High aspect ratio fillers

Aspect ratio. Aspect ratio is the length of the particle divided by its diameter. A high aspect ratio (typically provided by fibres) provides reinforcement of polymers. The majority of fillers have a low aspect ratio (below 10). 

Filler architecture Filler geometry to some extent is influenced by the way in which the fillers are extracted and processed. The aspect ratio (the ratio of filler length to diameter) is an important characteristic for any material to be used as filler. Thus fillers with high aspect ratio are long and thin, while those with low aspect ratio are shorter in length and broader in the transverse direction [1]. 

Fillers with extremely high aspect ratios (1000-10,000) such as carbon nanotubes (Figure 32.5) have a much lower percolation threshold (lower amount is required for equivalent reinforcement). 

Where a is the composite conductivity, a0 a proportionally coefficient, Vfc the percolation threshold and t an exponent that depends on the dimensionality of the system. For high aspect ratio nanofillers the percolation threshold is several orders of magnitude lower than for traditional fillers such as carbon black, and is in fact often lower than predictions using statistical percolation theory, this anomaly being usually attributed to flocculation [24] (Fig. 8.3). 

Celzard A, McRae E, Deleuze C, Dufort M, Furdin G, Mareche JF. Critical concentration in percolating systems containing a high-aspect-ratio filler. Physical Review B. 1996 Mar 1 53(10) 6209-14. 

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]. 

MW 0/1mud Mica. Wei ground mica is used because of its unique properties, i.e.. luster, slip and sheen, and high aspect ratio. It is used in wallpaper and coated paper, nacreous pigments, as a coating for rubber, in ouidoor house paint, and in aluminum paints. Mica is used in all types of scalers for porous surfaces, such as vvallboard masonry, and concrete blocks, to reduce penetration and improve holdout and as a filler in plasties to improve its electrical and thermal resistance and its insulating qualities. See also Sealants. 

Organic Extenders. Organic extenders are primarily of two types (1) fillers derived from organic materials and (2) low-cost, naturally occurring or synthetic resins. Of the first type, wood flour, shell flour, and other cellulosic fillers are the most common. They also provide a margin of mechanical property reinforcement because of their relatively high aspect ratio. Of the resinous types these are petroleum-based derivatives as well as soluble lignin and scrap synthetic resins. 

This factor approaches 1 for aspect ratios above 10, pointing out the importance of using high aspect ratio fillers. 

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. 

The crystal quality can be varied by careful adjustment of bismuth concentration, temperature, pH, pressure, reactor geometry, and by addition of surfactants. The usually tetragonal bipyramidal structure can be flattened to platelets with a high aspect ratio. Products with an aspect ratio of 10-15 show low luster and a very good skin feel and are used as fillers in cosmetics. Crystals with higher aspect ratios show an exceptional luster and are mainly used for nail polish [5.122, 5.123, 5.128, 5.130]. 

Mica fillers are obtained by separation of mica from other minerals which might compose 10-20% of mineral content. Mica is diy or wet milled and classified. The mechanical grinding produces flakes with a low aspect ratio in a range from 20 to 40. The process may include ultrasonic delamination which leads to a high aspect ratio of over 200. Flakes of mica fillers have a thickness in a range from 1 to 3 im and a width in a range from 10 to 450 pm. 

The most common reasons for adding fillers, especially high aspect ratio fillers, to polymers is to improve physical properties such as increased modulus (stiffness) or reduced creep. 

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). 

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