Compounding with medium thermal black.

The unique morphology and chemistry of medium thermal can be translated into excellent rubber compound properties. Thermal blacks were once used as low cost rubber compound extenders in order to reduce the pound volume cost of the product. This rubber function of medium thermal black has in many cases been replaced by less expensive non-black filler systems. Today thermal black is utilized as a functional filler in a wide range of rubber products. Many rubber goods require medium thermal black in order to meet highly demanding performance specifications. When medium thermal is incorporated into a rubber compound, not only is the performance of the rubber compound improved but many rubber processing advantages are realized.

Processing

* Short mixing times: The large particle size of medium thermal facilitates fast incorporation of the black into the thermal facilities fast incorporation of the black into the polymer stock. While ease of mixing would be expected to be hindered by low structure, thermal black's large particle size is the overriding factor. For many low viscosity elastomers, powder grades of MT are used due to their ease of incorporation.

* Cool processing: Lack of interaction between the thermal particles and elastomer chains prevents heat buildup in the mixer and allows for lower batch temperatures.

* Less polymer breakdown: Medium thermal black reduces the nerve of the polymer and does not increase the viscosity to any large degree. Lower shear forces result which are less likely to destroy the long chain molecules.

* Less power consumption: Since medium thermal black does not greatly increase the viscosity of the compound, less energy is consumed mixing the compound.

* Long mixer life: Reduced power consumption and cooler internal mixer temperatures will prolong the life of a mixer running predominantly thermal black compounds as opposed to various furnace compounds. The graphitic layer plane surface nature of thermal is essentially non-abrasive (less abrasive than soft clay). In contrast, furnace blacks are considered abrasive. Lack of abrasion reduces the amount of wear on the surfaces of an internal mixer.

* Flowability: Compounds containing medium thermal readily flow into intricate molds due to low compound viscosities.

* Good extrusion characteristics: Thermal use in extrusion compounds provides for fast extrusion rates, low die swell and excellent dimensional control.

Performance

* Dynamic properties: Elastomers compounded with medium thermal black exhibit good dynamic properties. As a result, medium thermal is often the black of choice in numerous dynamic rubber applications. Seals, o-rings, hoses, rolls, motor mounts, automotive body mounts, calendered goods, extrusions, sponge and belting all benefit from the incorporation of thermal black into the compound. Compounds that require high rebound resilience and low hysteresis often contain medium thermal black to achieve these properties. Even at high loadings of thermal black high resilience and low heat build-up can be maintained.

Compared to other carbon blacks, thermal provides lower transmissibility (defined as the ratio of the transmitted force to the applied force) at high frequencies. This property is important for rubber parts that must support vibrating machines such as automotive engines. Hence the widespread use of thermal black in engine mounts.

Thermal black is often incorporated to provide a specific rubber property (good rubber to metal bonding, flex resistance, low hardness, etc.) and at the same time dynamic performance, characteristics can be maintained.

* Sealing characteristics: Many dynamic rubber articles often must support a load and maintain their original height within tolerances. Under a constant load or deformation a rubber part will change in dimension. This property is measured by the compression set test. In order to maintain a good seal, compression set must be minimized. Elastomers compounded with medium thermal black exhibit a very low degree of compression set. Diaphragms, sponge, gaskets, seals, o-rings and hoses are examples of compounds requiring good sealing characteristics and hence good compression set.

The softness of a rubber article can also play a role in sealing characteristics. Hardness is not greatly increased via the addition of thermal black and hence its incorporation into many soft rubber compounds. For soft rubber applications, such as sponge, powder grades of MT are often employed.

Elastomer compounds containing thermal black often have a very smooth finish. This property combined with the non-abrasiveness of thermal black can reduce the wear of rubber parts and materials in which they move against. The performance of o-rings and seals that are in constant contact with a moving part can benefit from the above properties.

* Low gas permeability: Low air or gas permeability can be achieved in hoses, membranes, thin calendered goods, various rubber liners, plugs and tire inner liners when medium thermal is used. Medium thermal black has the widest particle size distribution (80 - 550 nm) of any carbon black. This coupled with the fact that the particles are essentially spheres allows it to minimize void spaces at high loadings. In effect, thermal black at high loadings creates longer, more torturous diffusion paths for gas molecules.

* Chemical and heat resistance: The stable, inactive surface of medium thermal and relatively low surface area prevents or reduces its chemical interaction with oils, fuels, fuel additives and down hole oil drilling chemicals. Thermal black also has excellent heat resistance. Many high performance elastomer compounds, such as nitriles, hydrogenated nitriles and fluoroelastomers, can incorporate medium thermal without serious degradation of their properties and the resulting compound can thereby withstand harsh environments. Many automotive and aerospace rubber parts benefit from the marriage of high performance elastomers and medium thermal carbon black. In fluoroelastomers, it is recognized as the best general purpose filler.

