Rubber Compounds 2018-05-17T09:15:02+00:00

Rubber Compounds / Elastomers

Elastomers are the third most important group of plastics after thermoplastics and thermosets. Elastomers are dimensionally stable, but elastically deformable plastics, whose glass transition point is below room temperature. They can deform under tensile, shear and compressive load, but then return to their original undeformed shape.

Chemically, elastomers are macromolecules that are irreversibly connected to each other only by a few wide-meshed crosslinking bridges. With thermoplastic elastomers, these crosslinking bridges can be annealed under the influence of heat so that they exhibit a thermoplastic behaviour (see separate document / description). Modifications can be used to produce elastomers with tailored hardness degrees, crosslinking densities and application temperatures.

Compounding requirements

Excavations in Guatemala have shown that already in the 3rd century, the Maya knew rubber as a material. Since the 18th century, more and more applications are described. Invention of the vulcanization process by Charles Goodyear in 1839 enabled stable elastic properties both in cold and hot conditions, and thus the breakthrough to technical applications as well. At the beginning of the twentieth century, German chemists succeeded in producing synthetic rubbers.

Traditionally, elastomers have been produced on internal mixers or rolling mills. These established processes offer advantages such as maximum flexibility, precise offline dosing of even the smallest formulation proportions, as well as variable dwell and mixing times. However, limitations such as high specific energies, interim storage times and also property fluctuations between individual batches, have underlined the advantages of continuous processing. These typically include uniform process conditions, narrow residence time distribution, integration of process steps and improved product quality consistency. The Buss Kneader can offer even more advantages with its precise temperature control, the ability to mix high proportions of filler in an excellent and yet gentle way, and also the ability to inject liquid ingredients such as softener oils or reagents directly into the process zone at the optimal position.

The widespread and highly successful use of the Buss Kneader for silicone and fluoroelastomers has been known for decades. Recently, a wide range of other application fields has been added. The integration of process steps, the expansion of requirement profiles and the use of alternative recipe components play the main roles.

Sophisticated processes are used to accommodate and implement these high standards. In the Buss technical centre, processes can be developed, optimized, and the first scale-up steps realized with a throughput factor of 10. With the Buss Kneader the compounding and pressure build-up stages can be directly flanged or decoupled so that they can be independently optimized. In the case of elastomers, pressure build-up is usually by a flange-mounted discharge extruder for pelletizing or further processing. The split barrel of the Buss Kneader ensures fast access and high system availability. Together with broadly based Buss process expertise, the modular design and therefore adaptable arrangement of the entire compounding line makes the Buss Kneader an excellent choice for compounding elastomers.

Typical plant layout for Calendering

BUSS compounding systems offer the following specific benefits

  • Large number of mixing cycles
    A larger number of mixing cycles is achieved with the latest Buss multiple-flight Kneaders. The unique new screw designs enable maximized splitting and recombining of the compound mass, with numerous striations and excellent mixing over a very short process length.

  • High filler loadings
    Filler loadings up to 90% are possible with Buss technology thanks to 2 or 3 feed openings, separate gravimetric feeding of filler, removal of trapped air by back venting, and excellent conveying efficiency. The moderate shear rates allow perfect handling of the highest viscosities at such high loadings.

  • Uniform, moderate shear rates
    Uniformly moderate shear rates allow controlled mixing at low-temperatures while imparting only the required shear for the task at hand. The narrow shear rate distribution compared to alternative systems ensures uniform shear histories for every individual particle. This results in high quality compounding with reduced energy input.
  • Narrow temperature range
    A narrow temperature range can be maintained with a Buss Kneader due to consistent and moderate shear rates. Therefore, the typically excessive temperature peaks of alternative systems are eliminated. This enables precise temperature control. Temperatures are monitored by thermocouples inserted in the mixing pins, which are surrounded by polymer along the barrel.

  • Degassing of volatiles
    Volatiles are typically removed by a vacuum degassing opening at the end of the barrel or optionally in the discharge unit. Continual highlevel compound surface renewal is achieved with the large number of mixing cycles, striations and foldings created by the Buss Kneader, thus enabling entrapped air or volatiles to be removed completely.

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