Filler particle size effect

Chow demonstrated theoretically [143] that for anisodiametrical particles, the ultimate tensile stress is inversely proportional to square root of the effective or characteristic filler particle size (in this case by effective particle size the ratio of particle volume to surface area is implied). 

Effect of Filler Particle Size and Shape on the Rheological Properties of Composites... 

In general, the filler industry recognises these limitations, and tries to use a few relatively simple parameters that, taken in combination, give an approximate, working definition of morphology appropriate to the application in mind. The parameters that are most likely to be encountered are specific surface area, average particle size, effective top size and oil adsorption. The measurement and application of these are discussed in more detail below. 

Fig. 3. Effect of filler particle size on the adhesion and shear forces acting during the homogenization of PP/CaC03 composites. Symbols (O) shear, (A) adhesion [25]... 

There is evidence to show that the particle size of the filler also plays a significant role in flammability resistance. For example, below a certain particle size (about 1-2 pm), in many tests, including oxygen index, aluminum hydroxide shows enhanced fire-retarding performance,34 which may be associated with the rate of filler decomposition and/or with the formation of a more stable ash. However, it has been found that the particle size effect is absent, or less evident, in the cone calorimeter test.35 Similarly, particle size reduction has been shown to enhance fire retardancy in magnesium hydroxide-filled PP in this case, samples were characterized by the UL94 test.36 This raises the question as to whether further reductions in particle size to the nanoscale will lead to an additional increase in flammability performance, and perhaps enable filler overall levels to be significantly reduced. This aspect is considered in a later section. 

Equation (70) is a scaling invariant relation for the concentration-dependency of the elastic modulus of highly filled rubbers, i.e., the relation is independent of filler particle size. The invariant relation results from the special invariant form of the space-filling condition at Eq. (67) together with the scaling invariance of Eqs. (68) and (69), where the particle size d enters as a normalization factor for the cluster size only. This scaling invariance disappears if the action of the immobilized rubber layer is considered. The effect of a hard, glassy layer of immobilized polymer on the elastic modulus of CCA-clusters can be de-... 

Figure 18.9 shows the effect of filler particle size on extruder throughput for PP filled with talc. Several reasons account for reduction in throughput as the particle size decreases. Increasing the surface area of filler makes mixing more difficult because of agglomerate formation. Smaller particle sized talc has lower bulk density which decreases the conveying efficiency of screw. The relatively large amount of air supplied with the particles decreases the conveying efficiency and increases the time required to extract air. The amount of talc added affects the ratio of throughput, Q, to the screw speed. Ns (Figure 18.10). As the concentration...

Fillers significantly increase the tensile properties of polysulfides. This is related to the type of filler, its particle size and the type of cure. A balance of filler particle size and type is required to achieve the optimum wetting and rheology to produce the most cost effective compounds. Consideration must be given to the pH of the filler, since this affects shelf stability or well as cure properties of the compound. Fillers must be inert and insoluble in the sealants s environment. Care must be taken that the filler is adequately dispersed to ensure optimum thixotropy and barrier properties. 

With regard to particle size effects contradictory results have been published (11,12). Assuming that the filler particles change the properties of the resin In the Immediate neighbourhood (13), an Influence of particle size should be expected. A reduction of particle radius from 25 to 1 means a 1600 fold number of particles and a 300 fold surface area. Even if actually Important aspects like agglomeration of filler particles and filler/matrix adhesion are neglected it seems almost impossible to deduce effective mechanical properties from constituent properties without knowing more about bondary layers. 

As it is known [13, 14], the scale effects are often found at the study of different materials mechanical properties. The dependence of failure stress on grain size for metals (Holl-Petsch formula) [15] or of effective filling degree on filler particles size in case of polymer composites [16] are examples of such effect. The strong dependence of elasticity modulus on nanofiller particles diameter is observed for particulate-filled elastomeric nanocomposites [5], Therefore, it is necessary to elucidate the physical grounds of nano- and micromechanical behavior scale effect for polymer nanocomposites. 

Non-linear viscoelastic behaviour of NR filled with surface-modified nanosilica was prepared with a range of silica compositions. Non-linear viscoelastic properties were measured with change in temperature. A Payne effect was observed at higher silica loadings and evaluated with filler particle size, specific surface area and surface characteristics that were found to be the source of the Payne effect. Storage modulus decreased non-linearly with increasing strain even for unfilled NR. The mechanism included disruption of networks such as... 

Figure 6.2 The effect of filler particle size on the oxygen index of PMMA filled with...

Thus, the beneficial effect seen in oxygen index has been shown, in this system at least, to be mirrored in a range of other tests. Furthermore, the particle size effect is also observed in all of the tests, although it appears more marked in some than in others. In an attempt to explain the particle size effect, the fillers were all examined by differential scanning calorimetry (DSC) but only minor differences, insufficient to account for tbe observed variation in performance, were found (see the section on ATH in Chapter 2 for a detailed discussion of particle size effect on kinetics of decomposition). 

Further limitations to Lange s model are revealed when the effect of filler particle size on G (c) is considered. Lange himself [53] measured GJc) against Vf for three series of sodium borosilicate glass/alumina composites in which the average alumina particle size was 3.5 Xm, 11 pm and 44 pm, respectively. For any given value of Vf, GJc) increased... 

---