Performance-driven black selection system.

The diverse and demanding performance requirements of today's industrial rubber products market have created the need for new solutions for achieving these enhanced product specifications. Cabot IRB has developed new, advanced carbon blacks to meet the specific applications needs of this market. These innovative new products have been introduced under the Sterling trademark.

Prior to development of these products, the choice of carbon black grades for a given application was limited, with compounders selecting from among the standard ASTM grades as shown in the traditional carbon black product map for industrial rubber products (figure 1). Additionally, the choice between two closely related carbon black grades was not always perceived as having a significant impact on overall compounding or end-product performance.

That perception is changing, with rubber chemists utilizing differences in carbon black characteristics to manipulate compounds in terms of processing, performance and cost. Thus, the new product development efforts reflect both the increased awareness of carbon black's impact on industrial rubber applications and the growing demand throughout the industrial rubber market for enhanced product quality and performance.

Each Sterling carbon black is designed to meet precise performance specifications for a range of industrial rubber product applications, which allows rubber chemists to select the grade that best meets their end-use performance requirements. In addition, selection of the appropriate custom grade can contribute to lower overall compounding cost through a number of factors that range from ease of dispersion to reduced process scrap [ref. 1].

As new products that fall outside of the traditional carbon black product map are introduced, the overall number of carbon blacks available specifically for industrial rubber applications is increasing. While this, in theory, means that rubber chemists have more carbon black options in achieving product specifications, it was recognized that the optimum benefits of enhanced carbon black choice would not be realized unless the rubber chemist was provided with a reliable and accurate way to assess a given grade's effect on rubber properties.

Thus, in developing these new carbon black products for the industrial rubber market, we have also sought to take some of the uncertainty out of carbon black selection by creating a unique product selection system that provides information on a product's rubber performance properties - that is, the relationship between the carbon black's properties and those of rubber compounds in which it is used.

Carbon black selection system

The four-digit carbon black selection system addresses a long-standing problem for carbon black customers - determining what carbon black to use for specific applications and performance requirements. It does so by indexing a given carbon black's effect on key rubber performance properties: hardness, extrusion and damping. In this way, the system speaks in terms meaningful to the rubber chemist rather than in terms related to carbon black properties (figure 2).

Hardness index

In recognition of its importance to the rubber chemist, the first digit of the system is the hardness index, where numerical designations from 1-9 correspond to an increasing ability to build hardness in rubber compounds. Hardness is a complex function of many carbon black properties - all of which make it difficult for a rubber chemist to predict the ability of a carbon black to build hardness and the effects on other compound properties. With the carbon black selection system, the process becomes easier: The higher the hardness index, the greater the ability of the carbon black to build hardness at equal loadings. Conversely, a lower index denotes the ability to increase loadings of a given carbon black to achieve an equivalent hardness level.

Figure 3 demonstrates the relationship between the hardness index and actual hardness measurements for selected carbon blacks at 160 phr in EPDM.

Extrusion index

The second digit of the system is the extrusion index. In general, the higher the number (again, ranging from 1-9 and assuming equal loading in the comparison), the lower the extrusion shrinkage or die swell. Lower shrinkage facilitates both die design and process stability. Extrusion shrinkage also has a major impact on the aesthetics of extruded parts.

Achieving good dimensional stability in the extruded part is related to the elastic behavior of the rubber compound, exemplified by the degree of die swell/extrusion shrinkage. Selecting the appropriate carbon black for a given polymer can effectively minimize the die swell influence, resulting in good dimensional control of the extruded profile. The system provides guidance in choosing a carbon black that achieves desired extrusion properties while maintaining comparable hardness and/or resilience.

Figure 4 demonstrates the relationship between the extrusion index and actual extrusion shrinkage measurements for selected carbon blacks at 160 phr in EPDM.

Damping index

The third digit of the system is the damping index. Several terms are used interchangeably to describe damping, including tan delta and hysteresis. These terms are directly related, as higher damping corresponds to higher tan delta and higher hysteresis. Damping is also characterized as the inverse of rebound - the higher the damping index for a given carbon black, the greater the compound's damping and energy absorption characteristics at constant carbon black loading. On the other hand, the lower the index number, the lower the damping and higher the rebound the resulting compound will have at equal carbon black loadings.

A rubber compound's damping characteristics are also greatly affected by carbon black properties and by carbon black loading, with higher loadings giving higher damping for any given carbon black. Damping may vary as carbon black loading is adjusted to maintain constant hardness. In terms of rubber compound applications, the damping level of a compound can also relate to other properties such as heat build-up. In general, the higher the Damping Index, the higher the heat build-up and abrasion resistance at equal carbon black loadings.

Figure 5 demonstrates the relationship between the damping index and actual rebound measurements for selected carbon blacks at 75 phr in neoprene.

