Silane Cross-Linkable Cable Compounds 2018-06-21T10:25:14+00:00

Silane 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. Silane cross-linkable cable compounds are also known as PEX-b or SioplasĀ®.

SioplasĀ® was developed in 1968 by Midland Silicones (now Dow). In the first stage of this two-stage process, vinyl silane is grafted onto the polymer chain with peroxide activation. In the second stage, pipe is conventionally extruded from the grafted compound with a catalyst masterbatch. Cross-linking takes place offline in an aqueous or steam environment. Ever since this process was developed, Buss Kneaders have been used in the first stage and for producing the catalyst masterbatch.

For cable insulation up to 10 kV, silane cross-linkable PE compounds have been globally proven as the most cost-effective materials and are produced in large quantities.

Compounding requirements

The grafting process requires precise control of the temperature profile and retention time. A liquid silane-peroxide mixture is fed into the polymer melt by a membrane or piston pump and intensively mixed distributively. The silane molecules are linked to the polymer chains by radical grafting according to precisely defined residence time and temperatures.

For this reactive extrusion process, the Buss Kneader characteristics are of key importance. The silane-peroxide mixture can be directly injected into the melt pool at the process-optimized position. Distributive mixing immediately afterwards enables a uniformly high graft yield, and minimal scorching effects compared with other systems. The process can be monitored at any time by temperature readings at several locations in the processing section. This enables online quality assurance and control. A pressure-regulated melt pump is utilized to feed the underwater pelletizer. Each processing stage can therefore be optimized independently. The hinged housing of the Buss Kneader gives fast and easy access to ensure high system availability. High throughputs are easily attainable.

The modular design of the entire Buss Kneader line makes it easily adaptable to all requirements. Together with the widely based Buss processing expertise, the Buss Kneader has always been first choice for compounding Silane cross-linkable cable insulation with the highest possible quality and investment security.

Typical plant layout for SioplasĀ®

BUSS compounding systems offer the following specific benefits

  • 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.

  • 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.

  • 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.

  • Precise temperature control
    The Buss Kneader has been well known for precise temperature control since more than 6 decades. This is achieved via controlled energy input due to uniform, moderate shear rates and precise temperature monitoring by thermocouples mounted in pins along the barrel.

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