Paste extruded insulator with air channels

Abstract

The invention provides an insulated wire having a conductor and an insulator of paste extruded PTFE fine powder around the conductor. The paste extruded insulator has at least one closed continuous longitudinal air channel spaced apart from the conductor. The invention further provides a process for forming an insulator around a conductor by paste extruding lubricated PTFE fine powder in an extrusion device comprising a die, mandrel, and at least one channel-forming member. The die and mandrel form a converging chamber leading to an extrusion orifice and the channel-forming member is positioned in the orifice. The mandrel has a central bore for supplying the conductor. Lubricated PTFE fine powder is forced through the chamber and out of the exit of the orifice as a lubricated green extrudate around the conductor forming an insulator with at least one closed longitudinal air channel spaced apart from the conductor.

Description

FIELD OF THE INVENTION [0001] This invention relates to an insulated wire and a method for manufacturing it by paste extruding an insulator of polytetrafluoroethylene. BACKGROUND OF THE INVENTION [0002] Polytetrafluoroethylene (PTFE) fine powder has been used for many years as a wire insulator. However, despite the desirable properties of high molecular weight PTFE, namely low dielectric constant and low dissipation factor, other polymers have been used in forming insulation for wires, especially in large diameter, high frequency coaxial transmission cable because of the processing limitations of PTFE. [0003] Polytetrafluoroethylene (PTFE) fine powder is a type of PTFE that is made by aqueous dispersion polymerization, followed by coagulation of the dispersion and drying of the resultant coagulated solids to obtain the fine powder. Because the PTFE fine powder does not flow in the melt condition sufficiently to enable melt processing, the powder has been fabricated into articles and...

AI Technical Summary

Benefits of technology

[0011] In one preferred embodiment, the channel forming-member is supported upstream of the orifice. In another preferred embodiment, the extrusion device has multiple channel-forming members which form multiple closed longitudinal channels spaced apart from each other. In a more preferred embodiment, the lubricated PTFE fine powder is forced around the channel-forming member into a space between the conductor and the channel forming member pressure coalescing the lubricated PTFE fine powder into the lubricated green extrudate around the conductor. Preferably, the extrusion device has a high shear region formed at the juncture of the chamber and the orifice which causes substantial fibrillation and the pressure coalescing is performed upstream of the high shear region. Preferably, the lubricated green extrudate is dried and, most preferably, gas is circulated though the air channel of the lubricated green extrudate to facilitate lubricant removal during drying. For most applications, it is also preferred to sinter the dried green extrudate.

Problems solved by technology

However, despite the desirable properties of high molecular weight PTFE, namely low dielectric constant and low dissipation factor, other polymers have been used in forming insulation for wires, especially in large diameter, high frequency coaxial transmission cable because of the processing limitations of PTFE.

Nonetheless, PTFE has been difficult to form into large diameter insulators for wire because of difficulties in drying the green extrudate to remove the lubricant from thick structures of paste extruded PTFE fine powder.

Large diameter PTFE insulators have also been expensive because of the quantity of PTFE fine powder required.

Another significant requirement for high frequency communication cable is noncombustibility that fluoropolymers can provide but structures of polyethylene, polypropylene or polyvinyl chloride cannot.

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