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Role of carbon black in lowering overall compound costs.

In today's industrial rubber parts business, manufacturers are under intense pressure to control and, in most cases, reduce the cost of their products. Perhaps the most telling illustration of this trend is in the automotive parts market, where car makers are demanding better pricing from their parts suppliers without sacrificing part performance. In many cases, auto manufacturers are pushing for both improved performance and lower costs.

Expanded role for carbon black

Given the fluctuations in pricing for various components of a formulation, compounders looking to achieve overall cost reduction goals must either reduce the cost of materials through reformulation or find savings through improved processing efficiency and minimized scrap rates. In both instances, carbon black can - and should - play a role in helping manufacturers meet their cost goals. Until recently, however, carbon black suppliers have not kept pace with the changing needs of this market.

For example, in the mechanical molded goods market, carbon blacks have often been selected first for their compound extension capabilities and second for their resulting reinforcement properties. This is because: specialty polymers for molded products range in price per pound from three times to 30 times that of carbon black, meaning that compound costs can go down significantly as carbon black loading increases; and, most static and some dynamic molded goods do not require high levels of physical strength, which is often compromised by compound extension.

Traditionally, semi-reinforcing grades such as N762 and N754 carbon blacks have been the choice for delivering some level of compound extension capability while providing required reinforcement properties for molded applications. MT (N990) grades have also been used for higher levels of compound extension. However, manufacturers have had to balance improvements in compound extension with further compromises in physical properties such as tensile and tear strength. In some formulations, a 50-50 blend of N990 and N762 has been used to attain some of the compound extension benefits of the former and the physical properties of the latter - a compromise that introduces new mixing and processing complications from using two blacks instead of one.

However, these conventional grades have clear limitations in their ability to provide cost savings through compound extension - and molded parts manufacturers have long since taken their formulations to those limits. So, in an environment where price and performance pressures in this market continue to escalate, conventional carbon blacks simply cannot deliver additional compound extension capabilities without sacrificing performance.

This same concept holds true for other industrial rubber product applications. Whether for extruded automotive parts such as door seals and glass run channels or for any number of demanding under-hood applications, most carbon black manufacturers continue to offer the same products that were in use 20, 30 or even 40 years ago. As a result, rubber parts manufacturers have come to see carbon black's price as its key influence on compounding costs, rather than looking at carbon black as a source of new technical solutions for meeting their cost reduction challenges.


When discussing carbon black's effect on overall compound costs, there are several factors that must be considered. As noted above, the compound extension capability of a given carbon black is a key indicator of its ability to influence over-all costs. However, additional compound extension must also be balanced by processing and end-product performance considerations.

If, for example, a new carbon black can offer additional compound extension compared to conventional grades without sacrificing product performance or without introducing additional processing variables, then that carbon black may in fact have real potential for reducing overall compound costs. However, if the new product not only provides additional compound extension capabilities but also delivers improved processing and performance compared to conventional grades, the ability of this carbon black to meet manufacturers' cost and performance requirements becomes more pronounced. In developing the Sterling line of advanced carbon blacks, Cabot IRB has sought to address all aspects of the cost/processing/performance equation. The result is carbon black products that deliver added value and cost-saving potential on several levels.

Sterling 2320 carbon black

Sterling 2320 carbon black delivers excellent value in molded products such as seals and other mechanical goods components through the combination of greater formula extension capabilities, better processability and improved physical properties. Together these performance features can result in overall lower compound costs compared to conventional carbon black grades (ref. 1).

Because it is engineered to build hardness more slowly, ST-2320 carbon black can be used at significantly higher loadings than conventional grades at equal hardness. In some polymers, ST-2320 carbon black can be used at loadings of 20 to 40% higher than SRF grades. Higher loadability translates into increased compound extension capabilities and cost savings for manufacturers.

