How Different Carbon-Black Types Influence the Processing Behavior of a Rubber Compound

A major portion—up to 15%—of the gas in a car’s tank is used to overcome the tires’ resistance to the road. So with green tires—or low rolling resistance tires—the fuel economy can increase greatly.

It is essential to comprehend the process ability of new rubber compound formulations to develop more fuel-efficient tires. The ideal equipment to study this behavior is a torque rheometer with laboratory extruders and laboratory mixers because it mimics the processing conditions in a small-scale testing environment.

The following article explains how to examine the influence of three different carbon black types on the process ability of a rubber compound formulation used for tire production.

Test Samples

The following are the rubber compound tire formulations. These are based on a branched cobalt butadiene rubber, specifically, Buna® CB 1220 from ARLANXEO, using three different kinds of carbon black:

  • N234 Rubber Carbon Black:
    • Fine reinforcing carbon black with increased structure
    • Iodine adsorption: 120 g/kg, Nitrogen surface area: 118 m2/g
  • N326 Rubber Carbon Black:
    • Fine reinforcing carbon black with low structure
    • Iodine adsorption: 82 g/kg, Nitrogen surface area: 78 m2/g
  • N339 Rubber Carbon Black:
    • Fine reinforcing carbon black with increased structure
    • Iodine adsorption: 90 g/kg, Nitrogen surface area: 91 m2/g

Testing Equipment

Thermo Scientific™ HAAKE™ PolyLab™ OS modular torque rheometer platform with:

  • Single screw extruder: Thermo Scientific™ HAAKE™ Rheomex™ 19/10 OS rubber
  • Drive unit: Thermo Scientific™ HAAKE™ RheoDrive 7 OS
  • Screw diameter 19 mm, length L/D 10, compression ratio 1:1

Schematic drawing of a Garvey die and its profile.

Figure 1. Schematic drawing of a Garvey die and its profile.

Test Method 1: Garvey Test

To conduct the Garvey test, the extruder was fitted with an extrusion die with a Garvey profile adhering to ASTM D2230 as shown in Figure 1 and a conveyor belt take-off.

The Garvey die generates a profile with four different angles, which looks like a scaled-down interpretation of half of a tire tread. A well-flowing rubber compound will show a polished profile with no defects even in the smallest corners. A poor flowing rubber compound will give a ripped, uneven, and swollen profile, as depicted in Figure 2. The quality of an extruded profile is then graded as per the ranking system mentioned in the ASTM standard.

Extruded Garvey die profile.

Figure 2. Extruded Garvey die profile.

Figure 3 lists the results of the Garvey test with the three rubber compounds.

Garvey profile examples of the three rubber formulations.

Figure 3. Garvey profile examples of the three rubber formulations.

Figure 3 reveals that the type of the carbon black has a powerful influence on the quality of the profile. The carbon blacks with increased structure (CB N234, CB N339) produce a much-polished profile compared to the sample with the low structure carbon black (CB N326).

Test Method 2: Die-Swell Measurement

Die swell, which is also referred to as the Barus effect, is a familiar phenomenon in rubber and polymer processing. Die swell occurs when a polymer stream is constricted by entrance into a die and is followed by a “swell” back or partial recovery to the initial volume and shape of the polymer after exiting the die.

To conduct this test, the extruder was fitted with a vertical rod die, a rod die nozzle D = 2 mm, L/D = 0 and a laser die-swell tester.

PolyLab OS setup with Die-Swell measurement.

Figure 4. PolyLab OS setup with Die-Swell measurement.

The system continuously calculates the diameter of the expanded strand. From the relation between the calculated diameter and the actual diameter of the rod die nozzle, the die swell is calculated.

Schematic and calculation of Die-Swell measurement.

Figure 5. Schematic and calculation of Die-Swell measurement.

The three samples were tested at three different screw speeds of 20 rpm, 40 rpm, and 60 rpm.

Figure 6 displays the results of these tests.

Die-Swelling phenomena for the three rubber formulations.

Figure 6. Die-Swelling phenomena for the three rubber formulations.

The powerful influence of the type of the carbon black can be explicitly seen. The compounds with the carbon black with increased structure (CB N234, CB N339) show a much lower swelling behavior when compared to the sample with the low structure carbon black (CB N326).

The Die-Swell test also exhibits clear differences between the compounds with the carbon blacks with the increased structure (CB N234 and CB N339).

Test Method 3: Extruder Capillary Rheology

To test the rheological characteristics of the rubber compounds, the extruder was fitted with a horizontal slit capillary die with a measuring geometry of W = 20 x H = 2.0 mm. This is shown in Figure 7.

horizontal slit capillary die.

Figure 7. Schematic of a horizontal slit capillary die.

A balance connected to the PolyLab control computer with an RS232 connection was used to measure the output of the extruder.

The PolySoft™ OS Capillary Rheometry software package is conceived to implement the measurement sequence spontaneously after programming. The software drives the extruder at different speed steps. At each speed step, it calculates the pressure drop inside the slit capillary to measure the shear stress, and it uses the output information from the balance to measure the shear rate. Based on this measurement data, the compound viscosity is measured at different shear rates as shown in Figure 8.

Viscosity calculation for the three different rubber formulations.

Figure 8. Viscosity calculation for the three different rubber formulations.

The results of the viscosity measurements.

Figure 9. The results of the viscosity measurements.

Again, it is revealed that the type of the carbon black has a powerful influence on the quality of the profile. The compounds with increased-structure carbon black (CB N234, CB N339) demonstrate a lower shear thinning effect and a higher viscosity. The rubber compound with low-structure carbon black (CB N326) reveals a much lower viscosity, especially at higher shear rates.

Conclusion

The growing demand for high-quality products at a low price drives the requirement for accurate, meaningful, and simple test methods as a quality control and development instrument.

This article demonstrates how the torque rheometer system HAAKE PolyLab OS can be essentially used to address the aforementioned challenges. Using a single measuring system, three different test methods can be conducted. Changeover time from one test to another is reduced due to the modular nature of the PolyLab system. This time-efficient workflow permits a high number of experiments to be conducted in a short duration of time.

Linking the real world manufacturing process with the test results is crucial to success. As a scaled-down production system, the PolyLab OS can realize meaningful processing parameters in a laboratory setting that closely simulates full-scale production. With a single instrument, it is thus feasible to formulate, generate test specimen, and characterize the samples.

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

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

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