Isotropic fillers

Fig. 9.1 Top view on two variants of C3 materials. The carbon fibers (a) themselves exhibit a complex inner microstructure that needs carful optimization for strength and stability. The isotropic filler phase (b) should be free of pores and other weak points caused by uneven distribution in the composite body. The ordered graphitic BSU (c) can provide a very strong but still flexible anchoring of the fibers in the isotropic matrix.

Other equations have been proposed, though testing appears to have been limited. Turner (1946) suggested the following equation for isotropic fillers, where a is independent of size and shape ... 

For any given application, certain mechanical properties will be of more importance than others. It is therefore, essential to identify and rank the most relevant properties and formulate or purchase the least expensive composite material that satisfies the requirements. The key mechanical properties for most applications are modulus (tensile or flexural), yield strength, impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere in between. As shown later, it is not appropriate to define reinforcement in terms of particle shape, because that definition breaks down with variations in anisotropy and particle size. In agreement with Ram [59], the definition of reinforcement as the simultaneous improvement of both modulus and yield strength will be used in this chapter... 

Isotropic fillers increase the modulus least but they do so equally in all three directions, x, y, and z. As an example, adding 40 wt.% calcium carbonate to PP homopolymer will increase the modulus from 1.5 to 3 GPa. 

Platy fillers such as talc, clay, and mica increase the polymer modulus more strongly than do isotropic fillers, such as, calcium carbonate, dolomite, silica, and fly ash. In general, the higher the aspect ratio of the filler, the greater the increase is in modulus. 

Isotropic fillers and fine fillers are least deleterious to elongation to break. Highly anisolropic and coarse fillers are worst. Dispersants help prevent agglomeration, decrease the effective particle size, and thereby... 

Low sulfur and ash levels are required for high GTE, isotropic cokes used for carbon and graphite specialty products. Highly isotropic cokes are also the filler materials for producing graphite for nuclear reactors. The purity, particularly the boron content, is critical in this appHcation. Properties of typical needle and isotropic (regular) cokes are summarized in Table 1. 

It is demonstrated in Figure 22.11 that the quasi-static stress-strain cycles at different prestrains of silica-filled rubbers can be well described in the scope of the above-mentioned dynamic flocculation model of stress softening and filler-induced hysteresis up to large strain. Thereby, the size distribution < ( ) has been chosen as an isotropic logarithmic normal distribution (< ( i) = 4> ) = A( 3)) ... 

The filler route has proved to be very efficient to obtain isotropic composites with relatively large improvements in matrix properties at small mass (volume) fractions of nanocarbon. For example, electrical percolation in epoxy has been obtained with only 0.0025 wt% of multi-wall nanotubes (MWNTs) [12]. Similarly, a 2.7-fold increase in matrix modulus has been observed on addition of 0.6 vol% MWNTs to polyvinyl alcohol (PVA) [13]. Although more modest compared to the previous two examples, a... 

Apart from particular cases of resins filled with fibres and other acicular or lamellar fillers, the parts are isotropic if there are no residual constraints. 

The main technological advantage of microspheres is that the viscosity of systems with spherical fillers is always less than that of a system with fillers of any other shape, because a sphere has the smallest surface. Moreover the isotropic materials with the best strength properties are those with spherical gas inclusions10). 

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