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Electrically conductive

Electrically and thermally conductive polymer composites by shashak
Posted: 16 Jul 03

Typical electrical resistivity for most polymers is of the order of 10^12 to 10^14 ohms.

Adding amines or quaternary ammonium compounds, phosphate esters and polyethylene glycol ester or inherently ESD polymers that can be compounded into the polymer will impart electrical resistivity in the range of 10^4 to 10^9 ohms rendering it electrostatically dissipative. ESD with inherently conductive polymer compounds is clean, controlled and moisture insensitive. However, improvement in thermal conductivity is not substantial.

Carbon black based compounds (highly conductive blacks like Raven or Printex XE2) when loaded up high enough (10 wt% or higher) can lower electrical resistivity to 100 - 10000 ohm range. Lower loadings can result in an ESD composite. Thermal conductivity is slightly improved but leaves much to be desired.

Using PAN based or pitch based carbon fibers, nickel coated graphite fibers (NCGF), vapor grown carbon fibers (VCF), silver coated graphite fibers and carbon nanotubes (CNT) can lower resisitivity to 10^-4 to 10^-5 ohms which is excellent for EMI shielding. Short carbon fibers composites are much more effective in EMI shielding than particulate composites and relatively less loading is required. Compounding can be tricky due to problems with feeding fibers consistently to the mixing process.

Importantly, it has been found that a 50/50 blend of elastomers having different polarity (for e.g. EVA/EPDM) is quite effective in forming a well-defined conductive particulate or fibrous blend interface.

Most importantly, if the objective is to get good electrical AND thermal conductivity, use of a fiber filled composite (loading upwards of 10wt%) is often a viable solution. The use of a properly configured low shear compounder is very crucial to get most oomph out of the composite. Using a high shear compounder is known to be detrimental to the mean fiber length in the composite, which directly affects the physical properties. Use of aluminum flakes or metal particles though effective is not very desirable due to wear on the compounder and loss of mechanical (impact) properties.

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