In addition to increased compound extension capabilities, ST-2320 carbon black is designed to deliver improved physical strength compared to conventional grades. This has an important impact on both manufacturing efficiency and end-product performance. Compared to compounds with conventional grades at equal hardness, compounds containing ST-2320 carbon black can exhibit better tear strength both at room temperature and the higher temperatures associated with demolding operations. Improved hot tear strength can result in several processing advantages, including:

* faster cycle times due to easier part removal from the mold;

* reduced in-process scrap rates for additional material savings; and

* less need for 100% part inspection.

In terms of end-product advantages, the improved tear strength achieved with ST-2320 carbon black will most often increase the functional performance life of molded components. ST-2320 carbon black also delivers desired stress/ strain performance - even at the significantly higher loading levels used to achieve equal hardness.

Another unique processing advantage exhibited by ST-2320 carbon black is its effect on compound viscosity. Normally, higher carbon black loading results in increased Mooney viscosity and more difficult processing characteristics. However, when used at higher loading levels to achieve equal hardness, compounds containing ST-2320 carbon black exhibit equal or lower Mooney viscosity compared to compounds with conventional grades. This means manufacturers can enjoy improved mixing, handling and improved extrusion and mold flow characteristics in addition to the cost and performance benefits outlined above.

ST-2320 carbon black in NBR

In a variety of polymers used in molded applications, ST-2320 carbon black exhibits higher loadability at equal hardness compared to conventional grades. For example, in NBR compounds commonly used for a variety of oil-resistant seals, o-rings and gasket applications, ST-2320 carbon black shows increased loadability at all hardness levels compared to N762 and N754 grades. When compared to a 50-50 blend of N990 and N762, this product can be used at equal loading while also delivering improved processing and performance features.

The relationship between hardness and tear strength in NBR is illustrated in figure 1. ST-2320 carbon black exhibits higher tear strength than the N754 grade and N990/N762 blend at all hardness levels and comparable tear strength to N762.

When viewed together, the combination of loadability and physical performance at a given hardness provides a clear indication of ST-2320 carbon black's advantages over competitive grades. Figure 2 illustrates that in NBR compounds at 70 durometer, ST-2320 carbon black exhibits improved tear strength compared to N754 and the 50-50 blend of N990/N762, and significantly higher loadability compared to N762 and N754 grades.


ST-2320 carbon black in polychloroprene

Because of the relatively high cost of polychloroprene (CR), there is a clear benefit to achieving maximum compound extension. Figure 3 shows that ST-2320 carbon black exhibits higher loadability than N762 and comparable load-ability to N990/N762 blend at equal hardness.


However, physical strength is also a key requirement for many CR applications. ST-2320 carbon black delivers significantly better tear strength (room temperature and 100 [degrees] C) and tensile strength compared to N762 and the 50-50 blend of N990/N762.

ST-2320 carbon black in other polymers

Comparative testing in other polymers like EPDM, NR and IIR confirms similar extension properties to those seen in NBR and CR. Loadings as high as 30% more for ST-2320 carbon black are reasonable, with negligible change in physical strength compared to N762 and no compromise in strength when replacing N754.

Sterling 4620 carbon black

Sterling 4620 carbon black was developed to deliver a number of processing and performance advantages over conventional grades such as N550 and N650 in extruded and hose products. As with the previous example, these advantages mean that in many instances manufacturers can achieve both lower compound costs and improved performance.

One of the ways in which ST-4620 carbon black can help lower compound costs is by allowing for higher carbon black loading and increased compound extension. As shown in figure 4, ST-4620 carbon black can be used at loadings as much as 20% higher than conventional grades at equal hardness without any loss of processability or end-product performance.

Manufacturers are also looking for savings through improved productivity and lower scrap rates. ST-4620 carbon black can contribute to improved processing and performance even at higher loadings. For example, compounds using this product exhibit lower die swell than those using conventional grades. This, along with the lower viscosity provided by ST-4620 carbon black, allows manufacturers to increase their extrusion rate for greater productivity.

This black can also play a key role in reducing scrap rates for extruded profiles because of the ease with which it is dispersed. Undispersed carbon black is a major contributing factor to surface defects in many extruded products, such as high show-surface profiles used in automotive door seal applications, and can lead to scrap rates that have a decidedly negative impact on manufacturing costs. By using a better dispersing carbon black, compounders not only achieve greater process efficiency through less mixing time but also smoother surface profiles for their extruded products.

