**1. Introduction**

Having many advantages over other conventional polymer coatings, such as good adhesion, transparency, high gloss, and weathering resistance, acrylic polyurethane coating is used widely as a protective and decorative coating for metal structures, interior and exterior wood, and automotive paint [1,2]. Two-component acrylic polyurethane paint can be cured at room temperature through the step-growth polymerization of isocyanate groups with hydroxyl groups in the acrylic polyol resin forming crosslinked urethane.

In order to improve the weathering resistance of the outdoor coatings, in addition to selecting the main components such as resins and pigments with high weathering durability, organic and inorganic light stabilizing additives are usually added to the paint formulation [3–7]. Previous published works showed that organic light stabilizers (such as Tinuvin 384, Tinuvin 1130, Tinuvin 292) exhibited an excellent light stabilizing efficiency. Adding these additives at appropriate content could increase the weathering durability of the polymer coatings by 2–3 times [3]. Besides, as reported in literature, some metal oxides (TiO2, ZnO, ZrO2) can absorb ultraviolet (UV) rays, therefore having an ability to protect the polymer substrates from harmful UV light [4–9].

During UV absorption, an electron from the valence band jumps onto the conduction band, leaving a positively charged hole. These electrons and positively charged holes move onto the particle surface, where they recombine to each other or react with oxygen and water to form •OH activated free radicals [4]. These free radicals can be the agent promoting the degradation of polymers. Thus, metal oxides exhibit the dual effects: (i) Blocking UV rays, and (ii) causing photo-catalytic degradation of the polymer (depending on their content, sizes, structure, and surface modifications) [5].

Currently, the addition of inorganic particles/nanoparticles (at appropriate content) into the polymer substrate can enhance its property [10–16]. Nano-SiO2 is the most used among the common inorganic nanoparticles. For example, by adding nano-SiO2 into the coating formulation, the mechanical, thermal, [2,17] weathering resistance [18,19], and anticorrosion properties [20,21] of the organic coatings were significantly improved. Nano-SiO2 is also used for both super hydrophobic coatings [22–25] or hydrophilic coatings [26].

In our previous work [3], acrylic polyurethane coating with photostabilizers exhibited the lifespan over 12 years under the marine weathering condition (natural exposure). On the other side, as reported in literature, nano-SiO2 has only been considered as a reinforcer (nanofiller) for polymer matrix. In this study, the combination of nano-SiO2 and organic light stabilizers (Tinuvin 384, Tinuvin 292) are expected to enhance simultaneously the abrasion resistance and weathering durability of coatings. We will try to explore the role of nano-SiO2 in this expected enhancement.
