Anisotropy fillers

The analytical expressions of micromechanics are generally most accurate at low volume fractions of the filler phase. The details of the morphology become increasingly more important at higher volume fractions. This fact was illustrated by Bush [64] with boundary element simulations of the elastic properties of particulate-reinforced and whisker-reinforced composites. The volume fraction at which such details become more important decreases with increasing filler anisotropy, as was shown by Fredrickson and Bicerano [60] in the context of analytical models for the permeability of nanocomposites. 

Tensile measurements were taken in most cases to determine the mechanical properties of NR/CNT nanocomposites. Initial modulus, determined from stress-strain curves, was observed to remarkably increase with the filler content. At 1 wt%, the increase was 25.9%, compared to pure NR, ° at 3, 5, 7 and 10 wt% the increase % was 142, 306, 680 and 850, respectively.It was commented that the modulus increase is due to the hydrodynamic effect, further increased by the filler anisotropy, and by the presence of occluded rub-ber.i° For composites with 37 wt% of CNT, the storage modulus was about three orders of magnitude higher than that of the pure rubber. CNT modification with resorcinol caused an increase in modulus at all CNT loadings, indicating improved filler-matrix adhesion. 

Figure 1 Effect of filler anisotropy on the flexural modulus of PP composites ...

Filler anisotropy has a similar effect on yield stress and tensile strength as well as on modulus both increase with increasing aspect ratio and orientation. The two factors are interconnected here too, so both must be known to predict composite properties accurately. The important effect of fiber orientation is shown by Fig. 5, where the strength of glass fiber reinforced PP composites is plotted against the alignment of the fibers [32]. A considerable increase in strength is observed when the fibers have parallel orientation to the direction of the external load (0°), as expected. 

B. Pukanszky, K. Belina A. Rockenbauer, F. Maurer. Effect of nucleation, filler anisotropy and orientation on the properties of PP composites. Composites, 25, 205-214,1994. 

Such is the anisotropy that flexural modulus may be four times as high in the flow direction as in the transverse directions. This difference may be reduced by incorporating fillers such as glass fibre or mica. 

The orientation of an anisodiametric filler which results from pressure molding is known to bring about anisotropy of properties in different directions, the extent of which is determined by the matrix polymer [367], This is not always a plausible result from the technological standpoint, if only because orientation, frequently nonuniform, may build up to cause buckling of articles [368-370]. 

Anisodiametrical particles of filler appearance of anisotropy of properties and relaxation phenomena, determined by the turn of solid particles in a flow... 

Liquid-solid transitions in suspensions are especially complicated to study since they are accompanied by additional phenomena such as order-disorder transition of particulates [98,106,107], anisotropy [108], particle-particle interactions [109], Brownian motion, and sedimentation-particle convection [109], Furthermore, the size, size distribution, and shape of the filler particles strongly influence the rheological properties [108,110]. More comprehensive reviews on the rheology of suspensions and rubber modified polymer melts were presented by Metzner [111] and Masuda et al. [112], respectively. 

Wollastonite is a preferred filler in some instances due to its fibrous form. While not as effective in improving the mechanical properties as glass fibers, it will give more strength than spherical fillers and less anisotropy than longer glass fibers. 

Grain - The unidirectional orientation of rubber or filler particles occurring during processing (extrusion, milling, calendering) resulting in anisotropy of a rubber vulcanizate. 

Anisotropy of mechanical properties at the increase of filler concentration [70, 78] rises significantly in the items produced from mineral media-filled thermoplasts. When a mineral filler is replaced for a mineralorganic, the dfect of mechanical anisotropy increase fades considerably [18]. At the same time cp values, durability of cold seal moulded products, made of kerogenes is higher than in products of mineral materials-filled compositions [18]. 

Volumetric CTE appears to be preserved for a specified formulation, independent of gate or part geometry. Analysis of CTE behavior for the filled LCP compositions indicates some anisotropy remains even at relatively high loadings of filler. 

As expected, the anisotropy and volumetric CTE for the highly filled experimental compound (entry 15 in Table I) are much lower than the other XYDAR formulations. This material is particularly suited for applications where lower anisotropy and volumetric CTE match are needed and highlights the importance of adjusting filler loading in LCP s to meet shrinkage and CTE requirements for an end-use application. 

Most micromechanical theories treat composites where the thermoelastic properties of the matrix and of each filler particle are assumed to be homogeneous and isotropic within each phase domain. Under this simplifying assumption, the elastic properties of the matrix phase and of the filler particles are each described by two independent quantities, usually the Young s modulus E and Poisson s ratio v. The thermal expansion behavior of each constituent of the composite is described by its linear thermal expansion coefficient (3. It is far more complicated to treat composites where the properties of some of the individual components (such as high-modulus aromatic polyamide fibers) are themselves inhomogeneous and/or anisotropic within the individual phase domains, at a level of theory that accounts for the internal inhomogeneities and/or anisotropies of these phase domains. Consequently, there are very few analytical models that can treat such very complicated but not uncommon systems truly adequately. 

The general types of behavior predicted for systems following equations 20.1, 20.2 and 20.3 are compared in Figure 20.3. The Young s modulus of the filler in Equation 20.1 was assumed to be 100 times that of the matrix and calculations were performed for Af=10, Af=100 and Af=1000 to compare the effects of discrete filler particles with differing levels of anisotropy. It was assumed that E(hard phase)=100, pc=0.156 and (3=1.8 in Equation 20.2. For simplicity, it... 

---