The improved cleanliness and excellent dispersability of ST-4620 carbon black results in fewer surface defects compared to N550 and N650 grades in an extruded EPDM compound at 65 Shore A hardness. This improved performance means that many extruding operations using ST-4620 carbon black will experience scrap rate reductions significant enough to have a positive impact on the bottom line.

ST-4620 carbon black adds value not only through its contribution to process efficiency and lower costs, but also for the improved end-product performance it can deliver. For example, one area of improvement sought by extruded parts manufacturers is lower compression set. ST-4620 carbon black provides improved compression set compared to conventional grades at all durometer levels in many compounds.

Added value in demanding applications

Other unique performance attributes of ST-4620 carbon black bring added value to certain demanding applications. For example, many dense extruded parts in high-visibility automotive applications incorporate multi-metal inserts. A common occurrence with such parts is galvanic corrosion of the metal, resulting in an oxidized insert and eventual part failure. One way of mitigating this problem is to increase the electrical resistance of the extruded rubber compound. ST-4620 carbon black is an excellent fit for this situation because of its higher resistance compared to conventional grades.

Another application where the product's lower conductivity can help prevent premature product failure is in automotive radiator hose. Electrostatic charges in the under-hood environment can cause cracking and eventual product failure in radiator hose. Compared to N550 and N650, ST-4620 carbon black can reduce the incidence of cracking by lowering the conductivity of the hose. Similarly, ST-4620 carbon black exhibits a lower receptivity to microwave (WET) energy. This feature can be used to minimize the temperature differential between sponge and dense components for dual-durometer extruded parts that are microwave cured (ref. 2).

Cost reduction with carbon black

When considering raw materials for use in formulations, one intuitively might look at the cost of individual components on a strict price-per-pound basis. However, the true measure of cost for a given rubber compound is the cumulative price per pound-volume of the compound itself. This is why extension of expensive polymers with less expensive carbon black is important to cost-control measures. The ability to load more carbon black per volume of compound means that less polymer will be required per finished part.

From this perspective, the "in-the-door" price of a given carbon black does not give a complete view of the cost-saving opportunities that the product delivers. For example, the Sterling products used in illustrations above are priced a few cents per pound higher than the conventional grades to which they are compared. As shown above, however, these products can be used at significantly higher loadings than conventional grades in many applications. Because this additional carbon black displaces a comparable volume of the more expensive polymer, the compound with Sterling carbon black shows a lower "out-the-door" cost for the finished product than the compounds using conventional grades. A lower "out-the-door" cost allows manufacturers to mitigate pricing pressures on other raw materials (i.e. rising polymer prices) and/or pass on price reductions to their customers. This now gives a more complete view of material cost-savings opportunities. However, material costs are only part of the story. Because Sterling carbon blacks also deliver improved processing and performance in many applications (i.e., improved hot tear strength, better dispersion, etc.), manufacturers may also see a reduction in scrap rates for even greater savings and efficiency. This allows the Sterling products to compare favorably on an overall cost basis with competitive grades independent of compound extension considerations.


These Sterling products have demonstrated that there is room for innovation in the manufacture of carbon blacks for the industrial rubber products market. Moreover, the need for such innovation from carbon black suppliers can only increase as the challenges facing the market continue to grow.


(1.) "Sterling-2320 carbon black for molded applications: The new alternative for improved formula extension and performance," Cabot IRB Technical Bulletin, September 1996.

(2.) "Carbon blacks for automotive extrusion applications," Cabot IRB Technical Bulletin, September 1995.

Jeff lmmel is marketing manager for Cabot's Industrial Kubber Blacks business. He joined Cabot in 1995 as applications development manager for Molded Products after 10 years at Goshen Rubber.
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Title Annotation:Tech Service; Cabot Carbon Ltd.'s Sterling 2320 industrial rubber carbon black
Author:Immel, Jeff
Publication:Rubber World
Date:Jan 1, 1997
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