Silicone-acrylic hybrid aqueous dispersions of core-shell particle structure and corresponding silicone-acrylic nanopowders designed for modification of powder coatings and plastics. Part I - Effect of silicone resin composition on properties of dispersio

Aqueous silicone-acrylic dispersions with core-shell particle structure can be obtained in the process of emulsion polymerization of acrylic or methacrylic monomers in previously synthesized silicone resin dispersions. If the glass transition temperature (Tg) of the shell is around +120 °C or higher, drying of such dispersions leads to “nanopowders” which can be applied as impact modifiers for powder coatings and plastics due to the presence of low Tg silicone resin contained in the hybrid nanoparticles. The aim of our study was to investigate the effect of silicone resin composition on the properties of dispersions and the corresponding nanopowders what, in turn, was expected to influence the properties of powder coatings modified with such nanopowders. Silicone resin dispersions (DSI) were synthesized by emulsion polymerization of three silicone monomers: octamethylcyclotetrasiloxane (D4), methyltrimethoxysilane (METMS) and methacryloyltrimethoxysilane (MATMS) in the presence of dodecylbenzenesulphonic acid playing the role of both surfactant and polymerization catalyst. Silicone-acrylic hybrid dispersions (DASI) having core-shell particle structure confirmed by TEM were further obtained by emulsion polymerization of methyl methacrylate in DSI, and eventually nanopowders (NP-DASI) were produced by spray-drying of DASI. A designed experiment was conducted where the different proportions of D4, METMS and MATMS were used in DSI synthesis and a range of properties of DSI, DASI and NP-DASI were tested. A significant effect of starting silicone monomers composition (reflected in silicone resin structure) on dispersion particle size was observed what could be explained by differences in their hydrophobicity. SEM investigations revealed that NP-DASI were produced in the form of 1-10 μm agglomerates of round-shaped nanoparticles of ca. 120 nm in size. Two clear glass transition temperatures (Tg) of NP-DASI were identified by DSC: one attributed to silicone part - around −120 °C - and the other attributed to poly(methyl methacrylate) (PMM) part - around +120 °C. Tg attributed to silicone part decreased with increased share of D4 and MATMS in the silicone monomers composition while Tg of PMM part showed a minimum for specific composition of silicone monomers.