Intrinsically conductive polymer-carbon plastics

Thermally Stable Intrinsically Conductive Polymer-Carbon Black Composites as New Additives for Plastics... 

When the first intrinsically conductive polymer (ICP) was discovered by Hideki Shirakawa, Alan G. Mac-Diarmid and Alan J. Heeger at the University of Pennsylvania in Philadelphia in the late seventies, it was thought in the initial euphoria that it would not be long before such materials could be put to practical use. The idea was that it ought to be possible to process them more easily and in larger quantities than classical metallic conductors and compared with carbon-blackfilled plastics they were expected to possess better and more uniform conductivity and better mechanical properties. 

In order to render a plastic conductive although it is, by nature, an electrical and thermal insulator (with the exception of intrinsically conducting polymers, ICPs), we need to dope it with electrically conductive fillers such as steel microfibers (pFSs) [FEL 06], CNPs [FEL 01] or indeed carbon nanotubes [FEL 11]. By gradually varying the proportion of fillers in the polymer matrix, we see that its resistance goes... 

One of the main limitations of intrinsically conductive polymers (ICP s) towards their wide application as conductive additives for thermoplastics is their poor thermal-oxidative stability at typical melt processing temperatures (i.e., above 200 °C). On the other hand, the use of high surface area carbon blacks (CB) as conductive additives is limited due to the increased melt viscosity of their blends with thermoplastics. Eeonomers are a new class of thermally stable, chemically neutral, and electrically conductive composites made via in-situ deposition of conductive polyaniline (PANI) or polypyrrole (PPY) on CB substrates. Eeonomer composites are more stable (up to 300 °C) than pure ICP s and more easily processible with thermoplastics than CB. Use of Eeonomers as conductive additives for plastics lead to compounds with improved electrical, mechanical, and processing properties. By varying Ae conductive polymer to CB ratio, it is possible to fine tune the polarity of Eeonomer composites and achieve very low percolation thresholds. This control is possible because of preferred Monomer localization at the 2D phase boundary of the immiscible polymer blends. 

Conductive polymer composites can be defined as insulating polymer matrices which have been blended with filler particles such as carbon black, metal flakes or powders, or other conductive materials to render them conductive. Although the majority of applications of polymers in the electrical and electronic areas are based on their ability to act as electrical insulators, many cases have arisen more recently when electrical conductivity is required. These applications include the dissipation of electrical charge from rubber and plastic parts and the shielding of plastic boxes from the effects of electromagnetic waves. Consequently, materials scientists have sought to combine the versatility of polymers with the electrical properties of metals. The method currently used to increase the electrical conductivity of plastics is to fill them with conductive additives such as metallic powders, metallic fibres, carbon black and intrinsically conducting polymers such as polypyrrole. 

Eeonomers are a new class of conductive additives for thermoplastics made via in-situ deposition of intrinsically conductive polyaniline or polypyrrole on carbon black. Eeonomers are highly thermally stable, pH neutral conductive materials that are compatible with the chemistry and melt processing conditions of acid sensitive polymers. Compounding studies with thermoplastics indicate better electrical, mechanical, and melt flow properties of Eeonomer blends as compared to blends with traditional carbon blacks. In co-continuous plastic blends it was possible to fine tune the polarity of Eeonomer by varying the conductive polymer to CB ratio. The same variation affords very low percolation thresholds due to preferred Eeonomer localization at the 2D phase boundary. 

When one thinks of polymers, one perhaps envisions common plastics, such as polythene, that one may encounter in everyday life. If one then conjures up a conducting polymer, one may perhaps envision these plastics filled up with conductors such as metal or carbon particles. The Conducting Polymers (CPs, also sometimes called Conductive Polymers or Conjugated Conductive Polymers or Organic Polymeric Conductors), which are the subject of this book, are quite a different beast, in the sense that they are intrinsically conducting, and do not have any conductive fillers as such. 

The fact that plastics are good insulators does not mean that plastics are inert in an electrical field. They can in fact, be made to conduct electricity by the addition of fillers such as carbon black and metallic flake. The type and degree of interaction depends on the polarity of the basic resin material and the ability of an electrical field to produce ions that will cause current flows. In most applications for plastics, the intrinsic properties of the polymer are related to the performance under specific test conditions. The properties of interest are the dielectric strength, the dielectric constant at a range of frequencies, the dielectric loss factor at a range of frequencies, the volume resistivity, the surface resistivity, and the arc resistance. The last three are sensitive to moisture content in many materials. These properties are determined by the use of standardized tests described by ASTM (Table 16-1). These properties of the plastics are temperature dependent as are many of their other properties. Temperature dependence must be recognized to avoid problems in electrical products made of plastics. 

Biobased plastics, in which the fossil carbon is replaced by biobased carbon from plant-biomass resonrces, offer the intrinsic value proposition of a sustainable, zero material carbon footprint which is in balance with the rates and timescale of the biological carbon cycle. The process carbon and environmental footprint using LCA methodology is important and needs to be conducted as well. However, it does not capture nor convey the true, intrinsic value proposition of the zero material carbon footprint arising from the selection of the plant-biomass carbon resources. Identification and quantification of biobased content is based on the radioactive C-14 signature associated with (new) biobased carbon. Not all biobased plastics are biodegradable and not all biodegradable polymers are biobased. 

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