INSIGHTS ON CONDUCTIVE PLASTICS - Modern Dispersions

INSIGHTS ON CONDUCTIVE PLASTICS

Overview
Most plastics are inherently electrically insulating materials,
they do not conduct electricity. In some applications, imparting
electrical conductivity adds significant value and utility. For
example by imparting electrical conductivity to plastics enables
electrostatic painting in automotive bumpers, reduces dust
collection of household molded articles and facilitates

fabrication of some types of plastic films – reducing “telescoping” or the creation and
storage of static electricity. Electrically conductive plastic compounds are used as jacket
coverings for high power transmission cables – reducing induced currents from nearby
cables. Additionally, electrically conductive plastics are used for shielding sensitive
electronic components.

There are three performance regions of electrical conductivity of plastics; 1) antistatic, 2)
electrostatic dissipation (ESD) and 3) electromagnetic dissipation (EMI.) Antistatic
applications involve materials that exhibit surface resistivity of 1012 to 106 ohm-cm, ESD
applications involve imparting sufficient conductivity to plastics to reduce their surface
resistivity to the range of 106 to 104 ohm-cm. The EMI range is below 104 ohm-cm.

Sterate additives are typically used to achieve antistatic performance levels, carbon black
and specialty additives are frequently used to achieve ESD performance, whereas, to
achieve EMI performance metal powders or wires are needed to achieve this level of
conductivity. This Modern Dispersions Insight Article will focus on compounds used to
achieve ESD performance and the importance of dispersion on achieving electrical
conductivity.

Factors influencing conductivity: A variety of factors influence conductivity of plastic
compounds including, the inherent conductivity of the plastic, the level of dispersion
achieved for the conductive additive, the intrinsic conductivity of the additive and the

applied electric potential. In the case of antistatic compounds, the conductive mechanism
is surface conductivity thorough bridges formed between water molecules absorbed onto
the polymer surface enabled through the use of surface active agents like glycerol
derivatives.

For ESD performance, carbon black is frequently used as the active agent and
conductivity is achieved by forming conductive bridges through a “conduction zone” of
overlapping electronic structures enabling nearest-neighbor transfer of electrons.
Consequently, to achieve conductivity using carbon black, there needs to be sufficient
carbon black present so as to form conductive bridges for the electrons.

Percolation: Carbon black when used to impart electrical conductivity to plastics exhibits
a phenomenon known as percolation – where the level of carbon black is sufficient to
cause a significant and abrupt increase in electrical conductivity. As the loading of the
carbon black in the compound increases, the plastic compound remains initially
insulating, as the loading increases the conductivity passes through a sharp and abrupt
rise over a very narrow black concentration (loading) range. Further increases in loading
past this threshold cause little increase in the conductivity. This narrow range is known as
the percolation threshold.

Structure influences conductivity: Carbon black structure, surface area and loading have
significant influences on the compound conductivity. The Janzen equation, a widely used
model, can be used to predict the percolation threshold concentration based on the density
and structure (CDBP) of a carbon black.

The Janzen equation is shown as follows:
ϕcrit = 1 / (1 + 4 ρυ)

ϕcrit = the critical volume fraction (threshold concentration)
ρ = density of carbon black
υ = the DBP absorption on the crushed carbon black expressed in cm3/g.

The conductivity efficiency of carbon black is a function of primary particle size,
structure and porosity. Small particle size blacks have high surface area and higher inter-
aggregate attractive force which result in agglomerates and a pseudo “secondary
structure.” Consequently, the pseudo-structure results in higher conductivity than would
have been predicted based on the intrinsic structure of the nascent carbon black.
However, this secondary structure can cause a reduction in mechanical property and an
increase in melt viscosity.

An ideal carbon black compound for the industrial users should have the following
desirable attributes:

• low percolation threshold (efficiency)
• minimal degradation of mechanical properties
• minimal effect on compound melt rheology
• low compound moisture absorption (CMA)
• cost effective

Achieving the desired balance of properties frequently involves compromises.

Importance of dispersion quality: Considering that ESD conductivity is achieved by
creating bridges among the conductive additives, a high quality dispersion is essential to
distribute the conductive additives homogeneously within the polymer matrix and to
maintaining the balance of properties desired by the end-use application. Carbon black
loadings of over 20% are frequently needed to achieve ESD performance in most
thermoplastic resins. At this loading level physical properties of the polymer are
frequently compromised, thus, the selection of the right carbon black to impart
conductivity but not compromise properties or processing is critical. Skill and
knowledge, achieved through years of experience, are essential in developing the
appropriate compound for the specific resin and specific end-use application.

Modern Dispersions, Incorporated offers a family of products for the static dissipative
and conductive plastics markets. Our products are marketed under the Realstat® brand.
Please visit our website for more information on our antistatic products.