Fillers shapes

Unfortunately, the percolation approach is also not really able to predict accurately the critical volume fraction in real composites, because so many different factors, like filler shape, size, distribution and particle agglomeration, come into play. Lux (1993) has reviewed the various percolation models that have been proposed in theoretical treatments of the problem. 

Kogan et al. (1988) examined the chemorheology of silica- and carbon-fibre-filled epoxyresin systems. They found unusual effects of carbon fibre on the uncured rheology and chemorheology of filled epoxy-resin systems, and related these to the anisotropic namre of the filler shape and the effect of filler surfaces on the kinetics. 

In filled polymer systems, it has been observed that the effect of filler content on viscosity decreases as shear rate increases [14, 49]. In the case of nanocomposite flllers, this effect has been explained in terms of a detachment/reattachment mechanism [49]. With respect to the dimensions of the flllers, it has been observed that as the surface area of the filler increases so does the viscosity of the filled polymer melt [18, 48]. For particles with similar shapes, an increase in the surface area means a reduction in particle size. In this sense, nanoflllers are expected to significantly increase the viscosity of polymer melts in relation to flllers with sizes in the range of micrometers. An analysis of filler shape and other relevant aspects of polymer flllers can be found in the work by Shenoy [50]. 

From the above, it is clear that the specific surface area of the filler, which is closely related to filler shape, is of fundamental importance in the xmderstanding of the structtue-property relationship of nanocomposites. The change in particle diameter, layer thickness, or fibrous material diameter from micrometer to nanometer, changes the ratio by three orders in magnitude. At this scale, there is often distinct size dependence of the material properties. Furthermore, with the drastic increase in interfadal area, the properties of the composite are dominated more by the properties of the interface or interphase [2] and they play a much more important role in enhancing the mechanical properties of nanocomposites than in conventional composites or bulk materials [ 1 ]. 

In addition, newer aspects, such as the effects of sustainable materials based on starch on the macro or nanostructure and subsequent processing, thermomechanical properties and performance properties of plasticized starch pol5mciers have been examined (10). Specific structures and the resulting properties are controlled by many specific factors, such as filler shape, size and surface chemistry, processing conditions and environmental aging. In case of nanosized biocomposites, the interfadal interactions are extremely important to the final nanostructures and performance of these materials. 

B. Fisa, J. Dufour and T. Vu-Khanh, Weldfine integrity of reinforced plastics effect of filler shape. Polymer Composites, 8,408 18 (1987). 

Many models have been proposed (117) to explain the electrical conductivity of mixtures composed of conductive and insulating materials. Percolation concentration is the most interesting of all of these models. Several parameters, such as filler distribution, filler shape, filler/matrix interactions, and processing technique, can infiuence the percolation concentration. Among these models, the statistical percolation model (118) uses finite regular arrays of points and bonds (between the points) to estimate percolation concentration. The thermodynamic model (119) emphasizes the importance of interfacial interactions at the boimdary between individual filler particles and the polymeric host in the network formation. The most promising ones are the structure-oriented models, which explain condnctivity on the basis of factors determined from the microlevel stmctin-e of the as-produced mixtures (120). 

Taking these factors into consideration, MFCs could be considered a natural choice of barrier material for several reasons. First, in situ formation of microfibrils removes the need for exfoliation or intercalation of the reinforcing medium while at the same time creating a near perfect fibrillar dispersion. Furthermore, an element of control over the filler shape and aspect ratio can be achieved by changing the extrusion and drawing parameters. Finally, manipulation of the film preparation technique can produce films with either well aligned or randomly oriented fibrils. 

The case of fillers which adhere to the polymer matrix is important in most applications. This is general whether the adhesion is intrinsic to the materials or enhanced by surface treatment of the fillers or additives to the polymer. When the adhesion between the filler and the resin matrix is adequate, the resin and the filler coact under stress. From the standpoint of visualizing how these effects alter the performance of the composite to make the whole greater than the parts, we will examine the combination of a fibrous filled fiber glass and a suitable resin matrix such as an adhering polyester resin. The case of a simplified configuration of a bundle of parallel fibers before and after the addition of the resin matrix is illustrated. The analysis will be extended to random fiber orientation and then to other filler shapes. 

In this section we are going to discuss the effect high aspect ratio fillers have in the polymer matrix and subsequent properties. We will discuss the effect the filler shape, size, distribution and orientation have on the polymer matrix focusing especially in the resulting morphology. 

Filler Shape Density Mohs Hardness Uses... 

Funabashi M, Hirose S, Hatakeyama T, Hatakeyama H (2003) Effect of filler shape on mechanical properties of rigid polyurethane composites containing plant particles. Macromol Symp. 197 231-241... 

TaiM 2. Maximum padcing fractions of common filler shapes... 

Nanoclay fillers are categorized as platelet-like nanoclays or layered silicates and tubular nanoclays in terms of filler shape. With the configuration of two tetrahedral sheets of silicate and a sheet layer of octahedral alumina, platelet-like nanoclays or phyllosilicates are formed, which include smectite, mica, vermiculite, and chlorite. In particular, smectite clays are widely employed with further subcategories of MMT, saponite, hectorite, and nontronite. The typical MMT clays are regarded as one of the most effective nanofillers used in polymer/clay nanocomposites due to their low material cost and easy intercalation and modification (Triantafillidis et al., 2002). On the other hand, the fundamental structure of tubular nanoclays contains an aluminum hydroxide layer and a silicate hydroxide layer. They are also known as dio-ctahedral minerals with two different types of halloysite nanotubes (HNTs) and imo-golite nanotubes (INTs). Notwithstanding their material role as clay minerals, these two types of tubular nanoclays resemble the hollow tubular structure of carbon nanotubes (CNTs). In this section, three different types of clay nanofillers, namely MMTs, HNTs, and INTs are reviewed in detail along with the development of clay modification. 

Mineral fillers are naturally occurring or synthetic nonblack, nonmetallic solid-surface particles. Such fillers have assorted shapes, from nodular to platy to aci-cular. When describing filler shape, the term aspect ratio is employed to describe the relationship of one dimension to another. Mineral fillers for PVC have historically included calcium carbonate (ground and precipitated), alumina trihydrate (ATH), barytes, talc, mica, kaolin, feldspar and nepheline syenite, and wollastonite. 

Interparticle contact is of critical importance to the behavior of lithium batteries. Most lithium-ion electrodes contain 2 to 15 wt% conductive filler, such as carbon black, in order to maintain contact among aU the particles of active material and in order to reduce ohmic losses in the electrodes. Presently, there are few models available for predicting contact resistance, and the effect of the weight fraction of conductive filler on the overall electronic conductivity of the composite electrode must be determined experimentally. Doyle et al. [35] demonstrate how the fuU-cell-sandwich model can be used to determine what minimum value of effective electronic conductivity is needed to make solid-phase ohmic resistance negligible. Then, one need only measure the effective conductivity of the composite electrode as a function of filler content, and one need not run separate experiments on complete cells to determine the optimum filler content. Modeling techniques for predicting effective electroitic conductivities of composite electrodes are under development, and hold promise to aid in optimizing filler shape and volume fraction [85]. 

---