Product identifier

The fourth number in the system, nominally designated as 0, is intended for new application-specific carbon black solutions.

With this new product selection system, customers will be able to determine at a glance how a given carbon black will affect the performance of industrial rubber products. A new carbon black's performance properties can readily and easily be compared with a product already in use, with no more uncertainty or confusion as to the effect carbon black properties have on rubber performance properties.

The system is based upon actual rubber compound performance properties of carbon blacks over a range of carbon black loadings and in a variety of polymer systems. Its development was based upon correlation of several years of performance data with carbon black properties [ref. 2].

We found that, while the magnitude of difference in performance properties between grades of carbon is affected by carbon black loading and changes in polymer systems, the overall ranking of carbon blacks for hardness, extrusion and damping remains consistent. This allows the use of the selection system by all users of carbon black in industrial rubber applications.

Practical applications

Until now, the rubber chemist has only been equipped with general guidelines for relating the distinctive properties of carbon black that influence the basic processing and vulcanization properties of rubber compounds.

While the old guidelines are accurate and easily applied as long as change in only one carbon black property is involved, it is usually the case that many properties change when choosing one carbon black over another in an effort to enhance rubber compound performance. As a result, the effects on hardness, extrusion and damping are often difficult to predict. The new system helps the rubber chemist to determine the extent of these effects relative to the desired outcome. Below, a few practical examples are shown using the system.

Perhaps the simplest illustration is to compare S 6640 carbon black and S 6740 carbon black, two new products that differ primarily in a single property. The system indicates that this difference between these carbon blacks will result only in a difference in extrusion shrinkage. In a comparison of the two products at equal carbon black loadings of 150 phr in EPDM (table 1), S 6740 carbon black has lower extrusion shrinkage than S 6640 carbon black, while hardness and rebound are equivalent.

  Table 1 - comparison of S 6640 and S 6740
      carbon blacks at 150 phr in EPDM

Carbon black  Hardness     Extrusion    Rebound (%)
              (Shore A)  shrinkage (%)

S 6740          71.0          6.1          36.0
S 6640          70.0         10.3          35.0

For a rubber chemist using S 6630 carbon black and desiring higher damping, the system indicates that S 6640 carbon black would be an appropriate choice. S 6640 carbon black offers greater damping than S 6630 carbon black without significant changes in either extrusion shrinkage or hardness properties. By contrast, this choice could not easily be made if selecting a carbon black solely on the basis of its analytical properties.

If the rubber chemist is concerned with minimizing damping while maintaining good extrusion stability properties, he or she would choose S 4620 carbon black over S 5630 carbon black as it offers lower damping with comparable good extrusion shrinkage. Table 2 shows a comparison of S 4620 and S 5630 carbon blacks at 160 phr in EPDM. In this case, the extrusion shrinkage does not change. Because of the relatively low carbon black loading, the magnitudes of difference in hardness and rebound are not large. However, as predicted by the hardness and damping index numbers, S 4620 carbon black has lower damping (higher rebound) and lower hardness than S 5630 carbon black.

  Table 2 - comparison of S 4620 and S 5630
      carbon blacks at 60 phr in EPDM

Carbon black  Hardness     Extrusion    Rebound (%)
              (Shore A)  shrinkage (%)

S 4620          58.0         33.6          71.4
S 5630          60.0         33.6          68.6

For the rubber chemist using an ASTM semi-reinforcing (SRF) carbon black but needing better resilience, the system would indicate that the new product, S 1120 carbon black, offers the lowest damping (or best resilience) and lowest hardness at equal carbon black loading of any existing, standard furnace grade. An example of this is shown in table 3. In addition, S 1120 carbon black allows the rubber chemist to use higher loadings of carbon black at equal hardness and damping compared to an SRF grade, thereby reducing overall material costs.

  Table 3 - comparison of S 1120 carbon black
          and ASTM N762 carbon black

Carbon black  Loading  Hardness   Tan delta
               (phr)   (shore A)

S 1120          40         49       0.060
ASTM N762       40         55       0.072
S 1120          60         58       0.078
ASTM N762       60         63       0.105

Summary

With the dual introduction of advanced, new carbon blacks for industrial rubber applications and a product selection system that allows rubber chemists to easily match rubber product specifications with the appropriate carbon black, we are striving to meet today's market needs and to pave the way for future advances in industrial rubber applications.

While Cabot IRB's new product selection system can be applied to Sterling carbon blacks that are now commercially available, its broader application ties in with the company's ongoing product development program through which additional custom grades are being designed and produced to meet precise customer specifications.

References

[1.] Monthey, S., Duddleston, B. and Podobnik, J., "New blacks address compounding challenges," Rubber World 210, No. 3, June 1994. [2.] Funt, J.M. and Chung, B., "Industrial rubber products: Carbon black and compound performance," Rubber World 202, No. 2, Max. 1990.