**3. Results**

#### *3.1. Flow Behavior and Specific Gravity of Paint Powders*

Flow behaviors of the powder paints were characterized by AOR and BER measurements. Figure 4 shows the AORs of the powder samples dry-blended with fluidization additives with respect to the fillers' types and contents. It is clear that the type of resin system affects the AORs of the paint samples. AORs of the samples based on polyester are much higher than that of the hybrid. However, for the same resin system, the type of filler only makes a small difference in AOR, if the same amount of Al(OH)3 and BaSO4 are used. As mentioned before, the particle size distribution, shape and humidity of powder samples can be ignored because of the same manufacturing processes, operating parameters and conditions, as shown in Figure 5. The slightly higher AORs of samples with BaSO4 is partially due to its smaller D50, as shown in Table 3, which results in a larger tendency to the formation of agglomerates and poor dispersion in powder paints.

**Figure 4.** AORs of samples.

**Figure 5.** SEM micrographs of powder paints.

From our prior paper and industrial experience, when AORs are higher than 42◦, the paint powders tend to agglomerate and thus exhibit poor flowability during the spraying process [40]. Accordingly, the filler contents in this study for Al(OH)3 and BaSO4 are quite acceptable for hybrid-based coatings with the addition of fluidization additives; the AORs of most of the paint samples vary below 42◦, except for those based on polyester resins.

BER was used to characterize flow behavior based on the belief that a higher BER indicates more gas in the interstitial void among particles, implying more uniform gas-solid contact and thus better flow behavior and fluidization quality. Figure 6 presents the result of BER measured from samples based on hybrid and polyester. Evidently, filler-free powder paints exhibit the highest BERs in almost the whole range of superficial gas velocity. With the increase of filler contents, BERs of powder paints deteriorate and decrease to about 1.8 and 1.7 for samples of H-B3 and PE-B3, respectively. From previous investigations, the BERs for regular powder coatings (D50 <sup>&</sup>gt;30μm) showed a maximum value of 1.6 when the superficial gas velocity was 1.0 cm/s [1]. Compared to regular powder coatings, the ultrafines normally exhibit poorer flowabilities, such as lower BER and higher AOR, because of the stronger inter-particle forces [32–36], while with the aid of the fluidization additives the BERs of the ultrafine powder coatings in this research vary from 1.7 to 2.4 at a similar superficial gas velocity of 0.93 cm/s (Figure 7). A higher BER leads to a better gas-solid contact, which results in a better film appearance and gas saving in the electrostatic spraying process.

Besides, PE-powders are much sensitive to filler's incorporation, BERs of polyester powders decrease more significantly in contrast to those of hybrid. With the increase of filler contents, the BERs of PE-B and PE-A deteriorate very fast and decrease by more than 30% and 20% in contrast to that of PE-C, while the reduction of the BERs of H-B and H-A are 19% and 10%, respectively.

Figure 8 shows the specific gravities of paint powders respect to different contents of two fillers. It is observed that all samples possess similar specific gravities of 1.2 when no filler is incorporated. Obviously, the addition of fillers into the powder coatings directly raise its specific gravity and the samples with the same filler contents have close values. The overall density of samples incorporated with BaSO4 is much higher than that of Al(OH)3, which is due to the difference of specific densities between the two fillers.

**Figure 6.** Bed expansion ratios (BERs) of samples respect to superficial gas velocity.

**Figure 7.** BERs of samples at gas velocity of 0.93 cm/s.

**Figure 8.** Specific gravities of samples.

#### *3.2. Physical Properties, UV and Corrosion Resistances of Coating Films*

## 3.2.1. Physical Properties

Surface hardness, impact resistance, and flexibility are all crucial properties for typical protective coating films. Figure 9 exhibits the pencil scratch hardness test comparisons of samples respect to fillers' types and contents. As expected, owing to the similar resin and curing agent, samples exhibit similar pencil scratch hardness of HB when no filler is incorporated in. It is observed that the pencil scratch hardness of samples with Al(OH)3 increase to H when the filler content is 30.6%. While, for samples incorporated with BaSO4, the pencil scratch hardness increased to H at the maximum BaSO4 loading. Compared to BaSO4, Al(OH)3 is more efficient to increase the hardness of coating film at higher loadings. The H-A3 sample exhibited the highest hardness of 2H.

