Non-hydrolytic sol–gel synthesis of polypropylene/TiO₂ composites by reactive extrusion

An original way to synthesise polypropylene/titanium dioxide (PP/TiO₂) composites combining non-hydrolytic sol–gel chemistry and reactive extrusion was reported. The non-hydrolytic sol–gel reaction between titanium alkoxide as a titanium dioxide precursor and an acid anhydride as an oxygen donor at 240 °C was first evaluated and optimised in an alkane liquid medium. TiO₂ was present essentially in crystalline anatase form when synthesised in squalane with a domain size around 10 nm. Based on these encouraging results, the synthesis was adapted to reactive extrusion that means at high temperature in molten viscous polypropylene and for short reaction times (few minutes maximum). The proof of concept was evidenced through the formation of amorphous TiO₂-based inorganic domains of size around 1 µm within the polymer matrix from the reaction between titanium isopropoxide and acetic anhydride. A condensation degree of 79% was obtained. To optimise the polymer/filler interface, titanium dioxide precursors and oxygen donors with different alkyl chain lengths were evaluated. Hexanoic anhydride as substituent of acetic anhydride and titanium tetrakis 2-ethylhexyloxide as substituent of titanium isopropoxide were used. The oxygen donor contribution appeared to be dominant for the particle dispersion state. The polypropylene (PP)/TiO₂ composite synthesised from the reaction between titanium isopropoxide and hexanoic anhydride showed the narrowest TiO₂ particle diameter distribution with a mean particle size around 700 nm.