Mixing Carbon Black with Polyethylene Pellets

Generally, conductive carbon black is employed in a variety of polymers to achieve permanently dissipative, antistatic, or electro-conductive properties in paints, plastics and rubber. The carbon black not only influences thermal and electrical conductivity but also impacts the coloration and electromagnetic properties of varnishes and paints and even the coloration of rubber and plastics. Blown and cast films, extruded profiles, sheets, pipes, and injection-molded parts are common application areas.

Different types of carbon black come in different conductive properties to serve in numerous applications. They are also made in batches and can differ from one batch to another. As a result, a reliable characterization method is important to measure the conductivity for varied types and batches of carbon black.

Test Equipment

Three consecutive experiments were performed in which high-density polyethylene, also known as HDPE, was first melted in a laboratory mixer and then blended with three different types of carbon black (CB). Three HDPE + CB samples were prepared using a Thermo Scientific™ HAAKE™ PolyLab™ OS Modular Torque Rheometer equipped with an electrically heated Thermo Scientific™ HAAKE™ Rheomix Lab Mixer (600 OS version) with roller rotors, pneumatic ram, and the option for measuring comparative electrical conductivity. The sensor designed for measuring both conductivity and temperature (see Figure 1) can easily penetrate the sample and is separated from the mixing chamber. The system determines the resistance between the mixing chamber and the conductivity sensor for an output value of Siemens S = 1/Ω. If there is a higher comparative conductivity value in Siemens, the conductance in the final mixture will be better.

Figure 1. The chamber of the Rheomix Lab Mixer (600 OS version) with combined melt temperature and conductivity sensor

Test Conditions

33 g of HDPE pellets were initially placed into the mixer chamber and then mixed at a rotor speed of 100 rpm for 5 minutes and a mixer temperature of 150 °C. During this time, the conductivity of the polymer and the mixer torque were determined and shown in a rheogram (Graphs 1, 2, and 3). Once the polymer became molten, 4 g of carbon black was added by lifting the pneumatic ram. The mixer chamber was then closed by lowering the ram, and mixing and measuring went on for another 10 minutes.

Materials and Results

By using three different types of carbon black (CB1, CB2, and CB3) and the same grade of HDPE, three different samples were analyzed. In the case of CB1, batch-to-batch variations were checked by testing two manufacturing batches.

Table 1 shows the mixer results for the various batches and samples. The electrical conductivity (right y-axis) and the torque (left y-axis) of all tests are shown with the same scaling for the y-axis in all rheograms.

Table 1. HDPE + CB samples, their electrical resistance and productivity. Electrical conductivity obtained by van der Pauw method¹.

Tests with carbon black type 1 batch 2 — that is, sample numbers 2 and 3 — were repeated twice (see Graph 1) to demonstrate the reproducibility of the test method. Both test results demonstrated the same comparative conductivity baseline (red curves) of 230 mS, and when the HDPE became fully molten, the torque signal displayed the usual loading peak and decreases to 13.4 Nm.

Graph 1. Repeatability of the mixer test with samples 2 and 3.

The carbon black was added after a period of 5 minutes. Due to the location of the conductivity sensor in the mixer chamber, the measure of conductivity showed an instant increase upon adding the carbon black. After the carbon black was mixed more thoroughly, the comparative conductivity was observed to decrease again because particles of carbon black were better distributed inside the polymer matrix. After a period of 15 minutes, the output value was 620 and 550 mS, respectively. Upon adding the carbon black, a higher torque signal (blue curves) was obtained as it reinforced the polymer melt, but this leveled out toward the end of the mixing time at 17 Nm. The results demonstrate good repeatability for the test method. Graph 2 shows the variation between the two batches of carbon black type 1, that is, sample 1 and sample 2.

Graph 2. Variance of two batches of carbon black type 1 (sample 1 and sample 2).

Comparative conductivity readings demonstrate a variation, whereas the torque for both samples (blue curves) was found to be equal with 17 Nm toward the end of the test. After mixing for 15 minutes, Batch 1 of carbon black type 1 (sample 1) was found to have a value of 1890 mS (light red curve) when compared to batch 2 of carbon black type 1 (sample 2) with just 620 mS (dark red line). All conducted tests were compared (see Graph 3), which showed good agreement with the electrical conductivity of the raw carbon blacks given in Table 1.

Graph 3. Rheograms of all four types of carbon black tested.

At the end of the mixer test, carbon black type 2 (pink line) displayed the highest comparative conductivity with 12,200 mS. Carbon black type 3 (violet line) with a baseline value of 245 mS after mixing for 15 minutes was not conductive. Similar torque curves were observed in all conductive carbon blacks. However, after the carbon black was added to the polymer melt, the torque curve of the nonconductive CB3 was slightly different.

Conclusion

By using the HAAKE PolyLab OS system equipped with a batch mixer, researchers can get reproducible results of comparative conductivity data easily and rapidly. This article demonstrated how that can be accomplished by using HDPE samples with different types of carbon black. Variations in different types and batches of carbon black can be reliably tracked. The quantified results showed good correlation with the values achieved through the van der Pauw method¹ for electrical conductivity (see Table 1). Furthermore, with comparative conductivity measurement, a more precise characterization of the value can be realized to aid material scientists to further improve their formulations. Moreover, this test method can also be applied to rubber applications².

References and Further Reading

1. A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shapel. - J. van der Pauw: Philips Res. Reports 13 (1), 1958, p. 1-9.

2. Macro- and microdispersion of carbon black in liquid silicone rubbers Le, H. H.; Ilisch, Sybill; Radusch, Hans-Joachim; Steinberger, H.; Plastics, rubber and composites. - London: IOM, Bd. 37.2008, 8, S. 367-375.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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