In polymer industries, as well as material science, the use of inorganic fillers becomes more and more important. For many decades, low-cost fillers (e.g. those based on CaCO3, clay, quartz, carbon black) have been used to reduce material costs of monomers or polymers. In addition to this cost-reducing effect, suitable inorganic fillers can provide an improvement of material properties like tensile strength, hardness, abrasion resistance, thermostability, coefficient of expansion and gas-barrier effect. However, the applicability of such fillers, particularly in coatings, is restricted because of some major disadvantages (e.g. loss of transparency, high viscosity build-up, sedimentation). A solution for this problem can be the use of monodisperse nanoscaled SiO2 particles that don't inherently have these drawbacks.
Currently, silica nanoparticles suitable for industrial use are mainly made from silicon tetrachloride by a flame hydrolysis process, followed in many cases by a silane treatment of the SiO2 surface.1 The size of the primary particles of this fumed silica is in the range of 7 to 40 nm, but during the flame hydrolysis much larger agglomerates are formed. These agglomerates are only partially redispersable to the original primary particles, which is the reason for the dramatic viscosity increase and clarity loss of most dispersions containing fumed silica in amounts of more than a few percent. Therefore, the achievable filler load is usually limited below the level desirable from a performance standpoint. Surface modification of the silica with organofunctional alkoxysilanes together with mechanical aftertreatment may help up to a certain level and shows promising results,2,3 but at the expense of significantly increased cost and certain processing inconveniences.