*3.1. Morphology Study of Coating*

Figure 2 presents the FESEM and TEM images of the as-received nano-SiO2. As can be seen in this figure, the average size of these nanoparticles ranges from 15 to 20 nm. It was a challenge to disperse homogenously these small nanoparticles in polymer matrix. In this study, a 25 KHz supersonic bath was used to prepare the formulations of the nanocomposite coatings.

**Figure 2.** Field emission scanning electron microscopy (FESEM) (**a**) and transmission electron microscopy (TEM) (**b**) images of used SiO2 nanoparticles.

Figure 3 presents the FESEM images of the neat coating (0% nano-SiO2), nanocomposite coating without organic photostabilizers (2 wt % nano-SiO2), nanocomposite coating with the photostabilizers (2 wt % nano-SiO2, 2 wt % T384, and 1 wt % T292), before and after 36 aging cycles.

**Figure 3.** FESEM images of the coatings before (**left**) and 36 cycles of aging (**right**).

Before aging test, as shown in Figure 3, nanoparticles were dispersed evenly inside the coatings, providing a tight structure. Thus, incorporation of nano-SiO2 in polymer matrix should enhance the abrasion resistance of coating [27]. Besides, the light stabilizer, as viscous liquid—plasticizer [3]—might cause the softening effect to the coating. Thus, the abrasion resistance of coating containing stabilizer should decrease as compared to that of the one without stabilizer.

After 36 testing cycles (right side on Figure 3), the neat coating was destroyed seriously with the presence of many pits and pores (ranging in size from 100 nm to μm) appearing on both surface and inside the coating. Whereas, by the presence of 2 wt % nano-SiO2, only few pits (size of a few micrometers) could be observed on the nanocomposite surface. The size of these pits was reduced (up to a few hundred nanometers) when 2 wt % T 384 and 1 wt % T 292 were added into the coating formulation.

#### *3.2. IR Spectra Study*

The changes in chemical structures of coatings (under weathering test) can be evaluated by using IR spectra measurement. Under impacts of the weathering factors, chemical bonds in the polymer chain can be broken, leading to the loss of bonds or the formation of new bonds. Therefore, the investigation of these chemical variations can clarify the degradation mechanism of organic coatings during the aging process. In this study, the variations in chemical structures of the coatings were evaluated by IR method. The IR spectra of the coatings before and after 36 testing cycles are presented in the Figure 4. Figure 5 presents the changes of alkane CH groups and CNH groups in the coating samples, which were deducted from the quantitative IR spectra analysis. Three coating samples were tested, such as (i) neat coating (ACPU), (ii) nanocomposite coating with 2 wt % nano-SiO2 (ACPU/SiO2), (iii) nanocomposite coating with 2 wt % SiO2, 2 wt % T384, and 1 wt % T292. In addition, as the reference, we added the changes of alkane CH groups and CNH groups in the coating contained only 2 organic stabilizers (i.e., 2 wt % T384 and 1 wt % T292 -ACPU-T) [3], shown in Figure 5.

**Figure 4.** Infrared (IR) spectra of neat coating (ACPU), nanocomposite coating containing 2 wt % nano-SiO2 (ACPU/SiO2), and nanocomposite coating containing 2 wt % nano-SiO2, 2 wt % T 384, and 1 wt % T 292 (ACPU/SiO2-T), before and after 36 aging cycles.

**Figure 5.** Chemical changes of the alkane CH (**left**) and CNH (**right**) groups in the various coatings under accelerated weathering condition.

As can be seen in Figures 4 and 5, during the testing process, the peaks at 2950 and 1527 cm−<sup>1</sup> characterizing the alkane CH groups and CNH groups in the coatings both decreased. Their intensity reduced the most strongly in the neat coating, but least in the coating containing the nanoparticles and organic light stabilizers. After 36 testing cycles, the remaining content of alkane CH groups were 51.6%, 68.6%, and 97.1%, in the neat coating (ACPU), the nanocomposite coating with 2 wt % nano-SiO2 (ACPU/SiO2), and the nanocomposite coating with 2 wt % nano-SiO2, 2 wt % T384, and 1 wt % T292 (ACPU/SiO2-T), respectively. In the case of CNH groups, their remaining content was 25.4%, 52.1%, 93.5% in ACPU, ACPU/SiO2, and ACPU/SiO2-T coatings, respectively.

For comparative study, the chemical changes in the ACPU/SiO2-T coating were lower than that in the ACPU-T coating.
