Peroxide Cross-Linkable Cable Compounds 2018-05-17T09:06:24+00:00

Peroxide Cross-Linkable Cable Compounds

Polyethylenes (PE) have outstandingly good insulation properties. A temporary temperature rise, e.g. due to peak loading, can however cause a complete breakdown of PE cable insulation. The relatively low softening or melting point leads to thermo-mechanical failure or even dripping of the Insulation. Cross-linked polyethylene (PEX) is more thermosetting than thermoplastic. It can therefore be used at significantly higher operating temperatures, is mechanically stronger, more resistant to organic liquids, and often enables thinner wall thicknesses. Various cross-linking methods are used.

Typical applications

Peroxide cross-linkable cable compounds, are also known as PEX-a, have been used for the cable industry ever since the process was patented in the 1960s. The base polymers, peroxide and additives are first mixed intensively at temperatures maintained below 130°C. The cross-linking process takes place directly afterwards on a pressurized high-temperature CV (Continuous Vulcanisation) line. Due to its relative complexity, this technology is used above all for heavy-duty cables. Thanks to the development of water tree resistant formulations and processes, peroxide cross-linkable XLPE is increasingly used for medium voltage (MV) and high voltage (HV) AC cable insulation and for HV DC cables they are the preferred insulation material. Underground power cables for urban supplies are increasingly replacing overhead lines. The same insulation material is very often used for cables connecting offshore wind farms to the grid, and for undersea cables in general. These are global megatrend markets, with high volume and high growth rates accordingly.

Compounding requirements

The formulation components for these applications have to be intensively mixed at strictly controlled temperatures, with high plant and product purity. For MV applications the liquid peroxide is directly injected into the melt pool and intensively mixed at low temperatures. The compound is then melt filtered and pelletized. For HV applications, the compounding line is designed and operated to ensure the highest purity. The base polymers are prefiltered and contamination sources are eliminated both internally and externally, so that production takes place under cleanroom conditions.

The success factors of the Buss Kneader in this connection are as follows. Peroxide is directly injected into the process zone and optimally distributed by intensive mixing immediately afterwards. Uniformly moderate shear rates make sure that this occurs at low process temperatures with the least possible polymer breakdown. The cascaded line arrangement of the Buss Kneader and discharge extruder, furthermore enables product-considerate melt filtering and pelletizing as well as optimal and efficient equipment start-up and operation.

As a reference for this industry, all the compounding lines successfully realized so far exemplify the perfect fusion of Buss expertise in process technology, plant engineering and design.

Typical plant setup for peroxide cross-linkable cable compounds

BUSS compounding systems offer the following specific benefits

  • 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.

  • Liquid injection at any position
    Injection pins that can be mounted at any pin position along the process section allow liquid additives injection directly into the molten polymer. These are encapsulated and mixed in instantly.

  • Intensive distributive mixing
    The Buss Kneader achieves intensive distributive mixing because the combined rotation and axial motion of the Kneader screw generates extensional flow, a large number of shear interfaces, and cross channel mixing.

  • Temperature monitoring at any position
    Temperature control within the limits of the compound can be monitored by replacing mixing pins with thermocouple pins anywhere along the process section. Accuracy is assured since the thermocouples are constantly surrounded by molten compound. This feature enables online quality control.

  • Engineered scale-up process
    Buss Kneader scale-up processes, from laboratory to pilot to production size, are known for their reliability and robustness. The latest fluid dynamics and numerical simulation technologies, plus over 60 years of experience allow exact prediction and scale-up process engineering, whether surface-based or volumetric scale-up.

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