## 4 Composite Formation Techniques
### Drawbacks in composite formation  
In case of functional polymer based composites, the degree of tailored property adjustment follows mainly the volume amount of dispersed filler introducing the aspired physical property like refractive index or electrical conductivity change. Interestingly, in literature most filler amounts are not given in vol %, but in wt %. As part of a MRS Bulletin issued in 2007 [PERSON] and [PERSON] collected the use of selected micro and nanosized fillers like carbon fibers and nanotubes, alumosilicates, clay as well as Al\({}_{2}\)O\({}_{3}\) and TiO\({}_{2}\) for commercial applications [93]. In the following the impact of spherical nanoparticles on the composite flow behavior, which determines the maximum accessible solid load and shaping process significantly, will be discussed. Many _ex situ_ processes generally suffer extremely from the high agglomeration tendency of nanoparticles, as it is rather difficult to destroy the nanoparticle agglomerates even using high external shear forces. In case of chemically identical materials, the interaction energy between two particles increases significantly from zero dimensional spherical particles to two-dimensional nanoscaled sheets and therefore the required dispersion effort in a polymer matrix is raised, also. Considering solvent-free composites the composite viscosity depends on the used polymer matrix (curable low viscous reactive resins or polymer melts) and the suspended filler. The expected composite viscosity determines the applied dispersing technology. Low viscous reactive resin based mixtures can be processed with dissolver stirrers or by sonification under ambient or slight elevated temperatures avoiding monomer evaporation. Different dispersing techniques with increasing shear forces like simple laboratory dissolver stirrers generate only small mechanical forces while high speed stirrers (up to 25000 rpm) and high pressure homogenizers (up to 10\({}^{8}\) Pa) enable a pronounced deagglomeration [94]. These methods can only be applied at composite viscosities below 10 Pas. Mixer-kreader and extruders allow for the processing of high viscous polymer melts. In the latter case the resulting shear forces depend also on the configuration of the used extruder screws. As a measure of the deagglomeration capability of a dispersing method, TEM-images as well as the measurement of the optical transmittance can be used. [PERSON] and coworkers showed the influence of the dispersing method on the optical transmittance in the NIR-range of highly agglomerated nanosized SiO\({}_{2}\), ZrO\({}_{2}\), and Al\({}_{2}\)O\({}_{3}\), dispersed in a MMA/PMMA based reactive resin after polymerization [95]. They found, that with increasing shear forces the optical damping was significantly reduced, e.g., in case of 5 wt % amorphous SiO\({}_{2}\) in PMMA from 4 dB/mm (blade stirrer) down to 0.44 dB/mm (high speed stirrer). The use of the high pressure homogenizer enabled a further improvement down to values around 0.26 dB/mm [96]. TEM-images of samples applying either the high speed stirrer or the high pressure homogenizer showed a difference in the agglomeration behavior of the hydrophobic SiO\({}_{2}\) in PMMA (see Figure 3). A comparable optical damping decay was measured for ZrO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), dispersed in PMMA [95, 96]. After dispersing, a reagglomeration forming micron sized soft agglomerates has to be prevented. A surface hydrophobization using physisorption or chemisorption causes a steric stabilization enabling a repulsive interaction of the particles [12]. The treatment of the hydroxyl-terminated SiO\({}_{2}\), ZrO\({}_{2}\) or Al\({}_{2}\)O\({}_{3}\) with organosilanes yields via chemisorption a hydrophobic surface [12, 97]. If the organosilanes carry a reactive, polymerizable functionality, the surface modified particle can be attached to the resulting polymer backbone or network. A short review dealing with these grafting techniques was published by [PERSON] and coworkers in 2006 [98].