Antistatic and conducting composites of polypropylene

From the viewpoint of electrical behavior, materials can be divided into two groups conductors and insulators. Sometimes a third group, that of semiconductors, is also distinguished. The specific resistance (p) of a material can be calculated as  

The conductivity of composites containing conductive fillers is usually field, frequency and temperature dependent. Field dependence can be explained by the nonlinearities of charge transfer between particles separated by thin insulating layers (tunneling, charge injection, microbreakdown). Frequency dependence is caused, among others, by the 

Antistats are low molecular or macromolecular compounds which are either sprayed onto the surface of the plastic (external antistats) or migrate from the bulk to the surface (internal antistats). These compounds are in several respects simihir to detergents (surfactants), as they contain both nonpolar and polar groups. The latter attract water 

There are two kinds of carbon-based fillers used to produce antistatic and conducting PP composites carbon black and carbon fiber. 

Carbon black filled antistatic and conducting composites usually contain 10-30 wt.% filler. (Addition of 1-2 wt.% carbon black for improving the weatherability essentially does not change the conductivity, although it somewhat increases the dielectric loss). The addition of conducting carbon black to thermoplastics increases not only the electrical conductivity, but also the modulus, tensile strength, hardness, melt viscosity and the heat distortion temperature of the compound. It reduces, however, the elongation to break and impact properties. 

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The exponent n is dose to 0.5 for pure polypropylene and points to diffusive carrier transport. For the composites, n changes from about 0.5 at the low frequencies to 1 at the high frequencies. This fact proves the existence of diffusive as well as hopping carrier transport. In the lowest frequency range the compiosite samples show the frequency-independent behavior pointing to the ohmic conduction. This property makes them to be better antistatic material than pure propylene. 

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