additives | Carbon black


Moisture effect on conductivity for Imerys Ensaco carbon black and equivalent surface area furnace black


possibility of diluting with a cheap virgin polymer or with regrind. “This is a trend that [is increasingly popular] for the more commodity polyolefi ns and polystyrene-based injection moulding applications,” she says.


Source: Imerys


Comparison of the resistivity of EVA-based compounds charged with AkzoNobel’s Ketjenblacks with a competitive electro- conductive carbon black


However, Van Bellingen adds that the risks associated with this the practice should not be overlooked. “Any dilution should be carefully managed, considering the sharp slope of a percolation curve,” she points out. “Half a percent too low in carbon black is enough to make the material becoming insulating rather than conductive.” There are other technical and commercial consid- erations too, says Van Bellingen. “Diluting a very viscous product (the concentrate) with a fl uid polymer can lead to very inhomogeneous conductive end products. Last but not least, that route will require the use of higher carbon black loadings as a strong reduction in the structure of the black structure takes place during the realisation of the conductive concen- trate.” This structure reduction is caused by high shear forces applied on the CB aggregates to reduce viscosity. “At each step of the manufacturing process, one should consider that the carbon black structure—the main factor in providing the electrical conductivity—is ‘evolutive,’ says Van Bellingen. “By far, using the split feeding direct compounding route is the safest and best performing route.”


Watch out for water


compounds. “When conductive compounds are made using a twin screw extruder, the split feeding technique [carbon black added downstream to the molten polymer] will lead to higher conductivities than the upfront single feeding technique where the solid-solid contact reduces too much the CB structure and conductivity,” says Van Bellingen. For compounders, she says, an apparently attractive


solution could be to use conductive concentrates to make conductive end products, since this leaves them with the


Imerys Graphite & Carbon uses its own production process, different from the most widely used furnace process, to produce its Ensaco specialty conductive carbon blacks. Van Bellingen says the process delivers carbon blacks with a unique combination of high structure and low surface area, which guarantees very good dispersion in plastics compounds.


“It is known that lower surface area carbon blacks


exhibit lower moisture pick-up than higher surface area carbon blacks,” she says. “However carbon blacks of equal surface area may still differ from each other. When compared to a furnace black of equally low surface area, Ensaco250G [the company’s workhorse conductive grade] still shows a lower moisture pick-up.” The Imerys Graphite & Carbon production process


Impurity levels in compounds containing AkzoNobel’s Ketjenblacks and a competitive electro-conductive carbon black ( target resistivity of 200 Ω·cm) compound


KB EC 300J KB EC 600JD


Other carbon black


Target resistivity [Ω·cm] Required carbon black [wt%] ash [wt%] 200 200 200


12 5


27


1.3 E-3 0.8 E-3 2.7 E-3


grit [ppm] sulphur [wt%] 0.31 0.19 0.54


3.1 E-3 3.0 E-3 5.4 E-3


Impurity levels in compounds containg AkzoNobel’s Ketjenblacks and a competitive electro-conductive carbon black ( target resistivity of 200 Ω·cm) 30 COMPOUNDING WORLD | November 2015 www.compoundingworld.com


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