Other Carbon-Based Conductive Fillers

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. 

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Carbon black is the most widely used conducting filler in composite industry. Carbon black filled immiscible blends based on polar/polar (65), polar/nonpolar (63,66), nonpolar/nonpolar thermoplastics (67,68), plastic/rubber and rubber/mbber blends (69,70) have already been reported in the literature. The properties of carbon black filled immiscible PP/epoxy were reported recently by Li et al. (60). The blend system was interesting because one of the components is semicrystalline and the other is an amorphous polar material with different percolation thresholds. The volume resistivity of carbon black filled individual polymers is shown in Fig. 21.23. 

Graphene-polymer nanocomposites share with other nanocomposites the characteristic of remarkable improvements in properties and percolation thresholds at very low filler contents. Although the majority of research has focused on polymer nanocomposites based on layered materials of natural origin, such as an MMT type of layered silicate compounds or synthetic clay (layered double hydroxide), the electrical and thermal conductivity of clay minerals are quite poor [177]. To overcome these shortcomings, carbon-based nanofillers, such as CB, carbon nanotubes, carbon nanofibers, and graphite have been introduced to the preparation of polymer nanocomposites. Among these, carbon nanotubes have proven to be very effective as conductive fillers. An important drawback of them as nanofillers is their high production costs, which... 

In case of other fillers, the nanofillers can introduce new functionality into the polymer, e.g. electrical conductivity in case of carbon based nanoparticles, barrier properties in case of platelet like nanofillers (nanoclay, expanded graphite), enhancement of mechanical properties, enhanced flame retardancy, and many others. 

Composite-based PTC thermistors are potentially more economical. These devices are based on a combination of a conductor in a semicrystalline polymer—for example, carbon black in polyethylene. Other fillers include copper, iron, and silver. Important filler parameters in addition to conductivity include particle size, distribution, morphology, surface energy, oxidation state, and thermal expansion coefficient. Important polymer matrix characteristics in addition to conductivity include the glass transition temperature, Tg, and thermal expansion coefficient. Interfacial effects are extremely important in these materials and can influence the ultimate electrical properties of the composite. 

Among carbon fillers, carbon black is most commonly used due to good conduction performance, and metallic oxides are often used to make fiber white. Du Pont produced a composite nylon fiber made up of nylon sheath and conductive polymer core formed by dispersing about 30% carbon Hack in LDPE matrix. When the conductive core content was ca. 4%, the was around 10 cm[96,97]. Toray[98] developed a composite nylon fiber made up of nylon-6 sheath and conductive polymer core formed by dispersing about 30% carbon black in nylon-6 matrix. When the conductive core content was ca. 5%, the was 10 to 10 cm. Other conductive nylon fiber was reported by Unitika[99,100], in which 25% acetylene black was dispersed in nylon-6, which was combined with the same nylon 6 base polymer at a ratio of20/80. The conductive polymer was exposed onto the fiber surface to increase efficiency. A white-colored conductive nylon fiber was also obtained by using titanium dioxide particles with diameters of 2 pm or less coated with tin oxide. A heat resistant conductive nylon fiber was obtained by dispersing carbon black in an aromatic polyamide[101]. 

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