**Figure 9.** Pencil scratch hardness of samples.

Figure 10 exhibits the results of the impact resistance for samples in respect to filler types and contents. It is observed that the addition of fillers caused a minor decrease in the impact resistance of the coating films. This may be due to the discontinuities in the film matrix introduced by inorganic fillers, which makes the film less flexible and therefore lose adhesion to the panels when receiving an impact. Al(OH)3 shows a slight influence on the impact resistance of coatings. For samples incorporated with Al(OH)3, the decrease of impact resistance occurs at the maximum filler loading (40.2%), with the reduction of impact resistance for about 1.1 J (from 21.2 J to 20.1 J). As for samples added with BaSO4, the reduction of impact resistance starts at a filler content of 17.3%.

**Figure 10.** Impact resistances of samples.

The coating flexibility was tested with the conical mandrel bend device. The results show that samples exhibit good flexibility up to filler content of 40.2%, and there is no sign of cracking when the bending exceeds 3 mm mandrel diameter, which is the minimum radius of the cone used in this study.

In general, specular gloss is one of the most commonly used parameters for evaluating surface optical quality. Figure 11 presents the result of the evaluation of specular gloss at 60◦. When no filler is incorporated, the specular gloss varies from 95.8 (PE-C) to 98.4 (H-C). The increase of filler contents leads to reduction of the specular gloss. At the maximum load of fillers, the specular glosses decrease by about 14.3% (H-B3) and 21.9% (PE-B3), while they fall further down from 47.8% (H-A3) and 49.5% (PE-A3) in contrast to H-C and PE-C. At the same filler content, samples containing Al(OH)3 exhibit much lower levels of gloss than those with BaSO4. The specular glosses of H-A and PE-A decrease significantly to about 50 at the maximum loadings of Al(OH)3, while samples H-B and PE-B show glossy films with a specular gloss of about 80.

**Figure 11.** Specular gloss of samples with different fillers and filler contents at 60◦.

#### 3.2.2. UV and Corrosion Resistances

Ultraviolet light degrades the coatings by imparting energy into the films. The energy of UV light destroys the crosslink by generating heat or breaking chemical bonds. The heat and breakage of bonds will lead to the loss of physical and chemical properties of coating films, bringing in chalking, color change and gloss reduction.

Figure 12 presents the gloss retention at 60◦ after a UV accelerated test against 1000 h of UV exposure. PE based samples exhibit better overall UV resistances in contrast to H ones when no filler is incorporated in. The gloss retention of hybrid-based samples decreased rapidly to 90% of the initial gloss within 200 h. Polyester-based samples have the best UV resistances, the gloss retention keeps 90% of the initial gloss for about 700 h in the test chamber, which is 3.5 times of that of H based samples.

Besides, it is observed that with the use of Al(OH)3, the PE-based films exhibit much higher gloss retentions than those with BaSO4, as shown in Figure 13. Compared with Al(OH)3, the gloss retentions of most samples incorporated with BaSO4 deteriorate significantly with the increase of filler contents. Samples PE-A1 and PE-B1 keep 90% of the initial gloss for about 630 h and 530 h against UV exposure. Sample PE-A2 keeps 90% of initial gloss for more than 600 h, which is 1.5 times of that of PE-B2. When increasing the fillers' loading to the maximum, the gloss retention of PE-A3 decreases quickly, which is partially due to the much more significant consumption of resin and curing agents in samples incorporated with Al(OH)3. A similar trend can be observed in samples based on hybrid resin systems.

A salt spray test provides a method by which to evaluate the corrosion resistance in terms of loss of adhesion at a scribe mark. The rust creepages for scribed samples can be evaluated by rating grade between 10 and 1 (Table 6). Figure 14 presents the results of corrosion resistance of samples with different fillers and filler contents. Samples filled with Al(OH)3 exhibit better overall

corrosion resistance especially for H-A samples, which keeps rating number of grade 9 throughout the whole tested filler contents, while the rating number of H-B samples decrease to grade 7 when the filler contents exceed 30.6 wt.%. The corrosion resistances of samples added with BaSO4 deteriorate significantly with the increase of fillers contents, especially at the maximum loadings, except for H-A samples.

**Figure 12.** Gloss retention of samples after 1000 h of UV accelerate test.

**Figure 13.** Time consumed of samples at 90% of gloss retention.

**Figure 14.** Rating number of failures at scribe of samples after 1000 h of salt spray test.
