by gloss, seed density and surface roughness were systematically investigated in two powder coating systems. It was found that the encapsulation of additives with the same resin of the host powder coating can enhance both flowability and film properties, compared to commercial nano-silica. With the increase of resin content, both the AOR and AVA first decrease to a minimal value at resin content of 5 % to 10 % and then increase when applying excessive resin encapsulation. Similar to the effect of resin content, there is also an optimal additive dosage at 0.3 to 0.5 wt% for the best flowability, while excessive additives lead to "bridge effect". The negative effect raised by the additives on film quality can be mitigated by resin encapsulation. Higher gloss and lower roughness are obtained at larger resin content, while the smallest seed density is at resin content of 10 %. The study provides an effective and facile way to ameliorate the function of the flow additive and enhance the properties of powder coating films. • A new strategy of encapsulation of flow additive with coating resin is proposed. • The encapsulation improves the compatibility between coating and flow additive. • The modified additives perform better in improving powder flowability. • The modified additives maintain better coating film properties. [ABSTRACT FROM AUTHOR]

Abstract: The characterization of powder flow properties is often required for reliable design and proper operation of industrial processes. The effect of the state of compaction and bed voidage on bulk solids flowability is probably the most critical area of understanding. The goal of the present study is to compare traditional characterization techniques with methodologies provided by the FT4 Powder Rheometer (Freeman Technology). The data from six different methods, covering low to high stress levels, were compared to examine the hypothetical relationships between them. These techniques were also evaluated with regard to their ability to discriminate between different powders. To make a comparison of the testing methods, a range of seven materials was selected to cover the entire range of fine powders, i.e., from nanoparticles to group B powders. The results showed that the characterization techniques clearly have different working ranges depending on the level of cohesiveness of the powder. The powder rheometer was found to allow quick and reproducible measurements of the powder response to various environments. The different blade testing methods provided data that were in good agreement with traditional characterization techniques. However, the powder rheometer measurements were difficult to interpret because they depend on many physical properties and environmental parameters. They were particularly useful to compare similar materials but did not allow good discrimination between very different materials. A more detailed understanding of the physical phenomena involved in blade testing techniques is still needed. Finally, this study showed that powders and bulk materials cannot be viewed as invariant entities. Their flow properties cannot be predicted by only one indicator. The connection of several characterization methods is required to ensure a complete understanding of the powder flow properties over a wide range of conditions. This approach allows better insight into the powder/process relationship. [Copyright &y& Elsevier]