**2. Experimental**

## *2.1. Materials*

The acrylic polyol solution (HSU 1168) obtained from A&P Industrial Resins Company (Taiwan) contained 65 wt % solid, in which the content of hydroxyl groups was 4.3 wt %. The curing agent was polyisocyanate N-75 (75 wt %, Germany) in which the weight percentage of isocyanate groups was 17%. The organic UV absorber, benzotriazole-UV (2-(2-hydroxy-3-tert-butyl-5-propionic acid isooctyl ester)-2H-benzotriazol, or Tinuvin 384 and thelight stabilizerHALS, bis (1, 2, 2, 6, 6-pentamethyl-4-piperid502) sebacate or Tinuvin 292, were provided by Ciba (Switzerland). The chemical structures of these materials are presented in Figure 1.

The nano-SiO2 used in this study is non-porous silica (Aerosil 200 F, Antwerp, Belgium). Their average particle size and surface area are 15 nm and 200 m2/g, respectively.

The toluene and butyl acetate solvents which were used were the Chinese P-type.

**Figure 1.** Chemical structures of used compounds.

## *2.2. Preparation of Coatings*

For fabrication of coating, 2 wt % T384, 1 wt % T292, and 2 wt % SiO2 nanoparticles (by the total solid resin) were selected at the optimal values in our previously published works [5,27]. Weight ratios of materials in coating formulations are presented in Table 1.

At the first step, the nanoparticles were dispersed in the solvent using ultrasonic bath TP 25 for 1 h, then this as-prepared solution was mixed with acrylic polyol resin solution, T384, and T292 by stirring for 15 min. The obtained mixtures were continuously homogenized by ultrasonic bath for 3 h to ensure that the nanoparticles were well dispersed.

The coating samples (with quasi-constant thickness of ~25 μm) were prepared by using a Quadruple Film Applicator (Erichsen model 360, Hemer, Germany). For infrared (IR), ultraviolet-visible (UV-Vis), and field emission scanning electron microscopy (FESEM) analysis, coating samples were prepared on the Teflon sheets, then after 6 days of curing at room temperature separated and attached to aluminum windows. The coating samples for monitoring the weight loss were prepared on glass sheets (100 × 70 × 2 mm) whereas for measuring the gloss and the abrasion resistance, coating samples were prepared on CT3 steel sheets (100 × 100 × 2 mm).


**Table 1.** Weight ratios of components in coating formulations.

Note: The contents of nano-SiO2, T 384, and T 292 were 2%, 1.5%, and 1.0% by weight of the total solid resin, respectively.

## *2.3. Accelerated Weathering Test*

The accelerated weathering test was carried out in the Atlas UV/CON chamber (model UC-327-2, Chicago, IL, USA), under UV-A 340 fluorescent lamp, according to ASTM D 4587-05 standard (every testing cycle consists of 8 h for UV exposure at 60 ◦C and further 4 h for condensate water at 50 ◦C).

## *2.4. Characterizations*

2.4.1. Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM) Analysis

The morphology and size of nano-SiO2 were observed by FESEM S 4800 (Hitachi, Tokyo, Japan) and TEM-JEM 2100 (JEOL, Tokyo, Japan). FE-SEM technique was also used to assess the aging of the coatings under impact of accelerated weathering factors.