* Electrical insulation: Medium thermal black has excellent dielectric properties and when used as a filler it allows various elastomers to maintain their high volume resistivity. The large particle size and low structure of medium thermal is not conducive to the mechanism of semi-conductivity, namely electron tunneling. The insulating value of many polymers is maintained when thermal black is added. SBR insulating materials use thermal black as do neoprene automotive wire compounds and butyl rubber capacitor plugs. One of the largest applications of medium thermal carbon black is in low voltage cross-linked polyethylene power cable. In this application not only does thermal black extend the compound, maintain dielectric properties and reduce the nerve of the polymer, but it provides ultra-violet radiation protection and flame retardancy. Thermal black is also used as a filler in heat shrinkable electrical connectors.

* Highly extended compounds: Since medium thermal black does not seriously degrade inherent rubber properties, it can often be compounded at considerably high loading levels. This practice of high loading levels can lower the pound volume cost of elastomers in which thermal black is cheaper than the elastomer. Neoprene mechanical rubber goods quite often contain 100-200 parts per hundred of thermal black

In many rubber applications medium thermal is combined with a more reinforcing carbon black such as N330, N550, N660 or N762. By adding medium thermal, a reinforced rubber compound's dynamic properties can be improved. To a limited extent, thermal black is used as a colorant in rubber and plastics. Its low surface area provides a blue-grey tone. Typical applications include polyethylene, ABS, PVC, polypropylene battery separators and silicone automotive engine head gaskets.

While all thermal blacks have similar physical properties, not all thermal blacks have the same quality characteristics. One of the key features of all carbon blacks is sieve residue or "grit." Sieve residue is a measure of non-dispersible contaminants, such as coke, refractory particles, water soluble salts and metal oxides, as expressed as a percent of the carbon black by weight. High grit levels can create dispersion problems. Non-dispersed particulates can result in the catastrophic failure of certain rubber compounds under critical demanding performance parameters. In low voltage power cable, non-dispersed particles can result in the complete dielectic breakdown of the entire cable. Flawless surfaces are naturally required for applications such as o-rings, seals and gaskets. Non-dispersed particles can also ruin very thin extrusions or calendered goods. Maintaining extremely low levels of sieve residue is crucial in many special rubber applications.

Other characteristics which can affect the processing or performance of rubber include pH, extractables and pellet hardness. The pH of carbon blacks can affect the cure rate of compounds. Generally, a more basic pH speeds the rate of cure. Thermal blacks have a pH range of 4-11. It should be noted that pH is a function of the amount of water salts found in water used in the production process.

Trace amounts of highly unsaturated hydrocarbon oils, found absorbed on the surface of thermal blacks, are usually a dark color in nature. These oils are referred to as extractables since they can be extracted from the surface of the carbon black with a suitable solvent such as a toluene. There is the potential that these extractables can migrate to the surface of a rubber compound and stain the materials they contact. Certain rubber applications, such as jar and bottle seals and automotive weatherstripping, have staining limitation requirements. Hence the non-staining or stainless grades of thermal black.

Pellet hardness and size can affect the quality of compounds in terms of batch uniformity. Harder pellets require more energy for dispersion into a rubber compound and larger pellets tend to be harder. Therefore, a relatively soft (i.e. small), uniform pellet size is most desirable for consistent ease of processing into the polymer stock. A narrow pellet size distribution is also required for good flow characteristics. A readily free flowing carbon black can then be used in automated pneumatic conveying systems.

Conclusion

Today, more and more demands are being placed on rubber goods, especially in automotive and aerospace applications. The unique characteristics of medium thermal carbon black give it advantages in the manufacturing of many high performance and specialty rubber goods. While the largest use of carbon black will continue to be for the manufacture of tires (MT is not a preferred carbon black for this application), medium thermal should continue to find specialized niches. Either alone or in combination with other furnace blacks, it gives unique physical and processing advantages in many elastomer compounds.

References

1. Carbon black: Physics, chemistry and elastomer reinforcements, J. Donnet, Marcel Dekker, Inc., New York 1976. 2. Vanderbilt Rubber Handbook, 12th Edition, ed. by R. Babbit, R.T. Vanderbilt Co. Ltd., Norwalk, CT 1978. 3. Rubber Technology, 3rd Edition, ed. by M. Morton, Van Nostrand Reinhold Co., New York 1987. 4. Vanderbilt Rubber Handbook, 13th Edition, ed. by R. Ohm, R.T. Vanderbilt Co. Ltd., Norwalk, CT 1990. 5. Analysis of carbon black, B Schubert, Encyclopedia of Industrial Chemical Analysis, Volume 8, John Wiley & Sons, Inc., New York 1969.