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We investigated in this work the impact of particle size, size 'distribution, and siUca loading on film properties of UV-cured hybrid organic-inorganic thin coatings. Commercial silica organols of
hexanediol diacrylate monomer (HDDA) containing monodisperse silica nanospheres (13, 25 and 50 nanometers) were mixed with a polyester tetraacrylate (PEA) to give coatings with controlled particle size distribution and silica loading (0 to 40%). Dynamic mechanical thermal analysis (DMTA) measurements showed that complex modulus (E*) increased and loss tangent (tan5) decreased with small particle size and high silica content, but the dynamic glass transition temperature (Tg) was unaffected by size and size distribution . Coatings with mixtures of 50 and 13 nm particles at 75/25 weight ratio obeyed to volume packing theory and gave the highest values of E* and tan5. Resistance to abrasion and friction were more effective with 50 nm particles, while gloss was highest with 13 nm particles. These properties were best seen when silica content is superior than 15 % where strong interactions at' the silica-polymer interface and
particle/particle were detected. AFM observation showed that the surface of the coating was well
with silica particles, protecting it from aggressive physical and chemical attacks. In all cases, the properties of hybrid coatings were found superior to the pure organic coating. Key words: acrylic colloidal silica, nanocomposite, nanofillers, UV-curing, 'size and size distribution, filler concentration, viscoelastic properties, friction, abrasion resistance, gloss.