**1. Introduction**

During the last few decades, engineers and researchers have attached grea<sup>t</sup> importance to powder coatings due to their economic and environmental benefits. Compared with solvent-borne coatings, they eliminate the use of volatile organic compounds (VOC) that are both expensive and environmentally unfriendly. Moreover, the overspray paint powders can be reclaimed and reused, resulting in a nearly 100% transfer e fficiency [1,2]. However, the particle sizes of regular powder coating are generally in the range from 30μm to 60μm, which leads to a thicker film with rougher appearance in comparison with solvent-borne coatings.

Recently, there has been a strong trend to reduce the particle size to improve the surface quality of the final coatings and lead to the considerable cost savings of material and energy [2–7]. Therefore, ultrafine powder coatings (D50 < 25 μm) have attracted more and more attention from the finishing industries, such as oral drug delivery systems, dental and orthopedic implants and the pharmaceutical industry [8–10]. In contrast to regular powder coatings (D50 > 30 μm), the ultrafine powder coatings are with a higher sensitivity to any incompatibilities from coating components and application environment due to their smaller mean particle size, poorer flowability and the thinner film they can form [1,2,7]. Considering their potential utilization and sensitivity, study on the e ffects of coating components, e.g. fillers on their performance, is more valuable compared to the regular powder coatings. By including fillers in the ultrafine powder coatings, the agglomerates formed may cause a decrease in its flowability as well as defects during spraying and curing process [2,7,11]. Therefore, the study of the e ffects of filler addition on the performance of the ultrafine powder is of grea<sup>t</sup> significance.

Fillers are one of five principal components of powder coatings that include polymer resins, curing agents (also called hardeners or cross-linkers), pigments and additives. They are usually inorganic substances with chemical stability and are produced artificially or naturally like minerals. Fillers have two functions in coatings: one is to reduce costs by decreasing the dosage of the polymer resin, normally the main-cost part in the formulation; the other is to modify certain physical, chemical or visual properties of the coating [12–16]. The commonly used fillers in powder coatings include blanc fixe (BaSO4), lithopone (ZnS·BaSO4), talc (Mg3Si4O10(OH)2), zinc white (ZnO) and carbon black (C), etc. [17–22]. Above all, BaSO4 is used in a high percentage of powder paints because of its electric conductivity and light transparency in thin coatings.

Al(OH)3 is an extensively used filler in liquid coatings, and its usage in the polymer industry has been reported [23–26]. Owing to its higher thermal conductivity, it can be used in room temperature vulcanized (RTV) silicone rubber coatings for the improvement of tracking and corrosion resistances [27]. Several researchers and Al(OH)3 suppliers have also noted the superiority of Al(OH)3 in improving the mechanical and other properties of powder coatings [28–30]. However, little systematic research is so far reported on the comparisons of the e ffects of Al(OH)3 and BaSO4 on the physical properties, ultraviolet (UV) and corrosion resistances in powder coatings.

In this study, Al(OH)3 and BaSO4 were compared as fillers in the most widely used polyester-epoxy (hybrid) and polyester ultrafine powder coatings. The paper investigated the e ffects of Al(OH)3 as well as BaSO4 on flowability of powder coatings, mechanical and other physical properties of coating films, plus their performances under corrosive environments and other external circumstances. In addition, the results were discussed regarding the di fference of physical and chemical properties between both fillers.
