Polymer composites electrical property

Nearly every polymeric system absorbs some moisture under normal atmospheric conditions from the air. This can be a difficult to detect, very small amount as for polyethylene or a few percent as measured for nylons. The sensitivity for moisture increases if a polymer is used in a composite system i.e. as a polymeric matrix with filler particles or fibres dispersed in it. Hater absorption can occur then into the interfacial regions of filler/fibre and matrix [19]. Certain polymeric systems, like coatings and cable insulation, are for longer or shorter periods immersed in water during application. After water absorption, the dielectric constant of polymers will increase due to the relative high dielectric constant of water (80). The dielectric losses will also increase while the volume resistivity decreases due to absorbed moisture. Thus, the water sensitivity of a polymer is an important product parameter in connection with the polymer s electrical properties. The mechanical properties of polymers are like the electrical properties influenced by absorption of moisture. The water sensitivity of a polymer is therefore in Chapter 7 indicated as one of the key-parameters of a polymeric system. 

CNTs could have either metallic or semiconducting properties, depending on their diameters and chiralities. Due to their exceptional electrical properties, CNTs have become one of the most attractive materials to be used as conductive fillers in polymer composites. Electrical conductivity in insulating polymer composites is well described by means of the percolation threshold. The percolation threshold is the filler concentration at which the electrical conductivity increases sharply by orders of magnitude, indicating that conductive paths span the macroscopic sample [87]. 

As filler particles in the composite matrix are separated from each other by thin layers of polymer, the electrical properties of the matrix will influence electron transport, and thus the contact resistance, between particles. It has been shown theoretically that the characteristic trrrmeling length of electrons is larger in a host medirrm with a higher dielectric... 

Antioxidants are used to retard the reaction of organic materials with atmospheric oxygen. Such reaction can cause degradation of the mechanical, aesthetic, and electrical properties of polymers loss of flavor and development of rancidity ia foods and an iacrease ia the viscosity, acidity, and formation of iasolubles ia lubricants. The need for antioxidants depends upon the chemical composition of the substrate and the conditions of exposure. Relatively high concentrations of antioxidants are used to stabilize polymers such as natural mbber and polyunsaturated oils. Saturated polymers have greater oxidative stabiUty and require relatively low concentrations of stabilizers. Specialized antioxidants which have been commercialized meet the needs of the iadustry by extending the useflil Hves of the many substrates produced under anticipated conditions of exposure. The sales of antioxidants ia the United States were approximately 730 million ia 1990 (1,2). 

Fillers may be broadly defined as solid particulates or fibrous materials, substantially inert chemically, incorporated in polymer compositions to modify the properties and/or to reduce cost. Cost reduction is not the primary reason to incorporate fillers in adhesives but they are used to impart specific properties such as flow, improved adhesion, mechanical, thermal, electrical and optical properties, chemical and weather resistance, and rheological behaviour. 

We conclude that the preparation of the samples of the polymer composites with the corresponding electrical properties in the form, say, of the plates, bars, hollow cylinders, etc., that are usually used for the purpose of research in the laboratories, and of real articles should be considered as two interrelated problems. This is important and should be stressed, as the values of the conductivity and other parameters obtained for the simple forms might prove different for the forms that may be used as constructional elements. Therefore, these circumstances should be taken into account at the design stage of a conducting composite as well as the optimum technological techniques of molding of practically important articles. 

The Emerman model described in the previous section is hardly applicable to the carbon black-filled CCM as the black particles have sizes of hundreds angstrom and such a composite, compared with the molding channel size, may be considered as a homogeneous viscous fluid. Therefore, the polymer structure, crystallinity and orientation play an important role for such small particles. The above-given example of manufacture of the CCM demonstrates the importance of these factors being considered during processing of a composite material to and article with the desired electrical properties. 

Particular examples of using polymer composites as screens are given in [14-16, 67-75], The present review does not touch the properties of the composite materials based on fabrics of conducting fibres due to the fact that manufacturing techniques for such materials are specific and differ greatly from the mixing processes considered above. However, these materials also have an application field, say, in contacts for calculator and computer keyboards [9] and even in small-power electric motor commutators as a partial substitute for copper [76, 77]. 

Alignment of CNTs markedly affects the electrical properties of polymer/CNT composites. For example, the nanocomposites of epoxy/MWCNTs with MWCNTs aligned under a 25 T magnetic field leads to a 35% increase in electric conductivity compared to those similar composites without magnetic aligned CNTs (Kilbride et al., 2002). Improvements on the dispersion and alignment of CNTs in a polymer matrix could markedly decrease the percolation threshold value. 

Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing. J. Appl. Phys. 94 6034-6039. 

Spitalsky, Z., et al., Carbon nanotube-polymer composites Chemistry, processing, mechanical and electrical properties. Progress in Polymer Science, 2010. 35(3) p. 357-401. 

The research on nanocarbons dispersed in polymer matrices in recent years has shown that this route is very efficient at small volume fractions above electrical percolation, where it can be the basis for new composite functionalities in terms of processing and properties. It is also clear that there is an inherent difficulty in dispersing these nanoscopic objects at high volume fractions, which therefore limits composite absolute properties to a very small fraction of those of the filler. Independent of their absolute properties, composites based on dispersed nanocarbons have served as a test ground to understand better the basic interaction between nanocarbons and polymer matrices, often setting the foundation to study more complex composite structures, such as those discussed in the following sections. 

Such polymer composites (that will not be treated in this chapter) can be used as precursors to the C3 materials where the polymer is converted into a carbon phase with a low content of heteroatoms. A well-developed sp2 structure is desired, with its basic structural units being oriented perpendicular to the fiber axis. The required excellent mechanical and transport properties in the weak direction of the initial fiber can thus be delivered. This material is now called carbon and finds widespread application in energy-related structural material applications such as electric passenger cars, as construction material for airplanes and as the core structure of turbine blades for windmills and compression turbines. 

---