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served in this experiment were very different for the different formulations, but what becomes apparent is that the performance of the various carbon blacks also was affected.

This clearly indicates that the performance of a conductive or conventional carbon black grade will strongly depend on the formulation used.

Sidcon 159, however, did provide the lowest volume resistivity in every formulation presented in the above example.

Compounding differences

Because of high surface areas of typical “conductive” carbon blacks, including Sidcon 159, it is often appropriate to use them in combination with another black in the formulation.

This usually is done in order to control other essential rubber compounds properties, such as modulus, tensile, hardness, Mooney viscosity, hysteresis and compound economics.

In the example presented in Fig. 12, Sidcon 159, Sidcon 119, Sidcon 419 and conductive N472 were compounded in ASTM D3191 formulation where blends of N550 and the conductive black (CB) was used in the following ratios: 40 phr N550/10 phr CB, 30 phr N550/20 phr CB, and 20 phr N550/30 phr CB.

To establish the control baseline for this example, the rubber compound with 50 phr N550 was prepared and the volume resistivity was determined ( 10^{+ 13} cm). With the addition of conductive carbon black, the resistivity of the compound was reduced dramatically.

Once again, Sidcon 159 provided the highest conductivity (lowest resistivity), and Sidcon 119 was equivalent to the conductive N472 regardless of large surface area differences (more than 100 m²/g) between these two grades.

Replacing 10 phr of N550 with Sidcon 159 reduced the resistivity 8 orders of magnitude to the level of 10^{+ 5} m.

This reduction was achieved while maintaining good processibility but with some effect on other rubber properties.

A summary of rubber properties obtained for the compounds in this example is presented in Fig. 13. All blacks evaluated improved abrasion resistance, with the high surface area Sidcon 159 having the most pronounced effect. All also increased mooney viscosity, with Sidcon 419 and 119 having the least effect.

the majority of conducting electrons are flowing through the relatively poorly dispersed carbon black network.

With increasing mixing time the N660 aggregates separate, and the number of direct connections between them decrease, causing the resistivity to increase.

This result has a significant practical application in compounding rubber for a given resistivity level. It shows that not only the type of carbon black and/or carbon black loading but also carbon black dispersion can alter the level of resistivity. In this example the changes were significant: 10 orders of magnitude.

For practical purposes, poorer dispersion will lead to lower resistivity values.

Because it is not always practical in the factory environment to change the mixing time, other techniques may be employed. One possible technique is to change the order of how carbon black is added to the mixer during the mixing process.

Typically, in the masterbatch stage of mixing, the conventional black is added to the mixer. Next, in the final stage of mixing, the conductive black is added.

The chart in Fig. 15 represents the sequence when conductive black was added in the masterbatch stage of mixing versus when it was added during the final stage.

The volume resistivity was lowered one order of magnitude over a wide range of loadings. If further reduction in resistivity is necessary, the conductive carbon black could be added at the end of productive stage mixing, further reducing dispersion and decreasing resistivity.

resistivity and at the same time maintaining the required rubber processing and dynamic properties.

The influence on polymer type was also discussed. Next, the authors presented examples of how a rubber engineer or compounder could use various mixing and compounding techniques to further reduce or control the compound resistivity.

It was shown that poor dispersion usually leads to lower resistivity. Dispersion can be reduced by reducing mixing time and/or adding the conductive carbon black in the last, productive stage of mixing.

This paper focused on the influence of carbon black on compound volume resistivity, but it should be noted that many other important factors can influence this property.

These include but are not limited to: temperature, humidity, applied pressure and strain history of the sample.

This paper also illustrated the significant influence that carbon black can have on compounds beyond its well known role as a reinforcing filler.

Acknowledgments

The authors would like to acknowledge Sid Richardson Carbon & Energy Co. management for their support and authorization to publish this paper. We wish to thank Mitch Sanders and Patrick Nichols for help in sample preparation and testing.

References

1. M. Gerspacher, C.P. O’Farrell, L. Nikiel and H.H.

Yang, “Furnace Carbon Black Characterization: Continuing Saga,” Rubber Chem. Technol. 69 (1996) 569-576.

