## 4 Composite Formation Techniques
### Drawbacks in composite formation
Dispersants or surfactants are amphiphilic molecules with a polar and a nonpolar molecular moiety. They are attached physically (physisorption) via van-der-Waals-forces or hydrogen bridges to the particles forming a hydrophobic surface also. A comprehensive overview of the different surfactants and the application possibilities was given by [PERSON] [99]. The rheological behavior of polymer-nanoparticle composites was in the main research focus in the last 20 years. In an early work [PERSON] and coworkers [100] described the impact of the particle size distribution of nanosized SiO\({}_{2}\) (primary particle size 20 nm, agglomerates 50 nm), dispersed in a low viscous methacrylate mixture for dental applications on the rheological behavior. They compared these dispersions with related ones containing coarse and medium sized particles. All systems showed a non-Newtonian flow; the addition of the nanosized SiO\({}_{2}\) caused a larger viscosity increase than the medium sized at the identical solid loads. Applying the different empirical descriptions for the estimation of the critical filler load, the lowest values were found for nanoparticles [100]. Wetting agents, also named surfactants or dispersants, possess a strong influence on the composite rheology due to a reduced inter-particle friction [101, 102]. [PERSON] and [PERSON] measured the influence of different dispersants on wax-nanosized ZrO\({}_{2}\)-dispersions. Despite that ZrO\({}_{2}\) possessed a small average particle size of 70 nm, the specific surface area was relatively low with a numerical value around 22 m\({}^{2}\)/g [103]. For comparison, ZrO\({}_{2}\) grades with larger average particle sizes and small specific surface areas around 4-7 m\({}^{2}\)/g were considered also. The use of different dispersants, here stearic acid and a commercial product (KD5, ICI Surfactants, UK), showed a pronounced influence on the solid load dependent viscosity of the nanosized ZrO\({}_{2}\)-based composites, in case of the coarse ZrO\({}_{2}\) the composite viscosity was less affected. At constant load the composites containing the nanofiller showed significant higher viscosities than the composites with the coarse filler. This behavior can be attributed to the large specific surface area of the nanosized filler [103]. With respect to the lithographic (ink-jet) printing of ceramics, nanosized TiO\({}_{2}\) (average particle size around 200 nm) was dispersed in an acrylic-based ink with the aid of different commercial dispersants (concentration 2.0 mg/m\({}^{2}\) filler specific surface area) up to a solid load of 79 wt % (45 vol %) [104]. All composites exhibited a pronounced pseudoplastic flow, which required a printing at high shear rates of the ink. The change of the viscosity with load was at low TiO\({}_{2}\) concentrations moderate and increased disproportionately at concentrations higher than 60 wt % [104]. A comparison of the particle size, particle size distribution and specific surface area of different commercially available micro- and nanosized Al\({}_{2}\)O\({}_{3}\) on the composite rheology using an unsaturated polyester resin as matrix was published in [105]. Mainly the filler's specific surface area determined the resulting accessible load and the composite flow behavior. While micron sized Al\({}_{2}\)O\({}_{3}\) with specific surface areas below 10 m\({}^{2}\)/g allowed composites with a solid load around 40 vol %, nanosized Al\({}_{2}\)O\({}_{3}\) with a specific surface area of 107 m\({}^{2}\)/g enabled only mixtures with 8 vol % using an unsaturated polyester resin as polymer matrix [105]. The flow activation energy, which is a measure of the temperature influence on the viscosity, showed a pronounced dependency on the nanosized Al\({}_{2}\)O\({}_{3}\) content and increased with load significantly. The strong impact of the nanosized Al\({}_{2}\)O\({}_{3}\) on the flow properties can be deduced from the very large specific surface area, the resulting large interfacial layer to the binder and the reduced polymer chain mobility [35].