*3.5. Coating Abrasion Study*

Figure 8 presents the abrasion resistance of the ACPU, ACPU/SiO2, and ACPU/SiO2-T coatings, before and after 36 aging cycles.

As seen in Figure 8, adding 2 wt % nano-SiO2 to the paint formulation increased significantly the abrasion resistance of coating from 130 to 240 lite/mil. When added 2 wt % T 384 and 1 wt % T 292, the abrasion resistance of coating slightly decreased from 240 to 235 lite/mil. After 36 aging cycles, the abrasion resistance of the neat coating reduced strongly (down to 38.46%), from 130 to 80 lite/mil. However, the abrasion resistance of coating containing 2 wt % nano-SiO2 reduced slightly from 240 to 220 lite/mil (8.33%). Whereas in the case of ACPU/SiO2-T coating, reduction was only 2.12% (235 to 230 lite/mil) after aging test.

**Figure 8.** Values of abrasion resistance for coatings, before and after 36 cycles of aging.

As can be seen in Figure 8, addition of nano-SiO2, UV absorber T384, and light stabilizer HALS T292 into coating matrix significantly improved the weathering durability of the coating. As a complement data, Figure 9 presents the UV-Vis spectra of aqueous solution containing nano-SiO2 (0.5 wt % dispersion), ACPU, ACPU/SiO2, and ACPU/SiO2-T coatings.

**Figure 9.** Ultraviolet-visible (UV-Vis) spectra of the various coating samples and the aqueous solution of nano-SiO2.

As shown in Figure 9, the absorption of UV radiation was much lower (less effective) for the ACPU coating, as compared to the ACPU/SiO2-T coating. In particular, the UV absorption of ACPU/SiO2-T coating was the strongest among these coating samples. As expected, due to the UV absorption of nano-SiO2, the polymer coating was protected under UV irradiation. Thus, the nanocomposite coating containing 2 wt % SiO2 exhibited a smaller value in chemical change, leading lower loss in weight, higher gloss, and better abrasion resistance than that of the ACPU coating.

These findings are consistent with the data reported by Yari et al. [18] for the photo-protective effect of nano-SiO2 on acrylic melamine coating. Moreover, nano-SiO2 could also react with isocyanate groups [28] to form a tight organic-inorganic hybrid structure, which contributes to preventing the invasion of external environmental factors [29], leading the coating to reduce its degradation effectively. By the presence of 2 wt % T 384 and 1 wt % T 292, the polymer coating is double-protected by the combination of T 384 (as UV absorber) and T 292 (as free radical cleaner). In our previous paper [3], we discussed their photo-protection mechanism. Consequently, ACPU/SiO2-T coating exhibited an excellent weathering resistance after 36 testing cycles.

The obtained data indicate that incorporation of nanoparticles into the acrylic polyurethane coating (with the presence of photostabilizers) enhances its weathering resistance.

#### **4. Conclusions**

The synergistic effects of nano-SiO2 and organic light stabilizers on enhancing the weathering resistance of acrylic polyurethane coating have been investigated and discussed.

The main findings of this study are:


220 lite/mil (8.33%). In the case of ACPU/SiO2-T coating its reduction was only 2.12% (from 235 to 230 lite/mil) after aging test.

These enhancements could be explained by two possible reasons: (i) UV absorption of nanofillers (nano-SiO2), could photo-protect the polymer coating, and (ii) a tight organic-inorganic hybrid structure was formed under curing reactions. Besides, addition of organic light stabilizer into paint formulation has provided excellent weathering resistance for the nanocomposite coating. These findings indicated the promising application of this nanocomposite coating as a multi-functional durable material.

**Author Contributions:** Conceptualization and writing—the original draft: T.V.N.; Experimentals and data analysis: T.H.N.; Writing—review and editing: T.A.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the Vietnam National Foundation for Science and Technology Development (NAFOSTED, Grant # 104.02-2018.19).

**Conflicts of Interest:** The authors declare no conflict of interest.
