Composite conducting filler loaded

Janzen, J. (1975) Onthe critical conductive filler loading in antistatic composites. 

In many composites, conducting fillers (carbon black, carbon nanotubes, or metal nanoparticles) are added to make material conductive. The relationship between composite morphology and electrical conductivity has been studied extensively, especially in the context of carbon black filled polymers [156-162]. It is well known that the dependence of conductivity on the loading of conductive filler, percolation theory there is some threshold filler loading below which there is no conductive pathway through the system and conductivity is zero above the threshold, conductivity grows very rapidly as ... 

The thermal properties of fillers differ significantly from those of thermoplastics. This has a beneficial effect on productivity and processing. Decreased heat capacity and increased heat conductivity reduce cooling time [16]. Changing thermal properties of the composites result in a modification of the skin-core morphology of crystalline polymers and thus in the properties of injection molded parts as well. Large differences in the thermal properties of the components, on the other hand, lead to the development of thermal stresses, which also influence the performance of the composite under external load. 

Fig. 30 a Shielding effectiveness of various composite systems (at 12 GHz), b EMI shielding effectiveness as a function of conductivity at 16 wt% filler loading... 

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. 

Another common reason to add a filler to a polymer is to increase either electrical conductivity or thermal conductivity. Polymers typically have electrical conductivity from 10 to 10 S/cm though the addition of a moderately conductive filler such as carbon black conductivities of lO -lO S/cm are possible highly conductive fillers such as silver can raise this value to 10 -10 S/cm. Applications include static dissipative devices and surge protectors. The impact of adding a highly thermally conductive filler to a polymer is much smaller at low-volume fractions vs. the impact of an electrically conductive filler on electrical conductivity. However, if a highly loaded stiff product is acceptable, polymer composites are capable of dissipating substantial amounts of heat. 

For carbon nanotubes, discussed in detail in Chapter 10, conductivity is achieved at lower loadings (by weight) but these materials are difficult to disperse in molten polymers. Methods of surface functionalization and lower cost manufacturing must be developed before carbon nanotubes will find wider use as conductive fillers [52, 53]. As an alternative to nanotubes, Fukushima and Drzal [54] have observed conductivity thresholds of less than 3 vol% in composites containing acid-etched or othervdse functionalized exfoliated graphite. These composites retain or improve upon their mechanical properties compared to other carbon-filled polymers. 

The cheapest and most-often-employed materials have been polymer blends or composites containing carbon black as filler (428,429). However, high loading levels of conductive fillers are often required, leading to significant decreases in durability and transparency. In these cases, the use of EAPs, either as coatings or as fillers, is becoming more common (426). 

At low filler loading, CNTs are dispersed independently as individual fillers in the polymer matrix. The electrical behavior of composites is limited by the polymer matrix, having conductivity in the order of 10 S.cm . When sufficient nanotube content is loaded, the distance between the fillers is reduced considerably, leading to the formation of a network of connected nanotube paths through the insulating matrix. This critical filler content is commonly termed as the percolation threshold . At this filler content, the conductivity rises drastically by several orders of magnitude. In this respect, the insulating polymer becomes electrically conductive. 

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