## 2 Experimental Investigations
### Gloss  
Gloss is an optical property that indicates how the surface reflects light in a specular (mirror-like) direction [33]. The gloss of a colored powder system depends on the degree of dispersion and particle size distribution of the pigments, and gloss has little effect on the infrared absorptivity of coatings [34]. However, gloss affects the appearance of coatings. Therefore, the average gloss values for ten points on each sample were measured for analysis. The coating gloss decreased with increasing temperature for each curing time, as shown in Figure 10. The coating gloss significantly declined at curing temperatures higher than 220 \({}^{\circ}\)C. As shown in Figure 11, the curing time slightly affected the gloss at temperatures below 210 \({}^{\circ}\)C. The gloss values achieved at these low curing temperatures were within the gloss range required for the coatings (90 \(\pm\) 5). When the temperature exceeded 210 \({}^{\circ}\)C, the gloss sharply decreased with increasing curing time. When the curing time was 5 min and the curing temperature was 240 \({}^{\circ}\)C, the average gloss of the coatings decreased to below 85. This demonstrated the importance of controlling the temperature of the coating during curing. Figure 11: Changes in the coating gloss with curing time with different curing temperatures. Figure 10: Changes in the coating gloss with curing temperature after different curing times. Hardness  
Coating hardness increased with increasing curing time and temperature, as shown in Figure 12. Most of the tested curing parameters resulted in coating hardness ratings that were better than H (meeting the minimum requirement for these coatings). It was determined during the testing that thinner coatings exhibited higher pencil hardness ratings than thicker coatings with the same curing parameters. This was related to the physical properties of the coatings and the substrates. Impact Resistance  
Impact resistance, also known as impact strength, refers to the ability of a film on a substrate to rapidly deform under a heavy hammer impact without cracking or peeling away from the substrate. This test was performed to evaluate the flexibility of the coatings and their adhesion to the substrate. In this test, the impact height was 50 cm. As shown in Figure 13, the impact test did not generate cracks when the coating was cured at 200 \({}^{\circ}\)C for 5 min. Thus, the shortest curing time required for the coating at 200 \({}^{\circ}\)C was 5 min. Similarly, at curing temperatures of 210 \({}^{\circ}\)C, 220 \({}^{\circ}\)C, 230 \({}^{\circ}\)C, and 240 \({}^{\circ}\)C, the shortest curing times were 4 min, 3 min, 2 min, and 2 min, respectively. The laboratory color model developed by the International Commission on Illumination (CIE) can be used to describe human visual effects digitally. This lab color model consists of three elements: brightness (L*) and the color channels a* and b*. The term L* describes brightness and has potential values ranging from 0 (black) to 100 (white). The a* coordinates represent the redness and greenness of a sample. The b* coordinates represent the yellowness and blueness of a sample [35]. The coordinate ranges of a* and b* are approximately [-100, 100]. In this model, it was determined that the color of the tested coatings did not significantly change when varying the curing time at a fixed curing temperature. Therefore, the average values of L*, a*, and b* measured at different temperatures were assumed to be representative test values for those temperatures. Figure 14 shows that,  
Figure 12: Coated pencil hardness test results for the coatings. Figure 13: Impact resistance test results for the coatings. as the temperature increased, the a* value of the coating gradually decreased--that is, the \"red\" value decreased. The b* and L* values initially increased with increasing temperature. However, these values decreased when the curing temperature exceeded 230 \({}^{\circ}\)C. In particular, the value of L* sharply decreased at excessively high curing temperatures.