Semi Conductive Cable Compounding

Semi conductive compounds for cable applications

Medium-voltage (MV) and high-voltage (HV) cable insulation is enclosed between two concentric semi conducting layers to homogenize the electrical field in the insulation material. These layers generally comprise ethylene-based polymers like EVA, EBA or similar, with a high content of conductive materials such as carbon black, graphite, or carbon nanotubes to effect semiconductivity.

Not enough attention was paid at first to how these semi conductive layers affect the insulation material. It was soon discovered, however, that ions in the semiconductors can infiltrate the insulation between the layers and lead to the formation of water trees.

By suitably adjusting the chemical composition of the polymers and respective additives, the outer semi conducting layer can easily be removed from the insulation. This can be useful for cable installation in some cases, and is still required by the regulations in France and the USA. Other countries are increasingly dispensing with the requirement for an easily removable semi conducting layer, whether for cost reasons or because excellent peeling tools are now available. For EPR based compounds, an easily removable semi conducting layer is still required because mechanical removal is difficult. Volumes are rising in pace with the development and growth of polymer MV/HV cable insulations.

Compounding requirements for semi conductive cable compounding

The process requirements for semiconducting cable compounds are very demanding. Conductivity values must be dependable both at room temperatures and at higher operating temperatures. Highly structured carbon black must remain unchanged and be distributed extremely homogeneously to form the required conductive network. The polymers used must be degraded as little as possible. The surfaces of the coextruded layers must be very smooth and uniform. Usually peroxide-based cross-linking is required.

The BUSS Kneader within the semi conductive cable compounding systems meet these complex requirements thanks to its specific strengths: The high content of conductive additives can be introduced at several feed points along the processing section. Immediate mixing at moderate shear rates, with intermediate relaxation phases, ensures excellent distribution without damaging the internal structure of the polymers and conductive materials. The system design flexibility also enables specific attention to the increased viscosity in the process zones through targeted configurations. This allows custom-tailored monitoring and control of the process conditions.
Since the Buss Kneader’s two-stage system separates compounding and pressure build-up, processing is optimized for best possible product quality and throughput, independently of pressurization, melt filtration and pelletizing.

The modular design of the entire BUSS Kneader line makes it easily adaptable. Together with the widely based BUSS processing expertise, the BUSS Kneader has always been a good choice for almost all needs of semi conductive cable compounding in many parts of the world and for many output rate requirements.

Typical plant layout for semi conductive cable compounding systems

Typical plant layout for semi conductive cable compounding systems

BUSS semi conductive compounding systems offer the following specific benefits

  • Intensive mixing at low specific energy input
    BUSS multiple-flight compounders of the latest generation achieve better mixing at 15-40% lower overall specific energy input. This is because of an increased number of mixing cycles according to the needs of each individual process section. The energy for melting and mixing is provided almost entirely mechanically and optimally dissipated according to the imparted shear rates.

  • Limited damage to polymer and high-structured fillers such as carbon black
    The BUSS Kneader’s uniform and moderate shear rates result in controlled shear and low temperature profiles, stressing the structure of fibres and highly-structured fillers such as carbon black much less than with other systems. This leads to better mechanical and electrical properties, improved flow characteristics, and lower consumption of expensive formulation components.

  • High filler loadings
    Filler loadings of up to 90% are possible with BUSS compounder technology owing to partitioning into two or three feeder openings, the use of feed-in vessels such as side-feed screws, 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.

  • 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, with numerous striations and excellent mixing over a very short process length.

  • Uniform, moderate shear rates
    Uniformly moderate shear rates allow controlled mixing in the compounder at lower 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.

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