2. W.A. Wampler, T.F. Carlson and W.R. Jones, “Rubber Compounding – Chemistry and Applications,” edited by B. Rodgers, Chapter 6, Carbon Black, p. 239-284, Marcel Dekker Inc., 2004. 3. “Carbon Black. Science and Technology,” edited by J-B. Donnet, R.C. Bansal and M-J. Wang, Marcel Dekker Inc. 1993. 4. N.R. de Tacconi, C.C. Ramannair, K. Rajeshwar, W-Y Lin, T.F. Carlson, L. Nikiel, W.A. Wampler, S. Sambandam and V. Ramani (2008). “ Photocatalyti-cally Generated Pt/C-TiO2 Electrocatalysts with Enhanced Catalyst Dispersion for Improved Membrane Durability in Polymer Electrolyte Fuel Cells”. J. Electrochem. Soc. 155( 11), B1102-B1109. 5. C.P. O’Farrell, M. Gerspacher and L. Nikiel, “ Carbon Black Dispersion by Electrical Measurements,” Kautschuk Gummi Kunsts. 53 (2000) 701-710. 6. ASTM D991 -89 (2005) – “Standard Test Method for Rubber Property – Volume Resistivity of Electrically Conductive and Antistatic Products.” 7. R.L. Powell and G.E. Childs, American Institute of Physics Handbook, (1972) pp. 4-142 to 4-160. 8. L. Nikiel, W. Wampler, H. Yang and T. Carlson, “Carbon Blacks for Low-Hysteresis Applications,” Rubber & Plastic News, March 10, 2008. Pages 16- 19. 9. C.P. O’Farrell, M. Gerspacher and L. Nikiel, “Electrical Resistivity of Rubber Compounds. Role of Carbon Black,” ITEC ’98 Select, July 1999, 71-77. 10. L. Nikiel, H. Yang, M. Gerspacher and G. Hoang, “Electrical Resistivity and Thermal Expansion Coefficient of Carbon Black Filled Compounds Around Tg,” Kautschuk Gummi Kunsts. 57 (2004) 538-540. 11. M. Gerspacher, L. Nikiel, H.H. Yang, C.P. O’Farrell and G.A. Schwartz, “Flocculation in Carbon Black Filled Rubber Compounds,” Kautschuk Gummi Kunsts. 55 (2002) 596-604. 12 L. Nikiel, M. Gerspacher, H. Yang, and C.P. O’Farrell, “Filler Dispersion, Network Density and Tire Rolling Resistance,” Rubber Chem. Technol. 74 (2001) 249-259. 13. A.I. Medalia, “Electrical Conduction in Carbon Black Composites,” Rubber Chem. Technol. 59 (1986) 432-454. 14. R. H. Norman, “Conductive Rubbers and Plastics”, Elsevier Publishing Co., New York, 1970.

Summary and conclusions

One of the most important points the authors attempted to accomplish in this paper was to present a simple technique to measure the volume resistivity of rubber compounds which can be employed in the majority of rubber laboratories.

This procedure uses standard curing molds for sample preparation and the measuring instrumentation is just a simple multimeter.

Also, by changing the sample geometry, the range of resistivities can be measured that cover the entire orders of magnitude that have been observed for carbon black filled rubber compounds.

Because carbon black provides the conductivity in rubber compounds by forming the conductive network within the elastomer matrix, the authors presented the measurements of the intrinsic volume resistivity of carbon black itself.

This resistivity was established to be in the order of 10-2 cm and is the same for all carbon blacks including conductive grades. The schematic of the apparatus designed for these measurements was presented.

Volume resistivity measurements of rubber compounds presented in this paper were designed to demonstrate how resistivity depends on carbon black loading, carbon black grade, carbon black specific surface area and structure.

It was shown that resistivity decreases sharply with increased carbon black loading and with increased specific surface area of carbon black.

The performance of conductive black also was discussed. The authors demonstrated how conductive grades like Sidcon 159, Sidcon 119 and Sidcon 419 can influence compound resistivity as well as other properties.

The range of Sidcon conductive grades presented in this paper provides choices to the rubber compounder charged with designing rubber compounds with low

The authors

Leszek Nikiel is manager of the Fort Worth Research Center for Sid Richardson Carbon & Energy Co. He received his doctorate from Texas Christian University in physics in 1991 and began working for Sid Richardson in 1995. E-mail LNikiel@sidrich.com.

Wesley Wampler is vice president of research and development for Sid Richardson. He received his doctorate in chemistry from the University of Texas at Arlington and has been with the company for 24 years. E-mail WAWampler@sidrich.com.

Joel Neilsen is technical service manager for Sid Richardson. He received a degree in chemical engineering from Cleveland State University and has worked in the rubber industry for more than 32 years. Joel has been with Sid Richardson since 2000. E-mail JGNeilsen@sidrich.com.

Nicole Hershberger is the marketing analyst for Sid Richardson. She received her degree from Kent State University in 1999 and has been with the company for three years. E-mail NMHershberger@sidrich.com.

Fig. 15. The volume resistivity versus conductive black loading obtained for two
different mixing sequences as described in the graph legend.
Carbon black dispersion

One of the most effective ways to affect compound resistivity is to change the degree of dispersion by modifications to the mixing time or mixing sequence.

In the following example, the carbon black dispersion was increased by increasing the mixing time (Fig 14).

N660 carbon black at 50 phr in ASTM D3191 formula reaches a point where relatively small differences in mixing time leads to significant differences in resistivity.

One would conclude from this that at 50 phr loading N660 is in the proximity of its percolation point.

In order to demonstrate the same effect for tread grades of carbon blacks, lower carbon black loading should be used, and to achieve the same effect with N700 series black, higher carbon black loadings would be necessary.