Filler effects high aspect ratio

The coefficents of thermal expansion of mineral fillers are considerably less than those of thermoplastic polymers and thus their incorporation can significantly reduce that of a composite material. This is a generally useful effect. High aspect ratio fillers, when aligned by processing, will often give rise to anisotropic effects, leading to problems of warp age [69]. 

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. 

As mentioned, inorganic fillers with high aspect ratios are expected to lead to more effective permeability reduction in membranes [19]. According to Fig. 9.2, fillers in the form of flakes would be much more effective in reducing the diffusion and therefore the permeability of membranes than spherical particles. 

Interfacial structure is known to be different from bulk structure, and in polymers filled with nanofillers possessing extremely high specific surface areas, most of the polymers is present near the interface, in spite of the small weight fraction of filler. This is one of the reasons why the nature of the reinforcement is different in nanocomposites and is manifested even at very low filler loadings (<10 wt%). Crucial parameters in determining the effect of fillers on the properties of composites are filler size, shape, aspect ratio, and filler-matrix interactions [2-5]. In the case of nanocomposites, the properties of the material are more tied to the interface. Thus, the control and manipulation of microstructural evolution is essential for the growth of a strong polymer-filler interface in such nanocomposites. 

This paper represents an overview of investigations carried out in carbon nanotube / elastomeric composites with an emphasis on the factors that control their properties. Carbon nanotubes have clearly demonstrated their capability as electrical conductive fillers in nanocomposites and this property has already been commercially exploited in the fabrication of electronic devices. The filler network provides electrical conduction pathways above the percolation threshold. The percolation threshold is reduced when a good dispersion is achieved. Significant increases in stiffness are observed. The enhancement of mechanical properties is much more significant than that imparted by spherical carbon black or silica particles present in the same matrix at a same filler loading, thus highlighting the effect of the high aspect ratio of the nanotubes. 

In addition to this major use to improve mechanical properties, high aspect ratio flake-type fillers have been added to polymers for a variety of other purposes. They include improved thermal stability (3), high voltage resistance (4), electrical conductivity, radiation shielding (5) and optical and aesthetic effects (6). 

Hoffmann et al. [2000] demonstrated that the low-frequency modulus of exfoU-ated PS-based nanocomposites was higher than for intercalated nanocomposites. This conclusion was confirmed by Mohanty and Nayak [2007], who studied the effect of the MMT exfoliation in PA-6-based CPNCs. The large increase in contact surface between the two phases resulted in improved mechanical properties. The high aspect ratio, p = 200 to 1000, the high tensile modulus of the inorganic filler E 170 GPa), and the large specific surface area (Asp 150 m /g) all play a role in the confinement of the polymer chain—hence in mobility under stress [Yasmin et al., 2006 Utracki, 2009],... 

In thick, high-volume PO applications such as extruded pipe, standard (20-pm) talcs provide effective stiffness. But finer talc grades may allow reduced resin use by allowing thinner parts with equivalent beam stiffness. Reduced talc particle sizes (near 1 pm), for instance, can create a better balance of properties in thinner parts to help justify increased filler cost that results from the use of very fine talcs. Lower talc loadings may also be possible the newest high-aspect-ratio talcs are said to create the same mechanical properties at 3%-7% loadings in PP than in PP compounds loaded with 20% standard talc. Cases 8.1 and 8.2 cover applications in which different talcs supply these different functional needs [7-10]. 

The trend is to micronized types (nanocomposites) with high aspect ratios up to 1,000. With a degree of filling of 5 % of these materials, the obtainable properties resemble those obtained with a 30 % degree of filling with conventional fillers, for example nanoclays. The challenge is to distribute the nanoparticles in the plastic matrix uniformly, finely, opened up and with a skeletonizing effect. 

High aspect ratio fillers, such as woUastonite and glass fibers can have an even stronger effect than talc and mica on increasing the modulus of elasticity, tensile strength, and heat-distortion temperature of poly(lactic acid) (PLA)-based systems. 

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